Effect of brittle TiN coating on fatigue performance of TC11 titanium alloy under rotating bending and tension-tension

In this paper, the effect of brittle TiN coating on fatigue performance of TC11 titanium alloy (Ti-6.5Al-3.1Mo1.05Zr0.35Si) under rotating bending and tension-tension were investigated. TiN coating causes a reduction in high and low cycle fatigue performance of TC11 under both conditions. The median fatigue strength at 1 x 107 cycles of TC11 decreases from 582.5 MPa and 896.0-547.5 MPa and 531.0 MPa under rotating bending and tension-tension fatigue loading, respectively. In scenarios involving rotating bending fatigue, fatigue failure initiation for both TiN coated and uncoated TC11 specimens reside on the droplet in coating surface, irrespective of high or low stress conditions. As for tension-tension fatigue loading, both TiN coated and uncoated TC11 specimens experience surface-based fatigue initiation points under high fatigue stress. However, during lowstress tension-tension loading, the incorporation of a TiN coating prompts a shift in TC11's fatigue initiation source from the interior to its surface. Furthermore, under both fatigue loading condition, fatigue cracks initiate at droplets on the coating surface and propagate to the surface of the substrate and eventually its interior. And the fatigue failure of the TC11 substrate is accelerated due to the coating causing balanced tensile stress. The inherent compressive characteristics and lack of tensile strength in TiN ceramic materials render them more susceptible to premature cracking when subjected to tensile fatigue loads. This vulnerability leads to heightened damage to the fatigue performance of TC11.