Microstructural, mechanical, and corrosion improvement of friction-surfaced AZ91–bioactive glass composite using the fast multiple rotation rolling process

This study investigates the influence of underwater fast multiple rotation rolling (FMRR) on the microstructural evolution, mechanical performance, and corrosion resistance of friction-surfaced AZ91 magnesium composite coatings reinforced with 10 wt.% bioactive glass particles (64SiO₂–31CaO–5P₂O₅). Composite surface layers were first fabricated by friction surfacing and subsequently subjected to one, two, and three FMRR passes to evaluate the effect of deformation intensity. FMRR significantly modified coating geometry, increasing deposition efficiency from 42.4% to 48.6% while simultaneously reducing coating thickness from 4.52 to 1.54 mm. Severe plastic deformation promoted extensive particle fragmentation, decreasing the average bioactive glass particle size from 5.2 μm in the friction-surfaced condition to 0.8 μm after three FMRR passes. Concurrently, substantial grain refinement was achieved, with the minimum average grain size of 1.3 μm obtained after two FMRR passes, representing a 71% reduction compared with the untreated coating. The average β-Mg₁₇Al₁₂ precipitate size decreased from 0.62 μm in the as-deposited condition to 0.46 μm after one FMRR pass and reached a minimum value of 0.18 μm after two passes, before slightly increasing to 0.43 μm following the third pass due to thermally induced coarsening. The two-pass condition also exhibited the highest fraction of high-angle grain boundaries (88.1%). These microstructural modifications markedly enhanced mechanical properties, increasing nano-hardness from 7.72 to 9.54 GPa. Electrochemical measurements in simulated body fluid demonstrated a pronounced improvement in corrosion resistance. The optimum condition reduced the corrosion current density from 57.4 to 33.6 μA cm⁻ 2 and decreased the corrosion rate from 3.02 to 1.74 mm year⁻ 1 , while polarization resistance increased from 168.5 to 267.8 Ω cm 2 . Electrochemical impedance spectroscopy further confirmed the enhanced protection, with charge-transfer resistance increasing from 372.4 to 724.3 Ω cm 2 . Overall, two-pass FMRR provided the most favorable combination of microstructural refinement, surface hardening, and corrosion protection, demonstrating its effectiveness as a post-processing strategy for advanced magnesium-based bioactive composite coatings.