A qualitative strain-indicator for preliminary diagnostics of a-C based coatings

The analysis of the C sp2 mode region of a traditional 2.41 eV Raman spectra, usually finalized to the investigation of film-bonding properties for growth-process optimization purposes, is shown to also provide relevant information about bonding topology, structural-order degree and related stress-level, characteristics of basic importance in view of the film utilization as protective coatings. The example considered here is of the commonly used ablated hydrogen-free amorphous-carbon (a-C) and reactively sputtered hydrogenated amorphous carbon-nitride (a-CN:H) coatings, among which a large variety of behaviors are evidenced, from graphite- (GLC) to diamond-like carbon (DLC). Strains affecting coatings are indirectly evidenced by investigating the formation of defective rings within the trigonally bonded domains. Two structural components are singled out, whose competing formation within the film is shown to ultimately determine the average structural-disorder level. A strain indicator is introduced, which is able to monitor the changes in stress level at the C sp2 sites, or even to account for the overall behavior of a-C based materials with a limited sp 3 C bonding component. Finally, the reliability of the novel parameter is preliminarily tested.