HVOF Hydroxyapatite/Titania-Graded Coatings: Microstructural, Mechanical, and In Vitro Characterization

The present contribution aimed at exploring the HVOF deposition process of bioactive multilayered HAp/titania composite coatings on Ti-6Al-4V substrates. These coatings can be regarded as functionally graded as the weight fraction of the constituent phases gradually changes layer by layer, from pure titania at the substrate-coating interface to pure HAp at the outer surface of the coating. Microstructural investigations were carried out on the graded coatings using scanning electron microscopy coupled with EDS microanalysis to confirm that the compositional gradient met the initial specifications. On the other hand, the in vitro properties of the coatings were studied in simulated body fluid (SBF) for periods ranging from 1 to 14days. Moreover, mechanical characterization of both as-sprayed and soaked coatings in SBF was carried out by performing Vickers microhardness measurements through their cross section. The apparent interfacial toughness (K-Ca) of HAp/titania coatings, which is representative of their interfacial crack initiation resistance, was determined by performing indentation tests at the coating-substrate interface. Fracture toughness of both pure hydroxyapatite and functionally graded coatings was also calculated. The results revealed that the graded coatings produced in this work exhibited good reactivity and mechanical stability after being immersed in SBF indicating their potential for biomedical applications.