Nanomechanical behavior of MoS2 and WS2 multi-walled nanotubes and Carbon nanohorns

Abstract

Nano-objects have been investigated for drug delivery, oil detection, contaminant removal, and tribology applications. In some applications, they are subjected to friction and deformation during contact with each other and their surfaces on which they slide. Experimental studies directly comparing local and global deformation are lacking. This research performs nanoindentation (local deformation) and compression tests (global deformation) with a nanoindenter (sharp tip and flat punch, respectively) on molybdenum disulfide (MoS2) multi-walled nanotubes (MWNTs), ~500 nm in diameter. Hardness of the MoS2 nanotube was similar to bulk and does not follow the “smaller is stronger” phenomenon as previously reported for other nano-objects. Tungsten disulfide (WS2) MWNTs, ~300 nm in diameter and carbon nanohorns (CNHs) 80–100 nm in diameter were of interest and also selected for compression studies. These studies aid in understanding the mechanisms involved during global deformation when nano-objects are introduced to reduce friction and wear. For compression, highest loads were required for WS2 nanotubes, then MoS2 nanotubes and CNHs to achieve the same displacement. This was due to the greater number of defects with the MoS2 nanotubes and the flexibility of the CNHs. Repeat compression tests of nano-objects were performed showing a hardening effect for all three nano-objects.

Friction, wear and mechanical behavior of nano-objects on the nanoscale

“…As described by Maharaj and Bhushan [16], the nano-objects can be deformed locally, where they may be indented, or globally, where the entire nano-object is compressed. Asperities present on mating surfaces come into contact during sliding and deform to support the load.…”

“…For thin films, the thicknesses are given and for nano-objects, the diameters are given. Pillars described in the studies have low aspect ratios (2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20) compared to wires (>20).…”

“…Other mechanisms of interest which help explain deformation involve the creation of stacking faults and the types of dislocations associated with them. The mechanisms are given in the following sections and have been previously discussed by Maharaj and Bhushan [16,50].…”

“…Deformation, instead occurs through grain rotation and grain boundary sliding which becomes easier in the absence of dislocations. A combination of material defects, such as existing cracks and spaces within nano-objects made of layered material and dislocations formed with stacking faults can also prevent observation of enhanced hardness [16,51].…”

“…For all indentation experiments contact depth does not exceed 30% of the nano-object height or film thickness, to prevent substrate hardening effects [95]. All experiments were performed at room temperature (23 8C) and 50-55% relative humidity [16,51]. The large diameter of the nanoparticle compared to the 100 nm radius of the sharp tip provides as flat a surface as possible for nanoindentation.…”