Mineaki Matsumoto

Triple-layered, thick glassy grain boundaries in porous cordierite ceramics

Mechanical tests and microstructural investigations were performed on porous cordierite ceramics. Four-point bending tests were carried out both in air and in water. It was shown that the mechanical strength was strongly influenced by the experimental conditions. Triple-layered, thick amorphous grain boundaries were observed by high-resolution transmission electron microscopy. The composition of each layer as measured by energy dispersive X-ray spectroscopy was shown to be different. It is concluded that applying an external stress causes water-induced stress corrosion cracking preferentially occurring at the amorphous grain boundaries and at the triple grain junctions. This may have caused the dramatic decrease in the mechanical strength in water compared with that in air.

Thermal Conductivity of Zirconia for Thermal Barrier Coatings: A Perturbed Molecular Dynamics Study

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.

La2O3 Doped Y2O3 Stabilized ZrO2 TBC Prepared by EB-PVD

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.

Gas evolution behavior during fracture of alumina

Since ceramics are generally produced by sintering of compacted powders, a certain quantity of gases which are in the sintering atmosphere or absorbed on the surfaces of the raw powders must be retained in the sintered body. These gases have been believed to inhibit pore shrinkage at the final stage of sintering. Large residual pores are known to deteriorate the strength of sintered compacts because they act as a fracture origin, and small pores may decrease properties such as hardness, wear resistance, thermal conductivity, etc. It is probable that the gases themselves are deleterious to strength or wear resistance by reducing grain boundary strength. Thus, it is important to analyze gases contained in the sintered body and to decrease their amounts in order to improve the properties of ceramics. This paper describes gas evolution behavior during fracture of Al{sub 2}O{sub 3} ceramics as a first step in elucidating gas related problems in ceramics.