• Composites Research
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• v.33 no.4
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• pp.205-212
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• 2020

In this study, HfC was coated on two types of carbon/carbon composites coated with SiC by vacuum plasma spraying(VPS). The experiment was performed using a plasma wind tunnel with heat flux of 5.06 MW/㎡ for 120 s heat flux before and after the coating. The mass ablation rate was calculated through the mass change before and after the test, and the length change was measured by using calipers and high speed camera. The oxidation/ablation behavior were observed by FE-SEM with EDS analysis of the specimens cross section. The plasma wind tunnel test results showed that the coated specimens had low weight loss and length change, and high oxidation/ablation resistance. However, two types of the specimens tested under the same conditions were different in the ablation behavior and ablation rate, and it was evaluated that the cylindrical type had higher oxidation/ablation resistance.

• Korean Journal of Materials Research
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• v.32 no.12
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• pp.553-559
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• 2022

In this work, the ablation behavior of monolith ZrB₂-30 vol%SiC (Z30S) composites were studied under various oxy-acetylene flame angles. Typical oxidized microstructures (SiO₂/SiC-depleted/ZrB₂-SiC) were observed when the flame to Z30S was arranged vertically. However, formation of the outmost glassy SiO₂ layer was hindered when the Z30S was tilted. The SiC-depleted region was fully exposed to air with reduced thickness when highly tilted. Traces of the ablated and island type SiO₂ were observed at intermediate flame angles, which clearly verified the effect of flame angle on the ablation of the SiO₂ layer. Furthermore, the observed maximum surface temperature of the Z30S gradually increased up to 2,200 ℃ proving that surface amorphous silica was continuously removed while monoclinic ZrO₂ phase began to be exposed. A proposed ablation mechanism with respect to flame angles is discussed. This observation is expected to contribute to the design of complex-shaped UHTC applications for hypersonic vehicles and re-entry projectiles.

• Korean Journal of Metals and Materials
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• v.50 no.7
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• pp.539-544
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• 2012

Thermal shock behavior of TiN-coated SUS 304 substrate was investigated using a laser ablation method. By short surface ablation with a pulse Nd-YAG laser, considerable surface crack and spalling were observed, whereas there were few oxidation phenomena, such as grain growth of TiN crystallites, nucleation and growth of $TiO_2$ crystallites, which were observed from the coatings quenched from $700^{\circ}C$ in a chamber. The oxygen concentration of the ablated coating surface with the pulse laser also had a lower value than that of the quenched coating surface by Auger electron spectroscopy and electron probe micro analysis. These results were attributed to the fact that the properties of the pulse laser method have a very short heating time and so the diffusion time for oxidation was insufficient. Consequently, it was verified that the laser thermal shock test provides a way to evaluate the influence of the thermal shock load reduced oxidation effect.

• Proceedings of the Korean Society of Precision Engineering Conference
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• 2006.05a
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• pp.671-672
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• 2006

We have investigated the behavior of the ultrafast laser ablation of chromium films (200nm) on the silicon and pyrex-glass(corning 7740) substrate with respect to the laser fluence and the number of laser pulses. In addition, several experiments about ITO thin film were carried out with femto-second Ti:Sapphire laser (150fs). Finally, we introduce the ablation characteristic in accordance with materials of thin film and substrate.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• v.y2005m4
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• pp.119-122
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• 2005

Ablation phenomena is very complicated because it includes momentum, energy and mass transfer, chemical reactions as well as phase change. In this paper, ablation at the rocket nozzle throat is modeled as unsteady one dimensional axi-symmetric with proper boundary conditions and field equation is solved numerically. Analytical results are compared with measured ablation data from firing experimental liquid rocket engine. Test variables are combustion pressure and mixture ratio. for low combustion pressure and low mixture ratio, the discrepancy between analysis and experiments are large but for the normal rocket operation range, two results show a simliar trend with maximum discrepancy of $100\%$.

• Proceedings of the Korean Society of Laser Processing Conference
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• 2004.10a
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• pp.90-94
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• 2004

• Transactions of the Korean Society of Mechanical Engineers B
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• v.23 no.9
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• pp.1201-1212
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• 1999

A numerical analysis is performed to predict the thermal response and ablation rate for charring or non-charring material which is designed to be used as thermal protection system (TPS). The numerical program composed of in-depth energy balance equation and the aerotherm chemical equilibrium (ACE) program. The ACE program calculates various thermochemical state from ablation products. The developed numerical program is verified by comparing the reported results from literature. The sensitivity tests for input parameters are performed. The thermal behavior of ablating material is mainly affected by density of ablating material, convective heat transfer coefficient and recovery enthalpy of flow field.

• Nuclear Engineering and Technology
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• v.32 no.4
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• pp.410-423
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• 2000

A two-dimensional numerical model is developed and applied to the LAVA-EXV tests performed at the Korea Atomic Energy Research Institute (KAERI) to investigate the external cooling effect on the thermal margin to failure of a reactor pressure vessel (RPV) during a severe accident. The computational program was written to predict the temperature profile of a two-dimensional spherical vessel segment accounting for the conjugate heat transfer mechanisms of conduction through the debris and the vessel, natural convection within the molten debris pool, and the possible ablation of the vessel wall in contact with the high temperature melt. Results of the sensitivity analysis and comparison with the LAVA-EXV test data indicated that the developed computational tool carries a high potential for simulating the thermal behavior of the RPV during a core melt relocation accident. It is concluded that the main factors affecting the RPV failure are the natural convection within the debris pool and the ablation of the metal vessel, The simplistic natural convection model adopted in the computational program partly made up for the absence of the mechanistic momentum consideration in this study. Uncertainties in the prediction will be reduced when the natural convection and ablation phenomena are more rigorously dealt with in the code, and if more accurate initial and time-dependent conditions are supplied from the test in terms of material composition and its associated thermophysical properties.

• Journal of the Korean Society for Precision Engineering
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• v.29 no.5
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• pp.572-577
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• 2012

Recently porous polymer has widely been applied to packaging, heat isolation, and sound absorption in various fields from the electrics to the automobiles industry. A lot of micro porosities inside foamed polymer provide lower heat conduction and lighter weight than non-porous polymer, because they involve gas or air during foaming process. In this paper experimental approaches of the UV laser micro machining behavior for Expanded Polypropylene (EPP) foamed polymer materials, which have different expansion rates, were investigated. From these results, the ablation phenomena were finally observed that the ablation is depended upon stronger photo-chemical than photo-thermal effect. This study will also help us to understand interaction between UV laser beam and porous polymer.

• Journal of the Korean Ceramic Society
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• v.55 no.5
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• pp.498-503
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• 2018

Using the thermal ablation method, the oxidation behavior of $SiC_f/SiC$ composites was investigated in air and in the temperature range of $1,300^{\circ}C$ to $2,000^{\circ}C$. At the relatively low temperature of $1,300^{\circ}C$, passive oxidation, which formed amorphous phase, predominantly occurred in the thermal ablation test. When the oxidation temperature increased, SiO (g) and CO (g) were formed by active oxidation and the dense oxide layer changed to a porous one by vaporization of gas phases. In the higher temperature oxidation test, both active oxidation due to $SiO_2$ decomposition on the surface of the oxide layer and active/passive oxidation transition due to interfacial reaction between oxide and base materials such as SiC fiber and matrix phase simultaneously occurred. This was another cause of high temperature degradation of $SiC_f/SiC$ composites.