Abstract
The tribological behaviour of HVOF-sprayed Co-28%Mo- 17%Cr-3%Si coatings, both as-sprayed and after heat treatments at 200°C, 400°C, 600°C for 1h, has been studied. Microstructural and micromechanical properties were characterized by SEM, XRD, depth-sensing indentation (microhardness, elastic modulus, indentation fracture toughness); pin-on-disk dry sliding wear tests were performed against 100Cr6 and sintered alumina spherical pins. The as-sprayed coating contains splat boundary oxide stringers and is mostly amorphous (splat quenching). It has low hardness (≈6.7GPa) and toughness (≈1MPa*m1/2); thus, much adhesive wear occurs against 100Cr6 steel. The friction coefficient increases up to ≈0.9, raising the contact point temperature up to a critical coating oxidation value; then, it decreases, increases again and finally settles. Much of the adhesive wear occurs in the first stage of the test. Abrasive wear prevails with the alumina pin: the coating wear rate is lower because the metallic alloy possesses intrinsically good plasticity. Adhesive phenomena still occur, leading to the same thermal effects. After the 200°C and 400°C heat treatments, no major changes occur. The 600°C treatment causes the formation of some submicrometric crystals. The hardness increases (≈8.8GPa), adhesion to steel pin is prevented, and the friction coefficient soon settles at ≈0.8 with no peaks. The coating undergoes no wear loss and the pin wear rate largely decreases. With the alumina pin, coating and pin wear rates remain quite low. However, adhesion is reduced, so that the friction coefficient has no peaks and its final value is lowered (from 0.84 to 0.75).
