Pavel Ctibor

Microstructure and abrasion resistance of plasma sprayed titania coatings

Agglomerated titania nanopowder and a “classical” titania were sprayed by the high throughput water-stabilized plasma (WSP) and thoroughly compared. Optical microscopy with image analysis as well as mercury intrusion porosimetry were used for quantification of porosity. Results indicate that the “nano” coatings in general exhibit finer pores than coatings of the “conventional” micron-sized powders. Mechanical properties such as Vickers microhardness and slurry abrasion response were measured and linked to the structural investigation. Impact of the variation in the slurry composition on wear resistance of tested coatings and on character of the wear damage is discussed. The overall results, however, suggest that the “nano” coatings properties are better only for carefully selected sets of spraying parameters, which seem to have a very important impact.

Unmelted particles in plasma sprayed coatings

Abstract A technique is described for investigation of unmelted particles in plasma spraying and for porosity investigation in coatings. A study of unmelted particles is made. The technique is based on large area section (LAS) images, superimposed to obtain a densely packed “column” of images as a 3-D map of a certain volume of the plasma sprayed coating. Optical micrographs of a polished cross section of the coating were used. The LAS technique combines two parameters—magnification and field size. Such a combination is normally beyond the range of optical microscopy. The method makes it necessary to polish the sample again and again and after each polishing step align precisely the new LAS with the previous one. The unmelted particles are considered to have been spheroidized in the plasma plume during plasma spraying, but to have cooled below the melting point before reaching the substrate. Such particles are embedded in the coating as previously solidified, i.e. with globular shape. The 3-D map makes it possible to observe their distribution inside the coating. These globular particles represent defects in the deposited coating. Moreover they have a tendency to behave isotropically under the influence of stress or other physical factors and in this way serve in the anisotropic structure of the coating as centers of secondary defects deteriorating the functional properties of the coatings.

Dielectric relaxation in calcium titanate-containing ceramics prepared by plasma spraying

Abstract The relative permittivity and loss factor frequency dependence in a weak electric field of plasma sprayed perovskite and ilminite polycrystalline ceramic materials—pure synthetic calcium titanate, as well as magnesium titanate—calcium titanate mixture, were studied. It was observed that plasma-sprayed titanates exhibit a strong relaxation of permittivity and loss factor decrease when they are measured in an as-sprayed state. The frequency dependence of the earlier properties is influenced by chemical composition and spraying parameters, but the relaxation character in general is preserved in all cases. The volume resistivity of the samples in an as-sprayed as well as subsequently annealed state was studied also. Several aspects of the structural features of plasma deposits and their possible influence on dielectric properties are discussed.

The Role of Spraying Parameters and Inert Gas Shrouding in Hybrid Water-Argon Plasma Spraying of Tungsten and Copper for Nuclear Fusion Applications

Tungsten-based coatings have potential application in the plasma-facing components in future nuclear fusion reactors. By the combination of refractory tungsten with highly thermal conducting copper, or steel as a construction material, functionally graded coatings can be easily obtained by plasma spraying, and may result in the development of a material with favorable properties. During plasma spraying of these materials in the open atmosphere, oxidation is an important issue, which could have adverse effects on their properties. Among the means to control it is the application of inert gas shrouding, which forms the subject of this study and represents a lower-cost alternative to vacuum or low-pressure plasma spraying, potentially applicable also for spraying of large surfaces or spacious components. It is a continuation of recent studies focused on the effects of various parameters of the hybrid water-argon torch on the in-flight behavior of copper and tungsten powders and the resultant coatings. In the current study, argon shrouding with various configurations of the shroud was applied. The effects of torch parameters, such as power and argon flow rate, and powder morphology were also investigated. Their influence on the particle in-flight behavior as well as the structure, composition and properties of the coatings were quantified. With the help of auxiliary calculations, the mass changes of the powder particles, associated with oxidation and evaporation, were assessed.

Dielectric properties of plasma sprayed titanates

Abstract 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.

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.

Plasma Spraying and Characterization of Tungsten Carbide-Cobalt Coatings by the Water-Stabilized System WSP

Tungsten carbide-cobalt powders (WC-17wt% Co) were plasma sprayed by a water-stabilized system WSP. Experiments with variable feeding distances and spray distances were carried out. Thinner coatings were deposited on carbon steel substrates and thicker coatings on stainless steel substrates to compare different cooling conditions. Basic characterization of coatings was done by XRD, SEM, and light microscopy plus image analysis. Microhardness was measured on polished cross-sections. The main focus of investigation was resistance against wear in dry as well as wet conditions. The appropriate tests were performed with set-ups based on ASTM G65 and G75, respectively. The influence of spray parameters onto coating wear performance was observed. The results of mechanical tests were discussed in connection with changes of phase composition and with the quality of the coatings microstructure. The results show that for obtaining the best possible WC-17Co coating with WSP process, from the viewpoint of wear resistance, the desired parameters combination is long feeding distance combined with short spray distance.

Dielectric properties of CaTiO3 coatings prepared by plasma spraying

Abstract This paper presents a study of dielectric properties, namely the relative permittivity and loss factor dependence on the frequency of a weak electric field. Perovskite CaTiO3 was studied in the form of coatings and self-supporting plates made by plasma spraying. A conventional gas stabilised plasma gun (GSP) as well as a water stabilised plasma gun (WSP) were employed. It was observed that plasma sprayed titanates exhibit a strong relaxation of permittivity and loss factor decrease with a frequency rise. These properties are influenced by spray technique and spraying parameters, but the relaxation character in general is preserved in all cases. The volume resistivity of the samples was studied as well. Several aspects of the structural features of plasma deposits, especially the phase composition, porosity character, and their influence on dielectric properties are discussed.

Spraying of Metallic Powders by Hybrid Gas/Water Torch and the Effects of Inert Gas Shrouding

A hybrid DC arc plasma torch, combining water and gas stabilization, offers a high flexibility in plasma characteristics. These can be controlled in a wide range by the torch operational parameters, such as arc current and secondary gas flow rate. In this study, their influence on plasma spraying of tungsten and copper was investigated. To suppress the in-flight oxidation of the metals, inert gas shrouding was applied. In-flight particle diagnostics and analysis of free-flight particles and coatings was performed for spraying experiments in the open atmosphere and with argon shrouding. Both in-flight particle behavior and coating properties were found to be sensitive to the torch parameters. The application of shrouding was found to affect particle in-flight parameters, reduce the oxide content in the coatings and generally improve their properties, such as thermal conductivity. However, a different degree of these effects was observed for copper and tungsten.

Photocatalytic and electrochemical properties of single- and multi-layer sub-stoichiometric titanium oxide coatings prepared by atmospheric plasma spraying

We studied the photocatalytic and electrochemical impedance properties of two different sub-stoichiometric titania powders deposited using plasma spraying. Two different commercial powders with markedly diverse mean size as well as size distribution were chosen. Thermal oxidation of these as-received powders was carried out to restore the O/Ti ratio to be 2.0. By this way, another two kinds of feedstock powders were obtained. Total of four kinds of feedstock powders were used in the experiments. All powders were sprayed using identical plasma spraying parameters. For some spraying runs, carbon steel served as the substrate, and for others, the previously sprayed titania coating from a different kind of feedstock. Combinations of single- and double-layer coatings were studied. Porosity, microstructure, phase composition, chemical composition, band gap based on reflectance measurement, and photocatalytic activity were examined. Electrochemical impedance spectroscopy demonstrated substantial differences between samples sprayed from the fine and coarse powders. Coatings from oxidized powders were slightly more photocatalytic. High spraying distance used for several of them was good for obtaining low band gap, whereas surface roughness and phase composition were not substantially different compared to standard spraying distance coatings.