Cavitation erosion performance of CVD W/WC coatings

In the aviation industry, water droplet erosion (WDE) takes place when an aircraft takes off on as wet runway or flies through rain or clouds. The leading edge of turbofan blades suffers from high-speed (300–400 m/s) impingements of water droplets, resulting in material removal that subsequently changes the leading edge profile and surface roughness. This affects the aerodynamic performance of turbofan blades, which eventually leads to efficiency drop of the aircraft engine and the need to replace and/or recondition the blades. A coating solution is targeted, such that, not only that it resists the high impact pressure but also inhibits stress wave reinforcements at the coating-substrate or interlayers interfaces. Past studies indicate similar damage mechanisms to WDE are generated by cavitation erosion (CE) during the early stages (incubation). Hence, CE is introduced in this study to predict the WDE performance. The coatings studied were nanostructured CVD tungsten/tungsten carbide coatings, either hierarchical or monotonic in design, on Ti6Al4V alloy grade 5 substrates. In-depth understanding on the coating damage mechanisms are established by correlating the coating performance with microstructure, crystallographic texture, interface design, coating deposition conditions and mechanical properties for the first time. A particular crystalline texture was found that gives optimum performance. The effect of the initial coating topography on the CE performance is effectively characterised by the surface parameters Ssk and Sku. The damage was found to initiate at the grain boundaries of the exposed surfaces. The hierarchical coating microstructure demonstrated enhancement in CE performance compared to a monotonic columnar grain structure. Additionally, it is found that the coating performance under dynamic compressive loadings could not be predicted by a simple H/E approaches. However, combining the H/E ratios with the factors of microstructure and crystal orientations might further facilitate the understanding of coating performances along with better understanding of the role of compressive residual stresses and stress waves propagation or reinforcement through coating depth and at the top surface of the coating.