Processing and mechanical properties of silicon nitride/silicon carbide ceramic nanocomposites derived from polymer precursors

Creep deformation of silicon nitride and silicon carbide ceramics is dominated by a solution-precipitation process through the glassy interface phase at grain boundary regions, which is formed by the reaction of oxide additives with the silicon oxide surface layer of the ceramic powder particles during liquid phase sintering. The ultimate approach to increase the creep resistance of these materials is to decrease the oxide content at the grain boundaries, rendering the solution-precipitation process non-effective. This research presents a new method of enhancing the creep properties of silicon nitride/silicon carbide composites by forming micro-nano and nano-nano microstructures during sintering. Starting from amorphous Si-C-N powders of micrometric size particles, powders were consolidated in three ways: (1) Consolidation of pyrolyzed powders without additives, (2) Electric Field Assisted Sintering (EFAS) of pyrolyzed powders with and without additives and (3) High pressure sintering. In all three cases, nanocomposites with varied grain size were achieved.; High temperature mechanical creep testing was performed on the samples sintered by EFAS. Creep rates ranged from 1 × 10−8/s to 1 × 10−11/s depending on method in which powders were prepared and total oxide additive amount. For samples with high oxide contents the stress exponent was found to be n ∼ 2 with an activation energy of Q ∼ 600kJ/mol*K, indicating the typical solution precipitation process of deformation. But for the nano-nano composites sintered with little to none oxide additive, the stress exponent was found to be n ∼ 1 with and activation energy of Q ∼ 200kJ/mol*K, hinting at a diffusion controlled mechanism of creep deformation.; For the nano-nano composites sintered without oxide additives, oxygen was found in the microstructure. However, oxygen contamination was found to distribute at grain boundary regions especially triple junctions. It is suggested that this highly dispersed distribution of oxygen may prevent the formation of glassy phases that are effective as fast route for mass transfer during creep deformation, therefore leading to extraordinarily high creep resistance in these nanocomposites, as observed by compression creep testing.