Picosecond time scale imaging of mechanical contacts

By means of an ultrafast opto-acoustic technique we study the nanoindentation of thin chromium films on sapphire substrates by a ceramic ball bearing. Acoustic echoes at ∼40 GHz returning from the film-indenter interface allow the film indentation profiles to be probed to sub-nanometer resolution over contact areas ∼25 μm in radius. The deformation of the films during loading is thereby revealed. Furthermore, thermal wave imaging of the contact at megahertz frequencies is simultaneously achieved.