Understanding the tribological chemistry of chlorine-, sulfur- and phosphorus-containing additives

It is demonstrated that the surface chemistry of simple-model extreme-pressure lubricant additives, measured in ultrahigh vacuum, corresponds to that measured at higher pressures, where film growth rates are monitored using a microbalance. This chemistry and reaction kinetics are used to explain the extreme-pressure lubrication behavior by successfully modeling the measured seizure load vs. additive concentration curves. It is also demonstrated, by growing ferrous chloride films on iron substrates in ultrahigh vacuum, that these have the same friction coefficients as those found for model extreme-pressure lubricants. It is found that a monolayer of a solid boundary lubricant film is sufficient to lower the interfacial friction coefficient to its minimum value. These results demonstrate that the chemistry measured under conditions of thermodynamic equilibrium at some temperature can be successfully applied to the formation of a boundary film, in the extreme-pressure regime.