Volatility and wear characteristics of a variety of liquid lubricants for space applications

The vapor pressures and wear characteristics are critical properties for li quid lubricants to assure long-term reliability and performance in space ap plications. Vapor pressures, obtained using a Knudsen cell technique, and w ear properties, obtained using a vacuum four-ball apparatus, were measured for a series of unformulated liquid lubricants. These included: two multipl y alkylated cyclopentanes (MACs) (X-1000 and X-2000). two linear perfluorop olyalkylethers (PFPAEs) (Z-25 and 815Z), and four silahydrocarbons (a tri, a tetra- and two pentas). Vapor pressures were measured at three elevated t emperatures (423, 448 and 498K) and extrapolated to room temperature 298K. The lowest 298K vapor pressure of 5.7 x 10(-14) Pa, was obtained with the P FPAE fluid (815Z) and the highest value with the low, molecular weight MAC (X-1000) at 3.6 x 10(-7) Pa. In addition, vacuum wear rates were determined for some of the lubricants. The lowest wear rates (approximately 3 x 10(-1 1) mm 3 / mm) were observed for three of the silahydrocarbons while the hig hest wear rates (approximately 2 x 10(-9) mm(3) / mm) were observed with th e two PFPAE flu ids (Z-25 and 815Z). The MAC (X-2000) yielded a wear rate o f about 10(-10) mm(3) / mm. The results indicated that the silahydrocarbon class of liquid lubricants offers the better potential for space applicatio ns.