Gas-stabilized Plasma Research Articles

Structure and electrical properties of yttrium oxide sprayed by plasma torches from powders and suspensions

Yttrium oxide was sprayed by a plasma torch using a coarse thermal spray powder, which must be in size range of tens of micrometers to ensure good penetration into the plasma stream. Thick coatings on steel substrates were produced with two sprays systems facilitating gas stabilized plasma (GSP) and hybrid water-argon stabilized plasma (WSP–H) techniques. Additionally, an ultra-fine yttrium oxide powder was sprayed from a suspension. Hybrid water-argon stabilized plasma system was used for this purpose. Markedly thinner compact coatings were produced this way. All three sorts of plasma sprayed deposits were studied by the same methods. Dielectric properties were studied in a broad range of frequencies and temperatures. The microstructure aspects as well as crystallite size were analyzed and discussed in relation to electrical properties. All coatings exhibited stable dielectric parameters versus changing frequency and temperature, comparable with literature values for various samples. Concerning sintered bulks, and especially their thermal stability of capacitance, the plasma sprayed coatings were slightly worse. However due to shape and size variability of the plasma spraying are yttria coatings prospective for technical applications.

Dielectric properties of Ce-doped YAG coatings produced by two techniques of plasma spraying

Yttrium aluminum garnet (YAG) with cerium admixture (Ce:YAG) was plasma sprayed using two different devices – gas-stabilized plasma (GSP) torch and water-stabilized plasma (WSP) torch. Coatings on stainless steel as well as self-standing plates were produced. Besides microstructure and crystallographic characterizations, dielectric tests were performed on these coatings. They included capacitance (i.e. relative permittivity), loss tangent and volume resistivity measurements. After spraying, the YAG crystal phase was preserved without any decomposition, but an amorphous fraction was detected in the as-sprayed coatings deposited by both processes. The dielectric behavior of the coatings was influenced by imperfections like splat boundaries, pores and thin cracks. The Ce:YAG samples were successfully plasma sprayed by both spray techniques. Selected aspects of wear were measured and compared with a single-crystal. The dielectric properties are comparable with the single-crystal and highly promising, particularly the loss tangent with values so low that they were not found in any other as-sprayed ceramic coating.

Structure and properties of plasma sprayed BaTiO3 coatings after thermal posttreatment

Previously published results on electrical and mechanical properties of BaTiO3 coatings prepared by atmospheric plasma spraying showed anomalies in their dielectric response. This paper provides a study of electrical and mechanical properties of BaTiO3 coatings after thermal posttreatment. The spraying was carried out by a direct current gas-stabilized plasma gun. BaTiO3 was fed into the plasma jet as a feedstock powder prepared by reactive sintering of micrometer-sized powders of BaCO3 and TiO2. In the next step the coatings were annealed in air. Microstructure and phase composition are reported and discussed in relation to electric and mechanical properties. Dielectric properties are reported for the radio frequency (RF) range.

Plasma spraying of cerium-doped YAG

Abstract

Feasibility of suspension spraying of yttria-stabilized zirconia with water-stabilized plasma torch

Thermal spraying of suspensions allows preparation of coatings from submicron-sized powders but demands a source of heat with a high enthalpy in order to provide an appropriate thermal treatment of the liquid feedstock during the in-flight stage so that the solvent may be evaporated, dispersed particles melted and accelerated towards the substrate to form a coating. Water-stabilized plasma (WSP) torch developed at the Institute of Plasma Physics AS CR, v.v.i. provides such a heat source with high enthalpy, high velocity of the plasma and, when compared to high-enthalpy gas-stabilized plasma (GSP) torches, relatively cheap operation. In this study, results of our experiments with suspension spraying of yttria-stabilized zirconia (YSZ) with WSP torch are presented and demonstrate that coating deposition with a high feed rate is possible with WSP technology. Formation of both columnar “cauliflower” microstructure and segmentation cracks was achieved. Variation of the deposition conditions was observed to modify coating microstructure in terms of splat morphology, porosity and thickness per pass, which is promising for further coating development.

Properties of the BaTiO3 coating prepared by supersonic plasma spraying

Plasma spraying technique enables to create layers with thickness in a millimeter range adhering to various substrates. The supersonic plasma spraying can spray the powders at acoustic speed, which can increase the coating density and improve the bond strength between the coating and substrate. In the present study, a BaTiO3 coating with good physical property and preliminary piezoelectric index was prepared by supersonic plasma spraying. The spraying process was carried out by a direct current gas-stabilized plasma gun, which was manufactured by the Science and Technology on Remanufacturing Laboratory. It was found that the high temperature could make the BaTiO3 possess the piezoelectric nature. The surface of the BaTiO3 coating was smooth and dense, and the mechanical bond existed between the substrate and the coating. The average bond strength and microhardness were 47.4 MPa and 650 HV0.1, respectively. The BaTiO3 phase changed from cubic to tetragonal phase in the course of supersonic plasma flame flow. About 25% (mol) BaTiO3 was decomposed into BaO and TiO2. The decomposition and original defects of the spraying coating could decrease its piezoelectric activity. The piezoelectric performance of the BaTiO3 coating was lower than that of the sintering ceramic, but obvious piezoelectricity existed in the sprayed BaTiO3 coating.

Phase stabilization in plasma sprayed BaTiO3

Abstract This paper presents a comparison of properties of BaTiO3 ceramics prepared by two different production methods: gas-stabilized plasma spraying (GSP) and spark plasma sintering (SPS). Samples of both materials were evaluated by various techniques, the goal being to detect the Curie temperature of the ferroelectric transformation between the tetragonal and the cubic phase. All tests, resonant ultrasound spectroscopy, dielectric measurements, differential scanning calorimetry and temperature-resolved X-ray diffraction (XRD), used in combination, proved the absence of this transformation in the case of GSP coating up to 500 °C. Similarly, the tetragonal-to-orthorhombic transition temperature is shifted downwards, this transition probably taking place in a small fraction of the volume of coating. The SPS samples exhibit several anomalies, such as a strong anisotropy of relative permittivity, but their phase transformations were detected in the usual temperature ranges.

Structure and properties of plasma sprayed BaTiO 3 coatings: Spray parameters versus structure and photocatalytic activity

Plasma spraying enables the creation of layers with thickness in a millimeter range adhering on various substrates. This paper provides a study of phase composition, optical properties and photocatalytic activity of BaTiO 3 coatings prepared by atmospheric plasma spraying. The spraying was carried out by a direct current gas-stabilized plasma gun. BaTiO 3 was fed into the plasma jet as a feedstock powder prepared by a reactive sintering of micrometer-sized powders of BaCO 3 and TiO 2. Microstructure and phase composition are reported and discussed in connection with optical properties and photocatalytic activity. The spraying was carried out by a direct current gas-stabilized plasma gun which normally utilizes spray distance (SD) in frames from 100 to 150 mm. Besides conventional SD 100 mm also extremely high SD 190 mm was used. The color of the sprayed coating is different for each SD and also differs from sintered BaTiO 3. X-ray diffraction and also SAD mode of HR-TEM show certain content of amorphous fraction in the coating. The hydrogen content in the coating was found to be higher in the coatings than in the sintered bulk. The diffuse reflectance was measured by UV–VIS spectrophotometry and corresponding band-gap energy was estimated. X-ray photoelectron spectroscopy confirms specific stoichiometric and structural disorder observed also at bang-gap evaluation and by Raman spectroscopy. HR-TEM images for crystalline and amorphous zones are given. Photocatalytic decomposition of acetone was tested and BaTiO 3 coatings compared with a sintered bulk.

Structure and properties of plasma sprayed BaTiO 3 coatings

Plasma spraying enables to create layers with thickness in a millimeter range adhering on various substrates. This paper provides a study of electric and mechanical properties of BaTiO 3 coatings prepared by atmospheric plasma spraying. The spraying was carried out by a direct current gas-stabilized plasma gun. BaTiO 3 was fed into the plasma jet as a feedstock powder prepared by a reactive sintering of micrometer-sized powders of BaCO 3 and TiO 2. Microstructure and phase composition are reported and discussed in connection with electric and mechanical properties. The ability of the used techniques to detect precisely the phase transformation temperatures of BaTiO 3 plasma sprayed coatings is discussed as well. A depth-sensing indentation measurement was done between 290 and 520 K to provide local mechanical characterization. The elastic modulus has shown slightly higher values than that reported typically in papers focused on BaTiO 3 ferroelectric thin films. The average Vickers microhardness is tested to characterize the samples in larger scale. A wear resistance in a slurry environment is reported as well. Dielectric properties are reported for the temperature window of existence of the tetragonal ferroelectric phase. Relative permittivity and loss factor are studied at frequency from 50 Hz to 1 MHz and temperature from 260 to 400 K.

Atmospheric plasma sprayed (APS) coatings of Al2O3–TiO2 system for photocatalytic application

The goal of this study is to examine the photocatalytic ability of coatings produced by atmospheric plasma spraying (APS). The plasma gun used is a common gas-stabilized plasma gun (GSP) working with a d.c. current and a mixture of argon and hydrogen as plasma-forming gas. The TiO(2) powders are particles of about 100 nm which were agglomerated to a mean size of about 55 mum, suitable for spraying. Composition of the commercial powder is 13 wt% of TiO(2) in Al(2)O(3), whereas also in-house prepared powder with the same nominal composition but with agglomerated TiO(2) and conventional fused and crushed Al(2)O(3) was sprayed. The feedstock materials used for this purpose are alpha-alumina and anatase titanium dioxide. The coatings are analyzed by scanning electron microscopy (SEM), energy dispersion probe (EDS) and X-ray diffraction. Photocatalytic degradation of acetone is quantified for various coatings. All plasma sprayed coatings show a lamellar structure on cross section, as typical for this process. Anatase titania from feedstock powder is converted into rutile titania and alpha-alumina partly to gamma-alumina. Coatings are proven to catalyse the acetone decomposition when irradiated by UV rays.

Comparison of the structure and wear resistance of Al 2O 3–13 wt.%TiO 2 coatings made by GSP and WSP plasma process with two different powders

The goal of this study is to compare two Atmospheric Plasma Spraying (APS) systems for elaborating alumina/titania coatings with good wear resistance. Both torches used were a common d.c. current Gas-Stabilized Plasma gun (GSP) with argon and hydrogen as plasma forming gases, and a Water-Stabilized high-throughput Plasma gun (WSP) working with water as plasma forming substance. The feedstock were either cladded alumina/titania powder or a mixture of conventional fused and crushed Al 2O 3 powder with agglomerated TiO 2 nanometric particles, the resulting mean size in both cases being about 50 µm. Feedstock material phases were α-alumina and anatase titanium dioxide and the composition of both powders was 13 wt.% of TiO 2 in Al 2O 3. Coatings obtained were analyzed by scanning electron microscopy (SEM). They presented lamellar structures with titania uniformly distributed in the alumina matrix, for both spray techniques. X-ray diffraction showed that anatase phase of titania was converted into rutile phase. The wear resistance tested by slurry abrasion (SAR) has shown that the best coating was that obtained with cladded powder sprayed by GSP process. This finding matched with the highest microhardness of this coating (Vickers indentation). It is worth noting that with these two relatively dissimilar torches similar results were obtained for the wear resistance of alumina–13 wt.% titania coating.

In-flight oxidation of iron particles sprayed using gas and water stabilized plasma torch

This paper is devoted to the study of the oxidation of iron (low carbon) particles during their flight in d.c. plasma jets flowing in air as well as the oxidation after the impact and solidification of particles onto the substrate. Two types of torches have been studied: a conventional one: gas stabilized plasma (GPS) with an effective power P eff=18 kW and a more exotic one: water stabilized plasma (WSP) with P eff=139 kW. Besides their power levels, their main difference lies in the plasma gas specific mass downstream of the anode attachment almost 10 times lower in GSP than in WSP with correspondingly the plasma temperature 10,000 K higher for WSP. The GSP oxidation is promoted by the convective movement induced within the liquid metal droplets by the plasma flow in the jet core. In-fight oxidation shows strong similarities for both techniques with mainly the formation of non-stoichiometric Fe x O. In contrast in coatings depending on the temperature during spraying, Fe 2O 3 or Fe 3O 4 are observed.

Real-time imaging of the plasma spray process—Work in progress

A LaserStrobe Control Vision system was employed to examine water-stabilized plasma (WSP), gas-sta-bilized plasma (GSP), and single-wire arc plasma (SWAP) technologies. Visualization of the plasma spray process in each of these technologies has been made possible, in some instances, for the first time. Parame-ter optimization for the three processes was accomplished. This technology has significantly added to the theoretical and scientific knowledge of plasma diagnostics and plasma processing.

Plasma-spray forming ceramics and layered composites

In a program at the Thermal Spray Laboratory at the State University of New York at Stony Brook, both conventional gas-stabilized plasma and high-throughput water-stabilized plasma spray torches are being used to produce thick free-standing structures of oxide ceramics, metal-reinforced ceramic laminates, and multilayered or functionally graded ceramic materials. The results of the processing, microstructure, and properties of freeforms produced from ceramics in the alumina, zirconia, and alumina-zirconia system are reported.