Effects of silica nanoparticles addition on formation of oxide layers on Al-Si alloy by plasma electrolytic oxidation: The origin of stishovite under ambient conditions

The effects of different amorphous silica nanoparticles (NPs) with average particle size of 48 nm - n-SiO2(48) and 100 nm - n-SiO2(100) added to the electrolyte under plasma-electrolytic oxidation (PEO) of Al-Si alloy 361.0 (9.8 wt. % Si) on the oxide layers structure, phase composition and properties were investigated. Unexpected formation of high pressure (HP) SiO2-phase - stishovite was detected in the oxide layers formed using n-SiO2(48) under ambient conditions. This HP-phase was not detected while using n-SiO2(100) and the electrolyte without nanoparticles. It was found that the addition of the n-SiO2 resulted in significant increase in the oxidizability, and, accordingly, in the thickness of the PEO-formed oxide layers, their wear resistance (by -1.5 times) and thermal resistivity (by -4 times) as compared to the oxide layers obtained without NPs. It also resulted in crystallite size reduction and an increase of the amorphous phase fraction, both effects are more pronounced while using n-SiO2(48) despite the equal initial concentrations of the nanopowders in the electrolyte (3 g/l). To explain these effects, a novel approach to the NPs/oxide layer interaction and the corresponding quantitative model were introduced. The interaction was considered as the colliding with the rigid oxide layer surface of rapidly moving charged NPs accelerated by electric field in the vapour-gas bubbles (VGB) formed in porous channels prior micro-arc discharge. During elastic/inelastic collision, the particle kinetic energy transforms to compressive strain energy, heating of the particle and/or to kinetic energy of the bounced off particle providing different interaction scenarios depending on the particle size. The developed model makes it possible to predict the HP SiO2-phase formation, an increase in the fraction of amorphous phases and reduction in the crystallite size in the PEO-formed layers in the presence of NPs, which can result in improvement of protective and mechanical properties of the layer.