Nano-infiltration And Transient Eutectic-phase Research Articles

Effect of the fabrication process on the microstructural evolution of carbon fibers and flexural property on C/SiC composites by the NITE method

Nano-infiltration and transient eutectic phase (NITE) SiC matrix composites are designed for application in aerospace propulsion systems, particularly in fasteners and thrusters. A variety of carbon fibers with different properties have been selected as reinforcements for SiC matrix composites. Carbon fibers are known to be stable at high temperatures; however, the effects of high applied pressure at high temperatures on the fiber microstructure evolution and mechanical properties are not well-known. As a scoping study for fabricating NITE C/SiC composites, the behaviors of various carbon fibers in SiC composites. Pitch-based fibers, namely, GRANOX XN-05 and YS-90A, and a polyacrylonitrile-based fiber, namely, TORAYCA T-300B, were selected for matrix reinforcement. The 3-point bending test results indicated pseudo-ductile behaviors in the cases of YS-90A and T-300B fiber reinforcements. Fracture resistance evaluation based on the single-notch bending test indicated that the YS-90A fiber reinforced composite afforded the highest fracture resistance among the three C/SiC composites. The microstructure evolution on YS-90A and T-300B fibers was limited to near the fiber surface. Therefore, YS-90A and T-300B carbon fibers are potential candidates for reinforcement in NITE C/SiC composites.

Silicon dissolution and morphology modification of NITE SiC/SiC claddings in pressurized flowing water under neutron irradiation

Claddings of silicon carbide (SiC)-based composites for light water reactors are expected to be a key component of accident-tolerant fuel technology for enhancing nuclear safety. Muroran Institute of Technology has developed technologies for producing SiC/SiC claddings by a nano-infiltration and transient eutectic phase (NITE) method in the MEXT-funded project named “SCARLET.” As part of this program, neutron irradiation experiments were performed in a pressurized cooling water flow at the Halden Reactor in Norway. During the irradiation experiments, silicon dissolution from the SiC/SiC specimens was measured, and the relationship among the dissolution, properties of SiC/SiC claddings and reactor operating conditions was studied. The morphology of the SiC/SiC claddings was not significantly modified by the irradiation experiments; however, the joint parts were corroded and suffered damage. Observation of irradiated specimens and a rough estimate of the SiC dissolution rate under each irradiation condition indicated that the NITE SiC/SiC composite has potential as a structural material for pressurized water reactors (PWRs) that are operated under hydrogen water chemistry conditions. It is also shown that chemical-vapor-deposited (CVD)-SiC and other coating techniques are necessary for the realization of SiC reactor core. Joining and coating technologies were also tested, but appropriate methods could not be identified experimentally; thus, joining and coating techniques should be further developed.

Residual stress variation in SiCf/SiC composite during heat treatment and its effects on mechanical behavior

Residual stress originated from thermal expansion mismatch determines the mechanical properties of ceramic matrix composites (CMCs). Here, continuous SiC fiber reinforced SiC matrix (SiCf/SiC) composites were fabricated by nano-infiltration and transient eutectic-phase (NITE) method, and the residual stress of the composites was investigated using high-temperature Raman spectrometer. With temperature increasing from room temperature to 1400 °C, the residual stresses of the matrix and the fiber decrease from 1.29 to 0.62 GPa and from 0.84 to 0.55 GPa in compression respectively, while that of the interphase decreases from 0.16 to 0.10 GPa in tension. The variation of residual stress shows little effect on the tensile strength of the composites, while causes a slight decrease in the tensile strain. The suppression of fiber/matrix debonding and fiber pulling-out caused by the residual stress reduction in the interphase is responsible for the decreasing tensile strain. This work can open up new alternatives for residual stress analysis in CMCs.

Nano-infiltration and transient eutectic (NITE) phase joining SiC ceramics at 1500oC

By using nano-infiltration and transient eutectic (NITE) phase with nanosized SiC and the addition of CeO2–Al2O3 as joining material, liquid-phase-sintered SiC ceramics were joined at the relatively low temperature of 1500 °C and 1700 °C. For comparison purpose, Al2O3–Y2O3 and SiO2–Al2O3–Y2O3 were used as the additives to join SiC ceramics. The shear strength of SiC assembly with the interlayer thickness of ~70 μm and addition of CeO2–Al2O3 after joining at 1700 °C was measured to be 163.9 MPa but decreased to 57.4 MPa when joining at 1500 °C. It was revealed that the joining status could be improved by decreasing the thickness of the interlayer to ~35 μm and SiC joint finishing at 1500 °C possessed the comparatively high shear strength of 113.2 MPa. NITE phase containing other additives (Al2O3–Y2O3 and SiO2–Al2O3–Y2O3) showed the much lower shear strength after joining at 1500 °C.

Appropriate thickness of pyrolytic carbon coating on SiC fiber reinforcement to secure reasonable quasi-ductility on NITE SiC/SiC composites

Pyrolytic carbon (PyC) coating of silicon carbide (SiC) fibers is an important technology that creates quasi-ductility to SiC/SiC composites. Nano-infiltration and transient eutectic-phase (NITE) process is appealing for the fabrication of SiC/SiC composites for use in high temperature system structures. However, the appropriate conditions for the PyC coating of the composites have not been sufficiently tested. In this research, SiC fibers, with several thick PyC coatings prepared using a chemical vapor infiltration continuous furnace, were used in the fabrication of NITE SiC/SiC composites. Three point bending tests of the composites revealed that the thickness of the PyC coating affected the quasi-ductility of the composites. The composites reinforced by 300 nm thick coated SiC fibers showed a brittle fracture behavior; the composites reinforced 500 and 1200 nm thick PyC coated SiC fibers exhibited a better quasi-ductility. Transmission electron microscope research revealed that the surface of the as-coated PyC coating on a SiC fiber was almost smooth, but the interface between the PyC coating and SiC matrix in a NITE SiC/SiC composite was very rough. The thickness of the PyC coating was considered to be reduced maximum 400 nm during the composite fabrication procedure. The interface was possibly damaged during the composite fabrication procedure, and therefore, the thickness of the PyC coating on the SiC fibers should be thicker than 500 nm to ensure quasi-ductility of the NITE SiC/SiC composites.

Microstructure and hydrothermal corrosion behavior of NITE-SiC with various sintering additives in LWR coolant environments

Nano-infiltration and transient eutectic phase (NITE) sintering was developed for fabrication of nuclear grade SiC composites. We produced monolithic SiC ceramics using NITE sintering, as candidates for accident-tolerant fuels in light-water reactors (LWRs). In this work, we exposed three different NITE chemistries (yttria-alumina [YA], ceria-zirconia-alumina [CZA], and yttria-zirconia-alumina [YZA]) to autoclave conditions simulating LWR coolant loops. The YZA was most corrosion resistant, followed by CZA, with YA being worst. High-resolution elemental analysis using scanning transmission electron microscopy (STEM) X-ray mapping combined with multivariate statistical analysis (MVSA) datamining helped explain the differences in corrosion. YA-NITE lost all Al from the corroded region and the ytttria reformed into blocky precipitates. The CZA material lost all Al from the corroded area, and the YZA − which suffered the least corrosion −retained some Al in the corroded region. The results indicate that the YZA-NITE SiC is most resistant to hydrothermal corrosion in the LWR environment.

Evaluation of strength anisotropy and fracture behavior of UD NITE-SiC/SiC composites with various fiber orientations

A SiC/SiC composite by Nano-infiltration and Transient Eutectic-phase (NITE) process is the attractive candidate materials for advanced energy systems, aero-space system. The NITE process is improved to the industrialization grade process from the laboratory grade process. In order to ensure reliability of products by NITE-SiC/SiC composites, understanding of the strength anisotropy is important. This paper presented the basic strength anisotropy knowledges of unidirectional (UD) type NITE-SiC/SiC composites with various fiber orientations by correlation evaluation of microstructure and mechanical properties. Also, the strength anisotropy evaluation of UD NITE-SiC/SiC composites utilizing the strength anisotropy prediction theories were discussed. The axial/off-axial mechanical properties of UD NITE-SiC/SiC composites by the industrialization grade process were evaluated by axial/off-axial tensile test. The mechanical properties of NITE-SiC/SiC composites fabricated tended to decrease with increasing of fiber orientation angle. The experiment results by axial/off-axial tensile test were consistent with the strength anisotropy prediction theories by the maximum normal stress theory and the Tsai-Hill criterion. Also, the failure mode of SiC/SiC composites fabricated with each fiber orientation angle was consistent with fracture surface observation results. The strength anisotropy of UD NITE-SiC/SiC composites was suggested to be able to predict by Tsai-Hill criterion. The basic strength anisotropy of UD SiC/SiC composites was understood.

Microstructural modification of W–SiC bonded plate during high temperature exposure

R&D and technology integration of high performance materials for plasma facing components (PFCs) are important to realize in fusion reactor. Tungsten is becoming a prime candidate as a PFCs material because of the low plasma spatter rate, low hydrogen retention, and the high melting point. The utilization of tungsten is expected to suppress the plasma contamination. Although ITER employs full tungsten divertors, many issues are still remaining and need to be researched for fusion reactor after DEMO. The SiC/SiC composite is a candidate material for components of after DEMO reactor, and especially, SiC/SiC composites fabricated by Nano-Infiltration and Transient Eutectic phase (NITE) method have various advantages. From the view of divertor structural materials, because of small mismatch in coefficient of thermal expansion between tungsten and SiC is a great advantage, the application of SiC/SiC composites armored by tungsten will be an attractive option of divertor concepts in future. This paper provides the stability of W–SiC bonded plate at high temperature. The W–Si and W–C phases are observed around the interface of W–SiC bonded plate. Microstructural modification of W–SiC interface before and after high temperature exposure test is characterized using shear test and microscopes.

Effects of neutron irradiation on mechanical properties of silicon carbide composites fabricated by nano-infiltration and transient eutectic-phase process

Unidirectional silicon carbide (SiC)-fiber-reinforced SiC matrix (SiC/SiC) composites fabricated by a nano-infiltration and transient eutectic-phase (NITE) process were irradiated with neutrons at 600°C to 0.52dpa, at 830°C to 5.9dpa, and at 1270°C to 5.8dpa. The in-plane and trans-thickness tensile and the inter-laminar shear properties were evaluated at ambient temperature. The mechanical characteristics, including the quasi-ductile behavior, the proportional limit stress, and the ultimate tensile strength, were retained subsequent to irradiation. Analysis of the stress–strain hysteresis loop indicated the increased fiber/matrix interface friction and the decreased residual stresses. The inter-laminar shear strength exhibited a significant decrease following irradiation.

Mechanical properties of NITE–SiC/SiC composites fabricated after neutron irradiation

UD–SiC/SiC composite is SiC matrix reinforced with near-stoichiometric silicon carbide (SiC) fiber reinforcements, pyrolytic carbon (PyC) layer interphases. The composite was fabricated by nano infiltration and transient eutectic phase (NITE) process, and the specimens were irradiated under a flux of 7.0×1025n/m2 that corresponded for the SiC composites under the adopted experimental conditions to a damage dose of “7” dpa, with E>0.1MeV at high exposure temperatures of 1073K and 1573K, and loading/reloading test for tensile properties at room temperature. The elastic modulus has insignificant changes after irradiation, which can be considered by the irradiation resistance of sintering additives in NITE–SiC matrix. The misfit stress presents obvious effects of neutron irradiation on matrix cracking in a relatively thick specimen, but it is difficult to recognize the detrimental factor on the matrix cracking in a thin specimen. The interfacial sliding properties of thick specimens might be also influenced by neutron irradiation. Moreover, the improvement of UTS might be caused by the weakened interfacial layer after neutron irradiation, and the effective dimension of specimens for irradiation and tensile tests are guaranteed to obtain high reliable data.

Effects of Pressure during Preform Densification on SiC/SiC Composites

The nano-infiltration and transient eutectic-phase (NITE) method is one of the most attractive methods for fabrication of SiC and SiC/SiC composites. In the NITE method, preform densification is essential option for damage less near-net shaping technique. However, optimization of preform densification is insufficient yet. The objective of this study is to evaluate the effects of pressure during preform densification on SiC/SiC composites. The preform before preform densification has many pores in the inter-prepreg sheets. These pores were disappeared by preform densification. As the effects of pressure on preform, densification in the intra-fiber-bundle was improved due to increasing pressure. Flexural strength of the preforms with 1 MPa and 17 MPa indicated almost same value. The result suggested that increasing of pressure did not cause any change in fiber properties. In the effects of pressure on the composites, the composites with 17 MPa was exhibited improvement in bulk density and mechanical property, compared with that with 1 MPa.

Effects of Preform Densification on Near-Net Shaping of NITE-SiC/SiC Composites

Large volumetric shrinkage (~50 Vol%) occurs during ceramic matrix composites fabrication by hot-pressing due to infiltration and densification process of powder for matrix formation, resulting in unfortunately significant fiber-architecture and -strength damage. This study tries to explore damage-less near-net shaping technique by preform densification before hot-pressing in Nano-Infiltration and Transient Eutectic-phase (NITE) process. In particular, effects of preform densification for the damage-less near-net shaping, important influencing elements such as fiber-architecture, microstructural integrity and homogeneity, and composite's mechanical properties were evaluated using simple shaped (plate) and complex shaped composites. The preform densification demonstrated protective fiber-architecture in complex shaped composites and enhanced composites' density to 2.77/cm3 and ultimate bending strength to ∼200 MPa in simple shaped composites, owing to significantly reduced volumetric shrinkage.

Contact angle measurement of molten lead–lithium on silicon carbide surfaces

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.

プリフォーム高密度化処理によるSiC/SiC複合材料の微細組織および強度特性への影響

As an advanced liquid phase sintering method of SiC/SiC composites production, Nano-Infiltration and Transient Eutectic-phase (NITE) process has been intensively studied, where large volumetric shrinkage during sintering process is an issue to be solved. The objective of this work is to investigate the effects of preform densification on microstructure and mechanical properties of SiC/SiC composites fabricated by hot-press of preforms. The significant improvement seen in preform is the suppression of macro-pores at inter-/intra- fiber-bundles. Finally, this is contributing the reduction of volumetric shrinkage to suppress fiber damage and micro pore formation and eliminate macro pore formation after sintering. And, the composites fabricated presents microstructure homogeneity. The improvement in structure is reflected in high flexural strength. However, pseudo-ductility was worth than anticipated. The brittle behavior of the composites is interpreted to be caused by the damage of PyC interphase.

Development of the tailored SiC/SiC composites by the combined fabrication process of ICVI and NITE methods

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.

Influence of pyrolytic carbon interface thickness on microstructure and mechanical properties of SiC/SiC composites by NITE process

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.

SiC/SiC composites through transient eutectic-phase route for fusion applications

Factors that may limit attractiveness of silicon-carbide-based ceramic composites to fusion applications include thermal conductivity, applicable design stress, chemical compatibility, hermeticity, radiation stability and fabrication cost. A novel SiC/SiC composite, which has recently been developed through nano-infiltration and transient eutectic-phase (NITE) processing route, surpasses conventional materials in many of these properties. In this paper, the latest development, property evaluation and prospect of the NITE SiC/SiC composites are briefly reviewed. The topics range from fundamental aspects of process development to industrial process development. Elevated temperature strength, fracture behavior, and thermo-physical properties in various environments are summarized. Future directions of materials and application technology development are also discussed.