R-Curve Behavior and Flaw Insensitivity of Ce-TZP/Al2O3 Composite

A ceria-partially-stabilized zirconia-aiumina (Ce-TZP/Al₂O₃) composite optimized for transformation toughening was used to demonstrate its flaw insensitivity due to R -curve behavior. Four-point bend specimens fabricated with a controlled distribution of spherical pores showed nearly the same characteristic strength and strength variability (Weibull modulus) as specimens fabricated without the artificial pores. In situ observations confirmed stable growth of cracks initiated at pores and the crack lengths at fracture instability were much greater than the pore sizes, thus resulting in fracture strengths insensitive to the pores. The small variability in the fracture strength was found to be associated with variability in the R -curve and the instability crack lengths. An analysis based on the fracture instability criterion for rising crack growth resistance accounted for the strength variability due to variability in the R -curve. Comparable four-point bend experiments were also conducted on a sintered yttria-partially-stabilized zirconia (2Y-TZP) ceramic. This ceramic showed significant degradation of strength due to the presence of the pores. This flaw sensitivity is attributed to its steep rising R -curve over short crack lengths.     

Relationship between the Strength of Ceramic/Metal Joints in Tensile and Three-Point Bending

The tensile strength and the three-point bending strength of Si₃N₄/Al/Invar joints were compared. The fracture modes in both tests were almost the same. Statistical treatments of the strengths showed that both strengths had a similar Weibull slope of ∼6. The three-point bending strength was 2.5 times greater than the tensile strength. This magnification was slightly greater than that obtained from the relation of the effective areas in both tests.     

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.     

Evaluation of Brazed Silicon Nitride Joints: Microstructure and Mechanical Properties

Sintered Si₃N₄ has been bonded to itself and to AISI 316 steel by the active-metal brazing route. A commercial Ag-35Cu-1.6Ti filler has been used with joining taking place during a 30 min hold at 850°C under vacuum. Si₃N₄/ Si₃N₄ joints have been produced with strength distribution (average bend strength = 773.5 MPa, Weibull modulus = 11.2) similar to that of the monolithic ceramic. Direct brazing of the Si₃N₄ to AISI 316 steel was unsuccessful. However, reliably strong (bend strength of 250–400 MPa) ceramic/ steel joints with 20 × 20 mm² cross sections were fabricated by using Cu, Mo, or Nb interlayers. The most potent inter-layer used in this work was Mo, whose coefficient of thermal expansion matches best that of the ceramic.     

Tensile Strength of Yttria-Stabilized Tetragonal Zirconia Polycrystals

Tensile strengths of 2.0 to 5.0 mol% Y₂O₃-stabilized ZrO₂ polycrystals were described using the newly developed tensile testing method. The tensile test was conducted by attaching three strain gauges on both sides of a rectangular bar that was 10 mm by 1 mm by 200 mm. The tensile strength of tetragonal ZrO₂ polycrystals (TZP) containing 2.0 mol% Y₂O₃ showed 745 MPa, whereas the bend strength of this material was 1630 MPa. Inelastic behavior of the stress-strain curve was observed at critical stresses and strains of 500 to 700 MPa and 0.25% to 0.35%, respectively. Although deviation from proportionality was observed to be small, it increased with the increase of temperature from −100° to 200°C.     

Reaction-Bonded Three-Layer Alumina-Based Composites with Improved Damage Resistance

Three-layer alumina-based composites with Al₂O₃-containing mullite as an outer layer and ZrO₂-containing alumina as an inner layer were fabricated by die-pressing and reaction-bonding. The strength of samples with Vickers indents made at loads up to 300 N was investigated. When compared to monolithic ceramics, three-layer composites exhibit excellent damage resistance due to compressive stresses in the outer layer. Hot isostatically pressed samples show a four-point bend strength of 800 MPa after indentation at 300 N.     

Effects of Specimen Size on Strength of Sintered Silicon Nitride

The strength of sintered silicon nitride was measured by three-point bending, four-point bending, and expanded-ring tests at room temperature. The data obtained were analyzed using Weibull statistics. The results indicate that effective volume is the major factor determining strength.     

Synchrotron X-Ray Study of the Crystal Structure and Hydrothermal Degradation of Yttria-Stabilized Tetragonal Zirconia Polycrystal

Structural study with synchrotron X-ray diffractometry was made on phase separation phenomena in 2, 3, and 4 mol% Y₂O₃-stabilized tetragonal ZrO₂ polycrystal (abbreviated as 2, 3, and 4Y-TZP, respectively). The sintered body of 3Y and 4Y-TZP underwent phase separation into high and low yttrium regions as sintering temperature increased, and the tetragonal phase was assigned to both regions. The sintering body is less separated, and a large monoclinic phase was detected in 2Y-TZP. Analysis of aging kinetics of tetragonal- to monoclinic-phase transition showed that the fraction of the transformable phase agreed with that of the low yttrium region.     

Analysis of Fracture Probabilities in Nonuniformly Stressed Brittle Materials

The results of theoretical studies on the effect of nonuniform stress fields encountered in prismatic beams under bending on the fracture of brittle materials are described. Derivations were carried out to determine the risk of rupture of bending specimens subjected to a symmetrical four-point load of arbitrary spacing, the symmetric three-point loading and pure bending forming limiting cases of this more general loading. The analysis was based on materials obeying the Weibull distribution function with assumptions for either volumetric or surface flaw dispersion conditions. The predicted strengths of bending and tensile specimens are compared. An analytical method for the determination of the three Weibull parameters from a pure bending test is proposed. This method, based on the best fit of a theoretical curve to the experimental data, was applied successfully to experimental results on Columbia Resin, a brittle amorphous polymer.     

Properties and Microstructure of Molybdenum Disilicide–β';-SiAlON Particulate Ceramic Composites

Particulate ceramic composites that were composed of a combustion-synthesized β';-SiAlON matrix and dispersed MoSi₂ particles were hot pressed at 1600°C in a nitrogen atmosphere. The physical and mechanical properties of the composites that contained 15, 30, and 45 vol% MoSi₂ were evaluated. The average four-point bend strength, fracture toughness, and Vickers hardness of the composites were in the ranges of 500-600 MPa, 3-4 MP·am^1/2, and 11-13 GPa, respectively. The measured mechanical strength and hardness were very similar to the values that were predicted from the rule of mixtures. The fracture toughness of the combustion-synthesized β';-SiAlON (2.5 MPa·m^1/2) was apparently enhanced by the MoSi₂ particles that were added. The increase in the fracture toughness was predominately attributed to the residual thermal stress that was induced by the thermal expansion mismatch between the MoSi₂ particles and the β';-SiAlON matrix. The composites showed improved electrical conductivity and oxidation resistance over monolithic β';-SiAlON. High-resolution transmission electron microscopy examination of the composites indicated that the MoSi₂ was chemically well compatible with the β';-SiAlON.     

Strength and Microstructure of Brazed Alumina-Silicon Carbide Ceramic-Matrix Composites

Alumina-silicon carbide whisker (Al₂O₃-SiC _w ) composite pieces were brazed under high vacuum with Ag-Cu-based active braze alloys. Joined specimens exhibit mean fracture strengths of 480 MPa (4.5 wt% titanium in braze) and 591 MPa (2.0 wt% titanium in braze) in four-point bend tests; these strengths are 78%−96% of the measured strength of the composite. The interfacial phases have been identified, by using transmission electron microscopy and electron probe microanalysis, as being Ti₃(Cu,Al,Si)₃O, Ti₅Si₃, TiC, and γ-TiO, and the relative locations of the interfacial phases are described. The high strengths are believed to result from both good wetting behavior under high vacuum and the composite nature of the reaction product morphology.     

Mechanical Properties of Infiltrated Alumina-Y-TZP Composites

Small Al₂O₃ additions (∼ 4 vol%) made to Y-TZP using an infiltration technique increased the fracture toughness and strength by ∼15% and the amount of transgranular fracture. Ionic conductivity measurements showed decreased grain-boundary conductivity, confirming a change in the grain-boundary composition. The predominant failure origins for both the unmodified Y-TZP and the Al₂O₃—Y-TZP were surface flaws related to agglomerates in the original powder. Finishing reduced the severity of these flaws and substantially increased the strength of both materials (>50%). The infiltration approach introduced a new flaw population in some specimens; however, this problem was overcome by a simple processing modification.     

Hot Isostatic Pressing of a Presintered Yttria-Stabilized Zirconia Ceramic

The effect of hot isostatic pressing on the bend strength of ZrO₂–3 mol% Y₂O₃ was critically dependent on the presintering process. Optimally sintered bodies contained no open porosity and exhibited large increases in strength following hot isostatic pressing. When open porosity of as little as 0.3% persisted after sintering, hot isostatic pressing increased the bulk density, but little or no increase in strength was realized. Two-parameter Weibull analysis of the strength data was used to quantify the strength improvement obtained following hot isostatic pressing. Typical fracture-initiating flaws were identified through optical and electron microscopy.     

Microstructure and Bending Strength of 3Y-TZP/12Ce-TZP Ceramics Fabricated by Liquid-Phase Sintering at Low Temperature

With the addition of 1 wt% of MgO–Al₂O₃–SiO₂ glass as a sintering aid, 3Y-TZP/12Ce-TZP ceramics (composed from a mixture of 3Y-TZP and 12Ce-TZP powder) have been fabricated via liquid-phase sintering at 1250°–1400°C. In the sintered bodies, the grain growth of Y-TZP is almost unaffected, whereas that of Ce-TZP is inhibited. MgO·Al₂O₃ spinel and an amorphous phase that contains Al₂O₃ and SiO₂ (from the sintering aid) fully fill the grain junctions. The bending strength of 3Y-TZP/12Ce-TZP, when sintered at 1250°–1300°C, is ∼800–900 MPa, which is greater than that of 3Y-TZP ceramics without Ce-TZP particles. Ce-TZP grains and MgO·Al₂O₃ spinel in 3Y-TZP/12Ce-TZP ceramics may impede crack growth, and the bending strength is enhanced.     

Fatigue and Static Crack Propagation in Yttria-Stabilized Tetragonal Zirconia Polycrystals: Crack Growth Micromechanisms and Precracking Effects

The influence of precracking techniques in the crack growth behavior of yttria-stabilized tetragonal zirconia polycrystals (Y-TZP) is investigated. Load-bridge and cyclic-compression precracking enhance subsequent tensile crack growth rates, in comparison to results that are found with precracks that are extended under four-point bending prior to testing. The actual influence of these precracking techniques in the near-threshold crack growth regime is remarkably different. Although load-bridge precracking produces a pattern of crack growth fluctuations for stress intensity factors, K , lower than the effective crack-growth threshold of the material, compression-fatigue precracks start to propagate under far-field tensile loads at very fast growth rates and for K values that are slightly higher than the effective threshold. Crack-tip shielding by tetragonal-to-monoclinic transformation develops gradually, influencing the crack growth behavior in Y-TZP. Proposed fatigue crack growth micromechanisms involve damage accumulation at the crack-tip region. For K _max > 3 MPa·m^1/2, fatigue crack growth rates are strongly affected by environmental interactions at the crack tip, and postulated fatigue micromechanisms include the cyclic degradation of crack-tip shielding.     

In Situ Preparation of Si3N4/SiC Nanocomposites for Cutting Tools Application

Silicon nitride–silicon carbide nanocomposite has been prepared by an in situ method that utilizes the formation of SiC nanograins by carbothermal reduction of intentionally added fine SiO₂ during the sintering process. The mean value of room-temperature four-point bending strength is 675 MPa with the Weibull modulus of 6.4 and an indentation fracture toughness of 7.4 MPa·m^1/2. A significantly enhanced creep resistance was achieved by the incorporation of SiC nanoparticles into the matrix up to 1400°C. The tribological properties of the material were tested using a ball-on-disk configuration and showed a friction coefficient of about 0.7. The cutting inserts machined from this composite had three times longer lifetime compared with those available on the market. On the other hand, the scatter of results is much larger compared with those measured for the commercial inserts.     

Fracture Resistance Behavior of Silicon Carbide Whisker-Reinforced Alumina Composites with Different Porosities

Fracture resistance behavior was characterized for SiC-whisker-reinforced alumina composites with porosities ranging from 0.6% to 11.5% The composites were hot-pressed from an Al₂O₃ powder with 25 wt% SiC whiskers. Strengths of individual specimens were measured in four-point flexure either for natural flaws or for Vickers-indentation flaws as a function of radial crack size. Indentation crack sizes were controlled with indentation loads which varied between 2 and 200 N. A novel method of analysis of these measurements indicates that the fracture resistance of these composites increases as a function of crack extension, a rising R curve. This behavior is interpreted in terms of tractions from both crack-bridging whiskers and interlocking grains, which develop in the wake of the crack tip as it extends. A decrease in porosity raises the level of fracture resistance, but has a negligible effect on the relative steepness of the R curve. The sizes of natural flaws which causes failure in flexure testing were also estimated from analysis of the data.     

Processing and characterisation of three-layer alumina-based composites with enhanced damage tolerance

Three-layer alumina-based composites reinforced with iron in the inner layer and with chromium in the outer layers were fabricated by first uniaxially pressing the three-layer assembly, followed by cold isostatic pressing at 300 MPa and sintering in a graphite vacuum furnace at 1500 °C for 1 h.

The residual compressive stresses in the outer Cr–Al/Al₂O₃ layers and the residual tensile stresses in the Fe–Al/Al₂O₃ inner layer were predicted as a function of composition and the thickness ratio of the outer and inner layers. Theoretical calculations showed that the compressive stresses in the outer layers increased while the tensile stresses in the inner layer decreased with decreasing outer layer thickness. The existence of compressive stresses was verified by microscopic evidence, which showed that propagation of cracks perpendicular to the interface is suppressed in the outer layer, but promoted in the inner layer.

Indentation and subsequent strength testing showed that these layered composites exhibited improved damage tolerance. Three-layer composites showed four-point bend strengths exceeding the bend strength of unindented monolithic Al₂O₃ even after indentation at 300 N.     

Fracture Toughness of a Silicon Nitride/Silicon Carbide Nanocomposite at 1350°C

The fracture toughness of a hot-pressed silicon nitride/silicon carbide (Si₃N₄/SiC) nanocomposite and reference monolithic Si₃N₄ has been investigated in four-point bending at 1350°C in air, using different loading rates (0.01-1 mm/min). Single-edge V-notched bend specimens that were prepared by polishing the notch tip to a radius of <10 µm, using 1 µm diamond paste, were used for the fracture toughness measurement. Slow crack growth (SCG) prior to catastrophic failure was detected at all applied loading rates at 1350°C. The fracture toughness at 1350°C, as calculated using the actual crack size measured on the fracture surface after the bend test, increased in both ceramics with decreasing loading rate and increasing area of the SCG region.     

Microstructure and Phase Stability of Yttria-Doped Tetragonal Zirconia Polycrystals Heat Treated in Nitrogen Atmosphere

The low-temperature degradation of zirconia (ZrO₂) that was doped with 3 mol% yttria (Y₂O₃) (3Y-TZP) was prevented by the heat treatment of sintered specimens in nitrogen. The heat treatment of sintered specimens resulted in a surface layer that was stabilized by nitrogen ions, whereas the interior was only slightly affected by the heat treatment. X-ray diffractometry and transmission electron microscopy analyses revealed that the stabilized surface layer consisted of cubic grains with tetragonal precipitates. Although the presence of the surface layer decreased the strength of the sintered 3Y-TZP, the strength of nitrified specimens was maintained when low-temperature annealing was applied.