Additive manufacturing of flexible polymer-derived ceramic matrix composites

ABSTRACT
 It remains challenging to broaden the application fields of ceramics, largely because the hardness and brittleness of ceramics mean that they cannot undergo shape reconfiguration. In this study, we developed an ultraviolet light-curable preceramic polymer slurry, and this slurry was used for digital light processing printing of flexible green parts in designed shapes. These parts were subsequently transformed into complex structures by an assisted secondary molding strategy that enabled the morphology of their green and pyrolyzed forms to be well controlled. The collapse of bulk pyrolyzed parts was avoided by impregnating their precursors with silicon nitride (Si3N4) particles. The effects of different proportions of Si3N4 on the weight loss, shrinkage, density, porosity, and mechanical properties of the pyrolyzed composites were investigated, and the bending strength and Vickers hardness of the composites with 10 wt.% Si3N4 were found to be 130.61 ± 16.01 MPa and 6.43 ± 0.12 GPa, respectively.


Introduction
In recent years, the development of additive manufacturing (AM), also known as three-dimensional (3D) printing, has enabled the fabrication of a wide range of advanced ceramic components that are difficult or impossible to fabricate via traditional ceramic manufacturing processes (Berman 2012;Zhang et al. 2022).AM generates 3D objects by the layer-by-layer deposition of materials, whereas traditional ceramic manufacturing generates 3D objects by the molding of materials (Tofail et al. 2018).AM technologies used in the fabrication of ceramic materials include selective laser sintering (SLS) (Bertrand et al. 2007), binder jetting printing (BJP) (Fu et al. 2013), direct ink writing (DIW) (del-Mazo-Barbara and Ginebra 2021), and stereolithography (SLA) (Li et al. 2020) and digital light processing (DLP) (Guo et al. 2019;Chen et al. 2021).However, the static and rigid structures produced by these AM technologies cannot exhibit the dynamic functionality required in structures for advanced applications, the demand for which is increasing (Zhang, Demir, and Gu 2019).
To address this problem, four-dimensional (4D) printing-which is described as '3D printing + time'-has been devised (Momeni, Liu, and Ni 2017;Azlin et al. 2022).A 4D-printed object is defined as a 3D printed object that can change its shape, properties, or function over time in response to a predefined external environmental stimulus (Khoo et al. 2015).Well-developed 3D printing technologies and suitable stimulus-responsive materials are the two key elements of 4D printing, which will enable the generation of novel parts and components for innovative applications in many fields, such as aerospace, soft robotics (e.g.flexible actuators), and biomedical fields (Mitchell et al. 2018;González-Henríquez, Sarabia-Vallejos, and Rodriguez-Hernandez 2019;Daerden and Lefeber 2002).
Thus far, research on 4D printing technology has mainly focused on the macroscopic deformation behaviours of parts made from shape-memory alloys, polymers, and hydrogels, such as their bending, elongation, twisting, and corrugation (Zhang, Demir, and Gu 2019), as these materials respond to stimuli such as heat, light, water, electrical currents, and magnetic fields (Zafar and Zhao 2020;Joshi et al. 2020).In comparison, ceramic materials have strong ionic or covalent bonds and thus a stable structure, so neither printed green ceramic objects nor sintered ceramic objects are flexible, and thus they cannot undergo further shape reconfiguration (Zhang et al. 2019).It is therefore critical to develop elastomeric ceramic materials and associated 4D printing processes to expand the range of components available for use in advanced applications.
Polymer-derived ceramics (PDCs) are a class of ceramic materials that can be formed directly from precursors by pyrolysis, without the need for sintering (Colombo et al. 2010;Xia et al. 2020).For instance, polycarbosilane, polysiloxane, polysilazane, and polyborosilazane can be pyrolyzed to silicon carbide (SiC), silicon oxycarbide (SiOC), silicon carbon nitride (SiCN), and silicon boron carbon nitride (SiBCN), respectively.Moreover, the pyrolysis of PDCs is completed at relatively low temperatures (typically 800-1300°C) (He et al. 2020;Zanchetta et al. 2016), and PDCs are resistant to oxidation, creep, and phase separation at temperatures up to 1500°C and higher (Colombo et al. 2010;Zanchetta et al. 2016).Furthermore, preceramic polymers can be modified such that they can be converted to ceramic parts with the compositions and microstructures necessary to exhibit desired performances and functionalities (Riedel et al. 2006;Zhou et al. 2020).In particular, flexible preceramic polymer materials can be easily designed to meet the demand for 3D-printed and deformable green ceramic parts, which provides the opportunity to realise the 4D printing of deformable ceramic structures that can be stably transformed into various shapes.
The few studies that have been performed on the 4D printing of PDCs have primarily used DIW technology.Liu et al. (Liu et al. 2018) developed a new composite ink containing nanoparticles doped with a preceramic polymer, and found that the green parts generated via DIW with this ink were soft and elastic.Thus, this ink was used to construct various complex ceramic origami structures and elastomer-derived ceramics with excellent properties.This work of Liu et al. was the first reported exploration of the 4D printing of ceramics, and laid foundations that broadened the applications of ceramics.Huang et al. (Huang et al. 2020) fabricated 2D silicone elastomer objects by DIW, and converted these to geometrically complex 3D objects via filament-assisted origami or self-adhesive origami deformation.Moreover, the pyrolysis of these 3D objects to ceramic forms did not affect their structure.(Zhang et al. 2019) formed flexible green parts by DIW and photopolymerization of yttria-stabilized zirconia powder.They also applied self-assembly-assisted shaping and mold-assisted shaping for the post-printing reconfiguration of green parts, which were then converted into fine-featured ceramics via pyrolysis.
Photocuring technologies such as SLA and DLP are based on the formation of cross-linked polymerisation networks under ultraviolet light (UV) irradiation, and generate objects with higher surface quality and resolution than those generated by DIW (Schmidt and Colombo 2018).However, although the preparation of PDCs by photocuring has been demonstrated (Liu et al. 2018;Li et al. 2020;Wang et al. 2019;Zanchetta et al. 2016), the 4D photocuring of flexible green parts has not been reported.
In this work, we developed a UV-curable preceramic polymer slurry for the DLP printing of photocured flexible green parts.We found that auxiliary constraint treatment transformed the flexible green parts into objects with complex secondary structures, and that when these objects were pyrolyzed to PDCs they retained these structures.In addition, we used Si 3 N 4 powders as inert fillers in PDCs to generate PDC composites, and demonstrated that bulk samples of these PDC composites were immune to cracking and collapse during pyrolysis to ceramic composites.The effects of Si 3 N 4 content on printing slurries and curing behaviours were studied, and the microscopic morphologies and mechanical properties of the PDC composites were investigated.

Materials
A liquid photocurable polysilazane (PSZ, HTA 1500SC, AZ Electronic Materials Co., Ltd., UK) and vinyltrimethoxysilane (VTMS, Macklin, China) in this research were used as the preceramic polymers.A di-functional aliphatic urethane acrylate (U600, Royal DSM, the Netherlands) with high good flexibility and high photopolymerization reactivity was used as the photocuring resin.2,4,6-Trimethylbenzoyldiphenylphosphineoxide (TPO, IGM, the Netherlands) was used as the photoinitiator for initiation of the radical polymerisation of blended slurries upon exposure to UV light.α-Si 3 N 4 powder (d 50 = 700 nm, Denka, Japan) was used as the inert filler, and a scanning electron microscopy image of this material is presented in Figure S1.Sudan III (Macklin, China) was used as a photoabsorber.

Preparation of unfilled UV-curable slurry
A mixture comprising a 1.5:1.0mass ratio of PSZ and VTMS was magnetically stirred for 4 h at room temperature.And then add the U600 to the pre-solution with a mass ratio of 2:1 And then blended with the weight ratio of pre-solution and U600 in 2:1.Additionally 3wt.%TPO of U600 and 0.025 wt% Sudan III were added and the resulting mixture was magnetically stirred for 4 h to form a homogeneous mixture.

Preparation of Si 3 N 4 -filled UV-curable slurry
Si 3 N 4 powder was added to a pre-solution prepared as above, and the resulting mixture was ball-milled by an omni-directional planetary ball mill (PMQW04, Chishun Tech, China) for 2 h at 350 r/min to form a pre-slurry.This was then treated with a 2 wt% Oil-based polyurethane defoamer (Datian chemical, China) to eliminate the bubbles generated during milling.Subsequently, the U600 was added to the pre-slurry with a mass weight ratio of 2:1 and additionally, 3wt.%TPO of U600 was also added.The resulting mixture was ball-milled for 2 h and then evacuated to remove bubbles, which afforded a homogeneous UV-curable slurry.A certain mass ratio gradient (10, 20, 30, 40 wt.%) was calculated based on the system, note that, to ensure that the UV curing is complete, the content ratio of the U600 is fixed, and the composition ratio in the pre-slurry is to be changed with fillers content.

DLP printing
The principles of and equipment used in the DLP-based stereolithography in this study have been described (Lin et al. 2022;Liu et al. 2016).The DLP printer (SprintRay, Soonsolid, China) emitted UV light at an intensity of 10 mW/cm 2 , and exposure time and slice thickness were easily adjusted and its structure schematic was shown in Figure S2.
A pre-designed 3D model was used to generate STL files that were outputted via 3D design software.These files were imported into the DLP printer, which re-combined them into a 3D model that it then sliced into a series of adjacent 2D images.Adding the slurry with a spoon to the vat after removing the air bubbles until it is submerged past the lowest point of the blade.The building platform then moved down to one-layer-thickness away from the vat, and immediately afterward the light source projected a beam of UV light containing the sliced pattern information of the first layer to solidify (by cross-linking) the slurry on the building platform.Subsequently, the building platform returned to its initial position, and the UV-curable slurry was recoated by the blade as the vat rotated.The DLP printer automatically repeated the above steps, thereby performing a layer-by-layer construction process that ultimately yielded the flexible target green parts.These were carefully removed from the building platform and rinsed with cyclohexane to remove the residual slurry on their surfaces.

Shape transformations
Low-level stresses were manually applied to the flexible and thin green parts, which caused them to undergo simple transformations such as bending, twisting, folding, and compression and thus adopt new 3D shapes.In addition, appropriately designed flexible and thin net structures were assembled into the corresponding regular polyhedrons, namely a cube and a tetrahedron.Wire or tape was used to fix the shape of some transformed parts.The shape transformations process is shown in Figure 1e.

Pyrolysis
The 3D-shaped green parts were placed in a corundum tube, and then subjected to pyrolysis under the N 2 atmosphere to accomplish the desired polymer-toceramic transformation (Figure 1d).
Each part was pyrolyzed by being successively (i) heated from room temperature to 600 °C at a constant rate of 0.5 °C/min, with heating for 1 h at each of the critical temperature nodes of 300 and 600 °C; (ii) heated at a constant rate of 1 °C/min to 1000 °C, and then held for 1 h at this temperature; (iii) cooled to 500 °C at a rate of 3 °C/min; and finally (iv) cooled to room temperature under furnace cooling.

Characterisation
Fourier-transform infrared (FTIR) spectroscopic analyses of samples were performed on an infrared spectrometer (Nicolet IS50, Thermo Fisher, USA).The viscosity of the UV-curable slurries was measured by a rheometer (Physica MCR 301, Anton Paar, Austria) at room temperature.The cured-slice depth of the slurry was measured with a digital micrometer after a single exposure (adjust the exposure time to output different exposure energy).The mass loss of samples during pyrolysis was determined by thermogravimetric analysis (TGA) (TGA 4000, PE, the Netherlands) at a ramping rate of 5°C/min under a N 2 atmosphere.The phase of the samples after pyrolysis was characterised by X-ray diffraction (XRD; D8 Advance, Bruker, Germany).The microstructures and morphology of the samples after pyrolysis were observed via scanning electron microscopy (SU8220, Hitachi, Japan), and an energy dispersive spectroscopy (EDS) probe was used to determine the composition of the PDCs.The three-point bending strength of the cuboid PDC was measured on an electronic universal mechanical testing machine (WDW-5E, Shidaishijin, China).The density and porosity of PDCs were tested by the Archimedes method.The ceramic yield and shrinkage of PDCs were measured using an electronic balance and digital micrometer, respectively; a minimum of six samples were measured and the average value was calculated.The Vickers hardness and Vickers hardness imprints of samples were obtained by applying a test force of 3 Kg using a Vickers hardness testing machine (HVS-30Z, Shanghai Precision Instrument Co., Ltd., China).

Photopolymerization process
Figure 2 depicts the FTIR spectra of PSZ, VTMS, and U600.In the PSZ spectrum, the peak at ∼885 cm -1 represents Si-N-Si stretching and the peak at ∼2120 cm -1 represents Si-H stretching; The Si-H group would participate in photopolymerization with the acrylate groups represented by the peaks at ∼1619 cm -1 , ∼1636 cm -1 , and ∼1720cm -1 in the spectrum of U600 (Liu et al. 2018;Xiao et al. 2020;Ma et al. 2020).In the spectrum of VTMS, the peak at ∼764 cm -1 represents Si-O-Si symmetric stretching, and the peaks at ∼1410 cm -1 and ∼1600 cm -1 represent the vinyl groups on the side chain of the structure, which would also participate in the photopolymerization reaction (Abidi, Hequet, and Tarimala 2016).
To examine the cross-linking of slurries, FTIR spectra of PCPs-0 and PCPs-20 before and after single-layercuring were obtained.The spectra of PCPs-0 cured and PCPs-20 cured had much smaller peaks for Si-H, vinyl, and acrylate groups than the spectra of two photoactive uncured blends, confirming that cross-linking (i.e.photopolymerization) had occurred under UV exposure.In addition, the similarity of the PCPs-0 cured and PCPs-20 cured spectra indicates that Si 3 N 4 did not participate in the photopolymerization process.

Characterisation of rheological behaviour and curing ability
A preceramic polymeric UV-curable slurry must exhibit excellent fluidity and printability to be applicable in DLP AM.Thus, slurries must have low viscosity and high homogeneity, so that they respread and self-level during DLP (Huang et al. 2021).
Figure 3a shows the rheological behaviours of the slurries containing different proportions of Si 3 N 4 powder.As preceramic polymers have low viscosity, PCPs-0 had a low viscosity that was approximately maintained at 0.09 Pa•s.With increasing proportions of Si 3 N 4 powder, the slurries' viscosity and extent of shear-thinning behaviour gradually increased.That is, as shown in Figure 3b, as the proportion of Si 3 N 4 powder in slurries increased from 10 to 40 wt%, their viscosity increased from 0.19 Pa•s to 3.93 Pa•s at a shear rate of 10 s -1 .The PCPs with 50 wt% Si 3 N 4 had a viscosity of 22.8 Pa•s at 10 s -1 , which prevented its use in DLP printing.
The curing behaviours of slurries must also be optimised to enable DLP printing, according to the Beer-Lambert model: (Halloran 2016;Li et al. 2019;Wang et al. 2019). (1) where C d is the measured cure depth, S d is the depth sensitivity, E is the exposure energy dose, E d is the depth critical energy dose, P is the light power, and T is the exposure time.The cure depth of the five slurries was measured after single-layer curing at various E dosages, and the obtained relationships are shown in Figure 4a.The cure depth of each slurry increased with E dosage.In addition, the cure depth of the slurries dramatically decreased with increasing Si 3 N 4 powder content, so the PCPs-10-40 had a significantly weaker curing ability than that of PCPs-0, which is attributed to the high scattering and UV absorbance of Si 3 N 4 particles (Li et al. 2020;Zakeri, Vippola, and Levänen 2020).
The fitting equation (see the legend in Figure 4b) and the corresponding image (Figure 4b) were obtained by Meanwhile, E d decreased from ∼107.4-4.5 mJ/cm 2 for these slurries, this may be explained by the fact that when the ratio of photosensitive resin and photoinitiator are kept constant during slurry preparation.When Si 3 N 4 content (ρ about 3.2 g/cm 3 ) increases, the mass of preceramic polymers (ρ about 1 g/cm 3 ) decreases relatively, leading to a decrease in the total volume of various system with the same quality, and thus the concentration of photoinitiator per unit volume increases making it possible for the photopolymerization reaction to occur at lower energy.
During the DLP printing process, good cohesion between layers without delamination can be achieved by setting the sliced thickness to less than S d (Hinczewskia, Corbela, and Chartierb 1998;Huang et al. 2021).In addition, to trigger the photopolymerization reaction, the E dose must be greater than E d , which can be achieved by adjusting the exposure time at a certain power density.Therefore, in addition to considering the rheological behaviours and curing abilities, the specific printing parameters of PCPs-0, PCPs-10, PCPs-20, PCPs-30, and PCPs-40 slurries need to be determined in experiment.

Shape transformation of various green parts
A complete 4D printing strategy of ceramics must include the following four steps: (1) the design of a geometric model; (2) the 3D printing of green parts; (3) the shape transformation of green parts; and (4) the pyrolysis of green parts.The design of basic shapes is another critical step, and in this study, a reasonably specific geometry was easily achieved via the expected shape transformation The design of the basic model is another critical step, in this study, designing various basic models so as to achieve the expected shape transformation.In addition, the thickness of the printed green parts was adjusted to adapt to the deformation process,  as a printed flat sheet can theoretically be folded provided that its width is much greater than its thickness.In this study, a desktop DLP 3D printer was used for the fabrication of flexible 1-mm-thick green parts that could be folded into a predesigned shape under the action of external force (4D printing).
As illustrated in Figure 5, several flat green parts of various shapes were printed.These exhibited significant deformations under external force, such as bending, rolling/twisting, folding, and compression, thereby forming various target 3D structures.These aspects are summarised below.

1) Bending
The bending angle of a six-petal flower, with a wire constraint, could be dynamically modified to represent various phases of blossoming (Figure 5a).

2) Rolling/Twisting
A 36 mm × 5 mm rectangle was rolled into a cylinder or twisted into a spiral (Figure 5b); a rope-like strip was knotted without the use of restraints (Figure 5c); two long strips of the same length were rotated and intertwined to form a twisted braid, and the ends were taped (Figure 5e); and a rectangular sheet was twisted into a spiral (Figure 5f).

3) Folding
An axisymmetric sheet was folded along its horizontal axis of symmetry in the opposite direction, and the resulting bow shape was retained with wire or tape at the folds (Figure 5d).

4) Simple assembly
Regular polyhedra (a cube and a tetrahedron) could also be formed by the simple assembly of appropriate nets.These polyhedra retained their structure during pyrolysis; no collapse or defects were visible (Figure 5g, h).

5) Compression
Some porous complex parts, such as a porous sphere (buckyball) and a porous cube (Figure S3a, c), were also printed, demonstrating the good deformation-recovery capability of the PCPs-0 slurry.That is, these shapes reverted to their original configuration when the compression force was removed, indicating the shape transformation process was reversible (Movie S1, S2).The pyrolyzed porous ceramic structures also retained their precursors' morphology (Figure S3b, d).
Wire or tape was applied to specific areas of deformed structures to preserve their shape.Moreover, as described above, they retained their deformation during pyrolysis at 1000 °C.The wire loosened or even fell off, and the organic-polymer tape vaporised, leaving no residue on the resulting PDCs.Furthermore, the PDCs' structures were maintained without visible collapse or macro-cracks.Thus, although the use of wire or tape caused local stress concentrations in the structure, resulting in small deformations at the corresponding locations, no severe residual stresses were generated.This is attributable to the perfect bonding between the adjacent layers, which maintained the overall integrity of the structures.In addition, PCPs-20 could be printed, and exhibited shape transformations under the same external forces (Figure S4 and Movie S3, S4).
These results will enable various approaches to be explored for increasing the degrees of freedom and complexity in printed structures in future.

Polymer-to-ceramic conversion
To obtain the desired pyrolysis products, the thermal stability behaviours of materials must be analysed.Therefore, as shown in Figure 6a, the printed green parts of PCPs-0 and PCPs-20 were subjected to TGA.The two curves have a similar profile, indicating that the pyrolysis did not involve Si 3 N 4 .However, the part printed using PCPs-20 exhibited a higher residual weight than that printed using PCPs-0, which was attributed to the presence of Si 3 N 4 in PCPs-20 reducing its polymeric content.
The process of weight loss could be divided into three stages.The first stage occurred from room temperature to 300 °C, which is attributable to the evaporative loss of low-molecular-weight oligomers and physically adsorbed water.The second stage occurred from 300 to 500 °C, it involved a more rapid decrease in weight than the first stage, and is attributable to the thermal decomposition of the organic resin and the decomposition of macromolecular copolymers.The third stage occurred at temperatures above 600 °C, although no obvious weight loss of printed parts was observed; this represents the polymer-to-ceramic conversion step.Based on these results, a pyrolysis heating-regime curve was designed to ensure the integrity of parts after pyrolysis (Figure 6b).
EDS analyses of the surface of the pyrolyzed samples were performed to determine the elemental composition of the amorphous matrix.SEM images and corresponding EDS maps of the surface of the pyrolyzed  specimens are shown in Figure S5.The EDS results revealed that the matrix was composed of SiCNO compounds (Figure S5a), and the SEM and EDS results indicated that the Si 3 N 4 particles were encapsulated by amorphous compounds (Figure S5b).Additionally, there was no significant difference between the elemental distribution on the PDCs-0 samples and that on the PDCs-10 samples, but the presence of Si 3 N 4 led to an enrichment of Si and N in the particle distribution in the PDCs-10 samples.
Unlike the pyrolyzed thin parts fabricated from the PCPs-0 slurry (Figure 5), the bulk samples fabricated from the PCPs-0 slurry collapsed and deformed after pyrolysis (Figure S6a).This has been described in many other studies, and has been attributed to the extreme volume shrinkage caused by the great increase in density and porosity resulting from the release of gases during a polymer-to-ceramic conversion.This results in extended cracking and pore formation in pyrolyzed products, and thus causes structural instability (He et al. 2020;Li et al. 2020;Xiong et al. 2019;Colombo et al. 2010).In contrast, in the current study, the thin or porous parts did not exhibit crack formation or structural collapse as their thin profiles facilitated the release of gases from the matrix during the pyrolysis process (Greil 2000;Bernardo et al. 2014).Hence, polymer-to-ceramic conversion of the bulk PDCs with structural damage occurring was common.
However, the filler volume effect was exploited to solve this problem (Bernardo et al. 2014;Zhou et al. 2020).Accordingly, Si 3 N 4 powder was introduced into a mixture of precursors, and the resulting pyrolyzed bulk PDCs exhibited no obvious external flaws.The bulk PDCs-10 and PDCs-20 are shown in Figure S6b  and c, respectively, to illustrate the feasibility of inertfiller treatment of pure precursors.
The typical parameters of PDC samples with various proportions of Si 3 N 4 powder are summarised in Table 1.With increasing Si 3 N 4 content, the shrinkage of samples along all three axes decreased.The z-axis displayed a larger shrinkage than that along the x/y axes, which may be due to pyrolysis eliminating the voids in a green printed part caused by the large tensioning forces that occur when each cured layer is separated from the release film (Zhang et al. 2021).The existence of tiny gaps between the layers of PDCs-0 and PDCs-20 samples pyrolyzed at low temperature by 500°C (Figure S7a, c) disappeared when pyrolyzed at high temperature at 1000°C (Figure S7b, d) also verified this explanation.In addition, the ceramic yield gradually increased with increasing Si 3 N 4 content; the ceramic yield of PCDs-20 was 47.52%, which is approximately consistent with the final residual weight determined from the TGA result in Figure 7a.
However, as the Si 3 N 4 content increased, the porosity of the PDCs increased, due to their numbers of pores and inter-particle voids increasing.This may be attributable to two phenomena.First, the matrix would have undergone violent shrinkage during pyrolysis, leading to more rigid particles colliding with the matrix and the interface, generating high local stresses and eventually leading to the formation of cavities (O'Masta et al. 2020).Second, owing to the presence of tiny bubbles in the slurry, pore defects were increased in the structures, even after pyrolysis.These bubbles may have been present for the following reasons: (1) PSZ is sensitive to moisture, so the exposure of the slurry to air is likely to have caused hydrolysis reactions that released gas molecules, which formed bubbles during the DLP printing process; (2) The constant coating of the blade with slurry also produced tiny bubbles.Furthermore, any bubbles generated in the lower-viscosity slurries could have easily achieved overflow and burst independently, without affecting the quality of curing molding.However, bubbles generated in the higher-viscosity slurries would have been locked into the slurries, so they would have been unable to overflow or break independently, and thus may have caused increases in porosity.As a result, the density of PDC composites containing more than 20 wt.% Si 3 N 4 was lower than the density of those with less than 20 wt.% Si 3 N 4 , due to the greater number of pores in the former composites.
Overall, the introduction of Si 3 N 4 into the pure preceramic polymer not only reduced linear shrinkage but also increased the ceramic yield in the pyrolyzed products.More importantly, it prevented the collapse of bulk samples during the polymer-to-ceramic conversion process.However, an excessive proportion of Si 3 N 4 caused the porosity of the PDCs to be too high, which drastically decreased their density.
The fracture surface of the samples pyrolyzed at 1000 °C was characterised by scanning electron microscopy (Figure 8).As seen in Figure 8a, the fracture surface of the PDCs-0 sample was very flat and the matrix was uniform, suggesting that this was a typical brittle fracture.In contrast, the fracture surface of the PDCs-10 sample was uneven (Figure 8b), indicating the presence of Si 3 N 4 particles affect the fracture behaviour and thus increased the fracture toughness.due to the presence of Si 3 N 4 particles, indicating that the mechanical behaviour had begun to change from brittle fracture to ductile fracture.However, with further increases in the content of Si 3 N 4 , the continuity of the amorphous matrix was weakened, and the Si 3 N 4 particles in the matrix had an agglomerated and exfoliated morphology, which caused more pores to be formed (Figure 8d, e).The polished surface of the samples is presented in Figure S8.The Si 3 N 4 particles were evenly dispersed throughout the matrix, and there were few pores in the PDCs-10 sample (Figure 8b).However, the number of pores on the surface increased with increasing Si 3 N 4 content, and these pores were randomly distributed.Moreover, there were large-porosity defects on the surface of the PDCs-40 sample (Figure S8e), which were possibly formed by the aggregation of tiny bubbles during printing.These results support the  explanation provided above for the increase in porosity, and correspond to the porosities listed in Table 1.

Mechanical properties of the PDCs
Figure 9 presents the mechanical properties of the PDCs.Si 3 N 4 particles acted as the reinforcement phase in the network of the amorphous matrix, and thus contributed to the establishment of the pinning effect in the amorphous matrix.This improved the mechanical properties of the PDCs.PDCs-10 had excellent mechanical properties, such as a bending strength and vickers hardness as high as 130.61 ± 16.01 MPa and 6.43 ± 0.12 Pa, respectively.However, as the Si 3 N 4 content increased, the bending strength and vickers hardness of the samples gradually decreased; in particular, the bending strength and vickers hardness of PDCs-40 was very low (19.61 ± 3.16 MPa and 1.38 ± 0.04 GPa, respectively).Moreover, representative bending stress-strain curves of the PDCs composites as shown in Figure S9 revealed the typical brittle fracture behaviour of all the samples.As discussed, increases in the content of Si 3 N 4 in PDCs increased their porosity and decreased the continuity of their matrix phases, which directly affected their mechanical properties.Therefore, although the Si 3 N 4 particles acted as a reinforcement phase that greatly improved the mechanical properties of the PDCs at suitable contents, the use of an excessive Si 3 N 4 content severely degraded the mechanical properties of the PDCs.
In addition, the Vickers hardness imprints of surfacepolished PDCs-0 and PDCs-10 samples were observed by SEM (Figure 10).Compared with the local indentation on the SEM image of the PDCs-0 sample, the indentation on the PDCs-10 sample was shallower (Figure 10b), and there was a local deflection of the tip of the radial crack, due to the presence of Si 3 N 4 particles (Figure 10b, c), which was the expected result of particle toughening.This also indicates that the addition of an appropriate proportion of Si 3 N 4 particles increased the strength and toughness of PDCs, which enhanced their mechanical properties.

Conclusion
A UV-curable slurry was prepared by mixing a preceramic polymer with a photosensitive resin.The green parts fabricated from these slurries using DLP printing technology exhibited good flexibility, and could be transformed by the manual application of various forces into complex 3D structures, whose forms were preserved with wire or tape.The pyrolysis of these structures generated ceramic parts that retained the shape of their polymeric precursor, and which no longer needed to be wired or taped.
The common problem of the collapse of bulk samples during pyrolysis was solved by introducing Si 3 N 4 particles into PDCs to function as inert fillers.The linear shrinkage and weight loss of the Si 3 N 4 -containing PDCs decreased with increasing Si 3 N 4 content.The bending strength and Vickers hardness of the PDCs containing 10 wt.% Si 3 N 4 were 130.61 ± 16.01 MPa and 6.43 ± 0.12 GPa, respectively.However, above a certain wt.% of Si 3 N 4 , the PDCs exhibited worse mechanical properties than those with less than this wt.% of Si 3 N 4 , due to the presence of more void defects and particle agglomerates in the former PDCs.
This novel DLP printing-based strategy for the fabrication of PDCs or ceramic composites with deformable structures will broaden the opportunities for manufacturing ceramic parts with unique and complex structures.It will also promote the application of ceramic materials in many demanding applications.

Disclosure statement
No potential conflict of interest was reported by the author(s).

Notes on contributors
Jun Ou, the first author of this manuscript, is a master's student at Guangdong University of Technology, and his main research topic is 4D printing ceramic materials.
Minzhong Huang, the corresponding author of this manuscript, is a postdoctoral researcher at Guangdong University of Technology, mainly engaged in additive manufacturing research of advanced structural ceramics and functional ceramics.
Yangyang Wu, the third author of this manuscript, is a master's student at Guangdong University of Technology, and his main research topic is 3D printing ceramic materials.
Shengwu Huang is a master's student at Guangdong University of Technology, and his main research topic is 3D printing porous ceramic materials.
Jian Lu works at City University of Hong Kong.He is an academician of the National Academy of Technologies of France and a Fellow of Hong Kong Academy of Engineering Science.His research interests focus on the fabrication and mechanical properties of nanomaterials and advanced materials, experimental mechanics, materials surface engineering, and simulation.
Shanghua Wu works at Guangdong University of Technology.His research interests include additive manufacturing of ceramic materials, the development and fabrication of ceramic components for integrated circuits, and the highspeed and efficient processing of difficult-to-machine materials.

Figure 1 .
Figure 1.Schematic illustration of the fabrication process of PDCs or PDC composites.

Figure 3 .
Figure 3. (a) Rheological behaviours of printing slurries with various Si 3 N 4 powder contents; (b) viscosity of printing slurries containing various Si 3 N 4 powder contents at a shear rate of 10 s -1 .

Figure 4 .
Figure 4. (a) Relationship between printing slurries and cure depth at different exposure energy dosages (E); (b) linear fitting curves of the dependence of cure depth on the natural logarithm of the exposure energy dose (LnE) in slurries with different contents of Si 3 N 4 .

Figure 5 .
Figure 5. Illustration of various flexible printed structures, transformed structures, and pyrolyzed structures.(a) A flat sixpetalled 'flower' and the same structure transformed into a convex six-petalled flower; (b) a flat rectangular sheet and the same structure transformed into a cylinder; (c) a long strip and the same structure transformed into a knot; (d) a thin structure and the same structure pinched into a bow; (e) two long flat strips and the same structures twisted together to form a braid; (f) a flat rectangular sheet and the same structure twisted into a spiral.Flat net structures and the same structures folded into a cube (g) and a tetrahedron (h).

Figure 6 .
Figure 6.(a) TG curves of printed green parts; (b) curve of pyrolysis heating regime.

Figure 8 .
Figure 8. SEM images of the fracture surface of pyrolyzed samples.

Funding
This work was supported by Key-Area Research and Development Program of Guangdong Province (Grant No. 2020B090923002), Local Innovative Research Team Project of Guangdong Pearl River Talents Program (Grant No. 2017BT01C169), and Xijiang Innovation Team Introduction Program of Zhaoqing.

Table 1 .
Typical parameters of PDC samples pyrolyzed at 1000 °C, where x-, y-, and z-values indicate the shrinkage on the corresponding axes of samples.