Low-Energy-Electron-Stimulated Degradation of a Multiply Alkylated Cyclopentane Oil and Implications for Space Bearings

The volatile and liquid-phase products of low-energy electron irradiation of a multiply alkylated cyclopentane oil in vacuum were analyzed using mass spectroscopy, infrared spectroscopy, supercritical fluid chromatography, and gel-permeation chromatography. Both alkanes and alkenes are formed in each phase; in the fluid both primary and trans-alkenes are detected. Smaller and larger molecules than are present in the original oil (product molecular weights up to at least 10,000 g/mol) are found, and alkenes are present over the entire molecular weight range. The dependence of the amount of products found on electron beam energy and current suggest that the first step of the process is fragmentation of the oil molecules and their initial transformation into both volatile and non-volatile alkenes and alkanes, followed by further reaction to form larger molecules. Combining these results with an estimate of the properties of the triboelectrons that would probably be produced during wear in either all-steel or hybrid (steel raceways with ceramic balls) ball bearings in spacecraft, we find that degradation in vacuum due to triboelectronic reactions alone, with this type of lubricant in this application, is insignificant. Oil degradation in these systems in vacuum is likely due to chemical reactions with fresh metal surfaces produced during wear.