IMPROVEMENT OF ENVIRONMENTAL DURABILITY AND STRENGTH OF SIALON CERAMICS BY TAILORING THE GRAIN-BOUNDARY PHASE DUE TO SOLID-SOLUTION NATURE

为了确定晶界相对si_3n_4基陶瓷耐蚀性的裁剪作用，采用高压灭菌法研究了si_3n_4基陶瓷的水热腐蚀和强度退化。在超声波水浴中可以很容易地从衬底上去除硅烷陶瓷中的水热腐蚀产物。在Si_3N_4陶瓷中，可以明显地观察到腐蚀坑。而alpha-sialon的腐蚀基底表面没有明显的腐蚀坑，属于均匀腐蚀。断裂强度随暴露时间的变化表现为腐蚀初期断裂强度快速下降，随后在10 d内基本保持不变。而Si_3N_4中的点蚀和α -sialon中的均匀腐蚀的强度下降程度不同。尽管后者在长时间暴露后强度下降了40%，但前者仅下降了15%。腐蚀坑的形成可以解释较大程度的强度降低。因此，由于其固溶体性质，α -莎龙的耐腐蚀性和强度退化优于其他类型的莎龙。

Hydrothermal corrosion and strength degradation of Si_3N_4-based ceramics has been investigated using an autoclave method in order to assure the tailoring effect of grain-boundary phase on corrosion resistance of alpha-sialon. The hydrothermal corrosion products in sialon ceramics can be removed easily from the substrate in an ultrasonic waterbath. SEM micrographs for corroded substrate after removal of corrosion products revealed the nature of forms of corrosion for each ceramics In Si_3N_4 ceramics, corrsion pits were clearly observed. In contrast, the corroded substrate of alpha-sialon revealed the surface with no identifiable corrosion pits, which could be designated as uniform corrosion.Fracture strength degradation as a function of exposure time for these sialon ceramics is showed a rapid decrease in fracture strength occured in the early stage of corrosion, and then was almost cnstant up to 10 days. However, the degree of strength degradation was different between the uniform corrosion in alpha-sialon and the pitting corrosion in Si_3N_4. Although the strength decrease after extended exposure in the latter was 40%, thatin the firmer was only 15%. Formation of corrosion pits may explain the larger extent of strength reduction. Thus, it has been found that corrosion resistance and strength degradation in alpha-sialon are superior to other types of sialon due to the solid-solution nature.