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 °C for 1 h, has been studied. The as-sprayed coating contains oxide stringers and is mostly amorphous. It has low hardness (∼6.7 GPa) and toughness and undergoes adhesive wear against 100Cr6 steel. The friction coefficient increases up 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 °C treatment causes formation of submicrometric crystals. Hardness increases (∼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 to0.75).