Y₂O₃첨가량에 의한 Al₂O₃복합 세라믹스의 균열치유거동과 고온강도 특성

Abstract

Alumina (Al₂O₃) is one of the most cost effective and widely used material among engineering ceramics. However, it doesn't have enough bending strength, fracture toughness and heat-resistance limit temperature to be used as structural components. This study aims to overcome these weaknesses by endowing a self crack healing ability to Al₂O₃. Al₂O₃/SiC composite ceramics were prepared using a mixture of 85 wt.% Al₂O₃ (mean size 0.3 μm), 15 wt.% SiC powder with Y₂O₃ as an additive powder (1, 2 and 3 wt.%). To investigate crack-healing behavior, a crack was made on the surface of test specimen by Vickers indentation and the specimen was treated by heating at 1,373 K, 1,473 K, 1,573 K or 1,623 K for 1 h in air. The crack-healing ability of Al₂O₃ monolithic was very small. But, in the case of Al₂O₃/SiC composite, the strength of crack-healed specimen increased 112% than cracked specimen. The bending strength of heat-treated Al₂O₃/SiC composite at room temperature was higher than that of Al₂O₃ monolithic. Crack-healing ability was depended on the amount of Y₂O₃. Al₂O₃/SiC composite ceramic containing 3 wt.% of Y₂O₃ which treated at 1,573 K showed highest bending strength. The elastic waves released by inducing vickers indentation were detected by fracture wave detector. From the result of wavelet analysis of elastic wave signal, the smooth specimen and heat-treated crack healing specimen of 0.3 μm Al₂O₃ monolithic and Al₂O₃/SiC composite ceramics had characteristics of frequency about 58 kHz. The dominant frequencies increased with increasing the amount of Y₂O₃. It is known that wave frequency of homogeneous materials is converged on around dominant frequency. The dominant frequency has direct connection with the bending strength. The four kinds of Al₂O₃/SiC composite ceramics were prepared using a mixture of 85 wt.% Al₂O₃ (mean size 0.5 μm), 15 wt.% SiC powder with Y₂O₃ as an additive powder (0, 1, 3 and 5 wt.%). The crack-healing strengths were studied as functions of crack-healing temperature and amount of Y₂O₃. The in-situ crack-healing behavior was observed from 1,273 K to 1,573 K for 1 h in the air. The heat treated specimen with 3 wt.% of Y₂O₃ showed better crack-healing ability than specimen with 1 or 5 wt.% of Y₂O₃. The bending strength of the crack-healed specimen at 1,473 K was recovered to the bending strength of smooth specimen treated at 1573 K. The heat-resistance limit temperature of Al₂O₃/SiC composite ceramics was 1,073 K, 1,373 K, 873 K for the specimen with 1, 3, 5 wt.% of Y₂O₃. The structure of before and after the heat treatment was observed. The micro-structure of the heat treated specimen with 1 wt.% of Y₂O₃ showed the growth of columnar structure which contribute to the increase of strength. The strength of the specimen with 3 wt.% of Y₂O₃ was affected by the crack healing of mico-structured crack occurred in processing. The specimen with 5 wt.% of Y₂O₃ had insufficient strength because of the over-growth of grain. The structure of before and after the point of inflection was observed when the specimens were tested at high temperature. The micro-structure of the specimen with 1 wt.% of Y₂O₃ did not nearly changed and a few columnar structure was observed. The strength of the specimen with 3 wt.% of Y₂O₃ decreased due to the large grain resulted from the growth and agglomeration of columnar structure. The strength of the specimen with 5 wt.% of Y₂O₃ decreased suddenly because of the oversized grain in high temperature.