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Boundary Lubricating Film: Formation and Lubrication Mechanism



Stephen M. Hsu, Richard S. Gates


Boundary lubricating films are critical in effective lubrication of engine components. Their formation mechanism has not been fully understood because of the large variations of chemistry and machinery in operation. Various reports on boundary lubricating films tend to highlight the differences rather than the common characteristics. This paper will examine the various chemical reactions occurring under the boundary lubrication regime that eventually lead to the formation of high molecular weight organo-metallic compounds. The rate of formation of organo-metallic compounds depends on the molecular structure, functional groups, and the ability to competitively adsorb onto the surface (polarity). Polar molecules in a base oil form high molecular weight organo-metallic compounds (friction polymer) much faster and in larger amounts. This friction polymer is essential in achieving effective lubrication. When combined with reaction films from antiwear additive such as ZDDP, the soft friction polymer provides a shearable sacrificial layer and the inorganic phosphate glassy structure provides the load carry capacity. The resultant boundary film is stronger and more durable. Mechanistic study of boundary films suggested that the most effective films possess strong adhesive strength, cohesive strength, and self-healing properties. This concept is being explored for the design of new boundary lubricating films.
Tribology International


boundary lubricating films, film formation, lubrication, organo-metallic, surface reaction, ZDDP


Hsu, S. and Gates, R. (2005), Boundary Lubricating Film: Formation and Lubrication Mechanism, Tribology International (Accessed July 23, 2024)


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Created March 4, 2005, Updated February 19, 2017