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Polyol ester lubricants are used in a wide range of temperature conditions, normally from −54 up to +220 °C, and their application can be limited by high temperature oxidation leading to viscosity increase, sludge, and/or formation of deposits. This paper is concerned with the chemical processes which take place during the oxidation of esters, particularly polyol esters, and discusses the selectivity of oxygen attack on ester molecules, and the formation and decomposition of hydroperoxides as well as other products formed as a result of ester oxidation. Proposed mechanisms of polyol ester lubricant oxidation are discussed, and unresolved problems which require additional research efforts are revealed.

The tribological behavior of surface-capped MoS 3 nanoparticles (nano-MoS 3 ) in hydrocarbon oils was studied both alone and in combination with ZDDP. It was found that the nano-MoS 3 additive alone demonstrates pro-wear properties and decreases the friction coefficient only at high temperatures. The combination with ZDDP demonstrates synergism in antiwear and antifriction activity even at low ZDDP content. X-ray Absorption Near Edge Structure (XANES) spectroscopy at the sulfur, molybdenum and phosphorus edges was used to identify the chemical species in tribochemical films. It was established that the nano-MoS 3 -formed tribofilms are composed of oxidized sulfur and molybdenum species while tribofilms formed by combination with ZDDP are composed of phosphate layers incorporating MoS 2 -type fragments providing friction reduction.

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