Processing of alumina-zirconia composites by surface modification route with enhanced hardness and wear resistance

Zirconia toughened alumina (ZTA) materials are frequently used in mechanical engineering and biomedical applications due to their enhanced toughness, strength and wear resistance compared to monolithic alumina. In this study, a submicron size alumina powder was modified via wet chemical route: the alumina particles surface was coated with zirconium chloride, to yield 10 vol% zirconia by subsequent thermal treatment. From this powder, several ZTA materials were produced by slip casting, sintered at different temperatures from 1475 to 1575 °C. In all materials, a full characterization of their mechanical properties, microstructure and phase composition was carried out, together with wear tests carried out in a linear-reciprocating mode using a Y-TZP ball counterpart under environmental conditions. The results show low wear at sintering temperatures below 1525 °C and high wear at higher sintering temperatures, which can be well correlated to the hardness, microstructure and phase evolution. The microstructure of the materials is initially extremely homogeneous and fine grained. The grain size increases moderately both for the zirconia and alumina components with the sintering temperatures considered. The grain shape of alumina gradually changes from isometric to elongated. Up to 1525 °C, the size of zirconia grains stays below 400 nm. The zirconia transformability was evaluated and it was observed that the zirconia dispersion remains vastly untransformable up to that sintering temperature. In this condition, the alumina matrix is under compressive hydrostatic stress and fracture resistance is moderate. At higher sintering temperatures, grain growth induces higher zirconia transformability and fracture resistance but at the expense of hardness and wear resistance. The simultaneous evolution of tabular morphology in matrix grains also contributes to toughness but facilitates grain breakout and disruption of the surface during final machining and under tribological load.