Tribo-characteristics of cryogenic hybrid ceramic ball bearings for rocketturbopumps: Bearing wear and transfer film

The self-lubricating performance of the two types of modified, angular-cont act 25-mm-bore ball bearings, i.e., a hybrid ceramic bearing with Si3N4 cer amic balls and an all-steel bearing, was evaluated based on wear inspection as well as X-ray photoelectron spectroscopic (XPS) analysis of transfer fi lms. These bearings had been tested in liquid hydrogen (LH2), liquid oxygen (LO2), or liquid nitrogen (LN2) at speeds to 50,000 rpm and thrust loads t o 2840 N. In LH2, both the hybrid ceramic and all-steel bearings demonstrated excelle nt wear performance made possible by the thick transfer film consisting of PTFE (polytetrafluoroethylene)/CaF2 or CaF2/FeF2, in which CaF2 and FeF2 we re tribocatalytically formed by the reducing ability of LH, This film was g enerated from a glass cloth-reinforced PTFE retainer. In LO2 the all-steel bearing exhibited excellent wear performance as the result of the intense f ormation of a Cr2O3 layer beneath an extremely thin PTFE transfer plm. By c ontrast, the hybrid ceramic bearing showed marked wear of the ceramic balls because of an extremely thin, weakly adhering PTFE transfer film. In LN2, the hybrid ceramic bearing exhibited better wear performance provided by th e thick transfer film consisting of FeF2/iron oxide, as compared with seizu re of the all-steel bearing under identical operating conditions.