High‐Temperature Microstructure of a Hot‐Pressed Silicon Nitride

The outstanding question as to the microstructure of silicon nitride at temperatures associated with potential high‐temperature applications of the material is addressed experimentally by quenching thin (transmission electron microscopy) samples from 1450°C and examining them in the microscope. The morphology of the microstructure is qualitatively unchanged compared to the materials slowly cooled, for example, after hot‐pressing, to room temperature. The most significant difference is that the thickness of the intergranular phase is larger, typically 2 to 10 nm, as compared to the ∼ 1 nm observed in the hot‐pressed material. In addition there is an apparent increase in the volume fraction of the intergranular phase at the three‐grain junctions. On the basis of a number of supporting experiments including both hot‐stage transmission electron microscopy (up to 1000°C) and Auger electron spectroscopy of material fractured and examined at 850°C, the change in microstructure is concluded to occur at temperatures above about 1000°C.