Digital light processing fabrication of mullite component derived from preceramic precusor using photosensitive hydroxysiloxane as the matrix and alumina nanoparticles as the filler

By taking advantage of the low sintering temperatures of the preceramic polymers, stereolithography printed mullite components derived from preceramic polymer precursor containing alumina particles can be sintered at low temperatures. However, due to their high specific surface, nano alumina particles are difficult to be dispersed into the photocurable polysiloxane. Herein, to prepare mullite slurry, a photosensitive hydroxysiloxane was employed as the preceramic polymer matrix while γ-Al₂O₃ nanoparticle was added as the active filler. The introduction of photocurable hydroxysiloxane not only improved the homogeneity and rheological properties of mullite slurry but also shorted the ionic diffusion distance of Si-ion and Al-ion during the sintering process. Therefore, 3D mullite preceramic precursor stereolithography printed from hydroxysiloxane-Al₂O₃ slurry was endowed with a low sintering temperature around 1400 °C. During the sintering process of preceramic precursor, sintering aid AlF₃ can participate in the reaction and further promote the formulation of mullite grains.     

Effect of MgO sintering additive on mullite structures manufactured by fused deposition modeling (FDM) technology

An optimized recipe for 3D printing of Mullite-based structures was used to investigate the effect of MgO sintering additive on the processing stages and final ceramic properties. To achieve dense 3:2 mullite, ceramic filaments were prepared based on an alumina powder, a methyl silicone resin, EVA elastomeric binder and MgO powder. Using 1 wt% MgO and a dwell time of 5 h at 1600 °C, a dense mullite structure could be obtained from filaments with a diameter of 1.75 mm. Ceramic structures with and without sintering additive were printed in vertical and horizontal direction, to investigate the effect of printing direction on mechanical strength after sintering. Using four-point bending test, it was demonstrated that by using MgO, the printing orientation did not affect the mechanical strength significantly anymore. The low Weibull modulus could be explained by the closed porosity that emerge during the degassing of the preceramic polymer due to cross-linking.     

Additive manufacturing of silicon carbide by selective laser sintering of PA12 powders and polymer infiltration and pyrolysis

In this work, we propose a novel hybrid additive manufacturing technique, which combines selective laser sintering (SLS) of polyamide powders and subsequent preceramic polymer infiltration and pyrolysis to manufacture Silicon Carbide components for complex architectures. By controlling the porosity of the sintered polymeric preform we are able to control the shrinkage upon the first infiltration and pyrolysis. This enabled the manufacturing of smaller features than those achievable with other manufacturing techniques. The mechanical strength of the resulting ceramic increased with the number of reinfiltration cycles up to 24 MPa, inversely the residual porosity decreased to 10 vol%. The microstructure showed two distinct phases of SiOC and SiC. The first was attributed to the interaction between the porous polyamide and the ceramic precursor during the first infiltration. SiC derived from the pyrolysis of the preceramic precursor alone.     

Preparation and characterization of Nextel 720/alumina ceramic matrix composites via an improved prepreg process

In this work, the Nextel 720 continuous fiber reinforced alumina ceramic matrix composites (CMCs) were prepared by an improved prepreg process. The alumina matrix was derived from aqueous slurry, which consisted of organic glue, alumina sol, nanometer alumina powders, and micrometer alumina powders. This design provided a densely packed matrix for the CMC, and made the whole process relatively simple. The ratio of different alumina components in aqueous slurry was optimized to obtain good sintering activity, high thermal resistance, and excellent mechanical properties simultaneously. Furthermore, a preceramic polymer of mullite was used to strengthen the ceramic matrix through a multiple infiltration process. The final CMC sample achieved a high flexural strength of 255 MPa and a good high-temperature stability. After 24 h of heat treatment at 1100°C, 85% of the maximum flexural strength still had been retained.     

聚合物制备工程陶瓷的研究概况

随着聚碳硅烷、聚硅氮烷、聚硅氧烷以及聚硅硼烷等先进前驱体材料的开发,由含硅陶瓷预制体聚合物制备的工程陶瓷在Si-O-C-N-B体系中占有重要的地位。耐高温的SiC和SiN陶瓷纤维增强陶瓷基复合材料(CMC)已在航空、航天结构中获得应用,而耐中、低温的新型涂层、单向带,泡沫和复杂形状的构件在未来将在能源、环境、运输和通讯领域占有重要的地位。综述了陶瓷预制体聚合物的合成、聚合物制备陶瓷的性能、聚合物制备陶瓷的方法以及影响聚合物热解的主要因素。     

Processing and properties of Nextel™ 720 fiber reinforced alumina-zirconia ceramic matrix composites

The continuous Nextel? 720 fiber-reinforced zirconia/alumina ceramic matrix composites (CMCs) were prepared by slurry infiltration process and precursor infiltration pyrolysis (PIP) process. The introduction of submicron zirconia powders into the aqueous slurry was optimized to offer comprehensively good sintering activity, high thermal resistance and good mechanical properties for the CMCs. Meanwhile, the zirconia and alumina preceramic polymers were used to strengthen the porous ceramic matrix through the PIP process. The final CMC sample achieved a high flexural strength of 200 MPa after one infiltration cycle of alumina preceramic polymer and thermal treatment at 1150 °C for 2 h. The flexural strength retention of the improved CMC sample was 104% and 89% respectively after thermal exposure at 1100 °C and 1200 °C for 24 h.     

Powder bed fusion of polyamide powders combined with different preceramic polymers infiltration and pyrolysis to produce complex-shaped ceramics

The fabrication of a wide range of polymer-derived ceramic parts with high geometric complexity through a novel hybrid additive manufacturing technique is presented in this article. The process that we introduced in a previous work uses the powder bed fusion technology to manufacture high porous polymeric preforms to be then converted into ceramics through preceramic polymer infiltration and pyrolysis. The cellular architectures of a rotated cube (strut-based) and a gyroid (sheet-based) with 25 mm diameter, 44 mm height and 67 % of geometric macroporosity were generated and used for the fabrication. The complex structures were 3D printed and polycarbosilane, polycarbosiloxane, polysilazane and furan liquid polymers were used to produce SiC, SiOC, SiCN and glassy carbon, respectively. Despite a linear shrinkage of about 24 %, the parts maintained their designed complex shape without deformations. The significant advantages of the proposed method are the maturity of powder bed fusion for polymers with respect to ceramic additive manufacturing techniques and the possibility to fabricate net-shape complex ceramic parts directly from preceramic precursors.     

Silicon carbide particle reinforced mullite composite foams

Silicon carbide particle reinforced mullite composite foams were produced by the polymer replica method using alumina and kaolin to form in situ mullite matrix. Up to 20 wt.% silicon carbide particles (SiCp) were added to aqueous ceramic slurry to explore its effect on the rheological behaviour of ceramic slurries and also properties of as sintered products. By means of solid loading optimisation and sintering enhancement by silicon carbide, mullite based ceramic composite foams of higher strength were obtained. The strength of the as sintered foams was found to depend greatly on the phase composition, relative density of the structures and the amount of SiCp addition. By studying the effect of the additive concentration, on the mechanical properties of the ceramic matrix, it is found that the optimal silicon carbide addition is 20 wt.%.     

先驱体转化法制备多孔陶瓷的研究现状

多孔陶瓷作为重要的陶瓷材料,广泛应用于冶金、化工等众多领域,其制备工艺的改进一直是研究重点。先驱体转化法是20世纪末提出的制备多孔陶瓷新型工艺,利用陶瓷先驱体高温裂解产生气体的特性,可将其作为粘结剂、骨料、发泡剂制备多孔陶瓷,具有成型工艺简单,烧成温度低等特点,拥有广泛的应用前景。本文主要从以上几个方面简要介绍先驱体转化法制备多孔陶瓷的工艺、结构和性能的研究现状。     

Properties of multiple oxide-bonded porous SiC ceramics prepared by an infiltration technique

Multiple oxide-bonded porous SiC ceramics were fabricated by infiltrating a porous powder compact of SiC and alumina with cordierite sol followed by sintering at 1300-1400°C in air for 3 hours. The microstructures, phase components, mechanical properties, and air permeation behavior of the developed porous ceramics were examined and compared with materials obtained by the traditional powder processing route. The porosity, average pore diameter, and flexural strength of the ceramics varied from 33 to 37 vol%, ~12-14 μm and ~23-39.6 MPa, respectively, with variation in sintering temperature. The X-ray diffraction results reveal that both the amount of cordierite and mullite as the binder increased with increase in sintering temperature. In addition, it was found that the addition of alumina in powder form effectively enhanced the strength due to formation of mullite in the bond phase in contrast to the samples prepared without alumina additive. To determine the suitability of the material in particulate filtration application, particle collection efficiency of the filter material was evaluated theoretically using single collector efficiency model.     

Preparation and indirect selective laser sintering of alumina/PA microspheres

Indirect selective laser sintering (SLS) is a promising additive manufacturing technique to produce ceramic parts with complex shapes in a two-step process. In the first step, the polymer phase in a deposited polymer/alumina composite microsphere layer is locally molten by a scanning laser beam, resulting in local ceramic particle bonding. In the second step, the binder is removed from the green parts by slowly heating and subsequently furnace sintered to increase the density. In this work, polyamide 12 and submicrometer sized alumina were used. Homogeneous spherical composite powders in the form of microspheres were prepared by a novel phase inversion technique. The composite powder showed good flowability and formability. Differential scanning calorimetry (DSC) was used to determine the thermal properties and laser processing window of the composite powder. The effect of the laser beam scanning parameters such as laser power, scan speed and scan spacing on the fabrication of green parts was assessed. Green parts were subsequently debinded and furnace sintered to produce crack-free alumina components. The sintered density of the parts however was limited to only 50% of the theoretical density since the intersphere space formed during microsphere deposition and SLS remained after sintering.     

Fabrication of dense SiSiC ceramics by a hybrid additive manufacturing process

In this work, we report the fabrication of Silicon infiltrated Silicon Carbide (SiSiC) components by a hybrid additive manufacturing process. Selective laser sintering of polyamide powders was used to 3D print a polymeric preform with controlled relative density, which allows manufacturing geometrically complex parts with small features. Preceramic polymer infiltration with a silicon carbide precursor followed by pyrolysis (PIP) was used to convert the preform into an amorphous SiC ceramic, and five PIP cycles were performed to increase the relative density of the part. The final densification was achieved via liquid silicon infiltration (LSI) at 1500°C, obtaining a SiSiC ceramic component without change of size and shape distortion. The crystallization of the previously generated SiC phase, with associated volume change, allowed to fully infiltrate the part leading to an almost fully dense material consisting of β-SiC and Si in the volume fraction of 45% and 55% respectively. The advantage of this approach is the possibility of manufacturing SiSiC ceramics directly from the preceramic precursor, without the need of adding ceramic powder to the infiltrating solution. This can be seen as an alternative AM approach to Binder jetting and direct ink writing for the production of templates to be further processed by silicon infiltration.     

Design and preparation of high permeability porous mullite support for membranes by in-situ reaction

The natural mineral kaolin combined with alumina additives Al(OH)₃,α-Al₂O₃ and AlF₃ was used to prepare porous mullite ceramic membrane supports using an in-situ reaction. The effects of composition and sintering temperature on the sintering behavior, pore structure, permeability and microstructure of the resulting porous mullite supports were extensively investigated. The experimental results showed that excess SiO₂ in kaolin can be consumed by adding alumina precursors, which resulted in a stiff skeleton of interlinked needle-like mullite crystals in-situ during the sintering. The needle-like mullite crystals touched each other and formed a short network, which acted as a porous skeletal network structure. This network resulted in a highly permeable porous structure. The resulting support is suitable for the preparation of asymmetric ceramic membranes. The densification and pore structure of the support can be effectively adjusted by control of the quantity of alumina precursors in the composition and the sintering temperature. Sintering the subject mullite compositions at 1500 °C for two hours resulted in support structures with an average porosity of 45.9%, an average pore size of 1.3 µm and a penetrating porosity of 35.9%.     

Hybridization of additive manufacturing processes to build ceramic/metal parts: Example of HTCC

Stereolithography is an additive manufacturing process, which makes it possible to fabricate useful complex 3D ceramic parts with a high dimensional resolution, a good surface roughness and properties close to those obtained by classical routes. Previous work concerning LTCC components, demonstrates that it is possible, by coupling the stereolithography with robocasting additive processes, to obtain multi-material components (e.g. ceramic / metal components). On the base of this previous work, the manufacturing of HTCC components using this innovative hybrid additive manufacturing process is described. Various complex and innovative geometries of HTCC alumina/tungsten components, in order to improve the characteristics of current circuits, are built and the mechanical and electrical properties characterized. Finally, hyper-frequency parameters of simulated HTCC complex micro strip resonators were compared to measured values on components manufactured by additive manufacturing.     

In situ mullite foam fabrication using microwave energy

In situ mullite ceramic foams were fabricated using polymeric sponge replication method from ceramic slurry containing alpha alumina and kaolin mixtures. Ceramic preforms were processed using microwave energy and conventional heating. The sintered foam samples were characterized by SEM and XRD observations, density measurements and compression tests in order to observe the effect of two different sintering techniques on the structure and properties. It was found that the microwave processing was completed in a shorter burning out and sintering cycle and produced structures having higher mullite transformation ratio and fine grains.     

Novel fabrication route for porous silicon carbide ceramics through the combination of in situ polymerization and reaction bonding techniques

For the first time, an in situ polymerization technique was applied to produce mullite‐bonded porous SiC ceramics via a reaction bonding technique. In this study, SiC microsized particles and alumina nanopowders were successfully coated by polyethylene (PE), which was synthesized from the particle surface in a slurry phase reactor with a Ziegler–Natta catalyst system. The thermal studies of the resulting samples were performed with differential scanning calorimetry and thermogravimetric analysis. The morphology analysis obtained by transmission electron microscopy and scanning electron microscopy (SEM) confirmed that PE was successfully grafted onto the particle surface. Furthermore, the obtained porous ceramics were characterized in terms of their morphologies, phase composition, open porosity, pore size distribution, and mechanical strength. SEM observations and mercury porosimtery analysis revealed that the quality of the dispersion of nanosized alumina powder into the microsized SiC particles was strongly enhanced when the particles were coated by polymers with in situ polymerization. This resulted in a higher strength and porosity of the formed ceramic porous materials with respect to the traditional process. In addition, the X‐ray diffraction results reveal that the amount of mullite as the binder increased significantly for the samples fabricated by this novel method. The effects of the sintering temperature, forming pressure, and polymer content on the physical and mechanical properties of the final porous ceramic were also evaluated in this study. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40425.     

Optimized preceramic polymer for 3D structured ceramics via fused deposition modeling

3D structured ceramics stemmed from preceramic polymers via additive manufacturing have attracted much attention recently. However, these polymers with high ceramic yield are so brittle that extrusion-based additive manufacturing techniques are hardly able to be utilized for assembling 3D structures. Herein, we developed a strategy to prepare feedstocks for these manufacturing techniques, i.e., utilizing a small amount of thermal-plastic polymer to optimize the preceramic polymer while good compatibility is required between the two polymers to ensure a homogeneous mixture. Polycarbosilane and polypropylene were selected as the representative materials. Polypropylene occupied a small proportion (≤5wt.%) and significantly improved the formability of the precursor. Three-dimensional SiC were obtained via fused deposition modeling combined with crosslinking and pyrolysis. The SiC ceramic filaments showed a mean tensile strength of 471 MPa. The strategy is also applicable to a large field of ceramic systems with corresponding precursor, such as sialon ceramic and multicomponent Si-based ceramics.     

Effects of SiC addition on the structure and properties of reticulated porous mullite ceramics

Reticulated porous mullite ceramics (mullite RPCs) were produced by the polymer replica method using alpha alumina and kaolin to form in situ mullite. Up to 30% particulate silicon carbide (SiCp) was added to ceramic mixtures to explore its effect on the structure and properties of the sintered products. The ceramic slurry was coated onto polyurethane sponge using preset roller method, then recoating was applied by the spray coating technique. The strength of the mullite RPCs was found to depend greatly on the phase composition of the structures and on the amount of SiCp addition.     

Fused deposition modeling of mullite structures from a preceramic polymer and γ-alumina

Thermoplastic filaments for Fused Deposition Modeling fabrication of mullite ceramic components were produced from a polymethylsiloxane ceramic precursor, γ-Al₂O₃ powder and ethylene vinyl acetate (EVA) as an organic, elastomeric binder. Two micron-sized γ-Al₂O₃ powders with different particle size distributions were used. Both type of powders could be successfully 3D printed, however γ-Al₂O₃ with finer particle size resulted in better quality of printed parts with much smoother surface.Sintering experiments showed that mullite starts forming already at 1250 °C, but a final sintering of 1550 °C is necessary to achieve pure mullite using the coarse powder, while some residual α-Al₂O₃ was present when using finer powder.Results demostrate that Fused Deposition Modeling can be used to shape preceramic polymers to generate large scale components with a complex shape in which, during firing, the siloxane produces highly reactive silica that allows for the in-situ formation of silicate ceramic phases, such as mullite.     

Digital light processing additive manufacturing of in situ mullite-zirconia composites

Digital light processing (DLP) can produce small series ceramic parts with complex geometries and tiny structures without the high cost of molds usually associated with traditional ceramic processing. However, the availability of feedstock of different ceramics for the technique is still limited. Mullite-zirconia composites are refractory materials with diverse applications, nevertheless, their 3D printing has never been reported. In this work, alumina and zircon were used as raw materials for additive manufacturing by DLP followed by in situ mullite and zirconia formation. Thus, coarse zircon powder was milled to submicrometric size, alumina-zircon photosensitive slurries were prepared and characterized, parts were manufactured in a commercial DLP 3D printer, debound, and sintered at different temperatures. The printed parts sintered at 1600 °C completed the reaction sintering and reached a flexural strength of 84 ± 13 MPa. The process proved capable of producing detailed parts that would be unfeasible by other manufacturing methods.