Laser-induced acoustic surface wave spectroscopy allows quick and non-destructive access to elastic properties of coatings, surfaces and surface-near bulk materials down to the nanometer scale. Furthermore, cracks, pores and delamination can be measured, as they influence the propagation of surface waves as well. Therefore, the method is a quick and powerful tool for surface characterization and can be found today in research and development, quality control and as a precise and scientific reference method.

Generation of surface acoustic waves is accomplished contact-less with a short laser pulse on the surface of interest. Even though contact-less measurement systems exist, e.g. based on laser interferometry, piezo-mechanical contact sensors show significant advantages in signal intensity, roughness tolerance and cost. The conventional high frequency contact sensor, a steel wedge pushing down a polymer-based piezo foil, however, has a limited temperature range of about 80°C so that many coating scenarios with higher application temperature cannot be measured under such condition.

In this study, the development and application of a new mechanical surface wave sensor unit is presented, where the polymer piezo foil was replaced by a piezoelectric AlN coating. The coating was directly deposited on the sensor and can withstand temperatures of more than 1000°C. Major coating challenges like high film thickness, thermal mismatch and extreme substrate geometry were overcome by development of an adapted magnetron sputtering process.

The coated sensor was mounted in a modified measurement system, featuring temperature-resistant optics and a heating table. The new setup was compared to the conventional foil sensor at room temperature, as well with temperatures up to 600°C, where the limit of the heating table was reached. It was found that for both frequency range and signal intensity results were satisfying for most measurement scenarios.

Finally, the potential of the new measurement setup for the application of PVD coatings is shown by a systematic study of temperature resistance of hydrogen-free tetrahedral amorphous carbon coatings in air. Young’s Modulus, an thus indirectly the sp3 content, was measured up to maximum temperature, showing that softer a-C coatings degenerate much earlier than superhard ta-C coatings, which retain their full mechanical properties up to 500°C.

This work extends the benefits of surface acoustic wave spectroscopy for higher temperatures to at least 600°C, opening up the possibility to study the influence of thermal activated processes such as oxidation, diffusion or phase changes on mechanical properties.