Sang Kuk Woo

Influence of Subsurface Layer on the Indentation Damage Behavior of YSZ Thermal Barrier Coating Layers Deposited by Electron Beam Physical Vapor Deposition

The thermal barrier coating must withstand erosion when subjected to flowing gas and should also maintain good stability and mechanical properties while it must also protect the turbine component from high temperature, hot corrosion, creep, and oxidation during operation. In this study we investigated the influence of subsurface layer, Al₂O₃ or NiCrCoAlY bond coat layer, on the indentation damage behavior of YSZ thermal barrier coating layers deposited by electron beam physical vapor deposition (EB-PVD). The bond coat is deposited using different process such as air plasma spray (APS) or spray of high velocity oxygen fuel (HVOF) and the thickness is varied. Hertzian indentation technique is used to induce micro damages on the coated layer. The stress-strain behaviors are characterized by results of the indentation tests.

Preparation of YSZ Electrolyte for SOFC by Electron Beam PVD

Yttria stabilized zirconia (YSZ) films with the thickness of up to 12 μm were prepared on alumina and NiO-YSZ substrates by electron beam physical vapor deposition (EB-PVD). The films showed nano-scaled columnar structures depending on the substrate temperature. Electrical conductivity of the YSZ films on alumina was also investigated at the temperature between 700 and 1000oC in oxidizing atmosphere. High activation energy of the conductivity (>1.03eV) indicated that the conduction via grain boundary controlled the ionic conduction in the films prepared by EB-PVD. La0.6Sr0.4CoO3-δ as a cathode was applied on the YSZ/NiO-YSZ in order to evaluate the performance of the YSZ electrolyte.

Hardness and Wear Resistance of Reaction Bonded SiC-B4C Composite

Hardness and wear resistant characteristics of reaction-bonded silicon carbides with boron carbide additions are evaluated relative to those of reaction bonded silicon carbide (RBSC). The reaction-bonded SiC-B4C composites exhibit a distinctive improvement of hardness and wear resistance, indicative of high resistance against wear environment. Removal rates for the wear tests are decisively reduced by the addition of boron carbide in the composites. Controlling the amount of carbon content in the starting composition more enhances the hardness of the reaction-bonded composites. Implications concerning the partial decomposition of B4C during reaction process are considered.

Strength Degradation of Silicon Carbide Hot Gas Filter in the Simulated PFBC Condition

Strength degradations of porous silicon carbide hot gas filters bonded with clay and calcium carbonate are investigated in the simulated condition of the pressurized fluidized bed combustion (PFBC). Thermal cycling, static fatigue, thermal shock, and hot corrosion tests are conducted to differentiate the independent parameter that affects the strength degradation during the filtration using silicon carbide filter in PFBC condition. The results indicate that hot corrosive gas mainly causes the strength reduction because of the degradation of grain boundary region.

Improvement of Strength in Porous Ceramics by Control of Additives

The strengths of porous ceramics are improved by the control of additives with maintaining appropriate porosity, 30∼40vol%. The addition of small amount of CaCO 3 additive improved the strength of porous silicon carbide hot gas filters, which collects the dust particles in the pressurized fluidized bed combustion system. The strength of NiO-10mol%YSZ sintered at 1500°C, which applicable for the anode of SOFC, was higher than that of NiO-8mol%YSZ materials due to the increase in the fraction of intergranular fracture. The result indicates that the additives are one of the crucial factors for the enhancement of strength in the porous ceramics.

Preparation and Properties of Silicon Nitride Ceramics by Nitrided Pressureless Sintering (NPS) Process

Silicon nitride ceramics were prepared by new nitrided pressureless sintering (NPS) process in this study. The microstructures, strengths and thermal properties of the NPS silicon nitride ceramics containing three types of Al2O3 and Y2O3 sintering additives were investigated. Additionally, we have investigated the effect of silicon metal contents changing with 0, 5, 10, 15 and 20 wt% in each composition. The silicon nitride was successfully densified using NPS process, particularly at the starting composition of 5 wt.% Al2O3, 5 wt.% Y2O3, and 5 wt.% Si addition. The maximum flexural strengths and relative densities of these specimens were 500 MPa and 98%, respectively. The flexural strength of sintered specimens after the thermal shock test between 30oC and 1300oC for 20,000 cycles was maintained with the original laboratory strength of 500MPa by low thermal expansion coefficient, 2.9 × 10-6/oC, and high thermal conductivity, 28 W/m⋅oC.

Fabrication of YSZ Thin-Film for SOFC Applied by Electron Beam PVD

Thin film of yttria stabilized zirconia (YSZ) was prepared on alumina substrates by applying an electron beam physical vapor deposition (EB-PVD) method. The morphology and deposition behavior of the coating layer were investigated using atomic force microscope, secondary electron microscope, and X-ray diffractometer. The electrical conductivity of the YSZ coating layer was also evaluated. The fracture microstructure of YSZ electrolyte film, which was deposited with thickness of 10 μm, has showed a columnar structure. Also, the activation energy of the coating layer was similar to that of bulk YSZ.

Surface Coating of Reactive Layer with Inner Nano-Pores on Lantanium Gallium Based Oxide

A layer consisting of La0.6Sr0.4CoO3-d particles with inner nano-pores is coated on the La0.7Sr0.3Ga0.6Fe0.4O3-d oxide to improve the surface reaction resulting in high oxygen permeability. The La0.6Sr0.4CoO3-d particles with inner nano-pores are synthesized by an ultrasonic spray pyrolysis method. The permeated oxygen contents from air to helium gas were measured using gas chromatography after the synthesized particles were coated on the La0.7Sr0.3Ga0.6Fe0.4O3-d oxide by the conventional screen-printing. The results demonstrated that the reactive free surfaces of the nano-pores are an important factor in determining the oxygen permeation for application such as gas separation membrane and ceramic fuel cell.

Development of the Reaction-Bonded Si/SiC Ceramic Tubes for High-Temperature Heat Exchangers

The Ministry of Commerce, Industry and Energy has contracted with the KIER including the ceramics company to localize of the ceramic heat exchanger tubes and its components. The basic objective of this paper is to develop a ceramic formulation and process which would yield a cost-effective and long-life Si/SiC tube materials by extrusion and reaction sintering process. Thus,KIER is currently developed and localized heat exchanger tubes with 40mm out diameter, 1500mm length. This paper was tested the mechanical and thermal properties of the localized Si/SiC tubes obtained from reaction sintering. Also, the effect of energy savings of the heat exchanger after exposure to forging furnace with hot corrosive environment was investigated.

Preparation and Morphology of C/C-SiC Composites by Si Melt Infiltration

Two different types of carbon fibre bundles were used for filament winding to obtain C/C preforms. C/C-SiC composites were produced from the C/C preforms by a silicon melt infiltration technique. The effect of the type of carbon fibre bundle on the mechanical and thermal properties of the resultant C/C-SiC composites was compared. The spun fiber preform yields C/C-SiC composites of better mechanical properties than the unidirectional continuous fiber preform. The strength of the composites from the SFP was 1.8 times higher than that from the CFP. The flexural strength and the O-ring strength of the composites from the SFP with a density of 2.35 g/cm3 were about 160 MPa and 170 MPa, respectively.