Nanoindentation studies of multilayer amorphous carbon films Original Research Article

The mechanical properties of multilayer amorphous carbon thin films, consisted of sequential sp3- and sp2-rich layers developed on c-Si, were investigated by nanoindentation. The total (∼100 nm) and bilayer (14–45 nm) thickness, and the sp3 and sp2 carbon bonded content, were determined by spectroscopic ellipsometry. The continuous stiffness measurements technique was used to measure hardness (H) and elastic modulus (E), as a function of depth. The Bhattacharya–Nix equation, which takes into account the substrate effect on the measured values of H, was applied in order to extract the filmʹs hardness (Hf). Large deviations between Hf and H were found in hard multilayers. The dependence of Hf on the sp3 content and the bilayer thickness, was also studied. An almost linear relation was found between Hf and the sp3 content. In addition, the multilayer structure improves filmsʹ hardness, and this depends on the bilayer thickness, in agreement with Koelerʹs model for crystalline materials. Finally, it was found that the thickness reduction of the sp2-rich layer increases the multilayer hardness.