The steady-state volume erosion rate of the substrate along with that of coating are shown in Fig. 1 at both 90◦ and 30◦ impact angles. It clearly shows that the erosion rate at 30◦ impact is higher than that at 90◦ for the steel substrate which is common for ductile materials where material removal takes place predominantly by plastic deformation. The Fig. 1 also shows the volume wear rate for WC–CoCr coating which reveals that the erosion rate at 90◦ impact angle was higher than that at 30◦ which is the characteristic erosion behaviour of brittle materials . This is also expected as the WC–CoCr material is known to have ‘brittle’ type of erosion behaviour where material removal takes place predominantly by carbide particle fracture and fracture involving crack initiation at particle-matrix interface and its propagation to the free surface. The extent of cracking is most severe when the impact direction is normal to the surface leading to higher erosion rate. In as-coated form, the erosion rate at both the impact angles were higher in the case of HVOF coating as compared to DS coating. In brittle materials, higher porosity is also known to result in higher erosion rate. Given the fact that HVOF coatings have marginally higher porosity and lower residual compressive stresses as compared to DS coatings , higher erosion rate in HVOF coating compared to DS coating might be expected.
Fig. 1. Steady-state volume erosion rate
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