TANG Cong ¹, WU Haidong ¹, GE Shuai ¹, FAN Hanjing ¹, FENG Bocong ¹,

CHEN Zhongchun ², CUI Huachen ³, DENG Xin ¹, WU Shanghua ¹

(1. School of Electromechanical Engineering, Guangdong University of Technology, Guangzhou 510006, Guangdong, China;2. Faculty of Engineering, Tottori University, 4-101 Koyama-cho, Tottori-shi, Tottori Prefecture, Japan; 3. Smart Manufacturing Thrust, Systems Hub, The Hong Kong University of Science and Technology (Guangzhou), 

Guangzhou 511458, Guangdong, China)

Extended abstract:

[Background and purposes] Zirconia ceramics have attracted extensive attention in aerospace, biomedical engineering and precision manufacturing, due to their outstanding mechanical strength, excellent chemical stability and favorable biocompatibility. With the increasing demand for ceramic components featuring complex geometries, high dimensional accuracy and customized designs, conventional ceramic fabrication methods based on molding and subtractive machining have become increasingly inadequate. These traditional techniques are often associated with high tooling costs, long processing cycles and limited flexibility in producing complex structures, which severely restrict the practical application of high-performance zirconia ceramics. Digital light processing (DLP), a photopolymerization-based additive manufacturing technique, provides a promising alternative for fabricating complex ceramic components with high precision and excellent surface quality. However, the successful application of DLP to zirconia ceramics is strongly constrained by challenges related to slurry formulation and curing behavior. In particular, the large refractive index mismatch between zirconia particles and photosensitive resins causes severe light scattering during exposure, leading to excessive curing depth and reduced dimensional accuracy. Meanwhile, insufficient solid loading or poor dispersion stability of ceramic slurries result in cracking during debinding and non-uniform shrinkage during sintering, thereby degrading densification and mechanical performance of the final ceramic products. Therefore, it is essential to develop a high-solid-loading zirconia ceramic slurry with promising dispersion stability and controllable curing behavior for DLP-based fabrication. The purpose of this study is to develop and optimize a photopolymerizable zirconia ceramic slurries suitable for DLP additive manufacturing, aiming to achieve both high solid loading and high curing precision. The effects of sintering temperature on the microstructure and mechanical properties of the fabricated zirconia ceramics are systematically studied.

[Methods] In this study, 3Y-TZP zirconia powder was used as the ceramic raw material. To improve compatibility between the ceramic particles and the photosensitive resin, the zirconia powder was surface-modified using the silane coupling agent KH570 through ball-milling. The photosensitive resin system consisted of ethoxylated pentaerythritol tetraacrylate (PPTTA) and 1,6-hexanediol diacrylate (HDDA), while dioctyl phthalate (DOP) was introduced as a plasticizer. A commercial dispersant (BYK110) was added to enhance particle dispersion and adjust the rheological behavior of the slurry. In addition, a small amount of curcumin was incorporated as organic dye to increase light absorption and regulate the curing depth during photopolymerization. The effects of dispersant content on slurry viscosity and flow behavior were systematically studied using a rotational rheometer to determine optimal formulation of the slurry. Curing behaviors, including curing depth and overcuring width, were evaluated under controlled exposure conditions to assess printing precision. High-solid-loading zirconia ceramic slurries with solid content of up to 50 vol.% were prepared for DLP printing. The printed green bodies were subsequently subjected to debinding and pressureless sintering at temperatures ranging from 1350 ℃ to 1500 ℃. Phase composition was analyzed by using X-ray diffraction (XRD), while microstructural evolution was examined by using scanning electron microscopy (SEM). Relative density of the sintered ceramics was measured by using the Archimedes method. Mechanical properties, including flexural strength, hardness and fracture toughness, were evaluated by using three-point bending tests and indentation methods.

[Results] Surface modification with KH570 significantly increased compatibility between the zirconia particles and resin matrix, leading to enhanced dispersion stability and reduced slurry viscosity. The optimal content of BYK110 was determined 2 wt.%, at which the slurry exhibited the lowest viscosity and favorable shear-thinning behavior, enabling smooth recoating during DLP printing. The introduction of curcumin effectively increased optical absorbance of the slurry, resulting in a reduced curing depth and overcuring width, which markedly improved the dimensional accuracy of the printed structures. A stable zirconia ceramic slurry with a solid loading of 50 vol.% was successfully prepared, which exhibited good printability and curing controllability. Sintering temperature was found to have a pronounced influence on microstructure and mechanical properties of the final ceramics. As the sintering temperature increased from 1350 ℃ to 1500 ℃, grain growth became more pronounced, accompanied by a decrease in hardness and an increase in fracture toughness. The zirconia ceramics sintered at 1450 ℃ exhibited the highest overall performance, achieving a relative density of 98.53% and a flexural strength of 924.64 MPa. No macroscopic defects such as cracking or warping were observed in the sintered components, indicating good shape retention and process stability.

[Conclusions] A high-solid-loading photopolymerizable zirconia ceramic slurry was developed for DLP additive manufacturing through the synergistic optimization of particle surface modification, dispersant regulation and curing behavior control using an organic dye. The optimized slurry enabled high printing precision, while maintaining excellent processability and sintering performance. The zirconia ceramics exhibited high density and superior mechanical properties, meeting the typical performance requirements for structural and biomedical ceramic applications. The proposed slurry formulation strategy and processing route provide a reliable materials design framework and practical guidance for the DLP-based fabrication of high-performance zirconia ceramics with complex geometries.

Key words: photopolymerization; additive manufacturing; zirconia ceramics; rheological properties; mechanical properties