Yoshiyuki Yasutomi

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.

Improvement in Oxidation and Thermal Shock Resistance of Molten Glass-Coated Carbon Materials by Interfacial Control

The coating of molten silicate glass on a porous carbon substrate was developed, without the formation of cristobalite at the carbon-glass layer interface, in order to improve the steam oxidation and thermal shock resistance. Initially, suitable conditions for coating were assumed from thermodynamic analysis. Based on these calculations, the wettability of the carbon to molten glass was modified by infiltration and pyrolysis of a Si-N precursor, and the coating with glass was carried out under higher N2 partial pressures. As a result, carbon substrates were completely sealed with glass, without the production of cristobalite at the interface, and the glass was infiltrated into the substrate. In contrast, coating with glass at lower N2 partial pressures, such as in Ar, were followed by the formation of cristobalite along with many pores at the interface. The structural changes occurring as a result of variation of the N2 partial pressure during sealing with glass are in good agreement with the thermodynamic analysis. The glass-coated carbon materials, which were fabricated at higher N2 partial pressure, possessed excellent steam oxidation resistance and thermal shock resistance.

Self Reinforced Behavior of Silicon Nitride Ceramics by Reaction Bonding Method

Microstructure control of Si3N4 ceramics using reaction bondiing method with mixtures of the two types of Si powder is discussed. Specifically, the effect of the Si powder size on mechanical properties of reaction bonded Si3N4 ceramics is investigated. In order to increase the fracture toughness without decreasing the bending strength of the ceramics, it is effective to mix fine and coarse Si powders. The fracture toughness of nitrided bodies using mixtures of 0.4μm Si powder and 3μm or 11μm Si powder was 3.7MPa. m1/2, which is 1.5 times higher than that of using fine Si powder alone. The bending strength (300MPa) was the same as the nitrided body using fine Si powder alone. The fracture toughness was improved by the dispersing coarse Si3N4 grains in fine grained Si3N4 matrix.

Reaction Sintering Mechanism of Submicrometer Silicon Powder in Nitrogen

Knowing the nitridation mechanism of Si is important to obtain near-net-shape high strength reaction bonded ceramics. In this report, comparison of nitridation mechanism of small spherical Si powder produced by plasma arc method and large faceted Si powder, is discussed on the basis of microstructural analyses by SEM and TEM. From the analyses, nitrogen diffuses through the oxide film of Si powder and forms Si 3 N 4 under the oxide films during the initial stage of sintering. At higher temperature, the oxide film transform into fine Si 3 N 4 grains when heated to the 1350°C. The Si 3 N 4 has morphology of hollow shell and a size, which corresponds to that of the original Si particles.