Phase transformation of alumina coating by plasma assisted tempering of aluminized P91 steels

alpha-Al2O3 coating on aluminized surfaces are considered candidate coatings for blanket applications in fusion reactor. In order to generate alpha-Al2O3, aluminized P91 steel samples were subjected to normalizing and tempering treatments at 980 degrees C and 750 degrees C respectively. Oxygen plasma has been used during tempering treatment of aluminized P91 steel samples at 750 degrees C for 1 h. The resulting alumina coating on plasma tempered samples were compared with those of thermally tempered samples. The alumina films were characterized using XRD, XPS, and SEM-EDS techniques. Results indicate that the thermally tempered samples had theta-Al2O3 coating while the plasma tempered samples had alpha-Al2O3 coating after heat treatment. Such transformation of alumina phase was not visible without plasma. A hypothesis of theta to alpha-Al2O3 transformation in plasma is proposed. This paper emphasizes the role of plasma processing on generation of an improved insulation coating for TBM applications in fusion reactors. (C) 2015 Elsevier B.V. All rights reserved