UC Berkeley

Coextrusion is shown to be a viable processing route for the fabrication of bioinspired ceramic materials that exhibit improved damage tolerance. This provides one of the few examples of the synthesis of "nacre-like" ceramic hybrid structures with “brick-and-mortar” architectures using a method that includes a metallic “mortar” (i.e., the compliant phase) during the entire fabrication process, instead of infiltrating it into a pre-fabricated ceramic scaffold. Detailed examination shows how manipulation of such processing can lead to improved mechanical performance in the resulting biomimetic ceramics by obtaining a model high volume fraction (~90 vol. %) ceramic with a brick-and-mortar architecture containing a metallic compliant phase. Specifically, brick-and-mortar alumina hybrid structures were synthesized containing small volume fractions (<10 vol. %) of nickel (Al2O3/Ni) which were made by the coextrusion of alumina and nickel oxide in a thermoplastic (polyethylene-ethyl acrylate) suspension. Furthermore the contributions from brick morphology as well as size are examined to extrapolate on the mechanical properties of idealized metal compliant phase brick-and-mortar structures. Flexural strength and crack-initiation fracture toughness values were used to compare the performance of various ceramic architectures, with full crack-growth resistance curves (R-curves) measured and compared to similar bioinspired ceramics for the most promising alumina structures. These mechanical tests were performed as a function of temperature to explore the evolution of toughening contributions of the mortar and structure as it relates to changes in the mortar plasticity and to consider the viability of these structures in elevated temperature environments. It was found that even though these structures are significantly coarser than those made with other processing methods, they still exhibit comparable crack-growth resistance despite their lower strength. Toughening was induced by marked crack deflection as the crack path followed the metallic mortar phase coupled with significant crack bridging and brick pull-out in the image of the toughening mechanisms seen in nacre. These effects were shown to be more pronounced as brick size reduced closer to natural structures and additionally it was observed that crack growth resistance improved as a function of increasing temperature due to greater ductility in the metallic mortar.