Friction behavior and wear analysis of SiC sliding against Mo in SO2, O-2 and H2S at gas pressures between 4 and 40 Pa

Friction tests were performed in an ultrahigh vacuum chamber with an BC pin sliding against an Mo flat during and after exposure to gases at pressures between 4 and 40 Pa. Wear tracks, scars and debris were analyzed ex situ with scanning electron microscopy (SEM), energy-dispersive X-ray analysis (EDX) and transmission electron microscopy (TEM). Steady-stale friction coefficients were between 0.10 and 0.15 in SO2 and O-2 gases and less than 0.01 in H2S gas; in contrast, tests performed at <10(-5) Pa after H2S exposure gave friction coefficients of about 1. SEM/EDX of wear features formed in SO2 and O-2 tests showed that tracks on Mo and sears on SiC were covered and surrounded by small (1-5 mu m), flakelike Mo particles. In contrast, tracks formed in H2S had long stretches of a smoothly burnished surface with very little debris. TEM of wear debris collected from both pin and flat also found only metallic Mo particles. The lubricant afforded by the gases is discussed in terms of the third-body products formed during sliding, and the friction and wear behavior in SO2 and O-2 is accounted for by the adhesive junction theory of Bowden and Tabor.