Polycrystalline SiC Fibers Research Articles

Long-time oxidation behavior of the nearly stoichiometric polycrystalline SiC fibers under air atmosphere at different temperatures

SiC fibers are important reinforcements due to their excellent oxidation and high-temperature resistance. In this work, near-stoichiometric polycrystalline SiC fibers were oxidized in air at 1200 °C to 1600 °C for up to 100 h. The thickness of the oxide layer reached 1.9 µm after oxidation at 1400 °C for 100 h. Wide cracks appeared on the surface due to residual stress. Activation energy was comparable to that of commercial SiC fibers. The results in this work could provide important reference for such SiC fibers to be applied in the fields of aviation, aerospace, and nuclear power.

Flexible ultrafine nearly stoichiometric polycrystalline SiC fibers with excellent oxidation resistance and superior thermal stability up to 1900 °C

Flexible ultrafine SiC fibers with superior high-temperature stability and excellent oxidation resistance are regarded as one of the most promising materials for high-temperature applications. However, excess oxygen and carbon in the ultrafine SiC fibers limit their thermal stability due to decomposition of the SiCxOy phase. In the present work, flexible ultrafine nearly stoichiometric polycrystalline SiC fibers were fabricated by combining the electrospinning technique and polymer-derived ceramic method. The ultrafine SiC fibers exhibited superior high-temperature stability and oxidation resistance. The retention rates of tensile strength were 90.0 %, 94.2 % and 86.4 % after heat treatment in argon at 1800 °C, 1900 °C and 2000 °C, respectively. TG results of the fibers showed little weight loss of only 1.52 % at 1900 °C in Ar and the weight gain of only 4.1 % up to 1500 °C in air. Such improved thermal stability was achieved through sintering at high temperature for elimination of excess oxygen and carbon with Al doped as the sintering aid to restrain the grain coarsening. The ultrafine SiC fibers still exhibited excellent flexibility without obvious damage when they were heated by the butane blowtorch flame of about 1100 °C in air. Furthermore, the infrared thermography illustrated that the ultrafine SiC fiber membrane also had good thermal insulation performance. The outstanding mechanical properties and thermal stability of ultrafine SiC fibers suggest their potential applications at the high temperature and harsh environment.

Effect of Impurities Control on the Crystallization and Densification of Polymer-Derived SiC Fibers.

The polymer-derived SiC fibers are mainly used as reinforcing materials for ceramic matrix composites (CMCs) because of their excellent mechanical properties at high temperature. However, decomposition reactions such as release of SiO and CO gases and the formation of pores proceed above 1400 °C because of impurities introduced during the curing process. In this study, polycrystalline SiC fibers were fabricated by applying iodine-curing method and using controlled pyrolysis conditions to investigate crystallization and densification behavior. Oxygen and iodine impurities in amorphous SiC fibers were reduced without pores by diffusion and release to the fiber surface depending on the pyrolysis time. In addition, the reduction of the impurity content had a positive effect on the densification and crystallization of polymer-derived SiC fibers without a sintering aid above the sintering temperature. Consequently, dense Si-Al-C-O polycrystalline fibers containing β-SiC crystal grains of 50~100 nm were easily fabricated through the blending method and controlled pyrolysis conditions.

Revealing the formation mechanism of the skin-core structure in nearly stoichiometric polycrystalline SiC fibers

The polymer-derived SiC fibers have broad application prospects in the fields of aerospace, nuclear industry and high-tech weapon. Oxygen plays an essential role in adjusting the composition, structure and tensile strength of SiC fibers. Our studies have found that introducing too much oxygen during air curing process will form the skin-core structure in nearly stoichiometric polycrystalline SiC fibers. In order to reveal the formation mechanism of the skin-core structure, gradient oxygen was introduced into the fibers. The morphologies, phase distributions and defects of the fibers were well characterized. By strictly controlling the introduction of oxygen, the polycrystalline product fiber exhibits intragranular fracture behavior and excellent high-temperature resistance. The retention rate of its tensile strength can reach up to 91% and 61% after exposure at 1800 °C for 1 h and 10 h, respectively. The present results give valuable insights into the structural optimization of the nearly stoichiometric polycrystalline SiC fibers.

High-Performance SiC-Polycrystalline Fiber with Smooth Surface

Polymer-derived SiC-polycrystalline fibers show excellent heat-resistance up to 2000 °C, and relatively high strength. Up to now, through our research, the relationship between the strength and residual defects of the fiber, which were formed during the production processes (degradation and sintering), has been clarified. In this paper, we addressed the relationship between the production conditions and the surface smoothness of the obtained SiC-polycrystalline fiber, using three different raw fibers (Elementary ratio: Si1Al0.01C1.5O0.4~0.5) and three different types of reactors. With increase in the oxygen content in the raw fiber, the degradation during the production process easily proceeded. In this case, the degradation reactions (SiO + 2C = SiC + CO and SiO2 + 3C = SiC + 2CO) in the inside of each filament become faster, and then the CO partial pressure on the surface of each filament was considered to be increased. As a result, according to Le Chatelier’s principle, the surface degradation reaction and grain growth of formed SiC crystals would be considered to become slower. That is to say, using the raw fiber with higher oxygen content and closed system (highest CO content in the reactor), a much smoother surface of the SiC-polycrystalline fiber could be achieved. Furthermore, the similar effect obtained by simple oxidation of the SiC-polycrystalline fiber was confirmed, and the advantageous points of the aforementioned process were also considered.

Fabrication of nearly stoichiometric polycrystalline SiC fibers with excellent high‐temperature stability up to 1900°C

AbstractDue to the extensive applications of SiC fiber‐reinforced composite materials in the fields of aviation, aerospace, and nuclear power, there are increasing demands for SiC fibers with both excellent mechanical performance and high‐temperature stability. In this work, nearly stoichiometric polycrystalline SiC fibers were fabricated using amorphous Si–C–Al–O fibers with excess carbon and oxygen (C/Si = 1.34, O content: 7.74 wt%). The nearly stoichiometric composition (C/Si = 1.05) of the product fibers was achieved by thermal decomposition of the starting fibers. The fibers were well‐crystallized with grain sizes of ~200 nm due to sintering at a high temperature of 1900°C. The fibers exhibited a high tensile strength and a high elastic modulus and were composed of SiC grains with twins and stacking‐faults, exhibiting intragranular fracture behavior. Furthermore, the fibers maintained their original tensile strength after being maintained at 1800°C for 5 hour or at 1900°C for 1 hour under an inert atmosphere, and they exhibited a high strength retention (97%) after exposure at 1300°C for 1 hour under air. The high‐temperature stability and creep resistance of the fibers were comparable to that of commercial Hi‐Nicalon S and Tyranno SA fibers.

Structural Control Aiming for High-performance SiC Polycrystalline Fiber

SiC-polycrystalline fiber (Tyranno SA, Ube Industries, Ltd.) shows very high heat-resistance and excellent mechanical properties up to very high temperatures. However, further increase in the strength is required. Up to now, we have already clarified the relationship between the strength and the defect-size of the SiC-polycrystalline fiber. The defects are formed during the conversion process from the raw material (amorphous Si-Al-C-O fiber) into SiC-polycrystalline fiber. In this conversion process, a degradation of the Si-Al-C-O fiber and a subsequent sintering of the degraded fiber proceed as well, accompanied by a release of CO gas and compositional changes, to obtain the dense SiC-polycrystalline fiber. Since these changes proceed in each filament, the strict control should be needed to minimize residual defects on the surface and in the inside of each filament for achieving the higher strength. In this paper, the controlling factors of the fiber strength and the fine structure will appear.

Conversion Process of Amorphous Si-Al-C-O Fiber into Nearly Stoichiometric SiC Polycrystalline Fiber

Tyranno SA (SiC-polycrystalline fiber, Ube Industries Ltd.) shows excellent heat-resistance up to 2000℃ with relatively high mechanical strength. This fiber is produced by the conversion process from a raw material (amorphous Si-Al-C-O fiber) into SiCpolycrystalline fiber at very high temperatures over 1500℃ in argon. In this conversion process, the degradation reaction of the amorphous Si-Al-C-O fiber accompanied by a release of CO gas for obtaining a stoichiometric composition and the subsequent sintering of the degraded fiber proceed. Furthermore, vaporization of gaseous SiO, phase transformation and active diffusion of the components of the Si-Al-C-O fiber competitively occur. Of these changes, vaporization of the gaseous SiO during the conversion process results in an abnormal SiC-grain growth and also leads to the non-stoichiometric composition. However, using a modified Si-Al-C-O fiber with an oxygen-rich surface, vaporization of the gaseous SiO was effectively prevented, and then consequently a nearly stoichiometric SiC composition could be obtained.

Defect control of SiC polycrystalline fiber synthesized from poly-aluminocarbosilane

SiC-polycrystalline fiber (Tyranno SA) shows excellent heat-resistance up to 2000°C. Aiming to enlargement of the future application, increase in the mechanical strength of the fiber is eagerly required. Tyranno SA was produced via lots of heat-treatment procedure. This production process is very similar to that of carbon fiber containing decomposition and densification processes at high temperatures in inert gas atmosphere. As these production processes are accompanied by the gaseous by-products, it’s easy to remain pores in each filament along with some other defects. These pores would also become “defect” as well as both residual carbon and abnormal crystalline growth become remarkable defects. In this study, first, the relationship between the defect size and the tensile strength of SiC-polycrystalline fiber was obtained. From this result, it was clarified that the smaller the defect size became, the higher the strength became. By controlling the processes, smoother surface of SiC-polycrystalline fiber was obtained.

Microstructure design and control for improvement of thermal conductivity of SiCf/SiC composites

We focused on microstructure design and control of SiC f /SiC composite based on our fabrication process and the simple model of thermal conductivity of the SiC f /SiC composite, and the improvement of their thermal conductivity was investigated. Submicron-sized α-SiC with coarse α-SiC particles addition was used as the starting materials for SiC matrix layers between SiC fiber cloths because it showed higher thermal conductivity . The thermal conductivity of PCS-composite, EPD-composite and Untreated-composite was 18, 45 and 56 W/m K, respectively, and these values were much higher than that of the composites reported in our previous papers. Untreated composite is simply considered as a multilayered composite consisting of the SiC fiber layers with high thermal conductivity and the SiC matrix layers with high thermal conductivity. The experimental thermal conductivity of the Untreated composite well agreed with the theoretical thermal conductivity calculated by series model. Thermal conductivity of EPD-composite was lower than that of Untreated composite. In EPD-composite, the thermal conductivity of SiC fiber layers with the SiC matrix should be lower than that of SiC fibers themselves due to the SiC matrix with slightly lower thermal conductivity in SiC fiber cloths. The SiC matrix formed in SiC fiber cloths in PCS-composite was derived from PCS, and this matrix would show much lower thermal conductivity due to its low crystallinity . PCS-composite is considered as a multilayered composite consisting of the SiC fiber layers with very low thermal conductivity and the SiC matrix layers with high thermal conductivity, and thus the PCS-composite has low thermal conductivity. In this study, higher thermal conductivity of SiC f /SiC composite was successfully achieved by EPD process and using microstructure-controlled SiC matrix and polycrystalline SiC fibers.

Estimation of statistical strength distribution of Carborundum polycrystalline SiC fiber using the single fiber composite with consideration of the matrix hardening

The statistical strength distribution of Carborundum polycrystalline SiC fibers was derived from the fragmentation process in a single fiber/epoxy composite. We conducted Monte Carlo simulations of the fragmentation process using an elastic–plastic hardening shear-lag model. The Monte Carlo simulation with the estimated fiber strength distribution reproduces the fragmentation seen in the experiments very well. We also compared these simulated results with those calculated by the elastic shear-lag model with the shear-lag parameter β tuned as proposed in Curtin et al. [Curtin WA, Netravali AN, Park JM. Strength distribution of Carborundum polycrystalline SiC fibers as derived from the single-fiber-composite. J Mater Sci 1994;29:4718–28] and with an elastic–plastic finite element model. The fiber axial stress distributions in all three models are in close agreement, with the characteristic length β −1 used in Curtin et al. consistent with the plastic length around a fiber break in an elastic–plastic hardening shear-lag model.

Heat treatment effects on creep behavior of polycrystalline SiC fibers

Polycrystalline SiC fibers are being considered as potential reinforcement for ceramic matrix composites (CMCs). For these fibers with fine grain size, basic issues arise concerning thermo-mechanical properties and microstructural instability during fabrication and application of CMCs. To examine these issues, three commercially available SiC fibers were heat treated at elevated temperatures for 1 h in an Ar atmosphere. The creep resistance of SiC fibers was evaluated by the bend stress relaxation (BSR) method, and it was found that the creep resistance could be improved by heat treatment. Combining the results of the BSR tests with the results of X-ray diffraction examinations indicated that the creep resistance of SiC fibers is mainly related to the β-SiC grain size and the composition at or adjacent to the grain boundaries. Also, the apparent activation energy of creep for both Hi-Nicalon™ and Hi-Nicalon™ type S fiber increased with increasing heat treatment temperature. In the case of Tyranno™-SA fiber, the apparent activation energy of creep did not show an obvious dependence on the heat treatment temperature.

焼結ＳｉＣ繊維強化ＳｉＣ複合材料の作製と高温曲げ強度

A sintered SiC polycrystalline fiber, which has been developed by UBE Industries, Ltd., possesses high modulus, excellent thermal stability and oxidation resistance. A SiC matrix composite reinforced by the sintered SiC fiber was fabricated by polymer impregnation and pyrolysis (PIP) method. The composite consisted of 26vol% 3-D sintered SiC fabric and polymer-derived SiC matrix. Carbon coating was applied by chemical vapor deposition on the 3-D sintered SiC fabric prior to the matrix densification. Three point flexural tests of the sintered SiC fiber reinforced SiC composite up to 1400°C in Ar were carried out, and the results were compared with a SiC matrix composite reinforced by an amorphous Si-Zr-C-O fiber. The flexural strength of the sintered SiC fiber reinforced SiC composite at 1400°C retained 84% of the strength at room temperature, while that of the amorphous Si-Zr-C-O fiber reinforced SiC composite retained only 56%. In addition, the sintered SiC fiber reinforced SiC composite showed higher stiffness than that of the amorphous Si-Zr-C-O fiber reinforced SiC composite. The higher retention ratio of the strength at 1400°C and higher stiffness of the sintered SiC fiber reinforced SiC composite were attributed to excellent heat resistance and high tensile modulus of the sintered SiC fiber.

Development of a new Si—C—O—N—B ceramic fiber using a hybrid precursor of polycarbosilane and polyborosilazane

Development of a new Si—C—O—N—B ceramic fiber using a hybrid precursor of polycarbosilane and polyborosilazane

Silicon carbide polycrystalline fibers and single-crystal whiskers

The origin of the high inhomogeneity of powders of nonoxide refractory compounds, related to the chemical nature of the compounds and the preparation procedure (usually, solid-state reactions) is considered. Conclusive evidence is presented that vapor transport reactions have considerable potential for the reproducible synthesis of such compounds. It is shown that carbothermic reduction of silicon dioxide can be run as a vapor transport reaction using activated carbon fibers as a reductant. The forming polycrystalline SiC fibers are similar in particle morphology to the parent carbon fibers, suggesting that the reaction proceeds through the incorporation of silicon into the lattice of amorphous carbon. Experimental data are presented for the first time which demonstrate that, under similar process conditions, silicon carbide can be obtained in the form of polycrystalline fibers, single-crystal whiskers, or fine powder. The reaction products differ not only in morphology but also in the crystal structure of SiC (α or β form). The mechanisms of α- and β-SiC nucleation and growth are discussed.

Creep and rupture of an advanced CVD SiC fiber

In the as-produced condition the room temperature strength (~6 GPa) of Textron Specialty Materials' 50 μm CVD SiC fiber represents the highest value thus far obtained for commercially produced polycrystalline SiC fibers. To understand whether this strength can be maintained after composite processing conditions, high temperature studies were performed on the effects of time, stress, and environment on 1400 °C tensile creep strain and stress rupture on as-produced, chemically vapor deposited SiC fibers. Creep strain results were consistent, allowing an evaluation of time and stress effects. Test environment had no influence on creep strain but 1 hour annealing at 1600 °C in argon gas significantly reduced the total creep strain and increased the stress dependence. This is attributed to changes in the free carbon morphology and its distribution within the CVD SiC fiber. For the as-produced and annealed fibers, strength at 1400 °C was found to decrease from a fast fracture value of 2 GPa to a 100-hour rupture strength value of 0.8 GPa. In addition a loss of fast fracture strength from 6 GPa is attributed to thermally induced changes in the outer carbon coating and microstructure. Scatter in rupture times made a definitive analysis of environmental and annealing effects on creep strength difficult.

Structure and properties of ceramic fibers prepared from organosilicon polymers

This paper is a review of structure and properties of ceramic fibers derived from organosilicon polymers, with emphasis on the author's research. Ceramic fibers are prepared from organosilicon polymers by melt-spinning, cross-linking, and pyrolysis. Desirable polymeric precursors display the following properties: high char yield of desired composition, thermal stability at melt-spinning temperature, stable rheology, high purity and freedom from particulate impurities, and ability to undergo rapid cure (cross-linking). Ceramic fibers in the Si-C-O or Si-C-N-O systems display a rich nanostructure consisting of some or all of the following metastable phases: (1) an amorphous, continuous siliconoxycarbide or siliconoxycarbonitride phase; (2) dispersed carbon nanocrystallites; (3) dispersed β-SiC or Si3N4 nanocrystallites; and (4) closed, globular nanopores. The crystalline phases increase in volume fraction and crystallite size as stoichiometry approaches the crystalline composition and as pyrolysis temperature increases. The Si-C-N-O fibers are amorphous. Pore size increases and total pore volume decreases with increasing pyrolysis temperature. Considerable variation in ceramic fiber composition can be achieved by varying cure conditions and pyrolysis atmosphere. Polycrystalline SiC fibers can be produced by pyrolysis above 1600°C. Fiber diameters range from 7 to 20 µm. Elastic moduli vary from 140 to >420 GPa (20 to >60 Msi) and are controlled by composition, nanostructure, and fiber density. Fiber densities range from ∼2.2 to >3.1 g/cm3. Tensile strengths range up to ∼5 GPa (700 ksi) and are Griffith flaw-controlled.