  The frictional properties of individual multiwalled boron nitride nanotubes
(BN-NTs) synthesized by chemical vapor deposition (CVD) and deposited on a
silicon substrate are investigated using an atomic force microscope tip sliding
along (longitudinal sliding) and across (transverse sliding) the tube's
principal axis. Because of the tube transverse deformations during the tip
sliding, a larger friction coefficient is found for the transverse sliding as
compared to the longitudinal sliding. Here, we show that the friction
anisotropy in BN-NTs, defined as the ratio between transverse and longitudinal
friction forces per unit area, increases with the nanotube-substrate contact
area. Larger contact area denotes stronger surface adhesion, resulting in a
longitudinal friction coefficient closer to the value expected in absence of
transverse deformations. Compared to carbon nanotubes (C-NT), BN-NTs display a
friction coefficient in each sliding direction with intermediate values between
CVD and arc discharge C-NTs. CVD BN-NTs with improved tribological properties
and higher oxidation temperature might be a better candidate than CVD C-NTs for
applications in extreme environments.
