Wear of yttria-doped α-alumina: effects of grain-boundary composition and machine stiffness

Abstract

The study used four samples of α-alumina, with grain sizes 0.49–1.44 μm, dopant concentrations 0–1500 ppm yttria, and a sintering aid of MgO at a concentration of 500 ppm in all samples. Wear was measured in unlubricated sliding at low velocity under loads of 60–100 N in ambient air. This load range comprises the intermediate wear regime, which is sensitive to grain boundary fatigue and grain pull-out, and the regime of catastrophic wear characterized by macroscopic fracture. The results show that the transition between intermediate and catastrophic wear is determined by machine vibrations. In the intermediate wear regime, the wear rate is higher for material with an yttria grain boundary phase. Yttria promotes adhesion between the sliding surfaces and decreases the grain boundary strength. In this regime, the wear volume increases with a high power of the applied load, in apparent contradiction with the fact that the real contact area is proportional to the load and therefore the local contact stresses are independent of it. The contradiction is lifted by recognizing that the stresses at grain boundaries occur at a sufficient distance below the surface to cause a load dependence of the local stresses.