Microstructural Changes in β‐Silicon Nitride Grains upon Crystallizing the Grain‐Boundary Glass

Crystallizing the grain‐boundary glass of a liquid‐phase‐sintered Si 3 N 4 ceramic for 2 h or less at 1500° led to formation of δ‐Y 2 Si 2 O 7 . After 5 h at 1500°, the δ‐Y 2 Si 2 O 7 had transformed to β‐Y 2 Si 2 O 7 with a concurrent dramatic increase in dislocation density within β‐Si 3 N 4 grains. Reasons for the increased dislocation density are discussed. Annealing for 20 h at 1500° reduced dislocation densities to the levels found in as‐sintered material.