Carbon-based Tribofilms Research Articles

Gaseous Lubricity Additives for Hydrogen Gas

ABSTRACT There is great interest in using hydrogen as a gaseous fuel in combustion engines to eliminate CO2 emissions. Unfortunately, hydrogen gas is a poor lubricant for most engineering metals and an effective lubrication solution for pumping and injecting hydrogen is required. This study explores the possibility of additivating hydrogen with a low concentration of a lubricious gas to reduce friction and wear. We find that unsaturated hydrocarbon gas additives form protective carbon-based tribofilms, while gaseous ammonia and amine additives form nitrogen-based films on steel surfaces during rubbing in additivated hydrogen. Gaseous amines are particularly effective in reducing friction and wear, even at concentrations as low as 100 ppm mole/mole. This demonstrates that the addition of a small concentration of lubricous gas is a feasible way to improve the lubricity of gaseous hydrogen.

One-step ultrafast laser-induced graphitization on PS-SiC surfaces for superior friction performance

Pressureless sintered silicon carbide (PS-SiC) ceramics are widely used as friction materials in the aerospace industry, and enhancing the self-lubricating properties of PS-SiC ceramics under dry friction is highly significant. In this study, an infrared femtosecond laser was used to treat the surface of PS-SiC ceramics, and the effects of various processing parameters on surface microstructure, chemical composition, and graphitization degree were investigated. More importantly, SiC decomposes into amorphous carbon and stays on the surface of PS-SiC ceramics under the photothermal effect, and the amorphous carbon realizes the transition to the ordered graphite structure by controlling the laser energy. The highly graphitized, carbon-containing micro/nanostructures on the surface of laser-treated PS-SiC ceramics promote the formation of stable carbon-based tribofilms during sliding, which significantly enhances the tribological properties of PS-SiC ceramics under dry friction. This study proposes a method for inducing graphitization on the surface of PS-SiC ceramics using an infrared femtosecond laser, providing a manufacturing approach and theoretical support for the development of high-performance PS-SiC ceramic friction materials.

Tribological behavior and lubrication mechanism of polyalphaolefin with added Cu-Ni bimetallic nanoparticles on a TiN coating

In this study, the tribological behavior of TiN coatings lubricated with a polyalphaolefin (PAO)-based oil containing Cu-Ni bimetallic nanoparticles was investigated under a maximum contact pressure of 0.9 GPa in reciprocating ball-on-disk mode. The addition of 0.1 wt% Cu-Ni nanoparticles to PAO6 reduced the friction coefficient to 0.018 and also decreased the wear rate of TiN coatings to 4.23 × 10-7 mm3/Nm. X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy revealed the deposition of Cu-Ni nanoparticles and carbon-based tribofilms on the contact region of the tribopairs. These originated from the tribo-induced degradation of the polyalphaolefin molecule in PAO6 + 0.1 %Cu-Ni nanoparticles, which helped decrease the COF and wear. Moreover, a comparative analysis suggested that the presence of nickel in the Cu-Ni nanoparticles had a catalytic effect during sliding. They facilitated the in situ formation of carbon-based film due to the degradation of polyalphaolefins under the combined effects of frictional heat and contact stress, demonstrating a self-repair capability.

In-situ engineering catalytically active surfaces for tribocatalysis with layered double hydroxide nanoparticles

Ensuring long-lasting lubrication is vital for sustainable machinery operation, made possible by self-regenerating carbon-based tribofilms via tribocatalysis. Conventional methods use expensive catalytic coatings, posing challenges for replacement and maintenance in practice. Here, we are proposing catalytic layered double hydroxide (LDH) nanoparticles as cost-effective and easily replenished lubricant additives to engineer catalytically active surfaces in situ where binary and ternary LDHs with Ni2+, Co2+, and/or Cu2+ divalent cations alongside Al3+ trivalent cations are investigated for lubrication performance. Under 100 °C sliding condition equivalent to the lubricating temperature in an internal combustion engine, NiCoAl–CO3 LDH exhibits the lowest wear losses alongside the durable low-friction regime. This excellent performance is attributed to Co-containing spinel and oxide phases in the catalytic tribo-oxide layer which help stabilize and maintain the microstructures of the tribo-oxide layer. In contrast, deterioration in lubrication performance at this temperature was observed for copper-containing LDHs, especially NiCuAl–CO3 LDHs, which is due to the reduction of metallic oxides that drive phase separation in the catalytic oxide tribo-layers. The more stable tribo-oxide layers can result in thick, durable carbon-based tribofilm during sliding along with higher resistance to plastic deformation bulk interlayer. This study offers valuable insight into the synergy of catalytic oxide materials, opening avenues for a rational design of innovative catalytic nano-materials for tribocatalysis processes.

Ionic Nitrogen-Doped Carbon Dots as Nonpolar Lubricant Additives at Low Effective Addition.

The poor compatibility with nonpolar lubricant still hinders the application of carbon dots (CDs) in lubrication. In addition, research proves that the existence of ionic structure and active groups on CDs are conducive to their lubricity. In order to obtain the ionic structures and good oil compatibility synchronously, a kind of ionic nitrogen-doped CDs (NCDs) was synthesized via the alkylation of nitrogen in NCDs and anion exchange. The new material could exhibit good tribological performance as poly alpha olefins (PAO4) additives with low addition. Moreover, an ionic liquid, [N44HH][DEHP], with the same anion was chosen as a comparison to investigate the role of NCD cations. The surface analyses demonstrate that NCD cations and phosphate ester anions adsorb on the friction interface to play a synergistic lubrication role during the friction process, which could generate a superior carbon-based tribofilm.

In-situ tribo-induced formation of superb lubricious carbon-based tribofilms on the catalytical active MoN-Ag film surfaces

The establishment of environmentally friendly lubrication systems is of significant importance to the sustainable development. Here, we demonstrated an environmentally friendly approach, where superb lubricious carbon-based tribofilms formed in situ originating from base oil through tribochemical reactions, on the surfaces of catalytically active MoN-Ag film. We focused on the study of the tribological behavior of the MoN-Ag/oil solid-liquid system under varying loads. Through detailed characterization analysis of the carbon-based tribofilms, we elucidated the mechanism of friction reduction and wear resistance of the MoN-Ag film in the base oil, as well as the formation mechanism of carbon-based tribofilms. Furthermore, we provided a thermodynamic perspective to clarify the influence of load on the tribo-induced tribochemical reactions within the MoN-Ag/oil solid-liquid system.

Effect of Nickel Acetyl Acetonate as Lubricant Additive in Base Oils with Different Molecular Structure on In-Situ Formation and Tribomechanism of Carbon-Based Tribofilms of Steel-Steel Sliding Pair

Effect of Nickel Acetyl Acetonate as Lubricant Additive in Base Oils with Different Molecular Structure on In-Situ Formation and Tribomechanism of Carbon-Based Tribofilms of Steel-Steel Sliding Pair

CO2 Capture and Conversion into Carbon-Based Tribofilm by In Situ Tribochemical Reaction for Green Lubrication

CO₂ Capture and Conversion into Carbon-Based Tribofilm by In Situ Tribochemical Reaction for Green Lubrication

Pd nanoparticles lubricant additive catalyze the construction of carbon-based tribofilm to reduce graphitization-induced wear of DLC films under boundary lubrication

To explore approaches of reducing graphitization-induced wear on the friction surface of DLC films under boundary lubrication to increase their service life, Pd nanoparticles (PdNPs) prepared by chemical reduction method were used as lubricant additive of DLC films and GCr15 ball under boundary lubrication. Tribotests, reactive force field molecular dynamics (ReaxFF MD) and surface characterization methods, such as SEM, XPS, Raman and FIB-TEM, were used to study the effects of PdNPs as lubricant additive on the graphitization-induced wear of DLC films and the effect mechanisms. The results show that, PdNPs lubricant additive can reduce the friction coefficient (COF) of the tribosystems, and more importantly, they can obviously increase the lifespan of DLC films. Characterization results indicate that PdNPs can promote the tribochemical reaction on the friction surface and catalyze the formation of carbon-based tribofilm on the friction surface, which significantly reduces the graphitization-induced wear of DLC films. ReaxFF MD studies show that PdNPs effectively catalyze the massive dehydrogenation and chain-breaking of base oil hydrocarbon molecules, providing the basis for the formation of carbon-based tribofilm. These results indicate that PdNPs are promising as effective additive to prolong the service life of DLC films under boundary lubrication.

Tribochemistry and frictional properties of octene molecules confined between iron oxide surfaces

Reactive molecular dynamics simulations were conducted to investigate the tribological behavior between two amorphous iron oxide substrates with octene molecules confined between them. The results revealed that higher temperatures facilitated the degradation and oxidation of octene molecules, leading to an increased quantity of reaction products such as water and oxidation products like alcohols. Due to the obstructive action of water molecules, the octene and its oxidation products were able to adsorb onto the substrate surfaces, resulting in the formation of a stably adsorbed carbon-based tribofilm at elevated temperatures. In addition, the water layer also significantly reduces friction by preventing direct contact between the carbon-based tribofilm, even though only a small amount of octene molecules are present at the interface.

Tribocatalysis Induced Carbon-Based Tribofilms—An Emerging Tribological Approach for Sustainable Lubrications

To comply with the high demand for efficient and sustainable lubrications, carbon-based tribofilms and/or nanomaterials have emerged as a potential solution that can resolve the current major shortcomings of phosphorus- and sulphur-rich tribofilms and protective coatings. Although their employment is still in the early stages of realization and research, these tribofilms receive significant interest due to their capability to continuously and in situ repair/replenish themselves during sliding, which has been an ultimate goal of all moving mechanical systems. Structurally, these tribofilms are complex and predominantly amorphous or disordered with/without graphitic domains (e.g., graphene/graphite, onion-like carbon, etc.). Chemically, the compositions of these tribofilms vary significantly with environments, conditions, and material precursors. Yet, the structural properties of carbon-based tribofilms remain largely ambiguous, which precludes a full understanding of the mechanisms underlying the formation and lubrication performance. This review will summarize the current state-of-art research about the in situ carbon-based tribofilms that have been published since the pioneering works. Particularly, this work will highlight the recent approaches to generate these tribofilms, their associated lubrication performance, current understanding of the formation mechanics, common analytical approaches for these tribofilms, and the compatibility of these tribofilms with other additives. Together, the overall outlooks will be drawn, demonstrating the knowledge gaps and proposing further investigation tactics to tackle these emerging issues.

Tribocatalytically-active nickel/cobalt phosphorous films for universal protection in a hydrocarbon-rich environment

High-contact stresses generated at the sliding interfaces during their relative movement provide a unique combination of local heating and shear- and load-induced compression conditions. These conditions, when involving the sliding of surfaces with certain material characteristics, may facilitate tribochemical reactions with the environment, leading to the formation of a protective, damage-suppressing tribofilm directly at the contact. Here, we employ the electrodeposition process to design a coating composed of a hard cobalt-phosphorous matrix with the inclusion of tribocatalytically-active nickel clusters. The coating is optimized in terms of its relative composition and mechanical characteristics. We demonstrate the excellent tribological performance of the coating in the presence of a hydrocarbon environment, both in the form of a liquid lubricant and as a hydrocarbon-saturated vapor. Characterization of the wear track indicates that the origin of such performance lies in the formation of a protective carbon-based tribofilm on the surface of the coating during sliding. These results contribute to the advancement of knowledge on material transformations in the contact, thus providing a robust and versatile approach to addressing tribological challenges in mechanical systems.

Tribo-induced catalytically active oxide surfaces enabling the formation of the durable and high-performance carbon-based tribofilms

Carbon-containing tribofilms have attracted significant interest in the lubrication research despite a scarcity of information on their high-temperature performance under severe boundary conditions. In this study, high-temperature lubrication of the carbon tribofilm produced from cyclopropane carboxylic acid (CPCa) and NiAl-layered double hydroxide (LDH) nanoparticles was evaluated. NiAl-LDH nanoparticles significantly enhanced the friction stability and antiwear performance of CPCa by over 90% at 50°C and 100°C, comparable to the benchmark zinc dialkyldithiophosphates (ZDDPs). The highly graphitic amorphous carbon tribofilms and the fine-grain intermediate tribolayer constructed by the thermal decomposition products of NiAl-LDH contributed to such excellent lubrication performance. This study paves a pathway in developing functional anti-wear additives for the durable and high-performance carbon-containing tribofilms at high temperatures.

Revealing the structure-property relationships of amorphous carbon tribofilms on platinum-gold surfaces

Nanocrystalline metal alloys have shown great promise as electrical contact materials, given their mechanical and tribological properties. In particular, platinum-gold (Pt–Au) nanocrystalline alloys have demonstrated coefficients of friction as low as 0.01 and specific wear rates on the order of 10−9 mm3 N−1 m−1, largely due to the formation of carbon-based tribofilms at the sliding interfaces. In this study, we advance our understanding of the Pt–Au tribofilm structure-property relations and growth mechanisms via high-throughput and high-resolution measurements as a function of Pt–Au composition. As the solute content increased from 0 at. % to 10 at. % Au, cross-sectional and plan-view transmission electron microscopy demonstrated a decrease in average grain size d and an accompanied increase in grain boundary (GB) segregation. The decrease in d and increase in GB solute segregation translated to a decrease in modulus Er and an increase in hardness H as determined via nanoindentation; the Er trend was mainly described using a rule-of-mixtures approximation, whereas the H trend was ascribed to solid solution strengthening and GB stabilization. The steady state-friction μ and wear rate decreased with the addition of Au; low Au-content films showed substrate wear, while high Au-content films showed stable tribofilm growth in both macroscale and nanoscale friction tests. The carbon bonding configuration of the tribofilms was investigated by near-edge X-ray absorption fine structure spectroscopic analyses and found to be similar to that of hydrogenated amorphous carbon films. Altogether, the study provided insight into the mechanistic origins of the tribofilms, thus opening the door to tunable properties ranging from mitigation for electrical contacts to the creation of self-healing films for solid lubricants.

Aromatic molecules as sustainable lubricants explored by ab initio simulations

In pursuit of sustainable lubricant materials, the in situ formation of graphitic material has been shown to effectively reduce friction at metallic interfaces. Aromatic molecules are perfect candidates for the formation of carbon-based tribofilms due to their inertness and chemical structure. We selected a group of common aromatic compounds, which are still unexplored in tribology, and we investigated their capability to reduce the adhesion of sliding iron interfaces. Ab initio molecular dynamics simulations show that hypericin, a component of St. John’s wort, can effectively separate the mating iron surfaces better than graphene. The size of this molecule, combined with the reactivity and the hindrance of its moieties, plays an important role in maintaining large interfacial distances. Stacked hypericin molecules can easily slide on top of each other due to the intermolecular repulsion arising in the presence of load. The decomposition of the lateral groups of hypericin observed in the dynamic simulations suggests that the polymerization of several molecules can occur in tribological conditions. All these results pave the way for promising alternatives to commonly employed friction modifiers.

Synthesis and interfacial interaction of Ag2S quantum dots for enhancing the tribological behaviors of PTFE-based lubricating coatings

The small size and strong interfacial interaction are the key factor and challenge for quantum dots to play their reinforcing role as promising fillers of solid lubricating coatings. Herein, the minuscule Ag2S quantum dots (FQDs) of 1.71 nm with high dispersibility were synthesized via thermal decomposition of the precursor under the disturbance of polyamide-imide (PAI) molecules, and their interaction was investigated. Then the optimization of the mechanical and tribological properties of the polytetrafluoroethylene-based lubricating coatings depending on interfacial interaction was conducted, and their enhancement mechanisms were discussed. The results indicated that the PAI molecules were chemically adsorbed to the surface of the Ag2S FQDs and the interfacial interaction was improved based on the in-situ coordination reaction between the carboxyl group and Ag ions. The micro-hardness and wear life were thus increased by 45.70 % and 395.24 % compared with the original coating. Enhancing mechanical strength, inhibiting the frictional reaction of substrate and promoting the formation of sp2 carbon-based tribofilm were the crucial effects of Ag2S FQDs to improve the coating performance in terms of protection against friction and wear. The smaller size and strong interfacial interaction of Ag2S FQDs promoted the structural integrity and continuity of carbon-based tribofilm.

Near-Net Forging of Titanium and Titanium Alloys with Low Friction and Low Work Hardening by Using Carbon-Supersaturated SKD11 Dies

A new near-net forging procedure of titanium and titanium alloys was proposed by using a carbon-supersaturated punch and die. Due to the in situ formation of carbon-based tribofilm on the contact interface between the dies and work materials, a low frictional state was sustained through the forging process even in a high reduction in thickness. The work hardening was suppressed during forging; an additional annealing process was unnecessary through the whole process of near-net forging. Pure titanium and β-phase titanium alloy wires were utilized to describe their galling-free forging behavior when increasing the reduction in thickness. Wires with a diameter of 3 mm were upset in a single-shot forging. The reduction in thickness reached 58% when upsetting the pure titanium wire and 45% when upsetting the β-phase titanium alloys, without lubricating materials or oils at room temperature. The friction coefficient on the contact interface was estimated to be 0.05 by inverse analysis. The work-hardening behavior was described by the hardness mapping on the work cross section. The formation of carbon tribofilms was explained by microstructural analysis, element mapping, and Raman spectroscopy. This tribofilm was formed from the isolated carbon solute from the carbon-supersaturated punch and die to sustain the in situ solid lubrication on the contact interface.

Dependence of the lubrication enhancement of alkyl-functionalized graphene oxide and boric acid nanoparticles on the anti-oxidation property

The influence of anti-oxidation property of nanoparticles on the tribological properties is rarely reported. In this study, the alkyl-functionalized graphene oxide (FGO) and alkyl-functionalized boric acid (FBA) were fabricated as oil additives to explore their lubrication effectiveness. FBA exhibited remarkable lubricating properties which reduced friction coefficient by approximately 24 % and the wear depth by about 94 %. However, the friction coefficient and wear depth of FGO were reduced by only 9.5 % and 44 %, respectively. Worn surface analysis shows a bunch of deep-plowing on rubbing surface in the case of FGO, while the wear scar was smooth and almost wear-free for FBA. Lubrication mechanism analysis by X-ray Photoelectron Spectroscopy revealed the formation of carbon-based tribofilm for FGO and boron-based tribofilm for FBA during rubbing. In the thermal behavior analysis, graphene oxide (GO) nanoparticles were completely decomposed at 650 °C, whereas, B2O3 nanoparticles remained stable. It indicated that the excellent thermal stability and anti-oxidation property of boron-based tribofilm benefited to the lubrication enhancement. This study highlights the importance of oxidation resistance in designing novel nanoparticle additives, and the enhancement of oxidation resistance of GO will be a promising way to further application as oil additives.

Influences of iron and iron oxides on ultra-thin carbon-based tribofilm lubrication

A molecular dynamics (MD) simulation was used to investigate the influences of iron/iron oxide surface on the lubrication and tribochemistry of C-based tribofilms mixed with cyclopropane carboxylic acid (CPCa). The results revealed that the best tribological performance was found for mixed 1-decene lubricant confined between iron surfaces. In contrast, Fe3O4 was the most beneficial surface for C-based tribofilm formation as this surface resulted in the most significant number of species and molecules, as well as the largest portion of short C-chain molecules, which are intermediate products generated in C-based tribofilm growth. Interestingly, iron oxides resulted in a larger dissociation rate of C−C and C–H bonds than iron in the current thin-film lubrication. The degree of polymerization, reflected by the number and size of polymerized molecules, increases with CPCa concentration on a pure iron surface, while it decreases with an iron oxide surface.

Reactive in-situ formation and self-assembly of MoS2 nanoflakes in carbon tribofilms for low friction

Modern lubricants require additives for improving their frictional and wear performance. The most effective and widely used additives rely on organo-metallic compounds, which lead to ash formation and pose serious environmental concerns. Despite intensive research, a cost-effective alternative cannot be foreseen in the immediate future.On the quest for an alternative concept, the reactive formation and self-assembly of few-layer MoS2 nanoflakes in a carbon-based tribofilm is studied during reciprocating sliding contact of molybdenum substrates lubricated with oils containing sulfurized olefin extreme-pressure (EP) additive. Based on a combination of Raman spectroscopy and transmission electron microscopy it can be concluded that nanoflakes of well-adherent 002-oriented MoS2 layers form in the presence of S-containing EP additive. This leads to a reduction in friction from 0.3 to 0.08. The reaction rate to form MoS2 nanoflakes increases with temperature and EP concentration. At temperatures over 100 °C, the MoS2 nanoflakes are accompanied by carbon-based tribofilms. These carbon-based tribofilms are catalytically formed by dissociating hydrocarbon molecules of the lubricant.These results suggest that applying Mo alloyed materials with sulfur containing lubricants provides an alternative to conventional organo-metallic compounds. The presented lubrication concept can be utilized for further developments of materials (like protective hard coatings) and machine designs.