炭素蒸着窒化ケイ素粉末から作製したSi_3N_4/SiCナノ複合体の高温での機械的強度

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Highly densified sintered Si_3N_4 bodies are conventionally obtained through liquid phase sintering by adding sintering aids to overcome difficulty of sintering due to the strong covalent bonding in Si_3N_4. Sintered Si_3N_4 ceramics contain a weak grain boundary phase, which is produced from a reaction between the sintering aids and the raw Si_3N_4 powder, especially oxide phase of surface contamination on the Si_3N_4 powder. Therefore, the grain boundary phase influences mechanical properties of Si_3N_4 ceramics. The present paper discusses a method to enhance mechanical properties by forming nano SiC particles in Si_3N_4 system with additives, especially in grain boundary phase. The Si_3N_4 ceramics with dispersed nano SiC particles were obtained by using a Si_3N_4 powder coated by carbon, which was obtained by thermal decomposition of methane(CH_4)gas. The nano SiC particles were formed by in-situ reaction between the oxide phase on the surface of the Si_3N_4 powder and the coated carbon. The sintered bodies were obtained by hot-pressing at 1800℃ for 1h in N_2 atmosphere under 30 MPa of applied pressure. Results of mechanical property measurements showed that the bending strength and fracture toughness increased about 20-25%, and the high temperature mechanical strength was also improved by the carbon coating method. TEM observation showed that the precipitated SiC particles were dispersed in three different types of locations, i.e., in the grain boundary layer, at the Si_3N_4-Si_3N_4 interface penetrating the grain boundary layer and inside of the Si_3N_4 grain. SiC particles dispersed at the Si_3N_4-Si_3N_4 interface block grain boundary sliding and slow crack growth at high temperatures. For carbon coated samples, the quantity of grain boundary phase was smaller than the non-coated samples as shown by cryogenic specific heat measurements. The quantity of grain boundary phase decreases, which leads the improvement of high temperature mechanical strength also. Therefore, the high temperature strength was improved by precipitated SiC particles dispersion and decreasing the quantity of grain boundary phase.
社団法人日本セラミックス協会の論文
1995-10-01

炭素蒸着窒化ケイ素粉末から作製したSi_3N_4/SiCナノ複合体の高温での機械的強度

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