Nanoindentation: Unload-to-load work ratio analysis in amorphous carbon films for mechanical properties

In many applications and in particular those requiring protective coatings, material characterization in terms of mechanical properties is of importance. The most appropriate technique to obtain these properties is nanoindentation. As coating and substrate form a composite system, it is crucial to know the energy repartition during the indentation process, in order to find the intrinsic properties of the film or the composite properties of the whole system. In this work, the elastic and dissipated works of indentation, estimated from the load displacement curves in nanoindentation measurements and described by the unload-to-load indentation work ratio, were followed in function of indentation depth. This parameter was used to examine the behaviour of four carbon film/substrate systems: hard C film on silicon, hard C film on polyethylene terephthalate (PET) substrate. soft C film on silicon and soft C film on PET. The harder carbon films were sputter-deposited in a RF plasma in Ar-H(2) from a graphite target, while the so-called soft films consisted of polymer-like carbon growth by PECVD in CH(4)-N(2)-H(2) plasma. Different behaviours were observed for the four systems. The results allow to distinguish between cases of elasto-plastic behaviour of the materials and those where a fully plastic deformation of the film or of the substrate affect the determined hardness values. in the case of an elasto-plastic behaviour, a curve fit of the depth profile of unload-to-load indentation work ratio to a Hill function allowed to define a critical depth which limits the depth zone of a film only property. (C) 2009 Elsevier B.V. All rights reserved.