Preparation and characterization of Al2O3–25 mol% ZrO2 composites

Al₂O₃–ZrO₂ (AZ_x), with 25 mol% ZrO₂ content, was prepared using the co-precipitation method. Synthesized powders were characterized by thermal reaction using a differential thermal analysis technique (TG–DTA) and were investigated by phase formation using X-ray diffraction. It indicated that the reaction occurred at 850 °C; cubic (c)-ZrO₂ phase and Al₂O₃ were obtained. By increasing temperature to 1100 °C, tetragonal (t)-ZrO₂ phase was detected. The Al₂O₃–25 mol% ZrO₂ was sintered for 2 h in the temperature range of between 1300 and 1600 °C. The majority phases of ceramics were m-ZrO₂ and α-Al₂O₃, although a t-ZrO₂ phase also appeared as a minor phase and decreased with higher temperature. Moreover, morphology and particle size evolution have been determined via the SEM technique. SEM showed that the particles of powder are agglomerated and basically irregular in shape. An SEM micrograph of ceramics exhibits uniform microstructure without abnormal grain growth.     

Fabrication and microstructure of Al2O3–ZrO2(3Y)–SiC nanocomposites

Al₂O₃–ZrO₂(3Y)–SiC composite powder was prepared by the heterogeneous precipitation method. Calcinating temperature of the powder was important to obtain dense sintered body. The nanocomposites were got by hot-pressing, and addition of ZrO₂ did not raise the sintering temperature. Some Al₂O₃ grain shape was elongated, and Al₂O₃ grain size was about μm. Nano SiC particles were observed uniformly distributing throughout the composites, and most of them were located within the matrix grains. Because SiC particles located within ZrO₂ grains influenced the phase transformation of ZrO₂, the sintering of nanocomposites, which controlled grain size and transformable ZrO₂ amount, become important to get high performance. The strength of 80 wt% Al₂O₃–15 wt% ZrO₂–5 wt% SiC nanocomposites was 555 MPa, and toughness was 3·8 MPa m^1/2, which were higher than those of monolithic Al₂O₃ ceramics. ©     

Effect of adding Y2O3 on structural and mechanical properties of Al2O3–ZrO2 ceramics

The effects of adding 1–8 wt% Y₂O₃ on phase formation and fracture toughness of Al₂O₃–xZrO₂–Y₂O₃(AZY) ceramics were studied. Phase formations of the samples were characterized by the X-ray diffraction (XRD) technique. It was found that the major phase was rhombohedral-Al₂O₃, while the minor phase consisted of the monoclinic-ZrO₂, tetragonal-ZrO₂ and monoclinic-Y₂O₃. It was found that Y₂O₃ contents did not clearly influence grain shape of AZY ceramics. The results obtained from the microhardness test could be used to evaluate the fracture toughness. It was found that the smaller grains had high fracture toughness. The maximum fracture toughness of 4.827 MPa m^1/2 was obtained from 4 wt% Y₂O₃. Refinement of lattice parameters using Rietveld analysis revealed the quantitative phases of AZY ceramics. This shows that under adding Y₂O₃ conditions the proportion of tetragonal-ZrO₂ phase plays an important role for the mechanical properties of AZY ceramics.     

Superplastic deformation of directionally solidified nanofibrillar Al2O3–Y3Al5O12–ZrO2 eutectics

Nanofibrillar Al₂O₃–Y₃Al₅O₁₂–ZrO₂ eutectic rods were manufactured by directional solidification from the melt at high growth rates in an inert atmosphere using the laser-heated floating zone method. Under conditions of cooperative growth, the ternary eutectic presented a homogeneous microstructure, formed by bundles of single-crystal c-oriented Al₂O₃ and Y₃Al₅O₁₂ (YAG) whiskers of ≈100 nm in width with smaller Y₂O₃-doped ZrO₂ (YSZ) whiskers between them. Owing to the anisotropic fibrillar microstructure, Al₂O₃–YAG–YSZ ternary eutectics present high strength and toughness at ambient temperature while they exhibit superplastic behavior at 1600 K and above. Careful examination of the deformed samples by transmission electron microscopy did not show any evidence of dislocation activity and superplastic deformation was attributed to mass-transport by diffusion within the nanometric domains. This combination of high strength and toughness at ambient temperature together with the ability to support large deformations without failure above 1600 K is unique and shows a large potential to develop new structural materials for very high temperature structural applications.     

Assessing fracture toughness in sintered Al2O3–ZrO2(3Y)–SiC ceramic composites through indentation technique

Al₂O₃–ZrO₂(3Y)–SiC ceramic composites were prepared by spark plasma sintering. The fracture toughness (K_1C) of the material was evaluated by indentation technique using different types of empirical formulas between 29.4 N and 98 N. The calculated K_1C values depend on the crack profile and the applied load. By analyzing the relationship between the indentation radius and the crack length and the crack profile, the crack form of the composite is determined as a Palmqvist crack. Raman spectra result shows that ZrO₂ has a significant martensitic phase transformation when the applied load is 5 kg or higher. Additionally, the K_1C value decreases slightly as the load increases. The equation based on the Palmqvist crack system and the curve fitting equation are more suitable for the calculation of the K_1C value of the material. The composite exhibits a combination of various toughening mechanisms, such as crack deflection and bridging, thereby improving the fracture toughness of the material.     

Electrical resistivity of Cr–Al2O3 and ZrxAly–Al2O3 composites with interpenetrating microstructure

AC and DC resistivity of Cr–Al₂O₃ and Zr_xAl_y–Al₂O₃ composites with varying metal content were measured. A strong percolation behavior was observed in the Cr–Al₂O₃ system, where the AC resistivity varied nine orders of magnitude close to the percolation threshold of 28 vol.%. AC measurements were less dependant on the contact resistance than DC measurements. The best reproducibility was obtained at a frequency of 100 kHz. AC resistivity values of insulating composites differed from DC values and may also be frequency-dependant. DC measurements up to 600 °C indicate that the intermetallic phases ZrAl₃ and ZrAl are PTC conductors. The electrical properties of Zr_xAl_y–Al₂O₃ samples with a metal content of 29 vol.% were anisotropic, with a much higher resistivity in the pressing direction.     

Microwave hybrid sintering of Al2O3 and Al2O3–ZrO2 composites,and effects of ZrO2 crystal structure on mechanical properties,thermal properties,and sintering kinetics

Structural ceramics such as Al₂O₃ and Al₂O₃–ZrO₂ composites are widely used in harsh environment applications. The conventional sintering process for fabrication of these ceramics is time-consuming method that requires large amount of energy. Microwave sintering is a novel way to resolve this problem. However, to date, very limited research has been carried out to study the effects of different ZrO₂ crystal structures on Al₂O₃–ZrO₂ composites, especially on the sintering kinetics, when fabricated by microwave sintering.The microwave hybrid sintering of Al₂O₃ and Al₂O₃–ZrO₂ composites was performed in this study. Tetragonal zirconia and cubic zirconia were used as two different reinforcements for an α–alumina matrix, and the mechanical and thermal properties were studied. It was found that Al₂O₃ experienced a remarkable increase in fracture toughness of up to 42% when t-ZrO₂ was added. Al₂O₃–c-ZrO₂ also showed increased fracture toughness. The sintering kinetics were also thoroughly investigated, and the average activation energy values for the intermediate stage of sintering were estimated to be 246 ± 11 kJ/mol for pure Al₂O₃, 319 ± 71 kJ/mol for Al₂O₃–c-ZrO₂, and 342 ± 77 kJ/mol for Al₂O₃–t-ZrO₂. These values indicated that the activation energy was increased by the addition of either type of ZrO₂, with the highest value shown by Al₂O₃–t-ZrO₂.     

Mechanical and functional properties of Al2O3–ZrO2–MWCNTs nanocomposites

Multi-walled carbon nanotubes (MWCNTs) are often reported as additives improving mechanical and functional properties of ceramic composites. However, despite tremendous efforts in the field in the past 20 years, the results are still inconclusive. This paper studies room temperature properties of the composites with polycrystalline alumina matrix reinforced with 0.5–2 vol.% MWCNTs (composites AC) and zirconia toughened alumina with 5 vol.% of yttria partially stabilised zirconia (3Y-PSZ) containing 0.5–2 vol.% of MWCNTs (composites AZC). Dense composites were prepared through wet mixing of the respective powders with functionalised MWCNTs, followed by freeze granulation, and hot-pressing of granulated powders. Room temperature bending strength, Young's modulus, indentation fracture toughness, thermal and electrical conductivity of the composites were studied, and related to their composition and microstructure. Slight increase of Young's modulus, indentation fracture toughness, bending strength, and thermal conductivity was observed at the MWCNTs contents ≤1 vol.%. At higher MWCNTs contents the properties were impaired by agglomeration of the MWCNTs. The DC electrical conductivity increased with increasing volume fraction of the MWCNTs.     

Thermodynamic calculation of Ms in ZrO2–CeO2–Y2O3 system

The difference of Gibbs free energy between tetragonal and monoclinic phases in ZrO₂–CeO₂–Y₂O₃ as a function of composition and temperature is thermodynamically calculated from the three related binary systems. In 8 mol% CeO₂–0.5 mol% Y₂O₃–ZrO₂, the equilibrium temperature between tetragonal and monoclinic phases, T₀, is obtained as 832.5 K and the M_s temperature of this alloy with a mean grain size of 0.90 μm is calculated as 249.9 K using the approach derived by Hsu et al. [J. Mater. Sci., 18(1983)3206; 20(1985)23; Acta Metall., 37(1989)3091; Acta Metall. Mater., 39(1991)1045; Mater. Trans. JIM, 37(1996)1284], which is in good agreement with the experimental one of 253 K.     

The corrosion resistance of microsilica-containing Al2O3–MgO and Al2O3–spinel castables

Microsilica addition in Al₂O₃–MgO and Al₂O₃–spinel castables helps to improve their flowability and partially accommodate their residual expansion after firing. Nevertheless, there is a lack of conclusive statements in the literature regarding the effects of microsilica on one of the main requisites for steel ladle refractories: corrosion resistance. In the present work, the performance of alumina–magnesia and alumina–spinel with or without microsilica when in contact with a steel ladle slag was evaluated based on three aspects: the material's physical properties, its chemical composition and the microstructural features before the slag attack. According to the attained results, microsilica induced liquid formation and pore growth during sintering, favoring the physical slag infiltration. Moreover, due to this liquid, CA₆ was formed in the matrix, mainly for the Al₂O₃–spinel composition, which also favored the castable dissolution into the molten slag.     

Processing and crystallisation of rapidly solidified Al2O3–Y2O3 fibres

Abstract

Amorphous fibres of the Al₂O₃–Y₂O₃ system were prepared by a melt extraction technique, and subjected to crystallisation. The quality of the melt extracted fibres is controlled by the wheel edge and rotational speed, with both having a significant effect on fibre diameter and avoidance of irregularities and instabilities along the fibre length. Tensile strength in the glassy state varied from 0·6 to 1·0 GPa. Crystallisation activation energies calculated from scan-rate dependence of DTA peaks are 741 and 1374 kJ mol^-1 for E1 (Al₂O₃–yttrium aluminium garnet (YAG) eutectic), 390 kJ mol^-1 for YAG, and 438 kJ mol^-1 for E2 (YAG–yttrium aluminium perovskite (YAP) eutectic) by the Kissinger method; and 698 and 1346 kJ mol^-1 for E1, 352 kJ mol^-1 for YAG, and 399 kJ mol^-1 for E2 by the Augis–Bennett method.     

Pressure-less joining of SiCf/SiC composites by Y2O3–Al2O3–SiO2 glass: Microstructure and properties

In this study, Y₂O₃–Al₂O₃–SiO₂ (YAS) glass was prepared from Y₂O₃, Al₂O₃, and SiO₂ micron powders. Thermal expansion coefficient of as-obtained YAS glass was about 3.9 × 10⁻⁶, matching-well with that of SiC_f/SiC composites. SiC_f/SiC composites were then brazed under pressure-less state by YAS glass and effects of brazing temperature on microstructures and properties of resulting joints were investigated. The results showed that glass powder in brazed seam sintered and precipitated yttrium disilicate, cristobalite, and mullite crystals after heat treatment. With the increase in temperature, joint layer gradually densified and got tightly bonded to SiC_f/SiC composite. The optimal brazing parameter was recorded as 1400 °C/30 min and shear strength of the joint was 51.7 MPa. Formation mechanism of glass-ceramic joints was proposed based on combined analysis of microstructure and fracture morphology of joints brazed at different temperatures. Thermal shock resistance testing of joints was also carried out, which depicted decline in shear strength with the increase of thermal shock times. The strength of the joint after three successive thermal shock cycles at 1200 °C was 35.6 MPa, equivalent to 69% of that without thermal shock.     

Mechanical properties and thermal shock in thin ZrO2–Y2O3–Al2O3 films obtained by the sol-gel method

Thin multilayer coatings of ZrO₂–Y₂O₃–Al₂O₃ were prepared using the sol-gel method and dip-coating technique in order to advance in the study of what influence the incorporation of Al₂O₃ has on films of Y₂O₃-doped ZrO₂, investigating its role in the synthesis of the solutions and in the characteristics and properties of the coatings. After the characterization of the solutions used in the process, the microstructure of the films was studied and their mechanical behaviour and resistance to thermal shock were determined so as to optimize the characteristics and functionality of these coatings. With increased alumina content, 3YSZ-Al₂O₃ (20 mol%), the cubic phase of the zirconia disappeared completely at the sintering temperature used (700 °C), resulting in the tetragonal phase with Al in solution. There was also a decrease in the coatings' hardness and Young's modulus, and an increase in toughness and resistance to thermal shock. These results allow guidelines to be established for the design of multilayer structures that are, tougher, more resistant, and have improved surface properties.     

Enhanced thermal shock response of Al2O3–graphite composites through a layered architectural design

The use of ceramics such as alumina in moving components often requires the addition of low friction materials such as graphite. A new strategy for improving toughness, strength, and thermal-shock resistance of Al₂O₃–graphite self-lubricating composites was proposed in this study. Alumina layers embedded between Al₂O₃–graphite layers were fabricated and tested after thermal shock conditions ranging between 500 °C and 800 °C maximum temperature. Retained strength and apparent fracture toughness after the tests were compared to room temperature values. Results show that compressive residual stresses generated in the outer Al₂O₃–graphite layers during cooling down from sintering improve the failure resistance of the materials. The introduction of heat-resistant particles (Al₂O₃ particles) into graphite layers combined with a layered architecture can greatly decrease the oxidation degradation of the materials below 500 °C. In addition, the retained strength and toughness in the layered architectures after thermal shock between 550 °C and 800 °C remains constant, thus indicating that the new-developed Al₂O₃/Al₂O₃–graphite laminated composites may be reliable candidates for self–lubricating applications also for elevated temperatures.     

Thermal conductivity of single- and multi-phase compositions in the ZrO2–Y2O3–Ta2O5 system

Compositions in the ZrO₂–Y₂O₃–Ta₂O₅ system are of interest as low thermal conductivity, fracture resistant oxides for the next generation thermal barrier coatings (TBC). Multiple phases occur in the system offering the opportunity to compare the thermal properties of single, two-phase, and three-phase materials. Despite rather large variations in compositions almost all the solid solution compounds had rather similar thermal conductivities and, furthermore, exhibited only relatively small variations with temperature up to 1000 °C. These characteristics are attributed to the extensive mass disorder in all the compounds and, in turn, small interfacial Kapitza (thermal) resistances. More complicated behavior, associated with the transformation from the tetragonal to monoclinic phase, occurs on long-term annealing in air of some of the compositions. However, the phases in two of the compositional regions do not change with annealing in air and their thermal conductivities remain unchanged suggesting they may be suitable for further exploration as thermally stable TBC overcoats.     

Structural parameters and oxygen ion conductivity of Y2O3–ZrO2 and MgO–ZrO2 at high temperature

The oxygen ion conductivity of zirconia-based solid electrolytes doped with 8 mol% Y₂O₃–ZrO₂ (YSZ) and 9 mol% MgO–ZrO₂ (Mg-PSZ) at high temperature was investigated in terms of their thermal behavior and structural changes. At room temperature, YSZ showed a single phase with a fluorite cubic structure, whereas Mg-PSZ had a mixture of cubic, tetragonal and some monoclinic phases. YSZ exhibited higher ionic conductivity than Mg-PSZ at temperatures from 600 °C to 1250 °C because of the existence of the single cubic structure and low activation energy. A considerable increase in the conductivity with increasing temperature was observed in Mg-PSZ, which showed higher ionic conductivity than YSZ within the higher temperature range of 1300–1500 °C. A monoclinic-to-tetragonal phase transformation was found in Mg-PSZ and the lattice parameter of the cubic phase increased at 1200 °C. The phase transformation and the large lattice free volume contributed to the significant enhancement of the ionic conductivity of Mg-PSZ at high temperatures.     

Load-dependence of Knoop hardness of Al2O3–TiC composites

Nine samples of Al₂O₃–30 wt.% TiC composites were prepared by hot-pressing the Al₂O₃ powder mixed with TiC particles. The average sizes of the TiC particles used for preparing the nine samples were different with each other. Knoop hardness measurements were conducted on these nine samples, respectively, in the indentation load range from 1.47 to 35.77 N. For each sample, the measured Knoop hardness decreases with the increasing indentation load. The classical Meyer's power law and an empirical equation proposed originally by Bückle were verified to be sufficiently suitable for describing the observed load-dependence of the measured hardness. Analysis based on Meyer's law can not provide any useful information about the cause of the observed ISE while true hardness values, which are load-independent, can be deduced from the Bückle's equation. It was found that the deduced true hardness increases with the average size of TiC particles existing in the sample.     

Thermal properties of field-assisted-sintered SiCN–Y2O3 composites

Polymer-derived amorphous SiCN has excellent high-temperature stability and properties. To reduce the shrinkage during pyrolysis and to improve the high-temperature oxidation resistance, Y₂O₃ was added as a filler. In this study, polymer-derived SiCN–Y₂O₃ composites were fabricated by mixing a polymeric precursor of SiCN with Y₂O₃ submicron powders in different ratios. The mixtures were cross-linked and pyrolyzed in argon. SiCN–Y₂O₃ composites were processed using field-assisted sintering technology at 1350°C for 5 min under vacuum. Dense SiCN–Y₂O₃ composite pellets were successfully made with relative density higher than 98% and homogeneous microstructure. Due to low temperature and short time of the heat-treatment, the grain growth of Y₂O₃ was substantially inhibited. The Y₂O₃ grain size was ∼1 μm after sintering. The composites’ heat capacity, thermal diffusivity, and thermal expansion coefficients were characterized as a function of temperature. The thermal conductivity of the composites ceramics decreased as the amount of amorphous SiCN increased and the coefficient of thermal expansion (CTE) of the composites increased with Y₂O₃ content. However, the thermal conductivity and CTE did not follow the rule of mixture. This is likely due to the partial oxidation of SiCN and the resultant impurity phases such as Y₂SiO₅, Y₂Si₂O₇, and Y_4.67(SiO₄)₃O.     

Effect of ZrO2 content on the microstructure and flexural strength of Al2O3–ZrO2 composites with the stored failure energy-fragmentation relations

High flexural strength is an important mechanical property for a ceramic armor component to withstand high tensile stresses and protect its structural integrity against multiple hits. Also, larger fragments are required in fragmentation as larger fractured parts are harder to leave of the way for the penetrator and cause more abrasion and higher penetration resistance. In this study, the effect of different ZrO₂ content (0, 0.5, 1, 3, 5, 10, 20 vol%) on the flexural strength of Al₂O₃–ZrO₂ composites was investigated with relationship of the stored failure energy-crack length to evaluate the fragmentation behavior under possible impact conditions. Monotonic equibiaxial flexural strength test was used to measure the fracture strength. The highest strength was obtained for 20 vol% ZrO₂ containing composite as 435 ± 78 MPa, ~ 24% increase in comparison with the pure Al₂O₃. The transformation of tetragonal to monoclinic phase occurred during the strength test in the 10 and 20 vol% ZrO₂ content composites. 20 vol% ZrO₂ containing composite had the smallest total crack length accompanying the largest fragment size for a given fracture energy among all the composites due to the stress-induced transformation of ZrO₂ consumes energy that results in decreasing effective crack driving energy required for the crack branching.