FORMATION OF CARBON INTERPHASE ON POLYCRYSTALLINE AND AMORPHOUS SiC FIBERS IN SiC/SiC COMPOSITES BY ELECTROPHORETIC DEPOSITION

Continuous silicon carbide fiber-reinforced silicon carbide matrix (SiC/SiC) composites have been recognized as key materials for aerospace industries, high-temperature gas turbines and future nuclear and fusion applications because they show a pseudo-ductile fracture behavior and excellent fracture tolerance. Fiber/matrix interfaces act as an important role for toughening and strengthening SiC/SiC composites. Currently, carbon or hexagonal-boron nitride has been formed on SiC fibers as the interphases, and these interphases have been generally formed by chemical vapor infiltration (CVI), chemical vapor deposition (CVD) or solution-coating/pyrolysis process. However, these processes generally require long manufactuiring time and complicated apparatuses, and they use toxic, flammable or combustible reactant gases, resulting in much higher production cost and an increase in environmental load. Present authors paid attention to EPD process to form interphases on SiC fibers in SiC/SiC composites and demonstrated that EPD process was effective to form homogeneous C-interphase on SiC fibers [1-5]. In this study, EPD was applied for carbon interphase formation on polycrystalline (electric conductive) and amorphous (low electric conductive) SiC fibers in SiC/SiC composites, and microstructure and mechanical properties of the SiC/SiC composites were evaluated. From TEM micrograph of carbon interphase on the polycrystalline SiC fiber in the SiC/SiC composite formed by EPD, it was observed that flaky carbon particles were deposited parallel to the SiC fiber. Carbon has a layered structure and the layers bonded by weak van der Waals forces enable a low friction coefficient. As a result, the SiC/SiC composites with carbon interphases showed excellent mechanical properties with a pseudo-ductile fracture behavior. For amorphous SiC fibers, thin electric conductive polymer was coated on the amorphous SiC fibers, and carbon interphase formation was successfully achieved by EPD. The SiC/SiC composites reinforced with the carbon-coated amorphous SiC fibers also showed pseudo-ductile fracture behavior. References [1] K. Yoshida et al., Key Eng. Mater., 352 (2007) 133. [2] K. Yoshida et al., J. Nucl. Mater., 386-388 (2009) 643. [3] K. Yoshida et al., Mater. Sci. Eng. B, 161 (2009) 188. [4] K. Yoshida, J. Ceram. Soc. Japan, 118 (2010) 82. [5] K. Yoshida et al., Compos. Sci. Technol., 72 (2012) 1665. [6] K. Yoshida et al., Key Eng. Mater., 617 (2014) 213. [7] K. Yoshida, Chapter 18, in “MAX Phases and Ultra-High Temperature Ceramics for Extreme Environments” (2013)

Characterization of Macroscopic Properties and Crystalline Defects in Neutron-Irradiated Silicon Carbide

Length change, mechanical properties, helium release behavior, ESR observation and microstructure of neutron-irradiated silicon carbide (SiC) were investigated. Changes in those properties due to annealing were also measured to clarify the relationship between crystalline defects induced by neutron irradiation and macroscopic properties. Firstly, the effects of sintering aids on irradiation and annealing behavior of three kinds of SiC ceramics were described. Swelling, starting temperature to decrease in length during annealing and recovery rate up to 1300℃ were not affected by sintering aids. On the other hand, the bending strength and length change above 1300℃ were influenced by the kind of sintering aids. Secondary, the effects of external stress on defect annihilation and bubble swelling during annealing of neutron-irradiated specimens were investigated. From those experiments, it is clarified that the decrease in length by annealing below 1300℃ was not affected by the external stress. However, annealing above 1300℃ led to an increase in length in B-containing SiC, and the compressive stress retarded the expansion along the loading direction. In the third section, the helium release behavior of neutron-irradiated SiC containing B was observed and it was clarified that the helium release rate of ceramic and its powder was different from each other. Defects characterization was carried out by ESR measurement and high-resolution electron microscopy. ESR signal related to the vacancy type defects having unpaired electrons markedly increased by higher fluence irradiation over 5x10^n/m^2, and the intensity of signal decreased with increasing annealing temperature above irradiation temperature with slightly higher rate than that of length decrease. It indicated that the vacancies with unpaired electron detected by ESR selectively disappear at lower temperature than those detected by a macroscopic length measurement. High-resolution electron microscopy revealed that very small interstitial type Flank loops lying on {111}, having a Burgers vector b=1/3, were formed in β-SiC which was heavily-neutron-irradiated in a fast reactor. Defect nuclei, a few nanometer in diameter, in hexagonal α-SiC were induced by lower doses in a thermal reactor. They are on the (0001) basal plane and have a Burgers vector b=1/6[0001]

Development of a soft X-ray angle-resolved photoemission system applicable to 100 µm crystals

A soft X-ray angle-resolved photoemission system applicable to 100 µm crystals has been developed