Oxidation behavior of WC–Co hard metal with designed multilayer coatings by CVD

Abstract

Multilayer coatings composed of TiN, TiCN, α-Al2O3 and κ-Al2O3 were designed and then deposited on WC–Co alloy by chemical vapor deposition (CVD) technique, the samples of WC–Co alloy with multilayer coatings were oxidized in the temperature range of 600°C–950°C for various times in a Muffle furnace, and weight gain was measured by electronic balance. Phase component and microstructure evolution of coating samples after oxidation were investigated by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results showed that the oxidation resistance of WC–Co alloy was improved markedly after deposition of multilayer coatings. The oxidation kinetics of multilayer coating samples obeyed linear law and diffusion-controlled parabolic law at different oxidation conditions. The oxidation product of coatings was rutile TiO2. The sample exhibited excellent oxidation resistance when the outermost layer was α-Al2O3, which was consistent to the results of apparent activation energy of oxidation reaction. The oxidation resistance of multilayer coatings was improved with the increase of thickness of κ-Al2O3 layer. The κ-Al2O3 transformed into α-Al2O3 over 900°C. The interface between TiN and TiCN disappeared in the coating sample without α-Al2O3 and κ-Al2O3 layer.