WEAR TRANSITIONS IN MONOLITHIC ALUMINA AND ZIRCONIA-ALUMINA COMPOSITES

The tribological properties of composite materials can be improved by its specially designed microstructure. Four zirconia-alumina composites (ZrO2-Al2O3) and a monolithic alumina were compared under paraffin oil lubricated wear test to investigate the effect of grain size, the residual stress and the phase transformation on the wear transitions. The tensile stress on the contact surface has been identified as the dominant stress which initiates the fracture wear of brittle materials. Wear transition occurs in the brittle materials when the tensile stress at the contact exceeds the critical value. The effect of zirconia content on the mean grain size of alumina and the wear transition resistance of the materials are presented. The dominant factors for the high wear transition resistance of ZrO2-Al2O3 composites are attributed to the fine grains with narrow size distribution within the material, the compressive residual stress on the surface due to phase transformation of zirconia and the low elastic moduli of ZrO2-Al2O3 composites. The fine grain of the composites and compressive residual stress improve the wear transition resistance, while the low elastic moduli of the composites increase the contact area and reduce the contact tensile stress. The wear transition mechanism of alumina and ZrO2-Al2O3 composites are compared by transmission electronic microscopy. During the tribological contact, ZrO2-Al2O3 composites generate dense dislocations and twins within the fine alumina and zirconia grains to absorb input energy, while monolithic alumina generates the transgranular cracks.