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.     

Mode I Fracture Toughness of a Small-Grained Silicon Nitride: Orientation, Temperature, and Crack Length Effects

The Mode I fracture toughness ( K _{I C} ) of a small-grained Si₃N₄ was determined as a function of hot-pressing orientation, temperature, testing atmosphere, and crack length using the single-edge precracked beam method. The diameter of the Si₃N₄ grains was <0.4 µm, with aspect ratios of 2–8. K _{I C} at 25°C was 6.6 ± 0.2 and 5.9 ± 0.1 MPa·m^1/2 for the T–S and T–L orientations, respectively. This difference was attributed to the amount of elongated grains in the plane of crack growth. For both orientations, a continual decrease in K _IC was observed through 1200°C, to ∼4.1 MPa·m^1/2, before increasing rapidly to 7.5–8 MPa·m^1/2 at 1300°C. The decrease in K _IC through 1200°C was a result of grain-boundary glassy phase softening. At 1300°C, reorientation of elongated grains in the direction of the applied load was suggested to explain the large increase in K _IC. Crack healing was observed in specimens annealed in air. No R -curve behavior was observed for crack lengths as short as 300 µm at either 25° or 1000°C.     

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.     

Carbon Additions to Molybdenum Disilicide: Improved High-Temperature Mechanical Properties

C addition (2 wt%) to MoSi₂ acted as a deoxidant, removing the otherwise ubiquitous siliceous grain boundary phase in hot-pressed samples, and causing formation of SiC and Mo₅Si₃C₁ (a variable-composition Nowotny phase). Both hardness and fracture toughness of the C-containing alloy were higher than those of the C-free (and oxygen-rich) material; more significantly, the fracture toughness of the MoSi₂+ 2% C alloy increased from 5.5 MPa·m^1/2 at 800°C to ∼11.5 MPa·m^1/2 at 1400°C.     

Slow Crack Growth in Sapphire Fibers at 800° to 1500°C

Sapphire fibers with (near) c -axis orientation were tested in tension over a range of strain rates (10⁻⁵ to 0.5 min⁻¹) at elevated temperatures (800° to 1500°C). The strength of the fibers was dependent on the strain rate. Slow crack growth was confirmed as the degradation process by direct inspection of the fracture surfaces and estimation of fracture stresses from measured flaw sizes. The slow crack growth parameter, N , decreased with increasing temperature. At 1400°C a threshold in strength was observed; the threshold stress intensity factor was estimated to be ≅ 1 MP·m^1/2 at 1400°C. Thermally activated bond rupture (e.g., lattice trapping model) is postulated as the mechanism responsible for the slow crack growth.     

Subcritical Crack Growth in Y-TZP and Al2O3-Toughened Y-TZP

Crack velocity curves for Y-TZP and Al₂O₃-toughened Y-TZP were determined for long cracks in compact tension specimens with an in situ fracture device on the stage of an optical microscope. Indications for a crack velocity threshold were found for both materials. Above this threshold, at 2.6 MPa·m^1/2 for Y-TZP and 3.6 MPa·m^1/2 for Al₂O₃-toughened Y-TZP, chemically assisted subcritical crack growth occurs over an extended regime of applied stress intensity factors of width 2.1–2.8 MPa·m^1/2. It is recognized that the dependence of the shielding term on the crack-tip stress field renders transformation-toughened materials particularly susceptible to stress-corrosion cracking. This interrelation leads to the definition of a steady-state velocity at constant applied stress intensity factor. This velocity is obtained in the situation where the shielding term is fully defined by the present crack-tip stress field, not depending on prior loading history.     

Mechanical Properties of La1-xSrxCo0.2Fe0.8O3 Mixed-Conducting Perovskites Made by the Combustion Synthesis Technique

This paper examined the room-temperature mechanical properties of a mixed-conducting perovskite La_{1– x} Sr _x Co_0.2Fe_0.8O₃ ( x = 0.2–0.8). Powders were made by the combustion synthesis technique and sintered at 1250°C in air. Sintered density, crystal phase, and grain size were characterized. Young's and shear moduli, microhardness, indentation fracture toughness, and biaxial flexure strength were determined. The Young's and shear moduli slightly increased with increasing strontium content. Young's modulus of 151–188 GPa and shear modulus of 57–75 GPa were measured. Biaxial flexure strength of ∼160 MPa was measured for lower strontium content batches. Strength greatly decreased to ∼40 MPa at higher strontium concentrations ( x = 0.6–0.8) because of the formation of extensive cracking. Indentation toughness showed a higher value (∼1.5 MPa·m^1/2) for low strontium ( x = 0.2) content and a lower value (∼1.1 MPa·m^1/2) for the other batches ( x = 0.4–0.8). Materials with fine and coarse grain size were also tested at various indent loads and showed no dependence of toughness on crack size. In addition, fractography was used to characterize the critical flaw and fracture mode.     

Mechanical- and Physical-Property Changes of Neutron-Irradiated Chemical-Vapor-Deposited Silicon Carbide

Indentation and density measurements have revealed important changes in the mechanical and physical properties of silicon carbide (SiC) due to neutron irradiation. Specifically, the changes in the elastic modulus, hardness, fracture toughness, and density with irradiation have provided an understanding of the expected performance of SiC and SiC composites in nuclear applications. After the accumulated damage has saturated, these mechanical properties were affected primarily by the irradiation temperature. Chemical-vapor-deposited (CVD) SiC was irradiated above the saturation fluence and yielded volumetric swelling of 2.6% and 1.3% for irradiation temperatures of 100°-150°C and 500°-550°C, respectively. At the same respective temperatures, the elastic modulus decreased from an unirradiated value of 503 GPa to ∼420 and 450 GPa. Conversely, the hardness increased from 36 GPa for the unirradiated CVD SiC to 38 and 40 GPa for the samples irradiated at 100°-150°C and 500°-550°C, respectively. Interestingly, these two independent properties approached almost-constant levels after exposure to a fluence of 0.5 × 10²⁵ n/m², E > 0.1 MeV. Indentation fracture toughness measurements, which were within the range of values in the literature for conventional fracture toughness procedures for SiC, increased from ∼2.8 MPa·m^1/2 for the unirradiated samples to 3.7 and 4.2 MPa·m^1/2 for the samples that were irradiated at 100°-150°C and 500°-550°C, respectively.     

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.     

Effect of Heating Schedule on the Microstructure and Fracture Toughness of α-SiAlON—Cause and Solution

The effect of heating schedule on microstructure and fracture resistance has been investigated in single-phase Nd-, Y-, and Yb-α-SiAlON. Such effect is strongly system dependent, reflecting the strong influence of phase stability on α-SiAON nucleation and the amount of transient/residual liquid during processing. The addition of 1% of α-SiAlON seeds to the starting powders nearly completely obliterates such effect, while it simultaneously improves microstructure homogeneity and fracture resistance. SENB toughness of 7 MPa·m^1/2 and peak R -curve toughness of ∼11 MPa·m^1/2 have been obtained for seeded Y-α-SiAlON ceramics using heating rates from 1°C/min to 25°C/min.     

Microstructure and Mechanical Properties of Hot-Pressed α-Al2O3-Seeded γ-Alumina Ceramics

High-quality alumina ceramics were fabricated by a hot pressing with MgO and SiO₂ as additives using α-Al₂O₃-seeded nanocrystalline γ-Al₂O₃ powders as the raw material. Densification behavior, microstructure evolution, and mechanical properties of alumina were investigated from 1250°C to 1450°C. The seeded γ-Al₂O₃ sintered to 98% relative density at 1300°C. Obvious grain growth was observed at 1400°C and plate-like grains formed at 1450°C. For the 1350°C hot-pressed alumina ceramics, the grain boundary regions were generally clean. Spinel and mullite formed in the triple-grain junction regions. The bending strength and fracture toughness were 565 MPa and 4.5 MPa·m^1/2, respectively. For the 1300°C sintered alumina ceramics, the corresponding values were 492 MPa and 4.9 MPa·m^1/2.     

Subcritical Crack Growth in Lead Zirconate Titanate

Subcritical crack growth in terms of velocity–stress intensity factor ( v – K ) curves in lead zirconate titanate (PZT) were experimentally characterized on poled and unpoled compact tension specimens. The poled specimens were tested under open- and short-circuit electrical boundary conditions, which resulted in an increase in fracture toughness by 0.2 MPa·m^1/2 for the accessible velocity range ( v = 10⁻⁹ to 10⁻⁴ m/s) in the open-circuit case. Subcritical crack growth of unpoled specimens was obtained under ambient (relative humidity = 35%) and dry (relative humidity ∼ 0.02%) conditions over a regime in stress intensity factor of 0.5 MPa·m^1/2.     

Reactive Hot Pressing and Properties of Nb2AlC

Dense Nb₂AlC ceramic was synthesized from NbC, Nb, and Al powder mixture at 1650°C and a pressure of 30 MPa for 90 min using an in situ reaction/hot-pressing method. The reaction kinetics, microstructure, physical, and mechanical properties of the fabricated material were investigated. A thermal expansion coefficient of ∼8.1 × 10⁻⁶ K⁻¹ was measured in the temperature range of 30°–1050°C. At room temperature a thermal conductivity of ∼20 W·(m·K)⁻¹ and a Vickers hardness of ∼4.5 GPa were determined. The material attained Young's modulus, four-point bending strength and fracture toughness of ∼294 GPa, ∼443 MPa, and ∼5.9 MPa·m^1/2, respectively. The nanolayered grains with a mean grain size of 17 μm contributed to the damage tolerance of this ceramic. Quenching from 600°, 800°, and 1000°C into water at room temperature resulted in decrease in bending strength from 443 MPa for the as-synthesized Nb₂AlC to 391, 156, and 149 MPa, respectively.     

Ferroelastic Behavior of LaCoO3-Based Ceramics

LaCoO₃ and La_0.8Ca_0.2CoO₃ ceramics show a nonelastic stress–strain behavior during four-point bending experiments where hysteresis loops are observed during loading–unloading cycles. Permanent strain is stored in the material after unloading, and a mechanism related to ferroelastic domain switching in the rhombohedral perovskite is proposed. Domain switching in the materials has been confirmed using X-ray diffractometry. Fracture toughnesses of La_0.8Ca_0.2CoO₃ measured using single-edge notched beam and single-edge V-notched beam methods coincide and are equal to 2.2 MPa·m^1/2 at room temperature and decrease to ∼1 MPa·m^1/2 at temperatures >300°C. A decrease in fracture toughness is consistent with ferroelastic behavior, because the rhombohedral distortion decreases with increasing temperature.     

Fabrication and Mechanical Properties of Titanium Boride Ceramics

Dense TiB ceramics (99.6% of theoretical) with a grain size of ∼5 µm have been fabricated by reaction hot pressing of TiB₂ and titanium for 2 h at 1900°C and 28.5 MPa. The TiB ceramics exhibit a fracture toughness ( K_IC ) of 4.5 MPa·m^1/2 and a bending strength (sigma_b) of 360 MPa. Electrical resistivity (rho) is 3.4 × 10^-7 Omega·m at room temperature.     

Slow Growth of Microcracks: Evidence for One Type of ZrO2 Toughening

Alumina containing 15 vol% monoclinic ZrO₂ dispersed at the grain boundaries exhibited very high room-temperature fracture toughness (∼11 MPa·m^1/2) on cooling from 1275°C when microcrack precursors nucleated at Ts. With increasing time (up to ∼12 h) at room temperature, K_Ic and Young's modulus decreased when dilational and thermal-expansion strains subcritically propagated inter granular microcracks. Thus, transformation toughening of ceramics with inter crystalline ZrO₂ dispersions is to a great extent caused by microcrack nucleation and extension.     

ZrO2-Transformation-Toughened Glass-Ceramics Prepared by the Sol-Gel Process from Metal Alkoxides

Glasses of composition 3ZrO₂O · 2SiO₂ were prepared by the sol-gel process from metal alkoxides. Tetragonal ZrO₂ was precipitated by appropriate heat treatment at 1000° to 1200°C. The fracture toughness of these glass-ceramics increased with increasing crystallite size of the tetragonal ZrO₂, reaching ∼5.0 MN/m^3/2 at a size of ∼40 nm. The higher fracture toughness was attributed to tetragonal → monoclinic ZrO₂ transformation toughening.     

Processing, Fracture Toughness, and Vickers Hardness of Allylhydridopolycarbosilane-Derived Silicon Carbide

Bulk specimens of precursor-derived silicon carbide (SiC) suitable for mechanical-property measurements were prepared from allylhydridopolycarbosilane (AHPCS), which is a commercially available, hyperbranched polycarbosilane. Crack-free pellets were obtained by cold-pressing mixtures of finely ground, 1000°C pyrolyzed, "AHPCS-SiC" with neat AHPCS, followed by pyrolysis to 1000°C and ten subsequent reinfiltration/pyrolysis steps with the neat liquid AHPCS. Then, these pellets were heat-treated to 1200°, 1400°, and 1600°C, followed by additional reinfiltration/pyrolysis cycles to the final respective maximum temperatures. This fabrication process simulated the production of the matrix phase for ceramic-matrix composites via successive infiltration/pyrolysis cycles. The density of the material processed at these temperatures, measured via the Archimedes method, was 2.3, 2.5, 2.6, and 2.9 g/cm³, respectively, and the average open porosities of the samples were 2, 0.2, 1, and 9 vol%, respectively. The fracture toughness was measured using the single-edge V-notched-beam method, and the hardness was measured via Vickers indentation. The samples had an average toughness of 1.40 ± 0.08, 1.65 ± 0.09, 1.67 ± 0.07, and 1.46 ± 0.08 MPa·m^1/2 for the samples that were treated at 1000°, 1200°, 1400°, and 1600°C, respectively. The Vickers hardness for these samples, measured at a load of 1000 g, was 12 ± 1, 13 ± 2, 11 ± 1, and 9 ± 1 GPa, respectively.     

Formation, Powder Characterization and Sintering of MgCr2O4 by the Hydrazine Method

Picrochromite (MgCr₂O₄) crystallizes at 480° to 530°C from an amorphous material prepared by the hydrazine method. The MgCr₂O₄ powders were characterized for particle size and surface area. Individual particles tend toward a hexagonal morphology above 1000°C. Dense MgCr₂O₄ ceramics (99.5% of theoretical) with an average grain size of 2 μm have been fabricated by spark plasma sintering for 5 min at 1400°C and 30 MPa. Their fracture toughness and bending strength are 3.7 MPa·m^1/2 and 310 MPa, respectively.     

Heat-Resistant Silicon Carbide with Aluminum Nitride and Erbium Oxide

Fully dense SiC ceramics with high strength at high temperature were obtained by hot-pressing and subsequent annealing under pressure, with AlN and Er₂O₃ as sintering additives. The ceramics had a self-reinforced microstructure consisting of elongated SiC grains and a grain-boundary glassy phase. The strength of these ceramics was ∼550 MPa at 1600°C, and the fracture toughness was ∼6 MPa·m^1/2 at room temperature. The beneficial effect of the new additive composition on high-temperature strength might be attributable to the introduction of aluminum from the liquid composition into the SiC lattice, resulting in a refractive grain-boundary glassy phase.