Introduction

With a few important exceptions, engineering devices which involve the contact of loaded, sliding surfaces will only operate satisfactorily, that is, without giving rise to unacceptable amounts of surface damage or wear, when they are provided with adequate lubrication. The lubricant can act in two distinct, but not necessarily mutually exclusive, ways. The first of its functions may be to physically separate the surfaces by interposing between them a coherent, viscous film which is relatively thick (i.e. significantly larger than the size of likely surface asperities). In hydrostatic bearings this film is provided by an external pump and so its presence depends on the continuous operation of an external source of energy. In hydrodynamic bearings its generation relies only on the geometry and motion of the surfaces (hence the term dynamic) together with the viscous nature of the fluid. The second role of the lubricant may be to generate an additional thin, protective coating on one or both of the solid surfaces, preventing, or at least limiting, the formation of strong, adhesive and so potentially damaging friction junctions between the underlying solids at locations of particularly acute loading. If this protective coating has a comparatively low shear strength then the ultimate tangential force of friction can be much reduced: this mechanism of friction limitation is generally known as boundary lubrication. Such boundary films are generally very thin, perhaps only a few (albeit very large) molecules thick, and their formation and survival depends very much on the physical and chemical interactions between components of the lubricant and the solid surfaces.