Development of Low Expansion Glaze Coatings on As Fired Si3N4 to Enhance Room Temperature Flexural Strength
Majumdar, Nandita N.
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Silicon nitride (Si3N4) has the potential for use in various high-performance applications. However, surface defects such as voids/pits are commonly present on as processed Si3N4. When subjected to external forces, fracture originates at such flaws. To reduce or eliminate surface flaws, machining operations are required which constitute a major proportion of production costs. In order to offer an inexpensive alternative to machining and also to enhance the room temperature flexural strength of as fired Si3N4, low expansion glaze coatings of lithium aluminosilicate (LAS) and magnesium aluminosilicate (MAS) compositions were developed. Homogeneous and crack-free glaze coatings were successfully formed on as processed Si3N4. This ensured formation of compressive surface stresses on the as fired Si3N4 which, in turn, led to the reduction of the effects of surface flaws. When compared to the uncoated as fired Si3N4, both the glaze coatings helped achieve greater flexural strength. Analyses of the two glazes indicated better strength for the MAS coating compared to the LAS. Wear tests revealed that the MAS glaze exhibited higher wear resistance than the LAS glaze. These differences were attributed to the ability of the magnesium aluminosilicate glaze to achieve greater surface smoothness and better adherence to the substrate than the lithium aluminosilicate.
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