Biomedical investigation of CNT based coatings

Owing to their unique properties, carbon nanotubes (CNTs) have emerged as a promising material for biomedical applications. Especially, CNT based coatings have recently made great achievements, which have been mainly stated in this review. They can offer favorable condition to induce the cellular functions. They can possess excellent packing density without any macroscopic porosity within the film on metals. The good bonding strength between them and the metal substrates could be achieved. Addition of CNTs can reinforce the composite coatings and the CNTs-reinforced coatings are expected to be promising for high load-bearing orthopedic implants. Furthermore, based on their unique properties, CNTs can attach to the surfaces of dentin and cementum but not to the surface of enamel, suggesting that the teeth coated with CNTs should be a possible candidate for dental materials. In addition, CNTs can be coated on polymers, bioglasses, collagen, etc. to prepare bioactive, electrically conductive 3D scaffolds for tissue engineering. Besides the current achievements, CNT based coatings possess big space to further develop, such as functionalized CNT based coatings, mineralized CNT coatings, etc., which suggesting that CNTs coatings have emerged as promising methods and potentially reward opportunities to develop the next generation of engineered materials for biomedical applications.     

Diamond/WC bilayer formation mechanism by hot-filament CVD

A method for the growth of Diamond/WC bilayers in a single process is presented with interest for the production of well adhered electrical contacts to diamond surfaces. This process uses a common hot filament chemical vapor deposition (HFCVD) reactor, with W filaments as the source for the deposition of the metallic layer, and H₂ and CH₄ gasses as the reactive species for the diamond growth. The method begins by vaporizing the filaments in vacuum for a few minutes, followed by the chemical vapor deposition of diamond. The results have shown that by varying the filament vaporization time and temperature it is possible to deposit on the Si substrate tungsten containing coatings of different thicknesses. The process starts by vaporization of naturally oxidized filaments and deposition on the substrate. Afterward, the tungsten oxide carburises to W₂C and WC phases. The CVD growth of the diamond layers on these carbide layers is dependent on the CH₄/H₂ ratios, system pressure and substrate temperature. The seeding of the Si substrates with diamond powder before the CVD process, guarantees that diamond is nucleated inside the metallic carbide layer, anchoring the top nanocrystalline diamond layer.     

Investigation of the interfacial reaction between multi-walled carbon nanotubes and aluminum

Aluminum (Al)/carbon nanotube (CNT) composite films were fabricated by sputtering pure Al on the surface of aligned multi-walled CNT arrays. Heat treatment was performed in the temperature range 400–950 °C. The interfacial reaction between the Al and the CNTs was investigated by annealing the samples at various temperatures. The results indicated that aluminum carbide (Al₄C₃) was formed at the interface between the Al and CNT layers, and microscopy observation revealed that the reaction generally occurred at locations containing an amorphous carbon coating, at defect sites, and at open ends of CNTs. Because the nanosized CNTs are precursors for carbide formation, the Al₄C₃ formed is also nanoscale in size. The carbide formed on the surface as well as on the tips of the CNTs improves the interfacial interaction between the CNTs and the Al layers. This also contributes to the enhancement of the mechanical properties of the composite. Our investigation demonstrated that chemical vapor deposited CNTs are a suitable candidate as reinforcing material for Al and other metal matrices.     

Characteristics and biological responses of novel coatings containing strontium by micro-arc oxidation

The modification of micro-arc oxidation (MAO) technique is an effective method to improve biocompatibility of titanium. This study aimed to investigate the coatings formed in the electrolytes with different strontium content, which is beneficial for biological performance. The physicochemical characteristics and cell behavior were assessed. The physicochemical characteristics were investigated using scanning electron microscope (SEM) observation, energy dispersive X-ray spectrometer (EDX), thin film X-ray diffraction (TF-XRD) analysis, electron spectroscopy for chemical analysis (ESCA) and surface roughness test. Cell behavior included morphology observation by SEM and number count by methylthiazoletetrazolium assay of hFOB cells. The TF-XRD results indicated that phase of coatings was anatase and rutile. The calcium, phosphorus, and strontium were detected in the coatings by EDX and ESCA. Using the SEM, the surface morphology exhibited uniform porous structure on titanium. Cell culture experiments demonstrate that MAO coating formed in the electrolytes with different strontium content would not alter initial cell morphology and 1-day and 7-day cell numbers. The cell proliferation of coatings containing 1% or 5% strontium content at 14-day culture was higher than coatings without strontium content at 14-day culture, but the higher strontium content (10%) could not be beneficial to cell growth. Consequently, this study indicates that strontium incorporated into MAO coatings did not change the physicochemical characteristics but exhibited an effect on biological responses.     

Residual stress evaluation at the micrometer scale: Analysis of thin coatings by FIB milling and digital image correlation

In this report, an optimised method for residual stress determination at the microscopic scale is presented. The newly proposed approach involves incremental Focused Ion Beam (FIB) milling of annular trenches at material surface, combined with high resolution SEM imaging of a previously deposited marker pattern. Digital image correlation (DIC) analysis of the relative displacements between markers with respect to the undisturbed state provides a measure of strain relief. Results of finite element modeling show that the proposed configuration gives complete strain relief when the annular trench depth becomes comparable with the diameter of the remaining stub, thus allowing analytical calculation of the average residual stress from measured strain components. Basing on results of modeling, the experimental methodology has been developed and optimised for residual stress analysis in thin coatings. In order to cover a wide range of material properties and residual stress states, two different materials have been selected: TiN CAE-PVD coating (hard and stiff, with compressive residual stress) on WC-Co substrate, and also an Au MS-PVD coating (soft and compliant, with tensile residual stress). The procedure for the optimization of FIB milling parameters is reported. Results are validated by comparison with residual stress evaluation by X-ray diffraction and curvature measurement on the two different specifically selected PVD coatings.     

Evaluation of corrosion protection of carbon black filled fusion-bonded epoxy coatings on mild steel during exposure to a quiescent 3% NaCl solution

Carbon black (CB) was mixed with fusion-bonded epoxy (FBE) coatings to generate a series of formulations with 0.5-4% by weight of carbon black. The degradation of these FBE coatings on mild steel exposed to a quiescent 3% NaCl solution was monitored using electrochemical impedance spectroscopy (EIS). The experimental results showed that the electrochemical behaviour of coated systems changed dramatically when the CB concentration reached 3% by weight. This phenomenon was relevant to the formation of the percolation regime in the coating, at which a sharp drop in the electrical resistance of the coating was achieved by the generation of a continuous conducting network. A comparison of the protective properties of the FBE coatings filled with various CB loadings, along with the inspection of view underneath the coatings, indicated that the protective performance of the FBE coating was significantly improved when the CB loading exceeded the threshold concentration. This conclusion was confirmed by the results obtained from Thermogravimetric Analysis (TGA) and Differential Scanning Calorimetry (DSC) measurements.     

Cr含量对类石墨碳涂层硬度及其价键结构的影响

为了探讨Cr含量对类石墨碳(GLC)涂层硬度的影响机理,采用非平衡磁控溅射离子镀技术,通过调整Cr靶电流,制备出了具有不同Cr含量的GLC涂层.借助显微硬度计测量了涂层的硬度,利用X射线衍射(XRD)和X射线光电子能谱(XPS)分析了涂层的成分、相组成和主元素的价键结构.结果表明,Cr有显著软化GLC涂层的作用.随着Cr含量增加,GLC涂层中的sp3杂化碳的含量持续减小,sp2杂化碳的含量先增后减,而金属铬及碳铬化合物的含量则不断增大.上述因素对GLC涂层的硬度变化均有影响,而涂层中sp3杂化碳的含量随着Cr含量增加持续减小是造成GLC涂层硬度不断降低的主要原因.     

Diamond like carbon film as a protective coating for high strength steel and titanium alloy

Hard chromium plating is currently used to protect the surfaces of wear and fatigue sensitive aircraft parts such as the landing gear, flap tracks etc. At Boeing, this process has been targeted for elimination due to environmental and workplace safety considerations. Our objective is to develop a non-hazardous, non-line of sight (NLOS) dry process replacement for chromium plating that does not adversely affect the engineering properties such as fatigue and wear. In this work, we report the results for Diamond Like Carbon (DLC) coating as a replacement for chrome plating. Excellent adhesion of DLC film to the steel surface is achieved using a variety of surface modification techniques such as in-situ inert ion sputter cleaning and ion implantation or deposition of an intermediate bonding layer between the DLC film and the substrate. Our tests show that DLC coated 4340 M steel exhibit better wear and fatigue characteristics when compared to those of chromium-plated 4340 M. DLC coating improved the wear resistance of Ti-6Al-4V drastically and did not cause any coating induced fatigue degradation in this alloy.     

Crystallography of Fe2Al5 phase at the interface between solid Fe and liquid Al

In this study, the microstructures and crystallographic features of a η-Fe₂Al₅ phase formed on pure Fe hot-dipped in a pure Al melt at 750 °C were examined in order to understand the η phase layer formation having a saw-tooth morphology. A number of the columnar η grains (forming the η phase layer) grow towards the solid Fe (α-Fe) side along the [001] direction, resulting in a significant saw-tooth morphology at the interface between the η and α-Fe phases. The neighboring η grains have high-angle boundaries with a common [001] axis. In the η phase layer, the low-angle boundaries develop close to the liquid Al side, and their density becomes higher with longer dipping times, resulting in the development of a fine dislocation substructure in the η phase. In the α-Fe phase, fine substructure consisting of a high density of low-angle boundaries develops around the growth tips of the columnar η grains. These substructure developments are likely responsible for the α → η transformation strain. A possible mechanism for the formation of this η phase layer having a saw-tooth morphology will be discussed in terms of the stress field caused by the α → η transformation.     

Electrochemical impedance spectroscopy and morphological analyses of pyrrole, phenylpyrrole and methoxyphenylpyrrole on carbon fiber microelectrodes

In this study, N-pyrrole (Py), N-phenylpyrrole (PhPy), and 1[4-methoxyphenyl]-1H-pyrrole (MPhPy) homopolymers were synthesized electrochemically onto carbon fiber microelectrodes (CFMEs). The influences of the substituent effect on electrochemical impedance spectroscopy (EIS) were studied comparatively. All the monomers were electrodeposited in 0.05 M tetraethyl ammonium perchlorate (TEAP)/dichloromethane (CH₂Cl₂) solution and characterized by cyclic voltammetry (CV), Fourier transform infrared reflectance spectrophotometry (FTIR-ATR), scanning electron microscopy (SEM), and atomic force microscopy (AFM). The morphological study reveals that the polymers were deposited as a continuous and well adhered film to surface of the CFME. An equivalent electrical circuit for three different monomers on CFMEs was proposed and experimental data were simulated to obtain the numerical values of circuit components. All results support the high quality film deposition that resulted in desired electronic properties due to the electron donating behaviors of substituent group of phenyl and methoxy.     

Fabrication and adhesive properties of thin organosilane films coated on low carbon steel substrates

We investigated the fabrication and adhesive properties of thin organosilane films coated on the surface of low carbon steel substrates by curing γ-glycidoxypropyltrimethoxysilane (γ-GPS) solutions hydrolyzed in different solvents. This method appears to be a promising alternative treatment for the chromium and phosphate mainly due to its excellent corrosion protection and adhesion properties to organic coatings and a non-toxic pretreatment. In this work the elemental distribution in γ-GPS films coated on low carbon steel substrates was analyzed by an energy dispersive X-ray spectrometer (EDX), and the effect of curing temperature and solution concentration on the chemical structures of this silane was also carried out with reflection-absorption infrared (RAIR) spectroscopy. We systematically investigated the processing variables regarding adhesive strength of the film, including hydrolysis solvents, hydrolysis time, solution concentration and curing temperature. Compared with other experimental conditions, the present results provided a consistent demonstration that γ-GPS films exhibited improved adhesive strength to metal substrates in case of using DI water/methanol mixture as hydrolysis solvent, hydrolysis time of 48 h, solution concentration of 10%, curing temperature of 150 °C and curing time of 1 h. The fracture mode of γ-GPS films varied with different deposition parameters.     

Growth of carbon nanofiber coatings on nickel thin films on fused silica by catalytic thermal chemical vapor deposition: On the use of titanium, titanium-tungsten and tantalum as adhesion layers

Coatings of carbon nanofiber (CNF) layers were synthesized on fused silica substrates using a catalytic thermal chemical vapor deposition process (C-TCVD). The effects of various adhesion layers-titanium, titanium-tungsten and tantalum-under the nickel thin film on the attachment of carbon nanofibers and their morphological properties are presented. The diameter and the thicknesses of the CNF-coatings were analyzed by scanning electron microscopy, whereas the microstructure and crystallinity of the synthesized carbon nanofibers were investigated by transmission electron microscopy and Raman spectroscopy, respectively. Specific surface areas of CNF-coatings were determined with nitrogen adsorption-desorption isotherm measurements.Using C-TCVD of ethylene at 700 °C (1 h), well-attached, entangled, quasi-crystalline platelet carbon nanofibers were synthesized with tip-type growth mode on 25 nm thick nickel films with an adhesion layer of 10 nm Ta or Ti-W. The thickness of CNF-coating was ~ 3.5 μm, and the diameter of the fibers depended on the composition of the metallic thin film stack: 20-50 nm for Ni/Ta and 80-125 nm for Ni/Ti-W. The ultimate goal is the integration of these CNF-coatings as catalyst support in microfluidic devices, for which it is important to control CNF-coating characteristics such as fiber diameter, layer thickness, specific surface area and adhesion to the surface.     

FeAl and Mo–Si–B intermetallic coatings prepared by thermal spraying

FeAl and Mo–Si–B intermetallic coatings for elevated temperature environmental resistance were prepared using high-velocity oxy-fuel (HVOF) and air plasma spray (APS) techniques. For both coating types, the effect of coating parameters (spray particle velocity and temperature) on the microstructure and physical properties of the coatings was assessed. Fe–24Al (wt%) coatings were prepared using HVOF thermal spraying at spray particle velocities varying from 540 to 700 m/s. Mo–13.4Si–2.6B coatings were prepared using APS at particle velocities of 180 and 350 m/s. Residual stresses in the HVOF FeAl coatings were compressive, while stresses in the APS Mo–Si–B coatings were tensile. In both cases, residual stresses became more compressive with increasing spray particle velocity due to increased peening imparted by the spray particles. The hardness and elastic moduli of FeAl coatings also increased with increasing particle velocity. For Mo–Si–B coatings, plasma spraying at 180 m/s resulted in significant oxidation of the spray particles and conversion of the T1 phase into amorphous silica and -Mo. The T1 phase was retained after spraying at 350 m/s.     

Comparison between Ni-Cr-40vol%TiC wear-resistant plasma sprayed coatings produced from self-propagating high-temperature synthesis and plasma densified powders

Plasma sprayed Ni-Cr-40vol%TiC coatings produced from powders obtained by self-propagating hightemperature synthesis (SHS) and plasma densification (PD) processes are characterized. Chemical composition, microstructure, and mechanical properties, such as microhardness and wear resistance, are evaluated and compared. SHS coatings exhibit good sliding wear performance. The exact stoichiometry of titanium carbide inclusions in the metallic matrix affects the dimension of the crystal lattice parameter and was investigated by examining the shift of x-ray diffraction (XRD) peaks of the TiC. A value of the combined carbon/titanium ratio of about 0.6 was calculated for both powders, thus excluding the influence of the stoichiometry of the carbide inclusions on the wear properties of the coatings.     

Damage and failure mechanisms of a3-directional carbon/carbon composite under uniaxial tensile and shear loads

The mechanical behavior of a three-directional carbon/carbon (C/C) composite under tensile and shear loads is investigated in relation with the failure mechanisms and, the fiber architecture. This three-directional C/C composite was produced by Chemical Vapor Infiltration of a needled fiber preform of multiple layers of satin woven tows. The C/C composite exhibited several interesting features including an essentially non-linear stress–strain behavior and permanent deformations. Three families of matrix cracks were identified under tensile and shear loads, including microcracks in the tows, intertow delamination and cracks across the longitudinal tows. It was found that the delamination cracks affect preponderantly the stress–strain behavior and the mechanical properties. Similar features in the mechanical behavior and the failure mechanisms were highlighted under tension and under shear loading.

Résumé

Le comportement mécanique d’un composite Carbone/Carbone 3D sous chargement de traction et de cisaillement est étudié. Ce matériau composite C/C a étéobtenu par densification par C.V.I. (Chemical Vapor Infiltration) d’une préforme fibreuse aiguilletée normalement au plan du tissu. Les relations entre les courbes de comportement mécanique, les mécanismes de dégradation identifiés par microscopie optique et l’architecture fibreuse de la structure ont étéétablies. Ce matériau composite C/C présente un comportement mécanique non linéaire jusqu’àrupture avec déformations anélastiques. Les trois mêmes familles de fissures matricielles ont étéidentifiées sous chargement de traction et de cisaillement, àsavoir la fissuration intra-fil, la fissuration inter-fils, et, la fissuration intra et inter-fils. La fissuration inter-fil, et, la fissuration intra et inter-fils affectent majoritairement le comportement mécanique. Les similitudes importantes observées entre le comportement mécanique et les mécanismes de rupture sous sollicitations de traction et de cisaillement sont discutées.     

The grain-boundary segregation of phosphorus and carbon in an Fe–P–C alloy during cooling

Grain-boundary segregation in the Fe–P–C system has been examined using Auger Electron Spectroscopy on specimens which were cooled slowly to various intermediate temperatures after equilibration at 600°C, and then quenched prior to examination. The aim was to monitor the segregation of phosphorus and carbon through temperature ranges which offered the extremes of both solutes having long-range lattice mobility to that where only carbon was mobile. This approach revealed new complexities in the segregation process. It was found that phosphorus continued to segregate during the early stages of both furnace and air cooling, i.e. when long-range lattice diffusion remained possible. At intermediate temperatures, when the mobility of phosphorus was limited, an apparently competitive process occurred between the solutes, i.e. phosphorus de-segregated whilst the grain-boundary concentration of carbon increased. At temperatures less than approximately 200°C, carbon segregation continued under furnace cooling but not under air cooling but in neither case did phosphorus continue to de-segregate. A theoretical framework is developed which can account qualitatively for these segregation characteristics.