Non contact measurement of thin layer thickness by zero group velocity Lamb modes

Abstract : A homogeneous elastic plate can support non-propagating Lamb modes having a zero group velocity (ZGV) for nonzero values of their wave number, giving rise to sharp resonances. For plates covered with a thin layer, it was shown that the frequency of the S1S2 ZGV resonance was shifted proportionally to the mass loading through a factor which depends on the mechanical properties of the layer and of the substrate. As they are non-contact and broadband, Laser Based Ultrasonics techniques are very efficient to investigate mechanical vibrations. In our experiments, Lamb modes were generated by a Nd:YAG pulsed laser. An optical interferometer was used to measure the displacement normal to the plate surface at the same point on the non-coated face. Gold layers of a few hundred nanometers were detected through a 1.5-mm thick Duralumin plate. The shift of the 1.9-MHz resonance frequency of the fundamental ZGV mode is proportional to the layer thickness: typically 10 kHz per μm for a gold layer deposited on a Duralumin plate. Taking into account the influence of the temperature, a 240-nm gold layer was measured with a \pm 4% uncertainty. This thickness has been verified by scanning the coated face with an optical profiling system.
Type de document :
Communication dans un congrès
Société Française d'Acoustique. Acoustics 2012, Apr 2012, Nantes, France. 2012
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Maximin Cès, Dominique Clorennec, Daniel Royer, Claire Prada. Non contact measurement of thin layer thickness by zero group velocity Lamb modes. Société Française d'Acoustique. Acoustics 2012, Apr 2012, Nantes, France. 2012. 〈hal-00811086〉

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