Microstructure, Adhesion and Tribomechanical Performance of Zn–Mg–Ca- and Zn–Zr-Based Phosphate Conversion Coatings on Ti6Al4V Alloy

Ti6Al4V remains one of the most widely used metallic biomaterials for orthopedic and dental implants; however, the long-term performance of implantable devices is strongly influenced by the mechanical integrity and surface stability of the implant–tissue interface. In this study, novel phosphate conversion coatings based on Zn–Mg–Ca, Zn–Zr–Mg, and Zn–Zr–Ca systems were developed on Ti6Al4V substrates through a chemical conversion process and investigated from a tribomechanical perspective. The coatings were designed to incorporate biologically relevant elements capable of modifying crystal growth, coating compactness and interfacial characteristics. Surface morphology, coating architecture and elemental distribution were evaluated using scanning electron microscopy and energy-dispersive spectroscopy, while surface roughness was determined by profilometry. The mechanical response of the conversion-modified surfaces was assessed through instrumented microindentation and progressive-load scratch testing coupled with acoustic emission monitoring. Significant differences in coating morphology, thickness and compactness were observed as a function of phosphating chemistry. Dense and homogeneous coatings exhibited improved hardness, higher elastic recovery and enhanced resistance to plastic deformation, whereas heterogeneous structures containing large intergranular regions showed inferior mechanical performance. Scratch testing demonstrated that coating composition strongly influences damage initiation, interfacial stability and resistance to coating detachment. Among the investigated systems, Zn–Zr–Ca and selected Zn–Zr–Mg formulations provided the most favorable combination of compact microstructure, mechanical integrity and scratch resistance. The results establish clear relationships between phosphating chemistry, coating architecture and tribomechanical behavior, providing valuable insight for the design of conversion-treated Ti6Al4V surfaces intended for biomedical implant applications.