Microstructure evolution mechanisms and physical, mechanical properties of kinetic and thermal sprayed multi-walled carbon nanotube reinforced metal composite coatings

Abstract

Multi-walled carbon nanotube (MWCNT) reinforced metal matrix composite (MMC) coatings fabricated by kinetic and thermal spray processes were investigated, targeting the potential industrial applications. The MWCNTs were embedded in face-centered cubic (FFC) metals (Al, Ni alloy, and Cu) matrix powers with 0.0-1.0 wt% MWCNTs in order to efficiently strength and functionalize the matrix materials. The MWCNT reinforced MMC coatings were successfully fabricated by the consolidation of kinetic and thermal spraying using optimized process conditions. The property enhancements of the kinetic and thermal sprayed composite coatings such as electrical conductivity, elastic modulus, micro-hardness and wear resistance were found. The states of the composite powder, microstructural evolutions of the composite coatings, and interface and structure of MWCNTs in the composite coatings were analyzed. The correlations of property enhancements of the composite coatings with microstructural factors, and contributions of the embedded MWCNTs, which are predominantly influenced by the characteristics of kinetic and thermal spray processes, were investigated based on physical and mechanical metallurgies. The microstructure, and states (structure, interface, and dispersion) of the MWCNTs of the initial composite powders, which significantly influence the deposition behavior, and physical/chemical reactions during spraying process, were investigated by field-emission scanning electron microscopy, and transmission electron microscopy (TEM). The strain induced refinement (Al) or deformation band formation (Ni alloy, and Cu) in metal matrix were observed. MWCNTs were uniformed dispersed, and presented the structure and interface of quality, which satisfy the requirements of efficient reinforcements for MWCNT reinforced MMCs. Furthermore, thermal reactions of the composite powders were analyzed by differential scanning calorimeter (DSC) to simulate the thermally activated reactions during thermal spraying. The MWCNT reinforced MMC coatings were initially fabricated by thermal spray processes (high velocity oxygen fuel or plasma spraying). The deposition of thermal spraying is based on melting of particles in the gas flow field and rapid resolidification of melted splats upon impacts. Therefore, thermally activated reactions of MWCNTs such as destruction and gasification by oxidation were inevitable. Since the plasma spraying utilizes the higher process temperature, and in-flight velocity of particles is lower, the oxidation of MWCNTs significantly occurred. Microstructure of resolidified metallic matrix, destroyed MWCNTs, i.e., curling-up, peeling-off, and amorphization, and reactions of MWCNTs with metallic matrix were analyzed though intensive TEM examination of the thermal sprayed composite coatings. The destruction mechanisms of the MWCNTs during thermal spaying were investigated comparing with DSC analysis and thermodynamic calculation. Kinetic spraying of the composite powders was applied In order to avoid the aforementioned problematic phenomena of thermal spraying. Since the deposition of kinetic spraying is mainly based on supersonic collisions of particles and interfacial thermo-mechanical interactions that results from adiabatic shear instability, the thermal reaction of MWCNTs was avoidable. Furthermore, structure, dispersion, and interface of MWCNT in the kinetic sprayed composite coatings were sound, i.e., preserved their initial states of the composite powders. Supersonic velocity impacts of the composite powders resulted in plasticity induced grain refinements of the metal matrix. The grain refinement mechanisms of the kinetic sprayed composite coatings were investigated by comparing with microstructural evolutions of kinetic sprayed pure FCC metals. The microstructural evolutions of kinetic sprayed FCC metals were discussed in terms of the characteristics of kinetic spraying, and physical/mechanical properties of metals. Role of MWCNTs on grain refinements of the metallic matrix during kinetic spraying was explained. The measured physical and mechanical properties of the kinetic and thermal sprayed composite coatings were discussed. The properties were significantly affected by spray processes, and fractions of the embedded MWCNTs. The properties of the composite coatings were selectively evaluated considering fundamental data acquisitions and potential applications. The correlation of the measured properties of the composite coatings with the microstructural factors, such as the microstructure of the metallic matrix, MWCNT structure, and the interface between MWCNTs and the metal matrix, was investigated in order to provide an understanding of the contribution of MWCNTs to property enhancements of the composite coatings. |탄소 나노튜브 (carbon nanotube; CNT)는 나노 사이즈의 극도로 미세한 크기를 가지면서 우수한 물리 및 기계적 특성 보유하고 있는 소재로, CNT 자체 뿐만 아니라 여러 기지 소재 (유기물, 세라믹, 그리고 금속) 등의 삽입되어 복합 재료로 사용되어 기존의 기지 소재의 특성을 향상시키는 데에 널리 사용되어 왔다. 다중벽 탄소 나노튜브 (multi-walled carbon nanotube; MWCNT)는 graphene sheet가 20-50 겹으로 구성된 물질로서, 기존의 단일벽 탄소 나노튜브 (single-walled carbon nanotube; SWCNT)보다 전도성이나 기계적 특성이 떨어지지만, 복합 재료로 형성 시 화학 및 기계적 안정성이 우수하여 본 연구에서 사용되었다. 본 연구에서는MWCNT를 기계적 합금화법을 이용하여 금속 (Al, Ni alloy, Cu) 분말에 주입하여 복합재료 분말을 제조하여, 저온 분사 및 열용사 공정을 적용하여 복합재료 코팅을 형성하였다. 복합재료 코팅 내 MWCNT 분율을 0.0-1.0 wt% 범위로 다양하게 형성하여, 복합재료 코팅의 물리 및 기계적 특성 (전기 비저항, 탄성 계수, 미소 경도 및 마모 저항성)을 평가하였으며, MWCNT 분율과 공정 특성에 따라 물리 및 기계적 특성 향상에 차이를 보였다. MWCNT 복합재료의 특성향상에는 MWCNT의 균일한 분산, 구조 유지, 및 금속 기지와 치밀한 계면 형성이 요구된다. 따라서, 복합 재료 코팅의 미세 조직 및 MWCNT 상태에 대한 분석을 통하여 공정 특성와 특성 향상 대한 상관 관계를 물리 및 기계적 야금학을 바탕을 규명하였다. 본 연구를 통하여 형성된 MWCNT/금속 복합재료 분말은 금속 분말과 MWCNT를 정확한 비율로 혼합하여 초음속 분산 (ultra-sonication)과 기계적 혼합하여 MWCNT의 분산을 균일하게 한 후 기계적 합금화 (mechanical ball milling)을 통하여 MWCNT를 금속 기지 내 균일하게 분산시켰다. 제조된 복합재료 분말의 미세 조직, MWCNT 분산도 및 기지와의 계면 상태는 코팅 형성 이후 특성에 큰 영향을 미치기 때문에 투과 전자 현미경을 통하여 이에 대한 분석을 통하여 제조된 복합재료 분말의 기지 미세조직이 결정립 미세화 (Al) 또는 변형 밴드 (Ni alloy, Cu)를 형성하였으며, MWCNT가 균일하게 분산되어 기지와 치밀한 계면을 형성하여 분포하는 것을 확인하였다. Differential scanning calorimeter (DSC)를 이용하여 복합재료 분말 내에서 MWCNT의 산화 및 파괴 거동에 대하여 분석하였다. 제조된 MWCNT/금속 복합재료 분말을 열용사 공정을 통하여 복합재료 코팅으로 형성되었다. 열용사 공정은 플라즈마 또는 연료 연소 등의 열원 가스층을 이용하여 분말을 가속 및 용융시켜 모재에 충돌시킨 후 충돌 후 급속 응고를 거쳐 라멜라 구조 (lamellar structure)를 형성하는 공정으로, 높은 열원을 사용하며, 대기 중에 산소와 연료 가스 내 불연소 산소가 불가피하게 유입되므로, 장입 분말의 산화가 발생한다. 열용사 코팅 내 금속 기지는 용융 및 급속 응고를 통하여 200 nm 이하의 미세한 조직을 형성하였다. 그리고, 열용사 공정 중 발생한 산화는 복합재료 내 MWCNT의 산화를 통한 파괴 및 기화를 발생시킨다. 코팅 내 MWCNT는 curling-up 및 peeling-off 등의 파괴 현상을 겪었으며, 열원의 온도 범위가 가장 높은 plasma spraying은 MWCNT가 amorphization되는 현상을 나타내었다. 저온 분사는 열용사와 같이 높은 열원을 사용하지 않고, 초음속 이상으로 분말을 사용하여 충돌 시 발생하는 계면분의 높은 변형률에서 발생하는 변형을 기반으로 적층이 이루어지므로, 기지의 용융 및 MWCNT의 산화를 막을 수 있었다. 고속 변형을 통한 기지 금속의 조직 변화 (strain-induced grain refinement)는 순수한 저온 분사를 통하여 형성된 Al, Ni, 및 Cu 코팅의 미세 조직 구현 기구에 대한 연구를 바탕으로 분석되었으며, MWCNT의 분산 및 계면 상태가 우수함이 분석되었다. 저온 분사 및 열용사 공정을 통하여 형성된 MWCNT/금속 복합재료 코팅의 물리 및 기계적 특성 평가가 전기 비저항, 탄성 계수, 미소 경도 및 내마모성을 중심으로 수행되었다. 복합재료의 특성은 코팅 형성 공정 특성과 MWCNT 분율에 영향을 크게 받았다. MWCNT 분율이 증가할수록 특성이 향상되었으며, 저온 분사 공정에서 형성된 코팅이 열용사를 통하여 형성된 코팅보다 특성이 우수하였다. 본 연구에서는 복합재료 코팅의 특성과 기지 조직, MWCNT 구조 및 계면 등의 미세 조직적 요인과 공정 특성 간의 상관관계에 대한 연구가 수행되었다.; Multi-walled carbon nanotube (MWCNT) reinforced metal matrix composite (MMC) coatings fabricated by kinetic and thermal spray processes were investigated, targeting the potential industrial applications. The MWCNTs were embedded in face-centered cubic (FFC) metals (Al, Ni alloy, and Cu) matrix powers with 0.0-1.0 wt% MWCNTs in order to efficiently strength and functionalize the matrix materials. The MWCNT reinforced MMC coatings were successfully fabricated by the consolidation of kinetic and thermal spraying using optimized process conditions. The property enhancements of the kinetic and thermal sprayed composite coatings such as electrical conductivity, elastic modulus, micro-hardness and wear resistance were found. The states of the composite powder, microstructural evolutions of the composite coatings, and interface and structure of MWCNTs in the composite coatings were analyzed. The correlations of property enhancements of the composite coatings with microstructural factors, and contributions of the embedded MWCNTs, which are predominantly influenced by the characteristics of kinetic and thermal spray processes, were investigated based on physical and mechanical metallurgies. The microstructure, and states (structure, interface, and dispersion) of the MWCNTs of the initial composite powders, which significantly influence the deposition behavior, and physical/chemical reactions during spraying process, were investigated by field-emission scanning electron microscopy, and transmission electron microscopy (TEM). The strain induced refinement (Al) or deformation band formation (Ni alloy, and Cu) in metal matrix were observed. MWCNTs were uniformed dispersed, and presented the structure and interface of quality, which satisfy the requirements of efficient reinforcements for MWCNT reinforced MMCs. Furthermore, thermal reactions of the composite powders were analyzed by differential scanning calorimeter (DSC) to simulate the thermally activated reactions during thermal spraying. The MWCNT reinforced MMC coatings were initially fabricated by thermal spray processes (high velocity oxygen fuel or plasma spraying). The deposition of thermal spraying is based on melting of particles in the gas flow field and rapid resolidification of melted splats upon impacts. Therefore, thermally activated reactions of MWCNTs such as destruction and gasification by oxidation were inevitable. Since the plasma spraying utilizes the higher process temperature, and in-flight velocity of particles is lower, the oxidation of MWCNTs significantly occurred. Microstructure of resolidified metallic matrix, destroyed MWCNTs, i.e., curling-up, peeling-off, and amorphization, and reactions of MWCNTs with metallic matrix were analyzed though intensive TEM examination of the thermal sprayed composite coatings. The destruction mechanisms of the MWCNTs during thermal spaying were investigated comparing with DSC analysis and thermodynamic calculation. Kinetic spraying of the composite powders was applied In order to avoid the aforementioned problematic phenomena of thermal spraying. Since the deposition of kinetic spraying is mainly based on supersonic collisions of particles and interfacial thermo-mechanical interactions that results from adiabatic shear instability, the thermal reaction of MWCNTs was avoidable. Furthermore, structure, dispersion, and interface of MWCNT in the kinetic sprayed composite coatings were sound, i.e., preserved their initial states of the composite powders. Supersonic velocity impacts of the composite powders resulted in plasticity induced grain refinements of the metal matrix. The grain refinement mechanisms of the kinetic sprayed composite coatings were investigated by comparing with microstructural evolutions of kinetic sprayed pure FCC metals. The microstructural evolutions of kinetic sprayed FCC metals were discussed in terms of the characteristics of kinetic spraying, and physical/mechanical properties of metals. Role of MWCNTs on grain refinements of the metallic matrix during kinetic spraying was explained. The measured physical and mechanical properties of the kinetic and thermal sprayed composite coatings were discussed. The properties were significantly affected by spray processes, and fractions of the embedded MWCNTs. The properties of the composite coatings were selectively evaluated considering fundamental data acquisitions and potential applications. The correlation of the measured properties of the composite coatings with the microstructural factors, such as the microstructure of the metallic matrix, MWCNT structure, and the interface between MWCNTs and the metal matrix, was investigated in order to provide an understanding of the contribution of MWCNTs to property enhancements of the composite coatings.