Ceramics-Silikáty 55 (3) 294-300 (2011)

Phase Formation Control in Plasma Sprayed Alumina–Chromia Coatings

J. Dubskýy P. Chraska, B. Kolman, C. Ch. Stahr, L.-M. Berger

Alumina is an inexpensive material often used as feedstock for plasma spraying. Hard corundum (a-Al₂O₃) is desirable in coatings, but other phases, unstable at elevated temperatures, also form as result of the complexity of the spraying processes. When chromia (Cr₂O₃) is added to Al₂O₃ a complex corundum type a-(Al,Cr)₂O₃ phase forms in the deposits. Two types of Al₂O₃/Cr₂O₃ powder feedstock with 2%, 8%, 33% and 50% Cr₂O₃ were sprayed: fused materials and mixtures of Al₂O₃ and Cr₂O₃ powders. Coatings were made by the water-stabilized plasma (WSP^®) system and by commercial atmospheric plasma spraying (APS) and compared by various characterization techniques, including X-ray diffraction, scanning electron microscopy and microanalysis. Coatings made from fused feedstock by both spraying methods contain the corundum phase with its fraction increasing with increasing Cr₂O₃ content. In case of mixed feedstock the stabilization effect was observed for WSP^® spraying only. Explanation of this finding is offered on basis of differences in the spraying technologies, in particular the spraying temperature.

Keywords: Alumina, Chromia, Plasma spraying, Phase stabilization