Low Pressure Plasma Spraying Research Articles

Electrochemical Performance of Plasma Sprayed Metal Supported Planar Solid Oxide Fuel Cells

High material and production costs are key barriers to the widespread commercialization of solid oxide fuel cells (SOFCs). Thermal spray techniques are a low cost alternative for the production of SOFCs. The objective of this work was to fabricate single cells by thermal spraying and to evaluate the impact of electrolytes and anodes, differing in terms of thickness, composition and microstructure, on the cell performance. In this work, the anode layer was deposited on a planar ferritic steel support with in-plane dimensions 50 mm X 50 mm, thickness of 550 µm and 35-40% porosity by atmospheric plasma spraying (APS). The anode material consisted of nickel, yttria stabilized zirconia (YSZ) and a pore former. The anode layer thickness was around 60 µm. The electrolyte layer was deposited by a low pressure plasma spray technique called plasma spray-thin film (PS-TF) which can produce dense coatings at high deposition rates. The electrolyte material consisted of 3YSZ and 8YSZ consisting of different powder morphologies such as fused and crushed and agglomerated and sintered. In order to ensure sufficient gas tightness, electrolytes thicknesses of 50-90 µm were deposited. The cathode layer was screen-printed and sintered in-situ during the start-up of the electrochemical tests. The cathode material was La0.58Sr0.4Co0.2Fe0.8O3-δ (LSCF). It must be noted that no diffusion barrier layer was deposited between the YSZ electrolyte and the LSCF cathode which will affect the performance and stability of the cells. The electrochemical tests were performed in a temperature range of 600-800°C at atmospheric pressure under various gas compositions of H2, H2O and N2 on the anode side and of O2 and N2 on the cathode side while keeping the total gas flow constant. Current-voltage characteristics and impedance spectra were measured. Porous anodes and dense electrolytes were achieved in all cases. The measured open cell voltage indicated acceptable electrolyte gas tightness. Power density of produced cells at an operating voltage of 0.7 V were measured as high as 600 mW/cm2. In this work, the impact of spray parameters and applied layer thicknesses on the gas tightness of the electrolyte and the area specific resistance of the cell will be discussed. Keywords: plasma spraying, solid oxide fuel cells, electrochemical characterization, impedance spectroscopy

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.

Microstructure evolution of ZrB2–MoSi2 composite coatings at middle and high temperatures

In this work, ZrB2-based composite coatings with different contents of MoSi2 were fabricated by low pressure plasma spray (LPPS) technique. The oxidation behavior and microstructure evolution of the ZrB2–MoSi2 composite coatings were characterized at 1200°C and 1500°C. The results showed that the ZrB2–MoSi2 coatings exhibited lamellar microstrucuture with uniform MoSi2 distribution in the ZrB2 matrix. Silica, m-ZrO2, zircon and small amount of MoB were formed after the oxidation at 1200°C. While only silica, m-ZrO2 and zircon were formed at 1500°C. The increase of MoSi2 is helpful to improve the oxidation resistance of the composite coatings. The thickness of the total oxidized layer of ZrB2–40vol.% MoSi2 coating was about 17–22μm after 6h oxidation at 1200°C and 264–270μm after 6h oxidation at 1500°C.

Cavitation-resistant NiTi coatings produced by low-pressure plasma spraying (LPPS)

Cavitation is a severe wear mechanism in technical applications where parts are in contact with rapidly flowing liquids. Examples are turbine blades in hydropower plants or pump components. Coating exposed surfaces with wear-resistant materials is an effective measure for extending lifetime in the case of cavitation attack. NiTi is an attractive material for such coatings considering its clearly pronounced damping behavior based on its pseudoelastic properties. A promising processing route for coating net-shaped components with NiTi is low-pressure plasma spraying (LPPS). In the present work, NiTi layers were produced by LPPS, starting from pre-alloyed NiTi powder. Cavitation resistance was investigated in relation to LPPS parameters, layer thickness and specific surface treatment. Increased cavitation resistance was demonstrated compared to UTP 730, an established cavitation protection material. The study was accompanied by comprehensive characterization of microstructure and phase transformation behavior of the NiTi coatings.

Study of nanostructured coatings prepared by low-pressure plasma spraying with a supersonic jet

Zirconia coatings on copper prepared by the low-pressure plasma spraying of a ZrO2 + 5% Y2O3 powder through a mesh mask exhibit high resistance and low thermal conductivity in tests in the nitrogen plasma stream of a plasmatron. Data on the coating structure and its changes during thermal testing derived by scanning electron microscopy, X-ray diffraction analysis, and nuclear backscattering spectrometry are discussed.

Healing of the Interface Between Splashed Particles and Underlying Bulk Coating and Its Influence on Isothermal Oxidation Behavior of LPPS MCrAlY Bond Coat

The thermally grown oxide formed on the bond coat surface plays an important role in determining the lifetime of thermal barrier coatings (TBCs). The splashed particles on the thermally sprayed MCrAlY bond coat surface are weakly bonded to the underlying bulk coating, leading to the formation of mixed oxides and contributing to the TBC failure. In this study, the healing behavior of the weakly bonded interface between splashed particles and underlying MCrAlY bulk coating deposited by low pressure plasma spraying was examined, and the influence of interface healing on the isothermal oxidation behavior of the bond coat was discussed. Results show that the granular particles resulting from splashing of molten droplets were exposed on smooth splats which make up the surface of bulk coating. After the pre-diffusion treatment in vacuum, the small granular splashed particles are immersed into the bulk coating resulting from the element diffusion on the interface between splashed particles and underlying bulk coating. After the vacuum heat treatment, the formation of mixed oxides was effectively restrained due to the healing of the splashed particle/underlying bulk coating interface.

Low Pressure and Vacuum Plasma Spray Build Better Coatings for Engine Applications

Abstract Vacuum plasma and low pressure plasma spray technologies each provide unique solutions for achieving superior properties in metallic coatings that require low oxide content and high density. Applications include various turbine engine components, as well as biomedical and electronics components.

Microstructure of Thermal Barrier Coatings Deposited by Low Pressure Plasma Spraying and Chemical Vapour Deposition Methods on Rene 80 Nickel Superalloy

The paper presents results of research into thermal barrier coatings characterized by high oxidation resistance and hot corrosion. Bondcoats were formed by overaluminizing of an MeCrAlY–type coating deposited by low pressure plasma spraying. The outer ceramic layer of yttrium oxide stabilized zirconia oxide (Metco 6700) was deposited by plasma spray physical vapour deposition (PS-PVD). For comparison purposes additionally LPPS-sprayed were MeCrAlY bondcoats, which were not subsequently aluminized. Used as base material was Rene 80 nickel superalloy. The research has shown that as a result of aluminizing by the CVD method there was formed in the bondcoat a deposit zone built of the β-NiAl phase which protects from oxidation. Preserved below increased chromium content ensures resistance to hot corrosion. The outer ceramic layer was characterized by columnar structure similar to that obtained in the EB-PVD process.

Microstructure and Isothermal Oxidation Resistance of Thermal Barrier Coatings Deposited by LPPS, CVD and PS-PVD Methods on Inconel 617 Nickel Superalloy

The paper presents results of research into thermal barrier coatings characterized by higher oxidation resistance. Bondcoats were formed by overaluminizing of the MeCrAlY coating, deposited by low pressure plasma spraying (LPPS). The outer ceramic layer of yttrium oxide stabilized zirconia oxide was deposited by plasma spray physical vapour deposition (PS-PVD). For comparison purposes additionally formed were MeCrAlY bondcoats, which were not subsequently aluminized. The research showed that during CVD overaluminizing there was formed an additional layer built of the β-NiAl phase, which protects the base material from oxidation. Preserved below it increased chromium content ensures resistance to hot corrosion. The outer layer was characterized by columnar structure, similar to that obtained in the EB-PVD process. The isothermal oxidation tests showed that thickness of the TGO layer into overaluminized bondcoat significantly thicker in comparison with conventional LPPS-sprayed MeCrAlY bondcoats.

The Influence of Overaluminizing on TGO Formation on Thermal Barrier Coatings Deposited by Low Pressure Plasma Spraying and Chemical Vapour Deposition Methods on Rene 80 Nickel Superalloy

The paper presents results of research into isothermal oxidation test of thermal barrier coatings characterized by high oxidation resistance and hot corrosion. Bondcoats were deposited by overaluminizing of MeCrAlY–type coating deposited by LPPS method. The outer ceramic layer of yttrium oxide stabilized zirconia oxide (Metco 6700) was deposited by plasma spray physical vapour deposition (PS-PVD). For comparison purposes additionally LPPS-sprayed were MeCrAlY bondcoats, which were not subsequently aluminized.. The isothermal oxidation test at 1100oC for 1000h shown that thickness of the TGO layer in overaluminized bondcoat was significantly thicker in comparison with conventional LPPS-sprayed MeCrAlY bondcoats.

The Formation of TBCs Using LPPS, CVD and PS-PVD Methods on CMSX-4 Single-Crystal Nickel Superalloy

In the paper new type of thermal barrier coatings characterized by good oxidation and hot corrosion resistance were presented. Bond coats were formed by overaluminizing of an MeCrAlY type coating deposited by low pressure plasma spraying (LPPS). The outer ceramic layer of yttria stabilized zirconia (Metco 6700) was deposited by plasma spray physical vapour deposition (PS-PVD). Rene 80 nickel superalloy was used as base material. The research showed that double-layer bond coat was formed with external NiAl phase layer and inner MeCrAlY layer. The outer ceramic layer was characterized by columnar structure similar to that obtained in the EB-PVD process. The presence of secondary reaction zone (SRZ) was noted.

Equipment for Low Pressure Plasma Spraying Processes – A Review

The paper presents basic methods of plasma spraying at very low pressure (<2 mbar). Described in the text are conditions which influence microstructure of Thermal Barrier Coatings obtained by these methods. A review and characteristics of the LPPS-TF system applied around the world has been provided.

Influence of Deposition Parameters on Structure of TDCs Deposited by PS-PVD Method

The paper presents an advanced technology of Thermal Barrier Coatings (TBCs) deposition called Plasma Spray – Physical Vapor Deposition (PS-PVD). The PS-PVD is a low pressure plasma spray technology to deposit coatings out of vapor phase, which enables obtaining of columnar ceramic coatings. In this paper, the influence of various gas mixtures on properties of deposited coatings has been investigated. The measurement of coating thickness was conducted by a light microscopy method, followed by a statistical analysis. All processes had been conducted at a very low feed rate, which additionally affected ceramic microstructure.

Macroscopic and Microscopic Structure Evaluation of Al<sub>2</sub>O<sub>3</sub> Plasma Sprayed Coatings

Alumina (Al2O3) coatings by two types of plasma spraying method (APS: atmospheric plasma spraying and LPS: low pressure plasma spraying) were observed at the view point of coating structure in order to confirm superiority for electric isolation application. A breakdown voltage of dielectric layer is due to interlamellar gap continuous across Al2O3 coatings. The micron size evaluation of this gap was confirmed by macroscopic structure observation. The cross-sectional macroscopic structure was examined by copper electrolytic plating for visualization and by a scanning electron microscopy (SEM). Furthermore, dielectric volume resistivity was dominantly derived from microscopic structure and crystallinity of the coatings. The cross-sectional microscopic structure of the coatings was observed by a field emission SEM (FE-SEM). The crystallographic analyses were performed by electron back scatter diffraction (EBSD). As an evaluation result, there are no differences in macroscopic structures with gap of coatings between two spraying methods. The Al2O3 particles partially melted in spraying and made cracks in solidifying. The gap was derived from the cracking of molten particles and also exhibited the same structures between two spraying methods. Coating electric properties had no significant differences between two spraying methods. In this study, the factors which so significant differences were not observed in electric properties each spraying methods are investigated macroscopically and microscopically.

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).

Cavitation Erosion Resistance of Ti-Ni Intermetallic Coatings Prepared by Low Pressure Plasma Spray Process

In the current work, low pressure plasma spray process (LPPS) was applied to deposit Ti-Ni intermetallic coatings with Ni-clad Ti powder as feedstock. The microstructure and phase transition of LPPS sprayed Ti-Ni coating were investigated. Cavitation erosion resistance was examined using a standard ultrasonic cavitation test. The coating mainly consisted of TiNi phase with a certain amount of Ti2Ni, Ni3Ti phase and a few Ti phase. A few pores concentrated on the boundaries of the sprayed splats. The TiNi coating exhibited excellent cavitation erosion resistance.

Microstructure of 316L Stainless Steel Coating Deposited by the Low Pressure Plasma Spray

Original equiaxed 316L stainless steel coatings were successfully deposited by the low pressure plasma spray. For comparison, the coatings of 316L stainless steel with normal lamellar structure were also prepared by the air plasma spray (APS). The microstructures were investigated using optical micrograph (OM). The results show that the microstructures of LPPS 316L stainless steel coatings reveal the fine equiaxed microstructures like the solidified stainless steels,which are significantly different from that of APS coatings with lamellar structures.

Characterization of the Expanded Plasma Jet Generated at Reduced Chamber Pressure

Very low pressure plasma spraying has been attracted attention due to special properties of the expanded plasma jet which may probably deposit specific and unique structure coatings, such as columnar structure coatings. The forming of the coatings is affected by plasma generating and working conditions. These operating parameters such as arc power, plasma gas flow rate, and chamber pressure have influences on pressure, enthalpy and temperature of plasma jet. In this work, an enthalpy probe is applied to measure the enthalpy of argon-hydrogen plasma jet generated at chamber pressure of 3000 Pa and 50Pa. The results showed that the temperature of plasma jet reached to 11000 K at 25 mm from nozzle exit, and then dropped slowly to 7000 K at 400 mm.

Fabrication and Characterization of Thermal-Sprayed Fe-Based Amorphous/Nanocrystalline Composite Coatings: An Overview

This review focuses on the recent development of iron (Fe)-based amorphous/nanocrystalline composite coatings, which have attracted much attention due to their attractive combination of high hardness/strength, elevated abrasive wear resistance, and enhanced corrosion resistance. Accompanying the advancements in various thermal spray technologies, industrial application fields of Fe-based amorphous/nanocrystalline composite coatings are becoming more diverse. In the main part, the typical empirical rules for the design of amorphous alloys with high glass-forming ability are generalized and discussed at first. Then various thermal spray technologies for the fabrication of Fe-based amorphous/nanocrystalline composite coatings, such as high velocity oxygen/air spray (HVOF/HVAF), air plasma spray (APS), low-pressure plasma spray (LPPS), high-energy plasma spray (HPS), and high velocity arc spray (HVAS) processes, are introduced. The microstructures, hardness, wear resistance, and corrosion resistance of Fe-based amorphous/nanocrystalline composite coatings formed using these thermal spray technologies are reviewed and compared. Finally, the existing challenges and future prospects are proposed.

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.