High-Temperature Strength and Creep Behavior of Melt-Infiltrated SiC-Mo≤5Si3C≤1 Composites

Molybdenum carbosilicide composites (SiC-Mo_≤5Si₃C_≤1) were fabricated via the melt-infiltration process. The fracture behavior of the composites was studied from room temperature up to 1800°C in 1 atm (∼10⁵ Pa) of argon. The bend strength of the composites slightly increased at ∼1200°C, because of the brittle-ductile transition of the intermetallic phase. The composites retained ∼90% of their room-temperature strength, even at 1700°C. Compressive creep tests were performed over a temperature range of 1760°-1850°C and a stress range of 200–250 MPa. The creep rate of the SiC-Mo_≤5Si₃C_≤1 composites was approximately an order of magnitude higher than that of reaction-bonded SiC.     

Effect of Moisture on the High-Temperature Stability of Unidirectionally Solidified Al2O3/YAG Eutectic Composites

The corrosion resistance of a unidirectionally solidified alumina/yttrium aluminum garnet (Al₂O₃/YAG) eutectic mixture was investigated at high temperature. Samples were exposed to high temperature (1200°–1800°C) in different atmospheres, which included argon, argon/water vapor, air, and air/water vapor. The most important microstructural changes occurred at the interface between the YAG and the Al₂O₃. Those changes consisted of localized thermal grooving, especially when the corrosive atmosphere contained water vapor. The samples exhibited significant weight loss at high temperature (1800°C) after 20 h of exposure. The calculated volume gain that was induced by the increased surface relief was low and limited, except when the corrosive atmosphere contained air, which indicated that the presence of air (particularly oxygen) induced a more-active corrosion process. On the other hand, no change in the flexural strength was observed, even after 100 h at 1800°C in a humid atmosphere, because of the cross-linked structure of the composite, which limited propagation of the groove.     

High-Temperature Mechanical Properties of SiC–Mo5(Si,Al)3C Composites

SiC–Mo₅(Si,Al)₃C composites were fabricated by the melt infiltration process, and the infiltration characteristics were studied in detail. Fracture strength and toughness were measured up to 1600°C using a three-point bending test and indentation strength method, respectively. Both fracture strength and toughness significantly increased at 1400°C with respect to the values at room temperature. These increases were mainly attributed to plastic deformation of the infiltrated Mo₅(Si,Al)₃C phases at elevated temperatures, which acted as ductile toughening inclusions. Compressive creep tests were used to study the creep behavior of the composite in the range of 1550°–1650°C and 150–200 MPa. The stress exponent and activation energy were 1.3 and 277 kJ/mol, respectively. Preliminary oxidation tests showed that the composites exhibited good oxidation resistance at 1500°C because of the formation of a dense oxide scale.     

Microstructure and Creep Behavior of a Si3N4–SiC Micronanocomposite

The microstructure and its influence on the creep behaviour of carbon derived Si₃N₄-SiC micro/nanocomposite tested in bending at temperatures from 1200° to 1400°C in air has been studied. No phase and microstructure change after creep test implied that material is stable at tested temperature range. After creep test only partial crystallization of glassy intergranular phase has been observed. Creep parameters n close to 1, apparent activation energy around 350 kJ/mol together with TEM observation indicated that the main creep mechanisms is solution precipitation controlled by interface reaction in combination with grain boundary sliding caused by the amorphous intergranular phases present in microstructure. However, the grain boundary sliding is hindered by local SiC particles interlocking neighboring Si₃N₄ grains.     

Mechanical Behavior of MoSi2 Reinforced–Si3N4Matrix Composites

The mechanical behavior of MoSi₂ reinforced–Si₃N₄ matrix composites was investigated as a function of MoSi₂ phase content, MoSi₂ phase size, and amount of MgO densification aid for the Si₃N₄ phase. Coarse-phase MoSi₂-Si₃N₄ composites exhibited higher room-temperature fracture toughness than fine-phase composites, reaching values >8 MP·am^1/2. Composite fracture toughness levels increased at elevated temperature. Fine-phase composites were stronger and more creep resistant than coarse phase composites. Room-temperature strengths >1000 MPa and impression creep rates of ∼10⁻⁸ s⁻¹ at 1200°C were observed. Increased MgO levels generally were deleterious to MoSi₂-Si₃N₄ mechanical properties. Internal stresses due to MoSi₂ and Si₃N₄ thermal expansion coefficient mismatch appeared to contribute to fracture toughening in MoSi₂-Si₃N₄ composites.     

High-Temperature Failure Mechanisms of Hot-Pressed Si3N4 and Si3N4/Si3N4-Whisker-Reinforced Composites

The high-temperature flexural strength of hot-pressed silicon nitride (Si₃N₄) and Si₃N₄-whisker-reinforced Si₃N₄-matrix composites has been measured at a crosshead speed of 1.27 mm/min and temperatures up to 1400°C in a nitrogen atmosphere. Load–displacement curves for whisker-reinforced composites showed nonelastic fracture behavior at 1400°C. In contrast, such behavior was not observed for monolithic Si₃N₄. Microstructures of both materials have been examined by scanning and transmission electron microscopy. The results indicate that grain-boundary sliding could be responsible for strength degradation in both monolithic Si₃N₄ and its whisker composites. The origin of the nonelastic failure behavior of Si₃N₄-whisker composite at 1400°C was not positively identified but several possibilities are discussed.     

Creep of Polycrystalline MgO and MgO-Fe2O3 Solid Solutions at High Temperatures

Polycrystalline MgO and MgO-Fe₂O₃ solid solutions (0.10 to 8.08 wt% Fe₂O₃) were fabricated to almost theoretical density by vacuum hot-pressing. Specimens were creep-tested in air under four-point dead-load conditions between 1000° and 1400°C at stresses between 50 and 550 kg/cm². Steady-state creep was never achieved in the experiments, which sometimes lasted more than 50 h. The strain rate vs time ( t ) data were described by an equation of the form = c₁/(t+C₂)^p , which is consistent with the assumptions that creep occurs at least in part by a "viscous" mechanism and that grain growth occurs simultaneously. Doping MgO with Fe₂O₃ enhanced the viscous contributions to creep and inhibited the nonviscous ones. Creep rates in these specimens increased with increasing Fe₂O₃ additions. The occurrence of simultaneous grain growth during the high-temperature creep of magnesiowustite (i.e. MgO-Fe₂O₃ solid solutions) was used in establishing the strain rate vs grain size dependence. The results of this study are consistent with a transition between grain boundary and lattice diffusion mechanisms as the grain size increases (4 to 44 μan). The creep of polycrystalline MgO is a mixed process in that viscous and nonviscous (dislocation) contributions are present.     

High-Temperature Creep Behavior of Mixed Conducting La0.5Sr0.5Fe1−xCoxO3-δ (0.5≤x≤1) Materials

Steady-state compressive creep rate of La_0.5Sr_0.5Fe_0.5Co_0.5O_3−δ (LSFC) and La_0.5Sr_0.5CoO_3−δ (LSC) is reported in the temperature region 900°–1050°C and stress range 5–28 MPa. The stress exponents for the two materials were 1.71±0.18 and 1.24±0.15, respectively. The activation energy for creep was considerably higher for LSC (619±56 kJ/mol) than for LSFC (392±28 kJ/mol). The grain size exponent for LSC was 1.28±0.14. Considerably higher creep rates were observed for both materials in N₂ compared with air. Relaxation by creep of chemical-induced stresses in oxygen-permeable membranes is addressed, especially at low partial pressure of oxygen.     

Creep of Polycrystalline MgO-FeO-Fe2O3 Solid Solutions

Steady-state creep experiments were performed on hot-pressed polycrystalline MgO doped with Fe. Dead-load 4-point bend creep tests were conducted at stresses of 26 to 270 kg/cm², at temperatures of 1250° to 1450°C, in O₂ partial pressures of 1 to 10⁻⁹ atm, on specimens with grain sizes of 10 to 65 μm. Viscous steady-state creep was always observed when the grain size was stable. Experiments at variable P _O2's and temperatures were used to identify regimes of high (117 ± 10 kcal/mol) and low (81 ± 5 kcal/mol) activation energy. In the latter, creep rates were nearly independent of Fe dopant concentration and P _O2, whereas in the former creep rates were enhanced by increasing P _O2's and Fe dopant levels. The high- and low-activation-energy regimes were interpreted as diffusional creep controlled primarily by Mg lattice diffusion and O grain-boundary diffusion, respectively.     

Dielectric Behavior of Na0.5Bi0.5TiO3-Based Composites Incorporating Silver Particles

The dielectric properties of Na_0.5Bi_0.5TiO₃ (NBT) -based composites incorporating silver particles prepared by sintering at a low temperature of ∼900°C are reported. The dielectric constant increases with the amount of metal silver particles in the measured frequency range (150 Hz to 1 MHz), and could be enhanced up to ∼20 times higher than that of pure NBT ceramics, which was ascribed to the effective electric fields developed between the dispersed particles in the matrix and the percolation effect. Further investigation revealed that the dielectric constant of the composites has weak frequency and temperature dependence (−50°C to +50°C).     

Near-Net-Shape Fabrication of Ti3AlC2-Based Composites

A porous ceramic preform was fabricated by printing a powder blend of TiC, TiO₂, and dextrin. The presintered preforms contained a bimodal pore size distribution with intra-agglomerate pores ( d ₅₀≈0.7 μm) and inter-agglomerate pores ( d ₅₀≈30 μm), which were subsequently infiltrated by aluminum melt spontaneously in argon above 1050°C. A redox reaction at 1400°C resulted in the formation of dense Ti–Al–O–C composites mainly composed of Ti₃AlC₂, TiAl₃, Al, and Al₂O₃, which attained a bending strength of 320 MPa, a Young's modulus of 184 GPa, and a Vicker's hardness of 2.5 GPa.     

Energetics of La2O3–HfO2–SiO2 Glasses

A series of La₂O₃–HfO₂–SiO₂ glasses, approximately along the join 0.73SiO₂–0.27( x HfO₂–(1− x )La₂O₃), 0< x <0.3), was prepared using containerless processing techniques (aerodynamic levitation combined with laser heating in oxygen). The enthalpy of formation and enthalpy of vitrification at 25°C were obtained from drop solution calorimetry of these glasses and appropriate crystalline compounds in a molten lead borate (2PbO–B₂O₃) solvent at 702°C. The enthalpy of formation from crystalline oxides was exothermic and became less exothermic with increasing HfO₂ content. Heat contents were measured by transposed temperature drop calorimetry and depended linearly on the HfO₂ content. Differential scanning calorimetry showed that both the onset glass transition and the onset crystallization temperature of these glasses increased with increasing HfO₂ content. Upon slow cooling in air, the glasses crystallized to a mixture of baddeleyite, cristobalite, lanthanum disilicate, and hafnon.     

Zirconium Aluminum Carbides: New Precursors for Synthesizing ZrO2–Al2O3 Composites

ZrO₂–Al₂O₃ nanocrystalline powders have been synthesized by oxidizing ternary Zr₂Al₃C₄ powders. The simultaneous oxidation of Al and Zr in Zr₂Al₃C₄ results in homogeneous mixture of ZrO₂ and Al₂O₃ at nanoscale. Bulk nano- and submicro-composites were prepared by hot-pressing as-oxidized powders at 1100°–1500°C. The composition and microstructure evolution during sintering was investigated by XRD, Raman spectroscopy, SEM, and TEM. The crystallite size of ZrO₂ in the composites increased from 7.5 nm for as-oxidized powders to about 0.5 μm at 1500°C, while the tetragonal polymorph gradually converted to monolithic one with increasing crystallite size. The Al₂O₃ in the composites transformed from an amorphous phase in as oxidized powders to θ phase at 1100°C and α phase at higher temperatures. The hardness of the composite increased from 2.0 GPa at 1100°C to 13.5 GPa at 1400°C due to the increase of density.     

High/Low Modulus Si3N4-BN Composite for Improved Electrical and Thermal Shock Behavior

High-density Si₃N₄+6% CeO₂ composites with 5 to 50% BN were fabricated by hot-pressing. BN remained as a discrete phase. Dielectric constants were 4 to 8 and loss tangents were 0.0008 to 0.06 for the room temperature to 1100°C range for compositions with 10 to 50% BN. Thermal-expansion values perpendicular to the hot-pressing direction were somewhat less than those of hot-pressed Si₃N₄+6% CeO₂. Flexure strengths at room temperature were considerably lower than those of hot-pressed Si₃N₄+6% CeO₂ but values at 1000°, 1250°, and 1400°C in air were only slightly lower. Young's modulus values were found to decrease with increasing BN content at all temperatures. Better thermal shock resistance was found than for commercial hot-pressed Si₃N₄.     

Volatility of PuO2 in Nonreducing Atmospheres

The vapor pressure of plutonium dioxide (PuO₂) was investigated in the range 1450° to 1775°C in air, argon, and oxygen atmospheres by a transpiration technique. There were strong indications that PuO₂ can vaporize congruently or as a suboxide species, depending on the atmosphere. The δH°₂₉₈ for vaporization in 1 atm of oxygen is approximately 154,000 cal per mole. The estimated standard free energy of formation (δG°_f) of gaseous PuO₂ is −121,000 + 10.7 T from 1227° to 1827°C.     

Advantages of SiC Hi-Nicalon or NLM 202 Fibers in SiCf–SiBC Composites

SiC_f–SiBC composites were produced using NLM 202 and Hi-Nicalon SiC_f fibers. Tensile creep tests were performed under a reduced pressure of argon. These composites show a good creep resistance at 1473 K with a strain rate ranging from 10⁻⁹ to 10⁻⁷ s⁻¹, depending on the stress level. The creep strength was improved by using Hi-Nicalon instead of NLM 202 SiC_f fibers, to the extent of increasing the loadability by 50 MPa at the same operating temperature or increasing the temperature by 50 K for the same load. The damage observed in SEM micrographs and the use of the damage mechanics provide evidence that creep is governed by a damage-creep mechanism in two steps.     

Compressive Creep and Oxidation Resistance of an Si3N4 Material Fabricated in the System Si3N4-Si2N2O-Y2Si2O7

The compressive creep behavior and oxidation resistance of an Si₃N₄/Y₂Si₂O₇ material (0.85Si₃N₄+0.10SiO₂+0.05Y₂O₃) were determined at 1400°C. Creep re sistance was superior to that of other Si₃N₄ materials and was significantly in creased by a preoxidation treatment (1600°C /120 h). An apparent parabolic rate constant of 4.2 × 10⁻¹¹ kg²·m-⁴·s⁻¹ indicates excellent oxidation resistance.     

Crack-Healing Behavior of Al2O3 Toughened by SiC Whiskers

Al₂O₃ reinforced by SiC whiskers (Al₂O₃/SiC-W) was hot-pressed to investigate the crack-healing behavior. Semielliptical surface cracks of 100 μm in surface length were introduced using a Vickers indenter. The specimens containing precracks were crack-healed at temperatures between 1000° and 1300°C for 1 h in air, and their strengths were measured by three-point bending tests at room temperature and elevated temperatures between 400° and 1300°C. The results show that Al₂O₃/SiC-W possesses considerable crack-healing ability. The surface cracks with length of 2 c = 100 μm could be healed by crack-healing at 1200° or 1300°C for 1 h in air. Fracture toughness of the material was also determined. As expected, the SiC whiskers made their Al₂O₃ tougher.     

High-Temperature Creep of Y2O3-Stabilized ZrO2

The compression creep behavior of Y₂O₃-stabilized ZrO₂ (YSZ) was studied at temperatures to 2000 ° C. The function of Y₂O₃ content and grain size was tested in specimens with various impurity concentrations and porosity distributions. For relatively fine-grained specimens, creep rates increased with the 1.5 power of the applied stress at low stresses and with the third power at high stresses. The results for coarse-grained specimens can, in general, be fit by the cube dependence. The 1.5 power can be reduced to a linear dependence by correcting for an apparent threshold stress, which decreases with increasing temperature. Creep activation energies for YSZ are 128 ± 10 kcal/mol, independent of Y₂O₃ content, impurity level, grain size, and porosity distribution. In addition, over a broad range of temperatures and stresses the absolute values of the steady-state creep rates are influenced only by grain size and O₂ partial pressure.     

Reaction-Bonding Preparation of Si3N4/MoSi2 and Si3N4/WSi2 Composites from Elemental Powders

Si₃N₄/MoSi₂ and Si₃N₄/WSi₂ composites were prepared by reaction-bonding processes using as starting materials powder mixtures of Si-Mo and Si-W, respectively. A presintering step in an At-base atmosphere was used before nitriding for the formation of MoSi₂ and WSi₂; the nitridation in a N₂-base atmosphere was followed after presintering with the total stepwise cycle of 1350°C × 20 h +1400°C × 20 h +1450°C × 2 h. The final phases obtained in the two different composites were Si₃N₄ and MoSi₂ or WSi₂; no free elemental Si and Mo or W were detected by X-ray diffraction.