Abstract: Under liquid additive manufacturing has emerged as a transformative paradigm for advanced material processing, leveraging the unique liquid-mediated quenching effects to achieve accelerated solidification kinetics that enable the formation of unconventional microstructures and metastable phases. However, current under liquid additive manufacturing techniques remain predominantly constrained to macroscale engineering applications and water environment. In this study, photothermal-enhanced under liquid femtosecond laser sintering (PE-UL-FLS) in ethanol is proposed as an innovative approach for ultrafast sintering fabrication of refractory 3 mol. % yttria-stabilized tetragonal zirconia polycrystal (3Y-TZP) ceramics—a hard-to-process material system via conventional means. To overcome the inherent limitations of 3Y-TZP, including its poor near-infrared absorption and insufficient photothermal conversion efficiency, we strategically incorporate Fe₃O₄ nanoparticles in the 3Y-TZP matrix as multifunctional dopants, functioning simultaneously as optical sensitizers and photothermal nanoamplifiers. A systematic enhancement in melting dynamics is observed with increasing Fe₃O₄ concentrations (2-10 wt.%). Interfacial sintering of 3Y-TZP nanostructures into coarse-grained tetragonal zirconia (t-ZrO₂) with concurrent phase purification is identified. Remarkably, PE-UL-FLS induces a gradient-driven grain growth mechanism in t-ZrO₂, accompanied by Fe-ion segregation at grain boundaries and subsurface aggregation. This unique Fe-segregation redistribution fosters the formation of a transient molten layer atop the evolving t-ZrO₂ grains. Upon rapid solidification, this process yields highly anisotropic microstructures characterized by elongated grains with high aspect ratios and planarized surfaces with indiscernible grain boundaries-morphological features unattainable through traditional thermal annealing. These findings underscore the exceptional sintering kinetics and non-equilibrium solidification dynamics intrinsic to PE-UL-FLS.

Key words: Under liquid femtosecond laser sintering, Photothermal enhancement, 3Y-TZP ceramics, Anisotropic t-ZrO2 grains, Ultrafast solidification