Practical measurement of the residual stress in coatings

The method adopted to measure residual stress depends on the degree of detailed information needed, the size of the part, the coating thickness and the costs which can be incurred. Three of the main approaches used at the present time are discussed here. These are: first, the hole drilling method which returns a value of the macrostress and an indication of any anisotropy in coatings of thickness greater than about 0.3 mm; second, the cantilever beam methods applicable to thinner coatings on thin substrates; third, the X-ray diffraction methods applicable to crystalline coatings of a thickness which can be penetrated by the X-ray beam. This last method can be sub-divided into two groups where, in the first, the standard Bragg-Brentano diffractometer available in most laboratories is used, and, second, the more recent glancing incidence methods in which the stress in thin coatings or the surface of thicker coatings can be studied at depths of as little as 1 mu m.