High temperature behaviour of NbSi2 coating on the Nb-1Zr-0.1C alloy

High temperature behaviour of NbSi2 coatings on the Nb-1Zr-0.1C alloy, developed using the molten salt technique, has been investigated. For this purpose, samples were aged at different temperatures under vacuum and in the ambient atmosphere. High temperature annealing between 1200 and 1400 degrees C of the coating under vacuum led to the formation of a layered structure comprising various silicide phases and porous microstructures. The formation of the porous structure was rationalised on the basis of differential diffusion mechanism of elemental species. The high temperature annealing in an ambient atmosphere showed that the single layer of NbSi2 coating formed on the Nb alloy was found insufficient to provide protection at high temperatures under oxidising environments. By introducing intermediate coating, concentrations and effective size of pores were reduced. Based on the EDS analysis, density of the pores and their size distribution, a mechanism involving diffusion of Nb/Si in the formation of Nb5Si3 has been proposed.