Low Friction Properties Research Articles

Friction properties of partially polished CVD diamond films at different sliding speeds

The rate dependence of the friction properties between a partially polished diamond film and metal has been investigated experimentally in the present research. Partially polished diamond films have low friction and low wear properties in air, and it is expected that the friction coefficient becomes very small at high sliding speeds.Polycrystalline diamond films are deposited on SiC substrates by hot filament chemical vapor deposition using a gas mixture of CH4 and H2. Partially polished diamond films of two different surface roughnesses, as well as the as-deposited diamond film, were tested under different sliding speed conditions. The friction properties between the diamond films and an AISI440C pin under dry conditions were determined in ambient atmosphere. For the polished films, a definite reduction in the friction coefficient was observed as the sliding speed increased; the speed at which this reduction began depended on the surface roughness of the film. Extremely low friction and noiseless sliding between films and counter metals were realized on polished diamonds as the sliding speeds increased.

Ultrahigh Ductile Gels Having Inter-Crosslinking Network (ICN) Structure

Gels have unique properties such as low frictional properties, permeability and biocompatibility due to their high water content. In the last decade, several high-strength gels have been developed, which are promising for extending the application of gels as industrial materials. Previously we proposed novel ultrahigh ductile gels by defining inter-crosslinking network (ICN) structure. In this study, the mechanical properties of ICN gel was studied by experiments and theoretical estimations, and the mesh size of the internal network structure were determined. It is found that the Young’s modulus of ICN gels calculated by experiments well matches with the theoretical estimations. The appropriate ICN gels achieves more than 61% increase in the ductility in comparison with normal poly(N;N -dimethylacrylamide) (PDMAAm) gels, by introducing the ICN structure. For the ICN gels, the density of chains calculated from the mesh-size is well consistent with the other densities calculated from the modulus and the water content, in comparison with the PDMAAm gels. [DOI: 10.1380/ejssnt.2012.346]

TiN+MoS2複合ターゲットを用いた高潤滑性硬質スパッタ膜の創成とその密着性の改善

TiN films are superior in hardness and adhesive strength. However, friction coefficients of the TiN films are higher than those of DLC films. By adding molybdenum disulfide (MoS2) up to 40%, the TiN-MoS2 composite films with very low frictional properties could be obtained. In this study, the authors tried to reduce MoS2 content in the high lubricious TiN-MoS2 composite films by using a composite target of TiN and MoS2 for improving the hardness. In addition, Ti interlayer was deposited in order to improve the adhesive strength. The friction coefficient of the composite film with MoS2 content of 22% showed low friction coefficient which was almost equal to that of DLC film at room temperature and 200°C. The hardness of the newly developed high lubricious composite film was as high as 17GPa. In addition, the adhesive strength of the composite film was improved by forming the Ti interlayer.

Ultrahigh Ductile Gels Developed by Inter Cross-linking Network (ICN)

Gels have low frictional properties, permeability and biocompatibility due to high water content. In the last decade, several high-strength gels have been developed, which are promising for extending the application of gels as industrial materials. In this study, ultrahigh ductile gels are proposed by developing Inter Cross-linking Network (ICN), which is the novel internal structure of gels. The ICN gels can achieve more than 67.9% increase in the ductility in comparison with normal poly(N,N-dimethylacrylamide) gels, only by adding a little amount of fiber, hydroxypropyl cellulose, which is known as lyotropic crystalline polymer, while containing 97wt% water.

Electrochemical and mechanical properties of low friction nc-CrC/a-C:H and nc-WC/a-C:H coatings on construction materials deposited by magnetron sputtering.

The increased focus on the natural environment protection, the increasing fuel prices and the will to reduce the costs of machines’ maintenance - all that obliges us to search for innovative engineering solutions that would both limit the use of conventional lubricants and enhance the machines’ efficiency. To fulfil this challenging goal, the parts of machines are coated with special low friction and high wear resistant materials more and more often. The PVD methods play a great role in this field. As the examples of coatings deposited by a PVD method and assuring the above mentioned goals may serve the nc-CrC/a-C:H and nc-WC/a-C:H. This research focuses on the electrochemical and mechanical studies of low friction nc-CrC/a- C:H and nc-WC/a-C:H coatings deposited by magnetron sputtering on two kind of construction materials: Vanadis 23 steel and oxygen hardened Ti-6Al-4V alloy. The obtained coatings were investigated using various electrochemical and mechanical methods. The following parameters of coatings were determines: electrochemical (corrosion resistance), micro-mechanical (nanohardness, Young’s modulus) and tribological (wear resistance, friction coefficient). The results presented in this paper have been obtained within the project “KomCerMet” (contract no. POIG.01.03.01-00-013/08 with the Polish Ministry of Science and Higher Education) in the framework of the Innovative Economy Operational Programme 2007-2013.

Fate of methanol molecule sandwiched between hydrogen-terminated diamond-like carbon films by tribochemical reactions: tight-binding quantum chemical molecular dynamics study

Recently, much attention has been given to diamond-like carbon (DLC) as a solid-state lubricant, because it exhibits high resistance to wear, low friction and low abrasion. Experimentally it is reported that gas environments are very important for improving the tribological characteristics of DLC films. Recently one of the authors in the present paper, J.-M. Martin, experimentally observed that the low friction of DLC films is realized under alcohol environments. In the present paper, we aim to clarify the low-friction mechanism of the DLC films under methanol environments by using our tight-binding quantum chemical molecular dynamics method. We constructed the simulation model in which one methanol molecule is sandwiched between two hydrogen-terminated DLC films. Then, we performed sliding simulations of the DLC films. We observed the chemical reaction of the methanol molecule under sliding conditions. The methanol molecule decomposed and then OH-termination of the DLC was realized and the CH3 species was incorporated into the DLC film. We already reported that the OH-terminated DLC film is very effective to achieve good low-friction properties under high pressure conditions, compared to H-terminated DLC films. Here, we suggest that methanol environments are very effective to realize the OH-termination of DLC films which leads to the good low-friction properties.

A new model for thermodynamic analysis on wetting behavior of superhydrophobic surfaces

Superhydrophobic surfaces have shown inspiring applications in microfluidics, and self-cleaning coatings owing to water-repellent and low-friction properties. However, thermodynamic mechanism responsible for contact angle hysteresis (CAH) and free energy barrier (FEB) have not been understood completely yet. In this work, we propose an intuitional 3-dimension (3D) droplet model along with a reasonable thermodynamic approach to gain a thorough insight into the physical nature of CAH. Based on this model, the relationships between radius of three-phase contact line, change in surface free energy (CFE), average or local FEB and contact angle (CA) are established. Moreover, a thorough theoretical consideration is given to explain the experimental phenomena related to the superhydrophobic behavior. The present study can therefore provide some guidances for the practical fabrications of the superhydrophobic surfaces.

Tribological Characterisation of Air-Sprayed Epoxy-CNT Nanocomposite Coatings

Epoxy-carbon nanotube (CNT) composites are promising coating materials for wear and corrosion critical applications such as marine bearings, shafts, bolts and gears. However, there are insufficient tribological data available for design. This article described the fabrication and tribological testing of an epoxy-CNT composite coating composed of a commercial epoxy primer and commercial CNT filler. The CNT filler was pre-treated so that it was compatible with epoxy resin and was dispersed using a ball milling process. A reciprocating sliding test rig was built for the measurement of friction and wear of the coatings which were subjected to multi-pass testing using the ball-on-plate sliding geometry. The rig allowed testing with either constant or varying normal force, along with measurement of normal and tangential forces. Thus, the coefficient of friction (COF) under ramping or constant normal force could be determined. Following testing, the samples were examined using an optical microscopy to determine the severity of any galling which had taken place. The coatings were found to display encouraging properties in all aspects of testing. COF values of around 0.2 were recorded under a nominal contact pressure up to 1 GPa. This coating can be used for components which require anti-corrosion and low friction properties.

A study of electroless copper–phosphorus coatings with the addition of silicon carbide (SiC) and graphite (C g) particles

Copper composite coating with graphite (C g) and/or silicon carbide (SiC) particles were deposited by electroless plating. The surface morphology of the coatings that were analysed using scanning electron microscopy (SEM) showed that Cu particles were uniformly distributed. The obtained coating thickness was approximately ± 5 μm. X-ray diffraction (XRD) and differential scanning calorimetry (DSC) techniques were used to characterise the structure and to study the phase transition of the coatings, respectively. Phases such as Cu, Cu 2O, Cu 3P, Cu 3Si, SiC and C g were observed from X-ray diffraction patterns and the presence of Cu 2O, Cu 3P and Cu 3Si was confirmed by differential scanning calorimetry (DSC) studies. The results demonstrated that SiC and C g particles have little influence on the phase transition of the coating. The hardness and wear resistance of Cu–P composite coatings were improved with the incorporation of SiC particles. The friction coefficient of Cu–P composite coatings decreased with the incorporation of C g particles. Atomic force microscopy (AFM) results of coatings showed that the roughness of the coatings increased with the incorporation of SiC to the Cu–P coatings and decreased with the incorporation of C g. Cu–P–C g–SiC composite coatings showed a moderate roughness, hardness between Cu–P–SiC and Cu–P–C g coatings, had low friction and good anti-wear properties. The anti corrosion resistance of the electroless Cu–P composite coatings on carbon steel were studied in 3.5% NaCl and 1 M HCl solutions by the potentiodynamic polarisation technique. The study revealed that the corrosion resistance increased with the incorporation of SiC particles in the Cu–P and Cu–P–C g matrix but reduced with the incorporation of graphite.

Platelet Adhesion on Silicon Modified Carbon Nanotubes Films

Owing to its high mechanical strength, chemical inertness, low frictional coefficient, high wear and corrosion resistance properties and so on, carbon nanotubes (CNTs) films have emerged as a potential material for biomedical applications. In order to investigate the blood compatibility properties of CNTs films synthesized on the silicon (Si) substrate using thermal chemical vapor deposition, the blood compatibility was evaluated in vitro by the platelet adhesion and the activated partial thromboplastin time (APTT). The experimental results demonstrated that the Si-CNTs films reduced thrombus formation by minimizing the platelet adhesion, activation , aggregation and had a tendency to retard the intrinsic coagulation activities of blood due to a higher APTT compared to Si, lower temperature isotropic pyrolytic carbon (LTIC) and Polydimethylsiloxane (PDMS) materials. Further, we found that hydrophobic surfaces are more prone to direct cellular motility in comparison with hydrophilic surfaces. It is concluded that CNTs films on the Si substrate were effective for improvement of blood compatibility.

Friction characteristics and material transfer tendency in metal powder compaction

Abstract The friction characteristics and material transfer tendency between metal powder and die tool material in metal powder compaction play an important role in the production of near-net-shape components of high density. A natural step to further increase the green density and simplify the sintering process is to reduce the amount of internal lubricant in the powder since the volume fraction of an organic lubricant will result in a significant contribution to the resulting porosity. However, this will significantly increase the adhesive contact and thus the friction between the die and the powder/green body during the powder compaction process. As a result, the compaction and ejection forces as well as the wear rate of the die and punch surfaces will increase. Consequently, improved knowledge concerning the friction mechanisms prevailing at the metal powder/die tool material interface is needed. The present paper will present data regarding the influence of type of tool and coating material on the friction characteristics and material transfer tendency during simulated powder compaction of a water atomized plain iron powder under no or starved lubrication conditions using two different laboratory tribo tests. Tool materials investigated include ingot cast tool steel, powder metallurgy (PM) tool steel and TiAlN and DLC-type PVD coatings. Post-test characterization using scanning electron microscopy and energy dispersive X-ray spectroscopy was used to analyse the tribo surfaces and especially the tendency to material transfer and tribo film formation. The results show that the material transfer tendency is mainly controlled by strong adhesive metal–metal contacts and that a PVD coating showing intrinsic low-friction properties and a smooth surface topography may significantly reduce the interaction between the mating surfaces promoting a stable friction and a low material transfer tendency.

Tribological testing of some potential PVD and CVD coatings for steel wire drawing dies

The aim of this study was to investigate the possibility to replace cemented carbide wire drawing dies with CVD or PVD coated steel dies. Material pick-up tendency, friction and wear characteristics of four different commercial coatings – CVD TiC and PVD (Ti,Al)N, CrN and CrC/C – in sliding contact with ASTM 52100 bearing steel were evaluated using pin-on-disc testing. The load bearing capacity of the coating/substrate composites was evaluated using scratch testing. The results show that the friction characteristics and material pick-up tendency of the coatings to a large extent is controlled by the surface topography of the as-deposited coatings which should be improved by a polishing post-treatment in order to obtain a smooth surface. Based on the results obtained in this study, three different coatings – CrC/C, TiC and dual-layer TiC/CrC/C – are recommended to be evaluated in wire drawing field tests. CrC/C and TiC are recommended due to their intrinsic low friction properties and material pick-up tendency in sliding contact with steel. The dual-layer is recommended in order to combine the good properties of the two coatings CrC/C (low shear strength) and TiC (high hardness).

Tribological Aspects of Carbon-Based Nanocoatings – Theory and Simulation

Abstract Nanocoatings have the potential to improve the surface properties of various materials. They are of extreme importance for surfaces in sliding contact such as highly stressed engine parts. Here, nanocoatings have to be optimized with respect to low friction properties and a high wear resistance to enhance the energetic and environmental efficiency. An important example are diamond-like carbon (DLC) films, which exhibit high mechanical stability depending on their deposition process. We present an introduction to this field of tribology by giving a short overview on DLC films, on the influence of lubricants from a theoretical point of view and in a broader sense, on basic principles of modeling tribological processes with molecular dynamic methods.

Research on Lubrication and Wear-Resistant Mechanism of Ni60A/MoS<sub>2</sub> Composite Coating

The influence of MoS2 lubrication phase on the tribological properties of the Ni60A/MoS2 composite coating was conducted on UMT-2 micro-wear testing machine (USA), discussing the self-lubricating effect and mechanism. The result shows that with the increasing content of MoS2, the friction coefficient of the coating which changed with the increasing content of the MoS2 presents firstly decreases then increases, and the value reach the minimum when the quality percent of MoS2 wrapped with Nickel is 35%. Low-friction property of the Ni60A/MoS2 composite coating is due to the forming of MoS2 lubricating film in friction surface. The decreasing of the friction coefficient of the coating is in proportion to the coverage area of MoS2 lubricating film.

G-1-5 炭素系硬質薄膜の摩擦特性に及ぼす紫外線照射の影響(G-1 マイクロナノ理工学,口頭発表:マイクロナノ理工学)

The low friction coefficient of carbonaceous coatings under lubrication condition are desirable for industrial fields, however, these coatings, especially hydrogenated Diamond-Like Carbon (a-C:H), do not show low friction coefficient. In this study, the ultraviolet light irradiation method to obtain low friction property for carbonaceous coating was discussed. The a-C (hydrogen free DLC) did not show any UV effect for low friction property. On the other hand, a-C:H and CNx (Carbon Nitride) coatings showed lower friction coefficient rather than no-UV irradiation condition.

G040031 カーボンナノファイバー添加PTFE膜の電気的特性に及ぼす分散状態の影響([G04003]機械材料・材料加工部門一般セッション(3))

Fluoropolymers have very low friction and various unique properties. Therefore, they are expected to be applied for sliding materials. However, because of the insulation, it is easy to accumulate static electricity. Hence, to apply to sliding materials there is need to provide electrical conductivity. Nano Carbon is expected as one of the conductive fillers that can effectively improve the electrical property by a small amount of addition, so many researchers studied about composite materials filled with nano carbon. However, because nano carbon is easily agglomerated, the problem that steady electric conductive cannot be obtained is pointed out. In this study, we prepared Carbon nanofiber (CNF) filled Polytetrafluoroethylene (PTFE) films by spin coating method, and evaluated the surface resistivity of the composite film and dispersion status of CNF, and investigated relationship between electrical conductivity and dispersion status of CNF. As a result, when the condition of ultrasonic dispersion is optimized in the case of 2wt% CNF/PTFE composite film, by optimizing the conditions of ultrasonic dispersion process, the surface resistivity of composite film has decreased to 1/100 compared with no dispersion process.

Functional ionic gels formed by supramolecular assembly of a novel low molecular weight anticorrosive/antioxidative gelator

The present work reports a novel anticorrosive low molecular weight gelator (LMWG) that can self-assemble in normal ILs (II) through collective intermolecular interactions (ion pairing, H-bonding, π–π stacking). The LMWG is structurally imidazolium type “ionic liquid” bearing benzotriazole group. It can gelate ILs via supramolecular assembly to form a fibrous structure. The benzotriazole group endows the anticorrosion and antioxidation property of the gelator. The obtained ionic gels have good conductivity, making them potential anticorrosive solid electrolytes. The anticorrosion and antioxidation together with the thixotropic character also make them potential high performance semi-solid lubricants, especially for some electrical contacts, that can be used at low temperatures and liquify upon mechanical shearing with low friction and good antiwear property.

G-1-4 潤滑油中の窒化炭素膜の摩擦に及ぼすテクスチャリングの影響(G-1 マイクロナノ理工学,口頭発表:マイクロナノ理工学)

In the case of oil lubrication with low sliding speed under high load, the hydrodynamic lubrication barely occurs, causing the slid contacts between mating surfaces. This increases the friction and the wear of the sliding surfaces. To protect the surfaces in such severe conditions, carbonaceous hard coatings such as diamondlike carbon (DLC) or carbon nitride (CNx) coatings is well-known with their low friction and anti-wear properties. However, it is also known that those coatings are severely damaged due to the small particles generated by wear. Therefore, we proposed the CNx coatings with microgrooves to reject those wear debris. In this paper, we discussed the effect of microgrooves on the friction properties of CNx coating under oil lubrication with small particles.

J044094 研摩ダイヤモンド膜と金属の間の低摩擦摺動における速度依存性評価([J04409]知的材料・構造システム(9))

Institute of Fluid Science, Tohoku University Katahira 2-1-1, Aoba-ku, Sendai, 980-8577, JapanChemical vapor deposition (CVD) diamond film is well known as the material of high hardness, good wear resistance and so on. It is suitable for the maintenance free solid lubrication film for sliding bearing, but as-deposited CVD diamond films have a rough surface which induces high friction and severe abrasion. To realize the low frictional properties, diamond films have to be well polished or deposited as smooth surfaces. We have developed the deposition process for easy abrasable diamond films. These kinds of diamond films are moderately polished and show a stable low friction between films and steel pins in dry air. In addition, the extremely low friction phenomena of noiseless sliding between films and counter metals were realized as relatively higher sliding speed. These phenomena are similar to a certain type of air bearing. In the present study, we evaluate the sliding speed and contact pressure dependency of the friction properties on polished diamond films. The friction properties between polished diamond films and an AISI440C pin under dry condition were performed in the atmosphere. Definite reductions of the friction coefficient obtained by the sliding speed and contact pressure dependence measurements were clearly observed.

Tribological Properties of Thermo-Reversible Gel-Lubricants Containing Solid Lubricants

Previously, we have reported tribological characteristics of a new unique Thermo-Reversible Gel-Lubricants (TR Gel-Lube), which consists of base fluids and gel agent. TR Gel-Lube has low friction property compared with conventional greases under mild lubricating conditions. However, under severe lubricating condition, extreme pressure (EP) property must be improved by some EP additives. In this study, addition of solid lubricants was investigated. Tribological properties were examined by some tribometers and various enhancements by Molybdenum disulfide, Melamine Cyanuric Acid and Boron Nitrides were observed. Mechanisms of the lubricity improvements were proposed by results of some chemical/physical analyses.