Continuous stiffness techniques offer another means of assessing the mechanical properties of coated systems using nanoindentation. This paper will critically assess the differing types of sample information available from simple load-displacement (P-δ) curve approaches, established P-δ² analyses and, most recently, appraising P-S² (i.e. load-vs-(contact stiffness)²) data. As necessary background to establishing the P-S² approach, any effects (on the sample load-displacement response) of the small superposed AC signal used to continuously measure contact stiffness have also been explored. Apart from some small statistical variation in the loads at which crack initiation was observed in a SiC-on-Si coated system, TEM and HRSEM studies showed no detectable differences in deformation sub-structures between nominally identical indentations made with and without the AC signal.

While the parameter P/S² is independent of detailed tip shape and is a constant with displacement for monolithic systems, it was found to display unexpected variations with displacement for the coated systems. Since P/S² can also be related to the plasticity index (ψ) widely used to describe the balance between the elastic and plastic responses of materials subjected to contact damage, the observations of maxima in plots of P/S² with displacement, for at least some coated systems, suggests that there may be an optimum contact scale for maximising the elastic contribution to the contact response of such systems. The P/S² approach should also work well with the rougher surfaces often associated with real coated systems.

The concept of work-of-indentation arises naturally in the study of hardness. Similarly to the way in which toughness was introduced by Griffith to describe the energy expenditure per unit area during fracture, hardness can be understood in terms of energy expenditure per unit volume during indentation. However, a significant difference exists between the two cases. In the case of brittle fracture, the area concerned has a clear physical meaning of the crack surface. In the case of indentation, however, the volume in question IS NOT that displaced by the indenter. Not only is this volume poorly defined, but it is also difficult to measure.

Depth sensing indentation offers remarkable opportunities of analysis and interpretation. The detailed and accurate record of the applied load and the tip displacement during both the loading and unloading phases of the experiment allows the energetic characteristics of the process to be computed. It is then possible to choose as a basis for a hardness definition a combination of any two or more from the following parameters: maximum load P and depth d, total work of indentation W, as well as its elastic W_e, and plastic W_p parts.

In the present study, a family of possible hardness definitions were systematically applied to the results of nanoindentation in various bulk materials, and coated systems, with a view to identifying the most suitable formulations appropriate to each case and class of materials. A finite element study of the iindentation process, using various tip shapes and material response laws, has also been carried out. The results suggest some improvements that can be achieved in hardness measurement and interpretation, particularly for ultra low loads.

The carbon atom ability to make bonds with different hybridization allows the preparation of a large variety of materials with different structure such diamond, graphite, polymeric films, and nanostructures. The investigation of the carbon properties in different network helps us to understand its properties in addition to the development of new materials. In this work we investigated the properties of carbon-germanium alloys prepared by the sputtering technique, which allowed the preparation of thin films in the whole composition range, i.e. from 0 at.% up to 100 at.% carbon content range, using the same deposition conditions and varying the target composition. The elastic modulus, thermal expansion coefficient, stress, and nanohardnes of these films were investigated for the first time. It was observed that the stress of the alloys decreases in relation to the pure films, a-Ge:H and a-C:H, indicating the degradation of the structure of the alloys. Contrarily to what was expected, the hardness, as well as the elastic modulus, of the alloys with small concentration of carbon is smaller than that of pure germanium films. These properties are analyzed in terms of the structural properties of the films determined by infrared and Raman spectroscopies.

Supported by brazilian agencies: Fapesp and CNPq.