Effect of grain morphology and grain size on the mechanical properties of Al2O3 ceramics

Bending strength, fracture toughness, fracture energy and crack extension resistance were evaluated for Al₂O₃ ceramics with equi-axed and platelet grains. Bending strength was proportional to grain size^–1/2, but flaws with a size of 10 m controlled the strength when the microstructure was finer than 10 m. Fracture toughness, measured by the single etched precracked beam (SERB) method, was proportional to fracture energy^1/2, and increased with the grain size of Al₂O₃ ceramics with equi-axed and platelet grains. However, the toughness of platelet grain ceramics was higher than the ceramics with equi-axed grains, and increased up to 6.6 MPam^1/2 with grain size. Therefore, it is thought that fracture toughness not only depends on grain size, but also on grain morphology; equations were derived to account for this phenomenon.     

Formation and hot isostatic pressing of ZrO2 solid solution in the system ZrO2-Al2O3

In the system of ZrO₂-Al₂O₃, cubic ZrO₂ solid solutions containing up to 40 mol% Al₂O₃ crystallize at low temperatures from amorphous materials prepared by the simultaneous hydrolysis of zirconium and aluminium alkoxides. At higher temperatures, they transform into tetragonal solid solutions. Metastable ZrO₂ solid solution powders containing 25 mol% Al₂O₃ have been sintered at 1000–1150 °C under 196 M Pausing the hot isostatic pressing technique. The solid solution ceramics consisting of homogeneous microstructure with an average grain size of 50 nm exhibited a very high fracture toughness of 23 MN m ^–1.5. They have been characterized by X-ray diffraction and electron probe surface analyses.     

Formation of intergranular amorphous films during the microstructural development of liquid phase sintered silicon carbide ceramics

The microstructure of liquid phase sintered SiC ceramics was characterised by means of high resolution transmission electron microscopy (HRTEM). The SiC ceramics were pressureless sintered with the additions of Al₂O₃ and Y₂O₃ at sintering temperatures of 1800 and 1950°C, respectively. At a sintering temperature of 1800°C the microstructure of the SiC ceramics has no crystallised secondary phase and the SiC grains are separated by an intergranular amorphous film. In contrast, in the case of the microstructure of SiC ceramics sintered at 1950°C a clean interface without any amorphous layer between the SiC grains was observed. The secondary phase is crystallised into the Y₃Al₅O₁₂ phase and exhibits a clean interface between the SiC grains. An explanation for the existence or the absence of the intergranular glass films are given by an extended Clarke's model of the force balance of attractive van der Waals forces and repulsive steric forces. The chemical decomposition of the intergranular glass film at elevated temperature was considered.     

Effect of grain width and aspect ratio on mechanical properties of Si<Subscript>3</Subscript>N<Subscript>4</Subscript> ceramics

Sintering additives Y₂O₃ and Al₂O₃ with different ratios ((Y₂O₃/Al₂O₃) from 1 to 4) were used to sinter Si₃N₄ to high density and to induce microstructural changes suitable for raising mechanical properties of the resultant ceramics. The sintered Si₃N₄ ceramics have bi-modal microstructures with elongated β-Si₃N₄ grains uniformly distributed in a matrix of equiaxed or slightly elongated grains. Pores were found within the grain boundary phase at the junction regions of Si₃N₄ grains. The highest average aspect ratio (length/width of the grains) of ∼4.92 was found for Y₂O₃/Al₂O₃ ratio of 2.33 with fracture toughness and strength values of ∼7 MPam^1/2 and 800 MPa, respectively. The effect of microstructure, specifically grain morphology, on mechanical properties of sintered Si₃N₄ were investigated and found that the aspect ratio of the elongated grains is the most important microstructural feature which controls mechanical properties of these ceramics.     

Solid-solution range of mullite up to 1800 °C and microstructural development of ceramics

Tetraethoxysilane (TEOS) and Al-sec-butylate (Al-O-Bu) were used for the sol-gel synthesis of mullite ceramics. The starting materials had bulk compositions corresponding to values between 72 and 78 wt% Al₂O₃, and 28 and 22 wt% SiO₂, respectively, and were calcined at 400 °C (A-series) and 1100 °C (B-series). B-series samples, despite their higher green densities, could only be sintered to about 65–70% TD (theoretical density) at 1650 °C, whereas A-series samples achieve values of about 93–98% TD. Ceramics with relatively high amounts of glass phase from large tabular mullite crystals, which are embedded in a finer-grained mullite matrix. As soon as the bulk Al₂O₃ content increases, equiaxed mullite grains appear and the mean grain size becomes smaller, showing a significant difference between the nucleation and crystal growth mechanisms of mullites formed in samples with the lower and higher Al₂O₃-bulk compositions. Depending on the bulk composition of the samples, the temperature-controlled solid-solution of mullite ranges between about 72.7 and 74.3 wt% Al₂O₃ at 1600 °C and 74.1 and 75.4 wt% Al₂O₃ at 1800 °C, indicating that the solid-solution region bends over towards the Al₂O₃-side of the Al₂O₃-SiO₂ phase diagram.     

Roles of alumina in zirconia-based solid electrolyte

Up to 5 mol% Al₂O₃ was added to 9 mol% Y₂O₃-stabilized ZrO₂, and the roles of Al₂O₃ were systematically studied by means of the complex impedance approach, the positron annihilation technique, SEM, TEM, and electron probe microanalysis from the following aspects: (1) the existence of forms of Al₂O₃ in ZrO₂, (2) the effects of Al₂O₃ on the microstructure of ZrO₂, (3) the effects of Al₂O₃ on the resistance of ZrO₂, (4) the microstructure and property changes of ZrO₂ with Al₂O₃ addition during ageing at 940C. Two types of grain boundary phase, crystal and amorphous, were discovered. The Al₂O₃ segregation at grain boundaries can promote the mobility of the grain boundaries and thus results in a low density, because of entrapped pores. The Al₂O₃ addition decreases the grainboundary resistance in two ways: to scavenge the SiO₂ and CaO located at grain boundaries, and to form crystal grain-boundary phases with very high crystal defect concentrations. Ordered microdomains of Zr₃Y₄O₁₂ were precipitated from ZrO₂ grains during ageing, and aluminium was found to facilitate the precipitation.     

Microstructure and properties of the sintered composite prepared by hot pressing of TiN-coated alumina powder

Alumina powders (average grain size: 50 m) coated with TiN film of thickness 0.5 and 1.2 m were prepared by rotary powder-bed chemical vapour deposition for 15 and 90 min, respectively. These Al₂O₃-TiN composite powders were hot-pressed at 1800 °C and 40 MPa for 30 min. The microstructure of the Al₂O₃-TiN sintered composite was composed of a TiN network homogeneously distributed on the grain boundaries of alumina. The mechanical properties (hardness, bending strength and fractured toughness) and thermal conductivity of the sintered composite were found to depend on the composition and microstructure of the sintered composite, even with a small content (3–7 wt%) of TiN. The resistivity of the sintered composite was 10^–1-10^–3 cm. The relatively high electrical conductivity of the Al₂O₃-TiN composite was caused by the grain boundary conduction of TiN.     

Abnormal grain growth in alumina-doped hafnia ceramics

Hafnia (HfO₂) ceramics containing 0.0, 5.0, and 10.0 vol% Al₂O₃, respectively, were sintered at 1600°C for various periods from 2–24 h. Abnormal grain growth was found to occur in the Al₂O₃-containing compositions. Hafnia containing 5.0 vol% Al₂O₃ exhibits an average grain size of almost double that of the Al₂O₃-free hafnia matrix, coupled with a much wider grain-size distribution. The material containing 10.0 vol% Al₂O₃ shows a smaller average grain size than the composition containing 5.0 vol% Al₂O₃. However, its average grain size is still larger than that of the Al₂O₃-free hafnia on sintering at 1600°C for more than 8 h. Microstructural characterization, carried out using scanning electron microscopy (SEM) and transmission electron microscopy (TEM) equipped with an energy dispersive analysis facility (EDX), indicated that there existed a continuous segregant layer at the grain boundaries and grain junctions in the Al₂O₃-free hafnia. Hafnia exhibits a low solubility in the segregant layer phase which inhibits the growth of the hafnia grains. The Al₂O₃ particles act as a scavenger for the silicon-rich glassy phase, damaging the continuous nature of the boundary segregant layer and promoting grain growth in the Al₂O₃-doped hafnia ceramics. The microstructural development at the sintering temperature is an overall result of the concurrent scavenger effect and grain pinning by the Al₂O₃ particles.     

High temperature plastic flow and grain boundary chemistry in oxide ceramics

High temperature plastic flow or grain boundary failure in oxide ceramics such as Al₂O₃ and tetragonal ZrO₂ polycrystal (TZP) is sensitive to small levels of doping by various cations. For example, high temperature creep deformation in fine-grained, polycrystalline Al₂O₃ is highly suppressed by 0.1 mol% lanthanoid oxide or ZrO₂-doping. An elongation to failure in superplastic TZP is improved by 0.2–3 mol% GeO₂-doping. A high-resolution transmission electron microscopy (HRTEM) observation and an energy-dispersive X-ray spectroscopy (EDS) analysis revealed that the dopant cations tend to segregate along the grain boundaries in Al₂O₃ and TZP. The dopant effect is attributed to change in the grain boundary diffusivity due to the grain boundary segregation of the dopant cations. A molecular orbital calculation suggests that ionicity is one of the most important parameters to determine the high temperature flow stress, and probably, the grain boundary diffusivity in the oxide ceramics.     

Effect of erbium on varistor characteristics of vanadium oxide-doped zinc oxide ceramics

The effect of erbium addition on microstructure, electrical properties, and ageing behavior of vanadium oxide-doped zinc oxide varistor ceramics was systematically investigated. Analysis of the microstructure indicated that the ceramics added with erbium consisted of ZnO grain as a main phase and secondary phases such as Zn₃(VO₄)₂, ZnV₂O₄, ErVO₄, V₂O₅, and Mn-rich. The average grain size decreased from 5.5 to 5.2 μm up to 0.05 mol%, whereas a further addition gradually increased it to 5.7 μm at 0.25 mol%. The sintered density increased from 5.51 to 5.61 g/cm³ with an increase in the amount of Er₂O₃. With increasing the amount of Er₂O₃, the breakdown field increased from 4,800 to 5,444 V/cm up to 0.05 mol%, whereas a further addition decreased it to 4,061 V/cm at 0.25 mol%. The varistor ceramics added with 0.05 mol% Er₂O₃ additives induced excellent nonlinear properties, with nonlinear coefficient of 63.4 by properly adding the amount of Er₂O₃ (0.05 mol%). The study indicated that the erbium acted as a donor to increase the donor concentration with an increase in the amount of Er₂O₃.     

Thermal stability and mechanical properties of yttria-doped tetragonal zirconia polycrystals with dispersed alumina and silicon carbide particles

Yttria-doped tetragonal zirconia polycrystals in which were dispersed various amounts of Al₂O₃ and SiC particles were sintered at 1500° C for 3 h, and the mechanical properties and the thermal stability of the sintered bodies were evaluated. Dispersion of Al₂O₃ caused no significant effect on sinterability, and increased the hardness and elasticity of the composites. Dispersion of SiC particles decreased the relative density and the grain size of composites. Elasticity and hardness increased by dispersing less than 10 vol% SiC, but decreased above 10 vol% SiC due to the decrease of relative density. Dispersion of both Al₂O₃ and SiC particles slightly increased the fracture toughness of ZrO₂-3 mol% Y₂O₃ ceramics but significantly decreased that of ZrO₂-2 mol% Y₂O₃ ceramics. The rate of the tetragonal-to-monoclinic phase transformation decreased by dispersing both Al₂O₃ and SiC particles. The transformation depth increased rapidly and then slowly with increasing the annealing time. The rate of increase in the transformation depth greatly decreased by dispersing Al₂O₃ particles.     

Fabrication and microstructure of in situ toughened Al2O3/Fe3Al

Fe₃Al nano-particles and commercial purity Al₂O₃ powders were used as raw materials to fabricate in situ reinforced Al₂O₃/Fe₃Al nano/micro-composites. Densification and microstructure were studied. The Al₂O₃ matrix grains were characterized by platelet grains. The Fe₃Al particles inhibited the grain growth of Al₂O₃ grains and limited the densification of the composites. In Al₂O₃/Fe₃Al composites, the Fe₃Al particles were uniformly dispersed in the Al₂O₃ matrix. The major Fe₃Al micro-particles, about 1 μm in average size, existed at Al₂O₃ grain boundaries, and the Fe₃Al nano-particles were found embedded in the matrix grains. The grain size of the intragranular particles ranged from several to several hundred nanometers. The grain size and aspect ratio of Al₂O₃ platelet grains and distribution of intragranular Fe₃Al could be optimized by controlling the Fe₃Al contents and sintering process. The in situ formed Al₂O₃ platelet grains, as well as Fe₃Al dispersoids, were beneficial to the increase of the mechanical properties of alumina.     

Fracture toughness,strength and Vickers hardness of yttria-ceria-doped tetragonal zirconia/alumina composites fabricated by hot isostatic pressing

Yttria-ceria-doped tetragonal zirconia ((Y, Ce)-TZP)/alumina (Al₂O₃) composites were fabricated by hot isostatic pressing (HIP) at 1400–1600 °C and 147 MPa for 30 min in Ar gas using fine powders prepared by hydrolysis of ZrOCl₂ solution. The mechanical properties of these ceramic composites were evaluated. The fracture toughness and bending strength of the composites consisting of 25 wt% Al₂O₃ and tetragonal zirconia with compositions 4 mol% YO_1.5-4 mol% CeO₂-ZrO₂, 2.5 mol% YO_1.5-4 mol% CeO₂-ZrO₂ and 2.5 mol% YO_1.5-5.5 mol% CeO₂-ZrO₂ fabricated by HIP at 1400 °C were 6–7 MPa m^1/2 and 1700–1800 MPa. Fracture toughness, strength and hardness of (Y, Ce)-TZP/Al₂O₃ composites were strongly dependent on HIP temperature. The fracture strength and hardness were increased, and grain growth of zirconia grains and phase transformation from the tetragonal to the monoclinic structure of (Y, Ce)-TZP during HIP in Ar at high temperature (1600 °C) were suppressed by the dispersion of Al₂O₃ into (Y, Ce)-TZP.     

Sintering and characterization of Al2O3-B4C composites

The effect of B₄C on the densification, microstructure and mechanical properties of pressureless sintered Al₂O₃-B₄C composites have been studied. Sintering was performed without sintering additives with varying B₄C content from 0–40 vol %. Up to 20 vol % B₄C, more than 97% theoretical density was always obtained when sintered at 1850 °C for 60 min. On increasing the sintering time from 30–120 min, there was no change in density. The result of X-ray diffraction analysis showed that no reaction occurred between Al₂O₃ and B₄C. The grain growth of Al₂O₃ was inhibited by B₄C particles pinned at the grain boundary and the grain-boundary drag effect. The critical amount of B₄C to drag the grain boundary migration effectively was believed to occur at 10 vol % B₄C sintered at 1850 °C for 60 min. The maximum three-point flexural strength was found to be 550 MPa for the specimen containing 20 vol % B₄C, and the maximum microhardness was 2100 kg mm^–2 for 30 vol % B₄C specimen.     

Fracture and deformation behaviour of melt growth composites at very high temperatures

Unidirectionally solidified Al₂O₃/Y₃Al₅O₁₂ (YAG) or Al₂O₃/Er₃Al₅O₁₂ (EAG) eutectic composites, which are named as Melt Growth Composites (MGCs) has recently been fabricated by unidirectional solidification. The MGCs have a new microstructure, in which continuous networks of single-crystal Al₂O₃ phases and single-crystal oxide compounds (YAG or EAG) interpenetrate without grain boundaries. The MGCs fabricated are thermally stable and has the following properties: 1) the flexural strength at room temperature can be maintained up to 2073 K (just below its melting point), 2) a fracture manner from room temperature to 2073 K is an intergranular fracture different from a transgranular fracture of sintered composite with the same composition, 3) the compressive creep strength at 1873 K and a strain rate of 10^–4/sec is 7–13 times higher than that of sintered composites, 4) the mechanism of creep deformation is based on the dislocation creep models completely different from the Nabarro-Herring or Coble creep models of the sintered composites, and 5) it shows neither weight gain nor grain growth, even upon heating at 1973 K in an air atmosphere for 1000 hours. The above superior high-temperature characteristics are caused by such factor as the MGCs having a single-crystal Al₂O₃/single-cryatal oxide compounds without grain boundaries and colonies, and the formation of the thermodynamically stable and compatible interface without amorphous phase.     

Varistor behavior of the system SnO2ċCoOċTa2O5ċCr2O3

In this work the system (98.95–x)% SnO2+1%CoO+0.05%Ta2O5+x%Cr2O3 (mol %) was studied, with x=0.05 and 0.10, prepared by the conventional method of oxides mixture, and sintered at 1300 and 1350 °C for 2 h. The non-linear J versus E electrical characteristics (=25) were obtained in the Ta₂O₅ and Cr₂O₃–CoO doped highly densified SnO₂ ceramics. X-ray diffraction analysis showed that these ceramics are apparently single phase. Electrical properties and microstructure are highly dependent on the Cr₂O₃ concentration and on the sintering temperature. Excess of Cr₂O₃ leads to porous ceramics destroying the electrical characteristics of the material. Dopant solid solution formation in the SnO₂ may be responsible for the formation of electrical barriers in the grain boundaries.     

The influence of silicon on microstructure and electrical properties of La-doped BaTiO3 ceramics

BaTiO₃ positive temperature coefficient of resistance ceramics were prepared with the general composition (La_0.002Ca _x Ba_0.998–x ) (Ti_1.01–y Mn _y )O₃ ·zSiO₂ and sintered in air. The Ca content as well as the amount of doped Mn were varied ranging from 4–20 mol% and 0.02–0.04 mol%, respectively. Up to 4 mol% Si was added using a new method as well as by the classical means as solid silica. In the new method, the desired quantity of Si(OC₂O₅)₄ (Si(OEt)₄), contained in dry tetrahydrofurane (THF), was dropped under a slight stream of inert gas into the vigorously stirred aqueous slurry which was manufactured after the calcination process. The addition of Si by this technique was found to result in a homogenization of the microstructure and an improvement in electrical properties. The effect of Si on electrical properties is explained by the influence of the observed second phase on the equilibrium of Ba vacancies. The results of further variation of Ca and Mn contents are presented.     

The effect of rare-earth oxides on the crystallization of CaO–Al2O3–SiO2 glasses

CaO–Al₂O₃–SiO₂ –La₂O₃–Nd₂O₃ glass was prepared and their physical properties, such as density, glass transition temperature and crystallization temperature, were measured. The heat treatment of these glasses precipitated Ca₂La₈(SiO₄)₆O₂ oxyapatite (CLS) crystal for 20CaO–10Al₂O₃–60SiO₂–10La₂O₃ (mol%) glass and Ca₂Nd₈(SiO₄)₆O₂ oxyapatite (CNS) crystal was precipitated with Nd₂O₃ substitution. Crystallization of these glasses was observed at the surface and internally within the samples. The spherical and stick-like crystals were observed throughout the bulk of the glasses and the surface crystal layer of oxyapatite crystals were strongly oriented along the c-axis. The apparent activation energies of crystal growth were estimated as 360 kJ mol^–1.     

Transparency of LiTaO3-SiO2-Al2O3 glass-ceramics in relation to their microstructure

The glasses with various compositions in the LiTaO₃-SiO₂-Al₂O₃ system were heated from room temperature to temperatures ranging from 750° to 1050° C at a rate of 5° C min^–1. From the glasses in the LiTaO₃-SiO₂ system no transparent glass-ceramic was obtained even when their LiTaO₃/SiO₂ mole ratios were as high as 2.33. The diameter and number of the LiTaO₃ crystal grains precipitated in the glasses were 5–15 m and 10⁸–10¹⁰ grains cm^–3, respectively. On the contrary, transparent glass-ceramics were obtained from the glasses containing Al₂O₃; their compositions covered a fairly large area in the LiTaO₃-SiO₂ -Al₂O₃ system, which encompasses the compositions with the LiTaO₃/SiO₂+AlO_1.5 mole ratio as low as 0.25. The diameter and number of the LiTaO₃ crystal grains precipitated in the transparent glass-ceramics were as small as 10–20 nm and as many as 10¹⁶–10¹⁸ grains cm^–3, respectively. High nucleation rates of the LiTaO₃ crystals in the Al₂O₃-containing glasses were interpreted in terms of structural inflexibility induced in the glass-network by the addition of Al₂O₃ to the LiTaO₃-SiO₂ system.     

Effect of gadolinia addition on varistor characteristics of vanadium oxide–doped zinc oxide ceramics

The effect of gadolinia addition on microstructure, electrical and dielectric characteristics, and aging behavior of vanadium oxide–doped zinc oxide varistor ceramics was systematically investigated. The average grain size decreased from 5.6 to 5.2 μm with an increase in the amount of Gd₂O₃ up to 0.1 mol%, whereas a further increase caused it to increase to 5.7 μm at 0.25 mol%. The sintered densities decreased from 5.51 to 5.44 g/cm³ with an increase in the amount of Gd₂O₃. With increasing the amount of Gd₂O₃, the breakdown field increased from 4,800 to 5,365 V/cm up to 0.05 mol%, whereas a further increase decreased it to 4,781 V/cm at 0.25 mol%. The varistor ceramics modified with 0.05 mol% Gd₂O₃ exhibited excellent nonlinear properties, with 66.1 in the nonlinear coefficient, whereas a further increase caused it to decrease to 17.6 at 0.25 mol%. The gadolinium acted like a donor, based on the electron concentration increasing from 4.20 × 10¹⁷/cm³ to 7.38 × 10¹⁷/cm³ with an increase in the amount of Gd₂O₃.