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The use of biodiesel fuel converted from vegetable-based oil are continuously increasing due sustainability and environment concerns. It is important to increase the portion of converted fuel from non-edible sources to reduce the use of food stock. In this study, the tribological properties of stainless-steel material was investigated when lubricated by two kinds of methyl esters converted from non-edible vegetable oils, i.e. Cerbera oil and Rubber seeds oil. The friction and wear were investigated by using a ball-on-disk tribometer at severe friction condition. The results show that the wear of material lubricated by both methyl esters was much smaller than that lubricated by petrol diesel fuel. At temperature of 27°C, the worn area on the ball specimens for the case of Cerbera and Rubber seeds methyl esters were 0.396 mm2 and 0.391 mm2 compared to 3.642 mm2 for the case of petroleum diesel fuel. At 80°C, they were 0.410 mm2 and 0.429 mm2 compared to 4.152 mm2 for the case of petroleum diesel fuel. This shows that both Cerbera and Rubber seeds methyl esters has a good tribological properties comparable to other methyl esters sources used for mineral diesel fuel substitute.

Diamond-like carbon (DLC) has recently attracted much attention as a promising solid-state lubricant because it exhibits low friction, low abrasion, and high wear resistance. Although we previously reported the reason why H-terminated DLC exhibits low friction based on a tight-binding quantum chemical molecular dynamics (TB-QCMD) simulation, experimentally, the low-friction state of H-terminated DLC is not stable, limiting its application. In the present work, our TB-QCMD simulations suggest that H/OH-terminated DLC could give low friction even under high loads, whereas H-terminated DLC could not. By using gas-phase friction experiments, we confirm that OH termination can indeed provide much more stable lubricity than H termination, validating the predictions from simulations. We conclude that H/OH-terminated DLC is a new low-friction material with high load capacity and high stable lubricity that may be suitable for practical use in industrial applications.

We reveal the generation of the “Graphene Arch-Bridge” on a diamond (111) surface by Si doping via first-principles calculations. The “Graphene Arch-Bridge” is different from a simple graphene stru...

Nanodiamond particles (NDPs) have been considered as a potential lubricant additive to various tribological applications, such as water lubrication systems. In this study, the tribological properties of silicon carbide (SiC) lubricated by NDPs dispersed in water are investigated utilizing the ball-on-disk tribometer. It is found that the slight addition of NDP to water (i.e., 0.001 wt%) can distinctly accelerate the running-in process, which is necessary to achieve a friction coefficient (μ) as low as 0.01. This study also discusses two NDP functional terminations —hydroxyl and carboxyl. It is demonstrated that the use of carboxyl-terminated NDP over a wide range of concentration (0.001 wt%–1 wt%) yields a low friction force. In contrast, the ideal effective concentration of hydroxyl-terminated NDP is considerably limited because agglomeration in this material is more probable to occur than in the former. Meanwhile, when utilizing NDPs, the input friction energy (Pin, defined as the product of sliding speed and applied load) is found to have an essential function. Several sliding tests were implemented at various Pin values (50–1,500 mW) using carboxyl-terminated water-dispersed NDPs. It was observed that the μ and wear decreased with increasing Pin when 200 mW < Pin < 1,500 mW. However, when Pin < 200 mW, low friction with high wear occurs compared with the resulting friction and wear when pure water is used.

Because silicon-based ceramics show superlow friction in aqueous environments, these materials have attracted much attention for the development of water lubrication systems. The superlow friction ...

A mechanism to achieve superlow friction in water lubrication is still in the debate because friction is accompanied by complex mechanochemical processes at sliding interfaces. Here, we experimenta...

Diamond-like carbon (DLC) is one of the most promising solid lubricants for sliding against other materials, such as steel, alumina, and silicon carbide (SiC). During sliding, a DLC transfer film is usually formed on the counterpart surface, affording a low friction coefficient. It is well known that hydrogen in DLC strongly promotes the formation of the DLC transfer film. To further improve the lubricity of DLC, we investigate the formation mechanisms of the DLC transfer film on amorphous SiC and the influence of hydrogen on transfer film formation using reactive molecular dynamics simulations. In addition to the conventional transfer mechanism induced by surface adhesion, we herein propose the new transfer mechanism of “hydrocarbon-emission-induced transfer”. In the proposed transfer mechanism, hydrocarbon molecules are emitted from the DLC surface and subsequently adsorb on the counterpart surface during the continuous grinding of the sliding interface, ultimately generating the DLC transfer film. Furthermore, the addition of hydrogen atoms to DLC slightly increases the adhesion-induced transfer and greatly accelerates the “hydrocarbon-emission-induced transfer”, collaboratively contributing to substantial DLC transfer film formation. Thus, we suggest that the experimentally observed promotion of DLC transfer film formation by hydrogen is largely attributable to our proposed mechanism of “hydrocarbon-emission-induced transfer”.

Conventionally, several reports on quantification of wear of current collecting materials have been published. The objectives of these reports were to predict the wear of current collecting materials in field. However, the measure to reduce the wear of current collecting materials has not been proposed yet. In this paper, the authors focus on wear mode transition phenomena to propose the wear reduction measure, and carry out an electric potential distribution analysis and a temperature distribution analysis. From the analysis results, it is found that the relationship between electric potential and temperature is formed parabolic curve, and the curve depends on only a contact voltage. Further, the authors propose a “wear mode map” which shows transition conditions between the wear modes under electric flowing condition by formulating the parabolic curve. According to the wear mode map, the authors identify the three dominant parameters of wear mode transition as a contact voltage, contact resistances include film resistance and melting points of current collecting materials.

To clarify the wear mechanism of current collecting materials such as a contact wire and a contact strip under electric current condition, it is necessary to clarify temperature distribution around the contact spot. However, the traditional method of estimating the maximum contact temperature such as the φ-θ theory cannot estimate the temperature distribution. In this paper, we newly proposed an electric field analysis model of hard-drawn copper and iron-based sintered alloy considering a degenerated layer such as an oxide film and wear debris. Then, we newly proposed the heat conduction equation applying the Wiedemann-Franz law, and analyzed temperature distribution in electrodes. As a result, we find that the relation between the maximum temperature and the contact voltage even in dissimilar electrodes are based on the φ-θ theory. The temperature distribution in electrode changes depending on whether or not degenerated layers exist, and the maximum temperatures of each electrode are not necessarily the same as those estimated by the φ-θ theory. Finally, we clarify the influence of the degenerated layer on the melting condition of electrodes, and explain the electric wear phenomena observed in the previous wear test.

Non-empirical law depicting how atomic-scale friction behaves is crucial to facilitate the practical design of tribosystems. However, progress in developing a practically usable friction law has stagnated because atomic-scale friction arises from the continuous forming and rupturing of interfacial chemical bonds and such interfacial chemical reactions are difficult to measure precisely in experiments. Here, we propose a usable friction law for atomic-scale contact by using atomistic simulations to correctly measure the interfacial chemical reactions of a realistic rough surface, and confirm its applicability to predicting how atomic-scale friction varies with temperature, sliding velocity, and load.

In article number 2002827, Yang Wang, Momoji Kubo, and co‐workers succeed to propose a non‐empirical wear law for nanoscale contact and then validate its robustness and applicability by atomistic simulations. This proposed wear law unveils why the nanoscale wear behaviors do not generally obey the description by conventional macroscale wear law, leading to the development of design principles for nanoscale antiwear tribosystems. [Image: see text]

Tribological system with carbon nitride (CNx) coatings is promising to provide superlow friction in an inert gas environment without any specific lubricants. Such stable and superlow friction is key to both formation of an invisible transferred carbon layer of the thickness of several nanometers (so called as tribolayer) and termination of hydrogen and hydroxyl groups dissociated from water molecule in the inert gas environment to the tribolayer.

Wear of contact materials results in energy loss and device failure. Conventionally, wear is described by empirical laws such as the Archard's law; however, the fundamental physical and chemical origins of the empirical law have long been elusive, and moreover empirical wear laws do not always hold for nanoscale contact, collaboratively hindering the development of high‐durable tribosystems. Here, a non‐empirical and robustly applicable wear law for nanoscale contact situations is proposed. The proposed wear law successfully unveils why the nanoscale wear behaviors do not obey the description by Archard's law in all cases although still obey it in certain experiments. The robustness and applicability of the proposed wear law is validated by atomistic simulations. This work affords a way to calculate wear at nanoscale contact robustly and theoretically, and will contribute to developing design principles for wear reduction., A non‐empirical wear law for nanoscale contact situations is proposed and then its robustness and applicability on various contact states are validated by atomistic simulations. This work successfully unveils why nanoscale wear behaviors do not generally obey the description by conventional macroscale empirical laws and then contributes to the development of anti‐wear materials.

The original version of this article unfortunately contained a mistake. The affiliation information was incorrect for the co-authors Shigeru Tazawa, Tsuyoshi Urano, Shinji Kobayashi.

Mechanical seal is installed in EVAHEART®, one of left ventricular assist device, which supports the role of original human heart. Low friction and low leakage are both required for mechanical seal for ventricular assist device to realize longer battery lifetime and less maintenance. In this study, several types of surface texture by combination of laser-induced periodic surface structure (LIPSS) and μm-scaled undulation were fabricated as surface texture on sealing surface of SiC-made seal rung. Friction and sealing properties were investigated by test apparatus specialized for mechanical seal for ventricular assist device. As a result, surface texture which has parallel LIPSS and μm-scaled undulation shows decreasing friction coefficient in blood. Furthermore, decreasing leakage rate was detected simultaneously. On sealing surface, native and thin protein film which consists of uniformly distributed albumin and fibrinogen was made. Therefore, possibility for low friction and low leakage was suggested and control of protein adsorption on sealing surface of mechanical seal was found to be crucial to realize low friction and low leakage.

Diamond-like carbon (DLC) is a promising solid lubricant used as a protective coating to reduce friction against alumina. Friction properties of DLC/alumina are strongly affected by temperature. To improve the friction performance of DLC, we investigate the friction behaviors of DLC/alumina at various temperatures and reveal the mechanisms by using our tight-binding quantum chemical molecular dynamics method. We observe an interesting volcano-type temperature dependence of friction coefficients in our friction simulations. Friction coefficients of DLC/alumina are low and show little change at 300–600 K because no tribochemical reactions occur at the interface. However, as the temperature increases, friction coefficients increase at 600–800 K and subsequently decrease at 800–1000 K. At 600–800 K, interfacial C-O and C-Al bonds between two substrates are formed during friction, leading to a high friction coefficient. Interestingly, further increment of temperature to 800–1000 K induces the graphitization of DLC. The graphite-like surface suppresses the interfacial bond formation, reducing the friction coefficient. We reveal that the volcano-type temperature dependence of friction coefficients is due to the tribochemical reactions generating interfacial bonds at 600–800 K and the graphitization of DLC reducing the number of interfacial bonds at 800–1000 K.

This study aims to investigate the potential of double network hydrogel (DN gel) in a low friction mechanical system. In this study, the effect of the thickness of DN gel films on the friction property are clarified. An indentation test, contact pressure measurement, and series of friction tests in water using three different thickness (1.5, 3, 6 mm) of DN gel film and a SiC disc were carried out. Friction of DN gel film sliding against SiC in water can be classified into two types as “Friction region I: friction remains substantially unchanged during the test” and “Friction region II: friction decreases due to run-in”. Friction region transition can be related to the Hersey number region, which consists of viscosity of lubricant, sliding speed and apparent contact pressure. Hersey number larger than 1.5 × 10−10 is a necessary condition for friction region II where different lubrication regimes can be found. 1.5 and 6 mm thick DN gel films have exhibit super low friction (μ

Improving the tribological performance of polytetrafluoroethylene (PTFE) resin is important for industrial use of PTFE. An effective way to do this is to blend pure PTFE with another resin material to improve the quality of the transfer film, i.e., adhesion to a counter metallic surface. It is well-known that PTFE/polyetheretherketone (PEEK) polymer blends show significantly less wear than pure PTFE and PEEK. However, the structure and function of the transfer film formed from the blend, which is the key process for reducing friction and wear, have not yet been understood well. Accordingly, the tribological properties and the structure of transfer film of PTFE/PEEK polymer blends were investigated. The blends were prepared by compression and calcination of mixed powders, and a conventional pin-on-disk friction test was performed using pure aluminum as a counter material. Both the friction coefficient and wear rate of the polymer blends were lower than those of pure PTFE and PEEK, respectively. This is con...

This study aimed to clarify the shear strength of a protein film formed by friction of a SiC/SiC sliding pair in a plasma environment. A ball/disk-type tribometer was utilised to obtain the relationship between the friction force in the plasma and the area of protein aggregates formed on the textured SiC disk sliding against the SiC ball in the plasma. To control the area of protein aggregates adsorbed onto the SiC disk surface, different surface textures and ultraviolet (UV) irradiation for the SiC disk were introduced based on past research. Protein aggregates formed owing to protein denaturation and were adsorbed onto the fabricated surface texture of the SiC disk. A broad protein film formed on the SiC disk surface through conjugation of multiple protein films formed from closely distributed concaves. Therefore, the distribution of fabricated concaves determines the conjugation of the protein film formed from each concave, and the trends of frictional force for each SiC/SiC frictional pair in the plasma depends on the area of protein aggregates. In other words, the area of protein aggregates and the frictional force show an almost linear relationship, regardless of the surface texture on the SiC disk or UV irradiation. Finally, the shear strength of the protein aggregates against the sliding SiC ball in plasma was 7.14 MPa, as determined from the relationship between the area of the protein aggregates and the frictional force in the plasma environment.

Two running-in processes, i.e., polishing a diamond-like carbon (DLC) coated disk and pre-sliding in ambient air, were suggested as a delamination suppression method for self-mating DLC coatings in water, and the effects of these methods were verified using a ball-on-disk tribometer. An a-C:H-type DLC coating was deposited on SUJ2 bearing steel balls and disks using plasma chemical vapor deposition. It was confirmed that delamination of DLC coatings was suppressed because microcracks were not generated by removing droplets on the DLC coated disk using the polishing process. The friction coefficient in that condition was reduced to about one-fifth of that of the untreated disk. The running-in process of pre-sliding in ambient air also suppressed delamination of DLC coatings. Moreover, the friction coefficient after this process was reduced to half of that with the other polishing process in the boundary lubrication condition. It was found that these effects have a relationship with surface smoothness and hydrophilicity.

Triboemission of hydrocarbon molecules such as methane and ethylene induces chemical and mechanical wear of diamond-like carbon., Understanding atomic-scale wear is crucial to avoid device failure. Atomic-scale wear differs from macroscale wear because chemical reactions and interactions at the friction interface are dominant in atomic-scale tribological behaviors, instead of macroscale properties, such as material strength and hardness. It is particularly challenging to reveal interfacial reactions and atomic-scale wear mechanisms. Here, our operando friction experiments with hydrogenated diamond-like carbon (DLC) in vacuum demonstrate the triboemission of various hydrocarbon molecules from the DLC friction interface, indicating its atomic-scale chemical wear. Furthermore, our reactive molecular dynamics simulations reveal that this triboemission of hydrocarbon molecules induces the atomic-scale mechanical wear of DLC. As the hydrogen concentration in hydrogenated DLC increases, the chemical wear increases while mechanical wear decreases, indicating an opposite effect of hydrogen concentration on chemical and mechanical wear. Consequently, the total wear shows a concave hydrogen concentration dependence, with an optimal hydrogen concentration for wear reduction of around 20%.

Friction and wear of mechanical components (such as engines) is controlled by some kind of boundary films (also referred to as tribo-layer). In some cases, such boundary film is formed on the contact interface due to a tribo-chemical process in the presence of liquid media. In this paper, we demonstrate the formation of graphite oxide tribo-layer on the contact interface of steel in the presence of diesel fuel liquid media. A Raman spectrometer was used to analyse the tribo-layer structure. Although the intensity is low, it is shown that the tribo-layer has a structure of graphite oxide, demonstrated by the presence of D band peak at approximately 1350 cm−1 and G band peak at approximately 1550 cm−1 in the Raman spectra. Such graphite solid films on the sliding surfaces have ability to control friction and wear. This finding demonstrates the possibility of in-situ formation of solid protective film of a tribo-pair components using the working fluid where the use of lubricants is restricted as they contaminate the working fluid.