Very Low Pressure Plasma Spraying Research Articles

A novel reverse-polarity plasma torch with extended hot-wall nozzle for atmospheric plasma spraying of dense yttria-stabilized zirconia coatings

Due to the high heating temperature and low cost, atmospheric plasma spraying (APS) has been widely used in the preparation of ceramic coatings, but its coatings are normally formed by the stacked splats from fully or partially melted feedstock particles, which inevitably leads to some pores, unbonded interfaces or cracks. Thus, in this paper, a novel atmospheric plasma spraying technique using a reverse-polarity plasma torch (RPT) with extended hot-wall nozzle was proposed to fully melt and accelerate feedstock particles for the preparation of dense YSZ ceramic coatings. Firstly, the RPT spray system was designed in detail, and its main electro-thermal characteristics were experimentally studied. The results showed that the RPT exhibited higher arc voltage, thermal efficiency and lower arc voltage fluctuation than that of traditional rod-cathode/normal-polarity plasma torches (NPT) at the similar output power. Secondly, a numerical simulate was carried out to reveal the effect of the extended hot-wall nozzle on the temperature and velocity distributions of the plasma jet. The results showed that the extended hot-wall nozzle remarkably extended the high-temperature and high-velocity region of the plasma jet from the RPT exit to the downstream. Finally, the dense and uniform YSZ coatings with a porosity of about 5.3% were successfully prepared by the RPT with extended hot-wall nozzle. The results showed that the YSZ coating exhibited denser microstructure and fewer defects than that of the coating prepared by traditional APS process. Meanwhile, the hardness and Young's modulus of the YSZ coating were 11.9 ± 1.4 GPa and 197.9 ± 29.4 GPa, respectively, which were relatively higher than that of the coating prepared by traditional APS process, and were very close to that of the coating prepared by very low-pressure plasma spraying (VLPPS) process.

Nitrogen species in a thermal plasma under very low pressure (150 Pa): Application to reactive plasma spraying

Very Low-Pressure Plasma Spraying (VLPPS) is an emerging process allowing the manufacturing of metallic or ceramic coatings under reduced pressure conditions (i.e., in the 150 Pa range). It aims at vaporizing primary feedstock precursors in the warm core of the plasma flow and at carrying them, through the expanded plasma column, towards the surface to be coated, where they condense to form a deposit. This process also can be operated in a reactive mode. In such a case, a secondary reactive gaseous precursor is injected in the plasma flow where it can react with the primary vaporized precursors. In this work, nitrogen is considered as the secondary gaseous precursor. Thereby, nitriding reactions occur between this secondary precursor and the primary one, titanium in the considered case. Optical emission spectroscopy is implemented to identify the reactive nitrogen species in an Ar-H2-N2 plasma gas ternary mixture together with the reaction mechanisms giving rise to the nitriding processes. Two mechanisms have been identified. At first, a gas-gas reactions occur between titanium vapors and N2, N2+ and N species leading to the formation of TiN nitrides; At second, a solid-gas reactions occur between flattened titanium lamellae and N2 species, leading to the formation of Ti2N nitrides, which as evidenced by XRD analyses. Nevertheless, the number of nitrides is rather low, very likely due to the limited fraction of titanium vapors in the system: this is a consequence of a rather limited plasma energy (i.e., net power of about 26 kW, corresponding to an average plasma mass enthalpy of 15.21 kJ kg−1).

Study on Deposition Behavior of Less Than 5 μm YSZ Particles in VLPPS

A particular YSZ feedstock for very low-pressure plasma spraying (VLPPS) has been designed which can be automatically divided into less than 5 μm YSZ molten particles in the long plasma jet. Fully molten particles were deposited on the substrate at different deposition distances of 250, 350 and 450 mm, respectively. The deposition behavior of less than 5 μm YSZ molten particles were studied aiming to obtain a thin gastight YSZ coating. The flattening ratio of particles at different deposition distances and the gas permeability of YSZ coating prepared by VLPPS were investigated. The results revealed that the thickness of flattened particles was about 0.10-0.35 μm and the flattening ratio of molten particles was about 4.7. The flattened particles were bonded well with substrates and the width of vertical cracks appeared in flattened particles was 0.01-0.02 μm. The gas permeability of coatings prepared at 350 mm was 1.5 × 10−7 cm4 (gf)−1 s−1.

Very low-pressure plasma-sprayed dense yttria-stabilized zirconia coatings using an axial bi-cathode plasma torch

In order to increase the plasma power and enhance the heating of sprayed powder, a bi-cathode plasma torch with axial powder injection was developed for the very low-pressure plasma spraying (VLPPS). By using this torch, uniform and dense yttria-stabilized zirconia (YSZ) coatings were prepared at different spraying distances. The plasma characteristics, microstructures and mechanical properties of YSZ coatings were investigated in detail. Results show that the new plasma torch had the ability to vaporize part of the YSZ powder. Meanwhile, the YSZ powder underwent a distinct phase change, at a maximum of 50% monoclinic zirconia phase transformed to the tetragonal-prime zirconia phase. The sprayed coatings had hybrid microstructures, including unmelted particles, molten droplets, and vapor clusters. With increasing spraying distance, the melting state of YSZ coating got better, while the microstructure became looser, leading to the porosity increased from 2.7% to 7.4%. The best mechanical properties were found in the densest coating, whose microhardness and Young's modulus was 11.7 ± 0.6 GPa, 167.8 ± 4.0 GPa, respectively, indicating that the mechanical performance of the YSZ coating was more dependent on the density, rather than the melting state. The dense VLPPS ceramic coatings may be applied for corrosion and heat resistance against oxidation, as well as the solid oxide fuel cells.

Development of ScSZ Electrolyte by Very Low Pressure Plasma Spraying for High-Performance Metal-Supported SOFCs

Very low pressure plasma spraying (VLPPS) is an attractive method for metal-supported solid oxide fuel cells, wherein it can significantly avoid the oxidation of metal substrate. In this study, scandia-stabilized zirconia electrolyte was fabricated by VLPPS. To investigate the microstructure of coatings, the spraying distances were set to 150, 250 and 350 mm. The fractured morphology suggests that, at a relatively low deposition temperature (< 350 °C), typical lamellar structure was replaced by trans-granular structure. The apparent porosity of coatings ranges from 4.75 to 6.83%, as per the image analysis method. The ionic conductivity of coating at 250 mm was 0.068 S cm−1, which was ~ 68% of bulk. The ratio of surface area to projected area of coatings surface could reach 6.53-8.30 measured by a 3D confocal laser microscope. Finally, testing cells showed a maximum output power density of 1112 mW cm−2 and an OCV of 1.07 V at 750 °C. The ohmic resistance of whole cell was 0.1 Ω cm2, and the total resistance was 0.15 Ω cm2. The results suggest that very low pressure plasma spraying is a promising approach for the manufacturing of high-performance MS-SOFCs.

Environmental Barrier Coatings Made by Different Thermal Spray Technologies

Environmental barrier coatings (EBCs) are essential to protect ceramic matrix composites against water vapor recession in typical gas turbine environments. Both oxide and non-oxide-based ceramic matrix composites (CMCs) need such coatings as they show only a limited stability. As the thermal expansion coefficients are quite different between the two CMCs, the suitable EBC materials for both applications are different. In the paper examples of EBCs for both types of CMCs are presented. In case of EBCs for oxide-based CMCs, the limited strength of the CMC leads to damage of the surface if standard grit-blasting techniques are used. Only in the case of oxide-based CMCs different processes as laser ablation have been used to optimize the surface topography. Another result for many EBCs for oxide-based CMC is the possibility to deposit them by standard atmospheric plasma spraying (APS) as crystalline coatings. Hence, in case of these coatings only the APS process will be described. For the EBCs for non-oxide CMCs the state-of-the-art materials are rare earth or yttrium silicates. Here the major challenge is to obtain dense and crystalline coatings. While for the Y2SiO5 a promising microstructure could be obtained by a heat-treatment of an APS coating, this was not the case for Yb2Si2O7. Here also other thermal spray processes as high velocity oxygen fuel (HVOF), suspension plasma spraying (SPS), and very low-pressure plasma spraying (VLPPS) are used and the results described mainly with respect to crystallinity and porosity.

Fabrication of Dense Gadolinia-Doped Ceria Coatings via Very-Low-Pressure Plasma Spray and Plasma Spray–Physical Vapor Deposition Process

Gadolinia-doped ceria (GDC) is a promising electrolyte material for low-temperature solid oxide fuel cells (LT-SOFCs). Many works used ceramic sintering methods to prepare the GDC electrolyte, which was mature and reliable but presented difficulties in rapidly preparing a large area of GDC electrolyte without cracks. The low-pressure plasma spray (LPPS) process has the potential to solve this problem, but few studies have been conducted to date. In this work, submicron GDC powder was agglomerated by a spray drying method to achieve the proper granularity with D50 about 10 μm, and then two dense GDC coatings were fabricated with this agglomerated GDC powder using very-low-pressure plasma spray (VLPPS) and plasma spray–physical vapor deposition (PS-PVD), respectively. The results indicate that the two GDC coatings exhibited similar microstructure but with different densification mechanisms. The VLPPS coating was mainly built up in the form of liquid splats, which had lower mechanical properties due to the lower density and crystallinity, while the PS-PVD coating was co-deposited with the vapor clusters and liquid splats, which had higher density, crystallinity, and mechanical properties. It can therefore be concluded that the GDC coating prepared by PS-PVD is more appropriate for the LT-SOFC application.

YAlO3—A Novel Environmental Barrier Coating for Al2O3/Al2O3–Ceramic Matrix Composites

Ceramic matrix composites (CMCs) are promising materials for high-temperature applications. Environmental barrier coatings (EBCs) are needed to protect the components against water vapor attack. A new potential EBC material, YAlO3, was studied in this paper. Different plasma-spraying techniques were used for the production of coatings on an alumina-based CMC, such as atmospheric plasma spraying (APS) and very low pressure plasma spraying (VLPPS). No bond coats or surface treatments were applied. The performance was tested by pull–adhesion tests, burner rig tests, and calcium-magnesium-aluminum-silicate (CMAS) corrosion tests. The samples were subsequently analyzed by means of X-ray diffraction, scanning electron microscopy, and energy-dispersive X-ray spectroscopy. Special attention was paid to the interaction at the interface between coating and substrate. The results show that fully crystalline and good adherent YAlO3 coatings can be produced without further substrate preparation such as surface pretreatment or bond coat application. The formation of a thin reaction layer between coating and substrate seems to promote adhesion.

VLPPS: An Emerging Process to Create Well-Defined Components by Additive Manufacturing

Plasma spraying in controlled atmosphere as very-low-pressure plasma spraying (VLPPS) allows to work with different states of matter, particularly vapor. As a result, the coating microstructure is unique (lower-scale elements, pore architecture) and the properties are improved. Another benefit of VLPPS process is the mold filling. Unlike other thermal spray processes, the vapor mode can follow the gas flow and reach the mold walls to fill. The vapor mode deposition is very soft and protects the surface state of the mold/substrate. The coating microstructure is homogeneous on all the geometry. The choice of a soluble salt mold permits to easily get back the coating and use it as a functional part. The objective is to demonstrate that VLPPS process can be used as an additive manufacturing device to create well-defined objects/pieces.

Formation mechanisms of in-situ Al based intermetallic coatings manufactured by very-low pressure plasma spraying

This study presents the manufacturing of an AlFe based intermetallic coating by the Very Low Pressure Plasma Spraying (VLPPS) process. Coating microstructure and mechanical properties are carried on with a focus on the in-situ reaction. An aluminum powder was sprayed with a plasma torch under a working pressure of 150Pa on a stainless steel substrate to get the reaction. The microstructural analysis shows that the Al-based intermetallic coating, containing phases such as Al13Fe4, Fe3Al and Al8Cr5 is formed by the in-situ reaction between the high temperature of the aluminum coating manufacturing and the substrate. Moreover, the dense coating obtained, with a supposed metallurgical bonding to the steel substrate, featured a packed columnar microstructure mixed with nano-sized particles. Finally, the intermetallic coating presents a higher (438.9HV0.025 with SD: 53.5) than pure Al one (110HV0.025).

Study on the Fabrication and Performance of Very Low Pressure Plasma Sprayed Porous Metal Supported Solid Oxide Fuel Cell

Solid oxide fuel cell (SOFC) is a high efficiency clean device that directly converts chemical energy of the fuel gas into electric energy. The long-term degradation and stability are the main problems at high temperature, so how to reduce the working temperature becomes a hotspot of research. It is an important way to reduce the thickness of electrolyte. Therefore, how to produce dense and thin electrolyte films economically becomes particularly important. It has been reported that a uniform and dense coating is hopefully prepared by using very low pressure plasma spraying (VLPPS) technology. Due to low operating pressure (~100 Pa), the plasma plume length elongates and its diameter enlarges. Under this condition, the particles can been heated for more time, and it is helpful to form dense coatings. The current study focuses on this technology for application in porous metal supported solid oxide fuel cell (MS-SOFC), especially for preparation in electrolyte. Using SEM, it was found that the densification of the electrolyte was very well, and this could be confirmed by the open-circuit voltage (OCV) of the cell. In the temperature range of 550~750 ℃, the OCV of the cell fairly stabilized between 1.05 V and 1.1 V. The cell was also analyzed by using electrochemical impedance spectroscopy (EIS). The maximum power density of the cell could be up to more than 1200 mW/cm2 at 750 ℃. The long-term stability of the cell was also preliminarily explored, and the result has shown that it is a very promising method for application in MS-SOFC.

Yb2Si2O7 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.

Study on the Fabrication and Performance of Very Low Pressure Plasma Sprayed Porous Metal Supported Solid Oxide Fuel Cell

Very low pressure plasma spraying (VLPPS) technology was utilized to deposit electrolyte on porous metal supported solid oxide fuel cells (MS-SOFCs). The cross section of plasma sprayed electrolyte was characterized by SEM. The impedance spectra of sprayed single cell was measured by electrochemical impedance spectroscopy at different operating temperatures. The gas permeability was also analyzed, and the long-term stability of the cell was also investigated. The results shows that the OCV of the cell stabilized between 1.05 V and 1.1 V at the temperature range of 550~750 ℃. The maximum power density of the cell could be up to 1164mW/cm2 at 750 ℃. The plasma sprayed MS-SOFCs shows good stability.

Effects of the nozzle design on the properties of plasma jet and formation of YSZ coatings under low pressure conditions

How to control the quality of the coatings has become a major problem during the plasma spraying. Because nozzle contour has a great influence on the characteristic of the plasma jet, two kinds of plasma torches equipped with a standard cylindrical nozzle and a converging-diverging nozzle are designed for low pressure plasma spraying(LPPS) and very low pressure plasma spraying(VLPPS). Yttria stabilized zirconia(YSZ) coatings are obtained in the reducing pressure environment. The properties of the plasma jet without or with powder injection are analyzed by optical emission spectroscopy, and the electron temperature is calculated based on the ratio of the relative intensity of two Ar I spectral lines. The results show that some of the YSZ powder can be vaporized in the low pressure enlarged plasma jet, and the long anode nozzle may improve the characteristics of the plasma jet. The coatings deposited by LPPS are mainly composed of the equiaxed grains and while the unmelted powder particles and large scalar pores appear in the coatings made by VLPPS. The long anode nozzle could improve the melting of the powders and deposition efficiency, and enhance the coatings’ hardness. At the same time, the long anode nozzle could lead to a decrease in the overspray phenomenon. Through the comparison of the two different size’s nozzle, the long anode is much more suitable for making the YSZ coatings.

A Novel Plasma-Sprayed Nanostructured Coating with Agglomerated-Unsintered Feedstock

In this article, an unusual agglomerated powder of Y2O3-stabilized ZrO2 (YSZ) that did not undergo calcination was introduced as a feedstock for thermal spray deposition using internal injection atmospheric plasma spray (APS) and the very-low-pressure plasma spray (VLPPS) methods at an ambient pressure of 100-150 Pa. The results show that the microstructure of the coating is influenced not only by the spray parameters (such as arc gas composition, ambient pressure, and arc current) but also by the manufacture process of the agglomerates particularly the sintering process. The microstructure of the coating exhibited a bimodal structure if the APS method was used; in this case, the microstructure resembles that of other nanostructured coatings using regular agglomerated-sintered feedstock. A coating having a novel fully nano-equiaxed structure with a microporosity of 10-15% was first successfully deposited using VLPPS with 20Ar-30He SLPM plasma gas flows at a current of 500 A. The experimental results suggest that the nano-scale equiaxed structure in the coating is directly formed from original nanoparticles that had undergone melting, while inside the nozzle they were subsequently solidified on the substrate. The VLPPS method, which offers some unique advantages over the conventional plasma spray process, is generic in nature and can potentially be used to deposit a wide variety of ceramic coatings for diverse applications. The thermal conductivity values of the fully nanostructured and bimodal structured coatings were measured, and the microstructures of the coating both in the as-sprayed state and after heat treatment for 10 h at 1300 °C were investigated.

THERMAL SHOCK PROPERTIES OF YTTRIA-STABILIZED ZIRCONIA COATINGS DEPOSITED USING LOW-ENERGY VERY LOW PRESSURE PLASMA SPRAYING

Yttria-stabilized zirconia (YSZ) coatings have been frequently used as a thermal protective layer on the metal or alloy component surfaces. In the present study, ZrO 2-7% Y 2 O 3 thermal barrier coatings (TBCs) were successfully deposited by DC (direct current) plasma spray process under very low pressure conditions (less than 1 mbar) using low-energy plasma guns F4-VB and F100. The experiments were performed to evaluate the thermal shock resistance of different TBC specimens which were heated to 1373 K at a high-temperature cycling furnace and held for 0.5 h, followed by air cooling at room temperature for 0.2 h. For comparison, a corresponding atmospheric plasma spray (APS) counterpart was also elaborated to carry out the similar experiments. The results indicated that the very low pressure plasma spray (VLPPS) coatings displayed better thermal shock resistance. Moreover, the failure mechanism of the coatings was elucidated.

Study of metallic powder behavior in very low pressure plasma spraying (VLPPS) — Application to the manufacturing of titanium–aluminum coatings

In this study, metallic materials made of aluminum and titanium were manufactured implementing very low pressure plasma spraying (VLPPS). Aluminum was selected at first as a demonstrative material due to its rather low vaporization enthalpy (i.e., 381.9kJ·mol−1). Developments were then carried out with titanium which exhibits a higher vaporization enthalpy (i.e., 563.6kJ·mol−1). Optical emission spectroscopy (OES) was implemented to analyze the behavior of each solid precursor (metallic powders) when it is injected into the plasma jet under very low pressure (i.e., in the 150Pa range). Besides, aluminum, titanium and titanium–aluminum coatings were deposited in the same conditions implementing a stick-cathode plasma torch operated at 50kW, maximum power. Coating phase compositions were identified by X-Ray Diffraction (XRD). Coating elementary compositions were quantified by Glow Discharge Optical Emission Spectroscopy (GDOES) and Energy Dispersive Spectroscopy (EDS) analyses. The coating structures were observed by Scanning Electron Microscopy (SEM). The coating void content was determined by Ultra-Small Angle X-ray Scattering (USAXS). The coatings exhibit a two-scale structure corresponding to condensed vapors (smaller scale) and solidified areas (larger scale). Titanium–aluminum sprayed coatings, with various Ti/Al atomic ratios, are constituted of three phases: metastable α-Ti, Al and metastable α2-Ti3Al. This latter is formed at elevated temperature in the plasma flow, before being condensed. Its rather small fraction, impeded by the rather small amount of vaporized Ti, does not allow modifying however the coating hardness.

Manufacturing of composite titanium-titanium nitride coatings by reactive very low pressure plasma spraying (R-VLPPS)

Very Low Pressure Plasma Spraying (VLPPS) is an emerging spray process nowadays intensively studied by many research centers in the World. To date, studies are mostly focused on the manufacturing of ceramic or metallic coatings. None refers to composite coatings manufacturing by reactive plasma spraying under very low pressure (i.e., ~150 Pa).This paper aims at presenting the carried-out developments and some results concerning the manufacturing of composite coatings by reactive spraying. Titanium was selected as metallic material in order to deposit titanium-nitride titanium coatings (Ti-TiN). Nitrogen was used as plasma gas and was injected along an Ar-H2-N2 plasma jet via a secondary injector in order to reach the nitrogen content on the substrate surface. Thus, different kind of reactive mechanisms were highlighted.Resulting coatings were characterized by Scanning Electron Microscopy (SEM) observations. Porous microstructures are clearly identified and the deposits exhibit condensed vapours and molten particles. Glow Discharge Optical Emission Spectroscopy (GDOES) analysis evidenced nitrogen inside the deposits and X-Ray Diffraction (XRD) analysis confirmed the formation of titanium nitride phases, such as TiN and Ti2N, depending upon the location of the nitrogen injection. Microhardness values as high as 800 VHN were measured on manufactured samples (to be compared to 220 VHN for pure titanium VLPPS-manufactured coatings).

Vapors and Droplets Mixture Deposition of Metallic Coatings by Very Low Pressure Plasma Spraying

In recent years, the very low pressure plasma-spraying (VLPPS) process has been intensely developed and implemented to manufacture thin, dense and finely structured ceramic coatings for various applications, such as Y2O3 for diffusion barriers, among other examples. This paper aims at presenting developments carried out on metallic coatings. Aluminum was chosen as a demonstrative material due to its “moderate” vaporization enthalpy (i.e., 38.23 KJ cm−3) compared to the one of copper (i.e., 55.33 KJ cm−3), cobalt (i.e., 75.03 KJ cm−3), or even tantalum (i.e., 87.18 KJ cm−3). The objective of this work is primarily to better understand the behavior of a solid precursor injected into the plasma jet leading to the formation of vapors and to better control the factors affecting the coating structure. Nearly dense aluminum coatings were successfully deposited by VLPPS at 100 Pa with an intermediate power plasma torch (i.e., Sulzer Metco F4 type gun with maximum power of 45 kW). Optical emission spectroscopy (OES) was implemented to study and analyze the vapor behavior into the plasma jet. Simplified CFD modeling allowed better understanding of some of the thermo-physical mechanisms. The effect of powder-size distribution, substrate temperature and spray distance were studied. The phase composition and microstructural features of the coatings were characterized by XRD and SEM. Moreover, Vickers microhardness measurements were implemented.

Preparation of dense silicate electrolyte coating with low pressure plasma spraying and very low pressure plasma spraying for intermediate-temperature solid oxide fuel cells

Magnesium doped lanthanum silicate with apatite-type structure, as a solid electrolyte for IT-SOFC (intermediate-temperature solid oxide fuel cells), was prepared by solid state sintering. The electrolyte coatings were fabricated by LPPS (Low Pressure Plasma Spraying) and VLPPS (Very Low Pressure Plasma Spraying). Their microstructures and phase composition were characterized by SEM and XRD. The gas permeability of coatings was measured with pure H2 and O2 at room temperature. It is shown that dense lanthanum silicate coatings can be obtained by LPPS and VLPPS. Amorphous and trace phases existed in coating deposited by LPPS. Well-crystallized lanthanum silicate coating can be prepared by one step VLPPS. The specific permeability of coating fabricated by VLPPS is 1.79 × 10−19 m2.