Effect of TiN monolithic and Ti/TiN multilayer coating on the fatigue behavior of titanium alloy under tension-tension

This paper investigates the fatigue failure mechanism of mono- and multilayer coatings on the fatigue performance of TC11 titanium alloy under tension-tension. The morphology, phase composition, mechanical properties were measured by scanning electron microscope, X-ray diffractometer and nanoindentation. Electron back scatter diffraction was employed to investigated the failure mechanism. Fatigue limits obtained of uncoated TC11, TC11 with TiN coating, TiN/Ti multilayer (ML-6, ML-3, ML-1) and after 1 × 107 cycles are 855 MPa, 550 MPa, 525 MPa, 500 MPa and 400 MPa. Under fatigue loading, the hard-coating/TC11 substrate experiences fatigue failure through coating cracking hastens the substrate's fatigue failure. EBSD analysis results indicate that the main slip system of TC11 titanium alloy under tension-tension fatigue load is α phase (10-10)[-12-10]. After 1 × 107 cycles at fatigue limits, the average dislocation density on the surface of the TC11 with TiN coating is lower than that of TC11. Due to the surface defect induced by coating preparation and high crack propagation velocity, the hard coating significantly deteriorates fatigue property of TC11 by reducing fatigue crack initiation period. Therefore, instead of approaching from the perspective of coating structure design to increase the fatigue crack propagation cycles, it is more effective to reduce the surface roughness of the coating and enhance the fatigue crack initiation cycles.