Introduction
Lubrication is defined as the use of a material to improve the smoothness of movement of one surface over another. The materials used are liquids or semi-liquids, but may be solids and gases or any combination of solids, liquids and gases. The smoothness is improved by reducing friction. This is not, however, always the case, and there may be situations in which it is more important to maintain steady friction than to obtain the lowest possible friction. Lubrication has always been used to reduce friction and wear of a material. It has been dated to be used as early as 1400 B.C, when chariot wheels were lubricated with beef tallow for an improved rotary movement. Engines, transmissions, axles, hydraulic systems, gear assemblies and metal working operations need specially formulated lubricants to - protect critical components; provide reliable operation; lower maintenance costs; decrease downtime and increase equipment life. Nowadays, in manufacturing processes, the surfaces of tools and workpieces are subjected to: a) Forces and contact forces – which range from very low values to multiples of the yield stress of the workpiece material; b) Relative speeds – from very low to very high; c) Temperatures – which range from ambient to melting.
In order to accommodate and overcome external forces applied, friction & wearing and abnormal environmental circumstances – lubrication has provided manufacturers and consumers with a better-end product, that can withstand, to a certain degree, all of the above mentioned.

There are 4 regimes of lubrication: i) Hydrodynamic or Thick-Film Lubrication; ii) Hydrostatic or Thin-Film Lubrication; iii) Mixed Lubrication; and finally iv) Boundary Lubrication * Hydrodynamic Lubrication – the lubricant film is sufficiently thick to separate the moving surfaces completely, minimizing friction. This type of lubrication is present under the conditions of maximum fluid viscosity and minimum load. However it may fail under sudden increase in load; low speed + high torque operation or low lubricant viscosity. * Hydrostatic Lubrication – in contrast to hydrodynamic lubrication process, where there is no need to apply external fluid pressurization – the hydrostatic bearings rely on external fluid pressurization (i.e. pressure/oil pump) to generate load support and a large centering stiffness, even in the absence of journal rotation. Therefore as the load increases or the speed and viscosity decreases – the lubricant film becomes thinner. This condition raises the friction at the sliding interfaces and results in slight wear. * Mixed Lubrication – a significant portion of the load is carried by the physical contact (asperities) of the two surfaces. The rest is carried by the fluid film trapped in pockets, such as the valleys between asperities. * Boundary Lubrication – is the condition when the fluid films are negligible and there is considerable asperity contact. A thin lubricant layer is attracted physically to the metal surfaces, thus preventing direct metal-to-metal contact of the two bodies and reducing wear. The physical and chemical properties of thin surface films are of significant importance while the properties of the bulk fluid lubricant are insignificant. Boundary lubricants are typically natural oils, fats, fatty acids, esters or soaps.

Lubricants can also be classified into: 1) Liquids (i.e. vegetable, animal, mineral oil etc.); 2) Semi-solids (i.e. grease, soaps, waxes etc.); 3) Solids (i.e. graphite, molybdenum disulfide, metallic and polymeric films etc.); 4) Synthetics (i.e. synthetic oil, PAO, ester etc.)