Thermal Stability of a Linear Perfluoropolyalkylether in a Rolling Contact Fatigue Tester

A linear perfluoropolyalkylether (PFPAE) was tested under dynamic conditions at various temperatures using a ball-on-rod type rolling contact fatigue (RCF) tester. The test temperatures ranged from ambient to 343 °C for a period of 24 h at a stress of 4.8 GPa using VIM-VAR M50 steel bearing elements. The lubricant was stable up to a temperature of 288 °C. Its thermal stability had a significant effect on the tribological performance of the bearing material. Above 288 °C, the lubricant thermally degraded resulting in two orders of magnitude higher wear rate and total acid number (TAN) and heavy corrosion pitting of the specimens. Gas Chromatography (GC) analysis of samples tested above 288 °C showed lower molecular weight components formed as a result of the lubricants' degradation. These GC results correlate with the changes in viscosities. Fourier transform infrared (FTIR) and nuclear magnetic resonance (NMR) analysis of the 316 °C and 343 °C post test fluid showed the presence of acid fluoride and carboxylic acid. Also, iron fluoride as determined by X-ray photoelectron spectroscopy (XPS) was found in the deposits formed on the specimens tested at temperatures above 288 °C.