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A finite element model reproducing the mechanical behaviour during nanoindentation tests on Ti 0.67 Al 0.33 /Ti 0.54 Al 0.46 N multilayer coatings was developed. Coatings with two different nanoperiods (10 and 50 nm) were deposited by radio-frequency magnetron sputtering from a single sintered titanium/aluminium target using the reactive gas pulsing process. X-ray diffraction and transmission electron microscopy confirmed the multilayer stacking of the coatings. The finite element models of coatings with these two stacking periods were built considering successive hypotheses: equal thicknesses for metal and nitride nanolayers stacked without transition layer, equal thicknesses stacking with a transition layer between each metal and nitride nanolayer and finally imbalanced thicknesses stacking with a transition layer. The elastic and plastic properties of the stacked nanolayers were determined on thick monolithic coatings of metal and nitride using indentation testing and the finite element model updating method respectively. The elastic-plastic properties of the interface were introduced in the multilayer model as a rule of mixtures of the metal and nitride properties using two hypotheses: parallel or serial. For 50 nm-period film, the interface has a negligible effect on the overall indentation response. On the other hand, for the 10 nm period, the imbalanced stacking model with precise knowledge of the transition layer thickness obtained by N K-edge electron energy loss spectroscopy is required to reproduce the experimental indentation curve. Compared to classical analytical models accounting for hardness, not only the hardnesses but also the indentation moduli appear to be well predicted and evaluated in this work. [Display omitted] [ABSTRACT FROM AUTHOR]

Instrumented nanoindentation is widely used to extract the material properties from the measured force-displacement curves. In this work, the uniqueness/non-uniqueness of the intrinsic viscoelastic properties of materials determined by nanoindentation during load-unload tests is investigated. A four-parameter viscoelastic law with constant Poisson's ratio is used to model the mechanical behavior of a polymer material and a 2D-axisymmetric Finite Element Model (FEM) is used to simulate the nanoindentation test. Firstly, a nanoindentation experimental triangular load-unload test is performed on a bulk sample of polypropylene (PP) with a Berkovich indenter tip at a depth rate of 1000 nm/min. The values of the four material parameters are estimated by the Finite Element Model Updating (FEMU). The numerical results can accurately fit the experimental data. However, several quasi-solutions are shown to exist. These load-unload data allow to identify only three viscoelastic parameters if the Poisson's ratio is known. Secondly, the effect of nanoindentation depth rate, loading type (triangular, trapezoidal, exponential, sinusoidal) and apex angle is numerically investigated using an identifiability index based on the conditioning of the inverse problem. We show a correlation between the identifiability index and the energy dissipated by the material during the tests. The extraction of all material parameters remains impossible using a single test. Finally, some combinations of several nanoindentation triangular tests and indenter tip angles are also investigated. We show that a dual nanoindentation technique (cube corner and Berkovich tips) with triangular load-unload tests is an interesting combination to reliably extract all the viscoelastic parameters, provided that plasticity is taken into account. This result illustrates the interest of using this numerical identifiability index to design nanoindentation experiments to ensure the robustness of the intrinsic viscoelastic properties extraction. [ABSTRACT FROM AUTHOR]

Highlights: [•] A bio-sourced adhesive made of 75%w/w rosin and 25%w/w beeswax was studied. [•] Tack and mechanical properties were highlighted by experimental nanoindentations. [•] They depend strongly of the applied strain rate and are dictated by a size effect. [•] A modelisation of these phenomena is proposed. [•] The experimental measurements and the numerical results are in good agreement. [Copyright &y& Elsevier]

This work deals with the indentation analysis of nanocolumnar thin films and the difficulties encountered to deduce relevant mechanical parameters by this methodology. SiO 2 thin films prepared by physical vapour oblique angle deposition with different nanocolumnar microstructures have been subjected to indentation analysis. Despite the fact that the films had been made of the same material, deposited on the same substrate and had similar thickness, their indentation responses were different and depended on their particular microstructure. It has been also reali s ed that the measured hardness and elastic modulus variation with the indentation depth were length scale dependent and that there is not a unique analytical thin-film nanoindentation model to extract the mechanical properties from the experimental nanoindentation curves. To overcome these limitations a numerical finite element model (FEM) of the nanocolumnar coatings has been built to figure out the contributions of the different physical phenomena intervening in the indentation process. This FEM simulation relies on a description of the elasto-plastic microstructural units of the coatings and the contact friction interactions between them. Based on this simulation a parametrical representation, incorporating two length scales and the contributions of densification and/or the buckling of nanocolumnar units, has been developed to account for the evolution of the apparent elastic modulus deduced from numerical indentation tests. A Hall-Petch modification of this description considering two length scales instead of the common approximation considering a single length scale has rendered the best agreement with the elastic values determined experimentally. Although, at the present stage, the particular microstructure of the films can not be deduced from the evolution of their elastic moduli with the indentation depth, the obtained results and their interpretation constitute a first though essential step for the elaboration of an inverse analysis methodology capable of correlating microstructure and elastic response of nanocolumnar coatings. • The indentation response of multi-layered PVOAD columnar coatings greatly depends on their microstructure. • Buckling but also densification drive the evolution of indentation elastic modulus and hardness with indentation depth. • FEM of a columnar structure under indentation allow s to reproduce the experimental response of multi-layered PVOAD coatings. • Size dependent model allows to extract length scale parameters from elastic response of PVOAD coatings. [ABSTRACT FROM AUTHOR]

Abstract: The ageing of zirconia under hydrothermal conditions is a key issue in many applications, as the properties and thickness of the degraded layer influence the reliability of the material. This paper presents a study by nanoindentation of the hydrothermal degradation of tetragonal zirconia polycrystals doped with yttria with a tetragonal grain size of 0.3μm and of the same material after heat-treatment at high temperature (1650°C) in order to develop a duplex cubic-tetragonal microstructure with a large tetragonal grain size. Nanoindentation is used to evaluate the Young’s modulus and hardness of the degraded surfaces as a function of indenter penetration depth. The mechanisms of hydrothermal degradation are also studied. Using Raman spectroscopy, atomic force microscopy and focused ion beam (FIB) cross-sections, we show that hydrothermal degradation induces t–m phase transformation and microcracking under the surface. Microcracks in degraded zirconia, clearly observed in FIB cross-sections, are associated with the decrease in hardness and Young’s modulus of the degraded surface. Nanoindentation is used to detect and quantify the degradation of zirconia by calculating the decrease in hardness and Young’s modulus with penetration depth. Using a thin-film model, the depth of the degraded layer and its mechanical properties are also extracted. As expected, we demonstrate that duplex zirconia with a large tetragonal grain size is more sensitive to hydrothermal degradation than monophasic small-grain tetragonal zirconia. [Copyright &y& Elsevier]

Abstract: Incipient plasticity in magnesium oxide single crystals has been investigated by nanoindentation with a spherical indenter. For very low loads, the surface deformation can be limited to a single slip line, making modelling of the deformation easier. The recently developed nanoetching technique has been used to investigate the corresponding dislocation structure. Single pile-ups of a few dislocation half loops have been observed and fully characterised. A simple elastic model, based upon the calculation of the Peach Koehler force induced by the pile-ups and the spherical indenter stress field, has been used to determine the equilibrium positions of dislocations in these pile-ups. A comparison between the experimental and calculated structures has led to the determination of the lattice friction stress for edge and screw dislocations in magnesium oxide single crystals. This stress is found to be 65MPa for edge dislocations and 86MPa for screw dislocations. [Copyright &y& Elsevier]

In this paper, it is shown that combining AFM, chemical etching and controlled polishing, progressively removing thin layers of material, allows a three-dimensional reconstruction of the volume distribution of dislocations about and under nanoindentation imprints. To illustrate the method, results obtained in MgO, a material known for its simple plasticity, have been selected. It is shown, comparing surface deformation and etching pattern, that the entire dislocation distribution associated with small indents can be analysed in terms of individual dislocations, leading to a better understanding of the elementary mechanisms of plasticity associated with the early stages of indents formation in crystalline material. [Copyright &y& Elsevier]