Phase transformations in WC-Cr3C2-Ni thermal spray coatings under low, medium and high thermal input: Part 1 – Feedstock and as-sprayed coating characterisation

Conventionally, bulk WC and Cr3C2-based carbide compositions have been used independently of each other. However, recent investigations have begun to explore combining these carbides together within the same composite/hard metal coating system. This work explored the diversity of phases produced in the coatings from a 42 wt% WC-42 wt% Cr3C2–16 wt% Ni powder sprayed under “low”, “medium” and “high” thermal input conditions using high velocity oxygen-fuel (HVOF), an argon‑helium (ArHe) plasma and an argon‑hydrogen (ArH2) plasma respectively. During spraying, the Ni binder melted, leading to variations in the extent of carbide dissolution and peritectic decomposition as a function of thermal input. Low thermal input, typified by the HVOF technique, led to preferential dissolution/decomposition of the Cr3C2 grains into the melt, while retaining more of the higher melting point WC grains. The coating was dominated by a supersaturated Ni phase, along with the retained carbides. High thermal input, typified by the ArH2 plasma, was postulated to exceed the peritectic decomposition temperature of WC. The coating consisted of a supersaturated Ni alloy, along with the metastable phases WC1-X and β-W2C. No metastable Cr-carbides formed in any of the coatings. The mechanism of coating phase formation is discussed.