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.     

Static Fatigue Limit for Sintered Silicon Carbide at Elevated Temperatures

The modified static loading technique for estimating static fatigue limits was used to study the effects of oxidation and temperature on the static fatigue limit, K ₁₀ for crack growth in sintered silicon carbide. For as-machined, unoxidized sintered silicon carbide with a static load time of 4 h, K ₁₀× 2.25 MPa * m^1/2 at 1200° and ∼1.75 at 1400°C. On oxidation for 10 h at 1200°C, K ₁₀ drops to ∼1.75 MPam^1/2 at 1200° and ∼1.25 at 1400°C when tested in a nonoxidizing ambient. Similar results were obtained at 1200°C for tests performed in air. A tendency for strengthening below the static fatigue limit appears to result from plastic relaxation of stress in the crack-tip region by viscous deformation involving an oxide grain-boundary phase for oxidized material and, possibly, diffusive creep deformation in the case of unoxidized material.     

Thermomechanical Fatigue Behavior of a Silicon Carbide Fiber-Reinforced Calcium Aluminosilicate Composite

Isothermal fatigue and in-phase thermomechanical fatigue (TMF) tests were performed on a unidirectional, continuous-fiber, Nicalon®-reinforced calcium aluminosilicate glass-ceramic composite ([O]_16, SiC/CAS-II). Monotonic tensile tests were performed at 1100°C (2012°F) and 100 MPa/s (14.5 ksi/s) to determine the material's ultimate strength (σ_ult) and proportional limit (σ_pl). Isothermal fatigue tests at 1100°C employed two loading profiles, a triangular waveform with ramp times of 60 s and a similar profile with a superimposed 60-s hold time at σ_max. All fatigue tests used a σ_max of 100 MPa (40% of σ_pl), R = 0.1. TMF loading profiles were identical to the isothermal loading profiles, but the temperature was cycled between 500° and 1100°C (932° and 2012°F). All fatigued specimens reached run-out (1000 cycles) and were tested in tension at 1100°C immediately following the fatigue tests. Residual modulus, residual strength, cyclic stress-strain modulus, and strain accumulation were all examined as possible damage indicators. Strain accumulation allowed for the greatest distinction to be made among the types of tests performed. Fiber and matrix stress analyses and creep data for this material suggest that matrix creep is the primary source of damage for the fatigue loading histories investigated.     

Influence of Loading Frequency on the Room-Temperature Fatigue of a Carbon-Fiber/SiC-Matrix Composite

The influence of cyclic loading frequency on the tensile fatigue life of a woven-carbon-fiber/SiC-matrix composite was examined at room temperature. Tension-tension fatigue experiments were conducted under load control, at sinusoidal frequencies of 1, 10, and 50 Hz. Using a stress ratio (σ_min/σ_max) of 0.1, specimens were subjected to maximum fatigue stresses of 310 to 405 MPa. There were two key findings: (1) the fatigue life and extent of modulus decay were influenced by loading frequency and (2) the postfatigue monotonic tensile strength increased after fatigue loading. For loading frequencies of 1 and 10 Hz, the fatigue limit (defined at 1 × 10⁶ cycles) was approximately 335 MPa, which is over 80% of the initial monotonic strength of the composite; at 50 Hz, the fatigue limit was below 310 MPa. During 1- and 10-Hz fatigue at a maximum stress of 335 MPa, the modulus exhibited an initially rapid decrease, followed by a partial recovery; at 50 Hz, and the same stress limits, the modulus continually decayed. The residual strength of the composite increased by approximately 20% after 1 × 10⁶ fatigue cycles at 1 or 10 Hz under a peak stress of 335 MPa. The increase in strength is attributed in part to a decrease in the stress concentrations present near the crossover points of the 0° and 90° fiber bundles.     

Threshold Stress for Crack-Healing of Si3N4/SiC and Resultant Cyclic Fatigue Strength at the Healing Temperature

Si₃N₄/SiC composite ceramics were hot-pressed in order to investigate the crack-healing behavior under stress. Semi-elliptical surface cracks of 100 μm in surface length were made on each specimen. The pre-cracked specimens were crack-healed under cyclic or constant bending stress, and the resultant bending strength and cyclic fatigue strength were studied. The threshold stress for crack-healing was investigated at healing temperatures of 1000° and 1200°C. The cyclic fatigue strengths of crack-healed specimens were also investigated at healing temperatures of 900° and 1000°C. The main conclusions are as follows: (1) The threshold cyclic and constant stresses for crack-healing, below which pre-cracked specimens recovered their bending strength, were 300 MPa, which was 75% of the bending strength of the pre-cracked specimens and (2) the crack-healed specimens exhibited quite high cyclic fatigue strength at crack-healing temperatures of 900° and 1000°C.     

Crack-Healing Behavior of Si3N4/SiC Ceramics under Cyclic Stress and Resultant Fatigue Strength at the Healing Temperature

Si₃N₄/SiC composite ceramics were sintered and subjected to three-point bending. A semi-elliptical surface crack of 100 μm surface length was made on each specimen. The crack-healing behavior under cyclic stress of 5 Hz, and resultant cyclic fatigue strengths at healing temperatures of 1100° and 1200°C, were systematically investigated. The main conclusions are as follows: (1) Si₃N₄/SiC composite ceramics have an excellent ability to heal a crack at 1100° and 1200°C. (2) This sample could heal a crack even under cyclic stress at a frequency of 5 Hz. (3) The crack-healed sample exhibited quite high cyclic fatigue strength at each crack-healing temperature, 1100° and 1200°C.     

High-Temperature Stability of Lanthanum Orthophosphate (Monazite) on Silicon Carbide at Low Oxygen Partial Pressures

The stability of lanthanum orthophosphate (LaPO₄) on SiC was investigated using a LaPO₄-coated SiC fiber at 1200°–1400°C at low oxygen partial pressures. A critical oxygen partial pressure exists below which LaPO₄ is reduced in the presence of SiC and reacts to form La₂O₃ or La₂Si₂O₇ and SiO₂ as the solid reaction products. The critical oxygen partial pressure increases from ∼0.5 Pa at 1200°C to ∼50 Pa at 1400°C. Above the critical oxygen partial pressure, a thin SiO₂ film, which acts as a reaction barrier, exists between the SiC fiber and the LaPO₄ coating. Continuous LaPO₄ coatings and high strengths were obtained for coated fibers that were heated at or below 1300°C and just above the critical oxygen partial pressure for each temperature. At temperatures above 1300°C, the thin LaPO₄ coating becomes morphologically unstable due to free-energy minimization as the grain size reaches the coating thickness, which allows the SiO₂ oxidation product to penetrate the coating.     

Effect of Heat Treatments on the Crack-Healing and Static Fatigue Behavior of Silicon Carbide Sintered with Sc2O3 and AlN

Crack-healing behavior of silicon carbide ceramics sintered with AlN and Sc₂O₃ has been studied as a function of heat-treatment temperature and applied stress. Results showed that heat treatment in air could significantly increase the indentation strength whether a stress is applied or not. After heat treatment with no applied stress at 1300°C for 1 h in air, the indentation strength of the specimen with an indentation crack of ∼100 μm (≈2c) recovered its strength fully at room temperature. In addition, a simple heat treatment at 1200°C for 5 h under an applied stress of 200 MPa in air resulted in a complete recovery of the unindented strength at the healing temperature. However, higher applied stress led to fracture of the specimens during heat treatment. The static fatigue limit of the specimens crack healed at 1200°C for 5 h under 200 MPa was ∼450 MPa at the healing temperature. The ratio of the static fatigue limit of the crack-healed specimen to the unindented strength was ∼80%.     

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.     

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.     

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.     

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.     

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.     

Mechanical Behavior and High-Temperature Performance of a Woven Nicalon™/Si-N-C Ceramic-Matrix Composite

A modern ceramic-matrix composite (CMC) has been extensively characterized for a high-temperature aerospace turbine-engine application. The CMC system has a silicon-nitrogen-carbon (Si-N-C) matrix reinforced with Nicalon fibers woven in a balanced eight-harness satin weave fabric. Tensile tests have demonstrated that this CMC exhibits excellent strength retention up to 1100°C. The room-temperature fatigue limit was 160 MPa, ∼80% of the room-temperature tensile strength. The composite reached run-out conditions under cyclic (10⁵ cycles at 1 Hz) and sustained tension (100 h) conditions at a stress of 110 MPa, which was ∼35 MPa above the proportional limits at temperatures up to 1100°C in air. At stress levels >110 MPa, cyclic loading at 1000°C caused a more severe reduction in life, based on time, compared with sustained tension. Further life degradation was observed in the 1000°C fatigue specimens that were exposed to a salt-fog environment. This degradation decreased the fatigue life ∼85% at the stress levels that were tested.     

Stress Rupture in Tension and Flexure of a SiC-Whisker-Reinforced Silicon Nitride Composite at Elevated Temperatures

Stress rupture of a 20 vol% SiC whisker-reinforced Si₃N₄ composite processed by gas pressure sintering was investigated by both tension and flexure methods. The stress exponents for the stress rupture decrease with increasing temperature. The fracture surfaces of both tensile and flexural stress rupture at 1000°C consist of mirror, mist, and hackle regions. The size of the mirror region increases with decreasing stress. Crack propagation is a mixture of intergranular and transgranular modes at 1000°C. Both tensile and flexural fracture surfaces under constant stress at 1200°C were characterized by a rough zone and a mirror zone; the size of the rough zone increased with decreasing stress. Creep crack growth occurred at 1200°C, which is a process of cavity nucleation, growth, and interlinkage in front of a crack. The transition of fracture mechanisms with temperature is discussed.     

Determination of Threshold Stress Intensity for Crack Growth at High Temperature in Silicon Carbide Ceramics

A method for estimating the threshold stress intensity for crack growth is presented. The technique requires prior knowledge of the flaw population of a material and uses applied static loads followed by fast fracture to assess the effect of initial applied stress intensity on flaw behavior. The technique was applied to a hot-pressed Sic at 1200° and 1400°C in a nonoxidizing atmosphere. At 1400°C with a static load time of 4 h, the threshold stress intensity was determined to be ∼ 1.75 MPa·m^1/2 with a slight tendency toward higher fracture stress with increasing initial stress intensity below the threshold. At 1200°C for a static load time of 4 h, apparent strengthening was observed below a threshold stress intensity of ∼2.25 MPa·m^1/2. This strengthening effect appears to result from stress relaxation in the crack-tip region, probably by plastic deformation which involves the oxide grain-boundary phase.     

Cyclic-Fatigue Behavior of SiC/SiC Composites at Room and High Temperatures

Tension-tension cyclic-fatigue tests of a two-dimensional-woven-SiC-fiber-SiC-matrix composite (SiC/SiC) prepared by chemical vapor infiltration (CVI) were conducted in air at room temperature and in argon at 1000°C. The cyclic-fatigue limit (10⁷ cycles) at room temperature was ∼160 MPa, which was ∼80% of the monotonic tensile strength of the composite. However, the fatigue limit at 1000°C was only 75 MPa, which was 30% of the tensile strength of the composite. No difference was observed in cyclic-fatigue life at room temperature and at 1000°C at stresses >180 MPa; however, cyclic-fatigue life decreased at 1000°C at stresses < 180 MPa. The fracture mode changed from fracture in 0° and 90° bundles at high stresses to fracture mainly in 0° bundles at low stresses. Fiber-pullout length at 1000°C was longer than that at room temperature, and, in cyclic fatigue, it was longer than that in monotonic tension. The decrease in the fatigue limit at 1000°C was concluded to be possibly attributed to creep of fibers and the reduction of the sliding resistance of the interface between the matrix and the fibers.     

High-Temperature Failure of an Alumina-Silicon Carbide Composite under Cyclic loads: Mechanisms of Fatigue Crack-Tip Damage

Experimental results are presented on the mechanisms of tensile cyclic fatigue crack growth in an A1₂0₃-33-vol%-SiC-whisker composite at 1400°C. The ceramic composite exhibits subcritical fatigue crack propagation at stress-intensity-fator values far below the fracture toughness. The fatigue characterized by the stressintensity-factor range, ΔK, and crack propagation rates are found to be strongly sensitive to the mean stress (load ratio) and the frequency of the fatigue cycle. Detailed transmission electron microscopy of the fatigue crack-tip region, in conjunction with optical microscopy, reveals that the principal mechanism of permanent damage ahead of the advancing crack is the nucleation and growth of interfacial flaws. The oxidation of Sic whiskers in the crack-tip region leads to the formation of a silica-glass phase in the 1400°C air environment. The viscous flow of glass causes debonding of the whisker-matrix interface; the nucleation, growth, and coalescence of interfacial cavities aids in developing a diffuse microcrack zone at the fatigue crack tip. The shielding effect and periodic crack branching promoted by the microcracks result in an apparently benefcial fatigue crack-growth resistance in the A1₂0₃—SiC composite, as compared with the unreinforced alumina with a comparable grain size. A comparison of static and cyclic load crack velocities is provided to gain insight into the mechanisms of elevated temperature fatigue in ceramic composites.     

Impurity-Enhanced Intergranular Cavity Formation in Silicon Nitride at High Temperatures

The effect of trace impurities on high-temperature strength is examined in Si₃N₄ sintered without additives. Strength degradation above 1000°C occurs only in the low-purity material, which also exhibits an intergranular slow crack growth (SCG) rate 2 orders of magnitude faster than that of the high-purity material at 1400°C. A relaxation peak of internal friction is observed only in the low-purity material, at ≅1200°C, and the origin of this peak is ascribed to the initial stage of SCG—that is, the cavity-nucleation stage, enhanced by impurities. Based on the present results, a model for impurity-enhanced SCG is proposed.     

Synthesis and Magneto-mechanical Properties of Ce-TZP/La M-Type Hexaferrite Composite

An in situ composite composed of ceria-stabilized tetragonal zirconia polycrystals (Ce-TZP) and La{Co_0.5Fe_0.5(Fe_0.9Al_0.1)₁₁}O₁₉ was synthesized from a powder mixture of Ce-TZP, La(Fe_0.9Al_0.1)O₃, Fe₂O₃, Al₂O₃, and CoO. The dense Ce-TZP dispersed with platelike La{Co_0.5Fe_0.5(Fe_0.9Al_0.1)₁₁}O₁₉ crystals as a second phase were formed after sintering from 1250° to 1350°C. The saturation magnetization of the in situ composite Ce-TZP/La{Co_0.5Fe_0.5(Fe_0.9Al_0.1)₁₁}O₁₉ was proportional to the mass fraction of the hexaferrite second phase in Ce-TZP. The coercivity of the composite with a 20 mass% of second phase decreased from 9.14 to 2.52 kOe (from 728 to 201 kA/m) after the pulverization of the composite. The susceptibility (χ) increased by 15%–25% under uniaxial stress on the composite. The change of the susceptibility (Δχ/χ) value increased with decreasing the mass fraction of the second phase in the composite. The Δχ was found to increase linearly with applied stress and abruptly change on cracking, which is expected for the application in fracture sensing of the composite.