Dry sliding wear behaviour of HVOF thermal sprayed WC-Co-Cr and WC-CrxCy-Ni coatings

Abstract

High velocity oxy-fuel (HVOF) thermal spray process has shown obvious advantages over other surface hardening techniques when depositing WC-based layers, such as the laser cladding, electrodeposition and chemical/physical vapour deposition (CVD/PVD) methods, due to its versatility, survivability of hardening phase and low cost. HVOF thermal sprayed WC-based coatings are widely used in components that operate in harsh environments needing excellent sliding, fretting, abrasion and erosion resistance. WC-CrxCy-Ni coating shows better wear performance than the WC-Co-Cr coating at high temperature but inferior wear performance at room temperature at lower loads according to literature; however, the wear performance and relevant mechanisms of these two coatings under higher loads has not been reported. To fill this knowledge gap, wear testing of HVOF thermal sprayed WC-CrxCy-Ni and WC-Co-Cr coatings under high loads (96, 240 and 318 N) against a sintered WC-Co (6 mm diameter ball) counter-body was studied in this paper. For WC-CrxCy-Ni coating, decarburization of CrxCy rather than WC, took place during spraying. While the decarburization of WC to W2C occurred in the WC-Co-Cr coating. The major hardening phase (WC) dominated the wear performance of the coatings given its high hardness and small size, and Co also appeared to be a superior binder phase than Ni. At the maximum load, the specific wear rate of WC-CrxCy-Ni coating against WC-Co counter body was 17.92 × 10-7 mm3 N-1m-1, which is two times that of WC-Co-Cr coating (9.81 × 10-7 mm3 N-1m-1). The wear mechanisms for WC-CrxCy-Ni coatings included abrasion of the matrix, cracking of the secondary carbide phase and pulling out of WC particles. For WC-Co-Cr coatings, abrasion of the matrix was marginal, and cracking of the secondary carbide was not observed. The presence of CrxCy of lower hardness than the WC decreased the wear resistance of entire WC-based coating at room temperature, and improved oxidation resistance of WC at high temperatures due to the higher affinity of Cr to O. Hence, the secondary carbide hardening phase may be detrimental when considering the wear applications of HVOF thermal sprayed WC-based coatings.