A Study Of Passive Coatings For Surface Modification Of Biomedical Metal Materials

Biomedical metal stents have become one of main devices to treat vascular stenosis. However, the release of corrosion compounds, as well as pathological reaction, may influence its biofunctionability after the naked metal stents have been implanted. If a protective coating, with good biocompatibility, was used on the surface of metal stents, the biosafety, biocompatibility and biofunctionability would be enhanced. In the present study, aimed at NiTi alloy and AISI316L stainless steel, used for metal vascular stent, the functional diamond-like carbon graded, nano-multilayered Ti/TiN and TaN coatings have been prepared by different depositing methods, meanwhile the mechanical performance, corrosion behavior in simulated body fluid and biocompatibility have been measured and evaluated synthetically by the surface analysis technique, electrochemical measurement and hemocompatibility experiments. The main conclusions are as follows.In this study the DLC graded coating, with a graded layer of carbon implanted layer, has been prepared by hybrid plasma source ion implantation and plasma enhanced chemical vapor deposition techniques, and another DLC graded coating, with a graded layer of Si/SiC, has been deposited by combined magnetron sputtering and plasma enhanced chemical vapor deposition techniques. The carbon implanted layer and Si/SiC graded layer are beneficial to improving the bonding and antifriction properties of DLC films, and the greatest adhesion reaches 42.92 N, which is obtained form Si/SiC/DLC graded coating. It is found that the corrosion resistance of the metal substrates coated by two kinds of DLC graded coatings is greatly improved in vitro simulated body environment (37±1℃, Tyrode's simulated body fluid), which is indicated by the decrease of corrosion current density and the increase of the pitting corrosion potential. The results of electrochemical impedance spectroscopy measurements show that two kinds of DLC graded coatings have good long-term corrosion-resistant property, and the corrosion process of the graded DLC coating specimens mainly involves the galvanic corrosion between the coating and the substrate, and pitting corrosion of the substrate, however, the corrosion process is different. The dissolution of the carbon implanted layer causes the failure of carbon implanted layer/DLC graded coatings, and the debonding of Si/SiC layer involves the breakdown of Si/SiC/DLC graded coatings. A novel revolving technique for nano-multilayered Ti/TiN coating deposition has beendeveloped through pulsed bias Arc Ion Plating. Compared with TiN coatings, the surface quality of nano-multilayered coatings, as well as the packing factor and adhesion strength, is improved effectively. The results of electrochemical corrosion tests show that the thermodynamic stability, passivation stability and anti-breakdown property is also improved in vitro simulated body environment (37±rCTyrode's simulated body fluid), especially the Ti/TiN(45/45s) coating, and the reason is that the sandwich structure, composed of Ti layer-graded layer-TiN layer, plays an important role in inhibition of the penetration of test solution and causes the redistribution of interphase corrosion current density.Through pulsed bias Arc Ion Plating TaN coating has been prepared, mainly composed of a hexagonal TaNo.s and cubic TaN. Compared with TiN coating, it has fewer defects and smoother surface. Its surface consists of tantalum nitride, tantalum oxide and tantalum oxynitride. Its mechanical properties are worse than TiN coating by pulsed bias Arc Ion Plating, however, the TaN coated specimen performs better in vitro simulated body environment (37±l"CTyrode's simulated body fluid).The results of Fuzzy Hemocompatibility Degree (FHD) measurements indicate that the DLC graded coatings have best hemocompatibility among the above four kinds of coatings, especially the coatings by hybrid magnetron sputtering and plasma enhanced chemical vapor deposition techniques, and its FHD is about 0.709-0.809. Although the hemocompatibility of nitride coatings is poor, it is improved greatly in comparison with the substrates. Moreover, the above coatings have less cytotoxicity and do good to the conglutination and culture of rat pheochromocytoma PC12 cells.In this paper, the passive coatings, with good mechanical and biological properties, have been prepared through different plasma techniques. The results play an important role in promoting the development of the surface modification of biomedical metal materials by passive coatings.