Friction In Micro Research Articles

Dynamically tuning friction at the graphene interface using the field effect

Dynamically controlling friction in micro- and nanoscale devices is possible using applied electrical bias between contacting surfaces, but this can also induce unwanted reactions which can affect device performance. External electric fields provide a way around this limitation by removing the need to apply bias directly between the contacting surfaces. 2D materials are promising candidates for this approach as their properties can be easily tuned by electric fields and they can be straightforwardly used as surface coatings. This work investigates the friction between single layer graphene and an atomic force microscope tip under the influence of external electric fields. While the primary effect in most systems is electrostatically controllable adhesion, graphene in contact with semiconducting tips exhibits a regime of unexpectedly enhanced and highly tunable friction. The origins of this phenomenon are discussed in the context of fundamental frictional dissipation mechanisms considering stick slip behavior, electron-phonon coupling and viscous electronic flow.

Cumulative reduction of friction and size effects in micro milling through proper selection of coating thickness of TiAlN coated tool: Experimental and analytical assessments

It has been shown earlier that abrasion-resistant coatings on cutting tool can reduce tool wear and friction in micro milling and other machining processes. However, due to coating, the edge radius of micro end mill increases, and the influences of size effects become significant. Therefore, proper selection of the coating thickness is crucial to reduce friction and size effects together. In this article, thin-film coating of TiAlN of different thicknesses (0.5, 1, 2 and 3 μm) has been applied on the WC micro end mill using the physical vapour deposition (PVD) method. First, the enhanced tribological performance of TiAlN coating over un-coated WC material has been verified by evaluating the hardness and coefficient of friction values of both the materials. Next, the appropriate coating thickness value has been selected by experimental and analytical assessments of the machinability parameters and stability limit, respectively, in micro milling of P-20 steel. From the cumulative experimental and analytical investigations, TiAlN coated tool having coating thickness ≈ 1 μm has been found to be the appropriate thin film thickness by witnessing improved machinability and stability limit as compared to the uncoated WC tool and TiAlN coated tools of other coating thicknesses.

Friction reduction in a nanochannel with grafted poly(N-isopropylacrylamide) oligomers: A molecular dynamics study

Superhydrophobic surfaces have been used for reducing friction in micro- and nanochannels. In the present work, water flow between two carbon walls with nanostructures made of poly(N-isopropylacrylamide) via the molecular dynamics method has been studied. The structure of this polymer can change based on the temperature of the environment, so that by increasing the temperature the structure becomes hydrophobic. This property has been studied and the effect of multiple factors on the slip length is presented. The effects of the number of monomers in the polymer, the distance between the polymers, and the temperature on the flow field are investigated. The results reveal that the slip length and the flow rate increase with the temperature but both have a maximum with respect to the distance between the polymers and the number of monomers. For different temperatures considered in the study, the minimum friction occurs when the number of monomers and the distance are equal to 30 and 80, respectively. In this condition, the slip length increases by 180% and the flow rate rises by 200%. We also study the effect of air fraction in the fluid. The results indicate that by increasing the air fraction to 30%, the slip length and the flow rate increase by 250% and 310%, respectively.

Dynamic Sliding Enhancement on the Friction and Adhesion of Graphene, Graphene Oxide, and Fluorinated Graphene.

Graphene and functionalized graphene are promising candidates as ultrathin solid lubricants for dealing with the adhesion and friction in micro- and nanoelectromechanical systems (MEMS and NEMS). Here, the dynamic friction and adhesion characteristics of pristine graphene (PG), graphene oxide (GO), and fluorinated graphene (FG) were comparatively studied using atomic force microscopy (AFM). The friction as a function of load shows nonlinear characteristic on GO with strong adhesion and linear characteristic on PG and FG with relatively weak adhesions. An adhesion enhancement phenomenon that the slide-off force after dynamic friction sliding is larger than the pull-off force is observed. The degree of adhesion enhancement increases with the increasing surface energy, accompanied by a corresponding increase in transient friction strengthening effect. The dynamic adhesion and friction enhancements are attributed to the coupling of dynamic tip sliding and surface hydrophilic properties. The atomic-scale stick-slip behaviors confirm that the interfacial interaction is enhanced during dynamic sliding, and the enhancing degree depends on the surface hydrophilic properties. These findings demonstrate the adhesive strength between the contact surfaces can be enhanced in the dynamic friction process, which needs careful attention in the interface design of MEMS and NEMS.

Casimir friction force between a SiO2 probe and a graphene-coated SiO2 substrate

The possibility of mechanical detection of Casimir friction with the use of a noncontact atomic force microscope is discussed. A SiO2 probe tip located above a graphene-coated SiO2 substrate is subjected to the frictional force caused by a fluctuating electromagnetic field produced by a current in graphene. This frictional force will create the bend of a cantilever, which can be measured by a modern noncontact atomic force microscope. Both the quantum and thermal contributions to the Casimir frictional force can be measured using this experimental setup. This result can also be used to mechanically detect Casimir friction in micro- and nanoelectromechanical systems.

Influence of tribology size effect on Stribeck curve in micro deep drawing

The lubricant viscosity is one parameter for optimizing drawing processes. The influence is due to the velocity depending friction behaviour between work piece and tool, as well as the effective process forces and resulting pressures in the lubricant. Because of the tribological size effect, the knowledge gained in macro processes cannot be transferred directly to micro forming processes. To understand the influence of the drawing speed, several parameters as the drawing radius and viscosity of lubricants are varied in this work. The resulting Stribeck curves show a varying effect of the different kinematic viscosities. High viscosity lubricant reduces the occurring friction significantly at low Stribeck numbers, but increases friction with rising Stribeck number extremely. Concluding the results, it can be stated that the hydrodynamic friction demands a relevant portion of the overall friction in micro forming process especially at high drawing velocities, which cannot be explained by the lubricant pocket model.

Effect of Punch Surface Topography on Friction in Micro Combined Forward and Backward Extrusion of Brass

Friction is one of the key factors affecting the metal forming process. If the friction effects of the process can be accurately modeled, it is able to improve simulations and help the research and development of the metal forming process. This study used cylindrical brass (JIS C2600) billets with the height and diameter of 1.1 mm for conducting the experiments of the micro combined forward and backward extrusion. The purpose of the study was to investigate the effects of punch surface topography on friction in the process. Four surface topography conditions for 0.8 mm diameter punches were prepared by grinding, polishing, grooving and micro electrical discharge machining processes. By comparing the ratio of the cup height to rod length of the extruded cups with the calibration curves established by simulations, the friction factor was estimated in a range from 0.3 to 0.6. The results showed that the punch surface topography significantly affect the friction in the extrusion process. The predicted loads using the estimated friction factors were in good agreement with the experimental results.

Biomimetic patterned surfaces for controllable friction in micro- and nanoscale devices

Biomimetics is the study and simulation of biological systems for desired functional properties. It involves the transformation of underlying principles discovered in nature into man-made technologies. In this context, natural surfaces have significantly inspired and motivated new solutions for micro- and nano-scale devices (e.g., Micro/Nano-Electro-Mechanical Systems, MEMS/NEMS) towards controllable friction, during their operation. As a generic solution to reduce friction at small scale, various thin films/coatings have been employed in the last few decades. In recent years, inspiration from ‘Lotus Effect’ has initiated a new research direction for controllable friction with biomimetic patterned surfaces. By exploiting the intrinsic hydrophobicity and ability to reduce contact area, such micro- or nano-patterned surfaces have demonstrated great strength and potential for applications in MEMS/NEMS devices. This review highlights recent advancements on the design, development and performance of these biomimetic patterned surfaces. Also, we present some hybrid approaches to tackle current challenges in biomimetic tribological applications for MEMS/NEMS devices.

Phenomenological modeling of long range noncontact friction in micro- and nanoresonators

Motivated by the results of an experiment using atomic force microscopy performed by Gotsmann and Fuchs [Phys. Rev. Lett. 86, 2597 (2001)], where a strong energy loss due to the tip-sample interaction was measured, we investigate the potential implications of this energy loss channel to the quality factor of suspended micro- and nanoresonators. Because the observed tip-sample dissipation remains without a satisfactory theoretical explanation, two phenomenological models are proposed to generalize the experimental observations. In the minimal phenomenological model the range of validity of the power law found experimentally for the damping coefficient is assumed to be valid for larger separations. A more elaborate phenomenological model assumes that the noncontact friction is a consequence of the Casimir force acting between the closely spaced surfaces. Both models provide quantitative results for the noncontact friction between any two objects which are then used to estimate the energy loss for suspended bar micro- and nanoresonators. It is concluded that the energy loss due to the unknown mechanism has the potential to seriously restrict the quality factor of both micro- and nanoresonators.