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Coronary heart disease is still one of the most common sources for death in western industrial countries. Since 1986, a metal vessel scaffold (stent) has been inserted to prevent the vessel wall from collapsing. Most of these coronary stents are made from CrNiMosteel (316L). Due to its austenitic structure, the material shows a good combination of strength, ductility, corrosion resistance, and biocompatibility. However, this material has some disadvantages like its non-MRI compatibility and its poor fluoroscopic visibility. Other typically used materials are the CoBase alloys L-605 and F-562 which are MRI compatible as well as radiopaque. Another interesting fact is their excellent radial strength and therefore the ability to produce extra thin struts with increased strength. However, because of a strut diameter much less than 100 μm, the cross section consists of about 5 to 10 crystal grains (oligocrystalline). Thus, very few or even just one grain can be responsible for the success or failure of the whole stent. To investigate the relation between microstructure, mechanical factors and stent design, commercially available Cobalt-Chromium stents were investigated with focus on distinct inhomogeneous plastic deformation due to crimping and dilation. A characteristic, material related deformation behavior with predominantly primary slip was identified to be responsible for the special properties of CoCr stents.
The complex structural-phase composition, morphology and elemental composition of surface and nonmetallics in porous TiNi compounds produced by self-propagating high-temperature synthesis (SHS) in a flow reactor in the layer-by-layer combustion mode were analyzed. The samples were investigated using light microscopy (LM), X-ray diffraction (XRD), scanning electron microscopy (SEM), scanning transmission electron microscopy (STEM), and energy dispersive X-ray spectroscopy (EDS). The findings indicate that in addition to the TiNi and Ti2Ni intermetallic constituents, the pore's surface contains numerous Ti4Ni2(O,N,C) nanocrystalline oxycarbonitrides, spinels, polysilicates, and residual amorphous phases. The elemental composition of the surface and crystalline inclusions is investigated by the EDS method. LM, SEM, TEM, and EDS instrumental examination revealed the entire surface comprising a continuous shell of intermetallic superficial bulb-shaped structures, as well as crystalline inclusions of polysilicates and spinels in the intergranular peritectic phase. Prominent morphology was confirmed to appear throughout the pore's surface owing to the interaction of the peritectic liquid (PL) with reaction gases. The epitaxial, nanocrystalline strata of intermetallic oxycarbonitrides were shown to have the intricate nature. Reaction gases chemisorbed by the PL are responsible for the continuous and dense substrate, which ultrafine structure modulates a high corrosion resistance. On the contrary, the sparse and foamy overlay resulted from a convective transfer of the PL by reaction gases facilitates in vivo bio-integration of the alloy. Overall, this sheds light on and may be more indicative of the complex role of superficial strata and nonmetallic crystals in enhanced biocompatibility of the unwrought porous TiNi alloy.
The recovery of surfactants from organic mediums is of great economic and environmental interests in the field of petrochemicals sector. The aim of this study was to recover poly-dispersed nonionic surfactant (TX-100) from organic medium by the use of dehydrated and rehydrated kaolin. It was found that the presence of water after kaolin rehydration decreases considerably the amount of TX-100 adsorption from 28.7 μmol.g−1 onto dehydrated kaolin to 23.4 μmol.g−1 onto rehydrated kaolin. The estimation of the number of statistical layers of water deposited on kaolin was observed to be about 7 to 9. The effect of type of solvents (heptane, cyclohexane and benzene) on the adsorption onto rehydrated kaolin revealed a reduction in the adsorbed quantity; the adsorbed amounts were found to be 28.7 μmol.g−1, 26.7 μmol.g−1 and 24.2 μmol.g−1 in heptane, cycloheptane and benzene phases, respectively. On the other hand, it was observed that a temperature increased from 20 °C to 35 °C and 45 °C negatively affects the adsorption of TX-100 onto dehydrated and/or hydrated kaolin in heptane medium.
The biotribocorrosion behavior of newly developed nanocristalline near β-types Ti-15Nb and Ti-15Mo alloys surfaces, sintered by powder metallurgy and sequentially milled, has been investigated in SBF simulated body fluid (PBS solution) at OCP, an applied potential in the passive region and EIS. Reciprocating sliding tests using a ball-on-plate tribometer under differentes applied loads 3, 7 and 10 N load and anodic potentials were applied to evaluate the effect of applied lad and the effect of Nb and Mo elements on tribocorrosion behaviors of samples. Results showed that, Ti-Nb exhibited better anticorrosive properties than Ti-Mo. Under tribological action the nanostructured both of alloys showed similar friction coefficient, while Ti-Nb present lower tendency to corrosion compared to Ti-Mo. Furthermore, Nb diffusion increased the repassivation rate with respect to Ti-Mo surfaces due to its stable passive film. Due to the high chemical reaction rate in β-type Ti-15Mo alloy as compared to Ti-15Nb. The β- Ti-15Nb showed lower volume loss, lower friction coefficient values and exhibited better corrosion resistance during tribocorrosion tests than Ti-15Mo. Prevailing electrochemical conditions between -1 and 2 V influences the wear accelerated corrosion by increasing it with the applied potential and slightly increases the mechanical wear. Also, wear accelerated corrosion can be predicted by existing models as a function of electrochemical and mechanical parameters of the titanium alloys. However, considering biomedical applications, the β- Ti15 Mo and Ti15Nb alloys may be good candidates with low elastic modulus and without toxic alloying elements.
Porous TiNi alloys fabricated by self-propagating high-temperature synthesis (SHS) are biomaterials designed for medical application in substituting tissue lesions and they were clinically deployed more than 30 years ago. The SHS process, as a very fast and economically justified route of powder metallurgy, has distinctive features which impart special attributes to the resultant implant, facilitating its integration in terms of bio-mechanical/chemical compatibility. On the phenomenological level, the fact of high biocompatibility of porous SHS TiNi (PTN) material in vivo has been recognized and is not in dispute presently, but the rationale is somewhat disputable. The features of the SHS TiNi process led to a multifarious intermetallic Ti4Ni2(O,N,C)-based constituents in the amorphous-nanocrystalline superficial layer which entirely conceals the matrix and enhances the corrosion resistance of the unwrought alloy. In the current article, we briefly explore issues of the high biocompatibility level on which additional studies could be carried out, as well as recent progress and key fields of clinical application, yet allowing innovative solutions.
Aiming to develop alloys with better properties for orthopedic applications, the focus of the present research was to evaluate the effect of Mo at.% content on structural, mechanical, and tribological properties of hot isostatically pressed Ti-xMo (x = 4, 8, 12, 15, and 20 at.%) alloys. The structural evolution, mechanical properties, and tribological behavior of the nanostructured Ti-xMo alloys were evaluated using x-ray diffraction, scanning electron microscope, and ball-on-disk tribometer. Wear tests were conducted under different applied loads of 2, 8, and 16 N. Experimental results indicated that the structural evolution and morphological changes of the milled alloys were sensitive to their molybdenum (Mo) content. The morphological characterization showed that the crystallite size and the particle size decreased with increasing Mo content (at.%) reaching the lowest values of 27 and 26 nm in the case of Ti-15Mo and Ti-20Mo, respectively. On the other hand, the coefficient of friction and wear rates were found to be decreasing with increasing Mo content.
The bio-tribocorrosion behavior of newly developed near β-types Ti-15Nb and Ti-15Mo alloys was investigated in Phosphate-Buffered Saline (PBS) under different loads. Open-Circuit Potential (OCP), friction coefficient, wear volume and wear rate were evaluated. The results revealed that Ti-15Nb alloy exhibited lower wear rate, lower friction coefficient and better corrosion resistance during tribocorrosion than the Ti-15Mo alloy. This can be attributed to the diffusion of Nb which increases the repassivation rate (formation of a protective layer) in the Ti-15Nb alloy. In contrast Ti-15Mo shows a significantly higher rate of chemical reaction.