Rolling Contact Testing of Vapor Phase Lubricants—Part IV: Diffusion Mechanisms©

Auger depth profile data, obtained from vapor-lubricated T15 bearing steel, were modeled using the solution for diffusion in a semi-infinite pair. A tertiary-butyl phenyl phosphate was used as the vapor lubricant. The primary species undergoing diffusion are iron and carbon. The objective of the experiment was to determine the order of magnitude of the diffusion coefficient and to qualitatively assess what types of diffusion mechanisms are involved. The experimental results indicate that the diffusion profile travels at a velocity equal to the bearing wear rate under dynamic conditions. This is possible if iron diffusions at a faster rate than carbon, i.e., the Kirkendall effect. Analyses of the data were performed using Darken's equations. The results indicate that the diffusion coefficient of iron is of the order of 1 × 10−14 cm2/s at test temperatures of 370° and 430°C. Diffusion is thought to occur via the migration of iron cations through an anionic lattice of polyphosphate and phosphite, i.e., catio...