Simulation of Effervescent Atomization and Nanoparticle Characteristics in Radio Frequency Suspension Plasma Spray

In this paper, a comprehensive model was developed to investigate the suspension spray for a radio frequency (RF) plasma torch coupled with an effervescent atomizer. Firstly, the RF plasma is simulated by solving the thermo-fluid transport equations with electromagnetic Maxwell equation. Secondly, primary atomization of the suspension is solved by a proposed one-dimensional breakup model and validated with the experimental data. Thirdly, the suspension droplets and discharged nanoparticles are modeled in Lagrangian manner, to calculate each particle tracking, acceleration, heating, melting and evaporation. Saffman lift force, Brownian force and non-continuum effect are considered for nanoparticle momentum transfer, as well as the effects of evaporation on heat transfer. This model predicts the nanoparticle trajectory, velocity, temperature and size in the RF suspension plasma spray. Effects of the torch and atomizer operating conditions on the particle characteristics are investigated. Such operating conditions include gas-to-liquid flow ratio, atomizer orifice diameter, injection pressure, power input level, plasmas gas flow rate, and powder material. The statistical distributions for the multiple particles are also discussed for different cases.     

Influence of process parameters on the deposition footprint in plasma-spray coating

This paper presents an investigation of the influence of plasma spray process conditions on the in-flight particle behavior and their cumulative deposition to form a coating on the substrate. Three-dimensional computational fluid dynamics (CFD) analyses were performed to model the in-flight particle behavior in the plasma-spray process and their deposition on the substrate. The plasma spray was modeled as a jet issuing from the torch nozzle through the electrical heating of the arc gas. In the model, particles were injected into the plasma jet where they acquired heat and momentum from the plasma, some got melted and droplets were formed. By means of a droplet splatting model, the particle in-flight data generated by the CFD analyses were further processed to build up an imaginary three-dimensional deposition profile on a flat stationary substrate. It is found that the powder carrier gas flow rate influences the particle distribution on the substrate by imparting an injection momentum to the particles that were directed radially into the plasma jet in a direction perpendicular to the plasma jet. The larger sized particles will acquire higher injection momentum compared with the smaller sized particles. This causes particle distribution at the substrate surface that is elliptical in shape with the major axis of ellipse parallel to the particle injection port axis as illustrated in Fig. 1. Larger particles tend to congregate at the lower part of the ellipse, due to their greater momentum. The distribution of particle size, temperature, velocity, and count distribution at the substrate was analyzed. Further, based on the size and the computed particle temperature, velocity histories, and the impact sites on the substrate, the data were processed to build up a deposition profile with the Pasandideh-Fard model. The shapes of deposition profiles were found to be strongly driven by the segregation effect.     

Three-Dimensional Modeling of Suspension Plasma Spraying with Arc Voltage Fluctuations

In this study, a three-dimensional DC plasma torch is modeled using Joule effect method to simulate the plasma jet and its voltage fluctuations. The plasma gas is a mixture of argon/hydrogen, and the arc voltage fluctuation is used as an input data in the model. Reynolds stress model is used for time-dependent simulation of the oscillating flow of the plasma gas interacting with the ambient air. The results are used to investigate the plasma oscillation effects on the trajectory, temperature, and velocity of suspension droplets. Suspensions are formed of ethanol and yttria-stabilized zirconia submicron particles and modeled as multicomponent droplets. To track the droplets/particles, a two-way coupled Eulerian–Lagrangian method is employed. In addition, in order to simulate the droplet breakup, Kelvin–Helmholtz/Rayleigh–Taylor (KH–RT) breakup model is used. After the completion of suspension breakup and evaporation, the sprayed particles are tracked to obtain the in-flight particle conditions including trajectory, size, velocity, and temperature. The arc voltage fluctuations were found to cause more than two times wider particle trajectories resulting in wider particle temperature, velocity, and size distributions compared with the case of constant voltage.     

Parametric Study on Suspension Behavior in an Inductively Coupled Plasma

Numerical analysis is conducted for the evolution of suspension particles in an inductively coupled plasma (ICP). The mathematical model based on the Lagrangian tracking method incorporates a nanoparticle model into the ICP code. This comprehensive model considers entire physical phenomena of the in-flight particle such as injection, accelerating, solvent evaporation, solid particle discharge, heating, melting, and evaporation. After validating the computational results of the flow field with published experimental data, parametric analysis has been performed to find the way of controlling the operating conditions for desirable final particle status. The influences of injection position, carrier gas velocity, power level, particle initial size on particle size, temperature, and velocity evolution have been in detail discussed. The relationship between the predicted height of droplet complete evaporation and the droplet initial diameter is deduced. Finally, results also calculate the critical size of an ethanol droplet suspended with zirconia particles, which will be completely vaporized under present conditions.     

气体放电热源喷涂技术的数值模拟研究现状∗

热喷涂技术是表面工程领域中极为重要的一种装备强化修复技术，其中以气体放电形式为热源的喷涂技术包括等离子喷涂和电弧喷涂，两者更是占据热喷涂领域的绝大市场份额，采用数值模拟可以解决一些在试验上较为棘手的重点研究问题， 如等离子体流场和熔滴传热传质行为等，以期实现工艺参数的准确调控和优异涂层的制备。研究电弧及等离子喷涂模拟的模型差异化问题及流场速度、温度、电磁性质，归纳相关模拟的发展历程，并调查试验与模拟的吻合程度。结果表明：电弧喷涂中丝材原料会使阴阳极产生温度差，水平速度分布较发散，熔滴模型也多未考虑熔滴群间相互作用；等离子喷涂研究中常用的三维瞬态双温模型已十分贴近实际工况，对熔滴飞行中的加热、加速过程及破碎行为的研究已较为完备，但仍存在湍流模型计算精度不够、对鞘层弧柱区的研究不够深入等问题。后续应重点在电弧喷涂多液滴模型、等离子体电磁作用和等离子丝材喷涂工艺的数值模拟等方面进行深入研究。     

Numerical Study of Suspension Plasma Spraying

A numerical study of suspension plasma spraying is presented in the current work. The liquid suspension jet is replaced with a train of droplets containing the suspension particles injected into the plasma flow. Atomization, evaporation, and melting of different components are considered for droplets and particles as they travel toward the substrate. Effect of different parameters on particle conditions during flight and upon impact on the substrate is investigated. Initially, influence of the torch operating conditions such as inlet flow rate and power is studied. Additionally, effect of injector parameters like injection location, flow rate, and angle is examined. The model used in the current study takes high-temperature gradients and non-continuum effects into account. Moreover, the important effect of change in physical properties of suspension droplets as a result of evaporation is included in the model. These mainly include variations in heat transfer properties and viscosity. Utilizing this improved model, several test cases have been considered to better evaluate the effect of different parameters on the quality of particles during flight and upon impact on the substrate.     

Evaporation of Droplets in Plasma Spray–Physical Vapor Deposition Based on Energy Compensation Between Self-Cooling and Plasma Heat Transfer

In the plasma spray–physical vapor deposition process (PS-PVD), there is no obvious heating to the feedstock powders due to the free molecular flow condition of the open plasma jet. However, this is in contrast to recent experiments in which the molten droplets are transformed into vapor atoms in the open plasma jet. In this work, to better understand the heating process of feedstock powders in the open plasma jet of PS-PVD, an evaporation model of molten ZrO₂ is established by examining the heat and mass transfer process of molten ZrO₂. The results reveal that the heat flux in PS-PVD open plasma jet (about 10⁶ W/m²) is smaller than that in the plasma torch nozzle (about 10⁸ W/m²). However, the flying distance of molten ZrO₂ in the open plasma jet is much longer than that in the plasma torch nozzle, so the heating in the open plasma jet cannot be ignored. The results of the evaporation model show that the molten ZrO₂ can be partly evaporated by self-cooling, whereas the molten ZrO₂ with a diameter <0.28 μm and an initial temperature of 3247 K can be completely evaporated within the axial distance of 450 mm by heat transfer.     

In-flight characteristics of plasma sprayed alumina particles: Measurements,modeling, and comparison

The key phenomena controlling the properties of sprayed coatings are the heat and momentum transfer between the plasma jet and the injected particles. Modern on-line particle monitoring systems provide an efficient tool to measure in-flight particle characteristics in such a way that factors that could affect the coating quality can be identified during the spray process. In this work, the optical sensing device, DPV-2000 from Tecnar, was used for monitoring the velocity, temperature, and diameter of in-flight particles during the spraying of alumina with a Sulzer-Metco F4 plasma torch. Evolution of particle velocity, temperature, diameter, and trajectory showed well-marked trends. Relationships between the position of the in-flight particles into the jet and their characteristics were pointed out, thus delivering valuable information about their thermal treatment. Moreover, a numerical model was developed and predictions were compared with experimental results. A good agreement on particle characteristics was found between the two different approaches.     

Sensors in Spray Processes

This paper presents what is our actual knowledge about sensors, used in the harsh environment of spray booths, to improve the reproducibility and reliability of coatings sprayed with hot or cold gases. First are described, with their limitations and precisions, the different sensors following the in-flight hot particle parameters (trajectories, temperatures, velocities, sizes, and shapes). A few comments are also made about techniques, still under developments in laboratories, to improve our understanding of coating formation such as plasma jet temperature measurements in non-symmetrical conditions, hot gases heat flux, particles flattening and splats formation, particles evaporation. Then are described the illumination techniques by laser flash of either cold particles (those injected in hot gases, or in cold spray gun) or liquid injected into hot gases (suspensions or solutions). The possibilities they open to determine the flux and velocities of cold particles or visualize liquid penetration in the core of hot gases are discussed. Afterwards are presented sensors to follow, when spraying hot particles, substrate and coating temperature evolution, and the stress development within coatings during the spray process as well as the coating thickness. The different uses of these sensors are then described with successively: (i) Measurements limited to particle trajectories, velocities, temperatures, and sizes in different spray conditions: plasma (including transient conditions due to arc root fluctuations in d.c. plasma jets), HVOF, wire arc, cold spray. Afterwards are discussed how such sensor data can be used to achieve a better understanding of the different spray processes, compare experiments to calculations and improve the reproducibility and reliability of the spray conditions. (ii) Coatings monitoring through in-flight measurements coupled with those devoted to coatings formation. This is achieved by either maintaining at their set point both in-flight and certain spray parameters (spray pattern, coating temperature…), or defining a good working area through factorial design, or using artificial intelligence based on artificial neural network (ANN) to predict particle in-flight characteristics and coating structural attributes from the knowledge of processing parameters.     

An Insight into Suspension Plasma Spray: Injection of the Suspension and Its Interaction with the Plasma Flow

Yttria Stabilized Zirconia (YSZ) suspensions were injected in an atmospheric plasma jet using two designs of a home-made two-fluid atomizing nozzle. The sprays of drops were visualized and the behavior of the suspension in the plasma jet was investigated by implementing the Particle Image Velocimetry (PIV) method. The effects of the suspension formulation (surface tension, liquid viscosity, and relative gas-to-liquid mass ratio, GLR) on the distribution and median value of the drop size as well as on the velocity maximum value were evaluated. The interactions between the sprays and the plasma jet were studied. The differences in the behavior of the particle velocity along and radial to the torch axis were pointed out. The validity of PIV measurements was finally demonstrated by the relation established between the in-flight particle velocity and the coating structure.     

Engineering the Microstructure of Solution Precursor Plasma-Sprayed Coatings

This study examines the fundamental reactions that occur in-flight during the solution precursor plasma spraying (SPPS) of solutions containing Zr- and Y-based salts in water or ethanol solvent. The effect of plasma jet composition (pure Ar, Ar-H₂ and Ar-He-H₂ mixtures) on the mechanical break-up and thermal treatment of the solution, mechanically injected in the form of a liquid stream, was investigated. Observation of the size evolution of the solution droplets in the plasma flow by means of a laser shadowgraphy technique, showed that droplet break-up was more effective and solvent evaporation was faster when the ethanol-based solution was injected into binary or ternary plasma gas mixtures. In contrast with water-based solutions, residual liquid droplets were always detected at the substrate location. The morphology and structure of the material deposited onto stainless steel substrates during single-scan experiments were characterised by SEM, XRD and micro-Raman spectroscopy and were shown to be closely related to in-flight droplet behaviour. In-flight pyrolysis and melting of the precursor led to well-flattened splats, whereas residual liquid droplets at the substrate location turned into non pyrolysed inclusions. The latter, although subsequently pyrolysed by the plasma heat during the deposition of entire coatings, resulted in porous ??sponge-like?? structures in the deposit.     

Numerical Analysis of Multicomponent Suspension Droplets in High-Velocity Flame Spray Process

The liquid feedstock or suspension as a different mixture of liquid fuel ethanol and water is numerically studied in high-velocity suspension flame spray (HVSFS) process, and the results are compared for homogenous liquid feedstock of ethanol and water. The effects of mixture on droplet aerodynamic breakup, evaporation, combustion, and gas dynamics of HVSFS process are thoroughly investigated. The exact location where the particle heating is initiated (above the carrier liquid boiling point) can be controlled by increasing the water content in the mixture. In this way, the particle inflight time in the high-temperature gas regions can be adjusted avoiding adverse effects from surface chemical transformations. The mixture is modeled as a multicomponent droplet, and a convection/diffusion model, which takes into account the convective flow of evaporating material from droplet surface, is used to simulate the suspension evaporation. The model consists of several sub-models that include premixed combustion of propane-oxygen, non-premixed ethanol-oxygen combustion, modeling of multicomponent droplet breakup and evaporation, as well as heat and mass transfer between liquid droplets and gas phase.     

Deposition of nanoparticle suspensions by aerosol flame spraying: Model of the spray and impact processes

Aerosol flame spraying (AFS) combines the atomization of a colloidal suspension with the lateral injection of the aerosol in a flame. The aerosol droplets are partially dried when crossing the flame and then deposited as a coating onto a substrate. Afterwards, the coating is consolidated by heat treatment without extensive grain growth. In this paper a model of the trajectories, acceleration and vaporization of the droplets is used to predict the impact conditions of the in-flight dried droplets, as well as their size and water content when they impinge onto the substrate. From these calculations and the hydrodynamic properties (viscosity, surface tension, contact angle) of the suspensions, the morphology and size of the lamellae deposited on the substrate are determined by using classic impact models. In spite of the complexity of the mixing of the suspension spray with the flame and the diversity of the thermal histories of the droplets, the observation of the latter after impact shows that the results of the model are quite consistent with measurements. The relationship between droplet impact parameters and coating formation is discussed.     

Effect of in-flight particle properties on deposition of air plasma sprayed forsterite

Properties of forsterite coatings deposited by two DC-arc plasma spray guns were studied. The guns generate different types and shapes of plasma jets, resulting in different particle/plasma interactions and different microstructures in the coatings due to the different in-flight particle histories. The particle histories are characterized by cross-sectional maps of the plasma jet showing particle temperature, velocity, and particle size distributions and the number of particles correlated with the coating microstructures.     

PS-PVD等离子射流特性的光谱诊断研究进展

等离子喷涂-物理气相沉积（PS-PVD）作为一种新型喷涂技术融合了气相沉积与喷涂工艺两者的优点,其射流特性决定了涂层的结构与性能。光学发射光谱法（OES）作为一种等离子射流特性诊断技术,能实现射流特性原位检测,是判断射流内气化现象的有力手段。文中介绍了粉末送入前后及射流接触基体后射流特性的变化,展示了局部热力学平衡（LTE）假设及展宽理论下射流中各粒子状态的计算,探索了射流不同区域的传质传热等活动。Ar/He具有最高的温度,Ar具有高焓值对粉末的加热起主要作用,He凝聚射流能量对粉末的加热气化起关键作用,H₂会扩展射流宽度降低射流温度易形成带有致密层的柱状结构。结合射流数值模拟和射流光谱图可知,喷枪内复杂的热交互作用是粉末加热的主要原因,喷嘴处膨胀/压缩区发生热能与动能的交替转换,而射流中后段由于低压与高温继续发生气化现象,达到峰值后射流处于冷凝降温阶段,部分气相原子凝聚成团簇状粒子。文中还总结了功率、电流和送粉率等对等离子射流特性的影响等。     

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.     

Effect of direct-current plasma fluctuations on in-flight particle parameters: Part II

This paper is the continuation of previous work,^[1] in which plasma fluctuations were shown to produce significant time-dependent variations in the in-flight particle temperature and velocity, as well as in the number of detected particles. In this paper, the impact of the plasma fluctuations on the coating microstructure and deposition efficiency is demonstrated. Alumina coatings and deposition efficiencies, obtained with two sets of spray conditions showing similar in-flight particle conditions (velocity and temperature) with the DPV-2000 but displaying very different voltage fluctuations, are compared. The coating produced in the less stable plasma condition (C-I) is found to be more porous and contains a larger number of unmelted particles than the other coating produced in more steady plasma conditions (C-II). Moreover, condition C-I yields a significantly lower deposition efficiency. Such large discrepancies must be traced back to the physical characteristics of the particle jet. Laser illumination of the particle jet is used to probe particles too cold to be detected by pyrometric means. Cold particles are found in a much larger proportion in C-I than in C-II. They are ascribed to particles that are injected when the plasma is in a low enthalpy state. Periodic time-dependent variations in the in-flight characteristics of cold and hot particles, synchronous with the voltage fluctuations, are revealed.     

Plasma Spray Deposition on Inclined Substrates: Simulations and Experiments

In the plasma spray coating process, the coating’s profile and overall thickness are dependent on the number of overlapping traverses of the torch, the shape of the particle spray plume, the spatial distribution of the in-flight parameters of the particles within, and the orientation of the substrate. In this paper, a semi-empirical methodology for predicting three-dimensional deposits by the plasma spray process is developed. It comprises of three stages: first, spatial distributions of the in-flight parameters of multi-sized particles within the spray plume are determined by Computational Fluid Dynamics simulations. The size and shape parameters of the splats formed when individual droplets impact and spread out are obtained by experiments. Finally, a computer program is developed to integrate the particle parameters distribution and the empirical splat geometric data to generate a three-dimensional profile representing the deposit. The procedures predict the deposition volumes and thicknesses for different substrate inclinations with good agreement to experimentally sprayed deposits.     

Influence of suspension characteristics on coatings microstructure obtained by suspension plasma spraying

The growing interest for SOFC leads research towards new materials but also towards processes which could be more effective and less expensive to produce fuel cells. Plasma spraying is one of these interesting processes due to its ability to manufacture the whole cell with the same process. The present study uses suspension plasma spraying (SPS) process which consists in injecting a suspension of sub-micrometer particles dispersed in a liquid into a DC plasma torch. This process allows producing finely structured coatings. In the present study, suspensions containing YSZ particles were atomized into a spray by a bi-fluid atomizer. The droplets were then radially injected into an Ar-H₂ plasma jet where they were accelerated and heated. This paper aims at studying the importance of the suspension characteristics on the successive steps leading to the coating formation. Various suspensions based on three different media - methanol, azeotropic mixture of methyl-ethyl-ketone/ethanol and water - were evaluated. The amount of dispersant was adjusted in order to obtain well dispersed and stable suspensions. Plasticizers have been used to tailor the viscosity. Each suspension has thus been characterized versus stability, viscosity and surface tension. The influence of these characteristics on the droplets jet (size distribution and jet shape) issued from the atomizer-injector was assessed. In this purpose, two adimensional numbers (We, Re) were determined for the different atomization conditions and were correlated with the droplets jet. Finally, coatings were realised and characterized in terms of porosity by image analysis of SEM pictures. Powder velocity and temperature were evaluated implementing Accuraspray® and drop size distributions (after passing through the plasma jet) were determined as a function of the injection conditions.     

Modeling the Influence of Injection Modes on the Evolution of Solution Sprays in a Plasma Jet

Solution precursor plasma spraying (SPPS) is a novel technology with great potential for depositing finely structured ceramic coatings with nano- and sub-micrometric features. The solution is injected into the plasma jet either as a liquid stream or gas atomized droplets. Solution droplets or the stream interact with the plasma jet and break up into fine droplets. The solvent vaporizes very fast as the droplets travel downstream. Solid particles are finally formed, and the particle are heated up and accelerated to the substrate to generate the coating. The deposition process and the properties of coatings obtained are extremely sensitive to the process parameters, such as torch operating conditions, injection modes, injection parameters, and substrate temperatures. This article numerically investigates the effect of injection modes, a liquid stream injection and a gas-blast injection, on the size distribution of injected droplets. The particle/droplet size, temperature, and position distributions on the substrate are predicted for different injection modes.