Reversible radial deformation up to the complete flattening of carbon nanotubes in nanoindentation

The reversible flattening up to 60% deformation of individual multiwalled carbon nanotubes (MWCNTs) was revealed with nanoindentation. The nanoindentation induced, in terms of indentation force versus indentation depth, two linear deformation responses and a nonlinear one in between in the measurement of an ∼9 nm diameter MWCNT having six walls. A continuum shell model was applied and found to be fully capable of describing the observed behavior and extracting accurate mechanical properties of the MWCNT. The measured linear deformation persisted up to an indentation depth twice the shell thickness of the MWCNT, a behavior much like a macroscopic thin shell in classical shell theory. Nonlinear deformation was subsequently introduced due to the extended flattening of the MWCNT in the axial direction and the formation of high curvature "bulbs" along the edges of a squashed MWCNT. Finally, the elastic deformation of such bulbs initiated the second linear deformation response. The deformation behavior was found to be fully reversible, manifesting the remarkable mechanical performance of CNT as a nanoscale elastic shell in sustaining severe mechanical deformation.