Photopolymerization-Based Three-Dimensional Ceramic Printing Technology

Abstract

Ceramics have many applications in mechanics, electronics, aerospace, and biomedicine because of their high mechanical strength, high-temperature resistance, and excellent chemical stability. Three-dimensional (3D) printing is a fast, efficient, and intelligent technology that has revolutionized the manufacturing of complex structural parts. Among many ceramic 3D printing technologies, photopolymerization-based 3D printing techniques print out molded ceramic components with high molding accuracy and surface finish and have received widespread attention. This article reviews the current research status and problems experienced by three mainstream ceramic photocuring technologies, namely stereoscopic, digital light processing, and two-photon polymerization.

Introduction

As one of the most popular solid materials today, ceramics are widely used in aerospace, biomedical, and other industries due to their properties^1–3 of high hardness, strength, corrosion resistance, good insulation, biocompatibility, and high-temperature resistance. An increasing number of fields have started to incorporate this material, such as in ceramic cores for aero-engine turbine blade casting, artificial bone brackets, and other high-performance, complex structural ceramic parts.^4–6 The high hardness and brittleness of ceramic materials significantly increase their processing difficulties as ceramic block blanks are prone to internal damage during processing. The lack of nondestructive testing technology for ceramic-like materials also means that their structural and mechanical properties are more discrete and uncertain.

In addition, for some complex structures that cannot be processed, multipart combinations are required. Currently, grouting molding, gel injection molding, direct solidification injection molding, and other processes are used to manufacture complex ceramics structures.^7–9 However, it remains challenging to fabricate complex-shaped ceramics via these methods.

In recent years, the rapid development of three-dimensional (3D) printing technology (also known as additive manufacturing) has provided a new solution for manufacturing ceramic parts with highly complex structures¹ that can compensate for the shortcomings of traditional methods (e.g., casting and machining).¹⁰ According to the different process principles, the leading additive manufacturing technologies include fused deposition modeling, selective laser sintering, selective laser melting, laminated object manufacturing, stereolithography appearance, and direct ink writing.^11–14

Compared with other methods, the photopolymerization-based ceramic photocuring 3D printing molding process has the characteristics of high molding accuracy, fast printing speed, and good surface quality of the sample. As it can be used to design and manufacture high-precision complex structures such as cellular ceramic structures,¹⁵ it has a wide range of application prospects. This article focuses on the process history origins and technical principles of the three main ceramic photopolymerization-based 3D printing technologies and the most recent research progress on its application for manufacturing various ceramic parts and components. The characteristics of the raw materials, printing process, and ceramic part performance are summarized, and some of the challenges and possible solutions are discussed.

Introduction to Photopolymerization-Based 3D Ceramics Printing Technology

Ceramic photopolymerization-based 3D printing technology mainly includes stereolithography (SL), digital light processing (DLP), and two-photon polymerization (TPP).^16,17 The three processes generally apply a slurry system containing a mixture of ceramic powder and photosensitive solution or preceramic polymers (PCPs). Photosensitive polymerization is also known as SL and refers to the curing of a certain volume of liquid materials, such as polymer monomers, through crosslinking polymerization caused by light irradiation at a certain wavelength. For the 3D printing process of a slurry system, in which the ceramic powder is mixed with the photosensitive solution, the resin is polymerized and crosslinked into a network structure to evenly wrap the ceramic particles dispersed in the system so as to form the curing of the mixed material macroscopically.

After that, the print is subject to high-temperature degreasing and sintering, and the organic matter in the sample is discharged. The ceramic sample is further densified and the grain size is increased to form the final sample. This heat treatment stage shares many characteristics of the traditional ceramic manufacturing method. The PCPs technique is similar to the light-curing 3D printing process of ordinary photosensitive resin, which transforms into the required polymer-derived ceramic samples through high-temperature pyrolysis. This process is largely realized through chemical changes.

The formulation of photopolymerization-based ceramic pastes is relatively mature. Photopolymerization-based ceramic pastes mainly contain ceramic powder, photosensitive resin, photoinitiator, dispersant, and other components. Depending on the dispersion medium, they can be resin-based and water-based, and both systems have the same curing principle. There are two approaches to preparing water-based ceramic pastes: diluting a certain amount of oligomer or reactive monomer with water or replacing the oligomer in the photosensitive resin with an aqueous oligomer. Compared to various types of photosensitive resins, the viscosity of water is very low, so the water in the molded sample can be removed by simple drying.

Therefore, compared to resin-based materials, water-based ceramic pastes can be less toxic and environmentally polluting.¹⁸ At the same time, water-based ceramic pastes also have the disadvantage of lower strength plain blanks. As there is less research on the sintering process of water-based ceramic pastes, research on photopolymerization-based ceramic pastes is still dominated by resin-based pastes.^19,20

Resin-based photopolymerization-based ceramic pastes usually use acrylates as the dispersion medium. Photoinitiators absorb UV light at specific wavelengths, causing rapid crosslinking and curing of the photosensitive resin to form a solid polymeric mesh matrix containing ceramic particles, thus giving a particular strength to the plain blank.²¹ Ding et al²² selected a 1:1 mass ratio mixture of 1,6-hexanediol diacrylate and trimethylolpropane triacrylate resin as the reactive monomer. They blended 1% (mass fraction) nano-SiC particles into micron-sized SiC particles to significantly improve the stability of the slurry. Sun et al²³ formulated ZrO₂ nanopaste using a mixture of acryloylmorpholine and poly(ethylene glycol) diacrylate as the dispersing medium. They found that the addition of Disperbyk (BYK) could effectively enhance the stability and fluidity of the ceramic paste by adding different dispersants for comparison.

Ning et al²⁴ obtained well-dispersed water-based slurries by modifying 3Y-ZrO₂ nanopowder with silane coupling agent KH570. Single-line curing experiments revealed that the single-line cross-section of the water-based ceramic paste was broader and shallower than the single-line cross-section of the photosensitive resin. Wang et al²⁵ prepared a water-based hydroxyapatite ceramic slurry using a solid content of 52% (v/v) with an acute exposure of 20.3 mJ/cm², a transmission depth of 50.7 μm, and a final sintered component strength of 37 MPa, which had promising applications in bone tissue engineering.

Liu et al²⁶ prepared Si₃N₄ slurry with 40 vol% solid phase content and 0.8 Pa/s viscosity by extensively investigating the effects of the type of dispersant, pH, and solid-phase content on the rheological properties in the photopolymerization-based Si₃N₄ ceramic slurry system using 0.8 wt% ammonium methacrylate as dispersant and slurry pH = 10. The prepared turbine blanks had a microhardness of 13.76 ± 0.42 GPa and a fracture toughness of 5.32 ± 0.52 MPa-m^1/2 after pneumatic sintering at 1750°C.

The preparation of ceramics by PCPs has received much attention in recent years due to its good flowability, ease of molding, and processability. Tian et al²⁷ improved the fabrication process of composite SiC ceramic parts with phenolic resins and other carbon source precursors, using trimethylene glycol as a pore-forming agent and benzoin dimethyl ether as a photoinitiator, and developed a new photocurable resin with higher carbon yield after pyrolysis.

Stereolithography

Principle of SL

SL is one of the most widely used 3D printing technologies today. It was first proposed by Hull et al in the United States in 1986²⁸ and was then commercialized by 3D Systems. SL technology generally uses a specific wavelength light beam (normally UV) to irradiate and cure the photosensitive material system. The material is cured from point to line and from line to surface, followed by layer-by-layer stacking and molding (Fig. 1). Depending on the top-down or bottom-up printing method, the print platform is raised or lowered by one layer thickness when a layer has finished curing. The squeegee's reciprocating motion applies a new layer of ceramic paste, and then the next layer of the pattern is scanned and cured. The above steps are repeated to obtain a ceramic blank with a homogeneous composition.

FIG. 1.

Principle diagram of stereoscopic.

The beam used in SL has an excellent spot size (60–140 μm), making it possible to produce high surface quality parts with micron-level resolution. However, as the spot is not an ideal point, it is necessary to consider the influence of the actual shape and size of the site when processing tiny structures and contours. Domestic and foreign researchers have proposed numerous optimization solutions to improve this molding accuracy problem. Pan et al²⁹ proposed a spot scanning method with variable compensation amounts in the subregion, which effectively improved the forming accuracy of tiny structures in SL fabricated parts. Li³⁰ studied the inconsistent spot process and its accompanying path planning and proposed a continuous variable spot scanning method that avoided the local area overcuring phenomenon while improving accuracy and forming efficiency.

Application of SL

Ceramic SL has been widely developed in many fields and applied to the manufacture of dense/porous ceramic parts with complex structures, such as integral cores, microelectronic components such as sensors³¹ and photonic crystals, biomedical implant-bone scaffolds, and dental components (although most of them are in the stage of laboratory research and development). In addition, the study of ceramic SL has also made some progress regarding the influence of residual organic matter content, ceramic particle precipitation, light-curing scanning strategy, and degreasing processes.

Xing et al³² studied the sintering process of SL light-cured Al₂O₃ ceramic blanks. By adding plasticizer polyethylene glycol (PEG-400) to the slurry, the interlayer bonding strength of the blank was effectively improved so as to avoid the blank cracking caused by the interlayer internal stress in the process of discharging and sintering. It was determined that the addition of 22% (mass fraction) PEG-400 could make the sintered body equivalent in mechanical properties to the Al₂O₃ ceramic produced by the traditional dry pressing method. Zhou et al³³ added 0.4% (mass fraction) of polyvinylpyrrolidone to 30% (volume fraction) Al₂O₃ acrylamide aqueous solution as dispersant to prepare slurry. After blank printing by SL technology, a dense Al₂O₃ ceramic cutter was obtained following vacuum pyrolysis and air glue discharge, and its relative density was as high as 99.3%.

He et al³⁴ proposed a suspension enclosing projection SL technology based on SL. The method used ceramic slurry prepared by photosensitive resin flgpcl01, 30–40% (volume fraction) Al₂O₃ powder and 0.8% (mass fraction) dispersant phosphate ester ps-131 to balance the self-weight of the cured part with the inherent resistance between particles to without the need for building additional support structures.

Eckel et al³⁵ from HRL Laboratories, USA, analyzed the effect of the lattice structure on the mechanical properties of ceramics and obtained precursor polymers with a chemical composition of SiO_1.34C_1.25S_0.15 by adding siloxane to the resin and complex ceramic parts using the SL technique (shown in Fig. 2). Mechanical property tests illustrated that the porous ceramic parts produced by this process had higher compressive and flexural strengths than those obtained by conventional forming methods at the same relative density.

FIG. 2.

Ceramic parts converted from precursor ceramics of stereolithography.³⁵ (a) UV-curable preceramic monomers are mixed with photoinitiator. (b) The resin is exposed with UV light in a SLA 3D printer or through a patterned mask. (c) A preceramic polymer part is obtained. (d) Pyrolysis converts the polymer into a ceramic. (e) SLA 3D printed cork screw. (f, g) SPPW, self-propagating photopolymer waveguide technology formed microlattices. (h) Honeycomb. 3D, three-dimensional; SLA, stereolithography appearance.

Digital light processing

Principle of DLP

The basic principle of DLP technology is the same as that of SL (shown in Fig. 3). DLP is a light-curing technology developed using a digital micromirror device (DMD) (Texas Instruments) as the key processing element. This technology uses the light source to expose the whole layered printing shape to the photosensitive resin surface through the mask at one time for layer by layer curing. The working principle of DLP technology is shown in Figure 3. A DMD device is adopted to project the image of this layer directly from the bottom to top into the whole area to realize surface curing and molding.

FIG. 3.

Schematic diagram of digital light processing.

The ceramic slurry in the exposed area is rapidly solidified and adhered to the printing platform. After each layer of the pattern is exposed, the printing platform rises, and the process is repeated to obtain a ceramic blank with uniform components. DLP does not require point-to-point scanning as SL to cure a single layer, which greatly improves the printing rate and eliminates the need for high viscosity ceramic slurry. The forming accuracy of DLP technology is better than SL technology (shown in Fig. 4), and its accuracy mainly depends on the resolution of the DMD device.

FIG. 4.

Ceramic prototypes made by digital light processing technology.

Application of DLP

Due to the higher efficiency and lower cost of DLP technology compared to SL,^14,36 it has replaced a portion of ceramic SL in recent years. A research team from the University of Vienna, Austria, prepared complex ceramic structures with a characteristic resolution of 25 μm and a mechanical strength comparable to that of conventionally processed samples using materials such as alumina and bioactive ceramic glass in 2012.^37,38 Tumbleston et al³⁹ proposed a continuous liquid interface production technology similar to DLP technology in 2015, which used a highly light-transmissive and oxygen-permeable film as a release film, where the photocuring of the underlying resin was inhibited due to the oxygen-blocking effect of the photosensitive resin.

In 2019, Kelly et al⁴⁰ proposed the computer axial lithography technique, which was distinctly different from the standard face-exposure forming method. DLP projection equipment was used to project continuous images inside of the resin rotating around the axis, and the part was formed directly at the rotary axis, which significantly improved the forming speed of the region. However, its accuracy could only be controlled to the millimeter level.

DMD surface exposure allows DLP technology to achieve a molding accuracy of several microns. However, this process causes internal stress in the cured layer, which increases the chance of defects. Qiu et al⁴¹ added a grayscale compensation mask in the upper software of the DLP device so that the original exposure mask could be superimposed to obtain a more uniform light intensity distribution. However, this method dramatically increased the printing time, reduced printing efficiency, and was less portable for different device models. Based on this, Wu et al⁴² proposed a model adaptive light homogenization method, which first delineated the maximum exposure region for different model slices and then calculated the optimal grayscale distribution within this region. While this method was experimentally demonstrated to significantly improve light utilization, it required a sufficient number of sample points to be collected and detection instruments with high precision.

Chen⁴³ proposed a simple light compensation technique, which used a gradual disappearance of the projected image from the center to the surroundings instead of instantaneous disappearance. A longer exposure time was obtained at a smaller light intensity, which achieved the effect of compensating for the exposure energy.

Zhang et al designed and manufactured ceramic lattice structures (CLSs) with different structural configurations based on DLP technology, including body-centered cubic (BCC), octet, and modified body-centered cubic (MBCC) cells. The research showed that when the relative density was constant, the quasistatic compressive strength, quasistatic Young's modulus, and quasistatic energy absorption of a CLS with MBCC structure configuration was the best, that of the octet was the second, and BCC was the worst.³⁶

Overall, DLP technology with bottom-up projection has higher forming accuracy in the vertical direction than SL technology,⁴⁴ while saving raw materials. However, the separation pull of the cured layer from the release film during the printing process may damage the stock, resulting in a lower yield. In addition, the unevenness of the light intensity cannot be eliminated. As DLP technology will be applied to the manufacture of large-size ceramic parts production manufacturing in the future, this remains one of the main problems to overcome.

Two-photon polymerization

Principle of TPP

The rapid development of material and laser chemistry has significantly promoted the progress of nanomanufacturing technology in recent years. TPP is a typical example of nanomanufacturing technology. While the initiation polymerization mechanism of TPP and ordinary photopolymerization (SL, DLP) is basically the same, the TPP initiator absorbs photons in different ways. Ordinary photopolymerization is single-photon polymerization, where ultraviolet light is used as the excitation light source, and the polymerization is initiated after the initiator absorbs a light excitation. TPP is initiated using a near-infrared femtosecond laser as the excitation light source, where the initiator absorbs two photons at the same time. Two-photon absorption (TPA) is a third-order nonlinear optical effect, in which a molecule absorbs two photons simultaneously and transitions from a ground state to an excited state.

While ordinary photopolymerization uses an ultraviolet wavelength laser (250–400 nm) with high photon energy, where polymerization occurs wherever light passes, TPP uses a laser with a near-infrared wavelength (600–1000 nm). The photon energy of near-infrared wavelength is low, the linear absorption and Rayleigh scattering are small, and the penetration in the medium is high. The initiator or photosensitizer will produce TPA at the focus with high photon intensity, which will lead to the polymerization and curing of liquid resin (the curing area is called the voxel). A 3D structure of arbitrary shape can be processed by controlling the position distribution of the voxel in space (as shown in Fig. 5).

FIG. 5.

Principle diagram of two-photon polymerization.

Application of TPP

Maruo et al⁴⁵ first processed micron-scale helical 3D microstructures in photosensitive resin (SCR500) using the TPP technique (Fig. 6) in 1997. Kawata et al⁴⁶ processed a micron-sized bovine model in 2001, which was the smallest artificially manufactured animal model at that time at 10 μm long and 7 μm high (Fig. 7). In addition, Marino et al⁴⁷ processed a 200 × 200 × 27 μm bone trabecular cell scaffold using the TPP technique.

FIG. 6.

3D helix microstructure.⁴⁵

FIG. 7.

Bull sculpture produced by TPP.⁴⁶ TPP, two-photon polymerization. (a–c) Bull sculpture produced by raster scanning. (d–f) The surface of the bull was defined by two-photon absorption (TPA). (g) Achievement of subdiffraction-limit resolution. (h) Scanning electron micrograph of voxels formed at different exposure times and laser-pulse energies. (i) Dependence of lateral spatial resolution on exposure time. Scale bars, 2μm.

All of the above studies are based on polymer materials. There is a growing demand for the fabrication of high-performance ceramic parts with complex 3D microstructures and nanoscale feature sizes. The unique capability of TPP in submicron resolution manufacturing offers new possibilities to achieve this goal. However, it should be noted that as TPP technology requires deep penetration into the material for light curing, it is only applicable to “transparent” materials, which have very high requirements for liquid systems. PCPs resin is currently the most mature system used for TPP printing and can be converted into ceramic parts following high-temperature pyrolysis after printing. Therefore, TPP technology is also gradually becoming the most favored method for precision ceramic 3D printing manufacturing.

Pham et al⁴⁸ used TPP to fabricate a precursor SiCN ceramic lattice microstructure with submicron resolution and developed a PCP with high photosensitivity and high ceramic yield. The results showed that the linear shrinkage after pyrolysis was as high as 41%, and the shrinkage (20–40%, mass fraction) was then reduced by introducing 10 nm silica particles. Figure 8 shows the prepared SiCN sample with 40% silica. Park et al proposed using a new photosensitive bifunctional polymer⁴⁹ PCP to prepare a SiC microstructure with almost zero thermal shrinkage, while Colombo et al⁵⁰ used TPP to prepare a complex porous SiOC diamond structure based on micron-scale PCP in 2017. TPP can also be used for the preparation of 3D Zr Si organic ceramic bone tissue engineering scaffolds, making the design of structural porosity and pore size more flexible.⁵¹

FIG. 8.

SiCN based on TPP⁴⁸ 3D ceramic microstructures of spiral (a) microtube and (b) microcruciform with a twisting angle of 90° between their bottom and top.

Conclusion and Outlook

Compared with other ceramic 3D printing methods, the various photopolymerization-based 3D printing ceramics technologies described in this article show more significant advantages in terms of printing accuracy, part surface quality, and mechanical properties. As a result, they are considered highly promising ceramic 3D printing processes. SL technology uses point-by-point scanning to cure ceramic slurry or paste. By reasonably controlling the shape and size of the light spot, the forming accuracy and speed can be effectively improved. However, the quality of the slurry coating is difficult to control using this technique, and the squeegee movement reduces the structural integrity of the plain blank. DLP technology uses DMD chips for surface exposure. This method significantly increases the speed, and a variety of novel surface exposure additive manufacturing technologies have been derived.

However, problems such as uneven exposure and considerable separation tension between the cured layer and the release film remain unsolved. Despite its higher accuracy, TPP has a smaller molding range and requires more expensive equipment. It can currently only be used for the photocurable manufacture of “transparent” precursor conversion ceramic materials, which significantly limits its scope of use.

Table 1 provides a comprehensive comparison of the three ceramic photopolymerization-based 3D printing processes described in this article, listing the differences in the various factors involved in each process.

Table 1.

Comprehensive Comparison of Photopolymerization-Based Ceramic Three-Dimensional Printing Technologies

Technique	Applicable feedstock	Printing size	Resolution	Speed	Surface quality	Feedstock cost	Processing cost
Stereolithography	Slurries/PCPs	100 μm–100 cm	μm	Slow	High	High	Medium
Digital light processing	Slurries/PCPs	100 μm–100 cm	μm	Medium	High	High	Medium
Two-photon polymerization	PCPs only	1 μm–1 cm	nm–μm	Very slow	Very high	High	High

PCPs, preceramic polymers.

The main drawbacks of ceramic 3D printing technology, including photocuring, are the relatively low production efficiency and the subsequent steps for removing the support structure are more complex, which make it challenging to achieve automation. However, ceramic photocuring technology still has much room for improvement. It is expected to evolve around the following areas:

• 1.The manufacturing of large-size ceramic parts: The small size of the photopolymerization-based forming table is limited by the optical system, which seriously restricts the development of ceramic part size and hinders the transformation of research results in ceramic photopolymerization-based practical applications. As such, the photopolymerization-based manufacturing of large-size ceramic parts has become a pressing problem to solve.
• 2.The expansion of raw material types: At present, photopolymerization-based ceramic 3D printing technology is mainly used for the fabrication of oxide ceramics, such as Al₂O₃ and ZrO₂, and there is less research on some nonoxides with excellent performance. Due to the continuous progress in the aerospace field, the industry's demand for high performance and high precision complex structural ceramics is increasing, and the advantages of photocuring technology applied to nonoxide ceramics are becoming increasingly prominent. In addition, more environmentally friendly research into water-based slurry systems will become a significant trend in future research.
• 3.For some multilayer combined composite structural ceramic parts with multiple functional modules or multiple independent structures, there is a need to explore manufacturing methods that use a combination of various materials and multiple 3D printing processes. This can achieve excellent performance that is difficult to obtain with a single material, while also further enhancing manufacturing efficiency. For example, consider transforming a single slot in a DLP printer into multiple rotatable slots, where different ceramic pastes are placed in each space. Using this method, the printing process can be controlled to change the material of each printing layer freely, ultimately obtaining multimaterial ceramic prototypes.
• 4.Improving the performance of 3D printed ceramic parts: The 3D parts printed from ceramic precursors are generally accompanied by significant volume shrinkage during pyrolysis to produce ceramic pieces, which can quickly cause defects (e.g., collapse and cracks), which have an extremely negative impact on the performance of ceramic pieces. Optimizing the printing material system, printing technology, and pyrolysis and sintering processes to improve the quality of ceramic works requires further in-depth research.
• 5.The proposal of novel printing technologies: While most current research in ceramic photocuring focuses on materials and processes, some complex problems also continue to be faced regarding ceramic photocuring technology. More practical and efficient photocuring methods can significantly contribute to the development of photocuring technology in application.

Authors' Contributions

Writing—original draft, S.W.; Writing—review and editing and conceptualization, J.L.; Supervision, H.J.; Investigation, Y.Y., G.H,. and J.W. All authors have read and agreed to the published version of the article.

Author Disclosure Statement

No competing financial interests exist.

Funding Information

This work was supported by the National Natural Science Foundation of China (52173266, 51772253), the open fund of Fujian Provincial Key Laboratory of Functional Materials and Applications (Xiamen University of Technology) (fma2020001), and Scientific Research Climbing Program of Xiamen University of Technology (XPDKT20003), China.