  Nanoscale, single-asperity wear of single-crystal silicon carbide (sc-SiC)
and nanocrystalline silicon carbide (nc-SiC) is investigated using
single-crystal diamond nanoindenter tips and nanocrystalline diamond atomic
force microscopy (AFM) tips under dry conditions, and the wear behavior is
compared to that of single-crystal silicon with both thin and thick native
oxide layers. We discovered a transition in the relative wear resistance of the
SiC samples compared to that of Si as a function of contact size. With larger
nanoindenter tips (tip radius around 370 nm), the wear resistances of both
sc-SiC and nc-SiC are higher than that of Si. This result is expected from the
Archard's equation because SiC is harder than Si. However, with the smaller AFM
tips (tip radius around 20 nm), the wear resistances of sc-SiC and nc-SiC are
lower than that of Si, despite the fact that the contact pressures are
comparable to those applied with the nanoindenter tips, and the plastic zones
are well-developed in both sets of wear experiments. We attribute the decrease
in the relative wear resistance of SiC compared to that of Si to a transition
from a wear regime dominated by the materials' resistance to plastic
deformation (i.e., hardness) to a regime dominated by the materials' resistance
to interfacial shear. This conclusion is supported by our AFM studies of
wearless friction, which reveal that the interfacial shear strength of SiC is
higher than that of Si. The contributions of surface roughness and surface
chemistry to differences in interfacial shear strength are also discussed.
