The formation of a boundary films by reaction with a gas-phase lubricant or lubricant additive occurs in several steps that involve the initial reaction of the lubricant molecule on the surface, followed by shear-induced decomposition and formation of the boundary film.  Fully understanding this sequence of steps will enable the structure of the additive molecule to be related to the nature of the resulting boundary lubricating film.  Systems are studied for which the interface is at thermodynamic equilibrium to allow kinetic models to be applied.     <br/> <br/>The first regime for which the system is in thermodynamics equilibrium is when the interfacial temperatures are high (~1000 K), so-called extreme-pressure (EP) lubrication.  Here, the film formation pathways are dominated by thermal reactions at the surface and is illustrated for small chlorinated hydrocarbons reacting with iron.<br/> <br/>The second regime occurs under mild sliding conditions where the temperature rise is small. The tribochemistry in this regime investigated by carrying out friction measurements in ultrahigh vacuum, by analyzing the composition in the wear track region using Auger spectroscopy and by detecting gas-phase products evolved during sliding and is illustrated by the gas-phase lubrication of copper by dialkyl disulfides. They initially form thiolate species and shear causes sulfur and some carbon to penetrate the substrate to form a boundary film that reduces friction.  The reaction rate constants for these elementary step reactions can be measured for a monolayer of methyl thiolate species on the surface.  It is assumed that similar reactions occur during gas-phase lubrication where the clean surface produced by rubbing is replenished with l thiolate species by reaction with the gas-phase lubricant. The kinetic equation for this process are solved analytically to predict the variation in friction coefficient as a function of the number of sliding passes that is in excellent agreement with experiment.

Thermal and shear effects in boundary film formation

253rd National Meeting (2017)

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