Fundamentals of solid lubricants

Abstract

Solid lubricants technology is a flourish field that deserves the attention of the designer of machines and devices that will operate in ordinary as well as in extreme environments. This chapter describes solid lubrication processes, the mechanisms by which solid lubricants function, the properties of solid lubricants, and the materials involved in solid lubrication and techniques for their application. Reliability of solid lubrication and wear life of solid-film lubricants are being improved by designing machine elements specifically to employ solid lubricants and by careful matching of the solid lubricant with the substrate bearing material. Solid lubricants are applied either as surface coatings or as fillers in selflubricating composites. Tribological (friction and wear) contacts with solid lubricant coatings typically result in transfer of a thin layer of material from the surface of the coating to the counterface, commonly known as a transfer film or tribofilm. The wear surfaces can exhibit different chemistry, microstructure, and crystallographic texture from those of the bulk coating due to surface chemical reactions with the surrounding environment. As a result, solid lubricant coatings that give extremely low friction and long wear life in one environment can fail to do so in a different environment. Most solid lubricants exhibit non-Amontonian friction behavior with friction coefficients decreasing with increasing contact stress. The main mechanism responsible for low friction is typically governed by interfacial sliding between the worn coating and the transfer film. Strategies are discussed for the design of novel coating architectures to adapt to varying environments.