Wear of Ceramics and Metals

Abstract

The principal wear modes likely to occur in MEMS will be briefly described. Wear is a material’s response to the stresses occurring in a moving mechanical contact. The local contact stresses depend on the surface topography, which is usually modified by the wear processes, and by plastic or elastic deformation of contacting asperities. In metals, the contact stresses are roughly equal to the hardness of the softer material, wear occurs by plastic deformation and fatigue. The amount of material removed is roughly proportional to the contact load, sliding distance and inversely proportional to the hardness. In ceramics, three main wear modes occur, depending on the contact load: microfracture on a sub-grain scale at low loads, grain-boundary fatigue at intermediate loads and macroscopic fracture at high loads. (The latter is unlikely in MEMS). Wear is also influenced by chemical reactions of the sliding materials with the environment or with each other. Such reactions are often accelerated by friction, in which case they are called tribochemical. Tribochemical reactions can increase wear, when they increase fracture rates; they can decrease wear when they produce a smooth surface or a soft reaction product and reduce contact stresses; or they can produce a lubricating layer that decreases friction and wear. When dissimilar materials slide against each other, the softer material usually wears more and is transferred to the harder surface, except when the latter wears by tribochemical reaction. Practical examples are presented.