Fracture mechanism of TiN coatings on Ti-6Al-4V substrates: Role of interfaces and of the residual stress depth profile

The performance and integrity of coated engineering components rely on the time required for the appearance of defects and cracks and their propagation, leading to delamination of the coating and degradation of the substrate. Optimizing the coating's mechanical properties such as hardness, residual stress (RS), and adhesion is essential to delay crack onset and propagation. In the present work, we investigate the mechanical properties, and the failure mechanisms of model TiN coatings reactively sputtered onto Ti-6Al-4V substrates while comparing three interface engineering approaches to enhance the system durability: a) Argon plasma treatment, b) plasma surface nitriding, and c) Titanium implantation. The TiN coatings possessed a hardness of ~29 GPa and a Young's modulus of ~350 GPa. Multi-reflection grazing incidence X-ray diffraction was used to assess the compressive RS depth profile. Each interface engineering process induced RS variation in the coating and the adjacent interfacial area and in the substrate's near-surface layer ranging from −1 GPa to −2.2 GPa with different gradients. Cohesive failure, crack evolution and fracture toughness were studied using micro-scratch and micro-tensile tests, while the surface grain deformation behaviour and surface cracks on fractured Ti-6Al-4V with various interface treatments and TiN coatings were identified by SEM image and elasto-plastic property analyses. We found a close correlation between the RS of the coatings or the interface layers and the fracture mechanism. Specifically, higher compressive RS led to an enhanced interfacial shear strength and a critical energy release rate of around 550 to 790 MPa, and of ~18 J/m2, respectively.