The effect of grain-boundary devitrification on the wear of glass-bonded alumina ceramics

The steady-state wear behaviour of a 94 wt% alumina was investigated in the as-fired condition and after a post-sintering heat treatment. The post-sintering heat treatment yielded devitrification of the 6 wt% calcia-magnesia-alumino-silicate (CaO · MgO · Al₂O₃ · SiO₂) glass grain-boundary phase. In addition, the effect of surface finishing on the wear behaviour of as-fired and heat-treated samples was studied. Steady-state wear rates were determined using a single-pin-on-disc tribometer. The results indicated that heat treated, unfinished samples exhibit a higher steady-state wear rate than as-fired, unfinished samples. The differences observed may arise in response to one or more of the following mechanisms: (i) creation of intergranular thermoelastic stresses due to thermal-expansion mismatch among intergranular species, (ii) elimination of the lubricative glass phase in devitrified specimens, and (iii) elimination of the advantageous effects of viscoplastic deformation of the intergranular glassy phase on stress relaxation. Surface finishing further increased the steady-state wear rate of the heat-treated samples only, and it correlated with an increase in subsurface microcracking and grain pull-out. A lubricative glass film was found to persist on all sample wear tracks, suggesting that the differences in wear behaviour are dominated by intergranular fracture and grain pull-out.