Tribological response of hydrogen-charged Cantor high-entropy alloy: Near-surface hardening and transient wear protection

The tribological response of the equiatomic CoCrFeMnNi (Cantor) high-entropy alloy under electrochemical hydrogen charging was investigated to assess its suitability for hydrogen-facing tribological applications. Disc-shaped specimens were electrochemically charged in 0.1 M NaOH solution for 24 h at 90–100 mA/cm², introducing ~22 wppm of bulk hydrogen with a calculated near-surface concentration of ~1.8 × 10³ wppm, due to the intrinsically low hydrogen diffusivity of the face-centred cubic lattice. This near-surface hydrogen accumulation produced a significant hardness increase of up to 22%. Reciprocating sliding tests against an Al₂O₃ counterbody were conducted at three durations (15, 30, 60 min) to evaluate the temporal evolution of friction and wear. Hydrogen-charged specimens exhibited a transient reduction in the coefficient of friction during the first ~1500 cycles, followed by convergence towards the steady-state values of the uncharged alloy as the hardened surface layer was progressively consumed. Correspondingly, a ~49% reduction in specific wear rate was observed after 15 minutes, diminishing at longer durations as wear penetration exceeded the hydrogen-enriched zone. Hydrogen charging did not introduce wear mechanisms qualitatively distinct from those of the uncharged condition (both remained governed by abrasive grooving and progressive tribo-oxidation) but modulated their kinetics during the early stages of sliding. These findings indicate that, for tribological applications where contact stresses remain confined to the near-surface region, the Cantor alloy exhibits an inherent surface-hardening response under hydrogen charging that transiently improves tribological performance.