Comparison of the soft sparking behaviors in plasma electrolytic oxidation (PEO) of an Al-Cu-Li alloy in aluminate and silicate electrolytes: Oxide growth kinetics and phase transition

Plasma electrolytic oxidation (PEO) of Al-Cu-Li alloy was studied under cathodic polarization in aluminate and silicate electrolytes to compare their soft sparking behaviors. Interface temperature measurements were employed to assist understanding the α-Al₂O₃ transformation. Soft sparking (cathodic-to-anodic current ratio R=1.2) in silicate typically leads to thick coatings with a white amorphous outer layer; however, it only yielded thin black coatings in aluminate. Much higher α-Al₂O₃ content is normally detected for the coatings formed in aluminate, since the incorporation of Si species can suppress α-Al₂O₃ formation within the silicate coatings (I_α/I_γ is ∼1.38–7.26 and ∼0.81–0.51 for the outer part of the coatings formed in the aluminate and silicate electrolytes, respectively). However, high α-Al₂O₃ content is presented at the lower part of the silicate soft sparking coating due to thermal activation. Increased R in aluminate reduced α-Al₂O₃ in outer layers but enhanced its content in the inner layer due to cathodic Joule heating. A hydrogen complex incorporation hypothesis is proposed for the outer-layer α-Al₂O₃ suppression. This work clarifies electrolyte-dependent soft sparking behaviors and phase transformation mechanisms.