Browsing by Author "Kim, Bong-Gu"

Now showing 1 - 5 of 5
  • Thumbnail Image
    Item
    Improvement in the hygroscopicity of inorganic binder through a dual coating process
    (Elsevier, 2019-10) Choi, Hyun-Hee; Kim, Eun-Hee; Lee, Hye-Ju; Kim, Bong-Gu; Jung, Yeon-Gil; Zhang, Jing; Mechanical Engineering and Energy, School of Engineering and Technology
    The use of an anti-absorbent is proposed in this work to reduce the hygroscopicity of the inorganic binder in the casting mold, in which the anti-absorbent is coated on the mold prepared with an inorganic binder. Three types of polymers were used to select material with optimal water resistance. Polystyrene (PS) and polyvinyl alcohol (PVA) were used as a water-insoluble polymer and water-soluble polymer, respectively. In addition, polyurethane (PU) prepolymer has intermediate properties between PS and PVA. PVA and PU prepolymer were used for comparative testing with PS. For this testing process, the prepared green body was dipped into a solution of inorganic binder precursor mixed with tetraethyl orthosilicate (TEOS, SiO2 precursor) and sodium methoxide (NaOMe, Na2O precursor), and then dipped into a solution of coating reagent after a drying process. Thus, these series of coating processes in a green body is called a dual coating process. Finally the sample was heat-treated at 1000 °C to generate a glass phase by an organic–inorganic conversion process. In the sample prepared with PS, the highest contact angle and a high firing strength were exhibited, independent of polymer concentration, while the sample coated with PVA showed lower green and firing strengths. When prepolymer, PU, was applied, the green strength was remarkably improved, showing lower firing strength compared with that of PS. The green and firing strengths were optimized through the dual coating process with PS. Moreover, the moisture-proof effect of the dual coating process was verified through the moisture steam test.
  • Thumbnail Image
    Item
    Molecular dynamics modeling of mechanical and tribological properties of additively manufactured AlCoCrFe high entropy alloy coating on aluminum substrate
    (Elsevier, 2021-04-15) Yang, Xuehui; Zhang, Jian; Sagar, Sugrim; Dube, Tejesh; Kim, Bong-Gu; Jung, Yeon-Gil; Koo, Dan Daehyun; Jones, Alan; Zhang, Jing; Mechanical and Energy Engineering, School of Engineering and Technology
    In this work, an improved molecular dynamics (MD) model is developed to simulate the nanoindentation and tribological tests of additively manufactured high entropy alloys (HEA) AlCoCrFe coated on an aluminum substrate. The model shows that in the interface region between the HEA coating and Al substrate, as the laser heating temperature increases during the HEA coating additive manufacturing process, more Al in the substrate is melted to react with other elements in the coating layer, which is qualitatively in agreement with experiment in literature. Using the simulated nanoindentation tests, the calculated Young's modulus of pure Al and Al with HEA coating is 79.93 GPa and 119.30 GPa, respectively. In both our simulations and the experimental results in the literature, the hardness of Al with the HEA coating layer is about 10 times higher than the Al hardness, indicating that HEA can significantly improve the hardness of the metallic substrate. Using the simulated tribological scratch tests, the computed wear tracks are qualitatively in agreement with experimental images in literature. Both our model and experiment show that the Al with HEA coating has a much smaller wear track than that of Al, due to less plastic deformation, confirmed by a dislocation analysis. The computed average coefficient of friction of Al is 0.62 and Al with HEA coating is 0.14. This work demonstrates that the HEA coating significantly improves the mechanical and tribology properties, which are in excellent agreement with the experiments reported in the literature.
  • Thumbnail Image
    Item
    Oxidation Behavior of NiCoCrAlY Coatings Deposited by Vacuum Plasma Spraying and High-Velocity Oxygen Fuel Processes
    (MDPI, 2023-02) Kim, Junseong; Pyeon, Janghyeok; Kim, Bong-Gu; Khadaa, Tserendorj; Choi, Hyeryang; Zhe, Lu; Dube, Tejesh; Zhang, Jing; Yang, Byung-il; Jung, Yeon-gil; Yang, SeungCheol; Mechanical and Energy Engineering, Purdue School of Engineering and Technology
    To reduce the formation of detrimental complex oxides, bond coatings in the thermal barrier coatings for gas turbines are typically fabricated using vacuum plasma spraying (VPS) or the high-velocity oxygen fuel (HVOF) process. Herein, VPS and HVOF processes were applied using NiCoCrAlY + HfSi-based powder to assess the oxidation behavior of the bond coatings for both coating processes. Each coated sample was subjected to 50 cyclic heat treatments at 950 °C for 23 h and cooling for 1 h at 20 °C with nitrogen gas, and the weight change during the heat treatment was measured to evaluate the oxidation behavior. After the oxidation test, the coating layer was analyzed with X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy-dispersive spectroscopy (EDS). The VPS coating exhibited faster weight gain than the HVOF coating because the alumina particles generated during the initial formation of the HVOF coating inhibited oxidation and diffusion. The VPS coating formed a dense and thick thermal growth oxide (TGO) layer until the middle of the oxidation test and remained stable until the end of the evaluation. However, the HVOF coating demonstrated rapid weight loss during the final 20 cycles. Alumina within the bond coat suppressed the diffusion of internal elements and prevented the Al from being supplied to the surface. The isolation of the Al accelerated the growth of spinel TGO due to the oxidation of Ni, Co, and Cr near the surface. The as-coated VPS coating showed higher hardness and lower interfacial bonding strength than the HVOF did. Diffusion induced by heat treatment after the furnace cyclic test (FCT) led to a similar internal hardness and bonding strengths in both coating layers. To improve the quality of the HVOF process, the densification of the coating layer, suppression of internal oxide formation, and formation of a dense and uniform alumina layer on the surface must be additionally implemented.
  • Thumbnail Image
    Item
    Smoothed Particle Hydrodynamics Modeling of Thermal Barrier Coating Removal Process Using Abrasive Water Jet Technique
    (ASME, 2022-09) Zhang, Jian; Yang, Xuehui; Sagar, Sugrim; Dube, Tejesh; Koo, Dan Daehyun; Kim, Bong-Gu; Jung, Yeon-Gil; Zhang, Jing; Mechanical and Energy Engineering, School of Engineering and Technology
    In this work, a new smoothed particle hydrodynamics (SPH)-based model is developed to simulate the removal process of thermal barrier coatings (TBCs) using the abrasive water jet (AWJ) technique. The effects of water jet abrasive particle concentration, incident angle, and impacting time on the fracture behavior of the TBCs are investigated. The Johnson–Holmquist plasticity damage model (JH-2 model) is used for the TBC material, and abrasive particles are included in the water jet model. The results show that the simulated impact hole profiles are in good agreement with the experimental observation in the literature. Both the width and depth of the impact pit holes increase with impacting time. The deepest points in the pit hole shift gradually to the right when a 30-deg water jet incident angle is used because the water jet comes from the right side, which is more effective in removing the coatings on the right side. A higher concentration of abrasive particles increases both the width and depth, which is consistent with the experimental data. The depths of the impact pit holes increase with the water jet incident angle, while the width of the impact holes decreases with the increase in the water jet incident angle. The water jet incident angle dependence can be attributed to the vertical velocity components. The erosion rate increases with the incidence angle, which shows a good agreement with the analytical model. As the water jet incident angle increases, more vertical velocity component contributes to the kinetic energy which is responsible for the erosion process.
  • Thumbnail Image
    Item
    Three-dimensional analytical models for predicting coating thickness on non-axial symmetrical workpieces in electron beam physical vapor deposition
    (Elsevier, 2022-08) Li, Yafeng; Ji, Zhengzhao; Dhulipalla, Anvesh; Zhang, Jian; Yang, Xuehui; Dube, Tejesh; Kim, Bong-Gu; Jung, Yeon-Gil; Koo, Dan Daehyun; Zhang, Jing; Mechanical and Energy Engineering, School of Engineering and Technology
    In this work, three-dimensional (3D) analytical models for non-axial symmetric workpieces, including ellipsoid and cylinder, are derived to predict the coating thickness distributions in the EB-PVD process. Additionally, 3D analytical models for axial symmetric workpieces, including disk and sphere are presented, which will be used for deriving the non-axial symmetric workpiece solutions. The models are based on extending the two-dimensional (2D) models of a disk workpiece by Schiller et al. (1982) and a circular arc on a cylinder by Fuke et al. (2005). The 3D models for disk and sphere workpieces are also presented which are used to derive the non-axial symmetric models. The results show that the 3D analytical models are consistent with the 2D models, and also in excellent agreement with our finite element (FE) model predictions and experimental data in the literature.