Advanced Materials Research Vols. 15-17

Abstract: As some biomedical problems require only temporary intervention, there is a clinical need for degradable biomaterials with excellent mechanical properties and controllable degradation behaviour. Although several works were carried out on both polymeric and metallic materials, no proposed degradable biomaterial fully satisfied these requirements. Therefore a new Fe-35Mn alloy has been developed as a valid and well suited alternative. The alloy was fabricated through powder metallurgy route followed by successive cold rolling and sintering cycles. This austenitic alloy exhibits a high strength and ductility, comparable to that of type 316L stainless steel. Its antiferromagnetic behaviour is not changed by cold deformation process. The alloy shows suitable degradation behaviour with a uniform corrosion mechanism and a slow release of ions that make it particularly well suited for the development of a new class of biodegradable stents.

Abstract: The short-term need of scaffolding function of stent and the prevention of potential longterm complication of permanently implanted stent have directed to the original idea of biodegradable stent. Selecting and developing materials showing appropriate mechanical and degradation properties are key steps for the development of this new class of medical devices. Therefore, the study of their in vitro degradation behaviour is mandatory for the selection of potential candidate materials suited in vivo. In this work, the degradation behaviour of current studied biodegradable metals including three magnesium alloys (Mg, AM60B and AZ91D), pure iron and Fe-35Mn was investigated. The tests were performed in a simulated blood plasma solution at 37±0.1 oC, using three different methods; potentiodynamic polarization, static immersion, and dynamic test in a test-bench which mimics the flow condition in human coronary artery. Degradation rate was determined as ion release rate measured by using atomic adsorption spectroscopy (AAS) and also estimated from weight loss and corrosion current. Surface morphology and chemical composition of corroded specimens were analyzed by using SEM/EDS. The three degradation methods provide consistent results in corrosion tendency, where Mg showed the highest corrosion rate followed by AZ91D, AM60B, Fe-35Mn and iron. Potentiodynamic polarization gives a rapid estimation of corrosion behaviour and rate. Static immersion test shows the effect of time on the degradation rate and behaviour. Dynamic test provides the closest approach to the environment after stent implantation and its results show the effect of the flow on the materials degradation. In conclusion, the three investigated methods can be applied for screening, selecting and validating materials for degradable stent application before going further to in vivo assessments.

Abstract: Metallic intravascular stents are medical devices commonly made of 316L stainless steel or nitinol used to scaffold a biological lumen, most often diseased arteries, after balloon angioplasty. Stenting procedures reduce the risk of restenosis, but do not eliminate it completely. Indeed, restenosis remains the principal cause of clinical complications, leading to up to 30 % of failure after 3 months of implantation. During the last few years, several works have been focused on the development of an appropriate coating able to act as a carrier for specific anti-restenosis drugs. Moreover, this coating would act as an anti-corrosive barrier, thus inhibiting the release of potentially toxic ions. Actually, the main challenges in stent coatings are to synthesize a biocompatible polymer coating resistant to blood flow, wall shear stress and tensile force after the stent deployment which results in a permanent strain of up to 25%. The adhesion and chemical resistance after deployment are critical properties to investigate for the improvement of the long-term reliability of polymer coated stent. The aim of this study was to evaluate the effect of a 25% equivalent plastic deformation on chemical, mechanical and adhesion properties of Teflon-like films deposited on 316L stainless steel. These properties were studied by chemical spectroscopy and atomic force microscopy. Teflon-like films were deposited by pulsed plasma glow discharges on flat electropolished 316L stainless steel. An original method has been developed to induce the deformation, and preliminary results have showed that the 12 nm thick Teflon-like films successfully resist to deformations of up to 25%.

Abstract: High strength multiphase CMnSi steel is increasingly used in passenger cars. Si and Mn alloying levels are typically in the range of 1-2% in mass. While Si improves the mechanical properties, it considerably deteriorates the galvanisability of steel. Residual water vapour in the reducing gas atmosphere during the intercritical or austenitic annealing results in the selective oxidation of Si and Mn at the steel surface. Besides Mn and Si, C is oxidized as well at the steel surface, leading to the formation of CO gas and decarburisation of the steel surface. This decarburisation has a major influence on the phase composition in the steel surface region: it shifts the ferrite to austenite transformation to higher annealing temperatures, leading to differences in surface and bulk microstructure. The phase composition influences the solubility and diffusivity of all alloying elements near the surface. The evolution with temperature of the selective oxidation at the steel surface has been studied by interrupted annealing in a protective atmosphere containing residual water vapour. The influence of the annealing temperature on the selective oxidation of Mn and Si is characterized by XPS (X-ray Photo-electron Spectroscopy) analysis.

Abstract: The thermal sprayed coatings are widely used in waste incineration boilers and fossil fuel-fired boilers. However, the defects, such as porosity, cracks and unmelted particles, in these coatings are detrimental to corrosion performance. In this study, the nickel based self fluxing alloy coating was fused by YAG laser to improve performance of the coating. Under appropriate laser parameters, the nonporous, crack-free coating was produced. The rubber wheel type abrasion wear test and hot corrosion test conducted in the presence of a mixed salt of Na2SO4/NaCl/KCl at 650°C showed that the modified coating exhibited excellent wear and corrosion resistances compared with the as sprayed and gas fused coatings.

Abstract: The surface topography of Titanium implants modulates bone response and implant anchorage. For this reason great efforts yet will be made for new surface refinements. In this work Ti samples were surface structured by corundum blasting firstly and then chemically etched to remove the remaining corundum particles and further surface structuring. In following steps these samples were electrochemically etched or additionally structured and coated with various micro-plasma processes. As result we got various typical surface structures in micro- and nanoscale and also different coating layers in dependence on the composition of the electrolyte for the micro-plasma process. In all steps the properties of the modified Ti surfaces were characterised mechanically by surface profiling, optically by SEM and electrochemically by CV- (for testing the corrosion parameters), CA- (to give the enlargement of the real surface) and EIS-measurement in range of 100 kHz to 1 mHz (to give a survey of the changing of surface capacities and structures). A comparison of the measuring results demonstrated their reliability.

Abstract: The effect of autoclaving hydrothermal treatment on the characteristics of plasma-sprayed hydroxyapatite (HA) coatings on the Ti-6Al-4V substrate was investigated. The heating temperatures were 100°C, 150°C and 200°C with ambient saturated steam pressure in an autoclave. On the basis of quantitative analysis of crystallinity using x-ray diffraction (XRD), hydrothermal treatment was found to be effective for increasing the crystallinity and phase purity of the HA coatings. The prominent and sharp OH− and PO4 3− peaks detected from x-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FT-IR) spectra demonstrate a superior crystallized integrity of hydrothermal-treated HA coatings through the incorporation of water vapor. Moreover, the significant presence of OH− peak in XPS spectra represents a replenishment of water molecules which tends to reduce the dehydroxylation state of as-sprayed HA coatings. From the observation of microstructures, crystallized HA was found to diminish the spraying defects of hydrothermal HA coating layers, and finely-crystallized HA crystals, with a Ca/P atomic ratio of 1.67, were observed through transmission electron microscopy (TEM). Hydrothermal treatment could induce a low-temperature crystallization process, and the saturated steam pressure is thought to be a factor which reduces the activation energy and accelerates the HA crystallization. Experimental evidence confirmed that the ambient saturated steam pressure plays an important role in lowering heating temperatures and promoting HA crystallization.

Abstract: A hot dip aluminising process was carried out with a 1mm steel sheet dipped into the Al-10at.% Si melt in an automatic hot-dip simulator. When steel and liquid aluminium are in contact with each other, a thin intermetallic compound (IMC) is formed between the steel and the aluminium. The analysis and identification of the formation mechanism of the IMC is needed to manufacture the application products. Energy dispersive X-ray spectroscopy (EDX) and electron probe microanalysis (EPMA) are normally used to identify the phases of IMC. In the Al-Fe-Si system, numerous compounds with only slight differences in composition are formed. Consequently, EDX and EPMA are insufficient to confirm exactly the thin IMC with multiphases. In this study, transmission electron microscopy (TEM) analysis combined with EDX was used. The TEM sample was prepared with focused ion beam (FIB) sampling. The FIB lift-out technology is used to slice a very thin specimen with minimum contamination for TEM analysis. It is clearly shown that the IMC consists of Al-27 at. % Fe-10 at. % Si and is identified as Al8Fe2Si with a hexagonal unit cell (space group P63/mmc). The cell parameters are a= 1.2404nm and c= 2.6234nm.