On the wear of a cobalt-based superalloy in zinc baths

Journal bearings made of a cobalt-based superalloy, trade marked STELLITE 6, were evaluated in zinc baths with and without the presence of aluminum and iron. The sleeve and the bushing wore evenly when tested in a pure zinc bath. The surfaces were generally smooth and covered by some rather fine grooves after the test. The wear of the bearings was much more severe when tested in aluminum-containing zinc baths. The worn surfaces of the bearings were fully covered by wide and deep grooves. The coefficient of friction of the superalloy generally increased with increases in the aluminum content of the molten zinc. The iron addition to the molten zinc appeared to affect the friction and wear characteristics of the superalloy. Detailed metallographic and microchemical analyses were performed to elucidate the wear mechanisms. The superalloy was found to react readily with the molten zinc alloys to form intermetallic compounds. In a pure zinc bath, zinc-based intermetallic compounds formed on the bearing surfaces; in baths containing aluminum, a compact intermetallic layer, based on the cobalt-aluminide phase, was detected on the bearing surfaces. Evidence collected in this study indicated that wear debris reacted with aluminum in the molten zinc and transformed itself into hard and abrasive cobalt-aluminide particles. During the test, these particles reattached to the bearing surfaces and built up. Acting as blunt microcutters, these particles plowed the bearing surfaces and created deep grooves. The dominant wear mechanism in the aluminum-containing zinc baths was identified as abrasion; fatigue and corrosive wear, however, served as precursors of the dominant wear mechanism.