Effect of Porosity on Cavitation Erosion Resistance of HVOF Processed Tungsten Carbide Coatings

The phenomenon of cavitation is widely experienced in both pumps and under-water components of hydro power plants. The mechanism of material damage during cavitation is mainly attributed to implosion of high velocity bubbles on to the surface giving raise to local increase in stresses in excess of 1.4 GPa. In addition, material wastage due to particle induced erosion is observed in hydro plants utilizing silt laden water for power generation. In order to mitigate the severe erosion due to silt and cavitation, the components are presently given a hard coating of 300 to 500 microns using High Velocity Oxy Fuel ( HVOF ) technique. The present paper highlights the results of cavitation erosion resistance evaluated on two different tungsten carbide coating processed through HVOF. The samples of size 15 x 15 mm were cut and polished using various diamond grits to achieve a polished scratch free surface. The properties of coating such as chemical composition, phase analysis, hardness, porosity, surface roughness were evaluated. The indentation toughness of coatings was evaluated at a test load of 10 kg on the cross section of coatings. The cavitation erosion resistance was carried out on a vibratory type cavitation test rig according to ASTM G32 for duration of 10 hours. The weight loss of coating and surface roughness of cavitated surface was measured at different intervals. The progression of surface damage morphology at different locations of the coating was observed in Scanning Electron Microscope. The effect of porosity, initial surface roughness was studied with respect to the metal R.K. Kumar et al 308 loss during cavitation. The mechanism of metal removal has been identified based on the progress of surface degradation.