Transformation of amorphous phase in nanostructured WC-10Co4Cr coatings induced by heat treatment and accompanying micromechanical property enhancement

In this study, nanostructured WC-10Co4Cr coatings were prepared by an improved HVOF (high velocity oxy-fuel) process, with the average grain size of WC grains in as-sprayed coatings being less than 0.26 μm. It was observed that the segregation of the two phases of Co/Cr and dislocation slip of WC along the {101¯0} crystallographic planes were present in the sprayed coatings. The Co phase underwent a martensitic transformation during the heat treatment process, with the orientation of the two phases following the SN (Shoji-Nishiyama) criterion. The 900 °C heat-treated FCC-Co structure exhibits the presence of twinning boundaries within the grains, which serves to facilitate the absorption and movement of dislocations. During the cooling process, the fcc structure does not undergo complete transformation into the hcp structure, resulting in the coexistence of FCC-Co and HCP-Co at different heat treatment temperatures. During heat treatment, the Cr phase in the coating forms a carbide of Cr23C6 with carbon, which decomposes at higher temperatures and precipitates again during cooling. The amorphous phase of the coating was transformed into nano carbides and twins by heat treatment, and the woven network of three-dimensional carbides effectively enhanced the micromechanical properties of the coating. •Nanostructured WC-10Co4Cr coatings with a low degree of decarburisation were prepared.•Heat treatment promotes amorphous phase transformation in the coatings and leads to the generation of annealed twins.•Two structures of crystalline Co phase exist in the coating after heat treatment with a special orientation relationship.•The Cr element generates Cr23C6 carbides along the WC grain boundaries.•Heat treatment enhances the micromechanical properties of the coating.