Additive Manufacturing (AM), also known as 3D printing, is widely used due to its advantages such as high design freedom, no need for molds, mass customization, and the ability to construct complex shaped components. The existing 3D printing methods are developed, among which DLP technology has the advantages of fast printing speed, high dimensional accuracy, no need for molds, close to net shape, and high mechanical performance. Ceramic materials are widely used as high-temperature structural functional materials due to their high temperature resistance, oxidation resistance, corrosion resistance, and mechanical properties. However, compared with metal and polymer materials, ceramic materials are still immature in 3D print processing, facing some problems such as limited availability of suitable raw materials, difficulty in optimizing ratios, high costs, poor process stability, and limited mechanical properties. To overcome the problems above, combining photopolymerization 3D printing with ceramic sintering technology becomes an effective method. In this study, ceramic powder and photosensitive preceramic were ground to obtain a uniform slurry with a good flowability, and then high-precision green bodies were formed through DLP.

Polysilazane PSZ-ViH containing both vinyl and Si-H was prepared via ammonified of trimethylchlorosilane, methyltrichlorosilane, methylvinyldichlorosilane, and methylhydrogendichlorosilane. Referring to the reaction conditions above, a Vi-terminated/Si-H containing polysilazane PSZ-TViH was obtained with dimethylvinylchlorosilane, methyltrichlorosilane, and methyldichlorosilane as raw materials. Si-H containing polysilazane LSZ-H was obtained with trimethylchlorosilane, methyltrichlorosilane, and methyldichlorosilane as raw materials. When 2-isocyanatoethyl acrylate (ICA) was used as a modifier, the reaction occurred under stirring in dark conditions at room temperature for 24 h. 2,4,6-trimethylbenzoyl phenyl phosphonic acid ethyl ester (TPO-L) was added as a photosensitizer and stirred in dark conditions for 10 min. PSZ-ViH and Si₃N₄ powders with different mass fractions were ground, ICA and TMPTA were added, and then followed by TPO-L. Finally, a photosensitive polysilazane-based ceramic slurry was obtained after stirring in the dark for 10 min.

The photosensitive slurry was photo-cured and formed on the DLP printer Photon Mono 4k. The infrared spectra were determined by Fourier transform infrared spectroscopy (FT-IR). The curing thickness of photosensitive resin or slurry at different exposure durations was measured on a coating thickness gauge (HC622) by an iron-based F probe and an iron reference block. The rheological properties of photosensitive slurries were characterized by a rotary rheometer (DESKTOP BKLJ9S3), with steady-state shear rate scanning at room temperature and dynamic temperature, respectively. The thermo-behavior was characterized by differential scanning calorimetry (DSC) on a model Mettler Toledo DSC1 Q2000 thermal analyzer, and thermogravimetric analysis (TGA) on a model Discovery TGA 55 thermal analyzer with an argon gas flow rate of 40 mL/min and a heating rate of 10 ℃/min, respectively. The density of photo-cured printed parts was measured by the Archimede displacement method. The cross-sectional morphology of the material was characterized by field emission scanning electron microscopy (FESEM, Hitachi S4800), and the sample (ion sputter coated Pd layer) was fixed on the sample stage with conductive adhesive at an acceleration voltage of 5 kV and a vacuum degree of ≤ 10^-4 Pa.

PSZ-ViH with vinyl as side groups can be chemically modified with 2-Isocyanatoethylacrylate, and PSZ-ViH-ICA shows the maximum light-curing depth compared with PSZ-TViH-ICA and PSZ-H-ICA. Liquid photoinitiator TPO-L can equilibrate light reactivity and shelf-life via optimizing the TPO-L amount, leading to a good balance of transition depth (D_p=63.09 μm) and threshold exposure intensity (E_c=3.94 mJ/cm²). As a result, TPO-L can be chosen as a suitable photoinitiator in DLP processing for polysilazane slurry. Among the ceramic powders introduced in the slurry, the light-curing depth follows an order of SiO₂>Al₂O₃ > ZrO₂ > Si₃N₄ > AlN. SiC slurry shows a low light-curing depth, while ZrB₂ slurry cannot be light cured totally. Silicon nitride powder is applied as a slurry additive due to the proper light-curing depth and similar chemical components compared with polysilazane adhesive. Ideal light reactivity and higher material density can be realized by synergistic effect of combination addition of micro-sized and nano-sized Si₃N₄. Based on the model of the relationship between transition depth (D_p) and threshold exposure intensity (E_c), the processing parameters are determined (i.e., the layer width is 2/3 of transition depth, the exposure time is adjusted according to three times of threshold exposure intensity). DLP pringting with a high precision of polysilazane slurry can be realized for the production of complex structures (e.g., cellular structure). An interlayered adhesion occurs and the width of single layer is almostly equal to the setting value via analyzing the fracture morphology of light-curing samples with different powder contents and particle sizes.

The distribution of vinyl groups in polysilazane molecules significantly affected the crosslinking density and curing depth. Polysilazane PSZ-ViH containing vinyl side groups had a larger photopolymerization thickness than PSZ-TViH with terminated vinyl groups due to the uniform distribution of active sites in the former.

The photosensitizer screening experiment showed that liquid 2,4,6-trimethylbenzoyl phosphate ethyl ester TPO-L balanced the photo-curing activity and storage stability. The balance between transmission depth and critical exposure intensity could be achieved via optimizing the TPO-L dosage, making it a suitable photosensitizer for polysilazane slurry.

Oxide/nitride powders (such as SiO₂ and Si₃N₄) could ensure a light penetration due to their low refractive index and light color, while carbide/boride powders (such as SiC and ZrB₂) could suffer from severe light attenuation due to their high refractive index and dark color. The density of the photocured sample increased to 1.19 g/cm³ via mixing nano-/micro-sized ceramic powders (i.e., 5% 2 μm Si₃N₄ and 5% 20 nm Si₃N₄), and reducing the light scattering loss (i.e., transmission depth 52.80 μm), which verified the feasibility of micro-/nano-sized synergy in photosensitive polysilazane slurry.

Based on the theoretical model of transmission depth D_p and critical exposure intensity E_c, the processing parameters in DLP were proposed, i.e., the layer thickness was set at 2/3 of the transmission depth (h=2D_p/3), and the exposure time was calibrated to equal to three times of the energy of the critical exposure intensity (t=3E_c/I), thus achieving a high-precision DLP printing of complex structures such as honeycomb.