Adhesion and Interfacial Durability of Coatings on NiTi Alloys Under Tribocorrosion Conditions: A Review

The performance and long term reliability of nickel–titanium (NiTi) alloys in biomedical applications are strongly governed by surface related degradation processes, in which the adhesion and interfacial stability of protective coatings play a critical role. This review examines ceramic, polymer, and hybrid coatings applied to NiTi substrates, with particular emphasis on adhesion mechanisms, interfacial mechanics, and tribocorrosion behavior under coupled mechanical and electrochemical loading conditions. The analysis demonstrates that coating durability is controlled by the interplay between adhesion characteristics, mechanical compatibility, and functional thickness. Thin coatings provide favorable strain accommodation but limited wear resistance, whereas thicker layers improve barrier performance at the expense of increased susceptibility to interfacial stress accumulation and cracking. Ceramic coatings offer excellent corrosion and tribological performance but are prone to adhesion related failure under large deformation, while polymer coatings enhance interfacial compliance at the cost of reduced long term durability. Multilayer and graded architectures are shown to improve adhesion durability and delay delamination by redistributing interfacial stresses and decoupling protective and deformational functions. Coating degradation in NiTi systems is therefore a multiphysical phenomenon involving the coupled action of mechanical damage, electrochemical reactions, tribological interactions, and martensitic transformation. Despite significant progress, key challenges remain, including limited long term fatigue data, insufficient tribocorrosion studies under fully coupled conditions, and the lack of predictive models linking interfacial design with coating durability. This review highlights the need for integrated, adhesion oriented design strategies for advanced coating systems in NiTi biomedical applications.