Laser spray cladding of porous NiTi coatings on NiTi substrates

Laser spray cladding in N2-rich atmosphere is a competitive technique for manufacture of porous NiTi shape memory alloy (SMA) and TiN surface film. In order to gain expected porous coating and porosity size, TiH2 powder was considered to be used as porosity initiating agent, and nitrogen was used for shielding and active nitriding gas.A 2 kW continuous wave Nd:YAG laser equipped with a 3-way coaxial nozzle was used for laser spraying. NiTi alloy powder and TiH2 powder were used as additive materials. The mixed powder of NiTi and TiH2 was conveyed by a powder feeder, congregated at the laser interacting zone and sprayed onto the substrate with molten droplets. In order to improve the corrosion resistance of the surface of coating layer, a gas nitriding process is facilitated simultaneously during the laser spraying and cladding process in nitrogen-rich atmosphere.A gradient porous coating of NiTi alloy had been manufactured by laser spraying. The investigation shows that there are pores with size of 100–200 μm near the surface. XRD analysis shows that the coating consists of intermetallic compounds NiTi, NiTi2 and nitrides TiN, Ti2N. Corrosion test was carried out in Hank's balanced salt solution (H9269) by potentiostat/galvanostat model 273A. The polarization curves of the test result demonstrates the porous coating with TiN presents better corrosion resistance than NiTi substrate.