The surface and tribological chemistry of chlorine- and sulfur-containing lubricant additives

A model for the operation of chlorinated hydrocarbon extreme-pressure additives is outlined in which the chlorinated hydrocarbon thermally decomposes at the hot (~1000 K) interface forming a chloride+carbon film, which is simultaneously worn from the surface. Assuming that seizure occurs when the chloride film thickness decreases to zero reproduces the experimentally measured seizure load vs. additive concentration results. A similar model is found to apply to sulfur-containing additives. The thermal decomposition of model chlorinated hydrocarbons is investigated in ultrahigh vacuum (UHV) using constant-flux molecular beams and reveals that these react with similar activation energies as at higher pressures, forming films of identical compositions to those found tribologically. This allows the surface reaction chemistry of model extreme-pressure additives and the tribological properties of the films to be investigated separately in UHV. In this vein, measurements of the tribological properties of thin KCl films grown on iron in UHV show that the friction coefficient drops from a clean-surface value of ~1.9 to ~0.27 after completion of the first monolayer.