Surface properties and tensile bond strength of HVOF thermal spray coatings of WC-Co powder onto the surface of 420J2 steel and the bond coats of Ni, NiCr, and Ni/NiCr

Micron-sized WC-Co powder (powder) was coated onto an 420J2 steel substrate and the bond coats (BCs) of Ni, NiCr, and Ni/NiCr using high-velocity oxy-fuel thermal spraying to study the surface properties, friction behavior, and tensile bond strength of the WC-Co coating (WC-Co) on the 420J2 substrate (sub) and the BCs of Ni, NiCr, and Ni/NiCr. During the spray coating, a small portion of WC decomposed to the less-hard W2C, W, and free carbon above its decomposition temperature of 1250 °C, decreasing hardness and increasing porosity. The surface hardness of 1120 ± 100 Hv (10,980 ± 980 MPa) depended strongly on the spray parameters. It was three to four times harder than metals and alloys, but less than one-half the hardness of binder-less pure WC (2400 Hv). Free carbon reacted with the sprayed oxygen gas and formed carbon oxide gases, resulting in a coating of 4.3 ± 1.0% porosity. The friction coefficient of the coating increased about 17% with increasing surface temperature: 0.65 ± 0.03 at 25 °C to 0.76 ± 0.06 at 500 °C because of the increased local cold-welding of the asperities at the higher temperature of 500 °C. Sub/WC-Co, sub/Ni/WC-Co, sub/NiCr/WC-Co, and sub/Ni/NiCr/WC-Co had tensile bond strengths of 9600 ± 300 psi (66.2 ± 3.4 MPa), 6300 ± 200 psi, 6000 ± 200 psi, and 7500 ± 200 psi, respectively. The fracture locations of all coatings were at interfaces with the WC-Co coating, indicating that the adhesion of the WC/Co inside coating was higher than 9600 ± 500 psi and that the adhesion of WC-Co on the substrate (9600 ± 500 psi) was much higher than the adhesion on the BCs.