Now, I will describe the characteristics of hard alumite.

(1) Bath temperature and film hardness

The typical hardness of the films produced at different temperatures of various electrolytic baths is shown in Fig. 1.

The graph clearly reveals that the film hardness varies significantly depending on temperature and types of electrolytic baths. As a result, the method in which the electrolytes of sulfuric acid and organic acid are industrially used (mixed acid method) and electrolysis is performed at low temperatures close to 0°C is in the mainstream.

In addition, the properties of the anodic oxide film are significantly affected by the composition, heat treatment, working history of the raw material, and the aluminum alloy. Therefore, even if electrolysis is performed using the same electrolyte and under the same conditions, a different raw material cannot provide a film with the same properties. The relationship between the material and film hardness is shown in Fig. 2. This graph is based on periodic reverse electrolyzing developed by Keigo Okubo.

(2) Hardness of film cross-section

Fig. 3 shows the hardness distribution of the hard film cross-sections. As the film thickness increases, the initially produced film surface in contact with the electrolyte is provided with the porous property due to dissolution by electrolytes, resulting in lower hardness.

(3) Wear resistance

The hard alumite film has high hardness and excellent wear resistance. Table 1 shows the results of tests conducted using a Taber abrasion tester. It was found that the hard alumite film has wear resistance more than 32 times that of raw aluminum and is better than hard chrome plating.

[Table 1] Comparison of wear resistance of various materials

(Amount of wear after 20,000 revolutions, with wear wheel CS-17 subjected to loads of 1 kg)