Pyrocarbon performances and characterization

International audiencePyrocarbon is a key material in the field of C/C composites, for aircraft brake disks, exit cones and nozzles for rocket motors or nose shields for strategic missiles. This abstract published elsewhere [1] provides a comprehensive survey on pyrocarbon properties and characterization, say: transmission electron microscopy, optical microscopy and Raman spectroscopy

Pyrocarbon optical properties in reflected light

International audienceA new accurate method is presented for measuring the extinction angle of pyrocarbons and their related anisotropy. The method permits also the extraction of the main optical parameters. Firstly, it established that the wavelength of the measurement must be controlled. Secondly, the origin of the optical phase shift when a polarized beam is reflected by such an absorbing media is derived. Then, the physical meaning of the extinction angle is discussed on the basis of calculations in an ideal case. The optical phase shift, as well as the ordinary and extraordinary reflectance were obtained by fitting the experimental data to the theoretical model. The reflectance coefficients obtained are very close to those measured by a more common method. Finally, the extinction angle Ae and phase shift are proposed to distinguish the various families of pyrocarbons

Effects of Additives and Templates on Calcium Carbonate Mineralization in vitro

International audienceThe review focuses on the effects of several important additives and templates controlling the calcium carbonate crystals formation and the complexity of the crystal morphologies in vitro. Additives include soluble matrices extracted from shells and pearls, amino-acids, magnesium ions and collagen among others. Templates include modified single crystal silicon, natural biominerals among others. Mechanisms proposed to explain the phenomena are not systematic, further studies are necessary to explain how organic matrices mediate calcium carbonate mineralization

Mechanical properties of the elemental nanocomponents of nacre structure

Sheet nacre is a nanocomposite with a multiscale structure displaying a lamellar “bricks and mortar” microarchitecture. In this latter, the brick refer to aragonite platelets and the mortar to a soft organic biopolymer. However, it appears that each brick is also a nanocomposite constituted as CaCO3 nanoparticles reinforced organic composite material. What is the role of this “intracrystalline” organic phase in the deformation of platelet? How does this nanostructure control the mechanical behaviour of sheet nacre at the macroscale? To answer these questions, the mechanical properties of each nanocomponents are successively investigated and computed using spherical and sharp nanoindentation tests combined with a structural model of the organomineral platelets built from AFM investigations

La Nacre, les biominéralisations et la pharmacopée

International audienceLa Nacre n'a rien de commun avec les pierres précieuses comme le diamant qui est forgé à des centaines de kilomètres au cœur de la Terre. Rien de commun non plus avec le rubis né des forces tectoniques qui font surgir les montagnes. Non ! La Nacre est une structure fascinante produite par le monde vivant. Comment l'Évolution biologique a-t-elle réussi à produire ce joyau ? Cet article nous montre qu'il s'agit de l'aboutissement d'un long processus qui a commencé il y a des milliards d'années sur Terre avec la vie elle-même ! Les chercheurs l'appellent biominéralisation c'est-à-dire le processus que la vie a élaboré et maîtrisé pour développer les tissus minéralisés comme les coquilles, les dents ou les os

Biominéralisation vs Organominéralisation: Comparaison des processus de minéralisation dans la matière organique, vivante et "morte"

Les dépôts sédimentaires et les organismes fossilisés offrent de nombreux modèles où des minéralisations – essentiellement carbonatées et phosphatées - se développent au sein de matières organiques (m.o.) mortes ("Organominéralisation"1). L'observation au cryo-MEB (microscope électronique permettant l'observation de m.o. hydratées2) a montré que les minéraux néoformés se forment dans une étroite relation géométrique avec la structure de la m.o. Ceci exclut que la minéralisation soit le fait exclusif de populations bactériennes se développant aux dépens des constituants organiques en voie de décomposition, même si le métabolisme de ces populations a un rôle cardinal dans l'acquisition de la composition ionique des solutions en contact avec les constituants organiques

Evaluation of the real contact area in three-body dry friction by micro-thermal analysis

Many tribological properties and wear mechanisms occurring on the micro-and nanoscale are strongly controlled by the so-called real contact area (Ar) which is a small fraction of the nominal or apparent contact area (Aa). The determination of Ar is often based on either (i) a geometrical approach describing the real geometry of contacting surfaces or (ii) a mechanical approach involving contact mechanics and physical-mechanical properties. In addition some experimental methods have also been attempted but they generally do not take into account the presence of third body at the interface—i.e. the wear debris trapped within the contact. In this paper we propose an experimental approach to estimate the dynamic real contact area from the operating parameters (Fn, v, T) and the tribological responses (μ, Ft) in presence of third body. A scanning thermal microscope (SThM) is used for determining both the thermal conductivity of the third body and the relationship between the contact temperature and the thermal power really dissipated at the micro-asperity level. These results are combined with a thermal model of the macro-tribocontact for computing the real contact area and the real contact pressure. Validation of these results is carried out using a classical Greenwood Williamson model and finite element models built from the real AFM maps

A multiscale tribological study of nacre : Evidence of wear nanomechanisms controlled by the frictional dissipated power

Sheet nacre is a hybrid biocomposite with a multiscale structure, including nanograins of CaCO3 (97% wt.% – 40 nm in size) and two organic matrices: (i) the “interlamellar” mainly composed of β-chitin and proteins, and (ii) the “intracrystalline” mainly composed by silk-fibroin-like proteins. This material is currently studied as small prostheses with its tribological behaviour. In this work, the latter is studied by varying the frictional dissipated power from few nW to several hundreds mW, in order to study the various responses of the different nacre’s components, independently. Results reveal various dissipative mechanisms vs. dissipated frictional power: organic thin film lubrication, tablet’s elastoplastic deformations, stick-slip phenomenon and/or multiscale wear processes, including various thermo-mechanical processes (i.e., mineral phase transformation, organics melting and friction-induced nanoshocks process on a large range). All these mechanisms are controlled by the multiscale structure of nacre – and especially by its both matrices and respective orientation vs. the sliding direction

Nano-Composite Structure of Nacre Biocrystal.

Pucon - ChiliIntermittent-Contact Atomic Force Microscopy with phase detection imaging reveals a nanostructure within the tablet (Pinctada maxima). A continuous organic framework divides each tablet into nanograins. Their mean extension is 45nm. Transmission electron microscopy performed in the darkfield mode evidences that intracrystalline matrix is highly crystallized and responds like a ‘single crystal'. The organic matrix is continuous inside the tablet, mineral phase is thus finely divided but behaves in the same time as a single crystal. It is proposed that each tablet results from the coherent aggregation of nanograins keeping strictly the same crystallographic orientation thanks to an hetero-epitaxy mechanism

La croissance des pyrocarbones

Cet article fait la synthèse des recherches récentes dans le domaine des pyrocarbones. Ce sont les formes solides de carbone qui se déposent sur une surface chaude par craquage d'hydrocarbures liquides ou gazeux au-delà de 900 °C. Les applications touchent les matériaux composites, les biomatériaux ou les applications nucléaires. Très récemment, une étape importante a été franchie dans la connaissance de ces carbones grâce à une démarche pluridisciplinaire. Il en résulte une classification basée sur les mesures des défauts de réseau et d'anisotropie par spectroscopie Raman. Elle permet de relier de façon satisfaisante les mécanismes de croissance, les structures et les propriétés des pyrocarbones de basse température