SECTION 3.4
Understanding Lubrication and Adsorbed Film

The lubrication requirements of a coolant are also important, but are much more difficult to understand and accomplish. In lubrication practices, it often happens that sliding surfaces cannot be separated by a fluid film of lubricant because of a combination of high pressure between the parts, rough surfaces, and low viscosity. This is the case with most metal-cutting operations. Under such conditions, metal to metal contact can be reduced by using lubricants containing additives to encourage the formation of an adsorbed film.

This adsorbed film is composed of molecules of the lubricant which have become chemically combined with the surface atomic structure of the metal. This type of lubrication is referred to as boundary lubrication, since the lubrication is located at the boundary, or interface, of the two contacting surfaces. Such films are often three or four molecules thick--a few millionths of an inch. It is for this reason that intermittent point to point metal contact can still be expected. The best protection under this condition is an oil rich in oiliness and high in film strength.

Oiliness is an important concept, and can be thought of as internal lubrication of the oil. Film strength is a natural component of oils, but can be enhanced with additives such as organic compounds containing sulfur, chlorine, or phosphorus.