Cu 라이너의 반응성 향상을 위한 반응성 금속의 저온 분사 적층 거동 및 후열처리에 따른 특성 평가

Abstract

시가전에서 기동력 있는 보병 전투차량 및 건물 내에 은폐되어 있는 목표물을 제압하기 위한 무기 체계로써 관통 후 폭발 효과가 극대화될 수 있는 신개념의 이중층 라이너 (라이너 + 코팅층) 에 대해 연구가 대두되고 있다. 성형작약탄은 금속 라이너가 굉장히 빠른 변형속도로 metal jet이 형성되기 때문에 코팅 층은 고속변형거동 중에 탈락되지 않기 위해 충분히 견딜 수 있어야 하며, 관통 후 폭발하기 위한 반응성을 가져야만 한다. 따라서 본 연구에서는, 반응성 금속인 순수 알루미늄 (99.9%) 분말과 반응열로 계속적으로 반응을 일으키는 자전연소고온합성법 (Self-propagating high temperature synthesis (SHS))의 물질인 순수 알루미늄과 니켈 분말을 블렌딩 한 분말을 사용하여 구리 위에 적층한 코팅 층의 반응성, 기계적 특성, 적층 효율을 평가함으로써 공정최적화를 하였다. 또한, 좀 더 나은 기계적 특성을 얻고자 후열처리를 함으로써 코팅 층의 기계적 특성과 미세조직의 상관관계를 규명하고자 하였다. 먼저 알루미늄 분말은 시차 주사 열량 측정법 (DSC)를 통하여 분말크기가 작을수록 heating rate가 빠를수록 큰 발열반응을 보여 반응성 정도를 확인하였다. ABAQUS 유한요소 해석 툴을 사용하여 알루미늄 분말의 임계속도를 정의하고 적층하였다. 전산모사 결과로는 알루미늄 분말의 임계속도가 상온에서 780 m s-1로 산출 되었으며, 분말 온도가 올라감에 따라 알루미늄 분말의 임계속도는 감소되는 것을 알 수 있었다. 알루미늄의 경우 상대적으로 낮은 녹는 점을 가지기 때문에 공정 가스 온도와 압력에 의해 입자 온도가 영향을 받아 특정한 조건에서 좋은 특성을 보였다. 전산모사 결과를 바탕으로 각 공정 조건의 코팅 층마다 기계적 특성 (접합강도, 경도 등)을 평가하여 비교하였다. 이러한 특성들은 공정-미세조직-특성 관계를 통하여 규명하고자 하였다. 또한, 후열처리 함으로써 금속간 화합물 거동과 adhesive strength에 관하여 연구하였다. 금속간 화합물은 불 균일하게 생성되었으며, 상태도 에서 가능한 화합물 7가지 상 (Cu4Al, Cu3Al, Cu9Al4, Cu3Al2, Cu4Al3, CuAl, CuAl2) 중에서 4가지 상 (Cu9Al4, Cu4Al3, CuAl, CuAl2) 만 EDS를 통해 확인하였다. 나노인덴테이션 기법을 이용하여 금속간 화합물의 기계적 특성 (경도, 탄성 계수, 파괴 인성)을 평가하였다. CuAl 와 Cu9Al4이 가장 높은 경도와 탄성 계수를 가지고 서로 유사한 경도와 탄성 계수를 가지는 것으로 보여졌다. 그러나 파괴 인성에서는 Cu9Al4 가 확실히 높은 값을 보였다. CuAl2 는 낮은 경도와 탄성 계수를 가지나 가장 낮은 파괴 인성 값을 지니고 있는 것으로 보여진다. 이러한 brittle phase가 생성됨에도 불구하고 불 균일한 금속간 화합물 생성과 확산 접합과 어닐링으로 인해 adhesive strength 가 모두 증가하였다. 특정한 계면에서 파단이 일어났으며, 이러한 계면은 금속간 화합물의 열팽창계수에 의해 생성된 잔류응력 (인장) 방향과 거의 일치함을 알 수 있었다. 알루미늄 분말과 니켈 분말을 at%로 1:1과 3:1로 블렌딩하여 분말 예열과 함께 공정 최적화 하였다. |Pure Al coatings and blended Al+Ni coatings were fabricated on Cu substrates via kinetic spraying to produce a reactive Cu liner. The coatings need to endure severe high strain rate plastic deformation during explosion. Also, the coatings need to react with oxygen during penetration or after penetration. In this study, the Al powder underwent larger exothermic reactions with a smaller particle size and faster heating rate, as determined from the differential scanning calorimetry (DSC) data. Process optimization of the Al deposition was facilitated by defining the “critical velocity” of an Al particle in the kinetic spraying process based on numerical modeling and computations using ABAQUS finite element codes. The simulation results revealed that the critical velocity of an Al particle at room temperature (RT) is 780 m s-1 and the critical velocity decreases as the particle temperature increases. Certain process conditions resulted in improved coating properties as the temperature of the particles, which have a relatively low melting point, was affected by the process gas temperature and pressure. On the basis of the simulation results, the mechanical properties such as the bond strength of the coatings formed under various process conditions were evaluated and compared. The relationships among the resulting properties, processing conditions, and the structures of the coatings are discussed. Also, microstructure and adhesive strength between Al coating and Cu substrate were investigated after heat-treatments. Intermetallic compounds (IMC) between Al coating and Cu substrate were non-uniformly formed as heat-treatments. in the Al-Cu binary phase diagram, IMCs can be formed for 7 phases (Cu4Al, Cu3Al, Cu9Al4, Cu3Al2, Cu4Al3, CuAl, CuAl2). However, in this study, 4 phases (Cu9Al4, Cu4Al3, CuAl, CuAl2) were identified by EDS. Mechanical properties (such as hardness, elastic modulus and fracture toughness) of IMCs (CuAl2, CuAl, Cu9Al4) evaluated by nanoindentation method. CuAl and Cu9Al4 phases had similar and highest hardness and elastic modulus. However, fracture toughness of Cu9Al4 phase had obviously highest value among IMCs (CuAl2, CuAl, Cu9Al4). Fracture toughness of CuAl2 phase was measured lowest value, although CuAl2 phase had lowest hardness and elastic modulus between IMCs. Even though these brittle phases were formed, adhesive strength was increased because of non-uniformed intermetallic compounds and diffusion bonding. All conditions were fractured under certain interface. Good correlation was observed between actually fractured interface and residual stress direction (tensile) formed by difference among coefficient of thermal expansion of each IMC. Also, Al powder and Ni powder was blended by 1:1 at% and 3:1 at%. Process optimization of the blended Al+Ni deposition was performed with powder preheating.; Pure Al coatings and blended Al+Ni coatings were fabricated on Cu substrates via kinetic spraying to produce a reactive Cu liner. The coatings need to endure severe high strain rate plastic deformation during explosion. Also, the coatings need to react with oxygen during penetration or after penetration. In this study, the Al powder underwent larger exothermic reactions with a smaller particle size and faster heating rate, as determined from the differential scanning calorimetry (DSC) data. Process optimization of the Al deposition was facilitated by defining the “critical velocity” of an Al particle in the kinetic spraying process based on numerical modeling and computations using ABAQUS finite element codes. The simulation results revealed that the critical velocity of an Al particle at room temperature (RT) is 780 m s-1 and the critical velocity decreases as the particle temperature increases. Certain process conditions resulted in improved coating properties as the temperature of the particles, which have a relatively low melting point, was affected by the process gas temperature and pressure. On the basis of the simulation results, the mechanical properties such as the bond strength of the coatings formed under various process conditions were evaluated and compared. The relationships among the resulting properties, processing conditions, and the structures of the coatings are discussed. Also, microstructure and adhesive strength between Al coating and Cu substrate were investigated after heat-treatments. Intermetallic compounds (IMC) between Al coating and Cu substrate were non-uniformly formed as heat-treatments. in the Al-Cu binary phase diagram, IMCs can be formed for 7 phases (Cu4Al, Cu3Al, Cu9Al4, Cu3Al2, Cu4Al3, CuAl, CuAl2). However, in this study, 4 phases (Cu9Al4, Cu4Al3, CuAl, CuAl2) were identified by EDS. Mechanical properties (such as hardness, elastic modulus and fracture toughness) of IMCs (CuAl2, CuAl, Cu9Al4) evaluated by nanoindentation method. CuAl and Cu9Al4 phases had similar and highest hardness and elastic modulus. However, fracture toughness of Cu9Al4 phase had obviously highest value among IMCs (CuAl2, CuAl, Cu9Al4). Fracture toughness of CuAl2 phase was measured lowest value, although CuAl2 phase had lowest hardness and elastic modulus between IMCs. Even though these brittle phases were formed, adhesive strength was increased because of non-uniformed intermetallic compounds and diffusion bonding. All conditions were fractured under certain interface. Good correlation was observed between actually fractured interface and residual stress direction (tensile) formed by difference among coefficient of thermal expansion of each IMC. Also, Al powder and Ni powder was blended by 1:1 at% and 3:1 at%. Process optimization of the blended Al+Ni deposition was performed with powder preheating.