High-temperature interactions of refractory metal matrices with selected ceramic reinforcements

Abstract

Interdiffusion and reactions occurring at high temperatures between refractory metals (Nb and Ta) and ceramic materials (SiC and A1₂O₃) have been investigated. Diffusion couples were fabricated by depositing Nb and Ta films of ~l-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 NbC_xSi_y phase. In some instances, layered structures containing the phases NbC, Nb₂C, and NbC_xSi_y, were observed. The activation energies of formation for the NbC_x and NbC_xSi_y, 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 A1₂O₃ may be a promising diffusion barrier between Nb and Ta metal matrices and SiC ceramic reinforcements.