Kicheol Kang

Dynamic amorphization and recrystallization of metals in kinetic spray process

We reported dynamic amorphization and recrystallization processes of metals upon impact of micron-scaled particles at a high strain rate (109s−1) combining adiabatic heating with rapid cooling (1010Ks−1) in a kinetic spray process. At the interface of the particle/substrate, an amorphous zone with a thickness of about 3nm was observed after individual particle impact. It is consistent with the mechanism of amorphous shear lamella and adiabatic shear instability characteristics in kinetic spray process. At the interface of coating/substrate, a rapid phase transition from unstable amorphous to crystalline helps the formation of ductile joints of coatings.

Effect of powder state on the deposition behaviour and coating development in kinetic spray process

In this work, smooth spherical and irregular granular copper powders were deposited on copper and aluminium substrates through the kinetic spray deposition process in order to find the effect of the feedstock powder state on the deformation process and the deposition behaviour. The effect of the initial state of powder on the coating properties was analysed through scanning electron microscopy, Vickers microhardness tester, electrical resistivity and bond strength characterizations. Simulation for two kinds of powders was performed through ABAQUS explicit 6.7-2 finite element analysis using a dynamic explicit procedure. Powder–substrate interface properties were estimated for two kinds of powders and discussed in order to compare the experimental results. It is found that the coatings produced from irregular granular powder have higher bonding properties.

Deposition Behavior and Microstructural Features of Vacuum Kinetic Sprayed Aluminum Nitride

The vacuum kinetic spray (VKS) method is a relatively advanced technology by which thin and dense ceramic coatings can be fabricated via the high-speed impact of submicron-sized particles at room temperature. However, the actual bonding mechanism associated with the VKS process has not yet been elucidated. In this study, AlN powders were pretreated through ball-milling and heat-treatment processes in order to investigate the effects of microstructural changes on the deposition behavior. It was found that ball-milled and heat-treated powder with polycrystals formed by partially aligned dislocations showed considerably higher deposition rates when compared to only ball-milled powder with tangled dislocations. Therefore, in the VKS process, the deposition behavior is shown to be affected by not only the particle size and defect density, but also the microstructure of the feedstock powder.

Phase separation in kinetic sprayed bulk metallic glasses

During a kinetic spray process, a high strain rate (~109?s?1) induced phase separation of amorphous metals at an impact interface was observed using high resolution transmission electron microscopy. The decrease in viscosity induced by the high strain rate deformation effect acted as the driving force for this process. The phase was further separated by jetting due to differences in the viscosity of the separated phases. The amorphous phase was sustained against nanocrystallization due to a fast quenching rate within the melting jet, even though the composition was separated far from the glass-forming region.

Oxidation Effect on the Critical Velocity of Pure Al Feedstock Deposition in the Kinetic Spraying Process

In kinetic spraying process, the critical velocity is an important criterion which determines the deposition of a feedstock particle onto the substrate. In other studies, it was experimentally and numerically proven that the critical velocity is determined by the physical and mechanical properties and the state of materials such as initial temperature, size and the extent of oxidation. Compared to un-oxidized feedstock, oxidized feedstock required a greater kinetic energy of in-flight particle to break away oxide film during impact. The oxide film formed on the surface of particle and substrate is of a relatively higher brittleness and hardness than those of general metals. Because of its physical characteristics, the oxide significantly affected the deposition behavior and critical velocity. In this study, in order to investigate the effects of oxidation on the deposition behavior and critical velocity of feedstock, oxygen contents of Al feedstock were artificially controlled, individual particle impact tests were carried out and the velocities of in-flight Al feedstock was measured for a wide range of process gas conditions. As a result, as the oxygen contents of Al feedstock increased, the critical velocity increased.

Characterization of Ni/YSZ Anode Coating for Solid Oxide Fuel Cells by Atmospheric Plasma Spray Method

In this research, anode for SOFC has been manufactured from two different kinds of feedstock materials through thermal spraying process and the properties of the coatings were characterized and compared. One kind of feedstock was manufactured from spray drying method which includes nano-components of NiO, YSZ (300 nm) and graphite. And the other is manufactured by blending the micron size NiO coated graphite, YSZ and graphite powders as feedstock materials. Microstructure, mechanical properties and electrical conductivity of the coatings as-sprayed, after oxidation and after hydrogen reduction containing nano composite which is prepared from spray-dried powders were evaluated and compared with the same properties of the coatings prepared from blended powder feedstock. The coatings prepared from the spray dried powders has better properties as they provide larger triple phase boundaries for hydrogen oxidation reaction and is expected to have lower polarization loss for SOFC anode applications than that of the coatings prepared from blended feedstock. A maximum electrical conductivity of 651 S/cm at was achieved for the coatings from spray dried powders which much more than that of the average value.

Property Evaluation of HVOF Sprayed Multi-walled Carbon Nanotube Aluminum Composite Coatings

Multi-walled carbon nanotube (MWCNT) aluminum composite powders were deposited to form coatings using a high velocity oxygen fuel (HVOF) spraying process. High thermal energy and contact with atmospheric oxygen were supplied as the MWCNT aluminum composite particles were exposed to a gas flow field at high temperature ( K) during HVOF spraying. As a result, the particles underwent full or partial melting and rapid solidification due to the high thermal energy, and the exposure to oxygen induced the interfacial reaction of MWCNTs within the particle. The electrical and mechanical properties of MWCNT aluminum composite coatings were evaluated based on microstructure analysis. Electrical resistivity, elastic modulus, and micro-hardness, of the MWCNT aluminum composite coatings were higher than those of pure aluminum coating. The contribution of MWCNTs to the aluminum matrix can be attributed to their high electrical conductivity, dispersion hardening and anchoring effects. The relationship among the properties and the interaction of the MWCNTs with the aluminum matrix is discussed.

Deposition Behavior and Properties of Carbon Nanotube Aluminum Composite Coatings in Kinetic Spraying Process

Carbon nanotube (CNT) aluminum composite coatings were built up through kinetic spraying process. Deposition behavior of CNT aluminum composite on an aluminum 1050 alloy substrate was analyzed based on deposition mechanism of kinetic spraying. The microstructure of CNT aluminum composite coating were observed and analyzed. Also, the electrical resistivity, bond strength and micro-hardness of the CNT aluminum composite coatings were measured and compared to kinetic sprayed aluminum coatings. The CNT aluminum composite coatings have a dense structure with low porosity. Compared to kinetic sprayed aluminum coating, the CNT aluminum composite coatings present lower electrical resistivity and higher micro-hardness due to high electrical conductivity and dispersion hardening effects of CNTs.

Restoration of face-centered cubic metals subjected to kinetic spraying

Deformation mode and restoration of face-centered cubic (FCC) metal (Al, Ni, and Cu) particles subjected to kinetic spraying (KS) were investigated. The FCC metal particles were accelerated to supersonic velocity by high pressure process gas, and collided with substrates or previously deposited coating layer. The high velocity impact of in-flight particles and their successive deposition leads to severe plastic deformation at ultra-high strain rate and the dissipation of heat energy from the plasticity. Accordingly, highly strained interface undergoes restoration to stabilize strained area during KS. Although Al, Ni, and Cu have equivalent slip systems {111} 〈110〉, the different physical and metallurgical properties of the FCC metals differentiate the deformation mode and lead to variations in static recovery and recrystallization rates. The deformation and restoration behavior of KS FCC metals are discussed, taking into account the physical and metallurgical factors such as stacking fault energy, dislocation mobility, diffusivity, and melting point.

Microstructure Evolution of Cu-based BMG Coating during APS Process and Phase Analysis by Nano-indentation Test

In this study, Cu-based bulk metallic glass (BMG) coatings were deposited by atmospheric plasma spraying (APS) process with different process conditions (with- and without hydrogen gas). As adding the hydrogen gas, thermal energy in the plasma flame increased and induced difference in the melting state of the Cu-based BMG particles. The microstructure and mechanical properties of the coatings were analyzed using a scanning electron microscope (SEM) with an energy dispersive spectroscopy (EDS) and nano-indentation tester in the light of phase analysis. It was elucidated by the nano-indentation tests that un-melted region was a mainly amorphous phase which showed discrete plasticity observed as the flow serrations on the load.displacement (P - h) curves, and the curves of solidified region showed lower flow serrations as amorphous phase mingled with crystalline phase. Oxides produced during the spraying process had the highest hardness value among the phases and were well mixed with other phases resulted from the increase in melting degree.