Silicon Nitride Joining.
Annual rept. 31 Jan 82-31 Jan 83,
SRI INTERNATIONAL MENLO PARK CA
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Silicon nitride can be joined using an oxide glass which reacts to form a phase similar to the grain boundary phase found in hot pressed Si3N4. The microstructure and interfacial reactions were studied by scanning transmission electron microscopy and X-ray diffraction. Silicon nitride dissolves in the glass and silicon oxynitride crystals precipitate. The glass also penetrates the Si3N4 and is present as enlarged glass pockets in the bulk ceramic away from the joint. The optimal joining conditions were determined to be 1575-1650 C and 30 to 60 minutes. The maximum strengths at room temperature determined by 4 point bend tests of bars joined under these conditions were approximately 460 MPa, regardless of the specific conditions. The strength is related to the joint thickness, reaching a maximum for joint thicknesses of approximately 25 to 35 micrometers. Two fracture mechanisms were identified by SEM. Silicon oxynitride crystals grow across thin joints and the resulting microstructure is strong if few pores are present. Thermal expansion mismatch cracks are present in thicker joints and there are multiple fracture origins. Preliminary attempts to heat treat joints to promote crystallization resulted in vaporization of the glass and a low strength.
- Ceramics, Refractories and Glass