EFFECT OF GRAIN-BOUNDARY DOPANTS AND MEAN GRAIN-SIZE ON TRIBOMECHANICAL BEHAVIOR OF HIGHLY PURIFIED ALPHA-ALUMINA IN THE MILD WEAR REGIME

The mechanical properties and tribological behavior in the mild wear regime (9.8 and 29.4 N load on 6 mm diameter spheres) have been explored for highly purified alpha-alumina (> 99.5%) as a function of grain boundary dopant concentration from 0 to 1500 ppm Y2O3 and mean grain size from 0.49 to 1.44 mu m. A pin-on-disc tribometer was used for the evaluation of friction and wear. In pure alumina, Vickers indentations produce subsurface cracks detected by Nomarski interference contrast microscopy. No such cracks are formed in the yttria-doped materials. The hardness is higher when the mean grain size is small. The higher amounts of yttria dopant generate precipitates of yttrio-garnet (3Y(2)O(3).5Al(2)O(3)) in the grain boundaries and increase the fracture toughness; they also cause an increase in the friction coefficient. The wear rates decrease with increasing sliding distance at both loads, in agreement with previously reported results. At 29.4 N load, wear volumes in various aluminas are nearly the same; grain fall-out is observed and the wear surfaces are covered with an adherent, relatively soft, layer of wear debris. At 9.8 N, the wear rates scale with the friction coefficient; the wear surfaces are smooth and covered with a very thin layer of wear debris. Fourier transform IR spectroscopy revealed the formation of aluminum hydroxide which is enriched in yttria.