| The near-equiatomic NiTi alloy is widely applied in biomedicine, military equipment, aerospace engineering, mechanical engineering and other fields, due to its superior properties, such as shape memory effect, hyperelasticity, high specific strength, good wear and corrosion resistance and biocompatibility. Applications of TiNi alloy are concentrated mainly on machinery and building materials, such as adapting pieces, drive originals and SME dampers. But its hardness is low, which could be improved greatly for tribological applications. Refering to biomedicine materials, TiNi alloy acts as biomedical implants. However, the dissolution of Ni ion is still a major problem which may lead to severe allergy and cancerization of cells and organs.Diamond films have stable physicochemical properties, low coefficients of friction, good wear and corrosion resistance, excellent electric and optical properties, better biocompatibility than that of others. Therefore they are outstanding structural and functional materials and are widely used in optical, acoustic, electronic materials and mechanical and biomedical engineering.The atomic percent of Ni, a strong graphitizing element, is about50.8in the TiNi alloy used in this study, which prevents the growth of diamond films. Therefore, surface modification of TiNi alloy is an inevitable process in order to prepare diamond films on its surface.It is known that Mo has good high-temperature strength, high hardness, high density and good corrosion resistance. Mo is one of the trace metal elements which is needed for h μ man body. Because of its self-adhesion property, Mo has good adhesion strength with other metals as well. Besides, Mo is beneficial for development of diamond films, which contributes to fine adhesion between Mo and diamond films. Meanwhile, the appropriate and gradient mechanical properties of Mo, better than TiNi alloy and worse than diamond films, which enhance the adhesion strength between TiNi alloy and diamond films and bearing performance of TiNi alloy. The appearance of Mo surface modified layer diminishes the amount of Ni atoms of surface to a great extent. As a result, Mo is suitable as surface modified layer for TiNi alloy, which acts as the interlayer of TiNi-diamond composite coating.In this paper, the double glow plasma alloying/metallurgy (DG-PSM) technology was utilized to prepare Mo modification layer on surface of TiNi alloy.99.9%pure Mo and TiNi alloy worked as source electrode and negative electrode (workpiece electrode) respectively. Finally, the diamond films were fabricated on the modified surface of TiNi alloy by microwave plasma assisted chemical vapor deposition method (MPCVD).The effects of technological parameters on formation of surface modified layer were systematically analysed so that the parameters would be optimised purposefully. The surface modified layers were characterized and the surface hardness and the wear-resistance performance were compared among samples treated by different technological parameters. Meanwhile, diffusion mechanism of Mo atoms was analysed.The main results are shown as following:1) The optimized technological parameters were that the alloying temperature was950℃, the working air pressure was35Pa, the alloying time was2h, the voltages of source electrode and negative electrode were550V to800V and300V to500V respectively and the voltage difference of them was250V2) With depth of cross-section increasing, atomic percentage of Mo declined from100to zero, that of Ti ascended firstly and declined later and that of Ni increased gradually after surface modification. As the result of that, the surface modified layer consisted of triple layers structure. From the surface to the substrate, they were Mo deposition layer, Ti-rich alloying layer and Mo diffusion layer respectively. The main phases of surface modified layer were Mo, MoTi, MoNi and Ti2Ni.3) After surface modification, the surface hardness of all surface modified samples was improved. Under the optimized technological parameters, the surface hardness (832HV0.098) and were2.5times higher than that (241.3HV0.098) of matrix alloy. Metallurgical bonding existed between atoms of surface modified layer and TiNi alloy substrate.4) After surface modification, the wear-resistance performances of all surface modified samples were improved. Under the optimized technological parameters, the specific wear rate of surface modified samples was2orders of magnitude lower than that of matrix alloy. The improvement of surface hardness and hyperelasticity of TiNi matrix alloy contributed to optimize its wear-resistance performance.5) The microstructures of diamond films fabricated by MPCVD were uniform and dense. The grain size was small, about1.5μm. The growth speed of diamond films was high up to1.1μm/h. |
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