Effect of Porosity and Microstructure on the Strength of Si 3 N 4 : Designed Microstructure for High Strength, High Thermal Shock Resistance, and Facile Machining

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Abstract
The flexural strength of porous Si 3 N 4 ceramics with a variety of microstructures and porosities were evaluated, and the effect of microstructure on the flexural strength was investigated to obtain machinable Si 3 N 4 ceramics having both high strength and high thermal shock resistance. Porous Si 3 N 4 having three types of microstructure, consisting of (1) only spherical grains, (2) combinations of spherical and columnar grains, and (3) only columnar β‐grains connected randomly in three dimensions, were readied as specimens. Their mean pore diameters and porosities were 0.2 to 0.3 μm and 8% to 59%, respectively. The flexural strength of the porous Si3N4 (3) was much larger than that of the porous Si3N4 having the other microstructures, and the maximum flexural strength was 455 MPa at a porosity of 38.3%. The thermal shock resistance (ΔT), which was determined by a water quench test, of porous Si 3 N 4 with such microstructure and a porosity of 50% was 980 K. All of the porous Si 3 N 4 (3) was easily machined with cemented carbide drills.

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