Effects of iron fluoride layer on durability of cryogenic high-speed ball bearings for rocket turbopumps

摘要

Experiments were conducted to evaluate effects of an iron fluoride (FeF{sub}2) layer on the self lubrication and durability of the modified 25-mm-bore ball bearings made of SUS 440C. The modified bearings had a single outer land-guided, glass cloth-reinforced polytetrafluoroethylene (PTFE) retainers with elliptical ball-pockets. An FeF{sub}2 layer was chemically formed by means of a passivating surface treatment of fluoridation. The untreated and fluorine-passivated bearings were tested in liquid oxygen (LO{sub}2), liquid nitrogen (LN{sub}2) or liquid hydrogen (LH{sub}2) at a speed of 50,000 rpm with thrust loads up to 2740 N for total operation times up to 11.7 hours. Under sufficient cooling conditions, the untreated bearing tested in LO{sub}2, as well as the fluorine-passivated bearing tested in LN{sub}2, exhibited excellent self lubrication and durability, demonstrating sound surface conditions with hardly any wear. The fluorine-passivated bearing tested in LO{sub}2 showed a reduction in self lubrication. In contrast, even under poor cooling conditions in LH{sub}2, the fluorine-passivated bearing was superior to that of the untreated bearing which showed signs of seizure. Wear conditions were also evaluated based on X-ray photoelectron spectroscopic (XPS) analysis of transfer films.