  It is now well established that materials are stronger when their dimensions
are reduced to submicron scale. However, what happens at dimensions such as a
few tens of nanometers or lower remains largely unknown, with conflicting
reports on strength or plasticity mechanisms. Here, we combined first
principles molecular dynamics and classical force fields to investigate the
mechanical properties of 1-2 nm Si and SiC nanoparticles. These compression
simulations unambiguously reveal that the strength continues to increase down
to such sizes, and that in these systems the theoretical bulk strength can be
reached or even exceeded in some cases. Most of the nanoparticles yield by
amorphization at strains greater than 20%, with no evidence of beta-tin phase
for Si. Original and unexpected mechanisms are also identified, such as the
homogeneous formation of a dislocation loop embryo for the <111> compression of
SiC nanoparticles, and an elastic softening for the <001> compression of Si
nanoparticles.
