HIGH-TEMPERATURE INTERACTIONS OF REFRACTORY-METAL MATRICES WITH SELECTED CERAMIC REINFORCEMENTS

Interdiffusion and reactions occurring at high temperatures between refractory metals (Nb and Ta) and ceramic materials (SiC and Al2O2) have been investigated. Diffusion couples were fabricated by depositing Nb and Ta films of approx. 1-μm thickness onto polished ceramic substrates. These diffusion couples were vacuum annealed at high temperatures for various times. Interfacial reactions were evaluated using optical metallography, Auger electron spectroscopy (AES), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Kinetic studies in the 800 °C to 1200 °C temperature range for the Nb/SiC system indicated that Nb2C initially forms, followed by the more stable NbCxSiy phase. In some instances, layered structures containing the phases NbC, Nb2C, and NbCxSiy were observed. The activation energies of formation for the NbCx and NbCxSiy phases were determined from these measurements. Results from the Ta/SiC system were found to be similar to those from the Nb/SiC system. In both Nb/Al2O3 and Ta/Al2O3 diffusion couples, annealing for up to 4 hours in the 1100 °C to 1200 °C range did not result in any significant reactions. These results suggest that Al2O3 may be a promising diffusion barrier between Nb and Ta metal matrices and SiC ceramic reinforcements.