Sanghoon Yoon

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.

A novel time delay estimation using chirp signals robust to sampling frequency offset for a ranging system

We propose a novel time delay estimation algorithm using a training and data sequence composed of chirp signals for a ranging system with frequency offset. This training and data sequence can be used for SNR enhancement using the proposed denoising method. Then, a subspace-based algorithm, ESPRIT, is applied to estimate the time delay of the first arrival path. We analyze the estimation performance of ESPRIT based on the proposed denoising method and verify it through Monte-carlo simulations in AWGN and a multi-path channel.

Dependence of initial powder temperature on impact behaviour of bulk metallic glass in a kinetic spray process

In this work, the superplasticity and brittleness of CuNiTiZr bulk metallic glass (BMG) with different initial powder temperatures (below Tg, between Tg and Tx, and above Tx) are investigated. TEM analysis, together with focused ion beam prepared samples, showed that crystallization of BMG was promoted by increasing initial temperature. Interface heating and deformability upon impact enhance the deposition of BMG particles below Tx, while rebound and fracture occurred above Tx due to the formation of a thick pre-crystallized layer around the particles. Strain induced nanocrystallization of BMGs at the impact interface may improve the bonding between amorphous particles.

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.

Microstructure and Tribological Properties along with Chemical Composition and Size of Initial Powder in Fe-based BMG Coating through APS

In this study, two kinds of Fe-based bulk metallic glasses (BMG) powder were built-up through atmospheric plasma spray (APS) technique. The microstructure of two coatings was analyzed through X-ray diffraction, scanning electron microscopy and transmission electron microscopy. Crystallization and oxidation in coatings were affected by chemical composition and initial powder size. Then, both of them influenced the tribological property.

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.

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.

Effects of Impact Velocity on Crystallization and Activation Energy of Cu-based Bulk Metallic Glasses in Kinetic Spray Coating

In this paper, nanocrystallization of CuNiTiZr bulk metallic glass (BMG) subjecting to a kinetic spraying, dependent on impact velocity, was investigated by numerical and experimental approaches. The crystallization fraction and nucleation activation energy of initial feedstock and as-deposited coating were estimated by DSC and Kissinger method, respectively. The results of numerical modeling and experiment showed that the crystalline fraction and nucleation activation energy in BMG coatings were depended on kinetic energy of incident particle. Upon impact, the conversion of particle kinetic energy leads to not only decreasing free energy barrier but also increasing the driving force for an amorphous to crystalline phase transformation. The nanocrystallization of BMGs is associated with the strain energy delivered by a plastic deformation with a high strain rate.