Mechanochemistry at Solid Surfaces: Polymerization of Adsorbed Molecules by Mechanical Shear at Tribological Interfaces

Mechanochemistry: Fundamental Principles and Applications

Mechanochemistry is an emerging research field at the interface of physics, mechanics, materials science, and chemistry. Complementary to traditional activation methods in chemistry, such as heat, electricity, and light, mechanochemistry focuses on the activation of chemical reactions by directly or indirectly applying mechanical forces. It has evolved as a powerful tool for controlling chemical reactions in solid state systems, sensing and responding to stresses in polymer materials, regulating interfacial adhesions, and stimulating biological processes. By combining theoretical approaches, simulations and experimental techniques, researchers have gained intricate insights into the mechanisms underlying mechanochemistry. In this review, the physical chemistry principles underpinning mechanochemistry are elucidated and a comprehensive overview of recent significant achievements in the discovery of mechanically responsive chemical processes is provided, with a particular emphasis on their applications in materials science. Additionally, The perspectives and insights into potential future directions for this exciting research field are offered.

Shear-activation of mechanochemical reactions through molecular deformation

Mechanical stress can directly activate chemical reactions by reducing the reaction energy barrier. A possible mechanism of such mechanochemical activation is structural deformation of the reactant species. However, the effect of deformation on the reaction energetics is unclear, especially, for shear stress-driven reactions. Here, we investigated shear stress-driven oligomerization reactions of cyclohexene on silica using a combination of reactive molecular dynamics simulations and ball-on-flat tribometer experiments. Both simulations and experiments captured an exponential increase in reaction yield with shear stress. Elemental analysis of ball-on-flat reaction products revealed the presence of oxygen in the polymers, a trend corroborated by the simulations, highlighting the critical role of surface oxygen atoms in oligomerization reactions. Structural analysis of the reacting molecules in simulations indicated the reactants were deformed just before a reaction occurred. Quantitative evidence of shear-induced deformation was established by comparing bond lengths in cyclohexene molecules in equilibrium and prior to reactions. Nudged elastic band calculations showed that the deformation had a small effect on the transition state energy but notably increased the reactant state energy, ultimately leading to a reduction in the energy barrier. Finally, a quantitative relationship was developed between molecular deformation and energy barrier reduction by mechanical stress.

Induced Decomposition and Slip Interface Transformation of Oleic Acid Enables Ultralow Wear in Boundary Lubrication

The tribological behavior of carboxylic acids, especially oleic acid, in boundary lubrication conditions is a subject of interest. This study presents the results of four-ball tribological tests conducted under varying contact pressures and sliding speeds. The findings reveal a critical turning speed within a confined zone, which causes a significant change in the frictional performances of oleic acid, leading to the formation of an ultralow wear tribofilm. This tribofilm, predominantly composed of oxyhydrogen compounds and hydrocarbons with more than five carbon atoms, is generated by the molecular action of oleic acid. Reactive nonequilibrium molecular dynamics simulations demonstrate that the shear speed-dependent decomposition modes of oleic acid and the transformation of the lubrication slip interface are the fundamental processes underlying the formation of this ultralow-wear boundary tribofilm.

Effects of surface chemistry on the mechanochemical decomposition of tricresyl phosphate

The growth of protective tribofilms from lubricant antiwear additives on rubbing surfaces is initiated by mechanochemically promoted dissociation reactions. These processes are not well understood at the molecular scale for many important additives, such as tricresyl phosphate (TCP). One aspect that needs further clarification is the extent to which the surface properties affect the mechanochemical decomposition. Here, we use nonequilibrium molecular dynamics (NEMD) simulations with a reactive force field (ReaxFF) to study the decomposition of TCP molecules confined and pressurised between sliding ferrous surfaces at a range of temperatures. We compare the decomposition of TCP on native iron, iron carbide, and iron oxide surfaces. We show that the decomposition rate of TCP molecules on all the surfaces increases exponentially with temperature and shear stress, implying that this is a stress-augmented thermally activated (SATA) process. The presence of base oil molecules in the NEMD simulations decreases the shear stress, which in turn reduces the rate constant for TCP decomposition. The decomposition is much faster on iron surfaces than iron carbide, and particularly iron oxide. The activation energy, activation volume, and pre-exponential factor from the Bell model are similar on iron and iron carbide surfaces, but significantly differ for iron oxide surfaces. These findings provide new insights into the mechanochemical decomposition of TCP and have important implications for the design of novel lubricant additives for use in high-temperature and high-pressure environments.

Mechanistic model for quantifying the effect of impact force on mechanochemical reactivity

Conventional mechanochemical synthetic tools, such as ball mills, offer no methodology to quantitatively link macroscale reaction parameters, such as shaking frequency or milling ball radius, to fundamental drivers of reactivity, namely the force vectors applied to the reactive molecules. As a result, although mechanochemistry has proven to be a valuable method to make a wide variety of products, the results are seldom reproduceable between reactors, difficult to rationally optimize, and hard to ascribe to a specific reaction pathway. Here we have developed a controlled force reactor, which is a mechanochemical ball mill reactor with integrated force measurement and control during each impact. We relate two macroscale reactor parameters-impact force and impact time-to thermodynamic and kinetic transition state theories of mechanochemistry utilizing continuum contact mechanics principles. We demonstrate force controlled particle fracture of NaCl to characterize particle size evolution during reactions, and force controlled reaction between anhydrous copper(II) chloride and (1, 10) phenanthroline. During the fracture of NaCl, we monitor the evolution of particle size as a function of impact force and find that particles quickly reach a particle size of ∼100 μm largely independent of impact force, and reach steady state 10-100× faster than reaction kinetics of typical mechanochemical reactions. We monitor the copper(II) chloride reactivity by measuring color change during reaction. Applying our transition state theory developed here to the reaction curves of copper(II) chloride and (1, 10) phenanthroline at multiple impact forces results in an activation energy barrier of 0.61 ± 0.07 eV, distinctly higher than barriers for hydrated metal salts and organic ligands and distinctly lower than the direct cleavage of the CuCl bond, indicating that the reaction may be mediated by the higher affinity of Fe in the stainless steel vessel to Cl. We further show that the results in the controlled force reactor match rudimentary estimations of impact force within a commercial ball mill reactor Retsch MM400. These results demonstrate the ability to quantitatively link macroscale reactor parameters to reaction properties, motivating further work to make mechanochemical synthesis quantitative, predictable, and fundamentally insightful.

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.

Shear-activated chemisorption and association of cyclic organic molecules

Mechanochemical activation has created new opportunities for applications such as solvent-free chemical synthesis, polymer processing, and lubrication. However, mechanistic understanding of these processes is still limited because the mechanochemical response of a system is a complex function of many variables, including the direction of applied stress and the chemical features of the reactants in non-equilibrium conditions. Here, we studied shear-activated reactions of simple cyclic organic molecules to isolate the effect of chemical structure on reaction yield and pathway. Reactive molecular dynamics simulations were used to model methylcyclopentane, cyclohexane, and cyclohexene subject to pressure and shear stress between silica surfaces. Cyclohexene was found to be more susceptible to mechanochemical activation of oxidative chemisorption and subsequent oligomerization reactions than either methylcyclopentane or cyclohexane. The oligomerization trend was consistent with shear-driven polymerization yield measured in ball-on-flat sliding experiments. Analysis of the simulations showed the distribution of carbon atom sites at which oxidative chemisorption occurred and identified the double bond in cyclohexene as being the origin of its shear susceptibility. Lastly, the most common reaction pathways for association were identified, providing insight into how the chemical structures of the precursor molecules determined their response to mechanochemical activation.

The Effects of Physical-Chemical Evolution of High-Sulfur Petroleum Coke on Hg0 Removal from Coal-Fired Flue Gas and Exploration of Its Micro-Scale Mechanism

As the solid waste by-product from the delayed coking process, high-sulfur petroleum coke (HSPC), which is hardly used for green utilization, becomes a promising raw material for Hg⁰ removal from coal-fired flue gas. The effects of the physical-chemical evolution of HSPC on Hg⁰ removal are discussed. The improved micropores created by pyrolysis and KOH activation could lead to over 50% of Hg⁰ removal efficiency with the loss of inherent sulfur. Additional S-containing and Br-containing additives are usually introduced to enhance active surface functional groups for Hg⁰ oxidation, where the main product are HgS, HgBr, and HgBr₂. The chemical-mechanical activation method can make additives well loaded on the surface for Hg⁰ removal. The DFT method is used to sufficiently explain the micro-scale reaction mechanism of Hg⁰ oxidation on the surface of revised-HSPC. ReaxFF is usually employed for the simulation of the pyrolysis of HSPC. However, the developed mesoporous structure would be a better choice for Hg⁰ removal in that the coupled influence of pore structure and functional groups plays a comprehensive role in both adsorption and oxidation of Hg⁰. Thus, the optimal porous structure should be further explored. On the other hand, both internal and surface sulfur in HSPC should be enhanced to be exposed to saving sulfur additives or obtaining higher Hg⁰ removal capacity. For it, controllable pyrolysis with different pyrolysis parameters and the chemical-mechanical activation method is recommended to both improve pore structure and increase functional groups for Hg⁰ removal. For simulation methods, ReaxFF and DFT theory are expected to explain the micro-scale mechanisms of controllable pyrolysis, the chemical-mechanical activation of HSPC, and further Hg⁰ removal. This review work aims to provide both experimental and simulational guidance to promote the development of industrial application of Hg⁰ adsorbent based on HSPC.

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.

Mechanochemistry of phosphate esters confined between sliding iron surfaces

The molecular structure of lubricant additives controls not only their adsorption and dissociation behaviour at the nanoscale, but also their ability to reduce friction and wear at the macroscale. Here, we show using nonequilibrium molecular dynamics simulations with a reactive force field that tri(s-butyl)phosphate dissociates much faster than tri(n-butyl)phosphate when heated and compressed between sliding iron surfaces. For both molecules, dissociative chemisorption proceeds through cleavage of carbon-oxygen bonds. The dissociation rate increases exponentially with temperature and stress. When the rate-temperature-stress data are fitted with the Bell model, both molecules have similar activation energies and activation volumes and the higher reactivity of tri(s-butyl)phosphate is due to a larger pre-exponential factor. These observations are consistent with experiments using the antiwear additive zinc dialkyldithiophosphate. This study represents a crucial step towards the virtual screening of lubricant additives with different substituents to optimise tribological performance.

Toward a Greener World-Cyclodextrin Derivatization by Mechanochemistry

Cyclodextrin (CD) derivatives are a challenge, mainly due to solubility problems. In many cases, the synthesis of CD derivatives requires high-boiling solvents, whereas the product isolation from the aqueous methods often requires energy-intensive processes. Complex formation faces similar challenges in that it involves interacting materials with conflicting properties. However, many authors also refer to the formation of non-covalent bonds, such as the formation of inclusion complexes or metal–organic networks, as reactions or synthesis, which makes it difficult to classify the technical papers. In many cases, the solubility of both the starting material and the product in the same solvent differs significantly. The sweetest point of mechanochemistry is the reduced demand or complete elimination of solvents from the synthesis. The lack of solvents can make syntheses more economical and greener. The limited molecular movements in solid-state allow the preparation of CD derivatives, which are difficult to produce under solvent reaction conditions. A mechanochemical reaction generally has a higher reagent utilization rate. When the reaction yields a good guest co-product, solvent-free conditions can be slower than in solution conditions. Regioselective syntheses of per-6-amino and alkylthio-CD derivatives or insoluble cyclodextrin polymers and nanosponges are good examples of what a greener technology can offer through solvent-free reaction conditions. In the case of thiolated CD derivatives, the absence of solvents results in significant suppression of the thiol group oxidation, too. The insoluble polymer synthesis is also more efficient when using the same molar ratio of the reagents as the solution reaction. Solid reactants not only reduce the chance of hydrolysis of multifunctional reactants or side reactions, but the spatial proximity of macrocycles also reduces the length of the spacing formed by the crosslinker. The structure of insoluble polymers of the mechanochemical reactions generally is more compact, with fewer and shorter hydrophilic arms than the products of the solution reactions.

Effect of Native Oxide Layer on Mechanochemical Reaction at the GaN-Al2O3 Interface

Mechanochemical reactions at the gallium nitride-alumina (GaN-Al₂O₃) interface at nanoscale offer a significant beneficial reference for the high-efficiency and low-destruction ultra-precision machining on GaN surface. Here, the mechanochemical reactions on oxide-free and oxidized GaN surfaces rubbed by the Al₂O₃ nanoasperity as a function of the ambient humidity were studied. Experimental results reveal that oxidized GaN exhibits a higher mechanochemical removal rate than that of oxide-free GaN over the relative humidity range of 3-80%. The mechanical activation in the mechanochemical reactions at the GaN-Al₂O₃ interface is well-described by the mechanically-assisted Arrhenius-type kinetics model. The analysis indicates that less external mechanical activation energy is required to initiate the mechanochemical atomic attrition on the oxidized GaN surface compared with the oxide-free GaN surface. These results may not only gain a deep understanding of the mechanochemical removal mechanism of GaN but also provide the basic knowledge for the optimization of the oxidation-assisted ultra-precision machining.

Physical Origin of the Mechanochemical Coupling at Interfaces

We used density functional theory calculations to investigate the physical origin of the mechanochemical response of material interfaces. Our results show that the mechanochemical response can be decomposed into the contribution from the interface itself (deformation of interfacial bonds) and a contribution from the underlying solid. The relative contributions depend on the stiffness of these regions and the contact geometry, which affects the stress distribution within the bulk region. We demonstrate that, contrary to what is commonly assumed, the contribution to the activation volume from the elastic deformation of the surrounding bulk is significant and, in some case, may be dominant. We also show that the activation volume and the mechanochemical response of interfaces should be finite due to the effects on the stiffness and stress distribution within the near-surface bulk region. Our results indicate that the large range of activation volumes measured in the previous experiments even for the same material system might originate from the different degrees of contributions probed from the bulk vs interface.

The Influence of Sliding Speed on the Friction Behavior of Silica Surface

Studies have shown that the relative sliding speed of the silicon-based material surface has an effect on its friction behavior. In this study, the molecular dynamics method was used to simulate the sliding of the SiO₂ surface at different speeds. This is to explore the internal mechanism between SiO₂ surface friction behavior and the relative sliding speed. First of all, this study constructed a 3D model of the SiO₂ friction surface and simulated the sliding process under two different environments of absolute dryness and full wetness. Then, the sliding of the SiO₂ surface at different speeds in dry and wet environments is simulated and verified the rationality of the simulation through experiments. The final results show that the lattice distortion and tribochemical reactions that occur on the SiO₂ surface of the material have varying degrees of influence on the friction behavior of the material surface. In the dry environment, the coefficient of friction of the SiO₂ surface increases with the speed. On the contrary, in the humid environment, the SiO₂ surface decreases as the speed increases. The analysis results found that the speed has varying degrees of influence on the lattice distortion and tribochemical reaction of the SiO₂ surface. Eventually, this study quantifies the effect of speed on SiO₂ surface tribochemical reactions and lattice distortion in two different environments.

Effect of Temperature on the Deformation Behavior of Copper Nickel Alloys under Sliding

The microstructural evolution in the near-surface regions of a dry sliding interface has considerable influence on its tribological behavior and is driven mainly by mechanical energy and heat. In this work, we use large-scale molecular dynamics simulations to study the effect of temperature on the deformation response of FCC CuNi alloys of several compositions under various normal pressures. The microstructural evolution below the surface, marked by mechanisms spanning grain refinement, grain coarsening, twinning, and shear layer formation, is discussed in depth. The observed results are complemented by a rigorous analysis of the dislocation activity near the sliding interface. Moreover, we define key quantities corresponding to deformation mechanisms and analyze the time-independent differences between 300 K and 600 K for all simulated compositions and normal pressures. Raising the Ni content or reducing the temperature increases the energy barrier to activate dislocation activity or promote plasticity overall, thus increasing the threshold stress required for the transition to the next deformation regime. Repeated distillation of our quantitative analysis and successive elimination of spatial and time dimensions from the data allows us to produce a 3D map of the dominating deformation mechanism regimes for CuNi alloys as a function of composition, normal pressure, and homologous temperature.

Tribochemistry as an Alternative Synthesis Pathway

While reactions driven by mechanical force or stress can be labeled mechanochemical, those specifically occurring at a sliding interface inherit the name tribochemical, which stems from the study of friction and wear: tribology. Increased perception of tribochemical reactions has been gained through technological advancement, and the development of new applications remains on-going. This surprising physico-kinetic process offers great potential in novel reaction pathways for synthesis techniques and nanoparticle interactions, and it could prove to be a powerful cross-disciplinary research area among chemists, engineers, and physicists. In this review article, a survey of the history and recent usage of tribochemical reaction pathways is presented, with a focus on forging new compounds and materials with this sustainable synthesis methodology. In addition, an overview of tribochemistry’s current utility as a synthesis pathway is given and compared to that of traditional mechanochemistry.

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.

Mechanochemistry of Zinc Dialkyldithiophosphate on Steel Surfaces under Elastohydrodynamic Lubrication Conditions

Zinc dialkyldithiophosphate (ZDDP) is added to engine lubricants to reduce wear and ensure reliable operation. ZDDP reacts under rubbing conditions to form protective zinc/iron phosphate tribofilms on steel surfaces. Recently, it has been demonstrated that this process can be promoted by applied stresses in lubricated contacts, as well as temperature, and is thus mechanochemical in origin. In this study, a tribology test rig, capable of applying very high loads, has been developed to generate ZDDP tribofilms under full-film elastohydrodynamic lubrication (EHL) conditions in steel/steel ball-on-disk contacts. This provides a well-defined temperature and stress environment with negligible direct asperity contact in which to study mechanochemical processes. ZDDPs with branched primary and secondary alkyl substituents have been studied in three base oils, two with high EHL friction and one with low EHL friction. In the high EHL friction base oils, the tribofilm growth rate increases exponentially with shear stress and temperature for both ZDDPs, as predicted by a stress-augmented thermal activation model. Conversely, under otherwise identical conditions, negligible ZDDP tribofilm formation takes place in the low EHL friction base oil. This confirms that the ZDDP reaction is driven by macroscopic shear stress rather than hydrostatic pressure. The secondary ZDDP forms tribofilms considerably faster than the primary ZDDP under equivalent conditions, suggesting that the initial decomposition reaction is the rate-determining step for tribofilm formation. The rate of tribofilm growth is independent of ZDDP concentration over the range studied, indicating that this process follows zero-order kinetics. Under full-film EHL conditions, ZDDP tribofilm formation is promoted by macroscopic shear stress applied through the base oil molecules, which induces asymmetric stress on adsorbed ZDDP molecules to promote their decomposition and initiate rapid phosphate polymerization.

Measuring and modelling mechanochemical reaction kinetics

Quasi-static quantum calculations of the mechanochemical decomposition rate of methyl thiolate species on Cu(100) accurately reproduce the experimental kinetics measured in ultrahigh vacuum by an atomic force microscopy tip.

Mechanochemical Reactions of Adsorbates at Tribological Interfaces: Tribopolymerizations of Allyl Alcohol Coadsorbed with Water on Silicon Oxide

Mechanochemical reactions of adsorbed molecules at tribological interfaces can benefit or impede lubrication, depending on the type of reactions induced by the interfacial shear or friction. Shear-induced polymerization of oxidatively chemisorbed organic species can occur at tribological interfaces, and their products can mitigate the wear of the surface in the case of the intermittent cessation of the lubricant supply. In contrast, tribochemical reactions involving water molecules impinging from the ambient air could facilitate surface wear. In this study, we investigated how such processes are affected when a silicon oxide surface is exposed to the environment containing both water and polymerizable organic molecules. For the polymerizable organic moiety, allyl alcohol was chosen because it is known to have a good tribopolymerization activity and can compete with water for surface adsorption sites. The adsorbate composition can be divided into two regimes: water-rich and alcohol-rich. The tribopolymerization yield was found to be significantly enhanced, compared to the alcohol-only case, in both water-rich and alcohol-rich regimes. The coadsorbed water molecules appeared to be incorporated into the tribopolymerization product of allyl alcohol. The friction coefficient qualitatively correlated with the tribopolymerization yield. Surprisingly, a small degree of surface wear was observed in the alcohol-rich regime, although wear was completely suppressed in the water-rich regime and the alcohol-only condition. These results suggested that the wear prevention effect does not necessarily correlate with the tribopolymerization effects.

Effect of Ambient Chemistry on Friction at the Basal Plane of Graphite

Graphite is widely used as a solid lubricant due to its layered structure, which enables ultralow friction. However, the lubricity of graphite is affected by ambient conditions and previous studies have shown a sharp contrast between frictional behavior in vacuum or dry environments compared to humid air. Here, we studied the effect of organic gaseous species in the environment, specifically comparing the adsorption of phenol and pentanol vapor. Atomic force microscopy experiments and reactive molecular dynamics simulations showed that friction was larger with phenol than with pentanol. The simulation results were analyzed to test multiple hypotheses to explain the friction difference, and it was found that mechanically driven chemical bonding between the tip and phenol molecules plays a critical role. Bonding increases the number of phenol molecules in the contact, which increases the adhesion as well as the number of atoms in registry with the topmost graphene layer acting as a pinning site to resist sliding. The findings of this research provide insight into how the chemistry of the operating environment can affect the frictional behavior of graphite and layered materials more generally.

Surface Structure Dependence of Mechanochemical Etching: Scanning Probe-Based Nanolithography Study on Si(100), Si(110), and Si(111)

We employed a scanning probe-based lithography process on single-crystalline Si(100), Si(110), and Si(111) surfaces and studied the effects of crystallographic surface structures on mechanochemical etching of silicon in liquid water. The facet angle and etching rate of the mechanochemical process were different from those of the purely chemical etching process. In liquid water, the shape of the mechanochemically etched nanochannel appeared to be governed by thermodynamics of the etched surface, rather than stress distribution. Analyzing the etch rate with the mechanically assisted Arrhenius-type kinetics model showed that the shear-induced hydrolysis activity varies drastically with the crystallographic structure of silicon surface.

Formation and Nature of Carbon-Containing Tribofilms

Minimizing friction and wear at a rubbing interface continues to be a challenge and has resulted in the recent surge toward the use of coatings such as diamond-like carbon (DLC) on machine components. The problem with the coating approach is the limitation of coating wear life. Here, we report a lubrication approach in which lubricious, wear-protective carbon-containing tribofilms can be self-generated and replenishable, without any surface pretreatment. Such carbon-containing films were formed under modest sliding conditions in a lubricant consisting of cyclopropanecarboxylic acid as an additive dissolved in polyalphaolefin base oil. These tribofilms show the same Raman D and G signatures that have been interpreted to be due to the presence of graphite- or DLC films. Our experimental measurements and reactive molecular dynamics simulations demonstrate that these tribofilms are in fact high-molecular weight hydrocarbons acting as a solid lubricant.

Effect of Surface Silanol Groups on Friction and Wear between Amorphous Silica Surfaces

Reactive molecular dynamics (ReaxFF) simulations are performed to explore the tribological behavior between fully hydroxylated amorphous silica (a-SiO₂) surfaces as a function of surface silanol density. The results show that the interfacial friction and wear are greatly reduced by increasing surface silanol density, which originates from the suppression of the initial formation of interfacial Si-O-Si bridge bonds. Two different tribochemical reactions resulting in the formation of interfacial Si-O-Si bridge bonds are observed: i.e., one occurring between two silanol groups, which is insensitive to changes in silanol density, and the other occurring between a silanol group and a surface Si-O-Si bond, which is strongly suppressed with the increase of silanol density. We decouple the contributions of these two Si-O-Si bond formation mechanisms to the observed tribological behavior and find that the latter formation mechanism plays a dominant role. Furthermore, the changes in the geometry and structure of fully hydroxylated a-SiO₂ surface caused by the increased surface silanol groups also play an important role in the tribochemical reactions and the tribological performance of the a-SiO₂/a-SiO₂ system. This work provides a deeper insight into the effect of surface silanol groups on the tribological behaviors of silicon-based materials.

Mechano-chemical decomposition of organic friction modifiers with multiple reactive centres induces superlubricity of ta-C

Superlubricity of tetrahedral amorphous carbon (ta-C) coatings under boundary lubrication with organic friction modifiers is important for industrial applications, but the underlying mechanisms remain elusive. Here, combined experiments and simulations unveil a universal tribochemical mechanism leading to superlubricity of ta-C/ta-C tribopairs. Pin-on-disc sliding experiments show that ultra- and superlow friction with negligible wear can be achieved by lubrication with unsaturated fatty acids or glycerol, but not with saturated fatty acids and hydrocarbons. Atomistic simulations reveal that, due to the simultaneous presence of two reactive centers (carboxylic group and C=C double bond), unsaturated fatty acids can concurrently chemisorb on both ta-C surfaces and bridge the tribogap. Sliding-induced mechanical strain triggers a cascade of molecular fragmentation reactions releasing passivating hydroxyl, keto, epoxy, hydrogen and olefinic groups. Similarly, glycerol's three hydroxyl groups react simultaneously with both ta-C surfaces, causing the molecule's complete mechano-chemical fragmentation and formation of aromatic passivation layers with superlow friction.

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.

Nanotribological Printing: A Nanoscale Additive Manufacturing Method

Additive manufacturing methods are transforming the way components and devices are fabricated, which in turn is opening up completely new vistas for conceiving and designing products and engineered systems. Small-scale (submicrometer) additive manufacturing methods are largely in their infancy. While a number of methods exist, a particular challenge lies in finding methods that can produce a range of materials while obtaining sufficiently robust mechanical properties. In this paper, we describe a novel nanoscale additive manufacturing technique deemed "Nanotribological Printing" (NTP), which creates structures through tribomechanical and tribochemical surface interactions at the contact between a substrate and an atomic force microscope probe, where material pattern formation is driven by normal and shear contact stresses. The "ink" consists of nanoparticles or molecules dispersed in a carrier fluid surrounding the atomic force microscope (AFM) probe, which are entrained into the contact during sliding. Being stress-driven, patterning only occurs locally within regions which experience contact and sufficiently high stresses. Thus, imaging and measurement to characterize the morphology and properties of the deposited structures can be conducted in situ during the manufacturing process. Moreover, using local mechanical energy as the kinetic driver activating the solidification process, the method is compact and does not require application of a bias voltage or laser exposure and can be performed at ambient temperatures. We demonstrate (1) control of pattern dimensions with sub-100 nm lateral and sub-5 nm thickness control through variations in contact size and applied stress, (2) creation of amorphous, polycrystalline, and nanocomposite structures including sequential multimaterial deposition, and (3) formation of manufactured structures which exhibit mechanical properties approaching those of bulk counterparts. The ability to create nanoscale patterns using standard AFM cantilever probes and operation modes (contact mode scanning in fluid) with commercial AFM instruments, independent of substrate, establishes NTP as a versatile and easily accessible method for nanoscale additive manufacturing.

Tribochemical Competition within a MoS2/Ti Dry Lubricated Macroscale Contact in Ultrahigh Vacuum: A Time-of-Flight Secondary Ion Mass Spectrometry Investigation

Controlling and predicting the tribological behavior of dry lubricants is a necessity to ensure low friction, long life, and low particle generation. Understanding the tribochemistry of the materials as a function of the environment is of primary interest as synergistic effects exist between the mechanics, the physicochemistry, and the thermodynamics within a contact. However, in most studies the role of the coating internal contaminants in the process is often discarded to the benefit of a more common approach in which the performances of the materials are compared as a function of different atmospheric pressure environments. The study focuses on the understanding of the tribochemical processes occurring between the materials and their internal contaminants inside an AISI440C contact lubricated by a MoS₂/Ti coating. Time-of-flight secondary ion mass spectrometry is used to study at the molecular level, the material before and after friction. Friction tests with different durations are performed in ultrahigh vacuum at the macroscale to stay relevant to the real application (space). The adsorption/desorption of gaseous species during friction is monitored by mass spectrometry to ensure reliable study of the tribochemical processes inside the contact. The study shows that a competition exists between the Ti- and MoS₂-based materials to create the appropriate lubricating materials via (i) recrystallization of MoS₂ materials with creation of a MoS _xO _y material via reactions with internal contaminants (presumably H₂O), (ii) reaction of Ti-based materials with internal contaminants (mostly H₂O and N₂). The biphasic material created is highly similar to the one created in both humid air and dry N₂ environments and providing low friction and low particle generation. However, the process is incomplete. The study thus brings insight into the possibility of controlling friction via a rational inclusion of reactants in a form of contaminants to control the tribochemical processes governing the low friction and long life.

Mechanochemical Association Reaction of Interfacial Molecules Driven by Shear

Shear-driven chemical reaction mechanisms are poorly understood because the relevant reactions are often hidden between two solid surfaces moving in relative motion. Here, this phenomenon is explored by characterizing shear-induced polymerization reactions that occur during vapor phase lubrication of α-pinene between sliding hydroxylated and dehydroxylated silica surfaces, complemented by reactive molecular dynamics simulations. The results suggest that oxidative chemisorption of the α-pinene molecules at reactive surface sites, which transfers oxygen atoms from the surface to the adsorbate molecule, is the critical activation step. Such activation takes place more readily on the dehydroxylated surface. During this activation, the most strained part of the α-pinene molecules undergoes a partial distortion from its equilibrium geometry, which appears to be related to the critical activation volume for mechanical activation. Once α-pinene molecules are activated, association reactions occur between the newly attached oxygen and one of the carbon atoms in another molecule, forming ether bonds. These findings have general implications for mechanochemistry because they reveal that shear-driven reactions may occur through reaction pathways very different from their thermally induced counterparts and specifically the critical role of molecular distortion in such reactions.

Nanomanufacturing of silicon surface with a single atomic layer precision via mechanochemical reactions

Topographic nanomanufacturing with a depth precision down to atomic dimension is of importance for advancement of nanoelectronics with new functionalities. Here we demonstrate a mask-less and chemical-free nanolithography process for regio-specific removal of atomic layers on a single crystalline silicon surface via shear-induced mechanochemical reactions. Since chemical reactions involve only the topmost atomic layer exposed at the interface, the removal of a single atomic layer is possible and the crystalline lattice beneath the processed area remains intact without subsurface structural damages. Molecular dynamics simulations depict the atom-by-atom removal process, where the first atomic layer is removed preferentially through the formation and dissociation of interfacial bridge bonds. Based on the parametric thresholds needed for single atomic layer removal, the critical energy barrier for water-assisted mechanochemical dissociation of Si–Si bonds was determined. The mechanochemical nanolithography method demonstrated here could be extended to nanofabrication of other crystalline materials.

The continued scaling of silicon based electronic devices requires the development of increasingly innovative approaches for high-precision material removal. Here, the authors demonstrate subnanometre depth removal of silicon using scanning probe, shear-induced mechanochemical reactions.

Surface Chemistry Dependence of Mechanochemical Reaction of Adsorbed Molecules-An Experimental Study on Tribopolymerization of α-Pinene on Metal, Metal Oxide, and Carbon Surfaces

Mechanochemical reactions between adsorbate molecules sheared at tribological interfaces can induce association of adsorbed molecules, forming oligomeric and polymeric products often called tribopolymers). This study revealed the role or effect of surface chemistry of the solid substrate in mechanochemical polymerization reactions. As a model reactant, α-pinene was chosen because it was known to readily form tribopolymers at the sliding interface of stainless steel under vapor-phase lubrication conditions. Eight different substrate materials were tested-palladium, nickel, copper, stainless steel, gold, silicon oxide, aluminum oxide, and diamond-like carbon (DLC). All metal substrates and DLC were initially covered with surface oxide species formed naturally in air or during the oxidative sample cleaning. It was found that the tribopolymerization yield of α-pinene is much higher on the substrates that can chemisorb α-pinene, compared to the ones on which only physisorption occurs. From the load dependence of the tribopolymerization yield, it was found that the surfaces capable of chemisorption give a smaller critical activation volume for the mechanochemical reaction, compared to the ones capable of physisorption only. On the basis of these observations and infrared spectroscopy analyses of the adsorbed molecules and the produced polymers, it was concluded that the mechanochemical reaction mechanisms might be different between chemically reactive and inert surfaces and that the chemical reactivity of the substrate surface greatly influences the tribochemical polymerization reactions of adsorbed molecules.

Development of a ReaxFF Force Field for Cu/S/C/H and Reactive MD Simulations of Methyl Thiolate Decomposition on Cu (100)

It has been shown that the rate of decomposition of methyl thiolate species on copper is accelerated by sliding on a methyl thiolate covered surface in ultrahigh vacuum at room temperature. The reaction produces small gas-phase hydrocarbons and deposits sulfur on the surface. Here, a new ReaxFF potential was developed to enable investigation of the molecular processes that induce this mechanochemical reaction by using density functional theory calculations to tune force field parameters for the model system. Various processes, including volumetric expansion/compression of CuS, CuS₂, and Cu₂S unit cells; bond dissociation of Cu-S and valence angle bending of Cu-S-C; the binding energies of SCH₃, CH₃, and S atoms on a Cu surface; and energy for the decomposition of methyl thiolate molecular species on copper, were used to identify the new ReaxFF parameters. Molecular dynamics simulations of the reactions of adsorbed methyl thiolate species at various temperatures were performed to demonstrate the validity of the new potential and to study the thermal reaction pathways. It was found that reaction is initiated by C-S bond scission, consistent with experiments, and that the resulting methyl species diffuse on the surface and combine to desorb ethane, also as found experimentally.

Mechanochemistry of Physisorbed Molecules at Tribological Interfaces: Molecular Structure Dependence of Tribochemical Polymerization

Physisorbed molecules at a sliding solid interface could be activated by mechanical shear and react with each other to form polymeric products that are often called tribopolymers. The dependence of the tribopolymerization yield on the applied load and adsorbate molecular structure was studied to obtain mechanistic insights into mechanochemical reactions at a tribological interface of stainless steel. Three hydrocarbon precursors containing 10 carbon atoms-α-pinene (C₁₀H₁₆), pinane (C₁₀H₁₈), and n-decane (C₁₀H₂₂)-were chosen for this study. α-Pinene and pinane are bicyclic compounds with different ring strains. N-Decane was chosen as a reference molecule without any internal strain. By comparing the adsorption isotherm of these molecules and the total volume of tribopolymer products, the reaction yield was found to be proportional to the number of adsorbed molecules. An Arrhenius-type analysis of the applied load dependence of the tribopolymerization yield revealed how the critical activation volume (ΔV*) varies with the structure of adsorbed molecules. The experimentally determined ΔV* values of α-pinene, pinane, and n-decane were 3, 8, and 10% of their molar volumes, respectively. The molecule with the largest ring strain (α-pinene) showed the smallest ΔV*, which implies the critical role of internal molecular strain in the mechanochemical initiation of polymerization reaction. The tribopolymer film synthesized in situ at the sliding interface exhibited an excellent boundary lubrication effect in the absence of any external supply of lubricant molecules.