ABSTRACT
The use of lubricants by far predates da Vinci’s observation that interposing materials between sliding bodies can reduce their friction. Lubrication involves not only the selection and formulation of lubricants, but also the design of contact geometry and the methods to deliver, lter, and condition them. Broadly speaking, lubricants can be solids, liquids, or gases. Solid lubricants can be in the form of thin lms, constituent phases in composite materials, or powders. The de nition of lubricant, developed by the Organization for Economic Cooperation and Development (OECD), International Research Group on Wear of Engineering Materials,1 is as follows:
Note that the OECD de nition for lubricant contains the phrase “interposed … for the purpose of,” implying that a substance can be considered a lubricant only if it is applied intentionally. Some materials are considered lubricious (the more correct term is lubricative), that is, they tend to reduce friction, but by the OECD de nition, lubricative substances are not necessarily lubricants unless they are applied intentionally. Lubricative materials include soap, seawater, sputum (spit), blackberry jelly, perspiration, Te on™, and waxed paper. The oxide that forms on copper in air is lubricative,2 but it is not strictly a lubricant by some de nitions unless it was applied intentionally. It might also be argued that a “traction uid” used in wet clutches is not a lubricant per se even though it may have some lubricative properties.
