Papers by Author: Yukie Hayashi

Abstract: Thermal barrier coatings (TBCs) on turbine blades in gas turbine engines are in some cases damaged at high temperatures exceeding 1200°C by calcium-magnesium-alumino-silicates (CMAS) resulting from the ingestion of siliceous minerals. In this study, material interaction between molten CMAS and yttria-stabilized zirconia (YSZ) was investigated using a single crystal YSZ material and a synthetic CMAS product. Reaction between the molten CMAS and YSZ was significant at high temperature resulting in the infiltration of CMAS into the dense bulk-YSZ. The extent of interaction between CMAS and YSZ was found to be dependent on the crystallographic plane of the YSZ. The change in elastic stiffness due to the CMAS infiltration was also found by using a vibrating reed technique. The CMAS infiltrated layer had elastic stiffness higher by approximately five times of the non-infiltrated one. An attempt to detect the CMAS damage progression was also made through an AC impedance method. The proposed AC impedance technique is expected to be a useful technique to evaluate the CMAS infiltration as well as the associated delamination of TBC top coat.

Abstract: The mechanism of layer growth as well as defect formation in the SiC crystal is fundamentally important to derive its appropriate performance. The purpose of the present study is to investigate competitive adsorption properties of growth species on the various 4H-SiC polytype surfaces. Adsorption structure and binding energy of growth species in the experimentally condition on various SiC surfaces were investigated by density functional theory. For the SiC(000-1) and SiC(0001) surfaces, the adsorption energy by DFT follows the orders C > H > Si > SiC2 > Si2C > C2H2. Furthermore, based on the DFT results, amount of adsorption of each species in the experimental pressure condition were evaluated by grand canonical Monte Carlo method. H and Si are main adsorbed species on SiC(0001) and SiC(000-1) surfaces, respectively. The ratio of amount of adsorption of Si to H was depending on the surface structure that might explain different growth rate of the surfaces.