Grain-boundary viscosity of preoxidized and nitrogen-annealed silicon carbides

Internal friction experiments were conducted on a model SiC polycrystal pre pared from preoxidized (high-purity) SiC powder. This material contained hi gh-purity SiO2 glass at grain boundaries in addition to a free-carbon phase , which was completely removed upon powder preoxidation. Comparative tests were conducted on a SiC polycrystal, obtained from the as-received SiC powd er with the addition of 2.5 vol% of high-purity SiO2. This latter SIC mater ial was also investigated after annealing at 1900 degreesC for 3 h in a nit rogen atmosphere. Electron microscopy observations revealed a glass-wetted interface structure in SiC polycrystals prepared from both as-received and preoxidized powders. However, the former material also showed a large fract ion of interfaces coated by turbostratic graphite. Upon high-temperature an nealing in nitrogen, partial glass dewetting occurred, and voids were syste matically observed at multigrain junctions. The actual presence of nitrogen could only be detected in a limited number of wetted interfaces. A common feature in the internal friction behavior of the preoxidized, SiO2-added an d nitrogen-annealed SiC was a relaxation peak that resulted from grain-boun dary sliding. Frequency-shift analysis revealed markedly different characte ristics for this peak: both the magnitude of the intergranular glass viscos ity and the activation energy for grain-boundary viscous flow were much hig her in the nitrogen-annealed material. Results of torsional creep tests wer e consistent with these findings, with nitrogen-annealed SiC being the most creep resistant among the tested materials.