Fast formation of a black inner α‐Al₂O₃ layer doped with CuO on Al–Cu–Li alloy by soft sparking PEO process

Forming high-temperature α-Al2O3 phase under soft sparking is an intriguing phenomenon in plasma electrolytic oxidation (PEO) of Al alloys, which contradicts the low energy input of the process. In this study, α-Al2O3 doped with black CuO is formed beneath an amorphous white outer layer on Al–Cu–Li alloy by PEO in a dilute silicate electrolyte under soft sparking. In comparison, reddish coatings with dominating γ-Al2O3 are formed under the conventional plasma discharges, although blackish inner layer with α-Al2O3 can also be exposed by heavily polishing the samples. In order to know the underlying mechanism, temperatures at the coating surface and the underlying substrate have been monitored by a thermocouple under the conventional and soft sparking PEO regimes, respectively. Interestingly, high temperatures are detected in the case of soft sparking rather than PEO with strong discharges. The formation of CuO, quartz, and cristobalite within the soft sparking coating also supports the existence of high temperature. Hence, the formation of α-Al2O3 under soft sparking can be resolved to the conventional thermal activation mechanism, without the need of seeking other plausible explanations. Thermal condition evaluation for soft sparking PEO suggests that values of the effective thermal conductivity during PEO process for the outer layer and the barrier layer at the coating/substrate interface might be lower than ∼0.05 and ∼0.0017 W m−1 K−1, respectively. It is believed that the amorphous structure of the outer and barrier layers effectively blocks the heat dissipation, facilitating the formation of a highly wear-resistant inner layer with α-Al2O3, CuO, and the other high-temperature species under soft sparking.