Fracture toughness of normal high velocity oxygen-fuel flame and low temperature high velocity oxygen-fuel sprayed WC-10Co-4Cr coating

WC-10Co-4Cr splats and coatings were prepared with the ultrafine powder as a feed material by high velocity oxygen-fuel flame (HVOF) and low temperature high velocity oxygen-fuel (LT-HVOF) techniques, respectively. The microstruture, roughness, micro-hardness and fracture toughness were characterized by scanning electron microscope, micro-hardness tester, roughness tester and 3D topography tester, respectively. The results show that the LT-HVOF WC-10Co-4Cr splats are in a slightly-melted state, and the HVOF WC-10Co-4Cr splats are in a partially-melted state. The main crystal phases for the HVOF and LT-HVOF coatings are W2C and WC, respectively. Compared to the HVOF coating, the LT-HVOF coating has a lower surface roughness (i.e., Ra=1.22 μm), a higher micro-hardness (i.e., Hv0.3=1316±85) and a higher fracture toughness (i.e., KC=3.23 MPa·m1/2). The cracks in the HVOF coating propagate along the Cr-rich strip, and the cracks in the LT-HVOF coating deflect to the CoCr adhesive phase when propagated to the WC particles. In addition, it is also indicated that the fracture toughness of the coating could be decreased due to the presence of the Cr-rich strip.