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

J. Martin (1, 2) , A. Nomine (1, 3) , V. Ntomprougkidis (1, 2) , S. Migot (1) , S. Bruyère (1) , F. Soldera (4) , T. Belmonte (1) , G. Henrion (1, 2)

1 IJL - Institut Jean Lamour
2 Labex DAMAS
3 ITMO - National Research University of Information Technologies, Mechanics and Optics [St. Petersburg]
4 Saarland University [Saarbrücken]

This paper demonstrates the possibility of producing a lamellar ceramic nanocomposite at the topmost 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 × 10 8 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 × 10 7 K·s −1 as the micro-discharge extinguishes. Under such severe cooling conditions, the solidification process was non-equilibrium as predicted by the metastable SiO 2-Al 2 O 3 binary phase diagram. This resulted in phase separation where pure alumina lamellae alternate periodically with 1:1 mullite lamellae.

Domaines: Physique des plasmas [physics.plasm-ph]; Matériaux; Plasmas

J. Martin, A. Nomine, V. Ntomprougkidis, S. Migot, S. Bruyère, et al.. Formation of a metastable nanostructured mullite during Plasma Electrolytic Oxidation of aluminium in “soft” regime condition. Materials & Design, 2019, 180, pp.107977. ⟨10.1016/j.matdes.2019.107977⟩. ⟨hal-02182775⟩