Reaction between niobium and silicon carbide at 1373 K
- PDF / 3,595,344 Bytes
- 12 Pages / 593.28 x 841.68 pts Page_size
- 73 Downloads / 156 Views
The reaction zone formed between niobium and silicon carbide during heating for 4 h at 1373 K was examined by transmission electron microscopy (TEM) and Auger electron spectroscopy (AES). The typical reaction layer sequence is SiC/Nb5Si4C/Nb5Si3/Nb2C/NbO/Nb. However, in one area of the specimen, the first reaction layer was NbC rather than Nb5Si4C. The high oxygen and carbon concentrations near the outer surface were shown by AES depth profiling to result from carbon and oxygen contamination from the vacuum system during annealing. In order to determine if the observed reaction layer sequence is consistent with conditions of local thermodynamic equilibrium, the quaternary Nb-Si-C-O phase diagram was calculated from available thermodynamic data. A minimum (most negative) free energy of formation for the ternary compound Nb5Si4C of -582 kJ/mole was estimated assuming that the equilibrium between NbSi2 and SiC observed experimentally at 1573 K1 also exists at 1373 K. Except for the region immediately adjacent to the substrate, the observed reaction layer sequence was in agreement with the calculated quaternary phase diagram. However, it was noted that agreement with the quaternary phase diagram would be obtained if a thin layer of either SiO2 or NbC were present at the substrate surface.
I. INTRODUCTION
Niobium based alloys have been recognized for many years as promising materials for use in elevated temperature structural applications. Despite having a density that is comparable to that of iron and nickel, the melting point of niobium is approximately 1000 K higher than either of these elements. Niobium alloys are characterized by good workability and a relatively low ductile-to-brittle transition temperature, making them attractive for use in elevated temperature structural applications where nickel and iron based superalloys exhibit inadequate specific strength.2 In recent years, it has been recognized that reinforcement of niobium alloys with high strength, high stiffness ceramic fibers can produce a composite material which is significantly stronger and stiffer than the matrix. However, in order for such a composite to retain its strength during prolonged periods of elevated temperature exposure, extensive reaction between the fiber and the matrix must not occur. Only by understanding the nature of the interaction between reinforcement and matrix can appropriate diffusion barrier coatings capable of limiting such interaction be developed.
In the present study, the reaction between niobium and silicon carbide at 1373 K was investigated. Because of the limited thickness of the reaction zone following a few hours exposure at elevated temperature, both TEM and AES depth profiling techniques were used to examine and characterize the reaction products. In the sections that follow, the results of the TEM and AES studies are presented and analyzed in terms of calculated thermodynamic phase equilibria. II. EXPERIMENTAL
Niobium-silicon carbide composites were prepared by sputter deposition of niobium onto silicon carb
Data Loading...
