Interfaces and temperature stability of stepwise graded DLC films studied by nanoindentation and Raman spectroscopy

To investigate the temperature stability of stepwise graded diamond-like carbon (DLC) coatings the temperature development of the hardness depth profile measured by the small angle cross-section nanoindentation method (SACS) and of the Raman spectra has been studied in detail. Up to an annealing temperature of 600 °C the graded layer system remained stable, whereas after 1 h annealing in vacuum at 650 °C the uppermost layer completely delaminated, because the graphitisation in the lowest layer set in, thereby destroying its stress managing facility. Characteristic changes in the Raman spectra and linear correlations between characteristic Raman quantities such as the ID/IG ratio and the hardness in all layers as a function of annealing temperature were found, which are related to changes in bonding and nanostructure. In addition it was estimated from the Raman spectra that the films predominantly consist of sp2 bonds with a maximum sp3 content of about 12–14%.