QUANTITATIVE ACOUSTIC MICROSCOPY OF ANODIZED AND COATED ALUMINUM AT FREQUENCIES UP TO 1 GHZ

Quantitative acoustic microscopy (QAM) has been used to measure surfac e wave velocities on polished, anodized and coated aluminium substrate s, these materials being representative of those used for adhesive bon ding in the aerospace industry. Good quality acoustic measurements wer e obtainable at frequencies between 225 and 980 MHz, despite the inhom ogeneous nature of the oxide layer produced by phosphoric acid anodiza tion (PAA). Good agreement was obtained between the surface acoustic w ave dispersion measured on aluminium coated with 0.2 and 1.0 mu m PMMA , and that calculated by a simple isotropic layer model. The anodized aluminium was modelled as a transversely isotropic oxide layer on an a luminium substrate. At 0.2 mu m, the oxide layer was too thin for the comparison between measurement and calculation to be conclusive, but t he calculations suggest that a change in porosity of 10% in a 0.6 mu m oxide layer, as obtained with an industry standard PAA treatment, sho uld be readily detectable. The highly dispersive nature of some of the surface acoustic wave modes makes OAM extremely sensitive to small ch anges in the material parameters.