Failure mechanism and protective role of ultrathin ta-C films on Si (100) during cyclic nano-impact

Complex mechanical behavior with phase transformation and high brittleness limits the reliability of silicon-based microelectromechanical systems. Although very hard ultra-thin films are being considered as protective overcoats to improve the service life of substrate materials, their resistance to fatigue can be at least as important as hardness when exposed to cyclic loading. In this study repetitive nano-impact tests with a spherical diamond probe have been used to investigate the fatigue behavior and protective role of 5 and 80 nm tetrahedral amorphous carbon (ta-C) films on silicon. At the lowest load there was delamination of the 80 nm film but not for the 5 nm film. At higher loads failure involved lateral cracking of the silicon substrate. Single impact tests showed that this was preceded by ring and radial cracking. Changing contact pressure during the test provided further support for the degradation mechanism and the influence of phase transformation in the Silicon substrate. Under repetitive contact the thin film systems showed lower impact depth and greater impact cycles before substrate fracture than the uncoated Silicon. This is related to their enhanced load support which affects phase transformation in the substrate, with potentially delamination providing an additional impact energy dissipation mechanism.