Missions

The matrix of the latest generation of ceramic matrix composites (CMC) is partly produced by impregnating submicron SiC powders with added carbon and boron, followed by reactive melt infiltration (RMI). Prior to the RMI step, a vacuum deoxidation heat treatment is applied to the powder-filled SiC fiber texture.
The aim of the post-doc is to understand and control the physicochemical and structural processes involved in the thermal treatment of reactive fillers in the ceramic matrix, prior to the infiltration of liquid silicon. The expected results of this first stage are
(i) a detailed chemical and structural description of the granular matrix during the process, enabling a scenario to be established for the evolution of the material's phases and its initial state, prior to the RMI stage.
(ii) identification and validation of the chemical mechanisms associated with this evolution: the chemical nature of the particles used (mainly silicon carbide, carbon, boron) and the initial presence of residual oxygen give rise to coupled phenomena of deoxidation, carboreduction, pre-sintering, etc. Particular attention will be paid to clarifying the role of carbon and boron in these processes, compared with pure SiC powder.
The post-doctoral fellow will carry out the study on each component separately (SiC, B, C), then by combining them up to the granular matrix configuration chosen for the composite. He/she will use a dual thermodynamic and experimental approach. The thermodynamic study will be consolidated by in situ thermogravimetric measurements and chemical analysis of the gas phase. The detailed description of the granular matrix will be based on morphological, chemical and structural analyses using SEM-EDS, TEM, XRD and Raman spectroscopy. The objects of study will be single-, two- and three-component powder compacts, as model materials for the fillers in the composite matrix.
A second stage will involve applying a similar physicochemical and structural description approach to understanding the mechanisms of reactive infiltration by molten silicon. This step, aimed at improving the robustness of the CMC matrix preparation process, will be carried out in a second phase, at the end or following the first study, depending on the progress of the work.

Activités

- Couple HT/RMI heat treatment equipment with in situ monitoring (mass variation: TGA, gas release: FTIR, MS...)
- Carry out heat treatments (with in situ monitoring) of different formulations of green powder compacts
- Characterize pore size distribution (Hg porosimetry) and specific surface area (BET method) of powder compacts, before and after heat treatment
- Characterize the powder compact structure (XRD, Raman, TEM) and surface elemental composition (Auger nanoprobe: AES, EDS, EELS), before and after heat treatment
- Perform thermodynamic studies (equilibrium calculations: ThermoCalc)
- Establish relationships between kinetic (TGA) and thermodynamic data, and the structural and physico-chemical characteristics of the powder compacts
- Propose chemical mechanisms likely to be at the origin of the deoxidation/carboreduction process
- If applicable, carry out the RMI step and characterize SiC-Si,B ceramics (density, structure, microstructure, elemental composition).

Compétences

PhD in Physico-chemistry of materials synthesis and characterization, preferably non-oxide ceramics
An interest in the development of experimental devices, instrumentation, in situ techniques and the physico-chemical and structural characterization of ceramics (DRX, SEM-EDS, TEM, Raman, AES, etc.).
Experience in thermodynamic calculations would be appreciated
Ability to write reports and scientific publications, and to communicate orally
Ability to work in a team, adaptability and communication skills, rigor, organization, autonomy...

Contexte de travail

The LCTS is a laboratory located on the Bordeaux campus. It is a joint research unit with four supervisory authorities: CNRS, University of Bordeaux, Safran group and CEA. It is leader in France and internationally recognized in the field of ceramic matrix composites, high-performance materials used in aeronautics, space and energy. It collaborates closely with its non-academic partners, who make a major contribution to its research and operating budget. The laboratory has around thirty permanent staff and twenty PhD and post-doctoral students.
The position proposed concerns the development of the matrix of SiC/SiC CMCs designed to replace certain metal alloys in the hot parts of next-generation aero-engines. Their development and scaling-up to industrial scale require perfect control of the manufacturing process, in order to meet demanding criteria in terms of structure, composition, mechanical strength and corrosion resistance.
The matrix of the latest-generation CMCs is partly produced by impregnating submicron SiC powders with added carbon and boron, followed by reactive liquid silicon infiltration (RMI). A heat treatment is applied to the powder-filled SiC fiber texture prior to the RMI step. This step is crucial, as the structural and chemical state of the granular porous matrix strongly influences the interaction with the liquid, and thus the composition and structure of the final matrix. At the current stage of CMC development, the key process parameters have been established to optimize the matrix with regard to the desired behavior of the composite under application conditions. Making the process more robust now requires a precise understanding of the mechanisms governing the evolution of the material, from the powder-filled texture to the final densification of the matrix.

The position is located in a sector covered by the Protection of Scientific and Technical Potential (PPST), and therefore requires, in accordance with regulations, your arrival to be authorized by the competent MESR authority.

Le poste se situe dans un secteur relevant de la protection du potentiel scientifique et technique (PPST), et nécessite donc, conformément à la réglementation, que votre arrivée soit autorisée par l'autorité compétente du MESR.

Contraintes et risques

None