탄화텅스텐 및 코발트가 첨가된 붕화지르코늄의 치밀화 거동 및 미세구조

Abstract

2004년 미국 National Science Foundation과 Airforce Office Research가 공동으로 워크샵을 개최한 이후 초고온 세라믹스(UHTCs)라는 명칭으로 차세대 우주 왕복선, 차세대 초음속 여객기 등과 같은 극한 조건 하에서 사용되는 재료를 위한 연구가 활발히 진행되고 있다. 초고온 세라믹스 물질 중 하나인 ZrB2는 강한 공유결합성에 의한 낮은 자기확산 계수를 가지기 때문에, 소결 시에 높은 소결온도 및 높은 외부 압력을 필요로 하는 난소결성 물질이다. 따라서 ZrB2 소결성을 더욱 향상시키기 위한 소결 조제들에 대한 연구가 활발히 이루어지고 있는 실정이다. 본 연구에서는 소결조제로써 새로운 물질을 첨가하는 것이 아닌, high energy milling시에 ZrB2 powder에 섞여 들어가 소결성을 증진시키는 효과를 가져오는 것으로 생각되는, Co-bonded Tungsten Carbide(WC-Co)를 소결조제로 선정하였다. High energy milling시에 jar와 ball의 성분이 WC-Co인데, 높은 milling energy로 인하여 media로부터 WC-Co가 깎여서 들어가는 것을 XRF등의 분석방법을 통해 확인한 바 있다. 이러한 data에 기초하여 본 연구에서는 초경의 재질인 WC-Co와, 두가지 물질의 각각의 효과를 알아보기 위해 WC, Co 각각의 효과에 대하여 가압소결 연구를 진행하였다. 각각의 소결조제가 ZrB2에서 미치는 영향을 살펴보기 위해 Archimedes method로 밀도를 측정하였고, 소결체 내의 상을 분석하기 위해 X-ray diffraction(XRD)와 JCPDS diffraction files(PDF)를 사용하였고, 보다 정확한 상의 분석을 위해 EPMA분석을 실시하였다. 또한 소결체 내에서의 이차상 및 소결된 정도와 형상을 살펴보기 위해 FE-SEM을 이용하였다.| In the milling process, cobalt-bonded tungsten carbides(WC-Co) were included as a contaminant by 1~6wt% in zirconium diboride(ZrB2). The effect of tungsten carbide(WC) additive to improve densification, mechanical properties and oxidation resistance on ZrB2 have been studied. In contrast, the effect of cobalt in ZrB2 has not been studied much. In this work, the establishment of sintering mechanism of cobalt is discussed for comparing the effect of WC with Co. ZrB2 powders were densified by Spark Plasma Sintering(SPS) with WC, WC-Co, and Co additives on various compositions. The sintering process was carried out between at 1500°C and 1800°C for 15minutes in vacuum. Microstructure was observed by field emission scanning electron microscopy, and chemistry was analyzed by X-ray diffraction. The case of ZrB2-4wt%WC-0.6wt%Co achieved full-density at 1600°C. Analysis showed that the WC forming a solid solution which can enhance the densification of ZrB2. However, it seems to be the effect of Co better than WC on densification of ZrB2. Because, it is considered that the liquid phase of Co had effects on the densification of ZrB2.; In the milling process, cobalt-bonded tungsten carbides(WC-Co) were included as a contaminant by 1~6wt% in zirconium diboride(ZrB2). The effect of tungsten carbide(WC) additive to improve densification, mechanical properties and oxidation resistance on ZrB2 have been studied. In contrast, the effect of cobalt in ZrB2 has not been studied much. In this work, the establishment of sintering mechanism of cobalt is discussed for comparing the effect of WC with Co. ZrB2 powders were densified by Spark Plasma Sintering(SPS) with WC, WC-Co, and Co additives on various compositions. The sintering process was carried out between at 1500°C and 1800°C for 15minutes in vacuum. Microstructure was observed by field emission scanning electron microscopy, and chemistry was analyzed by X-ray diffraction. The case of ZrB2-4wt%WC-0.6wt%Co achieved full-density at 1600°C. Analysis showed that the WC forming a solid solution which can enhance the densification of ZrB2. However, it seems to be the effect of Co better than WC on densification of ZrB2. Because, it is considered that the liquid phase of Co had effects on the densification of ZrB2.