Initial Friction Coefficient Research Articles

TRIBOLOGY OF BIO-INSPIRED NANOWRINKLED FILMS ON ULTRASOFT SUBSTRATES

Biomimetic design of new materials uses nature as antetype, learning from billions of years of evolution. This work emphasizes the mechanical and tribological properties of skin, combining both hardness and wear resistance of its surface (the stratum corneum) with high elasticity of the bulk (epidermis, dermis, hypodermis). The key for combination of such opposite properties is wrinkling, being consequence of intrinsic stresses in the bulk (soft tissue): Tribological contact to counterparts below the stress threshold for tissue trauma occurs on the thick hard stratum corneum layer pads, while tensile loads smooth out wrinkles in between these pads. Similar mechanism offers high tribological resistance to hard films on soft, flexible polymers, which is shown for diamond-like carbon (DLC) and titanium nitride thin films on ultrasoft polyurethane and harder polycarbonate substrates. The choice of these two compared substrate materials will show that ultra-soft substrate materials are decisive for the distinct tribological material. Hierarchical wrinkled structures of films on these substrates are due to high intrinsic compressive stress, which evolves during high energetic film growth. Incremental relaxation of these stresses occurs by compound deformation of film and elastic substrate surface, appearing in hierarchical nano-wrinkles. Nano-wrinkled topographies enable high elastic deformability of thin hard films, while overstressing results in zigzag film fracture along larger hierarchical wrinkle structures. Tribologically, these fracture mechanisms are highly important for ploughing and sliding of sharp and flat counterparts on hard-coated ultra-soft substrates like polyurethane. Concentration of polyurethane deformation under the applied normal loads occurs below these zigzag cracks. Unloading closes these cracks again. Even cyclic testing do not lead to film delamination and retain low friction behavior, if the adhesion to the substrate is high and the initial friction coefficient of the film against the sliding counterpart low, e.g. found for DLC.

Research on the microstructure and space tribology properties of electric-brush plated Ni/MoS2–C composite coating

In order to expand the application field of MoS2 lubricant coatings (films) and to improve their adaptability in harsh environments, a Ni/MoS2–C composite coating composed of Ni metallic matrix and two lubricating components was prepared on AISI 52100 steel substrates with a nano electro-brush plating process. The relationship between microstructure and properties of the coating was analyzed via microstructure characterization. The tribological properties of the as-received coating samples and the samples treated in humid air (HA), high vacuum (HV), atomic oxygen (AO) erosion and ultraviolet (UV) irradiation exposure were investigated comparatively. The results show that the surface of the obtained Ni/MoS2–C coating is smooth and clean. The micro and nano MoS2–C grains are dispersed in the Ni matrix homogeneously. The MoS2 and C particles also showed good synergistic effects. The added nano graphite particles not only increased the moisture resistance of the composite coating, but also reduced the residual stress and enhanced the mechanical properties of the coating. The composite coating still showed good lubricating properties after long-duration treatments in HA, HV, AO and UV environments, and the friction coefficients during the stable running periods remained at about 0.1. The influence of vacuum outgassing on the properties of the lubricants is not obvious. The coatings were oxidized slightly by AO, and thus their initial friction coefficients in AO conditions also increased, but the coating still showed good friction reduction properties after a short running-in period. The changes in microstructure and properties of the coating after UV irradiation were also inconspicuous. All test results indicate that the Ni/MoS2–C composite coating has excellent space tribological properties and good adaptability in harsh environments. So it is promising to use Ni/MoS2–C composite coating in a space solid lubrication area.

Tribology of Self-lubricating SU-8+PFPE Composite based Lub-tape

Despite the fact that the SU-8 is a useful material for micro-fabrication of MEMS devices using the photo- lithography process, its poor tribological properties restrict its wider applications. It was postulated in our previous study that adding perflouropolyether (PFPE) lubricant to SU-8 possibly promoted chemical reaction between the molecules and also formed the boundary lubrication, which enhanced the wear durability of SU-8 by more than four orders of magnitude. The same SU-8+PFPE composite were used to fabricate a stand-alone laminate film called “Lub-tape”, which has two layers of ∼90μm thickness each. The top and bottom layers were made of SU-8+PFPE composite and pristine SU-8, respectively. The lub-tape has reduced the initial coefficient of friction by ∼7 times and increased wear life by more than five orders of magnitude of SU-8. It can also improve the tribology of any given surface. The Lub-tape can be used to protect all surfaces against friction and wear.

Frictional performance and near-surface evolution of nanocrystalline Ni–Fe as governed by contact stress and sliding velocity

While early reports on the wear performance of nanocrystalline alloys have suggested enhanced behavior consistent with their higher hardness compared to conventional microcrystalline alloys, there is still limited understanding of the mechanisms and limits of this enhanced behavior. In the present study, we examine the frictional response of a nanocrystalline Ni–20Fe alloy with 34-nm average grain size compared to the same film annealed to an average grain size of 500-nm. We examine the sliding friction performance of these films in contact with a 3.125mm diameter Si3N4 spherical counterface under a range of normal forces (0.1–1.0N) and sliding speeds (0.25–3.75mm/s) in a non-oxidizing dry nitrogen environment. Under all conditions, the initial break-in coefficient of friction (COF) starts high, μ≈0.5–0.8, typical of uncoated metallic friction. However, there is an evolution in the COF which depends on normal force and sliding speed. At low sliding speeds (or normal forces), the steady-state COF decreases to μ≈0.2 whereas at higher sliding speeds and normal forces, the steady-state COF remains high at μ≈0.8. Focused ion beam cross-sectioning and TEM imaging reveal that in all cases, a multilayer substructure is formed in the deforming film: a refined ultrananocrystalline layer at the top surface, over a region of coarsened grains, atop the parent nanocrystalline alloy. The key distinction between the high-friction and low-friction conditions appears to lie in the triggering of a delamination process: high-friction conditions are associated with a thickening of the UNC layer through repeated delamination, whereas low-friction conditions are associated with a thin UNC layer that does not delaminate. Finite element analysis is used to aid in the understanding of how the magnitude and location of stresses drive these two distinct regimes.

Self-lubricating SU-8 Nanocomposites for Microelectromechanical Systems Applications

SU-8 is an industrially useful photoresist polymer for micro-fabrication because of its unique UV-sensitive curing property. It is also used as a structural material for micro-machines such as micro-electro mechanical systems (MEMS). However, it has poor tribological and mechanical properties which make SU-8 inferior to Si, the mainstay MEMS material today. In this paper, we report the fabrication of SU-8 nanocomposites which are self-lubricating and have better mechanical properties. The liquid lubricant i.e., perfluoropolyether (PFPE) and nanoparticles such as SiO2, CNTs, and graphite were added into SU-8 for this purpose. These self-lubricating SU-8 + PFPE and SU-8 + PFPE + nanoparticle composites have shown a reduction in the initial coefficient of friction by ~6–9 times and increased wear life by more than four orders of magnitude. The mechanical properties such as the elastic modulus and the hardness have increased by ~1.4 times. These SU-8 nanocomposites can be used as a self-lubricating structural material for MEMS applications requiring no external lubrication. As well, these nanocomposites can find applications in many tribological components of traditional machines.

A DLC/diamond bilayer approach for reducing the initial friction towards a high bearing capacity

Sliding contact mechanics is used to account for the net increase of the bearing capacity of high wear-resistant CVD diamond coatings when an outer layer of a DLC lubricant film is deposited on their top surface. Experimental results, namely the critical failure load, taken from reciprocating ball-on-plate tribological testing of such DLC/diamond bilayers, are on the basis of this statement. DC magnetron sputtering was used to grow thin DLC lubricant coatings (non-hydrogenated and hydrogenated varieties) on the top of CVD diamond films (microcrystalline and nanocrystalline types). The application of the von Mises maximum yield parameter (J2) criterion demonstrates to be an adequate method for designing low-friction and high-bearing capacity systems for high-demanding tribological applications. Calculation of J2supports that the most significant increase observed on the coatings’ bearing capacity (from 20N to 60N) takes place when microcrystalline diamond is coated by hydrogenated DLC that decreases the initial friction coefficient peak (from 0.76 to 0.41).

The Tribological Performances of Multilayer Graphite-Like Carbon (GLC) Coatings Sliding Against Polymers for Mechanical Seals in Water Environments

Graphite-like carbon (GLC) coatings are being increasingly used in mechanical seals, especially under the water-lubricated condition, to improve component durability by providing a low friction coefficient and high wear resistance. We have addressed the friction and wear performances of multilayer Cr/CrN/GLC coatings sliding against polyether–ether–ketone (PEEK), polyimide (PI), and polytetrafluoroethylene (PTFE) in distilled water under various applied loads and sliding speeds using a ring-on-block test rig. Stribeck curves were developed to analyze the water lubrication mechanisms. Start–stop tests were also carried out to evaluate the tribological performances of the coatings. Our results showed that the friction coefficients and wear rates of the polymers decreased with increasing sliding speeds. Compared to the applied load, the sliding speed had a major effect on the friction and wear performance of the polymer. Three coating/polymer tribopairs demonstrated different tribological behaviors, which were attributed to their different molecular structures, the different mechanical properties of the polymers, and the different lubrication mechanisms. The transfer film determined the lubrication mechanism of the three tribopairs and also resulted in the low friction coefficient and wear rate. The initial and steady-state friction coefficient also decreased with increases in the repeated start–stop times. Based on our results, we concluded that the coating/PEEK tribopair presented an excellent tribological performance that we ascribed to the chemical properties of PEEK and the hydrodynamic lubrication regime of the coating/PEEK tribopair at high sliding speeds.

Friction and Wear on Single-Layer Epitaxial Graphene in Multi-Asperity Contacts

Friction and wear of single layers of graphene have been studied at the micrometer scale. Epitaxial graphene grown by thermal decomposition on SiC-6H(0001) is found to have an initial friction coefficient of 0.02, significantly lower than graphite under the same experimental conditions. During reciprocal sliding the graphene layer is damaged. The evolving friction coefficient of 0.08 for the carbon-rich interface layer terminating the SiC layer is still lower than that of graphite and five times lower than that of the hydrogen-etched SiC substrate. Micrometer-sized patches within the sliding track retain the low friction coefficient of graphene even after hundred sliding cycles.

The study on the stress and the friction coefficient of tetrahedral amorphous carbon films bombarded by energetic Ar ion

The ta-C films with sp3 bonds more than 80% in fraction are deposited by FCVA technique, and then they are bombarded by Ar ions. The composition and structures of the ta-C films before and after the bombardment of energetic Ar ions are analyzed by X-ray photoelectron spectroscopy. The surface morphology is investigated by AFM. The result shows that the bombardment of Ar ions induces the conversion of sp3 bond into sp2 bond, and the fraction of sp2 bonds increases with the energy of Ar ion increasing. The stress of the film decreases with the increase of the Ar ion energy. The RMS and etching pits on the surface of film increase with the increase of Ar ion bombarding energy.The friction test indicates that Ar ion bombardment has an important influence on initial friction coefficient, but just has little influence on steady state friction coefficient. The steady state friction coefficient of film keeps about 0.1, which shows a good antiwear property.

Control of seismically excited benchmark highway bridge with variable frequency pendulum isolator

Earthquake response of benchmark highway bridge, isolated with variable frequency pendulum isolator (VFPI) is investigated under six earthquakes, using a simplified lumped mass finite element model. The isolators are installed between deck-ends and abutments. Seismic response of the bridge with VFPI is compared with that of the bridge with the friction pendulum system (FPS). Parametric studies are carried to find the optimum values of coefficient of friction and isolation period of the FPS; and frequency variation factor, initial time period and coefficient of friction of VFPI. The response of the bridge with VFPI is compared with the uncontrolled case, with FPS, and that controlled by alternate sample control strategies. Significant reductions in base shear, base moment, deck displacements and bearing displacements are observed using FPS and VFPI. It is concluded that these isolators are quite effective in substantially reducing peak response quantities of the bridge, comparable to that of the sample controllers.

Contact Mechanisms of Transfer Layered Surface During Sliding Wear of Amorphous Carbon Film

The contact mechanisms of a transfer layered surface during sliding wear of a Si3N4 ball against the amorphous carbon film were investigated. In this study, amorphous carbon films were deposited by electron cyclotron resonance plasma sputtering technique. The dependence of friction coefficient and wear life of the films on transfer layer was tested with pin-on-disk tribometer. Wear tracks and the transfer layered surfaces at different friction coefficient stages were observed with scanning electron microscope and measured with energy dispersive spectrometer In order to clarify the contact mechanisms of a transfer layered surface, three contact models of initial high friction coefficient stage without transfer layer (state I), transfer layer forming stage with friction coefficient decreasing (state II), and transfer layered surface stable sliding stage with low friction coefficient (state III) were proposed, and the contact stresses (normal stress, shear stress, von Mises stress) of the three contact states were calculated by using finite element analysis. The results demonstrated that a transfer layer formed at the contact interface and gradually decreased the maximum contact stresses, which contributed to the long wear life of amorphous carbon films.

A wear particle-based model of friction in a polymer–metal high pressure contact

Sliding contact under high pressure between a more or less rough metal surface and a polymer occurs in diverse manufacturing processes. Transfer of the softer material on the harder counterpart is a common phenomenon under such conditions (high pressure) due to high level of polymer wear. The transfer of worn polymer particles on the metal surface generally has a large influence on friction. A model of wear, transfer and their consequences on friction is described. Based on the third body concept, it shares the created wear particles between the transfer film and particles escaping the contact. The model includes unknown parameters describing particles creation and movement on the one hand, and the friction–transfer relationship on the other hand. The parameters are determined experimentally on a linear reciprocating sliding tribometer, where a polymer-coated bar slides on a rough metallic cylinder in a point contact configuration, under conditions of large sliding length (10mm) and high pressure. Abrasive wear and transfer occur, the initial friction coefficient is shown to follow Coulomb's model (no variation with the applied average pressure); it increases with sliding length (or cycle number) due to increasing coverage of the metal surface by wear particles (or transfer film average thickness). It is shown finally that the model describes quite well the evolution of friction along several cycles.

Polymer Jet Printing of SU-8 Micro-Dot Patterns on Si Surface: Optimization of Tribological Properties

Tribological properties of optimized SU-8 patterns (micro-dots with varying pitch) on Si (silicon) were evaluated using a ball-on-disk tribometer. Sliding tests on the patterns were conducted against a 2-mm diameter Si3N4 ball at varying normal loads and sliding velocities. It was observed that the pitch of the SU-8 pattern on Si substrate had a significant effect on the initial coefficient of friction and wear durability. Initial coefficient of friction studies have concluded that the SU-8 polymeric micro-dots improved the tribological properties by sharing the normal force and reducing the contact area. For the wear durability test, ultra-thin layer of perfluoropolyether was over-coated onto SU-8 micro-dot specimens, and the optimized pitch specimens have shown wear durability of more than 100,000 cycles at a normal load of 350 mN.

Double variable frequency pendulum isolator for seismic isolation of liquid storage tanks

The paper describes the behaviour of liquid storage slender and broad tanks isolated by the double variable frequency pendulum isolator (DVFPI). The DVFPI is a double sliding isolation system having elliptical sliding surfaces. The geometry and coefficient of friction of top and bottom sliding surfaces can be unequal. The governing equations of motion and energy balance equation of the tank-isolation system subjected to bilateral ground excitation are derived and solved in the incremental form. In order to investigate the behaviour of the DVFPI, the response is obtained under different parametric variations for a set of 20 far-field earthquake ground motions. Four different combinations of the DVFPI design cases having different isolator geometry and coefficient friction at top and bottom sliding surfaces are studied and the criterion to optimize its performance is proposed based on minimum responses and energy quantities. Further, influences of the initial time period, coefficient of friction and frequency variation factors at the two sliding surfaces and the tank aspect ratio are investigated. It is found that the performance of the DVFPI can be optimized by designing the top sliding surface with high initial stiffness relative to the bottom one and the coefficient of friction of both sliding surfaces to be equal for a slender tank whereas both surfaces should be designed with equal initial stiffness and coefficient of friction for a broad tank.

Simulated three-dimensional transient temperature field during aircraft braking for C/SiC composite brake disc

In the present work, a theoretical model of three-dimensional transient temperature field for C/SiC composite brake discs was established by adopting a finite element method according to the theory of energy transformation and transportation. The variation regularities of transient temperature field and internal temperature gradient of the brake discs were obtained. The effects of initial velocity, deceleration and friction coefficient on the highest temperature of the brake discs were also discussed. The heat energy was mainly concentrated on the layer of friction surfaces. The highest temperature of the brake discs under normal landing, overload landing, and rejected take-off landing condition were 869.7 K, 1037.4 K and 1440.3 K, respectively. Furthermore, the highest temperature of the brake discs increased with the increase of the initial velocity and friction coefficient, but decreased with the increase of deceleration. Comparing simulation predictions with experimental results, it is found that the three-dimensional transient temperature field model was valid and reasonable.

Tribo-functionalizing Si and SU8 materials by surface modification for application in MEMS/NEMS actuator-based devices

Micro/nano-electro-mechanical-systems (MEMS/NEMS) are miniaturized devices built at micro/nanoscales. At these scales, the surface/interfacial forces are extremely strong and they adversely affect the smooth operation and the useful operating lifetimes of such devices. When these forces manifest in severe forms, they lead to material removal and thereby reduce the wear durability of the devices. In this paper, we present a simple, yet robust, two-step surface modification method to significantly enhance the tribological performance of MEMS/NEMS materials. The two-step method involves oxygen plasma treatment of polymeric films and the application of a nanolubricant, namely perfluoropolyether. We apply the two-step method to the two most important MEMS/NEMS structural materials, namely silicon and SU8 polymer. On applying surface modification to these materials, their initial coefficient of friction reduces by ∼4–7 times and the steady-state coefficient of friction reduces by ∼2.5–3.5 times. Simultaneously, the wear durability of both the materials increases by >1000 times. The two-step method is time effective as each of the steps takes the time duration of approximately 1 min. It is also cost effective as the oxygen plasma treatment is a part of the MEMS/NEMS fabrication process. The two-step method can be readily and easily integrated into MEMS/NEMS fabrication processes. It is anticipated that this method will work for any kind of structural material from which MEMS/NEMS are or can be made.

Preparation and Process Optimization of Ni-Tl-B Coating with Fretting Damage Resistance

Electroless plating technology used to alleviate fretting damage of titanium alloy was studied, Ni-Tl-B coatings were prepared, process optimization was worked out by the orthogonal table, deposition rate and solution consumption per hour and the initial friction coefficient and the bearing cycles under a low friction coefficient were chose as the index to calculate the range according to the economical efficiency and the application of coating under the surface morphology analysis and the fretting friction coefficient test, optimum process was finally worked out. Surface morphology was analyzed by TM-1000 scanning electron microscope and the fretting friction coefficient was tested by the DELTALAB-NENE fretting wear tester. The results shows that the surface morphology of the Ni-Tl-B coating are all typical cell structure which will be slightly different when the process changes and the optimum Ni-Tl-B coating has outstanding fretting damage resistance.

Dry Sliding Wear Behaviour of Oil Jet Peened Aluminium Alloy, AA6063-T6

Oil jet peening makes use of high-pressure oil jet to impart compressive residual stresses on the surface of metallic materials in order to improve the surface properties. This article reports the dry sliding wear characteristics of oil jet peened aluminium alloy, AA6063-T6. The presence of compressive residual stress and high hardness improves the wear resistance of oil jet peened surfaces. Changes in the hardness, surface morphology, and residual stress distribution due to peening affect the tribological behaviour. The initial and steady-state coefficient of friction are less in the treated samples compared to untreated samples. The scanning electron microscope images of the worn surfaces reveal the dominant adhesive and mild abrasive form of wear.

Control of lubricant transport by a CrN diffusion barrier layer during high-temperature sliding of a CrN–Ag composite coating

CrN–Ag composite coatings, 2 and 5 μm thick and containing 22 at.% Ag solid lubricant, were grown on Si(001) and 440C stainless steel substrates by reactive co-sputtering at T s = 500 °C, and were covered with 200 nm thick pure CrN diffusion barrier cap layers. Annealing experiments at T a = 625 °C, followed by quantitative scanning electron microscopy, energy dispersive x-ray spectroscopy, and Auger depth profile analyses indicate considerable Ag transport to the top surface for a barrier layer deposited at a substrate floating potential of −30 V, but negligible Ag diffusion when deposited with a substrate bias potential of −150 V. This is attributed to ion-irradiation induced densification which makes the cap layer an effective diffusion barrier. High temperature tribological sliding tests of this coating system against alumina balls at T t = 550 °C indicate an initial friction coefficient μ = 0.43 ± 0.04 which decreases monotonically to 0.23 ± 0.03. This is attributed to the development of wear mediated openings in the barrier layer which allow Ag lubricant to diffuse to the sliding top surface. In contrast, pure CrN exhibits a constant μ = 0.41 ± 0.02 while CrN–Ag composite coatings without cap layer show a low transient μ = 0.16 ± 0.03, attributed to Ag transport to the surface, that however increases to μ = 0.39 ± 0.04 after ~ 6000 cycles as the Ag reservoir in the coating is depleted. That is, the dense CrN cap layer reduces the Ag lubricant flow rate and therefore prolongs the time when the coating provides effective lubrication. This results in a cumulative wear rate over 10,000 cycles of 3.1 × 10 −6 mm 3/Nm, which is 3.3 × lower than without diffusion barrier layer.

Effects of friction on roller expanded tube–tubesheet joint strength

Roller expansion of new tubes in enlarged heat exchanger tubesheet holes requires higher rolling torques which may result in over-thinning of the tube wall. This affects adversely the tube–tubesheet joint strength, which is measured in terms of residual contact pressure between the expanded tube and tubesheet. The axial force required to cause the mechanical failure of the tube–tubesheet joint has therefore been considered as an indication of the integrity of the joint. This force is influenced by several parameters such as the type of tube and tubesheet materials, the initial clearance and coefficient of friction. In the present work, an axisymmetric finite element (FE) model is used to evaluate the combined effects of friction between tube and tubesheet, initial clearance and tube material strain hardening on the strength of the tube–tubesheet joint. The FE results show that the increase in friction between tube and tubesheet results in higher residual contact stress and lower cutoff clearances. The residual contact stress also increased linearly with increasing tube material strain hardening level for all friction coefficients.