The Experimental Study Of The Influence Of Ta Coating On The Corrosion Resistance Of Biomedical Titanium Alloy

Since the 1970s, with the development of biomedical materials science and metallurgical casting technology, medical titanium alloy has been widely used in joint surgery, orthopedic surgery and trauma surgery as human implantation because of its advantages, such as excellent biocompatibility, low density and low elastic modulus. Ti-6Al-4V alloy （TC4） and NiTi shape memory alloy （NiTi SAM） are the most common titanium alloy and the most representative biomedical metallic materials, from which joint implants, fracture fixation devices and orthopedic devices are made and used widely in clinical. However, it is found that these two alloys’corrosion resistance do not reach the ideal level in the long-term clinical application. After being implantated into the human body, they might be broken and flexible and their mechanical properties might decrease because of body fluids. More over, the Al, V, Ni and other ions disolving in the surrounding tissue will produce certain toxical effects. Therefore, the research of corrosion resistance of medical titanium alloys has great practical significance for the safe using in the human body. In recent years, the metal tantalum caused widespread concern in the biomedical field for its excellent corrosion resistance, stability, biological characteristics and unique structural properties. Trabecular bone, cranial prosthesis, cardiac pacemakers made from tantalum have been successful in clinical applications. If the biomedical composite material is made of tantalum coating and titanium alloy, it will get much more stability in human body because it makes up the shortcomings of poor corrosion and abrasion resistance. In this paper, the new Ta coating was applied on the surface of Ti-6Al-4V alloy and NiTi shape memory alloy specimen by plasma spraying technology. The corrosion resistance strength of the specimen before and after coating Ta by the electrochemical methods in simulated physiological environment was comppared, which will provid experimental basis for the medical composite tantalum titanium alloy used in clinical practice.Contents and Methods1. After coating Ta on the surface of the Ti-6Al-4V alloy specimen by plasma spraying method, the bond strength between Ta coating and the Ti-6Al-4V alloy was measurd by Hardness Tester. The free corrosion potential and Tafel curve of Ti-6Al-4V alloy before and after coating Ta were detected by using electrochemical method in 37℃simulated body fluid environment to study whether tantalum alloy can improve corrosion resistance of medical titanium alloy or not, and test the feasibility of Ta as medical metal coating material.2. After coating Ta on the surface of the NiTi shape memory alloy specimen using plasma technology, coating surface micro-morphology and elements were test using scanning electron microscopy and EDS spectrum analyzer,and the crystal structure of the coating was observed with X-ray diffraction instruments.The potentiodynamic polar curve of the specimen before and after coating Ta and the galvanic current density values between Ta and NiTi SMA was measured by electrochemical method in neutral and acidic simulated body fluid environment respectively.The physical and chemical properties of Ta coating and corrosion resistance in a complex physical environment was studied deeply to settle basis in experiments for the metal tantalum metal as a new generation of material in medical applications.Results1. By plasma spraying the Ti-6Al-4V alloy specimen was coated with a thickness of 500μm, dense and uniform layers of Ta on the surface. The hardness of bonding strength between the Ta coating and the Ti-6Al-4V alloy substrate reached HF1 by measuring through Hardness Tester. From the Tafel polarization curves, the specimen after coating Ta showed that both anode polarization curve and cathode polarization curve were flat, natural corrosion potential moved to the positive direction, while passive rangion got longer and passive current reduced.2. The thickness of Ta coating on the NiTi shape memory alloy sample surface was 400μm. Scanning electron microscopy（SEM） showed that the layer is dense and adherent to the surface and it contains fine crystallites. The electrodeposit was analysed as tantalum, titanium and nickel as revealed from the corresponding EDS profile,oxygen was also detected on the surface due to the oxidation of the deposited Ta film on exposure to air and water. The Phase components of the coating includedα-Ta with body-centered cubic structure,（B2）NiTi and Ta₂O,Ta₂O₅ generated by oxidation under X-ray diffraction’s analysis.The anodic polarization curves indicated that, after coating Ta the NiTi SMA specimen showed reduced passive current intensity, increased polarization resistance, and increased natural corrosion potential and breakdown potential. The galvanic current-time curve indicated that the galvanic current density of Ta-NiTi SMA was minimal, and keep inμA (10^-6A/cm²) level.Conclusions1.The surface modification of coating Ta on Ti-6Al-4V alloy can improve corrosion resistance of the base mental, solve the problem of aseptic loosening and provide optimal direction of Ti-6Al-4V alloy prosthesis.2.The surface modification of coating Ta on NiTi SMA can improve corrosion resistance of the base metal. There was no galvanic corrosion between Ta coating and NiTi SMA substrate.Providing a development direction for the optimization of nickel-titanium shape memory alloy implant.