Formation and properties of niobium silicide coatings on Nb-Ta alloy substrate

Abstract

Niobium silicide coatings have been noted in the aeronautical and aerospace industries due to their high resistance to oxidation and creep. Despite these desirable properties, there is a need for further improvements to make them more applicable in demanding scenarios. This study investigates niobium silicide coatings processed on a Nb-22Ta alloy substrate by Double Pack Cementation method, to assess the influence of tantalum in coatings. The pack mixture was composed by 15% Si, 5% salt halide activator and 80% Al₂O₃, with NH₄Cl and NH₄F used as activators. The coatings were processed at 1000°C for 6 h. Analysis of the processed surfaces included a correlation with similar diffusion coatings processed on pure Nb under similar conditions. The results obtained by SEM and EDS revealed a low presence of Ta in the coatings, regardless of the halide activator used. Coatings processed with NH₄F showed exclusive formation of NbSi₂, while those processed with NH₄Cl exhibited the presence of both Nb₅Si₃ and NbSi₂. Nanohardness analysis indicated hardness levels of the niobium silicide coatings processed with NH₄F similar to coatings processed with pure Nb substrate (1459 HV), different from the coatings processed with NH₄Cl which demonstrate a much higher hardness close to the surface, around 2038 HV. Furthermore, coatings on Nb-22Ta exhibited better oxidation resistance at elevated temperatures, demonstrated by thermogravimetric analysis up to 1350°C. This behavior confirms s that silicide coatings are influenced by elements from the substrate Nb-22Ta that can be enhanced by processing conditions such as the activator in the pack mixture.