Surface Modification Of Ni-Ti Shape Memory Alloy And Its Application As Medical Material

Amongst the most interesting medical alloys developed in the last few years is the NiTi shape memory alloy (NiTiSMA) with nearly equiatomic nickel and titanium distribution because of its shape memory effect, superelasticity and good biocompatibility. Laser gas nitrided (LGN) layer was fabricated on the surface of the NiTiSMA after irradiated by a continuous wave Nd-YAG laser in N₂ environment. SEM, EDX, XRD, FTIR, electrochemical, and wear tests were carried out for material characterization. The bone-like apatite formation on the specimens in body simulated fluid was also studied. The following conclusions are drawn:Laser gas nitrided layer, dense and free of crack, was fabricated on the surface of NiTi shape memory alloy (NiTiSMA) after irradiated by a continuous wave Nd-YAG laser in N₂ environment. The modified layer distributed by graduation. Test results indicated that there was a good metallurgical bonding between the modification layer and the NiTi alloy substrateThe hardness of the outmost surface of the modified layer was as high as 1300Hv. The hardness of the nitrided layer changed across the modified layer. The wear resistance was 4 times as much as that of the substrate.The electrochemical tests in the simulated body fluid (Hank's solution) at 37℃ showed that the corrosion and biocompatibility of the material were improved by means of laser gas nitriding. The corrosion potential was increased by 100mV and the breakdown potential by about 500mV while the corrosion current and capacitance decreased. EIS tests indicated that the transference resistance increased by about 50 times and the capacitance was 42% of the substrate.Biomineralization tests showed that the biocompatibility was effectively improved by laser gas nitriding. The modified layer have the ability to induce the formation of bone-like apatite during immersing in simulated body fluid(SBF) at 37℃ and thus are considered bioactive. The formed layer has the Ca/P atomic ratio of 1.47 very close to that of bone-apatite, which has the Ca/P atomic ratio of 1.67. The hydroxyl bond stretch, the vibration bands of phosphate andcarbonate are observed in FTIR examination. It is believed that the formed layer is composed of bone-like apatite.The results of the EIS tests coincided with those obtained from SEM very well. Time constant and the impedance of the bone-like apatite reflect the whole process of apatite nucleation and growth on the pretreated specimen surfaces.