Composite à matrice céramique

thumb|260px|Surface de cassure d'un composite constitué de fibres de SiC et d'une matrice de SiC.thumb|260px|Coussinets céramiques de paliers de diamètres de en CMC pour de grandes pompes. Les composites à matrice céramique ou CMC sont des matériaux composites faisant partie des céramiques techniques. Ils sont caractérisés par un ensemble de fibres céramiques incorporées dans une matrice également céramique. Fibres et matrice peuvent en principe être constituées de toutes les céramiques connues, en y incluant même le carbone. Cet article décrit les composites actuellement disponibles industriellement, les principaux procédés de fabrication, les propriétés essentielles ainsi que quelques exemples de développements et d'applications fructueuses de ce groupe relativement récent de matériaux. Introduction Dans les développements et applications des céramiques composites, on utilise principalement des fibres de carbone (C) et de carbure de silicium (SiC), et parfois des fibre

Spectroscopie mécanique de la relaxation des contraintes d'interface dans les composites à matrice métallique

Metal matrix composites are known for their excellent specific mechanical properties. Nevertheless, because of the thermal expansion mismatch between the matrix and the reinforcements, thermal stresses arise at the interfaces of such materials. In order to relax these stresses, dislocations are emitted from the interface and propagate in the matrix, which can influence the mechanical behavior of the composite material. Mechanical spectroscopy, which characterizes the ability of a solid to dissipate energy under an external excitation, was used to study the relaxation mechanisms occurring at the interface of MMCs. Indeed, the mobile matrix dislocations near the interface are the main sources of damping, making this technique very sensitive to these mechanisms. Composites studied here were processed by gas-pressure infiltration of the reinforcement preform by the molten metal. The processed composites were based on aluminum, magnesium and Mg-2%Si alloy, which is known for its high damping level. They were reinforced by short misorientated alumina fibers or long and aligned carbon and silicon carbide fibers. In the case of long fibers, two types of orientation were obtained: parallel or perpendicular to the composite axis. In order to modify the interface morphology, a heat treatment was carried out on aluminium matrix composites reinforced by alumina fibers during the infiltration process. By increasing the contact time between the molten matrix and the fibers, alumina crystals were formed on the surface of the fibers, whose size increased with time. The specific elastic moduli of the processed composites were clearly superior to those of matrix alone and their value agreed well with the existing theoretical models. Composites were submitted to thermal cycles from 120 K to 500 K and the internal friction and dynamical modulus were measured as a function of temperature. It was shown that the behavior of these two parameters, the large maximum at low temperatures and the modulus anomaly in the case of magnesium composites was driven by the motion of the dislocations activated in the matrix in order to relax thermal stresses. The internal friction was also characterized by a transient contribution, depending on the heating or cooling rate dT/dt and the excitation frequency ω. By using a model developed by Mayencourt and al., it was possible to determine two parameters C1 and C2 which were sensitive respectively to the mobile dislocation density relaxing the thermal stresses and the interface strength. The difference in the behavior of these two parameters as a function of temperature tended to show the great potential of the magnesium matrix composites. Indeed, the interface strength was decreasing at low temperature, allowing a better toughness whereas it was increasing at high temperature, improving the creep resistance. Finally, in the case of the aluminum matrix composites reinforced by short fibers with alumina crystals at the interface, it was observed that the elastic modulus was decreasing as crystal size increased. However, when the crystals exceeded a certain size, they were acting as pinning points between matrix and fibers, resulting in the increase of the modulus almost to its initial value. By determining the parameter C1 from the transient damping, it was shown that this parameter followed the same trend as the elastic modulus, which made it a good probe for the interface quality.

EPFL2004

Molecular modeling of cracks at interfaces in nanoceramic composites

William Curtin, Fabio Pavia

Toughness in Ceramic Matrix Composites (CMCs) is achieved if crack deflection can occur at the fiber/matrix interface, preventing crack penetration into the fiber and enabling energy-dissipating fiber pullout. To investigate toughening in nanoscale CMCs, direct atomistic models are used to study how matrix cracks behave as a function of the degree of interfacial bonding/sliding, as controlled by the density of C interstitial atoms, at the interface between carbon nanotubes (CNTs) and a diamond matrix. Under all interface conditions studied, incident matrix cracks do not penetrate into the nanotube. Under increased loading, weaker interfaces fail in shear while stronger interfaces do not fail and, instead, the CNT fails once the stress on the CNT reaches its tensile strength. An analytic shear lag model captures all of the micromechanical details as a function of loading and material parameters. Interface deflection versus fiber penetration is found to depend on the relative bond strengths of the interface and the CNT, with CNT failure occurring well below the prediction of the toughness-based continuum He-Hutchinson model. The shear lag model, in contrast, predicts the CNT failure point and shows that the nanoscale embrittlement transition occurs at an interface shear strength scaling as tau similar to epsilon(f),CNT sigma CNT, rather than tau s similar to sigma CNT typically prevailing for micron scale composites, where epsilon f,CNT and sigma CNT are the CNT failure strain and stress, respectively. Interface bonding also lowers the effective fracture strength in SWCNTs, due to formation of defects, but does not play a role in DWCNTs having interwall coupling, which are weaker than SWCNTs but less prone to damage in the outerwall. (C) 2013 Elsevier Ltd. All rights reserved.

Elsevier2013

Optimizing strength and toughness of nanofiber-reinforced CMCs

William Curtin, Fabio Pavia

Nanofibers used in current ceramic matrix composites (CMCs) are usually wavy and of finite length. Here, a shear-lag model for predicting the tensile strength and work of fracture of a composite containing a "single matrix crack", as a function of all the material parameters including constant plus Coulomb interfacial friction, is presented for a CMC containing wavy, finite-length nanofibers having a statistical distribution of strengths. Literature results are recovered for straight infinite fibers, with strength and toughness depending on a characteristic strength sigma(c) and a characteristic length delta(c) For nanofibers of finite length L, radius of curvature R, and with Coulomb friction coefficient mu, the strength and toughness are found to depend only on sigma(c), delta(c), and two new dimensionless parameters mu delta(c)/R and L/delta(c). Parametric results for a typical carbon nanotube CMC show an optimal region of morphology (L and R) that maximizes composite strength and toughness, exceeding the properties of composites with straight (R=infinity) and/or long (L=infinity) reinforcements. Therefore, while factors such as the nanofiber strength distribution and the nanofiber-matrix interface sliding resistance may not be easily controlled, the tuning, via processing, of purely geometrical properties of the nanofibers (L and R) alone can aid in maximizing composite properties. These results have important application in hybrid CMCs such as "fuzzy fiber" CMCs, where micron-scale fibers are covered with a forest of nanofibers such that the nanofiber array can bridge longitudinal cracks and thus improve delamination resistance. (C) 2012 Elsevier Ltd. All rights reserved.

Elsevier2012

Spectroscopie mécanique de la relaxation des contraintes d'interface dans les composites à matrice métallique

EPFL2004