Microstructure and fracture behaviour of biomimetic Al2O3-ZrO2 composite with 2-levels of structural hierarchy

Abstract

We report new class of natural nacre-like alumina toughened by zirconia fabricated by the simple two-step approach of unidirectional freeze-casting and spark plasma sintering. The biomimetic alumina-zirconia with a relative density of more than 98 % was achieved by consolidating at temperature of 1300–1425 °C with 50 MPa pressure. The resultant microstructure consists of two levels of structural hierarchy, i.e. level-1: bulk lamellar brick phase (alumina platelets) and level-2: mortar phase (yttria stabilized tetragonal zirconia polycrystals). The increase in volume fraction of zirconia (in tetragonal form) from 0 to 15 vol% was effective in progressively reducing the size of alumina platelets from 12 to 9 μm, and significantly improved the flexural strength of the composite by 60 % (from 172 to 270 MPa). By optimizing the hierarchical architecture from micro to macroscopic level, the structural performance of our synthetic nacre, where strength and toughness of 270 MPa and 13.5 MPa√m respectively are superior to that of natural nacre and many other engineering materials (illustrated in the form of Ashby map). The exceptional damage resistance is rationalized by both intrinsic (stress induced tetragonal to monoclinic phase transformation of zirconia) and extrinsic (crack deflection, crack branching and bridging, multiple cracking, platelets interlocking) toughening mechanisms. For 15 vol% ZrO2 composition, the contribution of intrinsic and extrinsic toughness was quantitatively evaluated as 0.9 ± 0.03 and 4.3 ± 0.07 MPa√m respectively (with experimentally measured toughness at crack initiation as 5.2 ± 0.2 MPa√m).