Search

Several technical R&D activities mainly related to the blanket materials are newly launched as a part of the Broader Approach (BA) activities, which was initiated by the EU and Japan. According to the common interests for these parties in DEMO, R&Ds on reduced activation ferritic/martensitic (RAFM) steels as structural material, SiCf/SiC composites as a flow channel insert material and/or alternative structural material, advanced tritium breeders and neutron multipliers, and tritium technology are carried out through the BA DEMO R&D program, in order to establish the technical bases on the blanket materials and the tritium technology required for DEMO design. This paper describes overall schedule of those R&D activities and recent progress in Japan carried out by JAEA as the domestic implementing agency on BA, collaborating with Japanese universities and other research institutes.

The combination of SiC and reduced activation ferritic/martensitic steels is attractive for fusion applications because it is expected to offer high thermal efficiency, high reliability and superior safety characteristic under a neutron irradiation environment. In this paper, diffusion bonding of SiC to F82H steel has been investigated. Direct joining of SiC to F82H was unsuccessful due to a large residual stress generated in the joint. A double W/Cu and a multiple W/Ni/Cu/Ni interlayer were used to reduce the residual stress, and encouraging results were obtained. The interfacial microstructure examination revealed that the various interfaces were bonded well. Diffusion products in the reaction zones were identified. The shear strength of the SiC/F82H joints measured by knife-edge tests at room temperature was found to increase with the increase in the joining temperature, and reached a maximum of 41.3 MPa. The fracture surfaces of the joints were also analyzed.

The effects of sintering additives on irradiation swelling and modulus for monolithic NITE SiC was studied using 5.1 MeV Si2+ ion irradiation up to 5 dpa at 553–1273 K. The elastic modulus of NITE SiC decreased less than ∼7% in the highest damage case of 553 K, 3 dpa, indicating that the additives did not negatively affect the modulus of irradiated NITE SiC. A sample containing 9% additives showed higher swelling rates than high purity CVD SiC at lower temperatures. At 553 K, 5 dpa, the swelling of NITE SiC (9 wt.%) was twice as large as that of CVD SiC. Analysis of the electron diffraction patterns showed amorphization of the secondary phase, which is likely a primary reason for the increased swelling.

Six different composite materials of various near-stoichiometric silicon carbide (SiC) fiber reinforcements and pyrolytic carbon or SiC/pyrolytic carbon multilayer interphases were neutron-irradiated to ∼6 × 10 25 n/m 2 ( E > 0.1 MeV) at nominal temperatures of 800 °C and 1300 °C, and tested for tensile properties at room temperature. Only insignificant or very minor modifications to the tensile strength were realized. However, statistical analysis on relatively large specimen populations revealed minor but statistically significant strength degradation for some composites. From 50 to 150 nm appeared to be within the optimum PyC interphase thickness range for the SiC fibers used in terms of tensile properties. The misfit stresses present in the unirradiated samples were significantly reduced after irradiation. The change in misfit stress may be attributed to the irradiation-induced modification of coefficient of thermal expansion or potential differential swelling between the fibers and the matrix. True matrix cracking stress estimated from the proportional limit stress and misfit stress did not appear to degrade by neutron irradiation.

Measurements of the contact angles at the different temperatures of a molten lead–lithium eutectic alloy (PbLi) droplet on a silicon carbide (SiC) wall are needed for the research and development both of a magnetic confinement fusion (MCF) and an inertia confinement fusion (ICF) blankets. PbLi coolant/breeder flows in the coolant channel, which is made of the SiC walls, and will experience a flow slip at the wall, called as a magnetohydrodynamic (MHD) slip flow. The ICF blanket adopts a molten PbLi film flow along the first wall made of SiC. The PbLi contact angle database is necessary as the thermal property for numerically predicting the behavior of the flowing molten PbLi film. This study attempts the measurement of the contact angles between the molten PbLi and the various SiC surfaces. For example, in order to examine the initial PbLi wettability, we measured the contact angles of a chemical vapor deposition (CVD) SiC, a nano-infiltration and transient eutectic-phase (NITE) SiC/SiC composites, and a NITE SiC in an inert atmosphere. We obtained the contact angle database of the molten PbLi, varying the temperature of PbLi from 250 to 400 °C, on a surface-polished as well as an unpolished SiC.

In this study, the evaluation of shear strength for joined SiC ceramics was conducted by using a torsion test. Two specimen types based on F734-95 ASTM Standard and one proposed for miniature test specimen in this study were prepared. The effects of specimen size and geometry were investigated by using finite-element stress method (FEM) analysis. The specimens were joined by the NITE process with mixed powder of Al 2 O 3 , Y 2 O 3 , SiO 2 and SiC as joining adhesive. Joining was carried out at 1800 °C for 1 h, under an applied pressure of 20 MPa by hot pressing. The fracture surfaces after torsion test were observed by using optical microscopy.

Japan-US cooperation program TITAN (Tritium, Irradiation and Thermofluid for America and Nippon) started in April 2007 as 6-year project. This is the summary report at the midterm of the project. H...

Permeability of helium gas through the NITE-SiC f /SiC composites after applying tensile stress was measured experimentally in a vacuum apparatus. Tensile stress equal to 1.1–1.2 times the proportional limit stress (PLS) was applied parallel to the direction of the reinforced fiber. Results of the permeability experiments revealed that the permeability rapidly increased when threshold stress was applied on the specimens. The permeability of helium gas was governed by the narrowest diameter of the permeation pathway. In the case of NITE composites, the diameter of the pathway was calculated to be below 0.65 μm. The NITE composites exhibited superior performance even when the applied stress was greater than 1.2 times the PLS. Fiber bundles considerably magnified the permeability of helium gas because of the relatively large pore size of the intra-fiber bundles. Transverse cracks propagated with increasing stress and they connected fiber bundles when the applied stress was 1.10–1.15 times the PLS.

SiC nanopowder infiltrated SiC f /SiC composites with a high fiber volume fraction above 50 vol.% were prepared at a relatively low fabrication temperature of 1800 °C by transient liquid phase sintering using Al 2 O 3 –Y 2 O 3 –SiO 2 additives. The effects of additive content with 6–18 wt.% were investigated, based on densification, microstructure, mechanical properties and fracture behaviors of the composites. The results showed that the densification and mechanical properties of the composites were greatly improved with the additive content. Microstructural observation indicated that the infiltration of SiC nanopowder inside fiber-bundles were enchanced with the increase of additive content due to the effectively widen space by the reaction between pyrocarbon (PyC) interface and the additives especially with the addition of SiO 2 . It proven that the enchanced matrix–PyC interface bonding by the high densification inside fiber-bundles played a key role in the improved mechanical properties and fracture behaviors of the composites.

We report the enchanced in situ performances of tensile strength and thermal conductivity at elevated temperatures of the PCS-free SiC/SiC composite with a high fiber volume fraction above 50% fabricated by NITE process for nuclear applications. The composite was fabricated by the optimized combination of the fiber coating, the matrix slurry and the pressure-sintering conditions, based on our previous composites’ study history. The composite showed the excellent tensile strength up to 1500 °C, that it retained approximately 88% of the room-temperature strength. Also, the thermal conductivity of the composites represented over 20 W/m K up to 1500 °C, which was enough high to take the advantage of the assumed design value for nuclear applications. Microstructural observation indicated that the excellent high-temperature performances regarding tensile strength and thermal conductivity up to 1500 °C were the contribution to the high densification and crystalline structure in matrix.

SiC/SiC composite and reduced activation ferritic/martensitic (RAFM) steels such as F82H are being considered as candidate structural materials for fusion reactors. Controlling the interfacial reaction between SiC and F82H is important for the development of advanced concept designs, e.g. dual coolant lead–lithium blanket. In this work, the interfacial stability between SiC and F82H was investigated. The results indicated that SiC is chemically incompatible with F82H at elevated temperature. Cracking was also observed in SiC due to the generation of large residual stress. To obtain a thermally stable interface between SiC and F82H, a proper diffusion barrier that not only suppresses the intense interfacial reaction but also reduces the residual stress between them is necessary, and three low activation materials of Si, Cr, and W for this purpose were evaluated in this paper. The results revealed that W is the most efficient diffusion barrier, although it reacted with F82H and formed brittle phase.

A diffusion bonding process, for joining of tungsten to ferritic steel using nickel as an interlayer, was developed for nuclear component application. The effect of joining temperature on the microstructure and tensile strength of the joint was investigated in this work. Metallographic analysis revealed that a good bonding was obtained at both the tungsten/nickel and nickel/steel interfaces, and the diffusion products were identified in the diffusion zone. Nano-indentation test across the joining interfaces demonstrated the effect of solid solution hardening in the diffusion zone. Tensile test showed that the maximum average tensile strength of not, vert, similar200 MPa was obtained for the joint diffusion bonded at 900 °C. The results were discussed in terms of the joining temperature and of the residual stress generated during joining process.

We explored the thermal relaxation creep characteristics of nanostructured SiC ceramics by bend stress relaxation (BSR) method. The effects of the differences in microstructure and secondary phases by liquid phase sintering at 1800 or 1900 °C were especially discussed, based on microstructural evolutions during the creep. The creep was characterized by the BSR ratio ( m ) of ∼0.80 up to 1200 °C, and the proportion of amorphous phase as a secondary phase was related to the creep resistance at 1300 °C. The microstructural evolutions during the creep consisted firstly in the re-distribution of amorphous phase, probably as a consequence of its viscous flow, and secondly in an extensive nucleation and growth of cavities. Furthermore, the former enhanced inter-diffusion of Al–Y among intergranular areas above the ternary eutectic temperature, which caused the significantly reduced creep resistance, and the latter reflected the crystalline YAG decomposition as another secondary phase near 1500 °C.

Tribological test was carried out using a pin-on-disc geometry with textured SKD11 pin on bearing steel disc, under sliding in paraffin oil. Micro-grooved crosshatch pattern has been fabricated with various angles and widths. The effects of geometrical parameters on friction were mainly examined in mixed and elastohydrodynamic lubrication. The results show that friction control can be achieved by fabricating the micro-grooved crosshatch pattern on a contact surface. It is observed that each geometrical parameter of texture influence on friction, especially decrease of groove aspect ratio and increases of groove sliding length show friction reduction performance. Crucial parameter Gl was proposed for micro-grooved crosshatch texture. The friction mechanism is explained by micro fluid flow with limited theoretical approach.

A solid-particle erosive wear test by impinging silicon carbide (SiC) powders was carried out at room temperature over a range of median particle sizes of 425–600 μm, speed of 100 m/s and impact angle of 90° and assessed by wear measurements and scanning electron microscopy. Erosive wear behaviour was examined on newly fabricated nano-powder infiltration and transient eutectoid (NITE) SiC/SiC composites and two commercial composites by the chemical vapour infiltration (CVI) and NITE fabrication route. Microstructural observation was performed to examine the correlation between erosive wear behaviours and fabrication impurities. Conspicuous defects were observed in the prototype materials as the forms of porosity, fibre deformation, residual oxide, pyrolytic carbon (PyC) deformation, PyC cleavage, among others. Erosive wear behaviour was rather serious in the prototype of fabricated composites, which employ pre-SiC fibre and phenolic resin. Two dominant erosive wear mechanisms were observed: delamination of constituents, mainly caused by erosive crack propagation, and fragmentation and detachment of constituents, which usually resulted from erosive impact. A unit size of delamination was the most decisive factor affecting wear volume. The bonding strength of each constituent was mostly affected by various forms of porosities. Therefore, the fundamental cause and subsequent results must be carefully elucidated. The correlation of microstructural defect and wear behaviour was investigated with the aim of reducing dominant wear by improving fabrication conditions. The final product of the cost-effective composite had a 2.5-fold higher resistance than the commercial CVI composite. Consequently, by controlling fabrication impurities, we have been successful in developing and improving a new fabrication technique; consequently, the known defects are rarely observed in final product. A schematic wear model of erosive wear mechanisms is proposed for the newly fabricated SiC/SiC composites under particle erosion.

This paper describes the design and examination of W/Ni double interlayer to produce a joint between SiC and ferritic stainless steel. Diffusion bonding was performed by a two steps solid state diffusion bonding process. Microstructural examination and mechanical properties evaluation of the joints show that bonding of SiC to steel was successful. EDS and XRD analysis revealed that W 5 Si 3 and WC were formed at SiC/W interface. The diffusion products at W/Ni interface, Ni-rich solid solution Ni(W) or intermetallic compound Ni 4 W, was found to be dependent on the second step joining temperature. Neither intermediate phases nor reaction products was observed at Ni/steel interface for the joints bonded at the temperature studied. The average tensile strength of 55 MPa which is insensitive to the second step process was measured for as-bonded SiC/steel joint and the failure occurred at SiC/W interface. The hardness near the various bonded interfaces was also evaluated.

Carbon nanofibers dispersed β-SiC (CNFs/SiC) nanocomposites were prepared by hot-pressing via a transient eutectic phase route at 1900 °C for 1 h under 20 MPa in Ar. The effects of additional CNFs content between 1 and 10 wt.% were investigated, based on densification, microstructure, thermal and mechanical properties. The CNFs/SiC nanocomposites by the CNFs contents below 5 wt.% exhibited excellent relative densities over 98% with well dispersed CNFs. However, the CNFs/SiC nanocomposites containing the CNFs of 10 wt.% possessed a relative density of 92%, accompanying CNFs agglomerates and many pores located inside the agglomerates. The three point bending strength gradually decreased with the increase of CNFs content, but the indentation fracture toughness increased to 5.7 MPa m1/2 by the CNFs content of 5 wt.%. The thermal conductivity was enchanced with the increase of CNFs content and represented a maximum value of 80 W/mK at the CNFs content of 5 wt.%.

Diffusion bonding between W and ferritic/martensitic steel F82H using a Ti interlayer was carried out in vacuum at temperature range of 850–950 °C for 1 h with 10 MPa. Metallographic analysis with field-emission scanning electron microscopy revealed excellent bonding at both W/Ti and Ti/F82H interfaces. The chemical compositions of the reaction products were analyzed by energy dispersive spectroscopy and their existence were confirmed by X-ray diffraction technique. α–β Ti solid solution was detected at W/Ti interface, while the reaction phases at Ti/F82H interface are dependent on the joining temperature. Joint strength was evaluated and the variations in strength of the joints were significantly related to the microstructural evolution of the diffusion zone. All the joints fractured at Ti/F82H interface during shear testing. The hardness distribution across the joining interfaces was also determined.

The microstructural development and mechanical properties of a tungsten/ferritic steel diffusion joint with a Ni interlayer, bonded at 900 °C under vacuum for 0.5–2 h, were investigated. Cross-sectional images of the W/Ni diffusion zone indicate the presence of a Ni-rich solid solution, Ni(W), for holding times up to 1.5 h. However, an intermetallic compound Ni4W grew as a distinguishable layer between the W and Ni(W) when the holding time was increased to 2 h. The growth behavior of diffusion layers and their growth mechanism is discussed. On the other hand, smooth changes in concentration of various elements across the Ni/steel interface were observed for the joints annealed at the holding time studied. An average bond strength of 215 MPa was obtained for the joint bonded for 1 h; this bond strength decreased as holding time increased. Variations in the strength of the joints was significantly related to the microstructural development of the diffusion zone. The formation of Ni4W and a solid solution phase enhanced hardness at the interfaces but reduced strength of the joints.

Unidirectional SiC/SiC composites are prepared by nano-powder infiltration and transient eutectic-phase (NITE) process, using pyrolytic carbon (PyC)-coated Tyranno-SA SiC fibers as reinforcement and SiC nano-powder with sintering additives for matrix formation. The effects of two kinds of fiber volume fraction incorporating fabrication temperature were characterized on densification, microstructure and mechanical properties. Densification of the composites with low fiber volume fraction (appropriately 30 vol%) was developed even at lower fabrication temperature of 1800 °C, and then saturated at 3rd stage of matrix densification corresponding to classic liquid phase sintering. Hence, densification of the composites with high volume fraction (above 50 vol%) became restricted because the many fibers retarded the infiltration of SiC nano-powder at lower fabrication temperature of 1800 °C. When fabrication temperature increased by 1900 °C, densification of the composites was effectively enhanced in the intra-fiber-bundles and simultaneously the interaction between PyC interface and matrix was strengthened. SEM observation on the fracture surface revealed that fiber pull-out length was accordingly changed with fabrication temperature as well as fiber volume fraction, which dominated tensile fracture behaviors. Through NITE process, SiC/SiC composites with two fracture types were successfully developed by tailoring of appropriate fabrication temperature to fiber volume fraction as follows: (1) high ductility type and (2) high strength type.

Porous silicon carbide (SiC) ceramics are being considered as functional materials for advanced energy systems due to their low thermal and electrical conductivity, low thermal-expansion coefficient, good thermal-shock resistance, and excellent mechanical and chemical stability at elevated temperature. However, conventional processing routes for SiC are complicated and conventional porous SiC shows poor mechanical and chemical stability at high temperature. Therefore, it is desirable to develop a simple fabrication method. In this study, porous SiC ceramic have been fabricated based on the NITE process, a recently developed processing technique for high performance SiC f /SiC composites. Ceramic porosity was calculated from relative and theoretical density, which was obtained by the rule of mixture. The port shape and size distribution were examined by optical microscopy and scanning electron microscopy. Mechanic properties were evaluated using three-point bend and tensile testing. Thermal conductivity was measured by the laser flash method from room temperature to 900 °C.

The new joining method of SiC components with channel was developed in this study by using hot-press. The SiC ceramics was joined by using mixed Al 2 O 3 , Y 2 O 3 , SiO 2 and SiC powders. Joining was carried out at from 1500 °C to 1900 °C for 1 h, under an applied pressure, range from 5 MPa to 20 MPa. Microstructural characterization was carried out for the joined materials by optical and scanning electron microscopy. The mechanical property of the joint was evaluated through a tensile test. The joint strength was increased with increasing joining temperature and pressure. In joining of complex shape SiC components, the serious deformation of substrate occurred because of high joining temperature and pressure. The low joining condition, In case of 1800 °C and 20 MPa, deformation of substrate not occurred. It is possible that the deformation of substrate was controlled by joining temperature. The joint layer of SiC component by using new joining method was cleaned and uniformed.

Radiation-resistant advanced silicon carbide (SiC/SiC) composites have been developed as a promising candidate of the high-temperature operating advanced fusion reactor. With the completion of the ‘proof-of-principle’ phase in development of ‘nuclear-grade’ SiC/SiC composites, the R&D on SiC/SiC composites is shifting toward the more pragmatic phase, i.e., industrialization of component manufactures and data-basing. In this paper, recent advances and issues in (1) development of component fabrication technology including joining and functional coating, e.g., a tungsten overcoat as a plasma facing barrier, (2) recent updates in characterization of non-irradiated properties, e.g., strength anisotropy and chemical compatibility with solid lithium-based ceramics and lead-lithium liquid metal breeders, and (3) irradiation effects are specifically reviewed. Importantly high-temperature neutron irradiation effects on microstructural evolution, thermal and electrical conductivities and mechanical properties including the fiber/matrix interfacial strength are specified under various irradiation conditions, indicating seemingly very minor influence on the composite performance in the design temperature range.

Two kinds of SiC fiber-reinforced SiC (SiC/SiC) composites (high strength type and high ductility type) were prepared by NITE process and their thermo-mechanical properties were investigated from room to high temperature. Furthermore, thermal stress figure of merit of advanced SiC/SiC composites including prepared NITE-SiC/SiC composites were discussed. Matrix densification, especially inter-fiber-tows, was effective way to restrict the oxygen diffusion through pores and hence fiber and matrix interface was protective against the oxidation. In NITE process, excess additional additives for matrix densification, like high ductility type, caused weight loss with pore formation derived from the oxide segregations over 1300 °C. There was no degradation in strength by 1500 °C and simultaneously high thermal conductivity was kept up to 1500 °C for high strength type, resulting in the excellent thermal figure of merit.

In order to improve the thermo-mechanical performances of SiC/SiC composite, process improvement and modification by the combination of nano-infiltration and transient eutectic-phase (NITE) method and chemical vapor infiltration (CVI) method were studied. Multilayered PyC/SiC fiber coating and matrix infiltration within fiber-tows were prepared with isothermal/isobaric CVI (ICVI) method and full-densification of SiC matrix was examined with NITE methods using four kinds of processing options. Applied pressure was useful for nearly-full matrix densification due to the promoting infiltration driving force of SiC nano-powder intra-fiber-tows, but simultaneously caused the sever degradation of fibers and interphase with fracture, resulting in lower strength. Increase of additives amount and additional polymer were effective ways for matrix densification by SiC nano-power infiltration intra-fiber bundles without pressure. Thermal conductivity was greatly improved with the decrease of matrix porosity. The tailoring of thermo-mechanical properties might be easily controlled by the SiC matrix porosity without process-induced fibers and interphases degradations.

Thermal and mechanical stabilities were investigated on Hi-Nicalon SiC fibers under annealing and creep in various oxygen partial pressures by mass change, mechanical properties as well as microstructural features. In the case of mechanical stability, the tensile strength of fiber is strongly dependent on the oxygen partial pressure of testing environment, but a weak dependence of BSR creep resistance on oxygen partial pressure is appeared. The analyses of surface morphology and mass change indicated the thermal stability of fiber under annealing in different environments was different. At different exposure temperatures and oxygen partial pressure levels, the different oxidation regimes are responsible for the strength degradation and microstructure change of fibers. Furthermore, the microstructure change is also affected by the stress applied through BSR test. This means the thermal stability of fibers is related to not only the exposed environments, but also the mechanical state of fibers.

Unidirectional SiC/SiC composites were prepared by Nano-Infiltration and Transient Eutectic-phase (NITE) process using SiC nano-powder infiltration technique, and the effects of pyrolytic carbon (PyC) interface thickness between fibers and matrix on density, microstructural evolution and mechanical properties were characterized. SiC fibers both with and without PyC interface were employed as reinforcement and SiC nano-powder was employed for matrix formation with 12 mass% sintering additives of the total powder. The thickness of PyC layer deposited by chemical vapor deposition (CVD) process was highly-accurately controlled at about 0.25, 0.50 and 1.00 μm. Nearly full-dense SiC/SiC composites with uncoated fibers caused strong interaction between fibers and matrix, resulting in a brittle fracture behavior without fiber pull-out. Higher strength with a pseudo-ductile fracture behavior could be obtained using 0.50 μm of PyC interface thickness, where a lot of deflects and branches of the propagating cracks and fiber pull-out were observed. Induced PyC interface conditions strongly affect the density, microstructural evolution, and therefore dominate mechanical properties and fracture behaviors.

The objective of this work is to develop manufacturing method of porous SiC ceramics with mechanical and chemical stability. The nanoinfiltration transient eutectic (NITE)-porous SiC ceramics were fabricated by decarburization of the hot-pressed NITE-SiC including carbon particles. Porosity could be controlled at less than ±0.5% by the amount of carbon particles. The NITE-porous SiC ceramics exhibited a substantially high strength in comparison with other conventional porous SiC ceramics, due to its robust microstructure consisted of spherical pores.

The F82H and Eurofer97 steel samples were irradiated at 300 K with 20 MeV W ions to the damage level of 0.54 displacements per atom (dpa) at the damage peak. Additionally, the F82H steel samples were irradiated at 523 K with 6.4 MeV Fe ions to various damage levels in the range from 0.02 to 12.5 dpa. The damaged samples were exposed to D2 gas at a pressure of 100 kPa and various temperatures in the range from 373 to 573 K for a certain length of time sufficient to fill ion-induced defects with deuterium. Trapping of deuterium at the ion-induced defects was examined by the D(3He, p)4He nuclear reaction with 3He energies between 0.69 and 4.0 MeV allowing determination of the D concentration up to a depth of 7 μm. It has been found that (i) at the damage level above 0.5 dpa, the concentration of the ion-induced defects responsible for trapping of diffusing D atoms does not depend practically on the numbers of displacements per atom, and (ii) the saturation value of the D concentration in the damage zone decreases with increasing D2 gas exposure temperature, Texp, and varies from about 10−1 at.% at Texp = 373 K to 10−3 at.% at Texp = 573 K. The deuterium-trap binding energy is estimated to be 0.7 ± 0.2 eV.

SiC nano-sized powder with high specific surface area is potentially of considerable interest to form fully dense SiC ceramics at lower sintering conditions (temperature/pressure). Surface structure of six kinds of commercially available SiC nano-sized powders produced by three different venders was analyzed in detail by X-ray photoelectron spectroscopy (XPS). The overall XPS spectra of all nano-sized powders detected O-based bond (O1s peak), C-based bond (C1s peak) and Si-based bond (Si2s and Si2p peak). Surface structure of nano-sized powders included one of three impurity phases: (1) free carbon, (2) silica and (3) silicon oxycarbide. Furthermore, the influences of surface structure by XPS were systematically investigated on basic powder characteristics, such as chemical composition, morphology, particle density and primary particle size. It was revealed that the basic powder characteristics had a close relationship to the surface structure of nano-sized powder each impurity.

The effects of fast neutron irradiation on SiC and SiC composites have been studied. The materials used were chemical vapor deposition (CVD) SiC and SiC/SiC composites reinforced with either Hi-Nicalon™ Type-S, Hi-Nicalon™ or Sylramic™ fibers fabricated by chemical vapor infiltration. A statistically significant population of flexural samples were irradiated up to 4.6 × 1025 n/m2 (E > 0.1 MeV) at 300, 500, and 800 °C in the High Flux Isotope Reactor at Oak Ridge National Laboratory. Dimensions and weights of the flexural bars were measured before and after the neutron irradiation. Mechanical properties were evaluated by four point flexural testing. Volume increase was seen for all bend bars following neutron irradiation. The magnitude of swelling depended on irradiation temperature and material, while it was nearly independent of irradiation fluence over the fluence range studied. Flexural strength of CVD SiC increased following irradiation depending on irradiation temperature. Over the temperature range studied, no significant degradation in mechanical properties was seen for composites fabricated with Hi-Nicalon™ Type-S, while composites reinforced with Hi-Nicalon™ or Sylramic fibers showed significant degradation. The effects of irradiation on the Weibull failure statistics are also presented suggesting a reduction in the Weibull modulus upon irradiation. The cause of this potential reduction is not known.

To indicate the feasibility of utilizing newly developed NITE-SiC/SiC composite materials in a fusion reactor, R&D on the NITE process with near-net shape forming has been carried out. In order to establish the large scale production of NITE-SiC/SiC, pilot grade NITE-SiC/SiC was fabricated and the baseline properties were evaluated. As the key elements, nano-powder fabrication and Tyranno-SA, SAK fabrication are extensively being developed. NITE-SiC/SiC composites of a cylindrical shape were also fabricated by a near-net shape process called pseudo-HIP, which was a new type HIP using a carbon powder as the pressure transmitter. The microstructure of NITE-SiC/SiC composites, such as fiber volume fraction, porosity and type of pores, can be controlled precisely. This makes it possible to produce test blanket modules (TBMs) or other components with proper thermal conductivity in response to the requirements of fusion reactor design.

Frank loop evolution in highly damaged polycrystalline cubic silicon carbide irradiated with 5.1 MeV Si 2+ ions at 1673 K was studied by transmission electron microscopy (TEM). Individual TEM images of Frank loops formed on each {1 1 1} plane revealed that their population strongly depended upon their orientation with respect to the incident beam direction. However, no significant difference in loop growth rates was observed between each habit plane. The anisotropic loop evolution has not been reported for neutron-irradiated SiC. Our examination shows that a grain, containing {1 1 1} planes nearly parallel to the irradiated surface, was largely strained toward the free surface by the preferential formation of Frank loops on the plane. Compressive stress following the anisotropic swelling in ion-irradiated specimen may affect the loop evolution. The possible mechanism of the anisotropic loop evolution observed here is accounted for by the stress induced preferential nucleation of Frank loops.

Charged-particle irradiation to highly pure SiC was performed to investigate the influence of irradiation-induced defects on its fracture behavior. To investigate the fracture behavior of SiC, the microstructural analysis was carried out by observing cross-sections of a residual indentation impression. An apparent increase of indentation fracture toughness was shown in the highly pure SiC upon ion irradiation. In the case of ion-irradiated SiC, a number of micro-cracks were created around the indentation-induced deformation region. Indentation-induced slip bands on the (1 1 1) crystallographic plane were observed in both the virgin and the ion-irradiated SiC. The indentation-induced slip bands and the pre-existing defects provided one of the sites for micro-crack creation for the ion-irradiated SiC. The formation of the micro-cracks may dissipate the fracture energy for brittle fracture, which seems to be one of the mechanisms for the toughening behaviors of SiC by an energetic particle irradiation.

The bend stress relaxation technique was applied for an irradiation creep study of high purity, chemically vapor-deposited beta-phase silicon carbide (CVD SiC) ceramic. A constant bend strain was applied to thin strip samples during neutron irradiation to fluences 0.2–4.2 dpa at various temperatures in the range ∼400 to ∼1080 °C. Irradiation creep strain at 0.7 dpa were estimated to be 2.7(±2.6) × 10 −7 and 1.5(±0.8) × 10 −6 (MPa dpa) −1 at ∼600 to ∼950 °C and ∼1080 °C, respectively, whereas linear-averaged creep compliances of 1–2 × 10 −6 (MPa dpa) −1 were obtained for doses of 0.6–0.7 dpa at all temperatures. Monocrystalline 3C SiC samples exhibited significantly smaller transient creep strain and greater subsequent deformation when loaded along 〈0 1 1〉 direction.

The effect of neutron irradiation on tensile properties in advanced 2D-SiC/SiC composites was evaluated. The composites used were composed of a SiC matrix obtained by the forced-flow chemical vapor infiltration (FCVI) process and either Tyranno™-SA Grade-3 or Hi-Nicalon™ Type-S fibers with single-layered PyC coating as the interphase. Neutron irradiation fluence and temperature were 3.1 × 10 25 n/m 2 ( E > 0.1 MeV) and 1.2 × 10 26 n/m 2 at 740–750 °C. Tensile properties were evaluated by cyclic tensile test, and hysteresis loop analysis was applied in order to evaluate interfacial properties. Both composites exhibited excellent irradiation resistance in ultimate and proportional limit tensile stresses. From the hysteresis loop analysis, the level of interfacial sliding stress decreased significantly after irradiation to 1.5 × 10 26 n/m 2 at 750 °C.

Tensile properties of unidirectionally reinforced Hi-Nicalon™ Type S SiC fiber, chemically vapor-infiltrated (CVI) SiC-matrix composites, with either pyrolytic carbon (PyC) or multilayered PyC/SiC interphase, were characterized following neutron irradiations to the maximum fluence of 7.7 × 1025 n/m2 at 380 and 800 °C. The stress–strain behavior of the multilayered interphase composites remained unmodified after irradiation. The PyC interphase composite increased in ultimate tensile stress and strain to failure following neutron irradiation, whereas the proportional limit stress exhibited a slight decrease. Potential mechanisms for these changes include accommodation of misfit stress through irradiation creep, reduced interfacial friction, and differential swelling among individual composite constituents.

Loading tests were performed on SiC fiber yarns by applying different dead loads at elevated temperatures in Ar atmosphere. After each loading test, the room temperature tensile properties as a function of load, exposure time and temperature, were evaluated on these fibers. Obvious strength degradation was occurred under the combined effect of exposure temperature and applied load. Meanwhile, the near stoichiometric SiC fiber, Hi-Nicalon™ Type-S, showed higher strength retention than did Hi-Nicalon™ fiber which contains excess carbon and small amount of amorphous phase. The microstructural observation revealed that the oxidation and loading accelerated new flaw nucleation and growth resulting in stress corrosion cracks. The latter acted as critical flaws and could be responsible for the strength degradation.

Four different shear test methods i.e. doubled notched shear test, asymmetrical four point bend test, Iosipescu test, and torsion test, were investigated for their ability to evaluate one standard SiC to SiC ceramic brittle joint while using small size specimens. Double notched shear test showed higher stress concentration at the notch base and a lower nominal shear strength. Both asymmetrical four point bend test and Iosipescu test utilized epoxy jointed metal extensors, which failed during test and caused misalignment and tensile type of failure. Torsion test can deliver true shear loading. However, base material failure was observed for the torsion joint samples in this study. None of the tests can successfully induce true shear failure of the joint because the joint is stronger and tougher than the SiC substrate. Torsion test appears to be promising because of the pure shear loading, less stress concentration, and easy alignment.

Chemically vapor infiltrated (CVI) silicon carbide (SiC) matrix composites with uni-directional and various two- and three-dimensional reinforcements by the near-stoichiometric SiC fibers or SiC/graphite hybrid fabrics were produced and evaluated for tensile, thermal and electrical properties. The parallel-serial approach models of these properties reasonably explained the experimental results. The experimental data and the model-based analysis suggested that, for the composite systems studied: (1) the strength properties are determined primarily by the volume fraction of longitudinal fibers, approximately following the theory that assumes the global load sharing; (2) presence of the axial fiber tows is the key factor in providing high thermal conductivity; (3) the maximum and minimum post-irradiation through-thickness conductivity of 10-15 W/m K at 800-1000 °C for 3D architecture and

Advanced SiC/SiC composites composed of highly-crystalline and near-stoichiometric SiC fibers, SiC matrix by CVI method, and PyC interphase were irradiated up to 1:0 � 10 25 n/m 2 (E > 0:1 MeV, � 1 dpa) at 1273 K. Unload/reload cyclic tensile test and hysteresis loop analysis were carried out in order to identify neutron irradiation effects on interfacial properties of advanced SiC/SiC. The composites exhibited good irradiation resistance in ultimate tensile strength (� 10% increase), but there were slight reduction of elastic modulus and proportional limit stress (PLS) (� 10%). Wider hysteresis loops and lower gradient of the curves beyond PLS from the strain–stress curves, and longer pullouts of fiber from the SEM observation were observed, which imply weaker F/M interactions after neutron irradiation. From the hysteresis loop analysis, degradation of interfacial sliding stress and negative increment of misfit stress after neutron irradiation were obtained.

Dense SiC ceramics fabricated by NITE process (NITE-SiC), using SiC nano-powder, were subjected to exposure tests from 1000 to 1800 � C in an argon-oxygen gas mixture with an oxygen partial pressure of 0.1 Pa. The thermal stability of NITE-SiC was examined through mass change, 3-point bending test, XRD analysis and TEM/SEM observation. The NITE-SiC presented excellent bending strength (above 800 MPa) up to � 1800 � C while the conventional liquid-phase sintered SiC ceramics (LPS-SiC), using SiC sub-micron powder, indicated severe degradation at 1300 � C due to volatilization and softening of amorphous grain boundary phase. The in situ 3-point bending test at 1300 � C was carried out to evaluate in-service fracture behavior, where excellent improvements in bending strength, elastic modulus and fracture behavior were confirmed, compared with the conventional LPS-SiC. These are interpreted by the modification with reduction and crystallization of grain boundary phase (Y3Al5O12). The decomposition of Y3Al5O12 were caused by the reactions with CO gas on the surface of NITE-SiC exposed at 1800 � C, but the severe degradation was not identified in strength.

Nous faisons le point des effets de l'irradiation par les neutrons sur les composites SiC/SiC et nous discutons les proprietes de ces composites necessitees par les applications nucleaires. Les effets d'irradiations a haute temperature, jusque 1600 °C, qui n'ont pas ete determines par irradiations aux neutrons, ont ete evalues en utilisant des irradiations par les ions, dans le dispositif DuET de l'Universite de Kyoto. Les effets de l'irradiation sur le gonflement et sur les proprietes mecaniques ont ete determines, et l'on a mis en evidence l'excellente resistance a l'irradiation a haute temperature du SiC de haute purete. La situation actuelle des composites SiC/SiC elabores par le procede NITE (Nano-powder Infiltration and Transient Eutectic-phase) en vue d'applications nucleaires, a cause en particulier de la formation d'une matrice hautement cristallisee, est passee en revue. Le procede NITE a ete perfectionne en vue des applications industrielles. On a ameliore les composites SiC/SiC prepares a l'echelle pilote et une resistance a la traction de 400 MPa a ete obtenue. La technique a ete appliquee a la fabrication de tubes aussi denses que les plaques initialement produites, et a ete utilisee avec succes pour des assemblages et des revetements de surface.

Tungsten (W) and molybdenum (Mo) were coated on silicon carbide (SiC) for use as a refractory armor using a high power plasma arc lamp at powers up to 23.5 MW/m 2 in an argon flow environment. Both tungsten powder and molybdenum powder melted and formed coating layers on silicon carbide within a few seconds. The effect of substrate pre-treatment (vapor deposition of titanium (Ti) and tungsten, and annealing) and sample heating conditions on microstructure of the coating and coating/substrate interface were investigated. The microstructure was observed by scanning electron microscopy (SEM) and optical microscopy (OM). The mechanical properties of the coated materials were evaluated by four-point flexural tests. A strong tungsten coating was successfully applied to the silicon carbide substrate. Tungsten vapor deposition and pre-heating at 5.2 MW/m 2 made for a refractory layer containing no cracks propagating into the silicon carbide substrate. The tungsten coating was formed without the thick reaction layer. For this study, small tungsten carbide grains were observed adjacent to the interface in all conditions. In addition, relatively large, widely scattered tungsten carbide grains and a eutectic structure of tungsten and silicon were observed through the thickness in the coatings formed at lower powers and longer heating times. The strength of the silicon carbide substrate was somewhat decreased as a result of the processing. Vapor deposition of tungsten prior to powder coating helped prevent this degradation. In contrast, molybdenum coating was more challenging than tungsten coating due to the larger coefficient of thermal expansion (CTE) mismatch as compared to tungsten and silicon carbide. From this work it is concluded that refractory armoring of silicon carbide by Infrared Transient Liquid Phase Processing is possible. The tungsten armored silicon carbide samples proved uniform, strong, and capable of withstanding thermal fatigue testing.

SiC-composites are candidate structural materials for high temperature applications such as gas turbines. For this purpose, a suitable coating against high temperature oxidation is essential. The coating material selection was made by phase stability/durability under service environment, CTE matching, low elastic modulus and chemical compatibility with SiC. This work is to evaluate the microstructural quality of mullite and erbium silicate coatings on SiC fiber bonded composites. In this work, SiC-fiber bonded 2D-composite (TyrannohexTM) by Ube Industries Ltd., were coated with mullite and erbium silicate by plasma spraying. The thickness of mullite and erbium silicate coatings was determined to be 120 microns and 220 microns, respectively. The fabricated samples were diamond polished for cross-sectional analyses by optical and scanning electron microscopy. The coating-substrate interfaces have been found bit undulating. The bonding between mullite and the substrate was found insufficient as compared to that observed in the erbium silicate coated substrate. The cross-sectional analyses of erbium silicate coating revealed the presence of throughthickness micro- and macro-cracks. The width of the macrocracks was found in the range of 1-3 microns. It is conceived that the evolution of large concentration of through-thickness-cracks is due to the stresses in the coating. The most probable sources of stresses are the precipitation of various second phases in the coating materials and the thermal expansion mismatches that arise during cooling. The mullite coating on the other hand is not found with macrocracks and on the whole the cohesion within the mullite coating is rather strong. The cross-sectional examination also revealed the irregularly distributed pores and cavities in both coating systems that are typical of thermal sprayed coatings. The surfaces of the coatings are characterized of typical lamellar microstructure formed by the flattened platelets of individual particles. The size of these droplets ranges from submicrometer to several micrometers.

Irradiation damage produced by neutrons or energetic particles lead to changes of physical- and mechanical-properties of SiC. Radiation hardening and fracture toughness changing of SiC were clarified by indentation method previously. However, the mechanism studies have received little alteration. The purpose of this study is to improve the understanding of the mechanisms of mechanical property changes under irradiation. In this paper, the microstructural observation beneath and near an indentation will be used to infer mechanisms of radiation hardening and toughening. Indenting polycrystalline SiC creates deformation and cracking in the plastically deformed region. In the case of irradiated SiC, however, small-sized deformation zone was observed below contact indent, which resulted in the restricted size of residual impression. Additionally, the indentation cracks showed complex propagation behaviors such as deflecting, branching and microcracking.

The microstructural evolution of SiC/SiC composites after Si{sup 2+} with/without He{sup +} ion irradiation was studied using transmission electron microscopy. The temperature, displacement damage level, and He/dpa ratio were 1273/1673K, 10/100dpa and 0/60appmHe/dpa, respectively. In 10dpa single-ion irradiation, no cavity was detected at 1273 and 1673K. But cavities were observed locally at 1673K, 100dpa. In dual-ion irradiation, cavities were observed at 1673K, 100dpa. Helium bubbles (d