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 rf 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 coating composition from Ti rich to HA rich toward the upper layer. When sprayed in plasma gas with 0.8% N-2 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 despite the other processing parameters being almost the same. When sprayed with 1.8% N-2 added at 27 kW, the bond strength reached 65.3 MPa. X-ray diffraction patterns of Ti coatings without HA showed that titanium nitrides were formed in the titanium deposits sprayed with plasma gas containing N-2. Scanning electron microscopic observation showed that these nitrided Ti deposits had an acicular texture. Transmission electron microscopic observation showed that Ti2N was formed at the interface between two nitrided Ti splats, while electron diffraction analyses showed that an alpha-titanium lattice was formed inside of the nitrided Ti splats. (C) 2003 American Vacuum Society.