Dr. Gwénaël Gouadec

THESIS SUMMARY
Micro-Raman scattering, the perfect tool for dynamics analysis, was used to characterize 'Oxygen-rich' (NLM-Nicalon, Tyranno), 'carbon-rich' (Hi-Nicalon, SCS-6) and 'stoichiometric' (Hi-Nicalon-S, SA & Sylramic) SiC fibers.

Main Results
* Assesment of the size and structural disorder effect on the optical modes of silicon carbide nano-precipitates.
* Carbon resonance (first, second and third order bands) allowed to propose a new assignment for 'D' band and a correction to the so-called 'Tuinstra and Koenig relationship' below 5 nm.
* Thermal history, mechanical properties (strength, Young's Modulus and micro-hardness) and microstructure 'radial gradients' can be derived from the Raman spectra for the fibers.
* On account of potential well anharmonicity, the wavenumber v associated with any of the vibration modes of a bond obeys a
{v = v0 + S%×e%} law, where e% is the macroscopic strain.
S% were measured in the fibers (first time ever for the stoichiometric grades) and a global model was proposed for carbonaceous fibers (taking the non-reversibility of S% in tension/compression into account).
* S% coefficients finally allowed residual strain assessment in NASA (Hi-Nicalon / celsian, with and without BN/SiC interphase) and ONERA (SCS-6 / Ti6242) composites. Quantitative results depended on :
- carbon bond resonance (wavelength and heating effects).
- the chemical moieties probed with any given band (sp2 and sp3
carbons behave differently under stress).

- interface/phase reactions.

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POST DOCTORAL RESEARCH

Microballoons are thin hollow spheres that are bonded together with resins to form ”syntactic foams”. Mechanical properties of the microballoons are integral to those of the foams (high compressive strength) and are needed in view of modeling the system. Nano-compression results were obtained for individual carbon microballoons (CMBs) tested between 5 and 50 mN on an XP nano-indentation device (MTS) customized with a special cylindrical tip.
* CMBs were randomly chosen, which allowed for a statistical analysis (140 tests on CMBs ranging in diameter from 5 to 80 µm ). Less than 25% of the CMBs were found to be 'perfect' spheres. CMBs smaller than 10 µm and greater than 50 µm were markedly ellipsoidal and about one third of the CMBs exhibited “sequential” cracking, revealing the existence of flaws.
* SEM and optical microscopy of the foams revealed the flaws were either voids in the wall thickness or compartments in some CMBs.
* Conventional Berkovich nano-indentation was performed on segments of CMB walls after nano-compresseion and yielded a value of approximately 31 GPa for Young’s modulus. The measured thickness was between 0.3 and 2.2 µm, showing no correlation with the diameter of the pristine CMBs.
* A power law was found to relate the maximum deformation to the diameters and a “pseudo-modulus” was defined for comparison between CMBs. The formula relates the load and deformation at each stage of the compression to the diameter of the CMB.

Raman and IR spectroscopies;
Scanning Electronic Microscopy (SEM);
Atomic Force Microscopy (AFM);
NanoIndentation;
Sol-Gel synthesis of interphase precursors;
Grazing X-rays;
Hot Pressing;

I can provide expertise in the field of Raman spectroscopy and would like to be involved in federative research projects in Materials Science.