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 -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 Si 3 N 4 (3) was much larger than that of the porous Si 3 N 4 having the other microstructures, and the maximum flexural strength was 455 MPa at a porosity of 38.3%. The thermal shock resistance (⌬T), 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.
and the diameter dependence of the strength was investigated. The Si 3 N 4 whiskers had diameters of 0.04 and 0.8 µm and dominant [1011] and [1010] growth directions. Chemical analysis showed that they contained Ti and O impurities. The tensile strength of six Si 3 N 4 whiskers increased from 17 to 59 GPa with decreasing whisker diameter.
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