ABSTRACT
In this chapter we discuss the tribological properties of the hydroxyapatite (HAp) coatings. The practical challenges of the biomedical application of a conventional dense high-power MAPS-HAp coating include (1) occlusion of the porous surface, (2) uncontrolled bioresorption, and (3) late delamination with formation of particulate debris [1-4]. The last factor is an issue of significant importance in the case of, e.g., hip implants, because particulate debris of HAp might well accelerate the polyethylene wear-induced granulomatous tissue response with an associated bone lysis [1, 5]. This last factor also constitutes the scope of the discussion on issues related to dynamic contact deformation and damage evolution in a HAp coating. It needs to be appreciated that due to repetitive loading during in vivo implantation, the HAp coating surfaces may often experience the micro/ nanoscale wear. Classically speaking, this is what tribology is all about. It needs to be recalled, at the same time, that the discussions presented in earlier chapters show that in spite of having about 11-20% porosity, the MIPS-HAp coatings have bonding strength, hardness, elastic modulus, and fracture toughness higher than those of the MAPS-HAp coatings. Therefore, for in vivo applications, the study of the nanotribological characteristics of the HAp coatings in general and MIPS-HAp coatings in particular emerges as the most important issue. This is what is attempted in this chapter.
