Direct observation of the two-layer structure of anodic oxide films on tantalum

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Abstract
Transmission electron microscopy of ultra-microtomed sections of anodic oxide films formed on sputtered tantalum substrates in phosphoric acid electrolyte reveals immediately that the films consist of two distinct layers of differing contrast. Elemental analysis of the film sections by energy-dispersive analysis of X-rays indicates the presence of phosphorus in the outer layer of lighter appearance, with phosphorus detected in the inner film layer of darker appearance. Direct interpretation of the data implies that the atomic density of tantalum in the outer layer is less than that within the inner layer owing to the incorporation of phosphate anions at the film-electrolyte interface during film growth. Furthermore, using the direct observation procedure, allowing precise film thickness measurements and revelation of differently contrasted film regions, it is possible to quantify the distribution of the field during anodic oxidation.