  The origin of ultralow friction and high wear resistance in
ultrananocrystalline diamond (UNCD) films is still under active debate because
of the perplexed tribochemistry at the sliding interface. Herein, we report a
comparative study on surface topography and nanoscale friction of tribofilms,
in wear tracks of two sets of UNCD films having different structural
characteristics. Despite both the films display ultralow coefficient of
friction, the UNCD films grown under Ar atmosphere (UNCDAr) exhibit a high wear
resistance while the wear rate is higher for the films grown in N2 (UNCDN).
Frictional force microscopic (FFM) investigations clearly reveal the
manifestation of shear induced graphitization on both the films. However, the
wear track of UNCDAr films have a large network of a few layer graphene (FLG)
structures over the amorphous carbon tribofilms while only isolated clusters of
FLG structures are present in the wear track of UNCDN films. Here, we
demonstrate the direct micro-/nanoscopic evidence for the formation of large
network of ~ 0.8 - 6 nm thick FLG structures, as a consequence of shear induced
graphitization and discuss their decisive role in ultralow friction and wear.
