Three-phase heterogenous microstructure and hardness enhancement in a TiZrHfMoW-O high-entropy coating prepared by reactive glow discharge sputtering

• A three-phase structure was developed in this coating: a metallic bcc phase, a spinel phase (HEO), and a fcc oxide phase. • Oxide formation → entropy stabilization → grain size refinement; from Ф~20 nm (inner region) to Ф~2 nm (outer region). • Ultra-fine (~2 nm) multiphase grain clusters in the outer region enhanced interfacial strengthening under load. • Hardness and residual modulus increased by 2.4× and 1.5×, respectively, with respect to a metallic TiZrHfMoW coating. A TiZrHfMoW–O high-entropy alloy (HEA) metal–metal oxide coating was fabricated via double-cathode glow discharge sputtering under an argon-to-oxygen (Ar:O 2 ) flow ratio of 30:1. Detailed microstructural analysis revealed a coating ∼23.6 μm in thickness, possessing a complex three-phase architecture consisting of a body-centered cubic (bcc) HEA metallic phase, a spinel-type high-entropy oxide (HEO) phase, and a face-centered cubic (fcc) oxide phase. The coating exhibited a heterogeneous microstructure across its cross-section, transitioning from equiaxed grains, ∼20 nm in diameter, in the inner region to ultra-fine grains, ∼2 nm in diameter, in the outer region. High-resolution TEM confirmed the co-existence of all three phases within individual microstructural domains, forming ultra-fine grain clusters together with intricate phase boundaries that enhance mechanical properties through interfacial strengthening. Additionally, the coating exhibited a compressive residual stress, further contributing to its hardness. The maximum hardness reached ∼22.6 ± 0.2 GPa—approximately 2.4 times higher than its fully metallic, oxygen-free TiZrHfMoW counterpart (∼9.6 ± 0.8 GPa). These findings demonstrate that partial oxidation during sputtering can significantly strengthen HEA coatings through multiple synergistic mechanisms.