Oxidation Resistance of High-Velocity Oxygen Fuel Sprayed WC-Cr3C2-Ni Hardmetal Coatings

In this paper, oxidation behavior of three hardmetal coating compositions with different Cr3C2 content deposited on 1.4828 (AISI 309) or 2.4856 (Alloy 625) substrates was investigated. Heat treatment was performed in a tube furnace in a slight air flow (2 l/h) in the temperature range 500-900 °C for 2-32 days.

The oxidation resistance of conventional WC-Co(Cr) coatings (maximum service temperature 450-500°C) and Cr3C2-NiCr coatings (maximum service temperature 900°C) differ significantly. However, coating compositions containing both carbides are expected to show more complex mechanisms of oxide scale formation. By this reason in this study the oxidation resistance of Cr3C2-25NiCr, 42Cr3C2-42WC-16Ni, and 73%WC-20”Cr3C2”-7Ni coatings (all compositions in wt.%) 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 in a slight 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 a correlative laser and focused ion beam (FIB) preparation by FESEM and EDS. The Cr3C2-25NiCr and 42Cr3C2-42WC-16Ni coatings form dense oxide scales of Cr2O3 and are stable at 900°C up to 32 days. The oxidation resistance of the 73WC-20”Cr3C2”-7Ni coating composition 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 grows rapidly when the temperature is increased.