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Surface free energy (SFE) is a property which depends on the chemical state and roughness of the surface and it is necessary to develop a reliable method to evaluate SFE value on a small area, taking into account these two different contributions. Today contact angle methods are the most used and they allow to evaluate the global mean value of SFE on areas of mm2 size. With these methods, it is not possible to evaluate the effects of roughness, surface defects, chemical contamination on SFE value. In addition, it is difficult to determine the surface free energy value on small components which have dimensions smaller than drop diameter. Nanoindentation and atomic force microscopy techniques provide alternative direct measurement methods to evaluate the SFE on small areas (on the order of μm2 or nm2) through a contact mechanism triggered by the contact of two bodies. In order to evaluate the adhesion properties, currently three models, Johnson- Kendall-Roberts, Maugis-Dugdale and Derjaguin-Muller-Toporov, use the value of pull-off force (force required to separate the indenter tip from the sample). All influences of surface morphology on SFE values are lost using these methods. In fact the adhesion value obtained refers to the energy balance between two conformal surfaces, which depends mainly on the morphology of the harder material (i.e., diamond tip). In this work we describe a new methodology for the SFE determination consisting in the modeling and quantitative evaluation of the interaction between the tip and sample surface during the approach phase in a nanoindentation test. During the test, the nanoindenter tip is attracted to the sample surface until the sample reaction forces become significant (in this case physical contact between two bodies is achieved). The SFE value is evaluated using experimental force of attraction and displacement of the nanoindenter spherical tip when it approaches the sample surface. In this method the sample surface is not altered by the tip, therefore unlike pull-off force method, it could be very useful to evaluate the actual SFE considering the effect of sample morphology (controlled roughness or pattern). [ABSTRACT FROM AUTHOR]

We present a new procedure for the determination of surface elastic residual stress by instrumented sharp indentation, based on nanoindentation testing on focused ion beam (FIB) milled micro-pillars. Finite element modeling (FEM) of strain relief after FIB milling of annular trenches demonstrates that full relaxation of pre-existing residual stress state occurs when the depth of the trench approaches the diameter of the remaining pillar. Considering this, the average residual stress present in the sample material can be calculated by the comparison of two different sets of load-depth curves, the first one obtained at the center of stress relieved pillars, the second on the undisturbed (residually stressed) surface. Analytical modeling of the contact stress distribution in non-halfspace conditions was adopted to take into consideration the additional boundary conditions given by the edges of pillars and the elastic properties of the substrate (in case of coatings). The results are presented for residual stress evaluation of a 3.8-µm TiN coating on WC-Co substrate obtained by cathodic arc evaporation-physical vapor deposition (CAE-PVD) techniques, showing an average compressive stress state of -5.63 GPa. This result is in close agreement with the estimation obtained by XRD (sin2 ψ method) analysis of -5.84 GPa of the same sample, adopting the same elastic constants. [ABSTRACT FROM AUTHOR]