ROLLING-CONTACT-FATIGUE WEAR CHARACTERISTICS OF DIAMOND-LIKE HYDROCARBON COATINGS ON STEELS

Adherent diamond-like hydrocarbon (DLHC) coatings were applied on AISI M-50, 52100, 4118 and 440C steel rods by using an approximately 0.2 mum amorphous-silicon-hydrocarbon (a-SiHC) bonding layer between the DLHC and the steel. Both the a-SiHC and DLHC coatings were applied using a single, broad-beam ion source. The rods were subjected to rolling-contact-fatigue (RCF) testing under high, cyclic Hertzian stress (5.5 GPa), low lubricant-film-thickness parameter (lambda=0.7) conditions. Order-of-magnitude increase in the fatigue lives of all four rod materials were observed. Systematic RCF tests coupled with microscopic examination after various test intervals show that micro-polishing by hard DLHC coating fragments may play an important role in prolonging fatigue lives. Raman spectroscopic measurements suggest that cyclic stressing of the DLHC layer causes it to transform from what was initially amorphous carbon into the more lubricous and stable graphite phase.