Introduction Deep sea equipment is an important guarantee for the exploration, development, and protection of deep-sea resources, including various types of deep-sea submersibles, landers, gliders, and their auxiliary equipment. In the deep-sea equipment above, the transparent protective cover for observation windows, cameras, and lighting must use high-strength transparent materials. The main transparent materials currently used are ultra-thick organic glass (observation window), inorganic glass, and sapphire (single crystal aluminum oxide). Their application in deep-sea equipment is possible due to the superior transparency of yttrium aluminum garnet (YAG) transparent ceramics like single crystals, as well as the characteristics of optical isotropy, high strength, high thermal conductivity, and stable physical and chemical properties. At present, there are many kinds of transparent ceramics, among which YAG transparent ceramics are the most representative. The existing researches on YAG transparent ceramics mainly focus on doping rare-earth elements to achieve optical functional applications, such as laser output, white LED luminescence, etc.. There are a few reports on the research of large-sized and complex shaped YAG transparent ceramics. In this paper, large-sized and complex shaped YAG transparent ceramics were prepared by a solid-state reaction sintering method, which could be applied to deep-sea equipment at 10 000 meter.Methods High-purity powders of α-Al2O3 (99.99% purity, Taimei Co., Japan), Y2O3 (self- synthesized by co-precipitation method) were used as starting materials. The starting powders were weighed according to a stoichiometric ratio of YAG, and then mixed with 0.5% tetraethyl orthosilicate (TEOS, 99.99%) and 0.05% MgO powder (analytical pure) as a sintering additive in ethanol by ball milling for 12 h. The mixtures were dried in an oven at 60 ℃ for 24 h and then sieved through 200-mesh screen. A portion of the dried powder was calcined at 900-1 500 ℃, and another part of the powder was pressed into circular discs with a diameter of 20 mm using a steel mold at 30 MPa, and then subjected to cold isostatic pressing at 200 MPa to obtain ceramic billets. The green body pellets were then sintered in a vacuum furnace with tungsten meshes as the heating elements under 10?3 Pa at 1?200-1 800 ℃ for 2-20 h. After sintering in vacuum, the pellets were further annealed in air at 1 400 ℃ for 10 h. Finally, the both surfaces of the as-prepared ceramic samples were mirror-polished with different grade of diamond slurries.The phase compositions of all the samples were qualitatively identified by a model D/Max-2550V X-ray diffractometer (Rigaku Co., Japan). The optical transmittances of all the samples of ceramics were measured by a model Carry 5000 UV-VIS-NIR spectrometer (Varian Co., USA). The surface microstructures of each group of samples after hot corrosion were characterized by a model JSM-6360 scanning electron microscope (JEOL Co., Japan).Results and discussion YAG transparent ceramics were fabricated by a solid-phase reaction sintering method. The phase evolution and microstructural changes during sintering were investigated. At high temperatures, Al2O3 and Y2O3 react to form intermediate phases YAM and YAP, eventually forming YAG. The effects of sintering temperature and holding time on the optical transmittance of YAG transparent ceramics were investigated. As the sintering temperature increases, the pores in YAG ceramics continue to discharge and the grains continue to grow. The optical transmittance firstly increases and then decreases, reaching its maximum value at 1 780 ℃. As the holding time prolongs, the transmittance of YAG ceramics gradually increases, but after more than 6 h, extending the holding time does not have a significant effect on improving the transmittance. The YAG ceramic dome is formed using a specially designed rubber mold and a stainless steel hemispherical mold by a cold isostatically press. The YAG ceramic dome is then vacuum sintered in the optimized process. The optical window under 127 MPa was analyzed by finite element analysis (i.e., a software named ANSYS). The analysis results show that the maximum compressive stress of the 120 mm diameter dome is 773.7 MPa and the maximum tensile stress is 21.9 MPa, both of which are less than the maximum allowable stress of YAG transparent ceramic material (i.e., approximately 1?000 MPa). The YAG transparent ceramic dome and titanium alloy cylinder are sealed and packaged. The transparent ceramic dome is undamaged without cracking or leakage at 127?MPa for 1.5 h. The high-power deep-sea LED lighting and deep-sea camera, using YAG transparent ceramics as protective covers, are installed on a deep-sea equipment “Canghai” lander. In November 2020, the lander sank to the depths of 10 000 meters in the Mariana Trench (the deepest part of the oceans) for several times, and a large amount of valuable video data were recorded. Conclusions Highly transparent YAG transparent ceramics were fabricated in vacuum via solid-state reaction sintering. Under high temperature conditions, Al2O3 and Y2O3 reacted to form intermediate phases YAM and YAP in sequence, and finally formed YAG at 1?500 ℃. As the sintering temperature increased, the pores in YAG ceramics continued to discharge and the grains continued to grow. The optical transmittance firstly increased and then decreased, reaching its maximum value at 1 780 ℃. As the holding time prolonged, the transmittance of YAG ceramics gradually increased. The transmittance of YAG ceramic samples sintered in vacuum at 1?780 ℃ for 20 h was 84.7% at 1 100 nm and 82.8% at 400 nm, respectively. The YAG transparent ceramic dome was prepared via optimization. It could be applied to high-power LED lighting and camera protection covers in the deep-sea equipment at 10?000?meters.