High resolution ultrasonic interferometry for quantitative nondestructive characterization of interfacial adhesion in multilayer (metal/polymer/metal) composites

We describe an ultrasonic method for the quantitative and nondestructive characterization of interfacial adhesion. Near an interface, such as polymer/metal, there exists a thin interfacial boundary layer at the nm length scale where the polymer may have properties different from those of the bulk polymer. On the other hand, ultrasonics measures linear mechanical properties of materials on a length scale which is that of the wavelength, usually in the range of several μm. Hence, it is usually stated that standard ultrasonic techniques cannot probe interfaces effectively. Here, we report on an ultrasonic interferometry technique that is built around metal/polymer/metal structures and that exhibits high sensitivity to interfacial properties, even at usual ultrasonic wavelengths. A rigorous mathematical model for the ultrasonic response of multilayered media that accounts for viscoelasticity is presented. Results of numerical calculations point out scaling features for interfacial properties in terms of speci...