Polymer precursor-based preparation of carbon nanotube-silicon carbide nanocomposites

Multiwalled carbon nanotubes were dispersed in a silicon carbide matrix to examine nanotube influence on mechanical properties of the resulting composite. The ceramic matrix was generated through high temperature conversion of poly(methylsilyne), a preceramic polymeric precursor. Nanotube alkylation was explored using two functionalization schemes: organic peroxide workup and alkyllithium displacement of fluorinated nanotubes, which promoted extensive mixing within precursor solutions, thereby ensuring nanotube dispersion within the polymer matrix while facilitating interfacial bonding. The former scheme was less effective at displacing inner nanotube shell bound fluorine and resulted in lower alkyl chain grafting density on the outer shell. Polymer nanocomposites were pyrolyzed and consolidated using an optimized spark plasma sintering scheme to generate fully densified ceramics. The pure polymer-derived ceramic displayed exceptional Young's modulus and Vickers microhardness of 126 ± 12 and 9.6 ± 0.5 GPa, respectively, while maintaining a fracture toughness of 2.8 ± 0.3 MPa·m ^1/2. Increased sintering time further augmented the fracture toughness to 3.6 ± 0.4 MPa·m ^1/2, approaching the 4 MPa·m ^1/2 that characterizes pure silicon carbide, while maintaining both Young's modulus and microhardness. Nanotube addition resulted in some loss of the intrinsic mechanical properties, but enhanced monolith damage tolerance behavior, raising the Vickers indent force needed to induce cracks to an excess of 98.1 N in contrast to the pure polymer-derived sample, which began crack propagation below 49.0 N.