Investigation into the thermal dehydroxylation and decomposition of hydroxylapatite during atmospheric plasma spraying: NMR and Raman spectroscopic study of as-sprayed coatings and coatings incubated in simulated body fluid

Hydroxylapatite (HA) is a frequently used bioceramic material for replacement of bone matter subjected to low loading conditions, for osseoconductive coatings on implants and for utilisation as a drug carrier. Plasma spraying is widely used to coat hydroxylapatite onto titanium alloys in hip endoprostheses. However, the high temperature of the plasma jet changes the crystallinity and decomposes hydroxylapatite. This affects in turn its bioconductivity. In this study, HA was coated onto Ti6Al4V substrates by atmospheric plasma spraying (APS). Also, a bionert TiO2 bond coat was applied between the HA coating and the titanium alloy. By means of some sensitive analytical techniques, notably nuclear magnetic resonance (NMR) and Raman spectroscopy, the structural decomposition of HA during plasma-spraying and the in vitro reconstruction mechanisms of distorted (oxy)hydroxylapatite to well-ordered hydroxylapatite were investigated. The advantages of such a bond coat were also shown