Abstract
ALUMINIUM surfaces in telephone apparatus are sometimes protected by an electrochemically deposited oxide coating. It is important to control the thickness of these deposits and several methods have been tried. A majority of them, such as scratching the surface, stripping the deposit to weigh it, and measuring the thickness of a cross-sectional cut under the microscope involve destruction of the sample. A quick and reliable method, which is not subject to this limitation, is described in the Bell Laboratory Record of July 1942. It measures the voltage required to break down the oxide coating and punctures the specimen with so small a hole that it is not appreciably marred. A chromium-plated sphere about in. diameter is pressed against a thin plate of oxide-coated aluminium until the force, as indicated by a calibrated spring, is 1-2 kgm. Increasing voltages up to 1,500 are then applied, and that at which breakdown occurs is noted, the current being limited by resistors. This method has been used in the Bell Laboratories to study the relation between film thickness and the time to make the deposits electrochemically. The thickness obtained by an average of several readings is generally within ten per cent of the value found by direct measurement with a microscope.
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Thickness of Aluminium Oxide Coatings. Nature 150, 428 (1942). https://doi.org/10.1038/150428b0
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DOI: https://doi.org/10.1038/150428b0