ULTRASONIC SPECTROSCOPY OF IMPERFECT CONTACT INTERFACES BETWEEN A LAYER AND 2 SOLIDS

Ultrasonic waves reflected from the front and back surfaces of a thin layer are often not separated in the time domain, and interfere. The s pectrum of the resulting interference signal depends on (a) the thickn ess of the layer and the elastic moduli and density of the layer and t he surrounding material (substrates), and (b) properties of the layer/ substrate interface which can be described in terms of the interfacial stiffness. In this paper the effect of interfacial stiffness is isola ted by considering the ultrasonic wave interaction with a solid layer compressed between two substrates of the same material, Since the laye r and the substrate have identical properties the effect of impedance difference on the layer reflection vanishes. An aluminum system is sel ected for the experiment; the contacting surfaces are roughened and va rying pressure is applied to model imperfect interface changes. It is shown both theoretically and experimentally that the contact pressure increase results in increase of the interfacial stiffness and spectral minima shift to higher frequency. A simple analytical expression rela ting the reflection minimum position to the interfacial stiffness is d erived and shows good agreement with experimental results. It is shown that in the high-interfacial-stiffness limit the resonance minima pos itions are given by the condition h=lambda/4 +n lambda/2, n=0,1,2,.... In the limit of low interfacial stiffness the first minimum shifts to zero and higher order resonances are given by h=n lambda/2, Since the resonance minima measurements can be done with high precision it is p roposed to use the frequency minimum shift for determination of interf acial stiffness and, consequently, the quality of the interfacial cont act. (C) 1998 Acoustical Society of America. [S0001-4966(98)06201-8].