8 - Boundary and Extreme Pressure Lubrication

Abstract

In many practical applications there are cases where the operating conditions are such that neither hydrodynamic nor EHL lubrication is effective. The models of lubrication which are thought to operate under such conditions are discussed in this chapter. The traditional name for this type of lubrication is “boundary lubrication” or “boundary and extreme-pressure lubrication.” Boundary and EP lubrication is a complex phenomenon. The lubrication mechanisms involved can be classified in terms of relative load capacity and limiting frictional temperature. Lubrication by chemical and physical interaction between an oil-based lubricant and a surface (usually metal) is essential to the operation of most practical machinery. Four basic forms of this lubrication are identified: the formation of an ultra-viscous layer close to the worn surface, the shielding of an oxidized metal surface by a mono-molecular layer of adsorbed linear surfactants, the separation of contacting surfaces by entrapped layers of finely divided and perhaps amorphous debris and the suppression of metal to metal contact at extreme pressures by the temperature-dependent formation of sacrificial films of corrosion product on worn metallic surfaces.