Eutectic Ceramics Research Articles

Microstructure tailoring and thermal stability of directionally solidified Al2O3/GdAlO3 binary eutectic ceramics by laser floating zone melting

Directionally solidified Al2O3/GdAlO3 eutectic composite ceramics with low solidification defects are prepared by laser floating zone melting to tailor homogeneous microstructure and thermal stability performance under high temperature gradient. At low solidification rate, the solidification microstructure shows periodical growth striations in which coarse colony microstructure and homogeneous ‘Chinese script’ irregular eutectic structure are coexisted. The growth striation interval L and solidification rate V follow the relationship of L/V = (238.8 ± 2.4) s. By further optimizing laser processing parameters, the eutectic spacing is greatly refined to submicron scale and microstructure oscillation is eliminated at high solidification rates, and fully regular lamellar/rod eutectic structures are obtained by faceted eutectic transition. The rod-like eutectic spacing (λ) and solidification rate (V) follow the linear relationship of λ·V1/2 = 6.21 µm3/2 s−1/2. Furthermore, the as-solidified eutectic composite even with fine eutectic structure exhibits excellent thermal stability after heat exposure at 1500 °C for 250 h.

Nanostructured Al <sub>2</sub>O <sub>3</sub>-YAG-ZrO <sub>2</sub> Ternary Eutectic Components Prepared by Laser Engineered Net Shaping

The present work proposed a novel 3D printing approach for fabricating ternary eutectic ceramics (Al2O3-YAG-ZrO2, or AYZ in short) using laser engineered net shaping (LENS) technique. Highly dense (≥ 98%) thin wall components with refined eutectic microstructures were successfully fabricated. The as-fabricated ceramic eutectics mainly consist of cellular eutectics with three phases interpenetrated. In each deposited layer, the solidified microstructure changes from planar to cellular along building direction. The evolution of characteristic dimension of microstructure can be well interpreted using Jackson-Hunt relationship. Irregular interpenetrating to regular fibrous eutectic structure transition occurs at boundary region of LENSed specimens, primarily due to much higher solidification rate. The texture of Al2O3 // YAG // ZrO2 is recognized at the interior region of LENSed eutectic part along building direction, but gradually tilts from central to edge due to change of cellular growth direction. The LENSed AYZ samples exhibit almost isotropic property, and have mechanical performances comparable to those fabricated using conventional direction solidification methods.

Processing, microstructure and performance of Al2O3/Er3Al5O12/ZrO2 ternary eutectic ceramics prepared by laser floating zone melting with ultra-high temperature gradient

Directionally solidified Al2O3/Er3Al5O12(EAG)/ZrO2 ternary eutectic/off-eutectic composite ceramics with high density, homogeneous microstructures, well-oriented growth have been prepared by laser floating zone melting at different solidification rates from 4 to 400µm/s. Uniform and stable melting zone is obtained by optimizing temperature field distribution to keep continuous and stable eutectic growth and prevent from cracks and defects. The as-solidified composite ceramic exhibits complexly irregular eutectic structure, in which the eutectic spacing is rapidly refined but dotted ZrO2 number inside Al2O3 phase is decreased as increasing the solidification rate. The formation mechanism of ZrO2 distributed inside Al2O3 matrix is revealed by examining the depression of solid/liquid interface. Furthermore, after heat exposure 1500°C for 200h, the eutectic microstructure only shows tiny coarsening, which indicates it has excellent microstructural stability. As increasing the ZrO2 content, the fracture toughness can be improved up to 3.5MPam1/2 at 20.6mol% ZrO2.

Microstructure transition and selection of Al2O3/Er3Al5O12/ZrO2 ternary eutectic ceramics from micronscale to nanoscale: The effect of rapid solidification

Microstructure transition and selection of Al2O3/Er3Al5O12/ZrO2 ternary eutectic ceramics with minimal eutectic spacing of 50nm are studied by laser zone melting under rapid solidification rate. When the eutectic spacings are in micronscale, the ‘Chinese script’ structure is the main microstructure. As increasing the solidification rate (≥ 50µm/s), the eutectic colony structure is obviously formed. The irregular-regular structural transition and coexistence are produced around eutectic spacing of 100–200nm. The mechanism of irregular/regular eutectic structure transition and coexistence is revealed by considering the relative kinetic undercooling of each phase. In addition, the strong formation tendency of amorphous/nano-eutectic structure is observed at the surface of as-solidified ceramic drops by rapid liquid-metal quenching.

Effective elastic properties and residual stresses in directionally solidified eutectic Al2O3/YAG/ZrO2 ceramics estimated by finite element analysis

Effective elastic properties and residual stresses were assessed in directionally solidified ternary eutectic ceramic, Al2O3/YAG/ZrO2, by finite element analyses. A 3D finite element model was generated from a CT scan, representative of the microstructure and with a similar volume fraction. Effective elastic properties were calculated by numerical homogenisation. They highlight a quasi-isotropic behaviour of the ternary eutectic ceramics. Despite the difficulties to measure the strain, the dispersion observed in the results and the limited reliability of the materials properties, the results constitute a step towards a better understanding of the material behaviour. Thermal residual stresses induced by the manufacturing were also evaluated. Tensile residual stresses in yttria-stabilised zirconia and compressive residual stresses in YAG and alumina were highlighted. This evaluation also shed light on the influence of the phase morphology in the microstructure. Indeed, the computed spatial distribution of the residual stresses showed that they are different from one position to another due to the variation in phase morphology and also to material properties variability. Therefore, it is important when numerically assessing the thermomechanical properties to take into account the microstructure morphology as well as the variability of material properties.

Mechanical testing of directionally solidified eutectic ceramics (DSECs): specific problems and limitations

The specific problems of DSECs mechanical testing result from the particularities of these 3-D interconnected eutectic ceramics. First of all, 4-point bending tests ensure pure bending loading, whereas 3 PB tests only lead to a tensile and shear stress combination. Consequently, due to the 3-D microstructure of DSECs, interfaces between the various phases are subjected to a mixed (tensile and shear) loading which makes the interpretation of the results (strength) and of the fracture surfaces, rather difficult. For usual ceramics, biaxial flexure testing offers many advantages over 3- or 4-point beam-bending testing. The coaxial-ring test is free of edge influences (flaws): cracks initiate in the central area and propagate outwardly. However, in the case of DSECs, due to the presence of high internal thermal stresses (especially for ternary eutectics), interfaces can be subjected to a strong radial tensile and shear (near the free surface) stress combination. In the presence of the radial tensile stress resulting from biaxial loading, this internal thermal stress combination can lead to premature crack initiation leading to failure. Specimen machining through grinding leads to the formation of a strongly damaged layer. Annealing of this layer leads to the formation of a rough surface: slightly protruding phases and stress concentrations at the interfaces. The measured strength is ≈20% lower after annealing than that directly after grinding. Concerning the effect of the microstructure size, four representative sizes have been selected in the ≈10 µm to submicrometre range. A classical crack propagation criterion has allowed explaining the corresponding strength values.

Microstructure, interfaces and creep behaviour of Al2O3–Sm2O3 (ZrO2) eutectic ceramic composites

New compositions in the melt-grown eutectic ceramics field are investigated for thermomechanical applications. This paper is focused on the Al2O3–Sm2O3–(ZrO2) system. The studied compositions give rise to interconnected microstructures without anisotropy along the growth direction. At variance with the binary eutectic Al2O3–SmAlO3, the homogeneity of the microstructure of the Al2O3–SmAlO3–ZrO2 ternary eutectic is less sensitive to the growth rate. Interfaces between the alumina and perovskite phases are investigated by high-resolution transmission electron microscopy (TEM). They are semi-coherent. In stepped interfaces, the facets are parallel to dense planes of each phase. The steps have a dislocation character and may accommodate both misfits. The ternary eutectic displays a very good creep behaviour with strain rates very close to those obtained on other previously studied eutectics in the Al2O3–RE2O3(RE = Y, Gd, Er)–ZrO2 systems. The deformation micromechanisms are analysed by TEM in the three eutectic phases. After creep, dislocations are present in every phase. The activation of unusual slip systems (pyramidal slip in the alumina phase) shows that high local stresses can be reached. The presence of dislocation networks with low energy configurations is consistent with predominance of dislocation climb processes controlled by bulk diffusion.

Microstructure, texture, and crystallography in Ni–GDC and Co–GDC porous cermets obtained from directionally solidified eutectic ceramics

Lamellar NiO–GDC (Gadolinium-doped Ceria) and CoO–GDC directionally solidified eutectic ceramics (DSECs) were produced by the laser floating-zone technique and subjected to reduction in order to obtain porous cermets of Ni–GDC and Co–GDC, which have potential applications as anodes in solid oxide fuel cells (SOFC). The reduction of these DSECs into porous cermets was studied at 650 °C in NiO–GDC and at 500 and 700 °C in CoO–GDC, all of them processed with similar reduction kinetics. In comparison to similar Ni–YSZ and Co–YSZ lamellar cermets previously studied, no sharp reduction front was observed. The interface between the reduced and nonreduced zones is broader, with pores homogenously distributed in wide areas. Afterwards, the microstructure, texture, and crystallography of the samples were studied by electron microscopy as well as by electron and X-ray diffraction when completely reduced. The single crystal NiO and CoO lamellae transformed into porous polycrystalline metallic lamellae. Moreover, microscopy observations revealed a porous nanostructure of Co particles obtained by reduction at low temperatures (500 °C). Many of the Co and Ni particles seemed to have roughly maintained the previous crystallographic orientation with respect to the GDC phase, although the disorder of the crystallographic orientation increased significantly. In addition, a significant amount of the Ni particles reoriented to form an epitaxial interface with the (100)–GDC surface.

Directionally Solidified Boride and Carbide Eutectic Ceramics

Borides and carbides generally have outstanding hardness, excellent wear resistance, and high melting points due to their covalent bonding. Directionally solidified eutectic (DSE) composites of boride and carbide constituent phases have been investigated since the 1970s as an approach to produce dense composite microstructures with added control over the microstructure. A variety of DSE ceramic composites have been developed and evaluated as potential materials for structural and functional applications due to their unique thermo‐electro‐mechanical properties. Renewed interest over the past few decades has been motivated, in part, by the needs for ultrahigh‐temperature composites for aerospace applications along with low‐density composites for armor applications. Some directionally solidified boride and carbide DSEs exhibit advantages in material properties over monolithic materials. This study reviews historical and recent research on processing methods, microstructure, crystallography, and material properties (mechanical, electrical, thermal properties, and oxidation resistance) of directionally solidified boride and carbide eutectic ceramic composites. Opportunities along with current limitations and needs for future developments are also reviewed and discussed.

Potential of innovative ceramics for turbine applications

Improving the thermal efficiency of aircraft gas turbines and of thermal power generation systems is of great interest for industry. Despite the various studies performed to increase their heat resistance, the use of nickel-based superalloys at temperatures beyond 1150 °C will be difficult. For higher temperatures, new high-temperature structural materials are required. In this context, ceramics offer many advantages compared to Ni-based superalloys: a lower density and a better resistance to oxidation and abrasion. Two kinds of ceramics are investigated at ONERA: Si3N4-based materials for use up to 1400–1500 °C and directionally solidified eutectic (DSE) ceramics up to 1700 °C. For both materials, improving toughness is a challenge. This paper presents the processing techniques, the microstructural features and the resulting mechanical properties.

Transitional/eutectic microstructure of Al2O3–ZrO2 (Y2O3) ceramics prepared by spark plasma sintering

Fully densified Al2O3–ZrO2 (Y2O3) eutectic ceramics without any binders were fabricated by a novel and efficient way, namely spark plasma sintering. The morphology of the specimen sintered at 1600°C shows a typical lamellar eutectic microstructure, while the one sintered at 1500°C manifests a transitional microstructure between sintering and casting. The latter specimen also exhibits high mechanical properties: the Vickers hardness and fracture toughness of it are 17.5±0.3GPa and 7.4±0.4MPam1/2, respectively.

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 10mm 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 12GPa and 6.1MPa·m1/2, 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.

Microstructure transformation from irregular eutectic to complex regular eutectic in directionally solidified Al2O3/GdAlO3/ZrO2 ceramics by laser floating zone melting

Directionally solidified Al2O3/GdAlO3(GAP)/ZrO2(GdSZ) ternary eutectic ceramics have been grown by laser floating zone melting. The microstructure evolution rule is investigated as a function of growth rate. The as-solidified samples present high density (>99%), smooth surface and high diameter stability free of pores and cracks. Increasing the growth rate from 4 to 100μm/s, the microstructures undergo a transformation from “Chinese script” irregular eutectic pattern to eutectic colony and to complex regular eutectic patterns (lamellae or rods). The average interphase spacing of the “Chinese script” microstructure presents an inverse square relationship according to λ=10.46×ν−1/2, where the average interphase spacing λ has the dimension of μm and growth rate ν is in μm/s. While the interphase spacing of the complex regular microstructure is finer. The mechanisms of the microstructure evolution and the effects of ZrO2 phase on the morphology and domain size of microstructure are discussed in detail.

Preparation of Al2O3–Y3Al5O12–ZrO2 eutectic ceramic by flash sintering

Al2O3–Y3Al5O12–ZrO2 ternary eutectic oxide ceramic was prepared by flash sintering the corresponding powder. The highly dense sample was obtained at relatively low temperature and very short time with the assist of dc electric field. As-prepared ceramics well preserve the phase composition and microstructure of the original eutectic powder. The ceramic exhibits relatively high hardness and high fracture toughness. These results clearly demonstrate the feasibility of employing the flash-sintering technique to fabricate eutectic ceramics.

Sintering densification and microstructure formation of bulk Al2O3/YAG eutectic ceramics by hot pressing based on fine eutectic structure

Highly-dense Al2O3/YAG bulk eutectic ceramics with fine irregular eutectic structure are produced by a facile way of hot pressing method based on a unique micro-eutectic structure. The densification behavior and microstructure formation are investigated. The eutectic particles with different sizes (<4μm, 4–10μm, <25μm) and homogenous micro-eutectic structure are first prepared by pulverizing Al2O3/YAG eutectic rods solidified by laser floating zone and then are consolidated by hot pressing under different sintering temperatures (1450–1650°C). The results show that the initial eutectic particle size and sintering temperature importantly affect the full-eutectic structure formation during hot pressing. Using the sintering temperature above 1650°C and eutectic particles of 4–10μm, dense bulk eutectic ceramics (99.1%) are obtained, particularly well remaining the initial eutectic structure without grain boundaries, comparable to eutectic ceramics prepared by melt growth method. The <25μm and <4μm eutectic particles are preferred to achieve higher densification (>99.5%) but are difficult to reproduce repeatable full-eutectic microstructure in whole composite. Moreover, the formation mechanism of the full-eutectic structure for the ceramics by sintering is discussed. This method paves a facile and cost-effective way to manufacture near net shaped large bulk eutectic ceramics for applications at elevated temperatures.

The intrinsic strength of triangular symmetrical composite eutectic

Composite ceramic has the excellent properties at normal and high temperatures, especially when the structure of the composite eutectic is triangular symmetrical. Obviously, mechanical behavior and fracture properties of composite ceramic closely relates to the micro-structure of symmetrical triangular eutectic. In order to reveal the mechanical properties of eutectic composite ceramic, it is necessary to determine the intrinsic strength of triangular composite eutectic. Since the fiber and matrix of triangular symmetrical composite eutectic sharing a same covalent bond, the theoretical cohesion strength of symmetrical triangular eutectic was obtained by the combination-separation displacement of intrinsic bond. Basing on micro-structure plastic deformation before fracture of composite eutectic matrix, the dislocation pile-up model of eutectic composite ceramics was established. And then intrinsic bond fracture shear stress of triangular symmetrical composite eutectic was given by using the theory of dislocation pile-up. According to the macroscopic structure properties of triangular symmetrical composite eutectic and the distribution of stress field of composite eutectic, intrinsic strength of eutectic was obtained. The results shows that intrinsic strength of triangular symmetrical composite eutectic possessed clear size-dependence and the stress decreases with the increases of the diameter of fiber inclusions.

Microstructure and Mechanical Properties of Al2O3/ZrO2 Directionally Solidified Eutectic Ceramic Prepared by Laser 3D Printing

Directionally solidified eutectic ceramics such as Al2O3/ZrO2 are promising structural materials for applications in harsh environment with an ultrahigh temperature. In this work, through adopting assistant heating laser 3D printing, Al2O3/ZrO2 eutectic samples were manufactured with suppressing the formation of cracks. The dependence of the average rod spacing (λav) on the scanning rate (V) follows a relation with λavV0.5 = 1 µm1.5 s–0.5. Typical eutectic microstructures, so-called complex regular, were analyzed with respect to its evolution with modulating the growth conditions. Formation mechanism of the solidification defect, shrinkage porosity, was discussed and the defect is found to be significantly suppressed by optimizing the solidification parameters. The maximum hardness and fracture toughness are measured to be 16.7 GPa and 4.5 MPa m1/2, respectively. The interplay between the propagation of cracks and the Al2O3/ZrO2 interface is discussed.

Phase Boundaries and Interfacial Energy in Quasibinary Boride and Metal Ceramic Eutectic Systems

The a priori pseudopotential method is employed to propose a model by which phase boundaries form in quasibinary eutectic systems. Interfacial energy for boride and cermet quasibinary eutectic composites, LaB 6 –TiB 2 and LaB 6 –ZrB 2 , is calculated for different temperatures.

Calculation of the Orientation Relationships of Directionally Solidified Eutectic Ceramics by a Modified Coincidence of Reciprocal Lattice Points Model (CRLP)

The Coincidence of Reciprocal Lattice Points (CRLP) method was used to predict, according to geometric considerations, the most favorable orientation relationships (ORs) between the component phases in a family of directionally solidified eutectic ceramics (DSEC) (NiO–YSZ, CoO–YSZ, NiO–CeO2, NiO–GDC, CoO–CeO2, and CoO–GDC) grown by the laser floating zone method. The ORs predicted by the CRLP model are consistent with those experimentally found in a previous work by means of Electron Backscatter Diffraction (EBSD). In this article, we also present a modification to the model with the aim of taking into account that the most stable ceramic–ceramic interfaces are usually formed between atomic planes with low Miller indices, due to their higher atomic density and bigger interplanar spacing. Thus, we introduce in the calculation of the overall coincidence volume a weighting factor which is a function of the interplanar spacing. This modified CRLP method has been applied to the aforementioned eutectic ceramics, and the results are presented and discussed in comparison with the traditional CRLP results and the experimental findings.

ZrB2–ZrCxN1−x Eutectic Composites Produced by Melt Solidification

Ceramic eutectics are naturally occurring in‐situ composites and can offer superior mechanical properties. Here, ZrB2–ZrCxN1−x quasi‐binary ceramic eutectic composites were produced by arc‐melting a mixture of ZrB2, ZrC, and ZrN powders in an N2 atmosphere. The arc‐melted ZrB2–ZrCxN1−x composites containing 50 mol% of ZrB2 (irrespective of the ZrC/ZrN ratio) showed rod‐like eutectic structures, where ZrCxN1−x single‐crystalline rods were dispersed in the ZrB2 single‐crystalline matrices. Multiple orientation relationships between the ZrCxN1−x rods and the ZrB2 matrices were observed, and one was determined as ZrB2 {} //ZrxN1−x {111} and ZrB2 &lt; &gt; //ZrCxN1−x &lt; &gt; . The rod‐like eutectic composites had higher hardness than the hypo‐ and hypereutectic composites and the 50ZrB2–40ZrC–10ZrN (mol%) eutectic composite showed the highest Vickers hardness (Hv) of 19 GPa.