Impact angle effects on the erosion - corrosion of superlattice CrN/NbN PVD coatings

Wastage due to the combined effects of erosion and corrosion is a serious issue in a wide range of industrial environments. The extent of wastage is dependent on a range of parameters relating to properties of the particle, target and the environment. In such cases, surface engineering (through providing a protective coating) may be used to avoid the deleterious effects of both processes. In recent years, there has been an increasing interest in the potential of CrN/NbN superlattice PVD coatings to provide resistance to erosion-corrosion in such conditions. Initial studies on the effects of velocity have shown that they significantly outperform conventional coatings, although the behaviour depends on the conditions. There is however, poor understanding on the effects of many important tribological variables, such as particle size and impact angle, on the performance of such coatings. In this work, the effect of impact angle on the erosion-corrosion of a range of CrN/NbN superlattice coatings was assessed at a range of applied potentials in a carbonate/bicarbonate solution. The mechanisms of erosion-corrosion of the coatings, as a function of impact angle and applied potential, showed significant differences when these parameters were varied. Erosion-corrosion maps were constructed based on these results showing the change in mechanisms and wastage rates as a function of impact angle and applied potential. Possible reasons for the differences in erosion-corrosion regimes for coated and uncoated specimens are described in this paper