Computational design of novel carbon enriched Si1-xCx ceramics: A molecular dynamics simulation study

Silicon Carbide (SiC) exhibits excellent mechanical, thermal and electrical properties. Low fracture toughness is one of the limiting properties of SiC that hinders its widespread applications. Recent studies suggest that controlled alteration of local microstructure may lead to dislocation nucleation in certain SiC polytypes. Here, we report classical molecular dynamics simulations results to demonstrate thermodynamic viability of a new type SiC-based "C enriched'' ceramics where certain Si atoms are substituted by C atoms. We studied five different systems with different fraction of "C'' enrichments, namely 10%, 20%, 30%, 40% and 50%. We hypothesize that if "Si'' atoms from SiC are randomly substituted by "C'' atoms, then the bond lengths in the vicinities of the newly formed "C-C'' bonds (in place of "Si-C'' bonds) will be shortened but the overall crystal structure may not change significantly because both "C-C'' and "Si-C'' bonds are sp(3) type. After equilibrating all types of enriched systems as well as the control SiC system, we studied their equilibrium densities, free energy profiles and internal morphologies as a function of "C'' enrichment amount. The energy profiles suggest that all "C'' systems should be thermodynamically viable because total configuration energies for all systems were minimized and remained stable over a long period of time. The densities of different "C'' enriched systems drop from 3.25 gm/cm(3) to 3.05 gm/cm(3) for "C'' enrichments up to 20%. For higher than 20% "C'' enrichments, densities then increase monotonically. Since C-C bond length is shorter than Si-C bond, in principle, the densities of "C'' enriched systems should have been increased with the increase of "C'' enrichments. The opposite trend in densities up to 20% enrichment suggests additional microstructural change due to C enrichment. We explore the microstructures by measuring the average coordination number and radial distribution functions of different systems. Both these studies confirm that the newly designed materials have local microstructure change. Visual inspections of the atomistic snapshots of all systems suggest that overall crystal structures of the new systems remain fairly unchanged. (C) 2014 Elsevier B. V. All rights reserved.