Ultrafast growth of dense HfB2 coatings via oriented attachment in molten salts

Hafnium diboride (HfB2) coatings are critical for protecting components in extreme environments; however, conventional PVD/CVD routes are often limited by slow growth and typically rely on high-vacuum infrastructure and hazardous precursors, leading to high cost and increased environmental burdens. Herein, we report a molten salt electrophoretic deposition strategy that enables the ultrafast and potentially cost-effective synthesis of high-performance HfB2 coatings, featuring an ultrafast growth rate, an oriented-attachment-governed densification pathway, and broad applicability to multiple substrates. This method begins with the in-situ formation of a stable colloidal suspension of ultrasmall (~4.6 nm) HfB2 nanocrystals (NCs) within a NaF-AlF3 melt. Driven by a low voltage (1.0 V), these NCs rapidly assemble on various substrates, including graphite, C/C composites, titanium, and molybdenum, forming fully dense and strongly adherent coatings at a rate of 57.6 μm·h-1—over an order of magnitude faster than that of traditional techniques. The resulting coatings exhibit exceptional microhardness (4247 HV0.1), a low friction coefficient (0.4), and excellent high-temperature oxidation resistance. By investigating the growth process, we reveal that the coating densification is governed by a non-classical crystallization pathway, that is crystal growth via oriented attachment (OA) of NCs at nearly 1000 °C. This work not only provides a scalable and versatile paradigm for manufacturing ultra-high temperature ceramic coatings but also extends the fundamental understanding of crystal growth by OA to high-temperature inorganic molten salt systems.