Strength of Nicalon Silicon Carbide Fibers Exposed to High-Temperature Gaseous Environments

The effects of exposures to high-temperature gaseous atmospheres on the strength of Nicalon SiC fibers were investigated. The exposure conditions were as follows: (1) H₂ with various P _H2O for 10 h at 1000° and 1200°C, and (2) air for 2 to 100 h at 800° to 1400°C. Individual fibers were tested in tension following each exposure. The strengths of the fibers were strongly influenced by the exposure atmosphere and temperature, but less affected by time at temperature. When exposed in air, a SiO₂ layer was formed on the surface, minimizing the degradation of strength. However, this beneficial effect was negated under conditions in which the SiO₂ layer became too thick. The most severe degradation resulted from exposure to a reducing atmosphere, presumably due to the reduction of SiO₂ inherent in the fibers.     

In Situ Reaction Synthesis of Silicon Carbide–Boron Nitride Composite from Silicon Nitride–Boron Oxide–Carbon

This work proposes a new approach, based on the reaction Si₃N₄+ 2B₂O₃+ 9C → 3SiC + 4BN + 6CO, to synthesize an SiC–BN composite. The composite was prepared by reactive hot pressing (RHP), at 2000°C for 60 min at 30 MPa under an argon atmosphere, following a 60 min hold at 1700°C without applied pressure before reaching the RHP temperature. TG-DTA results showed that a nitrogen atmosphere inhibited denitrification somewhat and retarded the reaction rate. The chemical composition of the obtained material was consistent with theoretical values. FE-SEM observation showed that in situ -formed SiC and BN phases were of spherical morphology with very fine particle size of ∼100 nm.     

In Situ Reaction Synthesis of Silicon Carbide–Boron Nitride Composites

SiC–BN composites were prepared via the proposed in situ reaction, which used Si₃N₄, B₄C, and carbon as reactants. Adding SiC powder to the reactants controlled the BN content in the composite. For comparison, SiC–BN composites with the same phase compositions were produced via conventional processing. The in situ process was advantageous for obtaining better composites with fine grain size and homogeneous microstructures. The in situ composite that had a BN content of 53.71 vol% exhibited considerably high strength (342 MPa) and a very low elastic modulus (107 GPa). The SiC–25-vol%-BN in situ composite had a peak strength of 588 MPa, which was 95% of that of monolithic SiC; however, the elastic modulus was as low as half that of monolithic SiC. These in situ SiC–BN composites can be expected to have excellent thermal shock resistance and mechanical strain tolerance.     

先驱体转化法制备含硼SiC纤维

通过二甲基胺硼烷（dimethylamine borane,DMAB）与低分子量聚碳硅烷（low-molecular mass polycarbosilane,LPCS）反应合成硼溶胶,将硼溶胶与高分子量聚碳硅烷（high-molecular mass polycarbosilane,HPCS）共混制备含硼聚碳硅烷先驱...     

Mirror Constant for Tyranno® Silicon-Titanium-Carbon-Oxygen Fibers Measured in Situ in a Three-Dimensional Woven Silicon Carbide/Silicon Carbide Composite

The strength, S , of ceramic and glass fibers often can be estimated from fractographic investigation using the fracture mirror radius, r _m, and the relationship S = A _m/( r _m)^1/2, where A _mis the "mirror constant." The present work estimates the value of A _mfor Tyranno^® Si-Ti-C-O fibers in situ in a three-dimensional woven SiC/SiC-based composite to be 2.50 ± 0.09 MPa·m^1/2. This value is within the range of 2–2.51 MPa·m^1/2 previously obtained for nominally similar Nicalon^® Si-C-O fibers.     

Thermogravimetry, Differential Thermal Analysis, and Mass Spectrometry Study of the Silicon Nitride–Boron Carbide–Carbon Reaction System for the Synthesis of Silicon Carbide–Boron Nitride Composites

Thermogravimetry, differential thermal analysis, mass spectrometry, and X-ray diffractometry were used to study the reaction process of the in situ reaction between Si₃N₄, B₄C, and carbon for the synthesis of silicon carbide–boron nitride composites. Atmospheres with a low partial pressure of nitrogen (for example argon + 5%–10% nitrogen) seemed to inhibit denitrification and also maintain a high reaction rate. However, the reaction rate decreased significantly in a pure nitrogen atmosphere. The experimental mass spectrometry results also revealed that B₄C in the Si₃N₄–B₄C–C system inhibited the reaction between Si₃N₄ and carbon and, even, the decomposition of Si₃N₄. The present results indicate that boron could be a composition stabilizer for ceramic materials in the Si-N-C system used at high temperature.     

Effect of Amount of Boron Doping on Compression Deformation of Fine-Grained Silicon Carbide at Elevated Temperature

The effect of the amount of boron doping in the range of 0 to 1.0 wt% on the high-temperature deformation of fine-grained β-silicon carbide (SiC) was investigated by compression testing. Flow stress at the same grain size increased as the amount of boron doping decreased. The stress exponent increased from 1.3 to 3.4 as the amount of boron doping decreased. The strain rates of undoped SiC were ∼2 orders of magnitude lower than those of 1.0-wt%-boron-doped SiC of the same grain size. The apparent activation energies of SiC doped with 1.0 wt% boron and of undoped SiC were 771 ± 12 and 884 ± 80 kJ/mol, respectively. These results suggest that the actual contribution of grain-boundary diffusion to the accommodation process of grain-boundary sliding decreased as the amount of boron doping decreased. Consequently, the apparent contribution of the dislocation glide increased.     

Oxidation of Low-Oxygen Silicon Carbide Fibers (Hi-Nicalon) in Carbon Dioxide

Polycarbosilane-derived low-oxygen SiC fibers, Hi-Nicalon, were heat-treated for 36 ks at temperatures from 1273 to 1773 K in CO₂ gas. The oxidation of the fibers was investigated through the examination of mass change, crystal phase, resistivity, morphology, and tensile strength. The mass gain, growth of β-SiC crystallites, reduction of resistivity of the fiber core, and formation of protective SiO₂ film were observed for the fibers after heat treatment in CO₂ gas. SiO₂ film crystallized into cristobalite above 1573 K. Despite the low oxygen potential of CO₂ gas ( p _O2= 1.22 Pa at 1273 K − 1.78 × 10² Pa at 1773 K), Hi-Nicalon fibers were passively oxidized at a high rate. There was a large loss of tensile strength in the as-oxidized state at higher temperatures because of imperfections in the SiO₂ film. On the other hand, the fiber cores showed better strength retention even after oxidation at 1773 K.     

Molybdenum Oxycarbide and Tungsten Oxycarbide Coatings on Silicon Carbide Fibers

The strength and toughness of fibrous composites depend on the interface properties which control the bonding between the fibers and matrices. One method of controlling the interface involves coating the fiber with an appropriate material. In a previous study, it was found that there is a definite advantage in using low coating temperatures to prevent fibers from degrading. We therefore were interested in a report that Mo₂C could be deposited from Mo(CO)₆ at temperatures as low as 300° to 475°C. Our studies indicated that the material was not Mo₂C, but an oxycarbide, which, with an analogous tungsten oxycarbide coating, was applied to SiC yarns. Both oxycarbides could be converted to the metals by heat-treating in N₂.     

Suppression of Active Oxidation of Polycarbosilane-Derived Silicon Carbide Fibers by Preoxidation at High Oxygen Pressure

Three types of polycarbosilane-derived SiC fibers (Nicalon, Hi-Nicalon, and Hi-Nicalon S) with different SiO₂ film thicknesses ( b ) were subjected to exposure tests at 1773 K in an argon-oxygen gas mixture with an oxygen partial pressure of 1 Pa. The suppression effect of a SiO₂ coating on active oxidation was examined through TG, XRD analysis, SEM observation, and tensile tests. All the as-received fibers were oxidized in the active-oxidation regime. The mass gain and the SiO₂ film development showed a suppression of active oxidation at b values of ≧0.070 μm for Nicalon, ≧0.013 μm for Hi-Nicalon, and ≧0.010 μm for Hi-Nicalon S fibers. Considerable strength was retained in the SiO₂-coated fibers. For Hi-Nicalon fibers, the retained strength was 71%–90% of the strength in the as-received state (2.14–2.69 GPa).     

Pressure Effects on the Thermal Stability of Silicon Carbide Fibers

Commerically available polymer-derived SiC fibers were treated at temperatures from 1000° to 2200°C under vacuum and at argon gas pressures of 0.1 and 138 MPa. Effects of increasing inert gas pressure on the thermal stability of the fibers were determined through studies of the fiber microstructure, weight loss, grain growth, and tensile strength. The 138-MPa argon gas treatment was found to shift the onset of fiber weight loss from 1200° to above 1500°C. Grain growth and tensile strength degradation were correlated with weight loss and were thus also inhibited by high-pressure treatments. Retreatment in 0.1 MPa of argon of the fibers initially treated in 138 MPa of argon caused further weight loss and tensile strength degradation, thus indicating that high-pressure inert gas conditions were effective only in delaying fiber strength degradation and that no permanent microstructural changes were induced.     

Strengths of Ceramic Fibers at Elevated Temperatures

Tensile strengths were measured and Young's moduli were estimated for two SiC-based and three oxide ceramic fibers for temperatures from 25° to 1400°C. The SiC-based fibers were stronger but less stiff than the oxide fibers at room temperature and retained more of both strength and stiffness to high temperatures. High-temperature strengths of the SiC-based fibers were limited by internal void formation and oxidation; those of the oxide fibers were limited by softening of an intergranular glassy phase.     

Models Proposed to Explain the Electrical Conductivity of Polymer‐Derived Silicon Carbide Fibers

Two types of silicon carbide fibers (SiC_f) were prepared employing different pyrolysis techniques. The relationship between the microstructure and the electrical resistivity of the fibers was investigated. The results indicated that the carbon layer present on the fiber surface acted as the main conductive phase in the SiC_f obtained by direct pyrolysis, whereas a free carbon phase determined the conductivity of the SiC_f prepared by the preheated pyrolysis method. A core‐shell model and a general effective media (GEM) theory were proposed to explain the conductivity of different types of SiC_f. Quantitative analysis based on these models indicated an electrical resistivity of ~10^?2 Ω·cm for the carbon layer on the surface of SiC_f obtained by direct pyrolysis. The electrical resistivity and the percolation threshold of the free carbon in SiC_f prepared by the preheated pyrolysis method were 10^?1 Ω·cm and 11.3% respectively.     

Growing SiC Nanowires on Tyranno-SA SiC Fibers

A new in situ process for growing SiC nanowires on Tyranno-SA SiC fibers (2-D, plain-woven) was developed using the thermal decomposition of methyltrichlorosilane in hydrogen. The process was performed using a chemical vapor infiltration system. β-SiC nanowires ∼100-nm thick and several tens of micrometers long were successfully synthesized on the fibers. The growing of the SiC nanowires suggests a conditions-dependent process.     

Carbothermal Synthesis of Boron Nitride Coatings on Silicon Carbide

Pure BN coatings have been synthesized on the surface of SiC powders and fibers by a novel carbothermal nitridation method. Three stages are involved in the process: first, formation of a carbon layer on the SiC by the extraction of Si with chlorine; second, infiltration of the resulting nanoporous carbide-derived carbon (CDC) coating by a saturated boric acid solution; and finally, nitridation in ammonia at atmospheric pressure to produce the pure BN coating. X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), and electron energy loss spectroscopy (EELS) were used to characterize the phase, elemental composition, and surface morphology of the coatings. The intermediate carbon layer acts as a template for BN growth, facilitates the formation of BN, and prevents the degradation of SiC fibers during nitridation. The whole process is simple, cost-effective, and less toxic due to the use of H₃BO₃ and NH₃ as precursors at atmospheric pressure compared with most commonly used chemical vapor deposition (CVD) methods. Uniform BN coatings obtained by this method prevent the bridging of fibers in the tow. The coating of powders is possible, which cannot be achieved by conventional CVD methods.     

Machinability of Silicon Nitride/Boron Nitride Nanocomposites

The machinability and deformation mechanism of Si₃N₄/BN nanocomposites were investigated in the present work. The fracture strength of Si₃N₄/BN microcomposites remarkably decreased with increased hexagonal graphitic boron nitride ( h -BN) content, although machinability was somewhat improved. However, the nanocomposites fabricated using the chemical method simultaneously had high fracture strength and good machinability. Hertzian contact tests were performed to clarify the deformation behavior by mechanical shock. As a result of this test, the damage of the monolithic Si₃N₄ and Si₃N₄/BN microcomposites indicated a classical Hertzian cone fracture and many large cracks, whereas the damage observed in the nanocomposites appeared to be quasi-plastic deformation.     

Strength Distribution of Reinforcing Fibers in a Nicalon Fiber/Chemically Vapor Infiltrated Silicon Carbide Matrix Composite

The strength distribution of fibers within a two-dimensional laminate ceramic/ceramic composite consisting of an eight harness satin weave of Nicalon continuous fibers within a chemically vapor infiltrated SiC matrix was determined from analysis of the fracture mirrors of the fibers. Comparison of the fiber strengths and the Weibull moduli with those for Nicalon fibers prior to incorporation into composites suggests that possible fiber damage may occur either during the weaving or during another stage of the composite manufacture. Observations also indicate that it is the higher-strength fibers which experience the greatest extent of fiber pullout and thus make a larger contribution to the overall composite toughness than do the weaker fibers.     

碳化硅纤维及其复合材料

碳化硅纤维及其复合材料是目前使用温度最高的增强材料和先进复合材料，本文简要介绍先驱体转换法和化学气相沉积法制备碳化硅纤维的工艺，不同工艺方法制备的碳化硅纤维的性能比较，碳化硅纤维及其复合材料的现状与应用。     

Fracture Toughness and Thermal Shock Behavior of Silicon Nitride–Boron Nitride Ceramics

Fracture toughness behavior, stress–strain behavior, and flaw resistance of pressureless-sintered Si₃N₄-BN ceramics are investigated. The results are discussed with respect to the reported thermal shock behavior of these composites. Although the materials behave linear-elastic and exhibit no R -curve behavior, their flaw resistance is different from that of other linear-elastic materials. Whereas the critical thermal shock temperature difference (Δ T _c) is enhanced by adding BN, the content of BN has no influence on the strength loss during severe thermal shocks.     

Layered Silicon Nitride-Based Composites with Discontinuous Boron Nitride Interlayers

The influence of a strong/weak interface ratio on the mechanical properties of Si₃N₄/BN-based layered composites was studied. The ratio was controlled by the number of BN spots between the adjacent Si₃N₄ layers. By increasing the BN interface area from 0% to 72%, fracture toughness increased from 7.7 to 10.9 MPa·m^1/2, and bending strength decreased from 1275 to 982 MPa. Fracture toughness was improved from 8.6 to 10.1 MPa·m^1/2 by additional heat treatment of samples containing 2 vol%β-Si₃N₄ seed particles. The bending strength of samples with 35% weak BN interfaces, measured perpendicular and parallel to layer alignment, was 1260 and 1240 MPa, respectively. This confirmed the two-directional isotropy of layered samples.