Jean-Claude Descamps

Silicon nitride/silicon carbide nanocomposite obtained by nitridation of SiC: fabrication and high temperature mechanical properties

Abstract Si 3 N 4 /SiC nanocomposites have been successfully synthesized according to a novel technique consisting in the nitridation of silicon carbide raw materials followed by their sintering in the presence of appropriate sintering aids. Depending on the kind of additive used, the microstructure of the final nanocomposite can be tailored from elongated silicon nitride matrix grains with silicon carbide nanoparticles dispersed into them, through completely nano-sized composites consisting of equiaxed silicon carbide and silicon nitride grains. Thermomechanical properties are related to those microstructural features and to the secondary phase composition and its subsequent crystallisation.

Creep behaviour of Al2O3–SiC nanocomposites

Abstract Compared with monolithic fine grained Al 2 O 3 , Al 2 O 3 nanocomposites reinforced with SiC nanoparticles display especially high modulus of rupture as well as reduced creep strain. Taking into account the fracture mode change, the morphology of ground surfaces showing plastic grooving, the low sensitivity to wear and the low dependence of erosion rate with grain size, it can be reasonably assumed that the strength improvement is associated with an increase of the interface cohesion (due to bridging by SiC particles) rather than with a grain size refinement involving substructure formation (as initially suggested by Niihara). In the present work, creep tests have been performed and the results agree with such a reinforcement of the mechanical properties by SiC particle bridging Al 2 O 3 –Al 2 O 3 grain boundaries. Indeed, particles pinning the grain boundaries hinder grain boundary sliding resulting in a large improvement in creep resistance. In addition, SiC particles, while counteracting sliding, give rise to a recoverable viscoelastic contribution to creep. Because of the increased interface strength, the samples undergoing creep support stress levels, greater than the threshold value required to activate dislocation motion. The high stress exponent value as well as the presence of a high dislocation density in the strained materials suggests that a lattice mechanism controls the deformation process. Finally, a model is proposed which fits well with the experimental creep results.

Determination and refinement of the crystal structure of M2SiAlO5N “B-phase” (M=Y, Er, Yb)

Abstract The crystal structure of the compound M 2 SiAlO 5 N “B-phase” (with M=Y, Yb, Er) is determined by Rietveld analysis of X-ray powder diffraction patterns. The pseudo α-wollastonite derived structure, often proposed in the literature [D.P. Thompson, The crystal chemistry of nitrogen ceramics, Materials Science Forum 47 (1989) 21–42], is used as a model for simulation of an X-ray diffraction pattern. The simulated pattern obtained with this model exhibits several peaks that are not observed on the experimental pattern. Moreover, when refining this model by Rietveld analysis, the final atom co-ordinates are strongly shifted with respect to their initial values and lead to aberrant bond lengths. A new model of the structure of B-phase is proposed: after refinement, the final reliability factors show that this new model is in very good agreement with experiments. According to this new model, the alternative layers of yttrium cations and (Si,Al)(O,N) 4 tetrahedra proposed in the pseudo α-wollastonite model is confirmed but the tetrahedra do not form rings as was initially suggested, but instead are randomly linked to each other.

Anisotropic properties in hot pressed silicon nitride - silicon carbide platelet reinforced composites

The anisotropic properties (microstructure, mechanical properties) of a hot-pressed platelet reinforced silicon nitride composite were compared with those of the monolithic material. The platelets appeared to be orientated with their basal plane in the compressive plane, and to be embedded in a silicon nitride matrix consisting of interlocked elongated b-Si3N4 grains with their c axis orientated in this plane. TEM analysis showed an interface, consisting of glassy phase and graphite at the platelet‐matrix grain boundary. Moreover the interfacial tensile stresses are in favour of a crack deflection mechanism. It was shown by TEM analysis that crack deflection occurs not only at the silicon nitride‐platelet interface, but also at silicon nitride‐silicon nitride grain boundaries. The eAciency of this reinforcing mechanism is highly orientation dependent. Because of their two dimensional geometry compared to the one-dimensional b-Si3N4 grains, platelets increase the toughness in two dimensions. # 1999 Elsevier Science Ltd. All rights reserved.

Anisotropic Properties in Hot Pressed Silicon Nitride - Silicon Carbide Platelet Reinforced Composites

The anisotropic properties (microstructure, mechanical properties) of a hot-pressed platelet reinforced silicon nitride composite were compared with those of the monolithic material. The platelets appeared to be orientated with their basal plane in the compressive plane, and to be embedded in a silicon nitride matrix consisting of interlocked elongated b-Si3N4 grains with their c axis orientated in this plane. TEM analysis showed an interface, consisting of glassy phase and graphite at the platelet‐matrix grain boundary. Moreover the interfacial tensile stresses are in favour of a crack deflection mechanism. It was shown by TEM analysis that crack deflection occurs not only at the silicon nitride‐platelet interface, but also at silicon nitride‐silicon nitride grain boundaries. The eAciency of this reinforcing mechanism is highly orientation dependent. Because of their two dimensional geometry compared to the one-dimensional b-Si3N4 grains, platelets increase the toughness in two dimensions. # 1999 Elsevier Science Ltd. All rights reserved.

A novel method to determine the R-curve behaviour of ceramic materials: Application to a ceria-partially stabilized zirconia

Abstract A new method, allowing the determination of R-curve behaviour of ceramic materials, is developed. This method, called ‘crack line wedge loading’ (CLWL), presents many advantages, especially the continuous observation of the crack growth, a mode 1 fracture specimen and plane stress, conditions favourable to show the rising R-curve effect. The R-curve behaviour of Ce-ZrO 2 is assessed by this new test procedure and the results are compared with those given by another classical method (Bluhms slice model). The data obtained by the two methods are in close agreement. However, the new method allows a broader range of crack lengths to be covered and thus is a more accurate technique to determine the stress intensity factor corresponding to the crack initiation. It is shown that the CLWL technique gives actual values of the stress intensity factor because it is possible to take into account the main crack, together with the microcracks constituting the process zone.