Asperity arrays

In this study, we clarified these friction-reduction effects under microload conditions by measuring the friction and pull-off forces for two-dimensional asperity arrays on silicon plates. First, two-dimensional asperity arrays were created using a focused ion bean (FIB) system to mill patterns on single-crystal silicon plates. Each silicon plate had several different patterns of equally spaced asperities. Then, the friction and pull-off forces were measured using an atomic force microscope (AFM) that had a square, flat probe. This report describes the geometry effects of creating asperity arrays and the chanical effects of depositing LB films or SAMs on the friction and puU-off forces. 

CREATION OF PERIODIC ASPERITY ARRAYS BY FOCUSED ION BEAM... 

Figures 2.1-2.4 show examples of periodic asperity arrays used in this experiment, which were measured by a conventional sharp probe that had a nominal radius of curvature of less than 40 nm. Figures 2.1 and 2.2 are two-dimensional...

FIGURE 2.4 AFM image of asperity array fabricated on platinum layer that was deposited by FIB (6.5-nm groove depth). 

Some asperity arrays were selected and the curvature radii of the asperity peaks were calculated to examine the effects of contact geometry in a different perspective. The patterns on test specimen no. 3 in table 2.1 were selected because the curvature radii for the array patterns in this specimen were distributed widely. 

TABLE 2.2 Geometry of Asperity Array Patterns used for determining geometry effects (specimen no. 3) ... 

In order to determine the effect of chemical modification on the friction and pull-off forces, self-assembled monolayers (SAMs) and Langmuir-Blodgett (LB) films were formed on the silicon surface. Films were formed on the asperity arrays having various groove depths, where the distance between the adjacent peaks was about 240 mn for each asperity array. The SAM and LB films were deposited as follows. 

Each plate was cleaned and then coated with a SAM of alkylchlorosilane in a three-step process. First, to remove chemically or physically adsorbed contaminants, the plate was cleaned with a so-called piranha solution (3/7 v/v mixture H2O2/ H2SO4) at about 70°C for 2 h and then with a 4/6 v/v mixture of benzene/ethanol for 48 h. After each cleaning process, the plate was rinsed with high-purity water. Second, to remove surface contaminants, the plate was placed in a UV (ultraviolet)/ O3 cleaner (UV output of 25 W) and exposed for 10 min. Finally, to form the SAM coating, the plate was immersed for about 5 s in a 0.5 mM solution of, for example, octadecyltrichlorosilane (CH3(CH2)nSiCl3 Cig) in hexane. We also prepared a control plate that contained an asperity array without a coating (Cq). 

LB films were also formed on the silicon plate after the asperity array was processed by FIB. Before depositing the LB film, we cleaned the silicon wafers in a mixture of benzene and ethanol, rinsed them in pure water, and then exposed them to a UV-ozone atmosphere. Then the plate was immersed in ultrapure water where a monolayer of stearic acid (Ci7H35COOH CH) or fluorocarboxylic acid (QFi3CiiH22COOH CFCH) was confined at a pressure of 30 mN/m. The monolayer on the water migrated onto the silicon surface and formed the LB film (CH-/CFCH-LB film) when the plate was removed from the ultrapure water. The temperature of the ultrapure water was 20°C. Table 2.3 shows the chemical modifications and the geometries of the asperity. 

FIGURE 2.10 Friction and pull-off forces measured on a silicon periodic asperity array, (a) Pull-off forces were measured on each pattern without surface scanning between the measurements. (b) Pull-off forces were measured before and after each friction measurement at the same scanning area and were averaged. 

FIGURE 2.12 Depth distribution of silicon asperity arrays. The relations between the area and depth are calculated from topography data of a scanning range of 2 x 2 pm. The underlined dimension shows the groove depth of each pattern. 

FIGURE 2.13 Relation between friction force and puU-off force measured on two kinds of asperity arrays. Friction forces are extracted from figs. 2.10b and 2.11 and are shown as solid circles and open circles, respectively. 

FIGURE 2.14 Friction and pull-off forces measnred on platinnm asperity arrays as shown in fig. 2.3. (a) Relation between pull-off force and groove depth and (b) effect of asperity height on friction and pull-off forces... 

FIGURE 2.17 Relation between friction force and pull-off force measured on a platinum asperity array as shown in fig. 2.15. Friction and pull-off forces are extracted from fig. 2.16. The maximum friction force of 100 nN (shown as O) was measured at the asperity height of 0 nm in fig. 2.6, which means the friction force was measured on a bare silicon surface without... 

FIGURE 2.18 Comparison of friction and pull-off forces between silicon and platinum asperity arrays. The platinum asperity arrays were fabricated near the silicon asperity arrays on the same silicon plate, which makes it possible to directly compare the difference in the friction and pull-off forces between two different materials. 

FIGURE 2.19 Comparison of friction force vs. pull-off force on platinum and silicon asperity arrays using data from fig. 2.18. The friction forces measured on each material were fitted with a line that passes through the origin. The gradient of each approximated line is shown in the parentheses in the inset box. 

FIGURE 2.22 Relation between pnll-off force and curvature radius of asperity peak measured on silicon asperity arrays of various groove depths. The pull-off forces were derived from fig. 2.10. The curvature radii were calculated from the AFM data by using a hemisphere approximation program. 

Pull-Off Forces on Asperity Arrays Covered with SAMs... 

The experiment in the previous section revealed that the capillary force was predominant between the flat probe and the periodic asperity array. In fig. 2.21, the capillary geometry shows that a capillary can be formed even if one of the surfaces is hydrophobic, and eq. (2.7) shows that the adhesion force exists between the surfaces when 01 H- 02 < 180°. Thus, the water capillary could form between the hydrophilic probe and SAM-coated asperity peaks, even though the SAM-coated surfaces are hydrophobic and the adhesion force is given by eq. (2.7). 

Figure 2.25 shows the pull-off force measured for the asperity arrays covered with two kinds of LB films. The curvature radius on the jc-axis was shown in table 2.3. The data for each plate with CHCCnHjjCOOHl-LB or CFCH(C6F,3C H22COOH)-LB film were fitted with a line passing through the origin. The pull-off force decreased with smaller curvature radius and was roughly proportional to the curvature radius. The pull-off force on the CH-LB film was about l/5th of the pull-off force on the CFCH-LB film for the same curvature radius. 

Figure 2.26 shows the pull-off forces on two kinds of asperity arrays as a function of the relative humidity. The average curvature radius of each asperity array was 150 and 440 nm for the CH-LB film and 95 and 370 nm for the CFCH-LB film. Each plot in fig. 2.26 is the average of 256 pull-off force measurements. The error bar shows the standard deviation for each data point. The average pull-off force clearly increased with higher relative humidity for the asperity array of 370-nm radius with the CFCH-LB film... 

FIG U RE 2.26 Relation between pull-off forces and relative humidity measured on asperity arrays. Each plot shows the average of pull-off forces from 256 measurements on (a) CH-LB film and on (b) CFCH-LB film. Two asperity arrays were selected from each plate, and their curvature radii are shown in the inset boxes. 

Figure 2.29 shows the friction force measured for the asperity arrays covered with LB films as a function of the curvature radius of the asperity peaks. The error... 

Various patterns of two-dimensional asperity arrays were created by using FIB to deposit platinum asperities and to mill patterns on silicon plates and on a platinum layer deposited on the silicon plate. The pull-off and friction forces between the respective patterns and a flat scanning probe of an AFM were measured. Our findings are as follows ... 

The pull-off force decreased due to the SAM or LB film coatings on the asperity arrays. The magnitude of the pull-off force approximately corresponded to the capillary force calculated using the contact angle of water on the surface. 

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