Surface modifications on NiTi shape memory alloys in order to improve their surface anti-corrosion capabilities and impede the out-diffusion of Ni from NiTi substrate in simulated body fluid

Abstract

Nickel-titanium shape memory alloys (NiTi) can have many applications due to their high mechanical strength, super-elasticity, and shape memory effects. One of the potential applications of the materials is in orthopedic implants. However, out-diffusion of harmful Ni from the NiTi substrate during prolonged use inside a human body is a serious issue and the problem must be solved before the materials can be more widely used in orthopedics. Therefore, there is a need to create a barrier layer to impede the leaching of Ni from the NiTi substrate in normal use inside human bodies. In my studies, we created such barrier layers by performing nitrogen, oxygen, and acetylene plasma immersion ion implantation - deposition into NiTi substrates to form TiN, TiOx, and TiCx implanted layers as well as amorphous carbon layers. All these compounds are chemically inert. Besides, TiN and TiCx have high stiffness and hardness while TiN, TiOx and carbon films have excellent biocompatibility. Except for carbon deposition, post-implantation vacuum annealing was conducted on all the implanted samples. All the treated and control samples surfaces were tested in three categories, namely, the anti-corrosion capability, the mechanical strengths, and the biocompatibility. My results show that all the treated and control samples surfaces favor the attachment and growth of seeded bone cells. Cells grow especially well on the carbon-coated and nitrogen-implanted surfaces in terms of amount and orientation. Nonetheless, this is just the short-term result in the first few weeks. Long-term investigation and comparison are just under way. Nonetheless, all the treated samples exhibit improved biocompatible at this preliminary stage. Our electrochemical and immersion tests results show that all the implanted and deposited samples possess much improved anti-corrosion properties, while the amounts of out-diffused Ni are also mitigated compared to the untreated control sample. Among the nitrogen-treated samples, annealing at 450ºC results in the best performance based on the electrochemical and immersion tests. Annealing at both 450ºC and 600ºC results in similarly good performance among the oxygen-treated samples, but in this case, post-implantation annealing usually results in better anti-corrosion properties than the as-implanted sample. Mechanically, the nitrogen-implanted surface possesses greater hardness and Young’s modulus, followed by carbon-implanted, oxygen-implanted, and then carbon-coated sample. All these treated surfaces are mechanically stronger than the untreated control surface. In summary, all the C, N, or O plasma implanted or deposited samples are biocompatible and possess improved corrosion resistances and mechanical strength. Possible mechanisms will be discussed in the thesis.

Date of Award	4 Oct 2004
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Paul Kim Ho CHU (Supervisor)