Fabrication of fine grained SiC and SiC-exfoliated graphene nanocomposites using spark plasma sintering of polymer derived SiC

Abstract

In spite of excellent thermo-mechanical properties of silicon carbide (SiC), its application is limited due to its inherent brittle nature. Inclusion of a high strength and high modulus second phase can potentially improve the fracture toughness of SiC with proper processing. This study presents a systematic approach to understand the effect of processing parameters on sintering of SiC matrix without binders first, and then effect of graphene reinforcement on mechanical and microstructural properties of SiC.

At first, nanostructured bulk SiC ceramics are processed using a novel approach that combines pyrolysis of preceramic polymer and spark plasma sintering (SPS). Allylhydridopolycarbosilane (AHPCS) is used as the preceramic polymer and pyrolyzed under inert conditions to produce SiC powder. Fourier transform infrared spectroscopy (FTIR) confirmed complete conversion of preceramic polymer to amorphous SiC at 1400°C. Subsequently, spark plasma sintering (SPS) technique is used to compact the SiC powder at temperatures ranging from 1600 to 2100°C at a uni-axial pressure of 70 MPa and a soak time of 10 minutes. In situ crystallization of amorphous SiC using SPS results in fine grained structure ranging from 97-540 nm. Close to theoretical density materials are obtained, at the higher sintering temperatures, with mechanical properties that exceed those of commercially available micron-sized sintered silicon carbide.

Later, the effect of graphene nanoplatelets on the microstructure and mechanical properties of silicon carbide (SiC) was investigated. Graphene nanoplatelets are dispersed in a liquid preceramic polymer by ball milling. Pyrolysis of the graphene nanoplatelet preceramic polymer slurry results in near-stoichiometric SiC graphene nanoplatelet powder. This method leads to improved dispersion of graphene in the SiC matrix as compared to conventional mechanical blending of dry powders and thereby significantly influences the resulting mechanical properties. Subsequently, spark plasma sintering (SPS) is used to consolidate dense bulk SiC-graphene composites with varying graphene content up to a maximum of 5 wt.%. X-ray diffraction (XRD) and scanning electron microscopy (SEM) investigations reveal that inclusion of graphene restricts grain growth of SiC matrix during SPS processing. Fracture toughness of SiC-graphene composite is increased by 40% with the inclusion of 2 wt. % graphene nanoplatelets. However, for higher graphene content the change in fracture toughness is limited. Improvement in fracture toughness is due to crack deflection mechanism provided by the graphene platelets. Similar graphene content also resulted in 20% improvement in flexural strength. Finally, Raman spectroscopy is used to understand the complex effect of sintering temperature and pressure on graphene nanoplatelet.