Phase Formation Control in Plasma Sprayed Alumina–Chromia Coatings

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