Whenever one body moves over the surface of another, it encounters a resistance to its movement from the other surface. This resistance is called friction. Although friction may exist between a solid and fluid surface, friction is primarily a surface phenomenon that arises between solids.

Lubricants are used for one or more of the following purposes: to reduce friction, to prevent wear, to prevent adhesion, to aid in distribution of a load, to cool moving parts, and/or to prevent corrosion. Besides oils and greases, many plastics, solids, and even gases are now used as lubricants. The chief limitations on many of these materials is their ability to replenish themselves, to dissipate frictional heat, to withstand high temperature environments, and to remain stable in working environments. Types of lubricants include petroleum fluids, synthetic fluids, greases, solid films, working fluids, gases, plastics, animal fat, metallic and mineral films, and...