Microstructure of the directionally solidified ternary eutectic ceramic Al2O3/MgAl2O4/ZrO2

The directionally solidified Al₂O₃/MgAl₂O₄/ZrO₂ ternary eutectic ceramic was prepared via induction heating zone melting. Smooth Al₂O₃/MgAl₂O₄/ZrO₂ eutectic ceramic rods with diameters of 10 mm were successfully obtained. The results demonstrate that the eutectic rods consist of Al₂O₃, MgAl₂O₄ and ZrO₂ phases. In the eutectic microstructure, the MgAl₂O₄ and Al₂O₃ phases form the matrix, the ZrO₂ phase with a fibre or shuttle shape is embedded in the matrix, and a quasi-regular eutectic microstructure formed, presenting a typical in situ composite pattern. During the eutectic growth, the ZrO₂ phase grew on non-faceted phases ahead of the matrix growing on the faceted phase. The hardness and fracture toughness of the eutectic ceramics reached 12 GPa and 6.1 MPa·m ^1/2, respectively, i.e., two times and 1.7 times the values of the pre-sintered ceramic, respectively. In addition, the ZrO₂ phase in the matrix reinforced the matrix, acting as crystal whiskers to reinforce the sintered ceramic.     

Metastability in the MgAl2O4–Al2O3 System

Aluminum oxide must take a spinel form (γ‐Al₂O₃) at increased temperatures in order for extensive solid solution to form between MgAl₂O₄ and α‐Al₂O₃. The solvus line between MgAl₂O₄ and Al₂O₃ has been defined at 79.6 wt% Al₂O₃ at 1500°C, 83.0 wt% Al₂O₃ at 1600°C, and 86.5 wt% Al₂O₃ at 1700°C. A metastable region has been defined at temperatures up to 1700°C which could have significant implications for material processing and properties. Additionally, initial processing could have major implications on final chemistry.     

Transparent polycrystalline nanoceramics consisting of triclinic Al2SiO5 kyanite and Al2O3 corundum

Transparent polycrystalline nanoceramics consisting of triclinic Al₂SiO₅ kyanite (91.4 vol%) and Al₂O₃ corundum (8.6 vol%) were fabricated at 10 GPa and 1200‐1400°C. These materials were obtained by direct conversion from Al₂O₃‐SiO₂ glasses fabricated using the aerodynamic levitation technique. The material obtained at 10 GPa and 1200°C shows the highest optical transparency with a real in‐line transmission value of 78% at a wavelength of 645 nm and a sample‐thickness of 0.8 mm. This sample shows equigranular texture with an average grain size of 34 ± 13 nm. The optical transparency increases with decreasing mean grain size of the constituent phases. The relationship between real in‐line transmission and grain size is well explained by a grain‐boundary scattering model based on a classical theory.     

High strength and hardness BNNSs/Al2O3 composite ceramics prepared by hot-press sintering of in-situ composite BNNSs/Al2O3 powder

In this paper, BNNSs/Al₂O₃ composite powder was prepared by in-situ reaction using borate nitridation method and BNNSs/Al₂O₃ composite ceramics were prepared by hot-pressing sintering. This method achieves uniform mixing of BNNSs and Al₂O₃ ceramic matrix and reduces the introduction of impurities in the processing process. The BNNSs/Al₂O₃ composite ceramics have excellent bending strength (549.4 MPa), fracture toughness (5.18 MPa m^1/2) and hardness (21.3 GPa). The high hardness of composite ceramics is attributed to high grain boundary strength and density. The reinforcing mechanisms of ceramics include BNNSs pull-out, BNNSs bridging, crack deflection as well as the transgranular fracture and intergranular fracture of Al₂O₃ matrix.     

Sintering,hardness and cation inversion of nanocrystalline Beryllium – Magnesium aluminate ceramics

Beryllium-magnesium aluminate (Be_0.1Mg_0.9Al₂O₄ and Be_0.2Mg_0.8Al₂O₄) nanoparticles are synthesized by a coprecipitation method and sintered using Spark Plasma Sintering (SPS) to achieve near full density ceramics with grain sizes at the nanoscale. The sintered nanoceramics display grain sizes ranging from 14 to 33 nm, which are analyzed for optical transmission, Vickers hardness, and cation site inversion. When compared to Be-free MgAl₂O₄ nanoceramics, both Be_0.1Mg_0.9Al₂O₄ and Be_0.2Mg_0.8Al₂O₄ show transmissions ～30% lower at wavelength in the infrared range. The samples show a Vickers hardness of ～19.2 GPa with no apparent dependence on grain size. These values are consistently lower to those reported for beryllium-free MgAl₂O₄ spinel with similar grain size. ²⁷Al and ⁹Be Nuclear Magnetic Resonance (NMR) spectroscopy reveals that beryllium does not have a significant effect on cationic site inversion in the spinel and, similar to beryllium-free MgAl₂O₄, inversion remains solely a function of grain size. The results indicate beryllium ions form solid-solutions with MgAl₂O₄ spinel structure and do not alter the grain boundaries significantly enough to influence the mechanical properties of nanocrystalline ceramics.     

Effect of metastable phase on the crystallization and mechanical properties of MgO-Al2O3-SiO2 glass-ceramics without nucleating agents

Glass-ceramics without nucleating agents usually undergo surface crystallization, which deteriorates the overall performance of the products. In this paper, we evaluated the effects of the metastable MgAl₂Si₃O₁₀ crystalline phase on the crystallization behavior of a MgO–Al₂O₃–SiO₂ (MAS) glass without nucleating agents and mechanical properties of the glass-ceramics obtained. The results demonstrated that the precipitation of metastable MgAl₂Si₃O₁₀ crystallites promotes the crystallization mechanism transformed from surface crystallization into volume crystallization with two-dimensional crystal growth. Furthermore, the grain size of MgAl₂Si₃O₁₀ near the surface of the prepared glass-ceramics was larger than that of MgAl₂Si₃O₁₀ inside, which helps to generate compressive stress and improves its mechanical properties. The glass-ceramics containing metastable MgAl₂Si₃O₁₀ phase exhibited an enhanced hardness in the range of 7.6 GPa–9.5 GPa for indentation loads ranging from 2.94 N to 98 N, and indentation size effect behavior was observed in Vickers hardness tests of both MAS glass and glass-ceramics. The load-independent hardness values for MAS glass and glass-ceramics were reliably evaluated by the modified proportional specimen resistance (MPSR) model of 7.1 GPa and 7.6 GPa, respectively, with a high correlation coefficient of more than 0.9999. This work reveals the unexploited potential of the metastable phase in improving the crystallization ability and mechanical properties of glass-ceramics.     

Effect of TiO2 addition on the sintering densification and mechanical properties of MgAl2O4–CaAl4O7–CaAl12O19 composite

Materials designed in the high-alumina region of Al₂O₃–MgO–CaO system have been widely used in many technological fields. However, their further applications are limited by the high sintering temperatures necessary to achieve densification due to the poor sintering ability of calcium hexaluminate (CaAl₁₂O₁₉) and spinel (MgAl₂O₄). Considering this aspect, the present work investigated the effect of TiO₂ addition on the sintering densification and mechanical properties of MgAl₂O₄–CaAl₄O₇–CaAl₁₂O₁₉ composite by solid state reaction sintering. The results showed that the CA₆ grains presented a more equiaxed morphology instead of platelet structure by incorporating Ti⁴⁺ into its structure, which greatly improved the densification after heating at 1600 °C. The flexural strength was greatly enhanced with increasing addition of TiO₂ due to the significant decrease in porosity and improvement in uniformity of grain size as well as the absence of microcracks in the presence of Al₂TiO₅. The increased content of TiO₂ also played an active role in toughening this composite attributed to the increase in resistance to crack initiation and propagation.     

Microstructure evolution and plastic mechanism in deformation of bulk amorphous Al2O3-ZrO2-Y2O3

In this work, compressive deformation is performed on bulk amorphous Al₂O₃-ZrO₂-Y₂O₃ at moderate temperatures. The amorphous samples display brittle fracture without any noticeable permanent strain at 500 °C. However, a large plastic strain of up to 15.1% is achieved at 600 °C. During the entire compressive deformation process, the samples remain amorphous, and shear bands start to form, accompanied by a stress drop. The amorphous AZY shows low Vickers hardness value of 2.8 GPa at 500 °C, and 2.2 GPa at 600 °C, due to the disordering microstructure. In the optical microscope images, local plastic deformation are detected around the indention without large cracks. Transmission electron microscopic observations and selected area electron diffraction analysis suggest that the shear band formation originates from the presence of free volume. Furthermore, the nucleation and propogation of shear bands lead to the large macroscopic plastic strain in the bulk amorphous Al₂O₃-ZrO₂-Y₂O₃.     

Relationship between sintering methods and physical properties of the low positive thermal expansion material Al2W3O12

Ceramic materials from the A₂M₃O₁₂ family with near-zero thermal expansion are good candidates for applications requiring high thermal shock resistance. Considering their inherently low thermal conductivity, the bulk forms of A₂M₃O₁₂ have to present Young's moduli and mechanical strength close to 100 GPa and 100 MPa, respectively, in order to compete with the state-of-the-art materials used to avoid thermal shock. The relationship between sintering, microstructure, and physical properties within the A₂M₃O₁₂ family is generally unknown while the preparation of bulks with high mechanical resistance remains a great challenge. Bulk samples of dense Al₂W₃O₁₂ (96%TD) have been obtained by pressureless three-stage sintering (TSS) and spark plasma sintering (SPS). The Young's moduli and hardness of samples prepared by SPS were 50% higher than that measured for TSS samples and more than 100% in comparison to the Al₂W₃O₁₂ bulk (91%TD). UV-Vis spectroscopy confirmed that A₂M₃O₁₂ phases are wide band-gap semiconductors (3.11 eV). When prepared by SPS, black Al₂W₃O₁₂ absorbed light within the visible spectrum due to the introduction of donor sites within the band-gap. No enhancement of the mechanism causing negative thermal expansion was observed for black Al₂W₃O₁₂. The mechanical properties achieved were significantly improved over those previously reported in literature for Al₂W₃O₁₂.     

B6O materials with Al2O3/Y2O3 additives densified by FAST/SPS and HIP

Fully densified B₆O materials with Al₂O₃/Y₂O₃ sintering additives amounts systematically varied between 0 and 15 vol.% and Al₂O₃/(Al₂O₃ + Y₂O₃) molar ratios of 0.05–1 were prepared by FAST/SPS and HIP at sintering temperatures between 1725 °C and 1900 °C. Their densification and microstructure were correlated with measured mechanical properties. The addition of low additive amounts in the range of 2–3 vol.% was found to increase the fracture toughness and strength from 2.0 MPa m^1/2 (SEVNB) and 420 MPa for pure B₆O to about 3.0 MPa m^1/2 and 540 MPa, but it had no effect on the hardness, which remained at a high level of 30–36 GPa (HV_0.4). Higher additive contents did not yield a further improvement in the toughness but resulted in a reduction in hardness and strength.     

Uniform and fine Mg-γ-AlON powders prepared from MgAl2O4: A promising precursor material for highly-transparent Mg-γ-AlON ceramics

High-purity and sinterability Mg-γ-AlON (Mg_0.1Al_1.53O_1.89N_0.27) powders were synthesized by gas pressure sintering (GPS) of mixed powders of commercial Al₂O₃ and AlN, and lab-made MgAl₂O₄. The Mg-γ-AlON powders exhibited a uniform particle morphology and a small particle size of d₅₀ = 3.4 μm, owing to the use of MgAl₂O₄ as the Mg source. Highly-transparent Mg-γ-AlON ceramics were fabricated using the synthesized Mg-γ-AlON powders by spark plasma sintering (SPS) at 1800 °C for 5 min under an axial pressure of 80 MPa, followed by hot isostatic pressing (HIP) at 1800 °C for 2 h under a nitrogen gas pressure of 190 MPa. The ceramics showed a high in-line transmittance of ～ 80.5% at 450 nm, ascribed to the high sinterability of the MgAl₂O₄ raw powder that leads to a pore-free and fully densified microstructure. This indicates that MgAl₂O₄ as sintering additive is superior over MgO and MgF₂ in the fabrication of Mg-γ-AlON transparent ceramic.     

Evolution of microstructure,mechanical, and optical properties of Y2O3-MgO nanocomposites fabricated by high pressure spark plasma sintering

A high-pressure spark plasma sintering (SPS) process was applied for consolidating Y₂O₃–MgO nanocomposites. This approach enabled to fabricate a fully dense infrared (IR) transparent nanocomposites, which possess an average grain size of ∼70 nm and high hardness, at a relatively low sintering temperature of 1130 °C under a high pressure of 300 MPa. The light transmittance was improved with increasing pressure and reached to the maximum transmittance of 64.5% at a wavelength of 0.2–1.6 μm owing to the fine-grained microstructure. The Vickers hardness exhibited 16.6 ± 0.7 GPa for the grain size of 74 nm, which is significantly higher than that of the sub-micro grains obtained at a conventional sintering pressure of 70 MPa (11.9 ± 0.8 GPa). The hardness rigorously followed the Hall–Petch relationship, that is, it is enhanced with a reduction of the grain size. Successful fabrication of the high-performance Y₂O₃–MgO nanocomposites indicates that the nanopowder processing followed by the high-pressure sintering process can be applied for fabricating fully dense fine-grained nanocomposites with excellent optical and mechanical properties.     

The effect of slip casting and spark plasma sintering (SPS) temperature on the transparency of MgAl2O4 spinel

MgAl₂O₄ bulk samples were fabricated by two different approaches to investigate the effect of slip casting and sintering temperature on their transparency. Three MgAl₂O₄ samples containing 1 wt% LiF, as the sintering aid, were prepared by the spark plasma sintering process (SPS) at 1400 °C and 1500 °C, under 100 MPa, for 15 min. Also, another MgAl₂O₄ sample was prepared by slip casting followed by SPS under similar conditions. It was observed that utilizing slip casting led to more transparency (10% in the visible region and 20% in the IR region) due to the more homogeneous structure. It was also observed that by reducing the SPS temperature from 1500 °C to 1400 °C, the transparency increased (20% in the IR region) because of the lower grain growth rate at the lower temperature.     

Synthesis and characterization of MgAl2O4 micro-rods by a molten salt method

Large amounts of MgAl₂O₄ micro-rods were successfully synthesized using the molten-salt technology. The effect of KCl contents on the formation of MgAl₂O₄ micro-rods was investigated. The structure and morphology of MgAl₂O₄ were investigated by means of powder X-ray diffraction, field emission scanning electron microscopy, and transmission electron microscopy, respectively. The experimental results showed that the contents of KCl significantly influenced the formation of MgAl₂O₄ micro-rods. MgAl₂O₄ micro-rods could be prepared at 1150 °C with a weight ratio of 100:1 between the salt and the starting materials. The formation of MgAl₂O₄ micro-rods could be suggested to be due to the inhomogeneous nucleation and orientated growth perpendicularly to the surfaces of Al₂O₃ grains. An impedance-type humidity sensor was finally fabricated based on the as-prepared MgAl₂O₄ micro-rods. According to tests of the humidity performance, MgAl₂O₄ micro-rods might be suitable for high-performance humidity sensors.     

Effect of high energy milling on microstructure and mechanical properties of Al2O3-10 wt% Co composites consolidated by spark plasma sintering (SPS)

Al₂O₃-10 wt% Co composites were prepared by high energy milling in the presence of ethyl alcohol and with subsequent spark plasma sintering (SPS). The powders milled for 5 and 30 h were sintered by SPS at 1350 °C for 5 min. The effect of milling time on the sinterability and mechanical properties was studied. The morphology and structure of milled powders were investigated by scanning electron microscopy, dynamic light scattering and X-ray diffraction. The Co phase forms plate-like particles of different sizes, while finely fragmented Al₂O₃ particles are incorporated in the Co phase, forming composite particles. The average size of the composite particles decreases with increasing milling time, achieving 1.33 μm after 30 h. Crystallite size and micro-strain are inversely proportional. Overall, all the samples display homogeneous microstructures, high density (85.29–91.60%) and microhardness in the range 11.41–14.37 GPa.     

Improved thermal shock resistance of low-carbon Al2O3–C refractories fabricated with C/MgAl2O4 composite powders

The addition of C/MgAl₂O₄ composite powders can improve the thermal shock resistance of low-carbon Al₂O₃–C refractories attribute to the formation of microcracks in the agglomerated structure, thus consuming more thermal stress and strain energy. Moreover, C/MgAl₂O₄ composite powders additive promote the formation of short fibrous ceramic phases in the refractories, which suggest a bridging role in the interior of the refractories and increase its toughness. Furthermore, the C/MgAl₂O₄ composite powders also result in a remarkable enhancement of the slag corrosion resistance in the refractories.     

High-strength TiB2-B4C composite ceramics sintered by spark plasma sintering

Spark plasma sintering (SPS) is an advanced sintering technique because of its fast sintering speed and short dwelling time. In this study, TiB₂, Y₂O₃, Al₂O₃, and different contents of B₄C were used as the raw materials to synthesize TiB₂-B₄C composites ceramics at 1850°C under a uniaxial loading of 48 MPa for 10 min via SPS in vacuum. The influence of different B₄C content on the microstructure and mechanical properties of TiB₂-B₄C composites ceramics are explored. The experimental results show that TiB₂-B₄C composite ceramic achieves relatively good comprehensive properties and exceptionally excellent flexural strength when the addition amount of B₄C reaches 10 wt.%. Its relative density, Vickers hardness, fracture toughness, and flexural strength reach to 99.20%, 24.65 ± .66 GPa, 3.16 MPa·m^1/2, 730.65 ± 74.11 MPa, respectively.     

Effect of volume ratio on optical and mechanical properties of Y2O3-MgO composites fabricated by spark-plasma-sintering process

The effect of Y₂O₃:MgO ratio on the microstructures, optical and mechanical properties of the Y₂O₃-MgO composites were investigated. Although the dense Y₂O₃-MgO composites were successfully fabricated in various Y₂O₃:MgO ratios using the spark-plasma-sintering (SPS) technique, the Y₂O₃:MgO ratio significantly influenced the microstructures and the optical/mechanical properties of the composites. Fine grain size was obtained in the composite with Y₂O₃:MgO = 50:50 owing to the effective pinning force caused by the homogenous two phase microstructure. The SPSed dense composites showed good transmittance in the wide wavelength range from visible to infrared (IR). The monolithic Y₂O₃ polycrystal, that is Y₂O₃:MgO = 100:0, showed the highest transmittance of 62.3 % at 600 nm and 84.3 % at 5 μm. Although the IR transmittance is independent of the Y₂O₃:MgO ratio, the visible transmittance decreased with the MgO particle dispersion. Among the Y₂O₃-MgO composites, the higher visible transmittance was obtained in the composite with Y₂O₃:MgO = 50:50 than those of the other two composites with Y₂O₃:MgO = 30:70 and 70:30 due to its smallest grain size. In contrast to the transmittance, the hardness H_v and toughness K_IC tend to increase with increasing the MgO fraction irrespective of the grain size; both H_v and K_IC increased from 9.6 GPa and 1.1 MPa m^1/2 for the monolithic Y₂O₃ to 12.7 GPa and 2.5 MPa m^1/2 for the composite with Y₂O₃/MgO = 30:70, respectively. The enhanced hardness and toughness of the composite can be interpreted dominantly by the mixture rule as a function of the volume fraction of the MgO phase.     

Mechanical properties and thermal conductivity of dense β-SiAlON ceramics fabricated by two-stage spark plasma sintering with Al2O3-AlN-Y2O3 additives

Fully dense β-SiAlON ceramics with excellent mechanical properties and good thermal conductivity were fabricated by two-stage spark plasma sintering (SPS) processes without and with applying pressure respectively, using α-Si₃N₄ powder and 6 Al₂O₃-3 AlN-6 Y₂O₃ (in wt.%, label with 636), 424 and 422 additives. In the first stage SPS process without pressure, the relative dense β-SiAlON ceramics with interlock microstructures of elongated grains and density of 3.14˜3.18 g cm⁻³, hardness of 14.00˜14.82 GPa and fracture toughness of 6.00˜6.63 MPa m^1/2 were obtained by sintering at about 1600 °C for 20 min. In the second stage SPS process at about 1425 °C for 5 min under pressure of 24 MPa, the fully dese β-SiAlON ceramics with density of 3.22˜3.24 g cm⁻³, high hardness of 15.68˜15.95 GPa, high fracture toughness of 6.38˜7.03 MPa m^1/2 and thermal conductivity of 13.5˜19.6 Wm^-1K^-1 were obtained. The reaction between the samples and the graphite mold can be avoided in this fabrication method.     

Effect of Al addition on creep resistance of the fired SiC-MgAl2O4 composite refractories

SiC-MgAl₂O₄ composite refractories were prepared by using SiC particles, MgAl₂O₄ powders, α-Al₂O₃ micro-powders and metal Al powders as raw materials under flowing nitrogen at 1450 °C. The creep test was conducted at 1400 °C under 0.2 MPa for 50 h and the effects of Al addition on the creep resistance of SiC-MgAl₂O₄ composite refractories were investigated. The results show that the creep rate of samples without Al addition increases gradually as time prolonging while the samples with Al addition has a better creep resistance and the creep rate always contains a low creep rate. The mechanism for improving creep resistance of the samples with Al addition could be concluded as follow. During the creep test, the samples have a oxygen concentration from surface areas to the inner areas. (Al₂OC)_x-(AlN)_(1-x) whiskers (generated during the sintering process) at the surface area preferentially reacts with oxygen to form Al₂O₃ and a little SiC oxidizes to SiO and even SiO₂, the formed Al₂O₃ and SiO/SiO₂ can react with the MgAl₂O₄ matrix to generate mullite, cordierite and MgO. MgO and SiO/SiO₂ could also transfer and react with Al₂O₃ to generate new MgAl₂O₄ particles and even plate β-Sialon dispersed in pores where (Al₂OC)_x-(AlN)_(1-x) whiskers ever existed. The formation of mullite, cordierite, MgAl₂O₄ particles and plate β-Sialon had a synergistic effect to improve the creep resistance. Besides, these new phases could fill the pores and inhibit the diffusion of oxygen inside the samples. With a relative low oxygen partial pressure inside, most of the (Al₂OC)_x-(AlN)_(1-x) whiskers still existed in samples with a network structure and only partly oxidized to Al₂O₃, the oxidation of SiC was inhibited and formation of liquid was avoided. Thus, the creep resistance of the sample was further enhanced.