Papers by Author: Je Hyun Lee

Abstract: The microstructural evolution related to the thickness of thermal barrier coating (TBC) and their thermal stabilities have been investigated with a specific attention to defect species as well as to its morphology with the thermal exposure time. The TBCs with different thicknesses of 600 and 2,000 µm were prepared by air plasma spray (APS) process and the thermal exposure tests were performed at 950^C in a furnace with a dwell time of 100 hrs till 500 hrs. The thickness of thermally grown oxide (TGO) layer in the TBC with 2,000 µm is thinner than that with 600 µm. Also, the TBC with 2,000 µm is more efficient in improving the oxidation resistance of bond coat than that with 600 µm. Vickers indentation methods are used to evaluate the interfacial stabilities. Indentation impression and crack formation of the TBC of 600 µm is easily occurred in comparison with that of 2,000 µm, showing relatively longer cracks, independent of thermal exposure. However, the crack formation and propagation through the interface does not observed in the TBC with 2,000 µm, showing crack propagation through the top coat near the interface. These results imply that the interfacial stability of TBC can be also improved with increasing the coating thickness.

Abstract: The effects of bond coat nature in thermal barrier coating (TBC) systems on the delamination or fracture behavior of the TBCs with different bond coats prepared using two different processes—air plasma spray (APS) and high velocity oxyfuel (HVOF)—were investigated by cyclic thermal fatigue tests. The TBCs with the HVOF bond coat were delaminated or fractured after 3–6 cycles, whereas those with the APS bond coat were delaminated after 10 cycles or show a sound condition. These results indicate that the TBC system with the APS bond coat has better thermal durability than the system with the HVOF bond coat under long-term cyclic thermal exposure. The hardness values of the TBCs (top coats) in both systems are dependent on applied loads, irrespective of the hardness of the bond coats and the substrate. The values are not responded to the bond coat nature or the exposure time. Thermally grown oxide (TGO) layers in both cases consist of two regions with the inner TGO layer containing only Al2O3 and the outer TGO layer of mixed-oxide zone containing Ni, Co, Cr, Al in Al2O3 matrix. The outer TGO layer has a more irregular shape than the inner TGO layer, and there are many pores within the outer layer. At failure, the TGO thickness of the TBC system with the HVOF bond coat is 9–13 m, depending on the total exposed time, and that of the TBC system with the APS bond coat is about 20 m. The both TBC systems show the diffusion layer on the side of substrate in the interface between the bond coat and the substrate. The relationship between the delamination or fracture behavior and the bond coat nature has been discussed, based on the elemental analysis and microstructural evaluation.

Abstract: The effects of the introduction of a buffer layer between the bond and top coats on the indentation stress-strain behavior and the contact damage were investigated in air-plasma sprayed (APS) zirconia (ZrO2)–based thermal barrier coatings (TBCs). The microstructure is relatively continuous in the TBC system with the buffer layer, showing Zr, Ni, Cr, and Mg elements between the top and bond coats, whereas the Zr element suddenly disappears by passing the interface between the top and bond coats. The TBC system with the buffer layer shows less strain than that without the buffer layer in the higher stress regions above about 1.3 GPa, while both TBC systems become soft by forming the top coat in the lower stress regions compared with the substrate. The stress–strain curve in both TBC systems is dependent on the dwell time of thermal exposure condition. The TBC system with the buffer layer shows the lower stress-strain curves than that without the buffer layer in thermal cycles with the relatively short dwell time of 1 h, showing the reverse trend with the relatively long dwell time of 10 h. Subsurface damage in substrate is reduced at both indentation loads of P = 500 N and P = 2000 N by introducing the buffer layer, independent of thermal exposure. Therefore, the TBC system with the buffer layer is more efficient in protecting the substrate from contact environments than that without the buffer layer, showing cracking or delamination between the top coat and the buffer layer in the TBC system with the buffer layer.

Abstract: The micro-hardness and electric conductivity was studied in as cast alloy Cu-Fe-P alloys which were solution treated at 980°C and aging treated at 400-800°C for 1-20 hours. The micro-hardness peak value was 180 HV and the conductivity peak value was 65% IACS. The hardness and the electric conductivity all depended on precipitation principally. The addition of C in Cu-Fe-P alloy reduced the grain size and accelerated precipitation which resulted in enhancing hardness and electric conductivity together.

Abstract: Ni3Al shows the unique feature of increasing strength with increasing temperature. However, it is too brittle to use as a structural material due to grain boundary weakness. Ductility could be enhanced by controlling grains using directional solidification. In order to increase the ductility or strength of Ni3Al alloys, a ductile γ (Ni-rich) phase of dendrite fibers or a strong β (NiAl) phase of dendrite fibers were arrayed in the γ´ (Ni3Al) matrix by directional solidification. The dendrite spacing could be controlled by varying the solidification rates, and the volume fraction of the γ or β phase could be changed by using alloy compositions, from 23 to 27 at. % Al-Ni alloy. With increasing solidification rates, the dendrite spacing decreased, which caused the tensile strength to be enhanced and the elongation to decrease, evidently due to the phase boundary augmentation. With increasing Al content, the γ dendritic microstructure changed to β dendrites in the γ´ matrix, which resulted in a decrease in elongation as a result of an increase in the volume fraction of the brittle β dendrites in the γ´ matrix.

Abstract: In this work, we report on the fabrication process of alumina (Al2O3) microtubes using carbon fibers and aluminum/alumina (Al/Al2O3) mixed powder via a solid-vapor (SV) reaction. Al and Al2O3 (α-Al2O3) were mixed in a 1:1 molar ratio, and heated to generate an AlO vapor. The carbon fibers were heat-treated in the pre-carburized Al2O3 crucible at 1400°C for 9h with a heating rate of 5°C/min in flowing argon (Ar) gas at 200 ml/min. Any carbon residues remaining in the core after the heat treatment at 1400°C were burned off by subsequent calcination at 700°C for 3 h in air. A post-heat treatment was conducted to convert the product to Al2O3. As a result, Al2O3 microtubes are successfully synthesized via the SV reaction between carbon fibers and Al/Al2O3 mixed powder. The TGA study shows that the AlO vapor is generated at temperatures above 750°C. As the calcination temperature increases, carbon residues and Al4C3 peaks disappear in XRD patterns. Al2O3 microtubes are synthesized at 1200°C, and show thinner wall thickness and undulating outer and inner surfaces arising from the partial decomposition of Al2O3 microtubes.

Abstract: In this work we have evaluated the effect of platinum (Pt) pre-coating on microstructure evolution of MCrAlY coating during isothermal oxidation, to improve high temperature oxidation and corrosion resistance of the nickel-based superalloy MAR M247-DS. Pt was deposited on the superalloy by electroplating before the MCrAlY coating by a vacuum plasma spray, which was compared to the superalloy without Pt pre-coating. The samples were subjected to isothermal oxidation in a box furnace at 1100°C for 25–150 hrs. The weight gain and loss caused by the oxidation of samples was recorded, and the variation of surface layer and interface morphology were observed before and after the oxidation tests. The length decrement of cross section is remarkably low on according to the precipitation of (Ni,Pt)Al phase by Pt pre-coating. Also, the amount of voids at interface between the coating and the superalloy, which would be introduced as a processing defect, is reduced by Pt pre-coating.

Abstract: The effect of microstructural features of A356 alloys on plastic deformation behavior was studied in the present study. To authors' knowledge, the microstructural effect on mechanical properties of A356 alloy has not been well understood even though this alloy system is one of the most widely used alloys for the industrial purpose. Specially, quantitative relationship between properties like ductility and fracture toughness with microstructural features is lacking. In the present study, four different processing methods were used to investigate how the size and distribution of primary alpha and eutectic phases influence the plastic deformation characteristics. The prcessing routes for A356 control arm include low pressure casting, squeeze casting, rheo-casting and cast-forging processes. Special focus was given to understand which microstructural features do the beneficial or detrimental effect and how they work.

Abstract: Silicon carbide (SiC) nano-powders are successfully synthesized by a reaction between carbon nano-powders (carbon black) and SiO gas at 1300°C for 9 hrs in dynamic argon atmosphere (flow rate; 400 cm3 min-1), using the solid-vapor reaction method. The particle size of synthesized SiC nano-powders is below 40 nm and the shape is uniform. Unexpectedly, SiC nano-fibers are also coexisted in the SiC nano-powders. The quantitative and qualitative properties of the SiC nanopowders and nano-fibers are analyzed by scanning electron microscopy (SEM), transmission electron microscope (TEM), and X-ray diffraction (XRD). Carbon residuals removed by heating over 700°C in air are estimated by thermogravimetry analysis (TGA). It is found that the SiC nanopowders are easily produced by direct synthesis via the solid-vapor reaction method. The morphological characteristics of the resulting SiC nano-powders are dependent upon the morphology of carbon black used as precursor.

Abstract: TiO2 coated open-cell mullite ceramics were fabricated with coating TiO2 sol on pores of mullite, and its processing parameters were investigated. Open-cell mullite ceramics were fabricated through a gel-casting process. Two kinds of mullite precursor powders were prepared by dissolution of two kinds of aluminum salts (Al(NO3)3·9H2O; type I and Al2(SO4)3·16H2O; type II) into colloidal silica sols. To produce porous mullite ceramics, both mullite precursor powders and PMMA beads (≈ 5μm) were co-dispersed by electrosteric stabilized mechanism in an aqueous system and then gel-casted. The green bodies were sintered above 1300°C for 3hrs in air. The PMMA was offered as pores in sintered mullite bodies. The prepared open-cell mullite ceramics were dipped in TiO2 precursor solution. The sintered bodies coated with TiO2 sols were re-sintered below 1000°C for 3hrs after drying at room temperature. The TiO2 was successfully coated into the open-cell mullite ceramics. The characteristics of each TiO2 coated porous mullite ceramics were investigated by XRD, SEM, porosimetry, as functions of aluminum salt and pH of sol. It was found that the synthesis behavior and the porosity of the mullite are strongly dependent upon aluminum salt species, resulting in different grain size, morphology, and pore size.