Optical properties of plasma sprayed ZnO+Al2O3 coatings

The powders of ZnO and ZnO+3 and 22 wt.% Al2O3 were agglomerated using spray drying and subsequently heat-treated at 1300 °C in air. The powders were plasma sprayed in controlled atmosphere (CAPS) into water and onto sand blasted steel substrates. The powder particles were characterized using laser sizer and their microstructure was tested with a scanning electron microscope (SEM) equipped in secondary electrons and back-scattered electron detectors. The phase analysis, made with X-ray diffraction (XRD) enabled to find ZnAl2O4 spinel in heat-treated powders and in some of sprayed coatings. This phase was transformed from α-Al2O3 and ZnO phases present in initially agglomerated powders. The infrared emissivity of sprayed coatings was depending on the following factors: wavelength, chemical composition of initial powders and atmosphere of spraying.