Study of the Splat Microstructure and the Effects of Substrate Heating on the Splat Formation for Ni-Cr Particles Plasma Sprayed onto Stainless Steel Substrates

The plasma spraying process is still poorly understood in term of the processes by which the coating is built up, especially coating interactions with the substrate. This present study enhances this understanding by studying, through a range of electron microscopy techniques, single NiCr splats plasma sprayed onto stainless steel substrates, which were first exposed to different heat treatments. The microstructure of the splats, particularly the splat-substrate interface, was characterized, and the formation of the observed features is discussed. Evidence of localized substrate melting and inter-mixing with the splat material was found, showing metallurgical bonding. The structures observed were also correlated to the treatment of the substrate, demonstrating how such treatments can influence the properties of the fully deposited coating by modifying the splat formation process. Most notably, heating the substrate during spraying was found to significantly modify splat formation by reducing splashing and increasing the extent of substrate melting.     

Splat morphology and microstructure of plasma sprayed cast iron with different preheat substrate temperatures

A cast iron coating is a prime candidate for the surface modification of aluminum alloys for antiwear applications because cast iron is inexpensive and exhibits superior wear resistance arising from the self-lubricating properties of graphite. In the present study, fundamental aspects of a plasma sprayed cast iron coating on an aluminum alloy substrate, including (1) the effects of preheat substrate temperature on the splat morphology, (2) the formation of a reaction layer and pores, and (3) the splat microstructure, were investigated in low-pressure plasma spraying. With an increasing substrate temperature, the splat morphology changes from a splash type to a disk and star shape. Deformed substrate ridges mainly resulting from the slight surface melting, are recognized adjacent to the splat periphery at high substrate temperatures. The flattening ratio of disk splats decreases with substrate temperature because the ridges act as an obstacle for splat expansion. A reaction layer composed of iron, aluminum, and oxygen is ready to form at high substrate temperatures, which, along with the deformed ridges, improves the adhesive strength of splats. However, the pores appear at the splat interface at low substrate temperatures, which hinder the formation of a reaction layer. The amount of graphitized carbon increases in cast iron splats with an increase in substrate temperature.     

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.     

Microtomographic Analysis of Splat Formation and Layer Build-Up of a Thermally Sprayed Coating

Thermal spraying is a material processing technique, which is based on the combination of thermal and kinetic energy. The used feedstock is melted in a hot flame. The melt is atomized and accelerated by means of atomization or process gases. As the formed particles hit a pre-treated substrate they are rapidly solidified and consolidate to form splats. The splats pile one-on-top-of-other forming lamellas creating the final coating. In the work presented here a combination of cored wire (WC as filling powder) and massive wire (copper) were simultaneously sprayed using the twin wire arc spraying process. 3D micro tomography was used in order to gain knowledge about splat formation and layer build-up. Due to the high attenuation coefficient of tungsten in comparison with copper and carbon, tungsten-rich particles and splats can easily be spotted in the tomogram of the coating layer. It turns out that besides irregular formed flat splats also ball-shaped particles exist in the coating layer which suggests that the spherical particles impacted on the substrate in an un-molten state. By 3D data processing tungsten-rich particles were visualized to analyze their spatial distributions and to quantify their geometric parameters. This work aims at contributing to the understanding of spraying processes.     

Process map for plasma sprayed aluminum oxide-carbon nanotube nanocomposite coatings

Process map has been developed for plasma sprayed aluminum oxide (Al₂O₃) ceramic nanocomposite coatings with carbon nanotube (CNT) reinforcement in varying content and spatial distribution. The process map was constructed using the temperature and velocity data of the in-flight powder particles exiting from the plasma plume. Process map elucidates the interdependence of powder feedstock pre-treatment, CNT content and dispersion behavior on the in-flight particle thermal history and subsequently evolving microstructure and coating properties. High thermal conductivity of CNTs alters the heat transfer characteristic during the splat formation. Microstructure of the coatings consists of fully melted zone (FM), partially melted or solid-state sintered zone (PM) and porosity. Process map provides a processing control tool for plasma spraying of Al₂O₃-CNT nanocomposite coatings.     

Formation of Splats from Suspension Particles with Solid Inclusions Finely Dispersed in a Melted Metal Matrix

A theoretical model has been developed to describe the splats formation from composite particles of several tens of micrometers in size whose liquid metal binder contains a high volume concentration of ultra-fine refractory solid inclusions uniformly distributed in the binder. A theoretical solution was derived, enabling evaluation of splat thickness and diameter, and also the contact temperature at the particle-substrate interface, under complete control of key physical parameters (KPPs) of the spray process (impact velocity, temperature, and size of the particle, and substrate temperature) versus the concentration of solid inclusions suspended in the metal-binder melt. Using the solution obtained, the calculations performed demonstrate the possibility of formulating adequate requirements on the KPPs of particle-substrate interaction providing a deposition of ceramic-metal coatings with predictable splat thickness and degree of particle flattening on the substrate, and also with desired contact temperature during the formation of the first coating monolayer.     

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.     

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。     

Process Conditions and Microstructures of Ceramic Coatings by Gas Phase Deposition Based on Plasma Spraying

Plasma spraying at very low pressure (50-200 Pa) is significantly different from atmospheric plasma conditions (APS). By applying powder feedstock, it is possible to fragment the particles into very small clusters or even to evaporate the material. As a consequence, the deposition mechanisms and the resulting coating microstructures could be quite different compared to conventional APS liquid splat deposition. Thin and dense ceramic coatings as well as columnar-structured strain-tolerant coatings with low thermal conductivity can be achieved offering new possibilities for application in energy systems. To exploit the potential of such a gas phase deposition from plasma spray-based processes, the deposition mechanisms and their dependency on process conditions must be better understood. Thus, plasma conditions were investigated by optical emission spectroscopy. Coating experiments were performed, partially at extreme conditions. Based on the observed microstructures, a phenomenological model is developed to identify basic growth mechanisms.     

Flattening of droplets and formation of splats in thermal spraying: A review of recent work—Part 1

Properties of the coatings developed during thermal spraying are essentially determined by rapid solidification of splats formed as a result of impingement of the melted powder particles onto a substrate surface. The processes of flattening droplets and formation of splats in thermal spraying have been studied intensively during the last two decades. The last review on this topic was published at the end of 1994. Since then many papers have been dedicated to investigating splat formation, taking into account such important issues as roughness of the substrate surface, wetting phenomena, and splashing. This review, consisting of two parts, includes the main results obtained since 1994 and examines the influence of solidification of the lower part of the splat, substrate roughness, wetting at the substrate-coating interface, substrate deformation, oxidation, and splashing on the dynamics of flattening of droplets and the formation of splats. Flattening of composite powder particles, splat-substrate interaction, and development of splat-substrate adhesion and splat porosity are discussed. Part 1 of the review covers the following issues, which significantly influence the droplet flattening and splat formation: droplet solidification during flattening and roughness of the substrate surface, composite morphology of the powder particles, and oxidation processes. The results provide a better understanding of the thermal spray processes to increase their efficiency.     

Splat Morphology of Yttria-Stabilized Zirconia Droplet Deposited Via Hybrid Plasma Spraying

The splat morphology of yttria-stabilized zirconia (YSZ) droplets deposited by dc-rf hybrid plasma spraying (HYPS) was studied. Two types of YSZ powder were used, namely fused and crushed powder (FC) and hollow spherical powder (HOSP). The three-dimensional shape of more than 600 disk-shaped splats on preheated substrates was evaluated using a laser-scanning microscope to determine the splat diameter, thickness, and their dimensionless forms. The HOSP showed a higher degree of flattening than the FC. Both the FC with a powder size distribution of 45-75 μm and the HOSP of 30-120 μm can be used as spray materials in the HYPS to achieve a coating design based on fully molten particles. The effect of the substrate temperature on the splat morphology was similar to that of atmospheric dc plasma spraying; however, the transition from a splashed shape to a disk shape gradually occurred at higher substrate temperatures.     

Suspension Plasma-Sprayed Alumina Coating Structures: Operating Parameters Versus Coating Architecture

Suspension plasma spraying (SPS) is able to process sub-micrometric-sized feedstock particles and permits the deposition of layers thinner (from 5 to 50 μm) than those resulting from conventional atmospheric plasma spraying (APS). SPS consists in mechanically injecting within the plasma flow a liquid suspension of particles of average diameter varying between 0.02 and 1 μm, average values. Upon penetration within the DC plasma jet, two phenomena occur sequentially: droplet fragmentation and evaporation. Particles are then processed by the plasma flow prior their impact, spreading and solidification upon the surface to be covered. Depending upon the selection of operating parameters, among which plasma power parameters (operating mode, enthalpy, spray distance, etc.), suspension properties (particle size distribution, powder mass percentage, viscosity, etc.), and substrate characteristics (topology, temperature, etc.), different coating architectures can be manufactured, from dense to porous layers. Nevertheless, the coupling between the parameters controlling the coating microstructure and properties are not yet fully identified. The aim of this study is to further understand the influence of parameters controlling the manufacturing mechanisms of SPS alumina coatings, particularly the spray beads influence.     

Porous Architecture of SPS Thick YSZ Coatings Structured at the Nanometer Scale (~50 nm)

Suspension plasma spraying (SPS) is a fairly recent technology that is able to process sub-micrometer-sized or nanometer-sized feedstock particles and permits the deposition of coatings thinner (from 20 to 100 μm) than those resulting from conventional atmospheric plasma spraying (APS). SPS consists of mechanically injecting within the plasma flow a liquid suspension of particles of average diameter varying between 0.02 and 1 μm. Due to the large volume fraction of the internal interfaces and reduced size of stacking defects, thick nanometer- or sub-micrometer-sized coatings exhibit better properties than conventional micrometer-sized ones (e.g., higher coefficients of thermal expansion, lower thermal diffusivity, higher hardness and toughness, better wear resistance, among other coating characteristics and functional properties). They could hence offer pertinent solutions to numerous emerging applications, particularly for energy production, energy saving, etc. Coatings structured at the nanometer scale exhibit nanometer-sized voids. Depending upon the selection of operating parameters, among which plasma power parameters (operating mode, enthalpy, spray distance, etc.), suspension properties (particle size distribution, powder mass percentage, viscosity, etc.), and substrate characteristics (topology, temperature, etc.), different coating architectures can be manufactured, from dense to porous layers, from connected to non-connected network. Nevertheless, the discrimination of porosity in different classes of criteria such as size, shape, orientation, specific surface area, etc., is essential to describe the coating architecture. Moreover, the primary steps of the coating manufacturing process affect significantly the coating porous architecture. These steps need to be further understood. Different types of imaging experiments were performed to understand, describe and quantify the pore level of thick finely structured ceramics coatings.     

Corrosion behavior of HVOF-sprayed and Nd-YAG laser-remelted high-chromium,nickel-chromium coatings

Thermal spray processes are widely used to deposit high-chromium, nickel-chromium coatings to improve high temperature oxidation and corrosion behavior. However, despite the efforts made to improve the present spraying techniques, such as high-velocity oxyfuel (HVOF) and plasma spraying, these coatings may still exhibit certain defects, such as unmelted particles, oxide layers at splat boundaries, porosity, and cracks, which are detrimental to corrosion performance in severe operating conditions. Because of the process temperature, only mechanical bonding is obtained between the coating and substrate. Laser remelting of the sprayed coatings was studied in order to overcome the drawbacks of sprayed structures and to markedly improve the coating properties. The coating material was high-chromium, nickel-chromium alloy, which contains small amounts of molybdenum and boron (53.3% Cr, 42.5% Ni, 2.5% Mo, 0.5% B). The coatings were prepared by HVOF spraying onto mild steel substrates. A high-power, fiber-coupled, continuous-wave Nd:YAG laser equipped with large beam optics was used to remelt the HVOF-sprayed coating using different levels of scanning speed and beam width (10 or 20 mm). Coating that was remelted with the highest traverse speed suffered from cracking because of the rapid solidification inherent to laser processing. However, after the appropriate laser parameters were chosen, nonporous, crack-free coatings with minimal dilution between coating and substrate were produced. Laser remelting resulted in the formation of a dense oxide layer on top of the coatings and full homogenization of the sprayed structure. The coatings as sprayed and after laser remelting were characterized by optical and electron microscopy (OM, SEM, respectively). Dilution between coating and substrate was studied with energy dispersive spectrometry (EDS). The properties of the laser-remelted coatings were directly compared with properties of as-sprayed HVOF coatings.     

Comparison Between High-Velocity Suspension Flame Spraying and Suspension Plasma Spraying of Alumina

Two different spray processes??suspension plasma spraying (SPS) and high-velocity suspension flame spraying (HVSFS)??are under focus in the field of suspension spraying. Both techniques are suitable for manufacturing finely structured coatings. The differences in the particle velocity and temperature of these two processes cause varying coating characteristics. The high particle velocity of the HVSFS process leads to more dense coatings with low porosity values. Coatings with a higher and also homogeneous porosity, which can be generated by SPS, have also high potential, for example, for thermal barrier coatings. In this study, both the processes??SPS and HVSFS??were compared using alumina as feedstock material mixed with different solvents. Besides the characterization of the microstructure and phase composition of the applied coatings, the focus of this study was the investigation of the melting behavior of the particles in-flight and of single splat characteristics.     

Operating parameters for suspension and solution plasma-spray coatings

The interest to manufacture on large surfaces thick (i.e., 10 to 20 μm, average thickness) finely structured or nano-structured layers is increasingly growing since about 10 years. This explains the interest for suspension plasma spraying (SPS) and solution precursor plasma spraying (SPPS), both allowing manufacturing finely structured layers of thicknesses varying between a few micrometers up to a few hundred of micrometers. SPS aims at processing a suspension of sub-micrometric-sized or even nano-metric-sized solid particles dispersed in a solvent. The liquid solvent permits to inject particles in the thermal flow (i.e., due to their size, a carrier gas cannot play this role). SPTS aims at processing a solution of precursors under the same conditions. Upon evaporation of the liquid, the precursor concentration increases until precipitation, pyrolysis and melting of small droplets. Compared to conventional plasma spraying, SPS and SPPS are by far more complex because fragmentation and vaporization of the liquid control the coating build-up mechanisms. Numerous studies are still necessary to reach a better understanding of involved phenomena and to further develop the technology, among which injection systems, suspension and solution optimizations, spray kinematics, etc. This review presents some recent developments in this field.     

Microstructure and Properties of Cu Coating Fabricated onto Diamond-Cu Substrate by Low-Temperature HVOF Process

Diamond-Cu composites have been considered to be the next generation of electronic packing materials. One of the key stumbles for such an application is the joining problem between diamond-Cu composites and other materials due to the poor wettability of the diamond particles in the composites. In order to overcome this hurdle, pure Cu powder was thermally sprayed onto diamond-Cu substrate by low-temperature high-velocity oxygen fuel spraying process. Microstructure and some fundamental properties of the coating obtained were systematically investigated, and morphologies of the single splat deposited on the diamond-Cu substrate were also observed. The splats obtained have good adhesion with the substrate as fine particles flattened sufficiently, while the coarse particles were significantly deformed. The coating was quite dense with porosity lower than 1%, oxygen content under 0.5% and thermal conductivity about 266 Wm^?1 K^?1 and still remained on the diamond-Cu substrate after 50 thermal shock cycles between 300 °C and water bath at room temperature. Meanwhile, the solderability of the coating was significantly improved. Therefore, Cu coating deposited on diamond-Cu substrate by low-temperature high-velocity oxygen fuel spraying process can be beneficial in electronic industry assisting with soldering and improved wettability for joining of other materials.     

Influence of surface laser cleaning combined with substrate preheating on the splat morphology

The morphology of sprayed splats influences the coating adhesion and properties, which are determined by the spraying parameters. Many studies in this field show that the substrate surface temperature is a very relevant factor for the splat shape: the hypotheses of substrate surface wettability and contamination or adsorption layer on the surfaces are supported by the fact that the near-disk-shaped splat can be obtained by increasing the substrate temperature. In this work, a short-duration pulsed laser was used to ablate the substrate just before powder spraying. This ablation was powerful enough to eliminate the contaminants on the substrate surface and to improve the adhesion. In this study the analyses of NiAl splat morphology on the polished TA6V (Ti-6Al-4V) substrate were carried out using laser ablation with different substrate temperatures and different heating modes: the flame and another laser. Results show that the temperature at which the disk-shaped splat can be obtained decreased dramatically by laser ablation. Moreover, laser ablation combined with another laser increased the adhesion strength of the coatings. The original version of this article was published as part of the ASM Proceedings, Thermal Spray 2003: Advancing the Science and Applying the Technology, International Thermal Spray Conference (Orlando, FL), May 5–8, 2003, Basil R. Marple and Christian Moreau, Ed., ASM International, 2003.     

Epitaxial Grain Growth During Splat Cooling of Alumina Droplets Produced by Atmospheric Plasma Spraying

Alumina splats were deposited on polished single crystal alumina substrates with two different crystalline facet orientations of [001] and [110] by atmospheric plasma spraying at a substrate preheating temperature of 900 °C to examine the epitaxy during splat cooling. The cross-sectional samples for high-resolution transmission electron microscopy examination were prepared by focused ion beam-assisted-scanning electron microscopy (FIB-SEM). The results show that the whole splats with a thickness ranging from ~600 to ~1000 nm exhibit the same crystalline structure as the substrate. Moreover, the splat deposited on the single crystalline alumina substrates exhibits exactly the same orientation as the substrate. The results evidently indicate that the epitaxial grain growth occurs after alumina droplets impact on the single crystal alumina substrate. The present results suggest that the crystalline structure of alumina deposit formed by plasma spraying can be possibly controlled by the coating surface temperature.     

液相等离子喷涂制备Au-WO3复合涂层及其气敏机理

目的提高WO_3基涂层的气敏性能。方法以WCl_6为前驱体原料,加入一定量的纳米Au颗粒制成稳定的喷涂浆料,采用液相等离子体喷涂技术制备出Au掺杂的WO_3基复合涂层。通过扫描电子显微镜(SEM)及其附带的能谱仪、X射线衍射仪(XRD)等对Au-WO_3复合涂层的微观结构进行表征。通过自主搭建的气敏性能测试系统对所制备Au-WO_3复合涂层的气敏性能进行测试,并探讨了涂层的气敏机理。结果前驱体液滴在等离子体热源作用下发生溶剂蒸发、WO_3形核、结晶和长大等一系列反应,随后形成的WO_3固体粒子发生熔化或半熔化,并加速撞击到基体表面形成涂层。在同等条件下,喷涂距离对WO_3气敏涂层的结晶度和形貌有很大影响,适宜的喷涂距离(170 mm)下获得的涂层结晶完整且晶粒细小(20～50 nm),有利于涂层气敏性能的发挥。Au-WO_3复合涂层的气敏性能均显著优于纯WO_3涂层。结论复合涂层气敏性能的改善归因于涂层中Au和WO_3界面处所形成的肖特基结使复合涂层的导电性降低,接触势垒高度增加,初始电阻值变大。