In-situ observations of lubricant chemistry at a sliding contact using ultraviolet Raman spectroscopy

In-situ analyses of lubricated sliding contacts were performed by interfacing an ultraviolet Raman spectrometer to a ball-on-flat tribotester. The sliding contact was simulated by rotating a sapphire window against a stationary chrome steel ball. Various loads were transmitted to the contact center through the chrome steel ball.;Four lubricants have been studied under various loads at a 10cm/s sliding speed: (1) a multialkylated cyclopentane (2001A), (2) a branched perfluoropolyaklyl ether (Krytox 479), and (3) two linear perfluoropolyalkyl ethers (Fomblin 491 and Fomblin 497). 2001A, Krytox and a Fomblin of lower viscosity, Fomblin 497, all decomposed to amorphous carbon upon sliding under all the loads used. No amorphous carbon was detected from Fomblin 491. The Raman spectra indicate that the amorphous carbon produced from 2001A, Krytox 479 and Fomblin 497 had the common structure of aromatic clusters of sp 2 carbons. In addition, the amorphous carbon produced from Krytox 479 might contain heterocyclic aromatic rings similar to that of furan while the amorphous carbon from Fomblin 497 might contain the aryl oxygen functionality. The amount of amorphous carbon at the contact area during sliding was a balance of formation and removal rates. After sliding, the amorphous carbon continued to form on the chrome steel ball.;It is postulated that surface activity of the chrome steel ball was the main cause for the lubricant degradation. The sliding action removed the protective oxide layer to expose the metal alloy underneath which reacted with the lubricant to form FeF3. FeF3 in turn catalyzed the decomposition of the lubricant to form amorphous carbon. When the chrome steel ball was replaced by a sapphire ball and the experiments were repeated, no amorphous carbon were detected with any of the four lubricants. The lubricant degradation on the chrome steel ball increased the coefficient of friction by 15%--20%. Catastrophic scuffing was not observed.;In-situ temperature and pressure of a silicone grease at the sliding contact were successfully measured with the UV Raman spectrometer for the first time. The silicone temperature was found to increase with the sliding speed while the pressure was independent of the sliding speed.