Study On The Techniques And Performances Of Surface Modification Composite Layer On Porous Niti Shape Memory Alloy

Porous NiTi SMA has been widely applied as a biomedical material in medical clinic forits unique shape memory effect, super-elasticity and controllable elastic modulus. However,its corrosion resistance （mainly about the Ni release） and bioactivity remain to be improved.In this thesis, the preparation process and the properties of the composite surface modificationlayer on porous NiTi SMA, whose bottom layer is TiO2（chemical oxidation） or TiN （in situnitride） film and surface layer is hydroxyapatite （HA）, was studied to improve both thecorrosion resistance and bioactivity. Metallographic microscope, scanning electronmicroscope （SEM）, energy dispersive X-ray spectrum （EDS） and X-ray diffraction （XRD） areused to characterize the surface morphology, composition and structure of the modifiedcomposite layer. The corrosion resistance of the surface-modified porous NiTi SMA wasevaluated by electrochemical corrosion test in physiological saline. The adhesive strength ofthe bottom layer was measured with nano-scratching experiment.The results have shown that the uniform oxide layer prepared by chemical oxidation in80℃,30wt.%H₂O₂solution for1h significantly enhance the corrosion resistance of thesubstrate and reduce Ni ion release, thus would greatly improve the biocompatibility of NiTiSMA. The uniform oxide layer could be prepared both on dense and porous NiTi SMA by thischemical oxidation process.HA layer, which would improve the bioactivity of NiTi alloy, was deposited on the oxidefilm layer. Alkali treatment was followed after chemical oxidation to improve thehydrophilicity. It was shown that the alkali treatment would be uncomplete when theconcentration of NaOH was smaller than3M, and the layer became spalling when the NaOHconcentration was bigger than7M. The layer morphology was uniformity and the surfacehydrophilic is excellent when the NaOH concentration was5M. Finally, the5M NaOH wasused in the alkali treatment both for the uniform surface morphology and good hydrophilicity.Research also showed that the pre-calcification could not only accelerate the depositionrate of HA, but also increase the thickness of HA layer. A composite layer, which was madeup of TiO2layer （bottom） and HA layer （surface）, was prepared on the surface of NiTi SMAafter HA deposition for14days. This composite layer suppressed the Ni releas effectively andenhanced the surface bioactivity of the NiTi SMA.In order to improve the adhesion strength of the bottom layer, the in-situ nitridingprocess during sintering of porous NiTi SMA was studied for stable process parameters. The results showed that sintering pressure was necessary for in-situ nitriding and should reach upto5MPa for good nitriding effect. What more, the pre-forming pressure should be controlledat200MPa, the sintering temperature be at1050℃and the NH₄HCO₃content in N supplyingsample be around15wt.%.The corrosion resistant of NiTi SMA could be improved both by in-situ nitriding andchemical oxidation. The corrosion resistant of the oxide layer obtained by chemical oxidationwas better than that of the nitriding layer with lower corrosion current density. However, theadhesion strength of the nitriding layer was much larger than that of the oxide layer.