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

Abstract The aims of this study were to develop Nb 4 Fe 4 Si 7 coating for Nb-silicide composite and to assess their oxidation resistance. The Nb 4 Fe 4 Si 7 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 Fe 2 O 3 and SiO 2 . Halide activated pack-cementation was successful to deposit the Nb 4 Fe 4 Si 7 compound as outer layer of the coating both on pure niobium and Nb-silicide composite. In both cases, the master alloy 10%FeSi 2  + 10%NbSi 2  + 80%Nb 4 Fe 4 Si 7 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 Nb 4 Fe 4 Si 7 compound. Nevertheless lifetime as long as 500 1 h-cycle at 1100 °C was obtained both on pure niobium and Nb-silicide composite.