Heterogeneity governs diameter-dependent toughness and strength in SiC nanowires

F Elahi and L Ma and ZM Hossain, PHYSICAL REVIEW B, 98, 174111 (2018).

DOI: 10.1103/PhysRevB.98.174111

Using a combination of density functional theory and molecular dynamics simulations, this paper reveals the atomistic origin of diameter- dependent extreme mechanical behavior of 111 3C-SiC nanowires obtained from an energy-based framework. Our results suggest that heterogeneity in atomic stress and variations in diameter-dependent potential-energy density have a profound impact on extreme mechanical properties in the nanowires. The heterogeneity in stress evolves from the nonuniform bond lengths mediated by low coordinated surface atoms-and it penetrates the entire cross section in thinner nanowires and constitutes the atomistic basis for their large reduction in fracture strain, toughness, and strength. Although stress heterogeneity is substantially higher in ultrathin nanowires, its intensity drops and saturates rapidly in larger nanowires following a nonlinear dependence on diameter. The maximum stress heterogeneity in a cross section localizes crack nucleation at the core in ultrathin nanowires but near the surface in larger nanowires. Moreover results show that stiffness, toughness, strength, and fracture strain of the nanowires increase nonlinearly with increasing diameter and saturate at a lower value compared to bulk SiC. In addition to resolving wide discrepancies in the reported values of the first-order elastic modulus in SiC nanowires, the findings highlight heterogeneity as a critical factor for inducing diameter-dependent extreme mechanical behavior in brittle nanowires.

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