Microstructural refinement and stability of Si-containing (Al,Mo,Ta,V,W)-carbide coatings

Many ceramic coatings suffer from grain coarsening and mechanical softening during high-temperature exposure, which limits their performance in extreme environments. High-entropy carbides (HECs), while thermodynamically metastable, offer opportunities to enhance kinetic microstructural stability through compositional complexity. Here, we investigate the effect of Si addition on sputtered (Al,Mo,Ta,V,W)C coatings. Increasing the Si content from 0 to 7 at.% systematically refines the columnar grain width from ~500 nm to ~20 nm and promotes a pronounced (200) texture. Upon annealing at 1200 °C, nanoscale SiC precipitates form preferentially along grain boundaries while the fcc carbide phase and nanocrystalline morphology remain largely preserved. These precipitates effectively impede grain-boundary motion and dislocation glide, resulting in only a moderate hardness decrease from 28 to 24 GPa and an indentation modulus of ~390 GPa upon annealing. The results demonstrate that low-solubility, strong carbide-forming elements can enhance microstructural coarsening resistance in metastable HEC coatings by promoting boundary-precipitation strengthening, providing a promising strategy for designing thermally robust ceramic coatings.