Pavel Chráska

Young's modulus-and fatigue behavior of plasma-sprayed alumina coatings

The fatigue behavior and Young’s modulus of plasma-sprayed gray alumina on low-carbon steel substrates were investigated. The investigation of the properties of composites that were defined as “coating-substrate” composites included measurements of the microhardness profile, the residual stress on the top of the coating, and the residual stress profile in the substrate. Fatigue samples were periodically loaded as a cantilever beam on a special testing machine. Failed samples were observed with a scanning electron microscope to determine the failure processes in the coating. The Young’s modulus of the coating was measured by the four-point bending method. Samples were tested both in tension and compression under low (300 N) and high (800 N) loads. The authors’ experiments revealed that the average fatigue lives of coated specimens were nearly two times longer than those of the uncoated specimens. The measurements of Young’s modulus of the coating yielded values that varied between 27 and 53 GPa, with an average value of 43 GPa. Loading in tension caused a decrease in the Young’s modulus of the coating, while loading in compression led to an increase in Young’s modulus. The increase in the lifetime of coated samples was likely due to compressive residual stresses in the substrate, originating during the spray process. The failure of the coating was due to several processes, among which the most important were splat cracking, splat debonding, and the coalescence of cracks through the voids in the coating.

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

Abstract 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 Mossbauer 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.

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.

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.

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.

Plasma sprayed coatings for RF wave absorption

Abstract High requirements for fusion reactor materials and for experimental fusion devices have led within the fusion community to the development and testing of various coatings of the surfaces of in-vessel components and biological shields for microwave heating systems. Based on contacts with ITER, W7X and the Spanish Stellarator TJ-II, IPP Prague has initiated a development, production and test program on various low- Z materials. This paper reports on the production, development and properties of B 4 C, Si and Al 2 O 3 coatings sprayed by water stabilized plasma. Main focus is on their radio frequency wave reflection properties. Further characterization includes the coating structure by microscopy, phase composition by X-ray diffraction and oxygen content measurement by atomic absorption spectroscopy. The results are discussed with respect to processing conditions as well as potential application.

Homogeneity studies of powders and plasma sprayed deposits

For production of plasma sprayed protective deposits and self-standing ceramic parts many different feedstock powders are used. The powders are very often of a composite type prepared by various technologies and mostly chemically very inhomogeneous. A method has been developed for evaluation of chemical homogeneity of both, the feedstock powders and the sprayed materials. The backscattered electron images along with the element mapping of dopant have been used as the basic measurement. The homogeneity coefficients were determined by quantitative point analysis with ZAF corrections. The results show that under all spraying parameters there is always a certain redistribution of all species leading to a better homogeneity. But generally, the more homogeneous the feedstock powder, the better the homogeneity of the deposits on both micro and macro scale.

Resistometric investigation of phase transformations in NiAl alloys

Abstract The paper represents a contribution to the martensitic transformation and to the existence regions of Ni2Al and Ni5Al3 phases in the Ni–Al phase diagram. Resistivity, optical microscopy and differential thermal analysis (DTA) investigations were performed on 63.4, 64.8 and 66.2 at.% Ni alloys in the temperature region between −190 and 850°C. The martensite–austenite transformation (reversible for the alloy with the lowest Ni content) takes place between 60 and 300°C according to the nickel content of the alloy. In the 63.4 at.% Ni alloy, the Ni2Al phase originates above 350°C and exists up to 580°C. After an incubation period, the Ni5Al3 phase starts to nucleate from austenite above 640°C. The Ni5Al3 phase then dissolves above 740°C forming Ni3Al+NiAl phases in a peritectoid reaction. In Ni-richer alloys, martensite rapidly changes into the Ni5Al3 phase which does not transform martensitically. Only very rapid heating to 600°C retains the possibility of martensitic transformation in these alloys.

Chemical inhomogeneity of silicates treated by plasma spraying

The properties of thermally sprayed deposits are directly influenced by the homogeneity of their chemical composition as well as by their microstructure. The presented work deals with the evaluation and quantification of the chemical homogeneity of silicates, i.e., basalt, cordierite, garnets, mullite, steatite and wollastonite. The deposits were manufactured by a water-stabilized plasma spray system WSP ® PAL160. A number of point chemical analyses were performed for each deposit on a polished cross-section by X-ray microanalysis and the results were statistically evaluated. For some of the selected materials, the homogeneity of deposits was compared with the homogeneity of appropriate feedstock powder used for spraying. The microstructure of the cross-sections was observed by back-scattered electron imaging. Electron probe X-ray microanalysis (EDS-SEM) results were combined with X-ray diffraction measurements and showed the presence of various crystalline and amorphous phases in the deposits. The mean concentration, its standard deviation, the minimum and maximum compound content as well as Weibull statistics were used to characterize the chemical homogeneity of the deposits. Generally, the materials showed changes in homogeneity after deposition. This was most notable in steatite and wollastonite.

Oxidation of nickel during plasma spraying and some properties of nickel oxide

The techniques of plasma spraying are suitable for deposition of metals, ceramics or composites. Atmospheric plasma spraying of metals is accompanied by their oxidation. The oxidation of nickel during its spraying gives rise to NiO. During the flight of molten nickel particles in the plasma plume, the first stage of the oxidation reaction takes place. To determine the amount of NiO grown during this stage, oxidation can be stopped abruptly by trapping and quenching the particles in liquid nitrogen. If, on the other hand, the flying molten particles are allowed to hit a solid substrate, a plasma deposit or coating is built up. The period starting at the moment of the particle impact and solidification corresponds to the second oxidation stage. This is finished by cooling down the system substrate — coating.Plasma spraying of nickel was conducted using a water-stabilized plasma gun. To study the structure and optical properties of the oxidation products, it is necessary to remove the metallic phase from the samples. This was done by a technique of metal dissolution described previously.After the first oxidation stage, if the particles are trapped in liquid nitrogen, NiO is obtained by rapid solidification of oxide melt grown on the surface of the flying particles as a result of a gas — molten Ni reaction. The colour of solid NiO formed in this way is green, which corresponds to a region of high reflectance between 1.9 and 2.7 eV. The green colour is typical of stoichiometric NiO and is due to octahedral Ni2+ ions.The second oxidation stage is characterized by a gas — solid Ni reaction. It results in black NiO, whose colour follows from strong absorption of light in the whole visible range. The oxygen content in this oxide exceeds slightly the stoichiometric value. The light absorption is due to free charge carriers, i. e. holes, whose presence is a consequence of the deviation of NiO from stoichiometry.