Ionic liquids as advanced lubricant fluids Bermúdez, María-Dolores; Jiménez, Ana-Eva; Sanes, José ...
Molecules,
08/2009, Volume:
14, Issue:
8
Journal Article, Book Review
Peer reviewed
Open access
Ionic liquids (ILs) are finding technological applications as chemical reaction media and engineering fluids. Some emerging fields are those of lubrication, surface engineering and nanotechnology. ...ILs are thermally stable, non-flammable highly polar fluids with negligible volatility, these characteristics make them ideal candidates for new lubricants under severe conditions, were conventional oils and greases or solid lubricants fail. Such conditions include ultra-high vacuum and extreme temperatures. Other very promising areas which depend on the interaction between IL molecules and material surfaces are the use of ILs in the lubrication of microelectromechanic and nanoelectromechanic systems (MEMS and NEMS), the friction and wear reduction of reactive light alloys and the modification of nanophases.
The tribochemistry and transfer film formation at the metal/polymer interface plays an essential role in surface protection, wear reduction, and lubrication. Although the topic has been studied for ...decades, challenges persist in clarifying the nanoscale mechanism and dynamic evolution of tribochemical reactions. To investigate the tribochemistry between iron and polytetrafluoroethylene (PTFE) in ambient and cryogenic environments, we have trained and expanded a ReaxFF reactive force field to describe iron–oxygen–water–PTFE systems (C/H/O/F/Fe). Using ReaxFF molecular dynamics simulations, we find that mechanical shearing of single asperity induced the degradation of PTFE molecules and radicals, showing subsequent oxidation and hydroxylation reactions of the radicals initiated by C–C bond cleavage, in agreement with previous experimental observations. Furthermore, we studied mechanisms of interfacial tribochemical reactions and formation of transfer films. We found that tribochemical wear and Fe–C and Fe–F bonding networks are important mechanisms for anchoring molecular chains to form a transfer film on the iron countersurface. Hydroxyl groups can dehydrogenate to form short and strong chelation bonds with the Fe2O3 countersurface. A friction-induced oriented molecular layer plays a key role in reducing friction, which is responsible for the excellent lubrication property. By varying temperatures in the range of 10–300 K, we found a nonmonotonic change in friction with a maxima at 100 K. At cryogenic temperatures, the molecular mobility was obviously suppressed, while the chain rigidity was enhanced, resulting in the less oriented interface and brittle-like shear interface, which is responsible for nonmonotonic friction. This work elucidates mechanisms of tribochemical reactions and transfer film formation between iron and PTFE at the atomistic level, facilitating design and development of self-lubricating materials, especially under harsh conditions.
Zinc dialkyldithiophosphate additives are used to control wear and inhibit oxidation in almost all engine oils as well as many other types of lubricant. They limit wear primarily by forming a thick, ...protective, phosphate glass-based tribofilm on rubbing surfaces. This film formation can occur at low temperatures and is relatively indifferent to the chemical nature of the substrate. There has been considerable debate as to what drives ZDDP tribofilm formation, why it occurs only on surfaces that experience sliding and whether film formation is controlled primarily by temperature, pressure, triboemission or some other factor. This paper describes a novel approach to the problem by studying the formation of ZDDP films in full film EHD conditions from two lubricants having very different EHD friction properties. This shows that ZDDP film formation does not require solid–solid rubbing contact but is driven simply by applied shear stress, in accord with a stress-promoted thermal activation model. The shear stress present in a high-pressure contact can reduce the thermal activation energy for ZDDP by at least half, greatly increasing the reaction rate. This mechanism explains the origins of many practically important features of ZDDP films; their topography, their thickness and the conditions under which they form. The insights that this study provides should prove valuable both in optimising ZDDP structure and in modelling ZDDP antiwear behaviour. The findings also highlight the importance of mechanochemistry to the behaviour of lubricant additives in general.
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•Tribological properties of a PESU-based tribocompound were studied under various load sequence conditions.•A significant change in the steel counterbody topography was observed when ...the load condition exceeded a threshold.•SO2 released by the PESU decomposition played an important role in the steel counterbody corrosion.•Tribological performance of the PESU nanocomposite was affected by the load histories due to variation of the counterbody surface.
In the present study, the impact of the loading history on the friction and wear behavior of a PESU-based nanocomposite/steel tribosystem was studied. Surprisingly, it was found that the supposedly stronger friction partner, the steel counterpart was also worn when the load conditions are above a critical load limit leading to significantly higher wear of the nanocomposite in the subsequent sliding process. Analysis of the steel worn surfaces substantiated the occurrence of the tribochemical reactions during sliding. These reactions were attributed to the redox between the sulfur dioxide (SO2) decomposed from PESU above its degradation temperature and the iron of the metallic counterbody, which led to pronounced changes of the steel surface. As a result, high wear was observed in the following frictional process. Therefore, it seems imperative to consider the possibility of tribochemical reactions when using PESU-based materials for tribological applications in their qualification.
In current work, the wear resistance of CuNiAl was improved by the application of an aromatic thermosetting co-polyester (ATSP)-based coating on its surface. Tribological behaviors of the ATSP ...coating at stresses of 1.52, 3.04 and 3.75 MPa under different lubrication conditions (dry friction and oil) were studied by the fretting wear test. The morphology and chemical composition of the coating were characterized by scanning electron microscopy, Fourier transform infrared spectroscopy and three-dimensional (3D) optical microscope. Results showed that at the same conditions, the friction torque of the ATSP coating was lower than that of CuNiAl. Significantly, wear was reduced by more than 90% after the application of the ATSP coating. The wear mechanism of the ATSP coating was slight deformation with pits induced by plastic deformation and fatigue crack propagation. The ATSP coating underwent molecular chain breakage during wear and chemical reaction with the friction pair to form a transfer film which played an important role in the reduction of wear.
•The friction and wear properties of ATSP coating under fretting wear were investigated.•ATSP coating can significantly improve the wear resistance of CuNiAl.•ATSP coatings undergo complex chemical changes during wear。.•Transfer film is an important factor in reducing ATSP friction and wear.
Ester-base oils are commonly used in environmentally adapted lubricants (EALs) because of their biodegradability. However, their antiwear performance is difficult to improve using the conventional ...lubricant additives developed for mineral-base oils. In this study, dialkyl phosphonates with various functional groups were designed as antiwear additives for ester-base oils. The antiwear performances of these phosphonates were evaluated using four-ball tests. The results suggest that dialkyl phosphonate with carboxylic acid (DAPA) has better antiwear performance in ester-base oils than that of other phosphonates and conventional phosphorus additives. Remarkably, DAPA exhibits better antiwear performance under severe test conditions in ester-base oils than in poly-alpha olefin. X-ray photoelectron spectroscopy, transmission electron microscopy, and Fourier transform infrared spectroscopy were used to study the chemical compositions of steel surfaces after friction and better understand the antiwear mechanism of DAPA, particularly in ester-base oils.
•Dialkyl phosphonates are proposed as antiwear additives for Environmentally Adapted Lubricants.•Dialkyl phosphonate with carboxylic acid (DAPA) has the best AW performance in ester-base oil (BO).•DAPA shows better AW performance in ester-BO than in poly-α-olefin.•The excellent performance of DAPA in ester-BO is due to a competitive adsorption effect between DAPA and ester-BO.
The nanowear tests of monocrystalline silicon against a SiO2 microsphere were performed using an atomic force microscope in air as a function of relative humidity (RH = 0%–90%) and in liquid water at ...a contact pressure of about 1.20 GPa. The experimental results indicated that RH played an important role in the nanowear of the Si/SiO2 interface. In dry air, a hillock-like wear scar with a height of ∼0.4 nm was formed on the silicon surface. However, with the increase of RH, the wear depth on the silicon surface first increased to a maximum value of ∼14 nm at 50% RH and then decreased below the detection limit at RH above 85% or in water. The transmission electron microscopy analysis showed that the serious wear on the silicon surface at low and medium RHs occurred without subsurface damage, indicating that the wear was due to tribochemical reactions between the Si substrate and the SiO2 counter surface, rather than mechanical damages. The RH dependence of the tribochemical wear could be explained with a model involving the formation of “SiOSi” chemical bonds (bridges) between two solid surfaces. The suppression of tribochemical wear at high RHs or in liquid water might be attributed to the fact that the thickness of the interfacial water layer is thick enough to prevent the solid surfaces from making chemical bridges. The results may help us understand the nanowear mechanism of silicon that is an important material for dynamic microelectromechanical systems.
In this work, we designed three series of tribo-couples based on amorphous carbon films including GLC, DLC and PLC that were modified by graphene quantum dots (GQDs). The tribo-testing environment ...was controlled at harsh conditions (like heavy load and high speed) in dry nitrogen atmosphere using bare and film-coated bearing steel balls as counterbodies, respectively. Through the tribochemical interactions, the self-mated DLC system obtained a surperlubricity state (μ = 0.01). During the whole sliding, the contact surface of the upper counterfacing ball was covered by 2D-layered carbon and graphitic lubricants induced via structural transformation of GQDs. Meanwhile, the tribofilm of the disc wear track was composed of a silica-like SiOx boundary layer and a multicomponent mixed-layer induced by tribochemistry. Compared to the self-mated DLC system, the structural boundary enriched with SiOx compounds was not formed at the bottom region of the tribofilm for the bare steel system; meanwhile, the disc wear track was covered by a thicker tribofilm containing plenty of degraded GQDs. This inferred the fact that the formation of a nanostructured sliding interface was the key to realize superlubricity. These discoveries successfully afforded a lubrication mechanism of GQDs for solid lubricant in applications of engineering and industry.
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•Graphene-quantum-dots lubricants with effective friction-reducing effects are achieved.•The lubricity depends strongly on the tribo-couple materials and surface strucutre.•The nanostructured tribofilms formed on the contacts govern the superlubricity mechanisms.
In this study, molybdenum dithiocarbamate (MoDTC) coatings were fabricated on 304 stainless steel substrates via an electrodynamic spraying process with the aim to clearly understand the evolutionary ...process from MoDTC to MoS2 during contact sliding. Experiments were performed using an in-situ tribo-tester equipped with Raman spectroscopy to continuously monitor the chemical state of the wear track. The tribochemical reaction and removal process of MoDTC were assessed, and the evolution process of MoDTC was determined systematically. Additionally, it was determined that the tribological performance of the MoDTC coating was dominated by a tribochemical reaction at the microscale and by surface topography at the macroscale.
•The tribochemical mechanism of MoDTC as a solid lubricant was first revealed.•Chemical evolution was monitored by an in-situ tribo-tester.•Tribochemical reaction affected by temperature and substrate roughness was studied.•Tribological performance of the MoDTC coating was highly dependent on scale.•The MoDTC coating exhibited low COF of 0.07 under severe conditions.
Tribological properties of AlCoCrCuFeNi and AlCoCrFeNiTi0.5 alloys under 90% H2O2 solution rubbing against 1Cr18Ni9Ti steel, ZrO2 ceramic and SiC ceramic were investigated. The tribological behavior ...is dependent on the counterparts and the structure of high entropy alloys. The high entropy alloys exhibit significantly lower friction coefficient and wear loss sliding against SiC ceramic than 1Cr18Ni9Ti steel and ZrO2 ceramic, which can be attributed to the lubricating colloidal film formed by tribo-chemical reaction between H2O2 and SiC. Moreover, the AlCoCrFeNiTi0.5/SiC tribo-pair preserves better tribological properties than AlCoCrCuFeNi/SiC tribo-pair, as results of high strength and hardness of AlCoCrFeNiTi0.5 alloy.
•Tribological properties of high entropy alloys under 90% H2O2 are investigated.•Mechanical wear dominates the wear behavior against 1Cr18Ni9Ti and ZrO2.•Rubbing with SiC, lubricating colloidal film formed offers low COF and wear loss.•AlCoCrFeNiTi0.5/SiC shows better tribological properties than AlCoCrCuFeNi/SiC.
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