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

The self-lubricating performance of the two types of modified, angular-contact 25-mm-bore ball bearings, i.e., a hybrid ceramic bearing with Si 3 N 4 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 (LH 2 ), liquid oxygen (LO 2 ), or liquid nitrogen (LN 2 ) at speeds to 50,000 rpm and thrust loads to 2840 N. In LH 2 , both the hybrid ceramic and all-steel bearings demonstrated excellent wear performance made possible by the thick transfer film consisting of PTFE (polytetrafluoroethylene)/CaF 2 or CaF 2 /FeF 2 , in which CaF 2 and FeF 2 were tribocatalytically formed by the reducing ability of LH τ This film was generated from a glass cloth-reinforced PTFE retainer. In LO 2 , the all-steel bearing exhibited excellent wear performance as the result of the intense formation of a Cr 2 O 3 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 LN 2 , the hybrid ceramic bearing exhibited better wear performance provided by the thick transfer film consisting of FeF 2 /iron oxide, as compared with seizure of the all-steel bearing under identical operating conditions. Presented as a Society of Tribologists and Lubrication Engineers paper at the ASME/STLE Tribology Conference in Toronto, Ontario, Canada, October 26-28, 1998