Ultrasonic Characterization of Surfaces and Interphases
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locity) and elastic-wave attenuation are affected by the microstructure. If one knows the effect of microstructure on mechanical properties such as strength, ultrasonics could be used for nondestructive evaluation of the mechanical properties of the material. While methods of ultrasonic characterization of materials in bulk are widely known in the material-science community, methods for characterization of thin surface and interphase layers are much more specialized and less developed. Here we will review some advances in this field.
DILATATION
COMPRESSION
Figure 1. (a) Deformation of a surface of a solid body with coating. (b) Longitudinal strain of a surface film on propagation of a Rayleigh wave.
Characterization of Thin Films Using Elastic Surface Waves Surface waves are often used to measure the properties of a very thin layer of material, the thickness of which is much smaller than the thickness of the substrates. Since the mechanical properties of the film-substrate system are controlled not only by the bulk properties of the film but also to a significant degree by the adhesion of the film to the substrate, the properties of the film-substrate interphase are also of prime interest. As shown by Rayleigh, an elastic surface wave may propagate along the surface of a solid body.2-3 The elastic energy of surface waves decays nearly exponentially from the surface and is localized in a subsurface layer with thickness of the order of wavelength A. At a depth of the order of A, there is virtually no transport of elastic energy. The presence of a thin film on the substrate surface changes the surface stiffness and also loads the surface. Figure 1 shows schematically that a thin film, whose thickness is much less than a wavelength, is subjected to a compression-tension strain in the course of surface-wave propagation. The strain arising in the film is similar to the strains in a thin plate upon propagation of a longitudinal plate mode. In this case, the velocity of the surface wave depends not only on the substrate properties but on the stiffness Eo/(l~vo2) and the density p0 of the film. (Eo is Young's modulus, and va is the film Poisson ratio.) Measurements of the surface-wave velocity can yield information on the coating properties and the condition of the bond between the coating and the substrate. Three methods are possible: (a) direct measurement of the velocity and attenuation of the surface wave, (b) the Rayleigh angle method, and (c) acoustic microscopy.
Direct Measurement of the Velocity and Attenuation of Surface Acoustic Waves There are no standard techniques or equipment for thin-film characterization using surface acoustic waves (SAW). However such measurements have been performed for a long time in different laboratories. Measurement of SAW attenuation4'5 has been used for the study of properties of thin films as a function of magnetic field or temperature. Measurements of the velocity and attenuation of SAW during deposition of thin films were also described.6'7 A noncontact laser ultrasonic metho
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