Karel Neufuss

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.

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.

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.

Effect of uniaxial pressure on ultrasound velocities and elastic moduli in plasma-sprayed ceramics

Microcracks and thin voids in plasma-sprayed ceramics are known to be responsible for elastic anisotropy and for small values of elastic stiffness constants (measured at small stresses), compared with well-sintered materials. The increase of ultrasound velocities with increasing uniaxial pressure up to 300 MPa in three types of plasma-sprayed ceramics was measured in two directions. The corresponding elastic stiffnesses increased from 1.4 to 4.7 times. The experimental results were explained by closing of intrasplat microcracks and intersplat thin voids by uniaxial pressure.