Evidence for the formation of distorted nanodomains involved in the phase transformation of stabilized zirconia by coupling convergent beam electron diffraction and in situ TEM nanoindentation

International audienceThe transformation of zirconia from its tetragonal to its monoclinic phase is an important feature of the zirconia system. First found to be an advantage due to its important toughening effect, it can also be very detrimental when it occurs in the framework of low-temperature degradation, particularly in the case of biomaterial applications. One way to avoid or to control this phase transformation is to understand how it initiates and more particularly the stress states that can trigger it. A new technique available inside a transmission electron microscope seems to be particularly well suited for that type of study: convergent beam electron diffraction, a well-known technique to reveal stresses, was coupled to in situ transmission electron microscopy mechanical nanoindentation. The experiments reveal the presence of sheared nanoregions at grain boundaries. These could act as embryos for tetragonal-to-monoclinic phase transformations. This is an important first step in the understanding of the earliest stage of zirconia phase transformation

Contribution of in situ nanoindentation in Transmission Electron Microscopy to the study of ceramics

La connaissance du comportement et des propriétés des matériaux est d’une grande importance pour optimiser leur mise en forme et adapter leur utilisation. Pour étudier ces propriétés de nombreuses techniques sont couramment utilisées : les essais de traction, la microindentation, la nanoindentation instrumentée… Aujourd’hui, un intérêt particulier est porté sur les nanomatériaux et matériaux nanostructurés car ils présentent souvent des propriétés différentes et plus intéressantes. La nanoindentation instrumentée, notamment, permet de déterminer des paramètres matériaux de manière locale. Cependant, le comportement en temps réel ne peut être observé et l’échantillon ne doit pas être de dimension trop faible (typiquement, l’étude de nanoparticules n’est pas envisageable). Le principal atout de la nanoindentation in situ en Microscopie Electronique en Transmission vis-à-vis des autres techniques existantes est la possibilité d’étudier le comportement de nano-objets ou des comportements très locaux et en temps réel, tout en observant les transformations subies par le matériau. Dans cette étude, nous avons évalué les potentialités de cette nouvelle technique via l’analyse de céramiques très étudiées au laboratoire notamment en tant que biomatériaux : la zircone stabilisée et l’alumine. Dans le cas de la zircone (stabilisée à l’yttrium ou au cérium), le but était de localiser à l’échelle nanométrique les contraintes responsables ou inhérentes à la transformation de phase quadratique-monoclinique, phénomène ayant une très grande influence sur les propriétés du matériau massif. Pour ce faire, après avoir déterminé une technique de préparation adaptée, nous proposons une voie d’étude pour la localisation des contraintes liées à la transformation de phase : le CBED (Convergent Beam Electron Diffraction) couplé à la nanoindentation in situ. Dans le cas de l’alumine, l’objectif était d’étudier le matériau (commercial et non un matériau modèle) dans sa forme originelle à savoir sous forme de nanoparticules d’alumine de transition. L’idée était d’étudier le comportement de ces nanoparticules sous compression. Nous avons notamment constaté que ces particules pouvaient subir une grande déformation plastique à température ambiante. Nous avons pu également, sur quelques particules, obtenir une série d’images en cours de compression ainsi que la courbe de charge-déplacement correspondante. Ces résultats ont ensuite été soumis à une analyse des images couplée à une simulation de type Eléments Finis (réalisées par le LAMCOS).Knowledge of the behavior and properties of materials is of great importance to optimize their processing and adapt their use. To study these properties, many techniques are commonly used: tensile tests, microindentation, instrumented nanoindentation ... Today, particular interest is focused on nanomaterials and nanostructured materials because they often have different and more interesting properties. Instrumented nanoindentation allow to determine material parameters. However, the real-time behavior can not be observed and the study of nano-objects is difficult (nanoparticles for example). The main advantage of in situ TEM (Transmission Electron Microscopy) nanoindentation is the ability to study the behavior of nano-objects in real time. In this study, we evaluated the potential of this new technique by analyzing ceramics extensively studied in the laboratory such as biomaterials: stabilized zirconia and alumina. In the case of zirconia (stabilized with yttrium or cerium), the goal was to locate at the nanoscale, the constraints responsible for the tetragonal to monoclinic phase transformation. This phenomenon having a great influence on the bulk material properties. To do this, after having determined a suitable preparation method, we suggest a way to study the localization of constraints: the CBED (Convergent Beam Electron Diffraction) coupled with in situ TEM nanoindentation. In the case of alumina, the goal was to study the material in its original form (nano powder of transition alumina). The idea was to study the behavior of these nanoparticles under compression. We particularly observed that these particles could undergo large plastic deformation at room temperature. We have also obtained during compression on few particles, series of images and the corresponding load-displacement curve. These results were then analyzed by image analysis coupled with Finite Element simulations (performed in LAMCOS lab)

Mechanical Properties of Nanoparticles: Characterization by In situ Nanoindentation Inside a Transmission Electron Microscope

International audienceThis chapter focuses on the nanoindentation of nanoparticles. It presents the most important works performed on thin sections of bulk materials. Most of the in situ mechanical experiments performed on nanoparticles are called “nanoindentation”. Such tests can also be called “nanocompression”, because they consist of a load under compression of the particles between two plateaus. Three main sample geometries can be envisaged for Transmission Electron Microscope (TEM) in situ nanoindentation experiments: nanopillars, thin sections, and nanoparticles. Isolated nanoparticles are widely studied by in situ nanoindentation to observe their behavior in real time and to determine their mechanical behavior. Two major results are obtained from an in situ nanoindentation experiment: images or movies and load–displacement curves. Numerical simulation can be used to complete and interpret the results obtained during in situ testing. Finite element (FE) simulations can be used to determine the constitutive law for nanosolicitation tests

A global investigation into in situ nanoindentation experiments on zirconia: from the sample geometry optimization to the stress nanolocalization using convergent beam electron diffraction

International audienceNanoindentation experiments inside a transmission electron microscope (TEM) are of much interest to characterize specific phenomena occuring in materials, like for instance dislocation movements or phase transformations. The key points of these experiments are i) the sample preparation and the optimization of its geometry to obtain reliable results and ii) the choice of the TEM observation mode, which will condition the type of information which can be deduced from the experiment. In this paper, we will focus on these two key points in the case of nanoindentation of zirconia, which is a ceramic material well known to be sensitive to stress since it can undergo a phase transformation. In this case, the information sought is the stress localization at the nanometer scale and in real time. As far as the sample preparation is concerned, one major drawback of nanoindentation inside a TEM is indeed a possible bending of the sample occurring during compression, which is detrimental to the experiment interpretation (the stress is not uniaxial anymore). In this paper, several sample preparation techniques have been used and compared to optimize the geometry of the sample to avoid bending. The results obtained on sample preparation can be useful for the preparation of ceramics samples but can also give interesting clues and experimental approaches to optimize the preparation of other kinds of materials. The second part of this paper is devoted to the second key point, which is the determination of the stress localization associated to the deformation phenomena observed by nanoindentation experiments. In this paper, the use of Convergent Beam Electron Diffraction (CBED) has been investigated and this technique could have been successfully coupled to nanoindentation experiments. Coupled nanoindentation experiments and CBED analyses have finally been applied to characterize the phase transformation of zirconia

Determination of the mechanical properties of ceramics nanoparticles via in situ nano-indentation in a transmission electron microscope and finite elements simulations

International audienc

Mechanical behavior law of ceramic nanoparticles from transmission electron microscopy in situ nano-compression tests

International audienceA methodology has been developed to determine constitutive laws of nanoparticles from in situ nanocompression experiments in a transmission electron microscope. It is based on image analysis to obtain relevant load–displacement curves, followed by finite element analysis associated to an inverse method. A transition alumina, stable only at the nanometer size, has been characterized as an example. The Young modulus and yield strength of this transition alumina, not available for such crystallographic structure, have been obtained