A Novel Method for Detecting Characteristic Parameters of Coating Layers by Ultrasonic Surface Wave Technique

An improved reversion method for ultrasonic surface wave characterization of coating layers is presented and validated by experiment. The experiment is implemented with seven specimens of thin Cu layers coated on Al substrates. 2-D Fourier transforms of the experimentally obtained surface wave pulse propagation signals were conducted to obtain frequency-wave number relations. The combined equations of two unknowns were derived using the relation. The two unknowns are the thickness h and the Young’s modulus E of the coating layer of the specimen. The h and E are obtained by solution of the combined equations. Comparing with that used for many years which are based on 1-D Fourier transform and non-linear regression, the new method is simpler, faster, and equally accurate and reliable. The standard error of $1.55~\mu \text{m}$ is reached in the measurements of the Cu films thicknesses which vary in the range from $23.6~\mu \text{m}$ to $224.8~\mu \text{m}$ . The standard error of 2.97GPa is reached in measurements of Young’s modulus of the Cu films and the exact value of Young’s modulus of pure Cu is 110GPa.