Kinetics and Mechanisms of High-Temperature Creep in Silicon Carbide: II, Chemically Vapor Deposited

Chemically vapor deposited (CVD) silicon carbide was subjected to constant compressive stresses (110 to 220 MN/m²) at high temperatures (1848 to 2023 K) in order to determine the controlling steady-state creep mechanisms under these conditions. An extensive TEM study was also conducted to facilitate this determination. The strong preferred crystallographic orientation of this material causes the creep rate to be very dependent on specimen orientation. The stress exponent, n , in the equation εασⁿ was calculated to be 2.3 below 1923 K and 3.7 at 1923 K. The activation energy for steady-state creep was determined to be 175 ± 5 kJ/mol throughout the temperature range employed. At temperatures between 1673 and 1873 K, the controlling creep mechanism for CVD Sic is dislocation glide, which is believed to be controlled by the Peierls stress. Although the activation energy does not change, the increase in the stress exponent for samples deformed at 1923 K suggests that the controlling creep mechanism becomes dislocation glide/climb controlled by climb.     

Kinetics and Mechanisms of High-Temperature Creep in Silicon Carbide: III, Sintered α-Silicon Carbide

The operative and controlling mechanisms of steady-state creep in sintered α-SiC have been determined both from kinetic data within the ranges of temperature and constant compressive stress of 1670 to 2073 K and 138 to 414 MPa, respectively, and from the results of extensive TEM and other analytical analyses. Dislocations in glide bands, B₄C precipitates, and the interaction of these two entities were the dominant microstructural features of the crept material. The stress exponent increased from 1.44 to 1.71 with temperature; it was not a function of stress at a given temperature. The curves of In ɛ vs 1/ T showed a change in slope at 1920 ± 20 K. The respective activation energies below and above this temperature interval were 338 to 434 and 802 to 914 kJ/mol. A synthesis of all this information leads to the conclusion that the controlling creep mechanism at low temperatures is grain-boundary sliding accommodated by grain-boundary self-diffusion; at high temperatures, the controlling mechanism becomes grain-boundary sliding accommodated by lattice diffusion. The parallel mechanism of dislocation glide contributes increasingly to the total strain as the number/volume of precipitates declines as a result of progressive coalescence with increasing temperature.     

反应烧结碳化硅研究进展

对有关反应结合碳化硅(RBSC)材料的研究进展作了综述,并对存在的问题和今后可能的发展方向提出了自己的见解,包括:进一步提高性能;降低游离硅含量,提高使用温度;提高材料的可靠性和稳定性;低成本化.     

High-Temperature Creep and Microstructural Evolution of Chemically Vapor-Deposited Silicon Carbide Fibers

The creep behavior of three types of silicon carbide fibers that have been fabricated via chemical vapor deposition is described. The fibers exhibit only primary creep over the range of conditions studied (1200°–1400°C, 190–500 MPa). A transmission electron microscopy study of the microstructural development that is induced by the creep deformation of SCS-6 silicon carbide fibers at 1400°C is presented. Significant grain growth occurs in all silicon carbide regions of the fiber during creep, in contrast to the reasonably stable microstructure that is observed after annealing at the same temperature and time.     

Reaction-Bonded and Superplastically Sinter-Forged Silicon Nitride–Silicon Carbide Nanocomposites

Silicon nitride–silicon carbide (Si₃N₄–SiC) nanocomposites were fabricated by a process involving reaction bonding followed by superplastic sinter-forging. The nanocomposites exhibited an anisotropic microstructure, in which rod-shaped, micrometer-sized Si₃N₄ grains tended to align with their long axes along the material-flow direction. SiC particles, typically measuring several hundred nanometers, were located at the Si₃N₄ grain boundaries, and nanosized particles were dispersed inside the Si₃N₄ grains. A high bending strength of 1246 ± 119 MPa, as well as a high fracture toughness of 8.2 ± 0.9 MPa·m^1/2, was achieved when a stress was applied along the grain-alignment direction.     

反应烧结碳化硅陶瓷注射成型工艺研究

以碳化硅(Si C)和炭黑(C)为原料,以石蜡(PW)、高密度聚乙烯(HDPE)、乙烯醋酸乙烯酯(EVA)为有机载体,以硬脂酸(SA)为表面改性剂,研究了反应烧结碳化硅陶瓷注射成型工艺对产品性能的影响。结果表明:在陶瓷粉体含量为80 wt%,有机载体以PW:HDPE:EVA:SA=9:3:3:1的比例,加入量为20 wt%,混炼1 h后,在100 MPa注射压力注射成坯,采用两步法脱脂,于真空烧结炉内1720°C下保温2h烧结,可获得结构致密的碳化硅陶瓷试样,其显气孔率为0.18%,密度为2.96 g/cm~3,抗弯强度达到290 MPa,断裂韧性值达到4.14MPa·m~(1/2),硬度达到21.6 GPa。     

氧化反应结合SiC基陶瓷的制备与性能

本文采用反应结合制备方法,通过对坯体进行预氧化使ＳｉＣ颗凿表面氧化形成ＳｉＯ２,而后在烧成中与添中剂ＡＩ２Ｏ３－Ｙ２Ｏ３反应,使坯体气化率减少,制备了多孔ＳｉＣ基陶瓷。文章探讨了坯体中ＳｉＣ的氧化特征、反应结合过程和相变化以及它们对烧结体性能的影响。     

Push-Out Tests on a New Silicon Carbide/Reaction-Bonded Silicon Carbide Ceramic Matrix Composite

Fiber push-out tests have been performed on a ceramic matrix composite consisting of Carborundum sintered SiC fibers, with a BN coating, embedded in a reaction-bonded SiC matrix. Analysis of the push-out data, utilizing the most complete theory presently available, shows that one of the fiber/coating/matrix interfaces has a low fracture energy (one-tenth that of the fiber) and a moderate sliding resistance τ∼ 8 MPa. The debonded sliding interface shows some continuous but minor abrasion, which appears to increase the sliding resistance, but overall the system exhibits very clean smooth sliding. The tensile response of a full-scale composite is then modeled, using data obtained here and known fiber strengths, to demonstrate the good composite behavior predicted for this material.     

反应烧结碳化硅的显微组织及其导电性的研究

研究了液态硅参与下的反应烧结碳化硅的工艺参数、显微组织对其电阻率的影响.随着烧结气氛压力和成型压力增加,反应烧结碳化硅中游离硅量减少,电阻率增加.其烧结机理以碳的溶解及碳化硅的淀析过程为主.     

Mechanisms and Kinetics of Reaction-Bonded Aluminum Oxide Ceramics

Reaction-bonded Al₂O₃ (RBAO) ceramics were fabricated starting from mechanically alloyed Al₂O₃/Al, Al₂O₃/ Al/ZrO₂, and Al₂O₃/Al/ZrO₂/Zr mixtures. Isopressed compacts were heat-treated in air up to 1550°C. Reaction-bonding mechanisms, kinetics, and the influence of ZrO₂ and Zr additions are investigated. Independent of additive, oxidation of Al proceeds both as solid/gas and liquid/gas reaction, and the reaction kinetics follow a parabolic rate law. The reaction rate depends strongly on the particle size of Al. The activation energy of the reaction depends essentially on green density. Below the melting temperature of Al, in samples containing 45 vol% Al and 55 vol% Al₂O₃, it is 112 and 152 kJ/mol at ∼64% and ∼74% TD, respectively, while above the melting temperature, it lies in the range ∼ 26–33 kJ/mol. Zr additions reduce the activation energy to some extent. Samples with only ZrO₂ additions exhibit nearly the same activation energies as ZrO₂-free samples, though ZrO₂ has a very positive effect on the microstructural development in RBAO ceramics. Microstructure evolution and some strength data of RBAO bodies are also reported.     

Oxidation Kinetics of Silicon Carbide Crystals and Ceramics: I, In Dry Oxygen

The oxidation kinetics of several single-crystal and polvcrystalline silicon carbide materials and single-crystal silicon in dry oxygen over the temperature range 1200° to 1500°C were fitted to the linear-parabolic model of Deal and Grove. The lower oxidation rates of silicon carbide compared to silicon can be rationalized by additional consumption of oxidant in oxidizing carbon to carbon dioxide. The (000J) Si face of the silicon carbide platelets exhibited lower parabolic oxidation rates than the (0001) C face, by a factor of 10 at 1200°C. Apparent activation energies increased from a value of ∼120 kJ/mol below 1400°C to a value of ∼300 kJ/mol above this temperature. The (0001) Si face exhibited this high activation energy over the entire temperature range. The controlled nucleation thermally deposited material exhibited the highest oxidation rates of the polycrystalline materials followed by the hot-pressed and sintered α-silicon carbides. In general, the oxidation rates of the polycrystalline materials were bracketed by the oxidation rates of the basal planes of the single-crystal materials. Higher impurity concentrations and higher density of nucleation sites led to a greater susceptibility to crystallization of the scale which significantly complicated the oxidation behaviors observed. When crystallization of the oxide scale occurred in the form of a layer of spherulitic cristobalite crystals, a retardation of the oxidation rates was observed. An accelerated oxidation behavior was found when this coherent layer was superseded by the formation of fine mullite crystals.     

Post-Hot-Pressing and High-Temperature Bending Strength of Reaction-Bonded Silicon Nitride-Molybdenum Disilicide and Silicon Nitride-Tungsten Silicide Composites

A hot-pressing technique was used for the further densification of reaction-bonded silicon nitride-molybdenum disilicide and silicon nitride-tungsten silicide (Si₃N₄-MoSi₂ and Si₃N₄-WSi₂, respectively) compacts that were prepared via a presintering step and a nitriding process from silicon-molybdenum or silicon-tungsten powders. After hot pressing was performed at 1650°C (25 MPa for 1 h), most of the alpha-Si₃N₄ that formed during the reaction-bonding process was transformed to β-Si₃N₄ and, moreover, a very small amount of Mo₅Si₃ (W₅Si₃) was formed in addition to MoSi₂ (WSi₂). Three- and four-point bend tests were performed at room temperature (25°C), 1000°C, 1200°C, and 1400°C. The bend strength of the Si₃N₄-WSi₂ composite increased slightly from room temperature up to 1000°C, whereas the Si₃N₄-MoSi₂ composite showed a more-pronounced increase up to 1200°C. Microstructural analysis was performed on the fracture surfaces of both composites that were tested at different temperatures.     

Formation of Reaction-Bonded Silicon Nitride from Silane-Derived Silicon Powders: Nucleation and Growth Mechanisms

The nucleation and growth of Si₃N₄ on silane-derived Si powders was investigated with transmission electron microscopy and FTIR spectroscopy. Thermogravimetric analysis (TGA) was also used to monitor the process through different stages of the reaction. The FTIR and TEM results provide clear evidence that the nucleation of crystalline Si₃N₄ coincides with the onset of rapid nitridation. Electron diffraction indicates that Si₃N₄ forms heteroepitaxially on the Si powder surfaces, with Si (111) || Si₃N₄(0001) and Si     || Si₃N₄     . Also, flat interfaces between the Si and Si₃N₄ (compared to the initial spherical surface of the Si powders) indicate that a significant rearrangement of the particle surface occurs during the initial stages of nitridation. The results reported here demonstrate that the rapid, low-temperature nitridation observed with silane-derived powders is possible because the Si/vapor surfaces are not covered with a continuous Si₃N₄ product layer. The measured nitridation rates are comparable to Si evaporation rates, which suggests that Si vaporization is rate limiting. This is significantly different from conventional RBSN, where nitridation is limited by solid-state diffusion through a Si₃N₄ product layer.     

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.     

Aqueous Processing of Sintered Reaction-Bonded Silicon Nitride: I, Dispersion Properties of Silicon Powder

An aqueous-based system (Si-Al₂O₃-Y₂O₃-Fe₂O₃) for processing sintered reaction-bonded silicon nitride (SRBSN) was investigated with an emphasis on chemical control of suspension component interactions. Chemical stability and dispersion properties of a commercial silicon powder were characterized using electroacoustic, adsorption isotherm, and rheological measurements. The interactions of silicon with nitriding agent, sintering aids, dispersants, and binder were considered. The effects of pH, electrolyte, aging, particle size, and solids loading were examined. The suspension properties of the silicon powder were influenced by the native oxide film and powder treatment history. The silicon-oxide composite particles exhibit dispersion behavior similar to silica, characterized by a negative surface potential above pH 2. A method to improve the dispersion and homogeneity of suspension components based on the use of quaternary amine dispersants is proposed.     

Flexural Creep of an in Situ-Toughened Silicon Carbide

Flexural creep behavior is reported for an in situ -toughened SiC between 1100° and 1500°C in four-point bending. The flexural creep rate of this SiC, sintered with aluminum, boron, and carbon (ABC-SiC), exhibits linear stress dependence, low apparent activation energy, and low incidence of cavitation and dislocation production. Most grain boundaries in this ceramic contain 1–5 nm intergranular films. The creep rate is consistent with a grain-boundary transport mechanism involving diffusion along the grain-boundary film-SiC interfaces. The microstructure and grain boundaries have been examined using transmission electron microscopy to assess possible changes during creep, particularly in relation to the applied stress direction.     

Formation of Reaction-Bonded Silicon Nitride from Silane-Derived Silicon Powders: Macroscopic Kinetics and Related Transport Phenomena

Reaction-bonded silicon nitride (RBSN) with little or no residual silicon was formed from laser-synthesized silicon powders and N₂, at low temperatures and in short times (<10 min at 1250°C, <1 h at 1150°C). The nitriding kinetics were studied with thermogravimetric analysis (TGA). Samples formed in the TGA furnace between 1150° and 1350°C were completely converted to silicon nitride near their centers, with residual silicon near the external surfaces. These unusual composition profiles were not observed in samples that were formed in a higher-purity atmosphere. Various experimental observations were compared to analyses of heat and mass transport, and it was determined that these composition in the nitriding gas. These analyses also indicated that heat transfer, N₂ diffusion, and Si evaporation and transport all affect the nitriding kinetics under certain conditions.     

Silicon Nitride/Silicon Carbide Nanocomposite Materials: II, Hot Strength, Creep, and Oxidation Resistance

The mechanical properties of Si₃N₄/SiC nanocomposite materials that contained nanosized intercrystalline SiC dispersions that originated from different starting powders and were made via different fabrication routes were studied in the temperature range of 1400°-1550°C. The strength retained at 1400°C was between 70% and ∼100% of the room-temperature strength. Both creep and oxidation resistance were very high and were comparable to or better than those of the best Si₃N₄-based materials published previously. The effect of SiC particles on the creep properties can be understood in terms of a recent model of dilatational creep; however, the model invokes a series of microstructural, micromechanical, and chemical modifications.     

Damage-Enhanced Creep in a Siliconized Silicon Carbide: Mechanics of Deformation

Continuum mechanics methods were employed to analyze creep deformation of a grade of siliconized silicon carbide at elevated temperatures. Three loading modes (tension, compression, and bending) are considered in this analysis. In tension, deformation is accompanied by cavitation at stresses in excess of a temperature-dependent threshold level, resulting in bilinear power-law creep. In compression, greater applied stresses are required to achieve the same rate of strain, and although bilinear creep behavior is also observed, a single power-law creep equation was assumed to simplify the mathematical analysis of the flexure problem. Asymmetrical creep in siliconized silicon carbide leads to a number of unique features in flexural creep. At steady state, a threshold bending moment exists below which no damage occurs. The neutral axis shifts from the geometric center toward the compressive side of the specimen by an amount that depends on the level of applied stress. Cavitation zone shapes, which are predicted to develop in a four-point bend specimen as a function of load, are found to be in qualitative agreement with those obtained experimentally. For transient creep under bending, the time-dependent neutral axes for stress and strain do not coincide, although they do converge toward a single axis at steady state. Quantitative predictions are given for relaxation of tensile stresses at the outer fiber, reverse loading in the midplane region, and the growth of the damage zone toward the compressive side of the flexural specimen. This load redistribution leads to a prolonged transient stage as compared to its counterpart in uniaxial creep.