Low Friction Properties Research Articles

Investigation on Wear and Friction Characteristics of Bi-Directional Silk Fiber Reinforced Nanoclay Added HDPE Composites

In the present day context ,the requirement of specific material properties cannot be met with the use of conventional metals and alloys.Therefore, the development of composite materials is gaining prominence.The readiness of composites for a specific purpose has been one of their greater advantages and more perplexing good challenges to adopt them as alternative materials to replace the existing ones.The polymer composites possessing low density, high strength to weight ratio, have the advantages of having low friction and good wear resistance property. Keeping this in mind, the naturally available silk fiber as reinforcement in HDPE matrix along with nano filler addition has been adopted in this work from the point of improving friction and wear resistance. In this study, the slide wear characteristics and the coefficient of friction of HDPE (high density polyethylene) containing bi-directional silk fiber (SF) composites at two levels of nanoclay (0.5 and 1.%NC) are experimentally investigated using pin on disc setup for three sliding distance. It is observed from slide wear data that increasing the sliding distance, the weight loss increases for pure HDPE, pure HDPE+SF, pure HDPE+SF+0.5%NC and pure HDPE+SF +1%NC of the load application of 10N,20N,30N respectively. Also among four samples studied, the pure HDPE+SF +1%NC shows the least weight loss and pure HDPE shows highest. As regards the coefficient of friction varies in the range of 0.12 to 0.37 for all the three sliding distances and load application 10N, 20N and 30N respectively. This work brings out the importance of nano-clay addition to silk fiber based HDPE composites shows the excellent wear resistance. Further the worn surface damage features have been examined and analyzed using scanning electron microscope (SEM) and it is noticed the wear data gets credence from the SEM photographs, thus complimenting each other.

Mechanical and frictional properties of aesthetic orthodontic wires obtained by hard chrome carbide plating

Background/purposeAlthough aesthetic wire coating has been increasing in demand, it has problems that changes in mechanical properties and increase in frictional force. The aim of this study was to evaluate the coating of the wire, as characterized by aesthetics, in terms of low and constant friction and mechanical properties. Materials and methodsHard chrome carbide-plated (HCCP) wires (HCCP group), commercially available polymer-coated wires (P group), rhodium-coated wires (R group), and uncoated wires (control group) were used. For all wire types, a stainless steel wire of dimensions 0.017 inch × 0.025 inch was used. They were evaluated by three-point bending, friction testing, surface observation, and colorimetric testing. ResultsThe HCCP group was not significantly different from the control group in terms of flexural strength (σ) and flexural modulus (E) (σ: p = 0.90, E: p = 0.35). However, it was significantly inferior compared to the three other groups in terms of the maximum static and kinetic frictional forces under both dry and wet conditions (p < 0.05). In the surface observation, scratches were observed on the wire after the friction test. In the colorimetric test, no significant difference was observed between the HCCP group and the R group (p > 0.05). ConclusionThe mechanical properties of the HCCP wire were not significantly different compared to the control group. The frictional force of the HCCP wire was significantly lower than the other group. Therefore, the HCCP wire was suggested to increase the efficiency of tooth movement in clinics.

Tribological properties of hydrogen free DLC in self-mated contacts against ZDDP-added oil

PurposeThe purpose of this study is to investigate the tribological properties of tetrahedral diamond-like carbon (DLC) films in self-mated contacts in the presence of additivated and non-additivated vegetable oils. DLC films have high practical value due to low friction and low wear properties. On the other hand, vegetable oils are considered to be lubricants for future due to its resource renewability and biodegradability. Sometimes different chemical agents are added to vegetable oils to further improve its tribological properties. Thus, the tribological study of DLC films against additivated oils becomes important.Design/methodology/approachThe tribology tests were conducted in a four ball tribo-meter under the boundary lubricated conditions.FindingsTa-C DLC exhibited 80 per cent lower wear rate under Zinc dialkyldithiophosphates (ZDDP)-added oil compared to that of base oil. In contrast, the friction coefficient under additivated oil was slightly higher than the base oil lubricated case. Moreover, the carbonyl band area as well as the viscosity change of ZDDP-added oil was much smaller than that of base oil. Therefore, ZDDP reduced the wear of DLC film and prevented the oxidation of base oil during tribotests.Originality/valueThis is the first work on the tribological properties of ta-C DLC lubricated with corn oil with and without anti-wear additives.

Demonstrating the benefit of self-ligating brackets in the management of severe dental crowding on a non–extraction basis

Abstract: There is a growing amount of literature describing the benefits of self-ligating brackets; one such benefit is their low friction properties. A clinical case is described in which dental extractions were avoided to alleviate severe crowding and the conclusion is drawn that this may, in part, have been due to the decreased friction in the orthodontic appliance afforded by the use of self-ligating brackets. CPD/Clinical Relevance: This article discusses the benefits of self-ligating brackets and describes a clinical case in which, potentially, the low friction properties of self-ligating brackets helped avoid dental extractions.

Tight-Binding Quantum Chemical Molecular Dynamics Study on the Friction and Wear Processes of Diamond-Like Carbon Coatings: Effect of Tensile Stress.

Diamond-like carbon (DLC) coatings have attracted much attention as an excellent solid lubricant due to their low-friction properties. However, wear is still a problem for the durability of DLC coatings. Tensile stress on the surface of DLC coatings has an important effect on the wear behavior during friction. To improve the tribological properties of DLC coatings, we investigate the friction process and wear mechanism under various tensile stresses by using our tight-binding quantum chemical molecular dynamics method. We observe the formation of C-C bonds between two DLC substrates under high tensile stress during friction, leading to a high friction coefficient. Furthermore, under high tensile stress, C-C bond dissociation in the DLC substrates is observed during friction, indicating the atomic-level wear. These dissociations of C-C bonds are caused by the transfer of surface hydrogen atoms during friction. This work provides atomic-scale insights into the friction process and the wear mechanism of DLC coatings during friction under tensile stress.

Experimental characterisation of carbon fibre brush seal leakage performance as a function of bristle pack geometrical parameters under dry conditions

ABSTRACTOver the last decades, it has been progressively acknowledged that reducing the specific fuel consumption and the emission of pollutants, as well as improving the thrust-to-weight ratio involves extensive research on advanced sealing technologies. Amongst these, brush seals are particularly well considered for their excellent leakage performance, their low friction properties, and their ability to cope with inevitable rotor excursions during flights. This paper presents the experimental work that has been carried on in order to characterise carbon fibre brush seals leakage rate in function of the bristle pack geometry, under different pressure loads and rotational speeds. The analysed parameters are the bristle-free length, the density and the inter-plate distance. The work, performed by the ULB (Université Libre de Bruxelles) in collaboration with French engine manufacturer Safran Aircraft Engines, highlights specific behaviour of carbon fibre brush seals under differential pressure, proposes a leakage prediction model developed through empirical equations, and discusses over the most influential parameters that influence the air consumption of a brush seal.

Effects of Machining History on Tribological Properties of Bronze Containing Micro-Sized Sulfide

Bronze based alloys have come to be used as bearing materials, and require low frictional properties. The present study describes the effects of surface finish machining history on the on tribological properties of sulfide-containing bronze sintered onto a steel disc. The sulfide phase consisted of copper, iron and sulfur, and acted as a solid lubricant. The relative density of the specimen was 90 % and the microstructure showed that the micro-sized sulfide dispersed into the bronze matrix and was accompanied by pores. Various processes, including cutting, burnishing, polishing were, applied to finish the surface. It was found that the area fraction of the sulfide phase depended on the cutting conditions. A further increase in the area fraction was achieved using roller burnishing after the cutting. Tribological properties were evaluated using chromium alloy steel (SUJ2) as the mating surface under dry conditions. The results showed that the friction coefficient depended on the surface finishing process. Therefore, the machining history had an effect on the frictional properties of the sulfide-containing bronze. Optimal cutting conditions for the sulfide containing bronze are also discussed.

Redifferentiated Chondrocytes for the Repair of Articular Cartilage Lesions

Objectives:Biological therapies such as autologous chondrocyte implantation which use serially passaged chondrocytes to repair cartilage defects, are often unsuccessful as they lead to the formation of fibrocartilage. Fibrocartilage is rich in collagen type 1 (Col1) and lacks the structural complexity, low friction, and load distribution properties of articular cartilage. Fibrocartilage forms as a result of the phenotype of passaged chondrocytes. When serially passaged in monolayer to increase cell numbers, chondrocytes lose their phenotype in a process termed “dedifferentiation”. This is characterized by a loss in the production of collagen type II (Col2) and aggrecan (Acan), the major extracellular matrix molecules of articular cartilage, and an increase in the production of Col1. Therefore, to increase the success of therapies which use passaged chondrocytes, these cells must be redifferentiated to re-establish an articular cartilage phenotype prior to implantation in a cartilage lesion. Transforming growth factor beta (TGFβ) signalling is known to play important roles in both cell differentiation and articular cartilage repair. However, intra-articular injections with TGFβ have been shown to result in osteophyte formation and fibrosis of synovial tissues. Therefore, redifferentiating passaged chondrocytes in vitro using TGFβ prior to their use in vivo may be a better alternative. Fibrin glue is commonly used in orthopaedics to maintain cells in a cartilage lesion, however the effects of fibrin glue on chondrocytes have not been fully investigated. Thus we hypothesize that passaged chondrocytes redifferentiated in vitro with TGFβ will maintain their phenotype and form articular cartilage-like tissue when embedded in fibrin glue in the absence of further exogenous supplementation with TGFβ in vitro.Methods:Chondrocytes were enzymatically isolated from rabbit articular cartilage and serially passaged in monolayer twice (P2) to increase cell number and facilitate dedifferentiation. P2 cells were cultured in scaffold-free 3D using a chondrogenic serum-free media supplemented with 10ng/ml TGFβ3 to facilitate redifferentiation. Redifferentiated chondrocytes were then embedded in fibrin glue (Tisseel, Baxter) at 1.5 x 106 cells/20uL and cultured for 2 weeks in vitro in the absence of TGFβ3. Controls were P2 cells that were not redifferentiated with TGFB3 prior to embedding in fibrin glue. Histology and immunohistochemistry were used to assess tissue formation and cell phenotype.Results:Redifferentiated chondrocytes accumulated more matrix rich in Col2 and Acan than controls (Figure 1). Furthermore, redifferentiated chondrocytes had a round cell shape characteristic of articular chondrocytes. The use of fibrin glue did not impair the ability of redifferentiated chondrocytes to accumulate matrix nor did it affect cell phenotype.Conclusion:Redifferentiated chondrocytes in fibrin glue may be a better alternative to passaged chondrocytes for cell based articular cartilage repair therapies as they accumulate more Col2 and Acan containing matrix than undifferentiated P2. In vivo studies are required to elucidate the potential for redifferentiated chondrocytes in cartilage repair.

Tribological Performance of Halogen-Free Ionic Liquids in Steel–Steel and DLC–DLC Contacts

ABSTRACTThe tribological performance of halogen-free ionic liquids at steel–steel and diamond-like carbon (DLC)–DLC contacts was investigated. Hydrogenated amorphous carbon (a-C:H) and tetrahedral amorphous carbon (ta-C) were used as test specimens. Friction tests were carried out on steel–steel, a-C:H–a-C:H, and ta-C–ta-C contacts by using a reciprocating cylinder-on-disk tribotester lubricated with two different types of halogen-free ionic liquids: 1-ethyl-3-methylimidazolium dicyanamide ([BMIM][DCN]) and 1-butyl-3-methylimidazolium tricyanomethanide ([BMIM][TCC]). From the results of friction tests, the ta-C–ta-C tribopair lubricated with [BMIM][DCN] or [BMIM][TCC] exhibited an ultralow friction coefficient of 0.018–0.03. On the other hand, ultralow friction was not observed at the steel–steel and a-C:H–a-C:H contacts. Measurements obtained with a laser scanning microscope and an atomic force microscope (AFM) showed that a chemical reaction film, derived from the ionic liquid lubricant used, was formed on the steel surfaces. However, this chemical reaction film was not observed on either of the DLC surfaces. The AFM results showed that there were high-viscosity products on the ta-C surfaces, that the wear tracks on the ta-C surfaces exhibited low frictional properties, and that the ta-C surfaces were extremely smooth after the friction tests. Based on these results, it was concluded that an ionic liquid–derived adsorbed film formed on the ta-C surface and resulted in the ultralow friction when lubricated with a halogen-free ionic liquid.

Low Friction Property of Boron Doped DLC under Engine Oil

Low Friction Property of Boron Doped DLC under Engine Oil

Low friction properties of associated carboxylic acids induced by molecular orientation

Molecular alignment is key to achieving an ultra-low friction coefficient. We prepared associated carboxylic acids using oleic acid and p-pentylbenzoic acid via hydrogen bonding, involving a benzene ring and an eight-membered cyclic carboxylic acid in the molecules. We investigated frictional properties when these were added into a base oil and found that the associated carboxylic acids decreased the traction coefficient in EHL and boundary lubrication, although the viscosity increased. An in situ micro Fourier transform infrared spectroscope showed that both the eight-membered cyclic carboxylic acid and the benzene ring were oriented parallel to the shearing force, leading to the low friction coefficient.

The Effects of Graphite Filler on the Tribological Properties of Polydopamine/PTFE Coatings

This investigation examines the effects of incorporating graphite as a filler in the polytetrafluoroethylene (PTFE) topcoat of a polydopamine/PTFE dual-layer coating. Polydopamine (PDA) is used to enhance the adhesion of PTFE coatings to the substrate, and graphite is used to improve the formation of a transfer film on the counterface and to take advantage of the low-friction properties of graphite. The results show that incorporating only 1.0 wt% graphite in PDA/PTFE coatings can increase their durability fivefold and reduce friction by 17 %. Examination of the wear morphology indicates that these improvements in tribological properties are in fact the result of the formation of a transfer film on the counterface that allows the coating to essentially slide against itself, creating a low shear strength interface that exhibits exceptionally low friction and low wear. Linearly increasing load scratch tests show that the improvement in durability is also the result of improved adhesion between the PTFE topcoat and the PDA basecoat, preventing large-scale delamination of the coating.

Tailoring hydrogel surface properties to modulate cellular response to shear loading.

Excessive mechanical loading is believed to be a major risk factor inducing pathogenesis of articular cartilage and other load-bearing tissues. Yet, the mechanisms leading to increased transmission of mechanical stimuli to cells embedded in the tissue remain largely unexplored. Here, we demonstrate that the tribological properties of loadbearing tissues regulate cellular behaviour by governing the magnitude of mechanical deformation arising from physiological tissue function. Based on these findings, we propose that changes to articular surface friction as they occur with trauma, aging, or disease, may initiate tissue pathology by increasing the magnitude of mechanical stress on embedded cells beyond a physiological level.

The role of calcinated sea shell particles on friction-wear behavior of electroless NiP coating: Fabrication and characterization

The present paper reveals the importance of calcinations of sea shell particles before their use to reinforce NiP composite coatings to achieve low friction and wear resistant property. NiP/calcinated sea shell particles (Cal.SSP) and NiP/non calcinated sea shell particles (SSP) composite coatings have been fabricated on En8 steel using electroless process with various dispersions of SSP and Cal.SSP. Thermo gravimetric analysis (TGA) of sea shell particles was carried out to find the optimum temperature for calcination. A comprehensive study on morphology, structure, hardness and friction - wear property of composite coatings was carried out using scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM) with SAED, X-ray diffraction analyzer (XRD), Vickers hardness tester and pin-on-disc apparatus. XRD analysis showed a shift in the peak for SSP and Cal.SSP, indicating a significant improvement in the property of the coating. Phase transformation of Cal.SSP from aragonite to calcite structure showed better friction and wear resistant properties with low hardness in contrast to SSP. The results accordingly demonstrate the friction and wear reducing ability of SSP after phase transformation.

Tribological behavior of diamond-like carbon thin films deposited by the pulse laser technique at different substrate temperatures

Diamond-like carbon (DLC) thin films were deposited by the pulse laser deposition at different substrate temperatures. They were deposited at the temperature range 100–500°C and evaluated for their tribological behavior. The as-synthesized DLC films were characterized by different techniques. Ultrafine nanoparticles were observed in the deposited DLC films. The morphology of the deposited DLC films was significantly modified by increasing the substrate temperature. The particle size was increased from around 10nm to approximately 30nm by increasing the substrate temperature from 100 to 500°C. The surface roughness was also increased, beside a formation of DLC clusters mainly at 500°C. Raman and XPS spectra showed a significant improvement in the graphitization along with pronounced defects in the amorphous carbon structures in these DLC films. These modifications, induced by increasing the DLC substrate temperature have increased values of the friction coefficient and specific wear rate. These results showed that the DLC films with ultrafine nanoparticles have good prospect to be used as protective layers for metallic surfaces as they show low friction property and high wear ability.

Moiré scaling of the sliding force in twisted bilayer graphene

The weak interlayer binding in two-dimensional layered materials such as graphite gives rise to distinguished low-friction properties if the atomic lattices at the interface are rotated with respect to one another. The lack of crystal symmetry leads to poorly understood correlations and cancelations of the interlayer atomic forces. Here we report on a powerful tiling method based on the moir\'e superstructure which allows us to study the intricate interplay of the interlayer forces in a systematic manner. Based on numerical simulation data for a circular graphene flake on an infinite graphene substrate, it is shown that the sliding force is dominated by a rim area consisting of incomplete moir\'e tiles. This rim force, which scales with the number of atoms in a moir\'e tile and as the radius to the power of 0.5, is minimal whenever the sliding structure can be approximated by a hexagon composed of an integer number ${N}_{t}$ of moir\'e tiles. Intriguingly, the corresponding area force scales as ${N}_{t}$ to the power of 0.25, i.e., it increases with size, whereas it has been often argued that interlayer forces should add up to a zero value for large twisted systems. However, at specific twist angles the moir\'e structure is commensurate with the graphene lattice, leading to a perfect force correlation in the moir\'e tiles. Correspondingly, the area force becomes dominant and scales as ${N}_{t}$, i.e., as the radius to the power of 2.

Low friction properties of Ti3AlC2/SiC tribo-pair in sea water environment

In this paper, the tribological property of Ti3AlC2/SiC tribo-pair in sea water was investigated. For comparison, the tribological property in deionized water was studied as well. The results revealed that the Ti3AlC2/SiC tribo-pair displayed better lubricating effect in the two aqueous liquids. The friction coefficient and wear rate decreased with the increment of reciprocating frequency. Herein, a layer of soft hydrated tribo-film, which was caused by tribo-chemical reaction in aqueous environment, was easily sheared and conceived to be liable for the lubricating effect. The lubrication and wear mechanism were boundary lubrication and tribo-chemical wear, respectively. When the soft hydrated tribo-film was exposed in air, the tribo-film could contract and yield rimose tribo-film on account of the loss of water.

Long-runout landslides and the long-lasting effects of early water activity on Mars: COMMENT

Long-runout landslides and the long-lasting effects of early water activity on Mars: COMMENT

Two Kinds of Contact Problems in Dodecagonal Quasicrystals of Point Group 12 mm

In this paper, two kinds of contact problems in 2-D dodecagonal quasicrystals were discussed using the complex variable function method: one is the finite frictional contact problem, the other is the adhesive contact problem. The analytic expressions of contact stresses in the phonon and phason fields were obtained for a flat rigid punch, which showed that: (1) for the finite frictional contact problem, the contact stress exhibited power-type singularities at the edge of the contact zone; (2) for the adhesive contact problem, the contact stress exhibited oscillatory singularities at the edge of the contact zone. The distribution regulation of contact stress under punch was illustrated; and the low friction property of quasicrystals was verified graphically.

Unprecedented self-assembly of precise molecular particles

Atomic self-assembly in pure metals and metal alloys can lead into various conventional (cubic, hexagonal closepacked (hcp), and icosahedron) and non-conventional (Frank–Kasper and quasicrystal [1]) structures. The nonconventional crystalline structures often lead to unexpected physical properties. For example, the A15 (A3B type) Frank– Kasper phase of Nb3Sn, Nb3Zr and Nb3Ti are found to be superconducting. Metallic quasicrystals are hard and have low-friction, low-thermal conductivity, and special electronic properties. Beyond the atomic self-assembly, computer simulations have predicted many complex self-assembled structures, such as Frank–Kasper phases and quasicrystals, as a function of shape, size, and interaction for nanoparticles (Fig. 1) [2]. However, it has been challenging to experimentally achieve these structures from both hard (colloidal particles and mixtures) and soft (hierarchically self-assembled surfactants, dendrimers, and micellar block copolymers) nanoparticles. This is because precise shape, size, and interaction for colloidal nanoparticles are still difficult to control. In a recent report by Huang et al. [3], this grand challenge has been conquered by precise synthesis of rigid giant tetrahedra from molecular nanoparticles i.e. polyhedral oligomeric silsesquioxane (POSS), by using two orthogonal ‘click’ chemistry techniques, namely, the azide-alkyne [3 + 2] cycloaddition reaction and the thiol-ene reaction. In this clever way, different numbers of Nanoparticle self-assembly