Natural and biomimetic artificial surfaces for superhydrophobicity, self-cleaning, low adhesion, and drag reduction

Friction, wear and lubrication between materials in contact are of fundamental importance in many pure and applied sciences. Owing to the development of experimental and computer-simulation techniques for studying these phenomena at the atomic scale, an understanding is beginning to emerge of the molecular mechanisms of tribology in thin films and at surfaces.

Nanotribology: friction, wear and lubrication at the atomic scale

is preferred over sliding, and it is expected to have an equally important role in the microscopic domain. Although progress has been made in our understanding of the dynamics of sliding at the atomic level 4 ,w e have no comparable insight into rolling owing to a lack of experimental data on microscopic length scales. Here we produce controlled rolling of carbon nanotubes on graphite surfaces using an atomic force microscope. We measure the accompanying energy loss and compare this with sliding. Moreover, by repro- ducibly rolling a nanotube to expose different faces to the substrate and to an external probe, we are able to study the object over its complete surface. The microscopic aspects of tribology have been explored by means of the surface force apparatus (SFA) 5 and atomic force microscopy (AFM) 6 , which have identified the intrinsic dependence of sliding friction on contact area 7,8 and crystallographic orientation 9,10 . Further, AFM has been used to perform friction mapping of surfaces with atomic resolution 4 and has identified stick-slip motion in the sliding of nanometre-scale objects 11 . Using AFM manipulation 12,13 of multiwall carbon nanotubes (CNTs), we have observed sliding and rolling. CNTs, with a range of available sizes down to the molecular scale, serve as interesting model systems for tribological studies. In addition, nanotubes are expected to play a part in future nanometre-scale electrical-mechanical devices, and rolling fullerenes have been proposed as ideal lubricants 14-16 . Our evidence for sliding and rolling comprises sequences of topogra- phical images of manipulated nanotubes and lateral force data acquired during the manipulation to measure energy loss. Samples were prepared by solvent evaporation on mica and graphite substrates from an ethanol solution of raw carbon soot produced by the carbon arc technique 17 . The multiwall CNTs were imaged and manipulated under ambient conditions. The nanomanipulator AFM system 18,19 , designed for manipulation, comprises an advanced visual interface, teleoperation capabilities for manual control of the AFM tip and haptic (touch) presentation of the AFM data (Topometrix Discoverer). Normal force, lateral force and AFM tip trajectory are recorded simultaneously for strict correlation. The AFM tip is used to apply lateral forces at locations along the tube to produce translations and rotations. These tubes are free of pinning material and move without deformation. The lateral force values have been calibrated from measured cantilever and tip geometry, literature values for the cantilever (Si) elastic moduli and the detector response from the z-translation of the cantilever. We estimate the absolute error in this calibration to be about 30%, with the largest contribution in the uncertainty coming

Nanometre-scale rolling and sliding of carbon nanotubes

Comparison of surface roughness measurements by stylus profiler, AFM and non-contact optical profiler

Nanotribology and nanomechanics

Friction, Wear, and Lubrication Physics of Tribological Materials Metals and Ceramics Solid Lubricants and Self-Lubricating Solids Coating Disposition and Surface Treatment Techniques - Classification Surface Preparation for Coating Deposition Coating Deposition by Hard Facing Coating Deposition from Vapour Phase Coating Deposition by Miscellaneous Techniques Surface Treatments by Thermal and Chemical Processes Soft Coatings Hard Coatings Screening Methodology for Materials, Coatings, and Surface Treatments Tribological Applications of Materials, Coatings, and Surface Treatments.

Handbook of Tribology: Materials, Coatings, and Surface Treatments

Despite the rapid progress being made in the synthesis of diamond films and recent interest in polishing of diamond films, no systematic measurements of friction and wear on polished diamond films have been reported. In the present study, chemomechanical and laser polishing techniques are used, and friction and wear data on the chemomechanically polished diamond films are presented. With the chemomechanical polishing technique used in this study, the rms roughness of hot filament chemical vapor deposited diamond films can be reduced from about 657 to about 170 nm with rounding off of sharp asperities with no change in the diamond structure. The polished films exhibit coefficient of friction (∼0.1) and wear rates much lower than that of unpolished films. Friction and wear properties of the polished films are comparable to that of single‐crystal natural diamond. Based on this study, it is concluded that polished films are potential candidates for tribological applications.

https://www.mecheng.osu.edu/netl/files/netl/TribologicalPropsPolishedDiamondFilm.pdf

Tribological properties of polished diamond films

Fullerenes take the form of hollow, geodesic domes, which are formed from a network of pentagons and hexagons. The C60 molecule has the highest possible symmetry (icosahedral) and assumes the shape of a soccer ball. At room temperature, fullerene molecules pack in a face‐centered‐cubic lattice bonded with weak van der Waals attractions. Fullerenes can be dissolved in solvents such as toluene and benzene and easily sublimed. The resilience, high load bearing capacity, low surface energy, high chemical stability, and spherical shape of C60 molecules and weak intermolecular bonding offer great potential for various mechanical and tribological applications. Sublimed films of C60 have been produced and friction and wear performance of these films in various operating environments are the subject of this letter.

Sublimed C60 films for tribology

Silicon is used in the fabrication of microelectromechanical systems (MEMS). The friction and wear characteristics are of major design concern for any mechanical device requiring relative motion. In the present investigations we have studied the influence of ion bombardment on the microstructure, crystallinity, composition, microhardness, friction, and wear behavior. The ion bombardment modifies the elastic/plastic deformation characteristics and crack nucleation that occurs during the indentation. C+ bombarded monocrystalline and polycrystalline Si exhibit very low coefficient of friction (0.025–0.05) and wear factors (10−7 mm3 /N m) while slid against 52100 steel and alumina in dry and moist air and dry nitrogen atmospheres. Ion bombardment resulted in the formation of an amorphized layer that consists of SiC, C, and Si. We have shown that the improvements in friction and wear are because of the formation of SiC and not because of amorphization alone.

Tribology of Ion Bombarded Silicon for Micromechanical Applications

Solid lubricant films of MoS2 with improved adhesion to a stainless‐steel substrate have been deposited by a combination of rf sputtering and ion implantation with inert gas ions in the 100‐keV region. When the sputter film had obtained a thickness of 500 A, it was subjected to the ionic bombardment prior to completion of the sputter deposition. Dependent on the ion dose an adhesion improvement and a sliding life enhancement of approximately a factor of 2 were obtained for the solid lubricant film.

B. K. Gupta

Papers

Handbook of Tribology: Materials, Coatings, and Surface Treatments

Tribological properties of polished diamond films

Sublimed C60 films for tribology

Tribology of Ion Bombarded Silicon for Micromechanical Applications

Enhancement of sliding life of MoS2 films deposited by combining sputtering and high‐energy ion implantation