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The membranes of anodic alumina sheets can be classified according to their hardness as follows: super (HV > 4500), semi-rigid (HV2500 ~ 3500), ordinary (HV1500 ~ 2500) and soft (HV < 1500).
Usually, the hard anodic oxidation is defined as the anodic oxidation whose thickness is more than 25 micron and hardness is higher than HV3500. The purpose of hard anodic oxidation is to obtain oxide film with high hardness, better wear resistance and corrosion resistance, which is much thicker than ordinary anodic oxide film. The maximum thickness of hard anodic oxide film can reach 250-300 um, so this method is also called thick film anodic oxidation method.
The technological process of hard anodic alumina plate is similar to that of ordinary anodic oxidation. The main difference lies in the formula of electrolyte and technological conditions.
Process flow of hard alumina plate: chemical degreasing - Cleaning - neutralization - Cleaning - hard anodic oxidation - Cleaning - sealing - drying
Hard oxide film characteristics:
1. High hardness;
2. Strong wear resistance;
3. Excellent corrosion resistance;
4. Strong heat resistance and good electrical insulation.
The disadvantage is that when the film thickness is too large, the fatigue strength of aluminium alloy decreases, and the stamping oxide film is easy to burst.