INFLUENCE OF GAS-PHASE COMPOSITION ON THE FORMATION OF TI-SI COATINGS ON NIOBIUM

The elemental and phase composition of diffusion coatings formed on niobium during high temperature gas-substrate interaction has been studied. The packs containing titanium, silicon and fluoride activator have been used for the generation of the gas phase. The coatings can be divided into two basic types: Nb-Ti solid solutions and niobium and titanium silicides. The coatings of the first type have been obtained in Ti-Ti5Si3 powder mixtures. The complex silicide layers were observed in the packs with higher silicon activity (Ti5Si3-TiSi and TiSi-TiSi2 mixtures). The equilibrium partial pressures of gaseous components and activities of titanium and silicon in packs and in niobium silicides have been calculated. The activities of titanium and silicon have been shown to be principal controlling factors in coating phase composition.