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Chemical-reaction-induced deformation mechanism change from FCC-to-HCP to FCC-to-BCC phase transition of high entropy alloy.

The definition of atomic-scale contact is a very ambiguous issue owing to the discrete atomic arrangement, which hinders the development of contact theory and nano-tribological techniques. In this work, we studied the atomic-scale contact area and their correlations with friction force based on three distinct contact definitions (interatomic distance, force, and interfacial chemical bonds) by performing large-scale atomistic simulations on a typical ball-on-disk contact model. In the simulations, the measured contact areas defined by interatomic distance, force, and interfacial chemical bonds (referred as to

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...

We investigated the effect of a cyclic chain structure on the wear resistance of polymer brushes via coarse-grained molecular dynamics simulations. Friction simulations were performed for cyclic an...

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 ...

The performance of carbon- and silicon-based solid lubricants is strongly affected by the tribochemical reactions, especially with hydrogen and oxygen. Even though understanding and unveiling these...

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”.

Ultraflat and damage-free single-crystal diamond is a promising material for use in electronic devices such as field-effect transistors. Diamond surfaces are conventionally prepared by the chemical mechanical polishing (CMP) method, although the CMP efficiency remains a critical issue owing to the extremely high hardness of diamond. Recently, OH radicals have been demonstrated to be potentially useful for improving the CMP efficiency for diamond; however, the underlying mechanisms are still elusive. In this work, we applied our previously developed CMP-specialized tight-binding quantum chemical molecular dynamics simulator to comprehensively elucidate the CMP mechanisms of diamond assisted by OH radicals. Our simulation results indicate that the diamond surface is oxidized by reactions with OH radicals and then a concomitant surface reconstruction takes place due to the distorted and unstable nature of the oxidized diamond surface structure. Furthermore, we interestingly reveal that the reconstruction of the diamond surface ultimately leads to two distinct removal mechanisms: (i) gradual atom-by-atom removal through the desorption of gaseous molecules (e.g., CO2 and H2CO3) and (ii) drastic sheet-by-sheet removal through the exfoliation of graphitic ring structures. Hence, we propose that promoting the oxidation-induced graphitization of the diamond surface may provide a route to further improving the CMP efficiency.

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]

Chemical mechanical polishing (CMP) is a key manufacturing process for applying gallium nitride (GaN), especially the Ga-face GaN, to semiconductor devices such as laser diodes. However, the CMP efficiency for GaN is very low due to its high hardness and chemical stability. Experimentally, OH radicals appear able to improve the CMP efficiency of GaN polished by a SiO2 abrasive grain, whereas the mechanisms of the OH-radical-assisted CMP process remain unclear because experimental elucidation of the complex chemical reactions occurring among GaN substrate, abrasive grain, and OH radicals is difficult. In this work, we used our previously developed tight-binding quantum chemical molecular dynamics simulator to study the OH-radical-assisted CMP process of the widely employed Ga-face GaN substrate polished by an amorphous SiO2 abrasive grain in an effort to understand how OH radicals assist the CMP process and then aid the development of next-generation CMP techniques. Our simulations revealed that the OH-radical-assisted CMP process of GaN occurs via the following three basic reaction steps: (i) first, all hydrogen terminations on the GaN surface are replaced by OH terminations through continuous reactions with OH radicals; (ii) after the substrate is fully terminated by OH, the hydrogen atoms of these OH terminations are removed by reacting with newly added OH radicals, which forms H2O molecules and leaves energetic oxygen atoms with dangling bonds on the surface; and (iii) finally, these energetic oxygen atoms intrude inside the substrate with concomitant dissociation of Ga-N bonds and the generation of N2 and gallium hydroxide molecules, which accumulatively lead to the removal of N and Ga atoms from the substrate.

The atomic-scale yielding mechanisms of body center cubic (BCC) iron in supercritical water have been an elusive problem that requires understanding the roles of chemical reactions with supercritical water in the mechanical behavior of iron. This work shows the combined effect of the supercritical water and tensile direction on the yielding mechanism of BCC iron using reactive molecular dynamics simulations. Our simulation results show that tensile strain along the [110] direction of BCC iron may potentially exhibit much higher tensile strength and lower sensitivity to the environment compared with other directions. This is because yielding of iron along the [110] direction originates from the homogenous generation of HCP precursors inside the iron bulk rather than at the surface, which limits the effects of surficial chemical reactions with supercritical water on the yielding behavior. This work is expected to contribute to the theoretical design of high-strength alloys in supercritical water.

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.

Thin film approaches are powerful methods for gaining a nanoscale understanding of interfacial ionic liquids (ILs) in the vicinity of solids. These approaches are used to directly elucidate the interfacial contributions to the physical properties of ILs as nanoscale thin films have significant proportions of the surface or interface region with respect to their total volume. Here, we report the growth of a uniform [emim][TFSA] thin film ionic liquid on a chemically modified, well-wettable sapphire, thereby allowing the in situ measurement of its ionic conductivity on the nanoscale. We observed the thickness-dependent behavior of the ionic conductivity, which gradually decreased especially when the thickness was less than 10 nm, and found it to be quantitatively analyzed well by using an empirical two-layer model. The molecular dynamics (MD) simulations show that the thickness-dependent ionic conductivity originates from the solid-like structuring of the IL near the substrate, reproducing a thickness-dependent ionic conductivity. The MD simulation results suggest that the thickness of the low conductivity region determined in the two-layer model should roughly correspond to the thickness of the solid-like structuring of the IL near the substrate.

The structural and energetic aspects of pentacene solvation in several imidazolium ionic liquids was studied by molecular dynamics simulations. The simulations revealed that the first solvation she...

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.

It is known that the wear of silicon-based materials is due to the tribochemical reaction with water at the sliding interface, but the detailed mechanisms remain under debate. In this study, we used a first-principles molecular dynamics method to investigate the tribochemical wear mechanism. When a small amount of water was present at the sliding interface, the formation of interfacial bridge bonds connecting the two surfaces was observed. These bonds transmitted shear force to the surfaces that induced strain therein. The strained surface Si–O bonds subsequently reacted with water (Si–O–Si + H2O → Si–OH + Si–OH), that is, the hydrolysis reaction occurred. Because the hydrolysis reaction resulted in dissociation of the surface Si–O bonds, water promoted tribochemical wear. However, when a large amount of water was present, it separated the two surfaces. The water thereby suppressed the formation of interfacial bridge bonds and in turn the hydrolysis of Si–O bonds and thus tribochemical wear. Our results i...

To recycle ethylene glycol (HOCH2CH2OH) fuel in alkaline fuel cells, active and selective catalysts for partially oxidizing HOCH2CH2OH to glycolic acid (HOCH2COOH) and oxalic acid ((COOH)2) are required at the anode; in other words, complete oxidation of HOCH2CH2OH to CO2 prevents ethylene glycol recycling. We investigate catalyst activity and selectivity for oxidizing HOCH2CH2OH to HOCH2COOH on Fe(0 0 1), Co(0 001), and Ni(1 1 1) via first-principles calculations. We calculate the oxidation reaction path from HOCH2CH2OH to HOCH2COOH without C C bond dissociation to avoid CO2 generation. Partial oxidation of HOCH2CH2OH to HOCH2COOH without C C bond dissociation proceeds as follows: O H bond dissociation of HOCH2CH2OH to generate HOCH2CH2O; C H bond dissociation of HOCH2CH2O to generate HOCH2CHO; C H bond dissociation of HOCH2CHO to generate HOCH2CO; and OH addition to HOCH2CO to generate HOCH2COOH. The activation energies for O H bond dissociation of HOCH2CH2OH and C H bond dissociation of HOCH2CH2O and HOCH2CHO on Fe(0 0 1) are 20.2, 22.8, and 35.2 kcal/mol, respectively, which are the lowest of the three surfaces. Thus, Fe(0 0 1) is most active. To determine the selectivity, we compare the bond dissociation activation energies. The activation energies for C C bond dissociation of HOCH2CH2OH and HOCH2CH2O on Fe(0 0 1) (66.7 and 39.5 kcal/mol, respectively) are higher than those for O H bond dissociation of HOCH2CH2OH (20.2 kcal/mol) and C H bond dissociation of HOCH2CH2O (22.8 kcal/mol), implying that the O H bond of HOCH2CH2OH and C H bond of HOCH2CH2O dissociate before the C C bond dissociation during oxidation on Fe(0 0 1). In contrast, the activation energies for C H and C C bond dissociation of HOCH2CHO (35.2 and 32.8 kcal/mol, respectively) are similar. The C H and C C bonds therefore dissociate during HOCH2CHO oxidation. On Co(0 0 0 1) and Ni(1 1 1), the activation energies for C C bond dissociation of HOCH2CH2O and HOCH2CHO are lower than those for their C H bond dissociation. Therefore, Fe(0 0 1) is more active and selective than Co(0 0 0 1) and Ni(1 1 1) for the partial oxidation of HOCH2CH2OH to HOCH2COOH.

Ni sintering in the Ni/YSZ porous anode of a solid oxide fuel cell changes the porous structure, leading to degradation. Preventing sintering and degradation during operation is a great challenge. Usually, a sintering molecular dynamics (MD) simulation model consisting of two particles on a substrate is used; however, the model cannot reflect the porous structure effect on sintering. In our previous study, a multi-nanoparticle sintering modeling method with tens of thousands of atoms revealed the effect of the particle framework and porosity on sintering. However, the method cannot reveal the effect of the particle size on sintering and the effect of sintering on the change in the porous structure. In the present study, we report a strategy to reveal them in the porous structure by using our multi-nanoparticle modeling method and a parallel large-scale multimillion-atom MD simulator. We used this method to investigate the effect of YSZ particle size and tortuosity on sintering and degradation in the Ni/YSZ anodes. Our parallel large-scale MD simulation showed that the sintering degree decreased as the YSZ particle size decreased. The gas fuel diffusion path, which reflects the overpotential, was blocked by pore coalescence during sintering. The degradation of gas diffusion performance increased as the YSZ particle size increased. Furthermore, the gas diffusion performance was quantified by a tortuosity parameter and an optimal YSZ particle size, which is equal to that of Ni, was found for good diffusion after sintering. These findings cannot be obtained by previous MD sintering studies with tens of thousands of atoms. The present parallel large-scale multimillion-atom MD simulation makes it possible to clarify the effects of the particle size and tortuosity on sintering and degradation.

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...

Coarse-grained molecular dynamics simulations can model the deformation and fracture processes of the lamellar structure in polyethylene on a molecular scale; however, the simulations have not been performed due to the difficulty in building the structure and the limitations of small-scale simulations on the order of 104 beads. Thus, we propose a crystallization method for a large-scale lamellar structure on the order of 106 beads. The highly oriented lamellar structure is stretched in a coarse-grained molecular dynamics simulation. The stress and variation of crystallinity during stretching parallel and perpendicular to the crystal direction agree with experimental results, confirming the validity of our simulation results. During stretching parallel to the crystal direction, the amorphous layers crystallize and the crystalline layers fragment. We also find that the movement of the polymer chain ends from amorphous to crystalline layers, which is difficult to observe experimentally, increases the compres...

We investigated the effect of fluorination on friction forces between polymer brushes in the dry state by means of an all-atom molecular dynamics simulation. An all-fluorinated brush produced less friction force than a non-fluorinated brush. In addition, a brush in which only the six monomers at the tips of the polymers were fluorinated produced a friction force comparable with that produced by the all-fluorinated brush. Fluorination was found to reduce the friction force by reducing entanglement between the brushes.

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%.