Nb4Fe4Si7 coatings to protect niobium and niobium silicide composites against high temperature oxidation

Abstract

The aims of this study were to develop Nb4Fe4Si7 coating for Nb-silicide composite and to assess their oxidation resistance. The Nb4Fe4Si7 was first manufactured as single phase and its oxidation behaviour was characterised from 1100 to 1300°C. Its oxidation resistance at 1300°C in air was high because of the formation of a duplex protective oxide layer of Fe2O3 and SiO2. Halide activated pack-cementation was successful to deposit the Nb4Fe4Si7 compound as outer layer of the coating both on pure niobium and Nb-silicide composite. In both cases, the master alloy 10%FeSi2+10%NbSi2+80%Nb4Fe4Si7 can be employed. Oxidation tests performed both cyclically and isothermally at 1100°C demonstrated that the oxidation resistance of coated Nb-silicide composite was not as good as that observed on the single phase Nb4Fe4Si7 compound. Nevertheless lifetime as long as 500 1h-cycle at 1100°C was obtained both on pure niobium and Nb-silicide composite.