The wear of a chromium carbide hard coating, LC1B, has been investigated in CO2, argon and air at pressures between 2 and 42 bar. In CO2 the wear rate of the shaft (but not the pin) is strongly gas pressure and load dependent. Post-test observations have suggested the presence of up to three wear regimes: (i) initial wear is short lived and probably results from the mechanical interlocking of asperities, (ii) adhesive wear involves the homogenization of surface layers and the preferential transfer of material to the static pin which then protects the pin from further wear, and (iii) mild wear which is in part abrasive, but in which oxidative wear provides the major contribution. Wear in argon is adhesive, that in air is primarily mild, but in CO2 a transition from adhesive to mild wear occurs after a sliding distance which is dependent upon sliding contact parameters and on gas pressure. The above observations and deductions have been developed analytically to explain the unusual results obtained in CO2. It has been shown how, in CO2, the sliding distance to the second transition (adhesive to mild wear), may be explained by the development of equations for the depletion of CO2 at the wearing interface. It is postulated that after its formation, wear debris is reactive to CO2 only for a very short time (∼ 5 × 10−9s) and that the transition to mild wear occurs when the depleted CO2 pressure increases to a level at which a monolayer of reaction product can form in this reaction time.

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