Radial compression of carbon nanotubes: Deformation and damage, super-elasticity and super-hardness

Molecular dynamics (MD) simulations of nanoindentation of multiwalled carbon nanotubes (MWCNTs) are carried out to study the deformation mechanism and the mechanical properties of MWCNTs in the radial direction. The MWCNT is found to be soft in its radial direction, with nanohardness rising slowly from about 6 to 15 GPa. The soft phase persists until all spaces between adjacent layers of the MWCNT are compressed beyond a critical value of about 1.9 Å. Beyond the critical stage the formation of new bonds between different layers starts to increase, producing a super-hard amorphous phase with a hardness of up to 94 GPa. Though locally compressed to a large radial strain of about 63%, the amorphous phase with a mixture of sp³ and sp² bonds is completely reversible upon unloading, showing super-elasticity. Further indentation afterwards leads to permanent sp³ and even sp rehybridization, the MWCNT is badly damaged and the hardness fluctuates with a maximum of about 124 GPa which is comparable to the microhardness of diamond.