直接烧结SiC循环氧化行为的试验研究

Experimental Study of Cyclic Oxidation Behavior of Direct-sintered SiC

采用管式炉对直接烧结SiC在1300和1400℃静止空气中进行不同循环周期下的循环氧化实验。使用分析天平记录样品质量变化,并使用扫描电镜对氧化产物进行表征以揭示循环氧化机理。结果表明,直接烧结SiC的循环氧化行为由前期接近抛物线氧化行为的氧化增重过程与长时间氧化产生氧化层剥落造成的氧化失重过程组成。循环氧化过程可分为氧化、扩散、开裂、剥落4个阶段,且4个阶段往复进行,最终导致了直接烧结SiC在循环氧化过程中的多层氧化层剥落。随氧化温度的升高,SiC氧化速率增加,且更快进入氧化失重阶段;循环氧化周期越短,越易发生局部剥落行为,循环氧化过程受氧化层开裂及剥落影响越严重;循环周期越长,受氧化层开裂及剥落影响推迟,但容易发生氧化层的整体剥落。

The cyclic oxidation performance of the direct-sintered SiC was assessed in air at 1300 ℃ with cyclic period of 5 h, and at 1400 ℃ with cyclic period of 1, 3 and 5 h, respectively, meanwhile, the mass change of the sintered SiC vs cycle nunber was measured with analytical balance. Then the morphology evolution and the formed oxide scales of the direct-sintered SiC were characterized by scanning electron microscope. The results indicate that during the cyclic oxidation, the direct-sintered SiC experienced a two-stage process, i.e. in the early stage the sintered SiC presented a course of parabolic mass gain and then turned to a course of mass loss companied by spallation of oxide scale for long-term oxidation. With the increase of cyclic oxidation temperature, the oxidation rate of direct-sintered SiC increases and the mass loss stage emerges more quickly. The shorter the cyclic period is, the easier the spallation of oxide scale takes place, and the cracking and spallation of oxide scale exerts a more serious influence on the cyclic oxidation process. As the increase of cyclic period, the occurrence of cracking and spallation of oxide scale may be postponed, but in that case, the overall spallation of oxide scale seems easy to take place.