A method was developed to determine simultaneously the thickness and the elastic modulus of surface layers from surface wave dispersion. Surface wave pulses with a broad bandwidth are generated by an impulse laser. The pulses are received with interdigital transducers in the frequency range 12-31 MHz. A Fourier Transform technique is used to determine the dispersion curve representing the phase velocity depending on frequency. The problem of the n2pi ambiguity has been overcome by a measuring procedure based on the determination of the phase shift for several increasing measuring distances between the sound source and the receiving transducer. To calculate thickness and elastic modulus of the layer, the inverse solution of the surface wave dispersion in a homogeneous, isotropic material coated with a homogeneous, isotropic layer is carried out by non-linear regression. Two materials, (W, Ti, Ta)C-Co cemented carbide coated with either nickel or TiC, represent the two cases of normal and anomalous surface wave dispersion and were investigated with the surface wave method. The layers had thicknesses of 3-80 mum. In the case of normal dispersion in nickel-coated cemented carbide, measurement of the dispersion curve in the range up to a ratio of layer thickness to wavelength d/lambda = 0.05 is sufficient to determine thickness and elastic modulus simultaneously. In the case of anomalous dispersion in TiC-coated cemented carbide, measurement in the range up to d/lambda = 0.21 is necessary. Good agreement has been found between the surface wave results for the layer thickness and those obtained with the stylus instrument and the weighing method. The measured values of the elastic modulus of the layer indicate structural changes.
