Fracture Toughness of a Toughened Silicon Nitride by ASTM C 1421

The fracture toughness of a sintered reaction-bonded silicon nitride was measured by the single-edge precracked beam and surface crack in flexure methods, which are two of the three complementary test methods in ASTM C 1421. Results were compared with chevron-notched beam results that were available from another source. Precracks ranged from tiny artificial flaws introduced by Knoop indentation to millimeter-long precracks in single-edge precracked beams. The fracture toughness values from the three methods were in good agreement at 5.6 MPa·m^1/2.     

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.     

Microhardness and Fracture Toughness Anisotropy in Cubic Zirconium Oxide Single Crystals

Vickers and Knoop indentation tests have been used to study the fracture and deformation characteristics of 9.4-mol%-Y₂O₃-stabilized ZrO₂ single crystals. K_c is anisotropic, with values of 1.9 and 1.1 MPa·m^1/2 for radial cracks propagating along (100) and (110), respectively. The toughness for these two orientations was also determined using the single-edge notched-beam geometry, and yielded values of 1.9 and 1.5 MPa·m^1/2.     

Processing and Characterization of SiC-Whisker-Reinforced Alumina-Matrix Composites

Brittle monolithic alumina can be reinforced with highstrength single-crystal SiC whiskers with the effect of increasing fracture toughness. In this study, well-mixed and nearly fully dense SiC whisker/alumina composites were fabricated by wet-blending the constituents and uniaxially hot-pressing the resulting powder. The alumina-matrix grain size depended on whisker volume fraction, whisker surface-oxygen content, and hot-pressing environment. Fracture toughness, measured by an indentation-fracture method, increased from 3.0 MPa·m^1/2 for the hot-pressed unreinforced alumina to 10.7 MPa·m^1/2 for the composite containing 25 vol% SiC whiskers. Fracture surfaces revealed evidence of toughening by the mechanisms of crack deflection, pullout, and crack bridging by the whiskers. The observed increase in fracture toughness of alumina due to the addition of SiC whiskers was correlated with existing models of toughening mechanisms.     

Low-Temperature Processing and Mechanical Properties of Zirconia and Zirconia–Alumina Nanoceramics

The 1.5- to 3-mol%-Y₂O₃-stabilized tetragonal ZrO₂ (Y-TZP) and Al₂O₃/Y-TZP nanocomposite ceramics with 1 to 5 wt% of alumina were produced by a colloidal technique and low-temperature sintering. The influence of the ceramic processing conditions, resulting density, microstructure, and the alumina content on the hardness and toughness were determined. The densification of the zirconia (Y-TZP) ceramic at low temperatures was possible only when a highly uniform packing of the nanoaggregates was achieved in the green compacts. The bulk nanostructured 3-mol%-yttria-stabilized zirconia ceramic with an average grain size of 112 nm was shown to reach a hardness of 12.2 GPa and a fracture toughness of 9.3 MPa·m^1/2. The addition of alumina allowed the sintering process to be intensified. A nanograined bulk alumina/zirconia composite ceramic with an average grain size of 94 nm was obtained, and the hardness increased to 16.2 GPa. Nanograined tetragonal zirconia ceramics with a reduced yttria-stabilizer content were shown to reach fracture toughnesses between 12.6–14.8 MPa·m^1/2 (2Y-TZP) and 11.9–13.9 MPa·m^1/2 (1.5Y-TZP).     

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.     

Translucent Al2O3/LaAl11O18 Composite

A translucent alumina composite containing 1 vol% LaAl₁₁O₁₈, prepared by the hot isostatic pressing (HIP) method, displays both high translucency and high fracture toughness. Its total forward transmission at 600 nm is 75% (thickness 1 mm), and its bending strength and fracture toughness are estimated to be 574±15 MPa and 5.9±0.46 MPa·m^0.5, respectively. Its high translucency is due to the similarity of refractive index between the additive phase (LaAl₁₁O₁₈) and the matrix (alumina).     

Method to Estimate Crack-Tip Toughness from Bending Tests on Prenotched Bars

A strain-gauge procedure that enables determination of the crack-tip toughness ( K _I0) from bending-strength tests is described. The procedure is applied to coarse-grained alumina and yields an average K _I0 value of 2.51 MPa·m^1/2, with a standard deviation of 0.16 MPa·m^1/2.     

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.     

Microcracking in B4C-TiB2 Composites

Boron carbide/titanium diboride composites with 20 and 40 vol% particulate TiB₂ and various amounts of free carbon were investigated with respect to microcrack toughening. In agreement with previous work, the mere addition of TiB₂ was found to raise the toughness from 2.2 MPa·m^1/2 up to 3.0 and 3.5 MPa·m^1/2, respectively. A further and very significant increase of composite toughness up to 6.0 MPa·m^1/2 was discovered upon the incorporation of free carbon. SEM and TEM observations reveal that this toughening is associated with microcracking at B₄C-TiB₂ phase boundaries. Microcracking is triggered by thin carbon interlayers, which are located at hetero interfaces and supply a weak fracture path.     

Combined Mode I–Mode II Fracture of 12-mol%-Ceria-Doped Tetragonal Zirconia Polycrystalline Ceramic

The mode I, mode II, and combined mode Imode II fracture behavior of ceria-doped tetragonal zirconia polycrystalline (Ce-TZP) ceramic was studied. The single-edge-precracked-beam (SEPB) samples were fractured using the asymmetric four-point-bend geometry. The ratio of mode I to mode II loading was varied by varying the degree of asymmetry in the four-point-bend geometry. The minimum strain energy density theory best described the mixed-mode fracture behavior of Ce-TZP with the mode I fracture toughness, K _IC= 8.2 ± 0.6 MPa·m^1/2, and the mode II fracture toughness, K_IIC= 8.6± 1.3 MPa·m^1/2.     

Effect of Rare-Earth Dopants on Mechanical Properties of Alumina

We report here about the effect of rare-earth dopants on the improvement of room-temperature mechanical properties of alumina. Rare-earth ions (RE = Yb³⁺, Er³⁺, and La³⁺) of different ionic radii in a minimum concentration of 1000 ppm were added as dopants individually to high-purity alumina and densified by pressureless sintering. High strength of about 700 MPa was attained for Yb-doped alumina sintered at 1400°C. And, high toughness of about 7.0 MPa·m^1/2 was attained for Er- and La-doped alumina samples.     

Pulse Electric Current Sintering of Al2O3/3 vol% ZrO2 with Constrained Grains and High Strength

The pulse electric current sintering technique (PECS) was demonstrated to be effective in rapid densification of fine-grained Al₂O₃/3Y-ZrO₂ using available commercial powders. The composites attained full densification (>99% of TD) at 1450°C in less than 5 min. The composites sintered at a high heating rate had a fine microstructure. The incorporation of 3 vol% 3Y-ZrO₂ substantially increased the average fracture strength and the toughness of alumina to as high as 827 MPa and 6.1 MPa·m^1/2, respectively. A variation in the heating rate during the PECS process influenced grain size, microstructure, and strength, though there was little or no variation in the fracture toughness.     

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.     

A ZrO2 (3Y) Matrix Composite Toughened with Fe3Al Intermetallic

Near fully dense ZrO₂(3Y)/Fe₃Al composites with significantly improved fracture toughness were synthesized by hot-press sintering at 1350°C. High fracture toughness and bending-strength values, 36 MPa·m^1/2 and 1321 MPa, respectively, were achieved in 40 vol% Fe₃Al composite ceramics, whereas those same values for ZrO₂(3Y) alone were 10 MPa·m^1/2 and 988 MPa, respectively. Microscopic observation of the crack path revealed that Fe₃Al particle uniformly dispersed in the matrix have obvious crack-bridging effect. Improved thermal-shock resistance was also obtained, which was attributed to higher toughness, thermal conductivity, and lower Young's modulus by adding of Fe₃Al particles.     

Toughening Behavior in Sic-Whisker-Reinforced Alumina

The development of hot-pressed Sic-whisker-reinforced alumina has resulted in composites with fracture toughness values ∼;8.7 MPa·m¹¹² at 20 vol% SiC. Whisker orientation during processing leads to anisotropy in both fracture toughness and fracture strength. Fracture strengths are also limited by the ability to disperse the Sic whiskers; however, use of both fine alumina powders and ultrasonic dispersion techniques yields strengths ∼;800 MPa.     

Strength and Toughness of Tape-Cast Yttria-Stabilized Zirconia

The biaxial flexural strength and fracture toughness of tape-cast yttria-stabilized zirconia, for application as the electrolyte in solid oxide fuel cells, have been measured at room temperature and at a typical operating temperature of 900°C. The flexural strength was measured in ring-on-ring loading and decreased from 416 MPa at room temperature to 265 MPa at 900°C. The fracture toughness was measured using two different techniques: indentation fracture and double-torsion loading. The latter was more reliable and gave a fracture toughness of 1.61 ± 0.12 MPa·m^1/2 at room temperature and 1.02 ± 0.05 MPa·m^1/2 at 900°C. The flexural strength and fracture toughness were quantitatively consistent with fracture being initiated at the observed surface defects. The lower fracture toughness at 900°C is partly due to a reduction in elastic modulus and partly due to a reduction in the work of fracture.     

High-Toughness Silicon Carbide as Armor

Silicon carbide, with single-edge precracked beam (SEPB) toughness greater than 7 MPa·m^1/2, was made by hot-pressing using Al–B–C (ABC) or Al–Y₂O₃ (YAG) as additives. The hardness of SiC processed with a liquid phase was always less than SiC densified without a liquid phase despite having a similar or finer grain size. With increasing Al content, the ABC system changed from trans- to intergranular fracture with a drop in hardness and a two- to threefold increase in SEPB toughness. Strength and Weibull modulus for materials processed with a liquid phase were higher than those of solid-state densified SiC. Ballistic testing, however, did not show any improvement over SiC densified with B and C additives. Depth of penetration was controlled by hardness of the SiC-based materials, while V ₅₀ values for 14.5 mm WC–Co cored projectiles were in the range of 720–750 m/s for all materials tested.     

Effect of Poling Direction on R-Curve Behavior in Lead Zirconate Titanate

R -curves of lead zirconate titanate (PZT) have been measured with compact tension (CT) specimens for different poling conditions and grain sizes. Depending on poling direction the plateau value of the R -curves ranged from 1.13 to 1.54 MPa·m^1/2 for a grain size of 6.4 μm and from 1.14 to 1.30 MPa·m^1/2 for a grain size of 5.2 μm. Poling in the thickness direction yielded the material with the highest fracture toughness while the direction parallel to the loading direction led to the lowest fracture toughness.     

Mechanical Properties of Dry-Pressed Powder Compacts: Case Study on Alumina Nanoparticles

Young's modulus and fracture toughness of dry-pressed powder compacts of nanocrystalline alumina powders have been determined for different relative densities and average grain size. The powder with a grain size of 40 nm yielded a maximum Young's modulus of 15 GPa and a peak fracture toughness of 0.12 MPa·m^1/2. These high values are rationalized using a model based on contact flattening between particle spheres, which allows a discussion of the influence of grain size and particle packing on the mechanical properties of green bodies.