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

Abstract

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 μm sized carbide particles. There are technical challenges to reduce the carbide particle grain size to the desirable <1 μm 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-H2 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.