1998 MRS Fall Meeting & Exhibit

Chairs

Richard Goettler, McDermott Technologies Inc
Michael Jenkins, University of Washington
Richard Lowden, Oak Ridge National Laboratory
Karren More, Oak Ridge National Laboratory

Symposium Support 

• Recent Advances in Ceramic Matrix Composites'' sponsored by the U.S. Department of Energy, Office of Industrial Technologies, Industrial Energy Efficiency Division, as part of the Continuous Fiber-reinforced Ceramic Composites Program. 

* Invited paper

SESSION PP1: STANDARD TEST METHODS, DESIGN CODES AND DATA BASES FOR CMCs 
Chairs: T.V. Narayanan and Marc Steen 
Monday Morning, November 30, 1998 
Dalton A/B (S)

8:30 AM PP1.1 STANDARDS, DESIGN CODES AND DATA BASES FOR CFCCS WITHIN THE US DOE's CFCC PROGRAM. Michael G. Jenkins , University of Washington, Seattle, WA; and Edgar Lara-Curzio, Oak Ridge National Laboratory, Oak Ridge, TN. Continuous fiber-reinforced ceramic composites (CFCCs) have been the focus of intensive developmental efforts over the last fifteen years. These efforts have been driven to a large extent by the promise of substantial economic and environmental benefits if CFCCs are used in aerospace and energy-related technologies, particularly at elevated temperatures. Because the commercial diffusion and industrial acceptance of CFCCs can be hampered by lack of standard test methods, data bases or design codes, special emphasis was placed on these within the US Department of Energy's (DOE's) CFCC Program which has been ongoing since 1991. National standardization efforts (primarily through American Society for Testing and Materials (ASTM) Subcommittee C28.07 on Ceramic Matrix Composites) concurrent with the development of CFCCs have focused on methods for the mechanical and thermal evaluation of test specimens and components/structures, and on the drafting of design codes. Harmonization of US national standards with both European (CEN) and international (ISO) standards for CFCCs is critical part of the standardization process for CFCCs. In addition to standard test methods, efforts have also focused on developing design codes such as a section for ceramic and ceramic matrix composite components as part of the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code and a set of volumes on ceramic matrix composites for Military Handbook-17 (Mil-Hdk-17) on composites. The Mil-Hdk-17 effort also includes statistically-qualified data bases for CFCCs which complement the design code. 

8:45 AM *PP1.2 
CEN STANDARDS FOR CFCCS AND TIME-INDEPENDENT BEHAVIOUR OF CFCCS: STOCHASTIC OR DETERMINISTIC? Marc Steen , European Commission, Joing Research Centre, Institute for Advanced Materials, Petten, THE NETHERLANDS. 

Standardization of test methods for CFCCs in Europe fits within a larger effort dealing with the development of standards for advanced technical ceramics including ceramic powders, monolithic, whisker-, particulate and continuous fiber reinforced ceramics, and ceramic coatings. In the current second stage of the CEN (Comit Europen de Normalisation) effort two types of activity are ongoing, namely a critical assessment of the standards that have been produced during the first stage, and the development of new standards. The paper presents the status of these activities for CFCCs and their reinforcements. It describes the operating mode of the CEN subcommittee responsible for CFCC test standards and the way in which the standard developers interact with pre- and co normative research in the European Union. Examples of some recent standard developments are given. Over the last years considerable attention has been paid to the mechanical characterization of CFCCs and the development of standard test methods. From the outset it was realized that mechanical testing, particularly at high temperatures where the potential benefits of using CFCCs are the largest, should be performed under uniaxial rather than under flexural loading. Substantial efforts have therefore been spent both in Europe and the US to develop room and high temperature tensile testing standards, particularly with respect to specifying allowable tolerances on test parameters to reduce the scatter in the results. However, when testing is performed respecting the tolerances imposed in the standards a large variation in the test results may still be observed, even for tests performed within the same laboratory by the same operator using the same equipment. This suggests that in some cases at least there is an intrinsic material contribution to the observed variability, which then conflicts with the expectation of reduced flaw sensitivity of CFCCs. However, an alternative explanation rnay be offered for the occurrence of this scatter. Indeed, the interpretation of the results of tensile tests is done in terms of the applied and measured stress and strain. This evaluation overlooks a critical factor, namely the presence of long range (axial) residual stress fields in CFCCs, who superimpose on the applied stress. The variation in mechanical response observed in different specimens could therefore also be attributed to the scatter in the average axial residual stresses. This hypothesis has been checked for a number of CFCCs. It is found that the variation in axial residual stress state from specimen to specimen can indeed explain the observed experimental scatter, and that moreover it allows to rationalize the temperature dependence of the mechanical response. 

9:15 AM PP1.3 
HIGH TEMPERATURE MECHANICAL TESTING OF CERAMIC MATRIX COMPOSITES. Ming Xie , AdTech Systems Research, Inc., Dayton, OH; Michael J. Verrilli, NASA Lewis Research Center, Cleveland, OH. 

Ceramic matrix composite (CMC) materials have been recently used to manufacture high temperature structural components. However, their stress-strain response, damage and failure mechanisms are still not well understood because of the difficulties in material characterization and structural testing at the high temperatures anticipated for CMC components. Also, only a few commonly-accepted standard test methods exist for these relatively new high temperature structural materials. 
General design and testing techniques of high temperature mechanical tests for ceramic matrix composite materials will be discussed based on more than three years of research sponsored by NASA Lewis Research Center and Air Force Research Laboratory. Detailed designs of a furnace, extensometry, and gripping system will be presented for static, fatigue and creep tests at temperatures of 800 to 1200C. In particular, the following two non-standard high temperature mechanical tests for CMCs will be highlighted. 
CMC's have been proposed for nozzle and combustor applications for advanced aeropropulsion systems. In order to investigate the durability of this class of materials, a special four-point flexural fatigue test is designed to characterize the response of a representative CMC coupon specimen. Currently, there is no standard test method available for flexural fatigue of CMC materials. This special test is based on the ASTM test method for flexural strength of advanced ceramics at elevated temperature and a consensus standard for flexure testing of ceramic matrix composite materials developed within NASA's High Speed Research Program. 
Another specially developed test for ceramic matrix composite materials is a spectrum loading thermomechanical fatigue (TMF) test. Compared to the idealized in-phase or out-of-phase TMF tests, this thermomechanical fatigue test system can apply both thermal spectrum and mechanical spectrum loading to the CMC specimens, which is a more realistic method to approximate the service conditions for high temperature CMC structures. This test method has been used to characterize the thermomechanical fatigue response of CMC coupons with a central hole. 
Typical high temperature experimental results for ceramic matrix composite materials and their sub-structures, such as coupons with holes and bolted single-lap and double-lap joints, obtained using these standard and non-standard mechanical test methods, will be presented, along with the discussion of general test method design and test techniques. 

9:30 AM *PP1.4 
TEST METHODS FOR ASSESSING THE ROLE OF MICRO AND MESOSTRUCTURE IN THE MACROSCOPIC MECHANICAL RESPONSE OF CFCCS. Edgar Lara-Curzio , Metals & Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, TN. 

Because of the availability of ceramic fibers in tows or woven fabric, today most CFCCs are fabricated by infiltrating the matrix into a fiber preform consisting of stacked unidirectional tapes or fabric layers. Consequently, the resulting laminated structure is highly anisotropic in the direction normal to the stacking sequence. In many applications involving out of plane loading and/or through-the-thickness thermal gradients, the interlaminar properties of these materials, and not their in-plane strength, very likely will dictate the operational envelope of components fabricated with these materials. Progress in the development of many CFCCs has been benchmarked by their toughness (area under the tensile stress-strain curve) and by their in-plane tensile strength. Unfortunately, microstructural changes in these materials that have led to increases in toughness and in-plane tensile strength have been achieved at the expense of inter- and translaminar properties. Results from parametric studies focused on the development, validation and standardization of test methods for the evaluation of interlaminar and translaminar properties of 2-D CFCCs and CFCC tubular structures will be presented. The role of the micro and mesostructure of these materials in their macroscopic in-plane and out-of-plane mechanical behavior will be discussed. 

10:30 AM *PP1.5 
INTERNATIONAL STANDARDS FOR CERAMICS AND CERAMIC MATRIX COMPOSITES. Takashi Kanno , Secretary of ISO/TC 206 on Fine Ceramics, Japan Fine Ceramics, Tokyo, JAPAN. 

In response to a proposal of ``global collaboration on standardization at early stages of technological innovation'' by the ISO/IEC President's Advisory Board on Technological Trends, the Japanese member body made a formal proposal to establish a new ISO Technical Committee on Fine Ceramics (Advanced Ceramics). Following a ballot among the ISO member bodies, the establishment of this new technical committee (ISO/TC 206) was approved in November 1992. The scope of this TC is the standardization in the field of fine ceramic materials and products in all forms; powders, monoliths, coatings and composites, intended for specific functional applications including mechanical, thermal, chemical, electrical, magnetic, optical and combinations thereof. The term ``fine ceramics'' its defined as ``a highly engineered, high performance, predominantly nonmetallic, inorganic material having specific functional attributes''. At present, the TC is consist of 10 P (participating)-members including Australia, Canada, China, Indonesia, Japan, Rep. of Korea, Malaysia, Russian Federation, Ukraine and USA, and 22 O (observer)-members. Recent progress of the standardization activities on monolithic ceramics and ceramic composites will be described. 

11:00 AM PP1.6 
SHEAR STRENGTH OF A UNIDIRECTIONAL SiC-FIBER REINFORCED (BaSr)Al₂Si₂O₈ CELSIAN COMPOSITE. O. Unal , N.P. Bansal^a and D.J. Barnard, Ames Laboratory, Ames, IA; ^aNASA Lewis Research Center, Cleveland, OH. 

In-plane and interlaminar shear strengths of a unidirectional Hi-Nicalon fiber reinforced celsian matrix composite containing about 40 volume % fibers were measured between room temperature and 1200C. Tests were conducted using double-notched shear specimens of identical dimensions. The specimen dimensions were 4x4x20 mm with the notch distance of 4.5 mm. Considerable differences in the fracture behavior of these two specimens were observed resulting in vastly different strength values. The factors leading to this discrepancy, particularly the stress concentration factor, were examined by finite element modeling (FEM). FEM models were made two-dimensional using orthotropic material properties. Temperature also had a strong influence on strength over 600C, but its effect on the strength of these two specimens was similar. 

11:15 AM *PP1.7 
ASME BOILER AND PRESSURE VESSEL CODE SECTION FOR CONTINUOUS FIBRE CERAMIC COMPOSITES (CFCCs). T.V. Narayanan , Foster Wheeler Development Corp., Livingston, NJ. 

Compliance with the ASME Boiler and Pressure Vessel Code is a legal requirement in 48 states within the United States. Materials suitable for use in ``Code¹¹-based designs must be specifically referenced and qualified within the ³Code². Since up until recently, the majority of accepted ``Code¹¹ materials have been traditional metals, monolithic and composite advanced ceramics must be qualified for inclusion in the ``Code¹¹ if they are to become serious candidates for advanced engineering designs involving pressurized equipment. The subtask group on ``Ceramic Pressure Equipment¹¹ of the task group on ``Graphite and Ceramic Pressure Equipment¹¹ has existed since 1995 with the stated purpose of introducing impervious graphite and ceramic composites as acceptable, performance-based materials in the ``Code.¹¹ Currently a new ``Code¹¹ section is in preparation which includes chapters on Material Requirements, Design, Fabrication Requirements, Qualification Requirements, Rules Governing Testing, Inspection Requirements, and Marking, Stamping, Reporting. Progress to date, specific issues, and future objectives will be reviewed and discussed. 

11:45 AM PP1.8 MECHANICAL AND THERMAL PROPERTY GENERATION FOR DEVELOPMENT OF A COMPREHENSIVE DATA BASE FOR A SINGLE CFCC FOR MIL-HDBK 17. Michael G. Jenkins , University of Washington, Seattle, WA; Stephen T. Gonczy, Gateway Materials Technology, Mt. Prospect, IL; Edgar Lara-Curzio, Oak Ridge National Laboratory, Oak Ridge, TN; Larry P. Zawada, Air Force Research Laboratory, Wright Patterson Air Force Base, OH. 

Commercialization of advanced ceramics and their composites requires standardization of test methods and design codes including data bases of material properties and performance both for bench marking processing improvements and for use as input for the design of components. National and international standardized test methods for continuous fibre ceramic composites (CFCCs) have been in place for over a decade. However, widely-available data bases are just now being developed for certain commercially-viable CFCCs. One such material, comprised of 8 plies of a ceramic grade Nicalon fabric in a symmetric 0/90 lay-up, a proprietary boron-containing interphase and a silicon nitrocarbide (Sylramic) matrix, was the subject of a multiple-laboratory round robin study to generate mechanical and thermal property data for the development of a comprehensive data base for a single CFCC. Mechanical properties and performance were characterized for in-plane and trans-thickness tensile behaviour (per ASTM Test Method C1275 and an advanced draft standard, respectively), in-plane and interlaminar shear behaviour (per ASTM Test Method C1292) and flexural response (per ASTM Test Method 1341). Physical properties included elastic constants and density. Thermal properties included coefficient of thermal expansion, thermal diffusivity, and thermal conductivity. Tests were conducted on up to 100 test pieces of a particular geometry by up to 10 laboratories. The statistically significant data sets will be included in evolving material data bases for CFCCs such as those being developed within in the Mil-Handbook-17 Ceramic Matrix Composite effort. These data sets will be accessible primarily in electronic form to facilitate their retrievability and/or dissemination. 

SESSION PP2: ENVIRONMENTAL EFFECTS 
Chairs: Karren L. More and Gregory N. Morscher 
Monday Afternoon, November 30, 1998 
Dalton A/B (S)

1:30 PM *PP2.1 
DEGRADATION OF BN INTERFACES IN CFCCs. P.F. Tortorelli , K.L. More, R.A. Lowden, E. Lara-Curzio and H.T. Lin, Oak Ridge National Laboratory, Oak Ridge, TN. 

The mechanical reliability of ceramic matrix composites (CMCs) at elevated temperatures in corrosive environments will be primarily dependent on the chemical and structural stability of the fiber and flber/matrix interface. The use of graphitic carbon fiber coatings is limited in high temperature oxidative environments due to the rapid loss of carbon and subsequent oxidation of the fiber and matrix. Different variations of BN are being evaluated as alternative interfacial coatings to carbon as a result of comparable room temperature properties with significantly improved oxidation resistance. The stability of BN interfacial coatings in several different Nicalon fiber-containing composites was investigated at 600C and 950C in either flowing oxygen or environments containing water vapor. Degradation mechanisms determined for the BN-containing, composites using techniques such as thermogravimetric analysis and transmission electron microscopy. The effect of several factors on BN stability, including crystallographic structure, extent of BN crystallization, and impurity content, were analyzed in this study. The individual roles of these factors evaluated for the BN within the composite systems (as opposed to evaluation of each constituent independently) is critical to understanding the overall composite behavior. 

2:00 PM PP2.2 
OXIDATION OF BN AND OXIDE INTERFACE LAYERS IN CERAMIC COMPOSITES. Brian W. Sheldon and Ram Krishnamurthy, Brown University, Division of Engineering, Providence, RI. 

Oxidation reactions which degrade fiber/matrix interfaces in ceramic composites impose serious limitations on the application of these materials. A number of complex systems have been developed to improve oxidation resistance, including BN and oxide interface layers in SiC matrix composites. A kinetic model was developed to describe the oxidation processes in these systems. This model describes the formation of both solid and volatile oxides, along with multicomponent gas-phase diffusion and solid-state diffusion through the solid oxide. These calculations were compared with experimental results. Model specimens with a nano-porous Al2O3 interface layers were prepared using a spray-deposition technique. In addition to measuring oxidation kinetics in these materials, fracture experiments were also conducted to study the effects of oxidation on crack deflection and interfacial fracture resistance. 

2:15 PM PP2.3 
KINETICS OF OXIDATION OF CERAMIC MATRIX COMPOSITES. Pavel Mogilevsky , Avigdor Zangvil, University of Illinois at Urbana-Champaign, Materials Research Laboratory, Urbana, IL. 

Internal oxidation of SiC reinforcement is a major factor affecting the environmental stability of SiC reinforced CMCs for high temperature applications. A model describing the oxidation of SiC reinforced oxide ceramic matrix composites is presented. The model allows to calculate the oxidation rate of the reinforcements, if the oxygen permeabilities of silica and the matrix are known. Alternatively, the model allows to evaluate the oxygen permeabilities of silica and the matrix from the experimental oxidation data. Moreover, the expected mode of oxidation, I or II, can be predicted depending on oxygen permeabilities and volume fraction of the reinforcement phase. Application of the model to the results of the microscopic study of the oxidation of SiC reinforced mullite/zirconia matrix composites is discussed. 

2:30 PM PP2.4 
EFFECT OF PYROLYSIS PROCEDURES ON OXIDATION RESISTANCE OF BLACKGLAS(TM) MATRIX COMPOSITES. Michael McNallan , D. Ersoy, CME Department, University of Illinois at Chicago, Chicago, IL; Y. Gogotsi, Department of Mechanical Engineering, University of Illinois at Chicago, Chicago, IL; P.J. Balin, R. Rao, Illinois Math and Science Academy, Aurora, IL. 

Blackglas(TM) is a silicon oxycarbide ceramic which is produced by pyrolysis of a preceramic polymer. Blackglas(TM) ceramic matrix composites can be produced by conventional polymer matrix composite fabrication techniques and converted into ceramic matrix composites by pyrolysis. Blackglas(TM) composites reinforced by Nextel(TM) 312 fibers have been subjected to long term oxidation in dry air at 600 C. At this temperature carbon, contained in the amorphous silicon oxycarbide matrix, is oxidized, producing a product with lower mechanical strength than the composites had immediately after fabrication. Oxidation leads to a gradual reduction in the flexure strength and toughness of the composite. The reduction in mechanical propeties occurs much more rapidly for composites pyrolyzed at 900 C than for composites pyrolyzed at 1000 C. The oxidation can also be observed by optical microscopy on polished sections of the composites. The mechanical and microstructural properties of oxidized Blackglas(TM) ceramic matrix composites pyrolyzed at 900 and 1000 C are compared and interpreted. 

3:15 PM *PP2.5 
INTERMEDIATE TEMPERATURE EMBRITTLEMENT OF Hi-NICALON, BN, SiC MATRIX COMPOSITES IN AIR. Gregory N. Morscher , Case Western Reserve University, Cleveland, OH. 

The embrittlement of SiC fiber reinforced SiC matrix composites at intermediate temperatures (600 to 1000C) has been determined for single tow minicomposites and woven macrocomposites consisting of Hi-Nicalon fibers, BN interphase, and SiC matrices. Data from tensile stress-rupture and low cycle fatigue will be presented for several systems. Modal acoustic emission is used during tensile testing to monitor the amount and location of damage accumulation in the composite. Scanning electron microscopy was performed on the fracture surfaces, including fiber fracture mirror analysis. The extent of oxidation ingress, load-bearing tow embrittlement, and the effect of stress, time, temperature, and environment on the fiber strengths were determined. The rate of loss in load carrying ability depends on the damage accumulation in the matrix, the stability of the interphase, and the mechanism which causes fiber breakage. Differences due to the geometry, loading sequence, and nature of cracking in the different samples will be discussed. 

3:45 PM PP2.6 
FAILURE MECHANISM MAP FOR ENVIRONMENT INDUCED CRACK GROWTH IN SIC/SIC COMPOSITES. Russ Jones , Charles Lewinsohn and Charles Henager, Jr., Pacific Northwest National Laboratory, Richland, WA. 

There are a number of high-temperature processes that can produce structural damage in ceramic matrix composites. Time-dependent matrix cracking as a result of fiber relaxation mechanisms (FRM) is an environmentally- independent process while interphase removal mechanism (IRM) and oxidation embrittlement mechanism (OEM) are two environmentally-dependent processes. FRM, IRM and OEM occur at different environmental conditions and exhibit different effects on the material. FRM occurs over a range of temperatures but at very low oxygen concentrations while both OEM and IRM require the presence of oxygen. IRM occurs at temperatures exceeding the glass transition temperature for silicon dioxide and oxygen partial pressures less than atmospheric while OEM occurs at temperatures below the glass transition temperature and at high oxygen partial pressures. A 2-dimensional failure mechanism map for temperature and oxygen partial pressure has been developed for SiC/SiC. Additional dimensions including time and interphase thickness have been examined but not fully mapped. A description of the failure mechanisms, the data and modeling to support the mechanisms and the resulting failure map will be presented. 

4:00 PM PP2.7 
ENVIRONMENTAL EFFECTS ON THE LIFETIME RESPONSE OF NICALON FIBER-SiC COMPOSITES. H.T. Lin , P.F. Becher, and P.F. Tortorelli, Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, TN. 

Stress-lifetime studies were carried out in four-point flexure on Nicalon fiber-reinforced FCVI SiC matrix composites with a carbon fiber coating at temperatures of 950 and 1150C in air and argon atmospheres. The objective of this study was to evaluate the effect of testing environments on the lifetime and long-term reliability performance. Two Nicalon/SiC composites containing 43 vol.  fiber content, but with differences in fiber cloth lay-up, carbon coating thickness, and open porosity were evaluated in the present study. Test results of both composites indicated that lifetimes in argon atmosphere were, at least, three orders of magnitude longer than those obtained in air at stress levels above the apparent fatigue limit. Also, lifetimes in argon atmosphere were not sensitive to the differences in material parameters observed between the two composites investigated. The extended lifetimes in argon environment were attributed to the extremely slow oxidation reaction of carbon fiber coating and fiber degradation, as confirmed by thermogravimetric analysis and SEM examinations. 

SESSION PP3: OXIDE/OXIDE COMPOSITES 
Chairs: Richard Goettler and Sankar Sambasivan 
Tuesday Morning, December 1, 1998 
Dalton A/B (S)

8:45 AM *PP3.1 
CERAMIC FIBER COATINGS AND FIBER STRENGTH. Randall S. Hay , Emmanuel Boakye, M. Dennis Petry, AFRL Materials Laboratory, Wright Patterson Air Force Base, OH. 

Extension of the T-t operation envelope probably requires an oxidation resistant analogue to C or BN interfaces for most CMC applications. Fiber or preform coating is necessary to make this interface. Coating uniformity and low cost without fiber degradation are desired coating characteristics. Porous oxide, monazite, and various other fiber coatings derived from sols and solutions are described for oxide and SiC fiber tows. Trends in coating thickness with coating speed, temperature, and precursor characteristics such as viscosity, density, concentration, and gas evolution are presented, and methods for modeling the coating process are discussed. Significant loss in fiber tensile strength is sometimes observed after fiber coating. Experiments that attempt to correlate strength loss with precursor stoichiometry, gas evolution, temperature, or coating thickness are described. Control experiments that attempt to isolate degradation phenomenon that are independant of final coating composition but dependant on the precursor chemistry are also described. Degradation mechanisms and methods that may avoid or minimize degradation are discussed. 

9:15 AM PP3.2 
EFFECT OF THERMAL DEGRADATION AND RESIDUAL STRESS STATE IN AN OXIDE/OXIDE CFCC. Christian X. Campbell and Michael G. Jenkins , University of Washington, Seattle, WA. 

Oxide fibre-reinforced / oxide matrix CFCCs have the potential of resisting the high-temperature degradation of these materials in the increasingly aggressive environments of the emerging applications of this still-evolving class of materials. An Al₂O₃ (Nextel ) fibre-reinforced / Al₂O₃ matrix composite with an oxidation resistant BN/SiC interphase was investigated in this study for its high temperature performance. Monotonic and load-unload tensile tests at room temperature conducted on specimens which had been exposed to 600, 800, 1000 and 1200.