Mechanistic insights into high strain rate deposition of tantalum carbide coatings via cold spray

• Cold spray is used to deposit superhard TaC ceramic coating. • Large TaC particles disrupt the bow shock and anchor ceramic into the substrate. • Sub-micron sized particles evade deflection and contribute to coating growth. • Grain refinement and dynamic recrystallization are observed due to the high strain rate. While cold spray (CS) is traditionally limited to metals and metal-ceramic composites, this study demonstrates its capability to consolidate a pure ultra-high temperature ceramic (UHTC), tantalum carbide (TaC), a superhard material considered highly challenging for processing. Using in-situ high-speed imaging and transmission electron microscopy (TEM), we investigated the consolidation mechanisms induced by high strain rate impacts during CS of TaC. The process begins with anchoring larger micron-sized TaC particles into the softer aluminum substrate, followed by buildup through finer sub-micron particles (∼200 nm to 1 µm). High-speed imaging reveals that large particles characterized by high Stokes numbers disrupt the bow shock front and anchor into the substrate, enabling finer particles to evade deflection and contribute to coating growth. Substrate topography and hardness critically influence particle retention. Softer substrate promotes mechanical interlocking, while pre-deposited TaC layers enhance adherence of finer particles. TEM and atomic force microscopy (AFM) confirm nanoscale grain refinement, dynamic recrystallization, and high dislocation densities, signifying plastic deformation and hardening in brittle ceramics. The resulting TaC coating exhibits a nanohardness of ∼31.6 GPa and shallow scratch penetration (∼2 µm), demonstrating exceptional wear resistance. This work establishes a new pathway for CS processing of pure UHTCs for extreme aerospace and hypersonic applications.