アルミニウム陽極皮膜の光学的研究 (II)

各種陽極化成皮膜の屈折率, 複屈折について

抄録

Optical observations facilitate the elucidation of the structure of anodic oxide films on aluminium which are mostly of amorphous nature and thus are difficult to be analyzed by X-ray diffraction.
In the previous paper, the authors clarified the process of formation and the crystal structure of α-alumina films anodically formed in bisulphate melts by the use of optical means.
In this paper, various anodic oxide films such as oxalic, sulphuric acid films formed at normal anodizing conditions and at lower temperature (hard-coating), chromic, sulphamic, phosphoric, boric (plus sulphuric) acid films, Ematal and Kalcolor films and boric acid-formamide films developed by the authors, were observed under polarizing microscopy and the refractive indices were determined and the existence of doubly-refracting properties was checked. Refractive indices of these films are usually lower than pure crystalline or amorphous alumina (1.67±1) and the films are composed mostly of amorphous alumina and including certain amounts of water and anions from the electrolyte. Formation at lower temperature and 'sealing' of the films lower the refractive indices.
The anodic oxide films are very unstable (or active) when they are formed (as is seen from the commercial 'sealing' or 'dyeing' operations) and tend to transform into γ-Al(OH)₃ (Gibbsite) by the stress produced during the growth of the films or by mechanical working applied externally, such as crushing or grinding. The phenomenon is notable in case of sulphuric acid films, especially of hard coating formed at lower temperature. The double refraction comes from the original properties of the compounds included in the films or from the porous structure of the film (form double refraction). The refractive indices of various films as compared with those of pure chemical substances are summarized in a chart. (Fig. 1)