On the measurement of strain in coatings formed on a wrinkled elastic substrate

Highly stressed coatings, such as those formed by oxidation can, on occasion, wrinkle. Such wrinkling has been suggested as a mode of deformation by which the overall strain energy in a compressively stressed coating can be reduced. One of the consequences of wrinkling is that the strain in the coating does not remain independent of position, but rather varies over the wavelength of the wrinkling. The strain variation caused by sinusoidal wrinkling is calculated using finite-element methods and the effects on both photostimulated Cr3+ luminescence piezospectroscopy measurements and X-ray measurements calculated. Wrinkling is shown to decrease the elastic-strain-energy density in the coating. A direct measure of the decrease is the shift in the R2 Cr3+ luminescence line and the X-ray diffraction peaks. Wrinkling of a compressive coating also causes stresses to be created perpendicular to the coating-substrate interface, tensile at the crests in the wrinkles, and compressive stress at the troughs.