High-Temperature Tensile Behavior of a Boron Nitride-Coated Silicon Carbide-Fiber Glass-Ceramic Composite

Tensile properties of a cross-ply glass-ceramic composite were investigated by conducting fracture, creep, and fatigue experiments at both room temperature and high temperatures in air. The composite consisted of a barium magnesium aluminosilicate (BMAS) glass-ceramic matrix reinforced with SiC fibers with a SiC/BN coating. The material exhibited retention of most tensile properties up to 1200°C. Monotonic tensile fracture tests produced ultimate strengths of 230–300 MPa with failure strains of ∼1%, and no degradation in ultimate strength was observed at 1100° and 1200°C. In creep experiments at 1100°C, nominal steady-state creep rates in the 10⁻⁹ s⁻¹ range were established after a period of transient creep. Tensile stress rupture experiments at 1100° and 1200°C lasted longer than one year at stress levels above the corresponding proportional limit stresses for those temperatures. Tensile fatigue experiments were conducted in which the maximum applied stress was slightly greater than the proportional limit stress of the matrix, and, in these experiments, the composite survived 10⁵ cycles without fracture at temperatures up to 1200°C. Microscopic damage mechanisms were investigated by TEM, and microstructural observations of tested samples were correlated with the mechanical response. The SiC/ BN fiber coatings effectively inhibited diffusion and reaction at the interface during high-temperature testing. The BN layer also provided a weak interfacial bond that resulted in damage-tolerant fracture behavior. However, oxidation of near-surface SiC fibers occurred during prolonged exposure at high temperatures, and limited oxidation at fiber interfaces was observed when samples were dynamically loaded above the proportional limit stress, creating micro-cracks along which oxygen could diffuse into the interior of the composite.     

Tensile Creep and Creep-Strain Recovery Behavior of Silicon Carbide Fiber/Calcium Aluminosilicate Matrix Ceramic Composites

The tensile creep and creep strain recovery behavior of 0° and 0°/90° Nicalon-fiber/calcium aluminosilicate matrix composites was investigated at 1200°C in high-purity argon. For the 0° composite, the 100-h creep rate ranged from approximately 4.6 × 10⁻⁹ s⁻¹ at 60 MPa to 2.2 × 10⁻⁸ s⁻¹ at 200 MPa. At 60 MPa, the creep rate of the 0°/90° composite was approximately the same as that found for the 0° composite, even though the 0°/90° composite had only one-half the number of fibers in the loading direction. Upon unloading, the composites exhibited viscous strain recovery. For a loading history involving 100 h of creep at 60 MPa, followed by 100 h of recovery at 2 MPa, approximately 27% of the prior creep strain was recovered for the 0° composite and 49% for the 0°/90° composite. At low stresses (60 and 120 MPa), cavities formed in the matrix, but there was no significant fiber or matrix damage. For moderate stresses (200 MPa), periodic fiber rupture occurred. At high stresses (250 MPa), matrix fracture and rupture of the highly stressed bridging fibers limited the creep life to under 70 min.     

Mechanical Properties of Polycrystalline Lithium Orthosilicate

Room-temperature mechanical properties and high-temperature creep deformation of lithium orthosilicate (Li₄SiO₄) were studied. Elastic constants, flexural strength, and fracture toughness were determined for specimens with densities between 68% and 98% of theoretical. Critical quenching temperature and thermal-shock resistance parameters for 90% dense specimens were also measured. High-temperature creep deformation was investigated by a constant-strain-rate test in an argon atmosphere at temperatures between 750° and 1025°C and strain rates ranging from about 10⁻⁶ to 10⁻³ s⁻¹. At 950°C and above, the stress exponent, n , was determined to be 3.6, with a creep activation energy of 715 kJ/mol. Selected results obtained for Li₄SiO₄ are compared with results obtained for other Li-containing ceramics that are under consideration as candidates for fusion reactor breeder blankets.     

Effect of Hydrogen-Water Atmospheres on Corrosion and Flexural Strength of Sintered α-Silicon Carbide

Sintered α-SiC was exposed for 10 h to H₂ containing various partial pressures of H₂O ( P _H2O from 5×10⁻⁶ to 2×10⁻² atm; 1 atm≅10⁵ Pa) at 1300° and 1400°C. Weight loss, surface morphology, and room-temperature flexural strength were strongly dependent on P _H2O. The strength of the SiC was not significantly affected by exposure to dry H₂ at a P _H2O of 5×10⁻⁶ atm; and following exposure at P _H2O >5×10⁻³ atm, the strength was even higher than that of the as-received material. The increase in strength is thought to be the result of crack blunting associated with SiO₂ formation at crack tips. However, after exposure in an intermediate range of water vapor pressures (1×10⁻⁵< P _H2O <1×10⁻³ atm), significant decreases in strength were observed. At a P _H2O of about 1×10⁻⁴ atm, the flexural strength decreased approximately 30% and 50% after exposure at 1300° and 1400°C, respectively. The decrease in strength is attributed to surface defects caused by corrosion in the form of grain-boundary attack and the formation of pits. The rates of weight loss and microstructural changes on the exposed surfaces correlated well with the observed strength changes.     

High-Temperature Deformation of SiC-Whisker-Reinforced MgO-PSZ/Mullite Composites

The effect of 33.5 vol% SiC whisker loading on high-temperature deformation of 1 wt% MgO-38.5 wt% zirconia-mullite composites was studied between 1300° and 1400°C. At strain rates of 10⁻⁶ to 5 × 10⁻⁴/s the creep resistance of zirconia-mullite composites without SiC reinforcement was inferior to monolithic mullite of similar grain size. Analysis of the results suggested that the decreased creep resistance of mullite-zirconia composites compared to pure mullite could be at least partially explained by mechanical effects of the weaker zirconia phase, increased effective diffusivity of mullite by zirconia addition, and to the differences in mullite grain morphology. With SiC whisker reinforcement, the deformation rate at high stress was nearly the same as that of the unrein-forced material, but at low stress the creep rates of the SiC-reinforced material were significantly lowered. The stress dependence of the creep rate of unreinforced material suggested that diffusional creep was the operative mechanism, while the reinforced material behaved as if a threshold stress for creep existed. The threshold stress could be rationalized based on a whisker network model. This was supported by data on other whisker-containing materials; however, the threshold stress had a temperature dependence that was orders of magnitude higher than the elastic constants, leaving the physical model incomplete. The effects of residual stresses and amorphous phases at whisker/matrix interfaces are invoked to help complete the physical model for creep threshold stress.     

Synthesis and Characterization of a Remarkable Ceramic: Ti3SiC2

Polycrystalline bulk samples of Ti₃SiC₂ were fabricated by reactively hot-pressing Ti, graphite, and SiC powders at 40 MPa and 1600°C for 4 h. This compound has remarkable properties. Its compressive strength, measured at room temperature, was 600 MPa, and dropped to 260 MPa at 1300°C in air. Although the room-temperature failure was brittle, the high-temperature load-displacement curve shows significant plastic behavior. The oxidation is parabolic and at 1000° and 1400°C the parabolic rate constants were, respectively, 2 × 10⁻⁸ and 2 × 10⁻⁵ kg²-m⁻⁴.s⁻¹. The activation energy for oxidation is thus =300 kJ/mol. The room-temperature electrical conductivity is 4.5 × 10⁶Ω⁻¹.m⁻¹, roughly twice that of pure Ti. The thermal expansion coefficient in the temperature range 25° to 1000°C, the room-temperature thermal conductivity, and the heat capacity are respectively, 10 × 10⁻⁶°C⁻¹, 43 W/(m.K), and 588 J/(kgK). With a hardness of 4 GPa and a Young's modulus of 320 GPa, it is relatively soft, but reasonably stiff. Furthermore, Ti₃SiC₂ does not appear to be susceptible to thermal shock; quenching from 1400°C into water does not affect the postquench bend strength. As significantly, this compound is as readily machinable as graphite. Scanning electron microscopy of polished and fractured surfaces leaves little doubt as to its layered nature.     

Creep Deformation in an Alumina–Silicon Carbide Composite Produced via a Directed Metal Oxidation Process

Flexural creep studies were conducted in a commercially available alumina matrix composite reinforced with SiC particulates (SiC_p) and aluminum metal at temperatures from 1200° to 1300°C under selected stress levels in air. The alumina composite (5 to 10 μm alumina grain size) containing 48 vol% SiC particulates and 13 vol% aluminum alloy was fabricated via a directed metal oxidation process (DIMOX(tm))† and had an external 15 μm oxide coating. Creep results indicated that the DIMOX Al₂O₃–SiC_p composite exhibited creep rates that were comparable to alumina composites reinforced with 10 vol% (8 (μm grain size) and 50 vol% (1.5 μm grain size) SiC whiskers under the employed test conditions. The DIMOX Al₂O₃–SiC_p composite exhibited a stress exponent of 2 at 1200°C and a higher exponent value (2.6) at ≥ 1260°C, which is associated with the enhanced creep cavitation. The creep mechanism in the DIMOX alumina composite was attributed to grain boundary sliding accommodated by diffusional processes. Creep damage observed in the DIMOX Al₂O₃-SiC_p composite resulted from the cavitation at alumina two-grain facets and multiple-grain junctions where aluminum alloy was present.     

Compressive Creep of Uranium Carbide

The compressive creep rates of slightly hyperstoichi-ometric uranium carbide were determined in vacuum at 1200° to 1600°C and 3000 to 10,000 psi. The equation best describing the minimum creep rates of uranium carbide over this range is:
é= 1.8 × 10⁻³σ³ exp (-90,000/RT)     

Microstructure and High-Temperature Corrosion Behavior of a Cr–Al–C Composite

A Cr–Al–C composite was successfully synthesized by a hot-pressing method using Cr, Al, and graphite as starting materials. X-ray diffraction, scanning electron microscopy, and transmission electron microscopy analyses revealed that the composite contained Cr₂AlC, AlCr₂, Al₈Cr₅, and Cr₇C₃. The orientation relationships and atomic-scale interfacial microstructures among Cr₂AlC, AlCr₂, and Al₈Cr₅ are presented. This composite displays both excellent high-temperature oxidation resistance in air and hot-corrosion resistance against molten Na₂SO₄ salt. The parabolic rate constants for the oxidation in air at 1000°, 1100°, and 1200°C are 3.0 × 10⁻¹², 6.2 × 10⁻¹¹, and 6.2 × 10⁻¹⁰ kg² (m⁴·s)⁻¹, respectively, while the linear weight gain rates for the hot corrosion of Na₂SO₄-coated samples at 900° and 1000°C are, respectively, 1.2 × 10⁻³ and 4.4 × 10⁻³ mg (cm²·h)⁻¹. The mechanism of the excellent high-temperature corrosion resistance can be attributed to the formation of a protectively alumina-rich scale.     

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.     

Tetragonal Zirconia Polycrystals Reinforced with SiC Whiskers

The microstructure and the mechanical properties of hot-pressed tetragonal ZrO₂ polycrystals (TZP) reinforced with up to 30 vol% SiC whiskers were studied. The SiC whisker-TZP composites were stable under the hot-pressing conditions at 1450°C. Annealing in an oxidizing atmosphere at ∼1000°C resulted in glass formation and microcracking caused by whisker oxidation and transformation of the ZrO₂ grains near the whiskers to monoclinic symmetry. The fracture toughness was markedly improved by the dispersed whiskers (∼12 Mpa·m^1/2 at 30 vol% SiC) compared to the values measured for the matrix (∼6 Mpa·m^1/2). The flexural strength of the hot-pressed TZP-30 vol% SiC whisker composite at 1000°C (∼400 MPa) was twice that of the TZP matrix.     

Shear Strength as a Function of Test Rate for SiCf/BSAS Ceramic Matrix Composite at Elevated Temperature

Both interlaminar and in-plane shear strengths of a unidirectional Hi-Nicalon™-fiber-reinforced barium strontium aluminosilicate (SiC_f/BSAS) composite were determined at 1100°C in air as a function of test rate using double-notch shear test specimens. The composite exhibited a significant effect of test rate on shear strength, regardless of orientation. The shear strength degraded by about 50% as the test rate decreased from 3.3 × 10⁻¹ to 3.3 × 10⁻⁵ mm/s. The rate dependency of shear strength was similar to that observed for ultimate tensile strength at 1100°C for the two-dimensional (2-D) SiC_f/BSAS composite, in which tensile strength decreased by about 60% when the test rate varied from 5 to 0.005 MPa/s. A phenomenological, power-law slow crack growth model is proposed and formulated to account for the rate dependency of shear strength of the composite. The proposed model has been validated with additional results of both constant stress-rate and constant stress testing in shear at 1100°C using a 2-D Nicalon-fiber-reinforced crossply magnesium aluminosilicate (SiC_f/MAS-5) ceramic matrix composite.     

Effect of a Silicon Carbide "Nano-Dispersion" on the Mechanical Properties of Silicon Nitride

The hypothesis that a synergistic effect by fine SiC dispersoids operating on the submicrometer scale is capable of enhancing the deformation and fracture properties of Si₃N₄ ceramics has been examined. In order to single out the effect of the SiC dispersion from other microstructural factors affecting the material properties, experiments were conducted on a highly pure and dense Si₃N₄ material, suitable for basic investigations. Fracture mechanics and creep characterizations were performed at room temperature and at 1400°C on composites containing 25 vol% submicrometer SiC particles, for which the intragranular fraction was varied by changing the sintering conditions. Despite the obtained difference in composite microstructure, almost no improvement in either the fracture toughness or strength, as compared with the monolithic material, was found. Similarly, the slow crack growth and creep resistance at 1400°C were still dictated by the inherent properties of the matrix. This study emphasizes the need for scientific rather than empirical approaches on simple systems, before deducing general rules for the microstructural design of structural ceramics.     

Low-Temperature Creep of Nanocrystalline Titanium(IV) Oxide

Nanocrystalline TiO₂ with densities higher than 99% of rutile has been deformed in compression without fracture at temperatures between 600° and 800°C. The total strains exceed 0.6 at strain rates as high as 10⁻³ s⁻¹. The original average grain size of 40 nm increases during the creep deformation to final values in the range of 120 to 1000 nm depending on the temperature and total deformation. The stress exponent of the strain rate, n , is approximately 3 and the grain size dependence is d ^{− q} with q in the range of 1 to 1.5. It is concluded that the creep deformation occurs by an interface reaction controlled mechanism.     

Creep Mechanism of Polycrystalline Yttrium Aluminum Garnet

The high-temperature deformation behavior of a fine-grained polycrystalline yttrium aluminum garnet (YAG) was studied in the temperature range of 1400° to 1610°C using constant strain rate compression tests under strain rates ranging from 10⁻⁵/s to 10⁻³/s. The stress exponent of the creep rate, the activation energy in comparison with that for single-crystal YAG, and the grain size dependence suggest that Nabarro–Herring creep rate limited by the bulk diffusion of one of the cations (Y or Al) is the operative mechanism.     

Creep Behavior of Uranium Carbide-Based Alloys

Compression creep tests were performed on fully dense specimens of UC_1.01, UC_1.05, UC_1.01.+ 4 wt% W, and U_0.9Zr_0.1C_1.01+ 4 wt% W. Steady-state creep rates were measured from 1400° to 1800°C in a vacuum of 1.33 × 10^-3 N/m² (1 × 10^-5 torr) at stresses of 4.55 to 69.0 MN/m² (660 to 10,000 psi). The data for UC_1.01 could best be fit by an expression of the form ɛ= 1773σ^6.024 exp (106.5/RT) , where σ is the steady-state creep rate (h^-l), σ is the applied stress (MN/m²), and the creep activation energy is given in kcal/mol. The stress dependence for creep of UC_1.05 decreased with decreasing temperature because of second-phase precipitation; therefore, a unique creep activation energy could not be established for this U/C ratio. At all temperatures, the creep strength of UC_1.05 exceeded that of UC_1.01. For example, at 1700 ° C steady-state creep rates for UC_1.05 are ∼1/4 those for UC_1.01, but at 1400°C the creep rates are ∼ 3 orders of magnitude less. At 1700°C, creep rates for UC alloys are ∼4 orders of magnitude lower than those for unalloyed UC_1.01.     

Fabrication of Mullite Body Using Superplastic Transient Phase

Mullite, an extremely creep-resistant ceramic, has been fabricated using a novel processing/forming approach taking advantage of superplastic transitional phases. Starting with a mixture of alumina, silica, and a small amount of lithia additive (0.8 wt%), a processing window of about 50°C around 1350°C has been found within which the material can be densified and superplastically deformed with negligible mullitization. The lithia additive promotes a transient lithium aluminosilicate glassy phase that greatly enhances sintering and deformation. The superplastic premullite maintains a nearly constant grain size during deformation between 1250° and 1400°C, over a strain rate from 6 × 10⁻⁷ to 10⁻¹ s⁻¹, and has unusually high activation energy values in the range of 1150 to 2086 kJ/mol. An increase in the transient glassy phase content due to the increased matrix dissolution at higher temperatures contributes in part to this anomaly. The mullite work pieces thus shaped become creep resistant again after a postforming annealing/mullitization treatment which decreases the creep rate by 6 orders of magnitude. The mechanical properties (hardness, toughness, and strength) of the finished mullite are compared to those of conventionally processed mullite.     

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.     

High-Temperature Creep and Kinetic Demixing in (Co,Mg)O

The creep behavior of fine-grained (Co_0.5Mg_0.5)O and (Co_0.25Mg_0.75)O has been characterized as part of an investigation of kinetic demixing in solid-solution oxides due to a nonhydrostatic stress. (i) For low stresses and small grain sizes, the dominant deformation mechanism for both compositions is diffusional creep limited by the transport of oxygen along grain boundaries. The oxygen grain-boundary diffusivity, D _o ^b is independent of oxygen partial pressure. The values of ω D _o ^b , where ω is the grain-boundary width, that have been determined from the steady-state diffusional creep rates are given by ω D _o ^b =4.7×10⁻⁸ exp[-230 (kJ/mol)/ RT ] (cm³/s) for (Co_0.5Mg_0.5)O in the range 950° to 1200°C and ω D _o ^b =7.4 × 10⁻⁸ exp[-263 (kJ/mol)/ RT ] (cm³/s) for (Co_0.25Mg_0.75)O in the range 1100° to 1250°C. Since oxygen diffusion controls the rate of diffusional creep, kinetic demixing is not observed in deformed samples of either composition. (ii) For high stresses and large grain sizes, the dominant deformation mechanism in both cases is dislocation-climb-controlled creep, where the rate of dislocation climb is controlled by oxygen lattice diffusion. Based on the positive dependence of creep rate on oxygen partial pressure, it is concluded that oxygen diffuses through the lattice by an interstitial mechanism.     

High-Temperature Stiffness and Damping Measurements to Monitor the Glassy Intergranular Phase in Liquid-Phase-Sintered Silicon Carbides

Detailed stiffness and internal friction ( Q ⁻¹) versus temperature curves were obtained for liquid-phase-sintered silicon carbides using advanced resonant beam analysis up to 1400°C. As-sintered materials display a stable Q ⁻¹-peak near 1100°C, superimposed on an increasing background. The change of stiffness associated with the damping peak is quantitatively related to the amount of matter in pockets of the amorphous intergranular phase in which the refractory SiC matrix grains are embedded. The successful removal of the amorphous pockets by annealing at 1900°C is deduced from the disappearance of the damping peak and confirmed with transmission electron microscopy.