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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]

Abstract: A novel method is presented for the assessment of the Poisson's ratio and residual stress on the micron scale, based on focused ion beam (FIB) two-step four-slot micro-milling and in-situ digital image correlation (DIC) analysis of the induced relaxation strains at the specimen's surface. The methodology has been fully validated through modelling and experiments on three different materials, namely, physical vapour deposition (PVD) chromium nitride (CrN), and as-deposited and annealed Cu thin films. The cases of non-equibiaxial stress state and non-isotropic elastic behaviour are discussed in detail, and a quantitative evaluation of the accuracy, error sensitivity and the effects of microstructure and anisotropy is presented. This method represents a substantial improvement over the existing state-of-the-art. It is suitable for application on amorphous and nanostructured materials, and provides a breakthrough in micro-scale Poisson's ratio analysis of thin films and small structures and components. [Copyright &y& Elsevier]

Abstract: Titanium and its alloys are extensively used in aerospace and mechanical application because of their high specific strength and high fracture toughness. On the other hand, titanium alloys often show low hardness, very low load bearing capacity and poor resistance to sliding wear, so that surface properties improvement is in many cases recommended, often by PVD processes. Present work deals with design, production and experimental characterisation of a duplex coating for Ti–6Al–4V components, consisting of a thick WC–Co interlayer deposited by High Velocity Oxygen Fuel (HVOF), followed by a Ti/TiN multilayer (two layer pairs, including the Ti bond layer) deposited by Cathodic Arc Evaporation (CAE) PVD. Before deposition, a preliminary coating design was carried out, based on finite element simulation of residual stress fields on the PVD coating for a range of configurations of its multilayered structure (Ti buffer layer position and thickness). Morphological properties of the produced coatings (thickness, grain size, surface defect size distribution, roughness) were measured by means of Digital Optical (DOM), Scanning Electron (SEM), Atomic Force (AFM) and Focused Ion Beam (FIB) microscopy techniques. Coatings mechanical properties were investigated by micro-scratch testing, Rockwell C adhesion test, nano-indentation techniques, Vickers/Knoop micro-hardness testing and composite hardness modelling. Results showed that the use of a CAE-PVD multilayer Ti/TiN top layer, whose thicknesses and Ti distribution were suggested by finite element modelling optimisation, leads to a significant increase (45%) in adhesion of PVD coating to the HVOF layer and load bearing capacity of the coated system, compared to monolayered TiN, without reduction in superficial hardness and load bearing capacity. [Copyright &y& Elsevier]

Abstract: Present paper deals with modelling, production and characterization of HVOF PVD (WC–Co and TiN or CrN, respectively) duplex coatings deposited on Ti–6Al–4V substrate for application in automotive industry. As a preliminary analysis, an analytical study of the contact stress distribution under spherical indenter in both coated and uncoated systems, and a finite element evaluation of residual stresses were performed: results showed that the presence of an interlayer with intermediate hardness and stiffness (such as the WC–Co coating) plays a fundamental role in the improvement of the load carrying capacity. Starting from the results of simulations, morphological and compositional characterization of the coatings were performed using SEM and AFM techniques. Mechanical properties were investigated by micro indentation techniques and composite hardness modelling; toughness of the system was qualitatively analysed by Rockwell C indentation. Wear rate of the coatings was measured by an implemented rotating wheel method. Results show higher superficial composite hardness, toughness, adhesion and lower wear rate, in comparison with the simple monolayer system. [Copyright &y& Elsevier]