Understanding Fracture Mechanisms in Tough Ceramic Composites Inspired from Nacre

Compared to metals and polymers, ceramics are great candidates for applications involving extreme environments like high temperatures, radiations, humidity or corrosion. The iono-covalent nature of their bonds make them very strong and stiff materials, although intrinsically brittle. This poor resistance to damage propagation greatly limits the use of ceramics in high-performance domains that cannot tolerate catastrophic failure, such as aeronautics or biomedical applications. For this reason, improving toughness has been a major axis of research in ceramic engineering over the past decades. To this day, the key mechanisms highlighted to improve ceramics’ resistance to crack growth mostly reside in a fine control of the microstructure. This includes reinforcing the bulk matrix with fibers and fillers to bridge the crack behind its tip or adding features that resist progression ahead of the crack tip such as residual compressive stresses and transformation toughening. More recently, other microstructures inspired from biological materials have been investigated, specifically ceramic-based materials such a bone, dentin, or nacre. Indeed, millions of years of evolution have gifted these materials with sophisticated hierarchical structures that enable them to resist fracture propagation despite being primarily composed of brittle ceramics. Among these materials, nacre stands out as having one of the simplest microstructures yet exhibiting many toughening mechanisms. Nacre is one of the main components of many seashells and is composed of 95 vol% calcium carbonate platelets stacked in a brick-and-mortar fashion. This microstructure provides nacre with intricate toughening mechanisms such as deflection, branching, interlocking, shearing, viscoelastic dissipation, all acting at different length scales. Taking inspiration from nacre, many processes and compositions have been developed to produce tough ceramic-based composites. Using the current standards of fracture mechanics, it has been proven that nacre-like composites exhibit higher values of fracture toughness and improved crack-resistance curves compared to bulk ceramics. However, characterizing fracture in such materials that exhibit high degrees of deflection and branching remains a challenge, and all the mechanisms that account for improved damage resistance in nacre-like composites have not been characterized or explained. We have worked on characterizing fracture propagation in ceramic-based nacre-like composites, with the aim of determining the role of microstructure and composition on the mechanical response. To this end, we have developed analytical tools validated by finite element analysis to measure crack resistance curves of highly deflected and branched cracks. We have used it to study and reveal the influence of specific parameters related either to the composition, such as the interphase and addition of residual stresses, or to the microstructure, with the presence of short- and long-range order, on the crack propagation behavior in brick-and-mortar structures.