Tribochemical Processes Research Articles

Ab initio informed machine learning potential for tribochemistry and mechanochemistry: Application for eco–friendly gallate lubricant additive

Mechanochemistry and tribochemistry processes involve multiple physical/chemical interactions induced by extreme conditions including molecular confinement, high temperatures and mechanical stress applied. Simulating these processes by molecular dynamics is very challenging. While force fields fall short reproducing the enhanced reactivity arising by quantum effects, ab initio molecular dynamics is severely limited by the complexity of the systems of interest, their sizes, and the long-time scale on which relevant events take place. In this work using an active learning approach, a landmark deep neural network potential has been developed which reproduces the accuracy of ab initio interactions at the classical molecular dynamics computational cost and permits to successfully simulate the tribochemical processes occurring at the interface between butyl gallate molecules and iron substrates under tribological conditions. The simulations of the dynamics of the Fe-gallate system when sliding under an imposed external load reveal the key atomistic mechanisms underlying the formation of the friction reducing lubricant tribofilm and permit to characterize the tribological properties of the explored systems, clearly exposing the shortcoming of reactive force field based approaches. The successful development of neural network potentials, making it possible to push the limits of molecular dynamics marrying accuracy with system sizes and long time scales, paves the route toward a new area in computational tribochemistry.

Tribological behaviour of Ti3C2Tx nano-sheets: Substrate-dependent tribo-chemical reactions

MXenes, a newly emerging class of layered two dimensional (2D) materials, are promising solid lubricants due to their 2D structure consisting of weakly-bonded layers with a low shear strength and ability to form beneficial tribo-layers. This work aims at evaluating for the first time MXenes lubrication performance and tribofilm formation ability on different metallic substrates (mirror-lapped Fe and Cu discs). After depositing MXenes via ethanol (1 wt%) on the substrates, pronounced differences in the resulting substrate-dependent frictional evolution are observed. While MXenes are capable to reduce friction for both substrates after the full evaporation of ethanol, MXenes lubricating effect on Cu is long-lasting, with a 35-fold increased lifetime compared to Fe. Raman spectra acquired in the wear-tracks of the substrates and counter-bodies reveal notable differences in the friction-induced chemical changes depending on the substrate material. In case of Fe, the progressive failure of MXenes lubrication generates different Fe oxides on both the substrate and the ball, resulting in continuously increasing friction and a poor lubrication effect. For Cu, sliding induces the formation of a Ti3C2-based tribofilm on both rubbing surfaces, enabling a long-lasting lubricating effect. This work boosts further experimental and theoretical work on MXenes involved tribo-chemical processes.

A Study of the Wear Mechanism of Composites Modified with Silicate Filler

The article considers the effect of a filler based on synthetic wollastonite (CaSiO3), which is introduced into a polymer matrix made of ultra-high molecular weight polyethylene, on the tribotechnical parameters of the produced polymer composite material. Behavioral features of composites after friction were investigated by infrared spectroscopy and scanning electron microscopy. It was found that the introduction of wollastonite into the polymer matrix contributed to a reduction in the friction coefficient by 23% and the wear rate by four times. In the micrographs of the friction surfaces of the obtained composite, the formation of new secondary structures oriented along the friction direction, different from the initial polymer matrix, was revealed. The presence of wear products (oxidized polymer groups) and CaSiO3 on the friction surfaces was recorded by infrared spectroscopy. It was established that the synthesized CaSiO3 particles were deformed under the action of shear forces and participated in tribochemical processes.

РЕАЛИЗАЦИЯ МЕХАНИЗМА ИЗБИРАТЕЛЬНОГО ПЕРЕНОСА В ПОДВИЖНЫХ СОПРЯЖЕНИЯХ СИЛОВЫХ ПЕРЕДАЧ ПРИ ЭКСПЛУАТАЦИИ СЕЛЬСКОХОЗЯЙСТВЕННЫХ МАШИН

Searching for new effective ways of increasing the wear resistance of power transmissions and improving their durability, the authors propose a device for enriching the lubrication system of power transmissions with copper-alloying elements. The tribochemical processes have been studied and the components of the lubricating composition have been selected to provide for the self-organized selective transfer in the “steel-steel” friction couples accompanied by the formation of a servovit film. The authors developed a utility model for enriching the lubricating system oil with copper-alloying elements for power transmission. For testing the effectiveness of the proposed additive and the method of forming the servovit film during operation, a roller test bench was used. It provided for synchronous measurement of the wear rate and the friction force moment during the whole experiment without disconnecting the friction zone. The friction zone (test contact) was formed by the cyclic surfaces of a roller with a diameter of 70.0 ± 0.1 mm made of steel 45 (HRs 50) and a homogenized pad with a radius of 35.0 ± 0.1 mm and dimensions (plan view) of 2.01 mm (along the sliding line) and 7.27 mm (across the sliding line). The friction area was 0.1461 cm2. A set of normal forces: 730; 925; and 1130 N – was experimentally determined to ensure that there was no seizure. The shaft speed of 100 min-1 (a linear velocity of 0.37 m) was experimentally chosen under the condition of guaranteed absence of the hydrodynamic lubrication mode. The presented method of cladding additive production is easily implemented in the transmission units and also prevents the sedimentary instability of the lubricating composition. The device for enriching the lubricating system oil with copper alloying elements in the process of operation provides for a stable and self-organized mode of selective transfer. The results of tribological studies have shown the high efficiency of the cladding additive – the wear rate in the range of the normal force of 720 to 1130 N has reduced in 2.8…7 times.

Comparative study of tribological properties of nanocomposite and nanolayer coatings containing low-friction MoSx and solid a-C(S,H) components

Reactive pulsed laser deposition was used to create solid lubricating coatings differing in the dimensions of MoSx and a-C(S,H) nanophases in the bulk of the coatings. For coating preparation, two deposition modes were used, including the co-deposition of Mo and C atomic fluxes in hydrogen sulfide and the successive deposition of Mo and C layers up to 12 nm thick in hydrogen sulfide. The two-phase composition of the coating formed by the co-deposition of Упрочняющие технологии и покрытия. 2022. Том 18, № 10 471 Strengthening technologies and coatings. 2022. Vol. 18, no. 10 УПРОЧНЯЮЩИЕ НАНОТЕХНОЛОГИИ atomic flows was recorded only by the method of micro-Raman spectroscopy. No fundamental difference in the tribological properties of these coatings was found during tests in humid and dry air, although some important differences in tribochemical processes on the surface of the coatings were revealed. The use of the technique of alternating deposition of thin layers of controlled thickness and composition opens more opportunities for controlling the nanostructure and phase composition of this type coatings than with the co-deposition of atomic flows. However, the successful implementation of the layer-by-layer deposition technique largely depends on the degree of adhesion between the nanolayers.

Environmental effects on superlubricity of hydrogenated diamond-like carbon: Understanding tribochemical kinetics in O2 and H2O environments

Hydrogenated diamond-like carbon (H-DLC) exhibits superlubricity in inert conditions, making it ideal for protective coating in machines, but its superlubricity is often lost in humid air. Knowing reaction mechanisms of such oxidation processes would be a prerequisite to incorporate environment tolerance into the H-DLC chemistry for superlubricious performance in ambient air. But mechanistic understanding has been hampered by air-oxidation of the tribo-tested H-DLC surface during the sample transfer for ex-situ analyses. As an alternative approach, this study derived a Langmuir-type kinetics model to analyze tribochemical processes occurring at the H-DLC surface in oxidative environments. The model unveiled very distinct environmental effects of O2 and H2O on tribochemistry of H-DLC. The O2-oxidized H-DLC surface retains low friction after the run-in period (μ≈0.057) but is susceptible to frictional wear (wear rate ≈ 32 μm3/(N·mm)). In contrast, the H2O-oxidized surface is susceptible to the adsorption of water which acts like boundary lubricant layers protecting the surface from wear (∼3.4 μm3/(N·mm)) but with relatively high friction after the run-in period (μ≈0.22). The oxidation probability of H-DLC by O2 or H2O (∼10−4 Torr−1 s−1) is found to be comparable to the reaction probability of hydrocarbons on catalytically-active metals (∼10−3 Torr−1 s−1).

Ultralow Wear Poly(tetrafluoroethylene): A Virtuous Cycle of Wear Reduction and Tribochemical Accumulation

Certain nanofillers have demonstrated a remarkable wear-reducing effect on poly(tetrafluoroethylene) (PTFE), a common solid lubricant polymer. In these unusual cases, ultralow wear rates are accompanied by tribochemical changes that stabilize surfaces by anchoring polymer chains to dispersed nanoparticles and the countersurface. The leading hypothesis is that nanofillers catalyze these favorable tribochemical processes, which ultimately causes ultralow wear rates. However, the extent to which reduced wear rates contribute to the accumulation of favorable tribochemistry remains untested. In this study, we used indexed reciprocation to eliminate the protective effects of transfer films, control the wear rate, and independently determine how wear rate and filler morphology affect these tribochemical outcomes. Despite large and important differences between the fillers tested (from nanoparticles to microparticles), we observed surprisingly little effect from the filler on tribochemistry when controlling the wear rate. By contrast, tribochemistry was extremely sensitive to the changes in wear rate regardless of the filler. Standard reciprocation, which promoted the development of protective transfer films, caused low wear rates and significant tribochemical accumulation for all three filler conditions. By removing the protective effects of transfer films, indexed reciprocation increased wear rates by 100-fold while effectively eliminating tribochemical accumulation and tribofilm formation on the polymer pin. In contrast to prior studies, which demonstrate that tribochemical accumulation enables ultralow wear rates, this study demonstrates that a pre-existing condition of low wear is needed to enable tribochemical accumulation. The results from this study and prior studies of PTFE wear appear to be most consistent with the hypothesis that protective transfer films initiate low wear rates, that low wear rates enable tribochemical accumulation, and that tribochemical accumulation reduces wear rates in a virtuous cycle.

Importance of the catalytic effect of the substrate in the functionality of lubricant additives: the case of molybdenum dithiocarbamates

Molybdenum dithiocarbamates (MoDTCs) are lubricant additives very efficient in reducing the friction of steel, and they are used in a number of industrial applications. The functionality of these additives is ruled by the chemical interactions occurring at the buried sliding interface, which are of key importance for the improvement of the lubrication performance. Yet, these tribochemical processes are very difficult to monitor in real time. Ab initio molecular dynamics simulations are the ideal tool to shed light on such a complicated reactivity. In this work, we perform ab initio simulations, both in static and tribological conditions, to understand the effect of surface oxidation on the tribochemical reactivity of MoDTC, and we find that when the surfaces are covered by oxygen, the first dissociative steps of the additives are significantly hindered. Our preliminary tribological tests on oxidized steel discs support these results. Bare metallic surfaces are necessary for a stable adsorption of the additives, their quick decomposition, and the formation of a durable MoS2 tribolayer. This work demonstrates the importance of the catalytic role of the substrate and confirms the full capability of the computational protocol in the pursuit of materials and compounds more efficient in reducing friction.

Sdq-Sdr Topological Map of Surface Topographies

The reliable characterization of surface topographies – especially in case of tribological processes – requires complex and comprehensive solution methods. The use of a combination of roughness parameters offers effective solutions in many cases. In our study, we found a close correlation between the Sdq-Sdr parameter pair and we applied the two parameters as a topological map. Surface topographies resulting from different machining techniques (turned, milled, ground, honed, electric discharge machined, and sintered) were placed in the topological map and their possibilities of use were explored. It was established that surfaces with an orientation characteristically have lower Sdq-Sdr value pairs than non-oriented or multi-oriented surfaces; however, surface defects, injuries and machining discrepancies increase the values of Sdq-Sdr parameters. Then the Sdq-Sdr topological map was applied for worn surfaces resulting from abrasive and tribochemical processes, and it was established that the worn surfaces examined are characterized by small Sdq-Sdr values. Test results showed that in case of manufacturing processes, the topological map is suitable for identifying manufacturing discrepancies and manufacturing defects, and it highlights structural changes in the microgeometry in the course of the wear process.

Influence of relative humidity on wear of self-mated 100Cr6 steel

The influence of relative humidity on friction and wear is subject of several studies in the last decades. A comprehensive understanding of physical and chemical phenomena affecting the tribology is hampered by the lack of reproducible experimental results, by the large number of variables, and by several difficulties in the detection of tribochemical processes and products.In the present work, we analyze the wear coefficient and the wear volumes of 686 unlubricated tests performed on different oscillating tribometers with 100Cr6 balls on 100Cr6 planes at different relative humidity.Aim of this work is to assess the repeatability and reproducibility of data, to determine the dependence of the wear coefficient on the relative humidity, to understand the underlying physicochemical phenomena and to build three dimensional maps of the wear coefficient as a function of both humidity and the product of normal force and sliding distance.

The evolution of subsurface micro-structure and tribo-chemical processes in cocrmo-ti6al4v fretting-corrosion contacts: What lies at and below the surface?

Titanium and its alloys are attractive biomaterials because of their desirable corrosion, mechanical, biocompatibility and osseointegration properties. Ti6Al4V alloy in particular remains a prominent biomaterial used in Total Hip Arthroplasty (THA). Recently, researchers have shown interest in understanding the degradation mechanisms and the subsurface implications of fretting-corrosion at the modular taper interface in THA. The purpose of this study was to utilise advanced microscopy and spectroscopy to characterise in-vitro fretting-corrosion induced subsurface refinement and microstructural changes in Ti6Al4V alloy. In-vitro fretting-corrosion tests were carried at four displacement: ±10, ±25, ±50 and ± 150 μm for a CoCrMo – Ti6Al4V ball-on-flat material couple. Subsequently, high resolution micrographs of the alloy microstructure were obtained using the Transmission Electron Microscope (TEM) together with Energy Dispersed X-Ray spectroscopy (EDX). The degree of subsurface microstructural changes was observed to be linked to the slip regime and magnitude of energy dissipated at the interface. Strain-induced orientation were observed at the stick regime. The mixed and gross-slip regimes were both characterised with mechanical mixing and formation of nano-crystalline structures. Specific to the mixed fretting regime, fluid ingression and material entrapment at the interface led to further refinement of nano-crystalline structures which resulted in the formation of an amorphous Ti6Al4V structure. The interwoven relationship between energy dissipation, contact condition and mechanisms of clinical failure in Ti6Al4V alloy are discussed.

Unveiling oil-additive/surface hierarchy at real ring-liner contact

Zinc-dialkyldithiophosphate (ZDDP) additive in oil has been extensively reported to form a protective tribofilm between the moving components of engine for preventing wear. In this study, surface and interface characterizations were conducted on real piston ring/cylinder liner contacts, the interacting mechanism between oil-additive and solid surfaces under realistic mechanical and thermal conditions was predicted according to the identification of the chemical composition and structure of the tribofilm. The results demonstrate the interesting laminated hierarchy of tribofilm. This tribofilm is composed mainly of dense carbon precipitation at the top region and a thinner polyphosphate intermediate layer in contact with deformed iron oxide substrate on the cylinder liner. The depletion of polyphosphate compounds and competitive growth of solid carbon in term of lubricant's degradation was speculated. These significant findings help manufacturer to understand possible tribochemical processes occurring in the real tribosystem from the practical view, and in particular inspire researchers to develop the new strategy to improve capability and efficiency of oil-additive at sliding contacts.

Interplay between microstructural evolution and tribo-chemistry during dry sliding of metals

Understanding the microstructural and tribo-chemical processes during tribological loading is of utmost importance to further improve the tribological behavior of metals. In this study, the friction, wear and tribo-chemical behavior of Ni with different initial microstructures (nanocrystalline, bi-modal, coarse-grained) is investigated under dry sliding conditions. In particular, the interplay be-tween frictional response, microstructural evolution and tribo-oxidation is considered. Friction tests are carried out using ball-on-disk experiments with alumina balls as counter-bodies, varying the load between 1 and 5 N. The microstructural evolution as well as the chemical reactions beneath the samples’ surface is investigated by means of cross-sections. The samples with finer microstructures show a faster run-in and lower maximum values of the coefficient of friction (COF) which can be attributed to higher oxidation kinetics and a higher hardness. It is observed that with increasing sliding cycles, a stable oxide layer is formed. Furthermore, initially coarse-grained samples show grain refinement, whereas initially finer microstructures undergo grain coarsening converging towards the same superficial grain size after 2,000 sliding cycles. Consequently, the experimental evidence supports that, irrespective of the initial microstructure, after a certain deformation almost identical steady-state COF values for all samples are achieved.

Cardo Copolymers: A Friction–Chemical Structure Relationship

Poly(arylene ether ketone)s containing pendent bulky cardo groups in the monomer unit, while having a high antifrictionality, do not obey the earlier established regularity of the relationship between chemical structure and friction. The necessity of taking into account the flexibility of macromolecules is an additional factor for these polymers. To XPS study the tribochemical processes of carded polymers, “flexible chain” and “rigid chain” copolyarylene ether ketones (co-PAEKs) were used. It was found that the flexible moieties of the macromolecule favor a decrease in the friction coefficient and an increase in the wear resistance. Impaired tribochemical indices of the rigid cardo polymer are caused by active tribodestruction of phthalimidine groups.

Tribochemical Processes in a Phenol Formaldehyde Polymer Modified by Polyformaldehyde Copolymer

Thermal and tribochemical processes in a phenol formaldehyde composite reinforced with polyoxadiazole fibers and modified with finely dispersed particles of a polyformaldehyde copolymer were studied by mass spectrometry and X-ray photoelectron spectroscopy. Under thermal exposure to polyformaldehyde, the liberated formaldehyde sharply reduces the amount of phenol that is released. In the case of friction, polyformaldehyde is a bifunctional modifier. It forms a discrete surface, which helps reduce the coefficient of friction. The released products can play the role of a “tribochemical lubricant” and, at the same time, a cross-linking agent. Tribological tests of the modified composite showed a decrease in the coefficient of friction (from 0.20 to 0.18) compared with the original composite, and a significant (approximately 2-fold) decrease in the vibration amplitude of the friction coefficient.

The Influence of Metal Cations of Mechanoactivated Bentonite on Tribochemical Processes in PTFE

The influence of metals of a bentonite composition on the processes of frictional interaction and tribotechnical properties of PTFE as a result of their preliminary mechanoactivation was studied. It is shown that mechanical activation liberates cations from octahedral layers of layered silicate. The formation of tribo-oxidation products and the influence of metals on tribochemical reactions have been shown and substantiated by IR spectroscopy. A correlation was obtained between the composition of the cations, the products of tribo-oxidation, and the wear resistance of composites.

Temperature Estimations in Elastohydrodynamic Contacts of Tribochemical Processes: A Comparative Study Using Modeling by Mechanical/Wear Process Mechanisms and Chemical Kinetic Modeling

Abstract This article deals with the subject of thermal Elastohydrodynamic Lubrication (EHL) and how temperatures can be estimated in elastohydrodynamic contacts. An attempt has been made to explain why it is essential to study temperature effects in EHL, and the process of thermal EHL modeling has been elucidated by solving a simple thermal EHL model. The temperatures obtained from this simple model seem to agree well with published literature. A comprehensive literature review of the numerous studies in thermal EHL modeling with particular emphasis on recent developments in this field has been presented. The second part of this comprehensive study includes a comparison between temperatures in the EHL contact as obtained by mechanical models and a chemical model, which will be discussed. The contact temperature required to consume the lubricant by oxidation during a microsample wear test is determined by a chemical kinetic model. Finally, the discrepancy between the temperatures obtained by both the chemical kinetic model and the mechanical model is discussed, presenting various plausible explanations to account for the temperature difference.

Effects of surface chemical groups and environmental media on tribochemical running-in behaviors of silicon surface

Abstract In order to offer insight into the removal of outmost silicon atoms, the various tribochemical running-in processes at the Si/SiO2 interfaces were investigated. The running-in of Si–SiOx surface in humid air exhibited a transition from protrusion to groove. By contrast, the initial wear of Si–H surface began with a nearly wearless state in air and revealed apparent material removal after only one sliding cycle in water. The variations in wear rate and friction force suggested the changes of interfacial dangling bonds and energy dissipation. Not only the removal of native oxide layer but also the condensation of interfacial water molecules and the change of surface chemical groups were proposed to interpret the tribochemical running-in mechanisms of silicon surface.

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.

Time-Resolved Characterization of Dynamic Tribochemical Processes for Dicationic Imidazolium Ionic Liquid

Dynamic tribochemical processes for dicationic ionic liquid containing a geminal imidazolium cation head group bridged by a poly(ethylene glycol) and a bis(trifluoromethylsulfonyl)imide anion were studied using time-resolved mechanically stimulated gas emission mass-spectrometry (MSGE-MS). In comparison with similar monocationic imidazolium ionic liquids with short alkyl or long polyether side chains, the dicationic ionic liquid had a lower coefficient of friction on Ti6Al4V alloy and smoother behavior. The analysis of volatile decomposition products suggested multiple tribochemical reactions in which both anionic and cationic moieties are involved. The tribochemical degradation of cations was mainly through the detachment of the side and bridging chains from the imidazolium head groups. The absence of volatile products containing nitrogen implies that the imidazole group remained unchanged. Hydrogen and water desorption were attributed to the reactions of hydrogen fluoride being a product of anion degradation with titanium and titanium oxide, respectively.