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

International audienceThe 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 sp 3 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 sp 2 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

ダイヤモンドライクカーボンの超低摩擦機構に関するマルチフィジックス・マルチスケールシミュレーション

要約のみTohoku University久保百司課

Accurate meso-scale dynamics by kinetic Monte Carlo simulation via free energy multicanonical sampling: oxygen vacancy diffusion in BaTiO3

A conceptually accurate method to connect the free energy multicanonical sampling to meso-scale kinetic Monte Carlo (kMC) dynamics is proposed. The required input parameters for kMC simulation are the attempt frequency and activation energy for each event, and the free energy multicanonical sampling enables to obtain the kinetic parameters as a function of temperature, which is the most significant difference from a conventional kMC approach that is based on fixed attempt frequency and activation energy. The present approach is applied to oxygen diffusion in single crystal BaTiO3 including Zn dopant (160 ppm) where an anomaly in the oxygen diffusion is experimentally confirmed; the oxygen diffusion coefficient is slightly dropped at around 1080 K. We carried out 1 μs kMC dynamics in the temperature range of 1020 to 1120 K, and obtained a diffusion anomaly at around 1060 K, which is not obtained in conventional kMC calculations. In addition, the calculated diffusion coefficients using the present approach are in the same order as those of experimental ones, whereas the calculated diffusion coefficients using the conventional method are larger than those of experiment by one order of magnitude at least. The results indicate the advantages of the present approach in comparison with the conventional ones because any assumption and fixation of kinetic parameters are not required in the dynamics simulation

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

Association Analysis of Genetic Variants with Type 2 Diabetes in a Mongolian Population in China

The large scale genome wide association studies (GWAS) have identified approximately 80 single nucleotide polymorphisms (SNPs) conferring susceptibility to type 2 diabetes (T2D). However, most of these loci have not been replicated in diverse populations and much genetic heterogeneity has been observed across ethnic groups. We tested 28 SNPs previously found to be associated with T2D by GWAS in a Mongolian sample of Northern China (497 diagnosed with T2D and 469 controls) for association with T2D and diabetes related quantitative traits. We replicated T2D association of 11 SNPs, namely, rs7578326 (IRS1), rs1531343 (HMGA2), rs8042680 (PRC1), rs7578597 (THADA), rs1333051 (CDKN2), rs6723108 (TMEM163), rs163182 and rs2237897 (KCNQ1), rs1387153 (MTNR1B), rs243021 (BCL11A), and rs10229583 (PAX4) in our sample. Further, we showed that risk allele of the strongest T2D associated SNP in our sample, rs757832 (IRS1), is associated with increased level of TG. We observed substantial difference of T2D risk allele frequency between the Mongolian sample and the 1000G Caucasian sample for a few SNPs, including rs6723108 (TMEM163) whose risk allele reaches near fixation in the Mongolian sample. Further study of genetic architecture of these variants in susceptibility of T2D is needed to understand the role of these variants in heterogeneous populations