Thickness limitations in carbon nanotube reinforced silicon nitride coatings synthesized by vapor infiltration

Chemical vapor infiltration is a convenient method for synthesizing carbon nanotube (CNT)-reinforced ceramic coatings. The thickness over which infiltration is relatively uniform is limited by gas phase diffusion in the pore structure. These effects were investigated in two types of silicon nitride matrix composites. With CNTs that were distributed uniformly on the substrate surface dense coatings were limited to thicknesses of several microns. With dual structured CNT arrays produced by photolithography coatings up to 400 μm thick were obtained with minimal residual porosity. Gas transport into these dual structured materials was facilitated by creating micron sized channels between “CNT pillars” (i.e. each pillar consisted of a large number of individual CNTs). The experimental results are consistent with basic comparisons between the rates of gas diffusion and silicon nitride growth in porous structures. This analysis also provides a general insight into optimizing infiltration conditions during the fabrication of thick CNT-reinforced composite coatings.     

Electrochemical and Reaction Bonding Processing of Thick ZrO_2/Al_2O_3 Composite Coatings

CERAMIC coatings are currently of much interest forapplications in high-temperature and highly corrosiveenvironments.Formation of ceramic coatings byelectrochemical processing is a relatively newtechnique.'1'It presents several advantages overalternative coating techniques.'2'Recently wedeveloped a novel fabrication technique for theproduction of ceramic/ceramic and metal/ceramiccomposite coatings by electrochemical processing.'31The technique combined two electrochemicaldeposition methods,…     

层流等离子体射流铝合金表面熔覆层中颗粒分布

采用层流等离子体射流在ZL104铝合金表面制备SiCp／Al-Si复合材料熔覆层。结果表明，熔覆层中SiCp颗粒与基体结合良好，但在熔覆层上颗粒分布不均匀，而且多数分布在枝状晶界上。通过提高层流等离子体射流扫描速度，可以显著提高SiCp颗粒在熔覆层中的均匀性。     

Overall micro-arc oxidation treatment for AZ31B–6061 magnesium–aluminium dissimilar metal connecting parts

The magnesium–aluminium connecting parts are important application form for lightweight structural materials. But the conductive connect of magnesium and aluminium will cause serious galvanic corrosion problems. Therefore, the overall corrosion protective treatment is necessary. A ceramic coating was prepared via overall micro-arc oxidation to wrap the magnesium–aluminium connecting part integrally. The surface morphologies and compositions of the coatings were analysed by scanning electron microscopy and X-ray energy dispersive spectroscopy. The corrosion behaviour of the coatings was investigated with potentiodynamic polarisation tests in 3.5?wt-% NaCl solution. The growth process of ceramic coating on aluminium and magnesium surface was investigated, which showed the micro-arc oxidation reaction priority and the balanced growth process of ceramic coating under unbalanced micro-arc distribution. The results demonstrated that the overall micro-arc oxidation treatment improved the corrosion resistance and reduced the corrosion potential difference of each metal of magnesium–aluminium connecting part.     

The effect of oxide coatings on fatigue properties of 7475-T6 aluminium alloy

The effect of hard anodic oxide and plasma electrolytic oxide coatings on the fatigue strength of 7475-T6 aluminium alloy has been investigated. The coated aluminium alloy was tested using constant load uniaxial tensile fatigue machine. Hard anodising led to an appreciable reduction in the fatigue strength of 7475-T6 alloy of about 75% for a 60 μm thick coating. Further, plasma electrolytic oxidation resulted in reduction of the fatigue strength of about 58% for a 65 μm thick oxide coating. The decrease in fatigue strength of the hard anodic oxide coatings was associated with the stress concentration at the microcracks in the coating. The better fatigue performance of the PEO coatings was attributed to the development of the compressive residual internal stress within the coatings. The reduction in the fatigue strength of the PEO coatings as compared to the uncoated material was associated with the development of the tensile residual internal stress within the substrate. This may cause an early crack initiation in the substrate adjacent to the coating.     

KF-KCl熔体中电沉积TiB2镀层

在铝工业中TiB2是一种非常有前途的阴极内衬取代材料.本研究首先通过热力学分析验证了在Ti-B-C体系生成TiB2的可能性,然后在K2TiF6和KBF4作为活性物质的KF-KCl熔体中以石墨为基体通过直流电沉积(CCP)和周期断开电流电沉积(PIC)技术制备了TiB2镀层,并且研究了电流密度和电镀技术对镀层表面平整度、致密度和晶粒尺寸的影响.结果表明,当电流密度为0.8 A/cm2时,能够得到厚度均匀且和基体具有良好附着的TiB2镀层;和CCP相比,采用PIC技术制备的TiB2镀层表面平整度和致密度都得到明显改善,并且晶粒也更为细小.XRD分析表明镀层由相对纯净的TiB2组成,并且镀层择优取向均为(001)面,这和二维晶核理论的预测相吻合.     

一起铝电解槽漏铝燃烧事故的分析

通过现场调研、理论计算和实验,对一起铝电解槽漏铝燃烧事故进行了研究和分析。认为事故发生的主要原因是在特定环境下,铝液表面氧化膜遭破坏,使铝液与空气充分连续接触,触发了铝氧燃烧反应,进而激发了铝铁还原反应、碳铝和铝氮等反应的发生,使燃烧进一步加剧。根据事故原因和机理提出了防止类似事故发生的建议。     

Studies on the Surface Treatment of Aluminium and Zinc

Comparative corrosion and adhesion tests have demonstrated the value of chemical surface treatments for improving the bare corrosion resistance and adhesion of paint to zinc and aluminium. Recent developments have enabled architectural green coatings to be produced on aluminium with consistent colour and chromate-fluoride coatings to be applied on high speed strip lines. It is now possible to correlate the colour of Chromate coatings on zinc with the ratio of the hexavalent chromium to the other anion present; best results are obtained when this anion is sulphate. Yellow Chromate films of very good protective power can be produced from comparatively dilute solutions. Colourless Chromate films can also be obtained from these solutions by adding ammonium sulphate to the processing solution.     

The influence of the surface ion implantation of aluminium and yttrium upon the oxidation behaviour of a Fe-15% Cr-4% Al Fecralloy® stainless steel,in air,at 1100°C

An increase in the concentration of aluminium in a 0.25 μm thick surface layer of a 15%Cr-4%Al yttrium free Fecralloy steel by the implantation of up to 10¹⁷ ions cm^?2 had no significant influence upon the oxidation behaviour of the steel, in air, at 1100°C. An alloy addition of 0.86% yttrium reduced the attack and oxide spallation for at least 3271 h, while the surface implantation of 3 × 10¹⁵ yttrium ions cm^?2, together with 10¹⁷ aluminium ions cm^?2 improved the oxidation behaviour of the steel for only a limited period (784 h). It is concluded that yttrium alloy additions in the Fecralloy steels exert their beneficial influence on the oxidation behaviour within the oxide film rather than the subscale.     

Exothermal oxidation of the bottom of discharge channels upon Microarc oxidation of aluminum alloys

It has been established empirically that microarc coatings, which are produced on the surface of aluminum alloys in electrolytes that contain relatively small concentrations of chemical components (which, after plasma-chemical and thermochemical transformations, form oxides that enter into the coating), contain metalloceramic layers. The latter fact confirms that one of the main mechanisms of the growth of such coatings is the exothermal oxidation of the metallic bottom of the microarc channels leading to the evaporation of metallic components of the alloy and their transfer into the coating or onto its surface. Therefore, the color of coatings at the relatively early stage of the process of microarc oxidation is a function of the phase composition of the bottom of channels of the microarc discharge. Original Russian Text ? A.G. Rakoch, Yu.V. Magurova, I.V. Bardin, G.M. El’khag, P.M. Zharinov, V.L. Kovalev, 2008, published in Korroziya: Materialy, Zashchita, 2007, No. 12, pp. 36–39.     

Determination of the protective properties of electrodeposited organic epoxy coatings on aluminium and modified aluminium surfaces

The electrochemical and transport properties and thermal stability of epoxy coatings electrodeposited on aluminium and modified aluminium surfaces (anodized, phosphatized and chromatized-phosphatized aluminium) have been investigated during exposure to 3% NaCl solution. From the results obtained from electrochemical impedance spectroscopy (EIS), gravimetric liquid sorption experiment and thermogravimetric analysis (TGA) it can be concluded that electrochemical and transport properties of epoxy coatings on anodized and chromatized-phosphatized aluminium are significantly improved with respect to the same epoxy coatings on aluminium and phosphatized aluminium: higher values of pore resistance and charge-transfer resistance and lower values of coating capacitance and double-layer capacitance, from EIS; smaller values of diffusion coefficient of water through epoxy coating, from sorption measurements and smaller amount of absorbed water inside the coating, from TGA. On the other hand, the somewhat lower thermal stability of these coatings was obtained.     

Effect of electrical parameters on the efficiency of the process of microarc oxidation of D69 aluminium alloy

Results of investigations of the effect of energy parameters on the process of micro-oxidation and the quality of produced oxidised coatings on the D16 aluminium alloy are presented. The coatings formed in oxidation in different working conditions of the power source with the capacitance regulation of power are compared. The microstructure of the surface of the specimens with the coatings produced in microarc oxidation in three sets of conditions is shown.     

2A12铝合金微弧氧化膜电化学腐蚀性能研究

采用微弧氧化技术对粉末冶金制备的2A12铝合金进行表面改性,表征改性层微观结构和相组成,评价改性后2A12铝合金的电化学腐蚀性能。研究发现:微弧氧化处理之后,2A12铝合金表面生成厚度约为80 μm的均匀氧化膜,其主要由α-Al₂O₃、γ-Al₂O₃及非晶组织构成;采用微弧氧化对2A12铝合金表面改性之后,改性层极大程度地抑制合金表面阳极反应和阴极反应的进行,自腐蚀电流密度明显下降,腐蚀速率显著降低,合金电化学腐蚀性能明显提高。     

预制膜对铝合金微弧氧化陶瓷层生长过程的影响

在磷酸盐体系电解液中,利用微弧氧化技术,分别对有、无高温氧化预制膜的铝合金进行表面陶瓷化处理,研究了预制膜对陶瓷层生长的影响规律.结果表明:高温氧化预制膜有利于提高陶瓷层的生长速率,降低起弧电压;陶瓷层的生长先是以初期形成的陶瓷颗粒为核心呈线状扩展,然后多条线接合呈网状,最后蔓延成面;陶瓷层生长的初期以高温氧化预制膜熔化生成为主,到后期,则是以铝合金基体熔化生成为主,此时预制膜对陶瓷层生长过程的影响较小,但由预制膜生成的陶瓷对陶瓷层生长的影响较大.     

Effect of Al Content on Microstructure and Cyclic Oxidation Performance of Pt-Aluminide Coatings

The effect of Al content, i.e., the amount of Al picked up during aluminizing, on the microstructure and cyclic oxidation properties of Pt-aluminide coatings has been investigated. The cast Ni-base superalloy CM-247 was used as the substrate material and a single-step, high-activity pack aluminizing process was used to produce the Pt-aluminide coatings. The Al content of these coatings was varied by using packs with different compositions of the Al source. Pt-aluminide coatings having three different Al contents, namely 6.5, 16, and 21 mg cm^-2, were evaluated for their cyclic oxidation resistance at 1200°C in air. It was found that the Pt-aluminide coatings, irrespective of their Al contents, evolve in the same manner during aluminizing and result in a three-layer structure with an outer PtAl₂+NiAl two-phase layer, an intermediate NiAl layer, and the inner interdiffusion layer. The stability of this three-layer coating structure over long periods of aluminizing, however, is dependent on the availability of Al from the pack during this period. Below a certain threshold Al availability, the two-phase outer layer transforms to a single-phase NiAl structure causing the coating to change from its three-layer structure to a two-layer one. Cyclic oxidation results indicate that, while a minimum Al content in Pt-aluminide coatings is essential for deriving the best oxidation performance, increasing the Al content beyond a certain level does not significantly enhance oxidation behavior. The effect of Al content on aspects, such as coating degradation and nature of coating–surface damage during cyclic oxidation, is also discussed.     

Formation of nanostructured YSZ/Ni anode with pore channels by plasma spraying

YSZ/Ni is the conventionally most used material for making the anode of a solid oxide fuel cell. Agglomerated nanostructured YSZ/NiO powders and plasma spray are applied to produce nanostructured YSZ/NiO coatings on porous support substrates. After reduction in an ambient atmosphere of 7% hydrogen and 93% argon at about 800 °C for 4 hours, a novel SOFC anode with nanostructured characteristics such as nano YSZ particles, nano Ni particles, nano pores and nano pore channels is produced. This new YSZ/Ni anode provides larger triple phase boundaries for hydrogen oxidation reactions. X-ray diffraction patterns of these YSZ/NiO coatings after 1 h of heat treatment at temperatures from 700 to 1100 °C are obtained and Scherrer analysis is conducted to study the effect of temperature on grain size. The results obtained from SEM, TEM, XRD and EDX measurements and analyses are presented in this investigation.     

铬铝涂层及其抗高温腐蚀的行为

本文作者采用粉末包装法制备Cr-Al二元涂层,对Cr-Al二元涂层的制备以及涂层的高温腐蚀性能进行了研究。实验结果表明,在纯Ni上制备Cr含量较高的Cr-Al共渗涂层是困难的,纯Ni上Cr-Al共渗时,Al会同时沉积到纯Ni试样和Cr粉表面上,由于Cr粉表而上铝涂层的形成,大大降低丁Cr的活度,从而抑制了Ni上渗Cr的过程;然而,在纯Ni上用二步法制备Cr-Al二元涂层是可行的,先渗Cr后渗Al能产生理想的Cr-Al二元涂层,涂层具有良好的抗高温氧化和抗热腐蚀性能;而先渗Al后渗Cr则产生有孔洞的Cr-Al二元涂层,涂层的抗高温腐蚀性能很差。     

Influence of thermodynamic activities of different masteralloys in pack powder mixtures to produce low activity aluminide coatings on TiAl alloys

Titanium aluminides are interesting high temperature materials, but show insufficient oxidation resistance as well as embrittlement at higher temperatures (>750 °C). Al-enriched coatings can be manufactured by pack cementation on many high temperature alloys to promote the formation of a protective alumina layer at high temperatures, which not only protects the alloy from oxidation but is also expected to impede embrittlement of TiAl at high temperatures. One drawback of such coatings is that Al-rich phases are very brittle. Therefore the major intermetallic aluminide phase in the coating plays a critical role for the protection behavior. Based on thermodynamic calculations different masteralloys were chosen to control the pack cementation process. Particular attention is given to the gradient between the aluminum activity of the different masteralloy powders and the aluminum activity of the substrate surface (alloy TNM^®-B1) in order to control the deposited phase at the surface. It is revealed that powder pack with Al as masteralloy provides a high Al activity and produces thick multi-layered coatings consisting of brittle TiAl₃ and TiAl₂ phase and aluminum-rich TiAl. By using different chromium aluminides as masteralloys, thinner, low-activity coatings could be produced, consisting of a bi-layer of brittle TiAl₂ phase and aluminum-rich TiAl or just the targeted pure aluminum-rich TiAl, which is known to have much better mechanical properties.     

Cyclic oxidation resistance of Ni–Al alloy coatings deposited on steel by a cathodic arc plasma process

The cyclic oxidation resistance of nickel-aluminide coatings deposited on steel using a cathodic arc plasma (CAP) process has been investigated. Our results show that nickel-aluminide films can be successfully deposited on carbon steel and stainless steel substrates by this process; NiAl₃ is the major phase in the deposited films. The thermal cycling behaviour suggests that such coatings can resist oxidation through physical blocking of oxygen, either by the coating itself or by the aluminium oxide scale subsequently formed in-service. Aluminium diffusion inwards to the substrate may also be beneficial to the thermal oxidation resistance. The coating protects stainless steel substrate materials at 500°C by transforming the NiAl₃ phase into NiAl, producing aluminium oxide on the open substrate surface. At 800°C, oxide flaking is suppressed by the trace amounts of nickel or aluminium which have partially diffused into the substrate.     

Oxidation Behavior of Pt–Ir Modified Aluminized Coatings on Ni-base Single Crystal Superalloy TMS-82+

The oxidation resistance of Pt–Ir modified aluminized coatings, prepared by magnetron sputtering, was investigated. Cyclic oxidation tests revealed that Pt–30 at%Ir and Pt–50 at%Ir modified aluminide coatings demonstrated a smaller mass change compared with Pt, Pt–80 at%Ir and Ir modified aluminide coatings. Cross-sectional analyses following cyclic oxidation tests showed that the TGO layer formed on the Pt modified aluminide coating surface is almost twice as thick as those on the Pt–30 at%Ir and Pt–50 at%Ir coatings. In addition, the Pt–30 at%Ir and Pt–50 at%Ir samples had a much smoother surface than the Pt modified coatings after cyclic oxidation, and the latter suffered from severe surface rumpling. However, when the Ir content exceeded 80 at% in Pt–Ir modified coatings, internal voids formed during cyclic oxidation. These results show that the addition of 30–50 at%Ir to Pt-modified aluminized coatings is most effective in enhancing oxidation resistance.