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

Abstract

The steady-state composition and microstructure of 42 wt% WC-42 wt% Cr3C2–16 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 Cr3C2 phase to form an interconnected, two-phase Cr3C2 + Ni-alloy matrix surrounding the retained WC grains, which did not achieve thermodynamic equilibrium with the precipitating phases. The high carbide dissolution plasma ArH2 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)3C2 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 WC system. The plasma ArHe 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.