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 Cr ₃C ₂ -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% Cr ₃C ₂–16 wt% Ni powder sprayed under “low”, “medium” and “high” thermal input conditions using high velocity oxygen-fuel (HVOF), an argon‑helium (Ar He) plasma and an argon‑hydrogen (Ar H ₂) 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 Cr ₃C ₂ 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 Ar H ₂ plasma, was postulated to exceed the peritectic decomposition temperature of WC. The coating consisted of a supersaturated Ni alloy, along with the metastable phases WC _1-X and β-W ₂C. No metastable Cr-carbides formed in any of the coatings. The mechanism of coating phase formation is discussed.