Formation of a metastable nanostructured mullite during Plasma Electrolytic Oxidation of aluminium in “soft” regime condition

This paper demonstrates the possibility of producing a lamellar ceramic nanocomposite at the top-most surface of oxide coatings grown with the Plasma Electrolytic Oxidation process (PEO). PEO was conducted on aluminium in a silicate-rich electrolyte under the so-called “soft” regime. Nanoscale characterisation showed that the transition from the “arcs” to the “soft” regime was concomitant with the gradual formation of a 1:1 mullite/alumina lamellar nanocomposite (≈120 nm thick) that filled the cavity of the PEO “pancake” structure. Combined with plasma diagnostic techniques, a three-step growth mechanism was proposed: (i) local melting of alumina under the PEO micro-discharges (≈3200 K at high heating rate ≈3 × 108 K·s−1); (ii) progressive silicon enrichment of the melt coming from the electrolyte; and (iii) quenching of the melt at a cooling rate of ≈3.3 × 107 K·s−1 as the micro-discharge extinguishes. Under such severe cooling conditions, the solidification process was non-equilibrium as predicted by the metastable SiO2 – Al2O3 binary phase diagram. This resulted in phase separation where pure alumina lamellae alternate periodically with 1:1 mullite lamellae.