Indexed on: 01 Jul '99Published on: 01 Jul '99Published in: Tribology Letters
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. However, 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 housings in rolling contact fatigue (RCF) tests with a linear perfluoropolyalkylether (PFPAE) lubricant and VIM-VAR M50 steel at 316°C and a stress of 4.8 GPa clearly show that significantly more wear, corrosion, and fluid breakdown occur in the closed system than in the open system. Under these conditions, PFPAEs catalytically decompose to corrosive products. These corrosive products are partially vented in the open system, but retained in the closed system, causing more extensive corrosion of bearing materials, and significantly affecting the lubricant's tribological performance. Post test lubricant was analyzed for viscosity, acid number, and metals to assess changes in the lubricant's physical and chemical properties. Changes were more severe in the closed system. The nuclear magnetic resonance (NMR) and Fourier transform infrared (FTIR) spectra also showed significantly more carboxylic acid buildup in the stressed fluid from the closed system. The films formed in the tribojunction were analyzed using X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES). Organic films formed in the open system reduced wear while inorganic films formed in the closed system resulted in higher wear. Consequently, we conclude that more attention needs to be given to the effect of decomposition products during bench type tribological testing of high temperature lubricants.