One-step additive manufacturing and microstructure evolution of melt-grown Al2O3/GdAlO3/ZrO2 eutectic ceramics by laser directed energy deposition

Synchronized-powder-feeding-based laser directed energy deposition (LDED) has great application potential for the rapid fabrication of large-scale composite ceramics with complex shapes. In this study, near-full-density Al₂O₃/GdAlO₃/ZrO₂ ternary eutectic ceramics with different shapes and smooth surfaces were directly prepared by using an improved LDED device. Spherical ceramic powders with eutectic composition and good flowability were obtained by centrifugal spray drying. The microstructure characteristics and microstructure evolution of the rapidly solidified 3D-printed eutectic ceramic were systematically elucidated. In particular, the formation mechanism of the observed periodic banded structures was revealed through a unique laser partial remelting technique. The result indicated that the appearance of the banded structure is attributed to the drastic abnormal coarsening of the nanoscale microstructures adjacent to the molten pool. On the basis these results, a physical model was proposed to illustrate the microstructure evolution of the 3D-printed Al₂O₃/GdAlO₃/ZrO₂ eutectic ceramic.     

Rapid Rate Sintering of Yttria Transparent Ceramics

This paper reports on the influence of rapid rate sintering (RRS) on densification and microstructure evolution of yttria transparent ceramics by using vacuum sintering. The presence of temperature gradient has been confirmed during the RRS process. The higher the heating rate (HR), the larger the temperature gradient in the samples would be. By using RRS, e.g., HR = 40°C/min, the samples could be densified very fast to a relative density of 99.6%. However, these samples could not be further densified, due to the presence of difference in densification caused by a heating rate‐induced temperature gradient. By using a two‐step RRS with an intermediate‐temperature thermal treatment, this problem has been successfully addressed. The intermediate‐temperature treatment allowed for the particle neck growth, so that effective thermal conductivity of the compacts was increased greatly. Therefore, the temperature gradient and differentiate densification were effectively prevented. Samples sintered using the two‐step RRS process could be fully densified and excellent in‐line optical transmittance was achieved. It is believed this strategy is applicable to other transparent ceramics, as well as other engineering ceramics.     

Temperature dependence of mechanical properties of aluminum titanate ceramics

Aluminum titanate (Al₂TiO₅: AT) is a synthetic ceramic material of potential interest for many structural applications. A critical feature, which greatly limits the mechanical properties of polycrystalline AT, is the considerable intergranular microcracking, which occurs due to the large thermal anisotropy of individual grains during cooling after sintering. This study discusses the temperature dependence of mechanical properties, and presents observations on the microstructure morphology. Both the fracture strength and fracture toughness increased considerably with increasing temperature. The critical frontal process zone (FPZ) size was estimated from the mechanical properties. A decrease in FPZ size was observed with increasing temperature in the first thermal treatment. These phenomena were explained on the basis of the stress redistribution and unique microscopic feature on the fracture surface of AT ceramics. The experimental results revealed that further thermal treatment increased the fracture strength and reduced the fracture toughness, while it had almost no effect on the FPZ size.     

Direct additive manufacturing of TiCp reinforced Al2O3-ZrO2 eutectic functionally graded ceramics by laser directed energy deposition

Laser directed energy deposition (LDED) provides an ideal manufacturing technique to produce ceramic matrix composites, which are endows with superior and customizable mechanical properties by the reinforcement of gradient distribution of rigid particles. In this paper, we for the first time manufactured TiCp reinforced Al₂O₃-ZrO₂ eutectic functionally graded ceramics with two different transition modes using LDED. The results show that the gradient transition realizes gradually increase of TiCp particles in Al₂O₃-ZrO₂ eutectic matrix. With the increase of TiCp content, the morphology of Al₂O₃-ZrO₂ eutectic matrix changes from lamellar or rod-shaped to irregular shape. Meanwhile, LDED realizes controllable fabrication of properties in different positions of gradient materials, and the wear resistance of TiCp rich regions has increased by 43.4% compared with pure Al₂O₃-ZrO₂ region.     

Surface Modification of Al2O3/TiC Cutting Ceramics

Cutting ceramics with a graded TiC-rich surface have been obtained by an in situ reaction. Al₂O₃/TiC ceramics, prepared by reaction sintering, have been subjected to various thermal posttreatments at atmospheric pressure. The temperature, dwelling time, and reaction atmosphere have been varied. It has been observed that a TiC-rich surface develops in CO-containing atmospheres. The microstructure of the bulk remains unaffected. Thermal treatment under a CO atmosphere at 1600 ° C for 1 h resulted in a dense TiC layer on the ceramic. A schematic model is proposed for this layer formation process.     

Coupled Thermomechanical Multiscale Modeling of Alumina Ceramics to Predict Thermally Induced Fractures Under Laser Heating

This study is concerned with thermally induced fractures and failure of high weight percentage alumina ceramics. A 3D coupled thermomechanical multiscale model has been developed to simulate thermally induced fractures. In laser heating of alumina ceramics, the temperature and stress distributions have been predominantly correlated with the interfacial glass phase within alumina microstructure. A coupled thermomechanical analysis with traction–separation law has been implemented in the finite element framework as a cohesive zone model (CZM). The alumina grains are modeled as thermomechanical continuum elements separated by CZM. A thermal and mechanical analysis has been conducted using Molecular Dynamics methods to obtain the thermal conductivities and parameterize traction–separation laws for the interface of alumina ceramics at different temperatures. The coupled thermal‐mechanical analysis achieved through a finite element model in Abaqus is compared with experimental results in laser‐heating tests. The model is successful in predicting temperature distributions and thermal fractures, which could help assist in selecting proper conditions in alumina applications and fabrication processes.     

Acoustic emission studies of low thermal expansion aluminum-titanate ceramics strengthened by compounding mullite

Mullite (Mu) with high strength was compounded into aluminum-titanate (AT) ceramics with low thermal expansion to increase their strength. For the AT–Mu system composites, thermal contraction and expansion and acoustic emission (AE) event count rate were measured during cooling using the AE technique and the characteristics of AT–Mu composites were evaluated. The expansion due to microcracking in the range of AE count peak temperatures to room temperature was obtained and the crack volume was estimated from the expansion by cracking. A linear relation with a very high correlation (r = 0.993) was observed between bending strength and crack volume at room temperature. From the linear plot, the bending strength at crack-free temperature in the best AT–Mu composite was shown to be 130 MP.     

Direct Laser Sintering of Al2O3–SiO2 Dental Ceramic Components by Layer-Wise Slurry Deposition

This publication presents a solid freeform fabrication technique for ceramics in the alumina–silica system by layering binder-free, high-loaded ceramic slurries, followed by selective laser sintering. The low melting silica phase and the reaction sintering between silica and alumina favor the rapid prototyping of pure ceramic parts. On the basis of electroacoustic and viscosity measurements, stable slurries from Al₂O₃/SiO₂ powder mixtures and water with a high fluidity have been prepared for the layer deposition with a doctor blade like in tape casting. Layers with a thickness of about 100 μm were processed. It was found in laser parameter studies that ceramic parts can only be obtained using special alumina contents and laser parameters. But the biphasic approach may allow greater flexibility in the processing regime than is afforded by the use of just one material. The microstructure of these parts depends mainly on the temperature gradient induced by the laser absorption and thermal conduction. The wet shaping facilitates laser-sintered parts with a relatively high density, which could be increased by a thermal post-treatment.     

Preparation of porous SiC ceramics skeleton with low-cost and controllable gradient based on liquid crystal display 3D printing

In recent years, the demand for gradient porous ceramics is increasing in engineering field. By traditional process, the disadvantage of prepared gradient porous ceramics is its low porosity and uncontrollable pore gradient, which limits the wide application of gradient porous ceramics. In this study, the gradient porous ceramic skeleton (GPCS) was prepared by combining liquid crystal display (LCD) 3D printer with liquid silicon infiltration (LSI). Experimental results showed that the mass of ceramic powder in the ceramic slurry with optimal printing performance accounts for 45% of the mass of photosensitive resin, and the thermal decomposition rate of photosensitive resin is faster in the range of 300–450 °C. Furthermore, the effect of LSI temperature on the composition, microstructure and mechanical properties of GPCS was investigated. The GPCS is expected to be applied in the fields of energy storage, heat transfer and biofouling, among others.     

Low sintering shrinkage porous mullite ceramics with high strength and low thermal conductivity via foam-gelcasting

Excessive sintering shrinkage leads to severe deformation and cracking, affecting the microstructure and properties of porous ceramics. Therefore, reducing sintering shrinkage and achieving near-net-size forming is one of the effective ways to prepare high-performance porous ceramics. Herein, low-shrinkage porous mullite ceramics were prepared by foam-gelcasting using kyanite as raw material and aluminum fluoride (AlF₃) as additive, through volume expansion from phase transition and gas generated from the reaction. The effects of AlF₃ content on the shrinkage, porosity, compressive strength, and thermal conductivity of mullite-based porous ceramics were investigated. The results showed that with the increase of content, the sintering shrinkage decreased, the porosity increased, and mullite whiskers were produced. Porous mullite ceramics with 30 wt% AlF₃ content exhibited a whisker structure with the lowest shrinkage of 3.5%, porosity of 85.2%, compressive strength of 3.06 ± 0.51 MPa, and thermal conductivity of 0.23 W/(m·K) at room temperature. The temperature difference between the front and back sides of the sample reached 710°C under high temperature fire resistance test. The low sintering shrinkage preparation process effectively reduces the subsequent processing cost, which is significant for the preparation of high-performance porous ceramics.     

Numerical modeling of thermally induced microcracking in porous ceramics: An approach using cohesive elements

A numerical framework is developed to study the hysteresis of elastic properties of porous ceramics as a function of temperature. The developed numerical model is capable of employing experimentally measured crystallographic orientation distribution and coefficient of thermal expansion values. For realistic modeling of the microstructure, Voronoi polygons are used to generate polycrystalline grains. Some grains are considered as voids, to simulate the material porosity. To model intercrystalline cracking, cohesive elements are inserted along grain boundaries. Crack healing (recovery of the initial properties) upon closure is taken into account with special cohesive elements implemented in the commercial code ABAQUS. The numerical model can be used to estimate fracture properties governing the cohesive behavior through inverse analysis procedure. The model is applied to a porous cordierite ceramic. The obtained fracture properties are further used to successfully simulate general non-linear macroscopic stress-strain curves of cordierite, thereby validating the model.     

Rapid fabrication of Si3N4 ceramics with gradient appearance and microstructure

Si₃N₄ ceramics with tailored gradient in color and microstructure were prepared by a rapid cost-effective one-step approach. The gradient microstructure was obtained by the manipulation of the dissolution-reprecipitation process, by controlling the sintering temperature and sintering additive content. In the Si₃N₄ ceramics, the β-phase content gradually changed from 84% to 11%. The Si₃N₄ ceramics exhibited white color on one side and showed a hardness of 19 GPa and fracture toughness of 7 MPa·m^1/2 and may be suitable for bio-implantation applications.     

Multiscale Genome Modeling for Predicting the Thermal Conductivity of Silicon Carbide Ceramics

Silicon carbide (SiC) ceramics have been widely used in industry due to its high thermal conductivity. Understanding the relations between the microstructure and the thermal conductivity of SiC ceramics is critical for improving the efficiency of heat removal in heat sink applications. In this paper, a multiscale model is proposed to predict the thermal conductivity of SiC ceramics by bridging atomistic simulations and continuum model via a materials genome model. Interatomic potentials are developed using ab initio calculations to achieve more accurate molecular dynamics (MD) simulations. Interfacial thermal conductivities with various additive compositions are predicted by nonequilibrium MD simulations. A homogenized materials genome model with the calculated interfacial thermal properties is used in a continuum model to predict the effective thermal conductivity of SiC ceramics. The effects of grain size, additive compositions, and temperature are also studied. The good agreement found between prediction results and experimental measurements validates the capabilities of the proposed multiscale genome model in understanding and improving the thermal transport characteristics of SiC ceramics.     

Direct additive manufacturing of melt growth Al2O3-ZrO2 functionally graded ceramics by laser directed energy deposition

Functionally graded ceramics (FGC), which combine properties of different ceramics in one part, usually have better comprehensive function and structural efficiency. In this study, four different gradient transition Al₂O₃-ZrO₂ FGC samples were prepared by laser directed energy deposition (LDED) method. The results show that there is an obvious interface in direct transition sample. The transition section bears tensile stress caused by difference of thermophysical properties of materials, resulting in significant longitudinal cracks. Element transition in interface region shows a step sharp transition. The direct transition sample shows intergranular fracture and the bonding strength is very low. Gradient transition mode can effectively suppress cracks, and avoid the step transition of microstructure and elements. Elements, microhardness of 25, 20 wt% FGC samples realized a nearly linear smooth transition. The interface fracture of FGC samples changed to transgranular fracture, bonding strength was significantly improved, and the maximum flexural strength reached 160.19 MPa.     

Numerical modeling of spark plasma sintering—Discussion on densification mechanism identification and generated porosity gradients

A multi-physical numerical model of the SPS process has been developed in order to evaluate electrical, thermal and mechanical fields undergone by the powder during sintering. The reliability of this numerical model has been validated by comparison with experimental data carried out on a submicrometric alpha alumina powder. This numerical approach is applied to achieve a critical analysis of the main methodologies used in the literature for the identification of the densification parameters, and the possible misinterpretations of involved deformation mechanisms. Moreover, as shown during sintering of alumina pellets of larger diameter (50 mm in diameter), the induced porosity gradient seems to be mainly correlated to the existence of thermal gradient within the powder, and in a lesser level to stress gradient. Consequently, the temperature gradient seems to be a crucial point to be controlled during SPS process of large size samples in order to obtain fine and homogeneous microstructure.     

Low/intermediate temperature pyrolyzed polysiloxane derived ceramics with increased carbon for electrical applications

This study focuses on the chemistry, thermal stability, and electrical conductivity of low/intermediate pyrolysis temperature (700?900 °C) polysiloxane derived ceramics. These ceramics were modified with additional carbon derived from divinylbenzene (DVB) added to the precursor. Their electrical properties were investigated for potential uses in micro-electrical mechanical systems (MEMS) and anodes for lithium batteries. The microstructure and chemical composition was investigated by attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), Raman spectroscopy, and x-ray photoelectron spectroscopy (XPS); thermogravimetric analysis (TGA) provided insight into the thermal stability; and electrochemical impedance spectroscopy (EIS) into the electrical properties of the material. The increase of pyrolysis temperature and carbon content lead to an enhancement of the electrical conductivity, higher than previously reported values for intermediate pyrolysis temperature SiOC polymer derived ceramics. A limit of the amount of DVB that can be added to PHMS to produce a hybrid precursor has also been obtained.     

Effects of holding time on structure and the properties of spodumene/mullite composite ceramics

In this paper, spodumene/mullite ceramics with good thermal shock resistance were prepared from spodumene, quartz, talc, and clay when the sintering temperature was 1270℃. In the sintering process, the effect of holding time on densification, mechanical properties, phase transformation, microstructure, and thermal shock resistance of the composite ceramics were investigated. The phase transition and microstructures of the ceramics were identified via X-ray diffraction (XRD) and scanning electron microscopy (SEM). The interaction between holding time and bulk density was studied by response surface methodology. The result show that an appropriate holding time can improve the mechanical properties of spodumene/mullite ceramics. When the holding time was kept 90 min, the spodumene/mullite ceramics with the apparent porosity was .47%, the bulk density was 2.28 g/cm³, and bending strength was 63.46 MPa. Furthermore, since no cracks formed after 20 thermal shock cycles for the composite ceramics with a bending strength decreasing rate of 12.66%, it is revealed that spodumene/mullite ceramics exhibit good thermal shock resistance. Therefore, this study can provide beneficial guidance for both industrial production and energy conservation.     

Direct laser additive manufacturing of high performance oxide ceramics: A state-of-the-art review

The implementation of additive manufacturing for ceramics is more challenging than for other material classes, since most of the shaping methods require polymer binder. Laser additive manufacturing (LAM) could offer a new binder-free consolidation route, since it is capable of processing ceramics in a direct manner without post-processing. However, laser processing of ceramics, especially high performance oxide ceramics, is limited by low thermal shock resistance, weak densification and low light absorptance at room temperature; particularly in the visible or near-infrared range. An extensive review focusing only on LAM (powder bed fusion – laser beam and directed energy deposition) of high performance oxide ceramics is currently lacking. This state-of-the-art review gives a detailed summary and critical analysis about process technologies, part properties, open challenges and process monitoring in the field of oxide ceramics. Improvements in accuracy and mechanical strength are proposed that could open LAM of oxide ceramics to new fields.     

From the research state of the thermal properties of graphene reinforced ceramics to the future of computer simulation

The poor thermal properties of ceramics limited its wide application. Recently, upgrading via reinforcing ceramics with graphene has been recognized as a feasible approach to enhance the thermal performance of ceramics, and then advance the applications due to the exceptional thermal properties of graphene. Thus, this review summarized the latest advances in manufacturing process, microstructure as well as the thermal properties of graphene reinforced ceramics (GRC), highlighting the impacts of sintering techniques, sintering parameters and graphene/ceramic matrix parameters on the thermal performance of GRC. Furthermore, microstructure model simulation was proposed as a supplement to the traditional trial and error method, so as to facilitate the research associated with GRC. Besides, the challenges coupled with outlook for GRC with the superior thermal properties were also envisioned to providing readers with deep understanding of future development of GRC.     

Porous Silicon Nitride Ceramics Prepared by Reduction–Nitridation of Silica

A new method for preparing porous silicon nitride ceramics with high porosity had been developed by carbothermal reduction of die-pressed green bodies composed of silicon dioxide, carbon, sintering additives, and seeds. The resultant porous silicon nitride ceramics showed fine microstructure and uniform pore structure. The influence of SiO₂ particle size and sintering process (sintering temperature and retaining time) on the microstructure of sintering bodies was analyzed. X-ray diffractometry demonstrated the formation of single-phase β-Si₃N₄ via the reaction between silicon dioxide and carbon at high temperature. SEM analysis showed that pores were formed by the banding up of rod-like β-Si₃N₄ grains. Porous Si₃N₄ ceramics with a porosity of 70–75%, and a strength of 5–8 MPa, were obtained.