Oxidation Resistance of HVOF-sprayed WC-Cr3C2-Ni Hardmetal Coatings

The oxidation resistance of conventional WC-Co(Cr) coatings (maximum service temperature 450-500°C) and Cr₃C₂-NiCr (maximum service temperature 900°C) coatings differs significantly. There are, however, several coating compositions containing both carbides, where the long-term oxidation resistance and the mechanism of oxide scale formation are not studied so far. By this reason in this study the oxidation resistance of 73%WC-20%”Cr₃C₂”-7%Ni, 42%WC-42%Cr₃C₂-16%Ni, 70%Cr₃C₂-10%WC-20%Ni (experimental composition) and Cr₃C₂-NiCr coatings was studied. The coatings were deposited on specimen of 25 mm diameter of 1.4828 (AISI 309) or 2.4856 (Alloy 625). Heat treatment was performed in a tube furnace with a small air flow (2 l/h) in the temperature range 500°C-900°C for 2-32 days. The oxide scale formation was studied by XRD, after conventional metallographic preparation or FIB-preparation by FESEM and EDS. The oxidation resistance of 73%WC-20%”Cr₃C₂”-7%Ni is much higher than for WC-Co(Cr), however, it was shown not to have a long-term oxidation resistance, due to formation of an oxide scale consisting mainly of loose NiWO₄. Such an oxide scale is formed at 500°C for 8 days, and growing rapidly when the temperature is increased. All other coatings form dense oxide scales of Cr₂O₃ at the surface and are stable at 900°C up to 32 days.