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International audience; Multiscale investigation of viscoelastic properties of biopolymer matrices Multiscale viscoelastic characterization • How to study at different scales the viscoelastic and structural properties of biopolymer networks to be used as matrix for biocompatible nanocomposites?

International audience; Time-domain Brillouin scattering (TDBS) is an experimental technique for generation and detection of nanometers-in-length acoustic pulses using ultrafast lasers, which was originally referred to as picosecond acoustic interferometry [1]. Experimental signals collected with the TDBS technique in transparent materials contain Brillouin oscillations, caused by interference of the probe laser pulses reflected from stationary interfaces and from the propagating ultrashort acoustic pulses. Frequencies of the Brillouin oscillations depend on the local refractive index and the local longitudinal sound velocity. Therefore, accurate estimates in a TDBS signal of the Brillouin frequency with respect to time give an access to the variations of the material parameters as a function of material depth. TDBS experiments could give access to acoustical, optical and acousto-optical parameters of the materials [2], by carrying experiments at several incidence angles for instance. Some applications of the TDBS technique for depth profiling of polycrystalline materials under very high-pressures [3,4] as well as its ability for real-time imaging of phase transition [5] will be presented. Limitations of this imaging technique, as well as the perspectives of its extension in multiple research areas [6] and industrial fields, will also be discussed. References[1]C. Thomsen, H. T. Graham, H. J. Maris, J. Tauc, Optics Communications 60, 55 (1986). [2]A. M. Lomonosov, A. Ayouch, P. Ruello, G. Vaudel, M. R. Baklanov, P. Verdonck, L. Zhao, V. E. Gusev, ACS Nano 6, 1410 (2012).[3]M. Kuriakose, S. Raetz, N. Chigarev, S. M. Nikitin, A. Bulou, D. Gasteau, V. Tournat, B. Castagnede, A. Zerr, and V. E. Gusev, Ultrasonics 69, 259-267 (2016).[4]M. Kuriakose, S. Raetz, Q. M. Hu, S. M. Nikitin, N. Chigarev, V. Tournat, A. Bulou, A. Lomonosov, P. Djemia, V. E. Gusev, and A. Zerr, Phys. Rev. B 96, 134122 (2017).[5]M. Kuriakose, N. Chigarev, S. Raetz, A. Bulou, V. Tournat, A. Zerr, and V. E. Gusev, New J. Phys., 19, 053026 (2017).[6]V. E. Gusev, P. Ruello, Appl. Phys. Rev. 5, 031101 (2018).

International audience

International audience; Water ice is a molecular solid whose behavior under compression reveals the interplay of covalent bonding in molecules and forces acting between them. This interplay determines high-pressure phase transitions, the elastic and plastic behavior of H 2 O ice, which are the properties needed for modeling the convection and internal structure of the giant planets and moons of the solar system as well as H 2 O-rich exoplanets. We investigated experimentally and theoretically elastic properties and phase transitions of cubic H 2 O ice at room temperature and high pressures between 10 and 82 GPa. The time-domain Brillouin scattering (TDBS) technique was used to measure longitudinal sound velocities (V L) in polycrystalline ice samples compressed in a diamond anvil cell. The high spatial resolution of the TDBS technique revealed variations of V L caused by elastic anisotropy, allowing us to reliably determine the fastest and the slowest sound velocity in a single crystal of cubic H 2 O ice and thus to evaluate existing equations of state. Pressure dependencies of the single-crystal elastic moduli C ij (P) of cubic H 2 O ice to 82 GPa have been obtained which indicate its hardness and brittleness. These results were compared with ab initio calculations. It is suggested that the transition from molecular ice VII to ionic ice X occurs at much higher pressures than proposed earlier, probably above 80 GPa.

Thin film metallic glasses (TFMGs) have recently attracted attention with a potential to mitigate the shear bands-induced brittle-like mechanical behavior of the bulk counterparts, leading to superior mechanical properties among which a rupture strength close to the theoretical limit and large elastic deformation. Most of studies deal with FIB-milled micro-pillars deformed in compression, but on-going debate on length-scale effects on shear band formation still remains unsettled. Here, we present a thorough study of size effects occurring in Zr65Ni35 (% at.) TFMGs deposited on Si substrate by DC magnetron sputtering with thickness ranging from 900 nm down to 100 nm. The amorphous structure was confirmed by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The elastic properties of Zr65Ni35 TFMGs were investigated by Brillouin spectroscopy and picosecond ultrasonics, while nano-indentation was used to extract hardness, pop-ins characteristics, and activation volume. Lastly, the thickness effect on the fracture behavior has been investigated by using an original cleavage method. Film elastic properties as well as the activation volume are found to be thickness independent, while hardness and the formation of pop-ins are affected by thickness even for very small indentation depths. Furthermore, the fracture surface shows significant changes for thicknesses below 500 nm wherein the corrugation pattern is no longer present. Finite element simulations of crack propagation in Zr65Ni35 TFMGs has been addressed to unravel the origin of the size effect. All the results suggest that no intrinsic size effects are present in TFMGs, while the thickness dependencies on hardness, pop-ins, and fracture surface are essentially extrinsic, namely arising from geometrical confinement effects.

A detailed structural analysis of ZrxNi100-x thin film metallic glasses (TFMGs) has been performed for various film compositions. TFMGs have received more and more attention these last years owing to their outstanding mechanical properties among which a fracture strength close to the theoretical limit, high hardness and elastic deformation [1]. Moreover, they exhibit fascinating functional properties which span in the electric, magnetic as well as corrosion and wear resistance fields [2]. Despite these promising properties, less attention has been paid to the atomic structure analysis of TFMGs, while many investigations have been conducted on bulk binary amorphous alloys [3]. Here, we systematically investigate the change in atomic structure for different ZrxNi100-x TFMGs compositions (42 ≤ % at. Zr ≤ 85) deposited by DC magnetron sputtering. An original method for designing a composite sputtering target has been used in which pure grade Ni slices are inserted in a Zr matrix, allowing a fine tuning of the film composition. The effects of this new target geometry on the film growth rate, thickness profile as well as film stoichiometry have been thoroughly investigated. The film structural analysis has been carried out by x-ray diffraction (XRD) and transmission electron microscopy (TEM) providing in-depth information about the evolution of the atomic structure for different film compositions, especially highlighting crystallites growth phenomena for Zr rich specimens. Furthermore, the compositional-induced changes in film elastic properties have been evaluated by Brillouin spectroscopy. Based on these results, we present an effective model that succeeds in describing the structural evolution of ZrxNi100-x TFMGs involving different compositions.

Bulk metallic glasses (BMGs) are currently among the most actively studied materials due to their outstanding mechanical and physico- chemical properties. Intense research is dealing with BMGs mechanical properties owing to their high yield strength and large elastic strain. Nevertheless, the shear band dominated mode of plastic deformation of BMGs leads to a macroscopic brittle behaviour. Thin film metallic glasses (TFMGs) are object of intense research investigating size-effects for which a brittle-to-ductile transition is expected below a certain thickness threshold (~ 500 nm). Our approach: 1) Deposition of ZrNi TFMGs using a composite DC-magnetron sputtering target 2) Investigation of mechanical properties for different thicknesses (100 up to 900 nm)

Bulk metallic glasses (BMGs) are known for their superior mechanical properties with respect to their crystalline counterparts owing to a high hardness and yield strength close to the theoretical limit, as well as to a large elastic deformation (around 2%) [1]. Nevertheless, the shear band instability that occurs during plastic deformation of BMGs leads to a macroscopic brittle behaviour, thus significantly undercutting structural applications [1]. Facing this issue, research is moving toward thin film metallic glasses (TFMGs) in order to investigate size-effects: namely a brittle-to-ductile transition below a thickness threshold [2]. Most size-effects studies are dealing with compression test of FIB-milled micro-pillars, reporting a correlation between the reduced size and several mechanical properties [3]. However, most of these results are inconsistent. Here, we present a detailed study of size-effects occurring in Zr65Ni35 TFMGs deposited by DC magnetron sputtering with thickness ranging from 900 nm down to 100 nm. A composite sputtering target – in which pure grade Ni slices are inserted in a Zr high purity matrix – has been designed to tune the composition. The structural investigation of Zr65Ni35 TFMGs has been carried out by x-ray diffraction (XRD) and transmission electron microscopy (TEM) showing the absence of crystalline phase. The effect of thickness on the mechanical behaviour of Zr65Ni35 TFMGs has been investigated by nanoindentation and Brillouin spectroscopy, and the TFMGs fracture surface after rupture test by scanning electron microscope (SEM). While nanoindentation and Brillouin spectroscopy show limited variations of the hardness and elastic modulus as a function of the thickness, the fracture surface analysis shows a dramatic change for thickness below 500 nm wherein the characteristic vein pattern is no more present. This transition – observed for the first time for TFMGs – has been thoroughly analysed with respect to crack propagation in metallic glasses. Based on the obtained results, we provide an effective analysis of size-effects for Zr65Ni35 TFMGs.