In situ Synthesizing Carbon-Based Film by Tribo-Induced Catalytic Degradation of Poly-α-Olefin Oil for Reducing Friction and Wear

Self-Lubricating Tribo-Catalytic Activity of 2D High Entropy Alloy Nanoflakes

High Entropy Alloys (HEAs) have garnered attention due to their remarkable tribological attributes. Predominantly, failure mechanisms in HEAs emanate from stress-induced dislocations, culminating in crack propagation and film delamination. In this study, we report on the synthesis of 2D HEA of (MoWNbTaV)0.2S2 which facilitates shear-induced energy dissipation at sliding interfaces. The ball-on-disk tribological investigations demonstrate unprecedentedly low average coefficients of friction (0.076) and wear rates (10-9 mm3 (N∙m)-1) under high contact pressures (0.936 GPa) within ambient conditions. Employing multi-scale characterizations alongside molecular dynamic simulations, we elucidate that the presence of the HEA triggers tribocatalytic activity under high contact pressures emerging as a pivotal factor in extending lubricant lifespan during tribological tests. The resilient lubriciousness coupled with the facile spray coating methodology of (MoWNbTaV)0.2S2 in ambient environments paves the way for the development of a new class of solid lubricants based on 2D HEA.

In-situ catalysis of green lubricants into graphitic carbon by iron single atoms to reduce friction and wear

Reducing friction and wear in moving mechanical systems is essential for their intended functionality. This is currently accomplished by using a large variety of anti-friction and anti-wear additives, that usually contain sulfur and phosphorous both of which cause harmful emission. Here, we introduce a series of diesters, typically dioctyl malate (DOM), as green and effective anti-friction and anti-wear additives which reduce wear by factors of 5-7 and friction by over 50% compared to base oil when tested under high pressures. Surface studies show that these impressive properties are primarily due to the formation of a 30 nm graphitic tribofilm that protects rubbing surfaces against wear and hence provides low shear stress at nanoscale. This graphitic tribofilm is prone to form from diesters derived from short-chain carboxylic acid due to their lone pair effect, which stabilizes the carbon free radicals. Furthermore, the formation of this tribofilm is catalyzed by nascent iron single atoms, which are in-situ generated due to the mechanochemical effects during sliding contact. Computational simulations provided additional insights into the steps involved in the catalytic decomposition of DOM by iron and the formation of a graphitic carbon tribofilm. Due to its anti-friction and wear properties, DOM holds promise to replace conventional additives, and thus provides a green and more effective alternative for next-generation lubricant formulations.

Study on the Friction Behavior and Mechanism of DLC Films in Waxy Oil Extraction Environment Based on Reactive Molecular Dynamics

Diamond-like carbon (DLC) film is considered a highly promising coating for reducing friction and wear in oil and gas exploitation equipment, owing to its exceptional wear resistance and antifriction properties. Herein, the friction and wear behavior of the Fe-Fe model (conventional steel plunger pump barrel) and the DLC-Fe model (DLC-coated plunger pump barrel) in a waxy oil recovery environment was studied by Molecular dynamics (MD) simulation. The results show that the DLC film can inhibit the lattice evolution of the friction interface and reduce the shear deformation between the friction pairs and the weak interaction between the DLC film and the waxy crude oil system, ultimately reducing the friction and wear. In addition, as the environmental temperature rises, the friction of the Fe-Fe friction system continues to increase. Interestingly, the average friction of the DLC-Fe friction system after the increase in environmental temperature is smaller than its average friction at 298 K, implying that the DLC film has great potential in high-temperature oil extraction environments. This work may provide theoretical support for the practical engineering application of DLC films on the surface of oil well pump plungers.

Perspective of Tribological Mechanisms for α-Alkene Molecules with Different Chain Lengths from Interface Behavior

Three α-alkene lubricants, differentiated by chain length, were selected as model compounds to investigate the influence of chain length on tribological properties. The novelty of this study lies in setting chain length as the sole variable to explore its impact on surface and adsorption energy. Based on the above findings, the study provides a unique explanation of the intrinsic relationship between chain length and tribological performance. The tribological properties of the three α-alkenes were compared, and subsequent characterization methods elucidated the wear mechanisms and explored tribochemical reactions. The study employed the Owens-Wendt-Rabel-Kaelble (OWRK) method and density functional theory (DFT) to investigate each compound's surface energy and adsorption energy. Experimental results revealed that the average friction coefficients (abridged as COF) for 1-decene, 1-tetradecene, and 1-octadecene decreased sequentially to 0.125, 0.099, and 0.075, respectively. The wear volume of 1-tetradecene decreased by 53.2% and that of 1-octadecene decreased by 64.0% compared to 1-decene. This can be attributed to the simultaneous enhancement of the surface energy and adsorption energy with increasing chain length. On the one hand, the increase in surface energy facilitates tribochemical reactions positively influencing the formation of tribofilms. On the other hand, the increase in adsorption energy enhances the adsorption of lubricants on the substrate surface. The synergy of these two effects allows 1-octadecene and 1-tetradecene (long-chain α-alkenes) to exhibit superior tribological performance compared to that of 1-decene (short-chain α-alkenes). Ultimately, this study offers unique insights into understanding lubrication mechanisms.

Tribological Properties of Phosphate Ester Confined between Iron-Based Surfaces

Recent studies have revealed that phosphate ester lubricant additives undergo decomposition and polymerization when confined between two iron-based surfaces, forming tribofilms that play a crucial role in antiwear and friction reduction. However, the behaviors of decomposition and polymerization, as well as the mechanisms behind the antiwear and friction-reducing properties of these tribofilms, remain largely unexplored. To address these gaps, we employed reactive force field molecular dynamics (ReaxFF-MD) to investigate the tribochemical reactions and tribological properties of tricresyl phosphate (TCP) and tri-n-butyl phosphate (TNBP) confined between three types of iron-based substrates with varying oxygen content: α-Fe⟨110⟩, amorphous FeO (a-FeO), and amorphous Fe2O3 (a-Fe2O3) surfaces. Our findings indicate that TCP and TNBP molecules primarily undergo dissociation of P-O and C-O bonds, a process influenced by both the oxygen content of the substrates and the molecular structure of the additives. Following the dissociation of these bonds, the released carbon-hydrogen groups adsorb onto the substrates, leading to the formation of adsorbed carbon films on the surfaces. Simultaneously, the dissociation of P-O and C-O bonds triggers polymerization reactions, resulting in the creation of organic polyphosphate iron groups confined between the surfaces coated with adsorbed carbon films. Due to the difference in the contact state and shearing behavior between the carbon films and the organic polyphosphate iron groups, substrates with higher oxygen content exhibit relatively higher friction forces for both TCP and TNBP molecules. Additionally, TCP molecules demonstrate lower friction forces compared to those of TNBP molecules on all three iron-based substrates.

Probing the Tribochemical Impact on Wear Rate Dynamics of Hydrogenated Amorphous Carbon via Raman-Based Profilometry

Solid-liquid lubricating systems have received significant attention as a promising way for energy saving and emission control. For deeply understanding their tribological behaviors, it is necessary to study interaction mechanisms between solid and liquid lubricants from the tribochemical viewpoint, as tribofilms formed by tribochemical products on contact surfaces critically affect the whole tribological process. Continually or periodically monitoring tribofilm formation and evolution can contribute significantly to clarifying its dominating role in tribological behavior under boundary lubrication. However, detecting tribofilms in situ remains a big challenge for conventional surface analytical approaches, mainly due to their limitations in accessing tribofilms or low signal intensities of thin tribofilms. In this study, highly sensitive Raman-based profilometry with in situ potential has been developed for detecting molybdenum dialkyldithiocarbamate (MoDTC)-derived tribofilms and exploring their effect on a-C:H wear over time. The optical properties of tribochemical products formed on the coating surface in different wear stages could result in extra attenuation of Raman signal intensities in the form of measurement deviations in wear depth. By monitoring the deviations, key information of tribofilm compositions was obtained and a two-stage wear progression mechanism was proposed for the first time to clarify the detrimental effect of MoDTC-derived tribofilms on a-C:H wear by combining detailed structure and composition analyses.

Achieving Ultralow Friction and Wear by Tribocatalysis: Enabled by In-Operando Formation of Nanocarbon Films

Under the high-contact-pressure and shear conditions of tribological interfaces lubricated by gaseous, liquid, and solid forms of carbon precursors, a variety of highly favorable tribocatalytic processes may take place and result in the in situ formation of nanocarbon-based tribofilms providing ultralow friction and wear even under extreme test conditions. Structurally, these tribofilms are rather complex and may consist of all known forms of nanocarbon including amorphous or disordered carbon, graphite, graphene, nano-onion, nanotube, etc. Tribologically, they shear readily to provide ultralow friction and protection against wear. In this paper, we review some of the latest developments in catalyst-enabled tribochemical films resulting from gaseous, liquid, and solid sources of carbon. Particular focus is given to the nature and lubrication mechanisms of such in situ derived tribofilms with the hope that future tribological surfaces can be designed in such a way to exploit the beneficial impact of catalysis in friction and wear control.

Tribocatalytically-activated formation of protective friction and wear reducing carbon coatings from alkane environment

Minimizing the wear of the surfaces exposed to mechanical shear stresses is a critical challenge for maximizing the lifespan of rotary mechanical parts. In this study, we have discovered the anti-wear capability of a series of metal nitride-copper nanocomposite coatings tested in a liquid hydrocarbon environment. The results indicate substantial reduction of the wear in comparison to the uncoated steel substrate. Analysis of the wear tracks indicates the formation of carbon-based protective films directly at the sliding interface during the tribological tests. Raman spectroscopy mapping of the wear track suggests the amorphous carbon (a-C) nature of the formed tribofilm. Further analysis of the tribocatalytic activity of the best coating candidate, MoN-Cu, as a function of load (0.25–1 N) and temperature (25 °C and 50 °C) was performed in three alkane solutions, decane, dodecane, and hexadecane. Results indicated that elevated temperature and high contact pressure lead to different tribological characteristics of the coating tested in different environments. The elemental energy dispersive x-ray spectroscopy analysis and Raman analysis revealed formation of the amorphous carbon film that facilitates easy shearing at the contact interface thus enabling more stable friction behavior and lower wear of the tribocatalytic coating. These findings provide new insights into the tribocatalysis mechanism that enables the formation of zero-wear coatings.