Tribo-Characteristics of Cryogenic Hybrid Ceramic Ball Bearings for Rocket Turbopumps: Bearing Wear and Transfer Film©

Abstract

The self-lubricating performance of the two types of modified, angular-contact 25-mm-bore ball bearings, i.e., a hybrid ceramic bearing with Si3N4 ceramic balls and an all-steel bearing, was evaluated based on wear inspection as well as X-ray photoelectron spectroscopic (XPS) analysis of transfer films. 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 to 2840 N. In LH2, both the hybrid ceramic and all-steel bearings demonstrated excellent wear performance made possible by the thick transfer film consisting of PTFE (polytetrafluoroethylene)/CaF2 or CaF2/FeF2, in which CaF2 and FeF2 were tribocatalytically formed by the reducing ability of LHτ This film was generated from a glass cloth-reinforced PTFE retainer. In LO2, the all-steel bearing exhibited excellent wear performance as the result of the intense formation of a Cr2O3 layer beneath an extremely thin PTFE transfer film. By contrast, 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 the thick transfer film consisting of FeF2/iron oxide, as compared with seizure of the all-steel bearing under identical operating conditions.