Sliding wear properties of HVOF sprayed WC–20%Cr3C2–7%Ni cermet coatings

WC–20 mass%Cr3C2–7 mass%Ni powder was sprayed onto low-carbon steel substrates by a commercial high velocity oxygen-fuel (HVOF) spray process as well as by an improved HVOF process equipped with a gas shroud attachment. The latter process utilizes a nitrogen gas flow to shield the region between the spray gun and the substrate in order to suppress the material's degradation caused by reaction with air such as oxidation and decarburization. Some coatings were further heat-treated in air at 773 K for 30 h to form a thin oxide film on the surface. The sliding wear properties of these coatings against an iron pin were evaluated by using a pin-on-disk wear tester. The specific wear rate of the as-sprayed cermet coatings prepared under the conventional spray condition was about three times higher than that of the chrome plating but by using the gas shroud, the wear rate was reduced to the same level with the chrome plating. The specific wear rate could be further decreased by the oxidation heat-treatment. It was found that a proper amount of oxides existing on the surface or within the coatings have a great beneficial effects on the wear properties such as to promote the transition from severe wear to mild wear and thus to reduce the wear rate remarkably. XPS analysis of the transfer particles collected from the wear track revealed a shift in the oxidation state of iron depending on the wear condition.