SURFACE-INITIATED SPALLING FATIGUE IN M-50 AND M-50 NIL BEARINGS

The initiation and development of spalling fatigue in M-50 and M-50 NiL thrust bearings running under partial EHD lubrication conditions have been studied. The inner and outer rings of the M-50 NiL bearings were plated with thin hard dense chromium (TDC), and the bearing was assembled with M-50 balls. A spectrum of initial surface flaws was introduced during a prescribed run in a separate test machine in which the oil had been contaminated with 2.5 ppm of 20-mu-m alumina. The initial flaws propagated by a micropeeling mechanism, and these developed into macroscopic spalls when the peeled region reached a critical size. The L10 life of the surface-damaged M-50 bearings was 87 hours, compared to a calculated AFBMA L10 of 24 hours. The M-50 NiL, TDC rings had an L10 of 480 hours, under the same test conditions. The enhancement of the life of M-50 NiL, TDC over M-50 by a factor of about six is ascribed in part to the presence of residual compressive stresses in the subsurface regions of the M-50 NiL rings.