Lubricant Molecules Research Articles

Molecular Probing of the Stress Activation Volume in Vapor Phase Lubricated Friction.

When two solid objects slide over each other, friction results from the interactions between the asperities of the (invariably rough) surfaces. Lubrication happens when viscous lubricants separate the two surfaces and carry the load such that solid-on-solid contacts are avoided. Yet, even small amounts of low-viscosity lubricants can still significantly lower friction through a process called boundary lubrication. Understanding the origin of the boundary lubricating effect is hampered by challenges in measuring the interfacial properties of lubricants directly between the two surfaces. Here, we use rigidochromic fluorescent probe molecules to measure precisely what happens on a molecular scale during vapor-phase boundary lubrication of a polymer bead-on-glass interface. The probe molecules have a longer fluorescence lifetime in a confined environment, which allows one to measure the area of real contact between rough surfaces and infer the shear stress at the lubricated interfaces. The latter is shown to be proportional to the inverse of the local interfacial free volume determined using the measured fluorescence lifetime. The free volume can then be used in an Eyring-type model as the stress activation volume, allowing to collapse the data of stress as a function of sliding velocity and partial pressure of the vapor phase lubricant. This shows directly that as more boundary lubricant is applied, larger clusters of lubricant molecules become involved in the shear process thereby lowering the friction.

Investigation of the running-in process in photoinduced superlubricity

Photoinduced superlubricity on TiO2 surfaces is a newfound phenomenon which draws researchers’ attention. This study provides a new method to achieve superlubricity (COF<0.01) with an external light field. However, photoinduced superlubricity can only be realized under specific conditions. Improper running-in conditions, such as speed, load, and pH value, will lead to superlubricity failure even after ultraviolet illumination on the TiO2 surface. In this paper, different running-in loads, speeds, or pH values were used in the experiment of photoinduced superlubricity, and the worn surfaces after running-in and testing in 70% v/v glycerol aqueous solution were investigated thoroughly. Results reveal that the morphology of worn scars differs under different running-in conditions. While the running-in speeds and loads are too low (<0.03 m/s and <2 N) or too large (>0.1 m/s and >9 N), the photoinduced superlubricity will fail because of wrong lubrication state. When the pH value of running-in solution is less than 4.5, photoinduced superlubricity is easier to achieve. In discuss, mixed lubrication is believed to be the key to success of photoinduced superlubricity, because the elastohydrodynamic effect, doublelayer effect and adsorption of glycerol molecules works at the same time. In addition, due to the formation of the SiO2 layer on the Si3N4 ball and better attraction to lubricant molecules with hydroxyl radicals on the TiO2 surface, running-in in solutions with low pH values contributes to the success of photoinduced superlubricity. In any event, the ultraviolet illumination can reduce the friction coefficient of the TiO2/Si3N4 tribological system and can realize photoinduced superlubricity under appropriate running-in conditions.

Calculating shear viscosity with confined non-equilibrium molecular dynamics: a case study on hematite - PAO-2 lubricant.

The behaviour of confined lubricants at the atomic scale as affected by the interactions at the surface-lubricant interface is relevant in a range of technological applications in areas such as the automotive industry. In this paper, by performing fully atomistic molecular dynamics, we investigate the regime where the viscosity starts to deviate from the bulk behaviour, a topic of great practical and scientific relevance. The simulations consist of setting up a shear flow by confining the lubricant between iron oxide surfaces. By using confined Non-Equilibrium Molecular Dynamics (NEMD) simulations at a pressure range of 0.1-1.0 GPa at 100 °C, we demonstrate that the film thickness of the fluid affects the behaviour of viscosity. We find that by increasing the number of lubricant molecules, we approach the viscosity value of the bulk fluid derived from previously published NEMD simulations for the same system. These changes in viscosity occurred at film thicknesses ranging from 10.12 to 55.93 Å. The viscosity deviations at different pressures between the system with the greatest number of lubricant molecules and the bulk simulations varied from -16% to 41%. The choice of the utilized force field for treating the atomic interactions was also investigated.

Tribological properties of chemical composite and physical mixture of ZnO and SiO2 nanoparticles as grease additives

Nano additives have been widely proven to significantly improve the tribological properties of traditional greases, and the synergistic effect of multiple nanomaterials to improve the performance of greases is a promising research direction. In this paper, based on the hardness of the bearing material GCr15 steel, harder SiO2 nanoparticles and softer ZnO nanoparticles were selected to synthesize hard-shelled soft-core composite nanoparticles (ZnO@SiO2) by chemical deposition, and it was used as grease additive for the first time. The composite ZnO@SiO2 nanoparticles have better dispersion and tribological properties than the physically mixed nanoparticles (ZnO/SiO2). When 1 wt% ZnO@SiO2 was added to the grease, the coefficient of friction and wear scar diameter were reduced by 11.5% and 25%, respectively. The special hard-shell-soft-core structure of the composite not only has good dispersion, but also forms a special solid friction film on the surface of the friction pairs. The SiO2 shell can absorb a large number of long-chain lubricant molecules and increase the viscosity of the lubricant in the friction zone, thus forming a solid-liquid phase composite lubricating film, which plays a significant role in friction reduction and anti-wear.

Molecular dynamics simulation of friction coefficient of Fe-Al during lubrication

Reducing wear and friction coefficient in the process of metal plate processing can save energy. In this work, the quantum chemical parameters of n-hexadecane, tetradecyl alcohol, triphenyl thiophosphate (T309) and dialkyl dithiophosphate (EAK) and the adsorption process on Al (111) plane were calculated based on density functional theory. The confined shear process of the lubricant system mixed with the above four molecules in Fe-Al system was simulated by molecular dynamics method, and the friction coefficient of the lubricant was calculated. The friction coefficients of lubricants were measured by reciprocating friction and wear test. The results show that T309 has the strongest reactivity and the most stable adsorption, and its adsorption energy on Al (111) plane is −12.08 eV. Shear force will affect the adsorption morphology of molecules, and long chain molecules will have a certain degree of crimp. When the pressure was 0.5 GPa, the minimum friction coefficient of lubricant was 0.24, mainly because the molecules would gather and adsorb to the metal wall during the shear process. With the increase of pressure perpendicular to the shear plane, the lubricant molecules on the metal wall would gather less, and the distribution of lubricant molecules between the metal walls was gradually uniform. The tested minimum friction coefficient (∼0.12) of the compound lubricant is attributed to synergistic friction reduction among molecules. Based on molecular dynamics simulation, the friction coefficient of aluminum lubricants can be predicted, and the simulated data are accurate and reliable.

Studies of frictional sliding contact by molecular dynamics assisted continuum mechanics

Lubricants are usually utilized to decrease wear, reduce friction, and release heat. Although the lubrication approximation originated from thin-film studies, it has limitations because of continuum mechanics assumptions. To overcome these limitations, for the first time, this article employs a hierarchical multiscale approach to study the frictional sliding contact considering elastohydrodynamic lubrication. This multiscale model consists of a molecular model of lubricant and a continuum model of sliding contact components. Specifically, the molecular dynamics method is used at the nanoscale to model the lubricant and analyze the friction coefficient in contacted surfaces. Through MD simulation, the effects of hydrocarbon chain lengths, temperature, and sliding velocity on a constant friction coefficient are studied in this article. The calculated friction coefficients from the molecular model are passed to the continuum model, in which the finite element method is employed to conduct stress and strain analysis of the sliding contact components. Although variations in the frictional sliding response are invariably coupled with the friction coefficients, the molecular-to-continuum hierarchical approach enables us to isolate the relative contributions from the chemical formulation of the lubricant and the temperature. This approach has further potential implications for the evaluation of tribological damage. The results demonstrate that an increase in temperature or the chain length of lubricant hydrocarbon molecules can significantly decrease the friction coefficient and, as a result, reduce contact shear stress, plastic deformation, and pile-up height.

Correlation between ordering and shear thinning in confined OMCTS liquids.

Despite decades of extensive research, the behavior of confined liquids, particularly in the mixed/boundary lubrication regime, remains unelucidated. This can be attributed to several factors, including the difficulty to make direct experimental observations of the behavior of lubricant molecules under nonequilibrium conditions, the high computational cost of molecular simulations to reach steady state, and the low signal-to-noise ratio at extremely low shear rates corresponding to actual operating conditions. In this regard, we studied the correlation between the structure formation and shear viscosity of octamethylcyclotetrasiloxane confined between two mica surfaces in a mixed/boundary lubrication regime. Three different surface separations-corresponding to two-, three-, and five-layered structures-were considered to analyze the effect of confinement. The orientational distributions with one specific peak for n = 2 and two distributions, including a parallel orientation with the surface normal for n > 2, were observed at rest. The confined liquids exhibited a distinct shear-thinning behavior independent of surface separations for a relatively low shear rate, γ̇≲108s-1. However, the shear viscosities at γ̇≲108s-1 depended on the number of layered structures. Newtonian behavior was observed with further increase in the shear rate. Furthermore, we found a strong correlation between the degree of molecular orientation and the shear viscosity of the confined liquids. The magnitude of the shear viscosity of the confined liquids can primarily be determined by the degree of molecular orientation, and shear thinning originates from the vanishing of specific orientational distributions with increasing shear rate.

Effect of functional groups on tribological properties of lubricants and mechanism investigation

Nine organic compounds were utilized as model lubricants to investigate the impact of functional groups on tribological performances. Nonanoic Acid with carboxyl showed the best lubrication properties, and fluid film and tribofilm were coexistent in its friction test, bringing a low friction coefficient and wear rate. In addition, the lubricant with low friction coefficient corresponded to high adsorption energy in density functional theory (DFT) calculations. And the lubricant forming adsorption film with large surface energy displayed small wear rate in friction test. Moreover, adsorption energies positively correlated surface energies. Based on the experimental results, the action mechanism of functional groups on tribological properties of lubricants was proposed. Various functional groups make lubricant molecules show different adsorption energies and surface energies. Lubricant molecules with high adsorption energy are more likely to adsorb on substrates and form a vertical monolayer, which can maintain a regular molecular brush structure during friction and bring a low friction coefficient. And lubricant molecules with high surface energy may be more prone having tribochemical reactions during friction and forming protective tribofilm, which leads to a low wear rate.

Relating Dry Friction to Interdigitation of Surface Passivation Species: A Molecular Dynamics Study on Amorphous Carbon.

Friction in boundary lubrication is strongly influenced by the atomic structure of the sliding surfaces. In this work, friction between dry amorphous carbon (a-C) surfaces with chemisorbed fragments of lubricant molecules is investigated employing molecular dynamic simulations. The influence of length, grafting density and polarity of the fragments on the shear stress is studied for linear alkanes and alcohols. We find that the shear stress of chain-passivated a-C surfaces is independent of the a-C density. Among all considered chain-passivated systems, those with a high density of chains of equal length exhibit the lowest shear stress. However, shear stress in chain-passivated a-C is consistently higher than in a-C surfaces with atomic passivation. Finally, surface passivation species with OH head groups generally lead to higher friction than their non-polar analogs. Beyond these qualitative trends, the shear stress behavior for all atomic- and chain-passivated, non-polar systems can be explained semi-quantitatively by steric interactions between the two surfaces that cause resistance to the sliding motion. For polar passivation species electrostatic interactions play an additional role. A corresponding descriptor that properly captures the interlocking of the two surfaces along the sliding direction is developed based on the maximum overlap between atoms of the two contacting surfaces.

Effect of lubricants on the rotational transmission between solid-state gears.

Lubricants are widely used in macroscopic mechanical systems to reduce friction and wear. However, on the microscopic scale, it is not clear to what extent lubricants are beneficial. Therefore, in this study, we consider two diamond solid-state gears at the nanoscale immersed in different lubricant molecules and perform classical MD simulations to investigate the rotational transmission of motion. We find that lubricants can help to synchronize the rotational transmission between gears regardless of the molecular species and the center-of-mass distance. Moreover, the influence of the angular velocity of the driving gear is investigated and shown to be related to the bond formation process between gears.

In-situ observation of molecular behavior of lubricant during application of electric field

Electric vehicles (EVs) have achieved considerable efficiency gains in terms of energy consumption, but further improvements remain a challenge. One example is electrical failure of bearings. The electrical environment of bearings, such as the sources of shaft voltage and bearing current and the different components, as well as typical modes of bearing electrical failure, such as various localized damage and poor lubrication, have been reported, but the mechanism of lubrication failure remains unclear. Therefore, the objective of this study was to investigate the molecular behavior of lubricating oil when an electric current is applied, especially molecular orientation and physical and chemical adsorption characteristics on sliding surfaces. A microscopic FT-IR in-situ observation system was applied to clarify the molecular behavior of the lubricating oil. Oleic acid was used as a typical lubricant. It was found that the friction coefficient increased on the charged surface. FT-IR spectra suggested that the dissociation of dimers of oleic acid due to surface charge. As a result, friction coefficient increased since molecular orientation of dimers was reduced. Moreover, lubricant molecules became difficult to adsorb on a negatively charged surface, thereby decreasing the boundary lubricant film thickness.

Influence of aldol condensation processes on the formation of surface films during friction in ester lubricants

The boundary lubrication mode is usually implemented in conditions of low sliding speeds and high loads. The formation of strong boundary lubricating films under this friction mode determines the operability and durability of the friction units. It is believed that the formation of surface boundary films during friction includes the stages of the lubricant oxidation, and the aldol condensation reaction of oxidized molecules. As a result, high-molecular substances called “friction polymers” are formed. The paper studies the formation of surface films in the presence of substances with different reactivity in the aldol condensation and Claisen condensation reactions. Sunflower oil, bis (2-ethylhexyl) sebacate (DEHS), triisodecyl benzene-1,2,4-tricarboxylate (TC) were used as lubricants. It is shown by ATR IR-spectroscopy of that the common thing for the studied oils is that the C=O and C-O groups participate in the formation of boundary films in these oils. The addition of substances, active in aldol condensation reactions, into lubricants does not accelerate the formation of boundary films. Additives that can chemically interact with iron contribute to the dissolution of the surface oxide film and accelerate the formation of boundary layers. The formation of “friction polymers” occurs when the lubricant molecules interact with the metal surface.

The mechanism of formation of boundary lubricating films during friction in a medium of di(2-ethylhexyl) sebacate

In this work, the mechanism of formation of boundary lubricating films during friction in synthetic oil di(2-ethylhexyl) sebacate (DEHS) was studied. The study of the kinetics of the interaction of DEHS with the metal surface showed that the formation of the boundary film proceeds in the first 40 s of friction. The unoxidized iron detected by the XPS method on the friction track accelerates the chemical transformations of lubricant molecules. Removing the oxide film from the friction surface is a mechanically activated process. When DEHS interacts with the iron surface, the reactions of decarboxylation of molecules proceed, as a result free radicals are formed. The sequence of transformations of free radicals and compounds with CC bonds leads to the formation of surface films upon friction. The retention of the film on the surface occurs due to the forces of interaction of the metal with free radicals or unsaturated bonds of organic molecules.

Effects of an articular cartilage lubrication with a viscosupplement in vitro and in vivo following osteochondral fractures in horses.

To assess whether the combination of hyaluronan, sodium chondroitin sulfate, and N-acetyl-d-glucosamine (HCSG) lubricates articular cartilage in vitro and modulates joint lubrication in vivo. 16 healthy adult horses. The effects of HCSG injections on SF lubricant properties and joint health, immediately after injury and 2 weeks later, were analyzed by use an equine osteochondral fracture model of post-traumatic osteoarthritis (OA). Middle carpal joints of adult horses were randomly assigned to 1 of 4 surgical treatment groups as follows: normal nonsurgical group (n = 8), normal sham-surgical group (8), OA-induced surgical group with HCSG injection (8), or OA-induced surgical group with saline (0.9% NaCl) solution injection (8). Synovial fluid was aspirated periodically and analyzed for boundary lubrication function and lubricant molecules. At 17 days, joints were screened for gross pathological changes. Induction of OA led to an impairment of SF lubrication function and diminished hyaluronan concentration in a time-dependent manner following surgery, with HCSG injection lessening these effects. Certain friction coefficients approached those of unaffected normal equine SF. Induction of OA also caused synovial hemorrhage at 17 days, which was lower in joints treated with HCSG. After induction of OA, equine SF lubricant function was impaired. Hyaluronan-sodium chondroitin sulfate-N-acetyl-d-glucosamine injection restored lubricant properties at certain time points and reduced pathological joint changes.

The effect of surface texturing in the sliding surface on tribological characteristics of alloy steel under wet condition

Surface texturing plays a significant role on the tribological performance of contact surfaces. The tribological characteristics are mostly dependent upon the different pattern and methods of surface texturing. This research works investigates lubricated wear behaviour on circular dimples textured EN-31 alloy steel materials using DUCOM linear reciprocating tribometer. The surface dimples are fabricated by micro-EDM and micro-drilling CNC methods. Initially plane surfaces are tested under different lubricants ISO-68 and ISO-220 oil on a couple of EN31 plates with EN8 steel pin. At loading conditions the results clearly shows that the ISO 68 oil gives better performance than ISO-220 oil in terms of frictional force and COF. The operation during test under ISO-220 oil observed is very noisy and wear rate is also more. Then after experiments under different load conditions on textured surfaces were investigated using optimized ISO-68 oil. An experimental results on m-EDM provides superior frictional characteristics with respect to m-CNC. Surface texturing may act as wear debris trapper, lubricant reservoirs, hydrodynamic lift, and retarding the lubricant molecules flow in a particular path where potential exists. Here micro-dimples EDM surface textures can enhance mixed lubrication and in mixed lubrication the friction is least and this may leads to decrement of wear loss.

Discovery of ester lubricants with low coefficient of friction on material surface via machine learning

The design of lubricants with excellent anti-wear performance is the focus of current tribological research. The anti-wear performance of lubricants is fundamentally determined by their chemical structures. Thus, the development of efficient quantitative structure–property relationship (QSPR) to accurately predict lubricant’s anti-wear performance can be a powerful approach toward the design of new lubricants. To this end, this work aims to establish an efficient QSPR model using Least Squares-Support Vector Regression (LS-SVR) method, which employs the screened important molecular descriptors as the input to represent the characteristics of lubricants. Specifically, 1444 molecular descriptors of 54 ester molecules are computed based on the molecular graphics and adjacency matrix. Multiple stepwise regression (MSR) algorithm is used to select important molecular descriptors, and 5 important and unrelated molecular descriptors are used as input in LS-SVR. The mean relative error (MRE) of the training set, test set and external set from the established QSPR model are 2.92%, 6.03% and 9.30% compared with experimental values, respectively, indicating the developed QSPR model exhibits good predictive ability in CoF values of certain molecules and can be used to predict the anti-wear performance of lubricants only through the 5 important molecular descriptors. In addition, it is shown that the anti-wear performance of lubricants can be enhanced by increasing the longest aliphatic chain, the relative molecular mass and van der Waals volume of the studied lubricant molecules via MSR analysis. In the end, we screened the compounds containing ester groups in the database and obtained 8 potential lubricants, which provided guidance for the discovery of new lubricants.

Tribological behaviors of DLC films with hierarchical surface textures under water lubrication: A molecular dynamic simulation

Tribological behaviors of diamond-like carbon (DLC) films with different levels of hierarchical surface textures with lubricant water molecules are investigated through molecular dynamics simulation. The friction forces stabilize at a small value for small normal loads, due to the complete separation between DLC films by water molecules, while friction forces with large normal loads show complicated changes under the cooperation of interfacial evolution and water behaviors. Under large normal loads, friction force increases firstly due to the direct contact of surface textures which are subsequently worn and graphitized, resulting in the temporary stabilization of friction force at a large value. With their further wearing, the amount of interfacial carbon clusters decreases and water molecules distribute evenly at interface, which leads to the gradual decrease and final stabilization of friction force. During the sliding, the water molecules show a restoration in the structure and amount of hydrogen bonds, thus making these molecules play different roles in various stages, i.e., these molecules demonstrate a better diffusion during the friction rise and an enhanced separating effect for DLC films during the friction stabilization. Furthermore, the same amount of water molecules in the one-level hierarchical (L1) model has a larger bearing capacity than that in the two-level hierarchical (L2) model. When the normal load exceeds the bearing capacity of water, the friction force for model L2 is more stable and smaller than that for model L1 after running-in periods due to flattened interfaces and evenly distributed water molecules.

Molecular Dynamics Simulation on the Interaction between Palygorskite Coating and Linear Chain Alkane Base Lubricant

Molecular dynamics (MD) simulations were conducted to investigate the interactions between a palygorskite coating and linear chain alkanes (dodecane C12, tetradecane C14, hexadecane C16, and octadecane C18), representing base oils in this study. The simulation models were built by placing the alkane molecules on the surface of the palygorskite coating. These systems were annealed and geometrically optimized to obtain the corresponding stable configurations, followed by the analysis of the structural changes occurring during the MD process. The interfacial interaction energies, mean square displacements, and self-diffusion coefficients of the systems were evaluated to characterize the interactions between base lubricant molecules and palygorskite coating. It was found that the alkanes exhibited self-arrangement ability after equilibrium. The interfacial interaction was attractive, and the electrostatic energy was the main component of the binding energy. The chain length of the linear alkanes had a significant impact on the intensity of the interfacial interactions and the molecular diffusion behavior. Moreover, the C12 molecule exhibited higher self-diffusion coefficient values than C14, C16 and C18. Therefore, it could be the best candidate to form an orderliness and stable lubricant film on the surface of the palygorskite coating. The present work provides new insight into the optimization of the structure and composition of coatings and lubricants, which will guide the experimental development of these systems for practical applications.

Lubricant-infused titania surfaces with high underwater transparency for antifouling applications: A combined experimental and molecular dynamics study

Marine optical instruments are usually blinded by biofouling. Biomimetic slippery lubricant-infused surfaces have been used for antifouling applications. Herein, lubricant-infused titania surfaces on glass substrates were fabricated, involving the formation of nanostructured surfaces via a sol–gel process of alkoxide, followed by fluorination treatment and then lubricant infusion. Molecular dynamics (MD) simulations directly suggested the adsorption behavior of fluorinate molecules on titania surfaces and the interaction between the fluorinated surface and lubricant molecules. The MD simulation results also illustrated the different interactions between as-prepared surfaces and water molecules, indicating the formation of stable lubricant-infused surfaces. The lubricant-infused surfaces showed a high transparency up to 87% throughout the visible-light region, and exhibited excellent antifouling performance by declining adhesion by 94% Phaeodactylum tricornutum. The results would show great potential of lubricant-infused surfaces for antifouling applications of marine optical instruments with high transparency.

Adsorption enhancement of a fatty acid on iron surface with Σ3(1 1 1) grain boundary

Nanocrystallization processes applied to metallic surfaces can enhance their tribological properties under boundary lubrication. To reveal the mechanism, the adsorbability of acetic acid as a fatty lubricant molecule on a nanostructured iron surface was investigated using first-principles calculations. The nanostructured surface was modeled as a surface slab including a grain boundary (GB) per unit cell. Comparing the adsorbability of the Fe (110) surface with symmetrical tilt Σ3 (111) grain boundaries to that of an Fe (110) surface without GBs, we found that the molecule tends to be adsorbed more strongly (0.76 eV in average) on sites close to the grain boundary than on the clean surface. A local energy analysis revealed that the adsorption was enhanced even at a relatively distant position (approximately 5 Å from the adsorbed molecule) and that the energy contributions of atoms of different types were qualitatively different for the adsorption on the surface with GBs.