A laser-acoustic method for testing and classifying hard surface layers

Abstract

The laser-acoustic method is accepted to be an interesting method of testing thin films. It is based on measuring the dispersion of surface acoustic waves which are generated by short laser pulses. A reliable test equipment was developed that allows a user-friendly operation. The method is non-destructive, the test takes little time and special sample preparation is not required. It is mainly applied to measure the Young’s modulus of thin films with thickness down to less than 50 nm. Recent studies showed these results to correlate with important microstructural and mechanical properties of hard and superhard films. The laser-acoustic technique was improved to test multilayer films consisting of two components. The approach of an effective medium of transversal symmetry is used to describe the elastic behavior of multilayer films. It enables the elastic anisotropy of the multilayer film to be evaluated. Applications are presented, performed at multilayers of diamond-like carbon and aluminum deposited by laser-arc on steel and silicon. The films consisted of four and twenty single layers, respectively. The Young’s modulus of the diamond-like carbon in the multilayer was determined with the laser-acoustic technique. The results reveal the reproducibility of the deposition technique and demonstrate the potential of the laser-acoustic technique to test multilayer films. The laser-acoustic method is shown to be sensitive to machining layers. The effect of grinding and polishing steel surfaces was studied. Studies were performed to compare the results of the laser-acoustic technique with those of membrane deflection and micro-indentation. TiN, CrN and TiCN films (thickness: 0.8–2.3 μm) were tested with laser-acoustics and micro-indentation, polysilicon films (thickness: 0.46 μm) with laser-acoustics and the membrane deflection technique.