: Molding process is recognized as one of the most critical procedures in the preparation of Y₃Al₅O₁₂ (YAG) transparent ceramics, affecting directly the densification of YAG transparent ceramics as well as their mechanical, thermal and optical properties. Various molding processes (i.e., cold isostatic pressing, slip casting, tap casting, and gel-casting) are applied to fabricate YAG transparent ceramics. However, the emergence of designing philosophy and applying technology promotes brand-new requirements on configurations or hybrid-functions of transparent ceramics. It is more difficult to realize the integrated molding of those transparent ceramics via traditional molding processes. Digital light processing (DLP) based 3D printing technology is a developed technology for ceramic forming. Some ceramic parts with complex shapes, high accuracy on sizes and high surface qualities can be obtained by this technology. The ceramic materials with a high performance are fabricated in recent years. Nevertheless, little work on YAG transparent ceramics have been reported yet. This paper dealt with DLP 3D printing of YAG transparent ceramics, and investigated the slurry preparation, molding, degreasing and sintering processes of the ceramics.

Commercial alumina (purity: 99.99%) and yttrium oxide (purity: 99.99%) powders with a small fraction of sintering additives (i.e., TEOS and MgO) were weighed stoichiometrically and mixed thoroughly in ethanol by ball-milling. The slurry was dried at 60 ℃ for 24 h, ground and sieved through a 200-mesh screen, subsequently calcined at 800 ℃ to obtain a homogenously distributed sub-micron-sized Al₂O₃-Y₂O₃ powder. The ceramic slurries with different solid loadings suitable for DLP 3D printing were prepared via mixing the mixed powder with photocurable resin, photoinitiator and defoamer. The green bodies with layers (layer thickness of 50 μm) were fabricated based on a computer-aided designed model in a model ADMAFLEX 130 instrument (ADMATEC Europe BV., the Netherlands). Afterwards, the green bodies were heat-treated in a muffle furnace at 600 ℃ for 5 h to eliminate organic components and further densified by sintering in vacuum at high temperatures. The specimens were double-surface polished to 1 mm for the coming characterizations.

The rheological properties of the photocurable ceramic slurries were characterized by a model MCR302 stress-controlled rotational rheometer (Anton Par GmbH, Austria) with a cone (the diameter of 20 mm). The curing depth was determined via collecting the thickness of three independent printed samples. The thermal properties of the green body were determined in air by thermogravimetry (TG) and differential scanning calorimetry in a model STA449C thermal analyzer (NETZSCH Co., Germany) at a heating rate of 10 ℃/min. The microstructures of the powders, biscuits, and sintered YAG transparent ceramics were measured by a model SU8220 field emission-scanning electron microscope (FE-SEM) and a model TM3000 scanning electron microscope (SEM). The in-line transmittances of the ceramics were examined by a model V-770 UV-Vis-NIR spectrophotometer.

The influence of solid loading on the rheological properties of the photocurable slurries was investigated by a stress-controlled rotational rheometer. The viscosity of the ceramic slurry increases from 7.9 to 11.7 Pa·s at 32 s^-1 as the solid loading increases from 39.5% to 42.0%. The viscosities are suitable for the spreading of the slurries on the PET film. However, further increasing the solid loading probably leads to an increase of the viscosity and the slurry cannot spread out fluently. Thus, the ceramic slurries with solid loadings from 39.5% to 42.0% are prepared. The influence of photo-curing parameters on the curing depth, accuracy and strength of the single-layer ceramic was analyzed. The proper curing parameter is 50 mW/cm²-3 s. According to the TG-DTA analysis, the degreasing procedure is precisely controlled. Especially, a low temperature rising rate of 0.3 ℃/min is set at 300-520 ℃. The degreasing process is accomplished after holding at 600 ℃ for 5 h to remove all organic components. Little defects such as layer-layer separation and inner-layer cracks appear due to the microstructure. A vacuum sintering method is applied to further densify the ceramic. The YAG transparent ceramic with a high in-line transmittance of 82.9% is finally obtained by DLP 3D printing technology. This work can provide a foundation for the applications of YAG transparent ceramics.

Ceramic slurries with different solid loadings suitable for DLP 3D printing were prepared with a mixed Al₂O₃-Y₂O₃ powder. The influence of photo-curing parameters on the green bodies was discussed. The ceramic slices with superior comprehensive properties (i.e., curing depth, accuracy and strength) were obtained at the curing parameter of 50 mW/cm²-3 s. This processing parameter was used to print the green bodies of YAG transparent ceramics. According to the results by TG-DTA, degreasing processing was decided, and a low temperature rising rate of 0.3 ℃/min was set at 300-520 ℃, due to the integrity of microstructures of the ceramic biscuits. YAG transparent ceramics with a high optical quality were obtained via high temperature sintering in vacuum. Increasing the solid loading and the sintering temperature favored the mass transfer process. The YAG transparent ceramic with a high in-line transmittance of 82.9%@600 nm was finally obtained with the slurry with 42% solid loading at 1 780 ℃ for 3 h.