Study of coupling fluids for the microacoustic characterization of high velocity materials

Mechanical characterization of materials using acoustic microscopy requires acquisition of acoustic signatures. These signatures or V(z) are not only characteristic of the investigated materials but are also influenced by the use of appropriate coupling fluids. The aim of this work is to demonstrate the modifications of the acoustic signatures recorded at 570 MHz and the d educed FFT curves versus the concentration of a series of coupling liquids. These liquids, chosen according to their acoustic properties, are aqueous solutions of potassium hydroxide KOH. Acoustic microechography and acoustic microscopy were used to determine absorption and velocity of these electro lyte solutions as a function of their concentration in a large range (0-19M ). The theoretical approach of the electrolyte/sample system enlightens on the part of the density of the coupling fluid. The experimental study was c arried out on three materials chosen according to their acoustic waves velo cities: steel, glass and silicon. The peaks appearing in the curves deduced by FFT treatment from the experimental acoustic signatures are assigned by comparison with theoretical models. The longitudinal mode velocity of stee l and the Rayleigh mode velocity of silicon which lie in the range 5500-750 0 mis can be measured using high concentrated solutions. The evolution of t he shape of the curves as a function of the solutions concentration confirm s that the increase in density of the coupling fluids improves the radiatio n of the surface waves in the liquid.