Enhancing oxidation resistance and thermal stability of (AlCrNbTaTi)C high-entropy thin films via Si alloying

High-entropy metal-sublattice (AlCrNbTaTi)C thin films were synthesized by reactive magnetron sputtering in an Ar–C 2 H 2 atmosphere and crystallized in the rock-salt structure, accompanied by varying fractions of amorphous a-C:H depending on the deposition conditions. Silicon alloying preserved the rock-salt structure yet produced pronounced grain refinement, shrinking the average column width from 113 nm to 18 nm at 8.3 at.% Si. This microstructural transformation caused only a modest hardness reduction from 32.8 ± 0.4 GPa to 29.5 ± 1.1 GPa. Vacuum annealing up to 1000 °C revealed that Si-containing films retained their hardness and nanostructure, whereas the Si-free reference suffered extensive softening above 800 °C. The oxidation behavior showed an even stronger dependence on Si content. The unalloyed film formed a thick (∼7.8 µm), microporous rutile-type (AlCrNbTaTi)O 2 solid solution upon exposure to 800 °C in ambient air. In contrast, the 8.3 at.% Si coating developed only a thin, dense, and passivating oxide scale and maintained protection during extended annealing at 1000 °C. The results demonstrate that Si alloying stabilizes the microstructure and suppresses oxygen ingress through the formation of Si-rich amorphous boundary regions, thereby substantially enhancing both the thermal and oxidative stability of high-entropy metal-sublattice carbide coatings while retaining high mechanical performance.