Doping of boron, nitrogen, aluminum and titanium in the SiC (310) twin boundary was investigated, and the first-principle calculation was used to analyze the underlying mechanism of excellent creep resistance and strength of Sylramic and Tyranno SA SiC fibers. The electronic structures were also analyzed and compared. The results of Mulliken overlap populations, electron density differences and density of states reveal that doping of B or N atom reinforces SiC GBs bonding, however, doping of Al or Ti atom weakens SiC GBs bonding. The reinforced SiC GBs will largely prevent atoms from sliding near GBs. The experimental results would be one of the reasons which lead to the reinforcement of either creep resistance or the strength of SiC fibers.
A particle preform was designed and prepared by conglomerating and cold-pressed process,which was condensed by chemical vapor infiltration(CVI)process to fabricate silicon nitride particles reinforced silicon nitride composites.The conglomera- tions are of almost sphericity after conglomerated.There are large pores among the conglomerations and small pores within them- selves in the preform according to the design and the test of pore size distribution.The pore size of the preform is characterized by a double-peak distribution.The pore size distribution is influenced by conglomeration size.Large pores among the conglomerations still exist after infiltrated Si_3N_4 matrix.The conglomerations,however,are very compact.The CVI Si_3N_4 looks like cauliflower- shaped structure.
Yongsheng Liu Laifei Cheng Litong Zhang Yongdong Xu Yi Liu
