Phase Transformation and Texture in Hot-Forged or Annealed Liquid-Phase-Sintered Silicon Carbide Ceramics

Starting from three powder mixtures of 80 vol% SiC (100α, 50α/50β, 100β) and 20 vol% YAG, liquid-phase-sintered silicon carbide ceramics were prepared by hot pressing at 1800°C for 1 h under 25 MPa, and then by hot forging or annealing at 1900°C for 4 h under an applied stress of 25 MPa in argon. The phase transformation and texture development in the as-hot-pressed, hot-forged, and annealed SiC ceramics were investigated via X-ray diffraction (XRD) and the pole figure measurements. The 6H → 4H polytypic transformation was observed in samples consisting of both α- and β-SiC phases when subjected to compressive deformation but absent in the case of annealing, suggesting the deformation-enhanced solubility of aluminum in SiC. Deformation was also found to enhance the 3C → 4H transformation in the sample containing entirely β-phase, which is due to the accelerated solution-precipitation process assisted by grain boundary sliding. The current study showed that the β- →α-phase transformation had little effect on texture development in SiC. Hot forging generally produced the strongest texture, with the calculated maximum of 2.2 times random in samples started with pure α-SiC phase. The mechanism for texture development was explained based on the microstructural observations.     

Effect of β-to-α Phase Transformation on the Microstructural Development and Mechanical Properties of Fine-Grained Silicon Carbide Ceramics

Ultrafine β-SiC powders mixed with 7 wt% Al₂O_3, 2 wt% Y₂O₃, and 1.785 wt% CaCO₃ were hot-pressed and subsequently annealed in either the absence or the presence of applied pressure. Because the β-SiC to α-SiC phase transformation is dependent on annealing conditions, the novel processing technique of annealing under pressure can control this phase transformation, and, hence, the microstructures and mechanical properties of fine-grained liquid-phase-sintered SiC ceramics. In comparison to annealing without pressure, the application of pressure during annealing greatly suppressed the phase transformation from β-SiC to α-SiC. Materials annealed with pressure exhibited a fine microstructure with equiaxed grains when the phase transformation from β-SiC to α-SiC was <30 vol%, whereas materials annealed without pressure developed microstructures with elongated grains when phase transformation was >30 vol%. These results suggested that the precise control of phase transformation in SiC ceramics and their mechanical properties could be achieved through annealing with or without pressure.     

Microstructural Analysis of Liquid-Phase-Sintered β-Silicon Carbide

The microstructures of fine-grained β-SiC materials with α-SiC seeds annealed either with or without uniaxial pressure at 1900°C for 4 h in an argon atmosphere were investigated using analytical electron microscopy and high-resolution electron microscopy (HREM). An applied annealing pressure can greatly retard phase transformation and grain growth. The material annealed with pressure consisted of fine grains with β-SiC as a major phase. In contrast, the microstructure in the material annealed without pressure consisted of elongated grains with half α-SiC. Energy-dispersive X-ray analysis showed no differences in the amount of segregation of aluminum and oxygen atoms at grain boundaries, but did show a significant difference in the segregation of yttrium atoms at grain boundaries along SiC grains for the two materials. The increased segregation of yttrium ions at grain boundaries caused by the applied pressure might be the reason for the retarded phase transformation and grain growth. HREM showed a thin secondary phase of 1 nm at the grain boundary interface for both materials. The development of a composite grain consisting of a mixture of β/α polytypes during annealing was a feature common to both materials. The possible mechanisms for grain growth and phase transformation are discussed.     

Effects of External Stress on Defect Annihilation and Bubble Swelling During Annealing of Neutron-Irradiated Silicon Carbide

Silicon carbide ceramics fabricated by three different methods were neutron-irradiated in the Japan Materials Testing Reactor and were subsequently annealed free from stresses or under compressive external stresses. The macroscopic length monotonically decreased with annealing above the irradiation temperature, when annealing was performed below ∼1300°C. This decrease was not affected by the external stresses. However, annealing above ∼1300°C led to an increase in length in B-containing SiC. The expansion was caused by the formation and growth of He bubbles at grain boundaries. The growth occurred by flow of vacancies into bubbles. The compressive stress retarded the expansion along the loading direction. This retardation was compensated by a length incrase along the lateral direction. The effect of external stresses was discussed by considering differently oriented He bubbles. One bubble was stress favored and the other stress unfavored. The stress determined the diffusional flow of vacancy-He atom complexes between the favored and unfavored bubbles which were caused by anisotropic expansion under a directional stress.     

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.     

Microstructural Changes in Liquid-Phase-Sintered Silicon Carbide during Creep in an Oxidizing Environment

The knowledge of the microstructural evolution during exposure to high temperatures is important to understanding the mechanisms responsible for the creep resistance of silicon carbide (SiC) ceramics. This includes not only the phase transformation of the SiC grains, but also the phase transformations of the oxynitride grain-boundary phases. For this study, a series of SiC specimens were prepared with varying molar ratios of AlN-Y₂O₃ additives. Increased creep resistance was observed in specimens with an additive system containing a 2:3 molar ratio or 60 mol% Y₂O₃. A continuous oxide layer of Y₂Si₂O₇ formed at the surface during elevated temperature testing in air. No blistering or cracking was observed in this oxide coating. Further increase of the creep resistance was achieved by a post-sintering nitrogen anneal.     

重结晶碳化硅烧成中碳化硅的分解现象、热力学条件及对蒸发凝聚的影响

重结晶碳化硅陶瓷烧成试验采用国内碳化硅微粉,对其烧成后的现象与结果进行分析检测,在热力学分析和试验的基础上得出含有较多杂质的国内碳化硅微粉在重结晶烧成中有显著分解挥发,而且影响碳化硅的蒸发凝聚过程。     

SiC基陶瓷高温相平衡——SiC-Si3N4-Gd203系统

通过热压烧结在1700℃,分别于Ar和N2气氛下制备了一系列SiC—Si3N4-Gd2O3陶瓷样品,采用X射线衍射技术分析了样品的相组成,确定了相关系。结果表明：在SiC-Si3N4-Gd2O3的二元子系统和三元系统中,除了二元化合物Si3N4-Gd2O3（M相）存在,尚有含氮稀土硅酸盐化合物SiEN20．2Gd2O3（J相）和Gd10（Si04）6N2（H相）生成。而这些化合物的生成主要是由于SiC、Si3N4粉料表面的杂质氧（以SiO2形式存在）参与了反应,由此引入的SiO2这一组分,使原三元系统扩大成为SiC-Si3N4-SiO2Gd2O3四元系统。通过对产物相组成的分析,确定了在此四元系统中存在8个相容性四面体。基于上述结果,提出了1700℃的SiC-Si3N4-Gd2O3准三元亚固相图、Si3N4-Gd2O3-SiO2三元亚固相图以及SiC-Si3N4-SiO2-Gd2O3四元亚固相图。     

Effect of Microstructure on the Creep of Siliconized Silicon Carbide

Mechanisms of creep deformation have been investigated for a commercial grade of siliconized carbide containing ≅33%/silicon. Microstructural studies of both tensile and compressive test specimens indicate dislocation damage generation in both the silicon carbide and the silicon phases as a consequence of creep. In the silicon carbide, dislocation damage was normally restricted to contact sites between the silicon carbide grains resulting from high intergranular contact stresses during deformation. Dislocation damage was also observed in the silicon. Although dislocation damage was heavy in some regions of the specimens, most regions of the specimens, most regions were free of dislocations. This result is consistent with the hypothesis that deformations occurs by the motion of clusters of grains during deformation. In tension, creep at high strain rates, 1 × 10⁻⁸_S⁻¹, was accompanied by the formation of cavities at Si/SiC interfaces within the intergranular silicon phase. As cavities were not associated with dislocations, their growth was probably controlled by diffusional processes. Based on observations of the microstructure, a model of deformation is proposed to explain the fact that siliconized silicon carbide creeps faster in tension than in compression, at the same applied stress. The model is based on soil mechanics concepts. It is suggested that creep is controlled by intergranular friction between aggregate particles of the composite.     

Oxidation of Silicon and Silicon Carbide in Ozone-Containing Atmospheres at 973 K

The oxidation behavior of a silicon wafer, chemically vapor-deposited SiC, and single-crystal SiC was investigated in an oxygen—2%–7% ozone gas mixture at 973 K. The thickness of the oxide film that formed during oxidation was measured by ellipsometry. The oxidation rates in the ozone-containing atmosphere were much higher than those in a pure oxygen atmosphere. The parabolic oxidation kinetics were observed for both silicon and SiC. The parabolic rate constants varied linearly with the ozone-gas partial pressure. Inward diffusion of atomic oxygen formed by the dissociation of ozone gas through the SiO₂ film apparently was the rate-controlling process.     

Some New Perspectives on Oxidation of Silicon Carbide and Silicon Nitride

This study provides new perspectives on why the oxidation rates of silicon carbide and silicon nitride are lower than those of silicon and on the conditions under which gas bubbles can form on them. The effects on oxidation of various rate-limiting steps are evaluated by considering the partial pressure gradients of various species, such as O₂, CO, and N₂. Also calculated are the parabolic rate constants for the situations when the rates are controlled by oxygen and/or carbon monoxide (or nitrogen) diffusion. These considerations indicate that the oxidation of silicon carbide and silicon nitride should be mixed controlled, influenced both by an interface reaction and diffusion.     

Silicon Carbide Monofilament-Reinforced Silicon Nitride or Silicon Carbide Matrix Composites

SiC-monofilament-reinforced SiC or Si₃N₄ matrix composites were fabricated by hot-pressing, and their mechanical properties and effects of filaments and filament coating layers were studied. Relationships between frictional stress of filament/matrix interface and fracture toughness of SiC monofilament/Si₃N₄ matrix composites were also investigated. As a result, it was confirmed experimentally that in the case of composites fractured with filament pullout, the fracture toughness increased as the frictional stress increased. On the other hand, when frictional stress was too large (>about 80 MPa) for the filament to be pulled out, fracture toughnesses of the composites were almost the same and not so much improved over that of Si₃N₄ monolithic ceramics. The filament coating layers were found to have a significant effect on the frictional stress of the SiC monofilament/Si₃N₄ matrix interface and consequently the fracture toughness of the composites. Also the crack propagation behavior in the SiC monofilament/Si₃N₄ matrix composites was observed during flexural loading and cyclic loading tests by an in situ observation apparatus consisting of an SEM and a bending machine. The filament effect which obstructed crack propagation was clearly observed. Fatigue crack growth was not detected after 300 cyclic load applications.     

碳化硅材料热导率计算研究进展

从声子散射机制出发,介绍了Si C热导率的温度特性和微观导热机理。综述了Si C单晶热导率的2种主要计算方法。Boltzmann-弛豫时间近似(RTA)适用于各个温度段的热导率计算,而分子动力学方法更适用于高温热导率计算。分子动力学方法相比于Boltzmann-RTA方法的优点在于它可以考虑所有高次项的非谐作用。介绍了3种Si C陶瓷热导率近似计算模型,包括界面热阻模型、Debye-Callaway模型及多相系统热导率模型。下一步研究的主要方向仍然是优化计算模型及减少拟合参数。     

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.     

Effect of Atmosphere on Weight Loss in Sintered Silicon Carbide during Heat Treatment

Heat treatment was performed on β-SiC with different sintering additives in the temperature range 1873–2073 K, in both argon and nitrogen-gas atmospheres. In the case of the specimens heat-treated at 2073 K in argon, the weight loss was more than the total weight of the sintering additives, except for B,C-doped β-SiC. On the other hand, weight loss was suppressed by about one-third to one-half in nitrogen gas. Weight loss depended mainly on the reaction at the interface between the SiC grains and the grain-boundary phase.     

硅、铝溶胶对碳化硅窑具的结构及性能的影响

以SiC颗粒为骨料,硅微粉为基体相,硅、铝凝胶在高温下形成纳米颗粒作为增强相制备了碳化硅窑具,分析了硅、铝溶胶的添加量对碳化硅窑具烧结特性、力学性能、物相组成及显微结构的影响规律.结果表明,硅、铝溶胶的添加可以增强碳化硅窑具的力学性能.当单独添加硅溶胶量为2.0％时,其抗弯强度可达19.67 MPa,当单独添加铝溶胶的量为1.1％时,抗弯强度为19.26 MPa,当同时添加硅、铝溶胶总量为3.06％时,其抗弯强度可达到18.10 MPa,但过多溶胶的引入会导致碳化硅窑具中气孔的增多从而影响碳化硅窑具的致密化.     

Complex Impedance Analysis on the Orientation Effect of Whiskers in Oriented Silicon Carbide Whisker/Silicon Nitride Composites

The effect of whisker orientation on the electrical properties of SiC_w/Si₃N₄ composites was investigated, using the principle of complex impedance. The impedance spectra of the composites exhibited significant dependence on whisker orientation. Based on the experimental results and the microstructure of the composites, the impedance mechanisms were discussed and a corresponding equivalent-circuit model was presented, which agreed well with the experiments.     

Effect of Silicon Carbide Whisker and Titanium Carbide Particulate Additions on the Friction and Wear Behavior of Silicon Nitride

A Si₃N₄/TiC composite was previously demonstrated to exhibit improved wear resistance compared to a monolithic Si₃N₄ because of the formation of a lubricious oxide film containing Ti and Si at 900°C. Further improvements of the composite have been made in this study through additions of SiC whiskers and improved processing. Four materials—Si₃N₄, Si₃N₄/TiC, Si₃N₄/SiC_wh, and Si₃N₄/TiC/SiC_wh— were processed to further optimize the wear resistance of Si₃N₄ through improvements in strength, hardness, fracture toughness, and the coefficient of friction. Oscillatory pin on flat wear tests showed a decrease in the coefficient of friction from ∼0.7 (Si₃N₄) to ∼0.4 with the addition of TiC at temperatures reaching 900°C. Wear track profiles illustrated the absence of appreciable wear on the TiC-containing composites at temperatures above 700°C. Microscopic (SEM) and chemical (AES) characterization of the wear tracks is also included to deduce respective wear and lubricating mechanisms.     

Stress-Corrosion Cracking of Silicon Carbide Fiber/Silicon Carbide Composites

Ceramic-matrix composites are being developed to operate at elevated temperatures and in oxidizing environments. Considerable improvements have been made in the creep resistance of SiC fibers and, hence, in the high-temperature properties of SiC fiber/SiC (SiC_f/SiC) composites; however, more must be known about the stability of these materials in oxidizing environments before they are widely accepted. Experimental weight change and crack growth data support the conclusion that the oxygen-enhanced crack growth of SiC_f/SiC occurs by more than one mechanism, depending on the experimental conditions. These data suggest an oxidation embrittlement mechanism (OEM) at temperatures <1373 K and high oxygen pressures and an interphase removal mechanism (IRM) at temperatures of ≳700 K and low oxygen pressures. The OEM results from the reaction of oxygen with SiC to form a glass layer on the fiber or within the fiber–matrix interphase region. The fracture stress of the fiber is decreased if this layer is thicker than a critical value ( d > d _c) and the temperature below a critical value ( T < T _g), such that a sharp crack can be sustained in the layer. The IRM results from the oxidation of the interfacial layer and the resulting decrease of stress that is carried by the bridging fibers. Interphase removal contributes to subcritical crack growth by decreasing the fiber-bridging stresses and, hence, increasing the crack-tip stress. The IRM occurs over a wide range of temperatures for d < d _c and may occur at T > T _g for d > d _c. This paper summarizes the evidence for the existence of these two mechanisms and attempts to define the conditions for their operation.     

Effect of Surface-Active Substances on the Rheological Properties of Silicon Carbide Suspensions in Paraffin

In this paper, the possibility of stabilization of SiC suspensions in paraffin (hot-molding slips) was investigated from the colloid science point of view. By considering the surface properties of finely dispersed silicon carbide, surface-active agents (fatty amine and alkylsuccinimide) which are suitable for surface modification by chemical adsorption on SiC were selected. Adsorption of fatty amines and alkylsuccinimide was carried out in model experiments in chloroform. Increasing the length of the carbon–hydrogen chain caused the adsorption to decrease, and the adsorption area of one molecule in the saturated adsorption layer to increase. The stabilizing effect of surface-active agents resulted in improved flow behavior of hot-molding slips. The efficiency of surface-active agents increased with increased adsorption layer thickness. In highly concentrated SiC hot-molding slips, a combination of alkylsuccinimide and fatty amine proved to be very efficient. This synergistic effect of long- and short-chained surface-active agents is discussed in terms of an adsorption layer model.