Binder-free additive manufacturing of ceramics using hydrothermal-assisted jet fusion

Ceramic additive manufacturing (AM) typically uses a high fraction of organic binders to form pre-sintered green parts that require a post de-binding process to remove. The de-binding process inevitably results in severe gas expansion and residual chars, leading to structural defects, accumulated stress, and compromised material properties in the final parts. Here we report a binder-free additive manufacturing process named hydrothermal-assisted jet fusion (HJF) that utilizes a hydrothermal method to create geometrically and compositionally complex ceramics under mild temperatures. The HJF process employs a selectively deposited volatile dissolving ink, high pressure, and mild heat to strategically fuse a ceramic powder bed into complex geometries. Compared to traditional AM methods for ceramics, the HJF process eliminates the need for organic binders in green part fabrication and offers the potential to directly co-print ceramics with other dissimilar materials, such as polymers and metals, enabling the development of novel multi-functional ceramic composites.     

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.     

Stereolithography-based additive manufacturing of polymer-derived SiOC/SiC ceramic composites

In order to overcome challenges typically encountered during additive manufacturing of ceramics via the polymer precursor route, a novel polymer-derived SiOC/SiC composite system suitable for advanced geometric designs achievable by lithography-based ceramic manufacturing was established. The photoreactive resin system filled with 20 wt% SiC exhibits suitable viscosity characteristics, adequate stability against sedimentation, and a fast photocuring behavior. After printing and pyrolytic conversion, SiC particulates were well-dispersed within the polymer-derived SiOC matrix. A direct comparison with the unfilled polysiloxane-based resin system showed that the addition of particulate SiC increases handleability, reduces shrinkage, and significantly increases critical wall thicknesses up to 5 mm. The biaxial Ball-on-Three-Balls testing methodology yielded a characteristic strength of 325 MPa for SiOC/SiC composites. The results highlight the high potential of particle-filled preceramic polymer systems toward the fabrication of high-performance SiC-based materials by lithography-based additive manufacturing.     

Polymer derived ceramic microparts from X-ray lithography—cross-linking behavior and process optimization

The fabrication of micro components made from ceramic materials is becoming more and more important because of their outstanding chemical stability. Different replication methods like low or high-pressure injection molding of ceramic feedstocks have been established. Various lithographic methods are being widely used for the direct fabrication of very precise plastic or metal micro structured surfaces.The presented work deals with the direct manufacturing of microstructures by using ceramic precursors in combination with X-ray lithography. This allows for a rapid fabrication of micro structured plastic components that are stable at high temperatures (400 °C), chemically resistant and transparent. Pyrolysis of these parts yields amorphous SiCN or Si₃N₄ ceramics.The applicability of different preceramic polymers based on polysilazane for direct structuring with synchrotron radiation was investigated.X-ray exposures of polysilazane polymer filled with Si₃N₄ ceramic powders were carried out, showing that it is possible to microstructure those composites. By inserting ceramic fillers, a reduction of the shrinkage during pyrolysis was possible. Further investigations were made to determine the maximum filler content, the required exposure dose and the cross-linking mechanism using IR- and NMR-spectroscopy. As established in the LIGA technique with PMMA, resist layers up to 1 mm should be possible by deep X-ray lithography on preceramic polymers. Admittedly for the pyrolytic conversion into the amorphous SiCN ceramic, film thickness should be limited as crack formation is deteriorating with increasing dimensions.     

One-step fabrication of Li2TiO3 ceramic pebbles using pulsed YAG laser

A large amount of Li-containing ceramic breeder pebbles is packed in the solid breeding blanket of a nuclear fusion reactor. Several pebble fabrication technologies have been proposed in previous studies, including wet process, emulsion method, extrusion spheronization, additive manufacturing, and melt process. However, a simple, energy-effective, and scalable fabrication technology remains to be developed for the automated mass production and reprocessing of used radioactive pebbles post-operation. Selective laser melting potentially enables the quick and automated fabrication of breeder pebbles. Herein, we employ a high-power density pulse laser to produce ceramic breeder pebbles. A pulsed YAG laser was irradiated over a lithium metatitanate (Li₂TiO₃) powder bed in air, and the corresponding temperature was monitored using fiber-type infrared pyrometers. Spherical Li₂TiO₃ pebbles were successfully fabricated in a single step with an average diameter of 0.78 ± 0.13 μm and the sintering density of 87.4% ± 5.6% (input power: 7.9 J/pulse). The irradiated Li₂TiO₃ powder melted and turned spherical under surface tension and rapidly solidified, resulting in uniaxial fine grains and a decrease in the degree of long-range cation ordering.     

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.     

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.     

Mechanical characterization of parts produced by ceramic on‐demand extrusion process

Ceramic On‐Demand Extrusion (CODE) is an additive manufacturing process recently developed to produce dense three‐dimensional ceramic components. In this paper, the properties of parts produced using this freeform extrusion fabrication process are described. High solids loading (~60 vol%) alumina paste was prepared to fabricate parts and standard test methods were employed to examine their properties including the density, strength, Young's modulus, Weibull modulus, toughness, and hardness. Microstructural evaluation was also performed to measure the grain size and critical flaw size. The results indicate that the properties of parts surpass most other ceramic additive manufacturing processes and match conventional fabrication techniques.     

Additive manufacturing of glass with laser powder bed fusion

Its transparency, esthetic appeal, chemical inertness, and electrical resistivity make glass an excellent candidate for small- and large-scale applications in the chemical, electronics, automotive, aerospace, and architectural industries. Additive manufacturing of glass has the potential to open new possibilities in design and reduce costs associated with manufacturing complex customized glass structures that are difficult to shape with traditional casting or subtractive methods. However, despite the significant progress in the additive manufacturing of metals, polymers, and ceramics, limited research has been undertaken on additive manufacturing of glass. In this study, a laser powder bed fusion method was developed for soda lime silica glass powder feedstock. Optimization of laser processing parameters was undertaken to define the processing window for creating three-dimensional multilayer structures. These findings enable the formation of complex glass structures with micro- or macroscale resolution. Our study supports laser powder bed fusion as a promising method for the additive manufacturing of glass and may guide the formation of a new generation of glass structures for a wide range of applications.     

Robocasting of dense 8Y zirconia parts: Rheology,printing, and mechanical properties

Advanced ceramics with complex geometry have become indispensable in engineering applications. Due to limitations of traditional ceramic fabrication processes, additive manufacturing represents a revolution for shaping and consolidation because of its unique capabilities for increasing shape complexity and reducing waste material. Among the additive manufacturing techniques, robocasting is often considered to yield fine and dense ceramic structures with geometrically complex morphology and high strength. Within this context, it is the objective to attain dense 8 mol% yttria-stabilized zirconia (8Y-ZrO₂) by evaluating the influence of solid loading and filament orientation on the physical and mechanical properties of sintered parts. In doing so, a printable ink was developed using an inverse-thermoresponsive hydrogel. Results revealed that ceramic charges of 67.5 and 70 wt% achieved the best balance regarding density, hardness, and compression strength. Furthermore, rectilinear geometry with a filament orientation at 45º displayed higher mechanical response than 0/90º and cylindrical ones.     

Additive manufacturing of ZrB2–ZrSi2 ultra-high temperature ceramic composites using an electron beam melting process

Owing to their high melting points and ability to resist extreme thermal stresses, ultra-high temperature ceramics (UHTCs) are important materials for critical applications such as hypersonic flights, space re-entry vehicles, and rocket engines. Traditional manufacturing processes restrict the freedom to manufacture UHTCs with complex geometries due to the limitations of die and mold designs. Electron beam melting (EBM) is an established powder-bed layer-by-layer additive manufacturing (AM) process for metal parts. In this research, an effort was made to evaluate the feasibility of EBM for the AM fabrication of UHTC-based materials, and to investigate the microstructures of the fabricated materials under different processing conditions. A mathematical model was developed to simulate and optimize the processing parameters for the fabrication of ZrB₂-30 vol% ZrSi₂ UHTC using EBM. The simulation results were compared with experimental observations. For EBM fabrication of ZrB₂-30 vol% ZrSi₂ composites, the optimal processing parameters are beam power of 500 W with scanning speeds of 500, 750, and 1000 mm/s, and beam power of 1000 W with scanning speed of 1000 mm/s. This study demonstrates the potential for additive manufacturing of UHTCs with complex geometries by the EBM technique.     

Power prediction for laser engineered net shaping of Al2O3 ceramic parts

Laser-aided additive manufacturing technique is a competitive method for direct fabrication of ceramic components. However, the optimal processing parameters are difficult to find because defects are easy to generate for ceramic parts. This paper proposes a mathematical model for predicting required laser power in direct fabrication of Al₂O₃ ceramic parts by laser engineered net shaping (LENS). The laser power model, which is derived based on energy balance of one deposition layer, reveals the relationship between laser power and other process conditions, such as powder flow rate, nozzle travel speed and physical properties of deposited material. The proposed model was then verified through a fabrication experiment of several single-bead wall Al₂O₃ ceramic parts with different laser power. Experimental results indicate that the laser power predicted by the model is accurate for different processing conditions. This model provides a new yet simple method for predicting required laser power accurately during LENS processes.     

Crack-reduced alumina/aluminum titanate composites additive manufactured by laser powder bed fusion of black TiO2−x doped alumina granules

Laser powder bed fusion is an emerging industrial technology, especially for metal and polymer applications. However, its implementation for oxide ceramics remains challenging due to low thermal shock resistance, weak densification and low light absorptance in the visible or near-infrared range. In this work, a solution to increase the powder absorptance and to reduce cracking during laser processing of alumina parts is given. This is achieved by the use of a homogeneously dispersed and reduced titanium oxide additive (TiO_2?x) within spray-dried alumina granules leading to formation of aluminum titanate with improved thermal shock behavior during powder bed fusion. The impact of different reduction temperatures on powder bed density, flowability, light absorption and grain growth of these granules is evaluated. Crack-reduced parts with a density of 96.5%, a compressive strength of 346.6 MPa and a Young's modulus of 90.2 GPa could be manufactured using powders containing 50 mol% (43.4 vol%) TiO_2?x.     

Preparation of low residual silicon content Si-SiC ceramics by binder jetting additive manufacturing and liquid silicon infiltration

Complex silicon carbide (SiC) ceramic components are difficult to fabricate due to their strong covalent bonds. Binder jetting (BJ) additive manufacturing has the outstanding advantages of high forming efficiency and no thermal deformation, especially suitable for printing complex structure SiC components. This study tried to obtain low silicon content silicon carbide ceramics by binder jetting followed by phenolic resin impregnation and pyrolysis (PRIP) and liquid silicon infiltration (LSI). BJ was used for the SiC green parts fabrication, and the highest compressive strength (7.7 ± 0.3 MPa) and lowest dimensional deviations (1.2–1.6 mm) were obtained with the printing layer thickness of 0.15 mm. Subsequently, PRIP treatments were introduced to increase the carbon content for the following LSI process. As the number of PRIP cycles increased, the carbon density of SiC/C preform increased and the porosity decreased. After the LSI treatment, the final Si-SiC composites processed with 2 PIRP cycles reached the highest flexural strength (257 ± 14.26 MPa) and the best wear resistance. This was attributed to the low residual silicon content (10.2 vol%) and almost no residual carbon. Furthermore, several complex structural components were fabricated using these methods. The preparation of complex components verifies the feasibility of BJ and LSI for manufacturing high-strength and high-precision SiC ceramics. Besides, this work hopes to provide technical guidance for the preparation of complex SiC composites in the future.     

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.     

Performance optimization of Si/SiC ceramic triply periodic minimal surface structures via laser powder bed fusion

SiC ceramic lattice structures (CLSs) via additive manufacturing (AM) have been recognized as potential candidates in engineering fields owing to their various merits. Compared with traditional SiC CLSs, SiC triply periodic minimal surface (TPMS) CLSs could possess more outstanding properties, making them more promising for wider applications. Since SiC CLSs are hard to be fabricated through stereolithography techniques because of inferior light performance, the laser powder bed fusion (LPBF) process via selective sintering is an effective method to prepare near-net-shaped SiC TPMS lattices. As the mechanical performances of lattice structures are the foundation for future practical applications, it is of great significance to optimize the preparation process, thus improving the mechanical properties of SiC TPMS structures. In this work, the optimal printing parameters of the LPBF and liquid silicon infiltration process for SiC ceramic TPMS CLSs with three different volume fractions were systematically illustrated and analyzed. The effects of the printing parameters and carbon densities on the fabrication accuracy, microstructure, and mechanical performance of SiC TPMS CLSs were defined. The mechanism of the reactive sintering process for the SiC TPMS lattice structure was revealed. The results reveal that Si/SiC TPMS CLSs with optimum preparation have superior manufacturing accuracy (most less than 6%), relatively high bulk densities (about 2.75 g/cm³), low residual Si content (6.01%), and excellent mechanical properties (5.67, 15.4, and 44.0 MPa for Si/SiC TPMS CLSs with 25%, 40%, and 55% volume fractions, respectively).     

Additive manufacturing of polymer-derived ceramic matrix composites

This study presents a fabrication method and identifies processing bounds for additively manufacturing (AM) ceramic matrix composites (CMCs), comprising a silicon oxycarbide (SiOC) ceramic matrix. A digital light projection printer was used to photopolymerize a siloxane-based preceramic resin containing inert ceramic reinforcement. A subsequent pyrolysis converted the preceramic polymer to SiOC. Particle reinforcements of 0 to 40% by volume in the green state were uniformly dispersed in the printed samples to study their effects on pyrolysis mass loss and shrinkage, and CMC notch sensitivity and strength. Both particle and whisker reinforcements toughened the glassy SiOC matrix (1 MPa m^1/2), reaching values >3 MPa m^1/2. Bending strengths of >300 MPa (>150 MPa (g cm⁻³)⁻¹) and a Weibull modulus of 10 were measured on AM samples without surface finish. We identified two pore formation mechanisms that placed processing bounds on sample size and reinforcement volume fraction. Methods for increasing these bounds are discussed. With properties commensurate to traditionally processed technical ceramics, the presented process allows for free-form fabrication of high-performance AM CMC components.     

Methodology for determining design rules for helical channels in glass components produced by selective laser sintering

The ability to accurately produce geometric features is one of the main hurdles in advancing additive manufacturing for the production of functional parts. Design rules can be found for parts produced using selective laser sintering for polymers with simple features such as thin walls and through-holes; however, these rules have not been studied for more complex features such as helical channels produced from glass or ceramic materials. In this study, feature resolution for helical channels is quantified for successful production of both green and final parts after debinding and sintering. We found the ability to clear loose powder was a major factor in determining whether a particular channel/part geometry could be produced. Our experiments suggest a linear relationship between the scaled diameter and tortuosity for successful clearing of channels. Using this, we propose a design rule that can predict success and failure in accurately forming internal helical channels. The proposed methodology was experimentally validated for parts created via indirect selective laser sintering from mixtures of glass and nylon powders.     

Additive manufacturing low shrinkage Si-based composite ceramics with SiCO microsphere/polyvinyl silicon acetylene slurry

In this paper, SiCO microsphere/polyvinyl silicon acetylene slurry was prepared for the first time with low viscosity and good rheological behavior. The dispersion of microspheres in the precursor resin and the UV-light curing performance of the slurry were investigated. After pyrolysis at 1000 °C, the properties of the three-dimensional (3D) printed samples were characterized. The introduction of SiCO microspheres reduced the specimen shrinkage of the pure precursor photosensitive resin used for additive manufacturing and improved their mechanical properties. When the addition of SiCO powder increased from 0 wt% to 10 wt%, the linear shrinkage decreased from 33 % to 26 %, and the flexural strength increased from 43.9 MPa to 79.8 MPa. This study realizes the preparation of Si-based composite materials through low-cost LCD additive manufacturing technology. It confirms the great potential of low-cost light-curing additive manufacturing to fabricate complex functional composite ceramic materials.     

Additive manufacturing of Al2O3 ceramic core with applicable microstructure and mechanical properties via digital light processing of high solid loading slurry

Ceramic cores are essential intermediate mediums in casting superalloy hollow turbine blades. The developing of additive manufacturing (AM) technology provides a new approach for the preparation of ceramic cores with complex structure. In this study, alumina oxide (Al₂O₃) ceramic cores with fine complex geometric shapes were fabricated by digital light processing (DLP) in high resolution. The maximum solid content of 70 vol% of ceramic slurry was adopted in the printing process, which is important for the regulation of deformations and mechanical properties. The effects of the printing parameters, including exposure intensity, printing layer thickness and sintering temperature on the microstructures and mechanical properties of printed samples were investigated. The decrease of residual stress and similar shrinkage in X, Y, and Z directions could be obtained by adjusting the printing parameters, which are crucial to prepare complex ceramic cores with high quality. Besides, the flexure strength and open porosity of ceramic cores reached 34.84 MPa and 26.94%, respectively, which were supposed to meet the requirement of ceramic cores for the fabrication of superalloy blades.