3D printing of honeycomb Si3N4 ceramic

In this paper, a honeycomb Si₃N₄ ceramic was fabricated by 3D printing with a well-preserved structure. The effects of Si₃N₄ content on the rheological properties of Si₃N₄/sol–silica ink and the printing resolution of products were investigated. The microstructure, phase composition, liner shrinkage rate, and fracture behavior of printed samples before and after sintering were systematically characterized in detail. The results showed that the modified inks had the optimized rheological properties, and the stress–shear rate curves corresponding to each slurry could be well described by Bingham and Herschel–Bulkley fluid models. The corresponding slump rates of the printed samples with different Si₃N₄ to sol–silica mass ratios were all lower than 4%, and the linear shrinkage rate of all of the samples after sintering was below 20%. The fracture behavior under compressive loading of the honeycomb Si₃N₄ ceramics tended to be non-catastrophic fractures both before and after sintering. The compressive strengths of all of the printed samples decreased with the increase of the Si₃N₄ content, and the highest compressive strength of the honeycomb ceramics could reach 131.2 MPa after sintering at 1600°C, which was about 366.9% higher than that of the samples in green state prior to the sintering.     

Influence of the content of polymethyl methacrylate on the properties of porous Si3N4 ceramics fabricated by digital light processing

Porous Si₃N₄ ceramics are highly regarded as ideal materials for radomes due to their unique characteristics. However, the slurry used for the preparation of porous Si₃N₄ ceramics suffers from a low cure depth, making it challenging to fabricate ceramic components using DLP technology. In this study, porous Si₃N₄ ceramics were prepared by combining DLP technology with pore-forming agent method. The addition of polymethyl methacrylate (PMMA) powders with lower refractive index than that of Si₃N₄ powders can improve the penetration depth of ultraviolet light in the Si₃N₄ slurry. A systematic study was conducted to investigate the influence of the addition of PMMA powders on the properties of Si₃N₄ slurries and porous Si₃N₄ ceramics. When PMMA powders were added at 10 wt%, the slurry with a lowest viscosity of 0.13 Pa s (the shear rate is 30 s⁻¹) and cure depth of 40.0 μm (the exposure energy is 600 mJ/cm²) was obtained. With the increase of PMMA content, porous Si₃N₄ ceramics experienced a gradual decrease in both the flexural strength and bulk density, while the porosity increased from 14.41% to 27.62%. Specifically, when 20 wt% PMMA was added, the resulting porous Si₃N₄ ceramics had a lowest bulk density (2.41 g/cm³), a maximum porosity (27.62%), and a flexural strength (435.87 MPa). The study is of great significance in establishing an experimental foundation for fabricating porous Si₃N₄ ceramics by using DLP technology.     

High-performance and high-precision Al2O3 architectures enabled by high-solid-loading,graphene-containing slurries for top-down DLP 3D printing

To date, obtaining the high-solid-loading Al₂O₃ slurry and overcoming the trade-off between high solid loading and printing accuracy and strength of printed green bodies to achieve high-performance and precision Al₂O₃ ceramic parts by DLP 3D printing remain challenging. In this study, an Al₂O₃ slurry with high solid loading of 60 vol% was developed through dispersant optimisation for top-down DLP 3D printing. Graphene was innovatively introduced during slurry fabrication to decouple the printing accuracy and strength of green bodies from such high solid loading. Simultaneously, graphene addition could considerably reduce slurry fluidity, thereby facilitating its coordination with top-down DLP. With 0.07 wt% graphene addition, the dimension deviations of printed green bodies improved from 90 to 880 µm to ≤ 70 µm, and the bending strength increased by 17.75%. High-performance and precise Al₂O₃ ceramic components with low sintering shrinkages were prepared. The density and microhardness were 99.7% and 18.61 GPa, respectively.     

Effect of polystyrene addition on properties of porous Si3N4 ceramics fabricated by digital light processing

Porous Si₃N₄ ceramics with high strength and high transmittance have been widely used in the field of defense and military. Additive manufacturing (AM) technology is one of the effective means to fabricate porous Si₃N₄ ceramics. Nevertheless, it is difficult to prepare porous Si₃N₄ ceramics by using digital light processing (DLP) because of the large refractive index difference between Si₃N₄ powders and photosensitive resin. In this study, the effects of the amount of polystyrene (PS) powders on the properties of Si₃N₄ ceramic slurries and sintered ceramics were systematically discussed. The addition of PS reduced the overall refractive index of powders and increased the average particle size of powders, thus improving the cure depth of Si₃N₄ ceramic slurries from 11.0 ± 2.0 μm to 55.7 ± 1.8 μm. With the increase of PS content, the shrinkage and porosity of Si₃N₄ ceramics gradually increased, and the bulk density and flexural strength showed the opposite trend. The slurry with low viscosity (2.38 Pa٠s at a shear rate of 30 s⁻¹) and high cure depth (51.2 ± 4.6 μm) was obtained when the content of PS was 15 wt%, which met the thickness requirements for printing. The total porosity of Si₃N₄ ceramics reached the maximum values at 28.21 ± 2.58%. The addition of PS solved the problem of low cure depth of slurries, and PS as a pore-forming agent could help Si₃N₄ ceramics form porous structure. This research provides valuable insights into the fabrication of non-oxide ceramics with high refractive index using DLP technology.     

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.     

Fabrication and properties of Si3N4 bioscaffolds with orderly–interconnected big pore channels and well–distributed small pores

This paper focuses on investigating the technical potential for fabricating porous ceramic bioscaffolds for the repair of osseous defects from trauma or disease by inverse replication of three–dimensional (3–D) printed polymer template. Si₃N₄ ceramics with pore structure comprising orderly–interconnected big pore channels and well–distributed small pores are successfully fabricated by a technique combining 3–D printing, vacuum suction filtration and oxidation sintering. The Si₃N₄ ceramics fabricated from the Si₃N₄ powder with addition of 10?wt% talcum by sintering at 1250?°C for 2?h have little deformation, uniform microstructure, low linear shrinkage of 4.1%, high open porosity of 58.2%, relatively high compression strength of 6.4?MPa, orderly–interconnected big pore channels and well–distributed small pores, which are promising bioscaffold in the field of bone tissue engineering.     

Fabrication and properties of Si2N2O-Si3N4 ceramics via direct ink writing and low-temperature sintering

Direct ink writing (DIW) and low-temperature sintering methods were applied to prepare Si₂N₂O-Si₃N₄ ceramics for radome materials. Lattices of Si-SiO₂ green body were printed by DIW with 78 wt % solid portion of water-based Si-SiO₂ slurry, in which silicon particles and silica fume were used as the solid portion and Methylcellulose (HPMC) was used as the dispersant. Effects of HPMC addition on stability and silica fume content on rheological properties of the slurry were studied, respectively. The pseudoplastic mechanism of the slurry was analyzed. The Si-SiO₂ green bodies were sintered at 1250 °C–1400 °C in nitrogen. The effect of temperature on phase composition, microstructure, mechanical and dielectric properties of samples was investigated. With the HPMC addition of 0.12 wt% and the silica fume proportion of 30 wt% in solid portion, a stable and pseudoplastic slurry with the yield stress of 110.9 Pa was obtained, which is suitable for DIW. With the decrease of initial holding temperature, more N₂ enters the sample and reacts with silicon and silica fume, promoting the generation of Si₂N₂O and Si₃N₄. The optimal condition yields Si₂N₂O-Si₃N₄ ceramics with apparent porosity of 42.73%, compressive strength of 24.7 MPa, dielectric constant of 4.89 and loss tangent of 0.0054. It is found that columnar Si₃N₄ comes from a direct reaction between silicon and N₂, and fibrous Si₂N₂O is mainly generated by the reaction between silicon, SiO(g), and N₂ through the chemical vapor deposition mechanism. Good dielectric properties are achieved due to high porosity, high proportion of Si₂N₂O phase and no residual silicon.     

Effect of print path process on sintering behavior and thermal shock resistance of Al2O3 ceramics fabricated by 3D inkjet-printing

In this study, Al₂O₃ ceramics parts were printed by inkjet printing technology with different printed paths distributions, such as the spiral printed path, round trip straight printed path and ladder lap printed path. The influences of inkjet printed paths on sintering performance and thermal shock resistance of the Al₂O₃ green bodies were investigated. The sintering performance of the green sample with the ladder lap printed path is the highest among the three samples. Sintered at 1550?℃, its bulk density and porosity reached 3.73?g/cm³ and 10.80%, respectively. In addition, the thermal shock resistance of the sample with the step print path reached 11 times. The results suggest that the optimization of the printed path provides an effective way to print 3D ceramics with good performances through 3D inkjet-printing technology.     

Stereolithographical fabrication of dense Si3N4 ceramics by slurry optimization and pressure sintering

Photocurable gray-colored Si₃N₄ ceramic slurry with high solid loading, suitable viscosity and high curing depth is critical to fabricate dense ceramic parts with complex shape and high surface precision by stereolithography technology. In the present study, Si₃N₄ ceramic slurry with suitable viscosity, high solid loading (45 vol %) and curing depth of 50 μm was prepared successfully when surface modifier KH560 (1 wt%) and dispersant Darvan (1 wt%) were used. The slurry exhibits the shear thinning behavior. Based on the Beer-Lambert formula, D_p (the attenuation length) and E_c (the critical energy dose) of Si₃N₄ ceramic slurry with solid loading of 45 vol % were derived as 0.032 mm and 0.177 mJ/mm², respectively. Si₃N₄ ceramic green parts with complex shape and high surface precision were successfully fabricated by stereolithography technology. After optimizing the debinding and sintering process for green parts, dense Si₃N₄ ceramics with 3.28 g/cm³ sintering density were fabricated. The microhardness and fracture toughness of as-sintered Si₃N₄ ceramics are ~14.63 GPa and ~5.82 MPa m^1/2, respectively, which are comparable to those of the samples by traditional dry-pressed and pressureless sintering technology. These results show that ceramic stereolithography technology could be promising to fabricate high performance ceramics, especially for gray-colored monolithic Si₃N₄ ceramics.     

Novel porous Si3N4 ceramics prepared by aqueous gelcasting using Si3N4 poly-hollow microspheres as pore-forming agent

In this paper, novel porous Si₃N₄ ceramics were prepared by aqueous gelcasting using Si₃N₄ poly-hollow microspheres as pore-forming agent. The effect of Si₃N₄ poly-hollow microsphere content on the phase composition, microstructure, shrinkage, porosity and mechanical properties of the prepared porous Si₃N₄ ceramics were investigated. It is found that there is only β-Si₃N₄ phase in all the prepared porous Si₃N₄ ceramics. Meanwhile, the SEM results show that the pores in the porous Si₃N₄ ceramics distribute uniformly, the added Si₃N₄ poly-hollow microspheres and the basal body contact closely. With the increase of Si₃N₄ poly-hollow microsphere content, the shrinkage of the porous Si₃N₄ ceramics decreases gradually, and the porosity of the porous Si₃N₄ ceramics decreases firstly and then increases. Furthermore, the flexural strength and fracture toughness of the porous Si₃N₄ ceramics decrease with the increase of the Si₃N₄ poly-hollow microsphere content.     

Influence of the ratio of sintering aids on the properties of porous Si3N4 ceramics fabricated by digital light processing

The sintering aids play an important role in affecting the properties of porous Si₃N₄ ceramics. However, there are few researches on the properties of porous Si₃N₄ ceramics fabricated by digital light processing (DLP) with different ratios of sintering aids. In this paper, porous Si₃N₄ ceramics with different ratios of sintering aids (Y₂O₃-Al₂O₃) were formed by DLP technology. The influence of Y₂O₃-Al₂O₃ ratios on the properties of Si₃N₄ slurry and porous ceramic was studied systematically. The ratio of Y₂O₃-Al₂O₃ had little effect on the rheology and cure depth of Si₃N₄ slurry due to the low addition of sintering aids. The increase of Y₂O₃-Al₂O₃ ratio promoted the anisotropic growth of β-Si₃N₄. When the ratio of Y₂O₃-Al₂O₃ was 9:1, the aspect ratio of β grains reached the maximum. As the ratio of Y₂O₃-Al₂O₃ powders increased, the linear shrinkage of porous Si₃N₄ ceramics showed an increasing and then decreasing trend in three directions. When the Y₂O₃-Al₂O₃ ratio was 3:7, the shrinkage rate in the length, width and height direction reached the maximum (27.03%, 30.27% and 40.02%, respectively). The bulk density and flexural strength exhibited an initial increase followed by a subsequent decrease, while the porosity showed the opposite trend. When the Y₂O₃-Al₂O₃ ratio was 9:1, the porosity reached a maximum of 28.1%. And the bulk density and flexural strength were 2.42 g/cm³ and 421.58 MPa, respectively. This study is of great significance as it lays the experimental foundation in the performance control of porous Si₃N₄ ceramics fabricated by DLP.     

Preparation of broadband transparent Si3N4-SiO2 ceramics by digital light processing (DLP) 3D printing technology

Wave-transmitting materials are a kind of multi-functional materials that protect the normal operation of communication and guidance systems of spacecraft in harsh environments. In this paper, we fabricate a broadband microwave transparent Si₃N₄-SiO₂ composite ceramic with excellent performance through digital light processing (DLP) 3D printing technology. The influences of sintering temperature on the weight increase rate, density, dimensional shrinkage, phase composition, microstructure, bending strength and dielectric properties of Si₃N₄-SiO₂ ceramic were all systematically studied. The strength of Si₃N₄-SiO₂ ceramic sintered at 1350 ℃ was 77 ± 5 MPa. The relative permittivity of the ceramic is within the range of less than 4, and the loss tangent can be below 0.003. The 3D printed Si₃N₄-SiO₂ ceramic material exhibited excellent wave-transparent performance.     

Preparation of Porous Si3N4 Ceramics with Controlled Porosity and Microstructure by Fibrous α‐Si3N4 Addition

Porous silicon nitride ceramics with various porosities were fabricated by liquid phase sintering of mixtures containing fibrous and equiaxed α‐Si₃N₄ powder with a various content ratios. The effects of the contents of the fibrous α‐Si₃N₄ powder (0%–100%) on the microstructure and mechanical properties of porous Si₃N₄ ceramics were studied. As the increase of the fibrous α‐Si₃N₄ powder content, both the density of green bodies and the linear shrinkage decreased, resulting in increased porosity due to the inhibited densification by the fibrous Si₃N₄ particle. XRD analysis proved the complete formation of single β‐Si₃N₄ phase. SEM analysis revealed that the microstructure of the low content of fibrous α‐Si₃N₄ porous ceramics was almost composed of fine elongated β‐Si₃N₄ grains with high aspect ratio while numerous coarse elongated β‐Si₃N₄ grains with low aspect ratio surrounding fine grains were formed as the content of the fibrous α‐Si₃N₄ powder increased. With the increase in content of the fibrous α‐Si₃N₄ powder from 0% to 100%, the porosity changed from 47.8% to 56.6%, and the flexural strength decreased from 146 to 62 MPa correspondingly, indicating a flexural adjustment of the porosity and mechanical properties.     

Porous Si3N4/SiC Ceramics Prepared via Nitridation of Si Powder with SiC Addition

Porous Si₃N₄/SiC ceramics with high porosity were prepared via nitridation of Si powder, using SiC as the second phase and Y₂O₃ as sintering additive. With increasing SiC addition, porous Si₃N₄/SiC ceramics showed high porosity, low flexural strength, and decreased grain size. However, the sample with 20wt% SiC addition showed highest flexural strength and lowest porosity. Porous Si₃N₄/SiC ceramics with a porosity of 36–45% and a flexural strength of 107‐46MPa were obtained. The linear shrinkage of all porous Si₃N₄/SiC ceramics is below 0.42%. This study reveals that the nitridation route is a promising way to prepare porous Si₃N₄/SiC ceramics with favorable flexural strength, high porosity, and low linear shrinkage.     

Fabrication of Si3N4 ceramics substrates with high thermal conductivity by tape casting and gas pressure sintering

In this paper, high thermal conductivity Si₃N₄ ceramics were successfully fabricated through exploring and optimizing the tape casting process. The impact of various organic additives on the rheological characteristics of Si₃N₄ slurry was explored, and the pore size distribution and microstructure of the green tapes at different solid loadings were investigated, as well as the microstructure of Si₃N₄ ceramics. Green tapes with a narrow pore size distribution, a small average pore size, and a high density of 1.88 g cm⁻³ were prepared by the investigation and optimization of the Si₃N₄ slurry formulation. After gas pressure sintering, Si₃N₄ ceramics with a density of 3.23 g cm⁻³, dimensions of 78 mm × 78 mm, and a thickness of 0.55 mm were obtained. The microstructure of the Si₃N₄ ceramics showed a bimodal distribution and a low content of glassy phases. The thermal conductivity of the Si₃N₄ ceramics was 100.5 W m⁻¹ K⁻¹, the flexural strength was 735 ± 24 MPa, and the fracture toughness was 7.17 MPa m^1/2.     

Pore structure control of Si3N4 ceramics based on particle-stabilized foams

Porous Si₃N₄ ceramics with open, closed pores and nest-like structures were prepared by direct foaming method, and the stability of bubbles and the microstructures of sintered Si₃N₄ foam ceramics were investigated. The bubbles produced by short-chain amphiphiles have higher stability as compared with that produced by long-chain surfactants. Si₃N₄ ceramic foams using short-chain amphiphiles are particle-stabilized one, porous Si₃N₄ ceramics with open and closed pores can be easily prepared with this method, and the nest-like microstructure in Si₃N₄ foam ceramics is achieved at high gas-pressure sintering conditions. The decrease of flexural strength due to the increase of porosity can be weakened by decreasing pore size.     

Photosensitive Si3N4 slurry with combined benefits of low viscosity and large cured depth for digital light processing 3D printing

Digital light processing 3D printing can be applied to fabricate complex silicon nitride (Si₃N₄) components. However, because of the surface hydroxyl groups and large refractive index, it is still a foremost challenge to realize a stable photosensitive Si₃N₄ slurry with combined benefits of low viscosity and large curing depth. In this study, we propose a new formulation strategy to prepare Si₃N₄ slurry. Starting from the optimization of monomer ratio, we have systematically optimized powder particle size, dispersant and photoinitiator on the rheological properties and curing properties of Si₃N₄ slurry. Specifically, we have fabricated a stable photosensitive Si₃N₄ slurry (48 vol%) with a viscosity of 2.09 Pa s (30 s^?1), a critical curing energy of 126.09 mJ/cm² and a maximum curing depth of 80 µm. Finally, based on this optimized slurry, we have successfully obtained complex Si₃N₄ green body with no defect, which demonstrates great potential to fabricate arbitrary complex ceramic components for various applications.     

Effects of AlN inorganic binder on the properties of porous Si3N4 ceramics prepared by selective laser sintering

Porous Si₃N₄ ceramics are widely used in the aerospace field due to its lightweight, high-strength, and high wave transmission. Traditional manufacturing methods are difficult to fabricate complex structural and functional ceramic parts. In this paper, selective laser sintering (SLS) technology was applied to prepare porous Si₃N₄ ceramics using AlN as an inorganic binder. And the effects of AlN content on the properties of the obtained ceramic samples were explored. As the AlN content increased, nano-Al₂O₃ and nano-SiO₂ formed the eutectic liquid phase, enhancing the sintering densification and phase transformation of Si₃N₄ poly-hollow microspheres (PHMs). The island-like partial densification structures in Si₃N₄ green bodies increased. During the high-temperature sintering, the eutectic liquid phase partially transformed into the mullite phase or reacted with AlN and Si₃N₄ to form the Sialon phase. With the increase of AlN content, the fracture mode of Si₃N₄ ceramics changed from fracturing along PHMs to fracturing across PHMs. The bonding depth between PHMs increased and the connection between the grains was tighter, so the Si₃N₄ ceramics became denser. With the increase of AlN addition, the total porosity of the porous Si₃N₄ ceramics tended to decrease and the flexural strength gradually increased. When AlN content was 20 wt%, the total porosity and the flexural strength were 33.6% and 23.9 MPa, respectively. The addition of AlN inorganic binder was carried out to develop a novel way to prepare high-performance porous Si₃N₄ ceramics by SLS.     

Application of polypropylene carbonate for the tape casting of silicon nitride ceramics

QPAC40 (polypropylene carbonate), with a little decomposition residue, is commonly used as a binder in aluminum nitride (AlN) tape casting. In this paper, we tried to explore its application in silicon nitride (Si₃N₄) tape casting. By studying the influence of dispersant, binder, plasticizer/binder ratio, and solid loading on slurry and green tape properties, the optimum formulation of the tape casting of Si₃N₄ slurry was determined, and the green tape with a uniform structure and relative density up to 63.16% was prepared. Si₃N₄ ceramics were obtained by debinding at 600°C for 1 h in vacuum and gas-pressure sintering at 1830°C for 2 h in N₂. The thermal conductivity and flexural strength of Si₃N₄ ceramics were 56.28 ± 1.21 W/(m·K) and 1130.67 ± 23.58 MPa, respectively. These results indicated that QPAC40 can be used to prepare Si₃N₄ sheets through tape casting.     

DLP 3D printing porous β-tricalcium phosphate scaffold by the use of acrylate/ceramic composite slurry

Digital light processing (DLP) 3D printing has been utilized to fabricate controlled porous β-tricalcium phosphate (β-TCP) scaffolds, which promote cell adhesion and angiogenesis during bone regeneration. However, the current limitation of DLP 3D printing for the fabrication of β-TCP scaffold is how to prepare a low viscosity ceramic slurry and remove the toxicity of residual non-polymerized slurry. The present study has developed a low viscosity ceramic slurry system by mixing β-TCP with photosensitive acrylate resin, and the viscosity of slurry is about 3 Pa s and the solid content of β-TCP can be as high as 60 wt%. After optimizing the ratio of slurry, printing, degreasing and sintering processes, the maximum compressive strength of the DLP printed scaffolds reaches up to 9.89 MPa, while the porosity keeps ca. 40%. According to the proliferation of cells, it confirms the preserved biocompatibility of DLP-fabricated β-TCP scaffolds. These porous scaffolds made by DLP 3D printing technology is of great significance for bone regeneration, and will also help to expand the application of DLP technology in biomedical field.