From ultrasmall nanocrystals to dense coatings: oriented attachment-driven growth of (Hf0.5Zr0.5)B2 coatings via molten salt electrophoretic deposition

Efficiently fabricating high-performance ultra-high temperature ceramics (UHTCs), especially ternary boride coatings, remains a critical bottleneck. Herein, a novel molten salt electrophoretic deposition (MS-EPD) strategy overcomes this challenge using ultrasmall (∼3.3 nm) (Hf 0.5 Zr 0.5 )B 2 nanocrystals (NCs) as building blocks. These (Hf 0.5 Zr 0.5 )B 2 nanocrystals are formed in situ via borothermal reduction of HfO 2 and ZrO 2 with B in the NaF-AlF 3 molten salt, and are subsequently used for MS-EPD to deposit dense coatings. This approach enables the rapid (1.02 μm min −1 ) and controllable growth of dense coatings on graphite. The resulting films exhibited a unique gradient columnar structure, yielding a high nano-hardness (31 GPa), robust adhesion (10.6 N), and effective oxidation resistance at 1200℃. Crucially, multi-scale characterization revealed that the growth is governed by an oriented attachment (OA) mechanism, where individual NCs crystallographically align and fuse at the growth front. This discovery of direct, cross-scale assembly in molten salt provides a new paradigm for the bottom-up fabrication of advanced materials with precisely engineered microstructures.