In this paper, yttria-stabilized zirconia (YSZ) coatings were prepared by plasma spraying of ready-to-spray suspensions provided by three different manufacturers. High-enthalpy hybrid water-argon plasma torch WSPH 500 was successfully used for deposition of coatings with porous and columnar microstructure consisting of tetragonal non-transformable phase. Sensitivity of the deposition process to variation of deposition conditions was also evaluated by the change of suspension injection point position. Slight differences in the microstructures of the deposited coatings (in particular character of porosity and mutual bonding of the microsplats) were reflected in slight but measurable differences in hardness and wear resistance of the coatings indicating changes in the coating cohesion. Tensile adhesion/cohesion strength of the coatings was found to be in the range of 9 to 15 MPa. High coating porosity desirable for low thermal conductivity combined with high suspension feed rate (from about 100 to 120 ml/min in this study) makes the WSP-H coatings promising for further development for example in thermal barrier coatings applications.

In this study, gas- and water-stabilized plasma torches were used to spray cesium-doped yttrium aluminum garnet (Ce:YAG) on different substrate materials and in large-area free-standing layers. The coatings were evaluated based on microstructure, crystallinity, and thermal stability, and tests were performed to measure porosity, hardness, phase composition, band-gap energy, and the presence of defects. Some coatings were also heat treated to determine how it changes their spectral response. The results of the examinations and tests are presented in the paper.

Titanium dioxide coatings were sprayed by a water stabilized plasma gun (WSP) 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. Majority of the tests was 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. Dominant phase present in the sprayed samples was rutile whereas anatase was the main minor component. 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 O 1s peak between both samples. The photocatalytic activity was tested by decomposition of acetone at UV illumination, whereas also the end products - CO and CO 2 - were monitored. The nitrogen-assisted coating was revealed as more efficient photocatalyst.

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, 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, different degree of these effects was observed for copper and tungsten.

The goal of this study is to find applicable spray conditions for producing tungsten (W), zirconium carbide (ZrC), and W-ZrC cermet layers. In the experiments, W and ZrC powder mixtures were fed into the plasma of a water-stabilized plasma gun and coatings approximately 1 mm thick were sprayed on graphite substrates. Pure W and pure ZrC were deposited under similar conditions. Microhardness, surface roughness, XRD, XRF, dilatometry, and spectroscopic techniques were used to characterize the coatings. The resulting coatings were found to be hard with a high elastic modulus.

An alternative method to produce bulk nanocrystalline materials and avoid the powder compaction step is to produce amorphous material by rapid solidification followed by controlled heat treatment to introduce nanocrystalline structure. The extremely high cooling rates in plasma sprayed particles give rise to formation of nonequilibrium phases, which may become amorphous for certain materials. Five different materials studied in this work are based on near-eutectic mixtures of alumina, zirconia and silica. The powder feedstock materials have been plasma sprayed using water stabilized plasma torch (WSP) and subsequently heat-treated to prepare nanocomposite materials with varying nanocrystallite size. The as-sprayed materials have very low open porosity and are mostly amorphous. The as-sprayed amorphous materials crystallize at temperatures around 950°C with an associated volume shrinkage of 1-2%. The resulting structure is best described as nanocomposite with very small crystallites (12 nm on average) embedded in inter-crystallite network. Role of the silica compound on phase composition, microstructure, and mechanical properties of the as-sprayed and annealed materials is discussed. Elastic properties were measured for the nanocrystalline materials. The as-sprayed amorphous materials exhibit high hardness and high abrasion resistance. Both properties are significantly improved in the heat treated nanocrystalline samples.

Tungsten carbide – cobalt powders (WC-17wt.%Co) were plasma sprayed by a water-stabilized system WSP. A matrix of experiments with variable feeding distances and spray distances was carried out. Thinner coatings were carried out on carbon steel substrates and thicker coatings on stainless steel substrates to compare fast cooling conditions – the former with slower 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 on resistances 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 are discussed in connection with changes of phase composition and with the character of the coating’s microstructure. The results show that for obtaining of 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.

A short survey is given of various techniques of 3D visualization of voids and other structural defects that are inherently present in thermally spray structures. It is proven that the only condition for visualization is possibility to set up a distinguishable threshold based on the gray scale of the microscopical image of the structure. Segmented images are then further processed by commercial software for image analysis, animations and anaglyph files creation. Several softwares were applied and combined, such as Lucia, Voxblast, Amira and Voxler. The visualization technique was demonstrated on a ceramic plasma sprayed coating, where two kinds of voids – semi-globular pores and large interlamellar pores could be separated. Similarly, this visualization technique can be used to distinguish between pores and oxide particles in case of metallic coatings. The visualization is done in a real Cartesian space without any transformation. The results can be useful mainly for: a) teaching to get a better insight into the microstructure of thermally sprayed coatings, b) research, where in combination with mathematical quantification the visualized structure can show important distinctions between coatings sprayed by different techniques and with variety of process parameters.

This work is concentrated on plasma sprayed cermet coatings consisting of stainless steel (SS) (17 wt % Cr and 12 wt % Ni) and chromium oxide (Cr 2 O 3 ). These powders were sprayed simultaneously, however being injected separately to account for the drastic difference in their thermo-physical properties. Chromium oxide was injected internally and stainless steel externally. The plasma parameters (arc current, hydrogen vol %, mass flow rate) were optimized as well as the injector positions. Coatings were achieved with different mass ratios of SS and Cr 2 O 3 . All exhibited a lamellar structure with well distributed Cr 2 O 3 and SS lamellae. They were then systematically characterized by their phase content (XRD), composition (EDS), Vickers micro hardness, morphology (SEM), slurry and dry wear resistances. Finally the best dry linear abrasion resistance was obtained for the pure chromium oxide coatings, while the best slurry wear resistance corresponded to coatings with a mass ratio SS/ Cr 2 O 3 of 56/44.

Agglomerated titania nanopowder and a “classical” titania were sprayed by the high throughput WSP and thoroughly compared. Optical microscopy with image analysis as well as mercury intrusion porosimetry were utilized 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 like 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 over-all 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.

TiO 2 nanopowder was used as a feedstock for spraying with the water stabilized plasma (WSP®) in search for superior mechanical properties and wear resistance of titania coatings. It has been proved that good quality coatings can be made even with the high throughput WSP®. Single splats evaluation and the free flight particles were used for the spray optimization. Phase compositions, stoichiometry and selected properties, such as density and elastic modulus, were then studied at the as-sprayed coatings. As for the phase composition of coatings, mainly rutile with possible traces of Magneli phases have been found. A comparison between tension and compression loading shows that values of the Elastic modulus for compression are slightly higher than these for tension, as it is usual in plasma sprayed coatings.

Alumina and stabilized Zirconia were plasma sprayed in air using the water-stabilized plasma torch. In the case of Alumina two different stand-off distances were applied at spraying. Nd-YAG laser was then used for additional treatment of plasma sprayed coatings. The laser was maintained in a quasi-continual regime and defocused from the surface to increase the treated coating's area. Energy density was varied together with the laser scanning velocity to ensure variance in thermal history of the treated surfaces. Microhardness, surface roughness and slurry abrasion resistance (SAR) were measured before and after the laser treatment. Results vary in dependence on the laser treatment's parameters. When the treatment results in substantial changes of the structure (up to a complete re-melting of the surface), enhancement of all measured properties was proven. It is demonstrated that the extent of change of mechanical properties can be correlated with optical properties of coating materials at the laser wavelength. Microstructural aspects of the laser treatment are discussed as well, especially at the boundary between the laser-annealed layer and the basic coating's microstructure. It is pointed out that laser overheating due to use of an extremely high energy density can cause delaminating of the coatings.

Plasma sprayed coatings, thanks to their specific structure, are considered for applications with thermal shocks. Alumina, Alumina-Titania as well as less common spraying materials like mullite, forsterite and ilmenite were selected for testing. Carbon steel coupons were used as substrate. Flame-sprayed bond coats of Ni-Cr and Ni-Al under plasma sprayed ceramic coating were applied for comparison in selected cases. Two different regimes were used to simulate high and low temperature thermal cycling. Before the test and after given number of cycles coating's adhesion was tested by a standard tensile test (EN 582). After the test also the microstructure of samples was observed to evaluate the damage mechanism. It has been proved that Titania addition to Alumina improves the adhesion of coatings under thermal cycling, especially for the coatings without any bond coat. All materials had higher adhesion after each given number of cycles with bond coat than without. Cracking occurred dominantly on the boundary with the base in the case of pure Alumina and mullite while a cohesive cleavage is preferred in ilmenite and combination of both types is typical for other tested materials.

The aim of this paper is to study the influence of plasma spray set-up parameters, such as the hydrogen percentage in the plasma forming gas and arc current, as well as powder injection parameters: the carrier gas flow rate and injected particle size on the melting, vaporization and velocity of particles of a Fe-Cr based alloy and stainless steel AISI 316L. Particles were collected in flight in a cylinder where they were quenched by argon jets to prevent there flattening at impact and chemical interaction with their environment. After collection, particles were analyzed by SEM, EDS and X-ray diffraction. When the injected particle size was between 60 and 100 µm, particles collected in flight exhibit complex shapes with a mean diameter of 40 µm, and the higher was the percentage of hydrogen the smaller was the resulting mean diameter. This probably corresponds to an inefficient cooling by the argon jets resulting in particles impacting at the bottom of the collector and partially exploding. When the injected particle size was between 10 and 35 µm, collected particle were nearly spherical with almost the same mean diameter, but then the oxidation changed with spray parameters. Measurements of temperature and velocity of the in-flight particles during plasma spraying were done using the Spray Watch (Oseir, Finland) equipment. Relationships between in-flight particle parameters and characteristics of the collected particles were found. Abstract only; no full-text paper available.

Amorphous metallic coatings are of high interest because of their good wear and corrosion resistance as well as their high hardness and toughness relative to the crystalline alloys with the same composition. Thermal spray that makes it possible to reach quenching rates in the order of 106-107 K/s, has the ability to deposit coatings with a high content of amorphous phase. However, very few studies dealt with the understanding of the spraying factors that affect the formation of the amorphous phase under thermal spray conditions. In this work, the relationship between temperature and velocity of the spray particles and coating characteristics is investigated. Special attention is given to the degree of amorphisation of the as-sprayed coatings. The latter were produced both by plasma and wire-arc spraying in order to get a larger range of particle parameters at impact and different particle heating history in the gas flow before impingement onto the substrate. A commercial iron-based alloy available both in powder and wire forms was used. Microstructural analyses show that the as-sprayed coatings are partially amorphous and that the proportion of the amorphous phases depends on the sizes of the sprayed particles as they control the heating and acceleration of particles in the gas flow and their cooling rate on the substrate.

By modification of parameters in plasma spraying it is possible to alter porosity of the deposits only in a relatively small range. Industrial applications may require to adjust the porosity in wider range, to double or triple the original value in some cases. Such changes can be achieved only by special procedures. One of them is plasma spraying of ceramic/metal deposit followed by removal of the metal within the coating. The material removal is performed by dissolving, by leaching or by its extraction with an appropriate process. The paper describes preparation of very porous Cr 2 O 3 coatings by this method from a composite Cr 2 O 3 /Al deposit. The initial porosity of approximately 13% (total porosity) was increased to 37% or 51% respectively by changing the initial volume of aluminum in the ceramic/metal plasma-sprayed composite. The discussion is complemented by observation of the resulting coating structure and detailed characterizations of the pore structure and porosity.

This paper examines the dielectric properties of silicate coatings including mullite (3Al 2 O 3 -2SiO 2 ), steatite (MgSiO3), spodumene (Li 2 O-Al 2 O 3 -4SiO 2 ), and olivine with near-forsterite (Mg 2 SiO 4 ) composition. The materials were sprayed using a water-stabilized plasma gun and the deposits were removed from the substrate, polished, and sputtered with aluminum on both sides. Electrical tests consisting of voltage, resistance, and capacitance measurements showed that the relative conductivity of plasma-sprayed silicates is stable between 200 Hz and 1 MHz, which is suitable for many insulation applications. Paper includes a German-language abstract.

An investigation was conducted to assess the potential of water-stabilized plasma (WSP) spraying for applying protective boron carbide coatings to fusion reactor components. This paper describes how test samples were produced and how coating quality was determined. The authors sprayed boron carbide powder onto steel and stainless steel substrates using different powder feeding and spraying distances, substrate preheat temperatures, and carrier gases. They also investigated methods for optimizing the plasma jet and improving coating adhesion. The boron carbide coatings were characterized based on phase composition, porosity, oxygen content, and flexural strength. Paper includes a German-language abstract.

Spraying distance (SD) is one of the main parameters that can affect the spraying process - its efficiency as well as deposit's character. The reason lies in a different thermal history of particles corresponding to different spraying distances. Variation in the structure, preferred orientation, variation of the phase contents and/or in the chemical composition of deposits can be then found for different SDs. Consequently, properties of coatings can greatly vary, not only in the resistance state but also on deposit's annealing. Some materials are, however, more and some less sensitive to that effect. The goal of this work is to compare CaTiO 3 samples produced by plasma spraying with WSP at SD = 350 and 450mm with sintered samples. The following properties were compared: microhardness, thermal expansion coefficient, permittivity and reflectivity. Porosity and the differential thermal analysis of resistance and annealed deposits were also compared. Perovskite CaTiO 3 belongs among materials very stable during spraying - neither chemical nor phase differences were found between the feedstock powder, free-flight particles, as-sprayed deposit and annealed deposit. Despite that there are significant differences in behavior of deposits and freestanding parts sprayed from different SDs. All the recorded differences for CaTiO 3 plasma sprayed deposits with varying SDs must be therefore accounted to the deposit's structural differences, such as pore and splat sizes and shapes and preferred orientation. Additional differences on annealing can be probably attributed to the different amount of "stored" thermal energy in deposits due to the varying SD. However, there is a lower limit for SD assuring a quality deposit on spraying.

Among candidate materials for plasma spraying titanates ATiO3, where A is an element from the alkaline earth group (11), were not systematically tested until today. This paper reports on plasma spraying of synthetic perovskite CaTiO3 and geikielite-perovskite system MgTiO3-CaTiO3. Perovskite CaTiO3 is well known as dielectric material and a basic component of complex dielectric ceramics. Since it is relatively chemically simple and inexpensive material it has been selected for the basic preliminary studies. Mixture of geikielite-perovskite MgTiO3-CaTiO3, with Mg:Ca ratio equal to 94:6, was chosen because its permittivity is independent of temperature. Plasma spraying was done with the water stabilized plasma gun WSP. Plasma spraying conditions were optimized using single splat observation for various substrates and varying substrate temperature. Standard experimental techniques were used for studying of microstructures, chemical and phase compositions and porosity of as-sprayed and annealed deposits. Mechanical properties such as Young’s modulus and microhardness were measured.