Additive manufacturing of electrofused mullite slurry by digital light processing

Mullite, one of the main refractory materials, has several applications that may demand tiny structures with complex geometries, and digital light processing (DLP) can produce such parts with outstanding dimensional precision and surface quality. In this work, electrofused mullite powder was used as a raw material for additive manufacturing by DLP. Photosensitive mullite suspensions were developed and their rheological behavior, stability, and thermal decomposition were investigated. Mullite parts were printed from suspensions with different ceramic loadings, debound, and sintered at different temperatures (from 1500 to 1650 °C). Density and strength increased with an increase in both solid loading and sintering temperature. Printed parts from slurry with 50 vol% of solid loading sintered at 1650 °C reached a relative density of 97.7 ± 0.3 % and flexural strength of 95.2 ± 5.0 MPa.     

Digital light processing of complex-shaped 3D-zircon (ZrSiO4) ceramic components from a photocurable polysiloxane/ZrO2 slurry

Explorations in the stereolithography fabricated polymer-derived ceramics are still far from maturity. Herein, a novel preceramic slurry, which consists of a photocurable epoxy-acrylic siloxane and particle-size gradated ZrO₂ fillers, was digital light processing shaped into 3D preceramic precursors. By taking advantage of the high reactivity of in-situ formed silica from the polysiloxane, 3D-zircon product can be synthesized through the sintering reaction between the polysiloxane and ZrO₂ particles in the precursor at a low temperature. During the sintering process, ZrSiO₄ phase starts to appear at the temperature of 1200 °C. A proper particle-size distribution of the ZrO₂ filler, 20 wt% of micropartciles and 4 wt% of nanoparticles, not only endowed the ceramic slurry with a low viscosity but also increase the purity of the zircon products. Besides, the addition of sintering aid NaF can promote the sintering reaction between the polysiloxane and ZrO₂ particles while increase the crystalize degree of the 3D-zircon products.     

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.     

Digital light processing of wollastonite-diopside glass-ceramic complex structures

In this work, the possibility of shaping a glass-filled photosensitive polymer resin with Digital Light Processing (DLP) into a complex 3D structure and transforming it subsequently into a bioactive glass-ceramic scaffold was investigated. The influence of the printing conditions and the heat-treatment was studied using a 41?vol% glass-filled acrylated polymer resin. Scaffolds with designed architecture were turned into a wollastonite-diopside glass-ceramic at 1100?°C. They completely maintained their shape, exhibited no viscous flow and showed a homogenous linear shrinkage of around 25%. At 83?vol% porosity structures with Kelvin cell design exhibited a compressive strength exceeding 3?MPa, demonstrating that the material is suitable for the fabrication of bioceramic scaffolds for bone tissue engineering applications.     

Digital light processing of ceramic components from polysiloxanes

Additive manufacturing using photocurable polymers is one method to answer the increased demand of ceramic structures with complicated morphology by fabricating ceramic parts with high resolution and good surface quality. We introduce here a new method to fabricate SiOC ceramic structures by utilizing a simple physical blend between two different preceramic polysiloxanes, one providing photosensitive acrylate groups while the other one a high ceramic yield. Different blend ratios have been realized and respectively optimized concerning the printing additives and setting times to fabricate exact replications of highly complex polysiloxane structures by Digital Light Processing. After pyrolysis, a uniform, homogenous shrinkage was observed yielding dense, pore- as well as crack-free SiOC ceramics. By adjusting the ratio between the different polysiloxanes, parameters such as the ceramic yield, shrinkage, chemical composition and resolution after pyrolysis could be tailored in a wide range of values.     

Polymer-derived silicon nitride ceramics by digital light processing based additive manufacturing

In this work, silicon nitride ceramic components with simple cuboid and complex honeycomb and lattice structures from preceramic polymers were fabricated by using digital light processing (DLP) based additive manufacturing and pyrolysis. The photosensitive precursor for DLP based additive manufacturing was prepared by mixing high ceramic yield polysilazane with commercial acrylic resin and photoinitiator. The material formulation and the structure of the green body were characterized by using FTIR. Si₃N₄ ceramic cuboid, 2D-structured Si₃N₄ ceramic honeycomb, and 2D-structured Si₃N₄ ceramic lattice with high precision were fabricated. The DLP-prepared specimens were pyrolyzed at different temperatures, and the crystalline phases after pyrolysis were analyzed by using XRD. The optimal pyrolysis temperature was found to be 1400°C.microstructures were characterized by using SEM. The compressive behavior of the complex-shaped Si₃N₄ ceramic structures was measured and discussed in detail.     

Digital light processing stereolithography of zirconia ceramics: Slurry elaboration and orientation-reliant mechanical properties

Digital Light Processing (DLP) is a promising technique for the preparation of ceramic parts with complex shapes and high accuracy. In this study, 3 mol% yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) UV-curable slurries were prepared and printed via DLP. Two different solid loadings (40.5 and 43.6 vol%, respectively) and printing directions were investigated to assess the influence of these parameters on physical and mechanical properties of the sintered parts. Zirconia samples were sintered at 1550 °C for 1 h, achieving a very high relative density (99.2%TD), regardless of solid loading and printing direction. FE-SEM micrographs shown a homogeneous and defect-free cross section with an average grains size of 0.56 ± 0.19 µm. Finally, mechanical properties were influenced by printing direction and zirconia vol%. Indeed, the composition with the higher solid loading (i.e. 43.6 vol%) had the highest three-point flexural strength (751 ± 83 MPa) when tested perpendicular to the printing plane.     

Sintering of lunar regolith structures fabricated via digital light processing

Architectural and functional structures composed of lunar regolith-simulant CLRS-2 were fabricated via digital light processing and sintered at 1100 °C and 1150 °C under an air or argon atmosphere. This work is to investigate effects of atmosphere and temperature on mechanical properties, microstructure, and chemical composition of lunar regolith products. Samples sintered at 1150 °C in air underwent the highest sintering shrinkage and showed the best mechanical properties, likely due to the formation of glassy phase and dense structure following sintering. Conversely, argon-sintered samples exhibited lower density resulting from the lack of glassy phase. Phase analysis revealed varying chemical composition and therefore different underlying reaction mechanisms under two sintering atmospheres, indicating that sintering atmosphere significantly influences the microstructure and macroscopic properties of lunar regolith products.     

Digital light processing of Si-based composite ceramics and bulk silica ceramics from a high solid loading polysiloxane/SiO2 slurry

Nowadays, 3D polymer-derived silicon oxycarbide ceramics (SiOC) can be fabricated by the stereolithography method successfully. However, due to their intrinsically poor ceramic content and the large thermal shrinkage during the pyrolysis, it is difficult for the bulk 3D polysiloxane precursors to be pyrolyzed into dense 3D-ceramics. Herein, the ceramic content of the photocurable polysiloxane precursors was increased by adding a large amount of SiO₂ powders into the low solid content epoxy-acrylic siloxane. After being added with proper dispersant, the viscosity of the high solid loading polysiloxane/SiO₂ slurry can reach a proper level. Bulk 3D-silica ceramics with the wall-thickness around 8 mm can be conveniently fabricated from the polysiloxane/SiO₂ slurry by stereolithography and a two-step sintering process. The addition of a proper sintering aid NaF can promote the 3D-PSO/SiO₂ precursor to be converted into dense and crack-free 3D-silica ceramics with good mechanical proprieties.     

Vat photopolymerization-based additive manufacturing of high-strength RB-SiC ceramics by introducing quasi-spherical diamond

Silicon carbide is one of the most important high-performance engineering ceramics. However, SiC ceramics with complex structure and high mechanical performance are difficult to shape, sinter, and process. Additive manufacturing is expected to solve the above problems, but the photosensitive slurry with low solid content leads to high residual Si content and low strength of final components. Here, we presented one novel strategy to prepare high-strength SiC components with complex structure by introducing quasi-spherical diamond powder as the high-density carbon source through vat photopolymerization 3D printing technology and reactive melt infiltration process. The final RB–SiC ceramics exhibited a specific flexural strength of 312.45 ± 18.75 MPa and elastic modulus of 359.16 ± 4.57 GPa, demonstrating one of the highest flexural strength and elastic modules among those reported for 3D-printed SiC composites. Owing to the high mechanical performance and simple fabrication process, this strategy has significant advantages in the manufacturing of structural SiC ceramics.     

Impact of processing conditions and sizing on the thermomechanical and morphological properties of polypropylene/carbon fiber composites fabricated by material extrusion additive manufacturing

For the 3D printed composites, fiber alignment is affected by the direction of melt-flow during extrusion of filaments and subsequently through the printing nozzle. The resulting fibers orientation and the fiber-matrix compatibility have a direct correlation with mechanical properties. This study investigates the impact of processing conditions on the state of the carbon fiber types and their orientation on the mechanical properties of 3D-printed composites. Short and long carbon fibers were used as starting reinforcing materials, and the state of fibers at the beginning and on the printed parts were evaluated. Strong anisotropy in terms of mechanical properties (flexural and impact properties) was observed for the samples printed with different printing orientations. Interestingly, the number of voids in the printed composites was found to be correlated with the fiber types. The present work provides a step towards the optimization of tailored composite properties by additive manufacturing.     

Three-dimensional printing of high-flux ceramic membranes with an asymmetric structure via digital light processing

In this study, a novel method was proposed for preparing high-flux ceramic membranes via digital light processing (DLP) three-dimensional (3D) printing technology. Two different alumina powders were well dispersed in a photosensitive resin to form a UV-curable slurry for DLP 3D printing. The effects of the grading ratio on the viscosity of the slurry and the porosity, pore size distribution, mechanical strength, roughness, and permeability of the ceramic membranes were systematically investigated. The thermal treatment conditions were also studied and optimized. The obtained ceramic membranes exhibited a uniform pore size distribution, a high porosity, a low tortuosity factor, and an asymmetric structure. The combination of these factors led to a high flux for the 3D-printed ceramic membranes. DLP 3D printing exhibited a good potential to be a strong candidate for the next generation of ceramic membrane fabrication technology.     

Fabrication of ZrO2 and ATZ materials via UV-LCM-DLP additive manufacturing technology

The development of photosensitive slurries for additive manufacturing is attracting high interest due to their correlation with the final properties of the fabricated parts. Lithography-based ceramics manufacturing (LCM), i.e. digital light processing (DLP), is one of the techniques that is receiving high attention. Within the different ceramic materials, ZrO₂ and ATZ are the most studied due to their potential applications including in the dental field. In this work, the fabrication of ZrO₂ and ATZ materials via LCM-DLP is shown. A comparison between one- and two-step procedures to develop UV-curable ZrO₂ and Al₂O₃-ZrO₂ slurries was performed. The printed parts with a relative density of ∼99% for both ZrO₂ and ATZ were subjected to 4-point bending measurements, obtaining flexural strength values around 800 MPa for both ZrO₂ and ATZ materials prepared by using the one-step slurry.     

Digital light processing-stereolithography three-dimensional printing of yttria-stabilized zirconia

Digital light processing (DLP)-stereolithography three-dimensional (3D) printing is a well known technique for fabricating components with complex geometries. However, the application of DLP 3D printing to functional ceramics such as 8 mol% yttria-stabilized zirconia (8YSZ), which is one of the most extensively used electrolyte materials for solid oxide fuel cells, is still a great challenge. Therefore, the fabrication of fully 8YSZ monoliths via DLP 3D printing was attempted herein, including the preparation of UV-curable ceramic suspensions, shaping of green bodies, and debinding and sintering. The results show that intact green bodies printed using a 30 vol% 8YSZ-photosensitive resin suspension with 0.1 wt% oleic acid as the dispersant under the optimized printing conditions was sufficiently dense without connected pores after vacuum debinding and sintering in air. The successful fabrication of 8YSZ monoliths with design flexibility via 3D printing provides a simple method for preparing functional ceramic components and may expand the application of 3D printing technology to the energy field.     

Complex mullite structures fabricated via digital light processing of a preceramic polysiloxane with active alumina fillers

By taking advantage of the multi-functional properties of preceramic polymers, their transformation into ceramic material at low sintering temperatures and the processing capabilities of polymer manufacturing processes, mullite components were fabricated by additive manufacturing. A photocurable silicone preceramic polymer resin containing alumina particles was shaped into complex structures via Digital Light Processing. Dense and crack-free, highly complex porous mullite ceramics were produced by firing a mixture of a commercially available photosensitive polysiloxane as the silica source, containing alumina powder as active filler, in air at a low sintering temperature (1300 °C). In particular, the developed formulations, coupled with the additive manufacturing approach, allow for precise control of the architecture of the porous ceramic components, providing better properties compared to parts with stochastic porosity.     

Fabrication of barium titanate ceramics via digital light processing 3D printing by using high refractive index monomer

A digital light processing (DLP) technology has been developed for 3D printing lead-free barium titanate (BTO) piezoelectric ceramics. By comparing the curing and rheological properties of slurries with different photosensitive monomer, a high refractive index monomer acryloyl morpholine (ACMO) was chosen, and a design and preparation method of BTO slurry with high solid content, low viscosity and high curing ability was proposed. By further selecting the printing parameters, the single-layer exposure time was reduced and the forming efficiency has been greatly improved. Sintered specimens were obtained after a nitrogen-air double-step debinding and furnace sintering process, and the BTO ceramics fabricated with 80 wt% slurry shows the highest relative density (95.32 %) and piezoelectric constant (168.1 pC/N). Furthermore, complex-structured BTO ceramics were prepared, impregnated by epoxy resin and finally assembly made into hydrophones, which has significance for the future design and manufacture of piezoelectric ceramic-based composites that used in functional devices.     

A review on the rheological behavior and formulations of ceramic suspensions for vat photopolymerization

Vat photopolymerization is an additive manufacturing process that produces high-performance ceramic parts. A critical step in the process is the preparation of a suspension that meets the requirements of high ceramic loading and proper rheological behavior, since an increase in solid loading might compromise the suspension rheology, resulting in non-uniform layer recoating. This review examines the rheological behavior of ceramic suspensions for vat photopolymerization, discussing the influence of the suspension formulation (solid loading, ceramic particle size and size distribution, monomers, diluents, and dispersants) on rheological aspects such as viscosity, shear-thinning/thickening behavior, critical shear rate, yield stress, and thixotropy. It provides a summary of the best formulations, which achieved low viscosity (<3 Pa.s) and high solid loading (>40 vol%), and reports the main trends and challenges of ceramic vat photopolymerization, suggesting general guidelines for the preparation of highly loaded photocurable ceramic suspensions with low viscosity.     

Three-dimensional complex construct fabrication of illite by digital light processing-based additive manufacturing technology

Illite is a group of clay minerals that are expected to be widely used in catalyst fabrication, radioactive element adsorption, and so forth, due to its excellent adsorption properties. However, the shape control limitation of the illite product should be overcome to maximize its utilization and properties. We herein propose additive manufacturing (AM) as one of the best solutions to solve this structural drawback. Digital light processing (DLP) technology with the film-type of the material supplying system was adapted instead of the general vat-type DLP system to increase illite printability. The photo-curability and printability of illite-contained photocurable suspension were optimized. The color effect due to different ferric oxide content in yellow- and white-illite which influence the photopolymerization process also adjusted thoroughly. White illite showed better photo-curability and could be increased solid loading than yellow illite. The defect-free illite products with three-dimensional complex structures, which cannot be produced by typical ceramic processes, were obtained by DLP technology for both yellow- and white-illite after sintering at 1100°C. The overcoming of shape control limitation of illites by ceramic AM proved in this study has excellent potential for expanding illite utilities in various applications.     

Yellow resistant photosensitive resin for digital light processing 3D printing

Digital light processing (DLP) has been studied and developed in the field of three-dimensional (3D) printing in recent years due to its fast curing rate and high resolution. To reduce the cost and viscosity of the resin system, the aromatic polyurethane acrylates (PUAs) were used as oligomer. The matrix resin called PUH₂ consists of oligomers (PUA, bisphenol A polyoxyethylene ether dimethyl acrylate) and active diluents (hydroxyethyl acrylate, hydroxyethyl methacrylate). However, the photosensitive resin containing aromatic isocyanate groups was easily yellowed under ultraviolet light. In this article, we developed a resin for DLP 3D printing with yellowing resistance, excellent mechanical properties and high heat resistance. The optimal ratio of 3DP-PUH₂ resin was PUH₂/TPO/RYOJI-292/dye/nanosilica = 100/5/0.4/0.01/0.1, and its viscosity was 500 cp, which is suitable for DLP 3D printing. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48369.     

Mechanical and dielectric properties of Si3N4-SiO2 ceramics prepared by digital light processing based 3D printing and oxidation sintering

Si₃N₄-SiO₂ ceramics are considered as the preferred high-performance wave-transmitting material in the aerospace field. However, traditional fabrication methods for Si₃N₄-SiO₂ ceramics have the disadvantages of high cost and complicated fabrication process. In this paper, Si₃N₄-SiO₂ ceramics with excellent mechanical and dielectric properties were fabricated by digital light processing-based 3D printing combined with oxidation sintering. Firstly, the curing thickness and viscosity of slurries with different solid loadings for vat photopolymerization-based 3D printing were studied. Then, the effects of the sintering temperature on the linear shrinkage, phase composition, microstructure, flexural strength, and dielectric properties of Si₃N₄-SiO₂ ceramics, and the influences of solid loading on them were explored. The curing thickness and viscosity of the slurry with a solid loading of 55 vol% were 30 μm and ∼1.5 Pa‧s, respectively. The open porosity and the flexural strength of Si₃N₄-SiO₂ ceramic with a solid loading of 55 vol% were 4.3 ± 0.61% and 76 ± 5.6 MPa, respectively. In the electromagnetic wave band of 8–18 GHz, the dielectric constant of Si₃N₄-SiO₂ ceramics was within the range of less than 4, and the dielectric loss remained below 0.09. The method of digital light processing-based 3D printing combined with oxidation sintering can be further extended in the preparation of Si₃N₄-based structure-function integrated ceramics.