B4C对Si3N4/Si_(2)N_(2)O结合SiC材料抗热震性能的影响

Effect of B_(4)C on thermal shock resistance of Si_(3)N_(4)/Si_(2)N_(2)O bonded SiC materials

首先以Si粉、SiO_(2)微粉为原料,先在700℃空气气氛处理,然后在1400℃氮气气氛下合成Si_(2)N_(2)O,研究了B_(4)C添加量(外加质量分数分别为0、1.0%、2.0%、3.0%、4.0%)对Si_(2)N_(2)O合成效果的影响。然后根据B_(4)C最优加入量,先在700℃空气气氛保温5 h,然后在1400℃氮气气氛保温5 h制备了Si_(3)N_(4)/Si_(2)N_(2)O结合SiC试样。采用1300℃风冷5次后试样的抗折强度保持率评价其抗热震性,分析了热震前后试样的物相组成和显微结构。结果表明:1)合成Si_(2)N_(2)O的B_(4)C最优添加量为3%(w);在700℃空气处理时,B_(4)C优先和气氛中O_(2)反应生成液相B2O3,为1400℃氮气气氛生成板片状Si_(2)N_(2)O提供充足液相。2)风冷热震后,添加B_(4)C的试样中Si_(2)N_(2)O结合相保持完整,强度保持率由未添加时的67.5%提高至88.5%;同时,生成的B2O3发生碳热还原氮化反应,生成导热性好、具有自润滑特性的BN,改善其抗热震性,并在使用过程中继续为Si_(3)N_(4)/Si_(2)N_(2)O结合SiC材料提供保护。

Si_(2)N_(2)O was synthesized using Si powder and SiO_(2)micropowder as raw materials and treating at 700℃in the air atmosphere and then at 1400℃ in the nitrogen atmosphere.The effect of the B_(4)C addition(0,1.0%,2.0%,3.0% and 4.0%,extra adding,by mass)on the synthesis of Si_(2)N_(2)O was studied.With the optimal B_(4)C addition,Si_(3)N_(4)/Si_(2)N_(2)O bonded SiC samples were prepared after firing at 700℃for 5 h in the air atmosphere and then at 1400℃for 5 h in the nitrogen atmosphere.The thermal shock resistance of the samples was evaluated by the retention ratio of the modulus of rupture after 5 cycles of thermal shock(1300℃,air quenching).The phase composition and the microstructure of the samples before and after thermal shock were analyzed.The results show that:(1)the optimal B_(4)C addition is 3%;B_(4)C preferentially reacts with O_(2)to generate B2O3at 700℃in the air atmosphere,which provides sufficient liquid phase for the formation of plate-like Si_(2)N_(2)O at 1400℃in the nitrogen atmosphere;(2)after thermal shock,the sample added with B_(4)C has complete Si_(2)N_(2)O bonding phase and increased strength retention ratio from 67.5%to 88.5%compared with the sample without B_(4)C;at the same time,B2O3transforms to BN of good thermal conductivity and self-lubricating property through the carbothermal reduction nitridation reaction,which improves the thermal shock resistance and continues to protect Si_(3)N_(4)/Si_(2)N_(2)O bonded SiC materials during service.