*2.1. Materials and PEO Treatment*

The 1060 aluminum alloy (99.6% purity) was used as substrate in this experiment. Specimens with the dimension of 20 mm × 20 mm × 2 mm were polished using silicon-carbide abrasive paper up to 1000 grit and cleaned with acetone for 5 min.

The PEO process was performed using a direct-current pulsed power supply (WHD-20, Harbin Institute of Technology, Harbin, China) at a constant current density of 15 A/dm2, a frequency of 500 Hz, and a duty ratio of 60%. Electrolyte, mainly comprised of 8 g/L Na2SiO3·9H2O and 3 g/L Na5P3O10, was prepared by deionized water and high-purity chemicals. A water-cooling system was used to maintain an electrolyte temperature below 300 K. The specimens were subjected to PEO treatment for 5, 15, 45, and 60 min, cleaned ultrasonically in deionized water and dried in warm air.
