Ultra thin films (50 nm and 180 nm) of amorphous diamond-like carbon on a silicon substrate produced by laser ablation are tested by nanoindentation with a new instrument deriving from a Surface Force Apparatus. Quasi-static measurements of the load and dynamic measurements of the contact stiffness are continuously and simultaneously recorded versus the penetration depth. Scanning lines on the tested surface before and after indentation are made by means of tangential displacement of the diamond indenter on the surface.
The tests are conducted with maximum loads from 50 μN to 2500 μN, which correspond to maximum indentation depths between 7 nm and 70 nm. The indentation curves show near elastic recovery but scanning lines and/or topographic images on the surfaces show detectable plastic prints. Despite the extremely small residual indentation depths for these ultra thin films, we show how the hardness value we calculate from the indentation curves with an elastoplastic theory is in good agreement with the hardness value we calculate from the indentation print profile. The determination of the Young's modulus, even at the smallest indentation depths, must take into account the mechanical properties of the substrate. The determination of both values, hardness and elastic modulus, also requires a calibration procedure for the geometry of the tip and knowledge of the piling-up effect.
We find that the apparent hardness and the apparent Young's modulus of the tested diamondlike films are high. They are underestimated in comparison with the real values. A rough correction which overestimates the Young’s modulus gives higher values than those of natural diamond.