The COMEFai project aims to understand the mechanisms of oxidation during atmospheric re-entry of composite materials with ultra-refractory ceramic matrix. Materials are used in severe environments under high flux (oxidation, matter, thermal, etc.) whether for space applications (rocket, space shuttle), or defence applications related to the deterrence policy. These materials have a complex architecture (fibrous reinforcement, embedded in a matrix with possibly a protective coating) allowing both mechanical and oxidation resistance. The materials in this project are composites, with a fibrous reinforcement that can be continuous or discontinuous; and a matrix of ultra-refractory materials in the Hf-Si-B-C or Zr-Si-B-C system. Two matrix manufacturing methods are used: (i) filling of porosity by submicrometric powder impregnation, and densification of the material by infiltration of metal or molten alloy or, (ii) fabrication by flash sintering. The formed phases within the material are known for their extreme refractory properties: HfB2, HfC and SiC or ZrB2, ZrC and SiC. The presence of silicon carbide improves the protective role of the oxide layer growing during oxidation. The oxidation resistance of materials is tested by means of tests in an oxyacetylene flame, varying the conditions: oxygen content, angle of contact, material flow, etc. In 2023, tests at the European Synchrotron Radiation Facility – Grenoble (ESRF, 6 shifts) and the LURE – Paris Intermediate Energy Optimized Light Source (SOLEIL, 15 shifts) will allow for in-situ tomography experiments. The originality of the COMEFai project lies in the strategy of understanding the mechanisms of oxidation, based on these in situ characterizations through synchrotron tests. This original and unique project will make it possible to monitor in situ the progression of oxidation at more than 2000°C to highlight all oxidation mechanisms as a function of time and validate the use of these materials under extreme stress conditions. At the same time, a classification of resistance to oxidation will be established between the various chemical compositions studied.

Self-healing Ceramic-Matrix Composites (SH-CMCs) have extremely long lifetimes even under severe thermal, mechanical and chemical solicitations. They are made of ceramic fibres embedded in a brittle ceramic matrix subject to multi-cracking, yielding a “damageable-elastic” mechanical behaviour. The crack network resulting from local damage opens a path to fibre degradation by corrosion and ultimately to failure of the composite, e.g. under static fatigue in high-temperature oxidative conditions. But these materials have the particularity of protecting themselves against corrosion by the formation of a sealing oxide that fills the matrix cracks, delaying considerably the fibres degradation. Applications encompass civil aeronautic propulsion engine hot parts and they represent a considerable market; however this is only possible if the lifetime duration of the materials is fully certified. Numerical modelling is an essential tool for such an aim, and very few mathematical models exist for these materials; fulfilling the needs requires a strong academic-level effort before considering industrial valorisation. Therefore, the ambition of this innovative project is to provide reliable, experimentally validated numerical models able to reproduce the behaviour of SH-CMCs. The starting point is an existing image-based coupled model of progressive oxidative degradation under tensile stress of a mini-composite (i.e. a unidirectional bundle of fibres embedded in multi-layered matrix). Important improvements will be brought to this model in order to better describe several physic-chemical phenomena leading to a non-linear behaviour: this will require an important effort in mathematical analysis and numerical model building. A systematic benchmarking will allow creating a large database suited for the statistical analysis of the impact of material and environmental parameter variations on lifetime. Experimental verifications of this model with respect to tests carried out on model materials using in-situ X-ray tomography – in a specially adapted high-temperature environmental & mechanical testing cell – and other characterizations are proposed. The extension of the modelling procedure to Discrete Crack Networks for the large-scale description of the material life will be the next action; it will require important developments on mesh manipulations and on mathematical model analysis. Finally, experimental validation will be carried out by comparing the results of the newly created software to tests run on 3D composite material samples provided by the industrial partner of the project. The project originality lies in a multidisciplinary character, mixing competences in physico-chemistry, mechanics, numerical and mathematical modelling, software engineering and high-performance computing. It aims creating a true computational platform describing the multi-scale, multidimensional and multi-physics character of the phenomena that determine the material lifetime. Important outcomes in the domain of civil aircraft jet propulsion are expected, that could relate to other materials than those considered in this study.

To face the energy, environmental and economic challenges of air transportation, significant efforts are devoted to the next generation of aircraft turbojets, particularly in the field of advanced materials and their elaboration processes. The main objective is a decarbonisation of air transportation by 2050. One of the actions to do consists of increasing the operating temperatures of turbomachines in order to increase their efficiency and thus reduce both specific fuel consumption and pollutant emissions while significantly reducing cooling air flow. This objective is in line with the transverse axis on energy. However, this action requires the replacement of the key materials - metallic superalloys – as components of turbine parts (turbine rings, combustion chamber, …) which are no longer the material of the future for this application. The substitution of these superalloys by ceramic matrix composites (CMC) sufficiently stable at high temperatures (1450 ° C) would avoid the use of internal cooling while allowing the temperature increase of internal and external parts of turbojets. Moreover, the density of CMC being much lower than that of metallic superalloys, the use of these materials would also allow lightening the structures. Thus, by entering the scope of the topic « Material » of the ASTRID call, the MATRIX project of the ASTRID2021 program - an exploratory and innovative project with a TRL < 4 - is based on a collaboration between two research laboratories with internationally recognized and complementary expertises: the research institute on ceramics (IRCER, University of Limoges-UMR CNRS 7315-coordinator) and the laboratory of thermostructural composites (LCTS, University of Bordeaux-UMR CNRS 5801) and an industrial partner (Safran Ceramics, SCe). The study focuses on the development of CMC whose composition is based on the ternary silicon-carbon-nitrogen (Si-C-N) system offering thermomechanical characteristics and a lifetime compatible with use at 1450°C (≈ 2700 F) for long periods of time to be used as internal engine components. The technology implemented in MATRIX is based on a liquid phase infiltration process - Polymer Impregnation Pyrolysis (PIP) - from two formulations of preceramic polymers selected for their rheological properties tailored for impregnation of fiber preforms and their capabilities to form oxygen free-SiC/Si3N4 ceramics without the presence of sp2 carbon and free silicon phases. This tailored composition allows the considered ceramic matrices to be very stable both under an inert atmosphere at high temperature (1450°C) and in air at intermediate temperature (700°C). The liquid phase PIP process will be hybridized with a gas phase infiltration process - Chemical Vapor Infiltration (CVI) - in order to optimize the density of the CMC. The MATRIX project - with a particular focus on the process-structure-properties relationship - is organized into five interconnected scientific tasks, ranging from the preparation of formulations to the characterizations of the properties of CMCs. This organization is expected to validate our approach and should enable us to tackle the various scientific and technological issues to use CMC in the next generation of turbojets for civil and military aircrafts. At the end of the project (30 months), they should lead to the technological building blocks required for the production of robust CMC 2700F as components suitable for further development in an industrial context, in accordance with the energy, environmental and economic targets of the market.

The aim of the OSCAR project is the development of an 100% biobased high performance carbon fiber (CF) based on a precursor derived from the biomass transformation industries. Thanks to its remarkable properties, polyacrylonitrile (PAN) is the mainly used precursor for high performance CF production. PAN is synthesized from the acrylonitrile monomer (AN). However, the industrial source of AN is today directly related to the non renewable petroleum ressources. In this context, the OSCAR project aims at exploring the technical, scientific, economical and environmental feasibility of the production process of biobased acrylonitrile (bioAN) and its transformation to high performance CF. Glycerol, a coproduct of the biodiesel industry, is the starting material considered for the preparation of bioAN. Based on the knowledge and the expertise of the consortium, the production process of bioAN from glycerol (patented by the project coordinator) will be developed and optimized. Then, the multi-steps transformation of bioAN in CF will be carefully demonstrated. In a second step, the OSCAR project will investigate a cost reduction strategy of the CF based on the fabrication of precursor from mixes of biobased polyacrylonitrile (bioPAN) and a byproduct of the paper industry : lignins. As lignins are also renewable ressources, the CF proposed with this strategy will be 100% biosourced too. OSCAR project fits in the thematic axis 7 (Materials) of the 2021 ASTRID call fo proposals. In addition to being part of sustainable development, the OSCAR project places particular emphasis on the strategic independence of the CF sector, an essential component of the civil and defense industry. The OSCAR project concerns both military and civilian fields where high performance CF find their use for aerospace, defense and aeronautical applications.

The CoMéCA project aims at modeling the mechanical behavior of carbon-based thermal protection systems with aerospace and defense applications. These multi-scale materials are made of complex architectures of carbon fibers and matrices, which both derive from graphite, yet being heavily disordered. Modeling the thermomechanical behavior of such structures requires knowledge of the individual behavior of each constituent, in addition to their interactions, from the atomic scale (typically one nanometer) to the largest scale of the complex architecture (sub-millimeter to centimeter). Our consortium, composed of LCTS (CNRS, U. Bordeaux, Safran, CEA/DAM), ISM (U. Bordeaux, CNRS) and CEA/DAM, proposes to approach this problem through a combined experimental/theoretical investigation. The experimental part will aim at preparing samples of the individual matrices in a suitable form, for detailed characterization and mechanical testing, and to perform the characterizations and tests. The objective will be to produce a large database of structures with associated structural and mechanical properties. The theoretical part will make use of recently developed bottom-up strategies based on atomistic modeling in order to build mechanical models with the help of numerical materials derived from actual ones. The principle is an image-guided atomistic reconstruction that makes use of experimental data obtained on real materials. Comparison between experimental and simulation results will allow establishing the laws governing the mechanical behavior at the mesoscopic scale, a necessary step towards the bottom up prediction of the behavior of composite materials at the macroscopic scale. Hence, important steps will be achieved in understanding the relationships between non-linear, anisotropic mechanical behavior and structural and textural features, which will enable a better design and optimization of future generations of carbon-based composite materials.