Solid particle erosion behaviour of CVD boron phosphide coatings

This paper describes the solid particle erosion behaviour of boron phosphide coatings deposited onto AISI type 316 stainless steel substrates. The coatings, which were 20–28 μm in thickness, were tested under low velocity impact (33 m s−1) in an air–sand erosion rig and the damage features were studied in order to determine the erosion mechanisms. The results show that the particle impacts cause the initiation and propagation of radial and lateral cracks, which leads to a gradual removal of the coating. Hertzian ring cracks are also observed, although they are fewer in number. Comparison of the ring crack diameters with Hertz theory reveals close agreement between theory and experiment. Also significant is the finding that the maximum sub-surface shear stress, which is generated by particle impacts, is close to the coating–substrate interface: this partially explains the rapid failure of the coating. It is thought that increasing the coating thickness and/or employing a harder substrate may improve the erosion resistance of the coating. However, the effect that these changes may have on the residual stresses and coating adhesion must be fully evaluated.