|Author:||Chan, On Ki|
|Title:||Synergistic enhancement of strength and wear resistance of titanium alloys by TiN surface network reinforcement|
|Advisors:||Man, H. C. (ISE)|
Hong Kong Polytechnic University -- Dissertations
|Department:||Department of Industrial and Systems Engineering|
|Pages:||xi, 152 leaves : illustrations (some color) ; 30 cm|
|Abstract:||Because of the unique properties as smart material and the biocompatibility, near-equiatomic nickel titanium (NiTi) was widely applied in making biomedical devices. Further improvements in the tribological and mechanical properties of NiTi could bring benefit in improving biocompatibility and durability of these devices. Various surface modification techniques such as PVD, CVD, plasma spraying, etc., have been developed and applied on NiTi over years but all of these techniques have different limitations such as poor interfacial bonding at the surface and long production time. This project investigates the feasibility of applying laser gas nitriding technique to improve the tensile stiffness, wear resistance and corrosion resistance of nickel titanium (NiTi). In the present work, a continuous wave fibre laser was applied to perform laser gas nitriding process under a pure nitrogen environment on the substrate of commercial NiTi plates. A Taguchi analysis was implemented in this project to optimize the process parameters for laser gas nitriding. Optimum parameters for nitriding in terms of output power, scanning speed, beam diameter and nitrogen flow rate were obtained. The optimum set of process parameter for laser gas nitriding was subsequently applied to fabricate titanium nitride (TiN) tracks on surface of NiTi with hardness more than 700HV. With the optimized parameters, different thickness of TiN single track could be produced at the surface of the NiTi alloy using laser gas nitriding. Two schemes of combining single TiN tracks were investigated: a) parallel longitudinal tracks for studying the effect of laser gas nitriding on the tensile stiffness of NiTi; b) network grid for studying the effect of laser gas nitriding on wear and corrosion resistance of NiTi. The results from scheme (a) indicate that the amount of TiN surface coverage may not be a linear relationship with the increase of the tensile stiffness of the NiTi. Sample with 40% TiN surface coverage has the highest tensile stiffness. Using scheme (b), with a 47% and 76% grid surface coverage, the average wearing volume are 0.00495 μm² and 0.00859 μm² respectively less than that of the untreated NiTi. The corrosion resistance of the NiTi is enhanced as well. The icorr for untreated, 47% and 76% grid surface coverage was 0.5μA/cm² and 0.34μA/cm² respectively. It could be concluded that reinforcement tracks or grid of TiN by laser gas nitriding could be effective in improving the tensile stiffness, wear and corrosion properties of NiTi. These innovative surfacing schemes enable NiTi to be treated in an efficient and more economical way.|
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