An adhesion tester for thin oxide films on a metal

We have developed an instrument to measure the relative adherence of oxide films on sheet steel. It is based on the observation that, when a sheet metal substrate is bent, a surface film may adhere for a gentle bend, but peel off under a sharp bend. The instrument bends the sample around a conical mandrel, allowing us to observe the adherence as a continuous function of the radius of curvature of the bend. We show the usefulness of the instrument in ranking a series of samples, quantitatively, according to the adherence of the film. The bending method should be useful for characterizing other films on metal substrates.