DEVELOPMENT OF A LOW-EXPANSION BOND COATING FOR NI-BASE SUPERALLOYS

Ternary NiCrAl alloys were modified by the addition of Ti and Si in order to adjust their coefficients of thermal expansion (CTE) to less than 15 x 10(-6) K-1 (from room temperature to 1000 degrees C) without sacrificing essential high temperature oxidation resistance. Vacuum induction melted cast alloy samples were investigated by dilatometry and thermogravimetry (TG). Oxidative TG was conducted isothermally at 900 and 1000 degrees C and cyclically between 600 and 1100 degrees C for 100 h respectively. The CTE is reduced mainly by Ti and secondly by Cr additions. Quinary alloys which showed optimal oxidation resistance essentially exhibit phase stability in the solid state at all temperatures. Approximately 4 at.% Si is needed to attain low oxidation rates and to prevent oxide spallation as well. The beneficial effect of Si on oxidation behavior is attributed hypothetically to its ability to initiate the formation of protective alumina and subsequently silica. High Cr contents lessen the beneficial effect of Si owing to the concurrent formation of chromia and/or titania.