Pressureless sintering of Si{sub 3}N{sub 4} with selected liquid compositions from the system Y{sub 2}O{sub 3}-AlN-SiO{sub 2} has been reported. Compositions were selected so that the grain-boundary liquid produced YAG(3Y{sub 2}O{sub 3}{center dot}5Al{sub 2}O{sub 3}) after crystallization. Studies on dynamic sintering indicated that the first liquid formation occurred between 1,278 and 1,305{degree}C and the maximum sintering rate was observed between 1,590{degree} and 1,610{degree}C depending on the compositions. Si{sub 2}N{sub 2}O, YAG, or YAG along with YSiO{sub 2}N were the crystalline phases obtained on devitrification as the composition moved toward the AlN corner. The grain boundary liquid phase could be reduced from 20 to 9% on crystallization. Highest flexural strength and fracture toughness obtained were 475 MPa (four-point 45 to 20 mm span) and 6.8 MPa {center dot} m{sup 1/2}, respectively. The effect of crystallization on high-temperature strength and toughness was beneficial in all cases. The creep rate of the sintered product varied from 10.0 {times} 10{sup {minus}7} to 4 {times} 10{sup {minus}5} h{sup {minus}1} between 1,200{degree} and 1,450{degree}C and at 200 to 350 MPa stresses.
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DAS et al. (1988) studied this question.
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