Introduction

Nano-objects have been investigated for applications that include drug delivery for cancer treatment, oil detection, contaminant removal and tribology on the macro- to nanoscale. In some applications, they are subjected to friction and deformation during contact with each other and the surfaces on which they are used. The nano-objects can be deformed locally where they may be indented, or globally where the entire nano-object is compressed. To determine their suitability for applications, fundamental understanding of interfacial friction and wear, their mechanical properties and deformation mechanisms is needed.

In targeted drug delivery applications, Au, Fe2O3, polymer and SiO2 nano-objects have been studied1,2,3,4. For applications requiring chemical sensors, oxidized carbon black coated with oil detecting agents and composites of collagen and iron oxide nanoparticles have been used5,6,7,8. In contaminant removal, magnetic shell cross-linked knedel-like nanoparticles (MSCKs) are of interest9. In tribological applications on the macro-to nanoscale, a large body of data exists on nano-objects and examples of materials are given along with the various studies. Nanoparticles including Au, CNHs, Fe2O3, Sb, MoS2 and WS2; nanorods including ZnS, ZnO and AgVO3; and nanotubes including MoS2, WS2 and CNT have been used. Studies have been carried out on the macroscale10,11,12,13,14,15,16,17, microscale18,19,20 and nanoscale15,16,17,21,22,23,24,25,26,27,28.

In many applications described, low friction and wear is desirable for efficient and sustained use. The nano-objects may come into sliding contact with each other and the surfaces in which they are used. During sliding contact, asperities present on the mating surfaces come into contact and deform, to support the load. Adhesive contacts are formed with the asperities by either physical or chemical interactions. When the surfaces move relative to each other a shear force is needed to break the adhesive bonds formed at the real area of contact. The friction measured is given as: Real area of contact is directly related to the nanomechanical properties relevant to nanoscale asperity contacts.

where Ar is the real area of contact and τ is the average shear strength of the contacts30. Both reduction in shear strength and contact area reduces friction as well as wear. Deformation of nano-objects increases the real area of contact and causes friction and wear to increase.

For friction and wear reduction on the macro- to nanoscle, nano-objects made of MoS2, WS2 and graphene are of interest. In tribology, MoS2 and WS2 coatings and nano-objects have been used due to their ability to reduce the real area of contact between sliding surfaces and reduce friction and wear. Nanotubes made of these materials can also reduce friction and wear through rolling, sliding and exfoliation of the outer layers (peeling of layers) to form a tribofilm on the macro – to nanoscale16,28. Nano-objects made of graphene such as CNHs have also been used for friction reduction on the macro – to nanoscale. The additional roughness provided by the nanohorns can further reduce the contact area in addition to rolling and sliding17.

Understanding the mechanical behavior of MoS2 and WS2 MWNTs and CNHs under varying loading conditions during sliding is important for friction and wear reduction. Previous compression studies on MoS2 and WS2 nanoparticles have been performed using a nanomanipulator with an AFM probe26,27. Lahouji et al26. suggested rolling, sliding and exfoliation (peeling of layers) of an IF-MoS2 nano-object, as the AFM probe was translated over it in dry conditions. These experiments were performed inside of a transmission electron microscopy (TEM) chamber. Tevet et al27. also suggested rolling, sliding and exfoliation of IF-WS2 and MoS2 nanoparticles inside of a high resolution scanning electron microscopy (HRSEM) chamber. They were also able to show that each mechanism takes place under different loading regimes. Studies that directly compare indentation (local deformation) and compression (global deformation) have been performed on Au nanoparticles (500 nm)29. Similar studies on other nano-objects have not been performed. Given the importance of MoS2 and WS2 MWNTs and CNHs in friction and wear reduction under various loading conditions, additional studies on deformation mechanisms of these nano-objects are needed.

In this study, MoS2 and WS2 MWNTs and CNHs were investigated to determine their mechanical behavior. Nanoscale measurements were made to study scale effects. By using a sharp tip and a flat punch, local deformation (indentation) and global deformation (compression) behavior were studied. Repeat compression tests of all three nano-objects were performed to study nanoscale hardening.

Methods

Mechanical behavior under local deformation (indentation) with a sharp tip and global deformation (compression) with a flat punch were studied. MoS2 and WS2 MWNTs and CNHs nano-objects were selected because of their ability to reduce friction and wear. It is important to understand their mechanical behavior under different loading conditions that simulate the environments in which they may be used.

The nominal diameters of MoS2 MWNTs were ~500 nm with lengths of 15–20 μm and for WS2 MWNTs the nominal diameters were ~300 nm and also 15–20 μm in length. The CNHs comprise of horn shaped sheets of single-walled graphene 2–3 nm in diameter with a length of 30–50 nm. These horns, aggregate to form nanoparticles with nominal diameters of 80–100 nm.

For the determination of mechanical properties and scale effects compared to bulk, the MoS2 MWNTs ~500 nm in diameter were selected. The large diameter of the nanotubes compared to the 100 nm radius of the sharp tip provides a flatter surface for nanoindentation compared to the other nano-objects. This allows for reduction of curvature effects. As nano-object size decreases there is less contact with the tip due to the curvature of both the tip and nano-object and this can lead to inaccurate determination of contact area. This results in errors in obtaining mechanical properties accurately. In determination of mechanical properties a displacement of 50 nm was selected. This was done to obtain well defined load-displacement curves where the contact depth does not exceed 30% of the nano-object thickness and result in substrate hardening effects31.

Compression studies were also performed on the MoS2 MWNTs to compare local and global deformation. Compression studies of WS2 MWNTs and CNHs to observe mechanical behavior during global deformation were also performed along with repeat compression tests to study nanoscale hardening. Further details are given in the section on nanomechanical characterization for indentation and compression.

Materials and Sample preparation

Si (100) silicon wafers with a native oxide layer (University Wafers, Boston, MA) were ultrasonically cleaned in deionized (DI) water, followed by isopropyl alcohol (IPA) and finally acetone for 15 min each.

Figure 1 shows representative images of the nanotubes and nanoparticles used for the experiments performed. MoS2 nanotubes were obtained from Josef Stefan Institute, Ljubljana, Slovenia and WS2 nanotubes from Weizmann Institute, Rehovot, Israel and were manufactured as discussed by Maharaj and Bhushan16. Figure 1a shows representative transmission electron microscopy (TEM) micrographs of a MoS2 nanotube with a broken edge (Top), a magnified image showing separated layers between the arrows (Left), magnified image showing broken surface layers (Center) and a magnified image of a nanotube showing stacking faults highlighted by the dashed lines (Right). Figure 1b shows agglomerated WS2 nanotubes (Left), a magnified image with an arrow pointing to exfoliated layers (Center) and a magnified image of a nanotube showing ordered layers. Single-walled graphitic CNHs (Meijo University, Japan) were created as described by Iijima et al32. and Kasuya et al33. CNHs are spherical nano-objects made of single layers of graphene in the shape of horns. The horns agglomerate through van der Waals attraction to form the nano-object. Figure 1c shows TEM micrographs of spherical CNHs (Left) with a magnified image of CNHs (center) and a magnified image of individual nanohrns (Right)32.

Figure 1
figure 1

TEM images of (a) MoS2 MWNT (Top), a magnified image showing separated layers between the arrows (Left), magnified image showing broken surface layers (Center) and a magnified image of a nanotube showing stacking faults highlighted by the dashed lines (Right) (b) agglomerated WS2 MWNTS (Left), a magnified image with an arrow pointing to exfoliated layers (Center) and a magnified image of a nanotube showing ordered layers. (c) Spherical CNHs (Left) with magnified image of CNHs (Right) (Iijima et al., 1999).

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CNHs added to ethanol and MoS2 and WS2 nanotubes added to isopropyl alcohol (IPA) were sonicated for an hour to disperse the nano-objects. For experiments involving nano-object coated surfaces in dry conditions, several droplets of MoS2 and WS2 nanotubes suspended in IPA and CNHs suspended in ethanol, were deposited onto clean Si (100) substrates using a syringe. A solution concentration of 0.01 mg/ml was used. The substrate was then placed on a hot plate and heated to a temperature of about 70–80°C and left until the liquid evaporated. For the micrographs the nano-objects are deposited on a lacey carbon support (Ted Pella, Redding, CA) and then placed in a holder and observed using a TEM system (Tecnai F20, FEI, Hillsboro, OR) operated at a voltage of 200 kV with a current of 1nA.

Nanomechanical characterization- Nanoindentation

All experiments were carried out using a probe based scanning nanoindenter head TS 75 Trboscope, (Hysitron, Inc., Minneapolis, MN) attached to an AFM (Dimension 3100, Bruker, Santa Barbara, CA) with a diamond tip.

For nanoindentation experiments a three-sided diamond pyramidal Berkovich tip of approximately 100 nm in radius was used as shown in Figure 2 (left). These experiments were only performed on the 500 nm diameter MoS2 nanotubes. It is necessary to provide as flat a surface as possible for nanoindentation. This allows for elimination of curvature effects and more accurate determination of the contact area and mechanical properties. This was not possible with the other nano-objects due to their smaller dimensions. Hardness and elastic modulus were obtained by indenting at maximum displacement of 50 nm perpendicular to the basal plane. This was done to obtain well defined load-displacement curves where the contact depth does not exceed 30% of the nano-object thickness and result in substrate hardening effects31. Displacements of 100 nm were also explored and were the maximum displacements possible before the Berkovich tip penetrated the walls of the nanotube into the hollow interior. This was done to explore deformation under increased depths of penetration.

Figure 2
figure 2

Schematic showing method of deformation using a Berkovich tip for nanoindentation (local deformation) and flat punch for compression (global deformation) with MoS2 MWNTs as an example.

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The Oliver and Pharr34 method was used to obtain the hardness and elastic modulus of MoS2 nanotube. A reduced modulus is obtained from the slope of the unloading part of the load-displacement curve. This is a function of the Young's moduli and Poisson's ratios of diamond and MoS2. The Young's modulus of elasticity and Poisson's ratio for diamond were taken as 1141 GPa and 0.07 respectively35. Poisson's ratio for MoS2 was taken as 0.2736. The unknown Young's modulus of MoS2 can now be determined. Hardness is found from the ratio of the maximum load to the contact area. The data from these experiments is the average of five measurements on five different nanotubes for each load. The duration for loading and unloading was 20 s for all experiments (unless otherwise stated) to safeguard against the nanotube slipping under the indenter during more rapid and unstable loading. Topography images were also taken before and after indentation with the same tip used for indentation. All experiments were performed at room temperature (23°C) and 50–55% relative humidity.

Nanomechanical characterization- Compression

For compression experiments a spherical diamond tip of approximately 3.5 μm in radius was used as shown in Figure 2 (right). This can be considered to be a flat punch due to the large radius of the diamond tip compared to the nano-objects. Compression was performed under displacement control for displacements of 50, 100 and 300 nm for the MoS2 MWNTs. These displacements were selected to understand how the nano-objects deform under various levels of compression. The lowest displacement was 50 nm similar to indentation. This was done to compare the deformation between indentation and compression and to investigate global deformation for MoS2, WS2 MWNTs and CNHs. For the CNHs displacements were 25, 50 and 75 nm due to smaller diameters of the nanoparticles which ranged from 80–100 nm. Repeat compression loading experiments, where the nano-objects are loaded several times for a fixed displacement, were performed. Experiments were carried out to explore hardening effects on the nanoscale as well as pop-in behavior. For the nanotubes the displacement was held constant at 50 nm. The number of compressions was limited by the nanotubes either being pushed during imaging or stuck to the diamond tip during compression. This makes imaging and location of the nanotube impossible for further compression. For the CNHs the displacement was held constant at 25 nm. The duration for loading and unloading was 20 s for all experiments similar to nanoindentation. Topography images were also taken before and after indentation with the same tip used for compression.

To ensure repeatability, each experiment was performed five times and representative data are shown in the results section. Experiments were performed at room temperature (23°C) and 50–55% relative humidity for the nanotubes and for the CNHs the relative humidity was 70–75% to ensure that the nanoparticles were securely held in place by adhesive meniscus forces due to its lubricious nature.

Mechanism

In order to understand the data presented for MoS2 MWNTs and WS2 MWNTs and explain mechanical behavior during deformation, it is important to provide a detailed explanation of the underlying mechanisms involved.

It is known that movement of dislocations by slip causes plastic deformation to occur. In lamellar structures as found in MoS2 and WS2, stacking faults that are created during material formation are accompanied by partial dislocations. Local regions in a crystal consist of layers of atoms on top of each other in a repeating pattern known as the stacking order. The first layer is designated as the ‘A’ layer and successive layers are given a new letter designation such as ‘B’ or ‘C’ if offset to the original layer and each other. Stacking faults results when this order is interrupted by either introduction or removal of an extra plane of atoms as shown in Figure 337. Where the fault ends within the crystal, the area separating the faulted region from the perfect crystal is a partial dislocation. Figure 3 illustrates the concept of stacking faults and partial dislocations with Frank and Shockley partials.

Figure 3
figure 3

Schematic showing (a) stacking faults resulting from the removal of a plane of atoms with sessile Frank partials dislocations in the region between the faulted and perfect crystal. The layer directly above the missing atoms changes internally from a ‘B to B’ to ‘B to C’ arrangement to prevent an energetically unstable arrangement (Hull and Bacon, 2001). The subsequent layer changes from an ‘A to A’ to ‘A to B’ arrangement and (b) stacking faults with glissile Shockley partials in the region between the faulted and perfect crystal. These faults are as a result of an insertion of a plane of atoms with ‘C’ arrangement between a layer with ‘A’ arrangement. The ‘C’ arrangement is more energetically favorable to a ‘B’ arrangement.

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Frank partials are sessile and do not move by dislocation slip and prevents plastic deformation by obstructing movement of dislocations that can slide. Figure 3a shows an example, where a plane of atoms are missing within a layer of atoms with ‘A to A’ arrangement. The plane of atoms above and below, shift and settle into the vacant area. It is energetically unstable to have two layers of atoms with similar arrangement stacked on top of each other. To return to a more stable state, the internal arrangement of atoms above the missing layer changes from a ‘B to B’ to a ‘B to C’ configuration. This causes subsequent internal layers above it, to also rearrange themselves to accommodate this change. Figure 3b shows a stacking fault resulting from a Shockley partial as an example. This results in a stacking fault where a ‘C’ arrangement of atoms is found within an ‘A’ layer shown within the dashed lines. In this case the ‘C’ arrangement is preferred to one with ‘B’ since it is more energetically favorable. Shockley partials are glissile and move leading to slip and plastic deformation37.

Results and Discussion

Results for nanoindentation using a Berkovich tip and compression using a flat punch are given for MoS2 MWNTs and only compression for WS2 MWNTs and CNHs. For both deformation methods, representative load-displacement curves are presented. Morphological characterization, before and after deformation, is also presented using a Berkovich tip. Finally load-displacement curves for repeat compression tests using the flat punch are presented.

Nanomechanical characterization- Nanoindentation

Indentation with a Berkovich tip was used to determine the mechanical properties of MoS2 MWNTs. This was done using displacement control at a maximum displacement of 50 nm. Figure 4a shows a typical load-displacement curve with topography maps of the MWNTs over a 20 μm × 20 μm scan area and 2-D profiles before and after indentation. The topography maps are used to confirm the indentation of the nanotubes and that the nanotube did not slip during loading and unloading. The vertical arrows on the load-displacement curves point to pop-in events during indentation. The horizontal white arrows indicate the nanotube of interest along with the section on which the profiles were taken. The indent is highlighted by the dashed square on the “after indentation” profile and is not present on the “before indentation profile” at the same location.

Figure 4
figure 4

(a) Typical load-displacement curve for low displacement of 50 nm for MoS2 MWNTs and topography maps and 2-D profiles, at sections indicated by the horizontal arrows before indentation (first row) and after indentation (second row). The dashed square highlights the indent in the “after indentation profile” (b) Typical load-displacement curve for high displacement of 100 nm. Vertical arrows point to pop-in events for both curves which occur due breaking of nanotube layers and sudden compression of separated layers. Indentation was performed on separate nanotubes for low and high displacement.

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Pop-ins have been observed in metallic and intermetallic materials by Göken and Kempf38 during initial steps of loading. They were reported to represent a sharp transition from pure elastic loading to a plastic deformation corresponding to an initial yield point. In the experiments with nanotubes pop-ins are believed to be as a result of displacement during breaking of the individual layers as well as sudden displacement through the spaces between the separated layers and already broken layers as shown in Figure 1a (Left and center). Figure 3a–b shows stacking faults where Frank and Shockley partial dislocations form at the edge of the fault. Since the Frank partials increase resistance to deformation, the breaking of the layers through plastic deformation occurs due to slip of Shockley partial dislocations which causes plastic deformation.

The hardness and Young's modulus were found at a displacement of 50 nm normal to the basal plane and are given in Table 1. The hardness and Young's modulus for MoS2 MWNTs was found to be 0.6 GPa ± 0.2 GPa and 12.5 GPa ± 4 GPa respectively. Bulk MoS2 has a similar hardness (0.6 GPa) and a Young's modulus of 52 GPa normal to the basal plane. This was determined by Feldman36 based on neutron data by Wakabayashi et al41. of a 2 mm thick MoS2 crystal. The lower Young's modulus of the MoS2 MWNTs occurs as a result of the broken and separated layers present in the nanotube walls which provide less resistance to deformation (Figure 1a). The hardness of the MoS2 nanotube does not follow the smaller is stronger phenomenon seen in various nano-objects made of materials including Au, Cu, Nb and Ni29,42,43,44,45,46,47.

Table 1 Mechanical properties of MoS2 MWNTs obtained from the nanoindentation experiments normal to the basal plane. For comparison, mechanical properties of naturally occurring MoS2 (normal to the basal planes) and single layer graphene are given
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In studies, for polycrystalline nano-objects, the grain sizes were smaller than those found in bulk, which leads to dislocation pile-ups and an increased density of dislocations which entangle and resists plastic deformation48,49,50,51,52. In the case of single crystalline nano-objects, dislocation starvation occurs where the absence of dislocations in the interior of the nano-object limits plastic deformation43. The thin layers that comprise the walls of the MoS2 nanotubes are single crystalline and mechanisms associated with grain sizes are not relevant. Dislocation starvation is also not possible since stacking faults are observed in Figure 1a (Right) and both Frank and Shockley partial dislocations are present. In this case, separated layers, broken layers and missing layers and Shockley dislocations decrease resistance to deformation and compete with Frank dislocations present which increase resistance to deformation, as explained previously in the mechanism section. These competing effects result in no appreciable increase in hardness.

To further understand deformation during indentation, higher displacements were also explored. Figure 4b shows indentation at a displacement close to 100 nm with a larger number of pop-ins compared to displacement of 50 nm. This is due to a greater number of defects, such as broken, separated and missing layers, encountered by the diamond tip as the layers are penetrated.

The broken and separated layers can lead to exfoliation as the layers are easier to peel apart at higher loads. In applications requiring friction and wear reduction this can be beneficial as exfoliation leads to formation of an easily sheared film which leads to lower friction and wear16,17.

In summary, the load-displacement curves for MoS2 MWNTs revealed several pop-in effects which result from slip as the sharp tip penetrates broken layers and spaces between separated layers. No scale effects were observed. Separated layers, broken layers and missing layers and Shockley dislocations decrease resistance to deformation and compete with Frank dislocations present which increase resistance to deformation. These competing effects result in no appreciable increase in hardness.

Nanomechanical characterization- Compression

Nano-objects were compressed to understand global deformation (compression) behavior and compare to local deformation (indentation). For this purpose, a tip approximately 3.5 μm in radius was used to carry out compression tests. Compression data for the MWNTs are presented first, followed by CNHs.

Figure 5 shows typical load displacement curves for compression at maximum displacements of 50, 100 and 300 nm for MoS2 and WS2 MWNTs. The vertical arrows point to some examples of pop-in events during compression of the nanotubes. These events similar to indentation are a result of breaking of the layers of the walls of the MoS2 and WS2 nanotubes where plastic deformation occurs as a result of slip of Shockley partial dislocations. In the case of MoS2 nanotubes, compression of spaces between separated layers also contributes to pop-ins. For the MoS2 MWNTs there are less pop-in events during compression compared to indentation (Figure 4). Here, the entire nanotube is compressed and there is more resistance to deformation and breaking of the layers of the nanotube walls. For WS2 MWNTs, a greater load is required for the same displacement (50 nm) as compared to MoS2 MWNTs. At a depth as low as 50 nm, hardening due to substrate effects are unlikely as the displacement is less than 30%31 of the height of the nanotubes. The higher resistance to deformation of the WS2 MWNTs, is due to fewer defects (Figure 1b), as there are no separated layers or broken layers present as in the case of MoS2 MWNTs. However, even though there are more pop-ins at higher displacement s (100 nm and 300 nm) for MoS2 MWNTs the difference in load between the MoS2 and WS2 MWNTS is less as the substrate begins to influence the resistance to deformation and creates a hardening effect.

Figure 5
figure 5

Compression load displacement curves for displacements of 50 nm, 100 nm and 300 nm for MoS2 (Left) and WS2 MWNTs (Right) with the vertical arrows depicting pop-in events.

Magnified images are shown inset for displacements of 50 nm to more clearly illustrate pop-in events which are a result of breaking of individual layers of the nanotubes and sudden compression of separated layers.

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The broken layers of MoS2 MWNTS shown in Figure 1a (Center) and can lead to exfoliation as the layers come apart under increasing load. This along with already exfoliated layers (center) on WS2 MWNTs as shown in Figure 1b (Center) can lead to formation of an easily shearable film during sliding in tribological applications, which reduces friction and wear. For WS2 MWNTs, greater resistance to compression can also be beneficial for reduction of friction and wear. In instances where there is high moisture content, MoO3 and WO3 can be formed as MoS2 and WS2 become oxidized in the presence of oxygen. In these situations the film formation is interrupted as layers clump together. The water vapor present can also condense in the defects of the crystal structure as shown in Figure 1a, b and would interrupt the easy shear of the basal planes53,54,55. Since the WS2 MWNTs offer better resistance to deformation compared to the MoS2 MWNTs, these may be more suitable for friction and wear reduction in humid conditions. In this case since the nanotube layers are not as easily exfoliated, the friction and wear reduction is as a result of reduced contact area, sliding and possible rolling16,17.

Compression data for CNHs are now presented and compared to the MWNTs. Figure 6 shows load-displacement compression curves for CNHs for displacements of 25, 50 and 75 nm with vertical arrows pointing to pop-in events. Lower displacements were chosen for the compression due to the smaller diameter of the nanoparticles (80–100 nm). Similar to the MoS2 and WS2 MWNTs, pop-in events occur with CNHs at all three displacements. The single layer graphene horns which comprise CNHs, have a high hardness of 950 GPa and Young's modulus of 1000 GPa as shown in Table 1, but at the same time is very flexible and can easily bend56. The pop-ins for CNHs occurs as a result of buckling of the individual nanohorns. For a displacement of 50 nm a lower load (80 μN) for the CNHs is required to produce the same displacement compared the MoS2 and WS2 MWNTs, shown in Figure 5 (108 μN and 150 μN), due to the flexibility of the CNHs.

Figure 6
figure 6

Compression load displacement curves for displacements of 25 nm, 50 nm and 75 nm for CNHs with the vertical arrows showing examples of pop-in events.

Here pop-in events are a result from buckling of individual nanohorns.

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For tribological applications under high loads, rolling and sliding of spherical CNHs may not be possible due to their flexible nature and ability to deform easily. The individual nanohorns can still reduce friction and wear since they will not break apart easily due to the high hardness and Young's modulus and would provide an easily shearble layer for sliding which would reduce friction and wear. In summary, MoS2 and WS2 MWNTs and CNHs, pop-ins are observed under low to high displacements. The mechanism for the MWNTs is different from the CNHs, where the individual walls breaks compared to buckling of the individual nanohorns which are flexible. This flexibility was further seen where, for a comparable displacement for all three nano-objects, CNHs required the lowest load. Further compression tests to study hardening effects of the nano-objects were also performed.

Repeat compression tests were performed at a constant displacement of 50 nm for the MoS2 and WS2 MWNTs and 25 nm for the CNHs. This provides an opportunity to study hardening effects on the nanoscale. Figure 7 shows load displacement curves for repeat compression for the nano-objects. For all cases higher consecutive loads are required to produce the same displacements. This depicts a hardening effect up to the loads used. The layers of the MoS2 and WS2 nanotube walls are pressed together and the spaces between them become smaller. As they are pressed into the substrate a higher load is required to produce the same displacement for each successive run. The vertical arrows point to examples of pop-ins as individual layers that comprise the nanotube walls break. For the CNHs, as individuals nanohorns buckle and are pressed onto each other and the substrate, further compression becomes more difficult, similar to the MWNTs. Pop-ins in this case occur due to buckling of the individual nanohorns.

Figure 7
figure 7

Examples of repeat load-displacement curves for MoS2 and WS2 MWNTs at a constant displacement of 50 nm and CNHs at a constant displacement of 25 nm.

Vertical arrows point to pop-in events. Pop-in events for the nanotubes occur due to breaking of walls and sudden compression of separated layers. For the CNH nanoparticle pop-in events occur due to buckling of individual nanohorns. Evidence hardening can be seen for all three cases where the loads increase at a constant displacement.

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In summary, all three nano-objects display pop-in effects during loading and increased resistance to deformation during repeat compression tests. CNHs required the smallest load for a displacement of 50 nm compared to the MWNTs. This is due to buckling of the individual nanohorns as a result of their flexible nature.

The increased resistance to deformation with the repeat compression can be useful in situations where repeated contacts occur in tribological systems on the macro- to nanoscale as deformation of the nano-objects becomes more difficult.

Conclusions

MoS2 (500 nm in diameter) and WS2 MWNTs (300 nm in diameter) and CNHs (80–100 nm in diameter) were investigated to determine their mechanical behavior. MoS2 MWNTs were investigated to study scale effects and local deformation (nanoindentation) with a sharp tip. Nanoindentation was performed only on the MoS2 MWNTs. Since decreasing nano-object size results in less contact with the tip due to the curvature of both the tip and nano-object this can lead to inaccuracies in determining contact area, consequently mechanical properties. The large diameter of the MoS2 MWNTs provides a flatter surface for nanoindentation compared to the other nano-objects and leads to less error in determination of mechanical properties. Global deformation (compression) studies were performed on all samples including MoS2 and WS2 MWNTs and CNHs with a flat punch using a depth sensing nanoindenter in order to compare mechanical behavior during both types of deformation. This was carried out displacements from low to high. Hardening was investigated by compression using repeat loads.

For indentation with a sharp tip, the load-displacement curves revealed several pop-in effects which results as the sharp tip penetrates broken layers and spaces between separated layers. The hardness of the MoS2 MWNTs was found to be similar to bulk and does not follow a smaller is stronger trend. Separated layers, broken layers and missing layers and Shockley dislocations decrease resistance to deformation and compete with Frank dislocations present which increase resistance to deformation. This results in no appreciable change in hardness. For compression, pop-in effects from the sudden displacement of material were observed in both MWNTs during loading similar to nanoindentation for MWNTs. For CNHs pop-ins were also observed and resulted from buckling of the horns of the nanoparticle. The defected layers of MoS2 MWNTs and the flexibility of the graphene results in less resistance to deformation compared to WS2 MWNTs which do not show separated and broken layers observed in MoS2 MWNTs. Repeat compression tests showed a hardening effect with each subsequent load at constant displacement. The resulting load for constant displacement was either the same or higher than the previous. For the nanotubes, this was due to increased resistance to deformation as a result of the layers of the nanotubes being pressed together. For the CNHs, individual horns buckle and are pressed against each other and onto the substrate making further compression more difficult.

All three nano-objects display pop-in effect during loading and increased resistance to deformation during repeat compression tests. CNHs required the smallest load for a displacement of 50 nm compared to the MWNTs. This is due to buckling of the individual nanohorns as a result of their flexible nature.

Deformation studies comparing local deformation (indentation) and global deformation (compression) on the nanoscale to examine scale effects of mechanical properties and deformation mechanisms of MoS2 MWNTs was performed. Global deformation studies onWS2 MWNTs and carbon nanohorns were also performed. These studies aid in understanding how nano-objects and materials behave during loading and unloading in tribological applications.

The knowledge gained will have far reaching effects when designing macro-to nanoscale systems that incorporate materials with nano-dimensions for reducing friction and wear.