Microstructure and High Temperature Oxidation Performance of Silicide Coating on Nb-Based Alloy C-103

The microstructure and oxidation resistance of NbSi(2) coating formed on Nb-based alloy C-103 by a pack siliconization process have been studied. The as-formed coating consists of an outer NbSi(2) layer and an inner Nb(5)Si(3) layer. A NbSi(2)-Nb(5)Si(3) two-phase zone is also present between the above two layers. Weight-change data obtained under isothermal and cyclic oxidation in air at 1100 and 1300A degrees C, suggests that the coating gives oxidation protection up to about 4 h. The oxide scale that formed on the coating during oxidation exposure consists of an outer glassy silica layer and an inner Nb(2)O(5)-silica mixed layer. Nb(2)O(5) phase is also present in the outer silica scale in the form of elongated particles. Oxidation protection is achieved primarily by the presence of the glassy silica layer on the surface. Spallation of this layer during thermal cycling causes significant reduction in the protective life of the coating.