Heat treatment effects on the tribological performance of HVOF sprayed Co-Mo-Cr-Si coatings

The tribological behavior of high-velocity oxyfuel sprayed Co-28%Mo-17%Cr-3%Si coatings, both as-sprayed and after heat treatments at 200, 400, and 600 degrees C for 1 h, has been studied. The as-sprayed coating contains oxide stringers and is mostly amorphous. It has low hardness (similar to 6.7 GPa) and toughness and undergoes adhesive wear against 100Cr6 steel. The friction coefficient increases up to similar to 0.9, so the flash temperature reaches a critical oxidation value; then, friction decreases and increases again. This phenomenon occurs periodically. Much adhesive wear occurs in the first stage. Abrasive wear prevails against alumina pin: the coating wear rate is lower because it possesses good plasticity. Thermal effects still occur. The 600 degrees C treatment causes formation of submicrometric crystals. Hardness increases (similar to 8.8 GPa), adhesive wear is prevented, the friction coefficient has no peaks. Against the alumina pin, wear rates remain similar to the as-sprayed case. Nevertheless, the friction coefficient has no peaks and its final value is lowered (from 0.84 to 0.75).