Research Progress on Digital Light Processing 3D Printing Forming of Silicon Nitride Ceramics

Silicon nitride (Si₃N₄) ceramics are widely utilized in mechanical engineering, semiconductors, aerospace, and biomedical fields owing to their outstanding properties, including high strength, high fracture toughness, excellent thermal conductivity, resistance to thermal shock and corrosion, low thermal expansion coefficient, and favorable biocompatibility. However, the high hardness and intrinsic brittleness of Si₃N₄ make it difficult to efficiently fabricate complex components using conventional machining methods. These limitations often result in prolonged processing cycles, high manufacturing costs, and challenges in achieving dimensional accuracy, thereby restricting its broader application.

To overcome these limitations, advanced 3D printing technologies have emerged as promising alternatives. Among them, photopolymerization-based techniques, such as digital light processing (DLP) 3D printing, have demonstrated significant progress in fabricating oxide ceramics including ZrO₂ and Al₂O₃. Nevertheless, their application to dark-colored Si₃N₄ ceramics remains challenging. Owing to the high refractive index and strong ultraviolet absorption of Si₃N₄ powders, the corresponding slurries typically exhibit poor curing performance, low forming efficiency, and difficulties in achieving high solid loading. Furthermore, sintered components are prone to defect formation, which further hinders practical implementation.

Therefore, the development of Si₃N₄ ceramic slurries that simultaneously exhibit favorable rheological properties and efficient photopolymerization behavior is crucial for advancing this technology. This review focuses on the challenges in slurry formulation, the key performance parameters, and the recent research progress in digital light processing 3D printing of Si₃N₄. It aims to provide guidance and insights for the digital fabrication of high-performance Si₃N₄ ceramics.

1. Powder Surface Modification

Enhancing the curing performance of Si₃N₄ powders has become a key research focus. Current strategies primarily focus on powder surface modification and slurry composition optimization.

Modifying the surface properties of powders can potentially reduce their refractive index and UV absorbance, thereby improving slurry photopolymerization behavior:

• Thermal oxidation treatment: Studies have shown that air oxidation of Si₃N₄ powders at 1150–1200 °C can form an amorphous SiO₂ coating on the particle surface. This treatment increases the curing depth by 38% and enables successful high-density sintering.
• Organic modifiers: Silane coupling agents such as KH560 can effectively link ceramic particles with the resin, reducing refractive index mismatch. Curing depths up to 50 μm have been achieved, and the sintered ceramics exhibit excellent hardness and fracture toughness.
• Inorganic additive coating: By introducing Al₂O₃–Y₂O₃ onto the powder surface via co-precipitation, the UV absorbance can be reduced by approximately 18%, further enhancing printing precision and forming quality.

Although surface modification improves curing behavior, the oxidation process may introduce secondary phases such as SiO₂, which can affect the final mechanical properties. In addition, the process is relatively complex, and achieving stable batch-scale production remains challenging.

Figure 1. TEM images of Si₃N₄ powders (a) Pre-oxidation and (b) Post-oxidation


Figure 2. Green bodies of Si₃N₄ ceramics fabricated via DLP
 

2. Slurry Composition Optimization

In addition to powder modification, optimizing the composition of the slurry is also crucial for enhancing forming performance:

• Particle size distribution optimization: Studies have shown that appropriate mixing of coarse and fine particles can effectively improve light transmittance and enhance mechanical strength. For instance, a coarse-to-fine mass ratio of 3:7 results in a maximum flexural strength of 728 MPa.
• Resin system optimization: Employing high-functionality or high-refractive-index monomers (e.g., TMPTA, OPPEOA) can enhance slurry stability and photosensitivity, achieving greater curing depth and lower critical exposure energy.
• Photoinitiator and dispersant control: Optimizing the type and dosage of photoinitiators (e.g., a molar ratio of 6:7 for 907 and ITX) can increase the curing depth up to 70 μm. Additionally, slurry formulations with optimized resin ratios (e.g., HEMA:HDDA:TMPTA = 3:4:3) exhibit stable viscosity and excellent curing performance, enabling the successful printing of complex green bodies.

The advantage of these approaches lies in their avoidance of additional chemical treatments, offering low cost and high efficiency, which is favorable for industrial-scale production. However, attention must be paid to differences in thermal behavior among components, which may affect resin flow and the sintering process; thermogravimetric analysis should be employed to determine compatible sintering protocols.

Figure 3. Schematic illustration of the effect of particle composition on the microstructure of Si₃N₄ ceramics
 

3.Other Approaches to Enhance Curing Performance
Beyond conventional powder modification and resin composition optimization, researchers have explored additional strategies, such as the incorporation of oxide sintering aids and low-refractive-index fillers, to further improve the curing performance of Si₃N₄ ceramic slurries.

• Incorporation of oxide sintering aids: Chen et al. demonstrated that introducing Y₂O₃–Al₂O₃ sintering aids into the slurry increased the curing depth of 50 vol.% solid-loaded Si₃N₄ slurries from 30–50 μm to 50–60 μm, effectively enhancing photopolymerization capability.
• Introduction of low-refractive-index particles: Tian et al. added polystyrene (PS) powders into the slurry, exploiting their low refractive index and pore-forming characteristics to reduce viscosity while improving photosensitivity. The curing depth was significantly increased from approximately 11 μm to over 55 μm. Subsequent studies replaced PS with polymethyl methacrylate (PMMA), achieving similar refractive index modulation of the powder and further improving slurry formability.

These methods are typically applied in the fabrication of porous Si₃N₄ ceramics. By introducing low-refractive-index pore-forming agents as temporary fillers, the optical mismatch between powders and resin can be mitigated, leading to increased curing depth and enhanced printing stability.

4. Sintering Processes and Performance Comparison of Si₃N₄ Ceramics

In the field of Digital Light Processing 3D printing of Si₃N₄ ceramics, the sintering process plays a pivotal role in determining the final properties. Currently, gas-pressure sintering is the mainstream approach for producing high-performance Si₃N₄ ceramics with complex geometries. The sintering atmosphere, applied pressure, and type of sintering aids significantly influence both the density and mechanical performance of the ceramics.

• Gas-pressure vs. Pressureless Sintering: Liu et al. compared the sintering outcomes of DLP-fabricated Si₃N₄ green bodies under ambient pressure and a 5 MPa nitrogen atmosphere. The results demonstrated that gas-pressure sintering substantially reduced both the number and size of pores, with the sintered density reaching approximately 3.28 g/cm³, significantly higher than the 2.95 g/cm³ achieved via pressureless sintering.
• Effect of Sintering Aid Composition: Wu et al. investigated the influence of varying the Y₂O₃–Al₂O₃ mass ratio on the properties of porous Si₃N₄ ceramics. They found that increasing the Y₂O₃ content promoted anisotropic growth of β-Si₃N₄ grains. At a Y₂O₃:Al₂O₃ ratio of 9:1, the grain aspect ratio reached its maximum, porosity was 28.1%, and the flexural strength attained 421.6 MPa, demonstrating a favorable balance between strength and porosity.

Recent studies indicate that, despite the suboptimal curing behavior of Si₃N₄ slurries, dense ceramics fabricated via photopolymerization have achieved flexural strengths up to 800 MPa and densification as high as 98.28%. This demonstrates that the final performance is governed by multiple factors, extending well beyond slurry curing behavior alone.

Therefore, the fabrication of high-performance Si₃N₄ ceramics requires a comprehensive optimization strategy:

• Powder selection, which influences sintering activity and phase transformation;
• Dispersants and resin systems, which affect resin flow behavior and green body structure;
• Photoinitiators and printing parameters, which determine green body quality and dimensional accuracy;
• Sintering aids and process parameters, which control grain growth and densification at high temperatures.

Figure 4. Comparison of photopolymerization-based forming and sintering performance of Si₃N₄ ceramics

Conclusion

In summary, this review has analyzed the challenges related to curing depth in digital light processing 3D printing of Si₃N₄ ceramics and systematically summarized strategies and recent progress in enhancing slurry performance. Although this technology remains at the experimental stage and faces challenges before practical implementation, it holds significant potential for future development.

Future research should focus on three aspects:

• Preparation of ceramic slurries with high solid loading and favorable rheological and curing properties, through optimization of powder selection, dispersant systems, and light absorption mechanisms;
• Improvement of green body quality, by adjusting printing parameters and cleaning procedures to minimize defects and enhance forming accuracy;
• In-depth investigation of resin flow and sintering processes, aiming to identify optimal thermal treatment protocols, achieve dense sintering, and enhance the final mechanical performance.

Through the synergistic optimization of processing techniques and materials, it is expected that photopolymerization-based Si₃N₄ ceramics can achieve practical applications in advanced manufacturing.