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. 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\u27s tribological performance. Post test lubricant was analyzed for viscosity, acid number, and metals to assess changes in the lubricant\u27s 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

VaporPhase Lubrication: Reaction of Phosphate Ester Vapors with Iron and Steel

Aromatic phosphate esters such as triphenyl phosphate, tricresyl phosphate (TCP), and tri(tert-butylphenyl) phosphate, have been degraded in the presence of pure iron or metal alloys such as M-50 or 52100 steel. Among these volatile degradation products are those generated from the addition of an aromatic ring to the phosphate ester. Other products, which have been identified, include substituted biphenyls and diphenyl ethers derived from the decomposition of the above-mentioned addition product. Still other products are fused ring aromatic compounds such as anthracene, which arise from secondary reactions of the initial decomposition reactions. The decomposition reactions leave a nonvolatile phosphate film on the surface of the metal. Characterization of the film with Auger spectroscopy suggests iron phosphate as the product. X-ray photoelectron spectroscopy shows the presence of a bound organic layer at the surface. A mechanism that explains many of the decomposition products and the formation of a bound glassy iron phosphate film is proposed

Temperature-resolved Molecular Emission Spectroscopy: An Analytical Technique for Solid Materials

Temperature-resolved molecular emission spectroscopy is described as a thermal analysis method for the analysis of solids and liquids. The technique uses an electrically heated graphite cup to decompose and/or vaporize the sample. The vapors are carried by a stream of argon into a cool hydrogen diffusion flame. Both the quantity and the nature of the decomposed species can be determined. The technique is particularly useful for the determination of sulfur, phosphorus, or nitrogen. Calibration curves for sulfur show the expected parabolic shape, and those for phosphorus are linear. The detection limit for elemental sulfur was determined to be approximately 50 ng. The evolution of sulfur is shown to be related to the decomposition temperature which is characteristic of the sulfur-containing species. Reproducibility of the decomposition temperatures is typically ±2%