Protective ceramic multilayer coatings for carbon fibers

Uniform, adherent, crack-free and non-bridging HfC and HfC/SiC coatings on carbon fibers have been synthesized by a reactive CVD (RCVD) process at low temperatures. To fabricate SiC coating on HfC-coated carbon fiber, an alternative approach using unsaturated organosilicon polymer solution was also proposed. The schemes describing the formation of carbide coatings on carbon fibers by RCVD and transformation of unsaturated organosilicon polymers into silicon carbide are discussed. The HfC and HfC/SiC coatings were studied by SEM, EDS and XPS techniques. Coatings are composed of hafnium, silicon and carbon as the main constituents and oxygen and fluorine as contaminants. As was proposed, the composition of the coatings is affected by several factors, among them a chemical attack of coated fibers by gaseous oxygen- and fluorine-containing by-products can be considered as dominant ones. The duplex HfC/SiC-coated carbon fibers exhibit more oxidation resistance at elevated temperatures than the initial and HfC-coated carbon fibers.     

Effects of deposition parameters on characteristics of carbon coatings on optical fibers prepared by thermal chemical vapor deposition

The effects of deposition parameters on characteristics of carbon coatings on optical fibers prepared by thermal chemical vapor deposition are investigated. The deposition parameters are selected as follows. The CH₄/(CH₄ + N₂) ratio is in the range between 20% and 100%; the temperature is set from 1173 to 1248 K; the working pressure is arranged between 50 and 100 kPa, and the residence time is ranging from 1.47 to 7.37 s. The deposition rate, microstructure, and electrical resistivity of carbon coatings are measured. The low-temperature surface morphology of carbon-coated optical fibers is elucidated. Experimental results indicate that the deposition rate increases with increasing the CH₄/(CH₄ + N₂) ratio, deposition temperature, working pressure, and residence time. The activation energy (= 456 kJ/mol) of carbon deposition from methane was shown to correlate to the activation energy of methane dissociation. The deposition rate is proportional to about first-order of partial pressure of methane, and thus, the deposition process is mainly controlled by the process to create mono-carbon species in the carbon film. As the deposition rate increases, the size and number of particles on the carbon coating surface and electrical resistivity of carbon coatings increase, while the ordered degree, nano-crystallite size, and sp² carbon atoms of the carbon coatings decrease. Additionally, the low-temperature surface morphology of carbon coatings shows that as the carbon coating thickness is large enough to sustain the thermal loading, decreasing the deposition rate is good for producing hermetic optical fiber coatings.     

Effect of sintering temperature on microstructure and properties of SiC coatings for carbon materials

SiC coating for the graphite materials was prepared by slurry-sintering technique in a vacuum. The phase, microstructure, thickness and resistance against irradiation of the SiC coatings prepared from 1500 to 1800 °C were investigated. Research results showed that, the porous β-SiC coating occurred at 1500 and 1600 °C, while compact β-SiC/Si coatings obtained at 1700 and 1800 °C. The thickness of the coatings was in most cases around 150 μm when the sintering was performed at 1500 and 1600 °C. However, the thickness was decreased, and the crystal size of SiC particles was increased when the sintering temperature was higher than 1600 °C. Thermal fatigue tests showed that, based on the surface morphology changes, the sintering temperature of 1700 and 1800 °C gave much improved irradiation resistance over that of coatings formed at 1500 and 1600 °C.     

Study of fluorhydroxyapatite coatings on carbon/carbon composites

Fluorhydroxyapatite coatings were prepared on the surface of carbon/carbon composites using the combined action of ultrasonic-electrodeposition and ion exchange. The morphology, structure and composition of the prepared coatings after ion exchange were investigated by scanning electron microscopy, X-ray energy spectrum analysis, infrared spectroscopy and X-ray diffraction. Results show that the crystallinity of the coating as well as the intensity of the diffraction peaks (112) and (300) of hydroxyapatite increased, whereas the intensity of the diffraction peaks of tricalcium phosphate decreased. The small lamellar crystals were converted into large ones. The fluorine content in the coating was 4.59%. The bonding strength between the coating and the carbon/carbon matrix increased slightly after immersion, reaching 4.12 MPa. The mechanism of the ion exchange during which hydroxyapatite turned into fluorhydroxyapatite is discussed.     

Microstructural characterisation of carbon doped CrAlTiN nanoscale multilayer coatings

Carbon doped CrAlTiN coatings for reducing the coefficient of friction sliding against a WC-Co ball in air were developed using a closed field unbalanced magnetron sputtering system (CFUMS). The carbon content was controlled by increasing the current of the carbon target power from 0 A to 5 A. The surface morphology, cross-sectional microstructure, phase constituent, chemical bonding energy and mechanical properties of coatings were characterised by means of XRD, SEM, XTEM, XPS, and nanoindentation. The results showed that the hardness, Young's modulus and coefficient of friction (CoF) of the coatings sliding against a WC-Co ball in air decreased with increasing carbon content. Microstructure characterisation revealed that the carbon doped coatings consisted of four sublayers from the substrate to the surface and the main phase formed is fcc B1-NaCl like (Cr, Al, Ti) N and (Cr, Al, Ti) (C, N) phase in sublayers III and IV respectively when the carbon content is below 3.96 at.%. When the carbon concentration exceeds this value, the excessive carbon will begin to form the amorphous carbon (or carbon-riched) phase which leads to a decrease in the coefficient of friction (CoF) of the coatings sliding against a WC-Co ball in air.     

Interactions between carbon coatings and tissue

The unique properties of thin diamond layers make them perspective candidates for producing advanced micro-electronic devices, coatings for cutting tools and optics. However, due to the highest biocompatibility of carbon resulting from the presence of this element in the human body, it appears to be a potential biomaterial.Carbon, especially in the form of the nanocrystalline diamond film, have found industrial applications in the area of medical implants.The studies of carbon films as coatings for implants in surgery were aimed at the investigations of biological resistance of implants, histopathological investigations on laboratory animals, tests of corrosion resistance, measurements of mechanical properties and a breakdown test in Tyrode's solution.Different medical implants are covered by Nanocrystalline Diamond Coatings (NCD). NCD forms the barrier diffusion between implant and human environment. The research on NCD proved that diamond layers are biocompatible with living organisms.Diamond Powder Particles (DPP) is an extended surface of NCD. Biological research with diamond powder can answer the basic question: what is the influence of DPP on cells, tissues and organs in human organism?     

Characterisation of amorphous phosphate coatings onto carbon steel surfaces

Abstract

This study continues the authors’ investigation of the newly developed product KAF-09OF and concerns the preparation and characterisation of amorphous phosphate coatings on carbon steels. KAF-09OF is a liquid concentrate of ammonium and alkaline phosphates, stabilising additives – buffers, softeners, activators (both inorganic and organic), as well as surfactants – a non-ionic and an anionic detergents. Gravimetric and polarisation methods, X-ray fluorescence analysis, transmission electron microscopy, X-ray photoelectron spectroscopy and atomic force microscopy have been used to determine the effect of pH (2–6), temperature conditions (20–80°C) and agitation rate (0–2000 rev min^?1) on the process of phosphating using this developed product. Coating thicknesses, compositions, structures and passivation characteristics in the phosphating baths with different concentrations of KAF-09OF have been analysed.     

Feasibility study on utilizing carbon dioxide during the processing of Mg-Al alloys

The feasibility of utilizing carbon dioxide (CO₂) during magnesium-aluminium (Mg-Al) alloys processing was investigated by incorporating CO₂ gas during melting and casting of the alloys. Mg-Al alloys containing ∼3 wt.% and ∼5 wt.% Al were processed with and without CO₂ atmosphere using the disintegrated melt deposition (DMD) technique. The cast alloys after extrusion were characterized for their structural, physical and mechanical properties to identify the utilization of carbon dioxide during processing. Results indicated that sound, defect-free Mg-Alloys were produced with CO₂ processing. Improvement in mechanical properties such as hardness, tensile strength and compressive yield strength were observed. The in situ formation of Al₄C₃ phase during processing was identified as the reason for the improvement in the properties, which indicated the utilization of carbon dioxide by the melt.     

Preparation and mechanism of calcium phosphate coatings on chemical modified carbon fibers by biomineralization

In order to prepare HA coatings on the carbon fibers, chemical modification and biomineralization processes were applied. The phase components, morphologies, and possible growth mechanism of calcium phosphate were studied by infrared spectrosenpy（IR）, X-ray diffractometry（XRD） and scanning electron microsenpy（SEM）. The results show that calcium phosphate coating on carbon fibers can be obtained by biomineralization. But the phase components and morphologies of calcium phosphate coatings are different due to different modification methods. Plate-like CaHPO4·2H2O （DCPD） crystals grow from one site of the active centre by HNO3 treatment. While on the para-aminobenzoic acid treated fibers, the coating is composed of nano-structural HA crystal homogeneously. This is because the -COOH functional groups of para-aminobenzoic acid graft on fibers, with negative charge and arranged structure, accelerating the HA crystal nucleation and crystallization on the carbon fibers.     

Self-healing properties of protective coatings containing isophorone diisocyanate microcapsules on carbon steel surfaces

Microcapsules containing isophorone diisocyanate (IPDI) were used as self-healing additives in the alkyd varnish coatings (AVCs), and their self-healing performance was evaluated in the case of artificial defects on Q235 steel surfaces, using scanning micro-reference electrode technique and Fourier-transform infrared spectroscopy. Comparison of the micromechanical properties between the water-insoluble self-healing products (polyurethanes) and AVCs indicates that the former significantly enhanced the capability of the scratched crevice to successfully endure outer stress. The electrochemical impedance spectroscopy experiments analysed the different stages in the self-healing process. This study successfully demonstrated the self-healing activity of IPDI-AVCs in protecting steel surfaces.     

The processing,properties, and structure of carbon fibers

This paper reviews the processing, properties, and structure of carbon fibers. Carbon fibers are derived from several precursors, with polyacrylonitrile being the predominant precursor used today. Carbon fibers have high strength (3–7 GPa), high modulus (200–500 GPa), compressive strength (1–3 GPa), shear modulus (10–15 GPa), and low density (1.75–2.00 g/cm³). Carbon fibers made from pitch can have modulus, thermal, and electrical conductivities as high as 900 GPa, 1,000 W/mK, and 10⁶ S/m, respectively. These fibers have become a dominant material in the aerospace industry and their use in the automotive and other industries is growing as their cost continues to come down. For more information, contact Satish Kumar, Georgia Institute of Technology, School of Polymer, Textile and Fiber Engineering, 801 Ferst Drive, NW MRDC-1, Atlanta, Georgia 30332-0295; (404) 894-7550; fax (404) 894-8780; e-mail satish.kumar@ptfe.gatech.edu.     

Effects of heating temperature and duration on the microstructure and properties of the self-healing coatings

The present work investigates how the heating temperature and duration affect the properties of the self-healing coating on martensitic steels. The coating composed of TiC + mixture (TiC/Al₂O₃) + Al₂O₃ is fabricated by means of air plasma spraying. The thermal shock test is performed at 600 °C, 700 °C and 800 °C, respectively, to evaluate the thermal-mechanical stability of the coating. The cross-section morphology of the samples after 1 h, 9 h, 18 h and 30 h of heat treatment shows that the porosity of the coating decreases with the increase of heating duration. The evaluation of electrochemical performance by electrochemical impedance spectroscopy shows that the corrosion resistance of the coating after being heated for 18 h is much better than the other samples due to the process of the inner layer being compacted in the coating. The adhesive tensile strength test between coating and substrate shows that the adhesive strength of the coatings is higher than 9 MPa within 40 h of heat treatment at 600 °C. The residual stress reaches a minimum value after the coating was heated for 9 h at 600 °C, then increases with the heating duration after 9 h. Energy dispersive X-ray analysis at the Vickers indentation indicates that the oxygen content at the crack position increases significantly after being heated for 30 h at 600 °C. These experimental results suggest that this coating can meet the requirement of application under the actual temperature conditions.     

搅拌摩擦加工原位反应制备Al3Ti-Al表面复合层

通过在铝合金1100-H14表面加工矩形凹槽并添加微米级钛粉再进行搅拌摩擦加工(friction stir processing,FSP)的方法,在铝合金表面获得Al3Ti-Al复合层.采用扫描电镜(SEM),能谱分析(EDS)以及X射线衍射(XRD)对表面复合层微观结构及相组成进行了分析,并对复合层的显微硬度进行了检测.结果表明,在FSP强烈的热、力耦合作用下,钛粉产生了碎化,破碎后的钛颗粒与铝产生快速原位反应,生成微米和亚微米级Al3Ti颗粒,残留的钛颗粒和细小的Al3Ti颗粒一同均匀地分布于铝合金基体中,从而使得铝合金表面的硬度得到提高,其平均值达到了71.39HV,为基体硬度的2.1倍.     

Surface modification of 6150 steel substrates for the deposition of thick and adherent diamond-like carbon coatings

Because of the high residual compressive stress normally accompanying the growth of diamond-like carbon (DLC) coatings and the large mismatch in the thermal expansion coefficient between DLC and steel, it is difficult to grow DLC coatings much thicker than 0.25 μm on steels. This paper describes our attempt to overcome this thickness limitation by a sequence of carbonitriding, carburizing and equilibration pre-treatments of the steel surface, followed by DLC coating deposition, all conducted within the same deposition system without breaking vacuum. These pre-treatments resulted in a surface with a graded composition and hardness profile. Such a graded interface is expected to reduce the interfacial energy, decrease thermal mismatch between the coating and the substrate, and thus improve coating adhesion. X-ray diffraction revealed the formation of various hard carbide and nitride phases. Raman spectroscopy showed that the modified steel surface just before DLC deposition exhibits local carbon bonding characteristics similar to DLC. Pulsed dc plasma-enhanced chemical vapor deposition was used to deposit one-micron thick DLC on these steel surfaces. The coating hardness was ~ 18-19 GPa. Its adhesion on the steel substrate was measured by scratch testing and was found to be comparable to thick, adherent DLC coatings deposited by other methods.     

Electrochemical and chemical characterization of polypyrrole/phosphotungstate coatings electrosynthesized on carbon steel electrodes in acetonitrile medium

Hybrid material polypyrrole/PW₁₂O₄₀³⁻ was potentiostatically electrosynthesized on carbon steel electrodes in acetonitrile medium. The obtained coatings were characterized by cyclic voltammetry, FTIR-ATR, XPS, SEM and EDX techniques. Phosphotungstate as dopant agent, Fe oxides and polypyrrole chains are the compounds that form the hybrid material structure. The influence of the electrosynthesis potential was analysed. Oxidized and reduced state of the coatings was also studied. When the electrosynthesis potential is increased up to 2.00 V and the polymeric coatings are in reduced state, the covering degree is very high and the coatings present a very homogenous and uniform surface.     

Laser-assisted synthesis of carbon nanofibers: From arrays to thin films and coatings

Carbon nanofiber assemblies in the form of non-aligned films, arrays of vertically aligned nanofibers, aligned nanofiber mats and composite coatings were produced by laser-assisted catalytic chemical vapor deposition. A visible argon ion laser was used to thermally decompose pure ethylene over alumina supported nickel catalysts. Straight, vermicular, beaded, branched and coiled individual nanofibers were observed. The effects of the laser irradiation time on individual nanofiber characteristics, thus on overall nanofiber assembly characteristics were investigated. The arrays, nanostructured films and coatings were examined by scanning electron microscopy. The individual nanofibers were examined by transmission electron microscopy. Nanofiber texture and nanotexture were assessed by lattice fringe analysis of high resolution transmission electron microscopy images. The observed variation in the interfringe distance along the nanofiber wall suggests a pulsed growth mode. This growth mode and the nanofiber shaping mechanism are discussed. Recommendations on how to control nanofiber characteristics such as shape and internal structure are provided.     

The influence of process parameters and heat treatment on the properties of cold sprayed silver coatings

Cold spray coating technology is a promising low temperature variant of thermal spray technology which can deposit pure, dense and thick coatings at a rapid rate. Unlike the other thermal spray coating techniques, cold spray is especially suitable for depositing coatings with high electrical and thermal conductivity as the integrity of the feedstock is maintained during the coating process. In the present study, the effect of process parameters and heat treatment on the properties of silver coatings has been investigated. An attempt has been made to correlate the powder particle velocity with the properties and microstructure of the coating. The effect of heat treatment temperature and atmosphere on the properties of the coatings, especially electrical conductivity, has been studied in detail in conjunction with a thorough analysis of the evolution of microstructure of the coatings.     

Effects of annealing on the microstructure and the mechanical properties of EB-PVD thermal barrier coatings

The effects of thermal annealing at 1000 °C in air on the microstructure and the mechanical properties (Young's modulus and hardness) of thermal barrier coatings consisting of a 4 mol% Y₂O₃ partially stabilized ZrO₂ top coat and a NiCoCrAlY bond coat, deposited by electron beam physical vapour deposition on nickel-based superalloy IN 625, have been investigated using X-ray diffraction, Raman spectroscopy, scanning electron microscopy (SEM), image analysis and nanoindentation. During annealing, the ceramic top coat undergoes sintering and recrystallization. These processes lead to stress relaxation, an increase of the intra-columnar porosity and the number of large pores as measured by image analysis of SEM micrographs. An increase of the grain size of the γ-phase in the bond coat, accompanied by changes in the morphology of γ-grains with annealing time, is also observed. Correlations between these microstructural changes in the top coat and the bond coat and their mechanical properties are established and discussed.     

碳纤维表面超声振荡辅助电镀镍涂层工艺及其参数的影响规律

碳纤维增强铝基复合材料具有诸多优良性能,在航空航天以及民用领域有广阔的应用前景。在碳纤维表面电镀镍涂层,可有效改善碳纤维与铝基体的润湿性,并抑制基体对纤维的损伤。镍涂层的质量与改善效果密切相关。通过预氧化处理并结合超声振荡辅助电镀的方法在纤维表面沉积镍涂层,解决碳纤维在电镀过程中因纤维束丝难以分散而出现的"黑心"问题。在此基础上,研究电镀工艺参数对涂层质量的影响规律,并对优化工艺参数后制备的镍涂层与碳纤维的结合强度进行评价。结果表明:使用添加剂可提高镍涂层与碳纤维之间的结合力,当电镀液pH值调整到3～4、电流密度大于0.3 A/dm²、电镀液温度控制在20～40℃时,所制备的镍涂层均匀致密。     

Effect of carbon fiber on microstructure evolution and surface properties of FeCoCrNiCu high-entropy alloy coatings

FeCoCrNiCu and C_f/FeCoCrNiCu high-entropy alloy coatings are prepared by laser cladding to study the effects of carbon fiber on the microstructure and surface properties of the FeCoCrNiCu alloy. It is found that the addition of carbon fiber has little effect on the phase structure of the FeCoCrNiCu alloy coating. However, an interesting finding is that the addition of carbon fiber could decrease the segregation of Cu-element along the grain boundaries in the FeCoCrNiCu coating. In addition, the addition of carbon fiber can significantly improve the corrosion resistance of the FeCoCrNiCu alloy coating. It can also reduce the intergranular corrosion in the coating. Moreover, the addition of carbon fiber has positive effects on the improvement in the hardness and wear resistance of the high-entropy alloys (HEAs) coatings. The decreased segregation of Cu-element along the grain boundaries is mainly attributed to the significant improvement in surface properties of the HEA coatings.