Jiří Dubský

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.

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.

Residual and Applied Stresses in Plasma Sprayed Cr2O3 Coatings

The results of stress measurements obtained by x-ray diffraction are often ambiguous and difficult to interpret. In this work both neutron and X-ray diffraction methods were used to determine the residual stresses in plasma sprayed ceramic coatings. In addition a four-point bending device was used to check the correctness of method used. The results of both methods were compared. The volume- and lattice-effective Youngs modulus were calculated from applied bending stress and lattice deformation, respectively. The results obtained are discussed from the point of view of the behavior of the defective coating structure influenced by tensile and compressive loading.