Effect of Microstructure on High‐Temperature Compressive Creep of Self‐Reinforced Hot‐Pressed Silicon Nitride

Abstract

An experimental self-reinforced hot-pressed silicon nitride was used to examine the effects of microstructure on hightemperature deformation mechanisms during compression testing. At 1575-1625°C, the as-received material exhibited a stress exponent of 1 and appeared to deform by steady-state grain-boundary sliding accommodated by solutionreprecipitation of silicon nitride through the grain-boundary phase. The activation energy was 610 ؎ 110 kJ/mol. At 1450 -1525°C for the as-received material, and at 1525-1600°C for the larger-grained heat-treated samples, the stress exponent was >1. Damage, primarily in the form of pockets of intergranular material at two-grain junctions, was observed in these samples.

A critical analysis and a recent improvement of the two-dimensional model for solution–precipitation creep: application to silicon nitride ceramics

“…The numerical estimate of equation (26) (with S ¼ 7 Â 10 À10 J m À1 , a ¼ 3 Â 10 À10 m and Á solidÀsat ¼ 3.7 Â 10 6 J m À3 , set 1 in figure 4) gives an apparent activation energy that is almost independent of the applied stress within the usual accuracy of the creep tests (AE 0.5 eV atom À1 ), in good agreement with the available experimental data (Backhaus-Ricoult et al 1995, Wilkinson 1998, Schneider and Mukherjee 1999, Boling-Risser et al 2000, Xie et al 2000, Yoon et al 2000, Zhan et al 2000a, Mele´ndez-Martı´nez et al 2002. (Set 1 is shown in figure 4 as a solid curve; the broken curves and the chain curve correspond to different choices of Á solidÀsat .)…”

“…Regarding the stress dependence, the Raj-Chyung model predicts n ¼ 1 for the aforementioned two rate-controlling processes, which is clearly inconsistent with numerous studies reporting stress exponents different from unity. In silicon nitride ceramics, for instance, stress exponents different from unity are commonly found in conditions where solution-precipitation is reported to operate (Backhaus-Ricoult et al 1995, Wilkinson 1998, Schneider and Mukherjee 1999, Boling-Risser et al 2000, Xie et al 2000, Yoon et al 2000, Zhan et al 2000a, Mele´ndez-Martı´nez et al 2002. Recently, Wakai (1994) has developed an alternative, and significantly improved, model for solution-precipitation creep: the so-called 'step model'.…”

The creep behavior of Si3N4/SiC nanocomposites

“…Similar data were also found in Si 3 N 4 monoliths. [26][27][28] The volume of data is insufficient to make a conclusion as to whether the basic creep mechanism in micro-nanocomposites is the same as in silicon-nitride monoliths. The best one can say at this point is that there has not been any evidence that the creep mechanism in silicon-nitride/siliconcarbide micro-nano is different from that in silicon-nitride monoliths.…”

Silicon nitride compressive creep behavior in argon atmosphere

“…Si 3 N 4 solution and subsequent ␤-Si 3 N 4 reprecipitation, leading to densification. The intergranular phase formation is strongly important to the final high temperature properties [4]. Good densification has been achieved using yttrium oxide as sintering aid [5][6][7][8].…”