저온 분사 공정의 입자 충돌 조건과 금속의 계면 반응 및 기계적 특성과의 상관관계

Abstract

저온 분사 공정은 입자의 운동 에너지를 이용하여 충돌 시 발생하는 고 변형률 소성 변형에 따른 계면의 국부 열적 활성화 또는 용융을 통해 소재를 저온에서 빠른 속도로 접합 및 적층시키는 기술로서, 다양한 표면 코팅뿐 아니라, 특히 고온 산화에 민감한 고 반응성 금속 소재의 진공 주조 공정을 대체하여 분말 소재를 이용한 bulk forming이 가능한 기술이다. 실험적 접근으로 분석 및 규명이 난해한 저온 분사 공정의 금속 입자 충돌 거동 및 이에 따른 단열 전단 불안정 현상을 이해하고자, 물리적/기계적 특성이 상이한 충돌 조합에 대한 유한요소모델링/전산모사를 수행하였다. 그 결과, 열적 상승 지역이라는 일반화된 정량적 개념을 도출하였고, 접합 인자 및 반발 인자와 실험적으로 관찰된 소재 조합 별 개별 입자 접합 효율과의 상관성을 입증하였다. 또한, 본 연구는 저온 분사 입자 충돌 조건 (입자 크기, 속도 및 온도)과 상이한 물리적/기계적 특성 및 결정구조를 갖는 순 금속 (FCC Ni / HCP Ti / BCC Ta)의 접합, 미세구조 및 기계적 특성과의 상관관계를 규명하는 것을 목적으로 하였다. 특히 타 소재 대비 고 융점으로 인해 상대적으로 접합 및 적층 효율이 낮은 문제를 해결하고자 재료 손실 최소화 및 공정 비용 저감을 우선 목표로 하였으며, 이와 동시에 미세조직의 초미세립화를 통해 접합 계면의 기계적 및 파괴 인성 특성을 향상시키고자 하였다. 각 소재 별 거시적 고온 및 고 변형률 열-기계적 반응을 고려한 구성 방정식을 유한요소모델 전산모사에 적용하여, 입자 고속 충돌에 따른 고 변형률 소성 변형 거동을 정량적으로 예측하였고, 계면 단열 전단 불안정 및 용융 발생을 근거로 입자 충돌 조건에 따른 접합 임계 속도를 산출하였다. 투과 전자 현미경 미세 조직 관찰, X-선 회절을 통한 구조 분석 및 나노 압입 경도 특성 평가를 통해, 각 소재 별 입자 충돌 조건과의 상관성을 규명함으로써, 최적 공정 효율 및 특성 구현을 위한 기술적 솔루션을 마련하였다. 거시적 관점에서 FCC 니켈의 경우, 공정 비용이 높은 고속 충돌 조건에서 잉여 운동 에너지에 따른 과도한 반발 및 침식 현상으로 인해 오히려 낮은 적층 효율을 보였고, 공정 비용 측면에서 경제적인 저속 충돌 조건에서 적정 온도의 분말 예열을 통해 95% 이상의 높은 적층 효율을 얻었다. 이 결과는 계면 응력 (반발 인자) 및 온도 (접합 인자) 측면에서 전산모사 결과와 잘 일치하였다. HCP 티타늄의 경우, 소재 고유의 높은 단열지수 및 온도 민감성을 갖는 열-기계적 특성이 반영된 온도 별 구성 방정식을 적용하여 다수 입자 충돌 모델링을 수행하였다. 실험 결과, 저속 충돌 조건에서는 다 기공의 부분 접합 양상을 보인 반면, 고속 충돌 조건에서는 계면 용융을 동반한 견고한 접합을 형성하였다. 분말 예열 저속 충돌 조건의 경우, 계면 활성화에 따른 접합 및 적층 효율의 증가 (>90%)를 보였으며, 향상된 미소경도 및 접합강도의 결과를 나타냈다. BCC 탄탈륨의 경우, 적정 분말 예열 조건에서, 저속 충돌 조건의 낮은 적층 효율 (~50%) 대비 월등히 향상된 적층 효율 (~99%)을 보였으며, 고속 충돌 조건 보다 더 높은 효율을 나타냈다. 또한, 고 변형률 고온 변형 시 이동 전위의 활동도가 급격히 증가하는 소재 고유의 특성에 따른 변형 경화 효과의 증대로 인해, 분말 예열 조건에서 적층 후 미소경도가 월등히 증가하는 결과를 보였다. 타 소재와 다르게 고속 충돌 조건에서 다소 낮은 미소경도의 증가를 보였다. 미시적 관점에서 FCC 니켈의 경우, 분말 예열 저속 충돌 조건에서 접합 계면 부근에 100 nm 이하의 나노 결정립이 형성되었고, 이는 고온 충돌 계면의 변형 촉진 및 이에 따른 열적 활성화에 기인하였다. 또한, 높은 적층 결함 에너지에도 불구하고 나노 결정립 (<100 nm)내에 변형 쌍정이 형성되었는데, 이는 고 변형률 및 나노 결정립 크기 효과에 따른 제한된 전위 활동도에 기인한 것으로 보인다. 이러한 미세조직의 나노 구조화에 따른 초미세립화는 나노 압입 경도 특성에도 반영되어, 특히 접합 계면 부의 높은 강화 현상이 증명되었다. HCP 티타늄의 경우, 열전도를 고려한 layer-by-layer 적층 모델을 적용하였고, 이에 따라 규명된 층 간 변형 및 온도 축적 효과에 의해, 접합 계면 부근에 높은 전위 밀도를 갖는 상대적으로 조대한 (>250 nm) 결정립과 전위 free하고 비평형 결정립계를 갖는 나노 결정립 (<100 nm)이 상당히 넓은 지역에 걸쳐 bimodal하게 분포하였다. 이는 접합 부의 변형 경화, 결정립계 강화 및 연성 파괴 양상과의 직접적인 연계성을 보였다. BCC 탄탈륨의 경우, 타 소재 대비 입자 충돌 조건에 따른 상이한 기계적 특성의 경향을 보인 바, Synchrotron 광 가속기 고 분해능 X-선 회절 실험을 통한 미세구조 분석을 실시하였고, 그 결과 전위 밀도 및 평균 도메인 (결정립 및 전위셀) 크기 등을 산출함으로써, 분말 예열 저속 충돌 조건의 경우 상대적으로 높은 전위 밀도에 따른 매우 높은 경도 증가를, 고속 충돌 조건의 경우 동적 회복 및 재결정 현상에 따른 상대적으로 낮은 경도 증가를 규명하였다. |In this decade, kinetic spraying, or cold gas dynamic spraying, has been developed as a novel deposition technology to obtain dense and high-quality coatings or deposits, which have low oxygen content and high bond strength. These properties can be achieved because the technique is a low-temperature and high-pressure deposition process and is therefore unique when compared to conventional thermal spraying processes. This dissertation aims to 1) develop a database for critical velocities of metals for successful bonding in kinetic spraying process by studying the particle impact behaviors and high-strain-rate thermomechanical responses, by means of finite-element modeling (FEM) and simulations, 2) explore the bonding features and mechanisms present in kinetic spray deposit of metals by linking experimental observations with FEM for the purpose of optimizing the deposition process, and 3) investigate the correlation of particle impact conditions with microstructural refinement and nanomechanical properties in kinetic sprayed metals. First, different engineering materials are classified into four impact cases according to their physical and mechanical properties, i.e., soft/soft, hard/hard, soft/hard, and hard/soft (particle/substrate). Based on FEM, impact behaviors of the four cases were numerically analyzed. For soft/soft and hard/hard cases, the size variation of the thermal boost-up zone (TBZ), accompanied with the different aspects of adiabatic shear instability (ASI), was numerically estimated and is theoretically discussed. Meanwhile, for soft/hard and hard/soft cases, the specific aspect of shear instability, which has a very high heat-up rate, is always observed on the relatively soft impact counterpart where a thin molten layer is expected as well. Based on these phenomenological characteristics, bonding aspects are characterized, and a database for critical velocities of different particle/substrate combinations was developed for kinetic spraying process. Second, the bonding features and mechanisms present in kinetic sprayed metals, i.e., face-centered cubic (FCC) Ni, hexagonal close-packed (HCP) Ti and body-centered cubic (BCC) Ta were explored on the basis of their high-temperature rate-dependent responses, by linking experimental observations with FEM with the aim to optimize the deposition process. The numerically predicted deformation features and interface thermomechanical responses of the particles adequately elucidate the deposition characteristics for different impact conditions (i.e., particle size, velocity and temperature). Prior to the experiments, the adhesion factors (interface temperature, contact time and contact area), rebound factor (relative recovery energy) and the resultant critical velocities of these metals for different particle sizes and temperatures were estimated by FEM. The microhardness of the deposit decreased with increasing particle size due to effect of deformation localization. In FCC Ni, owing to the specific high-strain-rate thermomechanical characteristics of Ni particle impact in kinetic spraying, the rebound phenomenon of the impacting particles hinders the formation of the first layer and impedes successful build-up of the deposit. Even at higher impact velocities, the deposition efficiency (DE) of the deposit was quite low because of excessive kinetic energy, which induces the rebound and/or erosion of the highly flattened particles. Noticeably improved bonding and deposition characteristics of Ni particles resulting from suppressed equivalent (von Mises) flow stress and enhanced interface heat-up as a result of powder preheating were revealed from experimental observations coupled with FEM. In HCP Ti, the formation of a porous deposit by local bonding at relatively low velocity and of a very dense deposit accompanied by interfacial melting at higher velocity resulted from deformation localization at high strain rates due to the relatively higher adiabaticity as compared with other materials, which is one of the unique bonding mechanisms in Ti. In BCC Ta, it is revealed that the particle impact temperature was more significant on the bonding and strengthening of the deposit rather than particle impact velocity, which can be quite beneficial for economical spraying processes. Based on the FEM results, the TBZ, increased by thermally accelerated ASI, is proposed as a crucial factor for enhancing bonding between the particles, which is essential in producing better properties. Third, in FCC Ni, by powder preheating, nanocrystal formation (<100 nm) in the deposit was more pronounced than cases previously reported in the literature, mainly because of the enhanced thermal activation and straining of the severely deformed particles, which was verified by transmission electron microscopy investigations and nanoindentation tests. Also, nanoscale twins were observed to be formed by deformation twinning in a nano-sized grain located at an interfacial region dominated by strain hardening over thermal softening upon impact. In HCP Ti, at a higher impact velocity, considerably homogeneous and randomly orientated equiaxed nanograins, including some recovered grains having a low dislocation density, were found to be formed over wide areas inside the deposit due to strain accumulation and the resultant thermal history enhanced by subsequent impacts of the particles. The bimodal grain structure consisting of both larger grains having high-density dislocations (>250 nm) and smaller dislocation-free grains with non-equilibrium grain boundaries (<100 nm) was determined to be associated with both the strain hardening and the ductile dimple fracture of the deposit. In BCC Ta, microstructural parameters (e.g., dislocation density and mean size of domains such as subgrains or dislocation cells) of the deposits formed under different particle impact conditions were estimated by performing high resolution X-ray diffraction experiments and found to be in good agreement with transmission electron microscopy investigations. At a higher impact temperature, high-density dislocation arrays, bands and cells were prominent, while, at a higher impact velocity, dynamic recovered and/or recrystallized grains were evidently observed especially in the vicinity of the bonded zone. Further, nanomechanical properties (i.e., nanohardness and elastic modulus) agreed well with these microstructural characteristics.; In this decade, kinetic spraying, or cold gas dynamic spraying, has been developed as a novel deposition technology to obtain dense and high-quality coatings or deposits, which have low oxygen content and high bond strength. These properties can be achieved because the technique is a low-temperature and high-pressure deposition process and is therefore unique when compared to conventional thermal spraying processes. This dissertation aims to 1) develop a database for critical velocities of metals for successful bonding in kinetic spraying process by studying the particle impact behaviors and high-strain-rate thermomechanical responses, by means of finite-element modeling (FEM) and simulations, 2) explore the bonding features and mechanisms present in kinetic spray deposit of metals by linking experimental observations with FEM for the purpose of optimizing the deposition process, and 3) investigate the correlation of particle impact conditions with microstructural refinement and nanomechanical properties in kinetic sprayed metals. First, different engineering materials are classified into four impact cases according to their physical and mechanical properties, i.e., soft/soft, hard/hard, soft/hard, and hard/soft (particle/substrate). Based on FEM, impact behaviors of the four cases were numerically analyzed. For soft/soft and hard/hard cases, the size variation of the thermal boost-up zone (TBZ), accompanied with the different aspects of adiabatic shear instability (ASI), was numerically estimated and is theoretically discussed. Meanwhile, for soft/hard and hard/soft cases, the specific aspect of shear instability, which has a very high heat-up rate, is always observed on the relatively soft impact counterpart where a thin molten layer is expected as well. Based on these phenomenological characteristics, bonding aspects are characterized, and a database for critical velocities of different particle/substrate combinations was developed for kinetic spraying process. Second, the bonding features and mechanisms present in kinetic sprayed metals, i.e., face-centered cubic (FCC) Ni, hexagonal close-packed (HCP) Ti and body-centered cubic (BCC) Ta were explored on the basis of their high-temperature rate-dependent responses, by linking experimental observations with FEM with the aim to optimize the deposition process. The numerically predicted deformation features and interface thermomechanical responses of the particles adequately elucidate the deposition characteristics for different impact conditions (i.e., particle size, velocity and temperature). Prior to the experiments, the adhesion factors (interface temperature, contact time and contact area), rebound factor (relative recovery energy) and the resultant critical velocities of these metals for different particle sizes and temperatures were estimated by FEM. The microhardness of the deposit decreased with increasing particle size due to effect of deformation localization. In FCC Ni, owing to the specific high-strain-rate thermomechanical characteristics of Ni particle impact in kinetic spraying, the rebound phenomenon of the impacting particles hinders the formation of the first layer and impedes successful build-up of the deposit. Even at higher impact velocities, the deposition efficiency (DE) of the deposit was quite low because of excessive kinetic energy, which induces the rebound and/or erosion of the highly flattened particles. Noticeably improved bonding and deposition characteristics of Ni particles resulting from suppressed equivalent (von Mises) flow stress and enhanced interface heat-up as a result of powder preheating were revealed from experimental observations coupled with FEM. In HCP Ti, the formation of a porous deposit by local bonding at relatively low velocity and of a very dense deposit accompanied by interfacial melting at higher velocity resulted from deformation localization at high strain rates due to the relatively higher adiabaticity as compared with other materials, which is one of the unique bonding mechanisms in Ti. In BCC Ta, it is revealed that the particle impact temperature was more significant on the bonding and strengthening of the deposit rather than particle impact velocity, which can be quite beneficial for economical spraying processes. Based on the FEM results, the TBZ, increased by thermally accelerated ASI, is proposed as a crucial factor for enhancing bonding between the particles, which is essential in producing better properties. Third, in FCC Ni, by powder preheating, nanocrystal formation (<100 nm) in the deposit was more pronounced than cases previously reported in the literature, mainly because of the enhanced thermal activation and straining of the severely deformed particles, which was verified by transmission electron microscopy investigations and nanoindentation tests. Also, nanoscale twins were observed to be formed by deformation twinning in a nano-sized grain located at an interfacial region dominated by strain hardening over thermal softening upon impact. In HCP Ti, at a higher impact velocity, considerably homogeneous and randomly orientated equiaxed nanograins, including some recovered grains having a low dislocation density, were found to be formed over wide areas inside the deposit due to strain accumulation and the resultant thermal history enhanced by subsequent impacts of the particles. The bimodal grain structure consisting of both larger grains having high-density dislocations (>250 nm) and smaller dislocation-free grains with non-equilibrium grain boundaries (<100 nm) was determined to be associated with both the strain hardening and the ductile dimple fracture of the deposit. In BCC Ta, microstructural parameters (e.g., dislocation density and mean size of domains such as subgrains or dislocation cells) of the deposits formed under different particle impact conditions were estimated by performing high resolution X-ray diffraction experiments and found to be in good agreement with transmission electron microscopy investigations. At a higher impact temperature, high-density dislocation arrays, bands and cells were prominent, while, at a higher impact velocity, dynamic recovered and/or recrystallized grains were evidently observed especially in the vicinity of the bonded zone. Further, nanomechanical properties (i.e., nanohardness and elastic modulus) agreed well with these microstructural characteristics.