In situ formation of a TiN/Ti metal matrix composite gradient coating on NiTi by laser cladding and nitriding

A TiN reinforced metal matrix composite (MMC) layer was fabricated in situ on a NiTi substrate aiming at improving the wear resistance and reducing the surface Ni content in view of potential medical applications. Ti powder was preplaced on a NiTi substrate and irradiated with a high-power CW Nd:YAG laser in N2atmosphere for laser nitriding and alloying. SEM micrographs of the cross-section revealed a gradient coating with an almost compact TiN film of about 1-2 μm thickness as the outermost layer and an MMC layer beneath with the amount of TiN decreasing with depth. The hardness was increased from 250 HV in the substrate to 600-900 HV in the modified layer due to the presence of the hard TiN phase. The wear resistance against a diamond ball was correspondingly increased by a factor of two. EDS analysis also indicated a low Ni content in the surface layer. These results suggest that the laser modification technique employed in the present study is capable of enhancing the feasibility and biocompatibility of NiTi samples used as orthopedic implants.