Size Effects on the Plastic Deformation of Nanocomposites Prepared from SiCN and SiCNAlYO Nanopowders Synthesized by Laser Pyrolysis

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.