Toward High Cohesion and Adhesion of Coatings on Titanium and Its Alloys

Titanium long-term implants are nowadays mostly inserted in the body with high mechanical pressures, e.g., by screwing for dental implants and insertion into the previously drilled femur bone. On the other side, modern titanium implants are, as a rule, surface-modified, usually by change within the surface layer or deposition of coatings. Thus, the high mechanical stresses of different values and signs can significantly damage the implant surface, the interface between a body and an implant, and also the fixation of the coating to the implant surface. In other words, the use of improper surface modifications can result in a significant loss of adhesion and cohesion of the surface, decreasing the expected corrosion resistance, bioactivity, antibacterial efficiency, if designed, and cytocompatibility. In this chapter, based on the so far achieved, the mechanical stresses are assessed and the possibility of damaging the implant surface is discussed for different layers and coatings. The industrial use of hardened surfaces by the tough oxides obtained by micro-arc oxidation or vacuum plasma spraying is explained by their toughness. The testing of adhesion and cohesion by different techniques is presented with an emphasis on quantification and validation of the obtained results to make them comparable to each other.