Structure Optimization and Failure Mechanism of Metal Nitride Coatings for Enhancing the Sand Erosion Resistance of Aluminum Alloys

In this study, TiN/Ti coatings with various modulation ratios (TiN/Ti-4:1, TiN/Ti-1:1, and TiN/Ti-1:4) were deposited on 2A70 aluminum to improve its sand erosion performance. The structural design of ion implantation + high thickness Ti transition layer + TiN/Ti coatings was applied to alleviate the differences in physical properties between hard nitride coatings and 2A70 aluminum. Surface roughness, XRD, elastic modulus, hardness, and the sand erosion failure mechanism of each coating were evaluated. The hardness of TiN/Ti-4:1, TiN/Ti-1:1, and TiN/Ti-1:4 on aluminum was 26.99 GPa, 21.70 GPa, and 10.99 GPa. Sand erosion test results showed that TiN/Ti-1:1 had the highest erosion rates due to its rougher surface. Under a 90° incident angle, TiN/Ti-4:1 and TiN/Ti-1:4 both exhibited vertical cracks parallel to the coating growth direction in the bottom TiN layer at the initial erosion stage. Also, a lateral crack caused by TiN layer crack deflection emerged due to a higher crack resistance in the thicker Ti layer of TiN/Ti-1:4. Furthermore, in comparison with the layer-by-layer spalling failure behavior of TiN/Ti-1:4, overall spallation induced by the crack coalescence of the TiN layer was exhibited in TiN/Ti-4:1. In addition, cracks formed and intersected in the inner TiN layer in TiN/Ti-1:1 and TiN/Ti-4:1, resulting in layer-by-layer spallation under a 45° incident angle.