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

Abstract

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.

Study of Coating Growth Direction of 6061 Aluminum Alloy in Soft Spark Discharge of Plasma Electrolytic Oxidation

“…The voltage decreases from 505 V to about 302 V—a voltage drop of 40.19%. He et al [ 22 ] found that the drop increases with increasing current density.…”

“…The voltage decreases from 505 V to about 302 V− a voltage drop of 40.19%. He et al [22] found that the drop increases with increasing current density. The fifth stage (Stage E) is the soft spark discharge stage, in which small snow-white sparks are always uniformly distributed on the surface of the sample, and the voltage will again increase with the treatment time.…”

Mutual Impact of Four Organic Calcium Salts on the Formation and Properties of Micro-Arc Oxidation Coatings on AZ31B Magnesium Alloys

“…Various surface modification and coating techniques have been applied to suppress the degradation of Mg alloys or even to endow them with improved cytocompatibility [1][2][3][4][5][6][7][8], osteogenesis ability [1][2][3][4][5][6], antibacterial ability [1][2][3][4][5][6], and antitumor ability [7,8]. Among these, micro-arc oxidation (MAO) has received considerable attention, since MAO coatings are generated via in situ oxidation and sintering on magnesium alloys [1,3,4,9], aluminum alloys [10][11][12][13], titanium alloys [14][15][16][17][18], and tantalum alloys [19], and therefore exhibit high hardness, good wear resistance, and corrosion resistance. Most importantly, the composition, structure, papers from 80 to 3000 grits, cleaned with tap and distilled water, and finally dried in an air stream.…”

Investigation of In Vitro Cytocompatibility of Zinc-Containing Coatings Developed on Medical Magnesium Alloys

“…Micro-arc oxidation (MAO) is a widely used surface treatment technique to modify magnesium alloys [ 3 , 4 , 5 , 6 ], aluminum alloys [ 7 , 8 , 9 , 10 , 11 , 12 ], titanium alloys [ 13 , 14 , 15 , 16 ], and tantalum alloys [ 17 ] by modulating electrolyte compositions and concentrations [ 18 , 19 , 20 ]. MAO technology can improve the corrosion resistance of magnesium alloys and introduce macro or trace elements such as phosphorus (P) and zinc (Zn) into MAO coating to produce functionalized coating.…”