  Frictional contacts lead to the formation of a surface layer called the third
body, consisting of wear particles and structures resulting from their
agglomerates. Its behaviour and properties at the nanoscale control the
macroscopic tribological performance. It is known that wear particles and
surface topography evolve with time and mutually influence one another.
However, the formation of the mature third body is largely uncharted territory
and the properties of its early stages are unknown. Here we show how a third
body initially consisting of particles acting as roller bearings transitions
into a shear-band-like state by forming adhesive bridges between the particles.
Using large-scale atomistic simulations on a brittle model material, we find
that this transition is controlled by the growth and increasing disorganization
of the particles with increasing sliding distance. Sliding resistance is at
first predominantly controlled by surface roughness, but upon agglomeration
becomes dependent on the real contact area in accordance with plasticity theory
of contact.
