B. Kolman

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.

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.

Structure and residual stresses in thermally sprayed steel coatings

Mechanical properties of thermally sprayed coatings are related to their structure and phase composition and, in addition, to the residual stresses. All of these factors depend on the spraying technology affecting the shapes of splats and amounts of oxides and pores as well as the state of residual stresses. It can be assumed that the thermally sprayed steels, in spite of oxide and porosity presence, behave similarly as bulk steels produced by conventional metallurgy. Two stainless steels (AISI 410 and AISI 316) were deposited by a high velocity oxy-fuel unit (HVOF). The microstructure of samples was analyzed using the light and scanning microscopy. Among other features, the contents of oxides and pores were determined. The X-ray diffraction method sin 2 ψ was used to determine residual stresses in the coatings. The same method was used for the measurement of surface tensile stresses in a four-point bend attachment installed in the X-ray diffractometer. Macroscopic residual stress on steel 410 coating was found to be compressive and about 97 MPa and surface effective Young modulus of coating was about 53 GPa evaluated from stresses slope during bending. The volume effective Young modulus evaluated from applied force for coating of steel 410 and steel 316 was 102 GPa and 88.5 GPa, respectively. Their relationships to the microstructure are discussed in comparison to bulk material.

Application of arc-plasma discharges to coating of screw taps

The paper reports on results of technological research on the coating of standard screw taps with TiN layers deposited by means of arc-plasma discharges, which were carried out within a BULAT-type device equipped with three identical metal-plasma sources. Particular attention is paid to morphology of the coated surfaces. Some data about thickness and microhardness of the coating layers, as well as results of laboratory wear tests of the coated tools, are also presented.