On the Measurement of Strain in Coatings Formed on a Wrinkled Elastic Substrate

Highly stressed coatings, such as those formedby oxidation can, on occasion, wrinkle. Such wrinklinghas been suggested as a mode of deformation by which theoverall strain energy in a compressively stressed coating can be reduced. One of the consequencesof wrinkling is that the strain in the coating does notremain independent of position, but rather varies overthe wavelength of the wrinkling. The strain variation caused by sinusoidal wrinkling iscalculated using finite-element methods and the effectson both photostimulated Cr³⁺ luminescencepiezospectroscopy measurements and X-ray measurementscalculated. Wrinkling is shown to decrease theelastic-strain-energy density in the coating. A directmeasure of the decrease is the shift in the R2Cr³⁺ luminescence line and the X-raydiffraction peaks. Wrinkling of a compressive coating also causes stressesto be created perpendicular to the coating-substrateinterface, tensile at the crests in the wrinkles, andcompressive stress at the troughs.