Enhancing mechanism of TC4 high-cycle fatigue performance by the introduction of hard nitride mono-and multilayer coatings

•The TiN coating increased the fatigue strength of TC4 substrate by 35%.•In high-cycle loading, fatigue cracks initiation dominates in coating/metal substrate system.•Enhancing coating plasticity boosts fatigue resistance in coating/metal substrate system. This paper investigates the enhancement mechanism of mono- and multilayer coatings on the high-cycle fatigue behavior of TC4 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 investigate the fatigue failure mechanism. Median fatigue limits obtained by stair case method of TC4 with TiN coating, TiN/Ti (7:1), TiN/Ti (3:1), and uncoated TC4 after 1 × 107 cycles are 553 MPa, 422 MPa, 480 MPa, and 411 MPa, respectively. Under high-cycle loading, the hard-coating/TC4 substrate experiences fatigue failure through two mechanisms: coating cracking hastens the substrate's fatigue failure, or the fatigue source within the substrate creates sliding steps, ultimately resulting in failure. After 1 × 107 cycles under fatigue strength, the average dislocation density on TC4 as well as coated TC4 is similar. The plastic deformation resistant of TC4 under cyclic load was enhanced with nanoscale hard TiN coatings. This finding underscored that the hard coating significantly improves fatigue resistance by inhibiting crack initiation and accommodating substantial cyclic plastic strain. Furthermore, our research provides strong support for the application of hard ceramic coating on the surface strengthening of titanium alloy.