Effects of Various Rare Earth Sesquioxide Additives on Grain Growth in Millimeter-Wave Sintered Silicon Nitride Ceramics

摘要

Using various rare earth sesquioxides as additives, silicon nitride (Si_3N_4) samples were sintered at 1700癈 for 4 h by millimeter-wave heating performed in an applicator fed by a 28 GHz Gyrotron source under a nitrogen pressure of 0.1 MPa. A comparative study of densification, grain growth behavior and mechanical properties of silicon nitride fabricated by millimeter-wave and conventional sintering was carried out. Bulk densities were measured by Archimedes' technique Except for the Eu_2O_3 containing sample, all samples were densified to relative densities of above 97.0%. Microstructure of the specimens was analyzed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). To investigate quantitatively the effect of millimeter-wave heating on grain growth, image analysis was carried out for grains in the specimens. Fracture toughness was determined by the indentation-fracture method (IF method) in accordance with Japan Industrial Standards (JIS). Fully dense millimeter-wave sintered silicon nitride presenting a bimodal mi-crostructure exhibited higher values of fracture toughness than materials processed by conventional heating techniques. Results indicate that millimeter-wave sintering is more effective in enhancing the grain growth and in producing the bimodal mi-crostructure than conventional heating. It was also confirmed that localized runaway in temperature, depending upon the sintering additives, can occur under millimeter-wave heating.