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.     

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.     

Effects of α-Sic versus β-Sic Starting Powders on Microstructure and Fracture Toughness of Sic Sintered with Al2O3-Y2O3 Additives

Dense Sic ceramics were obtained by pressureless sintering of β-Sic and α-Sic powders as starting materials using Al₂O₃-Y₂O₃ additives. The resulting microstructure depended highly on the polytypes of the starting SiC powders. The microstructure of SiC obtained from α-SiC powder was composed of equiaxed grains, whereas SiC obtained from α-SiC powder was composed of a platelike grain structure resulting from the grain growth associated with the β→α phase transformation of SiC during sintering. The fracture toughness for the sintered SiC using α-SiC powder increased slightly from 4.4 to 5.7 MPa.m^1/2 with holding time, that is, increased grain size. In the case of the sintered SiC using β-SiC powder, fracture toughness increased significantly from 4.5 to 8.3 MPa.m^1/2 with holding time. This improved fracture toughness was attributed to crack bridging and crack deflection by the platelike grains.     

SiC/ZTM Nanocomposite with Needlelike Mullite Grains and Enhanced Mechanical Properties

Zirconia-toughened mullite (SiC/ZTM) nanocomposites were prepared by a chemical precipitation method. The samples showed good sinterability and could be densified to >98.7% of the theoretical density at 1350°–1550°C. Because of the addition of mullite seeds in the starting powder and the pinning effects of ZrO₂ and SiC particles on mullite grain growth, a fine-grained microstructure formed. Mullite grains were generally equiaxed for the sample sintered at 1400°C; whereas, for the sample sintered at 1550°C, most mullite grains took a needlelike morphology, and SiC particles were primarily located within mullite grains. The strength and toughness increased with the increasing sintering temperature, and reached their respective maximum of 780 MPa and 3.7 MPa·m^1/2 for the sample sintered at 1550°C.     

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.     

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.     

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.     

Mechanical Properties and Fracture Toughness of α-Al2O3-Platelet-Reinforced Y-PSZ Composites at Room and High Temperatures

YPSZ/Al₂O₃-platelet composites were fabricated by conventional and tape-casting techniques followed by sintering and HIPing. The room-temperature fracture toughness increased, from 4.9 MPa·m^1/2 for YPSZ, to 7.9 MPa·m^1/2 (by the ISB method) for 25 mol% Al₂O₃ platelets with aspect ratio = 12. The room-temperature fiexural strength decreased 21% and 30% (from 935 MPa for YPSZ) for platelet contents of 25 vol% and 40 vol%, respectively. Al₂O₃ platelets improved the high-temperature strength (by 110% over YPSZ with 25 vol% platelets at 800°C and by 40% with 40 vol% platelets at 1300°C) and fracture toughness (by 90% at 800°C and 61% at 1300°C with 40 vol% platelets). An amorphous phase at the Al₂O₃-platelet/YPSZ interface limited mechanical property improvement at 1300°C. The influence of platelet alignment was examined by tape casting and laminating the composites. Platelet alignment improved the sintered density by >1% d _th , high-temperature strength by 11% at 800°C and 16% at 1300°C, and fracture toughness by 33% at 1300°C, over random platelet orientation.     

Effects of Annealing Temperature on Microstructural Development at the Interface Between Zirconia and Titanium

The interfacial reaction layers in the Ti/ZrO₂ diffusion couples, isothermally annealed in argon at temperatures ranging from 1100° to 1550°C for 6 h, were characterized using scanning electron microscopy and transmission electron microscopy, both attached with an energy-dispersive spectrometer. Very limited reaction occurred between Ti and ZrO₂ at 1100°C. A β'-Ti(Zr, O) layer and a two-phase α-Ti(O)+β'-Ti(Zr, O) layer were found in the titanium side after annealing at T ≥1300°C and T ≥1400°C, respectively. A three-phase layer, consisting of Ti₂ZrO+α-Ti(O, Zr)+β'-Ti (O, Zr), was formed after annealing at 1550°C. In the zirconia side near the original interface, β'-Ti coexisted with fine spherical c- ZrO_{2− x} , which dissolved a significant amount of Y₂O₃ in solid solution at T ≥1300°C. Further into the ceramic side, the α-Zr was formed due to the exsolution of Zr out of the metastable ZrO_{2− x} after annealing at T ≥1300°C: the α-Zr was very fine and dense at 1300°C, continuously distributed along grain boundaries at 1400°C, and became coarsened at 1550°C. Zirconia grains grew significantly at T ≥1400°C, with the lenticular t -ZrO_{2− x} being precipitated in c -ZrO_{2− x} . Finally, the microstructural development and diffusion paths in the Ti/ZrO₂ diffusion couples annealed at various temperatures were also described with the aid of the Ti–Zr–O ternary phase diagram.     

Effect of the Amount of Additives and Post-Heat Treatment on the Microstructure and Mechanical Properties of Yttrium–α-Sialon Ceramics

The yttrium–sialon ceramics with the composition of Y_0.333Si₁₀Al₂ON₁₅ and an excess addition of Y₂O₃ (2 or 5 wt%) were fabricated by hot isostatic press (HIP) sintering at 1800°C for 1 h. The resulting materials were subsequently heat-treated in the temperature range 1300–1900°C to investigate its effect on the α→β-sialon phase transformation, the morphology of α-sialon grains, and mechanical properties. The results show that α-sialons stabilized by yttrium have high thermal stability. An adjustment of the α-sialon phase composition is the dominating reaction in the investigated Y–α-sialon ceramics during low-temperature annealing. Incorporation of excess Y₂O₃ could effectively promote the formation of elongated α-sialon grains during post-heat-treating at relatively higher temperature (1700° and 1900°C) and hence resulted in a high fracture toughness ( K _IC= 6.3 MPa·m^1/2) via grain debonding and pullout effects. Although the addition of 5 wt% Y₂O₃ could promote the growth of elongated α grains with a higher aspect ratio, the higher liquid-phase content increased the interfacial bonding strength and therefore hindered interface debonding and crack deflection. The heat treatment at 1500°C significantly changed the morphology of α-sialon grains from elongated to equiaxed and hence decreased its toughness.     

High-Temperature Failure of Polycrystalline Alumina: II, Creep Crack Growth and Blunting

Creep crack growth in fine-grain alumina is measured by using surface cracks. A narrow power-law crack growth regime occurs at both 1300° and 1400°C, wherein the power-law exponent and activation energy are comparable to steady-state creep values. Asymptotic crack velocity behavior is exhibited near both the critical stress intensity factor, K_C , and the crack growth threshold, K_th . The threshold occurs near 0.4 K_1C at both 1300° and 1400°C and is associated with a transition in the size and distribution of damage. Displacement measurements indicate that crack tip damage exerts a strong influence on the displacement field, as predicted by recent theories. Furthermore, use of the stress intensity factor as a loading parameter does not produce adequate correlation with displacement measurements and is, therefore, not strictly suitable for nonlinear creeping ceramic poly crystals.     

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.     

Fabrication of New Cemented Carbide Containing Diamond Coated with Nanometer-Sized SiC Particles

A simple process for depositing a coating of silicon carbide (SiC) crystallites ∼10 nm in size onto diamond particles has been developed. SiO powders react with diamond in a vacuum at 1350°C to form a uniform β-SiC polycrystalline layer ∼60 nm thick. The SiC coating improves the oxidation resistance of the diamond. A cemented carbide material containing 20-vol%-SiC-coated diamond particles was sintered to a relative density of 99.5% by pulsed-electric-current sintering. A Vickers hardness and indentation fracture toughness of 15 GPa and 16.3 MPa·m^1/2, respectively, were obtained. This toughness is two times higher than that of cemented carbide containing no particles. The higher toughness is attributed to deflection and blockage of crack propagation by the diamond particles.     

Fine-Grained Silicon Carbide Ceramics with Oxynitride Glass

By using an oxynitride glass composition from the Y-Mg-Si-Al-O-N system as a sintering additive, the effect of atmosphere on densification was investigated during the liquid-phase sintering of SiC, and the resulting microstructure and mechanical properties of the sintered and subsequently annealed materials were investigated. SiC ceramics that were densified with 10 wt% oxynitride glass showed higher sinterability in a nitrogen atmosphere. Oxynitride glass enlarged the stability region of β-SiC and suppressed β→ alpha phase transformation, which resulted in an equiaxed microstructure. Grain growth of fine-grained SiC in some extent (up to ∼300 nm) was beneficial in improving both room-temperature strength and toughness. The best results were obtained when the ceramics were hot-pressed at 1800°C for 1 h in a nitrogen atmosphere and subsequently annealed at 1900°C for 3 h in an argon atmosphere. The room-temperature flexural strength and fracture toughness of the material were 847 MPa and 3.5 MPa·m^1/2, respectively.     

Inhibition of Sintering and Surface Area Loss in Phosphorus-Doped Corundum Derived from Diaspore

The influence of magnesium, phosphorus, and iron additions on the low-temperature (≤1000°C) sintering of nanocrystalline α-Al₂O₃ derived from α-AlOOH has been investigated. α-AlOOH powder with a surface area of 50 m²/g yielded α-Al₂O₃ products with surface areas of 150 and 80 m²/g after dehydration at temperatures of 400° and 500°C, respectively. However, these products were prone to sintering at >600°C, and the surface area was reduced to 15 m²/g within only 1 h at 1000°C. Although magnesium and iron doping had no discernible effect, the presence of phosphorus inhibited sintering and surface-area loss significantly. Samples doped with 1%–2% phosphorus had surface areas of >31 m²/g after 100 h at 1000°C. Atomic force microscopy studies of α-Al₂O₃ pseudomorphs derived from α-AlOOH single crystals also demonstrated the inhibiting effect of phosphorus, as the rate of crack elimination was reduced on phosphorus-modified surfaces. The effects of the dopants are discussed with regard to their potential influence on α-Al₂O₃ surface energy and diffusivity.     

Effects of Heat Treatment in a Wet Hydrogen Atmosphere on the Reliability of Sintered α-Silicon Carbide

Sintered α-SiC was exposed, for times up to 2 h, to a flowing wet H₂ atmosphere ( P _H2O= 1 × 10^-4 MPa) at temperatures of 1300°, 1400°, and 1500°C. The effect of such conditions on the reliability of the ceramic was estimated by comparing the Weibull modulus of the groups of specimens, tested in four-point flexure, before and after exposure. The Weibull modulus of as-polished specimens was 6.7, indicating a wide variation in room-temperature flexural strength. The Weibull modulus was increased to 14.2 by the heat treatment for 2 h in wet H₂ at 1400°C. The average strength was also improved from 347 to 446 MPa by such exposure. Heat treatment at 1300° and 1500°C also improved the reliability of the material, as indicated by increases in the Weibull modulus, but to less a degree than did exposure at 1400°C. The increases in reliability and average strength were attributed to the blunting of surface flaws by the formation of a thin SiO₂ layer on the sample surface.     

Pressureless Sintering and Properties of Titanium Diboride Ceramics Containing Chromium and Iron

The simultaneous addition of 0.5 wt% Cr and Fe was found to enhance the densification of TiB₂. The densities of specimens that were sintered for 2 h at 1800° and 1900°C were 97.6% and 98.8% of the theoretical value, respectively. The mechanical properties of the specimen sintered at 1800°C, which had a strength of 506 MPa and a fracture toughness of 6.16 MPa·m^1/2, were much better than those observed in the specimen sintered at 1900°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.     

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.     

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.