Modelling the Plasma Jet in Multi-Arc Plasma Spraying

Particle in-flight characteristics in atmospheric plasma spraying process are determined by impulse and heat energy transferred between the plasma jet and injected powder particles. One of the important factors for the quality of the plasma-sprayed coatings is thus the distribution of plasma gas temperatures and velocities in plasma jet. Plasma jets generated by conventional single-arc plasma spraying systems and their interaction with powder particles were subject matter of intensive research. However, this does not apply to plasma jets generated by means of multi-arc plasma spraying systems yet. In this study, a numerical model has been developed which is designated to dealing with the flow characteristics of the plasma jet generated by means of a three-cathode spraying system. The upstream flow conditions, which were calculated using a priori conducted plasma generator simulations, have been coupled to the plasma jet simulations. The significances of the relevant numerical assumptions and aspects of the models are analyzed. The focus is placed on to the turbulence and diffusion/demixing modelling. A critical evaluation of the prediction power of the models is conducted by comparing the numerical results to the experimental results determined by means of emission spectroscopic computed tomography. It is evident that the numerical models exhibit a good accuracy for their intended use.     

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.     

Plasma Spraying of Copper by Hybrid Water-Gas DC Arc Plasma Torch

Water-stabilized DC arc plasma torches offer a good alternative to common plasma sources used for plasma spraying applications. Unique properties of the generated plasma are determined by a specific plasma torch construction. This article is focused on a study of the plasma spraying process performed by a hybrid torch WSP500^®-H, which combines two principles of arc stabilization—water vortex and gas flow. Spraying tests with copper powder have been carried out in a wide range of plasma torch parameters. First, analyses of particle in-flight behavior for various spraying conditions were done. After, particles were collected in liquid nitrogen, which enabled analyses of the particle in-flight oxidation. A series of spraying tests were carried out and coatings were analyzed for their microstructure, porosity, oxide content, mechanical, and thermal properties.     

Modeling Multi-Arc Spraying Systems

The use of plasma as energy source in thermal spraying enables among others the processing of feed stock materials with very high melting temperatures as coating materials. New generation multi-arc plasma spraying systems are widely spread and promise several advantages in comparison to the conventional single-arc systems. Numerical modeling of multi-arc plasma spraying offers the possibility to increase the understanding about this process. This study focuses on the numerical modeling of three-cathode spraying systems, introducing the recent activities in this field and discussing the numerical aspects which influence the prediction power of the models.     

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。     

Plasma and Particle Temperature Measurements in Thermal Spray: Approaches and Applications

Growing demands on the quality of thermally sprayed coatings require reliable methods to monitor and optimize the spraying processes. Thus, the importance of diagnostic methods is increasing. A critical requirement of diagnostic methods in thermal spray is the accurate measurement of temperatures. This refers to the hot working gases as well as to the in-flight temperature of the particles. This article gives a review of plasma and particle temperature measurements in thermal spray. The enthalpy probe, optical emission spectroscopy, and computer tomography are introduced for plasma measurements. To determine the in-flight particle temperatures mainly multicolor pyrometry is applied and is hence described in detail. The theoretical background, operation principles and setups are given for each technique. Special interest is attached to calibration methods, application limits, and sources of errors. Furthermore, examples of fields of application are given in the form of results of current research work.     

Numerical modelling of in-flight particle dynamics of non-spherical powder

High velocity oxygen fuel (HVOF) is an important thermal spraying technology in depositing high quality coatings. Its ability to produce high particle velocities and relatively low particle temperatures is its most salient feature. Several computational fluid dynamic (CFD) models have been developed to study the in-flight particle behavior during thermal spraying. These models are limited to spherical particles, which are only appropriate for modelling gas atomised powders. On the other hand, hardmetal powders such as WC-Co are created using high energy ball milling and are not normally spherical. To examine the effect of particle morphology on particle dynamics, mathematical models are developed in the present paper to predict the in-flight particle behavior in a liquid fuelled HVOF thermal spray gun. The particle transport equations are coupled with the three-dimensional, chemically reacting, turbulent gas flow, and solved in a Lagrangian manner. The melting and solidification within the particles as a result of heat exchange with the surrounding gas flow are solved numerically. The results demonstrate that non-spherical particles gain more momentum and less heat during the HVOF process than spherical particles. Non-spherical particles are also predicted to stay closer to the center of the gas jet than spherical particles.     

基于响应曲面的Ar气氛超音速等离子喷涂钛涂层

采用超音速等离子喷涂可低成本、高效率制备钛涂层。采用响应曲面法（RSM）中的Box-Behnken（BBD）设计分析了Ar流量、功率、喷涂距离3个因素与超音速等离子射流中钛粒子飞行速度和温度的交互性,利用SEM和显微硬度计研究了钛涂层的微观结构和显微硬度。结果表明：建立的线性模型可靠,喷涂距离对粒子飞行速度和温度影响最大,且随喷涂距离增加粒子飞行速度减小温度增加,而Ar流量和功率对粒子飞行速度和温度的影响与喷涂距离相反。超音速等离子喷涂制备出的钛涂层硬度较低,且呈多孔结构,随粒子飞行速度增加孔隙率降低。     

Relationship Between Particle and Plasma Properties and Coating Characteristics of Samaria-Doped Ceria Prepared by Atmospheric Plasma Spraying for Use in Solid Oxide Fuel Cells

Samaria-doped ceria (SDC) has become a promising material for the fabrication of high-performance, intermediate-temperature solid oxide fuel cells (SOFCs). In this study, the in-flight characteristics, such as particle velocity and surface temperature, of spray-dried SDC agglomerates were measured and correlated to the resulting microstructures of SDC coatings fabricated using atmospheric plasma spraying, a manufacturing technique with the capability of producing full cells in minutes. Plasmas containing argon, nitrogen and hydrogen led to particle surface temperatures higher than those in plasmas containing only argon and nitrogen. A threshold temperature for the successful deposition of SDC on porous stainless steel substrates was calculated to be 2570?°C. Coating porosity was found to be linked to average particle temperature, suggesting that plasma conditions leading to lower particle temperatures may be most suitable for fabricating porous SOFC electrode layers.     

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.     

Temperature Measurement Challenges and Limitations for In-Flight Particles in Suspension Plasma Spraying

Suspension plasma spraying (SPS) acquires a significant interest from the industry. The deposited coatings using this technique were proved to have unique microstructural features compared to those built by conventional plasma spraying techniques. In order to optimize this process, in-flight particle diagnostics is considered a very useful tool that helps to control various spraying parameters and permits better coating reproducibility. In that context, the temperature of in-flight particles is one of the most important key elements that helps to optimize and control the SPS process. However, the limitations and challenges associated with this process have a significant effect on the accuracy of two-color pyrometric techniques used to measure the in-flight particle temperature. In this work, the influence of several nonthermal radiation sources on the particle temperature measurement is studied. The plasma radiation scattered by in-flight particles was found to have no significant influence on temperature measurement. Moreover, the detection of the two-color signals at two different locations was found to induce a significant error on temperature measurement. Finally, the plasma radiation surrounding the in-flight particles was identified as the main source of error on the temperature measurement of in-flight particles.     

基于熔融因子的纳米结构涂层制备工艺优化EI北大核心CSCD

在热喷涂制备微米/纳米双结构涂层的工艺优化研究中缺乏对颗粒状态与喷涂工艺和涂层结构性能之间关系的综合分析。针对大气等离子喷涂制备双模态微观结构的热障涂层过程开展工艺参数优化研究。首先基于试验测量和数值仿真模拟所获得的颗粒状态以及射流信息,计算可表征飞行粒子受热状态的熔融因子分布,同时开展试验研究获得给定喷涂工况下涂层微观结构和沉积效率。最后以熔融因子为中间参数,研究并建立喷涂工艺-飞行粒子状态-涂层结构特性相关关系。结果表明,数值仿真模拟得到的纳米团聚颗粒状态与试验测量结果基本一致。纳米结构涂层微观结构主要缺陷为未熔纳米团聚粒子及微裂纹。在纳米结构涂层制备过程中,随着喷涂距离增加,飞行粒子的熔融因子先增大后减小,涂层沉积效率先增加后降低,孔隙率和未熔粒子数则先减小后增大。主气流量的增加则会导致熔融因子减小。在喷涂功率相近情况下,采用低电流高电压的组合会使得飞行粒子熔融因子分布取值较大,进而使得涂层沉积效率增加、孔隙率降低。使用熔融因子分析喷涂工艺参数对纳米团聚粉末颗状态和涂层结构特性的影响关系,可用于指导纳米双结构涂层制备过程工艺控制。     

Investigation of particle in-flight characteristics during atmospheric plasma spraying of yttria stabilized ZrO2: Part 2. modeling

Yttria stabilized ZrO₂ particle in-flight characteristics in an Ar-H₂ atmospheric plasma jet have been studied using analytical and experimental techniques. In the previous article,^[1] the primary gas flow, plasma composition, current, and powder feed rate were systematically varied and particle surface temperatures, velocities, and size distributions measured and statistically analyzed. In this paper, a mathematical model for the plasma flow and particle characteristics is presented. Model predictions are compared with the experimental results in Ref 1 and a reasonable correlation is found. A statistical investigation (composite cubic face (CCF)) is performed on the particle predictions, giving fast and simple relationships between gun parameters and particle in-flight properties. The statistical and theoretical models that are presented here combine to form a powerful and cost-effective tool, which can be used in the evaluation and optimization of spray parameters off-line.     

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.     

From DC Time-Dependent Thermal Plasma Generation to Suspension Plasma-Spraying Interactions

This paper proposes an original route for modeling the time-dependent behavior of a plasma jet issued from a DC plasma-spraying torch operating with various kinds of gas mixtures. The hydrodynamic interactions between this jet and a liquid jet for suspension plasma-spraying or a classical particle injection for the deposition of coatings are studied. In a first step, the classical plasma spraying process was explored using the FLUENT CFD code. Zirconia particles, defined as Lagrangian particles, were injected in an Ar/H₂ flow and their positions, kinetic and thermal states were compared with experimental results. The trend and intensity of the values demonstrated a rather good agreement. In a second step, the suspension plasma spraying was investigated with the AQUILON CFD to simulate interactions between the plasma and aqueous jets. An Ar/H₂ plasma flow was simulated with the Large Eddy Scale turbulence model assumption, in which a liquid jet had been introduced. The behavior observed during the first stage of the interactions between the two fluids corresponded to expectations.     

Spraying of Metallic Powders by Hybrid Gas/Water Torch and the Effects of Inert Gas Shrouding

A hybrid DC arc plasma torch, combining water and gas stabilization, offers a high flexibility in plasma characteristics. These can be controlled in a wide range by the torch operational parameters, such as arc current and secondary gas flow rate. In this study, their influence on plasma spraying of tungsten and copper was investigated. To suppress the in-flight oxidation of the metals, inert gas shrouding was applied. In-flight particle diagnostics and analysis of free-flight particles and coatings was performed for spraying experiments in the open atmosphere and with argon shrouding. Both in-flight particle behavior and coating properties were found to be sensitive to the torch parameters. The application of shrouding was found to affect particle in-flight parameters, reduce the oxide content in the coatings and generally improve their properties, such as thermal conductivity. However, a different degree of these effects was observed for copper and tungsten.     

等离子喷涂NiCrCoAlY涂层氧化控制研究

利用等离子喷涂技术制备了NiCrCoAlY粒子和涂层,研究了等离子喷涂过程中NiCrCoAlY粒子的氧化行为以及屏蔽气体对NiCrCoAlY涂层抗高温氧化性能的影响。结果表明,粒子在飞行过程中存在对流氧化和扩散氧化两种氧化机制,对NiCrCoAlY粒子来说,在距喷嘴55 mm以内的射流中心处以对流氧化为主,在距离喷嘴55 mm以外将以扩散氧化为主;除飞行中的氧化外,粒子在喷涂过程中还发生形成涂层后的氧化,NiCrCoAlY粒子以飞行中的氧化为主;添加屏蔽气体能减少喷涂过程中涂层的氧化,提高涂层的抗高温氧化性能     

Latest Researches Advances of Plasma Spraying: From Splat to Coating Formation

The plasma spray process with solid feedstock, mainly ceramics powders, studied since the sixties is now a mature technology. The plasma jet and particle in-flight characterizations are now well established. The use of computer-aided robot trajectory allows spraying on industrial parts with complex geometries. Works about splat formation have shown the importance of: the substrate preheating over the transition temperature to get rid of adsorbates and condensates, substrate chemistry, crystal structure and substrate temperature during the whole coating process. These studies showed that coating properties strongly depend on the splat formation and layering. The first part of this work deals with a summary of conventional plasma spraying key points. The second part presents the current knowledge in plasma spraying with liquid feedstock, technology developed for about two decades with suspensions of particles below micrometers or solutions of precursors that form particles a few micrometers sized through precipitation. Coatings are finely structured and even nanostructured with properties arousing the interest of researchers. However, the technology is by far more complex than the conventional ones. The main conclusions are that models should be developed further, plasma torches and injection setups adapted, and new measuring techniques to reliably characterize these small particles must be designed.     

等离子喷涂ZrO2热障涂层工艺参数优化设计

为了深入研究等离子喷涂ZrO2粒子的飞行特征与涂层性能之间的关系，采用三水平四因素正交试验法对主气、辅气、电流及喷涂距离等4个主要参数进行了优化设计，并采用DPV2000热喷涂在线监测仪测定了ZrO2粒子的飞行特征参数，通过IA32定量金相分析软件对涂层的孔隙率进行了测试。结果表明，影响ZrO2粒子温度的主要因素为主气和辅气，影响ZrO2粒子飞行速度的主要因素为喷涂距离和辅气。