Elastic properties of hardness coatings using surface acoustic wave spectroscopy

Surface acoustic wave (SAW) spectroscopy is a nondestructive way to determine thin-film properties. Plane-wave SAWs are generated by a line-focused pulsed laser and detected by a Michelson interferometer. Phase velocity dispersion relations over hundreds of megahertz are obtained by measuring the wave displacement versus propagation distance. Data analysis employs a new Green´s function model that accommodates elastic anisotropy in the entire layered system. With this approach, quantitative values for film properties such as thickness d and Young´s modulus E are obtained. We evaluate two materials developed for enhanced wear resistance and show how different models containing one or more layers affect the results. In TiN films, E generally increased with increasing d and decreasing compressive residual stress σ, regardless of the analysis model used. In superhard Ti_1-xSi_xN_y films with a nanocomposite structure, SAW values for E were obtained. When combined with microhardness data from instrumented indentation techniques, these results showed that hardness-to-modulus ratios related to scratch and abrasion resistance were quite high.