Non-destructive evaluation of diamond and diamond-like carbon films by laser induced surface acoustic waves

Young's modulus of diamond-like carbon films varies in a wide range which suggests the use of this material parameter to characterize the film quality. It can be obtained non-destructively by means of surface acoustic waves. A reliable and quick photoacoustic technique has been developed which enables the film modulus to be determined for rather small samples such as cutting tools. The technique is based on the measurement of the surface wave velocity vs. frequency. This spectrum is derived from laser induced surface wave impulses by Fourier transformation. Comparison with the theoretical results may provide Young's modulus, density, and film thickness simultaneously. Less information is obtained for very thin films so that less than three parameters can be derived. By taking into account an empirical relation between Young's modulus and density which has been found for diamond-like carbon films, two parameters can be obtained even for films about 100 nm thick. The informational content of the measurement depends on the bandwidth of the equipment and on the substrate material. Diamond and diamond-like carbon films were studied. They were deposited using different technologies onto silicon single crystals, steel, and WC-Co cemented carbide. The film thickness was in the range 60 nm < d < 6 m.