Water-stabilized Plasma Gun Research Articles

Photoconductive TiO2 Dielectrics Prepared by Plasma Spraying

Titanium dioxide coatings (TiO2) were sprayed using a water-stabilized plasma gun (WSP) to form robust self-supporting bodies with the character of a ceramic disc capacitor (CDC). Agglomerated nanometric powder was used as feedstock. Argon was applied for powder feeding as well as coating–cooling to minimize the influence of ambient air. Stainless steel was used as a substrate, and the coatings were released after cooling. A more than three-millimeter-thick self-supporting TiO2 plate was observed using HR-TEM and SEM. Porosity was studied by image analysis on polished sections. Thermal post-treatment on the coating was conducted at a rather low temperature of 500 °C. The results of the subsequent dielectric measurement showed high permittivity, but this was strongly frequency-dependent and accompanied by a progressively decreasing loss tangent. On the other hand, the plasma-sprayed TiO2 exhibited persistent DC photoconductivity under and after illumination with a standard bulb.

Calcium titanate (CaTiO3) dielectrics prepared by plasma spray and post-deposition thermal treatment

This paper studies calcium titanate (CaTiO3) dielectrics prepared by plasma spray technology. A water stabilized plasma gun (WSP) as well as a widely used gas stabilized plasma gun (GSP) were employed in this study to deposit three sample sets at different spray conditions. Prepared specimens were annealed in air at atmospheric pressure for 2h at various temperatures from 530 to 1170°C. X-ray diffraction (XRD), Raman spectroscopy and porosity measurements were used for sample characterization. Dielectric spectroscopy was applied to obtain relative permittivity, conductivity and loss factor frequency dependence. Band gap energy was estimated from reflectance measurements. The work is focused on the explanation of changes in microstructure and properties of a plasma sprayed deposit after thermal annealing. Obtained results show significant improvement of dielectric properties after thermal annealing.

Improving dielectric properties of plasma sprayed calcium titanate (CaTiO3) coatings by thermal annealing

Self-supported CaTiO3 coatings prepared by plasma-spraying technology and effect of annealing on dielectric properties and microstructure of these coatings are studied in this work. A water stabilized plasma gun WSP (IPP ASCR, Prague) as well as a widely used conventional gas stabilized plasma gun GSP were employed. Prepared samples were annealed in air atmosphere up to 1170°C. The work is focused on dielectric properties of these specimens. Relative permittivity and loss factor were measured in a frequency range of 30Hz–30MHz. Microstructure was studied using light microscopy, microhardness measurements, X-ray diffraction – including high-temperature XRD measurements, and X-ray photoelectron spectroscopy. The work studies the influence of annealing temperature on dielectric properties, crystalline structure and porosity of plasma deposited calcium titanate. Results obtained show significant improvement in CaTiO3 coating properties after thermal annealing.

APPLICATION OF X-RAY DIFFRACTION IN DEVELOPMENT OF NEW TYPE OF MOULDS FOR PRODUCTION OF REFRACTORY AND HIGHLY ABRASIVE OBJECTS

Ceramic materials can be used due to their high hardness and chemical stability as both the wear and corrosion resistant coatings. Their deposition on moulds for manufacturing of objects from refractory and highly abrasive materials is challenging, because they are traditionally made from hard-to-blast casehardened tool steels with hardness exceeding 60 HRC. The pressing process imposes further stringent requirements on the coating microstructure; especially in respect to its homogeneity, cohesion and wear resistance. Considering results of laboratory analyses of residual stresses and roughness of blasted surfaces, mould plates from low-carbon C45 steel were sprayed with five different ceramic materials by water stabilized plasma gun. The testing of the moulds with plasma coatings was performed on-site in a factory under standard pressing conditions.

Titanium Dioxide Coatings Sprayed by a Water-Stabilized Plasma Gun (WSP) with Argon and Nitrogen as the Powder Feeding Gas: Differences in Structural, Mechanical and Photocatalytic Behavior

Titanium dioxide coatings were sprayed by a water-stabilized plasma gun to form robust self-supporting bodies with a photocatalytically active surface. Agglomerated nanometric powder was used as a feedstock. In one case argon was used as a powder-feeding as well as coating-cooling gas whereas in the other case nitrogen was used. Stainless steel was used as a substrate and the coatings were released after the cooling. Over one millimeter thick self-supporting bodies were studied by XRD, HR-TEM, XPS, Raman spectroscopy, UV-VIS spectrophotometry and photocatalytic tests. Selected tests were done at the surface as well as at the bottom side representing the contact surface with the substrate during the spray process. Porosity was studied by image analysis on polished cross sections where also microhardness was measured. The dominant phase present in the sprayed samples was rutile, whereas anatase was only a minor component. The hydrogen content in the nitrogen-assisted coating was higher, but the character of the optical absorption edge remained the same for both samples. Photoelectron spectroscopy revealed differences in the character of the O1s peak between both samples. The photocatalytic activity was tested by decomposition of acetone at UV illumination, whereas also the end products—CO and CO2—were monitored. The nitrogen-assisted coating was revealed as a more efficient photocatalyst. Certain aspects of a thermal post-treatment on the coatings are discussed as well. Color and electrical conductivity are markedly changed at annealing at 760 °C, whereas only very small changes of the as-sprayed coating character correspond to annealing at 500 °C.

Fatigue properties of Fe–Al intermetallic coatings prepared by plasma spraying

In this research, FeAl based coatings were deposited on low carbon steel using two different plasma spraying technologies – gas stabilized plasma gun (GSP) and water stabilized plasma gun (WSP) – resulting in different deposition conditions. Significant changes in local chemical composition compared to feedstock material were observed due to uneven evaporation of Fe and Al from the molten material during the interaction of feedstock with plasma. Influence of coatings on substrate fatigue properties was characterized at room temperature using special testing device “SF-Test”. Fatigue lifetimes of specimens with different coatings were compared with those of uncoated specimens. Fractographic analysis of failed specimens revealed that substrate fatigue cracks initiated at coating–substrate interface and grew by striation mechanism. The coatings failed due to intrasplat cracking and intersplat decohesion.

Influence of chemical composition on dielectric properties of Al2O3 and ZrO2 plasma deposits

Abstract Aluminum oxide, zirconium oxide and their mixture were plasma sprayed by a water-stabilized plasma gun. Resulting deposits were studied as dielectrics. Capacity and loss factor were measured for the frequency range 200–10 6 Hz and relative permittivity was calculated. Electric resistance was also measured and volume resistivity calculated for all studied materials. It was proved that dielectric behavior of plasma sprayed oxide ceramics like alumina- and zirconia-based materials at low voltage follows the expectations based on the knowledge of dielectric behavior of sintered ceramics.

Modeling of Oxidation During Plasma Spraying of Iron Particles

The plasma-sprayed metallic powder deposits contain usually some amounts of oxides originating from a gas–liquid reaction after melting the particles flying in the plasma plume, and a gas–solid reaction after the particle impact and solidification. In the present paper, a model of the in-flight oxidation of molten metallic paricles is described. Only the growth of a continuous oxide layer on the molten particle surface is assumed, whereas melt convection and oxide nodule formation inside the particles are not taken into consideration. The model is applied to spraying of pure iron powder by a water-stabilized plasma gun. After the presumed flight time, the calculated and measured values of the oxide thickness are of the same order of magnitude.

Dielectric properties of plasma sprayed titanates

This paper presents the study of the dielectric properties of three plasma-deposited titanates . The deposits were prepared from powders with the same starting composition as industrially produced dielectric ceramics . Influence of plasma spraying itself and of the subsequent annealing of sprayed deposits on electric resistivity, permittivity and the loss factor is reported. Pure synthetic perovskite (CaTiO 3 ) and two perovskite-related ceramic materials (MgTiO 3 –CaTiO 3 and LaMg 0.5 Ti 0.5 O 3 –CaTiO 3 ) were plasma sprayed to form specimens enabling various electric measurements. CaTiO 3 and their solid solution with LaMg 0.5 Ti 0.5 O 3 have perovskite crystal structure and MgTiO 3 have the ilmenite structure. Water-stabilized plasma gun WSP® as well as commercial APS (gas stabilized system) were used to form ceramic layers on stainless steel substrates as well as self-supporting ceramic discs. Surface of specimens was ground after spraying. Thin layer of aluminum as the counter-electrode was sputtered in reduced pressure on the ground surface. Micrometric capacitor and ASTM-convenient resistivity adapter were used for voltage applying. Permittivity and volume resistivity were calculated from the measured capacity and resistance respectively. Self-supporting ceramic deposits were annealed at two different temperatures below and above the sintering temperature of the given material. Properties dependence on annealing temperature was obtained and discussed in relation to porosity.

Metastable chromium-rich oxide formed during plasma spraying of high-alloy steel

During plasma spraying of metals in air, rapid-oxidation reactions occur, inmost cases. In oxidation products of Cr-rich steels, Fe–Cr spineloxide is often found as a dominant oxide phase. A thermodynamic analysisof a system composed of Fe–13%Cr alloy and water vapor or air showedthat the oxidation product in a wide range of high temperatures isFe3-wCrwO4 (w>2). This tetragonallydistorted spinel oxide is not stable at room temperature. Water vapor andair were considered as limits of the gaseous-phase composition inatmospheric spraying by a water-stabilized plasma gun, where the compositionof the plasma plume is modified by air entrainment. The equilibriumcalculations enabled determination of the effects of temperature andgas-to-solid ratio on w. To show the existence of chromium-rich,tetragonally distorted spinel oxide experimentally, a typical productresulting from oxidation of 13%Cr–steel particles during their flightin the plasma plume was studied after rapid solidific ation. This was madepossible by trapping and quenching the flying particles in liquid nitrogenat a distance from the plasma-gun nozzle corresponding to thenozzle–substrate distance in conventional plasma spraying. The resultsobtained by X-ray diffraction, Mossbauer spectroscopy, and X-rayfluorescence analysis showed that this oxide, in which w≈2.4, constitutedthe dominant phase in the oxidation product.

In-flight oxidation of high-alloy steels during plasma spraying

In-flight oxidation of high-alloy steel powder particles sprayed by a water stabilized plasma gun was studied. The particles were collected in liquid nitrogen, which enabled rapid cooling and prevention from further oxidation. Shrouding by two different gases (nitrogen–hydrogen mixture and acetylene) was carried out. The influence of both gases on the oxidation reaction was assessed by determining the oxygen content in particles quenched in liquid nitrogen. Acetylene was found to be significantly more effective in decreasing the oxygen content. The main method to study the composition of the oxidation products was Mössbauer spectroscopy . The dominant oxide phase found in the oxidation products was non-stoichiometric chromium-rich FeCr 2 O 4 . A comparison with previous results shows that if the particles are deposited onto a solid substrate instead of being quenched in liquid nitrogen, this oxide is converted in non-stoichiometric iron-rich FeCr 2 O 4 during the relatively slow cooling of the plasma sprayed deposit.

Oxides in plasma-sprayed chromium steel

Products of oxidation reactions accompanying plasma spraying of chromium steel by a water-stabilized plasma gun were separated from the metallic matrix by a chemical method. This enabled gravimetric determination of the oxide content in plasma- sprayed chromium steel and characterization of the composition and structure of oxides by diffraction and spectroscopic techniques. Both the random and inherent errors of the gravimetric method were estimated. The phase composition of oxides was compared with the results of thermodynamic calculations. It was shown that in spite of the highly nonequilibrium character of plasma spraying, the thermodynamic approach can yield useful predictions for the overall phase composition and for the stoichiometry of the prevailing spinel oxide phase.

Properties of alloy steel coatings oxidized during plasma spraying

Plasma spraying of metals in air is usually accompanied by oxidation reactions resulting in the presence of oxides in plasma deposited coatings. Oxides in plasma sprayed high-alloy steel coatings can strongly affect their protective and mechanical properties. Two high-alloy steels were plasma sprayed onto plain steel substrates using a water-stabilized plasma gun. Both coating types, composed as usual of metallic lamellae (splats), contained considerable amounts of oxides present mainly as thin oxide films enveloping the splats. The compressive behaviour of cubic samples cut out of the coatings was studied. The results indicate an appreciable anisotropy of the response to compressive loading. If the pressure is parallel to the coating surface, strong cracking occurs. For loading perpendicular to the surface, cracking is less pronounced and high values of plastic deformation are observed. Though the coating structure and the presence of oxides preclude a satisfactory tensile strength, the coating can withstand high purely compressive loading. In this respect, plasma sprayed steels are comparable to those produced by conventional metallurgy.

Erosion of zirconium-based cathodes in an arc

The specific erosion rates of cathodes from Zr and ZrB2 used in the water-stabilized plasma gun PAL 50 were measured. Very good values were obtained for ZrB2, establishing this material as one of the best prospects for such cathodes. X-ray microanalysis of the cross section proved that the emitting surface of ZrB2 contained zirconium oxide.