Topic supervisors:

Pierre-Olivier Renault (pierre.olivier.renault@univ-poitiers.fr / +33 5 49 49 67 45)
Sylvie Castagnet  (sylvie.castagnet@ensma.fr / +33 5 49 49 82 26)

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The scope of Theme 1 deals with the couplings between the response of materials and the heat and mass transfers induced by environmental and / or extreme conditions at the interface. In addition to the two other subjects devoted to the fluid phase and to surface functionalization, this research subject focuses solely on the way in which the surface layer of the material is affected.

Structural materials play an essential role in many applications such as transport or electronics. Taking into account their realistic conditions of exposure to the environment is therefore increasingly necessary. The different interactions with the environment discussed here are: combustion, exposure to diffuse species (inert or chemically aggressive) under different fluid flow conditions, specific local mechanical charges induced by the fluid flow, interaction with electrical charges , coating layers, irradiation, high pressure or low vacuum. Labex I has already addressed some of these situations (for example, high temperatures and thermal barriers, combustion, hydrogen embrittlement or oxidation.

The first objective of the subject is to understand how severe and specific external conditions of temperature, pressure, stress, chemical environment, electrical charges, passivation layer or radiation flux affect the physical, chemical and mechanical properties of the solid undercoat. through the addition of new phases (for example in physical vapor deposition coatings, oxidation film), microstructural changes (for example due to nanostructuring or ion implantation), the appearance defects (e.g. porosity resulting from chemical reaction or heterogeneity of gas content), etc. The second objective is to study the consequences of these processes on local stress, temperature and chemical species fields. We will then be interested in connecting them to the elementary mechanisms of thin film adhesion, plasticity initiation, genesis of internal stresses (for example during thermal cycles or loading or vacuum-air, etc.) to make the transition towards damage, kinetics of exits and coupling with mechanical behavior, crack growth, etc. The third objective is to link these mechanisms to the macroscopic property of the surface layer of the material and to provide certain ways to improve the resistance to fatigue, to corrosion (in particular from an original MAX phase track), to the resistance. radiation, fire and oxidation.

The studies carried out will be based on several advanced techniques in surface engineering: physical vapor deposition, implantation of ions, exposure to fire and gases. Modeling can also be used, not for interface functionalization (theme 2), but to control the mechanical fields in the first layer of the solid. In terms of characterization, projects will benefit from a wide range of micro and nano-scale hardware probes and tools, as well as advanced digital models providing access to large-scale IT facilities. The characterization of the microstructure – defined as the distribution of internal interfaces, phases and faults – of the sublayer will be carried out using different imaging techniques available at the Pprime Institute (FEG-SEM, EBSD, TEM, AFM and STM, X). X-ray tomography). Among these, the FIB implemented during Labex I, offers original ways to model surfaces from the micro scale to the nanometric scale, but also to develop metrology on a small scale. It could be used here to examine the displacement field at interfaces or microstructure scales (for example intra-granular heterogeneities in polycrystals) coupled with a correlation of digital images ex or in situ. The transition to higher scales is a fairly general challenge for modeling and experiments during this extension phase, in particular for the evaluation of the impact of elementary processes at higher scales and for the optimization of materials and structures. Progress could be made from finite element calculations, analytical modeling and / or atomistic / molecular simulations, depending on the temporal and spatial scale of the phenomena.

Experimental and numerical progress also aim at complete couplings, among which in situ characterization, the measurement of interdependent physical parameters, the capacity to simulate the effects of anisotropy in problems of chemical-mechanical diffusion, for example. in laminates or highly stretched materials.

Several of these interdisciplinary works should make it possible to predict the possible effects of feedback on the boundary layer of fluid and the transfers at the interface. In particular, the way in which the geometrical modifications of the solid surface on different scales (rather macroscopic on the surface covered in the event of combustion problems or on the micro-scale due to modifications of the roughness), partial liquefaction or deformation of the surface influence heat transfers, the transition from laminar to turbulent flow, thermoacoustics, tribo-fluidics or electro-fluidics could be studied.