  The impact of nanoparticles (NPs) comprised of atoms with covalent bonding is
investigated numerically and theoretically. We use recent models of covalent
bonding of carbon atoms and elaborate a numerical model of amorphous carbon
(a-C) NPs, which may be applied for modelling soot particles. We compute the
elastic moduli of the a-C material which agree well with the available data. We
reveal an interesting novel phenomenon - stress dependent adhesion, which
refers to stress-enhanced formation of covalent bonds between contacting
surfaces. We observe that the effective adhesion coefficient linearly depends
on the maximal stress between the surfaces and explain this dependence. We
compute the normal restitution coefficient for colliding NPs and explore the
dependence of the critical velocity, demarcating bouncing and aggregative
collisions, on the NP radius. Using the obtained elastic and stress-dependent
adhesive coefficients we develop a theory for the critical velocity. The
predictions of the theory agree very well with the simulation results.
