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Concept: Tribology


SignificanceThe goal of this study was to use the Surface Forces Apparatus to examine the effects of slip-stick friction on cartilage surface morphology under different loading and sliding conditions. Different load and speed regimes were represented using friction maps that separated regimes of smooth and stick-slip sliding. The finding of this work is that damage generally occurs within the stick-slip regimes and is not directly related to the friction coefficient. Prolonged exposure of cartilage surfaces to stick-slip sliding resulted in a significant increase of surface roughness, indicative of severe morphological changes (damage) of the cartilage surfaces.

Concepts: Force, Classical mechanics, Friction, Road, Tribology, Roughness, Normal force, Stick-slip phenomenon


Friction characteristics with respect to surface topographic orientation were investigated using surfaces of different materials and fabricated with grooves of different scales. Scratching friction tests were conducted using a nano-indentation-scratching system with the tip motion parallel or perpendicular to the groove orientation. Similar friction anisotropy trends were observed for all the surfaces studied, which are (1) under a light load and for surfaces with narrow grooves, the tip motion parallel to the grooves offers higher friction coefficients than does that perpendicular to them, (2) otherwise, equal or lower friction coefficients are found under this motion. The influences of groove size relative to the diameter of the mating tip (as a representative asperity), surface contact stiffness, contact area, and the characteristic stiction length are discussed. The appearance of this friction anisotropy is independent of material; however, the boundary and the point of trend transition depend on material properties.

Concepts: Force, Materials science, Orientation, Classical mechanics, Surface, Young's modulus, Friction, Tribology


The human finger is exquisitely sensitive in perceiving different materials, but the question remains as to what length scales are capable of being distinguished in active touch. We combine material science with psychophysics to manufacture and haptically explore a series of topographically patterned surfaces of controlled wavelength, but identical chemistry. Strain-induced surface wrinkling and subsequent templating produced 16 surfaces with wrinkle wavelengths ranging from 300 nm to 90 μm and amplitudes between 7 nm and 4.5 μm. Perceived similarities of these surfaces (and two blanks) were pairwise scaled by participants, and interdistances among all stimuli were determined by individual differences scaling (INDSCAL). The tactile space thus generated and its two perceptual dimensions were directly linked to surface physical properties - the finger friction coefficient and the wrinkle wavelength. Finally, the lowest amplitude of the wrinkles so distinguished was approximately 10 nm, demonstrating that human tactile discrimination extends to the nanoscale.

Concepts: Psychology, Perception, Sense, Mind, Wavelength, Philosophy of perception, Tribology


The nature of interfacial water is critical in several natural processes, including the aggregation of lipids into the bilayer, protein folding, lubrication of synovial joints, and underwater gecko adhesion. The nanometer-thin water layer trapped between two surfaces has been identified to have properties that are very different from those of bulk water, but the molecular cause of such discrepancy is often undetermined. Using surface-sensitive sum frequency generation (SFG) spectroscopy, we discover a strongly coordinated water layer confined between two charged surfaces, formed by the adsorption of a cationic surfactant on the hydrophobic surfaces. By varying the adsorbed surfactant coverage and hence the surface charge density, we observe a progressively evolving water structure that minimizes the sliding friction only beyond the surfactant concentration needed for monolayer formation. At complete surfactant coverage, the strongly coordinated confined water results in hydration forces, sustains confinement and sliding pressures, and reduces dynamic friction. Observing SFG signals requires breakdown in centrosymmetry, and the SFG signal from two oppositely oriented surfactant monolayers cancels out due to symmetry. Surprisingly, we observe the SFG signal for the water confined between the two charged surfactant monolayers, suggesting that this interfacial water layer is noncentrosymmetric. The structure of molecules under confinement and its macroscopic manifestation on adhesion and friction have significance in many complicated interfacial processes prevalent in biology, chemistry, and engineering.

Concepts: Electric charge, Fundamental physics concepts, Density, Water, Surfactant, Nature, Friction, Tribology


Arthritis is a leading cause of disability, and when nonoperative methods have failed, a prosthetic implant is a cost-effective and clinically successful treatment. Metal-on-metal replacements are an attractive implant technology, a lower-wear alternative to metal-on-polyethylene devices. Relatively little is known about how sliding occurs in these implants, except that proteins play a critical role and that there is a tribological layer on the metal surface. We report evidence for graphitic material in the tribological layer in metal-on-metal hip replacements retrieved from patients. As graphite is a solid lubricant, its presence helps to explain why these components exhibit low wear and suggests methods of improving their performance; simultaneously, this raises the issue of the physiological effects of graphitic wear debris.

Concepts: Critical thinking, Avicenna, Hip replacement, Graphite, Friction, Tribology, Lubricant, Molybdenum disulfide


Nano-objects have been investigated for drug delivery, oil detection, contaminant removal, and tribology applications. In some applications, they are subjected to friction and deformation during contact with each other and their surfaces on which they slide. Experimental studies directly comparing local and global deformation are lacking. This research performs nanoindentation (local deformation) and compression tests (global deformation) with a nanoindenter (sharp tip and flat punch, respectively) on molybdenum disulfide (MoS2) multi-walled nanotubes (MWNTs), ~500 nm in diameter. Hardness of the MoS2 nanotube was similar to bulk and does not follow the “smaller is stronger” phenomenon as previously reported for other nano-objects. Tungsten disulfide (WS2) MWNTs, ~300 nm in diameter and carbon nanohorns (CNHs) 80-100 nm in diameter were of interest and also selected for compression studies. These studies aid in understanding the mechanisms involved during global deformation when nano-objects are introduced to reduce friction and wear. For compression, highest loads were required for WS2 nanotubes, then MoS2 nanotubes and CNHs to achieve the same displacement. This was due to the greater number of defects with the MoS2 nanotubes and the flexibility of the CNHs. Repeat compression tests of nano-objects were performed showing a hardening effect for all three nano-objects.

Concepts: Molybdenum, Graphite, Materials science, Sulfide, Friction, Tribology, Tungsten, Lubricant


Friction, wear and the associated energy dissipation are major challenges in all systems containing moving parts. Examples range from nanoelectromechanical systems over hip prosthesis to off-shore wind turbines. Bionic approaches have proven to be very successful in many engineering problems, while investigating the potential of a bio-inspired approach in creating morphological surface textures is a relatively new field of research. Here, we developed laser-created textures inspired by the scales found on the skin of snakes and certain lizards. We show that this bio-inspired surface morphology reduced dry sliding friction forces by more than 40%. In lubricated contacts the same morphology increased friction by a factor of three. Two different kinds of morphologies, one with completely overlapping scales and one with the scales arranged in individual rows, were chosen. In lubricated as well as unlubricated contacts, the surface texture with the scales in rows showed lower friction forces than the completely overlapping ones. We anticipate that these results could have significant impact in all dry sliding contacts, ranging from nanoelectromechanical and micro-positioning systems up to large-scale tribological contacts which cannot be lubricated, e.g. because they are employed in a vacuum environment.

Concepts: Mass, Force, Classical mechanics, Friction, Tribology, Texture, Normal force, Lubricant


Hybrid nano-materials offer potential scope for an increasing number of novel applications when engineered to deliver usefully functional properties. Recent advancement in the design of new material products that result from interactions among different compositions at the nano and micro scale has led to innovative ways to fabricate and process hybrids with altered structural physicochemical properties. An example is the development of novel ‘lubricants’ that make use of ionic liquids (ILs) and their ability to induce exploitable molecular assemblies at the IL-graphene interface. In the present study, we report the potential of graphene-ionic liquid hybrid nanomaterials for engineering applications with a focus on ‘lubricant’ properties to reduce frictional forces to enhance tribological performance. The present contribution outlines the wear and tribological properties (friction and lubrication) of a highly viscous ionic liquid [BMIM][I] IL and its comparison with its nanohybrid material counterpart. Detailed structural-microstructural investigations of the nano-hybrid materials were performed using X-ray diffraction and microscopic techniques employing scanning electron (SEM) and transmission electron (TEM) microscopies. A comparative study of the morphology of friction track and wear behaviour was observed by SEM. These characteristic properties within and outside the friction track were further correlated with physical and chemical interactions obtained by contact angle measurements and Raman spectroscopy.

Concepts: Spectroscopy, X-ray, Liquid, Materials science, Ionic liquid, Friction, Tribology, Lubricant


A Ti-Cu coated layer on 316L stainless steel (SS) was obtained by using the Closed Field Unbalanced Magnetron Sputtering (CFUBMS) system to improve antibacterial activity, corrosion and tribological properties. The microstructure and phase constituents of Ti-Cu coated layer were characterized by using X-ray diffraction (XRD), scanning electron microscopy (SEM) and glow discharge optical emission spectrometry (GDOES). The corrosion and tribological properties of a stainless steel substrate, SS316L, when coated with Ti-Cu were investigated in a simulated body fluid (SBF) environment. The viability of bacteria attached to the antibacterial surface was tested using the spread plate method. The results indicate that the Ti-Cu coated SS316L could achieve a higher corrosion polarization resistance and a more stable corrosion potential in an SBF environment than the uncoated SS316L substrate. The desirable corrosion protection performance of Ti-Cu may be attributable to the formation of a Ti-O passive layer on the coating surface, protecting the coating from further corrosion. The Ti-Cu coated SS316L also exhibited excellent wear resistance and chemical stability during the sliding tests against Si3N4 balls in SBF environment. Moreover, the Ti-Cu coatings exhibited excellent antibacterial abilities, where an effective reduction of 99.9% of Escherichia coli (E.coli) within 12h was achieved by contact with the modified surface, which was attributed to the release of copper ions when the Ti-Cu coatings are in contact with bacterial solution.

Concepts: Bacteria, Escherichia coli, Steel, Stainless steel, Tribology, Coatings, Passivation, Corrosion


The number of revision joint replacements has been increasing substantially over the last few years. Understanding their failure mechanism is extremely important for improving the design and material selection of current implants. This study includes ten retrieved and four new mildly cross-linked ultra-high molecular weight polyethylene (UHMWPE) acetabular liners. Among them, most of the prostheses (n = 5) were reported to be revised and replaced due to aseptic loosening, followed by painful joint (n = 2), dislocation (n = 1), intra articular ossification (n = 1), combination of wear (liner) and osteolysis (stem) (n=1). Surface deviations (wear, material inflation and roughness), oxidative degradation and change of material properties were measured using micro-computed tomography (micro-CT) scan, 3D laser scanning microscopy, raman spectroscopy and nanoindentation, respectively. Prostheses having eccentric worn areas had much higher linear wear rates (228.01 ± 35.51µm/year) compared to that of centrically worn prostheses (96.71 ± 10.83µm/year). Oxidation index (OI) showed similar trends to the surface penetration depth. Among them, sample 10 exhibited the highest OI across the contact area and the rim of the cup liner. It also had the lowest hardness/elasticity ratio. Overall, wear and creep, oxidative degradation and reduced hardness/elasticity ratio all contributed to the premature failure of the UHMWPE acetabular cup liners.

Concepts: Redox, Raman spectroscopy, Materials science, Polyethylene, Tribology, Ultra high molecular weight polyethylene