The microstructure and creep deformation of hot-pressed Si 3 N 4 with different amounts of sintering additives
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The microstructure and creep deformation of hot-pressed Si3 N4 with different amounts of sintering additives Sang-Young Yoon, Takashi Akatsu, and Eiichi Yasuda Research Laboratory of Engineering Materials, Tokyo Institute of Technology, Yokohama, 226, Japan (Received 12 July 1995; accepted 18 August 1995)
Compressive creep deformation of hot-pressed silicon nitride with different amounts of grain boundary glassy phase was investigated at 1300–1400 ±C under 30–100 MPa. The stress exponent of the creep rate was determined to be nearly unity. The apparent activation energy of silicon nitride with a larger amount of glassy phase was measured to be about 700 kJymole, and that with a smaller amount of glassy phase was found to be 400 kJymole. In addition, the microstructural observation found that no cavity appeared and grain boundary glass was recrystallized during creep test. Thus, the rate-limiting steps in solution/precipitation creep mechanism change from the solution-reprecipitation of Si3 N4 grains to the diffusion through the grain boundary with increasing the amount of glassy phase.
I. INTRODUCTION
Hot-pressed silicon nitride is generally fabricated using the oxide additives to enhance the densification. However, the remaining glassy phase softens at high temperature and gives a deleterious effect on the high temperature strength.1 Hence, the crystallization and the increase in the viscosity of the glassy phase are often suggested to improve the creep properties.2 The creep deformation of materials containing intergranular glassy phase is generally thought to occur in combination of viscous flow,3 solutionreprecipitation,4 – 6 and caviation7–9 mechanisms. Creep deformation of silicon nitride could not be directly attributed to any single factor of composition of additive, liquid phase content, grain size, etc., since the grain size is also dependent on the composition and amount of sintering additives. The parameters in combination must be found to influence the creep behavior of silicon nitride. Thus, the creep behavior of silicon nitride must consider many factors, such as the amount and composition of glassy phase, the size and distribution of the matrix silicon nitride grains, the recrystallization of grain boundary glassy phase, and the oxidation. Some models have been developed based on these creep behaviors. The previous studies,7,10 –13 however, have been carried out only for silicon nitride with commercial grade additives and under high applied stress ranges. Moreover, although the amount of glassy phase is known to control the creep properties, few studies have focused on the amount and composition of intergranular glassy phase.14 The present study concerns the effect of the amount of glassy phase in the grain boundary on the creep deformation of silicon nitride at high temperature and under low applied stress. The creep properties of hot120
http://journals.cambridge.org
J. Mater. Res., Vol. 11, No. 1, Jan 1996
Downloaded: 11 Jul 2014
pressed silicon
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