The current study examined the tribological behavior of HVOF sprayed Co-28%Mo-17%Cr-3%Si coatings, both asdeposited and after heat treatments, correlating it with microstructural and micromechanical features. A significant degree of splat boundary oxidation exists in the as-sprayed coating, because of exothermic oxidative reaction occurring at T > 810 °C. This coating is mainly amorphous due to splat quenching; thus, it has low hardness and toughness, resulting in poor tribological performance—particularly, its low hardness promotes adhesive wear against 100Cr6 steel pins. Adhesion causes a rapid increase in friction coefficient, and consequently the contact point temperature reaches a critical value where rapid oxidation occurs. Oxides decrease the friction coefficient, but they are not particularly adherent to the contacting surfaces and mostly form debris. Therefore, friction increases again and continues to oscillate periodically because adhesive wear continues to raise flash temperature up to the critical value. Most of the wear loss occurs in the first stage, where adhesion is particularly severe due to direct contact between metallic surfaces. In the tests against alumina pin, the sample wear rate is smaller because less adhesion takes place; abrasive wear is prevalent, but the Co-base alloy has sufficient intrinsic plasticity to withstand it without undergoing too much cutting wear. However, the fast oxidation process, with peculiar friction coefficient behavior, still takes place.
While the 200 and 400 °C heat treatments do not cause any major change (the former one even degrading the coating properties), the 600 °C treatment causes the appearance of submicrometric crystalline regions improving hardness and elastic modulus. Adhesive phenomena between coating and steel pin are thus definitely reduced; the wear loss is negligible for the coating and decreased by two orders of magnitude for the pin; no friction coefficient peaks occur nor is fast oxidation started. Instead, friction coefficient soon gets to a steady value. The coating wear rate against alumina pin is not significantly changed because abrasive wear still prevails, so there are no major changes in the wear process. However, adhesive phenomena are further reduced, preventing the appearance of friction coefficient peaks and of fast oxidation. Thus, performing a 600 °C, 1 h heat treatment in air could be suggested as a way to improve the sliding wear performance of the present alloy at room temperature. The 600 °C heat treated coating wear rates are lower than those recorded by the authors for hard chrome platings at room temperature under the same testing conditions .
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