In this era, unusual bearing materials are lubricated by gases, liquids, or solids at extremes of operating conditions. Some examples are: gases lubricating red-hot metals, plastics lubricating solids at cryogenic temperatures, powder lubricating ceramics, and liquid metal lubricating at speeds of 2000 mph; the lubrication of animal joints is also of current interest.
Lubrication is a complex phenomenon, at the present time rather poorly understood. Lubrication research crosses many disciplines. In particular, it utilizes knowledge from the investigation of surfaces such as have been described in the previous papers of this symposium. The purpose of this paper is to review lubrication research, emphasizing surface phenomena. The scope is broad, ranging from the adhesion of clean metals to conditions where the sliding surfaces are completely separated by a fluid film. Many observed effects can be explained by the occurrence of physical adsorption, chemisorption, or chemical reaction on the surface. Under conditions of practical lubrication, these mechanisms can no doubt occur consecutively or simultaneously. The operating variables in a sliding system are temperature, load, and speed. Even if a machine is complex, the critical sliding system can usually be resolved into a simple sliding of one surface over another. Inadequate lubrication is manifested by a rise in temperature, increased friction, and the occurrence of wear.
Lubrication has been divided into two main regions, hydrodynamic and boundary. The term hydrodynamic suggests pressure resulting from motion. The term boundary refers to the solid-fluid interface. Hydrodynamic lubrication is a system in which the shape and relative motion of the sliding surfaces cause the formation of a continuous fluid film having sufficient pressure to separate the surfaces. The viscosity of the fluid is the most important parameter. Boundary lubrication is the condition where friction is determined by the properties of the surfaces and the surfactant properties of the fluid. Boundary lubrication occurs at low speeds, high loads, and small areas of contact. The coefficient of friction, which is the ratio of friction force to the applied load, will be above 0.03. Solid-to-solid contact may occur during boundary lubrication, and some of the encounters result in wear (1). Surface active agents reduce the free energy of the solid surfaces and reduce the number of encounters that result in damage. If the amount of metal-to-metal contact is not controlled, catastrophic wear or complete seizure may occur. The extreme of boundary lubrication is the sliding of unlubricated solids.