The effect of carbide decomposition and carbon loss on the steady state composition of WC-Cr3C2-Ni composite coatings at 900 °C.

The steady-state composition and microstructure of 42 wt% WC-42 wt% Cr ₃C ₂–16 wt% Ni coatings sprayed under “low”, “medium” and “high” thermal input conditions was explored following treatment at 900 °C for up to 30 days. None of the coatings achieved thermodynamic equilibrium compositions. Attempts at compensating for mechanisms of bulk carbon loss and selective carbon loss from the dissolution phases alone, were both unsuccessful in predicting the steady-state compositions based on equilibrium phase diagrams. The low thermal input HVOF coating underwent recovery of the dissolved Cr ₃C ₂ phase to form an interconnected, two-phase Cr ₃C ₂ + Ni-alloy matrix surrounding the retained WC grains, which did not achieve thermodynamic equilibrium with the precipitating phases. The high carbide dissolution plasma Ar H ₂ coating generated a homogeneous distribution of metallic and carbon elements such that the metal: carbon ratio exceeded 1:1, favouring the formation of (W,Cr) ₂C. The remaining material formed (Cr,W) ₃C ₂ within a Ni(Cr,W) alloy binder. Chromium stabilised the (W,Cr) ₂C phase such that it did not decompose into W + WC, as predicted in the W C system. The plasma Ar He response was a mix of these mechanisms. A distinctive feature of the heat-treated coatings was the lack of significant grain growth, with submicron carbide grains retained out to 30 days at 900 °C.