Strengthening of Porous Alumina by Pulse Electric Current Sintering and Nanocomposite Processing

Porous Al₂O₃ and SiC–dispersed-Al₂O₃ (Al₂O₃/SiC) nanocomposites with improved mechanical properties were fabricated using pulse electric current sintering (PECS). Microstructures with fine grains and enhanced neck growth, as well as high fracture strength, could be achieved via PECS of Al₂O₃. The incorporation of fine SiC particles into an Al₂O₃ matrix significantly increased the fracture strength of porous Al₂O₃. Based on microstructural observations, it was revealed that the refinement of Al₂O₃ grains and neck growth occurred by PECS and nanocomposite processing.     

Effect of Milling Conditions on the Strength of Alumina–Silicon Carbide Nanocomposites

Processing effects of wet ball-milling on the microstructure and fracture strength of Al₂O₃/5 vol% SiC nanocomposites were investigated. Homogeneous microstructure and a high fracture strength of 1200 MPa could be achieved under the milling condition of relatively low wear contents of Al₂O₃ grinding media, maintaining the homogeneity of the ball-milled powders. The fracture strength decreased with increasing wear content. Degradation of fracture strength was caused by abnormal grain growth related to wear particles from the Al₂O₃ balls.     

Homogeneous Distribution of Sintering Additives in Liquid-Phase Sintered Silicon Carbide

The addition of sintering additives to silicon carbide particles by electrostatic adsorption of colloidal A1₂O₃ and Y₂O₃ sols has been studied as a way to achieve an optimum homogeneity in the microstructure. The adsorption behavior of the sol particles was examined by electrophoretic measurements and X-ray fluorescence analysis. Both A1₂O₃ and Y₂O₃ sols could simultaneously be adsorbed on the SiC particle surfaces. Viscosity measurements showed that the colloidal sol particles had a stabilizing effect on the slip, and hence slips with relatively high solid loadings could be prepared without adding extra dispersing agent. Liquid-phase-sintered silicon carbide materials (LPS-SiC) with 2 wt% A1₂O₃ and 1 wt% Y₂O₃ were prepared by freeze granulation/ pressing and sintering at 1880^deg;C for 4 h. The homogeneity of the green compacts was quantified using a spot analysis technique in an electron probe microanalyzer. It was clearly shown that the addition of sols gave a more homogeneous microstructure than the reference sample with Y₂O₃ and A1₂O₃ added as powders. The addition of sintering additives as sols also enhanced the sintering behavior.     

Sintering ZrO2/TiC Composites with TiH2: The Role of Transient Liquid Phase and of Substoichiometry

Sintering tetragonal ZrO₂ with 35 vol% TiC results in a microstructure where all grain facets are free of amorphous interfaces independent of sintering aids as TiH₂ or MgO/ Y₂O₃; grain sizes are below 1 μm. With substoichiometric TiC_1-x, a relative density of 95% and closed porosity are obtained at a lower sintering temperature than with stoichiometric TiC, but subsequent cladless hot isostatic pressing (HIP) is required to achieve 99% density. High densities of 98% accompanied by good mechanical properties become possible by pressureless sintering with the use of TiH₂. MgO/Y₂O₃ doping also promotes densification, but results in less transformable zirconia and increases the number and size of amorphous triple junctions. The consequences are a lower fracture toughness and strength.     

Preparation and Analysis of a Silicon Carbide Composite Membrane

Silicon carbide (SiC) porous substrates, containing alumina (Al₂O₃) dopant levels of 3, 5, and 8 wt%, are prepared by slip casting and sintering in the temperature range of 1450°–1800°C. The linear shrinkage, bulk density, and pore size of the sintered substrate increase as the sintering temperature and the amount of dopant increase. A large amount of β-phase SiC is transformed to α-phase SiC if the dopant concentration is 5 or 8 wt%. The flexural strength of the substrate doped with 8 wt% Al₂O₃ is higher than that of the substrate doped with 3 wt% Al₂O₃; however, the Weibull modulus of the former is lower. SiC composite membranes of improved selectivity and strength are fabricated by coating the porous substrate with layers of lower Al₂O₃ contents at lower sintering temperatures.     

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     

Effect of Weight Loss on Liquid-Phase-Sintered Silicon Carbide

The evaporation of silicon carbide (SiC) ceramics during sintering has been studied by thermogravimetry in a graphite furnace filled with argon. The SiC powder compacts contained 7.5 wt% eutectic composition of Y₂O₃–Al₂O₃ to promote liquid-phase sintering. A weight loss of 1–11 wt% was observed during sintering, depending on the sintering temperature and sintering time. The weight loss severely influenced the final density and the microstructure of the SiC ceramics. Particularly, the oxide sintering aids, which were homogeneously distributed in the green ceramics, were observed to segregate and form particular patterns that were dependent on the temperature, sintering time, and the total weight loss. Possible heterogeneous reactions evolving volatile species have been discussed in relation to the experimental observations.     

Contact Damage of Silicon Carbide/Boron Nitride Nanocomposites

To investigate the deformation mechanism of silicon carbide (SiC)/boron nitride (BN) nanocomposites, Hertzian contact tests were performed on monolithic SiC, and nanocomposite and microcomposite SiC/BN. Monolithic SiC had the typical microstructure of hot-pressed SiC with Y₂O₃ and Al₂O₃ additives, composed of slightly large grains in small matrix grains. The microcomposite comprised large BN grains dispersed along the grain boundaries of elongated SiC grains, while the nanocomposite showed a finer microstructure with fine BN particles and small matrix grains. These microstructural differences led to differences in the mechanism of contact damage. The damage of the monolithic SiC and the SiC/BN microcomposite exhibited classical Hertzian cone fracture and many large cracks, whereas the damage observed in the nanocomposites appeared to be quasi-plastic deformation.     

In Situ-Toughened Silicon Carbide

A new processing strategy based on atmospheric pressure sintering is presented for obtaining dense SiC-based materials with microstructures consisting of (i) uniformly distributed elongate-shaped α-SiC grains and (ii) relatively high amounts (20 vol%) of second-phase yttrium aluminum garnet (YAG). This strategy entails the sintering of β-SiC powder doped with α-SiC, Al₂O₃, and Y₂O₃. The Al₂O₃ and Y₂O₃ aid in the liquid-phase sintering of SiC and form in situ YAG, which has a significant thermal expansion mismatch with SiC. During a subsequent grain-growth heat treatment, it is postulated that the α-SiC "seeds" assist in controlling in situ growth of the elongated α-SiC grains. The fracture pattern in the in situ -toughened SiC is intergranular with evidence of copious crack-wake bridging, akin to toughened Si₃N₄ ceramics. The elongate nature of the α-SiC grains, together with the high thermal-residual stresses in the microstructure, enhance the observed crack-wake bridging. This bridging accounts for a measured twofold increase in the indentation toughness of this new class of in situ -toughened SiC relative to a commercial SiC.     

Pressureless Sintering of SiC-Whisker-Reinforced Al2O3 Composites: II, Effects of Sintering Additives and Green Body Infiltration

Pressureless sintering of SiC-whisker-reinforced Al₂O₃ composites was investigated. In Part II of the study, the effects of Y₂O₃/MgO sintering additives and green body infiltration on densification behavior and microstructure development are reported. Both sintering additives and green body infiltration resulted in enhanced densification. However, the infiltration approach was more effective for samples with high SiC whisker concentrations. Samples with 27 vol% whiskers could be pressureless sintered to ∼93% relative density and ∼3% open porosity. Fracture toughness values and microstructural features (e.g., grain size) for the infiltrated samples remained approximately the same as observed in the uninfiltrated samples.     

Mechanical Properties of Alumina/Silicon Carbide Whisker Composites

The improvement of mechanical properties of Al₂O₃/SiC whisker composites has been studied with emphasis on the effects of the whisker content and of the hot-pressing temperature. Mechanical properties such as fracture toughness and fracture strength increased with increasing whisker content up to 40 wt%. In the case of the high SiC whisker content of 40 wt%, fracture toughness of the sample hot-pressed at 1900° decreased significantly, in spite of densification, compared with one hot-pressed at 1850°. Fracture toughness strongly depended on the microstructure, especially the distribution of SiC whiskers rather than the grain size of the Al₂O₃ matrix.     

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.     

Microstructure and Grain-Boundary Composition of Hot-Pressed Silicon Nitride With Yttria and Alumina

The microstructure of two hot-pressed silicon nitrides containing Y₂O₃ and Al₂O₃ was examined by electron microscopy, electron diffraction, and quantitative, energy-dispersive X-ray microanalysis. A crystalline second phase was identified in the material with additives of 5 wt% Y₂O₃+2 wt% Al₂O₃, as a solid solution of nitrogen mellilite and alumina. An amorphous third phase as narrow as 2 nm is discerned at all grain boundaries of this material by high-resolution dark-field and lattice imaging. The second phase in a material with additives of S wt% Y₂O₃+5 wt% Al₂O₃ was found to be amorphous. Some of the additional alumina additive appears in solid solution with silicon nitride. In situ hot-stage experiments in a high-voltage electron microscope show that the amorphous phase volatilizes above 1200°C, leaving a skeleton of Si₃N₄ grains linked by the mellilite crystals at triple points. The results show that intergranular glassy phases cannot be eliminated by the Y₂O₃/Al₂O₃ fluxing.     

Phase Relations and Microstructural Development of Aluminum Nitride–Aluminum Nitride Polytypoid Composites in the Aluminum Nitride–Alumina–Yttria System

AlN–AlN polytypoid composite materials were prepared in situ using pressureless sintering of AlN–Al₂O₃ mixtures (3.7–16.6 mol% Al₂O₃) using Y₂O₃ (1.4–1.5 wt%) as a sintering additive. Materials fired at 1950°C consisted of elongated grains of AlN polytypoids embedded in equiaxed AlN grains. The Al₂O₃ content in the polytypoids varied systematically with the overall Al₂O₃ content, but equilibrium phase composition was not established because of slow nucleation rate and rapid grain growth of the polytypoid grains. The polytypoids, 24 H and 39 R , previously not reported, were identified using HRTEM. Solid solution of Y₂O₃ in the polytypoids was demonstrated, and Y₂O₃ was shown to influence the stability of the AlN polytypoids. The present phase observations were summarized in a phase diagram for a binary section in the ternary system AlN–Al₂O₃–Y₂O₃ parallel to the AlN–Al₂O₃ join. Fracture toughness estimated from indentation measurements gave no evidence for a strengthening mechanism due to the elongated polytypoids.     

Oxidation Behavior of Liquid-Phase Sintered Silicon Carbide with Aluminum Nitride and Rare-Earth Oxides (Re2O3, where Re = Y, Er, Yb)

Silicon carbide (SiC) ceramics have been fabricated by hot-pressing and subsequent annealing under pressure with aluminum nitride (AlN) and rare-earth oxides (Y₂O₃, Er₂O₃, and Yb₂O₃) as sintering additives. The oxidation behavior of the SiC ceramics in air was characterized and compared with that of the SiC ceramics with yttrium–aluminum–garnet (YAG) and Al₂O₃–Y₂O₃–CaO (AYC). All SiC ceramics investigated herein showed a parabolic weight gain with oxidation time at 1400°C. The SiC ceramics sintered with AlN and rare-earth oxides showed superior oxidation resistance to those with YAG and Al₂O₃–Y₂O₃–CaO. SiC ceramics with AlN and Yb₂O₃ showed the best oxidation resistance of 0.4748 mg/cm² after oxidation at 1400°C for 192 h. The minimization of aluminum in the sintering additives was postulated as the prime factor contributing to the superior oxidation resistance of the resulting ceramics. A small cationic radius of rare-earth oxides, dissolution of nitrogen to the intergranular glassy film, and formation of disilicate crystalline phase as an oxidation product could also contribute to the superior oxidation resistance.     

Influence of the Y2O3 Content and Temperature on the Mechanical Properties of Melt-Grown Al2O3–ZrO2 Eutectics

The effect of Y₂O₃ content on the flexure strength of melt-grown Al₂O₃–ZrO₂ eutectics was studied in a temperature range of 25°–1427°C. The processing conditions were carefully controlled to obtain a constant microstructure independent of Y₂O₃ content. The rod microstructure was made up of alternating bands of fine and coarse dispersions of irregular ZrO₂ platelets oriented along the growth axis and embedded in the continuous Al₂O₃ matrix. The highest flexure strength at ambient temperature was found in the material with 3 mol% Y₂O₃ in relation to ZrO₂(Y₂O₃). Higher Y₂O₃ content did not substantially modify the mechanical response; however, materials with 0.5 mol% presented a significant degradation in the flexure strength because of the presence of large defects. They were nucleated at the Al₂O₃–ZrO₂ interface during the martensitic transformation of ZrO₂ on cooling and propagated into the Al₂O₃ matrix driven by the tensile residual stresses generated by the transformation. The material with 3 mol% Y₂O₃ retained 80% of the flexure strength at 1427°C, whereas the mechanical properties of the eutectic with 0.5 mol% Y₂O₃ dropped rapidly with temperature as a result of extensive microcracking.     

Field-Assisted Densification of Superhard B6O Materials with Y2O3/Al2O3 Addition

B₆O is a possible candidate of superhard materials with a hardness of 45 GPa measured on single crystals. Up to now, densification of these materials was only possible at high pressure. However, recently it was found that Al₂O₃ can be utilized as an effective sintering additive, similar to the addition of Y₂O₃/Al₂O₃ that was used in this work. The densification behavior of the material as a function of applied pressure, its microstructure evolution, and the resulting mechanical properties were investigated. A strong dependence of the densification with increasing pressure was found. The material revealed characteristic triple junctions filled with amorphous residue composed of B₂O₃, Al₂O₃, and Y₂O₃, while no amorphous grain-boundary films were observed along internal interfaces. Mechanical testing revealed on average a hardness of 33 GPa, a fracture toughness of 4 MPa·m^1/2, and a strength value of 520 MPa.     

Tensile Ductility in Zirconia-Dispersed Alumina at High Temperatures

High-temperature plastic flow in Al₂O₃-10 wt% ZrO₂ (2.5 mol% Y₂O₃) has been examined at temperatures between 1400° and 1500°C. Al₂O₃-10 wt% ZrO₂ (2.5 mol% Y₂O₃) exhibits much higher flow stress and smaller tensile elongation below about 1450°C than 0.1 wt% MgO-doped single-phase Al₂O₃. The suppression of grain growth with ZrO₂ dispersion into Al₂O₃ is not effective for improving the tensile ductility. The limited ductility in Al₂O₃-10 wt% ZrO₂ (2.5 mol% Y₂O₃) is associated with the increment of flow stress caused by ZrO₂. The ZrO₂ dispersion or segregation in Al₂O₃/Al₂O₃ boundaries suppresses the grain boundary sliding and hence results in the increased flow stress at high temperatures.     

Intragranular Particle Residual Stress Strengthening of Al2O3–SiC Nanocomposites

Al₂O₃/SiC ceramic nanocomposites were fabricated from nanocrystalline Al₂O₃ (10 nm in diameter) and SiC (15 nm in diameter) powders, and a theoretical model of intragranular particle residual stress strengthening was investigated. The SiC nanoparticles in the Al₂O₃ grains create a normal compressive stress at the grain boundaries and a tangential tensile stress in the Al₂O₃ grains, resulting in the "strengthening" of the grain boundaries and "weakening" of the grains. The model gives a good explanation of the experimental results of the authors and others which are difficult to be explained by the existing strengthening models, i.e. the maximum strength is normally achieved at about 5 vol% of SiC particles in the Al₂O₃–SiC ceramic nanocomposites. According to the model, there exists an optimum amount of SiC for strengthening, below which the grain boundaries are not fully "strengthened" and the fracture is mainly intergranular, above which the grains are "weakened" too much and the fracture is mainly transgranular, and at which the fracture is a mixture of intergranular and transgranular.     

Fabrication of Dense Nanostructured Silicon Carbide Ceramics through Two-Step Sintering

SiC powder compacts were prepared with Al₂O₃, Y₂O₃, and CaO powders. By two-step sintering, fully dense nanostructured SiC ceramics with a grain sizes of ∼40 nm were obtained. The grain size–density trajectories are compared with those of conventional sintering processes.