Measuring coating internal stresses by the curvature method applied to a beveled sample

The curvature method is widely used to measure the internal stress of coatings and films deposited on a substrate. The conventional Stoney's formulae then allow, in the case of a parallel-faced substrate, the stress to be deduced from the curvature measurement. We here propose an original technique using a substrate not parallel-faced but cut into a bevel, so that a large part of the specimen exhibits an arbitrarily small thickness, thus increasing the curvature to be measured. The analytical profile associated to a beveled specimen then is established in comparison with Stoney's formulae. In this way, residual stresses can be quantified in delicate cases of low residual stresses or thin coatings. The method is detailed by considering the case of a nitride coating on stainless steel which was previously characterized by the conventional method. Lastly, an amorphous silicon layer 65 nm thick deposited on crystalline silicon is studied. The stress in the layer is compressive and equals -1.3 +/- 0.4 GPa. (C) 2014 Elsevier B.V. All rights reserved.