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탄화환원과 액상소결 중 판상 WC의 형성 및 성장에 관한 연구
Study on the formation and the growth of platelet WC during carbothermal reduction and liquid phase sintering

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Authors
김진명
Advisor
강신후
Major
재료공학부
Issue Date
2012-02
Publisher
서울대학교 대학원
Description
학위논문 (석사)-- 서울대학교 대학원 : 재료공학부, 2012. 2. 강신후.
Abstract
WC-Co 초경재료는 뛰어난 내마모성, 파괴인성 등의 물성으로 인해 절삭공구 재료로 널리 사용되고 있다. Co 내에서 WC는 일반적으로 truncated triangular prism 형태를 띄고 있지만, 판상(platelet)형태의 WC도 보고된 바 있다. 특히 판상의 WC는 파단 시 crack의 전파를 방해하기 때문에 소결체의 파괴인성을 증가시키는 장점이 있다.
지금까지 알려진 바에 따르면, 판상의 WC 형성은 1) mechanical alloying된 WC-Co 또는 WC-TiC-Co의 액상 소결 과정에서 WC의 석출/재결정(Meadows-Shatov route), 2) Co3W3C중간상의 분해반응 후 액상 내에서의 판상 성장(reaction sintering) 등으로 설명되었다. 하지만 본 연구에서는 Co가 없이도 판상의 WC가 고에너지 밀링에 이은 탄화 환원 과정에서 급격하게 형성되는 것이 발견되었다.
특이할 만한 사실은 1200~1300도 사이에서 WC(irregular)→WC(platelet) 과정을 통해 판상의 WC의 생성이 빠르고 광범위하게 일어난 점이다. 또한 판상의 형성과 함께 WC의 잔류 응력이 완화되고 결정화되는 경향을 보였으며 동시에 (100)/(101) 면간의 XRD peak intensity 비가 역전되는 현상을 발견하였다. 환원 온도 외에도 판상 형성에 영향을 미치는 요인으로서 밀링, 탄화환원, 탄소량 및 다른 전이금속 탄화물 첨가 등의 실험을 실시하였으며, 각각의 조건에 따라 판상 형성이 억제되거나 촉진되는 것을 확인할 수 있었다.
또한 탄화환원을 통해 얻어진 판상의 WC 또는 (W,M)C 분말을 Co와 혼합한 후 액상 소결을 실시하였으며, 분말과 마찬가지로 소결체에서도 판상의 WC가 발견되었다. 액상 소결 중 판상의 WC는 edge로부터 용해되는 것을 XRD 결과로부터 확인하였다. 또한 액상 소결 중 다양한 첨가물에 따른 판상 성장에 대해서도 알아보았으며, 첨가물과 WC 면의 affinity에 따라 판상의 aspect ratio가 변하였다.
WC-Co, cemented carbide, is used for cutting tool application due to its excellent wear resistance and fracture toughness. During the liquid phase sintering, WC particles are embedded on Co binder and grown by Ostwald ripening. The morphology of WC in Co binder is a truncated trigonal prism, but the platelet type of WC was also investigated because it gives enhanced toughness by deterring crack propagation.
The formation of platelet WC has been reported to be occurred by 1) dissolution/reprecipitation of mechanically alloyed WC-Co or WC-TiC-Co (Meadows-Shatov route), 2) dissociation of Co3W3C and growth of WC in Co (reaction sintering). But in this investigation, it was observed that the platelet can be formed during carbothermal reduction without Co binder which is essential as medium for mass transfer of WC in the processes as mentioned in above. Thus, a new mechanism of platelet formation is necessary for elucidating the formation of WC by gas-solid or solid-solid mass transfer.
The starting materials were WO3 and graphite, mixed and crushed by high-energy ball milling. The carbothermal reduction was conducted for milled powder in graphite vacuum furnace in vacuum (10-4atm). In thermodynamic point of view, WC can be synthesized in the sequence of WO3+C→ W2C→ WC, which was experimentally confirmed. Interestingly, WC changed its morphology rapidly from irregular to platelet shape in a temperature range of 1200~1300°C. Coincidentally, the residual stress of WC was released and the XRD peak intensity ratio of (100)/(101) of WC was reversed. There were some factors which affect platelet formation, such as annealing, reduction atmosphere, and addition of secondary transition metal carbide. Each of factors has effect on the rate of platelet formation.
Liquid phase sintering was conducted on platelet WC or (W,M)C powders mixed with Co at 1450°C for 1 hour. The platelet WC was also observed in the sintered specimen. Further investigation is required to verify whether 1) WC was totally dissolved in Co then platelet was reformed by reprecipitation or 2) WC maintained its morphology of powder state and slightly grew during the sintering process.
Language
kor
URI
http://hdl.handle.net/10371/155448

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College of Engineering/Engineering Practice School (공과대학/대학원)Dept. of Material Science and Engineering (재료공학부) Theses (Master's Degree_재료공학부)
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