Evaluation of the real contact area in three-body dry friction by micro-thermal analysis

Many tribological properties and wear mechanisms occurring on the micro-and nanoscale are strongly controlled by the so-called real contact area (Ar) which is a small fraction of the nominal or apparent contact area (Aa). The determination of Ar is often based on either (i) a geometrical approach describing the real geometry of contacting surfaces or (ii) a mechanical approach involving contact mechanics and physical-mechanical properties. In addition some experimental methods have also been attempted but they generally do not take into account the presence of third body at the interface—i.e. the wear debris trapped within the contact. In this paper we propose an experimental approach to estimate the dynamic real contact area from the operating parameters (Fn, v, T) and the tribological responses (μ, Ft) in presence of third body. A scanning thermal microscope (SThM) is used for determining both the thermal conductivity of the third body and the relationship between the contact temperature and the thermal power really dissipated at the micro-asperity level. These results are combined with a thermal model of the macro-tribocontact for computing the real contact area and the real contact pressure. Validation of these results is carried out using a classical Greenwood Williamson model and finite element models built from the real AFM maps

A multiscale tribological study of nacre : Evidence of wear nanomechanisms controlled by the frictional dissipated power

Sheet nacre is a hybrid biocomposite with a multiscale structure, including nanograins of CaCO3 (97% wt.% – 40 nm in size) and two organic matrices: (i) the “interlamellar” mainly composed of β-chitin and proteins, and (ii) the “intracrystalline” mainly composed by silk-fibroin-like proteins. This material is currently studied as small prostheses with its tribological behaviour. In this work, the latter is studied by varying the frictional dissipated power from few nW to several hundreds mW, in order to study the various responses of the different nacre’s components, independently. Results reveal various dissipative mechanisms vs. dissipated frictional power: organic thin film lubrication, tablet’s elastoplastic deformations, stick-slip phenomenon and/or multiscale wear processes, including various thermo-mechanical processes (i.e., mineral phase transformation, organics melting and friction-induced nanoshocks process on a large range). All these mechanisms are controlled by the multiscale structure of nacre – and especially by its both matrices and respective orientation vs. the sliding direction

Multiscale structure of nacre biomaterial: Thermomechanical behavior and wear processes

Sheet nacre is a hybrid biocomposite with a multiscale structure, including nanograins of CaCO 3 (97% wt.% – 40 nm in size) and two organic matrices: (i) the interlamellar mainly composed of β-chitin and proteins, and (ii) the intracrystalline composed by silk-fibroin-like proteins. This material is currently contemplated for the manufacture of small prostheses (e.g. rachis and dorsal vertebra prostheses) which are subjected to micro-slip or fretting motion. In this work, the tribological behaviour of nacre is studied by varying the frictional dissipated power from few nW to several hundreds mW, in order to assess the various responses of the different nacre’s components, independently. Results reveal various dissipative mechanisms vs. dissipated frictional power: organic thin film lubrication, tablet’s elastoplastic deformations, stick-slip phenomenon and/or multiscale wear processes, including various thermo-mechanical processes (i.e., mineral phase transformation, organics melting and friction-induced nanoshocks process on a large range). All these mechanisms are controlled by the multiscale and anisotropy of its structure – and especially by its both matrices and respective orientation vs. the sliding direction

A (Constrained) Microstretch Approach in Living Tissue Modeling: a Numerical Investigation Using the Local Point Interpolation - Boundary Element Method

International audienceExtended continuum mechanical approaches are now becoming increasingly popular for modeling various types of microstructured materials such as foams and porous solids. The potential advantages of the microcontinuum approach are currently being investigated in the field of biomechanical modeling. In this field, conducting a numerical investigation of the material response is evidently of paramount importance. This study sought to investigate the potential of the (constrained) microstretch modeling method. The problem's field equations have been solved by applying a numerical approach combining the conventional isotropic boundary elements method with local radial point interpolation. Our resulting numerical examples demonstrated that the model is a good candidate for the mechanical modeling of living tissues

Comportement mécanique d'alliages pour couches de liaison de barrière thermique par microindentation instrumentée à haute température

Les systèmes barrières thermiques protégeant les aubes de turbine sont des multicouches constitués d une couche céramique isolante appliquée sur un superalliage par l intermédiaire d une couche de liaison qui, dans les systèmes actuels est à base de NiAl(Pt). Pour en comprendre et décrire le comportement thermomécanique, il est nécessaire de connaître le comportement de chaque couche, en particulier celui de la couche de liaison dont le rôle est critique. Nous avons employé une technique originale, la microindentation instrumentée à chaud (jusqu à 850C), pour obtenir des informations sur le comportement mécanique de matériaux de couches de liaison. Il a fallu d abord fiabiliser le dispositif pour minimiser les effets d oxydation et caractériser la stabilité thermique pour s assurer de la validité et la reproductibilité des résultats. Un second volet a consisté à mettre en place une méthode de traitement de données et une méthode d analyse inverse des résultats associant une approche analytique et une simulation de l essai par éléments finis. Les essais menés sur des matériaux massifs élaborés sous forme de couples de diffusion pour explorer une large gamme de compositions ont permis de déterminer la loi de comportement élastoviscoplastique du composé NiAl(Pt) sous forme [bêta] et sous forme martensitique. Des propriétés mécaniques ont été également été déterminées sur les composés NiAl(Ru) et NiAl(Zr) envisagés pour des systèmes futurs. L influence des divers éléments (Al, Pt et Ru) a pu ainsi être mise en évidence. Finalement des essais ont été effectués sur des couches de liaison de barrière thermique et les résultats corrélés à ceux obtenus sur matériaux massifsThermal barrier systems, which protect turbine blades, are multilayers constituted of an insulating ceramic layer applied on a metallic bondcoat itself in contact with the superalloy substrate. A widely used bondcoat is composed of a NiAl(Pt) compound. In order to understand and describe the thermomechanical behaviour of such systems, it is required to know the mechanical behaviour of each layer, in particular that of this bondcoat whose role is critical for maintaining the integrity of the systems. In this study, we have employed an original technique high temperature instrumented microindentation, up to 850C to extract information on the mechanical behaviour of bondcoat materials. A preliminary phase consisted in improving the experimental procedure - in particular to minimise oxidation phenomena - and in characterising the thermal stability of the equipment at high temperature to ensure the reliability, validity and reproducibility of the results obtained. We have then developed a systematic data treatment and an inverse problem analysis combining analytical approaches and a FEM simulation of the experiment to extract a mechanical behaviour law of the materials investigated. Tests performed on bulk diffusion couples, selected to explore a wide range of compositions representative of aging bondcoats, permitted to extract an elastic viscoplastic behaviour law of NiAl(Pt), both in the B2 phase and in the martensitic phase. Some mechanical properties could also be determined on NiAl(Ru) and NiAl(Zr) systems. Finally the results of a few tests performed on thermal barrier bondcoats could be correlated with the results obtained on bulk materialsNANCY-INPL-Bib. électronique (545479901) / SudocSudocFranceF

La caractérisation mécanique de systèmes film-substrat par indentation instrumentée (nanoindentation) en géométrie sphère-plan

L indentation instrumentée (nanoindentation) est une technique d analyse des données expérimentales utilisées pour atteindre les propriétés mécaniques de matériaux (dureté H, module de Young E) pour lesquels les techniques classiques sont difficilement applicables voire non envisageables. Ces paramètres mécaniques sont issus de l exploitation de la seule courbe expérimentale charge-décharge. L analyse de cette dernière repose sur des nombreux modèles reportés dans la littérature (Oliver et pharr, Field et Swain, Doener et Nix, Loubet et al.) qui considèrent la décharge purement élastique. De nombreuses expériences que nous avons menées, sur divers types de matériaux massifs (aciers inoxydables AISI304, AISI316, AISI430; aciers rapides HSS652; verre de silice SiO2) et revêtus de films minces de TiN et TiO2 ont montré que les propriétés mécaniques (E et H), déduites de la méthode de Oliver et Pharr, dépendent du pourcentage de la courbe de décharge considéré, de la charge appliquée et du rayon de la pointe. De plus, pour un système film-substrat, la technique est en général utilisée pour atteindre les propriétés in-situ du film ou du substrat, alors que la méthode de dépouillement fournit des paramètres composites qu il faut ensuite déconvoluer. Dans la recherche d une stratégie simple, permettant d accéder au module élastique d un film dur pour les applications mécaniques, nous avons fait appel à la simulation numérique. Le code de simulation numérique utilisé, est basé sur la méthode des éléments de frontière. Nos investigations numériques utilisant l indentation sphérique nous ont permis de mettre en évidence un certain nombre de résultats utiles pour l analyse des données expérimentales. Nous avons commencé par montrer que aussi bien pour un matériau massif homogène élastoplastique que pour un système film dur substrat élastoplastique, la relation [delta]=a2/R demeure valable (R étant le rayon de l indenteur, a le rayon de l aire projetée de contact). Cela permet de représenter les résultats de l essai d indentation sphérique par la courbe pression moyenne F/[pi]a2- déformation a/R . Au début du chargement, la pente cette courbe est proportionnelle au module de Young du film tandis que la pente initiale de la courbe de décharge est proportionnelle au module d élasticité du substrat. Une relation entre le déplacement de l indenteur et [delta] , puis une méthode d analyse d indentation ont été établies. Enfin, la procédure a été validée numériquement et expérimentalement sur les données issues de l indentation de divers combinaisons film-substrat (TiN/AISI430, TiN/HSS652 et TiO2/HSS652) avec succèsDepth sensing Indentation (nanoindentation) is an experimental technique increasing retained for the assessment of the mechanical properties of materials (hardness H, Young's modulus E) for which common homogeneous mechanical tests can not be performed or are extremely difficult to perform. The mechanical parameters are obtained from the indentation curve (the plot of the load vs penetration depth during both load and unload). Usually, some methodology reported in the literature (Oliver and pharr, Field and Swain, Doener and Nix, Loubet and al.) are used in order to assess E and H. We have performed a number of experiments on homogeneous materials (stainless steel AISI304, AISI316, AISI430; high-speed steel HSS652; glass SiO2) as well as a film-substrate system (TiN/AISI430, TiN/HSS652, TiO2/HSS652). Applying the Oliver and Pharr methodology, E end H vary with the applied load as well as the percentage of used unload curve retained for the analysis, as reported in the literature. Besides, in the case of the film-substrate system, only composite parameters are obtained instead of the in-situ films properties. In order to establish a simple strategy for the determination of the elastic modulus of a hard coating, we have carried out many simulations using a boundary element based numerical tool. Then a number of useful results have been identified. The well known elastic relation [delta]=a2/R between the relative approach [delta], the projected contact radius a and the punch radius R, remain valid in the plastic range for homogeneous as well as film-substrate specimens. This allows data indentation to be represented in term of mean pressure F/[pi]a2 vs indentation strain a/R . The initial slope of the loading part of the latter curve is proportional to the elastic modulus of the film, while the slope of the initial part of the unloading curve is proportional to the substrate elastic modulus. Our indentation procedure anlysis has been validated experimentally on a number of samples (TiN/AISI430, TiN/HSS652, TiO2/HSS652) after having established a relation between the punch displacement and the relative approach [delta]NANCY-INPL-Bib. électronique (545479901) / SudocSudocFranceF

In situ running-in wear assessment in multi-asperity nanotribology

International audienceWear analysis at the micro/nanoscale appears as a great challenge for MEMS/NEMS devices. At this scale classical post mortem analysis - like profilometry or AFM assessment - often failed because (i) the error in wear assessment owing to the elastic recovery is no longer negligible at this scale and (ii) the presence of nanometric tribolayer within the contact cannot be take into account when the contact is opened afterward. So, this paper deals with an in situ wear assessment based on a triboscopic approach where the final position z(f) of the ball is known without opening the contact because its vertical position is assessed at every instant of the process. This triboscopic assessment considers the initial approach of the surfaces z(o) and gives the wear rate by taking into account the presence of any tribolayer within the contact. It requires some corrections as (i) the tilt of the sample and (ii) the initial displacement of the surfaces, which is a function of the mechanical properties of the samples. The latter are determined by using an inverse method combining spherical nanoindentation and boundary element numerical simulations, which are both described too. Validation and application of this in situ approach to the running-in wear assessment of thin soft and hard coatings currently used in MEMS manufacturing are finally presented

The local point interpolation–boundary element method (LPI–BEM) applied to the solution of mechanical 3D problem of a microdilatation medium

International audienceA numerical solution procedure of three-dimensional constrained microstretch (microdilatation) elastic problem is presented. The approach called local point interpolation–boundary element method (LPI–BEM) uses a partition of the kinematical variables into complementary and particular parts. The complementary fields are obtained by isotropic boundary element method. The particular integrals are determined by solving the corresponding strong form differential equations using local radial point interpolation. The effectiveness and accuracy of the approach are proven on some simple examples. The latter are also used in order to highlight some peculiarities and potentialities of such extended continuum mechanical approach

A simple solution procedure to 3D-piezoelectric problems: Isotropic BEM coupled with a point collocation method

International audienceThis paper presents a simple strategy allowing to adapt well established isotropic BEM approach for the solution of coupled problems with anisotropic material parameters. The method which is illustrated on the case of a piezoelectric material is based on the partition of the primary fields into complementary and particular parts. The complementary fields solve the isotropic form of the partial differential equation while particular fields are obtained by a point collocation of the strong form equation. Using the local radial point interpolation method, the effectiveness and accuracy of approach is demonstrated on some examples allowing a comparison with literature results

On the deformation of a hard coating/soft substrate system under spherical nanoindentation : A boundary element numerical analysis

International audienceWe present the boundary element numerical analysis of spherical depth sensingindentation. The study aims to pinpoint some aspects of the deformation process thushighlighting simple and sufficiently accurate relations allowing a rapid analysis ofexperimental data. Results mainly concern elastic deformation of hard thin film coatingson an elastic-plastic substrate.First a well known and useful relation between the penetration and the projectedcontact area in the case of the elastic indentation of an isotropic homogeneous half spaceis shown to remain valid in the elastic-plastic deformation regime as well as in the case ofa thin film/substrate system. It is also shown that the radius of the pile-up forming duringunloading on the residual imprint is (approximately three times) larger than the contactradius at maximum load. Finally a careful analysis of the stress field evolution during thedeformation process is presented