• Tribology and Lubricants
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• v.11 no.5
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• pp.128-135
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• 1995

Atomic force microscopy/friction force microscopy (AFM/FFM) techniques are increasingly used for tribological studies of engineering surfaces at scales, ranging from atomic and molecular to microscales. These techniques have been used to study surface roughness, adhesion, friction, scratching/wear, indentation, detection of material transfer, and boundary lubrication and for nanofabrication/nanomachining purposes. Micro/nanotribological studies of single-crystal silicon, natural diamond, magnetic media (magnetic tapes and disks) and magnetic heads have been conducted. Commonly measured roughness parameters are found to be scale dependent, requiring the need of scale-independent fractal parameters to characterize surface roughness. Measurements of atomic-scale friction of a freshly-cleaved highly-oriented pyrolytic graphite exhibited the same periodicity as that of corresponding topography. However, the peaks in friction and those in corresponding topography were displaced relative to each other. Variations in atomic-scale friction and the observed displacement has been explained by the variations in interatomic forces in the normal and lateral directions. Local variation in microscale friction is found to correspond to the local slope suggesting that a ratchet mechanism is responsible for this variation. Directionality in the friction is observed on both micro- and macro scales which results from the surface preparation and anisotropy in surface roughness. Microscale friction is generally found to be smaller than the macrofriction as there is less ploughing contribution in microscale measurements. Microscale friction is load dependent and friction values increase with an increase in the normal load approaching to the macrofriction at contact stresses higher than the hardness of the softer material. Wear rate for single-crystal silicon is approximately constant for various loads and test durations. However, for magnetic disks with a multilayered thin-film structure, the wear of the diamond like carbon overcoat is catastrophic. Breakdown of thin films can be detected with AFM. Evolution of the wear has also been studied using AFM. Wear is found to be initiated at nono scratches. AFM has been modified to obtain load-displacement curves and for nanoindentation hardness measurements with depth of indentation as low as 1 mm. Scratching and indentation on nanoscales are the powerful ways to screen for adhesion and resistance to deformation of ultrathin fdms. Detection of material transfer on a nanoscale is possible with AFM. Boundary lubrication studies and measurement of lubricant-film thichness with a lateral resolution on a nanoscale have been conducted using AFM. Self-assembled monolyers and chemically-bonded lubricant films with a mobile fraction are superior in wear resistance. Finally, AFM has also shown to be useful for nanofabrication/nanomachining. Friction and wear on micro-and nanoscales have been found to be generally smaller compared to that at macroscales. Therefore, micro/nanotribological studies may help def'me the regimes for ultra-low friction and near zero wear.

• Journal of Mechanical Science and Technology
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• v.17 no.3
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• pp.350-356
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• 2003

In this study, a numerical analysis for the piston secondary dynamics of small refrigeration reciprocating compressors is performed. In general, the length of cylinder in this class of compressors is shortened to diminish the frictional losses of the piston-cylinder system. So, the contacting length between piston and cylinder wall is in variable with the rotating crank angle around the BDC of the reciprocating piston. In the problem formulation of the piston dynamics, the variation in bearing length of the piston and all corresponding forces and moments are considered in order to determine the piston trajectory, velocity and acceleration at each step. A Newton-Raphson procedure was employed in solving the secondary dynamic equations of the piston. The developed computer program can be used to calculate the entire piston trajectory and the lubrication characteristics as functions of crank angle under compressor running conditions. The results explored the effects of some design parameters and operating conditions on the stability of the piston, the oil leakage, and friction tosses.

• Tribology and Lubricants
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• v.22 no.6
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• pp.335-341
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• 2006

This paper reports the wear characteristics of two types of coating materials, which are $Cr_2O_3$ and $ZrO_2$, by coated plasma thermal spray method. The wear test was carried out under air, grease, and bearing fluid conditions. The wear testing machine of a pin-on disk type were used to measure friction forces, friction coefficients and the weight losses of the coating specimens on the various sliding velocity and loading condition. The wear surface of specimens were observed by scanning electron microscope (SEM) photographs.

• Tribology and Lubricants
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• v.19 no.3
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• pp.146-150
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• 2003

The natural refrigerant $CO_2$has attracted as an alternative refrigerant currently used in air conditioning system, which has high global warming potential. In this study, the tribological characteristics of the sliding surfaces between a fixed scroll and an orbiting scroll of the scroll compressor were investigated in $CO_2$/ POE mixed environment. The pin-on-disk type sliding tests were carried out under the various sliding speeds, normal loads. surface roughness, and pressures. During the test, friction forces, wear amount and surface temperature were monitored.

• Tribology and Lubricants
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• v.17 no.5
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• pp.386-390
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• 2001

Monolayers such as self-assembled monolayer (SAM) have received considerable attention to reduce stiction and friction in micro-devices and microelectromechanical systems (MEMS). Various organic molecular films were investigated to obtain better understanding of their tribological behaviors and adhesion property. The organic molecular films studied in this work are: epoxysilane SAMs, octadecyltricholosilane (OST), multi-layers composed of epoxysilane SAMs, poly[styrene-b-(ethylene-co-butylene)-b-styrene](SEBS) and compound of epoxy resin and poly (paraphenylene)(EP/PPP). The pull-off forces of these films were also obtained from force-distance curves measured in static mode of operation of atomic force microscope(AFM). Tribological tests were conducted with a ball-on-flat reciprocating friction tester. The OST showed the lowest pull-off force, indicating its low adhesion property. It was revealed that, the OST, EP/PPP and the multi-layer of epoxysilane SAMs, SEBS and EP/PPP exhibited good tribological properties at the lower load (0.3 N) whereas the OST showed best performance at the higher load (1.8 N).

• Proceedings of the Korean Society of Tribologists and Lubrication Engineers Conference
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• 2002.10b
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• pp.225-226
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• 2002

In this work, the friction and wear behavior under' various lubrication regimes were investigated. The objective of this work is to develop an Accelerated Life Test (ALT) method for the durability evaluation of a machine element which is operated under lubrication. Electric contact resistance and frictional forces were measured with respect to a wide range of the loads and speeds under various lubrication regimes using a pin-on-disk type tribotester. From the experimental results, it could be found that an effective and reliable ALT method could be achieved by controlling the lubrication regime through the measurements of friction coefficient and contact resistance with respect to load and sliding speed.

• Korean Journal of Computational Design and Engineering
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• v.20 no.3
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• pp.230-237
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• 2015

Soil deformation on the off-load ground is significantly affected by soil conditions, such as soil type, water content, and etc. Thus, the soil characteristics should be estimated for predicting vehicle movements on the off-load conditions. The plate-sinkage test, a widely-used experimental test for predicting the wheel-soil interaction, provides the soil characteristic parameters from the relationship between soil stress and plate sinkage. In this study, soil stress under the plate-sinkage situation is calculated by the DEM (Discrete Element Method) model. We developed a virtual soil bin with DEM to obtain the vertical reaction forces under the plate pressing the soil surface. Also parametric studies to investigate effects of DEM model parameters, such as, particle density, Young's modulus, dynamic friction, rolling friction, and adhesion, on the characteristic soil parameters were performed.

• Transactions of the Korean Society of Mechanical Engineers
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• v.15 no.2
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• pp.404-412
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• 1991

In the design of high speed machinery, designers must consider the problem of possible structural failure due to excessive dynamically varying stresses, which are induced by the varying external loads and internal inertia forces, in the links of the mechanism. A study of the dynamically induced stresses would indicate what values of the minimum permissible fatigue strength should be for safe mechanism operation. This paper investigates the nature of the stress fluctuation in high speed mechanism on the basis of the effects of both the loads and the friction. The latter is apt to be neglected in the usual analysis in spite of the fact that it is always generated in the operating machinery. The analysis is performed on the coupler of the slider-crank mechanism for illustrative purposes and the results are expressed in a non-dimensional form for design applications.

• Proceedings of the Korean Society of Precision Engineering Conference
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• 1993.10a
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• pp.22-27
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• 1993

The finite element method is applied to analyze the mechanism of metal cutting. This paper introduces some effects, such constitutive deformation laws of workpiece material, friction of tool-chip contact interfaces, tool rake angles and also simulate the cutting process, chip formation and geometry, tool-chip contact, reaction force of tool, cutting temperature. Under the usual [lane strain assumption, quasi-static analysis were performed with variation of tool-chip interface friction coefficients and rake angles. In this analysis, various cutting speeds and depth of cut are adopted. Some cutting parameters are affected to cutting force, plastic deformation of chip, shear plane angle, chip thickness and tool-chip contact length and reaction forces on tool. Cutting temperature and Thermal behavior. Several aspects of the metal cutting process predicted by the finite element analysis provide information about tool shape design and optimal cutting conditions.

• Proceedings of the Korean Society of Precision Engineering Conference
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• 2005.06a
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• pp.1151-1154
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• 2005

The size effects for adhesive and frictional characteristics were studied. The specimen was Mica and the AFM tips were SiO2. The radii of SiO2 tip were 280, 380, 930, and 2230 nm on which tribological tests had never been performed. It was found that the adhesive forces and the frictional coefficients increased non-linearly with tip radius. Compared with previous studies at nanoscale and microscale, the results showed behaviors bridging each previous result. It could be said that these results were clues to explain the material behaviors between nanoscale and microscale both in adhesion and friction.