Secondary Phases in Hot-Pressed Aluminum-Boron-Carbon–Silicon Carbide

Silicon carbide hot-pressed with aluminum, boron, and carbon as sintering aids (ABC–SiC), was studied by transmission electron microscopy. Both grain-boundary films and inclusions were prevalent in this material. The present study characterized the inclusions located in triple-junctions, grain boundaries, and the interior of the SiC grains, with emphases on phases not scrutinized before. These inclusions were crystalline, in contrast to the amorphous grain-boundary films. Two dominant types of boron-free triple-junction phases containing Al(Si)-O-C-(S) and Al(Si)-O were identified, where sulfur was an unexpected contaminant, and silicon came from SiO₂ or from dissolution of SiC. Boron-containing inclusions with a composition Al-O-B-C were frequently observed inside SiC grains. Although the boron-free aluminum-rich phases wet the grain boundaries completely and are, therefore, effective sintering additives, the boron-containing Al(Si)-O-B-C did not wet the grain boundaries. The structure and chemical composition of these boron-containing intragranular inclusions were determined, and their mechanism of formation is discussed.     

Microstructure and Fracture Toughness of Hot-Pressed Silicon Carbide Reinforced with Silicon Carbide Whisker

The effects of β-SiC whisker addition on the microstructural evolution and fracture toughness ( K _IC) of hot-pressed SiC were investigated. Most of the whiskers added disappeared during the densifcation process by transformation into the α-phase. The remaining whiskers acted as nuclei for grain growth, resulting in the formation of large tabular grains around the whiskers. The tabular grains around the whiskers were believed to be formed because of the extreme anisotropy of the interfacial energy between α- and β-SiC. The K _IC of the material was improved significantly by the whisker addition. The increase in the K _IC was attributed to crack bridging followed by grain pullout as a result of the formation of tabular grains in a fine matrix.     

Auger Analysis of Hot-Pressed and Sintered Silicon Carbide

Intergranular and transgranular chemistries of hot-pressed and sintered silicon carbides were investigated by Auger electron spectroscopy. Results indicated major differences in grain-boundary compositions between the two. Hot-pressed silicon carbide displayed a complex intergranular chemistry. Sintered silicon carbide displayed grain facets that were free of impurities and additives. The observed intergranular chemistries for both silicon carbides are discussed in terms of their relation to the processing methods.     

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.     

Effect of Alumina Content on the Oxidation of Hot-Pressed Silicon Carbide

The oxidation behavior at 1370°C of dense SiC, hot-pressed with the aid of Al₂O₃, has been investigated as a function of Al₂O₃ content. Increasing amounts of the Al₂O₃ hot-pressing aid increased the oxidation rate. Observations of the oxide surface show that a glassy phase (indicating formation of a liquid at the oxidation temperature) containing Si, Al, Fe, and K forms over the residual Al₂O₃ in the hot-pressed material. It is suggested that the oxygen transport through an impure aluminosilicate liquid is faster than that through a pure SiO₂ scale, thus causing an increased oxidation rate.     

Reactions between Hot-Pressed Calcium Hexaluminate and Silicon Carbide in the Presence of Oxygen

The reactions between hot-pressed calcium hexaluminate (CaAl₁₂O₁₉, hibonite) and silicon carbide (SiC) at 1100°-1400°C in air and nominal argon atmospheres were investigated. In inert atmospheres, there was no evidence of reaction at temperatures up to at least 1400°C. In air, the oxidation of SiC produced a layer of silica or a multicomponent amorphous silicate (depending on impurities) that reacted with CaAl₁₂O₁₉. At temperatures below 1300°C, the reaction resulted in the stratification of two distinct interfacial layers: a partially devitrified CaO-Al₂O₃-SiO₂ glass adjacent to SiC and a CaAl₂Si₂O₈ (anorthite) layer adjacent to hibonite. At 1400°C, a large amount of liquid was formed, the majority of which was squeezed out from between the reaction couple. No distinct layer of anorthite was present; instead, the anorthite was replaced by a layer of alumina between the glass-rich layer and hibonite. An activation energy of 290 kJ/mol was determined for the reaction, which is consistant with oxygen diffusion through a calcium aluminosilicate glass. The reaction between rare-earth hexaluminates and SiO₂ was predicted to produce a more-viscous glass than CaAl₁₂O₁₉ and SiO₂ and, therefore, have slower reaction kinetics, because of lower mass transport in the glass.     

Phase Transformation and Thermal Conductivity of Hot-Pressed Silicon Carbide Containing Alumina and Carbon

Phase transformation and thermal conductivity of hot-pressed β-SiC with Al₂O₃ and carbon additions were studied. Densification rate was a complex function of both Al₂O₃ and carbon. Simultaneous additions of Al₂O₃ and carbon accelerated the 3C → 4H phase transformation. Carbon additions lowered the thermal conductivity of the compact as did the high-temperature hot-pressing. The 3C → 4H transformation and the thermal conductivity were deduced to be related to each other.     

Tensile Creep Behavior of Alumina/Silicon Carbide Nanocomposite

Tensile creep and creep rupture behaviors of alumina/17 vol% silicon carbide nanocomposite and monolithic alumina Were investigated at 1200° to 1300°C and at 50 to 150 MPa. Compared to the monolithic alumina, the nanocomposite exhibited excellent creep resistance. The minimum creep rate of the nanocomposite was about three orders of magnitude lower and the creep life was 10 times longer than those of the monolith. The nanocomposite demonstrated transient creep until failure, while accelerated creep was observed in the monolith. It was revealed that rotating and plunging of intergranular silicon carbide nanoparticles into the alumina matrix increased the creep resistance with grain boundary sliding.     

Tensile Rupture Strength and Fracture Defects of Sintered Silicon Carbide

The tensile strength distribution of sintered silicon carbide was measured at room temperature and 1300°C in air and fracture defects were characterized. The measured strength was compared with strength obtained from flaw characteristics and fracture toughness assuming a peripherally cracked spherical void model.     

High-Temperature Microstructure of a Hot-Pressed Silicon Nitride

The outstanding question as to the microstructure of silicon nitride at temperatures associated with potential high-temperature applications of the material is addressed experimentally by quenching thin (transmission electron microscopy) samples from 1450°C and examining them in the microscope. The morphology of the microstructure is qualitatively unchanged compared to the materials slowly cooled, for example, after hot-pressing, to room temperature. The most significant difference is that the thickness of the intergranular phase is larger, typically 2 to 10 nm, as compared to the ∼ 1 nm observed in the hot-pressed material. In addition there is an apparent increase in the volume fraction of the intergranular phase at the three-grain junctions. On the basis of a number of supporting experiments including both hot-stage transmission electron microscopy (up to 1000°C) and Auger electron spectroscopy of material fractured and examined at 850°C, the change in microstructure is concluded to occur at temperatures above about 1000°C.     

Tensile and Fatigue Behavior of Silicon Carbide Fiber-Reinforced Aluminosilicate Glass

The onset of damage accumulation in ceramic-matrix composites occurs as matrix microcracking and fiber/matrix debonding. Tension tests were used to determine the stress and strain levels to first initiate microcracking in both unidirectional and cross-ply laminates of silicon carbide fiber-reinforced aluminosilicate glass. Tension–tension fatigue tests were then conducted at stress levels below and above the matrix cracking stress level. At stress levels below matrix microcracking, no loss in stiffness occurred. At stresses above matrix cracking, the elastic modulus of the unidirectional specimens exhibited a gradual decrease during the first 10 000 cycles, and then stabilized. However, the cross-ply material sustained most of the damage on the first loading cycle. It is shown that fatigue life can be related to nonlinear stress–strain behavior of the 0° plies, and that the cyclic strain limit was approximately 0.3%.     

Residual Stress Measurements in Alumina and Silicon Carbide

Residual stresses were measured in three types of ceramic components. Stresses were measured using X-ray diffraction and an advanced X-ray instrument. Measured stresses in alumina rods were shown to correlate well with breaking strength, and stress variations in an alumina tile were hypothesized to result from inhomogeneous cooling. The compressive stresses induced in a silicon carbide tube, by an outer steel sleeve, were seen to be balanced by tensile stresses in the sleeve.     

Tensile and Flexural Behaviour of Basalt Composites with Silicon Carbide Fillers

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Effect of Creep Damage on the Tensile Creep Behavior of a Siliconized Silicon Carbide

The tensile creep behavior of a siliconized silicon carbide was investigated in air, under applied stresses of 103 to 172 MPa for the temperature range of 1100° to 1200°C. At 1100°C, the steady-state stress exponent for creep was approximately 4 under applied stresses less than the threshold for creep damage (132 MPa). At applied stresses greater than the threshold stress for creep damage, the stress exponent increased to approximately 10. The activation energy for steady-state creep at 103 MPa was approximately 175 kJ/mol for the temperature range of 1100° to 1200°C. Under applied stresses of 137 and 172 MPa, the activation energy for creep increased to 210 and 350 kJ/mol, respectively, for the same temperature range. Creep deformation in the siliconized silicon carbide below the threshold stress for creep damage was determined to be controlled by dislocation processes in the silicon phase. At applied stresses above the threshold stress for creep damage, creep damage enhanced the rate of deformation, resulting in an increased stress exponent and activation energy for creep. The contribution of creep damage to the deformation process was shown to increase the stress exponent from 4 to 10.     

Silicon Carbide Platelet/Silicon Carbide Composites

α-silicon carbide platelet/β-silicon carbide composites have been produced in which the individual platelets were coated with an aluminum oxide layer. Hot-pressed composites showed a fracture toughness as high as 7.2 MPa·m^1/2. The experiments indicated that the significant increase in fracture toughness is mainly the result of crack deflection and accompanying platelet pullout. The coating on the platelets also served to prevent the platelets from acting as nucleation sites for the α- to β-phase transformation, so that the advantageous microstructure remains preserved during high-temperature processing.     

Influence of Stress Ratio on the Elevated-Temperature Fatigue of a Silicon Carbide Fiber-Reinforced Silicon Nitride Composite

The influence of stress ratio on the tensile fatigue behavior of a unidirectional SiC-fiber/Si₃N₄-matrix composite was investigated at 1200°C. Tensile stress ratios of 0.1, 0.3, and 0.5 were examined. Fatigue testing was conducted in air, at a sinusoidal loading frequency of 10 Hz. For peak fatigue stresses below the proportional limit of the composite (approximately 195 MPa at 1200°C) specimens survived 5 × 10⁶ cycles, independent of stress ratio. At peak stresses above the proportional limit, fatigue failures were observed; fatigue life decreased significantly as the stress ratio was lowered from 0.5 to 0.1. Creep appears to be the predominant damage mechanism which occurs during fatigue below the proportional limit. Both mechanical cycle-by-cycle fatigue damage and creep contribute to specimen failure at peak stresses above the proportional limit.     

Threshold Stress Intensity for Crack Growth in Silicon Carbide Ceramics

Measurements of threshold stress intensities for crack growth, K _h, of three polycrystalline SiC materials were attempted using interrupted static fatigue tests at 1200°–1400°C. Weibull statistics were used to calculate conservative K_th values from test results. The K _th of a chemically vapor deposited β-SiC could not be determined, as a result of its wide variations in strength. The K_th ≥ 3.3,2.2, and 1.7 MPa·m^1/2 for an Al-doped sintered α-SiC; and K_th ≥ 3.1, 2.7, and 2.2 MPa·m^1/2 for a hot isostatically pressed α-SiC, both at 1200°, 1300°, and 1400°C, respectively. A damage process concurrent with subcritical crack growth was apparent for the sintered SiC at 1400°C. The larger K_th 's for the HIPed SiC (compared to the sintered SiC) may be a result of enhanced viscous stress relaxation caused by the higher silica content and smaller grain size of this material. Values measured at 1300° and 1400°C were in good agreement with the K_th's predicted by a diffusive crack growth model, while the measured K_th 's were greater than the predicted ones at 1200°C.     

Properties of Isostatically Hot-Pressed Silicon Nitride

Hot isostatic pressing was studied for densification of reaction-bonded Si₃N₄ containing various levels of Y₂O₃. Near-theoretical density was achieved for com positions containing 3 to 7 wt% Y₂O₃. An Si₃N₄-5 wt% Y₂O₃ composition had a 4-point flexural strength at 1375°C of 628 MPa and survived 117 h of stress rupture testing at 1400°C and 345 MPa .