FABRICATION OF SILICON NITRIDE - MULTI-WALLED NANOTUBE COMPOSITES BY DIRECT IN-SITU GROWTH OF NANOTUBES ON SILICON NITRIDE PARTICLES

In this research, Silicon Nitride (Si3N4)-Carbon Nanotube (CNT) composites were fabricated by direct in-situ growth of CNTs on the Si3N4 mixtures using Chemical Vapor Depositon (CVD) followed by Spark Plasma Sintering (SPS). The SPS technique used to sinter these powders is characterized by high heating and cooling rates coupled with pressure which prevents grain coarsening and also allows for densification in a very short period of time compared to the conventional sintering methods. The CVD techniques for in situ CNT growth ensures a more uniform dispersion in the matrix than traditional ex-situ CNT mixing methods. The sintered samples were analyzed using Field Emission Scanning Electron Microscopy (FEGSEM), X Ray Diffraction (XRD), Raman Spectroscopy and High Resolution Transmission Electron Microscopy (HRTEM). FEGSEM analysis of the Si3N4-CNT powders show uniform distribution of multi-walled nanotubes (MWNTs) in the matrix without the formation of bundles seen with traditional ex-situ mixing of CNTs in ceramic compositions. FEGSEM analysis of the fractured surface shows a uniform distribution of CNTs in the ceramic matrix. The presence of CNTs in the matrix is confirmed by Raman Spectroscopy and HRTEM. The Si3N4-MWNT composite thus fabricated shows a more uniform distribution of CNTs in the matrix and excellent CNT retention after sintering at 1850 degrees C. FEGSEM analysis shows a finer grain size due to the presence of CNTs at grain boundaries which inhibit the diffusion related grain growth.