Chromia admixture to alumina feedstock is used to block the formation of γ-Al 2 O 3 in plasma sprayed coatings. In this work, a mechanical blend of alumina-chromia powders was used with the water stabilized plasma to make thick coatings. When additional post-treatment by a quasi-continuous laser was applied to the surface, the amount of the corundum phase substantially increased. Careful examination of the treated surface revealed that the individual splats at and near the surface were fully remelted and the structure is formed by a homogeneous phase with no visible splat boundaries. Segregation of Chromium has been detected in the remelted zones, which strongly affects the final properties of coatings. Fast melting and cooling of the ceramic material under certain laser parameters may result in the formation of undesirable cracks between traces of individual passes of the laser beam.

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.

A Ni-Al pseudo-alloy powder was studied from the point of view of spheroidization during spraying by a water-stabilized plasma gun. The powder particles of irregular shape were conglomerates of elemental Ni and Al, the average Al content being 9.7 %. To conserve the shape and composition of particles flying in the plasma stream, these were trapped in liquid nitrogen. Scanning electron microscopy and X-ray microanalysis were used to obtain information about particle shape and element distribution. Most plasma sprayed particles trapped in liquid nitrogen were composed of a Ni-Al alloy, where the Al content varied in a wide range. Spherical "caps" composed of Al-oxide covered partially their surfaces. It follows that on the interface between molten Ni and Al, the interaction of both components gave rise to a Ni-Al alloy. On the contrary, if Al was exposed to air, it oxidized rapidly during the flight of the particles. The X-ray diffraction lines of the metallic phase in the particles trapped in liquid nitrogen were shifted from the positions corresponding to pure Ni as observed in the feedstock powder. This, together with the line asymmetry, showed the presence of Ni-Al alloy containing varying amounts of Al. The X-ray diffraction did not find any elemental Al in the liquid nitrogen trapped powder, i.e. neither in the metallic phase nor in the "caps". This means that all Al accessible to the ambient oxygen was converted into oxide. The "caps" contained metastable γ- and δ- Al 2 O 3 . The mechanism of the "cap" formation appears to be based on the fact that after an acceleration and melting period, significant slowing down of a molten particle occurs. Due to the drag forces, the lighter Al 2 O 3 melt concentrates on the rear part of the droplet surface. The main condition, under which this mechanism holds, is the presence of two immiscible melts in the droplets and the significantly differing densities of both melts.

Calcium zirconate and magnesium zirconate prepared by synthesis and agglomeration were sprayed using a water stabilized plasma gun under varied spraying conditions. The same set of conditions was maintained during spraying of fused lime stabilized zirconia and magnesia stabilized zirconia. The intent of the study was to compare spraying behaviour and deposit properties of pairs of feedstock material that have just about the same chemical composition but differ considerably in the way of preparation. Free-flight particles as well as deposits were characterised by standard techniques, such as light microscopy, scanning electron microscopy with energy-dispersive spectroscopy, X-ray diffraction, differential thermal analysis, and pycnometry. The differences between relevant coatings were evaluated namely in terms of morphology, chemical and phase composition, density and porosity. Sources of observed differences are discussed.

Atmospheric plasma spraying of alloys often results in their composition changes. The main source of the changes is usually preferential oxidation of some elements composing the alloy. As a rule, these are the alloying elements whose affinity to oxygen is high. Changes due to this effect are well known from metallurgy; however, they were scarcely studied from the point of view of plasma spraying. Preferential evaporation of some elements may also contribute to the alloy composition changes. The aim of the present paper is to give quantitative data on composition changes of selected alloys sprayed by a water-stabilized plasma gun. Two Ni-base alloys and one high-alloy Cr-Ni-steel were studied. The main tool for determining the sample composition was electron probe X-ray microanalysis. To quantify the results and to eliminate the systematic errors, the data obtained by this method were calibrated by repeated chemical analysis of feedstock powders. The alloy composition was determined after both stages of plasma spraying, i.e. after the inflight stage of molten particles and after the stage comprising particle impact, solidification, coating formation and cooling. To study the situation after the former stage, the flying particles were trapped and quenched in liquid nitrogen. In the Ni-Cr alloy containing 20%Cr, strong Cr depletion was observed. The Fe depletion in the Ni-Fe alloy (47%Fe), though unambiguous, was less significant. The high-alloy steel (Czech equivalent of AISI 316) was also Cr depleted, whereas the concentrations of other alloying elements (Mo, Ni) remained effectively unchanged. In all cases, the depletion occurred at the first spraying stage and became more pronounced during the second stage. Strong air entrainment occurs not only in a plasma jet produced by a water-stabilized plasma gun, but also if gas-stabilized plasma guns are used in atmospheric plasma spraying. It follows that the dominant mechanisms of composition changes during plasma spraying by both techniques are similar.

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.

CaZrO 3 is a material for thermally sprayed ceramic coatings to which so far only a little attention was paid. This material has a high melting point, good thermal stability and a coefficient of thermal expansion close to that of steel. In this paper water stabilized plasma spraying (WSPR) and atmospheric plasma spraying (APS) were used to prepare CaZrO 3 coatings. The spraying feedstock was prepared from fine CaZrO 3 powder by agglomeration (spray drying) and sintering. Powders with three different particle sizes (- 45 + 20 µm, - 63 + 45 µm and -90+63 µm) were used in the experiments. The coarse fractions were used for WSP spraying, while the fine one was sprayed with the APS process. Plasma sprayed materials were studied from the point of view of phase changes and influence of the powder size on structure of coatings. The changes of phase composition were studied by X-ray diffraction on coatings as well as on free flight particles. Formation of a cubic phase with a reduced content of CaO in comparison to CaZrO 3 was observed. Its formation is probably connected with evaporation of CaO during spraying. This cubic phase is similar to the phase obtained by spraying of ZrO 2 +5%CaO. Plasma sprayed coatings were characterized by light and scanning electron microscopy (SEM) and by density and porosity. Coefficients of thermal expansion of plasma sprayed layers from CaZrO 3 were measured.

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.

Silicates represent a broad group of industrially important ceramic materials. The only silicate that is widely used for plasma spraying is zirconium silicate. Other silicates are generally not used, although they can offer interesting application properties. This paper presents results with other silicates: synthetic wollastonite, stoichiometric mullite, cordierite, and steatite. The input materials were produced in selected sizes from ceramic in industrial quality and applied with the water-stabilized plasma spray system PAL160 to form free-standing panels. The morphology of the microstructure, the thermal expansion, the bulk and filling densities, the open and closed porosity, the phase, and the chemical changes were evaluated. Paper includes a German-language abstract.

This paper aims to measure the residual stresses of plasma sprayed oxide ceramic deposits using the X-ray diffraction method and measure the Young’s Modulus in specially designed four-point bend test device. Aluminum oxide and chromium oxide coatings are made with a water stabilized plasma spray gun. The paper analyzes the microstructure and the phase composition. The X-ray diffraction method is used to determine residual stresses on the layer surfaces. The same method is used for the local measurement of surface tensions in a four-point bending device built into an X-ray diffractometer. This device also enables the force applied and the specimen deflection to be measured. Effective modules of elasticity are determined from the gradient from load to deflection during bending. The results of the tension measurements and the values calculated from the applied force are compared. Paper includes a German-language abstract.

The formation of the microstructure is influenced by a number of parameters. The spray angle is considered to be one of the most important and difficult to control parameters, especially for substrates with complex shapes. In this paper, the influence of the spray angle on the microstructure of gray aluminum oxide and yttrium oxide stabilized (8% by weight) zirconium oxide applications is investigated. Plots are made from each material at four spray angles (90 degree, 70 degree, 55 degree, and 45 degree). Their microstructure is characterized by means of intrusion porosimetry and SEM. The results show clear differences between the two materials. The aluminum oxide microstructure is significantly influenced by the spray angle, while the YSZ microstructure is almost independent of the spray angle. Paper includes a German-language abstract.

Phase transformations and/or decomposition of deposited compounds have an indisputable influence on materials properties of plasma sprayed deposits. Using water stabilized plasma, free-standing parts were manufactured from a mechanical mixture of zircon and alumina powders and annealed. The phase composition was determined by X-ray diffraction and the chemical composition was checked by x-ray microanalysis. ZrSiO 4 during plasma spraying decomposes into ZrO 2 and SiO 2 . In the as-sprayed condition, after a relatively fast quenching, the following phases can be found: a very fine eutectic mixture of tetragonal and monoclinic ZrO 2 , amorphous SiO 2 and a spinel phase of Al 2 O 3 . On annealing for 2 hours at 1300 and 1500 °C the spinel Al 2 O 3 transformed to corundum. At the same time, amorphous silica crystallized. Tetragonal ZrO 2 transformed to the monoclinic modification and together with SiO 2 formed again ZrSiO 4 . At the highest annealing temperature Al 2 O 3 and SiO 2 partialy reacted to form a small amount of mullite (3Al 2 O 3 .2SiO 2 ).

A search for cheap spraying materials offering interesting properties is conducted in connection with utilization of the high throughput water stabilized plasma. In this regard very promising materials are silicates. WSP PAL 160 was used for spraying garnets in their natural form and fused and crushed basalt. Chemical composition of both these materials is based generally on the same components but their contents are different. Paper reports on the spraying parameters used and then describes the structure and phase composition of deposits as well as their selected properties. Garnet of almandine type can be sprayed very well at a wide variety of parameters while pyrope type spraying is more difficult. Very interesting coatings were made of basalt.