Papers by Author: M. Sato

Abstract: For description of the mechanical performance of SiC/SiC composites and for safety design for practical use, it is needed to reveal the degradation mechanism especially of fiber under the oxygen atmosphere. In the present work, the fracture behavior and microstructure of the polycrystalline silicon carbide fiber exposed in air at 1173-1873 K for 20 and 3.6 ks were studied with monofilament tensile test, microstructure observation and fracture toughness determination test using newly developed FIB(focused-ion-beam)-method.

Abstract: SiC fiber reinforced SiC matrix (SiC/SiC) composites are one of the most promising materials for high temperature structural applications such as power generation and propulsion systems. SiC/SiC composites are, however, susceptible to accelerated attacks in water vapor environments through oxidation and volatilization reaction. For protection from such attacks, Environmental Barrier Coatings (EBCs) are indispensable. We have investigated some oxides and rare-earth silicates as topcoat candidate materials for EBCs. Topcoat materials must be stable in the high-water-vapor pressurized environments at high temperatures. Also, it is important that the thermal expansion coefficient of topcoat materials is similar to that of the SiC/SiC composites. In this study, first, zirconium oxides, lutetium silicates and yttrium silicates were selected as topcoat candidate materials. They were exposed in a water-containing atmosphere at a temperature of 1673 K for 100 h under a total pressure 0.96 MPa. Mass changes, structure of crystals and microstructures were investigated after the exposure experiments in order to evaluate the thermal stability of these materials. After their estimation, lutetium silicates were considered to be promising for topcoat materials. Then, lutetium silicates were coated as the topcoat of an EBC system on SiC/SiC composites, and their fracture toughness and microstructures were investigated after exposure to an oxidizing atmosphere. The evaluation results of the topcoat materials are reported in this paper.

Abstract: It has been reported that SiC/SiC composite has high strength and toughness, but is degraded when exposed in air at high temperatures due to the propagation of the crack made by the premature fracture of the SiO2 layer. The present work aimed to describe such a behavior with a computer simulation. For this aim, the shear lag - Monte Carlo simulation method was applied. The variation of strength of the composite as a function of thickness of the SiO2 layer and change of fracture morphology with progressing oxidation could be reproduced satisfactorily by this method.