Microstructure and Mechanical Properties of Tough Biomimetic Alumina-Zirconia Composites Inspired by Abalone Nacre Architecture

Abstract

Many natural materials demonstrate inspirations for the development of strong and tough lightweight composite materials for the applications in various fields like aviation, armour, construction, energy etc. One notable example is the nacreous part of the seashells. Inspired by the ‘brick-and-mortar’ architecture of abalone nacre, we prepared biomimetic all-ceramic composites, by taking α-alumina platelets as brick phase and 3 mol% yttria stabilized tetragonal zirconia polycrystals as an engineered mortar phase. The unidirectional freeze-casting with both brick and mortar phases (5, 10 and 15 vol% zirconia) was carried out at -40oC to produce large-scale ordered porous lamellar structure. The consolidation of the freeze-dried composite scaffold containing both alumina and zirconia was done using spark plasma sintering (SPS). The sintering parameters were optimized to achieve a relative density of >97% for all compositions of alumina/zirconia. The microstructure of the resultant composites reveled large-scale parallel layers of platy-alumina (bricks) with zirconia (mortar phase) well sintered between the alumina platelets, akin to natural nacre structure. The 3-point bending and single edge notch beam (SENB) tests were performed on the bending bar and micro-notched specimens respectively. The flexural strength was found to increase with zirconia vol%. A maximum strength of 270 MPa and fracture toughness of 13.4 MPa√m was obtained for nacre-like all-ceramic composite, which are much higher than monolithic alumina. The fractography analysis revealed that the bioinspired hierarchical alumina/zirconia composite failed by extrinsic toughening mechanisms, viz. large-scale cracks deflections, cracks branching, cracks bridging, similar to abalone nacre.