On the role of plastic deformation during the mild wear of alumina

Polycrystalline α-alumina was worn against Mg-partially stabilized zirconia (Mg-PSZ), using water lubrication, a sliding speed of 0.24 m/s and a load of 10 N. Differential wear between grains (maximum ∼33 nm) and fine (0.3-1.9 μm diameter) abrasive grooves were found on the worn surface. TEM of back-thinned samples indicated widespread dislocation flow at the surface, heterogeneously distributed between grains, and largely associated with abrasive grooves. Those grains standing proud of the surface invariably contained extensive dislocation damage. The dominant slip system was pyramidal ({112̄3}, {21̄1̄2}, {1̄012} and {101̄1}) although occasional (0001) basal slip was also found. No prism slip was observed. The pyramidal slip planes were concentrated at angles of ∼6-33° to the worn surface. Basal slip was frequently associated with basal twinning on planes at ∼72-73° to the worn surface. Dislocation pile-ups at grain boundaries often coincided with grain boundary cracking. The extent of damage from abrasive grooves varied from grain to grain and was dictated by crystallographic orientation more than the grain height. No evidence of mechanical damage was found in those grains that had suffered the highest wear, indicating that material removal had been controlled by tribochemical mechanisms. The origin of the differential wear between grains is considered and the implications of the experimental observations on the time-dependent transition to severe wear in aluminas are discussed.