Assessing Friction Stress on a Liquid Lubricant by Stable Isotope Analysis

Compound-specific stable isotope analysis (CSIA) was applied to a pentaerythritol ester lubricant to determine if predictable fractionation effects could be related to friction stress. A pin-on-disk tribometer was used to expose the lubricant to five doubly-increasing sliding distances (up to 36 km). PCA was used to ordinate multivariate data and test if significant isotopic shifts or FTIR spectral changes could be related to sliding cycles. 13C Isotope ratios only were found to be statistically different (P 0.05) between treatments. PCA gave a primary score (PC1) containing 44% of the 13C-isotopic variability, which correlated exponentially with sliding cycles (r2 = 0.96). A peak, not present in unexposed lubricant, became evident and showed increasing 13C depletion with increasing length of sliding exposure.
The original document contains color images. Prepared in collaboration with St. Mary s College of Maryland, St. Mary s City, MD. Sponsored in part by AFOSR.