Enhancing fatigue performance of TiN-coated Ti-6Al-4V by N/Ti ratio modulation

In this paper, we propose an approach to enhance fatigue properties of TiN-coated Ti-6Al-4V for aero-engine compressor blades by N/Ti ratio modulation of TiN. TiN coatings with N/Ti ratios of 0.70, 0.85, and 0.92 were prepared by modifying the nitrogen flow during coating deposition. The resulting coatings were evaluated for surface roughness, phase structure, microstructure, hardness and residual stress. Furthermore, the rotating bending fatigue test indicated that TiN coatings at different N/Ti ratios negatively affected the fatigue properties of Ti-6Al-4V. Specifically, the fatigue limit of the substrate dropped from 531 MPa to 480 MPa, 510 MPa, and 440 MPa when coated with TiN coatings at N/Ti ratios of 0.70, 0.85, and 0.92, respectively. However, the high-temperature insulation process during coating preparation enhanced the substrate's fatigue performance. The reduced fatigue performance of TiN coated Ti-6Al-4V was attributed to the cracking of TiN, led to the fatigue crack initiation on the substrate surface and accelerated fatigue failure. In the fatigue crack initiation stage, the TiN coating's higher stress concentration factor accelerated crack initiation by inducing stress concentration, reducing initiation cycles. During fatigue crack growth, the coating's residual compressive stress hindered propagation and reduced crack length caused by coating fracture in the substrate.