Shandan Bai

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.

Diamond-like carbon coating under oleic acid lubrication: Evidence for graphene oxide formation in superlow friction

The achievement of the superlubricity regime, with a friction coefficient below 0.01, is the Holy Grail of many tribological applications, with the potential to have a remarkable impact on economic and environmental issues. Based on a combined high-resolution photoemission and soft X-ray absorption study, we report that superlubricity can be realized for engineering applications in bearing steel coated with ultra-smooth tetrahedral amorphous carbon (ta-C) under oleic acid lubrication. The results show that tribochemical reactions promoted by the oil lubrication generate strong structural changes in the carbon hybridization of the ta-C hydrogen-free carbon, with initially high sp3 content. Interestingly, the macroscopic superlow friction regime of moving mechanical assemblies coated with ta-C can be attributed to a few partially oxidized graphene-like sheets, with a thickness of not more than 1 nm, formed at the surface inside the wear scar. The sp2 planar carbon and oxygen-derived species are the hallmark of these mesoscopic surface structures created on top of colliding asperities as a result of the tribochemical reactions induced by the oleic acid lubrication. Atomistic simulations elucidate the tribo-formation of such graphene-like structures, providing the link between the overall atomistic mechanism and the macroscopic experimental observations of green superlubricity in the investigated ta-C/oleic acid tribological systems.

Tight-binding quantum chemical molecular dynamics simulations of the low friction mechanism of fluorine-terminated diamond-like carbon films

The super-low friction mechanism of fluorine-terminated diamond-like carbon (F-terminated DLC) is investigated by using our tight-binding quantum molecular dynamics code and compared with that of hydrogen-terminated DLC (H-terminated DLC). Under a contact pressure of 1 GPa, F- and H-terminated DLC show smooth sliding and low friction coefficients of 0.07 and 0.04, respectively. The ion radius of fluorine is larger than that of hydrogen, which leads to the larger asperity of the F-terminated DLC surface. Thus, the friction coefficient of F-terminated DLC is slightly larger than that of H-terminated DLC. We also perform friction simulations under contact pressures of 3 and 7 GPa. Under a contact pressure of 3 GPa, the friction coefficients are 0.09 and 0.13 for F- and H-terminated DLC, respectively. F-terminated DLC shows the same friction behavior as seen under a contact pressure of 1 GPa, whereas the C–C bond formation reaction is observed at the interface of H-terminated DLC under a contact pressure of 3 GPa, leading to a slightly higher friction coefficient than when under a contact pressure of 1 GPa. Thus, under a contact pressure of 3 GPa, F- and H-terminated DLC show different friction behaviors. Furthermore, under a high contact pressure of 7 GPa, bond formation and dissociation are observed at the friction interface in F- and H-terminated DLC. C–C bond formation is observed more frequently in H-terminated DLC than in F-terminated DLC, and the lifetime of C–C bonds in H-terminated DLC is much longer. At this higher pressure, H-terminated DLC shows a high friction coefficient of 0.42 due to strong C–C bonds at the friction surface, whereas F-terminated DLC shows a low friction coefficient of 0.08. The strong repulsive interaction at the interface of F-terminated DLC that arises from the large negative charge and ion size of fluorine maintains the distance between DLC films under a high contact pressure. This prevents strong C–C bond formation at the friction surface, which results in the low friction properties of F-terminated DLC. We suggest that the friction properties of DLC films under a high contact pressure are improved by F termination.

Multi-nanoparticle model simulations of the porosity effect on sintering processes in Ni/YSZ and Ni/ScSZ by the molecular dynamics method

Understanding the sintering mechanism in porous anodes is necessary for developing durable anodes suitable for use in solid oxide fuel cells. A multi-nanoparticle sintering simulation method based on molecular dynamics (MD) calculation was developed for this purpose [J. Xu et al., J. Phys. Chem. C, 2013, 117, 9663–9672]. The method can be used to calculate the effect of the porous structure properties, such as the porosity and framework structure, on the sintering, unlike previous sintering simulations with conventional nanoparticle models. We revealed that, in a Ni/YSZ porous anode, the YSZ nanoparticle framework suppresses sintering of Ni nanoparticles by disrupting the growth of the neck between two Ni nanoparticles. In this paper, we used our method to reveal the effect of ceramic type on the sintering processes. We investigated the difference between the sintering and degradation processes in Ni/YSZ and Ni/ScSZ anodes. In the simulation, the degree of sintering of the Ni nanoparticles in Ni/ScSZ was smaller than that in Ni/YSZ. The stronger adhesion of Ni to ScSZ nanoparticles than to YSZ nanoparticles prevented the Ni nanoparticles from approaching each other in the Ni/ScSZ anode, inhibiting sintering. Our multi-nanoparticle sintering MD simulations revealed the different sintering processes for Ni nanoparticles in Ni/YSZ and Ni/ScSZ anodes. We also investigated the effect of sintering on degradation. The hydrogen adsorption sites and electrochemical reaction sites of the hydrogen oxidation decreased as the degree of sintering increased. A low degradation of the Ni/ScSZ anode relative to that of the Ni/YSZ anode was observed. Furthermore, we showed the effect of porosity on degradation induced by sintering in Ni/YSZ and Ni/ScSZ, and found an optimal porosity. These findings cannot be obtained by conventional two- or three-nanoparticle sintering MD simulations. Our multi-nanoparticle sintering simulation method is useful for revealing the types of ceramics suitable for inhibiting sintering and degradation in anodes, and can be used to design durable anodes.

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

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

Computational study on low friction mechanism of diamond-like carbon induced by oxidation reaction

Water lubrication has been attracting attention for environment-friendly society due to low CO2 emission. Furthermore, carbon-based materials such as diamond-like carbon (DLC) show the low friction properties in water lubrication due to the oxidation reaction on the surface in pre-sliding. However, the influence of oxidation reactions on low friction mechanism is still unclear. In this study, we clarify the structure change of DLC with the oxidation reaction in the pre-sliding using first-principles calculation, which suggests the low friction mechanism of DLC in water lubrication. The results show the structure change from sp3 carbon (Csp3) to sp2 carbon (Csp2) by the oxidation reaction on the surface. Furthermore, the Csp2 rich surface in water lubrication indicates the smooth sliding. We suggest that the structure change from Csp3 to Csp2 would affect low friction properties of DLC in water lubrication.

A theoretical study on sintering of Ni nanoparticles in the anode of solid oxide fuel cell under water vapor environment

Sintering of Ni particles in the Ni-based anode is a major obstacle to the widespread use of solid oxide fuel cell because the sintering induces the degradation in the anode. The large amount of water vapor in the fuel is known to accelerate the degradation during the operation. However, the detailed accelerated sintering mechanism is unclear. In this study, to clear the accelerated sintering mechanism during the initial stage of the sintering by water vapor, we investigated the adsorption and dissociation of water molecules on the Ni surface as well as the effect of the terminations of O, H, and OH on the interaction between the Ni clusters by density functional theory because the sintering of Ni particles is started by the Ni particles approaching each other due to the attractive interaction between Ni particles at the initial stage. In our adsorption and dissociation calculations, increasing the amount of water vapor facilitates the adsorption of H2O molecule on the Ni surface due to the H-bond interaction. Meanwhile, the activation energy for the dissociation of H2O molecules on the Ni surface is also lowered with increasing the amount of H2O molecules. Then, we calculated the interaction between Ni cluster, and O terminated, OH terminated, and H terminated Ni clusters. We observed that the attractive interaction between Ni cluster and O terminated Ni cluster is larger than that between two Ni clusters in vacuum. Enhancing the attractive interaction is not observed in the H terminated and OH terminated Ni clusters. It indicates that two Ni clusters approaching each other is faster under the water vapor environment than that in vacuum due to the strong attractive interaction caused by the termination of O. Thus, we suggest that the generation of O termination plays an important role in the sintering at the initial stage under the water vapor environment.