EFFECT OF GRAIN-BOUNDARY IMPURITIES ON THE MECHANICAL AND TRIBOLOGICAL PROPERTIES OF ZIRCONIA SURFACES

It is desirable to establish a relationship between the processing of ceramics, their mechanical properties that can be rapidly measured, fo r instance by indentation (hardness and toughness), their abrasion and scratch resistance, and their tribological performance. This paper ex amines such relationships for transformation-toughened zirconia. Exper iments were performed on two zirconium oxides of similar doping with y ttria (3%), one of high purity and one containing grain boundary impur ities. The results are compared with those of previous work, in which the composition of the zirconium oxide was changed, but the processing and grain boundary purity were constant. It is found that the relatio nship between mechanical properties and tribological performance is co mplex but understandable in terms of the scale of the material respons es (plastic deformation and fracture) with respect to its microstructu re. The yttria content of the zirconia has a large influence on the we ar resistance of the material (which increases with the fourth power o f toughness). At constant yttria content, impurities produce relativel y small changes in mechanical properties and wear behavior. The impuri ties weaken the grain boundaries with the following consequences: zirc onia with pure grain boundaries behaves much like a brittle continuum, the material is hard, and macroscopic cracks develop at the corners o f indentations and underneath wear tracks. Grain boundary impurities l ower the hardness and increase the apparent (macroscopic) toughness by crack diffusion. At low bearing load, wear occurs by microchipping, a nd grain boundary phases have no effect. In water, wear is increased b y intergranular fracture and the presence of grain boundary phases inc reases the wear rate. At high loads (44.5 N), macroscopic cracks under neath the wear track develop early in the pure material; these cracks are retarded by intergranular fissuration in the material with grain b oundary impurities.