Solid Lubricated Silicon Nitride Bearings at High Speed and Temperature - Phase 1

Abstract : Functional evaluations of solid lubricated 7205 bearings were conducted over the speed range, 10,000 rpm (1050 rad/sec) to 55,000 rpm (5760 rad/sec), using an air turbine drive system. The 7205 bearings were comprised of silicon nitride balls, M50 steel rings, and graphite containing cages. Graphite films transferred from the cage to the ball surfaces, then to the groove surfaces were the only source of lubrication. Liquid lubrication was not supplied to the test bearings. Inspection of the bearing components after the tests was very encouraging. The graphite films transferred to the silicon nitride balls and M50 steel grooves were effective in preventing wear of the bearing surfaces. The cage pocket wear and wear of graphite piloting surfaces were considered acceptable. Optical microscopy of etched M50 ring cross sections revealed that limited plastic deformation has occurred during the tests. Plastic deformation of the inner ring groove surface was believed to be significant, since plastic deformation would result in a higher heat generation at the inner ring surface contributing to bearing seizure. The silicon nitride balls did not exhibit any evidence of plastic deformation.