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Die vorliegende Studie stellt die vielfältigen Möglichkeiten der Transmissionselektronenmikroskopie (TEM) zur Charakterisierung dünner anodischer Barriereschichten vor. Dazu wurden magnetrongesputterte Aluminiumschichten bei unterschiedlichen Pulsfrequenzen oxidiert und mittels TEM analysiert. Unabhängig von den Herstellungsbedingungen sind die Oxidschichten porenfrei und amorph. Die Mikrostruktur des Aluminiums beeinflusst die Oxidschichten nicht. Mittels energiedispersiver Röntgenspektroskopie wurden keine Hinweise auf den Einbau von Elektrolytspezies in die Barriereschichten gefunden.

In a previous work, the authors investigated the formation of thin barrier-like oxide films on aluminium using pulse anodizing [1]. The electrochemical experiments have shown differences in the properties and behaviour of the anodic films depending on the pulse frequency. Furthermore, the results suggest a relationship to the microstructure (e.g. thickness or composition) of the oxide films. Therefore, the authors have completed the former investigation by new research using transmission electron microscopy (TEM) and ellipsometry. All results were compared with electrochemical measurements. Again, the anodic films were formed at 3VAg/AgCl with various pulse frequencies in an acetate buffer solution (pH5.9). The aluminium was chemical vapour deposited on silicon wafers to get a smooth and well-defined surface which is favourable for subsequent analytical analysis (TEM, ellipsometry). The TEM investigation confirms the tendency that the oxide thickness decreases with increasing pulse frequency. The film thicknesses determined by coulometry and microscopy fit very well assuming a native oxide layer of approximately 1nm. Additionally, the Al:O ratio across the film thickness clearly depends on the pulse frequency. A model concept explaining this fact will be presented. Furthermore, there are no indications to the formation of crystalline domains within the oxide layers deposited at various pulse frequencies. [1] M. Schneider, C. Lämmel, C. Heubner, A. Michaelis: Surface and Interface Analysis 45 (2013) 1497-1502

The temperature development during anodizing processes plays a key role in the case of industrial hard anodizing processes. Commonly, only the bath temperature is controlled; although it is well known that the bath temperature can significantly deviate from the temperature across the interface metal/oxide/electrolyte no investigation is known which shows the temperature deviation in this area. The authors have built a special experimental setup which allows a local temperature measurement across the interface. The results show the possibility of local resolved temperature measurements and the calculation of the heat flow into the metal and the anodizing bath. Based on this knowledge a target control of the current-voltage-regime avoids the formation of thermally caused cracks in hard anodized layers.

The present work is motivated by own studies about the heat impact during hard anodizing where the influence of voltage pulse modulation on the temperature development inside anodic films was investigated. For a better understanding of the interplay between pulse modulation and oxide formation, pulse experiments were carried out in an acetate buffer electrolyte with varying pulse parameters. Under these conditions, the oxide film only consists of a pore free barrier layer. The authors started from the presumption that the oxide film formation follows the high-field mechanism that is well evidenced for single pulse experiments. The oxide properties were characterized by electrochemical experiments. The authors found significant differences of the oxide properties depending on the pulse parameters that cannot be explained by high-field law of oxide formation. The results suggest a changing in the oxide properties (density, short-range order and content of electrolyte species) generated by pulse anodizing.