Effects of linear PFPAE decomposition products on the rolling contact fatigue performance of M50

In bench tribology tests, the influence of lubricant decomposition products on the lubricated system and on lubricant performance are often overlooked , primarily because testing is performed in vented, or open, systems. Howev er, the lubricant in a gas turbine engine might be expected to more closely follow the principles of interaction that are found in unvented, or closed , systems. Results reported here comparing vented and unvented bearing hous ings in rolling contact fatigue (RCF) tests with a linear perfluoropolyalky lether (PFPAE) lubricant and VIM-VAR M50 steel at 316 degrees C and a stres s of 4.8 GPa clearly show that significantly more wear, corrosion, and flui d breakdown occur in the closed system than in the open system. Under these conditions, PFPAEs catalytically decompose to corrosive products. These co rrosive products are partially vented in the open system, but retained in t he closed system, causing more extensive corrosion of bearing materials, an d significantly affecting the lubricant's tribological performance. Post te st lubricant was analyzed for viscosity, acid number, and metals to assess changes in the lubricant's physical and chemical properties. Changes were m ore severe in the closed system. The nuclear magnetic resonance (NMR) and F ourier transform infrared (FTIR) spectra also showed significantly more car boxylic acid buildup in the stressed fluid from the closed system. The film s formed in the tribojunction were analyzed using X-ray photoelectron spect roscopy (XPS) and Auger electron spectroscopy (AES). Organic films formed i n the open system reduced wear while inorganic films formed in the closed s ystem resulted in higher wear. Consequently, we conclude that more attentio n needs to be given to the effect of decomposition products during bench ty pe tribological testing of high temperature lubricants.