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Does your motor oil contain friction modifiers? Friction modifiers and mild antiwear agents are polar molecules added to lubricants for the purpose of minimizing light surface contacts (sliding and rolling) that may occur in a given machine design. These are also called boundary lubrication additives. Esters and natural and synthetic fatty acids, and some solid materials such as graphite and molybdenum disulfide, are used for these purposes. These molecules have a polar end (head) and an oil-soluble end (tail). Once placed into service, the polar end of the molecule finds a metal surface and attaches itself. If one could see the orientation of the molecules on the surface, it would appear something like the fibers of a carpet, with each molecule stacked vertically beside the others. As long at the frictional contact is light, these molecules provide a cushioning effect when one of the coated surfaces connects with another coated surface. If the contact is heavy, then the molecules are brushed off, eliminating any potential benefit of the additive. When the machine designer anticipates more than light surface contact (from shock loading for instance) then the designer would select a stronger type of friction modifier characterized as an antiwear additive. Zinc dialkyldithiophosphate is a common antiwear agent. This type of additive literally reacts with the metal surface when the reaction energy (temperature) is high enough. The reaction layer provides sacrificial surface protection. As the loading and metallic contact increase, the strength of additive and the strength of the reaction process increases. This leads to the use of sulphur-phosphorus based EP chemicals. The EP additives form organo-metallic salts on the loaded surfaces that serve as sacrificial films to protect against aggressive surface damage.
