J. Dubsky

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.

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.