High-Strength and High-Fracture-Toughness Ceramics in the Al2O3/LaAl11O18 Systems

Control of microstructure in the Al₂O₃/LaAl₁₁O₁₈ system was performed. Elongated alumina grains were formed by doping with small addition of silica, and 20 vol% lanthana- luminate was formed in situ by the reaction of LaAlO₃- A1₂O₃ in an alumina matrix. Strengths of over 600 MPa and a high fracture toughness (6 MPa.m^1/2) were achieved in the material with both elongated A1₂O₃ grains and LaAl₁₁O₁₈ platelets. Generally antagonistic properties such as strength and fracture toughness have been made compatible in the same ceramic system.     

Solid-State Kinetics of LaAl11O18

Synthesis of LaAl₁₁O₁₈ by the solid-state reaction of La₂O₃ and Al₂O₃ occurs in two stages, i.e. LaAlO₃ forms immediately at 1450°C in air but LaAl₁₁O₁₈ formation is very slow and requires up to 141 days. First-order kinetics were observed and an activation energy of 118±4 kcal was calculated. Formation of LaAl₁₁O₁₈ depends on external O₂-gas partial pressure. In an N₂-gas atmosphere, the very slow formation of LaAl₁₁O₁₈, observed in air, is slowed even further. Possible mechanisms are also discussed.     

Preparation and Microstructure of Multi-Wall Carbon Nanotubes-Toughened Al2O3 Composite

With multi-wall carbon nanotubes (MWNTs) as reinforcement, a 12 vol% MWNTs/alumina (Al₂O₃) ceramic composite was obtained by hot pressing. A fracture toughness of 5.55±0.26 MPa·m^1/2, 1.8 times that of pure Al₂O₃ ceramics, was achieved. Experimental results showed that the enveloping of carbon nanotubes (CNTs) with sodium dodecyl sulfate (SDS) is effective in changing the hydrophobicity of CNTs to hydrophilicity and improving the dispersion of CNTs in aqueous solution. Enveloped with SDS, CNTs can be homogeneously mixed with Al₂O₃ at a microscopic level by heterocoagulation. This mixing method can obviously improve the chemical compatibility between CNTs and Al₂O₃, which is important for enhancement of interfacial strength between them.     

R-Curve Behavior of In-Situ-Toughened Al2O3:CaAl12O19 Ceramic Composites

The mechanical behavior of reaction-sintered alumina: 30 vol% calcium hexaluminate (Al₂O₃:CaAl₁₂O_19,or A1₂O₃: CA₆) composites was evaluated using the indentation strength in bending technique. A composite in which the hexaluminate (CA₆) phase possessed a platelike morphology showed more-pronounced R -curve behavior than a composite in which the CA₆ phase consisted of equiaxed grains. Toughness curves derived from the indentation-strength data exhibited a "crossover," such that the platelet composite exhibited the lower toughness at small flaw sizes, but the higher toughness at large flaw sizes. Incorporation of the platelet CA₆ resulted in enhanced toughening, compared to single-phase alumina of comparable grain size, thus demonstrating the viability of the in-situ -toughening approach. A simple grain-pullout model was used to estimate the toughening increment due to bridging by the platelet grains; the value obtained was in good agreement with toughness curves derived from indentation-strength measurements. Finally, fabrication of trilayer specimens, whereby outer layers of equiaxed A1₂O₃:CA₆ composite were strongly bonded to the platelet A1₂O₃:CA₆ composite, demonstrated high strength over the range of tested flaw sizes.     

High-Temperature Stability of the Al2O3-LaPO4 System

The compatibility of Al₂O₃ and LaPO₄ at temperatures up to 1600°C is examined. Provided the ratio of La to P was close to 1:1, no reactions were observed after 200 h at 1600°C. Moreover, the Al₂O₃/LaPO₄ interface remained sufficiently weakly bonded to cause deflection of cracks, as reported previously. In the presence of excess P or La, reactions occurred as expected, forming AlPO₄ in the case of excess P, and LaAlO₃ and LaAl₁₁O₁₈ in the case of excess La.     

Slow Crack Growth Behavior in Transformation-Toughened Al2O3-ZrO2(Y2O3) Ceramics

Significant increases in the critical fracture toughness (K _IC ) over that of alumina are obtained by the stress-induced phase transformation in partially stabilized ZrO₂ particles which are dispersed in alumina. More importantly, improved slow crack growth resistance is observed in the alumina ceramics containing partially stabilized ZrO₂ particles when the stress-induced phase transformation occurs. Thus, increasing the contribution of the ZrO₂ phase transformation by tailoring the Y₂O₃ stabilizer content not only increases the critical fracture toughness (K_IC) but also the K _Ia to initiate slow crack growth. For example, crack velocities ( v )≥10^–9 m/s are obtained only at K _Ia≥5 MPa.m^1/2 in transformation-toughened ( K _IC=8.5 MPa.m^1/2) composites vs K _Ia≥2.7 MPa.m^1/2 for comparable velocities in composites where the transformation does not occur ( K _IC=4.5 MPa.m^1/2). This behavior is a result of crack-tip shielding by the dissipation of strain energy in the transformation zone surrounding the crack. The stress corrosion parameter n is lower and A greater in these fine-grained composite materials than in fine-grained aluminas. This is a result of the residual tensile stresses associated with larger (≥1 μm) monoclinic ZrO₂ particles which reside along the intergranular crack path.     

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.     

High-Temperature Strength and Fracture Toughness of Y2O3-Partially-Stabilized ZrO2/Al2O3 Composites

The temperature dependence of bending strength, fracture toughness, and Young's modulus of composite materials fabricated in the ZrO₂ (Y₂O₃)-Al₂O₃ system were examined. The addition of A1203 enhanced the high-temperature strength. Isostatically hot-pressed, 60 wt% ZrO₂ (2 mol% Y₂O₃)/40 wt% Al₂O₃ exhibited an extremely high strength, 1000 MPa, at 1000°C.     

Improvement in Mechanical Properties of Powder-Processed MoSi2 by the Addition of Sc2O3 and Y2O3

Additions of 1-20 mol% Sc₂O₃ or Y₂O₃ to MoSi₂ eliminate glassy SiO₂, which improves mechanical properties at both ambient and high temperatures. In particular, only 1 mol% ScO₃ additions dramatically enhance three-point bending strength from 521 to 1081 MPa. Vickers hardness, Young's modulus, fracture toughness, and high-temperature strength are also improved by this low level of additive. The improvement of mechanical properties is attributed to the formation of crystalline silicates: Sc₂Si₂O₇, Y₂Si₂O₇, Y₂SiO₅, and Y₄Si₃O₁₂, which are analyzed by XRD, SEM-EDS, and TEM-EDS methods.     

Phase Stability and Physical Properties of Cubic and Tetragonal ZrO2 in the System ZrO2–Y2O3–Ta2O5

The cubic ( c -ZrO₂) and tetragonal zirconia ( t -ZrO₂) phase stability regions in the system ZrO₂–Y₂O₃–Ta₂O₅ were delineated. The c -ZrO₂ solid solutions are formed with the fluorite structure. The t -ZrO₂ solid solutions having a c/a axial ratio (tetragonality) smaller than 1.0203 display high fracture toughness (5 to 14 MPa · m^1/2), and their instability/transformability to monoclinic zirconia ( m -ZrO₂) increases with increasing tetragonality. On the other hand, the t -ZrO₂ solid solutions stabilized at room temperature with tetragonality greater than 1.0203 have low toughness values (2 to 5 MPa · m^1/2), and their transformability is not related to the tetragonality.     

Enhanced Mechanical Properties of Machinable LaPO4/Al2O3 Composites by Spark Plasma Sintering

LaPO₄/Al₂O₃ composites were fabricated by spark plasma sintering. The effects of LaPO₄ contents on the mechanical properties of the composites were investigated. The bending strength and fracture toughness can reach the maximum value of 568.2±30 MPa and 4.8±0.5 MPa·m^1/2 for the composite with 16.4 vol% LaPO₄ addition, respectively. The elastic moduli and hardness of the composites decreased with increasing LaPO₄ content. Furthermore, the experimental results show that the composites can be machined by a tungsten carbide drill as the LaPO₄ volume fraction is higher than 34.4 vol%.     

In-Situ Alumina/Aluminate Platelet Composites

Alumina composites containing in-situ-formed hexaluminate (LaAl₁₁O₁₈, LaMgAl₁₁O₁₉, SrAl₁₂O₁₉, and Mg₂NaAl₁₅O₂₅) platelets can be pressureless-sintered to high density. The grain morphology of the aluminates can be controlled by composition. A peak toughness 50% higher than that of alumina is obtained at 30 vol% aluminates, with a modest reduction (10%) in hardness and Young's modulus. Although crack-bridging by aluminate platelets is apparently operating, the maximum toughness is intrinsically limited by the low cohesive strength of these layer compounds.     

Microstructures and Properties of Three Composites of Alumina, Mullite, and Monoclinic SrAl2Si2O8

Three composites that were 96% alumina were mixed and uniaxially dry-pressed into bars and pellets; all had monoclinic SrAl₂Si₂O₈ as an intergranular phase. The diffraction patterns, microstructure, density, dielectric properties, strength, and toughness were measured. The first composition, which contained crystalline SrCO₃, Al₂O₃, and SiO₂, in a 1:1:2 molar ratio, as the 4% component, densified but was generally inferior to the second and third compositions, which contained strontium aluminosilicate (SrAl _x Si _y O _z , SAS) glass as the 4% component, in terms of mechanical properties, defects, and monoclinic SrAl₂Si₂O₈ transformation. The second composition, which lacked B₂O₃, was very tough and was comparable to commercial alumina, in terms of the dielectric constant. The third, which contained 0.068% of B₂O₃ that was dissolved in the SAS glass as a sintering aid, had high strength and toughness and no macroscopically visible defects. Mullite formed, in addition to monoclinic SrAl₂Si₂O₈ in all three composites. Alumina–monoclinic SrAl₂Si₂O₈ composites of the third composition had room-temperature properties that were comparable to commercial aluminas that contained 96% alumina and also had potential for mechanical and refractory applications.     

Fracture Toughness of Al2O3-Particle-Dispersed Y2O3-Partially Stabilized Zirconia

The fracture toughness of 3 mol% Y₂O₃-ZrO₂ (3Y-PSZ) composites containing 10–30 vol% Al₂O₃ with different particle sizes was investigated. It was found that Al₂O₃ dispersion of up to 30 vol% increased the fracture toughness by 17% to 30%, and the toughness increase was more remarkable in the composite dispersed with Al₂O₃ particles of larger sizes. By combining the effects of the dispersion toughening and phase transformation toughening, the toughness change in the present materials was theoretically predicted, which was in good agreement with the experimental data.     

High-Toughness Ce-TZP/Al2O3 Ceramics with Improved Hardness and Strength

Simulataneous additions of SrO and Al₂O₃ to ZrO₂ (12 mol% CeO₂) lead to the in situ formation of strontium aluminate (SrO · 6Al₂O₃) platelets (∼0.5 μm in width and 5 to 10 μm in length) within the Ce-TZP matrix. These platelet-containing Ce-TZP ceramics have the strength (500 to 700 MPa) and hardness (13 to 14 GPa) of Ce-TZP/Al₂O₃ while maintaining the high toughness (14 to 15 MPa ± m^1/2) of Ce-TZP. Optimum room-temperature properties are obtained at SrO/Al₂O₃ molar ratios between 0.025 and 0.1 for ZrO₂ (12 mol% CeO₂) with starting Al₂O₃ contents ranging between 15 and 60 vol%. The role of various toughening mechanisms is discussed for these composite ceramics.     

Formation, Characterization, and Hot Isostatic Pressing of Cr2O3-Doped ZrO2 (0,3 mol% Y2O3) Prepared by Hydrazine Method

In the ZrO₂-Cr₂O₃ system, metastable t -ZrO₂ solid solutions containing up to 11 mol% Cr₂O₃ crystallize at low temperatures from amorphous materials prepared by the hydrazine method. The lattice parameter c decreases linearly from 0.5149 to 0.5077 nm with increased Cr₂O₃ content, whereas the lattice parameter a is a constant value ( a = 0.5077 nm) regardless of the starting composition. At higher temperatures, transformation (decomposition) of the solid solutions proceeds in the following way: t (ss)→ t (ss) + m + Cr₂O₃→ m + Cr₂O₃. Above 11 mol% Cr₂O₃ addition, c-ZrO₂ phases are formed in the presence of Cr₂O₃. The t -ZrO₂ solid solution powders have been characterized for particle size, shape, and surface area. They consist of very fine particles (15–30 nm) showing thin platelike morphology. Dense ZrO₂(3Y)-Cr₂O₃ composite ceramics (∼99.7% of theoretical) with an average grain size of 0.3 μm have been fabricated by hot isostatic pressing for 2 h at 1400°C and 196 MPa. Their fracture toughness increases with increased Cr₂O₃ content. The highest K _Ic value of 9.5 MPa·;m^1/2 is achieved in the composite ceramics containing 10 mol% Cr₂O₃.     

Room-Temperature Strength and Fracture of ZrO2-Y2O3 Single Crystals

Strength and fracture toughness results are presented for ZrO₂ single crystals stabilized with Y₂O₃. The crystals (2 cm in diameter by 5 cm long) were prepared by skull melting. The partially stabilized compositions with 4 to 6 wt% Y₂O₃ showed a dramatic improvement in mechanical properties over the fully stabilized samples containing 20 wt% Y₂O₃, i.e. a strength exceeding 1000 MPa and a fracture toughness of 8 Mpa,.m ^1/2 were achieved compared to 200 MPa and 2 Mpa.m^1/2, respectively, for fully stabilized ZrO₂ single crystals.     

Tape Casting of Al2O3/ZrO2 Laminated Composites

Fracture toughness of ZrO₂-toughened alumina could he increased by macroscopic interfaces, such as those existing in laminated composites. In this work, tape casting was used to produce A/A or A/B laminates, where A and B can be Al₂O₃, Al₂O₃/5 vol% ZrO₂, and Al₂O₃/l0 vol% ZrO₂. An increase of toughness is observed, even in the Al₂O₃/Al₂O₃ laminates.     

Phase Equilibria in the Ga2O3In2O3 System

Subsolidus phase relationships in the Ga₂O₃–In₂O₃ system were studied by X-ray diffraction and electron probe microanalysis (EPMA) for the temperature range of 800°–1400°C. The solubility limit of In₂O₃ in the β-gallia structure decreases with increasing temperature from 44.1 ± 0.5 mol% at 1000°C to 41.4 ± 0.5 mol% at 1400°C. The solubility limit of Ga₂O₃ in cubic In₂O₃ increases with temperature from 4.X ± 0.5 mol% at 1000°C to 10.0 ± 0.5 mol% at 1400°C. The previously reported transparent conducting oxide phase in the Ga-In-O system cannot be GaInO₃, which is not stable, but is likely the In-doped β-Ga₂O₃ solid solution.     

Strength and Fracture Toughness of Isostatically Hot-Pressed Composites of Al2O3 and Y2O3-Partially-Stabilized ZrO2

Composites of Al₂O₃ and Y₂O₃ partially-stabilized ZrO₂ were isostatically hot-pressed using submicrometer powders as the starting material. The addition of Al₂O₃ resulted in a large increase in bending strength. The average bending strength for a composite containing 20 wt% Al₂O₃ was 2400 MPa, and its fracture toughness was 17 MN·w^−3/2