Concurrent Flaw Populations in SiC

Concurrent surface and edge flaw populations were identified on fracture surfaces of as-machined specimens of sintered silicon carbide. The overall strength distribution and the individual concurrent distributions were estimated using Weibull statistics. Design implications of using a single flaw population and the difficulties associated with rigorously separating the concurrent distributions are discussed.     

A New Modified Weibull Distribution Function

The tensile strength of two groups of fused silica optical fibers, reported by previous workers, has been analyzed in terms of a modified Weibull flaw distribution function. The function provides upper and lower strength limits and is characterized by two shape and two location parameters. The long-length strengths predicted from this function are in close agreement with the experimental data.     

Evaluation of Bimodal Concurrent Flaw Distributions

Bimodal concurrent flaw distributions are often observed in strength testing of ceramics. A technique is proposed to evaluate the appropriate Weibull parameters of the individual concurrent distributions where the type of flaw causing failure can be identified. This technique involves separating the data according to flaw origin and then analyzing each of these two censored data sets .     

Strength Distribution in Commercial Silicon Carbide Materials

Four types of commercial silicon carbide samples from different sources were characterized in terms of baseline strength and strength distribution (reliability). Four-point flexural strength of each material was determined on 30 test bars, 5.1 by 0.64 by 0.32 cm, for a reliable estimate of the Weibull modulus values. The results show that the average strength of these sintered silicon carbide samples ranged from 380 to 482 MPa (55 to 69 ksi) at room temperature and 307 to 470 MPa (45 to 68 ksi) at 1370°C (2500°F). Considerable variations in strength were found among specimens of each material. Baseline Weibull modulus values ranged from 8 to 11 at room temperature and 7 to 11 at 1370°C (2500°F). The strength scatter clearly reflected flaw variability, which must be minimized to improve reliability in sintered silicon carbide materials.     

Estimation of Fracture Toughness by Intrinsic Flaw Fractography for Sintered Alpha Silicon Carbide

An attempt to apply three-dimensional flaw models to characterize fracture origins in sintered alpha silicon carbide is made. Some general improvements in the intrinsic flaw fractography results are obtained by using three-dimensional void models instead of two-dimensional crack models.     

Ultrasonic Evaluation of High-Density Silicon Carbide Ceramics

Nondestructive ultrasound testing has been evaluated as a technique for analyzing isolated bulk defects and microstructural inhomogeneities in silicon carbide (SiC). Three SiC samples of varying thickness, two of which were fabricated by hot pressing and a third that was fabricated by chemical vapor deposition (CVD), were characterized using pulse–echo ultrasound characterization at a frequency of 75 MHz. Point analysis techniques were utilized to measure variations in time-of-flight (TOF), or ultrasound travel time through each sample, for calculation of regional differences in material velocity and elastic properties. C-scan imaging was used to evaluate differences in both TOF and reflected signal amplitude over the area of each sample. Area-under-the-curve (AUTC) and full-width at half-maximum (FWHM) data were obtained from normalized histograms to establish trends for direct sample comparison. It was determined that lower AUTC and FWHM values correlated to higher density samples with fewer inhomogeneities. However, the histogram tail area and distribution were also important features, providing information about specific inhomogeneities and their distributions.     

Homogeneous Distribution of Sintering Additives in Liquid-Phase Sintered Silicon Carbide

The addition of sintering additives to silicon carbide particles by electrostatic adsorption of colloidal A1₂O₃ and Y₂O₃ sols has been studied as a way to achieve an optimum homogeneity in the microstructure. The adsorption behavior of the sol particles was examined by electrophoretic measurements and X-ray fluorescence analysis. Both A1₂O₃ and Y₂O₃ sols could simultaneously be adsorbed on the SiC particle surfaces. Viscosity measurements showed that the colloidal sol particles had a stabilizing effect on the slip, and hence slips with relatively high solid loadings could be prepared without adding extra dispersing agent. Liquid-phase-sintered silicon carbide materials (LPS-SiC) with 2 wt% A1₂O₃ and 1 wt% Y₂O₃ were prepared by freeze granulation/ pressing and sintering at 1880^deg;C for 4 h. The homogeneity of the green compacts was quantified using a spot analysis technique in an electron probe microanalyzer. It was clearly shown that the addition of sols gave a more homogeneous microstructure than the reference sample with Y₂O₃ and A1₂O₃ added as powders. The addition of sintering additives as sols also enhanced the sintering behavior.     

Synthesis of Silicon Carbide Ceramics Using Chemically Modified Polycarbosilanes as a Compaction Binder

A chemically modified polycarbosilane (PC) containing organofluoric groups (PCOCF) has been synthesized from PC and fluoroalkylmethyldimethoxysilane. PCOCF acts as an efficient compaction binder for SiC powders and as a coating material with excellent oxidation resistance in wet air. PCOCF-coated SiC powders also show excellent packing properties because of the organofluoric side chains, which give highly dense green compacts. PCOCF provides a high ceramic yield of 75% and highly dense SiC ceramics. Four-point bending strength increases and the scatter in strength values decreases significantly by PCOCF coating.     

Oxidation Resistance Evaluation of Silicon Carbide Ceramics with Various Additives

Five silicon carbide ceramics with various additives were evaluated for oxidation resistance at 1300°C in flowing dry and wet air. In the dry atmosphere, the oxidation of the five samples was diffusion-controlled, and in wet atmosphere they exhibited a linear relation beween weight gain by oxidation and water vapor content. Water vapor in the atmosphere strongly accelerated oxidation. The influence of oxidation on room-temperature strength was complex, but the samples were not as affected by oxidation.     

Tensile Creep Behavior of a Gas-Pressure-Sintered Silicon Nitride Containing Silicon Carbide

The tensile creep behavior of a gas-pressure-sintered silicon nitride containing silicon carbide was characterized at temperatures between 1375° and 1450°C with applied stresses between 50 and 250 MPa. Individual specimens were tested at fixed temperatures and applied loads. Each specimen was pin-loaded within the hot zone of a split-tube furnace through silicon carbide rods connected outside the furnace to a pneumatic cylinder. The gauge length was measured by laser extensometry, using gauge markers attached to the specimen. Secondary creep rates ranged from 0.54 to 270 Gs⁻¹, and the creep tests lasted from 6.7 to 1005 h. Exponential functions of stress and temperature were fitted to represent the secondary creep rate and the creep lifetime. This material was found to be more creep resistant than two other silicon nitride ceramics that had been characterized earlier by the same method of measurement as viable candidates for high-temperature service.     

Joining of Silicon Carbide/Silicon Carbide Composites and Dense Silicon Carbide Using Combustion Reactions in the Titanium–Carbon–Nickel System

A new ceramic joining technique has been developed that utilizes an exothermic combustion reaction to simultaneously synthesize the joint interlayer material and to bond together the ceramic workpieces. The method has been used to join SiC/SiC composites and dense SiC ceramics using TiC-Ni powder mixtures that ignite below 1200°C to form a TiC-Ni joining material. Thin layers of the powder reactants were prepared by tape casting, and joining was accomplished by heating in a hot-press to ignite the combustion reaction. During this process, localized exothermic heating of the joint region resulted in chemical interaction at the interface between the TiC-Ni and the SiC ceramic that contributed to bonding. Room-temperature four-point bending strengths of joints produced by this method have exceeded 100 MPa.     

Process and Mechanical Properties of in Situ Silicon Carbide-Nanowire-Reinforced Chemical Vapor Infiltrated Silicon Carbide/Silicon Carbide Composite

A SiC nanowire/Tyranno-SA fiber-reinforced SiC/SiC composite was fabricated via simple in situ growth of SiC nanowires directly in the fibrous preform before CVI matrix densification; the purpose of the SiC nanowires was to markedly improve strength and toughness. The nanowires consisted of single-crystal β-phase SiC with a uniform ∼5 nm carbon shell; the nanowires had diameters of several tens to one hundred nanometers. The volume fraction of the nanowires in the fabricated composite was ∼5%. However, the composite did not show significant increase in strength and toughness, likely because of strong bonding between the nanowires and the matrix caused by the very thin carbon coating on the nanowires. Little debonding and pullout of SiC nanowires from the matrix were observed at the fracture surfaces of the composite.     

Accumulation of Creep Damage in a Siliconized Silicon Carbide

The accumulation of creep damage in a siliconized silicon carbide was investigated as a function of applied stress, creep strain, and microstructure. At 1100°C, creep damage was observed to accompany deformation in specimens tested to creep strains greater than 0.10%, under applied stresses greater than 137 MPa. At low creep strains, creep damage occurred in regions of the microstructure of high silicon carbide content. As deformation progressed, creep damage extended into regions of the microstructure of lower silicon carbide content. The area density and area fraction of cavities were found to increase linearly with creep strain. From these results, a threshold stress for the formation of creep damage was determined to be 132 MPa at 1100°C. It was suggested that the formation of creep damage was controlled by the heterogeneous nucleation of cavities at the silicon-silicon carbide interface, with the aid of high localized stresses and iron impurities in the silicon phase.     

Silicon/Silicon Carbide Composites Fabricated by Infiltration of a Silicon Melt into Charcoal

Dense Si/SiC composites were fabricated via a conventional reaction-bonding process, using oak charcoal that exhibited a honeycomb structure. The silicon melt was infiltrated into the porous oak charcoal (density of ~0.6 g/cm³) while the sample was heated to 1700°C under vacuum (10^-3 torr (~0.133 Pa)), which resulted in in situ silicon-fiber/SiC composites. The reaction product had an average density of 2.8 g/cm³ and showed three-point flexural strengths of 330 MPa at room temperature and 280 MPa at 1300°C. Good oxidation resistance also was observed at temperatures up to 1300°C in flowing air. This process provided excellent shape-making capability, because the charcoal that was used as a preform was readily machinable.     

碳化硅在聚合物中的应用

综述了碳化硅（SiC）在用填充改性、表面包覆改性、离子注入改性和聚合物表面接枝改性等方法所制得的复合材料中的应用，并对纳米SiC改性环氧树脂、纳米微晶SiC改性聚乙烯基咔唑和香豆素共混物、聚吡咯包覆改性SC、硅离子注入改性聚对苯二甲酸乙二醇酯薄膜及丙烯酰胺接枝改性SiC等进行了详细讨论     

Effect of Reinforcement of a Silicon Nitride Matrix by Silicon Carbide Whiskers

A fine-grain high-density tough matrix is a general prerequisite for synthesis of high-strength and crack-resistant composite materials, as well as its self-reinforcement with extended grains of β-Si₃N₄ and reinforcement with β-SiC crystals. Both approaches are used simultaneously in the present work. It is shown that the reinforcement of a silicon nitride matrix based on ultradisperse powder compositions by SiC whiskers of grade TWS provides hot-pressed ceramics with a high ultimate bending strength (950 MPa) and crack resistance (10 MPa · m^1/2). Reinforcement by fine or coarse whiskers increases the crack resistance by 30% with respect to monolithic Si₃N₄. Composites reinforced by TWS silicon carbide whiskers preserve their high strength up to 1500°C.     

Joining of Silicon Carbide with a Cordierite Glass-Ceramic

A method for the joining of silicon carbide using a cordierite glass-ceramic has been developed. Cordierite, with glass-ceramic processing, remains amorphous and wets the SiC substrate to form a strong bond when rapidly fired. Subsequent heat treatment crystallizes a multiphase interlayer with a matching bulk thermal expansion coefficient (CTE). A benchtop tape casting method for depositing joining precursor films of varying thickness is described. The wetting characteristics of cordierite on SiC that are pertinent to the joining process are shown to be highly sensitive to processing atmosphere. Doping with a fluoride ion flux can lower the peak processing temperature without significantly altering the crystallization path. The effect of interlayer thickness is observed by monitoring indentation crack paths and with 4-point bending tests. Controlling the degree of crystallinity is shown to tailor the mismatches in thermal expansion coefficient and elastic moduli to produce joints of high strength (σ_F > 500 MPa). Characterization is accomplished with XRD, SEM, and TEM.     

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.     

Preparation of Silicon Carbide and Aluminum Silicon Carbide from a Montmorillonite–Polyacrylonitrile Intercalation Compound by Carbothermal Reduction

Both silicon carbide and aluminum silicon carbide have simultaneously been obtained directly from naturally occurring aluminosilicate by carbothermal reduction for the first time. A precursor of a montmorillonite–polyacrylonitrile (PAN) intercalation compound was heated at 1700°C in Ar. For comparison, montmorillonite–carbon mixtures were similarly heated. α-SiC, β-SiC, and Al₄Si₂C₅ formed from the montmorillonite–PAN intercalation compound. Mainly α-Al₄SiC₄ was obtained with ternary carbides from the montmorillonite–carbon mixtures in addition to a large amount of β-SiC. Hence, aluminum silicon carbide formation was affected by the mixing condition of the starting materials.     

Damage-Enhanced Creep in a Siliconized Silicon Carbide: Phenomenology

The creep behavior of a commercial grade of reaction-bonded silicon carbide was characterized at a temperature of 1300°C. Creep occurred more easily in tension than in compression. At a given applied stress, the steady-state creep rate in tension was found to be at least 20 times that obtained in compression. In both tension and compression, the stress exponent for steadystate creep was found to increase with increasing applied stresses. At low applied stresses, the stress exponent was ∼4, suggesting some kind of dislocation mechanism operating in the two-phase composite. At high stresses, the stress exponent was ∼11 in tension. The increase in the stress exponent was attributed to damage accumulation in the form of cavities. An effective threshold stress for cavitation of less than 100 MPa was suggested. In compression, the cause of the increase of stress exponent with stress cannot be attributed to cavitation.