Search

Abstract: Silicon (Si (100)) is a typically used material in micro/nano-scale devices, such as micro/nano-electromechanical systems (MEMS/NEMS). However, Si (100) does not have good tribological properties and hence its surface needs to be treated either chemically or topographically to enhance its tribological performance. In this paper, the micro/nano-frictional property of chemically and topographically modified Si (100) surfaces was studied. Chemically modified surfaces of Si (100) include coating of diamond-like carbon (DLC) films (two different thicknesses) and two self-assembled monolayers (SAMs). Topographically modified surfaces of Si (100) include nano-patterned poly(methyl methacrylate) (PMMA) on silicon wafer, fabricated by the process of a capillarity-directed soft lithographic technique. At the nano-scale, friction was measured using an atomic force microscope (AFM) and at the micro-scale it was measured using a ball-on-flat type micro-tribotester. Results showed that at both nano- and micro-scales, the modified Si (100) surfaces exhibited enhanced friction behavior when compared to bare Si (100) surfaces. The improved nano-friction behavior of the modified surfaces was attributed to their lower intrinsic adhesion and reduced real area of contact. In the case of nano-patterns, the physical (geometrical) reduction in contact area contributed in decreasing their friction. At micro-scale, wear was observed in the test samples (except in the case of SAMs), which influenced their friction behavior. Further, as a novel bio-mimetic approach for tribological application at micro-scale, the surface topography of natural leaves of Lotus and Colocasia were replicated by capillary force lithography using two different molding techniques. Interestingly, these bio-mimetically engineered surfaces exhibited superior micro-friction behavior. Indeed, this could be the first bio-mimetic approach of creating effective tribological materials by the direct replication of natural surfaces. [Copyright &y& Elsevier]

Abstract: In this paper, we report on the replication of surfaces of Lotus and Colocasia leaves onto thin polymeric films using a capillarity-directed soft lithographic technique. The replication was carried out on poly(methyl methacrylate) (PMMA) film spin coated on silicon wafer using poly(dimethyl siloxane) (PDMS) molds. The friction properties of the replicated surfaces were investigated at micro-scale in comparison with those of PMMA thin film and uncoated silicon wafer. The coefficients of friction of the replicated surfaces were almost five times lower than those of the PMMA thin film and four times lower than those of the uncoated silicon wafer. The superior micro-tribological properties of the replicated surfaces could be attributed to the reduced real area of contact projected by the surfaces. [Copyright &y& Elsevier]

Abstract: Friction characteristics of self-assembled monolayers (SAMs) coated on Si-wafer (100) by chemical vapor deposition technique were studied experimentally at nano and micro-scales. Four self-assembled monolayers, such as dimethyldichlorosilane (DMDC), diphenyldichlorosilane (DPDC), perfluorooctyltrichlorosilane (PFOTS) and perfluorodecanoicacid (PFDA) coated on Si-wafer (100) were used as test materials. Nano-scale friction was measured using atomic force microscopy (AFM) in the range of 0–40nN normal loads, in LFM (lateral force microscopy) mode, using a contact mode type Si3N4 tip. Results showed that the friction of SAMs at this scale was influenced by their physical/chemical properties, while that of Si-wafer by its inherent adhesion. Further, micro-scale friction tests were also performed with a ball-on-flat type micro-tribotester under reciprocating motion. Friction was measured in the range of 1500–4800μN applied normal loads using glass balls of varying radii, viz., 0.25, 0.5 and 1mm. It was observed that the performance of SAMs was more superior to Si-wafer even at micro-scale, except for PFDA. Evidences obtained using scanning electron microscope showed that Si-wafer and PFDA exhibited wear at this scale. Wear in the case of Si-wafer was due to solid–solid adhesion and that in the case of PFDA due to the influence of humidity (moisture). The micro-scale friction in both these materials was severely influenced by their wear. [Copyright &y& Elsevier]

Abstract: Diamond-like carbon (DLC) films deposited on silicon wafer with varying film thickness were investigated for their micro-scale friction behaviour. Films with three different thicknesses, namely 100 nm, 500 nm and 1000 nm, deposited by a radio frequency plasma-assisted chemical vapor deposition method on Si (100) wafer, were used as the test samples. The elastic modulus of the DLC samples increased with their film thickness. The micro-scale friction tests were conducted in a ball-on-flat type micro-tribotester, using soda lime glass balls with different radii (0.25 mm, 0.5 mm and 1 mm), and with varying applied normal load (load range: 1500 μN to 4800 μN). Results showed that the friction force increased with applied normal load, whereas with respect to the ball size, two different trends were observed. In the case of 100 nm thick sample, friction increased with the ball size at any given normal load, while for 500 nm and 1000 nm thick samples, friction had an inverse relation with the ball size at all applied normal loads. The friction behaviour in the case of the 100 nm thick film was adhesive in nature, whereas for the thicker films plowing was dominant. The friction behaviour of the test samples with the ball size, which was distinctly different, was discussed in terms of the contact area, influenced by their mechanical property, namely, the elastic modulus. [Copyright &y& Elsevier]

Abstract: The effect of contact area on nano/micro-scale friction was experimentally studied. Glass balls with various radii were used in order to change the contact area. Borosilicate glass balls (radii—0.32μm, 0.5μm, 1.25μm and 2.5μm) attached on the top of AFM tip (NPS, DI) were applied for nano-scale contact and Soda Lime balls with radii 0.25mm, 0.5mm and 1mm were applied for micro-scale contact. At the nano-scale, the friction between ball and surface was measured with the applied normal load using an atomic force microscope (AFM), and at the micro-scale it was measured using a ball-on-flat type micro-tribotester. All experiments were conducted on silicon wafer and diamond-like carbon (DLC) coated silicon samples, at controlled conditions of temperature of 24±1°C and relative humidity of 45±5%. Friction was measured with the applied load in the range of 0–160nN at the nano-scale and at 1000μN, 1500μN, 3000μN and 4800μN at the micro-scale. Results at the nano-scale showed that the friction increased with the applied normal load and tip size, for both kinds of samples. Similar behavior of friction with the applied normal load and ball size was observed for silicon at the micro-scale. However, for DLC friction decreased with the ball size. This distinct difference in the behavior of friction in DLC at the nano- and micro-scale was attributed to the difference in the operating mechanisms. At nano-scale, friction in DLC was affected by adhesion, whereas at the micro-scale it was affected mainly by plowing. Evidences of the operating mechanisms at micro-scale were obtained using scanning electron microscope (SEM). At micro-scale, solid–solid adhesion was dominant in silicon, while DLC showed plowing. Contrary to the nano-scale that is almost a ‘wear-less’ situation, wear was prominent at the micro-scale. At both the nano- and micro-scales, the effect of applied normal load and the tip/ball size on friction was discussed as the influence of contact area on these parameters. [Copyright &y& Elsevier]

Nano-adhesion characteristics between scanning probe microscope (SPM) tips of various radius of curvature and flats of different materials were experimentally studied. Adhesion and friction forces between Si-wafer (1 0 0) and Si3N4 tips were measured under various applied normal loads, and the results were compared to those of diamond-like carbon (DLC), tungsten incorporated diamond-like carbon (W-DLC) and octadecyltrichlorosilane (OTS) self-assembled monolayer (SAM) formed on Si-wafer surfaces. Also in order to study the effect of capillary force, tests were performed in various relative humidity. Results showed that the adhesion increased with the tip radius. When the applied normal load increased from 0 to 40 nN, the adhesion did not change, but the friction increased linearly. Results generally showed that surfaces of the more hydrophobic property revealed the lower adhesion. The adhesion forces increased with the relative humidity. The nano-adhesion phenomenon was discussed on the basis of JKR model and capillary force exerted by meniscus. [Copyright &y& Elsevier]

The tribological role of transfer layer was studied with silver coatings under various ranges of load and sliding speed. Silver coatings were performed with thermal evaporation, ion beam assisted deposition and functionally gradient coating method. Tests were performed in dry sliding conditions, using a ball-on-disk contact configuration, at the load of 0.02–17.64 N and the sliding speed of 20–1000 mm/s. Optical microscope and electron probe micro analyzer analyses showed that contact surfaces were covered with the transfer layers of agglomerated wear particles depending upon the contact conditions, and they greatly influenced the tribological characteristics of the surfaces. However, the formation of transfer layer was suppressed as the sliding speed increased above a critical sliding speed, and no transfer layer was able to form. For building up a general framework of tribological behavior of the silver coating films, all test data were summarized on a map whose axes are contact pressure and sliding speed. [Copyright &y& Elsevier]

A general relation between adhesion and friction was elusive. This was partly due to the limitations in experimentally configuring the wide variety of geometrical and chemical cues encountered at a sliding contact. We study the combined influence of capillary and van der Waals forces on the correlation between the adhesion (pull-off force) and friction of polymer patterns. We report the existence of master curves in plot of adhesion versus friction, spanning nearly two orders of magnitude, characteristic of the effective lateral contact stiffness of the contact determined by geometry and capillary forces. Further, we showed how nanocylindrical patterns, micropatterns and PTFE-coated PMMA flat surfaces subjected to varying capillary forces displayed similar sliding characteristics despite their large differences in contact characteristics. [ABSTRACT FROM AUTHOR]

[Display omitted] • MXenes are 2D lamellar materials suitable for nanoscale solid lubrication. • Friction decreases with an increase in the number of Ti 3 C 2 T x MXene layers. • A sliding probe on Ti 3 C 2 T x monolayers induces puckering, thereby increasing friction. • Thicker sheets reduce compliance, thereby reducing puckering and friction. • Interlayer binding properties enable enhanced frictional layer dependence in MXenes. MXenes are an emerging class of two-dimensional lamellar materials with exceptional architectural variety, tunable chemical compositions, and chemical inertness. Owing to their unique structure, with a highly tunable interlayer distance and binding properties, MXenes can potentially exhibit excellent nanoscale solid lubrication properties. Herein, we report the layer dependence of the frictional characteristics of atomically thin Ti 3 C 2 T x MXene nanosheets. The frictional properties of the isolated Ti 3 C 2 T x nanosheets deposited on a clean Si surface were investigated using friction force microscopy. The friction decreased with an increase in the number of layers, starting from monolayer Ti 3 C 2 T x. The layer dependence of friction was attributed to the reduced elastic compliance with an increase in the number of Ti 3 C 2 T x layers. A partially suspended Ti 3 C 2 T x monolayer exhibited a higher degree of friction than the substrate-supported Ti 3 C 2 T x monolayer, indicating the role of elastic compliance in the friction mechanism of Ti 3 C 2 T x nanosheets. The reduced elastic compliance of Ti 3 C 2 T x nanosheets decreases the localized out-of-plane puckering-type deformation surrounding the sliding indenter, thereby reducing the resistance to frictional sliding. The results indicate that tuning their out-of-plane mechanical properties by varying the chemical architecture, chemical composition, surface terminations, and interlayer intercalations can render MXenes as potential candidates for versatile nanoscale solid lubrication applications. [ABSTRACT FROM AUTHOR]

Abstract: With the invention of miniaturized devices like micro-electro-mechanical systems (MEMS), tribological studies at micro/nano-scale have gained importance. These studies are directed towards understanding the interactions between surfaces at micro/nano-scales, under relative motion. In MEMS devices, the critical forces, namely adhesion and friction restrict the smooth operation of the elements that are in relative motion. These miniaturized devices are traditionally made from silicon (Si), whose tribological properties are not good. In this paper, we present a short investigation of nano- and micro-tribological properties of diamond-like carbon (DLC) nano-dot surfaces. The investigation was undertaken to evaluate the potential of these surfaces for their possible application to the miniaturized devices. The tribological evaluation of the DLC nano-dot surfaces was done in comparison with bare Si (100) surfaces and DLC coated silicon surfaces. A commercial atomic force microscope (AFM) was used to measure adhesion and friction properties of the test materials at the nano-scale, whereas a custom-built micro-tribotester was used to measure their micro-friction property. Results showed that the DLC nano-dot surfaces exhibited superior tribological properties with the lowest values of adhesion force, and friction force both at the nano- and micro-scales, when compared to the bare Si (100) surfaces and DLC coated silicon surfaces. In addition, the DLC nano-dot surfaces showed no observable wear at the micro-scale, unlike the other two test materials. The superior tribological performance of the DLC nano-dot surfaces is attributed to their hydrophobic nature and the reduced area of contact projected by them. [ABSTRACT FROM AUTHOR]

Abstract: In miniaturized devices like micro/nano-electro-mechanical systems (MEMS/NEMS), the critical forces, namely adhesion and friction restrict the smooth operation of the elements that are in relative motion. MEMS/NEMS are traditionally made of silicon, whose tribological properties are not good. In this paper, we present an investigation on the approach of dual surface modification of silicon surfaces and their tribological properties at micro-scale. The dual surface modification is a combination of topographical and chemical modifications. As the topographical modification, micro-patterns with varying shapes of pillars and channels were fabricated on Si(100) wafer surfaces using photolithography method. Chemical modification included the coating of micro-patterns with diamond-like carbon (DLC) and Z-DOL (perfluoropolyether, PFPE) thin films. The surfaces with combined modification were evaluated for their micro-friction behavior in comparison with those of bare Si(100) flat surfaces and the topographically/chemically modified silicon surfaces. Results showed that the surfaces with dual modification exhibited superior tribological properties. These results indicate that a combination of topographical and chemical modification is very effective in enhancing tribological properties at small-scale. The combined surface treatments such as the ones investigated in the current work could be useful for tribological applications in small-scale devices such as MEMS/NEMS. The motivation for undertaking the dual modification approach comes from an earlier observation made on the significant influence of the surface characteristics of lotus leaf on its micro-friction behavior. [Copyright &y& Elsevier]

Abstract: Nano-scale adhesion and nano/micro-scale friction properties of one-component (pure) trichlorosilane self-assembled monolayers (SAMs) with different chain lengths and their two-component mixtures coated on silicon wafer (100) were experimentally studied. The tribological behaviour of these monolayer systems was compared with that of the uncoated silicon wafer. The nano-scale friction and adhesion forces were measured using atomic force microscopy (AFM) and the micro-scale friction tests were conducted using a custom-built micro-tribo tester. At both nano- and micro-scales, the pure SAMs and their mixtures outperformed the uncoated silicon wafer and were proved to be effective lubricants. At the nano-scale, both the friction and adhesion of pure SAMs decreased with the increase in chain length, and the mixed monolayers exhibited lower values of friction and adhesion than their pure counterparts. However, at the micro-scale, though the friction values of the pure SAMs decreased with the increase in the chain length, their mixtures exhibited values that were in between those of their individual components. The tribological behaviour of SAMs and their two-component systems observed in the present case has been explained by considering models available in literature. [Copyright &y& Elsevier]