Liquid Wetting on Metal Surfaces and the Heat of Immersion/Adsorption in Lubrication

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Abstract
Theories explaining the way in which liquid-surface interaction affect boundary lubrication are reviewed. In all views, adsorption is a critical step governing the end result—albeit through differing mechanism. This review collects persuasive qualitative experimental support to this effect. A quantitative treatment deriving the friction or wear coefficients as functions of the heats of adsorption of boundary lubricant molecules is discussed in more detail, and recommendations for testing existing models are made. Methods for measuring heats of adsorption are described and the interrelation between the different measurements is explained. Heat of adsorption values predicted from sliding friction and wear measurements are higher than the integral heats of adsorption measured for the lubricant. This discrepancy could mean that a differential heat (i.e., specific for a certain surface coverage) determines the friction and wear characteristics of the lubricant-surface combination or that the surface temperatures in the sliding friction experiments were underestimated. Values obtained for heats of adsorption of a variety of molecules onto steel are tabulated at the end of this article.