This project aims (i) to increase the crystal quality of AlN layers epitaxially grown by HT-HVPE (High Temperature Halide Vapor Phase Epitaxy) on patterned substrates and (ii) to assess HT-HVPE as an alternative process to produce freestanding AlN wafers with industrial grade. AlN layers will be grown by HT-HVPE in a cold wall reactor between 1200°C and 1500°C on patterned silicon (111) and sapphire (001) substrates. Two approaches will be proposed: a top down approach where the seed substrates are deeply etched to produce a dense forest of high aspect ratio micron-size pillars on which the growth will take place and a bottom-up approach where a dense forest of GaN pillars grown an sapphire are used as seed layer. The originality of the project is the use of a very high aspect ratio (>>10) dense pillar forest as an intermediate layer to grow thick continuous AlN layers (>100 µm). The presence of pillars, combined with optimal process parameters that promote the formation of voids at their bottom (diffusion limited growth at the length scale of the pillars), should decrease growth and cooling-down stresses in the AlN grown layer while ensuring that most of the AlN crystal volume grows freely by lateral overgrowth. The main idea is to decrease at most the mechanical interactions between AlN and the seed layer by using fragile pillars patterned in cheap, available and easy-to-process materials. The threading dislocations originating from lattice mismatch and primary AlN islands coalescence as well as the compressive stress during cooling-down will be reduced by this approach. If sufficiently thick, the grown layer would be easily removed from the pillars bed just by cooling down the assembly from process temperature to room temperature.

The aim of the MAGELAn project is to fabricate magnetic macroporous elastomers that demonstrate large and reversible magnetically-triggered deformation. Recent results have shown foam based materials with large pores. Here, we want to develop a scalable technique leading to a homogeneous porous structure with small pores (d ~ 1 µm), which will thus be compatible with microfabrication methods. This will allow the use of these materials as micro-sensors or micro-actuators. Our first objective is to develop synthesis methods to obtain a good control over porous structure, pore size and shape. For that purpose, inverse water/polydimethylsiloxane (PDMS) emulsions or suspensions of microparticles in PDMS will be formulated and used as templates for porous elastomers. Then, we will introduce magnetic nanoparticles in the PDMS phase and investigate their influence on the magnetorheological and magnetostrictive properties of the porous elastomer. Finally, as an exemple of applications, we will develop techniques to create patterned surfaces combining textures with magnetically-compressible blocks to control surface configurations with applications to switchable wetting and adhesion properties.

The present project is motivated by the flawed nature of current engineering ceramics and it aims at studying and developing new lightweight damage-tolerant ceramics. Designing cellular materials with a hierarchical structure similar to natural lightweight materials combined with the use of damage-tolerant ceramics as building blocks could provide new lightweight materials with unprecedented properties. The aim of MAD_Ceramics is to explore the chemical deposition techniques of MAX phases compounds, a class of damage-tolerant ceramics, by opening the “black box” of their synthesis and understanding the intricate phenomenon occurring at different length scales. This technique is today unexplored while it is the only viable method for deposition on complex structures, like hierarchical architectures. The hierarchical architectures to be coated will be lattice materials with designed periodic spatial architecture obtained by additive manufacturing. The possibility of marrying the structural control on several length scales like the one found in nature with the damage-tolerance, refractory nature and high stiffness of MAX phases could break the existing limitations in terms of strength-to-weight and stiffness-to-weight ratios and could lead to the development of totally new range of applications. An experimental approach based on innovative thin film chemical vapor deposition techniques of the MAX phases together with in situ observation of the synthesis will be integrated with numerical modeling of the deposition and investigation of mechanical properties at the relevant length scales. The material synthesis techniques proposed in this project are novel yet based on existing proofs.

The goals of this project are to study the self-assembly of polymer multi-layers at the oil-water interface and to produce polymeric capsules based on such multi-layers assembled on an oily core using microfluidic techniques. A large part of this project will be devoted to the characterization of the mechanical properties of the polymer layers because these are key parameters for the final properties of the capsules such as their stability and release ability. We will study multi-layers of oppositely charged polyelectrolytes which assembly is driven by electrostatic interactions. The first polymer adsorbing at the oil-water interface has to be amphiphilic: we will use hydrophobically modified polyelectrolytes that will be synthesized in the laboratory. For the subsequent layers, we will first study model systems of oppositely charged polyelectrolytes, such as PDADMAC (polydiallyl dimethyl ammonium chloride) and PSS (polystyrene sulfonate) which have been well studied at solid-liquid interfaces. To study the influence of the polymer hydrophobicity on the properties of the multi-layers, we will also synthesize hydrophobically modified polyelectrolytes of various hydrophobicities. In the first task we will investigate the building of the polymer multi-layers at the oil-water interface by surface tension measurements, ellipsometry and also by PM-IRRAS (Polarization-Modulated Infra-Red Reflection Absorption Spectroscopy). This technique, which is based on infra-red spectroscopy in reflexion, is sensitive to the degree of hydration of adsorbed layers at liquid interfaces. We expect that the hydration of the layers is a key parameter to understand the mechanical properties of the layers and it should depend on the hydrophobicity of the polymer molecules. In the second task, we will characterize the mechanical properties of the adsorbed multi-layers by adapting techniques of interfacial rheology in model geometries. The technical and scientific challenge is to be able to follow the transition from a liquid interface at low number of layers to a very rigid and solid interface, almost an elastic shell. We will adapt the pendant drop method which enables to measure the compression elasticity of the layers by oscillating a pendant drop of oil in water and measuring its shape or internal pressure. Moreover to measure the shear surface elasticity of the layers we will rotate a ring positioned at the oil-water interface and mounted on a classical rheometer to apply and measure the interfacial stress and deformation. We will study the frequency response of the layers as a function of the number of adsorbed layers and physico-chemical nature of the polymers and oils. The goal of the third task is to design microfluidics devices to produce capsules based on the same polymer multi-layers assembled on oil droplets studied in the previous tasks. The oil droplets in water will be produced in a micro-channel and forced to flow through successive zones of the chips containing polymers of alternating charges to build the capsules. We will also characterize the mechanical properties of the capsules by passing through constrictions and measuring the droplets deformation.

With the fast-growing development of electric mobility, there is a pressing need to improve the efficiency of tribological systems subjected to electrical stresses. Not to mention the fact that this development is associated with increasingly harsh environments: higher frictional speeds, higher voltages, higher temperatures. Similar challenges are also observed in wind turbine motors. More generally, the growing development of high power-density electrical systems (photovoltaic panels in solar plants, on-board electronics, power electronics, etc.) also requires the comprehensive understanding of electro-mechanical contacts under severe conditions (temperature cycling, intermittent contacts, arcing, etc.) METEORITE project is a highly multidisciplinary project at the crossroads of tribology, physical-chemistry and electronic transport in materials. It focuses on the development of dedicated instruments (with their associated methodologies) that combine in-operando electrical and tribological multi-scale characterizations. Electrical methods will be used either to monitor or to stress sliding interfaces placed in severe conditions. MoS2-based lubricants (both liquid and solid) will be used as application cases to illustrate the feasibility and the performance of the approach. Electrical measurements are currently under-exploited for the comprehensive understanding of the physical-chemistry of mechanical contacts. By coupling in-situ electrical, tribological and physical-chemical characterizations, this project proposes to enrich our understanding of contacts under severe conditions: mechanical and/or electrical cycling, high-temperature contact, etc. This coupling will be used either to better monitor the contact, or to stress it. The "Instrumentation" part will lie on the development of instruments that are already unique worldwide: at SIMaP, tribological measurements will be implemented on an in-situ SEM electrical-nanoindentation device; at LTDS, the environment controlled analytical tribometer will be coupled with advanced electrical measurements. These tools will complement each other in terms of spatial scale, mechanical sensitivity and in-situ characterization techniques. The "Characterization" part will focus on steels with MoS2-based lubricants (liquid or solid). The aim will be to better describe the kinetics of the nucleation/growth/microstructural processes of tribofilms, in relation to surface chemistry and tribological performance. The effect of electrical stresses on tribofilm growth and aging will also be studied. METEORITE combines two complementary areas of expertise: the tribological and physico-chemical study of contacts (LTDS) and the coupling of electrical and mechanical behaviors at small scales (SIMaP).