Reactive thermal spray by high velocity ceramic jet and characterization of the coatings

A ceramic jet composed of molten particles in an electrothermally exploded powder spray was identified by the flash, soft x-ray radiography technique. The velocity of the leading edge of the jet was estimated to be 900 m/s. The coating obtained by a ceramic jet of titanium diboride consisted of a mixing layer of the substrate material and sprayed ceramics. A coating, which exhibited no pores or cracks, was formed through the dense deposition and solidification of spray droplets. The successive impacts of the droplets caused melting and stirring of the substrate surface to form a mixing layer. Some of these layers were formed due to capillary movement of the molten substrate material into the fractures of coarse ceramic particles. Thermal spray by chemical reaction between titanium and boron nitride particles resulted in a composite coating of TiN and TiB₂. The character of the mixing layer indicated that the depth profiles depended on the substrate material.     

Splat Morphology and Influence of Feeding Rate During Reactive Plasma Spray of Aluminum Powder

Fabrication of aluminum nitride (AlN) coatings using conventional plasma spraying processes directly has been deemed impossible. It is attributed to the thermal decomposition of the AlN feedstock particles during spraying without a stable melting phase. Using the reactivity of the plasma (reactive plasma spraying: RPS) showed a promising consideration for in situ formation of AlN thermally sprayed coatings. Several AlN-based coatings were fabricated through the RPS of aluminum powders in the N₂/H₂ plasma. The focus of this study is in discussing the morphology of splat deposition during the nitriding of Al particles. Furthermore, the influence of the feeding rate during the RPS and nitriding of Al powders will be investigated. The nitride content, as well as the unreacted molten Al phase, strongly influences splat deposition and morphology during the RPS of Al. The collected splats can be divided into reacted, partially reacted, and unreacted splats. The reacted splats tend to show a disk or egg-shell shape. The partially reacted mainly had outside nitride shells and an unreacted molten Al part in the center. The unreacted splats tended to show a splash shape. The main controlling factor is the time of the droplet impact on the substrate during the reaction sequence. The particle size and spray distance showed significant effects on the splat formation due to their effect on the nitriding conversion and the melting behavior of the particles during RPS nitriding. The powder feeding rate was investigated through increasing the injection rate and by using a low carrier gas flow rate. Increasing the powder feeding rate significantly improved the coating thickness. However, it suppressed the nitriding conversion of the large Al particles. Thus, with increasing the amount of the powder in the plasma, the Al molten particles are easily aggregated and agglomerate together upon colliding on the substrate with an AlN shell on the surface. This prevents the N₂ from having access to all of the aggregated particles. Therefore, the fabricated coatings using large Al particles consist of surface AlN layers and the central parts of AlN and Al composite layers. On the other hand, it was possible to fabricate about 500-μm-thick AlN coatings using fine Al particles of 15 μm and increasing the feeding rate. Using the fine particles improved the nitriding reaction due to the improvement of the surface area (the reaction area). Moreover, the nitriding process of the Al particles with increasing the feeding rate was also investigated.     

电弧电压对低能等离子喷涂WC-Co涂层组织及性能的影响

使用烧结破碎的WC-12%Co粉末,采用轴向送粉等离子喷涂系统制备WC-Co涂层。保持电弧电流不变,增加工作气体中的氢气含量来提高电弧电压,以研究电弧电压对于涂层微观结构的影响。使用X射线衍射仪(XRD)分析WC-Co涂层的脱碳相变,使用扫描电子显微镜(SEM)观察粉末的熔化程度、扁平化状态和涂层的微观结构,使用MH-6维氏硬度计和MM200磨损试验机分别测量了涂层的显微硬度和耐磨性。结果表明,提高电弧电压有利于粉末的熔化。根据熔化程度的不同,粉末会呈现四种典型的扁平化状态。提高电弧电压促使碳化钨脱碳生成的W_2C和Co_3W_9C_4,涂层中硬质相体积增加,钴基体积减小。适当提高电弧电压有利于增加涂层的硬度和耐磨性,但过高的电弧电压会恶化涂层质量,反而降低涂层的硬度和耐磨性。     

Formation Routes of Nanocomposite Coatings in Detonation Spraying of Ti3SiC2-Cu Powders

In thermally sprayed coatings, nano-sized features of the microstructure may be either inherited from the nanostructured agglomerates of the feedstock powder or form as a result of rapid cooling of molten particles upon deposition. Applying a process of the computer-controlled detonation spraying (CCDS) to Ti₃SiC₂-Cu composite powders produced by high-energy mechanical milling, we show that both routes are possible depending on the spraying conditions. When the nanostructure of the Ti₃SiC₂-Cu coating is inherited from the feedstock powder—under very mild conditions of detonation spraying, which exclude melting, so is the phase composition of the coating. In higher-temperature conditions of spraying, a significant fraction of the copper matrix melts and the interaction between Ti₃SiC₂ and Cu occurs. The TiC _x -Cu(Si) coatings that form show crystallites of both phases in the nano-range. In this case, rapid solidification of the molten fraction of the particles is responsible for the formation of the coatings with a nanostructured matrix. Due to the flexibility of the CCDS process, conditions of spraying were found such that a composite coating with very fine crystallites of the Cu(Si) matrix (30 nm) and a hardness of 273 HV could be obtained.     

Base-metal saturation of refractory carbide coatings produced by enhanced ceramic jets in electrothermally exploded powder spray

Tungsten carbide and tantalum carbide were sprayed onto substrates of mild steel by the electrothermally exploded powder spray (ELTEPS) process. High-speed x-ray radiography revealed that tungsten-carbide jets of molten particles guided inside a nozzle exhibited denser flow than unguided jets at the substrate. The velocity of the jet was approximately 800 m/s at the early stage of jetting. The ceramic coatings obtained from the guided spray consisted of carbides of a few to tens of micrometers in size, which were saturated by the base metal up to the top of the coating. The coatings exhibited diffusion of the sprayed ceramics and base metal at the interface of the deposit and substrate. The enhancement of the jet flow formed a microstructure of the ceramic coating, which was saturated by the base metal even without post heat treatment.     

Cold Spray Coating of Submicronic Ceramic Particles on Poly(vinyl alcohol) in Dry and Hydrogel States

We report an approach using cold spray technology to coat poly(vinyl alcohol) (PVA) in polymer and hydrogel states with hydroxyapatite (HA). Using porous aggregated HA powder, we hypothesized that fragmentation of the powder upon cold spray could lead to formation of a ceramic coating on the surface of the PVA substrate. However, direct spraying of this powder led to complete destruction of the swollen PVA hydrogel substrate. As an alternative, HA coatings were successfully produced by spraying onto dry PVA substrates prior to swelling in water. Dense homogeneous HA coatings composed of submicron particles were obtained using rather low-energy spraying parameters (temperature 200-250 °C, pressure 1-3 MPa). Coated PVA substrates could swell in water without removal of the ceramic layer to form HA-coated hydrogels. Microscopic observations and in situ measurements were used to explain how local heating and impact of sprayed aggregates induced surface roughening and strong binding of HA particles to the molten PVA substrate. Such an approach could lead to design of ceramic coatings whose roughness and crystallinity can be finely adjusted to improve interfacing with biological tissues.     

Development of particulate treatments and coatings to reduce SiC degradation by liquid aluminum

SiC-particulate-reinforced aluminum alloys are potentially attractive materials for structural applications. Among the fabrication possibilities, the molten metal mixing technique is one of the most promising in terms of cost and production capacity. Degradation of SiC by chemical reaction with molten aluminum is a problem that could be, at least partially, overcome by treatments or coatings of the reinforcements. This article describes the various techniques that have been used to coat particles and presents results ob-tained during remelting of composites. Prior oxidation of particles, oxide coating by sol-gel, or dry mixing techniques are thus presented. Oxidation of particles and TiO₂ coatings slow the degradation of SiC. Analytical electron microscopy was used to characterize the interfacial reactions that occurred during fabrication and remelting of composites. Possible mechanisms of protection are then presented and discussed.     

粉末中C抑制FeAl熔滴氧化机制及其对等离子喷涂层组织与性能的影响*

大气等离子喷涂（APS）金属时,熔滴不可避免地发生氧化是难以获得粒子间结合充分的致密涂层的主要原因。以FeAl金属间化合物为例,提出一种在粉末中添加亚微米金刚石颗粒引入碳源,以期利用碳在高温下优先氧化的特性抑制等离子喷涂飞行粒子中Fe、Al元素的氧化,获得无氧化物的高温熔滴从而制备低氧含量（质量分数）、粒子间充分结合的FeAl金属间化合物涂层的新方法。采用APS制备Fe Al涂层,研究金刚石的添加对涂层氧含量、碳含量、涂层内粒子间结合质量与硬度的影响规律,探讨FeAl熔滴飞行中的氧化行为。采用商用热喷涂粒子诊断系统测量APS喷涂中的粒子温度,通过SEM与XRD表征了涂层的组织结构,并表征涂层的结合强度与硬度。结果表明,在等离子射流的加热和Fe、Al元素放热反应的联合作用下,飞行中FeAl熔滴的表面温度可达2 000℃以上,满足C原位脱氧的热力学条件。与不含碳的传统Fe Al涂层中的氧含量随喷涂距离的增加而显著增加的规律完全不同,用Fe/Al/2.5C粉末喷涂时涂层中的氧含量随距离的增加而减小,表明飞行中熔滴的氧化得到抑制,实现了C原位脱氧抑制金属元素氧化的自清洁氧化物的效应。FeAl/...     

Ceramic/fluoropolymer composite coatings by plasma spraying

Al₂O₃-TiO₂/perfluorinated vinylether (PFA) composite coatings are produced by plasma spraying. Because of the different thermo-physical characteristics of both materials, two systems of powder injection were studied: injection of mixed powders and separated injection (co-injection). The effect of the powder injection on the coating microstructure and on the PFA content was investigated using different methods, i.e., micrographic examination, image analysis and energy-dispersive spectrometry (EDS). From micrographic examination, it was observed that a composite structure can be obtained whatever the considered injection system. The coatings consist of a ceramic matrix in which rounded PFA particles are dispersed homogeneously. The content values of the polymer within the coatings evaluated by image analysis show a PFA increase. This trend is not observed for coatings realized by co-injection. On the other hand, powder injection system has no influence on the content of the polymer located at the coating surface. EDS analyses carried out for fluor and aluminium elements prove that PFA particles are homogeneously dispersed at the coating surface. The use of an appropriated thermal post-treatment allows polymer melting and its spreading.     

真空高频感应熔覆Ni60-WC复合涂层的耐蚀性

在不同的WC质量分数和不同的熔融状态下,用真空高频感应熔覆法制得Ni60-WC复合涂层,分析复合涂层的化学成分和组织形貌,研究WC质量分数和熔融状态对高频感应熔覆Ni60-WC复合涂层耐蚀性的影响。结果表明:复合涂层具有优良的耐蚀性能,其中在盐酸(质量分数10%)中最大耐蚀性是Q235-A的100倍左右,在硫酸(质量分数10%)中最大耐蚀性是Q235-A的20倍左右。在同种腐蚀溶液中的复合涂层材料耐蚀性一般随着WC质量分数的增加而降低。刚熔状态比过熔状态耐蚀性能要好,这是由于合金粉末中含有耐腐蚀元素Ni、Cr等,提高了涂层的耐蚀性。但随着熔烧时间的延长,涂层与基体之间产生扩散,涂层中Fe元素质量分数增加,腐蚀元素Ni、Cr的质量分数相对减少,导致涂层的耐蚀性降低。     

An investigation on selective laser melting of Al-Cu-Fe-Cr quasicrystal: From single layer to multilayers

In this study, the effects of processing parameters on the microstructure of Al-Cu-Fe-Cr quasicrystalline (QC) coatings fabricated by selective laser melting (SLM) are investigated. A qualitative analysis on the XRD patterns indicates that the phase composition for the SLM processed coating mainly consisted of Al-Cu-Fe-Cr quasicrystals and α-Al (CuFeCr) solid solution, and with increasing laser energy input or coating thickness, the volume fraction of QC i-Al₉₁Fe₄Cr₅ reduced and those of QC d-Al₆₅Cu₂₀Fe₁₀Cr₅ and crystalline θ-Al₂Cu increased. The formation of cracks during the coating building procedure from single layer to multilayers is also discussed. For the coatings with the same layer number, the pores and balling particles diminish as laser power increases, due to the growth of melting degree. At the early stage of fabrication, with increment of layer number (or coating thickness), pores and balling particles decrease considerably because the molten pool solidified more “slowly”. However, after the layer number increases continuously from 10 to 20, the porosity no longer decreases, and some big size pores, microcracks and fractures appear, especially for the sample obtained at lower laser power. A wavy-like pattern composed chiefly of Al and QC phases, is formed at the interfacial region between substrate and coating due to Marangoni effect.     

A Comparative Microstructural Investigation of Nanostructured and Conventional Al2O3 Coatings Deposited by Plasma Spraying

Nanostructured and conventional Al₂O₃ powders have been plasma sprayed to produce coatings. The parameters for retaining a fraction of the nanostructure were investigated. Dissimilarities were observed between the two types of coating, regarding properties and phase proportions, which are related with the different percentages of semimolten particles in the coatings. The nanocoatings retained a higher percentage of semimolten particles than the conventional coatings owing to the higher porosity of the nanoparticle agglomerates. The molten part of both conventional and nanostructured coatings consisted of γ-Al₂O₃ of columnar morphology. In order to investigate the mechanism of the melting front advance into the particle interior, the particles were sprayed directly into deionized water. The nanoparticles mainly formed hollow spheres, whereas the conventional particles mainly formed compact spheres. The internal porosity of the solidified nanoparticle agglomerates, which affected the overall coating porosity and, consequently, coating properties such as hardness, adhesion, and surface roughness, was linked to the hollow sphere phenomenon.     

表层绝热对定向凝固Ni60合金涂层凝固行为的影响

采用超音速等离子喷涂技术在45钢基体上制备Ni60合金涂层,在高频感应重熔+强制冷却处理条件下,研究了表面散热条件对涂层的组织结构、元素分布和性能的影响。结果表明:在自然散热条件,涂层形成了定向树枝晶结构组织,通过表面覆盖层的保护,改变表面散热条件,使涂层组织向具有熔结特征的等轴晶转变,并且形成了两个明显不同组织的区域,在靠近基体区域,组织细小致密,但组织和元素分布不均匀;而靠近涂层表面区域,组织具有长大现象,但元素和组织均匀性明显提高。相对于未加覆盖层涂层,附加覆盖层的涂层元素和基体元素在界面处具有更充分的扩散,基体元素向涂层中的扩散较涂层元素向基体扩散更加强烈。覆盖层的附加明显提高了涂层硬度,使涂层表面的宏观硬度由33.4 HRC提高到54.4 HRC,涂层截面的显微硬度由566.1 HV0.2提高到659.2 HV0.2。     

Microstructural Characteristics of Y2O3-MgO Composite Coatings Deposited by Suspension Plasma Spray

Dense composite Y₂O₃-MgO coatings have been deposited by suspension plasma spray. Ethanol-based suspensions of powders synthesized by thermal decomposition of precursor solutions containing yttrium nitrate (Y[n]) and magnesium nitrate (Mg[n]) or magnesium acetate (Mg[a]) were selected as the feedstock; this gave powders with both phases in each particle, to inhibit phase segregation during solvent evaporation. The influence of powder characteristics on the microstructures of the coatings was investigated. The Y[n]Mg[a] suspension was more stable, with a better dispersion of the component phases than the Y[n]Mg[n] suspension. The coatings deposited using each suspension type exhibited lamellar structures comprising Y₂O₃ and MgO phases in wavy alternating streaks, with unmelted/semi-melted particles entrapped in the lamellae; this indicates that phase segregation still occurred in the molten state. Eutectic structures were formed in the coating generated using the Y[n]Mg[a] suspension, resulting from improved mixing of the component phases in the suspension powder.     

Study of phase changes in plasma sprayed deposits

The formation of a plasma-sprayed coating that exhibits predictable properties requires the control of many process variables. The phase changes that take place during plasma spraying are significant material variables that should be controlled. Several different materials were deposited in air with a water-stabilized plasma torch (model PAL 160). Usually, air was used as a carrier gas for the powder; however, argon was also used for some coatings. The injected powders (NiAl, Ni, ZrSiO₄-based, Al₂O₃-based, etc.) as well as the coatings were studied for, among other properties, their structure, particle size, microhardness, and chemical and phase composition. Phase changes induced by the different cooling rates of molten particles after their impact on a substrate are illustrated for ZrSiO₄. It has also been found that the oxidizing power of the water-stabilized torch is less than previously believed. For example, coatings produced with nickel powder injected with argon as the carrier gas exhibited almost no oxides. Significant element redistribution during plasma spraying was demonstrated with a two-phase NiAl feedstock powder. The coating exhibited almost all the phases that are present in the binary NiAl alloy as well as envelopes of oxides and traces of amorphous phase.     

TiB2-SiC粉末喷雾造粒及其等离子喷涂沉积机理研究

利用喷雾干燥对TiB_2-SiC复合粉末进行造粒,研究了浆料固含量、粘结剂含量及SiC含量对喷雾干燥粉体颗粒形貌等的影响。采用大气等离子喷涂技术,以抛光的石墨为基体,在不同预热温度和不同喷距下对TiB_2-SiC粉末进行粒子收集,研究不同工艺参数对TiB_2-SiC粒子铺展形貌的影响,并制备了TiB_2-SiC涂层。结果表明:当浆料固含量为50%,粘结剂含量为5%,SiC含量为10%时,喷雾造粒获得球形度高、流动性好的TiB_2-SiC粉末;随着基体预热温度的升高,喷距的增大,扁平粒子的溅射逐渐减弱,形成规则的圆盘状粒子;在等离子焰流作用下,TiB_2-SiC粒子熔化加速并与基体发生碰撞,熔融粒子扁平化,急速冷却凝固,不断堆叠、搭接为宏观涂层。     

Annealing effects on the intermetallic compound formation of cold sprayed Ni,Al coatings

The annealing of Ni and Al coatings under various conditions on substrates fabricated by a cold gas dynamic spray process (CDSP) were investigated. The powder particles were accelerated through a standard De Laval-type nozzle with air used as the main carrying gas. The coatings were annealed at 450–550 °C in either argon or air atmospheres for 4 h. In the case of Ni coatings during annealing both in argon and air atmospheres, intermetallic compound layers such as Al₃Ni and Al₃Ni₂ were observed at the interfaces between the Ni coating and Al substrate. Also, the intermetallic layer formation of Al₃Ni and Al₃Ni₂ at the interfaces depended on the solid-state diffusion and the annealing temperature. The intermetallic compound AlNi was obtained at the interface of Al coating on a Ni substrate by low-temperature annealing under the melting temperature.     

Plasma Spray Coatings of Ni-Al-SiC Composite

Ni-Al-SiC powder mixture containing 12 wt.% SiC was prepared by conventional ball milling. Morphological and microstructural investigations showed that powder particles after 15 h of milling time had the optimum characteristics with respect to their size and microstructure. X-ray diffraction patterns of powder particles included only the elemental Ni, Al, and SiC peaks without any traces of oxides or intermetallic phases. The powder mixture was then deposited onto a steel substrate by atmospheric plasma spray (APS) process under different conditions. The results showed that under APS conditions used here, the coatings were composed of various intermetallics including Ni-Al and Ni₂Al₃. The mean hardness of coating was found to be about 567 HV. It was also found that by increasing current density of APS, the coating/substrate adhesive strength was increased.     

Carbon nanotube synthesis using magnetic fluids on various substrates

A new carbon nanotube synthesis method using a magnetic fluid is developed. The catalyst particles can be formed by simple spin coating of the magnetic fluid of Fe₃O₄ nano particles mixed with polyvinyl alcohol on substrates and subsequent heat treatment. A quantitative analysis of the catalyst particles distribution on Si substrate is carried out. The mixing of the magnetic fluid in polyvinyl alcohol gives viscosity to the solution, secures uniform particle distribution without agglomeration, and controls the catalyst particle density on the substrate. The distribution of the catalyst particles is mainly controlled by the NH₄OH concentration in the magnetic fluid solutions. Carbon nanotubes grown on various substrates of Si, alumina, and various metal plates revealed different morphologies due to the difference in the wetting of the catalyst particles on the substrate in the various particle-substrate systems.     

Influence of Impact Conditions on Feedstock Deposition Behavior of Cold-Sprayed Fe-Based Metallic Glass

Cold spray is a promising method by which to deposit dense Fe-based metallic glass coatings on conventional metal substrates. Relatively low process temperatures offer the potential to prevent the crystallization of amorphous feedstock powders while still providing adequate particle softening for bonding and coating formation. In this study, Fe₄₈Mo₁₄Cr₁₅Y₂C₁₅B₆ powder was sprayed onto a mild steel substrate, using a variety of process conditions, to investigate the feasibility of forming well-bonded amorphous Fe-based coatings. Particle splat adhesion was examined relative to impact conditions, and the limiting values of temperature and velocity associated with successful softening and adhesion were empirically established. Variability of particle sizes, impact temperatures, and impact velocities resulted in splat morphologies ranging from well-adhered deformed particles to substrate craters formed by rebounded particles and a variety of particle/substrate interface conditions. Transmission electron microscopy studies revealed the presence of a thin oxide layer between well-adhered particles and the substrate, suggesting that bonding is feasible even with an increased oxygen content at the interface. Results indicate that the proper optimization of cold spray process parameters supports the formation of Fe-based metallic glass coatings that successfully retain their amorphous structure, as well as the superior corrosion and wear-resistant properties of the feedstock powder.