Thin low shear strength layers of gas, liquid and solid are interposed between two surfaces in order to improve the smoothness of movement of one surface over another and to prevent damage. These layers of material separate contacting solid bodies and are usually very thin and often difficult to observe. In general, the thicknesses of these films range from 1 – 100 [μm], although thinner and thicker films can also be found. Knowledge that is related to
enhancing or diagnosing the effectiveness of these films in preventing damage in solid contacts is commonly known as ‘lubrication’. Although there are no restrictions on the type of material required to form a lubricating film, as gas, liquid and certain solids are all effective, the material type does influence the limits of film effectiveness. For example a gaseous film is suitable for low contact stress while solid films are usually applied to slow sliding speed contacts. Detailed analysis of gaseous or liquid films is usually termed ‘hydrodynamic lubrication’ by solids is termed ‘solid lubrication’.
A specialized form of hydrodynamic lubrication involving physical interaction between the contacting bodies and the liquid lubricant is termed ‘elastohydrodynamic lubrication’ and is of considerable practical significance. Another form of lubrication involves the chemical interactions between contacting bodies and the liquid lubricant and is termed ‘boundary and extreme pressure lubrication’. In the absence of any films, the only reliable means of ensuring relative movement is to maintain, by external force fields, a small distance of separation between the opposing surfaces.
Liquid lubrication is a technological nuisance since filters, pumps and cooling systems are required to maintain the performance of the lubricant over a period of time. There are also environmental issues associated with the disposal of the used lubricants. Therefore ‘solid lubrication’ and ‘surface coatings’ are the subject of intense research.
The principal limitations of, in particular, liquid lubricants are the loss of load carrying capacity at high temperature and degradation in service. The performance of the lubricant depends on its composition and its physical and chemical characteristics.
