Symposium Organizers
Eric Le Bourhis Universite de Poitiers
Dylan J. Morris National Institute of Standards and Technology
Michelle L. Oyen Cambridge University
Ruth Schwaiger Forschungszentrum Karlsruhe
Thorsten Staedler Universitaet Siegen
AA1: In-situ Methods I
Session Chairs
Andrew M. Minor
Benedikt Moser
Monday PM, November 26, 2007
Room 208 (Hynes)
9:30 AM - **AA1.1
Micromechanics Inside the SEM: What Can We Learn From In-situ Experiments?
Benedikt Moser 1 2
1 , Alcan Technology & Management AG, Neuhausen Switzerland, 2 , Empa, Materials Science and Technology, Thun Switzerland
Show AbstractA large variety of in-situ mechanical experiments have been developed over the last 10 years in a number of research institutes. Many of those experiments can be performed inside the scanning electron microscope, some of them even in the transmission electron microscope. Also instrumented indentation techniques have been developed to be performed in-situ. These techniques offer a wealth of new information not only about the behaviour of materials on a small scale but also about the indentation technique itself.In a first part of the presentation a short review of in-situ indentation experiments inside the SEM will be given. The experiments will be critically reviewed and benefits and limitations of the different methods will be evaluated.In a second part two applications of an in-situ indentation apparatus will be presented in more detail. The microindentation experiment has been developed at Empa over the last few years. The method allows the direct observation of the indenter tip surrounding during the process of indentation. In the presented example the method is used for the characterization of the discrete deformation behaviour of a bulk metallic glass. For the first time, it was shown that serrations in the load-displacement curve are correlated to the formation of new shear bands on the specimen surface around the indenter tip (Moser et al, Phil Mag 86 (2006) 5715).The same indentation apparatus can be used to perform compression tests on micron sized pillars. This is an interesting tool for studying size effects on mechanical properties and deformation and failure mechanisms. Performing these tests inside a scanning electron microscope allows for a very efficient and precise positioning. Additional information on deformation and failure processes can be gained from the observation of the pillar during the compression experiment. The method also has the potential for compliance-free strain measurement by video analysis. Results on a number of differently sized micromachined single crystalline Si-pillars (Moser et al, JMR 22 (2007) 1004) and FIB-machined Ni pillars will be shown.
10:00 AM - AA1.2
In-situ Nanomechanical Characterization of TiN/NbN, NbN/CrN and CrN/NbN Coatings.
Karolina Rzepiejewska - Malyska 1 , R. C. Major 2 , M. Parlinska - Wojtan 4 , H. Wrzesinska 3 , S. A. S. Asif 2 , J. Michler 1
1 Micro-/Nanomechanics Laboratory for Materials Technology, EMPA, Thun Switzerland, 2 , Hysitron Inc., Minneapolis, Minnesota, United States, 4 Nanoscale Materials Science, EMPA, Duebendorf Switzerland, 3 Department of Physics and Technology of Low Dimensional Structures, ITE, Warsaw Poland
Show AbstractTiN/NbN, NbN/CrN and CrN/TiN multilayer coatings with a total thickness up to one micron and different number of periods (between 5 and 20) with thicknesses of single layers in the range between 3 and 25 nm, were synthesized by magnetron sputtering. Hardness and Young’s modulus of the multilayers films were measured by nanoindentation and the results were compared to reference coatings of pure TiN, NbN and CrN. Some multilayer systems exhibited an increase of hardness, compared to the single layer reference coatings.To understand deformation mechanisms and phenomena that accompany the process of multilayer indentation, a new in-situ SEM-nanoindentation instrument was utilized. It is based on the Hysitron Picoindenter technology and has been integrated into a High Resolution Scanning Electron Microscope (SEM). The setup allows for the first time observation of pile-up, sinking-in and crack propagation as well as other phenomena with nanometer scale resolution that may occur during the indentation experiment. The in-situ nanoindentations revealed that the pile-up/sink-in behavior of the multilayer coatings is different compared to the single layer reference coatings of 1 um thick pure materials. To get a more complete understanding of the fundamental phenomena that occur in the layers during the process of indentation, transmission electron microscopy (TEM) samples that contain a cross-section of the indented area were prepared using a focus ion beam (FIB) technique. The TEM micrographs confirm that the thickness of the coating decreases within the residual indentation imprint. Possible deformation/compression mechanisms in combination with phenomena that accompany the indentation related to these observations will be discussed.
10:15 AM - AA1.3
In-situ Testing of Miniaturized Single Crystal Copper Tension and Compression Samples.
Daniel Kiener 1 2 , Reinhard Pippan 2 , Gerhard Dehm 2 3
1 , Materials Center Leoben, Leoben Austria, 2 Erich Schmid Institute of Materials Science, Austrian Academy of Sciences, Leoben Austria, 3 Department of Material Physics, Montanuniversität Leoben, Leoben Austria
Show AbstractRecently, the size-effect in flow-stress exhibited by miniaturized compression samples was investigated by several groups. In the majority of cases a focussed ion beam (FIB) microscope was employed to fabricate these specimens. We applied this technology to prepare miniaturized compression and tension samples from differently oriented copper single-crystals, with the critical dimensions ranging from 1 to 8 µm. These samples were loaded in-situ in a scanning electron microscope (SEM) by a microindenter equipped with modified tips to ensure proper alignment and to gain insight in the deformation mechanisms. Flow stresses up to ~900 MPa were measured for the smallest tested samples. Especially the tensile experiments revealed a strong influence of the aspect ratio (height/diameter) of the tested structures on the mechanical response. Furthermore, as the surface to volume ratio is large for submicron sized test structures, any surface modifications by ion bombardment and implantation may critically alter the mechanical properties. Therefore, transmission electron microscopy (TEM) and Auger electron spectroscopy (AES) measurements were performed to investigate this ion damage.The influence of specimen size, specimen geometry, specimen orientation, loading condition (compression – tension), and ion damage on the size-dependent mechanical properties will be discussed.
10:30 AM - **AA1.4
Mechanical Property Evaluation at the Micro-scale using FIB Fabrication and in-situ Testing Methods.
Michael Uchic 1 , Robert Wheeler 2 , Paul Schade 3 , Dennis Dimiduk 1 , Hamish Fraser 3
1 , AFRL, Wright-Patterson AFB, Ohio, United States, 2 , UES Inc., Dayton , Ohio, United States, 3 Dept. of Materials Science and Engineering, The Ohio State University, Columbus, Ohio, United States
Show AbstractFocused Ion Beam microscopes have become an indispensable and enabling tool for the testing of materials using micro-scale samples, where controlled ion beam milling is used for three primary functions. First, these instruments are used in the direct production of micron- and sub-micron sized test specimens, which can be studied either within the FIB chamber for in-situ experiments, or removed after fabrication for external testing. Second, FIB fabrication can be used to manufacture ancillary probes or test fixturing for the small-scale experiments; for example, specialized specimen grips for tensile testing. Third, the FIB can also be used after testing to characterize the tested structures in novel ways, either through fabrication and in-situ extraction of TEM lamellae or using 3D FIB tomography methods. This talk will present selected applications of the techniques listed previously to characterize the mechanical properties of single crystal Ni-base superalloys using micro-testing methods. In particular, we will highlight results from a prototype mechanical test system for performing micro-compression and tension experiments inside a SEM.
11:00 AM - AA1: In-situ
BREAK
11:30 AM - **AA1.5
Nanoscale Plasticity Phenomena Revealed Through Quantitative in situ TEM Compression Tests.
Andrew Minor 1 , Jia Ye 1 , Raj Mishra 2 , Zhiwei Shan 3 , S.A. Syed Asif 3 , Oden Warren 3
1 NCEM, Lawrence Berkeley National Laboratory, Berkeley, California, United States, 2 , General Motors Research and Development Center, Warren, Michigan, United States, 3 , Hysitron, Inc., Minneapolis, Minnesota, United States
Show AbstractThe technique of micro-pillar compression tests is quickly becoming a new paradigm for small-scale mechanical testing, particularly for examining size effects at small length scales. Through quantitative in situ nano-compression tests in a transmission electron microscope (TEM) we can directly correlate the dynamic deformation mechanisms in submicron pillars with simultaneous measurement of the imposed stresses. This talk will demonstrate this capability through the in situ compression of single crystal pure Ni, and Al alloys. In the pure Ni pillars the phenomenon of mechanical annealing leads to dislocation-free structures that allow for direct comparison of the deformation behavior across samples with dramatically different dislocation densities. In the Al alloy systems the deformation can be more complicated, where suppression of cross-slip can result in a three-dimensional dislocation network that leads to a rotation of the pillar structure under compression. These results will be discussed in relation to the size effects seen in ex situ pillar compression tests where a direct relation between the yield stress and the diameter of pillar structures has been confirmed.
12:00 PM - AA1.6
In Situ TEM Nanoindentation of Individual Nanoparticles: Observed Deformation Mechanisms and Theoretical Analysis.
Christopher Carlton 1 , Oleg Lourie 2 , Paulo Ferreira 1
1 Mechanical Engineering, University of Texas, Austin, Texas, United States, 2 , Nanofactory Instruments, Gothenburg Sweden
Show AbstractNanoindentation of nanostructured materials is a very rapidly growing area of research interest. Many experiments have tested nanostructured materials of various sizes, shapes, and compositions to determine the fundamental effects of length scale on the mechanical behavior of materials. In this context, in-situ nanoindentation experiments were performed on a single-crystal nanoparticle of silver with a diameter of approximately 50nm. Dislocation nucleation and motion was observed during in-situ TEM nanoindentation, while upon unloading dislocations were no longer visible. The TEM observations provide insight into the plastic deformation mechanisms available to nanoparticles and into the interpretation of other recent nanoindentation experiments. As during the nanoindentation experiments dislocations were observed to intersect the nanoparticle’s surface, previous models relying on both dislocation loop and image force only assumptions are inadequate for addressing the mechanisms operating herein. Additionally, several dislocation dynamics models developed for the nanoindentation of nanopillars are not directly applicable to explaining the current results because dislocations were not seen to nucleate from Frank-Read sources during the nanoindentation of silver nanoparticles and the nanopillars tested were generally of larger size. In this regard, a new analytical model for explaining dislocation instability in individual single-crystal nanoparticles is introduced. The application of this model to the deformation behavior of nanomaterials is also considered.
12:15 PM - AA1.7
Quantitative In situ TEM Nano-compression Tests of NiTi Shape Memory Alloys.
Jia Ye 1 , Raj Mishra 2 , Alan Pelton 3 , Andrew Minor 1
1 , Lawrence Berkeley National Laboratory, Berkeley, California, United States, 2 , General Motors Research and Development Center, Warren, Michigan, United States, 3 , Nitinol Devices and Components, Fremont, California, United States
Show AbstractThe shape memory effect found in Ni-Ti alloys has been studied for over 40 years and has been used to develop advanced mechanical and biomedical devices. It is well known that this thermoelastic behavior can also be induced by applying an external stress to drive the martensitic phase transformation in NiTi bulk material. As shape-memory technologies progress and these devices become smaller, questions remain as to whether or not the stress-induced martensitic phase transformation in NiTi progresses in the same manner or in fact exists at all when the critical dimensions approach nanometer scales.Quantitative in situ TEM mechanical testing is an ideal experimental method to probe the shape memory behavior of NiTi. By correlating mechanical data, microstructural analysis and crystallographic information we can explore the initial stages of deformation in this system at length scales difficult to approach with other testing methods. Here we present results from our in situ TEM uniaxial nano-compression experiments of microfabricated NiTi pillar structures. We start with a focused ion beam (FIB) to fabricate NiTi pillars with diameters ranging from 100 to 200nm, and then move to a 300kV TEM for mechanical testing. The in situ nano-compression test results clearly indicate that there is a stress-induced B2 to B19’ transformation upon initial compression. By running the experiments in either imaging or diffraction mode we are able to directly correlate the transformation with the compression data. Interestingly, the transformation starts at a very low stress level, and the load-displacement curves show a very distinctive characteristic that is indicative of the phase transformation at the beginning and at the end of the compression experiments. These encouraging discoveries demonstrate the successful application of the quantitative in situ TEM nano-compression method to study mechanically-induced phase transformations and to understand the characteristics of shape-memory behavior in nanoscale volumes.
12:30 PM - AA1.8
In-situ Laue Diffraction Providing a New View on Micro-compression.
Robert Maass 1 , Steven Van Petegem 1 , Helena Van Swygenhoven 1 , Peter Derlet 1 , Daniel Grolimund 1
1 , Paul Scherrer Institute, Villigen Switzerland
Show AbstractPlasticity in single crystal micron sized pillars has gained considerable interest since the development of a new micro compression technique with a flat punch equipped nanoindenter. For all fcc metals measured, the initial flow stress increases with decreasing pillar diameter. Plastic deformation is characterized by individual slip events that are reflected in discrete strain bursts. Large scattering among the data is reported, where pillars of similar orientation are deforming according to different slip systems, sometimes shearing-off or barrelling. Most information is obtained from post-mortem analysis, lacking any information on pillar’s initial microstructure as well as on the dynamics between microstructure and deformation. An in-situ micro compression device has been developed at the Swiss Light Source (SLS) allowing the continuous measurement of time-resolved white beam Laue diffraction patterns during compression of micron-sized pillars, capturing the initial microstructure and the changes in microstructure during deformation. Former measurements have demonstrated the presence of initial strain gradients and their role on the deformation mechanisms activated during compression. Here we present the evolution of Laue diffraction peaks and their direct relation to the measured stress strain-data for a series of Au pillars with different degrees of internal strain and different crystallographic orientations. Moreover, by deforming several pillars of the same diameter, using the same FIB conditions, the statistical aspect of the scattering is addressed. The results demonstrate that the pre-existing strain gradients as a result of FIB synthesis plays a determining role in the choice of the first activated slip system, which usually corresponds with a low Schmid factor slip system. Our results underline the importance of performing micromechanical testing in-situ to reveal the dynamics of small scale plasticity.
12:45 PM - AA1.9
Mechanical Behavior of Nanoscale Multilayers: Micropillar compression, Nanoindentation, and Tensile Testing.
Nathan Mara 1 , Dhriti Bhattacharyya 1 , Amit Misra 1 , Pat Dickerson 2 , Richard Hoagland 2
1 Center for Integrated Nanotechnologies, Los Alamos National Laboratory, Los Alamos, New Mexico, United States, 2 Materials Science and Technology Division, Los Alamos National Laboratory, Los Alamos, New Mexico, United States
Show AbstractUntil recently, mechanical testing of nanoscale multilayers has been largely limited to nanoindentation, with few studies carried out via tensile testing of freestanding samples. Testing methods such as tensile testing and micropillar compression directly determine properties such as yield stress, ductility to failure, and fracture behavior that cannot be directly determined through nanoindentation. In this work, the mechanical behavior of Cu/Nb multilayers are evaluated using three test methods: micropillar compression, nanoindentation, and tensile testing of freestanding samples. Individual layer thicknesses tested range from 100 nm to 5 nm, with flow stresses ranging from ~1.1 GPa to nearly 3 GPa, respectively. Remarkable ductility is demonstrated in these materials during micropillar testing, with 40 nm Cu/Nb exhibiting 30% strain to failure and 5 nm Cu/Nb exhibiting strains in excess of 15%. The unique behavior of these materials is attributed to the large fraction of Cu/Nb interfaces, and will be discussed in terms of interfacial effects on dislocation motion.
AA2: Nanomechanics,Tribology and Nanostructures
Session Chairs
Dave Bahr
Eric Le Bourhis
Monday PM, November 26, 2007
Room 208 (Hynes)
2:30 PM - **AA2.1
Probing Properties in Wear Tested Single Crystal Nickel Under Sliding Contacts - Tests and Simulations.
N. Moody 1 , M. Cordill 2 , C. Battaile 3 , J. Michael 3 , S. Prasad 3 , W. Gerberich 2
1 , Sandia National Laboratories, Livermore, California, United States, 2 , University of Minnesota, Minneapolis, Minnesota, United States, 3 , Sandia National Laboratories, Albuquerque, New Mexico, United States
Show AbstractMaterial behavior under sliding contacts often determines performance and reliability of many microsystem devices. The regions below the sliding contacts are severely stressed and give rise to acute gradients of plastic shear strain creating gradients of microstructures and deformation substructures. The gradients in microstructures and deformation substructures can lead to strong gradients in mechanical response. As a consequence, characterizing the mechanical behavior of material under wear surfaces is crucial to predicting the performance and reliability of microsystem devices. To address these effects on a fundamental level, we conducted a program using nanoscratch and nanoindentation to study wear on <001>, <011>, and <111> oriented single crystal nickel. Nanoscratch techniques were used to generate wear patterns as a function of load and number of cycles. AFM showed that surface deformation and roughness increased with applied load and number of wear passes for all samples tested. Nanoindentation was then used to measure properties in each wear pattern correcting for surface roughness effects on measured hardness values. The results for <001> and <011> single crystal nickel showed there was a strong increase in hardness with increasing applied wear load. An even stronger increase in hardness was observed for <111> single crystal nickel samples. Finite element simulations of the wear process show a similar progression in structure and development of plastic strain. In this presentation, we will combine experimental results with finite element simulations to show how crystal orientation affects properties under sliding contacts and its impact on microdevice performance. The authors gratefully acknowledge the support of Sandia National Laboratories under U.S. DOE contract DE-AC04-94AL85000. MJC and WWG gratefully acknowledge NSF support under Grants DMI 0103169 and CMS-0322436.Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company for the United States Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.
3:00 PM - AA2.2
Surface Nanowear Determination of Gold Coatings Using Atomic Force Microscopy and Digital Image Correlation Techniques.
Zhi-Hui Xu 1 , Xiaodong Li 1 , Michael Sutton 1
1 Department of Mechanical Engineering, University of South Carolina, Columbia, South Carolina, United States
Show AbstractSurface wear damage of coatings occurring at extremely low loads (in the range of nano-/pico-Newtons) and in nanocontacts is of great importance for the development and the reliability of nanocomponents such as micro-/nano-electromechanical systems (MEMS/NEMS). To date, appropriate tools for determining and visualizing the nanowear damage of coatings are still lacking. In this study, a new method combining atomic force microscopy (AFM) and digital image correlation techniques has been developed and applied for the determination and visualization of local nanowear damage field of a gold coating. It has been shown that this novel technique can successfully reveal the nanowear damage field of coating at an initial stage of wear, which cannot be detected by conventional AFM surface topography images. This technique can be used as an indicator for determining nano-/pico- wear damage and should find more applications in MEMS/NEMS
3:15 PM - AA2.3
High Cycle Contact Fatigue Studied by Cyclic Nanoindentation.
Knyrim Jorg 1 , Oliver Kraft 1 2 , Schwaiger Ruth 1
1 IMF II, Forschungszentrum Karlsruhe, Karlsruhe Germany, 2 IZBS, Universität Karlsruhe (TH), Karlsruhe Germany
Show Abstract3:30 PM - AA2.4
Comprehensive Mechanical and Tribological Characterization of Ultra-Thin-Films.
Norm Gitis 1 , Michael Vinogradov 1
1 , CETR, Inc., Campbell, California, United States
Show Abstract3:45 PM - AA2.5
Wear Behavior in SiC-Ti Nanocomposites.
Aaron Beaber 1 , Lejun Qi 1 , Joachim Heberlein 2 , Peter McMurry 2 , William Gerberich 1 , Steven Girshick 2
1 Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota, United States, 2 Mechanical Engineering, University of Minnesota, Minneapolis, Minnesota, United States
Show AbstractSiC-Ti nanocomposites were deposited through a hybrid process of nanoparticle impaction and chemical vapor deposition (CVD). Nanoparticles were synthesized in a process called hypersonic plasma particle deposition (HPPD). Reactants are injected into a thermal plasma and undergo a rapid expansion through a converging nozzle, resulting in gas-phase particle nucleation and hypersonic impaction onto a substrate. Excess vapor from the particle nucleation process forms a reactive boundary layer above the substrate for CVD growth. Particles with an average diameter of 5 nm both inhibit grain growth in the matrix material during the deposition and enhance fracture toughness under compression. Consecutive deposition of SiC and Ti creates a bilayered film with a unique transitional region that contains a composition gradient of SiC/Ti. This study focuses on the mechanical response of each of these layers and the interface as a function of both particle volume fraction and metallic-ceramic content using nanoindentation and pin-on-disk wear testing. SiC particles embedded in a SiC matrix show an increased hardness without a change in the modulus compared to bulk samples. In contrast to SiC/SiC, the SiC/Ti system is expected to show a significant decrease in the effective modulus (compared to bulk SiC) without a corresponding drop in hardness. These results demonstrate the potential for nanocomposites with the strength of a ceramic and the ductility of a metal.
4:00 PM - AA2: Tribology
BREAK
4:30 PM - **AA2.6
Point-load Testing of Freestanding Microfabricated Structures of Compliant Materials.
Matthew Begley 1
1 Dept. of Civil Eng., University of Virginia, Charlottesville, Virginia, United States
Show AbstractDeflection of freestanding thin film structures offers key advantages over traditional film-on-substrate indentation testing, particularly for compliant materials such as nano-porous metals/ceramics, polymers and biomaterials. This talk will describe experimental and theoretical aspects of determining mechanical properties via point-load testing, with a focus on utilizing instrumented nanoindenters. Two key issues will be addressed: (i) the use of dynamic testing approaches that accurately identify the instant of probe contact with ultra-compliant structures, and (ii) the design of specimens and testing ranges that allow for accurate closed-form expressions to determine properties. The advantages and challenges of testing freestanding structures will be illustrated in the context of experiments on nano-porous metals and nano-porous ceramics, which utilize micropatterned beams and circular drumheads to determine modulus, residual stress and strength.
5:00 PM - AA2.7
Indentation Response of µm-sized Semiconductor Structures.
Eric Le Bourhis 1 , Gilles Patriarche 2
1 LMP UMR CNRS 6630, Univ. Poitiers, Futuroscope-Chasseneuil France, 2 Laboratoire de Photonique et de Nanostructures, UPR 20 CNRS, Marcoussis France
Show AbstractPlasticity of III-V semiconductors has received much attention during the past two decades because of the needs from the optoelectronic industry. In this field, the indentation technique has proved to be a powerful tool to test small volume even at temperatures below the brittle-ductile transition (for a review refer to [1]). So far, contact mechanics has been developed for semi-infinite half space, this assumption being not fulfilled when the size of the plastic zone becomes of the order of one of the dimensions of the object. Thereafter, ‘small’ structures are expected to show a mechanical behaviour quite different from that of a bulk [2-4]. We report a size effect study on thin walls and membranes. Thin GaAs walls were formed by photolithography while thin InP membranes were etched free on top of their substrate. The mechanical behaviour was investigated by nanoindentation and the responses were compared to that of a standard bulk reference (flat surface). Dramatic differences in the indentation response are observed for both systems in terms of elastic-plastic and brittle behaviors. Interferential optical and transmission electron microscopies were extensively used to get further insight into the deformation of such small structures.[1] Le Bourhis E., Patriarche G, Prog. Cryst. Growth Charac. Mater., 2003 ; 47 : 1[2] Choi Y, Suresh S. Scripta Mater 2003; 48 : 249.[3] Le Bourhis E, Patriarche G. Appl. Phys. Lett. 2005; 86: 163107.[4] Michler J, Wasmer K, Meier S, Östlund F, Leifer K. Appl. Phys. Lett. 2007; 90: 043123
5:15 PM - AA2.8
Indentation Response of Nanostructured Turfs.
Ali Zbib 1 , David Bahr 1 , Sinisa Mesarovic 1 , Jun Jiao 2 , Devon McClain 2 , Erica Lilleodden 3
1 Mechanical and Materials Engineering, Washington State University, Pullman, Washington, United States, 2 Department of Physics, Portland State University, Portland, Oregon, United States, 3 , GKSS, Geesthacht Germany
Show AbstractThe mechanical properties of single wall and multi-walled carbon nanotubes (CNTs) have been documented in a wide range of conditions; both experimentally by testing a tube in tension, bending or compression, and theoretically using molecular dynamic and finite element simulations. However, little work has been performed on large assemblages of CNTs. When grown via vapor deposition, these aligned or random structures take on the form of a “turf”, consisting of many CNTs attached to an inflexible substrate. These turfs can be formed over large areas and with a range of heights (between 1 to 100 μm), and photolithographically patterned to from different aspect ratios. Prior results have demonstrated that the turfs are extremely compliant in compression (with an effective elastic modulus of 10-20 MPa), can buckle and deform permanently, and exhibit adhesion to a wide range of contact surfaces. Applications of these turfs include electrical and thermal switching and contacts using conduction, and therefore the mechanical response in compression will impact their performance. The current student is focused on an experimental effort using nanoindentation to assess the properties of turfs using a wide range of indentation geometries, including pyramidal, conical, spherical, and flat punch geometries with instrumented indentation. The onset of permanent deformation has been identified, and is approximately 2 MPa, similar to the stress required to form a collective buckle structure in the turf. Adhesion to diamond tips has been documented, and metallic coatings are shown to dramatically reduce the adhesion between the CNTs and diamond indenter tip. Finally, a model has been developed and will be presented to demonstrate the unique buckling behavior in these turfs. The buckling stresses do not follow an Eulerian response and are not dependent on turf aspect ratio, but instead are controlled by the CNT height and the relative elastic response of the turf, with buckles forming at the same applied stress and same height within the turf for height to width ratios of 2:1 to 1:40 in a wide range of systems.
5:30 PM - AA2.9
Comparing the Mechanical Response of Various Aluminum Nanogeometries.
William Mook 1 , William Gerberich 1
1 Chemcial Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota, United States
Show AbstractUnderstanding the mechanical response of common low dimensional geometries when subjected to indentation, compression and shear is necessary in order to predict and optimize their resultant nanotribological properties. From an experimental standpoint, nanoindentation has traditionally been used to investigate scale effects on the mechanical properties of flat surfaces such as thin films or single crystals. However recent years have seen an increase in the characterization of freestanding micro- and nanostructures by nanoindentation. Analysis of the similarities and differences of these geometries to indentation can lead to considerable insight. This talk will compare the mechanical response of aluminum thin films (from 30 nm to 100 nm in thickness) to freestanding aluminum nanoposts (from 30 nm to 100 nm in height). The elastic and plastic behavior for all of the geometries will be discussed. Differences in constraint and testing conditions which contribute to the observed range of mechanical behavior will also be addressed.
5:45 PM - AA2.10
The Effect of Plasticity Gradient on the Indentation Behavior of Ni-W.
In-Suk Choi 1 2 , Andrew Detor 1 , Ruth Schwaiger 2 , Ming Dao 1 , Christopher Schuh 1 , Subra Suresh 1
1 Materials Science & Engineering, MIT, Cambridge, Massachusetts, United States, 2 Forschungszentrum Karlsruhe, Institute for Materials Research II, 76344 Karlsruhe Germany
Show AbstractThe introduction of controlled gradients in plastic properties is known to influence the resistance to damage and cracking at contact surfaces in many tribological applications. In order to assess potentially beneficial effects of plastic property gradients, we develop a comprehensive and quantitative framework to understand the effects of yield strength and strain hardening exponent on contact deformation under the most fundamental contact condition: normal indentation. Then, universal dimensionless functions are extracted from extensive FEM simulations for the specific case of linear variation in yield strength with depth so as to predict the indentation load versus depth of penetration curves for a wide variety of plastically graded engineering metals and alloys. The effect of plasticity gradient on the residual indentation pile-up is also studied in conjunction with the stress and strain distribution under the indenter tip. Furthermore, for interpretation of, and comparisons with, experimental results, a systematic study of depth-sensing indentation was performed on nanocrystalline (nc) Ni-W alloys specially synthesized with controlled unidirectional linear gradients in plastic properties. A yield strength gradient and a roughly constant Young’s modulus were achieved in the nc alloys, using electrodeposition techniques. The force versus displacement response from instrumented indentation experiments matched very well with that predicted from the universal dimensionless functions extracted from FEM simulations. The experiments also revealed that the pile up of the graded alloy around the indenter is noticeably higher than that for the two homogeneous reference alloys that constitute the bounding conditions for the graded material. These trends are also consistent with the predictions of the computational analysis.
Symposium Organizers
Eric Le Bourhis Universite de Poitiers
Dylan J. Morris National Institute of Standards and Technology
Michelle L. Oyen Cambridge University
Ruth Schwaiger Forschungszentrum Karlsruhe
Thorsten Staedler Universitaet Siegen
AA3: Size Effects and Indentation of Thin Films
Session Chairs
Alexander Hartmaier
Oliver Kraft
Tuesday AM, November 27, 2007
Room 208 (Hynes)
9:30 AM - **AA3.1
The Indentation and Other Size Effects
Andy Bushby 1 , Tingting Zhu 1 , Xiaodong Hou 1 2 , Nigel Jennett 2 , David Dunstan 1
1 Centre for Materials Research, Queen Mary, University of London, London United Kingdom, 2 Materials, National Physical Laboratory, Teddington United Kingdom
Show AbstractThe ability to infer tensile properties from an indentation test is an exciting proposition offering extensive application of the test method for monitoring materials performance in critical industries. Understanding length scale effects is the key to harnessing the small scale capability of nanoindentation testing and to the design of advanced materials exploiting nano-scale structures. Many plasticity size effects have been observed for many years, each associated with a materials length scale. The hardness size effect is often explained in terms of strain gradient plasticity from the expected increase in hardness associated with steep plastic strain gradients beneath a small indentation. Here we show that for spherical indenters there is a clear size effect at the elastic limit, i.e. the initial yield point, that is proportional to the inverse square root of the contact radius. Furthermore, normalising the data by their macroscopic yield point, the data for metals and ceramics fall onto two lines with different slopes. This implies that the magnitude of the size effect is mostly driven by geometry while materials parameters play a role in determining the proportionality. The elastic moduli of the two classes of materials cover similar ranges from 70GPa to 400GPa. The difference in the slopes is presented in terms of parameters determining yield such as shear strain and Burgers vector and surface energy. Introducing a new length scale into the problem such as grain size changes the scaling of the combined size effects. The important experimental control parameter is the relative size of the indentation compared to the grain size. The interaction of the various length-scale effects is demonstrated (e.g. the indentation size effect and grain size effect in indentation). An ability to separate these effects is a key requirement for enabling step improvements in materials performance by exploiting nano-scale structuring.
10:00 AM - AA3.2
Strain Gradient Plasticity Modelling of Size Effects in Nanoindentation.
Bjoern Backes 1 , Karsten Durst 1 , Mathias Goeken 1
1 Department of Materials Science & Engineering, Institute I, University Erlangen-Nurnberg, Erlangen Germany
Show AbstractIn indentation testing an increasing hardness is found for decreasing indentation depth, which is referred to as the indentation size effect, ISE. Usually this effect is observed up to a few microns indentation depth. In this work, nanoindentation and compression experiments have been performed on Cu and brass. The conventional theory of mechanism-based strain gradient plasticity has been used for modelling the indentation size effect. This theory uses incremental plastic moduli for describing the strain gradient effects, and therefore no higher order stresses are involved. An analytical model for indentation is established to correlate the hardness with the penetration depth. This model was applied to indentation experiments on Cu and brass to determine the internal material length scale. The yield stress and the work hardening behaviour were determined from the uniaxial compression tests and were introduced in the finite element analysis and used for calculating the internal material length scale. The model will be discussed on the basis of the experimental and simulated load displacement curves as well as hardness results from nanoindentations.
10:15 AM - AA3.3
Microstructural Investigation of the Deformation Zone below Nano-Indents in Copper.
Martin Rester 1 , Christian Motz 1 , Reinhard Pippan 1
1 Erich Schmid Institute of Materials Science, Austrian Academy of Sciences, Leoben Austria
Show AbstractThe deformation zone below nano-indents in copper single crystals with an <1-10>{111} orientation is investigated. Using a focused ion beam (FIB) system, cross-sections through the middle of the indents were fabricated and subsequently analyzed by means of electron backscatter diffraction (EBSD) technique. Additionally, cross-sectional TEM foils were prepared and examined. Due to changes in the crystal orientation around and beneath the indentations, the plastically deformed zone can be visualized and compared to the measured hardness values. Furthermore, the hardness data were analyzed using the Nix-Gao model where a linear relationship was found for H^2 vs. 1/hc, but with different slopes for large and for shallow indentations. The found orientation maps indicate that this behaviour is associated to a change in the deformation mechanism. On the basis of possible dislocation arrangements, two models are suggested and compared to the experimental findings. The model presented for large imprints is similar to the dislocation pile-up model which explains the Hall-Petch effect, while the model for shallow indentations uses far-reaching dislocation loops to accommodate the shape change of the indenter.
10:30 AM - **AA3.4
Large-Scale Atomistic Simulations of Nanoindentation: Dislocation Microstructure and Indentation Size Effect.
Alexander Hartmaier 1
1 Dept. of Materials Science and Engineering, Friedrich-Alexander-University Erlangen-Nuremberg, Erlangen, Bavaria, Germany
Show AbstractLarge-scale molecular dynamics simulations of nanoindentation have been performed to study the influence of different length scales on the contact pressure, i.e. the normal force per projected contact area, corresponding to the hardness of the material. Typical numerical samples contained between 2 and 5 million atoms interacting with an embedded atom method (EAM) potential. The indenter was modeled essentially as a hard sphere with a purely repulsive potential. The indentation was performed in a displacement controlled fashion and at a constant finite temperature. The simulations show that the calculated hardness decreases with indentation depth, which is consistent with the indentation size effect found in experiments. To calculate the “true” material property hardness from nanoindentation experiments or simulations, a physical model of the deformation processes and their dependence on the microstructure is necessary. A careful analysis of the dislocation microstructure resulting from the numerical simulations reveals that the average dislocation density in the plastic zone increases with indentation depth. Hence, neither strain gradient theory nor Taylor hardening are able to explain this size effect in hardness. The origin of the size effect seems to be rather that the indentation is performed into a perfect, dislocation-free single-crystal, where the first dislocations nucleate homogeneously at the theoretical shear strength. Furthermore, to support the imposed strain rate in the material a large number of dislocations has to be produced, which becomes easier with ongoing plastic deformation via multiplication of dislocations.
11:00 AM - AA3:Size Effects
BREAK
11:30 AM - AA3.5
Microstructure and Mechanical Properties Characterisation of Nanocrystalline Copper Thin Films.
Nursiani Tjahyono 1 , Yu Lung Chiu 1
1 Chemical and Materials Engineering, University of Auckland, Auckland New Zealand
Show AbstractThe microstructure and mechanical properties of nanocrystalline copper with grain size ranging from 50 nm to 80 nm have been investigated. Thin films of nanocrystalline copper were electrodeposited from an additive-free acidified copper sulphate solution at room temperature. The effect of current density on the microstructure of deposited films has been studied by employing both the constant current and pulsed current of rectangular wave function, at different magnitudes between 20 and 80 mA/cm2. Both FCC and BCC steel substrates with the same surface finishing conditions have been used for the deposition. X-ray diffraction results show a strong (110) preferred orientation with the orientation index about 2 on the thin films deposited onto the BCC steel substrate, in contrast with those deposited on the FCC steel substrates using the same deposition parameters, where orientation indices smaller than 1 have been observed for all diffraction peaks. The microstructure of the thin films has been further studied using electron microscopy techniques, and the mechanical properties using nanoindentation technique. It has been noted that both the modulus and hardness measured following the Oliver-Pharr scheme show an apparent indentation size effect. The effect of indentation strain rate on the measured hardness and modulus of the thin films will be summarised and discussed in conjunction with the microstructure, particularly the grain size of the samples.
11:45 AM - **AA3.6
Mechanical Properties of Single and Polycrystalline SiC Thin Films.
Jayadeep Deva Reddy 1 , Alex Volinsky 1 , Christopher Frewin 2 , Stephen Saddow 2
1 Mechanical Engineering, University of South Florida, Tampa, Florida, United States, 2 Electrical Engineering, University of South Florida, Tampa, Florida, United States
Show AbstractThere is a technological need for hard thin films with high elastic modulus for harsh environmental conditions. Silicon carbide fulfills such requirements with a variety of applications in high temperature and power MEMS. A detailed study of SiC thin films mechanical properties was performed by means of nanoindentation. We report on the comparative study of the mechanical properties of 9 µm thick epitaxial cubic (3C) single crystalline <100> and polycrystalline SiC thin films grown on <100> Si substrates. The polycrystalline 3C-SiC films have a higher elastic modulus and hardness compared to the single crystalline films, which makes them more attractive for MEMS applications in harsh environments.
12:15 PM - AA3.7
Strength Evolution in Sub-Micron Thin Films.
Oscar Borrero-Lopez 1 , Mark Hoffman 1 , Avi Bendavid 2 , Phil Martin 2
1 Materials Science and Engineering, University of New South Wales, Sydney, New South Wales, Australia, 2 Industrial Physics, CSIRO, Sydney, New South Wales, Australia
Show AbstractIn this work we have investigated the strength variability of brittle thin films (thickness <1 μm) utilising a simple test methodology. Nanoindentation of as-deposited diamond-like carbon (DLC) and TiSiNx nanocomposite films on silicon substrates followed by cross-sectional examination of the damage with a Focused Ion Beam (FIB) Miller allows the occurrence of cracking to be assessed in comparison with discontinuities (pop-ins) in the load-displacement curves (Figures 1 and 2). Strength is determined from the critical loads at which cracking occurs using the theory of plates on a soft foundation. The small contact assumption between the indenter and the specimen is maintained by incorporating a buffer layer on top of the thin-film system, which increases the effective thickness of the film. The judicious selection of the elastic modulus mismatch between the system layers also ensures that fracture occurs in the thin film first. This methodology enables Weibull plots to be readily obtained, avoiding complex freestanding-film machining processes. More importantly, it opens up the possibility of introducing different defect populations at the interfaces to investigate their effect upon strength, which otherwise cannot be studied by means of bending of microbeams. This is of great relevance, since the mechanical strength of thin films ultimately controls their reliable use in a broad range of functional applications, and the strength of brittle materials is in turn governed by the presence of defects.
12:30 PM - AA3.8
Local Mechanical Properties of Thin ZnO Films.
J Swadener 1 , Jung-Kun Lee 2 1 , Jun Noh 3 , Kug Hong 3
1 MPA-CINT, Los Alamos National Laboratory, Los Alamos, New Mexico, United States, 2 Department of Mechanical Engineering and Materials Science, University of Pittsburgh, Pittsburgh, Pennsylvania, United States, 3 School of Materials Science & Engineering, Seoul National University, Seoul Korea (the Republic of)
Show AbstractWe have investigated the mechanical properties of ZnO and Al-doped ZnO (AZO) using nanoindentation and other techniques applicable to thin films. While the ZnO are softer and show greater work hardening, they are unexpectedly found to crack more easily. The as deposited 200 nm thick films were under tensile residual stresses: 490 MPa in ZnO and 590 MPa in AZO. Using spherical indentation at depths less than 50 nm, the compressive stress-strain behavior of the two films was determined. At low effective strains, the two materials show very similar elastic-plastic transition. At higher effective strains, ZnO shows lower flow stress and greater work hardening, which would be expected to result in increased ductility. However, at an effective strain of ~0.02, ZnO is found to have considerable scatter in the measured flow stress. This indicates that dislocation sources are relatively widely spaced in ZnO films and therefore the films plastic response is heterogeneous at the scale of 10s of nm. In a local region without dislocation sources, tensile stresses could lead to crack initiation, which would then propagate because of the residual stress in the film.
12:45 PM - AA3.9
Nanoindentation Studies of Al/TiN Multilayers with Unequal Thickness Ratio.
Dhriti Bhattacharyya 1 , Nathan Mara 1 , Richard Hoagland 2 , Amit Misra 1
1 MPA-CINT, Los Alamos National Laboraotry, Los Alamos, New Mexico, United States, 2 MST, Los Alamos National Laboraotry, Los Alamos, New Mexico, United States
Show AbstractAl/TiN multilayers comprise Al based interconnects with TiN diffusion barriers, which are used in multilevel metallization structures in submicron ultra-large scale integrated devices. With the continual decrease in the size of micro-electronic devices, the strength of these structures becomes an increasingly important factor for their integrity during manufacturing and handling. In this study, nano-scale multilayers of Al/TiN have been deposited with a ratio of Al:TiN equal to 9:1 using reactive sputter deposition, with TiN layer thickness varying from 50 nm to 1 nm. The microstructure of these nano-scale multilayers has been studied using Transmission Electron Microscopy (TEM) and high resolution TEM. The HRTEM images show clearly that both Al and TiN layers maintain a crystalline structure to the smallest layer thickness. Nano-indentation experiments have been performed on these multilayered thin films to measure their hardness, and these results have been compared with predicted values obtained from different existing models, such as the dislocation pile-up based Hall-Petch model for length scale dependence, and the dislocation image stress model following Koehler and Lehoczky that estimates the maximum strength of a multilayer based on differences in the elastic moduli of the constituent layers. This work clearly demonstrates that these multilayers are significantly stronger (peak hardness of approximately 4 GPa) than a simple rule of mixtures estimate would suggest (hardness of approximately 1 GPa). It is shown that the hardness follows the Hall-Petch scaling law to a bilayer period of 20 nm, below which the hardness value drops below that predicted by the Hall-Petch extrapolation. The maximum strength achieved is comparable with that estimated from the Koehler and Lehoczky model. Thus at the finest layer thicknesses, deformation mechanisms may involve transmission of single dislocation across the interfaces, without the mechanical advantage of a pile-up. TEM observations using focused-ion-beam sections taken from the regions underneath the indents illustrate the material flow patterns and deformation structures of these multilayers.This research is supported by the DoE, Office of Science, Office of Basic Energy Sciences.
AA4: Nanotribology and Friction
Session Chairs
T. A. Venkatesh
Alex A. Volinsky
Tuesday PM, November 27, 2007
Room 208 (Hynes)
2:30 PM - **AA4.1
Multi-scale Computational Approaches to Nanotribology.
Izabela Szlufarska 1
1 Materials Science & Engineering, University of Wisconsin, Madison, Wisconsin, United States
Show AbstractTribological properties of silicon, silicon carbide and diamond are of great interest because of potential applications of these materials in MEMS and NEMS devices. We employ ab initio calculations based on density functional theory to study adhesion in altered chemical environments. We choose Si and SiC to demonstrate the effect of oxidation on the work of adhesion. Ab initio energies are used as parameters for continuum level models of roughness to predict adhesion of surfaces with different topologies. We also employ massively parallel molecular dynamics (MD) simulations to study contact mechanics and friction at a single asperity level. Choosing diamond surfaces as an example, we study the effects of the tip size and stiffness, surface orientation, load, and the range of Van der Waals interactions on friction. Bringing together ab initio accuracy, the advantage of MD in atomistic modeling at length-scales comparable to experiments, with continuum level models of roughness, will allow unraveling most relevant energy dissipation mechanisms underlying atomic friction.
3:00 PM - AA4.2
Investigation of Tribological Properties of MPCVD Grown Nanocrystalline Diamond.
Abhishek Kothari 1 , Brian Sheldon 1 , Qungyang Li 1 , Xingcheng Xiao 2 , Leo Lev 2 , Erkan Konca 2 , Kyung Kim 1
1 Engineering, Brown university, Providence, Rhode Island, United States, 2 Materials & Process Laboratory, General Motors Research and Development, Warren, Michigan, United States
Show AbstractNanocrystalline diamond coated cutting tools have the potential to revolutionize the machining of Al alloys. With low wear rates and a low coefficient of friction (COF), these coatings may ultimately enable dry machining operations (i.e., with little or no lubricant). Understanding how the COF for nanocrystalline diamond can be engineered is of utmost importance in these efforts. Microwave plasma enhanced CVD was used to grow nanocrystalline diamond on Si substrate and on Ti interlayered Si substrate. The friction of these films was then investigated at different scales. Macroscopic friction tests were done using pin on disc tribometer, while lateral force microscopy was used to evaluate friction at smaller scales. For the latter, glass beads were coated with Al using PVD and then attached to AFM cantilevers. Changing the diameter of glass beads in the range of 20-100 μm was used to control the contact mechanics between the two surfaces. A Greenwood-Williamson model was then used to correlate the surface topography with the load carried by asperities and the resultant coefficient of friction. These studies were performed with diamond films, to study the effects of grain size, growth temperature, and surface termination. The same films were also subjected to pin on disc tests using Al pins of approximately 6 mm diameter. The friction measurements performed at these two length scales were compared to develop a better understanding of the inherent friction characteristics of nanocrystalline diamond.
3:15 PM - AA4.3
Effect of Scale on the Contact Area Dependence of Friction.
Kanaga Karuppiah Kanaga Subramanian 1 , Sriram Sundararajan 1
1 Mechanical Engineering, Iowa State University, Ames, Iowa, United States
Show AbstractIn this work, the friction behavior of two different materials (a) mica and (b) ultra-high molecular weight polyethylene (UHMWPE) at two length scales was investigated. The friction experiments were carried out using an atomic force microscope (AFM) at the nanoscale and using a custom-built ball-on-flat microtribometer at the microscale. The same counterface (Si3N4 probe) and the environmental conditions (25 °C, RH < 10%) were maintained for all experiments. The friction data obtained were analyzed for dependence on contact area. Friction between a silicon nitride and UHMWPE interface resulted in contact area dependence at both the length scales, for the applied load ranges of our experiment. The interfacial shear strength values were calculated for this material pair from the friction data at both scales using appropriate contact mechanics theory and found to be comparable. Friction between silicon nitride and mica at the nanoscale showed an initial non-linearity and then exhibited damage and linearity with normal load beyond certain loads. At the microscale, the mica-silicon nitride interface resulted in a linear friction behavior, which is attributed to the occurrence of deformation.
3:30 PM - AA4.4
Measurement of Ultrathin Film Mechanical Properties by an Integrated Nano-scratch/indentation Approach
Ashraf Bastawros 1
1 Aerospace Engineering, Iowa State University, Ames, Iowa, United States
Show Abstract3:45 PM - AA4.5
Tribological Contact and Friction Behavior of Carbon Nanopearls Synthesized by Nickel-catalyzed Chemical Vapor Deposition.
Chad Hunter 1 , Andrey Voevodin 1
1 AFRL/MLBT, Air Force Research Laboratory, Wright-Patterson AFB, Ohio, United States
Show AbstractMacroscopic quantites of carbon nanopearls, spherical particles with diameter ~150 nm containing both amorphous and graphitic material, were synthesized by CVD using nickel nanocluster-catalyzed dissociation of acetylene at 700 ○C. Test specimens were created by ultrasonically agitating a mixture of nanopearl material and methanol, applying the mixture to a specimen surface using a pipette dropper, and allowing the methanol to evaporate. Ball-on-disk tribometer (using silicon wafer and 440C stainless steel ball) and fretting wear (M50 steel to M50 steel contact) tests using carbon nanopearls dispersed onto the test surfaces were conducted. The nanopearl material effectively reduced the friction coefficient in both humid air and dry nitrogen environments in the ball-on-disk and fretting wear tests. Test surfaces were characterized using SEM and micro-Raman spectroscopy. Surface wear marks produced under 100-500 mN load by a reciprocal motion of a nanoindentor equipped with a spherical diamond tip were compared with macroscopic fretting contact wear. In both cases, compaction and aggregation of nanopearls in wear contacts were observed, the nature of which depended on the test environment humidity. These results and proposed lubrication mechanisms for humid and dry environments are discussed.
4:00 PM - AA4: Nanotribol
BREAK
4:30 PM - **AA4.6
Mechanics of Frictional Sliding.
Ming Dao 1 , Simon Bellemare 3 , Anamika Prasad 1 , Alan Humphreys 2 , Partha Ganguly 2 , Subra Suresh 1
1 Materials Science & Engineering, MIT, Cambridge, Massachusetts, United States, 3 , SGH Consulting Engineers, Waltham, Massachusetts, United States, 2 Mechanical and Materials Sciences, Schlumberger-Doll Research, Cambridge, Massachusetts, United States
Show AbstractThis presentation summarizes recent work on the mechanics of steady state frictional sliding of a hard indenter on the surface of an elasto-plastic material. Experimental measurements of instrumented frictional sliding are used to characterize indenter normal and tangential force versus penetration depth under different conditions. The evolution of material pile-up around the indenter tip (residual scratch profile) is evaluated using a 3D profilometer. Full three dimensional computational simulations are used to develop a quantitative measure of scratch hardness for different elasto-plastic properties of indented materials under different sliding conditions. The mechanics of instrumented frictional sliding is compared to depth-sensing instrumented normal indentation. Possible applications of the present work as a new quantitative indicator of the tribological response are also described. Frictional sliding in the presence of plastic gradients is also examined.
5:00 PM - AA4.7
Determination of Rolling Resistance Moment and Work-of-Adhesion of Microspheres on Surfaces through Nanomanipulation.
Weiqiang Ding 1 , Andrea Howard 1 , Murthy Peri 1 , Cetin Cetinkaya 1
1 Dept. of Mechanical & Aeronautical Engineering, Clarkson University, Potsdam, New York, United States
Show AbstractWith a custom-made nanomanipulator, the response of a microsphere adhered to a silicon substrate under lateral load is explored. Experimental evidence for the existence of rolling resistance moment and data on the critical rolling distance prior to detachment is found. Previously it has been argued that the critical rolling distance should be related to the lattice size and/or the molecular length of the particle and surface materials. However, there has been no theoretical prediction for the critical value and the reasons for the existence of the critical rolling distance are not fully understood. For polystyrene latex particles, measurement presented in current study on silicon surface suggests much higher values for the critical rolling distance than previous anticipated levels. The current approach can also be employed to measure the work-of-adhesion between a spherical particle and a flat surface. Experimental results are compared with the available data and good agreement between the theoretical predictions and the experimental values is found.
5:15 PM - AA4.8
Adhesion and Friction of Polyaromatic Self-Assembled Monolayers.
Marina Ruths 1 , Yutao Yang 1
1 Department of Chemistry, University of Massachusetts Lowell, Lowell, Massachusetts, United States
Show AbstractWe have studied the effects of adhesion on the boundary friction of self-assembled polyaromatic thiol monolayers. The strength of the adhesion between a flat surface and an AFM tip was altered by working in dry N2 gas or in ethanol. At low loads, low adhesion (in ethanol) results in a linear dependence of the friction force on load, whereas higher adhesion (in N2) gives an apparent area-dependence. Recent contact mechanics models for a compliant thin film confined between stiffer substrates are applied to the data obtained in N2. Systematic changes are found in the friction coefficient and critical shear stress with the packing density of the monolayers.
5:30 PM - AA4.9
Theory of Lubrication due to Poly-Electrolyte Polymer Brushes.
Jeffrey Sokoloff 1
1 Physics, Northeastern University, Boston, Massachusetts, United States
Show AbstractIt is shown using the mean field theory of Miklavic Marcelja that it should be possible for osmotic pressure due to the counterions associated with the two polyelectrolyte polymer brush coated surfaces (in the absence of excess salt) to support a reasonable load (i.e., about 10^5 Pa) with the brushes held sufficiently far apart to prevent entanglement of polymers belonging to the two brushes, thus avoiding large static and dry friction due to this entanglement. Adding salt ions actually reduces, rather than increasing the load carrying ability of polyelectrolyte brushes. Using counterions of higher valency will also not improve the load carrying because it will actually reduce the net charge on the polymers, by causing more counterions to undergo Manning condensation. This will in turn reduce the counterion concentration in solution. The load carying ability of the brushes could be improved by using a solvent with a higher dielectric constant. Using denser brushes or better solvents, which increase the ratio of brush height to polymer radius of gyration would improve the load carrying ability, since the minimum thickness of the interface region between the polymer brushes which avoids entanglements that lead to static friction, is approximately inversely proportional to the 1/3 power of the brush height. Making this region less thick increases the counterion osmotic pressure by increasing the counterion density.
5:45 PM - AA4.10
Adhesion and Failure Mechanism of Surfaces and Films: Solid to Liquid Transitions at the Micro and Nano Scales
Hongbo Zeng 1 2 , Jacob Israelachvili 1 2 3 , Matthew Tirrell 1 2 3 , L. Gary Leal 1 2 3
1 Chemical Engineering Department, University of California, Santa Barbara, California, United States, 2 Materials Research Laboratory, University of California, Santa Barbara, California, United States, 3 Materials Department, University of California, Santa Barbara, California, United States
Show AbstractThe classical Hertz, Johnson-Kendall-Roberts (JKR), Derjaguin-Muller-Toropov (DMT), and Maugis theories or models of ‘contact mechanics’ describe the adhesion and deformations of two initially curved elastic solid surfaces. All four theories are static (equilibrium) models, describing the equilibrium or end geometry, but not the dynamic growth of the contact immediately after adhesion or coalescence has occurred. The transients associated with adhesion and/or coalescence of surfaces from elastic solids to viscous liquids are still not clear. We studied the dynamic adhesion and fracture behaviors of surfaces and films using a surface forces apparatus (SFA) and an optical interference technique using fringes of equal chromatic order (FECO), which allowed us to observe the surface deformations at the micro and nano scales in real time. Interesting transient behaviors were found and investigated in detail when a polymer changed from the solid to the liquid state. Unlike polymers in the solid, elastic state which obey the classical theories of adhesive contact mechanics, for polymers in the melt or liquid state a new type of well-ordered transient surface fingering pattern is observed during both adhesive coalescence, and liquid-solid or liquid-liquid spreading. These ripples/waves eventually disappear, leaving smooth polymer-air or polymer-air-solid junctions. These new phenomena were observed not only with polymeric materials but also with small molecular materials such as glucose. The results therefore suggest a general transition in the adhesion (coalescence) and separation (detachment) processes, involving highly complex transient surface shapes, when solid-like materials become more liquid-like. We describe and show the different types of instabilities observed during the approach, contact, coalescence and detachment (snapping or failure) of surfaces as they transit from the solid to the liquid state, and propose an explanation for the observed phenomena in terms of fluid mechanics and the molecular forces between surfaces.References:1.Zeng, H. B.; Maeda, N.; Chen, N. H.; Tirrell, M.; Israelachvili, J. Macromolecules 2006, 39, 2350-2363.2.Zeng, H. B.; Tirrell, M.; Israelachvili, J. Journal of Adhesion 2006, 82, 933-943.3.Zeng, H. B.; Zhao, B. X.; Tian, Y.; Tirrell, M.; Leal, L. G.; Israelachvili, J. Soft Matter 2007, 3, 88 - 93.4.Zeng, H. B.; Tian, Y.; Zhao, B. X.; Tirrell, M.; Israelachvili, J. Langmuir 2007, 23, 6126-6135.5.Zhao, B. X.; Zeng, H. B.; Tian, Y.; Israelachvili, J. PNAS 2006, 103, 19624-19629.6.Zeng, H. B.; Tian, Y.; Zhao, B.X.; Tirrell, M.; Israelachvili, J. N. Macromolecules 2007, submitted.
AA5: Poster Session I
Session Chairs
Eric Le Bourhis
Dylan J. Morris
Michelle L. Oyen
Ruth Schwaiger
Thorsten Staedler
Wednesday AM, November 28, 2007
Exhibition Hall D (Hynes)
9:00 PM - AA5.1
Nanoindentation Analysis of Plasticity Evolution during Spherical Microindentation of Bulk Metallic Glasses.
Jae-il Jang 1 , Byung-Gil Yoo 1
1 Division of Materials Science and Engineering, Hanyang University, Seoul Korea (the Republic of)
Show AbstractUnlike most of crystalline metals, metallic glasses are known to exhibit a fully-plastic behavior or work softening during mechanical deformation. To analyze the characteristics of the deformed region, here a series of instrumented micro- and nano-indentation experiments were performed on Zr- and Pd-based bulk metallic glasses (BMGs) with geometrically self-similar sharp indenter as well as spherical indenters. First, we performed instrumented micro-indentation tests with a spherical indenter on the bonded interfaces of the BMGs. Although adhesive (used for bonding the interfaces) might significantly affect the deformation mode by reducing the constraint, the evolution of subsurface plasticity during spherical indentation was clearly observed. Subsequently, the subsurface plasticity underneath the hardness impressions was systematically examined through nanoindentation. The results are discussed in terms of major change in mechanical responses of BMGs before and after indentation-induced deformation.* This research was supported by the Korea Research Foundation Grant funded by the Korean Government, MOEHRD (Grant # KRF-2006-331-D00273).
9:00 PM - AA5.13
Atomistic Characterization of the Elastic Bending Behavior of Metallic Nanowires.
Matt McDowell 1 , Austin Leach 1 , Ken Gall 1
1 Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia, United States
Show AbstractAs the physical dimensions of a material are reduced to the nanometer size scale, experimental determination of mechanical becomes increasingly difficult. At the bulk scale, mechanical testing is performed with relative ease, by loading a specimen of defined geometry under uniaxial tension and recording values of force and displacement, then extracting relevant properties such as Young’s Modulus. For one-dimensional nanostructures (e.g., nanowires), mechanical testing under uniaxial tension is not yet feasible for many reasons. Recently, new techniques have been developed for the nanomechanical characterization of nanowires, based on the application of a transverse force on a nanowire by an Atomic Force Microscope (AFM). The applied force from the AFM and the tip displacement are monitored and used to extract the elastic properties of the nanowire using principles of continuum beam-bending theory. These tests can provide a quantitative understanding of nanomechanical behavior; however, they give little insight into the qualitative nature of elastic deformation, and specifically why the extracted Young’s modulus values may exhibit variations unseen in bulk materials. In addition, continuum beam-bending theory does not take into account surface energy and surface stress, both of which have been shown to strongly influence the mechanical behavior of nanometer scale materials. Unlike uniaxial tension, specimens subjected to a bending load will exhibit a distribution of axial stress through the cross-section from a maximum (tensile) value at the free surface, to a maximum (compressive) value at the opposite surface. Since in nanowires subjected to bending loads, the stresses are concentrated at the free surfaces, it is paramount to understand the impact of surface energy and surface stress on the resulting mechanical response. Using atomistic simulations we have investigated the bending behavior of metallic nanowires of varying axial and surface orientation to systematically determine the impact of nanowire structure and geometry on the resulting elastic response. Our results show that continuum, strain-energy methods are sufficient to determine the elastic properties of metal nanowires. The calculated elastic modulus values are shown to be independent of free-surface orientation and are consistent with the modulus extracted from simulated tensile deformation of identical nanowire geometries. Furthermore, as the nanowire diameter is increased, the elastic modulus was observed to approach that of a bulk specimen (of corresponding orientation). This research indicates that AFM bending test are an acceptable technique for nanomechanical characterization, and should be considered a sufficient replacement for tensile tests to determine the mechanical properties of nanometer-scale structures.
9:00 PM - AA5.14
Size Effect in the Fatigue Behavior of Amorphous Si Nanostructures.
Churamani Gaire 3 , Catalin Picu 2 , Gwo-Ching Wang 3 , Toh-Ming Lu 3
3 Department of Physics, Rensselaer Polytechnic Institute, Troy, New York, United States, 2 Department of Mechanical Engineering, Rensselaer Polytechnic Institute, Troy, New York, United States
Show AbstractMonotonic and fatigue tests have been performed at the nanoscale on amorphous Si nanostructures that were specially designed and grown for this purpose. The specimens in the form of nanorods and structures of nanorods (e.g., two-arm elbow-shaped) of high aspect ratio, are isolated from each other and are fixed at one end to the substrate. Their geometry was designed using finite element analysis such to obtain a desired stress state. The samples were grown according to these specifications by oblique angle vapor deposition. The AFM tip was used for imaging as well as to locate a position for loading the nanostructures in monotonic bending. Cyclic loading/unloading with controlled force was performed until the specimen failed (fatigue testing). A signature associated with the failure of a specimen during the fatigue test was identified. AFM topographic images of the sample area before and after the tests were taken to verify the failure of a specimen. The nanoscale specimens exhibit inelastic deformation before failure at room temperature, a behavior not seen in larger silicon specimens. They also exhibit fatigue susceptibility, the fatigue life increasing rapidly with the decrease of the applied force amplitude. The fatigue susceptibility also appears to become more pronounced as the specimen size decreases. Possible mechanisms of fatigue failure will be discussed. The work is supported by the NSF under Grant No. 0324492.
9:00 PM - AA5.15
Tip-Induced Calcite Single Crystal Nanowear.
Ramakrishna Gunda 1 , Alex Volinsky 1
1 Mechanical Engineering, University of South Florida, Tampa, Florida, United States
Show AbstractWear behavior of a freshly cleaved single crystal calcite was investigated by continuous scanning using the Nanoindenter in ambient environment as a function of scanning frequency (1–3 Hz) and contact load (2–10 µN) of the Berkovich diamond tip. Initiation of the surface wear takes place at the terrace edges of the cleavage planes of calcite surface which are acting as initial instability points. As the tip moves back and forth along the crystal surface, material slides towards the scan edges as the number of scans increases. Independent of the scanning speed at 2 µN normal load, the complete removal of the 40 nm thick layer was observed after 90 scans and new calcite surface is completely exposed in the scan region showing cleavage planes with terrace edges. After the top 40 nm calcite layer is removed, the wear process stops, as no further material removal is observed for extra 300 consequent scans. Material removal rate increases with contact load and the number of scans required to create a new crystal surface is inversely proportional to the contact load. The wear regime is due to frictional rather than abrasive wear. Single crystal calcite hardness of 2.5 GPa and elastic modulus of 63.8 GPa were measured using nanoindentation and used to model the wear process.
9:00 PM - AA5.16
Investigation of the Sliding Contact Properties of WC-Co Hard Metals Using Nanoscratch Testing.
Siphiliswe Ndlovu 1 , Karsten Durst 1 , Mathias Goeken 1
1 Department of Materials Science & Engineering, Institute I, University Erlangen-Nurnberg, Erlangen Germany
Show AbstractNanoscratch and nanoindentation tests were performed on a range of WC-Co hard metals with varying cobalt content and WC grain size. With low load nanoindentations the individual properties of the phases were determined. Single and multiscratch tests were conducted with various loads. The scratch friction coefficient for all the samples was found to be approximately 0.4 and was observed to fluctuate due to the hard and ductile phases in the material. The scratch width and depth were found to increase with increasing load for single scratches. Multiscratching with a constant load resulted in the widening and deepening of the scratches at each load with accumulative damage occurring as more tests were performed. Focused Ion Beam, FIB cross sections were used to study the damage mechanism in the wear scar. Damage was mainly attributed to a brittle mechanism occurring via the formation and interaction of subsurface cracks and the deformation of WC grains via slip. A deformed layer was formed on the surface of several of the hard metals during multiscratching and this was found to contain WC fragments which were formed during testing. Cobalt extrusion also took place and in the case of the 6 and 6.5% cobalt samples, led to the subsequent loss of loosely anchored WC grains. The scratch depth and width were found to increase linearly with load with more severe grain fracture taking place as the load increased.
9:00 PM - AA5.17
Mechanical Properties of Sputtered Silicon Oxynitride Films by Nanoindentation.
Yan Liu 1 2 , I-Kuan Lin 1 2 , Xin Zhang 1 2
1 Department of Manufacturing Engineering, Boston University, Brookline, Massachusetts, United States, 2 Photonics Center, Boston University, Brookline, Massachusetts, United States
Show AbstractSilicon oxynitride has been recently obtained great attention in integrating into micro-electro-mechanical system (MEMS) due to its composition dependent tunability in optical, dielectric and mechanical properties. In this work, 1 μm thick silicon oxynitride films with different composition of the oxygen and nitrogen content ranging from silicon oxide to silicon nitride were deposited on silicon substrates by direct sputtering of silicon oxide and silicon nitride targets. Creep behaviors of both as-deposited and post-annealed silicon oxynitride films have been investigated by nanoindentation load-controlled relaxation experiments at room temperature. The size effects on nanoindentation creep response of silicon oxynitride films were observed, and the mechanical properties such as Young’s modulus, hardness and creep stress exponents determined by nanoindentation measurements were found varying as a function of film composition and microstructure induced by sputtering and annealing processes. Energy dispersive X-ray spectroscopy (EDX), Fourier-transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD) were employed to characterize the silicon oxynitride films in respect to stoichiometric composition, types of Si-O and Si-N bonding and microstructure of the films. Correspondingly, a mechanism model based on atomic bonding and shear banding theory has been proposed to interpret the variations of both elastic and plastic mechanical properties of silicon oxynitride films.
9:00 PM - AA5.18
Does Small Size Always Favor Strong Adhesion?
Haimin Yao 1 , Pradeep Guduru 1 , Huajian Gao 1
1 Division of Engineering, Brown University, Providence, Rhode Island, United States
Show AbstractRecent studies on the biological adhesion systems revealed that the adhesion strength between two solids will increase and eventually saturate at the theoretical strength as the solid size decreases, implying that small size tends to promote adhesion strength. This finding is essentially based on an implicit assumption that the intermolecular adhesion between two solids can be treated as intersurface forces. Actually, this assumption is not always appropriate because for small solids the intermolecular forces will not just act on a small fraction but over a majority of the solid surfaces and sometimes even extend to the interior of the solids. In that case, the saturation of adhesion strength at small length scale remains undetermined. To solve this problem, here we investigate the size dependence of the adhesion strength by using a more fundamental description of the adhesion force, i.e., molecule-to-molecule interaction. Solids with different geometries are considered, including sphere, cylinder and spherical-shell. While cylinder and spherical shell exhibit strength saturation at small length scale, the strength between two spheres reaches the maximum at specific sphere sizes. Therefore, whether small size favors strong adhesion depends on the geometry of the contact solids. Although no deformation of solids is considered in our analysis, this conclusion also applies, at least in a qualitative manner, to the elastic cases.
9:00 PM - AA5.2
A Reexamination of the Extraction of Material Properties using Nanoindentation.
Benny Poon 1 , Daniel Rittel 2 , Guruswami Ravichandran 1
1 Aeronautics, California Institute of Technology, Pasadena, California, United States, 2 Mechanical Engineering, Technion, Haifa Israel
Show AbstractThe paper reexamines the extraction of material properties using nanoindentation for linearly elastic and elastic-plastic materials. The paper considers indentation performed using a rigid conical indenter, as follows.For linearly elastic solids: The reduction of nanoindentation test data of elastic solids is usually processed using Sneddon’s relation [1], which assumes a linearly elastic infinite half space and an infinitely sharp indenter tip. These assumptions are violated in practical indentation experiments. Since most of the research on the extraction of material properties relies heavily on numerical simulations, we investigate the specimen dimensions required for it to qualify as an infinite body, and how sharp the tip should be, for tip radius effects to be negligible. The outcome of this part is firstly, the definition of a “converged” 2D geometry so that additional magnification of the numerical model does not influence the load-displacement curve, and secondly, an explicit relationship between the measured load and displacement that takes into account the finite tip radius.For elastic-plastic solids: Here, the main data reduction technique was proposed by Pharr et al. [2], assuming elastic unloading of a plastic nanoindentation. We found that the accuracy of the prediction is currently limited by the accurate determination of the projected contact area. This point will be discussed and a new experimental technique to measure the projected contact area will be proposed. Finally, a technique to obtain an upper bound estimate of the yield stress for the indented elastic-plastic material (which is an exact estimation for non-hardening materials), will be presented.References[1] I. N. Sneddon, Proc. Cambridge Philos. Soc. (1948) 492-507.[2] G. M. Pharr, W. C. Oliver, F. R. Brotzen, J. Mater. Res. 7 (1992) 613-617.
9:00 PM - AA5.3
An Algorithm to Determine Plastic Properties using Single Sharp Indenter and Its Application for Thin Film.
Akio Yonezu 1 , Hiroyuki Hirakata 1 , Ye Jiping 2 , Minoshima Kohji 1
1 Mechanical Eng., Osaka University, Suita, Osaka, Japan, 2 Research Dept, Nissan ARC, Yokosuka, Kanagawa, Japan
Show AbstractMany Researchers have explored the algorithm to determine the plastic properties of materials based on the dimensionless function using dual or multiple sharp indenters. We proposed new algorithm to determine the plastic properties using just “one” sharp indenter. This single indentation method can be used for the material which obeys work hardening rules. The algorithm needs dimensionless functions for the parameters of an indentation curve. Finite element method (FEM) was employed to calculate indentation curves of 54 materials with different mechanical properties. These properties included Young’s modulus of between 100 and 300 GPa, yield stress of between 0.1 and 5GPa, and work hardening exponent n of 0.1 to 0.5. Since there are more than two independent plastic properties, we establish two dimensionless functions, which were independent of n, for the calculated indentation curves. One is the general Π function obtained from the loading curvature, C, or indentation work W, based on the approach established by Cheng et.al. The other function was newly proposed from the contact stiffness under unloading process. This function can determine the unique plastic parameter without the idea of representative stress and strain. The reverse analysis in FEM was found that this new algorithm successfully utilize to determine plastic properties of several materials. We, then, applied this method to the metallic film with 200 nm thickness to determine their plastic properties. Although it is difficult to measure the plastic properties of these thin films using a conventional loading test, indentation with our algorithm can obtain the information on plastic properties of the thin films
9:00 PM - AA5.4
On the Determination of Spherical Nanoindentation Stress-Strain Curves, Surface Zero Point, and their Applications.
Sandip Basu 1 , Alex Moseson 1 , Michel Barsoum 1
1 Materials Science and Engineering, Drexel University, Philadelphia, Pennsylvania, United States
Show Abstract9:00 PM - AA5.5
Direct Measurement of Surface Shape for Validation of Indentation Deformation and Plasticity Length-Scale Effects: A Comparison of Methods.
Xiaodong Hou 1 2 , Nigel Jennett 1 , Andrew Bushby 2
1 Materials, National Physical Laboratory, Teddington, Middlesex, United Kingdom, 2 Centre for Materials Research, Queen Mary, University of London, London United Kingdom
Show Abstract9:00 PM - AA5.6
Plasticity Characteristic Obtained by Instrumental Indentation.
Yuliy Milman 1 , Sergey Dub 2 , Alex Golubenko 1
1 Physics of High-Strength and Metastable Materials, Inst. for Problems of Materials Science, Kiev Ukraine, 2 , Institute of Superhard Materials, Kiev Ukraine
Show Abstract9:00 PM - AA5.8
Reversed Plasticity in Nanoindentation.
Yu Lung Chiu 1
1 Chemical and Materials Engineering, University of Auckland, Auckland New Zealand
Show AbstractIn this paper we summarise our recent studies of nanoindentation experiments, focusing on the unloading process. Nanoindentation tests have been carried out in a range of materials, including single crystals copper and aluminium, nanocrystalline copper, bulk metallic glasses and nanocrystalline/amorphous composites. The time-dependent components will be derived from the elastic recovery part in the unloading process and this is compared with the literature reference on creep tests. The reversed plasticity is then obtained by removing the elastic recovery and the time dependent components from the measured unloading data. The reversed plasticity derived will be discussed in the context of plastic deformation mechanisms involved in the nanoindentation of different materials.
Symposium Organizers
Eric Le Bourhis Universite de Poitiers
Dylan J. Morris National Institute of Standards and Technology
Michelle L. Oyen Cambridge University
Ruth Schwaiger Forschungszentrum Karlsruhe
Thorsten Staedler Universitaet Siegen
AA6: Modeling, Simulation and Analysis of Indentation Data
Session Chairs
Wednesday AM, November 28, 2007
Room 208 (Hynes)
9:30 AM - **AA6.1
Are Linear Continuum Mechanics Models Useful for Nano-scale Contacts?
Etienne Barthel 1 , Guillaume Haiat 2
1 Surface du Verre et Interfaces UMR 125, CNRS/Saint-Gobain, Aubervilliers France, 2 B2OA Laboratoire de Mécanique Physique UMR 7052, CNRS , Créteil France
Show AbstractIn macroscopic contact mechanics experiments, the direct observation of the contact zone is a powerful tool. It lies at the roots of traditional indentation testing as well as macroscopic adhesion measurements by the well-established JKR test. For small scale contacts, however, this direct observation is – most of the time – no longer possible. Then the most reasonable approach for routine data analysis is to rely on simple and robust contact models. Sophisticated results on nanoscale material behavior are usually obtained in that way.Clearly these simple and robust contact models derive from a continuum mechanics perspective. Indeed, within linear response, extremely simple relations between contact variables (force, penetration and contact radius) emerge out of the involved arithmetics underlying the Hertz model for the adhesionless contact of elastic homogeneous half-spaces. These simple relations are key to nanomechanical contact data analysis: for instrumented (nano)indentation, Hertzian contact mechanics is implicitly used for routine data analysis through the Oliver and Pharr or equivalent methods. Similarly, for nanoscale AFM adhesion measurements, pull-out forces are turned into adhesion energies through systematic use of the (macroscopic) JKR adhesive contact theory, which is a direct extension of Hertz’.Continuing developments starting in the 60s have shown how the simplicity of the Hertz result carries over into more elaborate systems. We are now in a position to provide exact generalizations of the Hertz model for1) non homogeneous substrates (thin films)2) time dependent materials (viscoelastic)3) adhesive contactsas well as combinations thereof. The aim of this paper is to show what benefit can be derived from these results in the analysis of small scale contact mechanics data. Two specific examples will be developed in details. In the context of the measurement of the intrinsic mechanical properties of thin film materials by nanoindentation, we will discuss recent models for contacts to coated substrates emphasizing two pathological behaviours relevant to experiments: a) elastomeric films b) films considerably more rigid than their substrate. In the context of the surface mechanics of adhesive soft viscoelastic materials, characterised through AFM measurements or small particle embedding, we will show how the competition between stress relaxation and material creep impacts the measured effective adhesion. Implications for the adhesion of viscoelastic materials to rough surfaces will be discussed.
10:00 AM - AA6.2
Contact Stiffness and Incipient Plasticity Studied by Multidimensional Nanocontact Experiments.
Yanfei Gao 1 3 , Warren Oliver 2 , George Pharr 1 4
1 Department of Materials Science and Engineering, University of Tennessee, Knoxville, Tennessee, United States, 3 Computer Science and Mathematics Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States, 2 , MTS Nanoinstruments, Oak Ridge, Tennessee, United States, 4 Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States
Show AbstractA multidimensional nano-contact system has recently been developed that can quantitatively examine tangential mechanical properties at the nano- and meso-scopic length scales (Lucas et al., J. Mater. Res. 19, 58-65, 2004). The ratio of normal to tangential contact stiffnesses can be used to deduce Poisson’s ratio, but the accuracy critically depends on the interfacial friction condition. The analogy between contact problem and bi-material crack helps us rigorously determine the dependence of stiffness correction factor on Dundurs parameter and friction coefficient. Experiments with this instrument have also shown a significant reduction of the tangential contact stiffness relative to the elastic prediction. The reduction occurs at contact sizes below about 50~200nm for aluminum single crystals and several other materials. This reduction can be used to understand the friction mechanism at small scale. The atomic stick-slip, as a consequence of the interatomic periodic potential, is only valid for the concurrent/homogeneous slip, while the initiation of inhomogeneous slip triggers the friction from the contact perimeter.
10:15 AM - AA6.3
Mapping of the Initial Volume at the Onset of Plasticity in Nanoindentation.
Ting Zhu 1 , Sandra Korte 2 , Andy Bushby 1 , William Clegg 2 , David Dunstan 1
1 Centre for Materials Research, Queen Mary University of London, London United Kingdom, 2 Materials Science and Metallurgy, University of Cambridge, Cambridge United Kingdom
Show AbstractUnderstanding the finite volume throughout which deformation begins is necessary to understand the mechanics of small-scale deformation. A proper understanding should enable the design of optimized materials exploiting nano-scale strength enhancement. Jayaweera et al (Proc. Roy. Soc. 2003) concluded that yield occurs over a finite volume at least 100 nm thick. Using semiconductor superlattices to incorporate known internal stresses and to vary the stress and thickness of individual layers opens up new possibilities for investigation that cannot be achieved by varying external stresses on a homogenous specimen. We have designed samples with bands of highly strained InGaAs superlattice, which is essentially a band of low yield-stress material devoid of other metallurgical artifacts, which can be scanned vertically through a series of samples. Different radius spherical indenters were used to determine the elastic-plastic transition. The stress field from different sized indenters interacts with the low yield-stress material at different depths below the surface to trace the size of initial yield volume. With comparing cross-section TEM, we map out the real vertical shape and position of the initial yield under the spherical indentation.
10:30 AM - AA6.4
Influence of Pre-existing Dislocation Density on Nanoindentation Pop-in Events in Single Crystal Ni.
Sanghoon Shim 1 3 , Hongbin Bei 1 3 , Yanfei Gao 2 3 , Easo George 1 3 , George Pharr 1 3
1 Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States, 3 Department of Materials Science and Engineering, The University of Tennessee, Knoxville, Tennessee, United States, 2 Computing and Computational Science Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States
Show AbstractThe discontinuity in the load-displacement curve in nanoindentation tests is referred to as a pop-in event. During this event the indenter sinks into the sample without any load increase for a load-controlled experiment. The pop-in event is often expected to be resulted from the nucleation of one or many dislocations, or more possibly from the activation of a dislocation source from pre-existing immobile dislocations. To our best knowledge, however, there has not been any quantitative approach to correlate pop-in loads to the pre-existing dislocation densities. Experimental observations of pop-in load on single crystal Ni with different amount of pre-strains are conducted and the probabilities of pop-in events are generated. We also constructed an analytical model that considers existing immobile dislocation density and the influence on subsequent dislocation nucleation. The theoretical prediction agrees well with the experiments.
10:45 AM - AA6: Analysis
BREAK
11:15 AM - **AA6.5
Modeling Indentation in Linear Viscoelastic Solids.
Yang-Tse Cheng 1
1 Materials and Processes Lab, General Motors Research and Development Center, Warren, Michigan, United States
Show AbstractInstrumented indentation is often used in the study of small-scale mechanical behavior of “soft” matters that exhibit viscoelastic behavior. A number of techniques have recently been proposed to obtain the viscoelastic properties from indentation load–displacement curves and from oscillatory indentations. We have recently studied indentation in linear viscoelastic solids using analytical and numerical modeling. In this presentation, we derive the initial unloading slope equation for indentation in linear viscoelastic solids using rigid indenters with arbitrary axisymmetric smooth profiles, including conical and spherical indenters. While the same expression is known for indentation in elastic and elasticplastic solids, we show that it is also valid for indentation in linear viscoelastic solids, provided that the unloading rate is sufficiently fast. When the unloading rate is slow, a “hold” period between loading and unloading can be used to provide a correction term for the initial unloading slope equation. Finite element calculations are used to illustrate the methods of fast unloading, “hold-at-the-peak-load,” and “hold-at-the-maximum-indenter-displacement” for determining instantaneous modulus by instrumented indentation. We also establish equations for obtaining storage and loss modulus from oscillatory indentations by performing a nonlinear analysis of conical and spherical indentation in elastic and viscoelastic solids. We show that, when the conical indenter is driven by a sinusoidal force, the square of displacement is a sinusoidal function of time, not the displacement itself, which is commonly assumed. Similar conclusions hold for spherical indentations. Well-known difficulties associated with measuring contact area and correcting thermal drift may be circumvented using the newly derived equations. These results may help improve methods of using indentation for determining viscoelastic properties of solids.
11:45 AM - AA6.6
Indentation of Nonlinearly Viscoelastic Solids.
Michelle Oyen 1
1 Engineering Dept., Cambridge University, Cambridge United Kingdom
Show AbstractMuch recent attention has been focused on the indentation of linearly viscoelastic solids, and analysis techniques have been developed for polymeric material characterization. However, there has been relatively little progress made in the development of analytical approaches for indentation of nonlinearly viscoelastic materials. Soft biological tissues tend to exhibit responses which are nonlinearly viscoelastic and are frequently modeled using a decomposition of the relaxation or creep function into a product of two functions, one time-dependent and the other stress- or strain-level dependent. Consideration here is for soft biological tissue-like responses, exhibiting approximately quadratic stress-strain behavior, which can be alternatively cast as linear dependence of elastic modulus on strain level. An analytical approach is considered in the context of indentation problems with flat punch, spherical and conical indenter shapes. Hereditary integral expressions are developed and solved for typical indentation experimental conditions including indentation creep, load-relaxation and monotonic constant load- or displacement-rate tests. Primary emphasis is on the deconvolution of material and geometrical nonlinearities during an indentation experiment. The simple analytical expressions which result from this analysis can be implemented for indentation characterization of soft biological tissues without the need for computationally- intensive inverse finite element approaches.
12:00 PM - AA6.7
On the Uniqueness and Sensitivity Issues in the Determination of the Elastic and Plastic Properties of Materials through Nanoindentation.
Hongzhi Lan 1 , T. Venkatesh 1
1 Mechanical Engineering, Tulane University, New Orleans, Louisiana, United States
Show AbstractNanoindentation as a technique for extracting the fundamental mechanical properties of materials has recently received considerable attention. Despite the relative simplicity of experimentation there is considerable complexity in extracting the elastic and plastic material properties. Depending on the choice of the geometry of the indenter (i.e., spherical or sharp), the nature of the computational analyzes (i.e., small or large deformation), and the number of indenters used in the experiments (i.e., single or multiple), several algorithms have been developed to capture the force-depth, indentation response of materials (i.e., forward analysis) and the elastic and plastic properties of the indented materials (i.e., reverse analysis). In determining the material elastic and plastic properties of materials through the reverse analyzes, issues concerning uniqueness and robustness of the extracted properties to variations in experimentally measured quantities have been recognized as being important. In the present study: (i) A uniform framework for quantifying and assessing the uniqueness and the sensitivity characteristics of the forward and reverse analyzes, for all the principal methods that have been developed so far, will be identified. (ii) Within a broad range of homogeneous, isotropic, power-law hardening, metal-based, engineering materials, domains where the indentation method would be applicable for an unambiguous identification of elastic and plastic properties through the reverse analysis, will be distinguished. (iii) The differences in the nature of the sensitivity of the indentation analyzes, as a function of the nature of the indentation method, i.e., spherical, or single- and dual – sharp indenters, will be characterized. (iv) Guidelines for selecting appropriate methods for accurate and robust determination of elastic and plastic properties will be provided.
12:15 PM - AA6.8
Determination of Residual Stress and Yield Stress Simultaneously by Indentation Method with Dual Indenters.
Xu Baoxing 1 , Wang Xinmei 1 , Yue Zhufeng 1
1 , Northwestern Polytechnical University, Xi'an, Shannxi, China
Show AbstractThe determination of residual stress with the indentation method has attracted intensive research interests recently. However, most of the literally available methods require the value of yield stress in advance and (or) calculating the contact area of the indenter and the sample. In the present study, we showed the possibility that residual stress and yield stress can be determined simultaneously by indentation method with dual sharp indenters. In addition, it can also avoid the calculation of contact area. In this method, firstly, a serial finite element calculation is done with coarsely pre- evaluated residual stress and yield stress put into the finite element model. A map of residual stress, yield stress and scale analysis constant is constructed. Secondarily, the direction of residual stress, i.e. compressive or tensile residual stress, can be determined by combing the indentation experimental result and the map. Thirdly, further finite element calculations are carried out by limiting the range of residual stress, and a new map of residual stress, yield stress and scale analysis constant is gotten. Fourthly, the relationship of yield stress and residual stress can be obtained by using cross-points of the indentation experimental result and the new map. Finally, yield stress and residual stress can be determined simultaneously by using the cross-points of two different relationships of yield stress and residual stress, which is built with two different size indenters. The results are in good agreement with those obtained by other methods. It should be pointed that only two indentation experiments with two indenters are required during the whole course. Provided the yield stress is known in advance, this method can also be simplified to a usual method.
12:30 PM - AA6.9
Instrumented Indentation Contact with Sharp Probes of Varying Acuity.
Dylan Morris 1 , Robert Cook 1
1 Nanomechanical Properties Group, National Institute of Standards and Technology, Gaithersburg, Maryland, United States
Show AbstractWhile elastic and plastic property extraction from instrumented indentation tests has been well-studied, similarly-based fracture property measurement remains difficult. Furthermore, estimation of fracture properties nearly always requires imaging of the contact, largely defeating the purpose of instrumented indentation. Frequently, small discontinuities in the load-displacement trace (so-called “pop-ins”) are associated with fracture of the material or coating under test, but relation of these events to material properties remains the subject of much conjecture. It is also known that initiation and propagation of cracks on a nano-scale requires a more acute indenter than a Berkovich or sphere, such as the cube-corner pyramid. As an attempt to elucidate some of the mechanisms of fracture at sharp, acute contacts, experiments were performed on a range of elastic, plastic and brittle materials with diamond indenters of acuity varying between the Berkovich and the cube-corner. These experiments reveal what is changed – and what remains the same – when the acuity of the probe is changed, when fracture is initiated at the contact, or both. The physical origin of the extra crack-driving power of acute probes will be also discussed with reference to complementary experiments performed on model materials at the macro-scale. The work provides the basis for the development of instrumented indentation-based nano-scale toughness measurement.
12:45 PM - AA6.10
Can Nanoindentation Measure Unique Elastoplastic Properties?
Xi Chen 1 , Nagahisa Ogasawara 2 , Manhong Zhao 1
1 Civil Engineering and Engineering Mechanics, Columbia University, New York, New York, United States, 2 Mechanical Engineering, National Defense Academy, Hashirimizu, Yokosuka, Japan
Show AbstractInstrumented indentation is widely used to extract material elastoplastic properties from the measured force-displacement curves. One of the most well-established indentation techniques utilizes dual (or plural) sharp indenters (which have different apex angles) to deduce key parameters such as the elastic modulus, yield stress, and work-hardening exponent for materials that obey the power-law constitutive relationship. However, the uniqueness of such analysis is not yet systematically studied or challenged. Here we show the existence of “mystical materials”, which have distinct elastoplastic properties yet they yield almost identical indentation behaviors, even when the indenter angle is varied in a large range. These mystical materials are therefore indistinguishable by many existing indentation analyses unless extreme (and often impractical) indenter angles are used. Explicit procedures of deriving these mystical materials are established, and the general characteristics of the mystical materials are discussed. In many cases, for a given indenter angle range, a material would have infinite numbers of mystical siblings, and the existence maps of the mystical materials are also obtained. Furthermore, we propose two alternative techniques to effectively distinguish these mystical materials. The study in this paper addresses the important question of the uniqueness of indentation test, as well as providing useful guidelines to properly use the indentation technique to measure material elastoplastic properties.
AA7/OO8: Joint Session: Nanomechanics and Tribology of Biological Materials
Session Chairs
Virginia L. Ferguson
Michelle L. Oyen
Wednesday PM, November 28, 2007
Room 208 (Hynes)
2:30 PM - **AA7.1/OO8.1
From Sharks to Gummi-Bears.
Susan Enders 1
1 , Max Planck Institute, Stuttgart Germany
Show AbstractNanoindentation and Nanotribology have been proven to be valuable tools for testing relatively stiff biological materials and polymers. But as always, the devil lies in the detail. For instance, biological samples are almost always composite materials, but only the properties of one particular component are of interest. This component often exists only in very small quantities and cannot easily be separated from the surrounding material. Second, bio-materials are extremely sensitive to desiccation. Testing can only be done under physiological conditions, for which equipment and methods are usually not designed for. Third, the majority of biological samples exhibit modulus and hardness values in the kPa-MPa range in which the accuracy of the measurements is limited.In my talk I will present and discuss experiments addressing all of the three problems above. Measurements on cartilage of fish are examples for tests under physiological conditions. The influence of sample preparation will be shown on samples of rubber-like proteins of insects.One conclusion from our tests on biomaterials is that still a lot of work has to be done to determine the influence of surface identification, viscoelasticity and adhesion on testing compliant materials. As a first step in this direction nanoindentation studies on polymers with different moduli will be presented, with focus on the initial contact identification during indentation testing and surface deformation outside the contact area.
3:00 PM - AA7.2/OO8.2
Nanoindentation Deformation Partitioning Reveals Insight into Mineral-modulus Relationships in Bone.
Virginia Ferguson 1
1 Department of Mechanical Engineering, University of Colorado, Boulder, Colorado, United States
Show AbstractNanoindentation testing of bone is highly sensitive to minute changes in bone composition (i.e., relative amounts of mineral, organic, and water) and microstructural organization. Biological variability, a critical factor in the study of mineralized tissues, is present within a single sample of bone between adjacent, closely spaced sites. Two dimensional indentation arrays were placed within a single region (e.g., osteonal or interstitial) in PMMA-embedded human subchondral bone. The relationship between indentation modulus and mineral volume fraction shows a familiar positive correlation, as has been previously reported for mechanical properties collected via larger scale testing methods, but with substantial variability in both modulus and mineral volume fraction. Elastic indentation modulus (spherical tip, R=5 microns) varied significantly with little change in mineral volume fraction within the spacing of several microns. Similarly, Berkovich indentations demonstrated that creep deformation at the maximum applied load (Pmax) varied spatially between adjacent indentation sites as well as within any single value for mineral volume fraction. These Berkovich data at each indentation site are deconvoluted into elastic, plastic, and viscous components to reveal the relationships of each with mineral volume fraction. Such partitioning of deformation measurements permits insight into one potential source of biological variability in bone physical properties and clarifies the relationship between local mechanical properties and mineralization.
3:15 PM - AA7.3/OO8.3
Dynamic Oscillatory Compression of Individual Cartilage Chondrocytes.
BoBae Lee 1 , Lin Han 1 , Eliot Frank 2 , Alan Grodzinsky 3 4 5 , Christine Ortiz 1
1 Materials Science and Engineering, MIT, Cambridge, Massachusetts, United States, 2 Center for Biomedical Engineering, MIT, Cambridge, Massachusetts, United States, 3 Electrical Engineering and Computer Science, MIT, Cambridge, Massachusetts, United States, 4 Mechanical Engineering, MIT, Cambridge, Massachusetts, United States, 5 Biological Engineering, MIT, Cambridge, Massachusetts, United States
Show AbstractChondrocytes are continuously loaded and unloaded as articular cartilage is deformed during ambulatory motion (from 1 Hz, walking, up to 1 kHz from higher harmonics of impact loading). While static compression of cartilage inhibits chondrocyte synthesis of the tissue’s extracellular matrix (ECM), oscillatory compression is known to stimulate ECM synthesis in animal and human cartilages. Thus, knowledge of the dynamic mechanical properties of living chondrocytes can help to understand how chondrocytes within their native dense ECM respond to physiological mechanical stimuli. In this study, we report the nanoscale dynamic mechanical properties (i.e. complex modulus and phase angle) of single chondrocytes cultured in vitro for increasing periods of time, measured using atomic force microscopy (AFM)-based nanoindentation. The effect of frequency (1-316 Hz), culture duration (freshly isolated (day 0) and cultured bovine chondrocytes (day 7, 14, 21, and 28)), and tip length scale and geometry was investigated (both pyramidal (end radius, Rtip~50 nm) and colloidal spherical (Rtip~2.5 μm) probe tips were employed). Each cell was first pre-indented to 1 μm in a microfabricated silicon well and then a small sinusoidal displacement (~5 nm amplitude) was applied via an external wave generator. Dynamic loading data were analyzed via Fast Fourier Transform by fitting the measured force and displacement to a sinusoidal function to obtain the magnitude and phase of the complex modulus. The complex moduli at each culture day varied with frequency for the spherical probe tip. On day 0, the magnitude increased from 1.52±0.26 kPa at 1 Hz (mean±SEM) to 12.1±1.73 kPa at 316 Hz. The magnitude at each frequency increased gradually with culture duration; e.g., from 13.6±2.07 kPa for day 7 to 21.9±6.63 kPa for day 28 at 316 Hz. Similarly, the phase angle changed with frequency and culture period. For day 0 chondrocytes, the phase angle increased from 18.8±3.78 deg at 1 Hz to 65.8±2.06 deg at 316 Hz. At 100 Hz, the phase angle for day 7 and day 28 cells was 29.5±2.15 deg and 43.8±3.41 deg, respectively. There was a significant effect of frequency on the magnitude and phase angle of the complex modulus at 316 Hz for both probe tips at each culture day (p<0.05, ANOVA with post-hoc Tukey-Kramer test). Similar trends were obtained with the pyramidal tip, but the values were slightly lower. Interestingly, a semi-log plot of the complex modulus vs. frequency showed sharp change at 100Hz, suggesting that different loading mechanisms may exist for higher frequency loading. Thus, the mechanical properties of single chondrocytes depended on frequency, but were not strongly dependent on culture duration in this study. This is first dynamic loading of chondrocytes to our knowledge. Also, the increase in magnitude and phase with frequency is indicative of intracellular energy dissipation; further work is needed to distinguish between visco- and poroelastic dissipation mechanisms.
3:30 PM - AA7.4/OO8.4
Analyzing Structure-Property Linkages in Bone using Spherical Nanoindentation and Raman Spectroscopy.
Siddhartha Pathak 1 , Melanie Patel 1 , Surya Kalidindi 1 , Hayden-William Courtland 3 , Karl Jepsen 3 , Haviva Goldman 2
1 Department of Materials Science and Engineering, Drexel University, Philadelphia, Pennsylvania, United States, 3 Department of Orthopaedics , Mount Sinai School of Medicine , New York, New York, United States, 2 Department of Neurobiology & Anatomy, Drexel University College of Medicine, Philadelphia, Pennsylvania, United States
Show AbstractBone is a composite material whose mechanical performance is strongly dependent on the complex details of its internal hierarchical structure. Using Raman spectroscopy (RS), in combination with a novel nanoindentation analysis technique, we seek to improve our understanding of the linkages of bone’s composition to its local properties at the lamellar level, thus enhancing our ability to predict bone fracture risk.Femora from three different mouse strains, known to differ in their whole-bone mechanical properties (A/J, C3H/HeJ [C3H], and C57BL/6J [B6], 16 wks of age) were embedded in PMMA, sectioned transversely below the third-trochanter and surface polished to 0.05µm. RS (data mapped across 60 x 60 μm ROI at 2 μm intervals) was used to assess the compositional details across the postero-lateral cortex of each sample. Specifically, the mineral to matrix ratio (defined as the phosphate to CH2-wag peak intensity ratio) was calculated to provide a measure of the degree of mineralization of the bone matrix. Nanoindentations (20µm apart, in a single row) were carried out across this same cortex with a 13.5 µm radius spherical diamond tip. In order to better elucidate trends in elastic and post-elastic behavior at the lamellar level in bone, we have developed and utilized a technique for translating nanoindentation generated load-displacement curves into indentation stress-strain (ISS) curves. These ISS curves were used in combination with our RS ratios to study property–composition relationships in these mice bone. Our results demonstrate inter-strain differences in nanomechanical properties which are consistent with those reported in macro-mechanical testing of these mouse strains. Specifically our experiments show a strong correlation between the mineral to matrix ratios obtained from RS and the elastic modulus values obtained from the ISS curves. The modulus values, which are low for regions of lower mineralization, increase across the cortex as the indenter encounters bone of higher mineralization. By using ISS curves we are able to obtain these modulus values from the loading segment itself, where the spherical indenter is pressed on to a flat, undeformed bone surface. The resulting modulus is therefore more accurate than a modulus calculated using unloading curves, which would reflect a surface altered by prior indentation. Moreover, examination of the ISS curves taken at a 40µm distance from the actively forming periosteal surface of each strain demonstrate that all three have similar slopes during (elastic) loading, but vary in post yield behavior; A/J demonstrates the highest hardening slope, while B6 demonstrates the lowest. Our combined property-compositional analysis of bone represents a significant advancement in the characterization of mechanical behavior of bone .As such this study constitutes a crucial first step in the formulation of a rigorous framework for establishing structure-property linkages in bone.
3:45 PM - AA7.5/OO8.5
Nanoindentation as Tool to Investigate Micro-mechanical Properties in the Hierarchical Structure of Biological Materials.
Christoph Sachs 1 , Helge Fabritius 1 , Dierk Raabe 1
1 Microstructure Physics and Metal Forming, Max-Planck-Institut fuer Eisenforschung, Duesseldorf Germany
Show AbstractIn biological materials like arthropod exoskeletons or bone the mechanical properties vary with the length scale due to their hierarchical organization. The exoskeleton of our model organism, the American lobster, represents a chitin-protein-based fibrous nano-composite hardened with nanoscopic particles of amorphous or crystalline calcium carbonate. Chitin protein nanofibrils cluster to form mineralized fibers which are arranged in horizontal planes where the long axes of the fibers are all oriented in the same direction. Theses planes are stacked and gradually rotate around the normal axis, leading to a plywood structure at the microscale. The organism adjusts the mechanical properties to its requirements on the macroscopic level by changing the stacking height of the planes and the grade of mineralization of the fibers. To examine the elastic properties of the fibers on the microscopic level, we performed nanoindentation on cross-sections of air-dried test specimens. Our results show a pronounced orientation dependency of the elastic properties of the fibers as well as differences between the two mechanically relevant layers of the exoskeleton, the endocuticle and the exocuticle, which are correlated with a different stacking height and grade of mineralization.
4:00 PM - AA7/OO8:Bio
BREAK
4:30 PM - **AA7.6/OO8.6
The Role of Non-Collagenous Bone Proteins in Determining the Nanomechanics of Mouse Bones.
Adrian Mann 1
1 , Rutgers University, Piscataway, New Jersey, United States
Show AbstractThe main constituents of mammalian bones are apatite crystals and collagen fibers, however there are also a number of non-collagenous proteins present in bone. These included osteopontin which is important in osteoporosis, osteocalcin which is taken to be an indicator of healthy bone and fibrillin which may play a direct role in determining bone mechanics because of its elastic properties. Using wildtype and knockout mouse models we have been studying the effects of these proteins on the nanomechanics of cortical femora bone. The changes in the nanomechanics can be related to changes in the apatite phase of the bone using micro-Raman spectroscopy. Each of the proteins has a statistically significant effect on the nanomechanics of the bones, but the nature of the effect varies for each protein. In young bones from wildtype mice the hardness and elastic modulus of the cortical bone are much higher than in the osteopontin knockout mice. This suggests that osteopontin is important in the early development and formation of bone, as well as in the loss of bone in older mice due to osteoporosis. Osteocalcin, in contrast, seems only to affect the hardness of cortical bone and not the elastic modulus. Specifically, bones from osteocalcin knockout mice show a statistically significant increase in hardness compared to wildtype mouse bones. The Raman data for these bones shows a possible link between hardness and the degree of carbonate for phosphate substitutions in the apatite. Lastly, the absence of fibrillin 2 is found to cause a significant reduction in the hardness and elastic modulus of the mouse bones, though this is seen predominantly in the central part of the cortical bone midway between the periosteal and endosteal regions. There is no discernible change in the Raman spectra with the presence/absence of fibrillin 2, which suggests the mechanical properties are directly affected by fibrillin 2 rather than indirectly through a change in the apatite phase during bone development and remodeling. In summary, the results show that the non-collagenous bone proteins are very important in determining the nanomechanics of cortical bone, but each of the proteins plays a distinctly different role. In some cases the bone’s formation and modeling is modified while in other cases the protein directly affects the bone’s mechanics.
5:00 PM - AA7.7/OO8.7
In vitro Dynamic Response of Porcine Brain Tissue in Uniaxial Compression and Indentation.
Asha Balakrishnan 1 , Thibault Prevost 1 , Simona Socrate 1
1 , Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Show Abstract5:15 PM - AA7.8/OO8.8
Viscoelastic Behavior of a Centrally Loaded Circular Film Being Clamped at the Circumference.
Kai-Tak Wan 1 , Michelle Oyen 2 , Kuo-Kang Liu 3
1 Mechanical and Industrial Engineering, Northeastern University, Boston, Massachusetts, United States, 2 Engineering Department, Cambridge University , Cambridge United Kingdom, 3 Institute of Science and Technology in Medicine, Keele University, Stoke-on-trent United Kingdom
Show AbstractWednesday, Nov. 28Transferred Poster AA8.6 to AA7.8/OO8.8 @ 4:15 PMViscoelastic Behavior of a Centrally Loaded Circular Film Being Clamped at the Circumference. Kai-Tak Wan
5:30 PM - AA7.9/OO8.9
Nanoindentation of PuraMatrix-Collagen Hydrogels.
Jessica Kaufman 1 , Catherine Klapperich 1 2
1 Biomedical Engineering, Boston University, Boston, Massachusetts, United States, 2 Manufacturing Engineering, Boston University, Boston, Massachusetts, United States
Show AbstractHydrogels composed of PuraMatrix(3DM, Cambridge, MA) and collagen have similar chemical and physical properties to native extracellular matrix. This makes them well suited for tissue engineering applications, in particular for mimicking the environment of soft tissues such as nerve. Mechanical testing of these artificial extracellular matrices poses several challenges. The hydrogels are often made in small amounts and are difficult to test using traditional methods. These very soft hydrogels, with a reduced modulus of 67.5 +/- 16.9 kPa (n=6), must be tested while hydrated to maintain their mechanical properties. Displacement controlled nanoindentation experiments with a 50 micron flat punch tip were performed on a Hysitron TriboIndenter. The indents were first used to calculate the contact stiffness and corresponding reduced modulus. The data were then fit using a 5-parameter Wiechert model to obtain the viscoelastic properties of these hydrogels. These methods indicate that biomaterials in the low-kPa range can be quickly and reliably tested using nanoindentation. The mechanical properties determined here will be used to analyze the cellular response to a material library of PuraMatrix-collagen hydrogels.
5:45 PM - AA7.10/OO8.10
The Graded Structure and Mechanical Properties of the Natural Armor of Polypterus Senegalus.
Ju Ha Song 1 , Benjamin Bruet 1 , Ilan Kalai 3 , Gadi Pelled 3 , Dan Gazit 3 , Mary Boyce 2 , Christine Ortiz 1
1 Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 3 , Hebrew University-Hadassah Medical Campus, Jerusalem Israel, 2 Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Show AbstractPolypterus senegalus are descendants of ancient fish that lived approximately 60 million years ago. Their body is covered by a natural armor consisting of mineralized scales which geometrically interlock and adhere to one another through a connective biomacromolecular fibrillar adhesive. This natural armor protects the fish from predators and harsh environments yet enables the fish to retain great flexibility in their body motion, using a whipping type motion to generate high speed movement. Each scale has a complex shape with “in-plane” dimensions of roughly 2.5 mm by 2.5 mm and a thickness that varies from 0.6 mm to 0.8 mm. Microcomputed tomography (μCT) was used to obtain full three-dimensional constructs of both a single scale and multiple interlocking scales and provided information on the density distribution, the flexible joints between two scales, and the scale-to-scale and scale-to-body interfaces. Optical micrography reveals the cross section to consist of four layers (from outer to inner): ganoine (8 ~ 20 μm thick), dentin (25 ~ 80 μm thick) isopedine (35 ~ 50 μm thick), and bone (200 ~ 500 μm thick). Material properties were reduced from instrumented nanoindentation data using a combination of Oliver-Pharr analysis and three-dimensional elastic-perfectly plastic finite element simulations giving (modulus, yield strength) pairs of (68.4 ± 2.0 GPa, 3.0GPa) for the ganoine, (29.9 ± 4.0 GPa, 750MPa) for the dentin, (17.7 ± 2.0 GPa, 265MPa) for the isopedine, and (17.6 ± 3.2 GPa, 225 MPa) for bone. Subsequently, the validity of the material property fit to the load-depth data was checked by comparison of simulation predictions of the residual depth profile after unloading to that measured by AFM. The properties are observed to transition from a very stiff and high yield strength outer layer to a more compliant lower strength bone region. The gradation in structure and properties is hypothesized to provide a penetration resistant yet tough protection to the fish. To understand the role of the graded layers macroscopically, the material properties for each layer were integrated into microindentation simulations of the scale which model the graded surface structure. In particular, a parametric study considering different thickness ratios of the various subsurface layers on the micro-hardness of the scale was conducted which also revealed the underlying stress and strain distribution in the layers and at the layer interfaces during indentation loading events. The computational analysis showed the load transfer and stress redistribution along the graded interfaces, which show the ratio of outer ganoine layer thickness to dentin thickness to provide a microhardness to resist penetration while minimizing the stress experienced in the layers and at the layer interfaces, giving the possibility of natural geometric optimization.
AA8: Poster Session II
Session Chairs
Eric Le Bourhis
Dylan J. Morris
Michelle L. Oyen
Ruth Schwaiger
Thorsten Staedler
Thursday AM, November 29, 2007
Exhibition Hall D (Hynes)
9:00 PM - AA8.1
Design and Instrumentation of a Micro-compression Apparatus in a Scanning Electron Microscope.
Stuart Smith 1 , Terry Xu 1 , Wei Qiuming 1 , Yaroslav Maksymiv 1
1 Department of Mechanical Engineering and Engineering Science, The University of North Carolina at Charlotte, Charlotte, North Carolina, United States
Show AbstractToday’s manufacturing processes are capable of producing a variety of micro/nanometer sized components from a similarly broad array of materials. It has been shown that properties of materials on the sub-micron scale in many cases differ from their properties on the macro scale. However, due to low availability of precision testing devices, many of the micro/nano scale material properties have yet to be explored. Here, we present our recent progress on design and implementation of an ultra high precision compression apparatus for probing micro/nano scale mechanical properties of materials. To visualize the deformation processes in real time, this device is designed to operate in a scanning electron microscope (SEM). The main features of this new instrument are its relatively tight measurement loop so that errors due to instrument frame distortions are minimized. Additionally, the apparatus is modular for easy accommodation to different sample sizes and also to change load cells to optimize the force range for maximum measurement precision. Modular design also enables disassembly within the SEM chamber while not in use. Dependent upon the load cell being used force ranges are expected to vary from 10 N down to 100 mN with a measurement precision in all cases in the region of 1 part in 10,000. A modified atomic force microscope (AFM) probe system is used to directly measure the relative displacement between the force anvil and the surface of the substrate upon which the material under test is mounted. The probe is designed for a range of approximately 5 micrometers with 0.5 nm resolution at a measurement bandwidth of 1 kHz (an equivalent noise density of approximately 16 pm Hz-½).
9:00 PM - AA8.10
Calibrated Lateral Force Reference Cantilevers for Precise Nanotribological Measurements.
Mark Reitsma 1 , Richard Gates 1 , Robert Cook 1
1 Materials Science and Engineering Laboratory, National Institute of Standards and Technology, Gaithersburg, Maryland, United States
Show AbstractWe present a microfabricated reference cantilever, designed for precise lateral force measurements using an Atomic Force Microscope (AFM). The cantilever device facilitates the application of both normal and lateral forces, comparable to those acting during a typical AFM friction experiment. An in-situ calibration procedure that utilizes the device produces an optical lever sensitivity for lateral forces, which can then be used to scale raw data to obtain precise friction force measurements. Calibration of the lateral force cantilever can be achieved with true force accuracy using materials and techniques developed at the National Institute of Standards and Technology (NIST) that allow for Systeme International (SI)-traceable force transfer to the device. The lateral force reference cantilever is an extension of, and related to, the Calibrated Reference Cantilever Array (CRCA) developed at NIST for SI-traceable flexural stiffness calibration for AFM cantilevers.
9:00 PM - AA8.11
Humidity Effect on the Tribological Behavior of Diamond-like Carbon Films Deposited on Nanopatterned Substrates.
Jin Woo Yi 1 2 , Se Jun Park 1 , Myoung-Woon Moon 1 , Kwang-Ryeol Lee 1 , Seock-Sam Kim 2
1 Computational Science Center, Korea Institute of Science and Technology, Seoul Korea (the Republic of), 2 Department of Mechanical Engineering, Kyungpook National University, Daegu Korea (the Republic of)
Show AbstractDLC films have been, recently, considered as a strong candidate for a protective layer on the medical implant materials such as the artificial hip or knee joint due to an atomically smooth roughness for a low friction coefficient and strong resistance against wear particle formation as well as a high mechanical performance. However, the application of DLC has been limited due to its sensitivity for environmental conditions, especially to relative humidity. In this work, the humidity effect on tribological behavior of the diamond-like carbon (DLC) films coated on nanopatterned substrate was investigated. Ni Nanopatterns ranged from 70 nm to 650 nm in diameters were fabricated on a Si (100) substrate by a rapid thermal annealing of Ni thin films. Using r.f. plasma enhanced CVD, DLC film were then deposited with the thickness of 100nm on the nano-patterned substrate, resulting the surface roughness from 3 nm to 18 nm in root mean square scale. Tribology test was performed for DLC coated surface as varied the sizes of Ni nanopatterns as well as the relative humidity ranged from 0% to 90%. The significant reduction of a friction coefficient for the high humidity was attributed to the formation of transfer layer on the wear scar. With increase of the relative humidity, the transfer layer containing the Fe-oxide wear debris was found to cover the ball surface around wear scar on DLC film with flat surface by analysis with Micro-Raman spectroscopy and Auger electron spectroscopy. However, in case of DLC film coated on Ni nano-patterns, Fe-oxide wear debris was removed from the sliding contact area and it accumulated around the wear scar, which might reduce the friction coefficient between DLC film and steel bearing ball under high humidity. Here we confirmed that by controlling the surface roughness of DLC film in nano-scale, the humidity effect on the frictional behavior between the DLC film and steel bearing ball could be minimized.
9:00 PM - AA8.12
WITHDRAWN 09/10/07 Friction and Adhesion of Controlled Roughness Surfaces in Single- and Multi-asperity Contacts.
Junhong Jia 1 , ChunChia Huang 1 , Yutao Yang 1 , Marina Ruths 1
1 Department of Chemistry, University of Massachusetts Lowell, Lowell, Massachusetts, United States
Show Abstract9:00 PM - AA8.13
Polyaromatic Thiols as Friction Modifiers.
Yutao Yang 1 , Marina Ruths 1
1 Chemistry, University of Massachusetts Lowell, Lowell, Massachusetts, United States
Show AbstractSome aromatic molecules are known to be natural lubricants in diesel fuel. They are stiffer than alkanes and have more complex intermolecular interactions, which also makes their lubrication properties interesting from a fundamental point of view. Friction force microscopy (FFM) was used to study the boundary friction of self-assembled monolayers of simple aromatic and polyaromatic thiols on gold substrates. The friction measurements were done in both ethanol (low adhesion) and dry N2 gas (higher adhesion). Linear relationships between friction force and load were found in ethanol; and non-linear behaviors were found in dry in N2 gas. Systematic differences are observed between systems with different packing density in both cases.
9:00 PM - AA8.14
Investigation of the Mechanical Properties of Different Polymers by Nanoindentation and Correlation of Such Properties with Variations in Relative Humidity.
Nicholas Randall 1 , Poonam Borgaonkar 2 , Emmanuelle Reynaud 3
1 , CSM Instruments, Needham, Massachusetts, United States, 2 Plastics Engineering, University of Massachusetts Lowell, Lowell, Massachusetts, United States, 3 Mechanical Engineering, University of Massachusetts Lowell, Lowell, Massachusetts, United States
Show AbstractTremendous theoretical and experimental efforts have led to important understanding in the indentation behavior of elasto-plastic materials. Viscoelastic or viscoplastic materials, on the other hand, are the focus of a lot of theoretical and experimental research work. This study aims at comparing the experimental behavior of three amorphous and transparent thermoplastics materials. In particular, it is foreseen to check the ability of the nanoindentation set-up to differentiate closely related viscoelastic materials. Previous nanoindentation studies on viscoelastic properties have revealed that the unloading rate, the nature of the experiment (either load-controlled or depth-controlled), the pause time at maximum peak are playing a major role on the contact area between the indenter and the material, the contact depth and therefore the evaluated reduced modulus. A number of authors have suggested adding a pause time between loading and unloading to avoid the occurrence of a bulge or nose on the beginning of the unloading curve, This bulge results from a competition between the loading induced viscous creep and the unloading elastic recovery that the indented polymer sample undergoes. The initial unloading slope is indeed a function of pause time and unloading condition. It has nevertheless been proved in recent studies that that the hold period becomes unnecessary if the unloading is performed at a sufficiently high rate.This study compares the experimental behavior of three different thermoplastics to the nanoindentation test. Attention was paid to the influence of the unloading rate, the pause time between loading and unloading and the relative humidity (RH) level.
9:00 PM - AA8.15
Time Dependent Nanomechanical Properties of Poly (ε-caprolactone)/Hydroxyapatite Nanocomposites for Bone Tissue Engineering Applications.
Rohit Khanna 1 , Kalpana Katti 1 , Dinesh Katti 1
1 Civil Engineering, North Dakota State University, Fargo, North Dakota, United States
Show AbstractRecently, poly (ε-caprolactone)/Hydroxyapatite (PCL/HAP) composites have found applications in the substitution, regeneration and repair of bone tissues and other orthopedic applications. The addition of organic additives such as polyacrylic acid (PAAc) with ionized surface has been reported to influence the nucleation and kinetics of HAP crystallization to a greater extent. In our previous work, the molecular interactions of PCL with insitu and exsitu HAP in PCL/HAP composites were studied using molecular dynamics simulation and FTIR experiments. Further investigation is undertaken to study the role of molecular interactions in influencing the nanomechanical properties of these composites. In view of the above, the objectives of present work are, (a) To compare the nanomechanical (using nanoindentation) and time dependent properties (using dynamic nanoindentation) of model systems: PCL and PCL/HAP composites, (b) To correlate the microstructure with nanomechanics results to better understand the mechanics at the nanoscale, (c) To study the role of crystallinity of polymer phase in influencing the nanomechanical properties of processed composites. Nanoindentation tests have been performed under the varied loads (100μN, 1000μN, and 5000 μN) using a Berkovich diamond tip. Dynamic indentation tests have been performed under a load of 500-1500μN, with a dynamic load of 10μN with 50 Hz frequency. Insitu HAP/PCL composite showed better viscoelastic properties than exsitu HAP/PCL composites. DSC analysis indicate that the crystallinity of PCL, Insitu HAP/PCL composite and exsitu HAP/PCL composite decreases in the following order: PCL (45.6%) > Insitu HAP/PCL composite (35.2%)> Exsitu HAP/PCL composite (25.4%). The influence of interfacial interactions on nanomechanical properties has been explained in the light of change in crystallization kinetics, and microstructural heterogeneity.
9:00 PM - AA8.2
Use of Nanoindentation-based Methods to Explore the Surface Driven Mechanical Behavior of Nanoporous Platinum Films.
Antonia Antoniou 1 , Dhriti Bhattacharyya 1 , Nathan Mara 1 , Elshan Akhadov 1 , S. Picraux 1 , Amit Misra 1
1 Center for Integrated Nanotechnologies, Los Alamos National Lab, Los Alamos, New Mexico, United States
Show AbstractWe have synthesized nanoporous platinum films by electrochemical dealloying of amorphous PtxSi1-x films that were electron beam co-evaporated on a silicon substrate. Varying the Pt to Si ratio in the evaporated films and annealing the films after dealloying produced a range of ligament and pore sizes. For 20 at.% Pt-80 at.% Si films, the average pore and ligament sizes were 20 nm and 15 nm respectively. The Pt ligaments in these nanoporous films were polycrystalline with a grain size on the order of 5 – 10 nm. The mechanical properties of nanoporous platinum films are investigated through nanoindentation, including compression testing of focused-ion-beam machined micro-pillars, and compared to the properties of fully dense platinum as well as macro-scale metallic foams. The dependence of strength is studied as a function of film density as well as ligament size to gain insight on the length scale effects. We observe ultra-high strengths in nanoporous platinum films that are not explained by the scaling laws for the strength of bulk metallic foams as a function of foam density. The mechanical behavior of nanoporous metallic films is discussed in terms of deformation mechanisms involving nucleation of plasticity events at free surfaces. This research is supported by DOE, Office of Science, Office of Basic Energy Sciences.
9:00 PM - AA8.3
Micromechanical Fracture Testing of Individual Grain Boundaries Using FIB Machining and Nanoindentation.
David Armstrong 1 , Steve Roberts 1 , Angus Wilkinson 1
1 Materials, Oxford University, Oxford, OxfordShire, United Kingdom
Show AbstractRecently much work has been carried out into the investigation of plastic and elastic properties of engineering materials on the micro and nanoscale using FIB machining to create micro test specimens, of varied geometries and design, which can then be manipulated using a nanoindenter. We have shown that using FIB machining it is possible to manufacture test specimens consisting of a microcantilever beam with a sharp notch at the root. Such specimens have been used to evaluate the fracture toughness of WCx coatings and nano-crystalline NiW alloys; Si was used as a calibration standard. It has been well documented that small amounts of Bi will segregate to Cu grain boundaries and cause grain boundary embrittlement. Pure copper samples were heat treated and swaged to give a microstructure consisting of large (40um+) columnar grains. Bismuth was introduced to the grain boundaries using a vapour phase transportation method at 250oC. FIB machining was used to create micro-fracture specimens consisting of cantilever beams (12um x 3um x 3um) containing a grain boundary, perpendicular to the major axis of the beam, of known character (measured using EBSD). A sharp notch was inserted at the grain boundary using the FIB. A nanoindenter was used to image and then deflect the cantilever so that failure occurred at the grain boundary. Analysis of the stress strain curve using FEA was used to determine the fracture toughness of the grain boundaries.
9:00 PM - AA8.5
Gold Nanopillar Compression: Strengthening Mechanisms with Nanoscale Growth Twins.
Frederic Sansoz 1 , Konstantin Afanasyev 1 , C. Deng 1
1 School of Engineering, University of Vermont, Burlington, Vermont, United States
Show Abstract9:00 PM - AA8.9
Comparative Study of Nanoindentation Creep in Polymeric Materials.
Ioannis Zyganitidis 1 , Spiridon Kassavetis 1 , Argirios Laskarakis 1 , Stergios Logothetidis 1
1 Lab for Thin Films Nanosystems & Nanometrology, Physics Department , Aristotle University of Thessaloniki, Thessaloniki Greece
Show AbstractThe investigation of the mechanical properties of polymers has become crucial during the last years, since they are increasingly used in a variety of application. Studying the mechanical properties of polymers is more complex compared to inorganic materials, since they display time-dependence response to load. In this work, a series of nanoindentation tests using a Berkovich diamond tip have been applied in the following polymeric materials: Polycarbocate-PC, Polytetrafluoroethylene-PTFE, amorphous and semicrystalline Poly(Ethylene Terephthalate)-PET, Poly(Ethylene Naphthalate)-PEN and polyvinyl-cloride-PVC.The main objective of the present study was the exploration of the time-dependent deformation under constant load. Thus, an appropriate nanoindention test, comprised by a constant loading rate, maximum applied load P>5 mN and a constant-load segment with a time range of 200 s, was chosen. The broad time range (200s) was set to assure the transition from time dependent to steady state creep rate, while the maximum applied load (P>5 mN) permits penetration of the indenter in such depth in order to avoid the influence from surface imperfections of the polymeric materials.In addition, the effect of the loading rate to the creep deformation during constant-load segment was also investigated.Analysis of the displacement-time curves has been done through the use of the Kelvin-Voight viscoelastic material model, a parallel connection of a purely viscous damper and a purely elastic spring. The combination of these two elements makes the model capable to simulate the viscoelastic behaviour of the polymeric material and to assess the modulus of elasticity and the viscosity. The results were comparatively discussed, in order to reveal the time-depended deformation due to nanoindentation creep of the polymeric materials.
9:00 PM - AA8: Poster 2
AA8.6 Transferrred to AA7.8/OO8.8
Show Abstract
Symposium Organizers
Eric Le Bourhis Universite de Poitiers
Dylan J. Morris National Institute of Standards and Technology
Michelle L. Oyen Cambridge University
Ruth Schwaiger Forschungszentrum Karlsruhe
Thorsten Staedler Universitaet Siegen
AA9: Nanomechanics of Polymers
Session Chairs
Thursday AM, November 29, 2007
Room 208 (Hynes)
9:30 AM - **AA9.1
Nanoindentation and Imprint Lithography of Thin Polymer Films.
John Pethica 1
1 , Trinity College Dublin, Dublin Ireland
Show AbstractAn important aspect of nano-imprint processes is the efficient extrusion of very thin films. In principle, residual layers should have negligible thickness and fully planar geometry, and also be reached with minimal imprint times. This talk will describe the limitations in practice, due to polymer properties and stress geometries. A flat punch nanoindentation method to characterise thin polymer films will be described. The method uses self-similar geometries, and has full control of temperature, strain rates and alignment. Most of the expected scalings of polymer response with molecular weight, temperature and strain rates are seen, along with some unusual behaviour at higher stresses and strain rates for films of thickness below Rg. Consequences for imprint material selection will be discussed. The talk will also describe the effects of shear as well as normal loading, and how small amplitude oscillatory shear can drastically increase the efficiency of extrusion below a punch. Unlike traditional forming, the method benefits from friction between indenter and polymer.
10:00 AM - AA9.2
Origin of Thermally Activated and Athermal Friction in Polymers.
Inkook Jang 1 , Peter Barry 1 , Susan Sinnott 1 , Simon Phillpot 1
1 Department of Materials Science and Engineering, Univeristy of Florida, Gainesville, Florida, United States
Show AbstractThermally activated friction and athermal friction behavior have both been experimental observed in tribological studies. Here classical molecular dynamics simulations are used to demonstrate that a single materials system, polytetrafluoroethylene (PTFE) the main component in Teflon™, can manifest both behaviors. In particular, by changing the relationship between the sliding direction and the polymer chain direction, the friction can be switched from a wear-free to a highly wearing friction mode. The simulations unequivocally attribute the thermally activated behavior to wear-free friction mediated by van der Waals forces, and the athermal behavior to wearing. Under wear free conditions, the friction coefficient increases monotonically as the sample temperature decreases from 400 to 100 K in a manner consistent with the Arrhenius equation with an activation energy of 4.0 kJ/mol, which is in excellent agreement with published experimental values of 3.7-5.0 kJ/mol for PTFE. The authors gratefully acknowledge the support of an AFOSR-MURI grant FA9550-04-1-0367.
10:15 AM - AA9.3
Dual Frequency Quantitative Dynamic Mechanical Property Measurements of Polymer Materials at Nanoscale.
Asif Syed 1 , Vitali Brand 2
1 Research and Development, Hysitron Inc., Minneapolis, Minnesota, United States, 2 Mechanical Engineering, Iowa State University, Ames, Iowa, United States
Show AbstractRecently there has been an increased interest in studying the mechanical properties of polymers and biomaterials at nanoscale. For nanoscale mechanical property measurements, the depth sensing indentation technique is very successful and gaining much attention. However, for time and temperature dependent mechanical property measurements, the depth sensing indentation technique has limited use as it is extremely difficult to make a reliable indenter displacement measurement due to unknown thermal drift over an extended period of time. The dynamic force modulation technique can be used to overcome the thermal drift problem to some extent as long as the contact area can be directly calculated from the contact stiffness measurement instead of the contact depth measurement. This method relies on knowing the modulus of the material at the frequency of the test so that the contact area can be calculated from the contact stiffness measurement. If the modulus of the material is frequency dependent and unknown, then the contact area can not be calculated from the contact stiffness measurement, and then the contact depth measurement must be relied upon. In this talk we will present a dual frequency dynamic testing technique using a reference frequency to track the contact area change independent of the indenter displacement measurement. We will show that this technique can be used to measure the quantitative dynamic mechanical properties, such as storage and loss moduli of polymer materials (PMMA, LDPE), over an extended period of time of more than 20 minutes without having much influence from the thermal drift.
10:30 AM - AA9.4
Nanomechanical Properties of Treated Polymers.
Spiridon Kassavetis 1 , Christoforos Gravalidis 1 , Argirios Laskarakis 1 , Stergios Logothetidis 1
1 Physics Department, Aristotle University of Thessaloniki, Thessaloniki Greece
Show AbstractPolymeric materials are of particular interest for many cutting edge applications like flexible optoelectronic devices, data storage etc. Several techniques were used for the modification of the polymer properties, especially in a thin surface layer, in order to optimize and/or functionalize them.This work studies the effect of the: (i) argon ion beam modification procedure and (ii) thermal annealing to the surface morphology and to the nanomechanical properties, such as hardness (H), elastic modulus (E) and deformation due to creep of the Polycarbonate (PC) and the Polytetrafluoroethylene (PTFE), respectively.For this purpose, nanoscale sensitive experimental techniques, such as Scanning Probe Microscopy (SPM) and depth–sensing Nanoindentation (NI) were used. The SPM in semicontact scanning mode was applied for the surface imaging of the polymers before and after the treatment procedure. On the other hand, NI, a well established technique for the nanomechanical characterization of several nanoscale materials, using a Berkovich type diamond indenter was employed to measure the hardness and the elastic modulus of the polymeric materials as well as to study the time depended deformation due to creep. More specifically, in the case of the PC, the Ar ion beam treated surface was firstly characterized in situ, by Fourier Transform IR Spectroscopic Ellipsometry (FTIRSE). The analysis of the FTIRSE spectra showed the preferential bond breaking and the formation of a treated overlayer with different properties from those of the as received PC material. The thickness of this overlayer depends on the Ar ions energy (500 to 1000 eV) and is ranging from 200 nm to 750 nm. As a result, the H and E values were found to significantly decrease for Ar ions with energy up to 750 eV. Further increase of the Ar ion energy results to the improvement of the H and E values, probably due to the crosslinking of the PC macromolecular chains. The nanoindentation creep was studied by applying 110 μΝ maximum load for t=100 s. Data were analyzed using the generalized Kelvin model and the results showed that the Ar ion modification procedure leads to a more viscous material. Finally, the nanomechanical properties and the surface morphology of the PTFE coatings, before and after the annealing, were discussed. It was found that the creep deformation is depended on the molecular weight of the PTFE macromolecule, while the H and E values are affected by the macromolecular conformation at the surface after annealing.
10:45 AM - AA9.5
Interfacial Characterization of Mechanical and Chemical Properties in Multiple Layer Polyurethane Coatings.
Aaron Forster 1 , Li-Piin Sung 1 , Justin Lucas 1 , Chris Michaels 2
1 Materials and Construction Research Division, National Institute of Standards and Technology, Gaithersburg, Maryland, United States, 2 Surface and Microanalysis Science Division , National Institute of Standards and Technology, Gaithersburg, Maryland, United States
Show AbstractThermoplastic polyolefins (TPO) are extensively used in the automotive industry for bumper fascia and interior panels. TPO’s are a family of polyolefin blends consisting of impact modified polypropylene and other polyolefins such as ethylene/propylene and ethylene/butane, along with an ethylene/octane elastomer dispersed phase. These semicrystalline polymers are advantageous because they are cheap, dissipate impact energy to reduce plastic deformation and allow for the incorporation of complicated aerodynamic designs. TPO materials have a low surface energy that complicates the adhesion of polyurethane automotive paints.Original equipment manufacturers solve the poor adhesion associated with TPO materials by using a chlorinated polypropylene (CPO) adhesion promoter. It is believed that solvent swells the TPO surface allowing diffusion of the CPO and thereby increasing wetting and adhesion of the paint. The diffusion length of CPO into the TPO for optimum adhesion has been an important question for coating suppliers and OEM manufacturers. Experimental techniques that have been used previously to quantify CPO diffusion have included Laser Scanning Confocal Microscopy, Atomic Force Microscopy, and Transmission Electron Microscopy. These different approaches have produced a range of interpenetration depths for different material systems. In this research, we investigate the impact of adhesion promoter on the surface mechanical properties of a model clear coat/base coat/adhesion promoter/TPO laminate coating manufactured using two industrially relevant application methods: wet-on-wet and bake. For these materials, the same clear coat, base coat, and TPO are used and the chlorine content or molecular weight of the adhesion promoter is varied. An instrumented indenter is used to scratch the surface of these materials to determine whether the scratch resistance of the coating is affected by the adhesion promoter. Finally, we use instrumented indentation, in conjunction with confocal Raman microscopy, to map the mechanical and chemical properties through the thickness of each layer. We find that the base coat/adhesion promoter/TPO interfaces are mechanically and chemically sharp despite differences in application method and/or adhesion promoter chemistry. In addition, we find that the polyurethane coatings increase in modulus and hardness closer to the air-coating interface.
11:00 AM - AA9.6
Nanoindentation Studies of the Graduated Region in a Polyurethane Spinal Disc Prosthesis.
Jessica Gwynne 1 , Michelle Oyen 2 , Scott Johnson 3 , Robert Snell 3 , Ruth Cameron 1
1 Department of Materials Science and Metallurgy, University of Cambridge, Cambridge, Cambridgeshire, United Kingdom, 2 Department of Engineering, University of Cambridge, Cambridge, Cambridgeshire, United Kingdom, 3 , Ranier Technology Ltd., Cambridge, Cambridgeshire, United Kingdom
Show AbstractThe CAdisc-L is a polyurethane lumbar intervertebral disc prosthesis manufactured using a novel single step injection moulding procedure. The design aims to replicate not only the motion but also the axial compliance provided by a natural spinal disc, by mimicking the structure as closely as possible. It consists of a lower modulus nucleus region surrounded by a higher modulus annulus region, with end-plates to allow both primary and secondary fixation. One of the main advantages of this design over other commercially available disc prostheses is the presence of a graduated region between the annulus and nucleus regions. A sharp interface between two different materials is likely to be a potential failure site due to the difference in mechanical properties, but any such interfaces have been eliminated from this device and the material properties instead change gradually from annulus to nucleus across the disc. This work utilises nanoindentation to study the graduated region in the CAdisc-L. A variety of disc samples were produced, ranging from having no graduated region (effectively a dual modulus disc, with a sharp interface between annulus and nucleus) to having up to 50% of the total disc volume incorporated into the interfacial region. Calibration samples were also produced, ranging in composition between pure annulus and pure nucleus material. The disc samples were cut in half laterally, mounted in resin and polished using an automatic polisher. Small blocks were cut from the calibration samples and these were also mounted and polished using the same polishing procedure. A spherical sapphire tip was used to carry out a series of indents, 100 microns apart, across the disc samples. This allowed the variation in modulus across discs containing different graduated regions to be assessed. Arrays of indents were carried out on the calibration samples to establish the variation of modulus with composition, which was then used to determine the variation in composition across the disc samples. It was observed that neither the modulus nor the composition varies linearly across the discs. The interfacial region becomes wider and the change in modulus becomes less sharp, the greater the degree of graduation. These results suggest that the presence of a graduated region is likely to be beneficial to the lifetime and performance of the device.
11:15 AM - AA9: Polymers
BREAK
AA10: Temperature and Time Dependent Characterization
Session Chairs
Thursday PM, November 29, 2007
Room 208 (Hynes)
11:30 AM - **AA10.1
Progress in High-Temperature Nanoindentation.
C. Schuh 1
1 Department of Materials Science and Engineering, MIT, Cambridge, Massachusetts, United States
Show AbstractNanoindentation is commonly used to (i) measure mechanical properties such as hardness and modulus, and (ii) explore details of deformation mechanisms at the nanoscale. In both of these arenas, the extension of the technique to elevated temperatures is highly desirable, allowing property measurements at true processing or service temperatures, and extraction of activation energies for deformation. This talk will briefly review the state of the art in high-temperature nanoindentation, and highlight our recent efforts to develop techniques for reproducible experimentation at temperatures up to and above 400° C. Applications in both the measurement of mechanical properties and assessment of deformation mechanisms will also be presented.
12:00 PM - AA10.2
Analysis of Indentation Creep.
Donald Stone 1 , Abdelmageed Elmustafa 2
1 Materials Science and Engineering, University of Wisconsin-Madison, Madison, Wisconsin, United States, 2 Mechanical Engineering, Old Dominion University, Norfolk, Virginia, United States
Show AbstractIncreasingly, indentation creep experiments are being used to characterize rate-sensitive deformation in specimens that, due to small size or high hardness, are difficult to study using uniaxial loading. In the present work we use finite element analysis to simulate indentation creep in a collection of materials whose properties vary across a wide range of hardness, strain rate sensitivity, and work hardening exponent. Our studies reveal that the commonly held assumption, that the strain rate sensitivity of the hardness equals the strain rate sensitivity of the flow stress, is violated except in materials with low hardness/modulus ratios like soft metals. Another commonly held assumption is that the area of the indent increases with the square of depth during constant load creep. This latter assumption is vital in an analysis where the experimenter estimates the increase in indent area (decrease in hardness) during creep based upon the change in depth. This assumption is also strongly violated so that, for instance, in a material like fused silica, which creeps a small amount beneath the indenter at room temperature, the area increases in proportion to the depth to the fifth power during creep rather than the second power. Fortunately, both of these violations are easily explained by noting that the “constants” of proportionality relating 1) hardness to flow stress and 2) area to depth squared, are actually functions of the hardness/modulus ratio. Based upon knowledge of these functions it is possible to accurately calculate 1) the strain rate sensitivity of the flow stress from a measurement of the strain rate sensitivity of the hardness and 2) the power law exponent relating area to depth during constant load creep. This work is supported by the NSF under award CMS-0528073.
12:15 PM - AA10.3
Haasen Plot Activation Analysis of Constant-Force Indentation Creep in FCC Systems.
Vineet Bhakhri 1 , Robert Klassen 1
1 Mechanical & Materials Engineering, The University of Western Ontario, London, Ontario, Canada
Show AbstractConstant-force (CF) indentation creep experiments were performed to investigate the length-scale and the temperature dependence of the deformation kinetics of several FCC metallic systems. The measured indentation creep rates were analyzed in terms of a deformation mechanism involving obstacle-limited dislocation glide. The data were used to generate Haasen plots showing the dependence of the reciprocal activation area (1/Δa) upon effective equivalent indentation shear stress (τeff).The length-scale dependence of the kinetics of indentation creep was studied with on annealed single-crystal gold and 20% cold-worked polycrystalline gold at 300K using 1-hour duration CF nanoindentation creep tests performed at initial indentation depths ranging from 200nm to 2000 nm. For both materials, the initial indentation stress (τind t =0) exhibited a clear indentation size-effect however the final indentation stress (τind t=1 hr) showed only a weak dependence upon indentation depth. The measured activation energy of the indentation creep process was insensitive to indentation depth or deformation state and was within the range typical of weak obstacles such as dislocation/dislocation interactions. Haasen plots of 1/Δa versus τeff showed linear trends passing through the origin for both materials at all the indentation depths. The similarity in the slopes of the Haasen plots for both materials suggests that the same type of dislocation/obstacle interactions govern the indentation creep rate of gold irrespective of indentation depth or deformation condition.CF microindentation experiments were also performed on three Aluminum-based alloys (commercially pure Al, PM fabricated- and Wrought-2024 Al alloys) at several temperatures from 473K to 833K to investigate the temperature dependence of the indentation creep deformation kinetics. Haasen plots from these tests yielded nonlinear trends for all the alloys and temperatures tested. We attribute this non-linearity to the recovery of the dislocation rate-limiting obstacles at the elevated temperatures.
12:30 PM - AA10.4
Softening Effect of Dynamic Indentation Techniques.
Megan Cordill 1 , Neville Moody 2 , William Gerberich 1
1 Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota, United States, 2 , Sandia National Laboratory, Livermore, California, United States
Show AbstractDynamic indentation techniques are often used to determine mechanical properties as a function of depth by continuously measuring the stiffness of a material. The dynamics are employed by superimposing an oscillation on top of the monotonic loading. Of interest was how the oscillation affects the measured mechanical properties when compared to a quasi-static indent run at the same loading conditions as a dynamic. Single crystals of nickel and NaCl as well as a polycrystalline nickel sample and amorphous fused quartz and polycarbonate have all been studied. With respect to dynamic oscillations, the result is a softening of the load-displacement curve and thus lower measured hardness values of the ductile crystalline materials. It has also been found that the first 100 nm of displacement are the most affected by the oscillating tip, an important length scale for testing thin films, nanopillars, and nanoparticles.
AA11: In-situ Methods II
Session Chairs
Thursday PM, November 29, 2007
Room 208 (Hynes)
2:30 PM - **AA11.1
Properties of Nanoindentation-induced End-phases in Silicon.
Jodie Bradby 1 , Simon Ruffell 1 , Bianca Haberl 1 , Naoki Fujisawa 1 , Jim Williams 1
1 Electronic Materials Engineering, The Australian National University, Canberra, Australian Capital Territory, Australia
Show Abstract3:00 PM - AA11.2
In situ Transport Measurements During Nanoindentation in Metallic Thin Films.
Douglas Stauffer 1 , David Vodnick 2 , R. Major 2 , J. Parker 1 , C. Leighton 1 , W. Gerberich 1
1 Chemical Engineering & Materials Science, U. of Minnesota, Minneapolis, Minnesota, United States, 2 , Hysitron Inc., Minneapolis, Minnesota, United States
Show AbstractElectrical nanoscale contact measurements can provide a unique insight into the material characteristics for mechanical contacts. Physicists employ metallic point contacts in a number of measurement techniques including superconductor-based techniques such as point contact spectroscopy. It is well known that the size of the contact determines whether it is ballistic or diffusive and controls the contact resistance. Additionally it is known that the resistance is also a function of pressure/volume effects, phases present, and dislocation density. These factors suggest that electrical nanoscale contact measurements could provide important new information in nanoindentation experiments. A series of nanoindentation experiments using the tip as an in situ two point electrical probe were performed using Au (111), Cr (001), Ni (111), and Al (001) thin films. The use of a highly characterized conductive tip allows the deconvolution of the transport measurements in order to give more accurate insight into contact area and dislocation formation during the nanoindentation process.We would like to thank UMN MRSEC for support.
3:15 PM - AA11.3
Influence of the Starting Material on Nanoindentation-Induced Phase Transformed Zones in Silicon.
Simon Ruffell 1 , Jodie Bradby 1 , James Williams 1
1 , Australian National University, Canberra, Australian Capital Territory, Australia
Show AbstractZones of phase transformed silicon created by nanoindentation exhibit a number of structural and electrical properties which are of great interest. Currently, the exact mechanisms behind the formation of these phases are not well understood. Furthermore, little work on the characterization of the properties of the end-phases created by the transformation process has been done. In this paper, we use in-situ electrical characterization to study the phase transformation mechanisms during indentation in both crystalline and amorphous forms of silicon as well as the electrical properties of the end-phases. Additionally, in-situ electrical measurements have been correlated with ex-situ measurements made on nanoindented silicon in specific test structures fabricated in silicon. The in-situ electrical characterization system proves to be extremely sensitive to changes in the silicon beneath the indenter tip. For example, in a regime in which only amorphous silicon is expected to form on unloading, detection of seed volumes (<5% volume fraction) of high pressure crystalline silicon (Si-III/XII) is achievable through the technique. We have probed the end-phases through such post-indentation electrical measurements and have shown differences in the formation of these phases between the two starting forms of silicon. Finally, we show that the introduction of impurity species by ion-implantation has a substantial effect on the transformation behaviour as well as the electrical properties of the end-phases. Interestingly, the conductivity of these end-phases can be controlled over several orders of magnitude by room temperature ion-implantation of traditional silicon dopants prior to nanoindentation. In addition, the formation of high pressure crystalline phases can be modified and we show that it is suppressed by introduction of species such as oxygen and nickel.
3:30 PM - AA11.4
Investigation of Mechanically Induced Crystallization and Shear Band Formation of Bulk Metallic Glass by Conductive-Nanoindentation.
David Vodnick 1 , Ryan Major 1 , Syed Asif 1
1 , Hysitron, Inc., Minneapolis, Minnesota, United States
Show AbstractDepth-sensing nanoindentation is a technique that has been used extensively for characterizing bulk metallic glass at the sub-micro/nanoscale. A new instrument has been developed which allows for the quantitative, simultaneous acquisition of both force-displacement and conductivity measurements. The nanomechanical and electrical measurements used in tandem (Conductive Nanoindentation) has proven to be highly sensitive to probe/sample contact conditions as well as material deformation behavior. This in-situ technique enhances the understanding of the localized material deformation behavior and provides insight into other nanoscale phenomena that may be occurring during the indentation process. Zr, Fe, and Ni- based bulk metallic glass samples of varying composition and volume fraction crystallinity have been analyzed by Conductive Nanoindentation. The correlation of the electrical and mechanical data to pressure-induced crystallization and shear band formation will be discussed.