Grain boundary engineering for enhanced corrosion resistance of Si3N4 in molten MgCl2-NaCl-KCl salts

Silicon nitride (Si 3 N 4 ) ceramics are promising structural materials for next-generation concentrated solar power (CSP) systems, while their long-term stability in aggressive molten chloride salts remains insufficiently understood. Here, we investigate the corrosion behavior and decomposition mechanism of a conventional Y 2 O 3 -sintered β-Si 3 N 4 ceramic in molten MgCl 2 –NaCl–KCl at 600 °C. The corrosion process proceeds through three sequential stages: preferential dissolution of the amorphous intergranular phase (IGP), formation of a detrimental Mg 2 SiO 4 layer, and subsequent chemical attack of β-Si 3 N 4 grains. Guided by this mechanistic understanding, we propose a grain boundary engineering strategy using YF 3 /MgF 2 sintering aids to fabricate corrosion-resistant Si 3 N 4 ceramics. The engineered ceramic exhibited a 25% reduction in average corrosion depth and excellent property retention after 800 h of exposure. Its bending strength loss is only 8.3%, compared with 13.8% for the reference material, and its thermal conductivity loss is 4.8%, compared with 8.4% for the reference material. The improved corrosion resistance is attributed to the combined effects of a chemically stable crystalline Y 4 Si 2 N 2 O 7 intergranular phase, elimination of residual SiO 2 impurities, and a coarsened microstructure. Salt purification experiments further confirmed the proposed corrosion mechanism by suppressing impurity-driven decomposition. In purified molten salt, the engineered ceramic shows an extrapolated corrosion rate of approximately 16.8 μm·year - ¹ , meeting the U.S. DOE corrosion target of < 20.0 μm·year - ¹ . These results demonstrate that grain boundary engineering is an effective strategy for improving the durability of Si 3 N 4 ceramics in high-temperature molten chloride salt environments.