Electrophoretic deposition of ZrB2 coatings in NaF-AlF3 molten salt through an oriented attachment mechanism

Emerging as a highly promising technique, molten salt electrophoretic deposition (MS-EPD) enables the fabrication of protective ultra-high-temperature ZrB 2 coatings. However, its practical application has been constrained by the slow deposition rates (<0.4 μm/min) and limited coating thicknesses (<25 μm) achieved in conventional chloride or chlorofluoride-based molten salt systems. Herein, we reported a novel electrophoretic deposition (EPD) strategy that utilized a high-temperature fluoride molten salt system (NaF-AlF 3 , 1100 °C), which successfully overcame this kinetic limitation. This method generated ultrasmall ZrB 2 nanoparticles (NPs) with a mean particle size of 7.9 nm in the molten salt via an in-situ borothermal reduction reaction. Using these NPs as building units, a high deposition rate of 0.74 μm/min was achieved, enabling the fabrication of a fully dense, high-quality coating over 50 μm thick within 1 h. The resulting coating exhibited outstanding comprehensive properties including a nanoindentation hardness of 16 GPa, an elastic modulus of 172 GPa, and exceptional protection capability in an oxidizing environment at 1000 °C. Furthermore, this work revealed that the coating growth was dominated by an oriented attachment crystallization (OA) mechanism. This study not only established a novel pathway for the efficient and low-cost production of thick, high-performance ZrB 2 coatings, but also provided new fundamental insights into understanding and controlling the assembly behavior of nanocrystals in molten salt systems.