• Tribology and Lubricants
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• 제36권2호
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• pp.55-63
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• 2020

Recently, graphene has attracted considerable attention owing to its unique electrical, optical, thermal, and mechanical properties. The broad spectrum of applications from optics, sensors, and electronics to biodevice have been proposed based on these properties. In particular, graphene has been proposed as a protective coating layer and solid lubricant for microdevices and nanodevices because of its high mechanical strength, chemical inertness, and low friction characteristics. During the past decade, extensive efforts have been made to explore the tribological characteristics of graphene under various conditions and to expand its applicability. In addition to the experimental approaches, the molecular simulations performed provide fundamental insights into the friction and wear characteristics of graphene resulting from molecular interactions. This work is a review of the studies conducted over the past decade on the tribological characteristics of graphene using molecular simulation. These studies demonstrate the principal mechanisms of the superlubricity of graphene and help clarify the influences of surface conditions on tribological behavior. In particular, the investigation of the effects of the number of layers, strength of adhesion to the substrate, surface roughness, and commensurability provides deeper insights into the tribological characteristics of graphene. These fundamental understandings can help elucidate the feasibility of graphene as a protective coating layer and solid lubricant for microdevices and nanodevices.

• Tribology and Lubricants
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• 제30권5호
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• pp.291-294
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• 2014

Multiple additives can help improve the performance of generally used lubricants. These additives include MoS2, cadmium, chloride, indium, sulfide, and phosphide, which are harmful to both humans and the environment. Thus, researchers in this industry have been trying to reduce the use of these additives by finding alternatives. Nanodiamonds are one of these candidates. Nanodiamond particles are very hard, chemically stable, and highly heat-conductive. This research involved uniformly dispersing nanodiamond particles in marine engine oils via a matrix synthesis method at various concentrations (0, 0.1, 0.3, 0.5, and 1.0 wt). Friction and wear tests involved constant loads on ball-on-disk specimens, where the ball was AISI 51200 steel, the disk was AISI 1020 steel, and the sliding speed was 0.217 m/s. The lowest wear occurred at a suitable concentration of nanodiamonds (0.3 wt). However, excessive amounts of nanodiamonds caused them to act as abrasive debris because of their hardness, which increased the wear amount. The friction coefficient decreased as the nanodiamond concentration increased because their octagonal, almost spherical shape caused them to act as rolling contact elements between two surfaces.

• Tribology and Lubricants
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• 제27권1호
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• pp.7-12
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• 2011

Interest in advanced machining process such as AJM(abrasive jet machining) and CMP(chemical-mechanical polishing) using micro/nano-sized abrasives has been on the increasing demand due to wide use of super alloys, composites, semiconductor and ceramics, which are difficult to or cannot be processed by traditional machining methods. In this paper, the effects of pressure, wafer moving velocity and fluid viscosity were investigated by 2-dimensional finite element analysis method considering slurry fluid flow. From the investigation, it could be found that the simulation results quite corresponded well to the Preston's equation that describes pressure/velocity dependency on material removal. The result also revealed that the stress and corresponding material removal induced by the collision of particle may decrease under relatively high wafer moving speed due to the slurry flow resistance. In addition, the increase in slurry fluid viscosity causes the reduction of material removal rate. It should be noted that the viscosity effect can vary with the shape of abrasive particle.

• Tribology and Lubricants
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• 제25권4호
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• pp.207-216
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• 2009

Friction tests were carried out in order to investigate the effects of temperature on the friction and wear behaviors between a PMMA film and a fused silica lens using a microtribometer. The friction forces on the PMMA film were measured under atmospheric condition as the temperature of the film was increased from 300 K to 443 K. The contact area between the film and the lens was observed. The tribological characteristics of the film were significantly changed as the temperature increased. The changes were discussed with the change of the film state from glassy to viscous flow. In addition, the results showed that the friction behavior can be varied with the thermal history of the PMMA film. Residual solvent in the PMMA film could emerge to the PMMA surface due to an additional heating and the solvent on the film surface decreased the friction force.

• Tribology and Lubricants
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• 제28권5호
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• pp.203-211
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• 2012

Atomic force microscopy (AFM) has been used as a tool, not only for imaging surfaces, but also for measuring surface forces and mechanical properties at the nano-scale. Force calibration is crucial for quantitatively measuring the forces that act between the AFM probe of a force sensing cantilever and a sample. In this work, the lateral force calibrations of a V-shaped cantilever were performed using the finite element method, multiple pivot loading, and thermal noise methods. As a result, it was shown that the multiple pivot loading method was appropriate for the lateral force calibration of a V-shaped cantilever. Further, through crosschecking of the abovementioned methods, it was concluded that the thermal noise method could be used for determining the lateral spring constants as long as the lateral deflection sensitivity was accurately determined. To obtain the lateral deflection sensitivity from the sticking portion of the friction loop, the contact stiffness should be taken into account.

• Tribology and Lubricants
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• 제39권5호
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• pp.216-219
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• 2023

This study employs molecular dynamics simulations to investigate the material behavior of workpieces in waterjet machining processes. To gain fundamental insights into waterjet machining, simulations were conducted using pure water, excluding abrasive particles. The simulation model comprised thousands of water molecules interacting with a single crystal metal workpiece. Water molecule clusters were imparted with various velocities to initiate collisions with the metal workpiece. The material behavior of the metal surface was analyzed with respect to the applied velocity conditions, considering the intricate interplay between water molecules and the workpiece at the atomic scale. The results demonstrated that the machining of the metal workpiece occurred only when water molecules were endowed with velocities above a certain threshold. In cases where energy was insufficient, the metal workpiece exhibited a slight increase in surface roughness due to mild plastic deformation, without undergoing substantial material removal. When machining occurred, the ejection of material revealed a 3-fold symmetric pattern, confirming that material removal in waterjet machining of the metal workpiece is primarily driven by plastic deformation-induced material ejection. This research provides crucial insights into the mechanisms underlying waterjet machining and enhances our understanding of material behavior during the process. The findings can be valuable in optimizing waterjet machining techniques.

• Tribology and Lubricants
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• 제23권6호
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• pp.288-297
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• 2007

Adhesion and friction tests were carried out in order to investigate the effect of temperature on the tribological characteristics of poly (methylmethacrylate) (PMMA) film using AFM. The pull-off and friction forces on the PMMA film were measured under a high vacuum condition (below $1{\times}10^{-4}$ Pa) as the temperature of the PMMA film was increased from 300 K to 420 K (heating) and decreased to 300K (cooling). Friction tests were also conducted in both high vacuum and air conditions at room temperature. When the temperature was 420 K, which is 25 K higher than the glass transition temperature $(T_g)$ of PMMA, the PMMA film surface became deformable. Subsequently, the pull-off force was proportional to the maximum applied load during the pull-off force measurement. In contrast, when the temperature was under 395 K, the pull-off force showed no correlation to the maximum applied load. The friction force began to increase when the temperature rose above 370 K, which is 25 K lower than the $T_g$ of PMMA, and rapidly increased at 420 K. Decrease of the PMMA film stiffness and plastic deformation of the PMMA film were observed at 420 K in force-displacement curves. After the heating to 420 K, the fiction coefficient was measured under the air condition at room temperature and was found to be lower than that measured before the heating. Additionally, the RMS roughness increased as a result of the heating.

• 한국윤활학회:학술대회논문집
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• 한국윤활학회 1999년도 제29회 춘계학술대회
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• pp.67-72
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• 1999

Scanning probe Microscope(SPM) such as Scanning Tunneling Microscope(STM) and Atomic Force Microscope(AFM) was shown to be the powerful tool for nano-scale characterization of material surfaces Using this technique, surface morphology of the cyclically deformed Cu or Cu-Al single crystal was observed. The surface became proportionately rough as the number of cycles increased, but after some number of cycles no further change was observed. Slip steps with the heights of 100 to 200 nm and the widths of 1000 to 2000 nm were prevailing at the stage. The slipped distance of one slip system at the surface was not uniform. and formation of the extrusions or intrusions was assumed to occur such place. By comparing the morphological change caused by crystallographic orientation, strain amplitude, number of cycles or stacking fault energy, some interesting results which help to clarify the basic mechanism of fatigue damage were obtained. Furthermore, applicability of the scanning tunneling microscopy to fatigue damage is discussed.

• 한국윤활학회:학술대회논문집
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• 한국윤활학회 2004년도 학술대회지
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• pp.283-288
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• 2004

The measurement of ultra low aspect ratio fluid film thickness is very crucial technique both for the verification of lubrication media characteristics and for the clearance design in many precision components such as MEMS, precision bearings and other slideways. Many technologies are applied to the measurement of ultra low aspect ratio fluid film thickness (i.e. elastohydrodynamic lubrication film thickness). In particular, in-situ optical interferometric method has many advantages in making the actual contact behaviors realized with the experimental apparatus. This measurement method also does the monitoring of the surface defects and fractures happening during the contact behavior, which are delicately influenced by the surface conditions such as load, velocity, lubricant media as well as surface roughness. Careful selection of incident lights greatly enhances the fringe resolutions up to $\~1.0$ nanometer scale with digital image processing technology. In this work, it is found that coaxial aligning trichromatic incident light filtering system developed by the author can provide much finer resolution of ultra low aspect ratio fluid film thickness than monochromatic or dichromatic incident lights, because it has much more spectrums of color components to be discriminated according the variations of film thickness. For the measured interferometric images of ultra low aspect ratio fluid film thickness it is shown how the film thickness is finely digitalized and measured in nanometer scale with digital image processing technology and space layer method. The developed measurement system can make it possible to visualize the contact deformations and possible fractures of contacting surface under the repeated loading condition.

• 한국진공학회:학술대회논문집
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• 한국진공학회 2013년도 제44회 동계 정기학술대회 초록집
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• pp.159-159
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• 2013

Atomically thin graphene is the ideal model system for studying nanoscale friction due to its intrinsic two-dimensional anisotropy. Furthermore, modulating its tribological properties could be an important milestone for graphene-based micro and nano-mechanical devices. Here, we report that the tribological properties can be easily altered via simple chemical modifications of the graphene surface. Friction force microscopy measurements show that hydrogenated, fluorinated, and oxidized graphenes exhibit, 2-, 6-, and 7-fold enhanced nanoscale friction on their surfaces, respectively, compared to pristine graphene. The measured nanoscale friction should be associated with the adhesive and elastic properties of the chemically modified graphenes. Density functional theory calculations suggest that, while the adhesive properties of chemically modified graphenes are marginally reduced down to ~30%, the out-of-plane elastic properties are drastically increased up to 800%. Based on these findings, we propose that nanoscale friction on graphene surfaces is characteristically different from that on conventional solid surfaces; stiffer graphene exhibits higher friction, whereas a stiffer three-dimensional solid generally exhibits lower friction. The unusual friction mechanics of graphene is attributed to the intrinsic mechanical anisotropy of graphene, which is inherently stiff in plane, but remarkably flexible out of plane. The out-of-plane flexibility can be modulated up to an order of magnitude by chemical treatmentof the graphene surface. The correlation between the measured nanoscale friction and the calculated out-of-plane flexibility suggests that the frictional energy in graphene is mainly dissipated through the out-of-plane vibrations, or the flexural phonons of graphene.