Ultrasonic characterization of mechanical properties of Cr- and W-doped diamond-like carbon hard coatings

Two ultrasonic nondestructive evaluation methods have been applied to systematically characterize the mechanical properties of nano-structured physical vapor deposited diamond-like carbon (DLC) ultra-hard coatings. A photoacoustic guided-wave technique is used to generate broadband surface acoustic waves (SAW) on the coating surfaces, and dispersion relations of the SAW for DLC coatings are measured with frequencies up to about 150 MHz. In addition, line-focus acoustic microscopy is used to investigate the dispersion curves of leaky SAW on coatings in the ultrasonic frequency range from 140 MHz to 240 MHz. A multi-layer material model is used to analyze the wave propagation phenomena in the DLC coating specimens and the transfer matrix method is used to numerically calculate theoretical dispersion curves. The measured acoustic dispersion curves by the two ultrasonic methods are therefore independently analyzed by a nonlinear optimization approach in the inverse problem. The derived Young's moduli using the two ultrasonic techniques are compared with nano-indentation tests and good quantitative agreement is found. An example application of photoacoustic methods to a curved gear component is also shown.