Bond strength improvement of hydroxyapatite/titanium composite coating by partial nitriding during RF-thermal plasma spraying

The bond strength of plasma-sprayed HA/Ti composite coatings on Ti substrate was significantly improved by the partial nitriding of Ti deposits in the coatings during plasma spraying. HA/Ti composite coatings were deposited on titanium substrates by a radio-frequency thermal plasma spraying method with RF input powers of 12–27 kW. The ratio of HA and Ti powders supplied into the plasma was precisely controlled by two microfeeders so as to change the composition from Ti-rich to HA-rich towards the upper layer of the coating. When sprayed in plasma gas with 0.8% N2 added, the bond (tensile) strength of the obtained HA/Ti composite coatings sprayed at 12 kW was 41.4 MPa. On the contrary, the bond strength of coatings sprayed with pure Ar at 12 kW was 33.6 MPa regardless of the processing parameters which were almost the same except for plasma gas composition. When sprayed with 1.8% N2 added at 27 kW, the bond strength reached 65.3 MPa. XRD patterns of Ti coatings without HA showed that titanium nitrides formed in the titanium deposits sprayed with plasma gas containing N2. Scanning electron microscopic observation showed that these Ti deposits had an acicular texture.