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.

In result of the spray process normally a phase change from α-alumina (corundum) in the feedstock powder to predominantly α-alumina in the coating takes place. This is a well known but often neglected fact in the preparation and use of alumina coatings. This is of special interest since the high usage properties of thermally sprayed alumina coatings are generally derived from sintered alumina ceramics which consist of corundum. It is expected that the prevention of this phase transformation will significantly change the mechanical, electrical and other properties of thermally sprayed alumina coatings. There is controversial information about the possibility of stabilization of α-alumina by additions of chromia in the literature. In this work the stabilization using different spray processes (water stabilized plasma WSP, gas stabilized plasma APS and HVOF) was studied. Mechanical mixtures of alumina and chromia were used, as well as pre-alloyed powders consisting of solid solutions. The coatings were studied by X-ray diffraction and metallographic cross-sections. It was shown that in the case of the mechanically mixed powders the stabilization predominantly depends on the applied spray process. The stabilization by use of the WSP process was confirmed.

Thermal spray is a flexible technique enabling deposition of wide choice of feedstock materials including ceramics, metals or the mixture of both. Various spray technologies are available, such as different types of thermal plasma methods, electric arc based methods, HVOF and other combustion methods. Obviously, the fatigue properties of coated bodies will differ based on the choice of feedstock material and on the spray technology used. Comparison was made of fatigue properties of bodies with Alumina, Ni-5wt%Al and composite coatings produced by gas and water stabilized plasma spray. It has been found, for instance, that Alumina coatings sprayed by water stabilized plasma torch (WSP) had substantially increased fatigue life times of specimens compared to specimens with coatings of the same feedstock deposited by the gas stabilized plasma torch. Measured fatigue data were related to a detailed structural characterization of the tested coatings. Obtained results are discussed with regard to our previous results obtained during fatigue tests of specimens with one-component coatings.

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.

It is generally known that plasma sprayed coatings exhibit rather a low strength thanks to their characteristic microstructure with porosity and microcracks. To determine the role of varying types of deformation in different parts of the coatings profile, 1.8 mm thick chromia coatings on a steel substrate were made using WSP plasma spray. Stress gradients were then measured "in-situ" on a four-point bending device during bend deformation of the coated beam using neutron diffraction. Only compressive loading in coatings increased the resulting stresses. From the plot of stresses vs. applied strain (linear dependence) the Young’s moduli of the substrate and the coating were determined. Both values agree well with those obtained by mechanical testing. In general it is believed that, in a simplified way, interlamellar voids (cohesion defects) roughly parallel to the substrate decrease the tensile strength in the direction perpendicular to the substrate, while intralamellar cracks, roughly perpendicular to the substrate, significantly affect the Young’s modulus of coating. It is supposed that the tensile deformation of coatings opens the cracks and no stress increase is observed. On the other hand the applied compressive deformation in the coating closes the intralamellar microcracks and the internal stresses increase, as suggested by obtained results.

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.

A novel material has been used for plasma spraying by WSP. The material is composed of three main phases, namely corundum (aluminum oxide), baddeleyite (zirconium oxide), and glassy phase (silicon oxide). The material is a refractory and exhibits very high hardness, extremely high abrasion resistance, and chemical resistance. Conventionally, the material is fabricated by melt casting and machining. Cast tiles of the material were ground and sieved to obtain the right powder cut size for plasma spraying by water stabilized plasma torch (WSP). Both dense coatings and free standing parts were achieved with the new material, which sprays very well with WSP. Spraying parameters were varied and molten particles were monitored in flight by DPV 2000. The coatings exhibit very low porosity and high hardness. The as-sprayed material is mostly amorphous with some nanocrystalline grains of aluminum and zirconium oxide present. The phase composition of the as-sprayed material is thus different from that of the feedstock material, which is mostly crystalline with a small fraction of amorphous silica glass. The microstructure of the newly sprayed material was studied by electron microscopy (SEM, TEM) and is very complex. Upon annealing, the as-sprayed material crystallizes around 950ºC. This result and other thermal properties were measured by TMA and DTA. The ease of plasma spraying and the coating properties make this material a suitable candidate for many industrial applications.

Plasma sprayed ceramic coatings have much lower stiffness in comparison to sintered ceramics. The reason for that is their characteristic microstructure with porosity and microcracks. Microcracks decrease the interlamellar cohesion in vertical direction, but also affect the individual splat properties in the horizontal direction. For that reason sealing treatments are often applied with plasma sprayed ceramic coatings in order to improve their corrosion resistance and mechanical properties. In this paper the effect of aluminum phosphate sealing treatment on the elastic properties of plasma sprayed Al 2 O 3 and Cr 2 O 3 coatings were studied. Residual stresses in the plane of coating surfaces were compared using the X-ray diffraction analysis (XRD). A special four point bending device, designed for the X-ray diffractometer, was used in determining the effect of additional load on coating elastic behavior. In as-sprayed alumina coatings tensile stresses of about 400 MPa were detected while only about 40 MPa of compressive stresses were measured in the as-sprayed chromia coatings. Microstructural characterization revealed that sealing treatment had apparently affected the coating microstructure and filled some microcracks and interlamellar spacings. As a result, in both sealed coatings, compressive stresses of about 100 up to 150 MPa were observed. Also a better stiffness of both materials was detected during the bending of specimens. In addition, the sealing treatment increased nearly ten times the Young’s modulus, determined by XRD analysis under various tensile loads.

During air plasma spraying, molten metal particles flying in the plasma jet are oxidized. As a result, a part of the metal melt is converted into oxide melt. After the particle impact and solidification, oxidation continues as a gas – solid reaction. The present paper deals with oxidation of two binary Ni-based alloys. One of them, Ni-20%Cr, is frequently used in thermal spray applications. Another one was a Ni-Fe alloy with an approximate proportion of both components 1:1. The feedstock powders were plasma sprayed by a water-stabilized gun WSP. PAL 160. To analyze the reaction products of the in-flight oxidation stage, the flying particles were trapped and quenched in liquid nitrogen. Oxides resulting from both oxidation stages were studied in the as-sprayed deposits after their cooling down to room temperature. The oxide amounts in the samples were determined indirectly by oxygen level measurement using "extractive fusion" (LECO-method). Structure of the oxides, separated by dissolution of the metallic phase, was investigated by X-ray diffraction. Iron-containing oxides were also characterized by Mössbauer spectroscopy. From the point of view of reaction kinetics, both alloys behaved in a similar way. The particles quenched in liquid nitrogen contained less than 2% of oxygen, whereas in the deposits these values were higher, up to 4.5 %. Two oxide phases were found in all plasma sprayed Ni-Cr samples: a rhombohedral phase similar to (Ni,Cr) 2 O 3 and a tetragonally distorted spinel phase (Ni,Cr) 3 O 4 , both of them very rich in chromium. Another oxide, NiO, was present mainly in the deposits. In the oxidation products of Ni-Fe alloy, the dominant phase was similar to nonstoichiometric wüstite FeO. The results of thermodynamic calculations are in sound agreement with the experiments except for the presence of the tetragonal phase, the composition of which is near to Cr 3 O 4 , in oxidation products of Ni-Cr alloy.

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.

In this paper, a four-point bending test is developed that makes it possible to load layers with defined tensile stresses. In the experiments, high-density stainless steel is applied using HVOF and ceramic layers are produced using water-stabilized plasma spraying. Both coating materials are then subjected to bend testing. The steel layers show a volume of elasticity twice that of the surface elasticity module, while in the ceramic layers, no surface tension was observed. This can be explained by the opening and closing of crack structures in the material. Paper includes a German-language abstract.

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.

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.

This paper examines ways to control the porosity of thermally sprayed deposits. All spraying was done with a water-stabilized plasma system using different combinations of alumina, zircon, Ni, and Al powders. Sandwiched structures with alternating ceramic and metal layers were sprayed as were thick deposits consisting of metal and ceramic mixtures. Porosity was characterized by methods such as gas permeability, water immersion, MIP, SEM, and SANS. In addition, several post-processing methods were tested to determine their effect on porosity volume. For example, removing metallic phases by leaching or by annealing at temperatures above the melting point was found to effectively increase porosity, while the use of sealing materials proved effective at reducing porosity. Another method tested was calcination, which resulted in an increase or decrease of porosity depending on the deposit's chemistry and annealing conditions.

Protective tubes ( Ø 90x1500 mm) were manufactured by spraying with a water stabilized plasma gun. Mixtures of zircon (ZrSiO 4 ) and alumina (Al 2 O 3 ) were used as feedstock powders. Products made of these powders exhibit very good properties during thermal cycling. Previous results of the phase composition and phase changes were obtained from as-sprayed and annealed samples using X-ray diffractometry (XRD) and scanning electron microscopy. During plasma spraying zircon decomposed into ZrO 2 and SiO 2 , which on impact and after rapid quenching formed a very fine eutectic mixture of tetragonal or monoclinic ZrO 2 and amorphous SiO 2 . During this process alumina, in feedstock as α-phase (corundum), formed the spinel γ-phase. On annealing the y-phase transformed into the a-phase, whereas amorphous SiO 2 crystallized and reacted with tetragonal ZrO 2 to form ZrSiO 4 . Mullite (3Al 2 O 3 .2SiO 2 ) was found at the highest annealing temperature of 1500°C when alumina reacted with SiO 2 . High temperature XRD was used for direct observation of phase changes during heating and cooling between room temperature and 1500°C in powdered as-sprayed deposits. This method confirmed the phase changes observed at room temperature in annealed samples, in particular the partial transformation of tetragonal to monoclinic ZrO 2 .