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α-Sialon耐蚀性的影响，用高压釜法研究了Si3N4基陶瓷的水热腐蚀和强度退化。在超声波水浴中，Sialon陶瓷中的水热腐蚀产物可以很容易地从衬底上去除。去除腐蚀产物后腐蚀衬底的扫描电子显微镜照片揭示了每种陶瓷在Si3N4陶瓷中腐蚀形式的本质，腐蚀坑清晰可见。与之相反，腐蚀后的α-sialon陶瓷表面没有可识别的腐蚀坑，可以认为是均匀腐蚀，断裂强度随暴露时间的变化而下降，断裂强度在腐蚀早期迅速下降，然后几乎保持到10d。然而，在α-sialon中的均匀腐蚀和在Si_3N_4中的点蚀，强度下降的程度是不同的。虽然后者在长时间暴露后强度下降了40%，但在较坚固的环境中，强度仅下降了15%。腐蚀坑的形成可能是强度下降幅度较大的原因。因此，人们发现α-sialon的耐腐蚀性和强度退化优于其他类型的sialon，这是由于其固溶体的性质。

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.