고강도 연자성 부품용 Fe 나노분말 소결체 제조 = Fabrication of sintered Iron nanopowder part for high strength soft magnetic components

국문 초록 (Abstract)

요 지
전기자동차와 같은 친환경자동차에 대한 수요가 증가하면서 차량의 동력기관이 내연기관에서 전기동력기관으로 바뀌고 있다. 코어를 포함하는 모터는 전기동력기관에서 가장 중요한 부품이며 코어는 모터를 구성하는 핵심 부품이다. 저속 주행 시 필요한 최적의 토크뿐만 아니라 차량의 진동으로 인해 모터는 주행 중에 힘을 받는다. 이러한 경우에 모터 코어의 기계적 강도가 주행 중 가해지는 힘을 견딜 수 없을 만큼 약하다면 모터 코어는 파괴된다. 코어의 또 다른 중요한 기술적 요구사항은 코어 부품의 복잡한 형상을 제조할 수 있는 net-shaping 공정이다. 최근 이러한 문제를 해결하기 위해 분말사출성형과 3D Printing 기술과 같은 신기술의 분말야금 기술에 의해 많은 노력이 이루어졌다. 재료 솔루션 관점에서 최근에 가격이 저렴한 철을 기반으로 둔 복합 재료, 즉 연자성 복합 재료(SMC)를 이용한 개발이 이루어지고 있다.
이러한 관점에서, 본 연구는 Fe 나노분말을 사용하여 고강도 연자성 부품용 Fe 나노분말 소결체 제조를 연구하는 것을 목적으로 한다. 이를 위해 두 가지 주요한 실험적 연구가 수행되었다. 먼저, 미세구조적 관점에서 Fe 나노분말 소결체의 기계적 특성을 조사하였다. 그 다음으로, 최적화된 공정 조건에서 제조된 Fe 나노분말 소결체 코어의 자기적 성질을 측정하였다.
고순도의 Fe2O3 분말을 1800r.p.m, 10시간의 조건으로 분쇄하였으며 슬러리를 분무 건조하여 구형의 응집체를 제조하였다. 구형의 Fe2O3 분말을 관상로에서 500oC 1시간 수소환원하였다. 대기 중 산화방지를 위해 환원된 분말에 5vol.%의 스테아린산을 코팅한 후 상용 압력인 600MPa에서 성형을 진행하였다. 최적의 소결온도 조건을 결정하기 위해 800oC와 900oC에서 승온 및 등온소결을 진행하여 기계적 특성을 측정하였다. 최적의 소결온도 조건을 정한 후 200oC와 400oC, 600oC에서 열처리를 진행하여 기계적 특성을 측정하였으며 표면과 파단면을 SEM으로 관찰하여 결정립의 크기를 조사하였다. 또한, 900oC에서 소결한 시편과 열처리 공정을 실시한 시편의 자기적 특성을 측정하여 상용 순철분말을 이용하여 제조한 시편의 자기적 특성과 비교하였다.
소결 및 기계적 시험 결과는 800oC에서 소결된 Fe 나노분말 소결체의 강도가 상업용 철제 분말야금 부품보다 높았지만 Fe 나노분말 소결체의 연신률은 이들보다 낮았다. 결정 미세화에 의한 결함뿐만 아니라 많은 양의 결정립은 800oC 소결체의 취성 성질에 영향을 미칠 수 있다. 이와 반대로, 900oC에서 1시간동안 등온 소결된 소결체의 강도와 연신률은 위에 기술된 재료보다 우수한 특성을 나타냈다. 열처리 실험의 결과는 열처리된 Fe 나노분말의 시편이 철계 분말야금 부품에 상응하는 기계적 성질을 가지는 것을 보였다. 자기적 특성 측정 결과, 900oC에서 소결된 Fe 나노분말 소결 코어의 최대자속밀도가 상용 Fe 분말 코어의 최대자속밀도와 일치함을 확인했다. 그러나 보자력은 상용 Fe 분말 코어보다 5배 높았다. 이는 높은 밀도의 결정립과 결정립 결함이 높은 보자력의 원인이 될 수 있다고 판단했다.
고강도 순철 자성 코어는 Fe 나노분말을 소결하여 제조할 수 있으며 그것의 자기적 특성은 충분한 열처리 공정에 의해 최적화되었다고 결론지었다. 그러나 보자력을 낮추기 위한 방법은 여전히 문제점이 있다. 본 연구는 재료 공정 관점에서 고강도 연자성 복합체를 설계하는 방법에 대한 기본적인 이해를 제공한다.

목차 (Table of Contents)

• 차 례
• 요 지 ········································································································ ⅰ
• 1. 서 론 ····································································································· 1
• 2. 실험 방법 ································································································9
• 2.1. Fe 나노분말의 제조 ·········································································· 9
• 2.2. 코팅 및 성형 ·················································································· 10
• 2.3. 소결 ······························································································ 14
• 2.3.1. 승온 소결 ················································································ 14
• 2.3.2. 등온 소결 ················································································ 14
• 2.4. 열처리 ··························································································· 14
• 2.5. 분석 및 평가 ·················································································· 15
• 2.5.1. 밀도 측정 및 상 & 조성 분석 ···················································· 15
• 2.5.2. 기계적 특성 평가 ······································································ 15
• 2.5.3. 미세구조 분석 ·········································································· 16
• 2.5.4. 자기적 특성 평가 ······································································ 16
• 3. 결과 및 고찰 ························································································ 19
• 3.1. 최적 소결온도 결정 ····································································· 19
• 3.2. 900oC 소결체 특성 ······································································ 24
• 3.2.1. 기계적 특성 ········································································· 30
• 3.2.2. 열처리에 따른 기계적 특성변화 ·············································· 36
• 3.3. 자기적 특성 ················································································ 43
• 4. 결론 ····································································································· 45
• 5. 참고 문헌 ····························································································· 47
• Abstract ··································································································· 52