Plasma spraying of Wc-Co part II: Experimental study of particle deposition and coating microstructure

WC-Co base wear-resistant coatings deposited by plasma spraying are widely used to enhance component longevity in a variety of wear environments. During spraying of WC-Co, ideally only the cobalt phase should melt and act as a binder for the WC particles. Although it is undesirable to fully melt WC because it can cause decarburization, complete melting of the cobalt phase and its satisfactory flattening on impacting the substrate is necessary to minimize porosity and achieve good substrate/coating adhesion. In this article, the influence of the primary plasma spray variables on the melting characteristics of WC-Co powders is investigated with respect to the microstructure of these coatings. This experimental work complements an analytical study on plasma spraying of WC-Co, and thus, observations are presented to support the predictions of the modeling effort.     

Prediction of powder particle behavior during high-velocity oxyfuel spraying

A mathematical model is developed to predict particle velocity and temperature during high-velocity oxyfuel (HVOF) spraying. This model accounts for internal heat conduction in powder particles; particle heating, fusion, cooling, and solidification; the influence of particle morphology on thermal behavior; and the composite structure of the particles. Analytical results are obtained that describe particle velocity and temperature variations. The dependence of fluid velocity on particle density and volume fraction is shown. The results agree with empirically established HVOF spraying practice. Physical Metal-lurgy—Materials Science     

Bonding mechanism in cold gas spraying

Cold gas spraying is a relatively new coating process by which coatings can be produced without significant heating of the sprayed powder. In contrast to the well-known thermal spray processes such as flame, arc, and plasma spraying, in cold spraying there is no melting of particles prior to impact on the substrate. The adhesion of particles in this process is due solely to their kinetic energy upon impact. Experimental investigations show that successful bonding is achieved only above a critical particle velocity, whose value depends on the temperature and the thermomechanical properties of the sprayed material. This paper supplies a hypothesis for the bonding of particles in cold gas spraying, by making use of numerical modelling of the deformation during particle impact. The results of modelling are assessed with respect to the experimentally evaluated critical velocities, impact morphologies and strengths of coatings. The analysis demonstrates that bonding can be attributed to adiabatic shear instabilities which occur at the particle surface at or beyond the critical velocity. On the basis of this criterion, critical velocities can be predicted and used to optimise process parameters for various materials.     

Oxidation control in atmospheric plasma spraying coating

Thermal spray coatings are formed by successive impingements and interbonding materials among the splats, solidified individual molten particles. Depending on the processing conditions employed during the spray process, deposits are produced with an assortment of microstructures and properties. This study highlights how the coating oxidation differences are influenced by the mechanisms involved during the spray process. The commercial steel powder referenced Amdry XPT 512 is chosen for a systematic study of comparison across different spraying techniques. Steel particles were sprayed with a F4 plasma torch and with a shrouded plasma spray process used in order to protect particles against atmospheric oxidation. The plasma jet was successively shielding by an inert gas shroud and by an oxidizing gas shroud. In-flight oxidation and post impact oxidation present in coatings are discussed in detail and the effects of these mechanisms on coating properties are addressed. The comparison was made on in-flight particle characteristics and on coating properties in terms of oxide content and porosity level. Using shrouded gas, in-flight characteristics are quite similarly independent of the nature of the shrouded gas. This way, the comparison of oxide contents present in the coatings corresponds to in-flight oxidation and is completely dependent on the nature of the shrouded gas. Comparing these results to those obtained by APS, a decrease in both velocity and temperature of in-flight particles was observed leading also to a significant decrease in oxide contents and to a slight increase in porosity level compared to coatings sprayed with air shrouding.     

Deposition of WC-Co Coatings by a Novel High Pressure HVOF

WC-Co coatings are primarily deposited using the high velocity oxy-fuel (HVOF) spray process. However, the decomposition and decarburization of carbides during spraying affects the wear performance and fracture toughness of the coatings. In this paper, a novel high pressure HVOF was developed to achieve lower particle temperature and higher particle velocity. It enables combustion chamber pressures up to 3.0 MPa. The influence of combustion chamber pressure and oxygen/fuel ratio on WC-Co particle velocity and temperature levels were analyzed by numerical simulation. The experimental results show that the combustion chamber pressure and the oxygen/fuel ratio have a significant influence on particle velocity and melting degree, as well as on the microstructure and microhardness of the coating. High velocity WC-Co particles in different states, i.e., molten, semi-molten, and non-molten can be readily obtained by changing the spraying conditions. A comparison to the conventional JP-5000 was also performed.     

碳化钨颗粒尺寸对超音速火焰喷涂WC-Co涂层形成的影响

通过探讨WC颗粒对扁平粒子厚度及喷涂后WC颗粒尺寸变化的影响，研究了超音速火焰喷涂过程中WC－Co深层的沉积过程。使用具有不同WC尺寸的四种WC-Co粉末，采用JET－KOTE喷枪系统喷制了WC－Co涂层。结果发现涂层中WC颗粒的大小主要取决于原始粉末中WC的尺寸．在粉末穿越火焰的过程中，大多数WC处于固态；WC-Co涂层的沉积涉及固液两相离子的扁平化，而不是象在优化条件下金属或陶瓷材料喷涂过程中仅存在单一液相的情况。很明显WC-Co粉末中的WC的大小对涂层的形成影响很大、在超音速火焰喷涂条件下当液固粒子碰撞到已形成的涂层表面上时，其中的大颗粒WC粒子容易被反弹脱落。基于实验结果，提出了计算由液相聚积固相形成的波固两相颗粒碰撞到表面时形成扁平粒子的厚度的模型。     

Particle melting behavior during high-velocity oxygen fuel thermal spraying

Particle melting behavior during high-velocity oxygen fuel (HVOF) thermal spraying was investigated using Inconel 625 powders. The powder characteristics and coating properties were investigated using scanning electron microscopy (SEM), x-ray, and microhardness studies. Results indicated that the volume fraction of unmelted particles in the coatings was dependent on the proportion of powder within a specified size range, in these experiments, 30 to 50 μm. This particle size range was primarily determined by the particle temperature, which was measured during spraying. Particle temperature significantly decreased as particle size increased. The microhardness values for the coatings containing unmelted particles were predicted by a simple rule-of-mixtures equation for the case of a low volume fraction of unmelted particles. However, for the condition of high volume fraction of unmelted particles, the measured microhardness values did not compare favorably with the calculated values, probably due to the presence of porosity, which occurred in the form of voids found among unmelted particles. The microstructure and characteristics of the feedstock powder were retained in the corresponding coating under certain spray conditions.     

Peening action and residual stresses in high-velocity oxygen fuel thermal spraying of 316L stainless steel

316L stainless steel powder was sprayed by a high-pressure high-velocity oxygen fuel (HVOF) process. Effects of powder size and the pressure in the combustion chamber on the velocity and temperature of sprayed particles were studied by using an optical instrument, first, at the substrate position. A strong negative correlation between the particle temperature and the diameter was found, whereas the correlation between the velocity and the diameter was not significant. The pressure in the combustion chamber affected the velocity of sprayed particles significantly, whereas the particle temperature remained largely unchanged. In-situ curvature measurement was employed in order to study the process of stress generation during HVOF spraying. From the measured curvature changes, the intensity of peening action and the resultant compressive stress by HVOF sprayed particles were found to increase with the kinetic energy of the sprayed particles. The results were further used to estimate the stress distribution within the coatings. X-ray stress measurement revealed that the residual stress on the surface of the HVOF coatings is low and often in tension, but the stress inside the coatings is in a high level of compression.     

多功能微弧等离子喷涂技术

根据空气动力学和等离子喷涂理论,按照"一专多能"的设计指导思想,采用IGBT逆变技术、微电脑控制技术、软开关变换技术、拉瓦尔喷嘴和中心轴向送粉等,设计了多功能微弧等离子喷涂系统,且具有体积小、重量轻、抗干扰强、控制精确、喷涂粒子速度高、粉末沉积效率高等特点,可制备各种金属合金、金属陶瓷以及氧化物涂层.通过试验测定,多功能微弧等离子喷涂制备的纳米结构Al2O3 13%TiO2涂层的结合强度、显微硬度等性能优于9M等离子喷涂制备的纳米结构的Al2O3 13%TiO2涂层,这表明通过改进电源设计、喷枪结构设计、送粉方式等,低功率多功能微弧等离子喷涂能够与大功率等离子叶涂性能相当甚至更优异性能的涂层.     

Reactive Atmospheric Plasma Spraying of AlN Coatings: Influence of Aluminum Feedstock Particle Size

Feedstock powder characteristics (size distribution, morphology, shape, specific mass, and injection rate) are considered to be one of the key factors in controlling plasma-sprayed coatings microstructure and properties. The influence of feedstock powder characteristics to control the reaction and coatings microstructure in reactive plasma spraying process (RPS) is still unclear. This study, investigated the influence of feedstock particle size in RPS of aluminum nitride (AlN) coatings, through plasma nitriding of aluminum (Al) feedstock powders. It was possible to fabricate AlN-based coatings through plasma nitriding of all kinds of Al powders in atmospheric plasma spray (APS) process. The nitriding ratio was improved with decreasing the particle size of feedstock powder, due to improving the nitriding reaction during flight. However, decreasing the particle size of feedstock powder suppressed the coatings thickness. Due to the loss of the powder during the injection, the excessive vaporization of fine Al particles and the completing nitriding reaction of some fine Al particles during flight. The feedstock particle size directly affects on the nitriding, melting, flowability, and the vaporization behaviors of Al powders during spraying. It concluded that using smaller particle size powders is useful for improving the nitriding ratio and not suitable for fabrication thick AlN coatings in reactive plasma spray process. To fabricate thick AlN coatings through RPS, enhancing the nitriding reaction of Al powders with large particle size during spraying is required.     

In-situ plasma spraying: Alumina formation and in-flight particle diagnostic

In-situ plasma spraying (IPS) is a promising process to fabricate composite coatings with in-situ formed thermodynamically stable phases. In the present study, mechanically alloyed Al-12Si and SiO₂ powder was deposited onto an aluminum substrate by atmospheric plasma spraying (APS) to obtain a composite coating consisting of in-situ formed alumina reinforced hypereutectic Al-18Si matrix alloy. The effects of spray parameters (arc current and spray distance) and in-flight particle characteristics (temperature and velocity) on in-situ reaction intensity (alumina and silicon) have been investigated. The results show that, in-situ alumina formation and silicon intensity strongly depend on in-flight particle characteristics, spray distance and substrate temperature.     

FeAl and Mo–Si–B intermetallic coatings prepared by thermal spraying

FeAl and Mo–Si–B intermetallic coatings for elevated temperature environmental resistance were prepared using high-velocity oxy-fuel (HVOF) and air plasma spray (APS) techniques. For both coating types, the effect of coating parameters (spray particle velocity and temperature) on the microstructure and physical properties of the coatings was assessed. Fe–24Al (wt%) coatings were prepared using HVOF thermal spraying at spray particle velocities varying from 540 to 700 m/s. Mo–13.4Si–2.6B coatings were prepared using APS at particle velocities of 180 and 350 m/s. Residual stresses in the HVOF FeAl coatings were compressive, while stresses in the APS Mo–Si–B coatings were tensile. In both cases, residual stresses became more compressive with increasing spray particle velocity due to increased peening imparted by the spray particles. The hardness and elastic moduli of FeAl coatings also increased with increasing particle velocity. For Mo–Si–B coatings, plasma spraying at 180 m/s resulted in significant oxidation of the spray particles and conversion of the T1 phase into amorphous silica and -Mo. The T1 phase was retained after spraying at 350 m/s.     

低温超音速喷涂团聚铁粒子沉积的SPH模拟

为了研究纳米团聚粒子形态对涂层特性的影响,应用SPH方法研究了低温超音速火焰喷涂金属团聚粒子的沉积行为.结果表明,纳米粒子团聚为微米级粒子后,基体碰撞出现了飞溅现象,应变变化明显.团聚粒子的等效塑性应变小于普通微米粒子,但团聚粒子的面积扩大比大于普通微米粒子.沉积过程中,当超过临界沉积速度后,金属团聚粒子与基体之间存在过渡区域,过渡区域随粒子速度的增加而扩大.     

Novel method for in-flight particle temperature and velocity measurements in plasma spraying using a single CCD camera

A novel, technically simple imaging system for individual, in-flight particle temperature and velocity measurements for plasma and other thermal spray processes is described. A custom double dichroic mirror is used to add spectral resolving capability to a single, black-and-white, fast-shutter digital charge coupled device (CCD) camera. The spectral double images produced by the individual in-flight particles are processed using specialized image processing algorithms. Particle temperature determination is based on two-color pyrometry, and particle velocities are measured from the length of the particle traces during known exposure times. In this paper, experimental results using the first prototype system are presented. Laboratory tests were performed using rotating pinholes to simulate in-flight particles, and plasma spraying experiments were performed with commercial, standard spraying equipment operated with Al₂O₃ and NiCrAlY powders. The prototype instrument can be readily used to determine velocity and temperature distributions of individual in-flight particles from the imaged region of interest of the plume. Dividing the imaged area into smaller sections, spatial distributions of particle temperature, velocity, and number of detected particles can be studied. The study aims to develop a technically simple, single imaging instrument, which can provide a visual overview of the spray plume in combination with quantitative evaluation of the most important spray particle parameters.     

Spray parameters and particle behavior relationships during plasma spraying

Using laser anemometry, laser fluxmetry, and statistical two-color pyrometry, the velocity, number flux, and surface temperature distributions of alumina and zirconia particles in dc plasma jets have been determined inflight for various spraying parameters. The flux measurements emphasized the importance of the carrier gas flow rate, which must be adjusted to the plasma jet momentum depending on the arc current, nozzle diameter, gas flow rate, and gas nature. It has also been shown that the particle trajectories depend both on the particle size and injection velocity distributions and that the position and tilting of the injector plays a great role. The particle size drastically influences its surface temperature and velocity, and for the refractory materials studied, only the particles below 45 μm in diameter are fully molten in Ar-H₂ (30 vol%) plasma jets at 40 kW. The morphology of the particles is also a critical parameter. The agglomerated particles partially explode upon penetration into the jet, and the heat propagation phenomenon is seriously enhanced, particularly for particles larger than 40 μm. The effects of the arc current and gas flow rate have been studied, and the results obtained in an air atmosphere cannot be understood without considering the enhanced pumping of air when the plasma velocity is increased. The Ar-He (60 vol%) and Ar-H₂ (30 vol%) plasma jets, when conditions are found where both plasma jets have about the same dimensions, do not result in the same treatment for the particles. The particles are not as well heated in the Ar-He jet compared to the Ar-H₂ jet. Where the surrounding atmosphere is pure argon instead of air (in a controlled atmosphere chamber), he radial velocity and temperature distributions are broadened, and if the velocities are about the same, the temperatures are higher. The use of nozzle shields delays the air pumping and increases both the velocity and surface temperature of the particles. However, the velocity increase in this case does not seem to be an advantage for coating properties.     

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。     

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.     

Effect of Powder Characteristics on Properties of Warm-Sprayed WC-Co Coatings

In high-velocity oxy-fuel (HVOF) spraying of WC-Co coatings, the decomposition and decarburization of WC during deposition are responsible for their much lower toughness compared with a sintered bulk WC-Co. In a previous study, Warm Spray (WS) process, which is capable to control the flame temperature used to propel powder particles, was successfully applied in an attempt to suppress such detrimental reactions by keeping particles’ temperature lower than their melting point. The coatings deposited by WS process showed no or little formation of W₂C and η phases and demonstrated moderately improved fracture properties. However, there is still a gap in fracture toughness between WS coatings and the corresponding sintered bulk. In order to optimize the properties of the WS coatings, the effect of original powder sizes were investigated. Microstructural characterization and phase analysis were carried out on deposited coatings by SEM and XRD. The results show that the feedstock powder size has substantial effects on the properties of the coatings, i.e., the smaller powder showed improved properties.     

等离子喷涂工艺参数对粒子速度和温度的影响

在G型喷嘴下，利用SprayWatch-2i喷涂粒子速度、温度测试仪在线测量粒子的飞行参数，研究了等离子喷涂工艺参数对喷涂粒子速度和温度的影响，并对等离子喷涂氧化锆粉末的主要工艺参数进行优化分析。在诸因素中．气体流量对粒子速度和温度的影响最为显著，随着主气流量的增加，粒子速度有所增加。