Mechanical property degradation of high crystalline SiC fiber–reinforced SiC matrix composite neutron irradiated to ∼100 displacements per atom

For the development of silicon carbide (SiC) materials for next-generation nuclear structural applications, degradation of material properties under intense neutron irradiation is a critical feasibility issue. This study evaluated the mechanical properties and microstructure of a chemical vapor infiltrated SiC matrix composite, reinforced with a multi-layer SiC/pyrolytic carbon–coated Hi-Nicalon^TM Type S SiC fiber, following neutron irradiation at 319 and 629?°C to ～100 displacements per atom. Both the proportional limit stress and ultimate flexural strength were significantly degraded as a result of irradiation at both temperatures. After irradiation at 319?°C, the quasi-ductile fracture behavior of the nonirradiated composite became brittle, a result that was explained by a loss of functionality of the fiber/matrix interface associated with the disappearance of the interphase due to irradiation. The specimens irradiated at 629?°C showed increased apparent failure strain because the fiber/matrix interphase was weakened by irradiation-induced partial debonding.     

Damage development of sintered SiC ceramics with the depth variation in Ar ion-irradiation at 600 ℃

Irradiation damage of the materials depends on the irradiation dose and the intrinsic property of the material. In this paper, the high purity hot pressing sintered SiC ceramics with very few second phase and excellent crystallinity were prepared as the target materials, and the high irradiation dose up to 0.95 and 3.16?dpa respectively were chosen. The as-sintered SiC ceramics were irradiated with a 160?keV Ar ion beam at 600?°C. X-ray photoelectron spectroscopy, Raman spectrum, transmission electron microscopy and nanoindentation tests were utilized to analyze the microstructure variations on the surface of irradiated SiC, and it was found that the irradiated crystals kept crystallinity, although amorphization of SiC was generated with 10–25?nm depth, following with a mixture of point defect clusters and extended defects. Furthermore, it is also evident that there is a balance between irradiated-induced damages buildup and dynamic annealing of defects in high temperature.     

Oxidation behavior of 3D SiCf/SiBCN composites at 800–1200 °C

3D SiC_f/BN–SiC/SiBCN composites were fabricated via precursor impregnation and polymer infiltration pyrolysis (PIP). Oxidation behavior of the composites heated in air at 800 °C, 1000 °C and 1200 °C for 50 h was investigated. Following the oxidation treatment, it was found that the bending strength of the composites at different oxidation temperatures was degraded. The weight loss of the composites decreased gradually over the range of oxidation times of 1–50 h. In order to clarify the oxidation mechanism of the composites, reconstructed images, microstructures, phase compositions, the oxide layer formed on the composites and main chemical reactions were all analyzed. It was revealed that the degradation in the fracture strength of the composites was closely related to the oxidation of SiBCN matrix and BN-SiC interphase, whereas there was no signs of oxidation products about SiC fiber, which indicated that SiC fiber could be protected from oxygen by SiBCN matrix at 800?1200 °C in air.     

Crystallization Behavior of Amorphous Si2BC3N Ceramic Monolith Subjected to High Pressure

The crystallization behavior of amorphous Si₂BC₃N monoliths by heating at 1000°C–1400°C and 5 GPa was investigated with the special attention to the nucleation mechanisms of β‐SiC and BN(C) phases. Nanoscale puckered structures arising in particle bridging areas were found and its evolution behavior well reflected the nucleation process of nanocrystallites. The temperature‐dependent crystallization of amorphous Si₂BC₃N monoliths at 5 GPa passes through four stages: The material remains amorphous below 1100°C. It undergoes partial phase segregation (1100°C–1200°C), followed by initiation of nucleation (1200°C–1250°C), and then nucleation and growth of β‐SiC and turbostratic BN(C) crystallites (>1250°C). The first principles calculation indicates the nucleation precedence of BN(C) phase over β‐SiC. BN(C) nucleates preferentially at bridges between ceramic particles causing SiC to concentrate in particle interiors thus forming capsule‐like structures.     

Microstructural evolution,swelling and hardening of CVD-SiC induced by He ions irradiation at 650 °C

In the present study, chemical vapor deposited (CVD) SiC samples were irradiated by 500 keV He ions to different doses at 650 °C. The microstructural evolution, surface swelling and nanohardness change caused by irradiation versus He ion dose were investigated using TEM, AFM and nanoindentation. Results showed that a high number density of He bubbles and dislocation loops were formed in the sample irradiated to the high dose, resulting in a swelling of 1.53%. Meanwhile, the hardness of the samples increased after He ions irradiation, which was attributed to the pinning effect of the irradiation induced defects and bubbles. The irradiation hardening degree increased and tended to be saturated with the increasing He ion dose.     

Electron spin resonance in glassy carbon

The ESR was investigated for a series of samples of glassy carbon heat-treated to various temperatures in the range 1000–3000°C using a Q and X band spectrometers. For the solid material, the width first increases slightly, then goes through a minimum at around 1400°C and increases greatly above HTT 2600°C. For a ground material, a large broadening in the transition range (~1300°C) is observed. The intensity for all samples passes through a minimum at HTT 1200°C and increases to a strong maximum at around HTT 1550°C after which it decays to a value 0·5 × 10¹⁹ spin/g. The influence of neutron irradiation and of subsequent anneal are also reported. The irradiated material shows a large broadening in the transition range (~ 1300°C) similar to the broadening observed for irradiated soft carbons. Temperature dependence measurements of the ESR signal were carried out for nonirradiated and for irradiated samples, in order to determine the contribution of the localized spin and of the conduction carriers to the total intensity.     

Deposition Rate,Texture, and Mechanical Properties of SiC Coatings Produced by Chemical Vapor Deposition at Different Temperatures

Silicon carbide (SiC) coatings were produced on carbon/carbon (C/C) composites substrates using chemical vapor deposition (CVD) at different temperatures (1100°C, 1200°C, and 1300°C). The deposition rate was found to increase with deposition temperature from 1100°C to 1200°C. From 1200°C to 1300°C, the deposition rate decreased. SiC coating produced at 1200°C exhibited a strong (111) texture compared with the coatings produced at other temperatures. Both hardness and Young's modulus were also found to be higher in the coating produced at 1200°C. The variation in mechanical properties with the increase in temperature from 1100°C to 1300°C showed a direct correlation with the change in deposition rate and (111) texture. Microstructure analysis shows that the change in CVD temperature leads to the change in grain size, crystallinity, and density of stacking faults of SiC coatings, which appears to have no significant effect on mechanical properties of SiC compared with the texture observed in SiC coating. For the coating deposited at 1200°C, both the hardness and Young's modulus increased gradually from the substrate/coating interface to the top surface. The nonuniformity of mechanical properties along the cross‐section of the coating is attributed to the nonuniform microstructure.     

High-temperature flexural strength of SiC ceramics prepared by additive manufacturing

Silicon carbide (SiC) ceramics, as a kind of candidate material for aero-engine, its high-temperature performance is a critical factor to determine its applicability. This investigation focuses on studying the high-temperature properties of SiC ceramics fabricated by using additive manufacturing technology. In this paper, SiC ceramics were prepared by combining selective laser sintering (SLS) with precursor infiltration and pyrolysis (PIP) technique. The microstructure, phase evolution, and failure mechanism after high-temperature tests were explored. SiC ceramic samples tested at room temperature (RT), 800°C, 1200°C, 1400°C, and 1600°C demonstrated bending strengths of 220.0, 226.1, 234.9, 215.5, and 203.7 MPa, respectively. The RT strength of this material can be maintained at 1400°C, but it decreased at 1600°C. The strength retention at 1400°C and 1600°C were 98% and 92%, respectively. The results indicate that the mechanical properties of SiC ceramics prepared using this method have excellent stability. As the temperature increases, the bending strength of the specimens increased slightly and reached the peak value at 1200°C, and dropped to 203.7 MPa at 1600°C. Such an evolution could be mainly due to the crack healing, and the softening of the glassy phase.     

In-situ TEM investigations on the microstructural evolution of SiC fibers under ion irradiation: Amorphization and grain growth

SiC/SiC composites are attractive candidates for many nuclear systems. As reinforcements, SiC fibers are critical to the in-service performance of composites. In this work, the temperature effects on the irradiation-induced microstructural evolution of Cansas-III SiC fibers were investigated using in-situ transmission electron microscopy (TEM). With in-situ 800 keV Kr ion irradiation, at room temperature (RT) the SiC fiber experienced heterogeneous amorphization and became completely amorphous at ～2.6 dpa. Above the critical temperature of crystalline-to-amorphous (T_c), SiC fibers underwent a simultaneous process of carbon packet disappearance and nano-grain growth at 300 °C and 800 °C. Possible mechanisms were discussed.     

Thermal stability of CVI and MI SiC/SiC composites with Hi-Nicalon™-S fibers

The influence of high-temperature argon heat-treatment on the microstructure and room- temperature in-plane tensile properties of 2D woven CVI and 2D unidirectional MI SiC/SiC composites with Hi-Nicalon?-S SiC fibers was investigated. The 2D woven CVI SiC/SiC composites were heat-treated between 1200 and 1600 °C for 1- and 100-hr, and the 2D unidirectional MI SiC/SiC composites between 1315 and 1400 °C for up to 2000 hr. In addition, the influence of temperature on fast fracture tensile strengths of these composites was also measured in air. Both composites exhibited different degradation behaviors. In 2D woven CVI SiC/SiC composites, the CVI BN interface coating reacted with Hi-Nicalon?-S SiC fibers causing a loss in fast fracture ultimate tensile strengths between 1200 and 1600 °C as well as after 100-hr isothermal heat treatment at temperatures > 1100 °C. In contrast, 2D unidirectional MI SiC/SiC composites showed no significant loss in in-plane tensile properties after the fast fracture tensile tests at 1315 °C. However, after isothermal exposure conditions from 1315° to 1400°C, the in-plane proportional limit stress decreased, and the ultimate tensile fracture strain increased with an increase in exposure time. The mechanisms of strength degradation in both composites are discussed.     

Interfacial microstructure evolution and mechanical properties of B4C-based composite joints bonded with Ti foil

The interfacial microstructure and mechanical properties of B₄C-SiC-TiB₂ composite joints diffusion bonded with Ti foil interlayer were investigated. The joints were diffusion bonded in the temperature range of 800–1200?°C with 50?MPa by spark plasma sintering. The results revealed that robust joint could be successfully obtained due to the interface reaction. B₄C reacted with Ti to form nanocrystalline TiB₂ and TiC at the interface at 800–1000?°C. Both the reactions between SiC and Ti and between TiB₂ and Ti were not observed during joining. A full ceramic joint consisted of micron- and submicron-sized TiB₂ and TiC, accompanied with the formation of micro-crack, was achieved for the joint bonded at 1200?°C. Joint strength was evaluated and the maximum shear strength (145?±?14.1?MPa) was obtained for the joint bonded at 900?°C. Vickers hardness of interlayer increased with increasing the joining temperature.     

Seeds-induced synthesis of SiC by microwave heating

α-SiC particles were used as heating seeds to prepare SiC from coal minerals with microwave heating method. Coating technique was carried out to prepare composite raw powders in three different methods. Heating temperatures were at 1000?°C, 1050?°C, 1100?°C, 1150?°C, 1200?°C for 10min, respectively. XRD, SEM techniques were carried out to characterize samples. It was found that different distribution between C and α-SiC particles from different mixing method leads to different microwave heating behavior and growth mechanisms. V-V reaction leads to in situ nucleation and grain growth on the surface of α-SiC seeds which contact with C particles. Well-grown β-SiC particulates appear. Hybrid V-V reaction V-L reaction lead to local microwave plasma and diffusion-precipitation with a very thin layer of SiO₂ between raw C particles and α-SiC seeds. Local fine β-SiC whiskers and particulates on the surface of α-SiC seeds co-exist. Primary V-V reaction leads to nucleation and grain growth along reacted C-SiO₂ interface with very thick layer of SiO₂ between raw C and α-SiC seeds. Substantial β-SiC whiskers appear. Transformation from β-SiC to α-SiC on the surface of as-formed whiskers will be enhanced by microwave plasma at high temperature.     

Fabrication of large diameter SiC monofilaments by polymer route

We demonstrate the possibility to fabricate SiC monofilaments with large diameters of 100 μm by a polymer route using a dry-spinning process. The properties of the spinning solution and the parameters of the spinning process were optimized to achieve a circular cross section of the spun filaments despite their large diameter. The evolution of the diameter and the mechanical properties of the filaments with pyrolysis temperature were studied. Filament shrinkage started above 400 °C. A radial shrinkage of about 25% was measured for pyrolysis temperatures of 1200 °C. The mechanical properties significantly start to increase at pyrolysis temperatures above 600 °C. At a diameter of 100 μm the filaments show a tensile strength of 620 MPa and a tensile modulus of 138 GPa after pyrolysis at 1200 °C. A decrease in the filament diameter leads to an improvement of the mechanical properties. We demonstrate the fabrication of these SiC monofilaments on spools.     

Samarium doped hafnium oxide cubic and monoclinic nanometric powders synthesized by hydrothermal route

Hafnium oxide nanoparticles doped with trivalent samarium (HfO₂:Sm³⁺) were synthesized by hydrothermal route from chloride reagents. Different samarium doping concentrations (0, 0.5, 1, 3, 5 and 10?at% with respect to Hf) and different post-annealing temperatures (200, 400, 600, 800 and 1000?°C) were evaluated. The resulting nanoparticles showed an oval morphology with average sizes below 30?nm. A sensitive relationship between the samarium concentration and the resulting crystal structure of the material was observed by X-Ray diffraction and further evidenced by the cathodoluminescence and photoluminescence spectra. Low concentrations of samarium (0, 0.5 and 1?at%) generated a monoclinic phase, whereas higher concentrations of samarium (5 and 10?at%) generated a metastable cubic phase. The average crystallite sizes calculated by the Scherrer's equation were close to 8?nm and 12?nm for cubic and monoclinic phases, respectively. The luminescent emission corresponded to the characteristic 4f-4f transitions of the samarium ion with a principal peak centered at 610?nm. The best luminescent properties were obtained with the sample doped with samarium at 0.5?at%, annealed at 600?°C.     

High-dose,intermediate-temperature neutron irradiation effects on silicon carbide composites with varied fiber/matrix interfaces

SiC/SiC composites are promising structural candidate materials for various nuclear applications over the wide temperature range of 300–1000 °C. Accordingly, irradiation tolerance over this wide temperature range needs to be understood to ensure the performance of these composites. In this study, neutron irradiation effects on dimensional stability and mechanical properties to high doses (11–44 dpa) at intermediate irradiation temperatures (?600 °C) were evaluated for Hi-Nicalon Type-S or Tyranno-SA3 fiber–reinforced SiC matrix composites produced by chemical vapor infiltration. The influence of various fiber/matrix interfaces, such as a 50–120 nm thick pyrolytic carbon (PyC) monolayer interphase and 70–130 nm thick PyC with a subsequent PyC (?20 nm)/SiC (?100 nm) multilayer, was evaluated and compared with the previous results for a thin-layer PyC (?20 nm)/SiC (?100 nm) multilayer interphase. Four-point flexural tests were conducted to evaluate post-irradiation strength, and SEM and TEM were used to investigate microstructure. Regardless of the fiber type, monolayer composites showed considerable reduction of flexural properties after irradiation to 11–12 dpa at 450–500 °C; and neither type showed the deterioration identified at the same dose level at higher temperatures (>750 °C) in a previous study. After further irradiation to 44 dpa at 590–640 °C, the degradation was enhanced compared with conventional multilayer composites with a PyC thickness of ?20 nm. Multilayer composites have shown comparatively good strength retention for irradiation to ?40 dpa, with moderate mechanical property degradation beginning at 70–100 dpa. Irradiation-induced debonding at the F/M interface was found to be the major cause of deterioration of various composites.     

Tribological performance of SiC coating for carbon/carbon composites at elevated temperatures

SiC coating was deposited on carbon/carbon (C/C) composites by chemical vapor deposition (CVD). The effects of elevated temperatures on tribological performance of SiC coating were investigated. The related microstructure and wear mechanism were analyzed. The results show that the as-deposited SiC coating consists of uniformity of β-SiC phase. The mild abrasive and slight adhesive wear were the main wear mechanisms at room temperature, and the SiC coating presented the maximum friction coefficient and the minimum wear rate. Slight oxidation of debris was occurred when the temperature rose to 300?°C. As the temperature was above 600?°C, dense oxide film formed on the worn surface. The silica tribo-film replaced the mechanical fracture and dominated the frication process. However, the aggravation of oxidation at elevated temperatures was responsible for the decrease of friction coefficient and the deterioration of wear rate. The SiC coating presented the minimum friction coefficient and the maximum wear rate when the temperature was 800?°C.     

Irradiation damage to pyrolytic graphites at very high temperatures

Six pyrolytic graphites of widely differing crystallite sizes (L_a = 1?120 μm) were irradiated at temperatures of 1020°C and 1475°C. The resulting damage was studied by transmission electron microscopy. Damage produced at 1020°C can be understood in terms of present nucleation ideas, assuming crystallite boundaries interfere with homogeneous nucleation when L_a is smaller than the separation of homogeneously nucleated defects. At the higher temperature there is an increase in defect density rather than a decrease as observed over the temperature range 150–1200°C. It is proposed that this is caused by the nucleating cluster being larger than in the lower temperature range, and hence having a larger migration activation energy, approximately 2 eV as opposed to 1·2 eV.     

Semiconductor-conductor transition of pristine polymer-derived ceramics SiC pyrolyzed at temperature range from 1200°C to 1800°C

This paper studies the effect of pyrolysis temperature on the semiconductor-conductor transition of pristine polymer-derived ceramic silicon carbide (PDC SiC). A comprehensive study of microstructural evolution and conduction mechanism of PDC SiC pyrolyzed at the temperature range of 1200°C-1800°C is presented. At relatively lower pyrolysis temperatures (1200°C-1600°C), the carbon phase goes through a microstructural evolution from amorphous carbon to nanocrystalline carbon. The PDC SiC samples behave as a semiconductor and the electron transport is governed by the band tail hopping (BTH) mechanism in low pyrolysis temperature (1300°C); by a mixed mechanism driven by band tail hopping and tunneling at intermediate temperature (1500°C). At higher pyrolysis temperatures (1700°C-1800°C), a percolative network of continuous turbostratic carbon is formed up along the grain boundary of the crystallized SiC. The samples demonstrate metal-like conductive response and their resistivity increases monotonically with the increasing measuring temperature.     

Kr ion irradiation study of polymer-derived SiFeOC–C–SiC nanocomposite

This study focuses on the pyrolysis and ion irradiation behaviors of polymer-derived SiFeOC–C–SiC ceramic. The pyrolyzed material is composed of SiO₂ and SiOC (amorphous), carbon (amorphous and turbostratic), and Fe₃Si and β-SiC (nanocrystalline). Irradiation was carried out at both room temperature and 600°C using 400 keV Kr ions with fluences of 4 × 10¹⁵ and 1 × 10¹⁶ ions cm⁻², respectively. The Fe₃Si and SiC nanocrystals are stable against irradiation up to 3 displacement per atom (dpa) at room temperature and up to 12 dpa at 600°C. The SiOC tetrahedrals show phase separation and minor carbothermal reduction. The high irradiation resistance and the dense, defect-free amorphous microstructure of SiFeOC–C–SiC after prolonged irradiation demonstrate its great potential for advanced nuclear reactor applications.     

Identification of a new oxidation/ dissolution mechanism for boria-accelerated SiC oxidation

Boria effects on accelerated SiC oxidation kinetics were investigated by conducting thermogravimetric analysis on SiC substrates coated with sol-gel derived borosilicate glass isothermally exposed to dry O₂ and argon at 800°C and 1200°C for 100 hours. Boria concentrations in the glass coatings were 0, 14-38, and 92-94 mol%, balance silica. Accelerated weight gain was observed for SiC exposures in dry O₂ at 800°C when boria concentrations were ≥ 92 mol%, corroborated by oxide thickness ranging from 3.5 to 10 µm. The oxide thickness predicted for pure SiC exposed to these conditions in the absence of boria is 0.15 µm. Microstructural analysis of SiC surfaces after oxide removal revealed that boria etched the underlying SiC substrate. Oxidation exposures at 1200°C in dry O₂ suppressed boria effects on accelerating SiC oxidation kinetics due to rapid boria volatilization coupled with the formation of a protective thermally grown silica scale. Accelerated weight gain or oxide growth did not occur with argon exposures at either temperature. A new mechanism for boria-accelerated SiC surface-reaction kinetics is presented based on evidence for boria etching of SiC.