Wear transitions and tribochemical reactions in ceramics

Abstract

Structural ceramics are used in diverse tribological applications due to their unique properties that include resistance to abrasion and erosion, resistance to corrosive wear, wear resistance at elevated temperatures, low density and unique electrical, thermal and magnetic properties. Applications include precision instrument bearings, cutting tool inserts, prosthetic articulating joints and engine components. Following a brief overview of the processing methods applied to alumina, silicon nitride and silicon carbide, the wear behaviour of these ceramics is reviewed. The wear behaviour changes as the load or the coefficient of friction is increased. This change is associated with a drastic increase in wear at a critical load that depends on the material and test conditions. This transition from mild to severe wear occurs through a microfracture process at the sliding contact. Contact mechanics is used to develop a simple model to determine whether contact failure occurs by a classical brittle process (i.e. cone cracking) or a quasi-plastic behaviour (i.e. small microcracks distributed below the contact surface). The model provides estimates of the transition load within a factor of two of the experimentally obtained values and indicates that the brittle behaviour dominates the observed transition from mild to severe wear in the three ceramics evaluated in this study. The limitations of the model and its use in design of tribological components are discussed. The role of lubrication and tribochemical reactions between ceramic surfaces and atmospheric moisture on the wear behaviour is discussed.