Ken Goto

日本複合材料学会

ZrB₂–SiC–ZrC (ZSZ) ternary composites of four differing compositions were fabricated by spark plasma sintering (SPS). The effect of the ZrB₂/ZrC ratio on the oxidation behavior was examined by thermogravimetric (TG) analysis and microstructural observations. In addition, in-situ observation above 1500°C was performed using a high-temperature observation system (HTOS). ZSZ composites with high ZrC contents were fully oxidized at temperatures below 1500°C. However, ZSZ composites with low ZrC contents formed protective oxide layers during heat exposure. These layers protected the unreacted regions from further oxidation. Island nucleation was successfully observed on the surface oxide layer at 1500°C using the HTOS. The nucleation of islands strongly depended on the ZSZ composition. The difference in oxidation behavior originated from the concentration and interparticle distance of the ZrC in the as-sintered compacts.

Fiber-matrix interface mechanical properties are the key design point for the composites, however, methodology for evaluating interface properties of C/Cs are not still well established. In this study, capability of the fiber bundle push-out method was demonstrated as a measurement technique of fiber-matrix interfacial mechanical properties of various C/Cs. First, to carefully determine the experimental conditions to give proper interface mechanical properties, the influences of a radius of an indenter and the thickness of specimens on the measured results were examined. From the experimental result, interface debonding stress, τ_d, and sliding stress, τ_s, could be obtained as a constant value independent from specimen thickness as long as tested using the same indenter size and the proper specimen thicknesses of < 400 µm. However, absolute value of especially τ_d showed dependence of on indenter size. This was shown to be the limitation of this method that τ_d obtained from this method cannot be treated as a quantitative value. To demonstrate the capability of the method, the change of fiber-matrix interface mechanical properties of C/Cs on the type of fibers and final heat treatment temperature. τ_d decreased with the heat treatment temperature in both C/Cs with pitch based carbon fibers, K633 and K321 and C/Cs reinforced with a carbon fiber with higher heat treatment temperature gave lower τ_d at the same heat treatment temperature. Main reason for the degradation of τ_d up to 2273K was attributed to the enlargement of debonding area at fiber-matrix interface. Degradation of τ_d was enhanced at the temperature range of > 2273 K by the structure change of the interface from the graphitization of matrices. Finally, The fiber bundle push-out method was proved to be a strong tool to investigate fiber-matrix interface mechanical properties of C/Cs.

Fracture behavior of three-dimensional reinforced C/C disk was investigated to establish design criteria for high temperature turbine disk. Fly-out objects were detected from the rotation speed of 3000 r.p.m and the vibration amplitude increased with increase of rotation speed. The observation of the surface after high speed rotation revealed that the object flown-out from the disk was θ-directional fiber bundles. The vibration increasing behavior was predicted with simplified model and the result agreed well in tendency.

Increased use of Carbon/Carbon (C/C) composites necessitates fabrication of complex structures, which require high temperature secondary bonding. Secondary bonding of C/C interfaces provides design flexibility and also alleviates complicated fabrication problems. In this study, resin-carbonizaition process was applied for the bonding of C/C composites. In this process, C/C composites were bonded with phenolic resin and the cured resin was then heat-treated to carbonize. Bonding strength of thus bonded layers was evaluated by a double-ends-notched method at elevated temperatures up to 2000℃ in vacuum. The results revealed that bonding-strength increase with increasing temperature. Since strength of graphite is known to be affected by absorbed gas, out gas test was also performed. Absorbed gas is shown to lower bonding strength. Therefore it is concluded that this effect was shown to be one of major factors for the strength enhancement at elevated temperatures.