IR study of the chemistry of boundary lubrication with high temperature and high pressure shear

Abstract

A unique diamond anvil cell has been constructed which permits the spectroscopic study of boundary layers under conditions of high temperature, high pressure and shear. We have used this cell in combination with Fourier transform IR spectroscopy to probe the function of stearic acid and zinc dialkyldithiophosphate (ZnDDP) as boundary layer enhancer. We have shown that thick boundary layers (about 1 μm) of stearic acid continue to exist at pressures as high as 5.0 GPa (725 000 lbf in −2). We have also demonstrated that the decomposition of ZnDDP in mineral oil under static high pressure and high temperature is first enhanced by increasing the pressure, reaching a maximum at about 0.7 GPa (about 10 000 lbf in −2), and is thereafter retarded by further increases in pressure. This result suggests that the decomposition of ZnDDP is triggered by pressure increases associated with boundary lubrication rather than by frictional heating. The decomposition results in the formation of a thick inorganic protective film at those surfaces associated with high wear.