Porosity effects of a Fe-based amorphous/nanocrystals coating prepared by a commercial high velocity oxy-fuel process on cavitation erosion behaviors

Coatings with different porosities were prepared by controlling high velocity oxy-fuel process parameters. Pores were distributed homogeneously along the thickness of the coatings. Cavitation erosion rate of the coating was obtained by a vibratory cavitation equipment following ASTM G32 standard. As porosity of the coating increases, the cavitation erosion rate increases. Significantly high cavitation erosion rate was obtained in the early stage of the test for the coating with high porosity. As cavitation erosion test proceeds, the cavitation erosion rate tends to decrease. Cracks initiated in the surface pore area propagate along powder boundaries and merge to pores near surface. Due to the cracks, large coating parts consisting of a bunch of powders with good bonding were detached from the coating increasing the cavitation erosion rate. Corrosion products were preferentially formed on the pore areas enhancing the cavitation erosion rate. Consequently, pores near coating surface significantly accelerate the cavitation erosion rate through mechanical as well as chemical manners.