Effect of TiN/Ti coating combined with laser shock peening pre-treatment on the fatigue strength of Ti-6Al-4V titanium alloy

The multilayer gradient TiN/Ti coating was deposited by using filtered cathodic vacuum arc (FCVA) deposition method. The laser shock peening (LSP) process was carried out before deposition. The surface morphology, mechanical properties were measured by atomic force microscope (AFM), nanoindentation and scratch tester. The fatigue strength was investigated on a vibration test platform and the fracture morphologies were observed by scanning electron microscope (SEM). The results showed that the compound process has little negative influence on the roughness. A hardened layer with high hardness and compressive residual stress was formed by the LSP pre-treatment. This layer can also suppress plastic deformation, making the adhesion strength between the coating and substrate increased from 54.9 N to 79.1 N. The TiN/Ti coating and the compound process improve the fatigue strength of TC4 alloy by 58.1% and 98.8%, respectively. The coating can restrain crack initiation on the surface and slow its propagation. The crack initiation location of the compound process sample moved into the deeper site and the improved adhesion strength further reduces the crack propagation rate. Thus, the compound process of TiN/Ti coating combined with laser shock peening pre-treatment greatly improves the fatigue strength of TC4 alloy.