Carbide dissolution in WC-17Co thermal spray coatings: Part 1-project concept and as-sprayed coatings

Tungsten carbide-cobalt (WC-Co) thermal spray coatings are widely used for ambient temperature wear applications and are typically deposited using agglomerated feedstock consisting of 1-5 mu m sized carbide particles. There are technical challenges to reduce the carbide particle grain size to the desirable <1 mu m because of carbide dissolution and carbon loss in-flight; i.e., in transit from the torch to the substrate. This work explores a novel processing route for the WC-Co system that has been conceptualised previously for the chromium carbide-nickel chromium (Cr3C2-NiCr) system. The approach is based on the tendency for carbides to dissolve in-flight and for precipitation of submicron carbide particles on subsequent heat treatment. Part 1 of this two-part series reviews the equilibrium Co-W-C system to assess this concept for the WC-17 wt%Co composition. WC-17 wt%Co thermal spray coatings, targeting varying degrees of carbide dissolution/decomposition and carbon loss, were sprayed using HVOF, plasma Ar-He and plasma Ar-H-2 techniques. The deposits were characterised by XRD pattern fitting, cross-sectional SEM and LECO carbon combustion analysis. The mechanisms of phase formation and carbon loss are discussed. (C) 2020 Elsevier B.V. All rights reserved.