Phase transformations in WC-Cr3C2-Ni thermal spray coatings under low, medium and high thermal input: Part 2 – DSC analysis and heat-treated 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. The thermally driven solid-state transformations in the coatings were investigated as a function of carbide dissolution/decomposition, using differential scanning calorimetry (DSC). All coatings exhibited a well defined exothermic peak attributed to the transformation of the metastable supersaturated Ni-alloy and WC1-X phases in the as-sprayed coatings, into the equilibrium Ni alloy and a mix of the intermediate metastable carbide phases β-W2C and Cr3C2-Y. At higher temperatures the metastable compounds transformed to the equilibrium phases of ε-W2C and Cr3C2. A defining feature of the XRD analysis was the significant shift in lattice positions of all phases, indicating substitutional alloying. The mechanisms of phase formation are discussed.