Nanoindentation

Résumé

vignette|Photo au microscope à force atomique d'une indentation par poinçon Berkovitch sur une céramique Zr-Cu-Al : l'écoulement plastique autour de l'indenteur est bien net. La nanoindentation (encore appelée indentation instrumentée) est une technique de caractérisation mécanique des matériaux. Cette technique suscite un intérêt considérable dans le domaine des films minces et des micro-objets Principe Cette technique de caractérisation est dite instrumentée dès lors que la force appliquée au contact et la profondeur de pénétration qu’elle engendre sont mesurées continûment. L'essai d'indentation consiste à faire pénétrer une pointe, encore appelée indenteur ou pénétrateur, de géométrie et propriétés mécaniques connues, dans un matériau (si possible plan) pour en déduire ses propriétés mécaniques. La technique de nanoindentation inventée au cours des années 1970 permet de pallier cette limitation en appliquant des charges de l’ordre de dizaine de micronewtons (μN) pour d

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This thesis comes within the scope of tribology studies at the nanometer scale. The experimental techniques used in this work are essentially related to atomic force microscopy, which gives a direct access to the topography and forces of the studied systems. Two principal aspects of the nanotribology have kept our attention. They are the nanofriction and the nanomechanics. These two domains divide the thesis in two main parts that remain however intimately bound. The first experimental part of the thesis describes the friction of a nanoscopic tip sliding on hydrophilic surfaces. The dependence of the friction force on the sliding velocity is studied for various applied normal loads and surrounding relative humidity levels. We found a logarithmic dependence of the friction force on both the scanning velocity and the relative humidity. For the first time, a transition from a positive to a negative slope of the friction force versus the logarithm of the sliding velocity has been observed for the very same tip-surface contact, by varying the relative humidity. The role of the relative humidity on the friction force has then been studied more deeply, leading to a two-thirds power law dependence of the capillary force on the normal load. Finally, analytical expressions for the friction phenomenon and the capillary condensation between the asperities of the tip and the surface have been developed to explain the phenomenological behaviors. The second experimental part describes themechanics of carbon nanotubes and tobacco mosaic viruses. In order to stay in the linear elasticity regime, small indentation amplitudes have been applied in the radial direction of multiwalled carbon nanotubes adsorbed on a silicon oxide surface. Using a theory based on the Hertz model, the radial stiffness has been evaluated and compared to molecular dynamics simulations. We found a radial Young modulus strongly decreasing with increasing radius and reaching an asymptotic value of 30 ± 10 GPa. The tobacco mosaic viruses have been adsorbed on a polyimide porous membrane. Evidence for the softness of the viruses has been obtained by imaging the tubes with the atomic force microscope in non-contact mode. Viruses hanging like ropes over the pores of the surface served as basis to measure their bending Young modulus. Using a model of a clamped beam loaded by a discrete gradient of van der Waals forces, we found a bending Young modulus of 3.1 ± 0.1 MPa.

EPFL2007

Chargement

Indentation fracture of gallium arsenide

Cédric Pouvreau

The scribe and break technique (or dicing) is a widely employed method in the industry of semiconductors to separate infrared laser diodes made from gallium arsenide (GaAs). The scribing step allows to create a precursor crack which is then propagated during the breaking step, along preferential {110} cleavage planes of GaAs. The main drawback of the scribing process is that it generates a lot of undesirable cracks and particles that degrade the performances of devices. In this dissertation, we have investigated the indentation process as a possible way to replace the scribing operation. For that purpose, we have investigated the morphology of the crack field and the cracking sequence as a function of the indenter geometry (shape, apex angle) and experimental conditions (maximal load, loading rate). Such investigations have been made with the help of a new tool: the in-situ SEM instrumented indentation which allows us to establish the cracking sequence and to correlate direct observations with the load-displacement histories. A new experimental technique has also been developed: cleavage cross-sectioning techniques allow us to determine the morphology of the crack field beneath the surface. The second research axis was focused on the interaction between deformation mechanisms and crack initiation. These investigations have been conducted with the help of Transmission Electron Microscopy (TEM) and cathodoluminescence. In the first part of the dissertation, we have shown that acute wedge indenters with an included angle of 60° promote the formation of a well defined half-penny crack, the nucleation of which is affected by load rate and indenter radius. The relation between the final half-penny crack size and the maximum indentation load was made with the help of a fracture mechanics model. The crack field has been compared for several indenter shape including conical (60° and 120° apex angles), cube corner and Vickers indenters. In the second part of this dissertation we have determined that the indenter apex angle influences the slip systems that are activated and the nature of dislocations that are found under the indenter. In particular, we have shown that below 60° wedge indenters, mainly diverging slip systems are activated whereas under obtuse wedge indenters, mainly converging slip systems are activated. The converging pattern predominant under obtuse indenters is correlated with a delayed half-penny crack formation and is so interpreted as a plastic shielding phenomenon. Some experiments have been performed on commercial devices under production conditions. Although some adjustments are needed to reduce the chipping-out effect that occurs at the indenter extremities, the results are encouraging.

EPFL2007

Chargement

Hexagonal OsB2: Sintering, microstructure and mechanical properties

Martin Guillermo Mueller, Nina Orlovskaya

The metastable high pressure ReB2-type hexagonal OsB2 bulk ceramics was produced by spark plasma sintering. The phase composition, microstructure, and mechanical behavior of the sintered OsB2 were studied by X-ray diffraction, optical microscopy, TEM, SEM, EDS, and nanoindentation. The produced ceramics was rather porous and contained a mixture of hexagonal (similar to 80 wt.%) and orthorhombic (similar to 20 wt.%) phases as identified by X-ray diffraction and EBSD analysis. Two boron-rich phases, which do not contain Os, were also identified by TEM and SEM/EDS analysis. Nanoindentation measurements yielded a hardness of 31 +/- 9 GPa and Young's modulus of 574 +/- 112 GPa, indicating that the material is rather hard and very stiff; however, it is very prone to crack formation and propagation, which is indicative of a very brittle nature of this material. Improvements in the sintering regime are required in order to produce dense, homogeneous and single phase hexagonal OsB2 bulk ceramics. (C) 2015 Elsevier B.V. All rights reserved.

Elsevier2015

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EPFL2007

Indentation fracture of gallium arsenide

Cédric Pouvreau

EPFL2007