Surface chemistry of new lubrication systems for high-speed spacecraft bearings

Abstract

Multialkylated cyclopentane (MAC) and silahydrocarbon (SiHC) are primary candidates for future spacecraft applications due to their high viscosity and good viscosity–temperature profile, low vapor pressure and good lubricating properties. In this work, the friction, wear and associated tribochemistry of these fluids, both unformulated and formulated with 2% aryl phosphate ester (TPP), were investigated. A Plint reciprocating wear rig equipped with an environmental chamber that was filled with dry air or nitrogen was used to produce boundary lubrication conditions. The resulting specimens were examined by X-ray absorption near-edge structure (XANES) spectroscopy in order to gain some understanding of how the base fluid and additive function. Several relationships were discovered among friction, wear and tribochemistry within the wear scar. First, the wear rate in both unformulated and formulated fluids was higher in a dry nitrogen environment than in dry air. Second, when tested in air, unformulated silahydrocarbon acts as its own antiwear additive by decomposing to a silicon oxide glass within the wear track thereby eliminating additive issues such as solubility, evaporation and concentration effects. Third, the antiwear properties of silahydrocarbon oil are hindered by the presence of a phosphate additive. Both the oil and additive form an oxide glass within the wear track and compete for active growth sites. Fourth, the chain length of the polyphosphate glass formed in the wear track controls the antiwear performance of the film. The phosphate additive in multialkylated cyclopentane decomposed to a polyphosphate glass in both dry air to generate a good antiwear film (short to medium chain length polyphosphate) and nitrogen to form a poor antiwear film (long chain length polyphosphate).