Role of OH Termination in Mitigating Friction of Diamond-like Carbon under High Load: A Joint Simulation and Experimental Study.

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.

Non-Empirical Law for Nanoscale Atom-by-Atom Wear.

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.

Triboemission of hydrocarbon molecules from diamond-like carbon friction interface induces atomic-scale wear.

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%.

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.

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.

Tribological properties of single-walled carbon nanotube solids.

Binder-free single-walled carbon nanotube (SWCNT) solids were evaluated for solid lubrication applications. The steady-state friction coefficients (mu) for the SWCNT solids were found to reach values as low as 0.22-0.24, according to unidirectional sliding friction tests using Si3N4 counterparts in air. The values were slightly higher than that of bulk graphite material (mu = 0.20). SEM and Raman analyses showed that most SWCNTs that existed in the friction surface transformed into SWCNT-derived transferred film made up of amorphous carbon during sliding. The resultant friction behavior may be related to the smearing of transferred film over the contact area, which was expected to permit easy shear and then help to achieve a lubricating effect during sliding.

Ear injury caused by a sticky-tipped applicator.

We here report a relatively rare case of traumatic injury of the tympanic membrane caused by a sticky-tipped applicator as well as some characteristics of this sticky-tipped applicator that were examined experimentally. This rare case was very unusual in that the injury was caused by external force applied from the medial to lateral direction during very careful cleaning (no unexpected force). Although removal of cerumen by a sticky substance seems to be an excellent idea, the present case shows that the stickiness that facilitates removal of the cerumen may be dangerous for thin, dry skin such as the tympanic membrane. Actually, the experimental considerations indicated that the stickiness was affected by the surface condition of the objective: soft, dry conditions increase it, while on the contrary, moist, dusty and oily conditions reduce it. The stickiness of the applicator, which was adjusted to use in the ear canal with an oily surface, appeared to be too large for a dry tympanic membrane. It is recommended that the sticky substance should be removed from the tip area or reduced by application of oil or powder before use.