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

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