Laser synthesis of silicon carbonitride nanopowders; structure and thermal stability

Si₃N₄/SiC nanocomposite materials are of great interest for structural applications at high temperature. In silicon nitride based ceramics, the small size and the spherical shape of the grains constituting the material are two important parameters in favour 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. SiCN nanopowders with different chemical compositions and characteristics can be prepared by CO₂ laser pyrolysis of organosilicon precursors. Laser pyrolysis of gaseous precursors is able to produce partly crystallised SiCN nanoparticles exhibiting a reasonable thermal stability and suitable for elaboration of ceramic materials. In order to reduce the cost and to improve the safety of the process, an aerosol generated from a liquid precursor, hexamethyldisilazane (HMDS), has also been used to synthesised SiCN nanopowders. However, in this latter case the powders obtained exhibit a high weight loss during heat treatment at high temperature. Therefore, in this study the effects of various synthesis parameters (chemical nature of the precursor and laser power) on the degree of crystallisation and on the thermal stability of nanopowders are investigated. Characteristics of powders such as chemical composition, morphology, structure and thermal stability are reported. A correlation between the synthesis conditions of powders and their thermal stability is established, and the synthesis parameters enabling improvement of thermal stability are determined.