Diamond-like carbon (DLC) films were tribochemically formed from ambient hydrocarbons on the surface of a highly stable nanocrystalline Pt-Au alloy. A sliding contact between an alumina sphere and ...Pt-Au coated steel exhibited friction coefficients as low as μ = 0.01 after dry sliding in environments containing trace (ppb) organics. Ex situ analysis indicated that the change in friction coefficient was due to the formation of amorphous carbon films, and Raman spectroscopy and elastic recoil analysis showed that these films consist of sp2/sp3 amorphous carbon with as much as 20% hydrogen. Transmission electron microscopy indicated these films had thicknesses exceeding 100 nm, and were enhanced by the incorporation of worn Pt-Au nanoparticles. The result was highly wear-resistant, low-friction DLC/Pt-Au nanocomposites. Atomistic simulations of hydrocarbons under shear between rigid Pt slabs using a reactive force field showed stress-induced changes in bonding through chain scission, a likely route towards the formation of these coatings. This novel demonstration of in situ tribochemical formation of self-lubricating films has significant impact potential in a wide range of engineering applications.
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Ultra-low-wear PTFE nanocomposites rely heavily on water-dependent tribochemistry, which reinforces surfaces by anchoring tribochemically-modified chains to nanofillers and the countersurface. In a ...recent study, we showed that trace nanofillers (0.1 wt%) reduced the wear rates of an already low wear PEEK-PTFE blend by 40-fold with minimal tribochemistry. Interestingly, wear rates increased by 2-fold at 5 wt% nanofillers despite increased tribochemical accumulation. This observation raises questions about the tribochemical and mechanical roles of nanofillers in this material system, particularly at the surface. This paper aimed to isolate these effects. Specifically, we varied environmental humidity to promote or inhibit favorable tribochemical accumulation while maintaining subsurface stability via PEEK reinforcement. When we discouraged tribochemical reinforcement using a dry environment, high loadings of nanofillers had severely detrimental effects on wear rates and tribofilm stability. For example, the addition of 5 wt% nano-alumina to 5 wt% PEEK-PTFE increased wear rates by > 100-fold in the dry environment. By contrast, the addition of trace amounts (0.1 wt%) of nano-alumina had no detrimental effect on wear rate (10−7 mm3/Nm) or tribofilm stability. These results suggest that the mechanical effects of nanofillers were primarily destabilizing rather than stabilizing and that these effects increased with filler loading. In humid environments, however, these adverse effects of nanofillers, particularly at loadings >1 wt%, were offset by the favorable competing effect of tribochemical accumulation. Trace nanofiller loadings (∼0.1 wt%) optimized surface reinforcement at both environmental extremes because they provided the tribochemical benefits of the nanofillers while minimizing their mechanical costs.
•Tested the hypothesis that trace nanofillers reduce PTFE wear by non-tribochemical means.•Rejected the hypothesis and demonstrated detrimental mechanical effects from nanofillers.•Showed that trace nanofillers optimized PTFE wear performance in dry and humid environments.•Trace nanofillers provide tribochemical benefits while minimizing mechanical costs.
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•Application of a QM/MM, DFT based, approach for the study of tribological systems.•QM/MM is as accurate as fully ab initio methods in description of tribochemical reactions.•Make it ...possible to simulate systems with large sizes without prohibitive computational costs.
Tribological phenomena, such as wear and boundary lubrication, are deeply influenced by tribochemical reactions, i.e., chemical reactions occurring in the presence of mechanical stresses. Atomistic descriptions of these processes are still in their infancies because of the high computational costs required to properly describe the buried sliding interface by theoretical methods. In this paper we outline the application of the Quantum Mechanics/Molecular Mechanics approach to simulate tribochemical reactions during sliding and show that it can accurately describe the tribochemistry of graphene interacting with water molecules, a system which is of relevance for technological applications. Comparison with ab initio (Car-Parrinello) and classical (using the ReaxFF force field) molecular dynamics calculations highlights the advantages of this hybrid approach both in terms of computational costs and accuracy of the results.
•The real interfacial mechanisms involved in the dynamics of the growth of ZDDP tribofilm has been studied in this work.•The results show that running conditions do not affect only the formation of ...the tribofilm but also its durability.•Chemical analyses show that chemistry of the film varies across its thickness.
Understanding the true interfacial mechanisms involved in the growth of tribofilms generated by Zinc Dialkyl Dithiophosphate (ZDDP) is important because it is the most widely used anti-wear additive and there is legislative pressure to find efficient environmentally-friendly replacements. The main focus of this study is to investigate the durability of the ZDDP tribofilm and correlate it to the chemical and physical properties of the glassy polyphosphates. A novel experimental method has been developed to study the effect of lubricant temperature and contact load on tribofilm growth and durability. Results show that physical parameters such as temperature and pressure significantly influence the tribofilm durability. XPS analyses were carried out before suspending the test and after changing the oil to assess the difference in chemical structure of the tribofilm before and after stopping the test. The chemical analyses suggest that there are different chemical properties across the thickness of the tribofilm and these determine the durability characteristics.
This study investigates the microstructure, the mechanical properties, the topography and the tribological behavior of a composite coating obtained by cold spray, as a potential candidate for its use ...in internal combustion engines in the automotive sector. The coating consists of a 410 L matrix and M2 tool steel reinforcements. For comparison, a single component 410 L cold sprayed coating and a wire arc coating, used in some of today's car engines, are also studied. Post-spraying, the coatings underwent specific surface finishing. Their microstructure and topographies were observed. A lubricated reciprocating sliding test was performed, resulting in an exceptionally low friction coefficient and negligible wear, even in harsh conditions for the composite coating. The analyses revealed that the surface finish creates protuberances due to the presence of hard M2 particles, activating a series of mechanisms bringing to the stabilization of a tribofilm. This can be considered responsible for the exceptional tribological properties observed. This study proposes the application of surface textures with protuberances, in particular, their use in lubricated contacts in presence of fully formulated oils, and how cold spray is a suitable process to easily produce them.
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•Metal-metal composite coatings were applied on cylinder bores by cold spray.•Protuberances emerge after the surface finishing of the composite coating.•Low friction and negligible wear were measured in tribological tests.•Coating performance is due to the formation of a particular tribofilm.•Protuberances are essential for the formation of the tribofilm.
A multifunctional hypothesis of filler selection for PTFE suggests ultralow wear (10−7 mm3/Nm) depends on the filler’s ability to both support the normal load by being large in size and to induce ...favorable tribochemistry by being weak in aggregate strength. This study is the first to test the independent effects of preferential load support and tribochemistry on PTFE wear by studying the wear reducing properties of a porous GaN fiber, a fully dense SiO2 fiber and microsized GaN particles. The results found the only filler that induce ultralow wear is the GaN fiber which was microns in size and friable under shear stresses. This study reinforces the leading hypothesis that a mechanistic synergy is required for ultralow wear PTFE composites.
•Gallium nitride fibers reinforced PTFE wear rates are over three orders of magnitude lower than unfilled PTFE.•Low wear behavior derived from nanostructured gallium nitride fibers and tribochemically altered PTFE.•Environmental tribology tests and molecular simulations were used for the mechanistic study.•A synergistic anti-wear mechanism of mechanics and tribochemistry was proposed.
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•The properties of graphene in a sliding electrical contact were investigated.•Conductive atomic force microscope probe was used to rub the graphene surface.•The applied bias led to ...tunneling triboelectricity and surface functionalization.•The structural defects in graphene enhanced the tunneling triboelectricity and surface functionalization.•Defects greatly impacted the physiochemical properties of graphene in dynamic contacts.
We investigated the physiochemical properties of single-layer graphene (SLG) with controlled structural defects that were used in sliding electrical contact. Conductive atomic force microscopy (cAFM) probes applied with electric biases were used to rub the SLG surface under ambient conditions. Structural and chemical properties of rubbed SLG were investigated using Raman spectroscopy, photothermal infrared atomic force microscopy (AFM), and X-ray photoelectron spectroscopy. The work function (WF) and conductivity were determined using Kelvin probe force microscopy and cAFM. Adhesive properties were examined using AFM in the force-volume mode. Biases applied to the cAFM probe resulted in the tunneling triboelectric effect (TTE), which modulated the WF of SLG. The formation of water menisci around the cAFM probe–SLG contact will be electrolyzed, leading to the surface oxidation of SLG, which also impacts the WF of SLG. Compared with pristine SLG with minimum defects, more-defective SLG enhanced the TTE and surface functionalization during the rubbing process, substantially affecting the morphological, electronic, chemical, and adhesive properties of rubbed defective SLG. Our results demonstrated that the physiochemical properties of SLG in a sliding contact are considerably affected by the presence of structural defects in SLG, which should be considered when SLG is used for electrode applications.
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•Tribological testing of pyranopyrazoles (PPz) with fused ring system studied by four-ball tester.•Best activity of PPz at a very low concentration (0.25%w/v) while that of ZDDP ...(standard) at 1%w/v.•Quantum chemical calculations based on Density Functional Theory(DFT) supported very well the observed tribological data.•The order of adsorption energies and radial distribution functions(from MD simulations) validated the experimental results.•A Boron-Nitrogen synergy was confirmed by XPS of the tribofilm.
The tribological properties of fused heterocyclic ring systems have been scarcely studied. Generally, increased heteroatoms in such molecules should make them better additives. Accordingly, antiwear behavior of substituted pyranopyrazoles (PPz-R, where R=H, methyl and methoxy), compounds with fused heterocyclic rings, have been investigated in the present research. These compounds were characterized by FTIR and NMR (1H and13C) spectroscopy. Their tribological activity followed the order PPz-OMe > PPz-Me > PPz-H > ZDDP (standard). Atomic force microscopy (AFM) and scanning electron micrographs (SEM) of wear scar lubricated with pyranopyrazoles exhibited surface smoothness according to tribological results. The energy-dispersive X-ray (EDX) spectroscopy analysis revealed the heteroatoms nitrogen and oxygen of additive on the surface. XPS analysis showed that interaction of the best additive, PPz-OMe with a borate ester (Van lube 289), increased the efficiency synergistically. Results of DFT calculations on the interaction between adsorbent and substituted pyranopyrazoles are in agreement with the experimental data. The order of adsorption energies found using molecular dynamics (MD) simulations, correlated very well with the additive activity order. Furthermore, MD studies gave the conformation of the additive molecules on the adsorbing surface, paving the way to an understanding of the mechanism of adsorption and triboactivity.
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As a hot topic in recent years, molecular dynamics (MD) simulation has become an effective tool in tribochemistry and lubrication investigations, which provides unique insight on ...dealing with these issues from atomic scale. This review paper presents an overview of recent MD simulation studies on revealing the friction laws, wear mechanism and lubrication performance of materials and lubricants, which aims to provide guidance and reference for future theoretical investigations on revealing the essence of friction, wear and lubrication. MD simulation researches upon the tribology, tribochemistry and lubrication are summarized, focusing on the field of friction and wear mechanism, nano-tribology, liquid lubricants, lubricant additives, superlubricity phenomenon, etc. Besides, the challenges and problems remain to be considered, as well as future development directions of MD simulation are briefly discussed. With the help of MD method, the obstacle to tribology research caused by insufficient experimental methods can be reduced in the future.
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