Preparation And Microstructure Of Porous NiTi Coating

In this paper, Ni/TiH₂MA alloy coating, Ni/TiH₂/Ti composite coating and Ni/Ticomposite coating were prepared repectively on1Cr18Ni9Ti stainless steel substrate by thetechnique of cold spraying and the powder of TiH₂acted as a pore-forming agent. The coatingswere researched by subsequent heat treatment. After heat treatment, the microstructure of thecoating and the morphology of the surface and the constitution of the phase were characterizedby scanning electron microscope and X-ray diffraction. The formation mechanism of NiTiintermetallic compounds as well as the pores in the coatings was discussed in detail.The results showed that three alloy powders with a high degree of refinement and layeredstructure were fabricated by cryo-milling but Ni/TiH₂alloy powder showing a poor mobility.The coatings sprayed with MA powders remained refinement and layered structure. With theincreasing of the holding time in heat treatment, the porosity of Ni/TiH₂/Ti composite coatingwas decreased. Enhancing the heating rate had a contribution to the formation of NiTiintermetallic compound and an important effect on the porosity of the coating. It was foundthat the porosity of the coating generally appeared in the Ti-rich region by the research of theheat treatment in Ni/Ti composite coating. The eutectic product （β-Ti+Ti₂Ni） was formedeasily in the interface between the zones of rich Ti and Ti₂Ni or the central of Ti nucleus. Theeutectic compounds were melted when the heat treatment temperature was above the eutectictemperature and the pores in the coating were formed in the regions of rich Ti. The studiesshowed that some main factors to affect the form of the porosity in coating: the densificationof the coating itself; the decomposition of TiH₂which act as the pore-forming agent; themicrostructure and Kirkendall effect formed by the diffusion reaction between Ni atoms andthe Ti atoms under the high-temperature, etc.. The phase of β-Ti which was generated in theheat treatment process was the reason of why the pores were formed in the region of rich Ti,while the content of the phase of β-Ti was also the main factors that affected the size of thecoating porosity.