Low Pressure Plasma Spray Process Research Articles

Correlation between microstructure, phase composition and oxidation behavior of CoNiCrAlY/nano-Al2O3 coatings using satellited feedstock deposited by LPPS process

In this study, satellited CoNiCrAlY/2, 4 wt% nano-Al2O3, and conventional CoNiCrAlY feedstocks were deposited through the low-pressure plasma spraying (LPPS) process on Inconel738 substrate. The oxidation test was done at 1050 °C for 10 min and 5, 50, 100, 150, and 200 h. The microstructure and phase composition of the powders and coatings were characterized by the FESEM and XRD, respectively. The results showed that the CoNiCrAlY and Al2O3 powders comprised γ-CoNiCr and β-(Co,Ni)Al, and α-Al2O3, respectively. In contrast, the as-sprayed coatings included supersaturated γ-CoNiCr and meta-stable γ-Al2O3 phases. With the increase in the percentage of nanoparticles (from 2 to 4 wt%), the amount of porosity (from 2 to 3.76 vol%), and the roughness of the coating (from 6.1 to 7.5 μm) increased. With oxidation time, dissolved β-(Co,Ni)Al was recovered and then gradually consumed to create a thermally grown oxide (TGO) layer. In the composite coatings, the transformation of γ-Al2O3 to stable α-Al2O3 occurred, creating defects across the TGO layer. Therefore, with the addition of Al2O3 nanoparticles, the oxidation rate was continuously increased. The oxidation rate constant (Kp × 10−13) of conventional and composite coatings (2, 4 %) was measured as 3.89, 4.35 and 5.08 cm2.s−1, respectively.

Thermal Spraying of MCrAlY Overlay Coating Using New Ethanol-Fueled HVOF Gun

The MCrAlY overlay coatings are widely used for high-temperature protection of hot section part of gas turbines and jet engines. This type of coatings are usually thermally sprayed using APS (Atmospheric Plasma Spraying), LPPS (Low Pressure Plasma Spraying) as well as HVOF (High Velocity Oxygen Fuel) methods. In present article the newly developed ethanol based HVOF gun was used for production of this type of coatings. The stainless steel 18-8 type was used as a base material. The AMDRY 386 (Oerlikon-Metco) NiCrAlY powder was used for coatings production. In the research different oxygen (400, 500, 600 NLPM) and ethanol (16.5, 18.3, 21.3, 23.6 and 26.6 dm3/h) flow ratio were selected for experimental processes. The powder feed ratio was also changed during process. After deposition the microstructural assessment using Scanning Electron Microscopy and chemical composition analysis using EDS method were conducted. The obtained results showed that coating was above 100 μm thick depending on the process parameters. The low concentration of pores and oxides was also observed on coatings cross-section. Using of ethanol HVOF gun enables to form good quality MCrAlY coatings with 50% reduction of oxygen consumption in comparison with conventional HP/HVOF torch using kerosene such as JP 5000. The other benefit of its using is lower CO2 emission and lower concentration of carbon in coating in comparison with classic JP 5000 HVOF gun. The ethanol HVOF is a promising technology and might be considered as an replacement of LPPS and HVOF process for production of MCrAlY type of coatings.

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.

Environmental barrier coatings using low pressure plasma spray process

AbstractYb2Si2O7/Si bilayer environmental barrier coatings (EBCs) on SiC ceramic substrate were produced by low pressure plasma spray (LPPS) process. Phase composition, microstructure, and thermal durability of LPPS Yb2Si2O7/Si coating were investigated. XRD analysis indicated that the coating is mainly composed of Yb2Si2O7 with ~15.5v% Yb2SiO5 phases. The LPPS EBCs have a dense microstructure with porosity less than 4%. Adhesion strength measurement indicated the LPPS EBCs have an average adhesion strength of 29.1 ± 0.8 MPa. Furnace cycle test (FCT) on the coatings in air at 1316°C was performed and the test ran for 900 cycles and there was no coating spallation/failure for LPPS Yb2Si2O7/Si EBCs. The FCT results demonstrated the excellent thermal cycle durability of LPPS EBCs. Oxidation kinetics investigation of LPPS EBCs in flowing 90% H2O (g)+10% air at 1316°C showed that the thermally grown oxide (TGO) growth rate is close to the oxidation rate of pure Si in dry air and is significantly lower than that in water vapor environment. The LPPS process is promising in making highly durable Yb2Si2O7‐based dense EBCs by impeding diffusion and ingression of water vapor/O2.

Isothermal Oxidation of Thermal Barrier Coatings Deposited Using LPPS, CVD, and PS-PVD Methods on MAR M247 Nickel Superalloy

Abstract The article presents the results of microstructural characterization of newly developed three-layer thermal barrier coating (TBC) after isothermal oxidation test. Bond coats were deposited by the overaluminizing of MCrAlY coating deposited by low-pressure plasma spraying (LPPS) process. The outer ceramic layer of yttria-stabilized zirconia was deposited by the plasma spray physical vapor deposition process. The TBCs with MCrAlY bond coat without aluminizing process was produced by LPPS as well. The isothermal oxidation test at 1,100°C for 1,000 h showed that the thickness of the thermally grown oxides alumina oxide layer on overaluminized bond coats was significantly thinner in comparison with conventional LPPS-sprayed MCrAlY bond coats. The possibility of the presence of NiAl and Ni3Al phases in the outer zone of overaluminized bond coat after the oxidation test was observed.

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.

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).

Supersonic Plasma Spray Deposition of CoNiCrAlY Coatings on Ti-6Al-4V Alloy

Plasma spray is a versatile technology used for production of environmental and thermal barrier coatings, mainly in the aerospace, gas turbine, and automotive industries, with potential application in the renewable energy industry. New plasma spray technologies have been developed recently to produce high-quality coatings as an alternative to the costly low-pressure plasma-spray process. In this work, we studied the properties of as-sprayed CoNiCrAlY coatings deposited on Ti-6Al-4V substrate with smooth surface (R a = 0.8 μm) by means of a plasma torch operating in supersonic regime at atmospheric pressure. The CoNiCrAlY coatings were evaluated in terms of their surface roughness, microstructure, instrumented indentation, and phase content. Static and dynamic depositions were investigated to examine their effect on coating characteristics. Results show that the substrate surface velocity has a major influence on the coating properties. The sprayed CoNiCrAlY coatings exhibit low roughness (R a of 5.7 μm), low porosity (0.8%), excellent mechanical properties (H it = 6.1 GPa, E it = 155 GPa), and elevated interface toughness (2.4 MPa m1/2).

Applying Low-Pressure Plasma Spray (LPPS) for coatings in low-temperature SOFC

Low-temperature solid oxide fuel cell (LTSOFC) has shown great potentials for commercial applications in clean energy generation. Seeking for low cost and easy fabrication method is one of the most important issues for LTSOFC investigations. This paper introduces a new coating spray technology, namely Low-Pressure Plasma Spray (LPPS), for efficiently manufacturing different functional coatings of LTSOFC. By applying the LPPS technique, uniform and dense Ni0.8Co0.15Al0.05LiO2−δ (NCAL) coatings were made on both solid bipolar plates and porous nickel foams to perform as protecting coatings and electrode catalyst coatings respectively. Microstructure study showed that multi phases were formed and in-situ nano-micro crystallization occurred in the coatings during the LPPS process. Around 30 W output was achieved in a 4-cell stack indicating that the LPPS sprayed NCAL coatings on bipolar plates worked well. A fuel cell based on the NCAL-coated Ni foam reached an open circuit voltage (OCV) at 1.08 V and a maximum power density of 717 mW cm−2 at 550 °C. This study reveals that LPPS is a promising technology for fabricating coatings of LTSOFC.

Microstructure and electric properties of low-pressure plasma sprayed β-FeSi2 based coatings

Thermoelectric material β-FeSi2 based coating was fabricated by the technique low-pressure plasma spray (LPPS) on the Al2O3 substrate from different alloy powders. During the process LPPS, the phase transformation had occurred through the peritectoid, eutectoid reaction and their inverse reaction. The grain size of the as-sprayed β-FeSi2 doped Co coatings was reduced comparing with the original feedstock powders, which implied the thermal conductivity could effectively decreased by the LPPS process. The room temperature electrical conductivity showed metal and semiconductor properties on the as-sprayed and annealed coatings. This method and the results could solve the problems of practical application especially on special-shaped surfaces such as the automobile exhaust pipe or other complex surface shape of the device.

The Influence of LPPS Process Parameters on Porosity and Microstructure of MCrAlY Coatings

In present article the influence of Low Pressure Plasma Spraying (LPPS) process parameters on microstructure of MCrAlY coatings was studied. Nowadays the Atmospheric Plasma Spray (APS) is the most widely used process to obtain MCrAlY-type coatings. The LPPS process may be consider as an alternative technology for deposition of this type coatings which have less porosity and smaller amount of oxides compared to APS coatings. This features are important to obtain coatings with better hot corrosion and oxidation resistance. In order to determine and optimize the process parameters the microscopic investigations were carried out. Assessment of microstructure, thickness of coating and porosity level was conducted as well. Correlation between LPPS parameters and obtained microstructure have been found. It has significant meaning for deposition of MCrAlY coatings which could be applied as a bond coat in thermal barrier coating (TBC).

Effects of defects and element diffusion at coating/substrate interface on damping capacity

In this work, the effects of the microstructures at the coating/substrate interface and in coating on damping and dynamic modulus by using low pressure plasma spray process and electro-deposition (ELD) technology to deposit the Ni coating onto the Cu substrate were studied, and also research was conducted to explore the influence of elements diffusion on damping after ELD Ni/Cu coating system was heat treated. The microstructure of the coating system was observed using a scanning electron microscopy (CS3400) with an energy dispersive X-ray spectrometer. The dynamic mechanical properties were tested using a dynamic mechanical thermal analyzer IV. The results indicated that there was a negative correlation between the modulus and the damping capacity for a coating structure. The difference between the damping values of the low pressure plasma spray coating system with more defects and ELD coating systems with few defects approximately equals to the damping value produced by defects. Defect can affect the value of pure damping peak (high temperature background damping subtracted) at around 150 °C. The higher the defect rate was, the higher the value of pure damping peak was. Elements diffusion at interface could change the amplitude and temperature of damping peak. Copyright © 2015 John Wiley & Sons, Ltd.

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.

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.

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.

Oxidation and Ablation Resistance of Low Pressure Plasma-Sprayed ZrB2-Si Composite Coating

In the present work, ZrB2-based coating containing Si additive was prepared by low pressure plasma spray process. The chemical composition and microstructure of the ZrB2-Si coating were characterized by XRD, EDS and SEM. The oxidation behavior of the coating was investigated in ambient air for different duration time. The ablation-resistant property of the coating was carried out using a plasma flame. The results obtained indicate that the ZrB2-Si composite coating exhibited compact lamellar microstructure with a porosity less than 5%. The silicon phase was uniformly distributed in the ZrB2 matrix. The composite coating presented excellent oxidation-resistance at high temperature of 1500 °C, which resulted from the formed continuous and dense glassy silicon oxide film on its surface. The ablation resistance of the ZrB2-Si coating has been proved to be excellent, which could withstand the plasma flame (above 2000 °C, atmosphere) for 10 min.

Production and modification of hollow powders in plasma under controlled pressure

Conducting plasma processes under high or low pressure is an efficient way to affect the heat, mass and momentum exchange in a two-phase flow and this technique is widely used in such well-developed technologies as low-pressure plasma spraying (LPPS) and high pressure plasma-chemical processes. In addition operating pressure is a key parameter in novel plasma process for modification of hollow powders properties. Plasma processing of porous ceramic powders is an effective method of producing hollow spheres (HOSP) with predefined properties. Regardless the method hollow powders were produced their geometric and structural properties can be adjusted by re-melting in plasma of certain pressure: low pressure processing will expand hollow spheres and high pressure – contract it. Regulating the outer diameter of hollow sphere allows adjusting its shell thickness, apparent density, gas pressure in the cavity etc. Preliminary experiments with zirconia hollow powders demonstrated good agreement with theoretical estimations of HOSP properties. The same technique can be used for adjusting properties of ceramic hollow powders produced by different methods, including cost effective fly-ash particles (cenospheres).

Characteristics of Ceramic Coatings Made by Thin Film Low Pressure Plasma Spraying (LPPS-TF)

The thin film low pressure plasma spray process (LPPS-TF) has been developed with the aim of efficient depositing uniform and thin coatings with large area coverage by plasma spraying. At high power input (~150 kW) and very low pressure (~100 Pa) the plasma jet properties change considerably and it is even possible to evaporate the powder feedstock material providing advanced microstructures of the deposits. This relatively new technique bridges the gap between conventional plasma spraying and physical vapor deposition. In addition, the resulting microstructures are unique and can hardly be obtained by other processes. In this paper, microstructures made by LPPS-TF are shown and the columnar layer growth by vapor deposition is demonstrated. In addition to the ceramic materials TiO2, Al2O3 or MgAl2O4, the focus of the research was placed on partially yttria-stabilized zirconia. Variations of the microstructures are shown and discussed concerning potential coating applications.

Very low pressure plasma sprayed alumina and yttria-stabilized zirconia thin dense coatings using a modified transferred arc plasma torch

As a novel thermal spray process, very low pressure plasma spray (VLPPS) process has been significantly used to deposit thin, dense and homogenous ceramic coating materials for special application needs in recent years. In this study, in order to enhance low-energy plasma jet under very low pressure ambience, a home-made transferred arc nozzle was made and mounted on a low-power F100 plasma torch to fully melt or evaporate powder feedstock. As a result, thin and dense alumina (Al2O3) and yttria-stabilized zirconia (YSZ) ceramic coatings with an average thickness of 30–40μm were successfully elaborated by the VLPPS process below 1mbar. An optical emission spectroscopy (OES) was used to analyze the plasma jet properties. The microstructures of the coatings were observed by means of a scanning electron microscopy (SEM). It was found that the YSZ coatings displayed a bimodal microstructure which was composed of splats formed by melted particles and a little amount of vapor condensation from evaporated particles. However, vapor condensation could not be observed in the Al2O3 coatings, and only lamellar splats were found. The mechanical properties of both coatings were also evaluated.

Very low pressure plasma sprayed yttria-stabilized zirconia coating using a low-energy plasma gun

In the present study, a more economical low-energy plasma source was used to perform a very low pressure plasma-spray (VLPPS) process. The plasma-jet properties were analyzed by means of optical emission spectroscopy (OES). Moreover, yttria-stabilized zirconia coating (YSZ) was elaborated by a F100 low-power plasma gun under working pressure of 1 mbar, and the substrate specimens were partially shadowed by a baffle-plate during plasma spraying for obtaining different coating microstructures. Based on the SEM observation, a column-like grain coating was deposited by pure vapor deposition at the shadowed region, whereas, in the unshadowed region, the coating exhibited a binary microstructure which was formed by a mixed deposition of melted particles and evaporated particles. The mechanical properties of the coating were also well under investigation.