Influence of PEO regime on sol-gel infiltration and performance of hybrid organic-inorganic coatings on AZ31 alloy

Magnesium alloys are considered for industrial applications due to their low density and high specific strength. However, their high corrosion susceptibility remains a limitation, necessitating the development of effective surface protection strategies. In this study, AZ31 alloy was modified using hybrid zirconium-silicon sol-gel coatings (ZTP), plasma electrolytic oxidation (PEO) layers produced under arc (PEO-AR) and soft (PEO-SR) regimes, and combined PEO/ZTP duplex systems. The coatings were characterized in terms of morphology, composition, and mechanical behaviour. FTIR analysis confirmed the formation of an inorganic-organic hybrid network based on Zr-O-Si, Si-O-Si and Zr-O-Zr linkages, whereas rheological measurements revealed constant behaviour with ageing time (∼11.8 mPa·s), favouring controlled pore infiltration. SEM/EDS analyses confirmed the formation of porous PEO layers and the deposition of a uniform sol-gel coating on AZ31, while the duplex systems displayed different structural architectures. The PEO-AR/ZTP coating reached 11.41 μm, reflecting extensive sol-gel penetration into the large, open pores of the oxide layer. While, the PEO-SR/ZTP coating exhibited a thicker duplex structure (28.4 μm), but sol-gel infiltration remained largely superficial due to the finer and more compact pore network of the PEO layer. Tribological testing revealed that the PEO-AR/ZTP coating achieved the lowest coefficient of friction (∼0.35) and superior wear resistance, whereas the PEO-SR/ZTP coating exhibited partial layer detachment and progressive degradation during sliding. These show that sol-gel infiltration depth and PEO pore architecture are key to the mechanical integrity and durability of duplex coatings, providing a framework for the design of hybrid protective systems for magnesium alloys.