Intrinsic mechanical properties of ultra-thin amorphous carbon layers

In this work, we extracted the film's hardness (HF) of ultra-thin diamond-like carbon layers by simultaneously taking into account the tip blunting and the substrate effect. As compared to previous approaches, which did not consider tip blunting, this resulted in marked differences (30-100%) for the HF value of the thinner carbon coatings. We find that the nature of the substrate influences this intrinsic film parameter and hence the growth mechanisms. Moreover, the HF values generally increase with film thickness. The 10 nm and 50 nm thick hydrogenated amorphous carbon (a-C:H) films deposited onto Si have HF values of, respectively, ∼26 GPa and ∼31 GPa whereas the 10 nm and 50 nm thick tetrahedral amorphous carbon (t-aC) films deposited onto Si have HF values of, respectively, ∼29 GPa and ∼38 GPa. Both the a-C:H and t-aC materials also show higher density and refractive index values for the thicker coatings, as measured, respectively by X-ray reflectometry and optical profilometry analysis. However, the Raman analysis of the a-C:H samples show bonding characteristics which are independent of the film thickness. This indicates that in these ultra-thin hydrogenated carbon films, the arrangement of sp2 clusters does not relate directly to the hardness of the film.