An in situ synchrotron XAS methodology for surface analysis under high temperature, pressure, and shear

Tribochemistry: A Review of Reactive Molecular Dynamics Simulations

Tribochemistry, the study of chemical reactions in tribological interfaces, plays a critical role in determining friction and wear behavior. One method researchers have used to explore tribochemistry is “reactive” molecular dynamics simulation based on empirical models that capture the formation and breaking of chemical bonds. This review summarizes studies that have been performed using reactive molecular dynamics simulations of chemical reactions in sliding contacts. Topics include shear-driven reactions between and within solid surfaces, between solid surfaces and lubricating fluids, and within lubricating fluids. The review concludes with a perspective on the contributions of reactive molecular dynamics simulations to the current understanding of tribochemistry, as well as opportunities for this approach going forward.

Interfacial Assembly Behavior of Alkylamine-Modulated Graphene Oxide with Different Oxidation Degrees

Multitudinous studies have been carried out on the controllable functionalization and performance evaluation of graphene oxide (GO). In this study, the correlation between the amount of grafted alkylamine on GO and its interfacial assembly behavior at liquid-liquid and liquid-solid interfaces was studied. GO was modified with n-octylamine through basal functionalization (bGO). The grafting amount of alkylamines was regulated using two GOs varied in oxidation degree (GO_L and GO_H). A study on the oil-water interfacial behaviors shows that bGO_L has better ability to modulate the interfacial tension than that of bGO_H. Grafting alkylamine on GO will not only increase the interaction strength with oil while weaken that with water but also do damage to the graphene lattice and weaken the interaction of π-π stacking; therefore, bGO_L displays a broader capability to modulate interfacial tensions than that of bGO_H. The bGO-based Pickering emulsion was prepared, and the interfacial behavior at the liquid-solid interface was investigated. A study on the interfacial anti-rust performances demonstrates that grafted alkyl chains in bGOs can form more compact and ordered protective films on the metal surface and enhance the hydrophobicity as a result of the similar structure to oil in the emulsion system, which makes Pickering emulsions show better anti-rust abilities than water dispersions. Meanwhile, the bGO_H emulsion shows a better anti-rust property than that of the bGO_L emulsion. A study on the interfacial tribological behaviors shows that the lubricity of bGO_L is better than that of bGO_H. X-ray photoelectron spectroscopy analysis shows that a high content of C-O-C/C-OH in lubricating films contributes to the improvement of lubricity. The modulated interfacial assembly properties of GO at both liquid-liquid and solid-liquid interfaces suggest their potential applications in surface protection, lubrication, controllable drug deliveries, absorption and separation, nanocomposites, and catalyst fields.

3D tribo-nanoprinting using triboreactive materials

Tribology: the science of friction, wear and lubrication has never been associated in a positive way with the ability to manufacture at the nanoscale. Triboreactivity, when the contact between two surfaces promotes a chemical reaction, has been harnessed in this study to create highly tenacious nano-features. The reported 3D tribo-nanoprinting methodology has been demonstrated for organic and inorganic fluids on steel and silicon substrates and is adaptable through the interface tribology. The growth rate, composition and shape of the printed features were all found to be dependent on the nature of the printing liquid and shearing interfaces in addition to the applied temperature and contact force. The reported methodology in this study opens unprecedented future possibilities to utilize the nanoprinted films for the expanding fields of microelectronics, medical devices, flexible electronics and sensor technologies.

On the Transient Decomposition and Reaction Kinetics of Zinc Dialkyldithiophosphate

Despite the ubiquitous use of the zinc dialkyldithiophosphate (ZDDP) as an antiwear additive, no complete information is yet available on its exact decomposition reactions and kinetics to form triboreactive protective films on contacting surfaces. This hinders the replacement of ZDDP with more environmentally friendly additives of similar antiwear capabilities. Using a multitechnique approach, this study shows that before the formation of a phosphate-rich protective film, the decomposition of ZDDP proceeds by forming intermediate zinc sulfide and sulfate species, which can be mechanically mixed with the iron oxides on the rubbing steel surfaces. The mixed sulfur-oxide layer can play different vital roles including binding the subsequently formed phosphate layers with the metal surface. These layers consist mainly of zinc thiophosphate of initially short chains, which are formed due to the excess concentration of metal oxide on the surface. As the concentration of the oxide decreases in the subsequent layers, the short chains start to polymerize into longer ones. The polymerization process follows first-order reaction kinetics with two distinctive phases. The first one is a fast transient burst phase near the steel surface, whereas the second phase dominates the formation process of the layers away from the substrate and is characterized by slow kinetics. The findings of this study provide new insights into the decomposition mechanisms of the currently most widely used antiwear additive and open future opportunities to find green alternatives with similar superior antiwear properties.

Reactive Molecular Dynamics Simulations of Thermal Film Growth from Di- tert-butyl Disulfide on an Fe(100) surface

Iron sulfide films are present in many applications, including lubricated interfaces where protective films are formed through the reactions of lubricant additive molecules with steel surfaces during operation. Such films are critical to the efficiency and useful lifetime of moving components. However, the mechanisms by which films form are still poorly understood because the reactions occur between two surfaces and so cannot be directly probed experimentally. To address this, we explore the thermal contribution to film formation of di- tert-butyl disulfide-an important extreme pressure additive-on an Fe(100) surface using reactive molecular dynamics simulations, where the reactive potential parameters are validated by comparison to ab initio calculations. The reaction pathway leading to the formation of iron sulfide surfaces is characterized using the reactive simulations. Then, the film formation process is mimicked by simulations where di- tert-butyl disulfide molecules are cyclically added to the surface and subjected to temperatures comparable to those expected due to frictional heating. The use of a reactive empirical potential is a novel approach to modeling the iterative nature of thermal film growth with realistic lubricant additive molecules.

In situ synchrotron XAS study of the decomposition kinetics of ZDDP triboreactive interfaces

One of the major obstacles in replacing the widely used zinc dialkyldithiophosphate (ZDDP) antiwear additive with a more environmentally friendly one is the difficulty of time-resolving the surface species resulting from its decomposition mechanism under high contact pressure and temperature. To tackle this issue, a newly developed miniature pin-on-disc tribotester was coupled with synchrotron X-ray absorption spectroscopy (XAS) to perform in situ tribological tests while examining the composition of the formed triboreactive films. The results showed that in the case of bare steel surfaces the initial decomposition products are mainly zinc sulfate species, which with further shearing and heating are reduced to zinc sulfide mixed with metal oxides. The mixed base layer seems to enhance the tenacity of the subsequently formed zinc phosphate layers composing the main bulk of the protective triboreactive film. This base layer was not observed in the case of coated substrates with hydrogenated diamond-like carbon (a-C:H DLC) coating, which results in the formation of less durable films of small volume barely covering the contacting surfaces and readily removed by shear. Comprehensive decomposition pathways and kinetics for the ZDDP triboreactive films are proposed, which enable the control and modification of the ZDDP triboreactive films.