Abstract
AbstractThe mechanical properties of thin films often differ from those of bulk material and depend on technological parameters. But they can be determined in the coating‐substrate composite directly by means of ultrasonic surface waves. For thin layers in relation to wavelength, the displacement field within the layer is expanded in powers d/Λ. The corresponding equation system given for the general case is evaluated for elastic and viscous coatings in lowest order of d/Λ. An elastic covering layer causes a relative change in velocity proportional to ω η d where the factor is determined by the elastic and inertial differences between film and substrate. By a viscous coating the surface wave is attenuated proportional to ω3ηd. The elastic case confirms in the first order the results of a plate theory approximation given formerly by Achenbach and Keshava. However, their higher order terms prove to be incomplete from the viewpoint of the present systematic expansion.
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