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, for instance by indentation (hardness and toughness), their abrasion and scratch resistance, and their tribological performance. This paper examines such relationships for transformation-toughened zirconia. Experiments were performed on two zirconium oxides of similar doping with yttria (3%), one of high purity and one containing grain boundary impurities. 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 relationship between mechanical properties and tribological performance is complex but understandable in terms of the scale of the material responses (plastic deformation and fracture) with respect to its microstructure. The yttria content of the zirconia has a large influence on the wear resistance of the material (which increases with the fourth power of toughness). At constant yttria content, impurities produce relatively small changes in mechanical properties and wear behavior. The impurities weaken the grain boundaries with the following consequences: zirconia with pure grain boundaries behaves much like a brittle continuum, the material is hard, and macroscopic cracks develop at the corners of indentations and underneath wear tracks. Grain boundary impurities lower the hardness and increase the apparent (macroscopic) toughness by crack diffusion. At low bearing load, wear occurs by microchipping, and grain boundary phases have no effect. In water, wear is increased by intergranular fracture and the presence of grain boundary phases increases the wear rate. At high loads (44.5 N), macroscopic cracks underneath the wear track develop early in the pure material; these cracks are retarded by intergranular fissuration in the material with grain boundary impurities.