Wear behavior of triode-sputtered MoS2 coatings in dry sliding contact with steel and ceramics

The endurance of MoS2 sputter-coated steel balls was measured in continuous and stop-start sliding tests in a four-ball wear tester. Test variables were counterface materials, uncoated steel and two ceramics (Co-bonded tungsten carbide and sapphire), and gaseous atmospheres, dry Ar and dry air. In continuous tests, coating endurance increased from 14 to 80 min; combinations were ranked as follows: steel in air=ceramics in air < steel in Ar < ceramics in Ar. Coating endurance in stop-start tests was the same as in continuous test for all combinations except steel in Ar, which failed sooner in stop-start tests. MoS2 wear tracks were analyzed at intervals from 1 min to failure by optical microscopy, energy-dispersive X-ray spectroscopy (EDX) and Auger electron spectroscopy (AES). Tracks run in Ar were smoother and showed ductile flow; tracks in air were rougher and developed ragged scratches. Blisters formed and contributed to failure in both atmospheres: in Ar they led to a slower ductile failure whereas in air they led to more rapid brittle failure. Abrasion by the worn steel ball accelerated coating wear in Ar. EDX of wear tracks showed that 2/3 of the coating was removed within the first 10 min in both atmospheres; AES of the same tracks found higher levels of oxygen in MoS2 run in air. Mechanisms of wear, transfer and lubrication are discussed, and a quantitative model for blister formation in ductile MoS2 coatings is presented. It is recommended that hard ceramics replace abrasion-prone steel to enhance the wear life of MoS2-coated steel in oxygen-free environments.