Sintered SiC Fiber Research Articles

Sliding wear resistance of sintered SiC-fiber bonded ceramics

Advanced SiC-based ceramics and fiber reinforced composites are interesting materials for a wide variety of applications involving sliding wear conditions because of their excellent thermomechanical properties. The microstructure and wear resistance of sintered SiC fiber bonded ceramics (SA Tyrannohex) were studied. The material is composed of SiC-fibers in two orientations, with polygonal cross sections and cores having higher carbon content than their surroundings, as observed with SEM. A thin layer of C exists between the fibers. This layer has been found to be a turbostratic-layered structure oriented parallel to the fiber surface. XRD shows that the material is highly crystalline and composed mostly of β-SiC. Unlubricated wear behavior of the SA-Tyrannohex material when sliding against a Si3N4 ball in air at room temperature was evaluated. Experiments were performed using a pin on disk apparatus, under different normal loads of 2, 5 and 10N at sliding speeds of 25, 50, 100mm/s. A decrease of the friction coefficient with load was found due to the presence of the turbostratic carbon layer between the fibers. Wear rates of the order of 100mm3/MJ were obtained, independently of sliding speed. Microfracture of the fibers is the main wear mechanism.

Preparation and Properties of SiC Fibers Containing Yttrium and Aluminum

SiC fibers containing Y and Al were prepared by sintering the amorphous KD-Y fibers at different temperatures from 1200 to 1800 °C. The tensile strength of fibers increases when the sintering temperature increases from 1200 to 1400 °C due to the dispersion strengthening of β-SiC microcrystal and decreases to the lowest point when being sintered at 1600 °C, then rose again with the further increase of temperature to 1800 °C. The tensile strength reached to 2.15 GPa. The oxygen content of KD-Y fibers determined the properties of the sintered SiC fibers and sintering process. Through chemical vapor curing (CVC), the oxygen content was controlled below 7.0 wt% and the tensile strength of KD-Y fibers reached to 3.08 GPa. Further-more, the sintering process of KD-Y fibers was discussed and the grain size of β-SiC grew up with the heat treatment temperature.

BN 界面相を有する焼結 SiC 繊維強化 SiO2-ムライト複合材料の作製と酸化挙動

As a potential material for high temperature services, unidirectional sintered SiC fiber reinforced SiO2-30 mol%Al2O3 composite with BN interphase was synthesized by liquid state hot pressing. The obtained composite had dense structure, and both the mullite and glassy SiO2 phases were found in the matrix. No chemical reaction among the constituents was observed. The composites were exposed to air at 1573 K up to 3600 ks and then oxidation behavior was investigated. Three-point flexural test was performed at room temperature after each exposure. Flexural strength decreased with an increase in the exposure time and then leveled off in 360 ks. This tendency corresponds to the crystallization of SiO2 glass in the matrix. Change in the stress field which is caused by the crystallization and phase transformation of SiO2 would lead to the reduction of the strength. It was confirmed that oxidation progressed along the fibers and the oxidized depth was limited within approximately 200 μm independent of exposure time (3.6∼3600 ks). The oxide layer with approximately 10 μm thickness including silicon, aluminum and oxygen was found on the surface of the composites. This layer will prevent the inner oxidation within 200 μm.

焼結ＳｉＣ繊維強化ＳｉＯ２‐ムライト複合材料の作製と特性評価

The sintered SiC fiber reinforced SiO2-mullite composites, which have three levels of added Al2O3 in the matrix, i.e., SiO2-3.7, 30, 50 mol%Al2O3, were synthesized by liquid state hot-pressing. Variation in the thermal residual stress fields in the composites caused by the difference in the thermo-mechanical mismatch between the fiber and matrices were investigated. The effects of the Al2O3 content in the matrix on the mechanical properties of the composites were then examined.The obtained composites had a very dense structure, and both the mullite and glassy SiO2 phases were found in the matrices. A higher Al2O3 content in the matrix lead to a higher mullite volume fraction. No chemical reaction among the constituents due to the fabrication process was observed.It was confirmed that the thermal residual stress field in the composites changed with an increase in the coefficient of thermal expansion of the matrix accompanied by an increasing Al2O3 content in the matrices.The network structure of the mullite crystals found in the SiC fiber/SiO2-30 mol%Al2O3 and SiC fiber/SiO2-50 mol%Al2O3 composites seemed to effectively prevent plastic deformation of the matrices during the flexural test at 1573 K.The SiC fiber/SiO2-30 mol%Al2O3 composite showed excellent mechanical properties up to 1573 K compared with previously reported ceramic matrix composites such as the SiC fiber/LAS composite or SiC fiber bonded ceramics in an inert atmosphere.

焼結ＳｉＣ繊維強化ＳｉＯ２‐ムライト複合材料のマトリックス亀裂発生ひずみと破壊挙動に及ぼす残留応力の影響

The sintered SiC fiber reinforced SiO2-mullite composites with matrix compositions, i.e., SiO2-3.7, 30, and 50 mol%Al2O3, had already been developed. In these composites, the thermal residual stress field changed with an increase in the coefficient of thermal expansion of the matrix accompanied by an increasing Al2O3 content (mullite volume fraction) in the matrices. A three-point flexural test was conducted for the composites at room temperature. The strain gauge method and AE monitoring were used to detect the matrix cracking strain during the test. The effect of the thermal residual stress on the matrix cracking strain and fracture behavior were investigated. The results are as follows.The matrix cracking strain observed in the composites with a higher tensile thermal residual stress in the matrix region parallel to the fiber axis was well predicted by the BHE theory. For the SiC fiber/SiO2-50 mol%Al2O3 composite, the first acoustic emission signal was detected just after the initial proportional limit of the stress-strain curve. For the other composites, the signals were detected below this limit. When the residual stress in the matrix region parallel to the fiber axis was compressive, the linear fracture behavior was found. On the contrary, the bi-linear fracture behavior was enhanced by the tensile residual stress in the matrix. This tendency agreed with the prediction method by Luh and Evans. The magnitude of the fracture energy obtained in each composite was qualitatively explained by the conventional theoretical formula.

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.

SiC/SiC複合材料の界面層のための連続CVD法によるBNコーティング厚さの均一化

Boron Nitride (BN) coating on sintered SiC fibers (Tyranno SA) by a continuous chemical vapor deposition (CVD) process was developed with the ultimate goal of controlling the interface in SiC/SiC composites. BN coating by a continuous CVD process was conducted at 1580°C under a pressure of approximately 1.3 x 10 2 Pa. Reactant gases were NH 3 and BCl 3 , both diluted by Ar; the yarn speed was 4.5 mm/s. Conventional experiments revealed that the BN coating thickness across the tow varied significantly. Hence, the CVD process was modified not only by adjusting the amounts of reactant gases, but also by reducing the yarn tension and setting a new furnace for removing the sizing agent of the fiber tow. The BN coating thickness obtained by the modified process was significantly more uniform than that achieved by a conventional process. The BN coating had a layered structure; it was nearly stoichiometric and pure, with little O or C. The BN-coated SA fiber retained 90% of the strength of the as-received SA fiber. Moreover, a minicomposite reinforced with a BN-coated SA fiber of 1600 filaments (fiber volume fraction: 10.8%) was fabricated by a polymer infiltration and pyrolysis (PIP) method. This minicomposite exhibited a higher fracture strength (199 MPa) at room temperature and a higher retention ratio (82%) of the fracture strength at 1400°C in air as compared to a minicomposite obtained by a conventional process (173 MPa and 73%).

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

Influence of Heat Treatment on Phases, Microstructures and Mechanical Properties of Laminated Fabric (SiC and Aluminosilicate)/Mullite Matrix Composites

Oxidation resistance of SiC satin fabric (25-29vol%) or aluminosilicate plain fabric (25-33vol%)/mullite matrix porous composites was investigated at 1000-1200°C in air for 24h using thermogravimetric analysis. The composites were produced by a polymer impregnation and pyrolysis (PIP) method using a mullite precursor solution of the mixtures of Si(OC2H5)4 and Al(NO3)3. The oxidation rate of the sintered SiC fibers with a stoichiometric SiC composition and well-crystallized microstructure was controlled by the diffusion process of gases through dense SiO2 layer formed on the SiC surface. The activation energy for the oxidation of the SiC fibers was 299kJ/mol. This value was close to the activation energy for the diffusion of oxygen atom in silica glass. The SiC satin fabric/mullite matrix composite heated in an Ar atmosphere showed pseudoductility due to the delamination along the fabric layers. The four point-flexural strength decreased slightly with increasing heat treatment temperature. However, the composite heated in air at 1100-1200°C showed brittle fracture behavior without delamination because of the increased strength of interfacial bond between the oxidized SiC fibers and mullite matrix. On the other hand, the amorphous aluminosilicate fibers/mullite matrix composite showed little weight change during the heat treatment at 1000-1200°C. Crystallization of mullite occurred in the mullite precursor-derived amorphous solid and aluminosilicate fibers at 1100 and 1200°C in air, respectively. However, no microstructural change was observed in the heat-treated composite. The significant pseudoductility and strength of the aluminosilicate plain fabric/mullite matrix composite were maintained after the heat treatment. The fracture energy increased with increasing heat treatment temperature.

Highly thermal conductive, sintered SiC fiber-reinforced 3D-SiC/SiC composites: experiments and finite-element analysis of the thermal diffusivity/conductivity

Chemical vapor infiltrated (CVI) and polymer impregnated and pyrolized (PIP) SiC/SiC composites were fabricated by using highly thermal conductive, sintered SiC fiber. These composites had relatively high thermal diffusivity/conductivity values, which were two or three times larger than those reinforced with Hi-Nicalon or Hi-Nicalon Type S fibers. The improvement of thermal diffusivity by using this fiber was more noticeable for PIP composites than for CVI composites. A 2D finite element thermal analysis supported the experimental results, and revealed that highly thermal conductive SiC fiber was much effective for PIP composites and that the increase of fiber volume worsened CVI composite thermal diffusivity if low thermal conductive SiC fibers were used.

Influence of fiber and grain bridging on crack profiles in SiC fiber-reinforced alumina-matrix composites

Initial crack extension and crack propagation in sintered SiC fiber reinforced alumina matrix composites were observed in situ in the scanning electron microscope (SEM). In these composites, coupled toughening mechanisms associated with both grain and fiber bridging are operative in the crack wake. Their contribution to toughness were obtained from crack profiles measured in the crack wake at various applied loads and crack lengths. The crack profiles of the fiber reinforced samples reveal much smaller crack opening displacements compared to the monolithic samples. The fibers act as ligaments which bridge and thereby exert closure stresses on the crack surfaces. The profiles revealed pronounced reduction in the crack opening displacement (COD) around the fiber positions. Accordingly, grain bridging in the vicinity of the fibers was still operative up to an applied stress intensity factor of 5.7 MPa m 1/2 which is 30% above the maximum toughness that could be obtained with the monolithic samples for the same crack length.

New Type of Sintered SiC Fiber and Its Composite Material

New Type of Sintered SiC Fiber and Its Composite Material

In-situ crack propagation in pressureless sintered fiber reinforced composites

Crack propagation in sintered SiC fiber reinforced alumina matrix composites was studied in situ inside a scanning electron microscope. Both crack front and crack wake toughening mechanisms were studied in isolation. In the crack front study, the crack propagation behavior varied with fiber coatings. For the uncoated fibers, a three-fold load increase was necessary to advance the crack over the fibers. A gradual load increase was necessary to propagate the crack over the fibers that were initially polymer coated. In samples where the fibers were either metal or polymer/metal coated, the crack becomes unstable before reaching the fiber positions and the fibers had no effect in resisting the crack propagation. However, the metal coated samples reached the highest toughness values during the crack wake study, with an increase in toughness of 75% over the monolithic samples. Crack profiles were measured and the crack opening displacements revealed direct evidence of the crack fiber interactions.

Creep limitations of current polycrystalline ceramic fibers

The objective of this paper is to present an overview of the issues, property status, and potential for use of creep-prone polycrystalline ceramic fibers in thermostructural ceramic composites. Issues arise because the fine-grained microstructures of high-strength fibers can result in creep-related property changes, often at temperatures as low as 800°C. The underlying mechanism is grain boundary sliding controlled by grain size and grain boundary character, and thus by the fiber processing method. With the assumption of upper and lower limit creep requirements, the creep properties of a variety of current SiC and Al 2O 3 polycrystalline fibers are reviewed and discussed. Property evaluation is based on the results of a simple bend stress relaxation test which allows predictive creep equations to be developed for each fiber type describing the effects of time, temperature, and applied stress. It is shown that sintered SiC fibers with grain sizes below 1000 nm appear to offer the best performance in terms of strength and creep resistance. However, even these fibers may not be capable of long-term service above 1400°C.