Hard anodizing
Characteristics of hard anodized films:
1) Appearance: The coating ranges in color from brown and gray to black.
2) Thickness: For aluminum–copper and aluminum–silicon alloys, which are difficult to anodize, the coating thickness is approximately 30 μm; for pure aluminum, which is easy to anodize, the thickness can reach 250–300 μm.
3) Hardness: Typical aluminum alloys exhibit a hardness of 400–600 HV, while high‑copper 2xxx series aluminum alloys can achieve an oxide film hardness of approximately 380 HV, whereas pure aluminum may exceed 1,000 HV.
4) Wear resistance: Due to the high hardness of the coating and its porous structure, it can effectively adsorb lubricants, thereby enhancing wear resistance.
5) Insulation: The coating exhibits high electrical resistivity, resulting in excellent insulating properties; its dielectric strength can reach 25 V/μm.
6) Heat resistance: The oxide film has a melting point of 2050°C and a thermal conductivity of only 67 kW/(m·K), providing excellent thermal insulation.
7) Corrosion resistance: When subjected to a 5% sodium chloride salt spray test, it can withstand thousands of hours without corrosion.
The thermal emissivity of the coating can reach 90% to 95%, making it suitable for use as a thermal control layer on spacecraft.
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