Studies On The Preparation And Properties Of Silver-based Composite Plating On The Nickel-titanium Alloy

Nickel-titanium alloy has become a very potential biomedical material for its uniqueshape memory, superelasticity and good biocompatibility. Metallic silver has a goodflexibility and scalability, good chemical stability, excellent electrical conductivity and astrong bactericidal capability. The purpose of this thesis is to electroplate silver on the surfaceof biomedical nickel-titanium alloy substrate, in order to change the biocompatibility ofnickel-titanium alloy in vivo and to improve the corrosion resistance of nickel-titanium alloy;to promote the further application of the nickel-titanium alloy in medicine, we improve thecorrosion resistance of Ag plating by composite depositing Ag/PTFE composite plating on thesurface of nickel-titanium alloy; meanwhile, to increase the corrosion property and frictionproperty of silver-plated layer by using the composite plating technology to deposit Ag/TiO₂composite plating on the surface of the nickel-titanium alloy, corrosion property, bloodcapatibility and antimicrobial property of Ag/TiO₂composite plating have been investigated.First, this paper optimized the process of silver electroplating on the surface ofnickel-titanium alloy, determines the basic composition and process conditions of our silverplating solution which are as follows: sulfosalicylic acid120g/L, potassium hydroxide10g/L,silver nitrate30g/L, ammonium acetate50g/L, and ammonia55ml/L；pH9, stirring speed250r/min, bath temperature25°C, current density0.4A/dm2, plating time1h. The effect ofplating time and current density on the coating properties is investigated. The morphology,composition and structure of coating surface are characterized by using SEM, EDS and XRD;while the open circuit potential-time curves, Tafel polarization curves and electrochemicalimpedance spectroscopy are used to evaluate corrosion resistance. The surface of Ag-coatedNiTi alloy is relatively smooth; the deposition rate is greater; the corrosion resistance is better.Ag/PTFE composite plating is prepared by adding PTFE powder wrapped in surfactantsto the electrolyte on the basis of silver plating. The corrosion property of three types’surfactants and their mixed systems on the composite plating is studied, and the results showthat when added non-ionic surfactant OP-101g/L the corrosion resistance can be the best. Onthis basis, the effect of the PTFE addition and plating time on the properties of Ag/PTFEcomposite plating is investigated. The effect of the plating time and PTFE concentration onthe coating surface morphology and crystal structure is investigated. The results show thatwith1h plating time it can get a more uniform coating; the increases in PTFE concentrationcould make the crystalline grain of the coating refinement and more dense, but when the concentration of PTFE is too high, particles in the coating are easy to gather so that the grainswould be coarse, and coating surface is uneven. The most appropriate PTFE added in the bathis8g/L. The electrochemical and abrasive resistance tests show that the8g/L PTFEconcentration in the bath has the best coating corrosion resistance and abrasive resistance.In order to improve the biocompatibility and corrosion property of the silver layer,Ag/TiO₂composite plating is prepared by using three methods which are adding TiO₂wrapped in the surfactant and then added to the bath, adding TiO₂powder directly and addingTiO₂sol to the plating solution. The effect of the adding amount of TiO₂powder, type andamount of surfactant, titanium sol dosage and other factors on the composite coatings hasbeen studied. Surfactant is used to coat TiO₂powder and then added to the bath to prepareAg/TiO₂composite coating. We design an orthogonal experiment to determine the optimumof the surfactant and TiO₂when adding TiO₂powder coated by surfactant to the bath toprepare Ag/TiO₂composite plating: anionic surfactant sodium dodecyl sulfate dose of1g/L,and TiO₂30g/L. According to the analysis of the surface morphology and electrochemicalcorrosion results of prepared plating obtained by adding different surfactants in the bath, wefound the coating prepared with1g/L anionic surfactant in the bath has the best corrosionresistance. But the corrosion resistance of Ag/TiO₂composite plating is slightly bad. Theeffect of TiO₂addition in the bath on the micro-surface morphology and crystal organizationalstructure of the plating is studied, when added TiO₂powder directly to prepare nano Ag/TiO₂composite plating. As the TiO₂concentration in the bath increased, the plating surfaceflattened, however, when TiO₂concentration level is too high, large particles would aggregateand the surface would be rough. When the concentration of titanium dioxide particle inplating solution is50g/L, we can have the most abundant particles in the composite coatingand the corrosion resistance of the coating can be the best. Biological performance tests of thecomposite platings show that: resistance to ion release is the best when bath concentration is50g/L; all the prepared composite platings with different TiO₂concentration in the bath havegood blood compatibility. Ag/TiO₂composite coating is prepared by joining with TiO₂sol andthe effect of sol addition on the coating properties is studied. Coating surface is most dense;and TiO₂concentration in the coating is the largest when the amount TiO₂sol is100ml/L--theplating time is1hour--much larger than the composite amount in the plating prepared byadding directly. At the same time, the corrosion resistance of composite plating is better thanadded directly. The plating has good antimicrobial property.