Nanotribological Properties Research Articles

Effects of surface texturing on nanotribological properties and subsurface damage of monocrystalline GaN subjected to scratching investigated using molecular dynamics simulation

Surface nanotribological properties and subsurface damage of flat and textured gallium nitride (GaN) substrates during both linear and circular nanoscratching processes are investigated using molecular dynamics simulation. Results show that the surface texture can remarkably reduce not only the surface friction and wear but also the scratching-induced subsurface damage regardless of the tip radius and penetration depth. Tangential force and friction coefficient decreased with an increase in groove width or depth primarily due to the reduced contact area. The effect of texturing on stress distribution can affect the development of dislocation to alleviate the nanoscale material deformation and removal and thereby reduce surface wear and subsurface damage. Increasing tip radius can reduce the friction coefficient and surface wear, but it promotes subsurface damage. By contrast, increasing tip penetration depth can increase the friction, wear, and subsurface damage for flat and textured GaN substrates. Compared with the c-plane, the m-plane GaN substrate exhibits higher friction but lower material wear, and it has a considerably thicker subsurface damage layer due to the difference in their dislocation networks. This work can provide useful insights into the design of GaN-based material with antifriction and wear properties through the surface texturing treatments.

Friction at nanopillared polymer surfaces beyond Amontons’ laws: Stick-slip amplitude coefficient (SSAC) and multiparametric nanotribological properties

Frictional and nanomechanical properties of nanostructured polymer surfaces are important to their technological and biomedical applications. In this work, poly(ethylene terephthalate) (PET) surfaces with a periodic distribution of well-defined nanopillars were fabricated through an anodization/embossing process. The apparent surface energy of the nanopillared surfaces was evaluated using the Fowkes acid-base approach, and the surface morphology was characterized using scanning electron microscope (SEM) and atomic force microscope (AFM). The normal and lateral forces between a silica microparticle and these surfaces were quantified using colloidal probe atomic force microscopy (CP-AFM). The friction-load relationship followed Amonton’s first law, and the friction coefficient appeared to scale linearly with the nanopillar height. Furthermore, all the nanopillared surfaces showed pronounced frictional instabilities compared to the smooth sliding friction loop on the flat control. Performing the stick–slip amplitude coefficient (SSAC) analysis, we found a correlation between the frictional instabilities and the nanopillars density, pull-off force and work of adhesion. We have summarised the dependence of the nanotribological properties on such nanopillared surfaces on five relevant parameters, i.e. pull-off force fp, Amontons’ friction coefficient μ, RMS roughness Rq, stick–slip amplitude friction coefficient SSAC, and work of adhesion between the substrate and water Wadh in a radar chart. Whilst demonstrating the complexity of the frictional behaviour of nanopillared polymer surfaces, our results show that analyses of multiparametric nanotribological properties of nanostructured surfaces should go beyond classic Amontons’ laws, with the SSAC more representative of the frictional properties compared to the friction coefficient.

Effect of tip size and experimental conditions on nano-scale friction testing

Assessment of nano-tribological properties (friction and wear) on polymer surface helps to understand their scratch, mar and wear properties. More polymers are used now for automotive, electronics, appliances, and building and constructions industries due to aesthetics and design freedom. Owing to their lower mechanical properties, polymers are vulnerable to surface damage and deteriorate their morphology, functionality, and aesthetics with time. In this study, nano-indenter was used for measuring friction and wear properties varying test parameters and indenter tips. A Berkovich tip (B-tip) with radius of ∼50 nm and a conical tip (C-tip) with 60° cone angle and tip radius of 1000 nm were used. Polyoxy methylene (POM) with additives were chosen for friction testing. Higher coefficient of friction (COF) for C-tip was observed compared to B-tip and this explained based on the higher tip-material interactional area for C-tip. COF was decreased with higher additive concentration and same trend was followed for wear depth for the B-tip. For C-tip, COF was increased with the testing loads and corresponding wear depth were higher for higher load. Change of COF with test velocity was not showing any specific trend. Shallow wear depth using B-tip compared to C-tip may be better for probing top surface of the materials. Nano-scale tribo-mechanical evaluation techniques are uniquely suited for fundamental understanding and screening many additives quickly. Observed wear properties not only depend on the additives added to the formulation, but may also be influenced by the geometry of the tip.

Study on laser shock imprinting nanoscale line textures on metallic foil and its application in nanotribology

The surface precision and mechanical properties were the bottleneck of laser shock imprinting extensive application in micro/nano manufacturing. In this work, the high-quality and precise nanoscale line textures were fabricated on aluminum foil, and the influence of pulse laser energy and laser pre-shock processing on the preparation of nanoscale line textures was also investigated experimentally. The morphology of nanoscale line texture was characterized by scanning electron microscope (SEM) and atomic force microscope (AFM), which obtained the best laser shock imprinting parameter. The microstructure and dislocation changes of laser shock imprinted aluminum foil were analyzed by transmission electron microscope (TEM), which explained the improvement of nanoscale line textured aluminum foil in electrochemical and mechanical properties. The nanotribological properties of nanoscale line textured aluminum foil were characterized by the AFM with sharp probe and SiO2 colloidal probe. The experimental results indicated that nanoscale line textured aluminum foil had the effect of reducing friction and adhesive force between AFM probe and aluminum foil. The laser shock imprinting can fabricate high-accuracy nanoscale line textures on aluminum foil. Compared with the original aluminum foil, the morphology, mechanical properties, electrochemical performance and nanotribological properties of nanoscale line textured aluminum foil were improved.

Superlubricity of Fullerene Derivatives Induced by Host-Guest Assembly.

Fullerenes have been recognized as good candidates for solid lubricants. In this study, the microscale superlubricity of fullerene derivatives was accomplished by the construction of regular host-guest assembly structures. Herein, the host-guest assembly structures of fullerene derivatives were successfully constructed on a highly oriented pyrolytic graphite (HOPG) surface by introducing the macrocycles as the templates and were explicitly revealed by scanning tunneling microscopy (STM). Meanwhile, the nanotribological properties of the host-guest assemblies were measured using atomic force microscopy (AFM), revealing ultralow friction coefficients of 0.003-0.008, which could be attributed to the restriction on removal of fullerene molecules after introducing the templates. The interaction energies were calculated by density functional theory (DFT) method, which indicates the correlation between friction coefficients and interaction strength in the host-guest assemblies. The effort on fullerene-related superlubricity could extend the solid superlubrication systems and provide a novel pathway to explore the friction mechanisms at the molecular level.

Adhesion and nanotribological properties of folded graphene prepared by mechanical exfoliation

Atomic force microscope was used to study the adhesion of mechanical exfoliated graphene under two different atmosphere conditions, air and nitrogen. It was found that the adhesion under nitrogen was smaller. The adhesion of graphene edge was larger than that of the inner region. The relationship between the adhesion of folded graphene and the number of layers along with its frictional properties were investigated under nitrogen atmosphere. The results showed that the adhesion was independent of the number of folded graphene layers. The frictional properties of each area of the folded graphene were far beyond the SiO<sub>2</sub> substrate. The friction coefficients of the single layer, the fold on single layer, the double layers and the fold on double layers regions were successively decreased, which were 0.049, 0.031, 0.023 and 0.021 respectively. The friction forces were successively decreased as well. The frictional property of the folded graphene was weaker than the unfolded graphene of same number of layers due to the weaker bonding force between the layers. When measuring the adhesion with a sharp tip or a ball tip, the measurement history of adhesion had little influence on subsequent adhesion. Studies on freshly folded graphene in the air showed that the friction force of the folded region was significantly higher than that of the unfolded region.

Nanomechanical and nanotribological properties of self-lubricating Ti/MoS<SUB align="right">2 nanocoating at nanoscale level

Ti/MoS2 coating of thickness 99.79 nm was prepared by pulse laser deposition method on Al-Si substrate. Mechanical and nanotribological properties of Ti/MoS2 coating were obtained by carrying out nanoindentation, nanoscratch and nanowear tests at low loads. It was observed that Young's modulus and nanohardness of Ti/MoS2 coating decrease with increasing load. The coefficient of friction also decreases with the increase in sliding distance, which proves that Ti/MoS2 coating have self-lubricating property. The wear rate of Ti/MoS2 coating increases from 5.7 × 10−10 mm3/Nm to 2.1 × 10−9 mm3/Nm with the increase in load. Scanning probe microscope images of Ti/MoS2 coating shows the plastic flow of coating with no debris and cracks on the surface. It indicates that the abrasive wear is the main wear mechanism.

Nanomechanical and nanotribological properties of self-lubricating Ti/MoS<SUB align="right">2 nanocoating at nanoscale level

Nanomechanical and nanotribological properties of self-lubricating Ti/MoS<SUB align="right">2 nanocoating at nanoscale level

Nanotribological Properties of Boundary Additives on DLCs

Nanotribological Properties of Boundary Additives on DLCs

Molecular dynamics simulation of abrasive characteristics and interfaces in chemical mechanical polishing

Molecular dynamics simulation is employed to analyze the effect of sliding, rolling and oscillating movements on nanotribology properties of a diamond abrasive on a silicon substrate. The abrasive oscillating mechanism is achieved by simulating megasonic vibration-assisted on the planarization process. In this paper, the effects of abrasive size, sliding velocity, depths of polishing, rolling velocity, rolling direction, oscillating amplitude and oscillating frequency on material removal are considered. The results showed that the rolling mechanism reaches the highest number of atoms removed while the suitable oscillating mechanism gains the lowest height of asperity. The oscillating movement has remarkable results in wiping out the asperity atoms, although at a high amplitude and a low frequency causing some atoms from the flat substrate stuck to the abrasive and left some surface defects. Moreover, a multi-asperities model is set up to simulate the global-scale ability of sliding, oscillating and rolling mechanisms on polishing. This model indicates the saturated behavior of the rolling mechanism, while the lowest value of the surface roughness attained by the sliding mechanism. Combining three types of removing mechanisms could create a smooth surface with a high effective removal rate on both local and global-scales.

Material removal and interactions between an abrasive and a SiC substrate: A molecular dynamics simulation study

In this paper, molecular dynamics simulation is applied to inspect the effect of sliding, rolling, vibrating and rolling plus vibrating movements of a diamond abrasive on nanotribology properties of a 4H–SiC substrate. The effect of the abrasive size, depth of polishing, sliding speed, rolling, and vibrating movements on material removal are surveyed. For the first time in literature, the rolling in x-axis, vibrating in horizontal direction, and rolling combined with vibrating movements are considered. The results present that a bigger abrasive mostly gains a lower asperity height and a higher number of atoms removed. The rolling motion in the x-axis achieves the highest number of atoms removed while the rolling motion in the reverse z-axis achieves the lowest asperity height. In the vibrating movements, the horizontal vibrating movement creates a lower asperity and a higher number of atoms removed than the vertical and the rolling plus vibrating motions. The number of atoms eliminated does not sensitive to the vibrating amplitude and the vibrating frequency. Moreover, a multi-asperities model is constructed to estimate the global-scale surface of the substrate after removing the asperities. The asperity height is removed effectively by the rolling plus vibrating motion. The surface roughness produced by rolling in x-axis movement and vibrating motions are better than other motions. This model also points out the saturated behavior of all types of movements, except the rolling plus vibrating motion. The interactions between the abrasive and the asperity could create a lengthened asperity, amorphization, new C–C bonding, clusters of atoms adhered on the abrasive surface, a high temperature at the interface, atomic strain, cleavage event and fly away atoms.

Bilayer graphene film synthesized by hot filament chemical vapor deposition as a nanoscale solid lubricant

In the present study, we synthesized homogeneous bilayer graphene (BLG) films on the Cu substrate by adopting the hot filament chemical vapor deposition (HFCVD) method. The precise control of the layer number of fabricated graphene films could be obtained by adjusting the flow rate of hydrogen or reactive pressure. Either increasing the flow rate of hydrogen under a fixed reactive pressure or decreasing the reactive pressure with a fixed CH4:H2 ratio resulted in an increase in the layer number of the synthesized graphene film. We further examined the nanotribological properties of BLG by Atomic Force Microscopy (AFM). The BLG film resulted in an 83%–92.5% reduction in friction when applied on the surface of either copper or SiO2/Si substrates. The obtained coefficients of friction (COFs) were as low as 0.025–0.085. Besides, the critical failure load of the BLG film on the SiO2/Si substrate was measured as 1266 ± 28 nN in a progressive-force scratching test, in which we used a diamond tip with a radius of 10 nm. The corresponding actual contact pressure was estimated at as high as 14.15 GPa, according to Derjaguin-Muller-Toporov model, which was enhanced by over one order of magnitude compared with that obtained for the bare SiO2/Si substrates (1.2 GPa). The results achieved in the present study demonstrate that homogeneous BLG films with promising lubricity and superior wear resistance could be synthesized in the HFCVD process, which, we believe, provides an effective and controllable approach for promoting the application of BLG films as a nanoscale solid lubricant in a variety of Nano/Micro Electromechanical Systems devices.

Nanotribological properties of 2-D MoS2 on different substrates made by atomic layer deposition (ALD)

The friction influence of 2-D molybdenum disulfide (MoS2) films deposited on different substrates (Si, SiO2 and Al2O3) by atomic layer deposition (ALD) has been investigated. All of the three substrates showed a trend of decreasing friction with increasing number of layers of MoS2, but the friction leveled off for more than 6 layers samples. Comparing these 2-D MoS2 films with each other, MoS2 on the Al2O3 possesses the lowest friction, while MoS2 on the Si possesses the highest one, owing to effect of substrate on the specific self-limiting reaction in ALD process. At the same time, the friction of the monolayer MoS2 is strongest affected by sulfur deficient. As the number of layers increases, the influence of the substrate on the friction of MoS2 can be approximately characterized by the work of adhesion. Greater work of adhesion corresponds to a higher friction force. When the thickness is more than 6 layers, the substrate effect disappears gradually. Moreover, the friction of 2-D MoS2 could be controlled near linearly by changing plasma treatment time within the range from 1 to 20 s, which suggests a possible way for the friction control of 2-D MoS2 on different substrate quasi-qualitatively or even qualitatively.

Nanomechanical Properties of a Bicomponent Epoxy Resin via Blending with Polyaryletherketone

In order to investigate the nanomechanical behaviors and nanotribological properties of bicomponent epoxy resin (BE) blends, which were filled with thermoplastic polyaryletherketone (PAEK) powders, nanoindentation and nanoscratch tests were performed. The brittle fractured morphologies of bicomponent epoxy resin blends were studied. The microhardness and elastic modules of the materials were measured using the nanoindentation technology. The hardness, elastic modulus, and other mechanical properties of materials on a nanoscale were determined. Nanoindentation and scratch experiments showed that the indentation response is dominated by plastic deformation. The microhardness is the lowest as the content of PAEK powders is increased to 30 parts per hundred parts of resin (phr), while that of the neat bicomponent epoxy resin specimen is the highest. Furthermore, the pristine bicomponent epoxy resin (BE) exhibited better load-carrying and indentation recovery capacity than the other three samples. The nanoscratch results indicate that the frictional coefficient of the BE/PAEK-30 blend is the lowest, and while that of the pristine bicomponent epoxy resin is the highest, with better scratch/wear resistance.

Nanotribological Properties of Ga- and N-Faced Bulk Gallium Nitride Surfaces Determined by Nanoscratch Experiments.

Through nanoscratch experiments with a spherical diamond indenter, a contrastive study of the nanotribological properties of Ga- and N-faced gallium nitride (GaN) samples was carried out. Nanoindentation results revealed that the elastic modulus of the Ga-faced GaN sample was slightly higher than that of N-faced GaN sample. Particularly, Ga- and N-faced GaN samples exhibited rather different nanotribological properties, and the Ga-faced sample showed a stronger wear resistance. The study indicated that the critical normal load required to cause material removal of N-faced GaN sample was almost two times that of Ga-faced GaN sample. Both Ga- and N-faces exhibited a rather low frictional coefficient at the elastic and elastoplastic stages of material, e.g., ~0.06 for Ga-face and ~0.075 for N-face when scratching under the progressive normal load. Combined with transmission electron microscopy and X-ray photoelectron spectroscopy, we speculated that, except for the intrinsic atomic arrangements attributed to the non-reverse crystallographic symmetry of c-plane wurtzite GaN, the difference of nanotribological properties between Ga- and N-faces may also be related to the preferential formation of a native oxide layer and a slight lattice damage layer on the N-faced GaN surface. This study can enrich the understanding of the nanotribological properties of Ga- and N-polar-faced bulk monocrystalline GaN materials fabricated by the conventional technique.

Brushes, Graft Copolymers, or Bottlebrushes? The Effect of Polymer Architecture on the Nanotribological Properties of Grafted-from Assemblies

Surface-grafted polyzwitterions (PZW) have gained a foothold in the design of synthetic materials that closely mimic the lubricious properties of articular joints in mammals. Besides their chemical composition, the architecture of PZW brushes strongly determines their morphological, nanomechanical, and nanotribological characteristics. This emerges while comparing the properties of linear poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC) brushes with those displayed by graft copolymer and bottlebrush brushes, either featuring a low or a high content of PMPC side chains. Surface-initiated atom transfer radical polymerization (SI-ATRP) enabled the synthesis of different branched-brush architectures from multifunctional macroinitiators via multiple grafting steps, and allowed us to modulate their structure by tuning the polymerization conditions. At relatively low grafting densities (σ), long PMPC side segments extend at the interface of bottlebrush and graft copolymer brushes, providing both morphology and lubrication properties comparable to those shown by loosely grafted, linear PMPC brushes. When σ > 0.1 chains nm-2 the effect of the branched-brush architecture on the nanotribological properties of the films became evident. Linear PMPC brushes showed the lowest friction among the studied brush structures, with a coefficient of friction (μ) that reached 1 × 10-4, as measured by atomic force microscopy (AFM). Bottlebrush brushes showed comparatively higher friction, although the high content of hydrophilic PMPC side chains along their backbone substantially improved lubrication compared to that displayed by the more sparsely substituted graft copolymer brushes.

Origin of Nanoscale Friction Contrast between Supported Graphene, MoS2, and a Graphene/MoS2 Heterostructure.

Ultralow friction can be achieved with 2D materials, particularly graphene and MoS2. The nanotribological properties of these different 2D materials have been measured in previous atomic force microscope (AFM) experiments sequentially, precluding immediate and direct comparison of their frictional behavior. Here, friction is characterized at the nanoscale using AFM experiments with the same tip sliding over graphene, MoS2, and a graphene/MoS2 heterostructure in a single measurement, repeated hundreds of times, and also measured with a slowly varying normal force. The same material systems are simulated using molecular dynamics (MD) and analyzed using density functional theory (DFT) calculations. In both experiments and MD simulations, graphene consistently exhibits lower friction than the MoS2 monolayer and the heterostructure. In some cases, friction on the heterostructure is lower than that on the MoS2 monolayer. Quasi-static MD simulations and DFT calculations show that the origin of the friction contrast is the difference in energy barriers for a tip sliding across each of the three surfaces.

Nanotribological Performance Factors for Aqueous Suspensions of Oxide Nanoparticles and Their Relation to Macroscale Lubricity

Quartz crystal microbalance (QCM) measurements of nanotribological properties of statistically diverse materials combinations of nanoparticles and substrate electrodes in aqueous suspensions are reported and compared to macroscale measurements of the same materials combinations for a subset of the nanoparticle combinations. Four ceramic nanoparticles, TiO2, SiO2, Al2O3, and maghemite (γ-Fe2O3) and ten substrate materials (Au, Al, Cr, Cu, Mo, Ni, Pt, SiO2, Al2O3, and SS304) were studied. The QCM technique was employed to measure frequency and motional resistance changes upon introduction of nanoparticles into the water surrounding its liquid-facing electrode. This series of experiments expanded prior studies that were often limited to a single nanoparticle - solid liquid combination. The variations in QCM response from one nanoparticle to another are observed to be far greater than the variation from one substrate to another, indicating that the nanoparticles play a larger role than the substrates in determining the frictional drag force levels. The results were categorized according to the direction of the frequency and motional resistance changes and candidate statistical performance factors for the datasets were generated. The performance factors were employed to identify associations between the QCM atomic scale results and the macroscale friction coefficient measurements. Macroscale measurements of friction coefficients for selected systems document that reductions (increases) in motional resistance to shear, as measured by the QCM, are linked to decreases (increases) in macroscale friction coefficients. The performance factors identified in the initial study therefore appear applicable to a broader set of statistically diverse samples. The results facilitate full statistical analyses of the data for identification of candidate materials properties or materials genomes that underlie the performance of nanoparticle systems as lubricants.

Microstructure Evolution and Nanotribological Properties of Different Heat-Treated AISI 420 Stainless Steels after Proton Irradiation.

In this paper, low-energy proton irradiation experiments with different cumulative fluences were performed on samples of AISI 420 stainless steel that were either annealed or tempered at 600 or 700 °C. The effects of the cumulative proton irradiation fluence on the evolution of the microstructure of AISI 420 were studied by transmission electron microscopy (TEM). Scratch tests were performed using a Tribo Indenter nanomechanical tester, in order to investigate the effects of the cumulative fluence on the tribological properties of the AISI 420 stainless steel. The results indicate that the dislocation density of the microstructure near the surface of the AISI 420 stainless steel increases with higher cumulative proton irradiation fluences. Under the same load, the nanoscale friction coefficient and wear rate both decreased with increasing cumulative proton irradiation fluence. This indicates that the surface hardening effect induced by proton irradiation can diminish the nanoscale friction coefficient and wear rate.

Studying Nanotribological Properties of Functional Materials via Atomic Force Microscopy

Abstract—Nanotribological tests via atomic force microscopy were optimized by recording the lateral interaction forces between the probe tip and the sample surface in the contact mode. In the case of thin film metal materials, the technique enabled us to determine independently the sliding force and the static friction force, which reduced the measurement error of tribological characteristics at the nanoscale by 3—5 times.