Asperity Arrays Research Articles

Interference effect on friction behavior of asperities on single crystal copper

By using Green function molecular dynamics method, we systematically study the friction behavior of a single asperity and asperity array over the (111) surface of single crystal copper. We find that internal plastic behavior (burst of stacking faults, dislocation emission and propagation) is a promising reason for the higher value of static friction coefficient than that of dynamics friction in non-adhesive scratch. For the rough surface, however, the difference between static and dynamic friction coefficients disappear due to the interference between asperities. The interference dramatically increases the friction coefficient by introducing atomic scale plastic features (pile-up atoms, stacking faults, and U-shape dislocation loop).

Recent studies on the application of microfabrication technologies for improving tribological properties

AbstractThe relationships between nanometer‐scale surface textures of periodic asperity arrays and friction force in unlubricated conditions and the relationships between micrometer‐scale surface textures of two‐tier structures and friction force in mixed lubrication conditions were investigated. The friction force and pull‐off force were measured on the asperity array in an unlubricated condition. Each force was proportional to the radius of the curvature of the asperity peak. The pull‐off force was determined by the Laplace pressure of the capillary condensed water. The friction force was measured on the micrometer‐scale surface textures of a two‐tier structure in lubricated conditions. The friction coefficient increased with higher bearing characteristic numbers at the higher B, though the friction coefficient was constant at the lower B. The roughness of the pin surface and the geometry of the two‐tier structure affected the friction coefficient at the lower B and higher B, respectively. Copyright © 2010 John Wiley & Sons, Ltd.

Surface Forces related to the Accumulation and Dispersion of Dust due to the Locomotion of a Cantilever

Since surface forces prevent micro-machines from moving in the air, there are a lot of difficult problems to be solved concerning the reliability of micro-machines. Surface forces depend strongly on dust and contaminants. Therefore, we should be considering the attachment and detachment of micro-applications between interactive contact surfaces related to the locomotion of micro-machines. We observed that dusts have effects upon contact surface forces via the locomotion of cantilever in the air. First, we made a stainless steel cantilever with a glass sphere (dry borosilicate glass sphere, curvature radius: R=10μm) glued on top. In addition, we created an asperity array at the micro-sphere's surfaces on the stainless steel cantilever using an FIB (Focused Ion Beam). The experiment was performed using an AFM (Atomic Force Microscope). From the results, we found out that contact surface forces are influenced remarkably by dust debris in the air. The spheres with asperity arrays tend to hinder the accumulation of dust than the spheres without asperity arrays in abrading work using the same normal spring constant (=kn) of the cantilever. Moreover, among the cases with non-asperity arrays, the accumulation of dust on the sphere's surface is lower in the thin cantilever than the wide cantilever. To prevent contact surface forces from increasing by the accumulation of contaminants, micro-machines must have locomotion and asperity arrays on any surface that comes in contact with severe natural conditions.

411 集束イオンビーム加工によるナノパターンの創製とその応用(T03-3 マイクロナノ理工学:nmからmmまでのテクスチャリングの創成と機能(3),大会テーマセッション,21世紀地球環境革命の機械工学:人・マイクロナノ・エネルギー・環境)

This paper describes the fabrication methods nano-scale patterns and their applications in tribology studies. Focused ion beam (FIB) was used to create two types of asperity patterns. One is the periodic asperity array. The other is the single asperity. In each pattern, the asperity has a hemispherical peak. Three kinds of experiments were carried out using the nano-patterns. First, the pull-off force measured on each asperity pattern was proportional to the curvature radius of asperity peak. Second, a periodic asperity array was used calibrate the output from lateral force microscopy. Image calculated from topography data corresponded well with the lateral force image due to torsion of the cantilever. Finally, periodic asperity pattern was used to form a flat part on the cantilever probe by friction-induced wear.

Development of a microlateral force sensor and its evaluation using lateral force microscopy

A microlateral force sensor (MLFS) was developed and evaluated using atomic force microscopy (AFM). The sensor was attached to a sensing table supported by a suspension system. The lateral motion of the sensing table was activated by a comb actuator. The driving voltage to the comb actuator was controlled to maintain a constant position of the sensing table by detecting the tunneling current at a detector, which consisted of two electrodes where the bias voltage was applied. An AFM was used to apply a lateral force to the sensing table of the sensor. When the probe of a cantilever was pressed against the sensing table and a raster scanning was conducted, the driving voltage of the comb actuator changed to compensate the friction force between the probe and sensing table. AFM measurements of an asperity array on the sensing table were conducted, and a lateral force microscopy image (LFM) was obtained from the change in driving voltage. The image by MLFS was very similar to the LFM image that was conventionally obtained from torsion of the cantilever. The LFM image strongly correlated with the gradient image calculated from the AFM topographic image. The force sensitivity of the MLFS was determined by comparing the LFM image obtained by using the MLFS with the tangential force derived from the gradient of the AFM image.

Tribological properties of asperity arrays coated with self-assembled monolayers

The effects of a self-assembled monolayer (SAM) coating on the friction and pull-off forces were determined by using two-dimensional asperity arrays on silicon wafers. The arrays were coated with SAM composed of one of five different alkylchlorsilanes. First, two-dimensional asperity arrays were created by using a focussed ion beam (FIB) system to mill patterns on silicon plates. Each silicon plate had different patterns of equally spaced asperities. Each pattern (5 × 5 μm2) had a different radius of curvature of the asperity peaks, ranging from about 200 to 2500 nm. Then, each silicon plate was immersed in a solution of a different alkylchlorsilane in hexane (either hexyltrichlorosilane, octyltrichlorosilane, dodecyltrichlorosilane, tetradecyltrichlorosilane, or octadecyltrichlorosilane), thus coating the asperity arrays with SAM. The friction and pull-off forces on the SAM-coated arrays were measured by using an atomic force microscope (AFM) that had a square flat probe. The pull-off force for SAM-coated silicon was roughly proportional to the radius of curvature of the asperity peaks. The magnitude of the pull-off force corresponded approximately to the capillary force calculated by using the contact angle of water on the surface of SAM. The friction coefficient correlated with the inverse of the alkyl-chain length of the SAM.

Tribological Properties of LB films Measured under Zero Applied Load

We examined the pull-off and friction forces between an atomic force microscope (AFM) probe and submicron-scale asperities covered with Langmuir Blodgett (LB) films. First, we used a focused ion beam to produce several kinds of asperity arrays on two silicon substrates. The average radius of curvature was 95 to 800 nm, depending on the asperity array. An LB film of stearic acid (C17H35COOH) or fluoro-carboxylic acid (C6F13C11H22COOH) was deposited on each substrate. The friction force was measured at each asperity array under a zero applied load. The pull-off force was measured at each asperity array and also at controlled relative humidities of 3 to 73%. The results showed that the pull-off and friction forces were almost proportional to the radius of the curvature of the asperity peak with both stearic acid and fluoro-carboxylic acid LB films. The gradient of the friction force against the pull-off force, which corresponded to the friction coefficient, was 0.021 and 0.14 for the stearic acid and fluoro-carboxylic acid respectively. The pull-off force increased with higher relative humidity on the fluoro-carboxylic acid. We concluded that differences in the pull-off and friction forces between the two kinds of LB films were mainly caused by differences in the stiffness of the LB film.

Development of Micro Lateral Force Sensor Using Tunnelling Current

A micro lateral force sensor has been developed. The sensor equipped a traveling table supported by suspensions. The lateral motion of the traveling table was activated by comb actuator. The applied voltage to the comb actuator was controlled to keep the position of the traveling table constant by detecting tunnelling current at a gap between two electrodes that were arranged on the traveling table and the fixed plate, respectively. An AFM (atomic force microscope) was used to apply a lateral force on the traveling table of the sensor. When the probe of a cantilever was pressed against the traveling table and a raster scanning was conducted, the driving voltage of the comb actuator was changed to compensate the friction force between the probe and traveling table. The driving voltage was captured into an external channel of the AFM system and recorded during the scanning. Friction force image of asperity array was obtained from the driving voltage, which was as same as the image obtained from torsion of the cantilever. The force sensitivity of the lateral force sensor was comparable to sensitivity from torsion of the cantilever.

周期的突起を有する表面間に作用する凝着力の測定(OS8-2 マイクロ機構とトライボロジー(II))

周期的突起を有する表面間に作用する凝着力の測定(OS8-2 マイクロ機構とトライボロジー(II))

Relation Between Friction and Plastic Deformation Examined by Using Periodic Asperity Arrays Fabricated on an AFM Multipurpose Cantilever

The relation between friction force and plastic deformation was determined by using FIB (focused ion beam)-processed multipurpose cantilevers for an AFM (atomic force microscope). First, a multipurpose cantilever consisting of a sliding block, parallel leaf spring, and hinge was made by using an FIB. Then, copper pyramids were rubbed against an unmodified area or periodic asperity arrays fabricated on the cantilever under loads of 23 to 960 nN. Under higher loads (∼900 nN), the friction coefficient measured for an asperity array was lower than that for an unmodified area and decreased with decreasing curvature radius of the asperity peak, and the specific volume change rate increased with decreasing curvature radius of the asperity peak.

Relationships of Friction, Pull-off Forces and Nanometer-scale Surface Geometry.

Friction and pull-off forces were measured between an atomic force microscope (AFM) probe made of Si3N4 and submicron asperity arrays on a single-crystal tungsten plate, a silicon wafer, and a platinum film. A focused ion beam was used to produce the two-dimensional asperity arrays. The tip of the AFM probe had a flat, square surface measuring 0.7×0.7 μm2. The pull-off force was proportional to the curvature radius of the asperity peak, but was not affected by micro-roughness at the peak. The friction force was proportional to the pull-off force. These findings suggest that the Laplace pressure of the condensed water was the predominant pull-off force and that the pull-off (adhesion) force worked as an external normal load for the friction. The friction coefficients calculated by regarding the pull-off force as an external normal load were compared for the platinum and silicon patterns, and this proved that the friction coefficient on the silicon was about twice as high as on the platinum.

The effect of relative humidity on friction and pull-off forces measured on submicron-size asperity arrays

The friction and pull-off forces were measured between atomic force microscope (AFM) probe and submicron-size asperity arrays on a silicon wafer in various relative humidities. First, a focused ion beam (FIB) was used to produce two-dimensional arrays in which asperities were arranged at uniform intervals of 240 nm. Independent asperities were also processed for comparison with the results for asperity arrays. The two kinds of arrays were made by milling grooves in two perpendicular directions at uniform intervals. Each asperity peak in contact with the probe had a radius of curvature of 168 and 452 nm, respectively. The tip of the AFM probe had a flat, square surface measuring 0.7×0.7 μm2. The pull-off forces were measured by using force curve mode in the AFM system. Measurements were carried out 256 or 1024 times in various humidities and on each array. The average pull-off force increased with relative humidity on each asperity array. On independent asperities, however, the pull-off force did not change with relative humidity. Histograms of pull-off force measured on the asperity arrays showed that the position of the maximum peak along the horizontal axis, i.e., the mode of pull-off force, did not change with relative humidity and that the other peaks at higher pull-off forces grew with relative humidity. The intervals between the adjacent peaks were almost the same. These findings suggest that the pull-off force on one asperity was the same but the number of contacting points between the flat probe and asperity array increased with relative humidity. The friction force was measured on the asperity arrays while changing the sliding speed in various relative humidities. The friction force increased with higher relative humidity and lower sliding speed. These findings can be explained by the viscous resistance and lubricating effect of condensed water.

Relations Among Friction and Pull-Off Forces and Surface Geometry in Nano Meter-Scale.

The friction and pull-ff forces were measured between an atomic force microscope (AFM) probe made of Si3N4 and submicron-size asperity arrays on a single crystal tungsten plate, a silicon wafer, and a platinum film. A focused ion beam was used to produce two-dimensional asperity arrays. The tip of the AFM probe had a flat, square surface measuring 0.7×0.7 μm2. The pull-off force was proportional to the curvature radius of asperity peak, but was not affected by microroughness on the Peak. The friction force was proportional to Pull-off force. These suggest that Laplace pressure of condensed water was predominant as the pull off force and the Pull-off (adhesion) force worked as an extemal normal load for the friction. The friction coefficients calculated by regarding the pull-off force as the external normal load were compared between the platinum and silicon patterns, which proved that the friction coefficient on silicon was about twice higher than platinum.

Recent Progress in Microtribology. Micromachines and Tribology.

As the dimensions of micromachine components decrease, the surface area decreases in proportion to the square of the linear decrease while the volume decreases in proportion to the cube power of the linear decrease. Therefore, the gravity and inertia force proportional to the volume are negligible but the influences of the attraction and friction forces proportional to the surface area are predominant in micromachines. This paper describes some examples of applications of micromachine technologies such as a microfactory and a microsatellite and introduces products of MEMS (microelectromechanical systems). The tribological problems in micromachines are then discussed. First, we show that the attraction force increases the friction force and tribological phenomena in a micro scale are different from the macro tribology. Second, we show a reduction of friction force by creating regular asperity arrays. Finally, the relation between the electrostatic charge and friction is discussed.

The Effect of Asperity Array Geometry on Friction and Pull-Off Force

The friction and pull-off forces between regular asperity arrays with various heights on a silicon wafer and a scanning probe of an atomic force microscope (AFM) were measured. We used two-dimensional periodic asperity arrays. The arrays were created by using a focused ion beam (FIB) to mill patterns on a silicon plate and on a platinum layer deposited on a silicon plate. For both materials, the distance between adjacent peaks was about 240 nm and the groove depth ranged from about 3 to 49 nm. The probe of the AFM was a square flat, 0.7 × 0.7 μm2. For the silicon array, the pull-off force decreased with increasing groove depth and was proportional to the radius of curvature of the asperity. The friction force also decreased with asperity height and was proportional to both the asperity curvature and the pull-off force. For the platinum asperity array, although both the pull-off and friction forces also decreased with groove depth, the friction coefficient (calculated by dividing the friction force by the pull-off force) was about half that of the silicon asperity array.