  Collisions between amorphous Fe nanoparticles were studied using
molecular-dynamics simulation. For head-on collisions of nanoparticles with
radii $R =$ 1.4 nm, $R =$ 5.2 nm, and $R =$ 11 nm, sticking was observed at all
simulated velocities. The results were compared to the description provided by
the JKR model. It was found that strong disagreement exists between the
predictions of JKR and the results of the molecular-dynamics simulation due to
the presence of additional dissipative processes which strengthen sticking
behavior. First, it is demonstrated that very strong dissipation into atomic
vibrations occurs during the collision. The dissipation is strong enough to
prevent significant rebound of the nanoparticles. Additionally, the morphology
of the adhered nanoparticles includes a ``neck'' that increases in radius with
increasing collision velocity which results in amplified irreversibility and
adhesion. Approximate calculation of the stress during the collision indicates
that stress levels are well above typical yield stress values even for low
velocity collisions, consistent with the observation of plastic deformation.
Furthermore, it is shown that for nanoparticles with $R \leq$ 11 nm, the
dominance of surface attraction results in large effective collision velocities
and plastic deformation. By obtaining scaling relations for computed
quantities, predictions are made for larger nanoparticles up to $R$ $\sim$ 1
$\mu$m. This work provides a new perspective on collisional dissipation and
adhesion with an important connection to the modern understanding of tribology
and friction.
