Tribological behavior of a multialkylated cyclopentane oil under ultrahigh vacuum conditions

Abstract

An ultrahigh vacuum, ball-on-flat test apparatus has been built to study the performance of candidate oils intended for spacecraft applications. Tests have been conducted on a multialkylated cyclopentane base oil using steel balls and disks. Different results are obtained when this oil is tested under vacuum conditions than when it is tested under a nitrogen environment. These differences are dramatic when the tests are conducted under starved conditions. Analyses of gases evolved during rubbing reveal that large quantities of methane are evolving from the process. A mechanism is proposed whereby oxide-free steel surfaces combine with tribological activity to crack the hydrocarbon oil to produce CHx radicals. These CHx radicals abstract hydrogen from the surrounding oil to produce methane. The increased volatility of the oil fragments remaining after methane formation leads to material loss by evaporation, thereby explaining the differences in vacuum and nitrogen performance of the oil.