Analysis of the unsteadiness of a plasma jet and the related turbulence

Plasma spraying using liquid feedstock allows realizing thin coatings (< 100 μm). In this process, the plasma jet fluctuations play an important role in the behavior of the liquid fragmentation and then in the final deposit characteristics. In this study, numerical simulations of two different plasma jets (argon or argon hydrogen mixture) issuing into air are investigated. The computations are based on a compressible model and a Large Eddy Simulation (LES) modeling for the turbulence. The aim of this work is to analyze the unsteady effects of the plasma jet regarding those of turbulence. In particular, the characterization of the turbulent zone inside the plasma jet is provided. A turbulence analysis is carried out in order to estimate the levels of turbulence and modeling effects.     

Fabrication of finely structured silicon-supported SOFC with anode deposited by multi-phase plasma spraying

The mechanically mixed NiO/YSZ powder was usually used as the anode material of atmospheric plasma sprayed (APS) solid oxide fuel cells (SOFC). Big particles and the non-uniform distribution of the pores were observed in the resultant anode layer. To overcome the limitations, a method of fabricating anode layer by multi-phase plasma spraying (MPS) was proposed in this paper. The NiO and YSZ powders were delivered into plasma jet by a separate injection, where nitrogen carrier was employed to feed micrometer-sized NiO powder and liquid carrier was to feed submicrometer-sized YSZ powder. Suspension plasma spraying (SPS) was applied to fabricate dense electrolyte layer. The microstructure and composition of coatings were characterized by SEM and EDS. The results showed that finely structured anode layer with small particle size (d ∼ 2 μm) was achieved by the MPS method. The MPS anode layer was porous with the porosity of 32.1% while the APS anode layer was 22.6%. Three kinds of elements (Ni, Y, Zr) were observed in the MPS anode layer and the NiO content was calculated to be 49.6 wt%. In the SPS process, the suspension flow rate was matched to the plasma gas flow rate to obtain proper injection condition.     

Characterization of suspension plasma-sprayed solid oxide fuel cell electrodes

Suspension plasma spray is a promising technique that uses fine particles dispersed in a liquid as feedstock material instead of dry powder as in conventional plasma spraying and has been implemented here to produce layers with appropriate morphologies and microstructures for SOFC applications.This study uses a pressurized gas delivery system to feed the slurry through a homemade two-fluid atomizing nozzle to a conventional plasma torch. The electrodes consist of porous NiO-YSZ as anode and lanthanum nickelate as cathode. The anode and respectively the cathode were deposited onto dense or porous ferritic steel substrates in order to be characterized and optimized. The cell components were examined by scanning electron microscopy (SEM), X-ray diffraction and leakage test. This paper aims at studying the influence of the suspension characteristics (surface tension and viscosity were selected as main parameters), the conditions of injection (nozzle design, gas to liquid ratio, injection angle have been identified as major parameters), the plasma conditions (plasma gas nature and flow rates, spray distance are of major importance) and finally the kinematics on the crystalline phases, the chemical composition (distribution of NiO particles into the layer), the thickness and roughness, the pore ratio and the gas permeability. Then the optimized electrodes have been deposited onto ferritic substrate to perform Open Circuit Voltage and impedance tests at a temperature around 800 °C. This work demonstrated the feasibility for the fabrication of electrodes with interesting performance using suspension plasma spraying technique.     

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.     

Contribution to the modeling of the interaction between a plasma flow and a liquid jet

A numerical simulation of the interaction between a plasma flow and a liquid jet was investigated, leading to the proposal of a compressible model, based on augmented Lagrangian, Large Eddy Simulation (LES) turbulence modeling and Volume of Fluid (VOF) approaches, capable of managing incompressible two-phase flows as well as turbulent compressible motions. The VOF method utilized volume markers to advect the local concentration of gas and liquid in a Eulerian manner. The numerical model was validated on single-phase plasma configurations as well as two-phase cross flow liquid jet interactions. Finally, an example of the first simulations of the interactions between a liquid jet and a plasma is presented. However improvements should be realized as to increase the speed of the VOF-SM algorithm or add specific subgrid models related to jet fragmentation and phase change.     

The effect of solids and dispersant loadings on the suspension viscosities and deposition rates of suspension plasma sprayed YSZ coatings

Suspension plasma spraying (SPS) is a promising modification of traditional plasma spraying techniques that uses small (≤ 2 μm) particles suspended in a liquid to fabricate coatings with fine microstructures and controlled porosity rapidly and without the need for post-deposition heat treatments. These qualities make SPS an interesting new technique to manufacture solid oxide fuel cell (SOFC) active layers. However, in order to be able to manufacture layers with good microstructures, the properties of the feedstock suspension must be optimized to enhance particle dispersion and improve feedability. This study uses a pressurized gas delivery system to feed aqueous YSZ suspensions containing an organic dispersant to a Northwest Mettech Axial III axial injection suspension plasma spray system. Three different dispersant types (polyacrylic acid (PAA), polyethylene imine (PEI) and 2-phosphonobutane-1,2,4-tricarboxylic acid (PBTCA)) were characterized and the effects of solid loadings, dispersant type, and dispersant concentration on suspension properties such as viscosity and feedability, and layer characteristics such as microstructure and deposited thickness were examined.     

Simulation of PYSZ particle impact and solidification in atmospheric plasma spraying coating process

In this work a numerical model of the impact and solidification of partially yttria stabilized zirconia particles on flat and rough substrate surfaces under plasma spraying conditions and the simulation results are presented. Results of the numerical simulation showed the influence of particle diameter and particle state prior to impact on splats spreading behavior and final morphology. The particles have a diameter range from 20 µm to 60 µm. Particle initial conditions prior to impact: speed, temperature and melting state are taken from previous simulation approaches of particle acceleration and heating. Simulations of fluid dynamics, heat transfer and solidification during the particle impact were performed using computational fluid dynamics. Tracing of free surfaces was determined by the volume of fluid method. The simulation results are compared with several numerical and experimental studies of other scientists and showed good agreement. Simulated splat morphologies are compared with experimentally obtained splats. The numerical model shows good results under real coating conditions and is suitable for the implementation in industrial applications. This model builds a basis for calculation of microstructure during real coating processes and can be used not only for coating under atmospheric plasma spraying conditions but also for similar coating processes and diverse materials.     

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.     

High density large area hydrogen plasma by hollow cathode plasma array

Hydrogen plasma becomes an alternative to the conventional oxygen plasma in stripping photoresist in the next generation semiconductor processing because the conventional oxygen plasma is known to degrade ultralow dielectric constant films by depleting carbons from the films. An array of hollow cathode plasma is designed to have uniform and high density hydrogen plasma. From many combinations of cavity size and distribution, it is found that cylindrical ceramic cavity with 6 mm inner diameter, 10 mm depth and 30 mm spacing between neighboring cavities shows the widest process window. Nineteen cavities are engraved into the cathode plate of 200 mm diameter. Ceramic cavities are needed to survive against energetic ion bombardment. Dependence of the stripping rate on mixture ratio of N₂/H₂, gas flow rate, chamber pressure and RF power is investigated, and we have found that a stripping rate of more than 260 nm/min with 7% uniformity can be achieved when chamber pressure is 213 Pa, gas flow rate 10000 sccm, N₂/H₂ mixture ratio of 3:7 and RF power 2.5 KW. This high density hydrogen plasma in the order of 10¹¹/cm³can be a very effective method of photoresist stripping in the dual damascene process of copper metal and low-k dielectrics where oxygen plasma cannot be used.     

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.     

Effect of substrate surface temperature on the microstructure and ionic conductivity of lanthanum silicate coatings deposited by plasma spraying

Apatite-type lanthanum silicate shows a high ionic conductivity and low activation energy at intermediate temperature (600-800 °C) compared with Yttria stabilized Zirconia (YSZ). In addition, its excellent stability in a wide oxygen partial pressure range meets the requirement for the electrolyte. Thus it is a potential candidate for intermediate temperature solid oxide fuel cell (IT-SOFC) electrolytes. Moreover, atmospheric plasma spraying is expected to be a promising alternative to other costly or low-deposition rate electrolyte processing methods. However, the amorphous phase and pores existing in the plasma sprayed coatings might impair the stability and the higher ionic conductivity of lanthanum silicate. The present work investigated the effect of substrate surface temperature from 545 °C to 900 °C on the microstructure and ionic conductivity of the lanthanum silicate coating. The crystallinity of as-deposited coatings increased with the increase of substrate surface temperature, whereas the porosity showed a contrary tendency. Ionic conductivity increased about four times with increasing substrate surface temperature, which is related to the low porosity and high crystallinity of coatings. Increasing the substrate surface temperature during plasma spraying is a feasible method to increase ionic conductivity of the lanthanum silicate coatings by reducing the porosity and enhancing the crystallinity.     

Cold gas dynamic spraying as a method for freeforming and joining materials

In this study cold gas dynamics spraying (CGDS) was successfully used as a joining process to deposit filler metal between two beveled plates. The technique was also employed to build 3-D freeforms. Verification of conceptual experiments was carried out using conventional zinc feedstock powder. SEM and optical microscopy confirmed that dense deposits were created. No significant difference was observed in the mechanical properties between the freeforms and the weldments. Hardness values of 34 ± 2.5 vs. 33 ± 3.2 HV and bending strength of 127 ± 17 vs. 162 ± 30 MPa were observed, respectively. Examination of the fracture surfaces showed that the strength of the interface is higher than the strength of the freeform itself. The results obtained demonstrate that CGDS will be a viable technique for rapid prototyping or joining assembly for thermally sensitive materials.     

A Particle-Tracking-Velocimetry (PTV) Investigation of Liquid Injection in a DC Plasma Jet

The present article describes experimental results of liquid injection in a thermal plasma jet by particle-tracking velocimetry (PTV). This technique delivers an in-situ real-time analysis of the liquid breakup and measures the velocities and the trajectories of the particles. The observations were done within the 10 mm surrounding the injection location where the plasma brightness is considerable. First, a validation of the proposed investigation method was carried out in a slower plasma jet. Subsequently, PTV measurements within faster plasma jets, resulting in a set of trajectories, were compared with trajectories achieved through optical diagnostics based on a simple shadow-graph technique proposed by Damiani et al. [Injection d’un liquide au sein d’un jet plasma thermique: optimisation de la trajectoire des particules, Proceedings of Congrès Francophone de Techniques Laser, CFTL 2010, Vandoeuvre lès nancy, France, 2010 (in French)]. These trajectories indicated that a higher plasma flow rate was required to spray all droplet sizes in the axis of the flow, thereby enabling an optimal spraying (then coating) application for producing nanostructured thin layers. This study showed that the liquid injection parameters are of main importance to obtain optimal injection and plasma parameters to achieve the required coating properties.     

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.     

Effects of heating temperature and duration on the microstructure and properties of the self-healing coatings

The present work investigates how the heating temperature and duration affect the properties of the self-healing coating on martensitic steels. The coating composed of TiC + mixture (TiC/Al₂O₃) + Al₂O₃ is fabricated by means of air plasma spraying. The thermal shock test is performed at 600 °C, 700 °C and 800 °C, respectively, to evaluate the thermal-mechanical stability of the coating. The cross-section morphology of the samples after 1 h, 9 h, 18 h and 30 h of heat treatment shows that the porosity of the coating decreases with the increase of heating duration. The evaluation of electrochemical performance by electrochemical impedance spectroscopy shows that the corrosion resistance of the coating after being heated for 18 h is much better than the other samples due to the process of the inner layer being compacted in the coating. The adhesive tensile strength test between coating and substrate shows that the adhesive strength of the coatings is higher than 9 MPa within 40 h of heat treatment at 600 °C. The residual stress reaches a minimum value after the coating was heated for 9 h at 600 °C, then increases with the heating duration after 9 h. Energy dispersive X-ray analysis at the Vickers indentation indicates that the oxygen content at the crack position increases significantly after being heated for 30 h at 600 °C. These experimental results suggest that this coating can meet the requirement of application under the actual temperature conditions.     

Modeling and visualization of plasma spraying of functionally graded materials and its application to the optimization of spray conditions

This paper presents a simulation and visualization system for plasma spraying of functionally graded materials (FGM). The recently modified CFD code, LAVA-P, that incorporates a well-verified model for plasma gas flow and chemistry is employed. The particle movement and its trajectory are described within a Lagrangian formulation by considering drag as the major driving force, and the particle melting, evaporation, and resolidification are considered using a recently developed model for particle-flame interaction. In addition to the noncontinuum and variable property effects associated with high-temperature plasma, the effects of particle evaporation on particle momentum and heat transfer are also taken into account. Calculations are performed for NiCrAlY and ZrO₂ powders for a wide range of size distributions. The influences of power levels and flow rate of H₂ on plasma flow field and, hence, on the particle velocity and temperature are investigated. The predicted velocity and temperature fields agree well with the measurements under similar spraying conditions. With the help of a special in-house built process animation and visualization algorithm, the powder injection conditions, such as the number of injectors, injector location, and injection velocity, are investigated and can be optimized to obtain coatings with a specified distribution of different species.     

Manufacturing of high performance solid oxide fuel cells (SOFCs) with atmospheric plasma spraying (APS)

The potential of atmospheric plasma spraying (APS) technology has been investigated for the manufacture of anode, electrolyte and cathode of a solid oxide fuel cell. As the substrate a tape-casted FeCr alloy was used. It turned out that all layers can be applied by this technique, however, the APS cathode layer, although applied by suspension plasma spraying led to cells with rather low performance. Much better cell characteristics could be obtained by using screen-printed LSCF cathodes, which do not need any additional thermal treatment.Anode layers with high electrochemical activity were produced by separate injection of NiO and YSZ powders. The manufacturing of gastight electrolyte layers was a key-issue of the present development. As APS ceramic coatings typically contain microcracks and pores their leakage rate is not sufficiently low for SOFC applications.Based on the understanding of the formation of defects during spraying an optimized spraying process was developed which led to highly dense coatings with the appearance of a bulk, sintered ceramic. Open cell voltages above 1 V proofed the low leakage rates of the rather thin (< 50 μm) coatings. With these cells having a screen-printed cathode an output power of 500 mW/cm² could be achieved at 800 °C.It turned out that the long-term stability of the metal substrate based APS SOFCs was rather poor. The aging of the cells was probably due to interdiffusion of anode and substrate material. Hence, diffusion barrier was applied by APS between substrate and anode. These layers were very effective in reducing the degradation rate. For these cells the output power reached 800 mW/cm².     

Liquid Precursor Plasma Spraying: Modeling the Interactions Between the Transient Plasma Jet and the Droplets

Plasma spraying using liquid feedstock makes it possible to produce thin coatings (<100 μm) with more refined microstructures than in conventional plasma spraying. However, the low density of the feedstock droplets makes them very sensitive to the instantaneous characteristics of the fluctuating plasma jet at the location where they are injected. In this study, the interactions between the fluctuating plasma jet and droplets are explored by using numerical simulations. The computations are based on a three-dimensional and time-dependent model of the plasma jet that couples the dynamic behaviour of the arc inside the torch and the plasma jet issuing from the plasma torch. The turbulence that develops in the jet flow issuing in air is modeled by a large Eddy simulation model that computes the largest structures of the flow which carry most of the energy and momentum. This article is an invited paper selected from presentations at the 2007 International Thermal Spray Conference and has been expanded from the original presentation. It is simultaneously published in Global Coating Solutions, Proceedings of the 2007 International Thermal Spray Conference, Beijing, China, May 14-16, 2007, Basil R. Marple, Margaret M. Hyland, Yuk-Chiu Lau, Chang-Jiu Li, Rogerio S. Lima, and Ghislain Montavon, Ed., ASM International, Materials Park, OH, 2007.     

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。     

Experimental characterization and numerical modeling of a micro-syringe deposition system for dispensing sacrificial photopolymers on particulate ceramic substrates

This work addresses the characterization of a UV-based micro-syringe deposition (μSD) system utilized in the micro-dispensing of photopolymers on particulate ceramic substrates. This methodology is used in embedding functionally graded and interconnected micro-features within constructs produced by a novel combined powder-based additive manufacturing (AM) and UV-based micro-syringe deposition (μSD) technique. The process is experimentally characterized using SEM and optical microscopy to study the effect of a wide range of process parameters on the geometrical quality of deposited tracks. Experimental data show that the system can produce features ranging from 200 to 575 μm in width and from 20 to 200 μm in height on particulate ceramic surfaces. To gain insight into the proposed micro-deposition process, a two-tier model is also developed. The first framework describes an analytical model for predicting the flow rate of the dispensed photopolymer fluid based on the piston displacement. The second model is a stochastic framework for predicting the line width of the features deposited on the substrate using a Monte Carlo probabilistic simulation to compensate for uncertainty in the system input parameters. A comparison between experimental and modeling line width predictions shows that the modeling results are 14–38% higher than the experimental results, depending on the system input variables. The proposed model is enhanced by introducing adjustment factors to compensate for UV exposure delay, fluid migration, and imbibition.