Synchrotron X-ray tomographic characterization of CVI engineered 2D-woven and 3D-braided SiCf/SiC composites

The properties of ceramic matrix composites strongly depend upon their complex internal structures. To better understand and improve the properties of the silicon carbide fiber-reinforced silicon carbide matrix composites (SiC_f/SiC), we explored the microstructural properties of composites reinforced with either two-dimensional (2D) woven or three-dimensional (3D) braided preforms using synchrotron X-ray computed microtomography. Transects and volumetric images of the composites were reconstructed from objection images and the microstructures were investigated in three spatial directions. The network of void space in a composites was visualized in 3D and quantitative analysis of the porosity was performed to characterize the fiber-tissue structures. 2D-woven SiC_f/SiC composite exhibited important fluctuations of porosity in different directions and the stacking of plies had a significant effect on the porosity distribution. In contrast, 3D-braided SiC_f/SiC composites showed much less variation of porosity. We found the degree of densification of the composite also influenced the porosity distribution.     

Correlations among sintering temperature,shrinkage, and open porosity of 3.5 YSZ/Al2O3 composites

Slip-cast ceramic samples of the system (100−x) (ZrO₂–3.5 mol% Y₂O₃)–xAl₂O₃ (abridged as (100−x) 3.5 YSZ/xAl₂O₃ composite, where x is expressed in wt%) were examined using dilatometry, isothermal sintering and electron microscopy methods. The shrinkage in the range 1100–1300 °C was found to be higher for the (100−x) 3.5 YSZ/xAl₂O₃ samples with prevailing fraction of PSZ than for the composites with a corundum matrix. When the weight fraction of corundum was increased, the relative shrinkage of the (100−x) 3.5 YSZ/xAl₂O₃ samples decreased and the open porosity of the ceramic materials grew. The effect of <gamma>-Al₂O₃ impurity on the sintering process and linear dimensions of ceramics is shown. Heat treatment of (50–40) 3.5 YSZ/(50–60) Al₂O₃ composites at 1300 °C are proposed as the optimum conditions for porous diaphragm formation.     

Synthesis of high surface area Al2O3-CeO2 composite nanopowder via inverse co-precipitation method

In the present work, Al₂O₃-CeO₂ composite nanopowder was synthesized by inverse co-precipitation method using metal chlorides, aluminum powder and NH₄OH as precipitant agent. The thermal decomposition of the precipitate and subsequent formation of Al₂O₃-CeO₂ were investigated by X-ray diffractometery, scanning electron microscopy, thermogravimetric and differential thermal analysis, Brunauer-Emmett-Teller surface area measurement and Fourier transform infrared spectroscopy. The results showed that the presence of ceria suppressed the formation of α-Al₂O₃. The BET-specific surface area was 173 m²/g for powders calcined at 800 °C. The particle size examined by using scanning electron microscopy was in the range 30-70 nm. The activation energy of Al₂O₃-15 wt.% CeO₂ nanocrystallite growth during calcination was measured to be 32.4 kJ/mol whereas that of Al₂O₃ was about 23.8 kJ/mol.     

3D printing boehmite gel foams into lightweight porous ceramics with hierarchical pore structure

We herein report a novel hierarchically porous ceramic foams derived from boehmite gel foams, which possess both high porosity and superior strength. The gel foams show excellent printability due to its predominant stability, high yield stress and storage modulus, which endows such foam material ideal ink for 3D printing lightweight and complex-shape materials via direct ink writing approach. The 3D printed ceramic foams possess programmable architecture assembled by porous filaments, uniform macro-pores with tunable size in the range of 4∼70 μm, as well as nanoscale pores in cell wall, after sintering at relatively low temperature of 1200–1300 °C. In this way, ceramic foams with high strength were achieved, attributed to the tiny grains, large amount of grain boundaries, uniform pores and hierarchical pore structure. Notably, the foams sintered below 1200 °C have significant advantage on specific surface area, which could reach up to 300-400 m²/g.     

High-performance B4C-LaB6 composite ceramics fabricated via hot-pressing sintering with La2O3 as sintering additive

The B₄C-LaB₆ composite ceramics were fabricated via hot-pressing sintering at 2050 °C and 20 MPa pressure with the mixture of boron carbide (B₄C) and 2–5 wt% lanthanum oxides (La₂O₃) as raw materials. The effects of additive La₂O₃ content on the microstructures and mechanical properties of composite ceramics were investigated, and reaction mechanisms of La₂O₃ and B₄C at different temperatures were studied in detail. La₂CO₅, La₃BO₆ and LaBO₃ were formed by the reactions of La₂O₃ and B₄C at different temperatures, and finally LaB₆ was formed below 1600 °C. The comprehensive mechanical properties of B₄C-LaB₆ composite ceramics were optimized by adding 4 wt% La₂O₃, the flexural strength, fracture toughness and Vickers hardness reached 350 MPa,4.92 MP am^1/2 and 39.08 GPa, respectively. The high densification and flexural strength of composite ceramics achieved in the present study were attributed to LaB₆ hindering the movement of grain boundary. However, the densification was reduced caused by CO as La₂O₃ content increased to 5 wt%. The fast channel was formed via B₄C reacting with La₂O₃, which accelerated migration of B₄C in the sintering process. The content of La₂O₃ played an important role in the fracture mode of the composite ceramics, and ultimately affected the fracture toughness of the composite ceramics.     

Microstructure and mechanical properties of cold sintered porous alumina ceramics

New innovative approach to fabricate porous alumina ceramics by cold sintering process (CSP) is presented using NaCl as pore forming agent. The effects of CSP and post-annealing temperature on the microstructure and mechanical strength were investigated. Al₂O₃–NaCl composite with bulk density of 2.92 g/cm³ was compacted firstly using CSP and then a porous structure was formed using post-annealing at 1200°C–1500°C for 30 min. Brazilian test method and Vickers hardness test were used to determine the indirect tensile strength and hardness of the porous alumina, respectively. Meanwhile, the phases and the microstructure were respectively examined using X-ray diffractometer and scanning electron microscope (SEM) complemented by the 3D image analysis with X-ray tomography (XRT). SEM structural and XRT image analysis of cold sintered composite showed a dense structure with NaCl precipitated between Al₂O₃ particles. The NaCl volatization from the composite was observed during the annealing and then complete porous Al₂O₃ structure was formed. The porosity decreased from 48 vol% to 28 vol% with the annealing temperature increased from 1200 °C to 1500 °C, while hardness and mechanical strength increased from 14.3 to 115.4 HV and 18.29–132.82 MPa respectively. The BET analysis also showed a complex pore structure of micropores, mesopores and macropores with broad pore size distribution.     

Microstructure and hardness scaling in laser-processed B4C-TiB2 eutectic ceramics

Surface layers of the pseudo-binary eutectic comprised of boron carbide (B₄C) and titanium diboride (TiB₂) were directionally solidified via direct laser irradiation in an argon atmosphere. The resulting surface eutectic layers had highly oriented lamellar microstructures, whose scale (i.e. interlamellar spacing) was controlled directly by the laser scan rate, following an inverse square root dependence for lower solidification velocities. Higher velocities (>∼4.2 mm/s) departed from this relationship, although well-ordered microstructures were still achieved. A concomitant increase in the Vickers hardness with decreasing interlamellar spacing was observed, although the trend did not correspond to traditional Hall-Petch behavior. The hardness of the eutectic composites became load-independent at indenter loads greater than 9.81 N, indicating a potential transition from plastic to fractural deformation during indentation. A Vickers hardness of 32 GPa was achieved in the highest solidification velocity samples (42 mm/s) which had interlamellar spacings of 180 nm.     

In situ fabrication of highly-dense Al2O3/YAG nanoeutectic composite ceramics by a modified laser surface processing

Alumina-matrix eutectic in situ composite ceramics present excellent high-temperature mechanical properties, which have been considered as promising next-generation ultra-high temperature structural materials. A modified laser surface processing is developed to in situ fabricate highly-dense Al₂O₃/YAG bulk nanoeutectic ceramics with large size and homogeneous three-dimensional network of nanoeutectic microstructure by introducing two-side remelting and high-temperature preheating. The crack and porosity are avoided, and the eutectic structure achieves a good continuous growth between two solidified layers. The eutectic phases show sharp interface bonding with a defined orientation relationship. The dislocations and crack deflection at high-density phase interfaces importantly contribute to the enhanced fracture toughness.     

Transmittance enhancement of spark plasma sintered CaF2 ceramics by preheating commercial powder

Spark plasma sintering (SPS) is a convenient approach for preparing transparent CaF₂ ceramics. However, carbon contamination is a key issue that should be addressed to achieve high transparency. In this study, a commercially available CaF₂ powder was preheated under vacuum before performing SPS to mitigate carbon contamination. During the preheating of the CaF₂ powder, impurities adsorbed on the particle surface, such as H₂O, CO₂, and O₂, are desorbed. Moreover, the interdiffusion of carbon contaminants is suppressed due to the pre-sintering of the raw powder. The in-line transmittance of the CaF₂ ceramic prepared from the preheated powder increased to 85 % at the wavelength of 1100 nm, which is 38 % higher than that of the ceramic prepared without preheating. In addition, the in-line transmittance increased with increasing grain size of the ceramic, possibly because of the decrease in the number of scattering sources with the reduction in the grain boundary fraction.     

Fabrication,microstructure and high-temperature plastic deformation of three-phase Al2O3/Er3Al5O12/ZrO2 sintered ceramics

The fabrication, microstructure and high-temperature creep behavior of chemically compatible, three-phase alumina/erbium aluminum garnet (Er₃Al₅O₁₂, EAG)/erbia fully-stabilized cubic ZrO₂ (ESZ) particulate composites with the ternary eutectic composition is investigated. The composites were fabricated by a solid-state reaction route of α-Al₂O₃, Er₂O₃ and monoclinic ZrO₂ powders. The final phases α-Al₂O₃, EAG and ESZ were obtained after calcination of the powder mixtures at 1400 °C. High dense bulk composites were obtained after sintering at 1500 °C in air for 10 h, with a homogeneous microstructure formed by fine and equiaxed grains of the three phases with average sizes of 1 μm. The composites were tested in compression at temperatures between 1250 and 1450 °C in air at constant load and at constant strain rate. As the temperature increases, a gradual brittle-to-ductile transition was found. Extended steady states of deformation were attained without signs of creep damage in the ductile region, characterized by a stress exponent of nearly 2 and by the lack of dislocation activity and modifications in grain size and shape. The main deformation mechanism in steady state is grain boundary sliding, as found in superplastic metals and ceramics. In the semibrittle region, microcavities developed along grain boundaries; these flaws, however, did not grow and coalescence into macrocracks, resulting in a flaw-tolerant material. Alumina is the creep-controlling phase in the composite because of the grain boundary strengthening caused by the (unavoidable) Er³⁺- and Zr⁴⁺-doping provided by the other two phases.     

Lattice misorientation at domain boundaries in β-Ga2O3 single-crystal substrates observed via synchrotron radiation X-ray diffraction imaging and X-ray reticulography

To evaluate the lattice misorientation at domain boundaries (DBs) in β-Ga₂O₃, we performed X-ray diffraction imaging (XRDI), X-ray reticulography (XRR), and X-ray topography (XRT) using a synchrotron radiation light source. Four reciprocal lattice vectors ( g -vectors) were applied, and the DBs showed different visibilities in the XRDI maps depending on the g -vector. By analyzing possible characteristics of the misorientation, the XRDI results suggested that the DB being investigated was associated with a misorientation on the ($\overline{10}05$) plane and contained twist and tilt components. The apparent peak change in XRDI caused by the two components was calculated. We further succeeded in separating the tilt and twist components using XRR images in conjunction with simulation. Dislocation arrays at the DBs were observed using XRT, and the average distance between the dislocations in the array was consistent with the misorientation obtained using XRDI and XRR. The distribution of DBs across a wide area was acquired by a combination of XRR images recorded on a charge-coupled device camera and X-ray films. The fringe-patterned XRR on X-ray films provided a powerful and nondestructive tool to characterize DBs distributed across a large-diameter wafer with an angular resolution on the order of several arc sec (low 10⁻⁵ rad).     

Three-dimensional characterization of the pore structures in SiC formed by binder jet 3D printing,polymer infiltration and pyrolysis (PIP)

Processing of binder jet printed refractory ceramics, including SiC, to high density remains a major challenge in additive manufacturing. Polymer Infiltration and Pyrolysis (PIP) has been applied to SiC made with large particle sizes and low sinterability, but the PIPed material struggles to reach high density even after many infiltration cycles. In this work, binder jetted α-SiC powders were PIPed up to 8 cycles and characterized after each cycle. By comparison with an exclude volume model, the infiltrated density showed a plateauing after 8 cycles. X-ray micro-computer tomography (μCT) was used to characterize the microstructure evolution in 3D. The reconstructed cross-sectional image indicated that large cracks, attributed to gas pressure build-up in burn-out, were formed as the number of infiltration cycles increased. Additionally, quantitative 3D data extracted from μCT images showed a large pore network existed in the interior of all samples and remained mostly open, even after 8 PIP cycles.     

Crack healing behaviour of Cr2AlC MAX phase studied by X-ray tomography

The autonomous crack-healing capability of Cr₂AlC MAX phase ceramic by surface oxidation at elevated temperatures has a huge potential for high temperature structural and protective coating applications. In this work we use time-lapse X-ray computed tomography (CT) to track the fine details of local crack filling phenomena in 3 dimensions (3D) with time. The maximum crack width that could be fully healed upon exposure to 1200 °C in air is 4.8 μm in 4 h and 10 μm after 12 h. Furthermore, during healing Cr₇C₃ phase is observed beneath the dense Al₂O₃ layer (average thickness of 1 μm on each crack surface) when the crack width exceeds 2 μm. The 3D image sequences indicated that the rate of healing is essentially independent of position along, or across, the crack faces. The crack healing kinetics of Cr₂AlC at 1200 °C broadly follows a parabolic rate law with a rate constant of 4.6 × 10⁻⁴ μm² s⁻¹. The microstructure, composition and thickness of the oxide scale in the healed crack area are characterized via post mortem SEM-EDS measurements and confirm the formation of an initial dense alumina layer on top of which a more porous layer forms. Impurity Cr particles appear to accelerate the oxidation process locally and correlative SEM imaging of the same region suggests this is by providing Cr₂O₃ nucleation sites.     

Micromechanical modeling of thermo-mechanical properties of high volume fraction particle-reinforced refractory composites using 3D Finite Element analysis

Previously, we have developed several particle-reinforced castable ceramic composites for refractory applications with exposure to thermal shock and measured their effective thermo-elastic properties experimentally. These composites contained silicon-carbide (SiC) solid particles, zirconia (ZrO₂) bubbles, and ZrO₂ solid particles, dispersed in an alumina (Al₂O₃) matrix. The present work aims to implement representative volume element (RVE) approach and periodic boundary condition (PBC) to accurately predict those properties, namely elastic and shear modulus, thermal conductivity, and coefficient of thermal expansion (CTE), using three-dimensional (3D) Finite Element (FE) simulations while accounting for the effect of porosity. In comparison to established micromechanical schemes and two-dimensional (2D) FE predictions, 3D FE simulations specifically show more accuracy in prediction of elastic properties and thermal conductivity. This novel and thorough comparison across various thermo-mechanical properties for complex microstructures (with up to three types of inclusions) can be valuable for designing comparable high volume fraction (VF) composites.     

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.     

Preparation of AlOOH/Al2O3 porous ceramics having CO2/N2 gas selectivity of less than 1

Porous Al₂O₃ bodies, having an average pore size of approximately 74 nm and a porosity of 40.4%, were prepared by the slip-cast method. The intended porous ceramics were prepared by hydrothermal treatment of the porous Al₂O₃ bodies. The obtained samples consisted of AlOOH and α-Al₂O₃ phases, and the α-Al₂O₃ surfaces were coated with AlOOH particles. The CO₂/N₂ gas selectivities of the samples depended on the conditions of the hydrothermal treatment and were found to decrease with increasing hydrothermal treatment time. However, the gas selectivities of the samples heated for 96 h were nearly equivalent to those of the samples heated for 72 h. A high concentration of Al(NO₃)₃, which was used as the aluminum source, leads to a decreased CO₂/N₂ gas selectivity. Moreover, CO₂/N₂ gas selectivity decreased slightly at low pressure drop. The minimum selectivity attained was approximately 0.69 at 0.02 MPa.     

Highly porous ZrO2 ceramics fabricated by a camphene-based freeze-casting route: Microstructure and properties

A camphene-based freeze-casting method was adopted to create ceramics with aligned, equiaxed pores applied so far exclusively for ceramics—is demonstrated for ZrO₂ porous ceramics. The pore volume fraction, channel size and pore shape were controlled by varying the freezing temperature, solid content and sintering condition. After sublimation of camphene, the samples were sintered for 2 h at elevated temperatures ranging from 1400 to 1550 °C. The initial level of solid loading played a primary role in the resulting porosity of the product. The porosity decreased from 82.5 to 65.5 vol.% when the solid loading was increased from 10 to 20 vol.%. The relationship of the compressive strength versus initial solid loading and sintering temperature was discussed. This technique is considered potentially useful in fabricating novel porous ceramics with special structure, and introduces a new application field of freeze-casting.     

Thermal properties of La2O3-doped ZrB2- and HfB2-based ultra-high temperature ceramics

Thermal properties of La₂O₃-doped ZrB₂- and HfB₂-based ultra high temperature ceramics (UHTCs) have been measured at temperatures from room temperature to 2000 °C and compared with SiC-doped ZrB₂- and HfB₂-based UHTCs and monolithic ZrB₂ and HfB₂. Thermal conductivities of La₂O₃-doped UHTCs remain constant around 55–60 W/mK from 1500 °C to 1900 °C while SiC-doped UHTCs showed a trend to decreasing values over this range.     

Microstructure and properties of nano-laminated Y3Si2C2 ceramics fabricated via in situ reaction by spark plasma sintering

A nano-laminated Y3Si2C2 ceramic material was successfully synthesized via an in situ reaction between YH2 and SiC using spark plasma sintering technology.A MAX...     

Microstructure evolution during reactive sintering of Y3Al5O12:Nd3+ transparent ceramics: Influence of green body annealing

The effect of green bodies’ mesostructure on the porosity, optical properties and laser performance of reactive sintered Y₃Al₅O₁₂:Nd³⁺ transparent ceramics was studied. Only minor changes in microstructure were revealed for green bodies without annealing and those annealed at 600, 800, 1000 °C, while average pore size increases to 140 nm for sample annealed at 1200 °C. Y₃Al₅O₁₂:Nd³⁺ ceramics sintered at 1750 °C for 10 hours possess significant differences in the final porosity, optical and laser characteristics. Despite all green bodies exhibit a similar phase evolution and sintering behavior on heating, the differences appear in the final stage, when the latest percentage of porosity is removed. The green bodies annealed at 600 °C have an optimal mesostructure from the standpoint of uniform densification. Y₃Al₅O₁₂:Nd³⁺ ceramics prepared using these green bodies exhibit porosity ≤0.001 vol% and yield efficient laser emission at 1.06 μm with slope efficiency as high as 67% in quasi-continuous pumping at 807 nm.