첨가제 SiO₂에 따른 SiC 세라믹스의 균열치유거동과 강도특성

Abstract

본 연구에서는 4종류의 SiC 세라믹스를 소결하고, SiO₂ 콜로이달 및 SiO₂ 분말 첨가에 따른 균열치유거동, 상온 굽힘 강도 및 고온 굽힘 강도 특성을 체계적으로 조사하였다. 얻어진 결과는 다음과 같다. 1) 본 연구에서 소결한 4종류의 SiC 세라믹스의 최적 균열치유조건은 대기 중, 1100 ℃에서 1시간이었다. 이 때 균열치유재와 열처리모재의 강도는 미열처리 모재보다 약 2배의 강도를 얻었다. 2) 소결 첨가제 SiO₂의 분말과 콜로이달은 SiC의 균열 치유 특성에 영향을 끼치지 못하였다. 그러나 SiO₂의 첨가로 소결온도의 저하가 예상되며, 특히 SiO₂ 콜로이달의 첨가는 분말에 비하여 가격면에서 상당한 절감효과가 기대된다. 한편, SiO₂ 콜로이달을 균열부에 도포하여 열처리한 균열치유재는 강도 상승이 없었다. 이것은 시험편을 열처리한 후 냉각과정에서 형성된 표면의 균열과 요철 때문이었다. 3) 최적 균열 치유 조건(1100 ℃, 1시간)에서 완전하게 치유 가능한 균열 길이(2C)는 4종류의 시험편에서 약 450 ㎛까지이고, 그 때의 균열 폭은 1.4 ㎛이었다. 더욱이 균열 폭이 1.4 ㎛이하라면, 길이 450 ㎛ 이상의 균열도 치유가 가능하리라 판단된다. 4) 미연마(연삭) 시험편의 표면 결함은 최적 치유 온도에서 열처리 시간이 증가하여도 완전히 치유할 수 없었다. 이것은 표면결함이 아주 크고, 열처리 시간의 증가에 따른 재료의 열화 때문이었다. 그러나 연삭 후 연마정도에 따라서 균열치유프로세스를 이용하여 가공 시 발생하는 결함을 충분히 치유 할 수 있었다. 5) 본 연구에서 소결한 4종류의 SiC 균열치유재의 고온 강도특성은 온도의 증가와 함께 감소하였으나, 균열치유재와 미열처리 모재의 내열한계는 1000 ~ 1100 ℃로 선연구보다 높은 수치를 나타내었다. 6) 균열치유는 대기 중에서 열처리를 통한 표면의 산화로부터 일어나고, EDX 및 XRD 분석결과 유리상의 SiO₂가 균열치유에 직접적으로 관여하는 물질이라고 판단된다.

Because of its good mechanical properties, radioactivity, and corrosion resistance, silicon carbide (SiC) constitutes a leading candidate material for high temperature structural applications such as future generation gas turbines and the inner containment of nuclear fusion reactors. However, it is saddled with low fracture toughness, which results in the rapid and easy propagation of cracks once an initial crack has been introduced. As a result, the structural integrity of a ceramic component may be seriously affected, and this low reliability has limited their application. To overcome this, many studies have attempted to achieve the following: (a) non-destructive inspection with very high ability to detect micro crack; (b) increase fracture toughness through fiber-reinforcement while also decreasing sensitivity to cracks; (c) introduce crack-healing properties. The introduction of crack-healing properties to the structural components used in engineering can be expected to yield significant results such as: (1) an increase in reliability, (2) decrease in machining costs, (3) decrease in maintenance costs, and (4) prolongation of the relevant item’s lifetime. Crack-healing has been observed in ceramics such as Si₃N₄, Al₂O₃-SiC composites, mullite-SiC composites, and Si₃N₄-SiC composites. In this regard, while SiC is a ceramic that has a high potential for crack-healing, very few studies have been conducted on its crack-healing behavior. In this study focuses on the crack-healing behavior and bending strength of SiC ceramics to which sintering additives have been added (Al₂O₃ + Y₂O₃ = 10 and 12 wt.% (Al₂O₃:Y₂O₃=60:40) and SiO2 = 0 and 3 wt.%). Both SiO2 additives feature nano-powder and nano-colloidal. The crack-healing behavior and bending strength of SiC ceramics were systematically examined, as a function of crack-healing temperature, crack size, surface condition, the SiO2 additive and the bending strength of the crack-healed sample from room temperature to 1300 ℃ was investigated. Three-point bending specimens were created and a vickers indenter was used to introduce semi-elliptical cracks on the specimens. Pre-cracked specimens were healed at various conditions. All fracture tests were performed on a three-point loading system with a 16mm bending span. The specimen surfaces were analyzed by electron probe microanalysis(EPMA), EDX line analysis by FESEM and X-ray diffraction to investigate crack-healing material. The main conclusions obtained were the following; (1) The optimized crack-healing condition was found to be 1h at an atmospheric level of 1100 ℃; (2) The maximum crack size that can be healed at the optimized condition was a semi-elliptical surface crack of 450 μm in diameter. (3) Strength could be recovered by healing at optimum temperature regardless of surface roughness. However, oxide evaporation and formation of gas were found to harmful for the crack-healing ability. (4) Limiting temperature for bending strength of crack-healed zone was about 1000 ∼ 1100 ℃. (5) Amorphous silica was revealed to be the principle material involved in crack-healing.