Friction Force Images Research Articles

Laser surface structuring of diamond-like carbon films for tribology

The paper overviews experimental findings of the direct laser processing and surface microstructuring (texturing) of various diamond-like carbon films (a-C:H, ta-C, DLN, metal-doped DLN), aimed at improvements of their tribological and nanotribological properties. The nanosecond UV and femtosecond IR/visible pulsed lasers were applied in microprocessing of the films, focusing on high precision surface structuring with fs-laser pulses. The studies were concentrated on the following tasks: (i) surface graphitization in laser microstructuring of the films under different irradiation conditions, (ii) lubricated friction performance of DLN films micropatterned with UV ns and visible fs pulsed lasers, and (iii) nanoscale friction of laser-structured DLN and metal-doped DLN films examined with contact-mode atomic force microscopy. The important findings of our studies are related to fabrication of highly-precise microgroove/microcrater patterns on DLN films and improvements of frictional properties of the laser-structured films at the macro, micro and nanoscale. The surface microstructures improved the film properties under oil-lubricated sliding in dependence on their geometrical parameters (size, depth, period) and ambient temperature. The nanoscale friction behavior of laser-structured films was shown to be controlled by the surface graphitization, nanoscale roughness, capillary forces and wear of AFM tips during friction force imaging.

In situ friction study of Ag Underpotential deposition (UPD) on Au(111) in aqueous electrolyte.

The electrodeposition of silver on Au(111) was investigated using lateral force microscopy (LFM) in Ag+ containing sulfuric acid. Friction force images show that adsorbed sulfate forms 3×7R19.1∘ structure (θsulfate=0.2) on Au(111) prior to Ag underpotential deposition (UPD) and (3×3R30∘) structure (θsulfate=0.33 ) on a complete monolayer or bilayer of Ag. Variation of friction with normal load shows a non‐monotonous dependence, which is caused by increasing penetration of the tip into the sulfate adlayer. In addition, the friction force is influenced by the varying coverage and mobility of Ag atoms on the surface. Before Ag coverage reaches the critical value, the deposited silver atoms may be mobile enough to be dragged by the movement of AFM tip. Possible penetration of the tip into the UPD layer at very high loads is discussed as a model for self‐healing wear. However, when the coverage of Ag is close to 1, the deposited Ag atoms are tight enough to resist the influence of the AFM tip and the tip penetrates only into the sulfate adlayer.

Engineering Graphene Wrinkles for Large Enhancement of Interlaminar Friction Enabled Damping Capability.

Graphene nanoplates are hoped-for solid lubricants to reduce friction and energy dissipation in micro and nanoscale devices benefiting from their interface slips to reach an expected superlubricity. On the contrary, we propose here by introducing engineered wrinkles of graphene nanoplates to exploit and optimize the interfacial energy dissipation mechanisms between the nanoplates in graphene-based composites for enhanced vibration damping performance. Polyurethane (PU) beams with designed sandwich structures have been successfully fabricated to activate the interlaminar slips of wrinkled graphene-graphene, which significantly contribute to the dissipation of vibration energy. These engineered composite materials with extremely low graphene content (∼0.08 wt %) yield a significant increase in quasi-static and dynamic damping compared to the baseline PU beams (by 71% and 94%, respectively). Friction force images of wrinkled graphene oxide (GO) nanoplates detected via an atomic force microscope (AFM) indicate that wrinkles with large coefficients of friction (COFs) indeed play a dominant role in delaying slip occurrences. Reduction of GO further enhances the COFs of the interacting wrinkles by 7.8%, owing to the increased effective contact area and adhesive force. This work provides a new insight into how to design graphene-based composites with optimized damping properties from the microstructure perspective.

Thickness-Dependent Hydrophobicity of Epitaxial Graphene

This article addresses the much debated question whether the degree of hydrophobicity of single-layer graphene (1LG) is different from that of double-layer graphene (2LG). Knowledge of the water affinity of graphene and its spatial variations is critically important as it can affect the graphene properties as well as the performance of graphene devices exposed to humidity. By employing chemical force microscopy with a probe rendered hydrophobic by functionalization with octadecyltrichlorosilane (OTS), the adhesion force between the probe and epitaxial graphene on SiC has been measured in deionized water. Owing to the hydrophobic attraction, a larger adhesion force was measured on 2LG Bernal-stacked domains of graphene surfaces, thus showing that 2LG is more hydrophobic than 1LG. Identification of 1LG and 2LG domains was achieved through Kelvin probe force microscopy and Raman spectral mapping. Approximate values of the adhesion force per OTS molecule have been calculated through contact area analysis. Furthermore, the contrast of friction force images measured in contact mode was reversed to the 1LG/2LG adhesion contrast, and its origin was discussed in terms of the likely water depletion over hydrophobic domains as well as deformation in the contact area between the atomic force microscope tip and 1LG.

S1160404 表面形状変化と摩擦力像による凝着摩耗のその場観察

S1160404 表面形状変化と摩擦力像による凝着摩耗のその場観察

Angular dependence of atomic friction with deformable substrate

The atomic stick-slip behavior of Prandtl-Tomlinson model sliding on 2D deformable substrate is studied. The influence of the shape of the interaction potential is investigated in details in the well-known phenomenon of the two-dimensional stick-slip friction. Numerical simulations are built to observe the influence of the geometry and orientation. The results show that the friction force has a maximum at r = − 0.6, this value of the shape parameter indicates a transition from a clear stick-slip at r> − 0.6 to a distorted stick-slip at r< − 0.6. We find a remarkable transition of the frictional force image pattern depending on the shape of the substrate.

Real-time deflection and friction force imaging by bimorph-based resonance-type high-speed scanning force microscopy in the contact mode.

We report herein an alternative high-speed scanning force microscopy method in the contact mode based on a resonance-type piezoelectric bimorph scanner. The experimental setup, the modified optical beam deflection scheme suitable for smaller cantilevers, and a high-speed control program for simultaneous data capture are described in detail. The feature of the method is that the deflection and friction force images of the sample surface can be obtained simultaneously in real time. Images of various samples (e.g., a test grating, a thin gold film, and fluorine-doped tin oxide-coated glass slides) are acquired successfully. The imaging rate is 25 frames per second, and the average scan speed reaches a value of approximately 2.5 cm/s. The method combines the advantages of both observing the dynamic processes of the sample surface and monitoring the frictional properties on the nanometer scale.PACS07.79.Lh; 07.79.Sp; 68.37.Ps

21am2-F6 AFM測定によるナノストライプ構造の潤滑特性の解明

In this study, we determined micro-scale friction force on the nano-patterned surface of "nanostripe" that has micro/nano-scale grooves. The nanostripe surface had the cross section of multilayer film consisting of two kinds of metal materials. We rubbed the nanostripe surface composed of Ag-Cu with the cantilever having a convex lens as a scanning probe using an AFM (atomic force microscope). The results showed that Stribeck curves differed by roughness of the probe and friction direction. A periodic changes appeared in the friction force images measured only on the nanostripe, which showed that the friction force decreased on nano-grooves region.

Tribological behavior of micro/nano-patterned surfaces in contact with AFM colloidal probe

In effort to investigate the influence of the micro/nano-patterning or surface texturing on the nanotribological properties of patterned surfaces, the patterned polydimethylsiloxane (PDMS) surfaces with pillars were fabricated by replica molding technique. The surface morphologies of patterned PDMS surfaces with varying pillar sizes and spacing between pillars were characterized by atomic force microscope (AFM) and scanning electron microscope (SEM). The AFM/FFM was used to acquire the friction force images of micro/nano-patterned surfaces using a colloidal probe. A difference in friction force produced a contrast on the friction force images when the colloidal probe slid over different regions of the patterned polymer surfaces. The average friction force of patterned surface was related to the spacing between the pillars and their size. It decreased with the decreasing of spacing between the pillars and the increasing of pillar size. A reduction in friction force was attributed to the reduced area of contact between patterned surface and colloidal probe. Additionally, the average friction force increased with increasing applied load and sliding velocity.

Elastic and frictional properties of graphene

AbstractWe describe studies of the elastic properties and frictional characteristics of graphene samples of varying thickness using an atomic force microscope. For tensile testing, graphene is suspended over micron‐sized circular holes and indented by atomic force microscope (AFM) tips. Fitting of the force‐displacement curves yields the prestress and elastic stiffness, while comparison of the breaking force to simulation gives the ultimate strength, which is the highest measured for any material. Experiments on samples with 1–3 atomic layers yield similar values for the intrinsic stiffness and strength of a single sheet, but also reveal differences in mechanical behavior with thickness. The frictional force between an AFM tip and graphene decreases with thickness for samples from 1 to 4 layers, and does not depend on the presence of a substrate. High‐resolution friction force imaging in stick‐slip mode shows the same trend, and allows direct imaging of the crystal lattice.

Surface Chemical Properties of Nanoscale Domains on UV-Treated Polystyrene−Poly(methyl methacrylate) Diblock Copolymer Films Studied Using Scanning Force Microscopy

This paper reports the surface chemical properties of ca. 20 nm wide domains on a UV-treated thin film of a polystyrene-poly(methyl methacrylate) diblock copolymer (PS-b-PMMA; 0.3 as the PMMA volume fraction). UV irradiation and subsequent acetic acid (AcOH) treatment were used for selectively etching horizontally aligned PMMA domains on a thin PS-b-PMMA film to obtain nanoscale trenches and ridges. The surface charge and hydrophilicity of the trenches (etched PMMA domains) and ridges (PS domains) were investigated using three approaches based on scanning force microscopy. Chemical force titration data with a COOH-terminated tip showed a prominent decrease in adhesion force from pH 3 to 4.5 due to electrostatic repulsion between negatively charged functional groups on the tip and film surface but could not clarify the difference in chemical properties between the two nanoscale domains. Friction force images in n-dodecane showed higher friction over etched PMMA and PS domains with an OH-terminated tip and a CH(3)-terminated tip, respectively, exhibiting higher hydrophilicity of the etched PMMA domains. In an atomic force microscopy image of a UV/AcOH-treated PS-b-PMMA film upon immersion in a ferritin solution, approximately 80% of the ferritin deposited on the film was found on the PS domains. The preferential deposition of ferritin on the PS domains was probably due to the electrostatic repulsion between negatively charged ferritin and negatively charged etched PMMA surface in addition to the hydrophobic interaction between ferritin and the PS surface. These results indicated that the etched PMMA domains were more hydrophilic than the PS domains due to the presence of acidic functional groups (e.g., -COOH groups) at a higher density.

Ion-selective Imaging by Atomic Force Microscopy with a Crown-ether-modified Tip

Abstract Ion-selective surface imaging was successfully attained by atomic force microscopy with a probe tip functionalized with a 15-crown-5 derivative. The frictional force images for nanopatterned substrates revealed enhanced contrasts attributable to the cation-binding interaction between the 15-crown-5 on the tip and potassium ions on the substrate.

Bulk phase separation study by atomic force microscope friction imaging of natural rubber/poly(methyl methacrylate) film

The first observation of bulk phase separation in immiscible natural rubber (NR)/poly(methyl methacrylate) (PMMA) film using atomic force microscopy (AFM) is reported. Three different forms of AFM measurements: topographic, friction force imaging, and nanoindentation have been effectively used to investigate combined morphological and compositional mapping of the NR/PMMA system. The fracture temperature during sample microtoming and material physical properties could be responsible for the observed topographic contrast. The stronger contrast of friction imaging, relative to topographic imaging, is ascribed to local variations in mechanical properties of the phase-separated domains. Friction force imaging associated with nanoindentation response, performed under AFM force mode, highlights the AFM's ability for probing local friction, adhesion, and elastic properties, and for compositional mapping of heterogeneous polymer film. The resulting friction force imaging along with the response of the nanoindentation are in good agreement, indicating that PMMA exists mainly near the modified NR surface.

High resolution molecular chain imaging of a poly(vinylidenefluoride–trifluoroethylene)crystal using force modulation microscopy

Topographic imaging or friction force imaging of a polymer lamella in molecularresolution using atomic force microscopy (AFM) is significantly disturbed by theexistence of an amorphous layer on top of the crystal. In this paper, we report thefirst successful molecular imaging of a polymer lamella using force modulationmicroscopy (FMM), which is capable of imaging crystalline molecules even ifthe crystal surface is covered with an amorphous layer. Molecular chains of apoly(vinylidenefluoride–trifluoroethylene) (P(VDF–TrFE)) lamella under an amorphouslayer were clearly imaged using FMM at room temperature in ambient conditions. Theorigins and mechanisms of the image contrast in molecular resolution are alsodiscussed.

Atomic-scale topographic and friction force imaging and cantilever dynamics in friction force microscopy

Friction force microscopy (FFM) is commonly used for micro-, nano-, and atomic-scale topographic and friction (lateral) force imaging of surfaces. The experimental-obtained topographic and friction force images are closely correlated to the cantilever dynamics since in FFM, the normal and lateral forces between the cantilever tip and sample surface are measured from the cantilever flexural and twist angles. To understand the cantilever dynamics under tip-surface interaction and its effects on the measured topographic and friction force maps, efficacious models that can accurately simulate the cantilever behavior in operating conditions of FFM are essential. In this paper, a three-dimensional (3D) finite element (FE) beam model is employed to simulate the atomic-scale topographic and friction force profiling process in FFM. The tip-sample interaction forces are modeled as the interatomic forces between the tip and sample surface. It is identified that the topographic and lateral force maps obtained in FFM experiments are the combined results of the real spatial distributions of 3D tip-sample interatomic forces and cantilever dynamics. The experimental-obtained hexagonal (full atomic structure) and trigonal (atomic resolution of every other atom) topographic images of graphite surfaces are reproduced in simulations with different combinations of cantilever geometries, applied normal loads, and scan directions. Based on the simulated results, the methods to realize the observation of the full atomic structure of graphite are discussed.

Investigation of micro-electro-mechanical processing characteristics of layered boron nitrideand carbon films

A microsquare fabrication process is developed for the investigationof the processing characteristics of boron nitride and carbon(C/BN)n and (BN/C)n films with a 4 nm nanoperiod multilayer structure. Simultaneous surface topography,friction and current measurements were performed on the multilayer films by conductiveatomic force microscopy (AFM) with force modulation, which permits the quantitativerecording of current and frictional force as functions of the applied force. Thecurrent image showed that highly conducting sites formed on carbon (C) layersand that nonconducting sites existed on boron nitride (BN) layers and/or mixedlayers. Furthermore, amplitude response (friction force) images show that theconducting sites have a low frictional force, whereas the nonconducting sites havea high frictional force. This is thought to be due to the structure phase of thenanostructure of the films. Further, the result that the nonconducting sites have a highamplitude response (friction) implies the existence of a mixed layer (interface)between the C and BN layers. It is suggested that friction and surface-currentmeasurements are effective methods of investigating multilayer nanostructural surfacesand fabricating micro-electro-mechanical processing systems of a high precision.

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.

The influence of electrostatic forces on protein adsorption

In this paper we investigate the importance of electrostatic double layer forces on the adsorption of human serum albumin by UV–ozone modified polystyrene. Electrostatic forces were measured between oxidized polystyrene surfaces and gold-coated atomic force microscope (AFM) probes in phosphate buffered saline (PBS) solutions. The variation in surface potential with surface oxygen concentration was measured. The observed force characteristics were found to agree with the theory of electrical double layer interaction under the assumption of constant potential. Chemically patterned polystyrene surfaces with adjacent 5 μ m × 5 μ m polar and non-polar domains have been studied by AFM before and after human serum albumin adsorption. A topographically flat surface is observed before protein adsorption indicating that the patterning process does not physically modify the surface. Friction force imaging clearly reveals the oxidation pattern with the polar domains being characterised by a higher relative friction compared to the non-polar, untreated domains. Far-field force imaging was performed on the patterned surface using the interleave AFM mode to produce two-dimensional plots of the distribution of electrostatic double-layer forces formed when the patterned polystyrene surfaces is immersed in PBS. Imaging of protein layers adsorbed onto the chemically patterned surfaces indicates that the electrostatic double-layer force was a significant driving force in the interaction of protein with the surface.

A surface topography-independent friction measurement technique using torsionalresonance mode in an AFM

The recently introduced torsional resonance mode (TR mode) technique has been appliedfor friction force measurements at the micro/nanoscale. In this technique, an atomic forcemicroscope (AFM) with a vibrating cantilever tip in torsional mode is used, andthe contact torsional vibration amplitude of the tip motion (TR amplitude) ismonitored at a contact resonance frequency under constant normal load. In thispaper, the TR mode friction force images are compared to the friction force imagesacquired by conventional contact mode friction force microscopy on three samples:a self-assembled monolayer (SAM) with two phase structure, a silicon ruler, and a metalevaporated (ME) tape. The results from those samples show that the TR mode frictionforce images are much less affected by the surface topography and are almostindependent of the scanning direction of the tip, whereas this is not the case forthe contact mode friction force measurements. Mechanisms responsible for theseobservations are also discussed in the paper. In order to predict coefficient of frictionas a function of the change in the TR amplitude, an energy balance model isproposed.

Elastic modulus, oxidation depth and adhesion force of surface modified polystyrene studied by AFM and XPS

AFM and XPS have been used to investigate the surface and near-surface properties of polystyrene (PS) substrates which have been subjected to one of three controlled surface modification processes performed in situ in a specially constructed cell. The cell was fitted to a Digital Instruments Nanoscope III AFM measuring head and allowed close control of the gaseous environment and made it possible to UV irradiate the sample during AFM measurements. Treatments were carried out using UV at 184.9 and 253.7 nm wavelengths, in oxygen (UV-ozone), and in nitrogen (UV-only). Polystyrene surfaces were also modified by an exposure to an atmosphere of ozone in the absence of UV (ozone-only). Data show that adhesion force is highest between tip and sample for the UV-ozone exposed surfaces and that the adhesion force increases with sample exposure time. Exposure to UV-only or ozone alone results in lower ultimate adhesion levels with a slower rate of increase with exposure time. Evaluation of Young's modulus for unmodified PS gave a value of 3.37 (±0.52) GPa which agrees well with the textbook value which ranges from 2 to 4 GPa depending on the measurement technique. A 60 s exposure to combined UV-ozone resulted in the formation of a surface layer with a modulus at the surface of 1.25 (±0.19) GPa which increased to 2.5 (±0.37) GPa at a depth of 3.5 nm. The sample exposed for 60 s to UV-only had a Young's modulus of 2.6 (±0.39) GPa but showed no reduced modulus layer at the surface. The modulus of the ozone-only treated material was the least affected with a decrease of around 0.75 GPa with some evidence for a surface layer with a modulus ranging from 2.6 (±0.39) GPa at the surface to 3.2 (±0.48) GPa at a depth of 2 nm. XPS analyses reveal that the oxygen content of the modified surfaces decreased in the order of UV-ozone > UV > ozone with approximate concentrations for a 60 s exposure of 5, 0.7 and 0.05 at.%, respectively. Friction force imaging of patterned surfaces reveals chemical heterogeneity. Statistical analysis of the friction force data shows that the resolution attainable with UV irradiation in nitrogen was higher than surfaces patterned by UV-ozone treatment.