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 semiinfinite 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 x 10(-14) cm(2)/s at test temperatures of 370 degrees and 430 degrees C. Diffusion is thought to occur via the migration of iron cations through an anionic lattice of polyphosphate and phosphite, i.e., cation diffusion. The diffusion mechanism provides a satisfactory explanation of how several hundred monolayers containing iron can exist in vapor deposition film grown from organophosphorus lubricants. The chemical gradient, ionic attraction, and the extreme stress gradients of the Hertzian contact are thought to contribute to diffusion under dynamic conditions.