Friction Behavior and Wear Analysis of SiC Sliding Against Mo in SO₂, O₂ and H₂S at Gas Pressures Between 4 and 40 Pa

Abstract

Friction tests were performed in an ultrahigh vacuum chamber with an SiC 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 O₂ 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 O₂ tests showed that tracks on Mo and scars on SiC were covered and surrounded by small (1–5 μm), 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 O₂ is accounted for by the adhesive junction theory of Bowden and Tabor.

KEY WORDS:

• Vapor Phase Lubrication
• Friction
• Wear