Papers by Author: Hideaki Matsubara

Abstract: Several kinds of thermal barrier coatings (TBCs) deposited by electron beam physical vapor deposition (EB-PVD) were produced as a function of electron beam power in order to evaluate their strain tolerance. The deposition temperatures were changed from 1210 K to 1303 K depending on EB power. In order to evaluate strain tolerances of the EB-PVD/TBCs, a uniaxial compressive spallation test was newly proposed in this study. In addition, the microstructures of the layers were observed with SEM and Young’s moduli were measured by a nanoindentation test. The strain tolerance in as-deposited samples decreased with an increase in deposition temperature. In the sample deposited at 1210 and 1268 K, high-temperature aging treatment at 1273 K for 10 h remarkably promoted the reduction of the strain tolerance. The growth of thermally grown oxide (TGO) layer generated at the interface between topcoat and bondcoat layers was the principal reason for this strain tolerance reduction. We observed TGO-layer growth even in the as-deposited sample. Although the thickness of the initial TGO layer in the sample deposited at high temperature was thicker, the growth rate during aging treatment was smaller than those of the other specimens. This result suggests that we can improve the oxidation resistance of TBC systems by controlling the processing parameters in the EB-PVD process.

Abstract: Thermal conductivity of pure and Y2O3-doped ZrO2 was calculated using a perturbed molecular dynamics method in order to analyze phonon scattering mechanism which is responsible for the reduction of thermal conductivity. Although absolute values of thermal conductivity were overestimated due to a simple model used in this study, relative values were in good agreement with experiment, which indicates that phonon scattering due to Y2O3 addition is reproduced well. It is found from quantitative analysis of the phonon scattering using the mean field theory that decrease of the thermal conductivity upon Y2O3 addition is attributed not only to the introduction of O2- vacancies but also to substitution of Y3+ ions for Zr4+ ions.

Abstract: ZrO2-4mol% Y2O3 coatings on zirconia substrate were deposited by EB-PVD. Influence of the coating thickness on thermal conductivity and thermal diffusivity of coated samples is examined. The fractured surface of the coated samples reveals a columnar microstructure consisting of feather-like structure. We have adopted a laser flash method for thermal diffusivity and specific heat capacity of coated samples. It was found that the thermal conductivity of the coating layers was strongly dependent on coating thickness. The thermal conductivity of coating layers shows increasing tendency with increasing the coating thickness.

Abstract: Effect of La2O3 addition on thermal conductivity and high temperature stability of YSZ coating produced by EB-PVD was investigated. La2O3 was selected as an additive because it had a significant effect on suppressing densification of YSZ. The developed coating showed extremely low thermal conductivity as well as high resistance to sintering. Microstructural observation revealed that the coating had fine feather-like subcolumns and nanopores, which contributed to limit thermal transport. These nanostructures were thought to be formed by suppressing densification during deposition.

Abstract: The conductive composites in which nano RuO2 particles are dispersed throughout a glass matrix have been successfully fabricated by sintering at 850°C. The sensing properties of conductive composites were investigated in real time during tensile testing by measuring the electrical resistance. It is shown that the excellent sensing ability based on electrical resistance changes in the low strain range was due to deformation of conduction paths between nano RuO2 particles by brittle fracture of the glass matrix. The change of electrical resistance depends strongly on the volume percent of Al2O3 fiber reinforcement.