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.     

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.     

Effective Parameters in Axial Injection Suspension Plasma Spray Process of Alumina-Zirconia Ceramics

Suspension plasma spray (SPS) is a novel process for producing nano-structured coatings with metastable phases using significantly smaller particles as compared to conventional thermal spraying. Considering the complexity of the system there is an extensive need to better understand the relationship between plasma spray conditions and resulting coating microstructure and defects. In this study, an alumina/8 wt.% yttria-stabilized zirconia was deposited by axial injection SPS process. The effects of principal deposition parameters on the microstructural features are evaluated using the Taguchi design of experiment. The microstructural features include microcracks, porosities, and deposition rate. To better understand the role of the spray parameters, in-flight particle characteristics, i.e., temperature and velocity were also measured. The role of the porosity in this multicomponent structure is studied as well. The results indicate that thermal diffusivity of the coatings, an important property for potential thermal barrier applications, is barely affected by the changes in porosity content.     

Nanoparticles Modeling in Axially Injection Suspension Plasma Spray of Zirconia and Alumina Ceramics

Suspension plasma spray (SPS) is a thermal spray method in which nanoparticles are injected into the plasma jet with the help of suspension droplets to achieve thin and finely structured nanocoatings. The nanoparticles experience three in-flight stages: injection within the suspension droplets, discharge of the nanoparticle agglomerates after the evaporation of the suspension solvent, and tracking of the nanoparticle or agglomerates during the momentum and heat transfer with the plasma jet before coating. A numerical model is proposed in this paper for nanoparticle injection, discharge, acceleration, heating, melting, and evaporation. Initial values of suspension droplet size and agglomerate size are selected according to typical experimental data. Noncontinuum effects on particle acceleration and heating, known as Knudsen effects, are considered, as well as the influence of evaporation on the heat transfer. After a comparison with the experimental data, this nanoparticle model is applied for zirconia and alumina axially injected into the suspension plasma spray. Trajectory, velocity, and temperature of the in-flight nanoparticles are predicted for different initial sizes ranging from 30 nm to 1.5 μm; the distributions of the particle characteristics for multiple particles in the spray are also presented. The effects of powder size and material, power input, plasma gas flow rate, and standoff distance on the nanoparticle characteristics have been investigated and discussed.     

Microstructure of Suspension Plasma Spray and Air Plasma Spray Al2O3-ZrO2 Composite Coatings

Al₂O₃-ZrO₂ coatings were deposited by the suspension plasma spray (SPS) molecularly mixed amorphous powder and the conventional air plasma spray (APS) Al₂O₃-ZrO₂ crystalline powder. The amorphous powder was produced by heat treatment of molecularly mixed chemical solution precursors below their crystallization temperatures. Phase composition and microstructure of the as-synthesized and heat-treated SPS and APS coatings were characterized by XRD and SEM. XRD analysis shows that the as-sprayed SPS coating is composed of α-Al₂O₃ and tetragonal ZrO₂ phases, while the as-sprayed APS coating consists of tetragonal ZrO₂, α-Al₂O₃, and γ-Al₂O₃ phases. Microstructure characterization revealed that the Al₂O₃ and ZrO₂ phase distribution in SPS coatings is much more homogeneous than that of APS coatings.     

Suspension Plasma Spraying of YPSZ Coatings: Suspension Atomization and Injection

Among processes evaluated to produce some parts of or the whole solid-oxide fuel cell, Suspension Plasma Spraying (SPS) is of prime interest. Aqueous suspensions of yttria partially stabilized zirconia atomized into a spray by an internal-mixing co-axial twin-fluid atomizer were injected into a DC plasma jet. The dispersion and stability of the suspensions were enhanced by adjusting the amount of dispersant (ammonium salt of polyacrylic acid, PAA). A polyvinyl alcohol (PVA) was further added to the suspension to tailor its viscosity. The PVA also improved the dispersion and stability of the suspensions. The atomization of optimized formulations is described implementing Weber and Ohnesorge dimensionless numbers as well as gas-to-liquid mass ratio (ALR) value. Drop size distributions changed from monomodal distributions at low We to multimodal distributions when We number increases. The viscosity of the suspensions has a clear influence on the drop size distribution and suspension spray pattern. The secondary fragmentation of the drops due to the plasma jet was evidenced and the final size of the sheared drops was shown to depend on the characteristics of the suspension. Rather dense zirconia coatings have been prepared, which is a promising way to produce electrolyte.     

Pressure-Based Liquid Feed System for Suspension Plasma Spray Coatings

Thermal spraying with liquid-based feedstocks demonstrated a potential to produce coatings with new and enhanced characteristics. A liquid delivery system prototype was developed and tested in this study. The feeder is based on the 5MPE platform and uses a pressure setup to optimally inject and atomize liquid feedstock into a plasma plume. A novel self-cleaning apparatus is incorporated into the system to greatly reduce problems associated with clogging and agglomeration of liquid suspensions. This approach also allows the liquid feedstock line to the gun to remain charged for quick on-off operation. Experiments on aqueous and ethanol-based suspensions of titania, alumina, and YSZ were performed through this liquid delivery system using a 9MB plasma gun. Coatings with ultrafine splat microstructures were obtained by plasma spraying of those suspensions. Phase composition and microstructure of the as-sprayed coatings were investigated.     

Study of the Synchronous Injection in a Controlled Pulsed Arc Plasma

In the field of plasma spray technologies, new processes are developing to obtain coatings with nanostructured architectures. Difficulties of understanding and controlling the process originate from the continuous injection of a liquid material and the power instabilities of the current torches which strongly affect the heat and momentum transfers to the nanometric particles. This paper reports an original method to make TiO₂ coatings by suspension plasma spraying. A direct current (DC) power supply applying time-modulated current amplitude to a custom DC torch is used to generate at low power (1.5 kW) a pulsed laminar plasma jet with periodic oscillations of its properties. To make best use of this pulsed mode, a synchronization device was developed. It allows triggering from the arc voltage an inkjet nozzle to deliver at a precise moment a single droplet to improve the control of plasma/material interaction. An ink of TiO₂ anatase solid particles is formulated to be compatible with a drop-on-demand printhead dispenser. In-flight diagnostic is made by optical emission spectroscopy and a fast shutter camera. TiO₂ coatings are characterized by scanning electron microscopy, x-ray diffraction and transmission electron microscopy. Results show that homogeneous TiO₂ coatings of nanostructured cauliflowers shapes are obtained thanks to the controlled injection system. A competition between nucleation mechanism and liquid particles deposition are also observed. These deposits correspond to a mixture of anatase and rutile phases.     

Highly Segmented Thermal Barrier Coatings Deposited by Suspension Plasma Spray: Effects of Spray Process on Microstructure

Effects of the ceramic powder size used for suspension as well as several processing parameters in suspension plasma spraying of YSZ were investigated experimentally, aiming to fabricate highly segmented microstructures for thermal barrier coating (TBC) applications. Particle image velocimetry (PIV) was used to observe the atomization process and the velocity distribution of atomized droplets and ceramic particles travelling toward the substrates. The tested parameters included the secondary plasma gas (He versus H₂), suspension injection flow rate, and substrate surface roughness. Results indicated that a plasma jet with a relatively higher content of He or H₂ as the secondary plasma gas was critical to produce highly segmented YSZ TBCs with a crack density up to ~12 cracks/mm. The optimized suspension flow rate played an important role to realize coatings with a reduced porosity level and improved adhesion. An increased powder size and higher operation power level were beneficial for the formation of highly segmented coatings onto substrates with a wider range of surface roughness.     

Nanocomposite Lanthanum Zirconate Thermal Barrier Coating Deposited by Suspension Plasma Spray Process

This work seeks to develop an innovative nanocomposite thermal barrier coating (TBC) exhibiting low thermal conductivity and high durability compared with that of current TBCs. To achieve this objective, nanosized lanthanum zirconate particles were selected for the topcoat of the TBC system, and a new process—suspension plasma spray—was employed to produce desirable microstructural features: the nanocomposite lanthanum zirconate TBC contains ultrafine splats and high volume porosity, for lower thermal conductivity, and better durability. The parameters of plasma spray experiment included two main variables: (i) spray distance varying from 40 to 80 mm and (ii) the concentration of suspension 20, 25, and 30 wt.%, respectively. The microstructure of obtained coatings was characterized with scanning electron microscope and x-ray diffraction. The porosity of coatings is in the range of 6-10%, and the single phase in the as-sprayed coatings was pyrochlore lanthanum zirconate.     

Erosion Performance of Gadolinium Zirconate-Based Thermal Barrier Coatings Processed by Suspension Plasma Spray

7-8 wt.% Yttria-stabilized zirconia (YSZ) is the standard thermal barrier coating (TBC) material used by the gas turbines industry due to its excellent thermal and thermo-mechanical properties up to 1200 °C. The need for improvement in gas turbine efficiency has led to an increase in the turbine inlet gas temperature. However, above 1200 °C, YSZ has issues such as poor sintering resistance, poor phase stability and susceptibility to calcium magnesium alumino silicates (CMAS) degradation. Gadolinium zirconate (GZ) is considered as one of the promising top coat candidates for TBC applications at high temperatures (>1200 °C) due to its low thermal conductivity, good sintering resistance and CMAS attack resistance. Single-layer 8YSZ, double-layer GZ/YSZ and triple-layer GZdense/GZ/YSZ TBCs were deposited by suspension plasma spray (SPS) process. Microstructural analysis was carried out by scanning electron microscopy (SEM). A columnar microstructure was observed in the single-, double- and triple-layer TBCs. Phase analysis of the as-sprayed TBCs was carried out using XRD (x-ray diffraction) where a tetragonal prime phase of zirconia in the single-layer YSZ TBC and a cubic defect fluorite phase of GZ in the double and triple-layer TBCs was observed. Porosity measurements of the as-sprayed TBCs were made by water intrusion method and image analysis method. The as-sprayed GZ-based multi-layered TBCs were subjected to erosion test at room temperature, and their erosion resistance was compared with single-layer 8YSZ. It was shown that the erosion resistance of 8YSZ single-layer TBC was higher than GZ-based multi-layered TBCs. Among the multi-layered TBCs, triple-layer TBC was slightly better than double layer in terms of erosion resistance. The eroded TBCs were cold-mounted and analyzed by SEM.     

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.     

钴基合金等离子转移弧喷焊组织结构和性能研究

应用光学金相、扫描电镜(SEM)、X射线衍射(XRD)、透射电镜(TEM)和显微硬度测试对钴基合金等离子喷焊的组织结构和显微硬度，以及时效过程中的物相和显微硬度变化进行了研究。结果表明，合金层主要是由γ（Co)和（Cr,Fe)7(C,B)3构成。表现出亚共晶的组织形态。它在时效过程中物相发生了变化，并且发生了由(Cr，Fe)7(C，B)3向Cr23(C，B)6的转变过程，时效后显微硬度平均提高约11％。研究表明，时效过程中新相的析出、物相类型的转变和聚集长大是喷焊层显微硬度发生变化的原因。     

Suspension Plasma Spray and Performance Characterization of Half Cells with NiO/YSZ Anode and YSZ Electrolyte

The use of a liquid feedstock carrier in suspension plasma spray (SPS) permits injection of fine powders, providing the possibility of producing sprayed coatings that are both thin and dense and have fine microstructures. These characteristics make SPS an attractive process for depositing highly efficient electrodes and electrolytes for solid oxide fuel cell (SOFC) applications. In this study, NiO-yttria stabilized zirconia (YSZ) anode and YSZ electrolyte half cells were successfully deposited on porous Hastelloy X substrates by SPS. The NiO-YSZ anode deposition process was optimized by design of experiment. The YSZ electrolyte spray process was examined by changing one parameter at a time. The results from the design-of-experiment trials indicated that the porosity of the as-deposited coatings increased with an increase of suspension feed rate while it decreased with an increase of total plasma gas flow rate and standoff distance. The deposition rate increased with an increase of total plasma gas flow rate, suspension feed rate, and standoff distance. The microstructure examination by SEM showed that the NiO and YSZ phases were homogeneously distributed and that the YSZ phase had a lamellar structure. It was observed that the density of the YSZ electrolyte layer increased as input power of the plasma torch increased. Electrochemical characterization of the fabricated cells indicated that an open cell voltage of 0.989 V at 500 °C and a peak power of 0.610 W/cm² at 750 °C were reached.     

Coating Bores of Light Metal Engine Blocks with a Nanocomposite Material using the Plasma Transferred Wire Arc Thermal Spray Process

Engine blocks of modern passenger car engines are generally made of light metal alloys, mostly hypoeutectic AlSi-alloys. Due to their low hardness, these alloys do not meet the tribological requirements of the system cylinder running surface—piston rings—lubricating oil. In order to provide a suitable cylinder running surface, nowadays cylinder liners made of gray cast iron are pressed in or cast into the engine block. A newer approach is to apply thermal spray coatings onto the cylinder bore walls. Due to the geometric conditions, the coatings are applied with specifically designed internal diameter thermal spray systems. With these processes a broad variety of feedstock can be applied, whereas mostly low-alloyed carbon steel feedstock is being used for this application. In the context of this work, an iron-based wire feedstock has been developed, which leads to a nanocrystalline coating. The application of this material was carried out with the Plasma Transferred Wire Arc system. AlMgSi0.5 liners were used as substrates. The coating microstructure and the properties of the coatings were analyzed.     

Microstructural Characteristics of Y2O3-MgO Composite Coatings Deposited by Suspension Plasma Spray

Dense composite Y₂O₃-MgO coatings have been deposited by suspension plasma spray. Ethanol-based suspensions of powders synthesized by thermal decomposition of precursor solutions containing yttrium nitrate (Y[n]) and magnesium nitrate (Mg[n]) or magnesium acetate (Mg[a]) were selected as the feedstock; this gave powders with both phases in each particle, to inhibit phase segregation during solvent evaporation. The influence of powder characteristics on the microstructures of the coatings was investigated. The Y[n]Mg[a] suspension was more stable, with a better dispersion of the component phases than the Y[n]Mg[n] suspension. The coatings deposited using each suspension type exhibited lamellar structures comprising Y₂O₃ and MgO phases in wavy alternating streaks, with unmelted/semi-melted particles entrapped in the lamellae; this indicates that phase segregation still occurred in the molten state. Eutectic structures were formed in the coating generated using the Y[n]Mg[a] suspension, resulting from improved mixing of the component phases in the suspension powder.     

A Study on Arc Instability Phenomena of an Axial Injection Cathode Plasma Torch

Axial injection in plasma gun through the cathode has clear benefit of longer particle residence time and optimum particle trajectory in the plume; however, accelerated wear of the cathode seem to be the major issue in this approach. This study investigates the arc instability phenomena in an axially injecting single cathode plasma torch design. Gun voltage measurements were used to evaluate the arc behavior. For comparison purpose, arc fluctuations with a standard solid cathode torch design under identical operating parameters have also been studied. A comparison of different internal hardware configurations is also done to understand and establish the important factors in the design of the axial injection and solid cathode systems. Further, this study presents the influence of plume elongation and accelerated gas velocities on the arc behavior in different configurations under low pressure environment.     

Effect of Powder Injection Location on Ceramic Coatings Properties When Using Plasma Spray

The effect of powder injecting location of the plasma spraying on spraying properties was studied. Three different powder-injecting methods were applied in the experiment. In the first method, the particles were axially injected into the plasma flow from the cathode tip. In the second method, the particles were radially injected into the plasma flow just downstream of the anode arc root inside the anode nozzle. In the third method, the particles were radially injected into the plasma jet at the nozzle exit. The alumina particles with a mean diameter of 20 μm were used to deposit coatings. Spraying properties, such as the deposition efficiency, the melting rate of the powder particles, and the coating quality were investigated. The results show that the spraying with axial particle injecting can heat and melt the powder particles more effectively, produce coatings with better quality, and have higher deposition efficiency. 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.     

Practical Aspects of Suspension Plasma Spray for Thermal Barrier Coatings on Potential Gas Turbine Components

Suspension plasma spray (SPS) process has attracted extensive efforts and interests to produce fine-structured and functional coatings. In particular, thermal barrier coatings (TBCs) applied by SPS process gain increasing interest due to its potential for superior thermal protection of gas turbine hot sections as compared to conventional TBCs. Unique columnar architectures and nano- and submicrometric grains in the SPS-TBC demonstrated some advantages of thermal shock durability, low thermal conductivity, erosion resistance and strain-tolerant microstructure. This work aimed to look into some practical aspects of SPS processing for TBC applications before it becomes a reliable industry method. The spray capability and applicability of SPS process to achieve uniformity thickness and microstructure on curved substrates were emphasized in designed spray trials to simulate the coating fabrication onto industrial turbine parts with complex configurations. The performances of the SPS-TBCs were tested in erosion, falling ballistic impact and indentational loading tests as to evaluate SPS-TBC performances in simulated turbine service conditions. Finally, a turbine blade was coated and sectioned to verify SPS sprayability in multiple critical sections. The SPS trials and test results demonstrated that SPS process is promising for innovative TBCs, but some challenges need to be addressed and resolved before it becomes an economic and capable industrial process, especially for complex turbine components.     

Numerical Modeling of Suspension HVOF Spray

A three-dimensional two-way coupled Eulerian-Lagrangian scheme is used to simulate suspension high-velocity oxy-fuel spraying process. The mass, momentum, energy, and species equations are solved together with the realizable k-ε turbulence model to simulate the gas phase. Suspension is assumed to be a mixture of solid particles [mullite powder (3Al₂O₃·2SiO₂)], ethanol, and ethylene glycol. The process involves premixed combustion of oxygen-propylene, and non-premixed combustion of oxygen-ethanol and oxygen-ethylene glycol. One-step global reaction is used for each mentioned reaction together with eddy dissipation model to compute the reaction rate. To simulate the droplet breakup, Taylor Analogy Breakup model is applied. After the completion of droplet breakup, and solvent evaporation/combustion, the solid suspended particles are tracked through the domain to determine the characteristics of the coating particles. Numerical simulations are validated against the experimental results in the literature for the same operating conditions. Seven or possibly eight shock diamonds are captured outside the nozzle. In addition, a good agreement between the predicted particle temperature, velocity, and diameter, and the experiment is obtained. It is shown that as the standoff distance increases, the particle temperature and velocity reduce. Furthermore, a correlation is proposed to determine the spray cross-sectional diameter and estimate the particle trajectories as a function of standoff distance.