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 r egime (9.8 and 29.4 N load on 6 mm diameter spheres) have been explore d for highly purified alpha-alumina (> 99.5%) as a function of grain b oundary dopant concentration from 0 to 1500 ppm Y2O3 and mean grain si ze from 0.49 to 1.44 mu m. A pin-on-disc tribometer was used for the e valuation of friction and wear. In pure alumina, Vickers indentations produce subsurface cracks detected by Nomarski interference contrast m icroscopy. No such cracks are formed in the yttria-doped materials. Th e hardness is higher when the mean grain size is small. The higher amo unts of yttria dopant generate precipitates of yttrio-garnet (3Y(2)O(3 ).5Al(2)O(3)) in the grain boundaries and increase the fracture toughn ess; they also cause an increase in the friction coefficient. The wear rates decrease with increasing sliding distance at both loads, in agr eement with previously reported results. At 29.4 N load, wear volumes in various aluminas are nearly the same; grain fall-out is observed an d the wear surfaces are covered with an adherent, relatively soft, lay er of wear debris. At 9.8 N, the wear rates scale with the friction co efficient; the wear surfaces are smooth and covered with a very thin l ayer of wear debris. Fourier transform IR spectroscopy revealed the fo rmation of aluminum hydroxide which is enriched in yttria.