Tribological Investigation of Nickel Titanium Shape Memory Alloy (NiTi SMA) Coatings
Abu Bakar, Tuty Asma (2010) Tribological Investigation of Nickel Titanium Shape Memory Alloy (NiTi SMA) Coatings. PhD thesis, Dublin City University.
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Nickel titanium shape memory alloy (NiTi SMA) coatings demonstrate shape memory effects, superelasticity and excellent biocompatibility. It has been widely used in various applications such as in dentistry, orthopaedics and micro electro mechanical system. However, the application of NiTi SMA coating in the tribological field is still limited due to its low hardness and low wear resistance properties. In this study, the aim is to design a NiTi SMA coating structure with excellent mechanical properties and high wear resistance for tribological applications. The approach was undertaken by considering the potential of the Ni rich NiTi SMA precipitations and creating the TiO2 rutile layer onto the NiTi SMA structure so as to improve their mechanical and wear properties. The physical vapour deposition (PVD) sputtering technique is the most commonly used method for the production of amorphous NiTi SMA coating with various composition. This material is very sensitive to its process parameters and to the process-structure-properties relationship. Thus, the post-sputtering annealing process was successful in producing a crystalline Ni rich NiTi SMA coating with excellent mechanical and wear properties for tribological applications. The existence of a TiO2 rutile layer with a combination of the Ni rich NiTi SMA (Ni3Ti) and NiTi B2 parent phase within the annealed NiTi SMA coatings produced a significant improvement in the adhesion, hardness and wear resistance performance compared to the as-deposited NiTi SMA coating. The post-sputtered annealing process succeeded in increasing the adhesion and wear resistance of the NiTi SMA coating. The adhesion properties of the NiTi SMA coating increased with a critical load to failure of twelve times higher than the as-deposited NiTi SMA coating. The major enhancement of the adhesion properties significantly influenced the wear of NiTi SMA with a decrease in wear track morphology (wear width/wear depth) four times lower than the as-deposited NiTi coating. The post-sputtering annealing parameters and the coating thickness were shown to be the main parameters that affected the NiTi SMA coating properties. In this study, an annealing temperature of 600°C for a period of 30 minutes provided the optimum adhesion at a coating thickness of 2 µm. However, the optimum wear resistance for the same coating was achieved at a temperature of 550°C for a period of 60 minutes. The findings show the potential the post-sputtering annealing process has, in creating an excellent structure for NiTi SMA coating which demonstrate significant adhesion and wear resistance properties for tribological applications.
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