Thin Yttria-Stabilized Zirconia Coatings Deposited by Low-Energy Plasma Spraying Under Very Low Pressure Condition

In recent decades, very low pressure plasma spraying (VLPPS) technology (less than 10 mbar), as a next-generation coating process, has been extensively studied, because it can fully evaporate the materials to deposit dense, thin, and columnar grain coatings. This research aims at applying VLPPS with low-energy plasma source to melt or evaporate ceramic materials to develop high-quality thermal barrier coatings. Thin and homogeneous yttria-stabilized zirconia coatings were deposited successfully on a stainless steel substrate using low-power plasma spraying torch F100 (23 kW maximal) under very low pressure (1 mbar). The optical emission spectroscopy was used to analyze the properties of the plasma jet. The phase composition and the microstructure of the coatings were characterized by x-ray diffraction and scanning electron microscopy. The results showed that the YSZ powder was fully melted and partially evaporated, and the coatings had a hybrid microstructure that was combined with the condensation of the YSZ vapor and the melted particles. In addition, the porosity and microhardness of the coatings were evaluated.     

Thin and Dense Ceramic Coatings by Plasma Spraying at Very Low Pressure

The very low pressure plasma spray (VLPPS) process operates at a pressure range of approximately 100 Pa. At this pressure, the plasma jet interaction with the surrounding atmosphere is very weak. Thus, the plasma velocity is almost constant over a large distance from the nozzle exit. Furthermore, at these low pressures the collision frequency is distinctly reduced and the mean free path is strongly increased. As a consequence, at low pressure the specific enthalpy of the plasma is substantially higher, but at lower density. These particular plasma characteristics offer enhanced possibilities to spray thin and dense ceramics compared to conventional processes which operate in the pressure range between 5 and 20 kPa. This paper presents some examples of gas-tight and electrically insulating coatings with low thicknesses <50 μm for solid oxide fuel cell applications. Furthermore, plasma spraying of oxygen conducting membrane materials such as perovskites is discussed.     

Plasma spray deposition of nylon 11 coatings

Nylon 11 coatings up to 2mm in thickness have been successfully deposited by plasma spraying. The work showed that control of the torch traverse speed and arc power were essential for the production of sound coatings. The coating quality deteriorated rapidly for torch traverse speeds below 100mms^?1 due to thermal degradation, under the conditions used. An optimum in arc power for maximum wear resistance and minimum porosity was found to exist: power levels below 21kW gave incomplete melting whereas those above caused thermal degradation. The presence of voids in the coatings had a major influence on properties and their formation is attributed to entrapped gas in the interstices between unmelted particles, gas evolution during degradation and porosity in the feed powder. The porosity and wear peformance of the coatings were controllable by the plama spraying parameters. Adhesive transfer was a principal wear mechanism under sliding contact with fracture occurring along the splat boundaries.     

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.     

Three Dimensional Modeling of the Plasma Spray Process

Results of simulations of three-dimensional (3D) temperature and flow fields inside and outside of a DC arc plasma torch in steady state are presented with transverse particle and carrier gas injection into the plasma jet. The results show that an increase of the gas flow rate at constant current moves the anode arc root further downstream leading to higher enthalpy and velocity at the exit of the torch anode, and stronger mixing effects in the jet region. An increase of the arc current with constant gas flow rate shortens the arc, but increases the enthalpy and velocity at the exit of the torch nozzle, and leads to longer jets. 3D features of the plasma jet due to the 3D starting conditions at the torch exit and, in particular, due to the transverse carrier gas and particle injection, as well as 3D trajectories and heating histories of sprayed particles are also discussed.     

Effects of Anode Arc Root Fluctuation on Coating Quality During Plasma Spraying

To obtain a coating of high quality, a new type of plasma torch was designed and constructed to increase the stability of the plasma arc and reduce the air entrainment into the plasma jet. The torch, called bi-anode torch, generates an elongated arc with comparatively high arc voltage and low arc fluctuation. Spraying experiments were carried out to compare the quality of coatings deposited by a conventional torch and a bi-anode torch. Alumina coatings and tungsten carbide coatings were prepared to appraise the heating of the sprayed particles in the plasma jets and the entrainment of the surrounding air into the plasma jets, respectively. The results show that anode arc root fluctuation has only a small effect on the melting rate of alumina particles. On the other hand, reduced air entrainment into the plasma jet of the bi-anode torch will drastically reduce the decarbonization of tungsten carbide coatings.     

Improvement of Plasma Spray Torch Stability by Controlling Pressure and Voltage Dynamic Coupling

The development of coating formation processes involving electric arcs depends on process stability and the capacity to ensure a constant reproducibility of coating properties. This is particularly important when considering suspension plasma spraying or solution precursor plasma spraying. Submicron particles closely follow plasma instabilities and have nonhomogeneous plasma treatment. Recently, it has been shown that arc voltage fluctuations in direct-current (dc) plasma torches, showing dominant fluctuation frequencies between 4 and 6 kHz, are linked to pressure oscillations in the cathode cavity of the plasma torch. In this study, first, a method to isolate the different oscillation modes in arc voltage and pressure signals using signal processing methods is presented. Second, correlations between the different modes of oscillations are analyzed following the plasma torch operating parameters. Lastly, it is shown that the use of an acoustic stub, mounted on the torch body, decreases the amplitude of arc voltage fluctuations and slightly increases the mean voltage.     

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.     

Comparisons Between Two Different Three-Dimensional Arc Plasma Torch Simulations

Several numerical models have been developed to study the characteristics of an arc inside the nontransferred plasma torch. A few of them have considered complete geometry of cathode and anode nozzle (type I) whereas others have considered only anode nozzle with cathode tip (type II). In this work, a three-dimensional model is developed to simulate Ar-N₂ arc in type I and type II geometries. Various combinations of the arc length and arc core radius are predicted for the torch power that corresponds to given gas flow rate and current. Various combinations of the same and minimum entropy production for all cases could not be predicted in type II geometry. The difference between velocities predicted in both geometries is larger than that between temperatures. Three-dimensional effect in the plasma jet thermo-fluid fields demises along the axial direction. Torch efficiencies and arc voltages predicted in both geometries are comparable with measurements.     

Plasma–Powder Feedstock Interaction During Plasma Spray–Physical Vapor Deposition

Plasma spray–physical vapor deposition is a new process developed to produce coatings from the vapor phase. To achieve deposition from the vapor phase, the plasma–feedstock interaction inside the plasma torch, i.e., from the powder injection point to the nozzle exit, is critical. In this work, the plasma characteristics and the momentum and heat transfer between the plasma and powder feedstock at different torch input power levels were investigated theoretically to optimize the net plasma torch power, among other important factors such as the plasma gas composition, powder feed rate, and carrier gas. The plasma characteristics were calculated using the CEA2 code, and the plasma–feedstock interaction was studied inside the torch nozzle at low-pressure (20-25 kPa) conditions. A particle dynamics model was introduced to compute the particle velocity, coupled with Xi Chen’s drag model for nonevaporating particles. The results show that the energy transferred to the particles and the coating morphology are greatly influenced by the plasma gas characteristics and the particle dynamics inside the nozzle. The heat transfer between the plasma gas and feedstock material increased with the net torch power up to an optimum at 64 kW, at which a maximum of ~3.4% of the available plasma energy was absorbed by the feedstock powder. Experimental results using agglomerated 7-8 wt.% yttria-stabilized zirconia (YSZ) powder as feedstock material confirmed the theoretical predictions.     

Suspensions Plasma Spraying of Ceramics with Hybrid Water-Stabilized Plasma Technology

Technology of water-stabilized plasma torch was recently substantially updated through introduction of a so-called hybrid concept that combines benefits of water stabilization and gas stabilization principles. The high-enthalpy plasma provided by the WSP-H (“hybrid”) torch may be used for thermal spraying of powders as well as liquid feedstocks with high feed rates. In this study, results from three selected experiments with suspension plasma spraying with WSP-H technology are presented. Possibility of deposition of coatings with controlled microstructures was demonstrated for three different ceramics (YSZ—yttria-stabilized zirconia, YAG—yttrium aluminum garnet and Al₂O₃) introduced into ethanol-based suspensions. Shadowgraphy was used for optimization of suspension injection and visualization of the liquid fragmentation in the plasma jet. Coatings were deposited onto substrates attached to the rotating carousel with integrated temperature monitoring and air cooling, which provided an excellent reproducibility of the deposition process. Deposition of columnar-like YSZ and dense YAG and Al₂O₃ coatings was successfully achieved. Deposition efficiency reached more than 50%, as evaluated according to EN ISO 17 836 standard.     

Simulation of Arc Root Fluctuation in a DC Non-Transferred Plasma Torch with Three Dimensional Modeling

It is well known that the coating quality of plasma spraying is strongly influenced by the instability of jets in the plasma spray, which is due to arc root fluctuation. Three dimensional (3D) unsteady-state modeling was employed in this research to analyze the arc root fluctuation in a DC non-transferred plasma torch. Numerical calculations on the distributions of gas temperature and velocity in the plasma torch were carried out using argon as the plasma gas. The electrical current density and potential were also discussed. The results indicate that the fluctuation of arc inside the plasma torch is mainly induced by the movement of the arc root on the anode surface. The arc root moves downstream with the flow of gas, and simultaneously the arc is bent by electromagnetic force. When the arc bends close enough to the anode boundary, a new arc root is formed somewhere upstream of the current attachment. In this paper the nature of the arc root fluctuation is presented, and also it is demonstrated that the voltage-drop calculated is larger than that measured experimentally because the plasma inside the torch has some deviation from the local thermodynamic equilibrium state hypothesis used in the current study.     

Yb<Subscript>2</Subscript>Si<Subscript>2</Subscript>O<Subscript>7</Subscript> Environmental Barrier Coatings Deposited by Various Thermal Spray Techniques: A Preliminary Comparative Study

Dense, crack-free, uniform, and well-adhered environmental barrier coatings (EBCs) are required to enhance the environmental durability of silicon (Si)-based ceramic matrix composites in high pressure, high gas velocity combustion atmospheres. This paper represents an assessment of different thermal spray techniques for the deposition of Yb₂Si₂O₇ EBCs. The Yb₂Si₂O₇ coatings were deposited by means of atmospheric plasma spraying (APS), high-velocity oxygen fuel spraying (HVOF), suspension plasma spraying (SPS), and very low-pressure plasma spraying (VLPPS) techniques. The initial feedstock, as well as the deposited coatings, were characterized and compared in terms of their phase composition. The as-sprayed amorphous content, microstructure, and porosity of the coatings were further analyzed. Based on this preliminary investigation, the HVOF process stood out from the other techniques as it enabled the production of vertical crack-free coatings with higher crystallinity in comparison with the APS and SPS techniques in atmospheric conditions. Nevertheless, VLPPS was found to be the preferred process for the deposition of Yb₂Si₂O₇ coatings with desired characteristics in a controlled-atmosphere chamber.     

Characteristics of Cu Film Deposited Using VLPPS

Cu coatings were obtained by the very low pressure plasma spray (VLPPS) process using a torch F4-VB. The tank pressure was varied from 1 to 5 mbar: these specific conditions can be allowed to obtain a higher vapor condensation fraction in the coating. Different sizes of powders are used to compare the vaporization level. The other possible influencing factors for obtaining compact film-like coating are also considered such as the distance between the torch and substrate, the orientation of the vapors and also the substrate temperatures. Microstructures of coatings are analyzed and combined with the results of plasma diagnostics. Jobin-Yvon spectrometer (type TRIAX190, UK) and Plasus Specline Spectroscopy software are both used for detecting and analyzing plasma spectrum data. The value of plasma electronic excited temperature T _e was calculated through choosing H_α and H_β two atom spectra. The results showed that the plasma belongs to cold plasma in the local thermodynamic equilibrium situation in VLPPS.     

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     

Noise emissions in thermal spray operations

Acoustic noise generation is an accompanying effect produced during thermal spraying. This type of noise is found both during the preparatory stages, such as grit blasting and compressed air cleaning, and during thermal spraying. A real-time noise meter was used to measure the noise level at frequencies between 63 and 8000 Hz during the operation of powder flame, wire flame, wire arc, air plasma, and high velocity oxygen fuel (HVOF) spraying processes. Noise was reported as either an A-weighted noise spectrum or an equivalent sound pressure level. The effect of different parameters, such as secondary plasma gas type, modes of wire flame torch operation, and use of compressed air cooling were investigated. The results indicated that the turbulence of the gas departing from the torch gives rise to jet noise. High gas flows mainly contributed to the lower frequencies, whereas combustion and plasma generation contributed to the higher frequencies. Noise level was the highest (123 dB(A)) with HVOF spraying and air plasma spraying with the use of a small-diameter nozzle and hydrogen as a secondary plasma gas. All manual operators of thermal spray equipment require hearing protection. The use of different hearing protection devices is discussed and the attenuation provided by each device is reported.     

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.     

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.     

An evaluation of the electric arc spray and (HPPS) processes for the manufacturing of high power plasma spraying MCrAIY coatings

The high power plasma torch (PlazJet) can be used to spray refractory ceramics with high spray rates and deposition efficiency. It can provide dense and hard coating with high bond strengths. When manufacturing thermal barrier coatings, the PlazJet gun is well adapted to spraying the ceramic top coat but not the MCrAIY materials that are used as bond coat. Arc spraying can compete with plasma spraying for metallic coatings since cored wires can be used to spray alloys and composites. In addition, the high production rate of arc spraying enables a significant decrease in coating cost. This paper discusses the performances of the PlazJet gun, and a twin-wire are spray system, and compares the properties and cost of MCrAIY coatings made with these two processes. For arc spraying, the use of air or nitrogen as atomizing gas is also investigated.     

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.