Abstract
Atmospheric plasma spraying of alloys often results in their composition changes. The main source of the changes is usually preferential oxidation of some elements composing the alloy. As a rule, these are the alloying elements whose affinity to oxygen is high. Changes due to this effect are well known from metallurgy; however, they were scarcely studied from the point of view of plasma spraying. Preferential evaporation of some elements may also contribute to the alloy composition changes. The aim of the present paper is to give quantitative data on composition changes of selected alloys sprayed by a water-stabilized plasma gun. Two Ni-base alloys and one high-alloy Cr-Ni-steel were studied. The main tool for determining the sample composition was electron probe X-ray microanalysis. To quantify the results and to eliminate the systematic errors, the data obtained by this method were calibrated by repeated chemical analysis of feedstock powders. The alloy composition was determined after both stages of plasma spraying, i.e. after the inflight stage of molten particles and after the stage comprising particle impact, solidification, coating formation and cooling. To study the situation after the former stage, the flying particles were trapped and quenched in liquid nitrogen. In the Ni-Cr alloy containing 20%Cr, strong Cr depletion was observed. The Fe depletion in the Ni-Fe alloy (47%Fe), though unambiguous, was less significant. The high-alloy steel (Czech equivalent of AISI 316) was also Cr depleted, whereas the concentrations of other alloying elements (Mo, Ni) remained effectively unchanged. In all cases, the depletion occurred at the first spraying stage and became more pronounced during the second stage. Strong air entrainment occurs not only in a plasma jet produced by a water-stabilized plasma gun, but also if gas-stabilized plasma guns are used in atmospheric plasma spraying. It follows that the dominant mechanisms of composition changes during plasma spraying by both techniques are similar.
