Size Effects on the Plastic Deformation of Nanocomposites Prepared from SiCN and SiCNAlYO Nanopowders Synthesized by Las
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Size Effects on the Plastic Deformation of Nanocomposites Prepared from SiCN and SiCNAlYO Nanopowders Synthesized by Laser Pyrolysis Nathalie Herlin-Boime1, Romuald Dez1, cecile Reynaud1, Jean Louis Besson2, and Jean Vicens3 1 DRECAM - SPAM, CEA, CEA Saclay, Bat 522 - SPAM LFP (CEA CNRS URA 2453), gif sur yvette cedex, 91191, France 2 CNRS, Limoges, 87060, France 3 SIFCOM - ENSICAEN, Caen, 14000, France ABSTRACT The aim of this work was to synthesise nanopowders by laser pyrolysis in order to elaborate Si3N4-SiC nanocomposites exhibiting a high ductility at high temperature. The synthesis of SiCN and SiCNYAlO nanopowders with a good thermal stability has been developed from a liquid mixture based on hexamethyldisilazane with a gaseous precursor (silane, SiH4) and ammonia. Si3N4-SiC nanocomposites exhibiting almost theoretical density have been elaborated by hot pressing under 35 MPa at 1600°C either from SiCNYAlO nanopowders or from SiCN nanopowders mixed with commercial sintering aids nanopowders (6 wt% yttria and 3 wt% alumina) in ethanol and de-agglomerated by milling with Si3N4 balls. The densification behaviour is better for nanopowders containing in-situ additives. In addition, using prealloyed powders enables to simplify the elaboration process since the mixing step of the nanopowders is eliminated. The nanocomposites elaborated from the mixture (SiCN + commercial sintering aids) exhibit a different microstructure, depending on the drying process (under vacuum or in an oven) of the nanopowders after the mixing step: When drying was performed in a confined atmosphere, a noticeable amount of Si2N2O was detected in addition to β-Si3N4 and the average grain size was 60 nm, whereas the crystalline phases were α- and β-Si3N4, when the slurry was dried under vacuum and the average grain size was 200 nm. The creep behaviour of the two grades was assessed under compressive loading in air and the deformation appears strongly influenced by this change of the microstructure: A strong ductility was observed for the smallest grain size: a deformation as high as 45 % was measured under a compressive load of 180 MPa at 1350°C under air. INTRODUCTION In material science, the ability to create nanoparticles with finely controlled characteristics is very important for the manufacture of nanostructured materials. This paper deals with the specific case of Si3N4/SiC structural ceramics, for which it is highly important to obtain hard materials exhibiting plastic deformation at high temperature [1] Such materials would make it easier to manufacture geometrically accurate parts, comparable to those produced for metal. In silicon nitride based ceramics, the small size and the "spherical" shape of the grains constituting the material are two important parameters in favor of high temperature deformation. Therefore, SiCN nano-sized powders are real candidates as starting materials to elaborate dense Si3N4/SiC nanocomposites exhibiting the microstructure required for ductility at high temperature.
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