Preparation and mechanical properties of composite diamond-like carbon thin films

Abstract

We have investigated mechanical properties of diamond-like carbon (DLC) thin films, particularly the internal compressive stress and ways to alleviate it. Foreign atoms such as copper, titanium, and silicon were incorporated into the DLC films during pulsed laser deposition. The chemical composition of the doped films was determined using Rutherford backscattering spectrometry (RBS) and x-ray photoelectron spectroscopy (XPS). Optical microscopy of the doped films showed that DLC films containing Cu exhibit much less particulate density as compared to the films containing Ti and Si. Visible Raman spectroscopy was used to characterize the films. The effect of dopants on the Raman spectrum was analyzed in terms of peak shape and position. Optical microscopy of the pure DLC of a certain thickness showed severe buckling. The mechanisms of adhesion associated with DLC coatings were discussed. Qualitative scratch tests on the specimens showed that pure DLC films have relatively poor adhesion due to a large compressive stress, while the doped DLC films exhibit much improved adhesion. Wear tests show improved wear resistance in the doped DLC coatings. Nanoindentation results suggest that pure DLC has an average hardness above 40 GPa and effective Young’s modulus above 200 GPa. The doped DLC films showed slightly decreased hardness and Young’s modulus as compared to pure DLC films. These results can be rationalized by analyzing the internal stress reduction as derived from Raman G-peak shift to lower wavenumbers. A preliminary interpretation of the stress reduction mechanism is discussed.