The present study investigates the local mechanical and frictional properties of Ag/MoS2-based self-lubricating claddings in order to unveil the contribution to friction reduction of the different ...phase constituents present in the cladding. The cladding microstructure is formed by nickel dendrites surrounded by borides and homogeneously scattered pure silver pockets encapsulated within molybdenum and chromium sulfides that arise from the thermal decomposition of MoS2 during deposition. Particular attention is devoted to the latter, since their local mechanical and frictional properties are completely unknown. The nanotribological results show that chromium sulfides have a high hardness and a low intrinsic friction. This implies a twofold role in friction reduction. They provide intrinsic low friction and support friction reduction by silver smearing thanks to their high hardness. The presented microstructure is able to effectively control and reduce friction down to a value of 0.25 in vacuum at room temperature and 300 °C by the smearing of silver over the chromium sulfides. This friction reduction mechanism is enhanced by thermal softening of the pure silver phase at elevated temperatures, contrary to air atmosphere, where smearing is hampered by silver oxidation. This overall tribological performance makes the presented claddings potential candidates for space applications.
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•Nickel-based self-lubricating cladding provide low friction in vacuum up to 300 °C.•Nanotribological characterisation shows a high nanohardness and low intrinsic friction for chromium sulfide phases.•Low friction attributed to silver smearing over hard chromium sulfides.•Friction reduction does not come at the expense of a higher wear in the cladding.•Adhesion to the counterbody is lowered about 20 fold at 300 °C.
The requirements for green and sustainable manufacturing mean that stamping lubricants must be continuously re-evaluated and re-designed. In this investigation, the tribological performance of four ...base oils with different chemical structures (paraffinic and naphthenic) and viscosities (2 and 20 cSt), as well as water, was evaluated for the stamping of steel sheets and compared with a non-lubricated contact. Most lubricants reduce the coefficient of friction and maintain a similar wear coefficient for steel sheets as in dry contacts. Low-viscosity (LV) naphthenic oil performs very like both high-viscosity (HV) oils. A surprising exception is the LV paraffinic oil, with several-times-higher friction and wear compared to dry contact. This is due to the excellent wetting-spreading and very low cohesion forces that enable oil to escape from extremely thin-film contacts because the viscosity is so low, leading to lubricant starvation. In contrast, HV oils provide a sufficiently thick lubricating film, while strong cohesive forces help in the film’s strength, lessening wear, and reducing friction. In thin-film lubrication with LV oils, such as when stamping, it is thus extremely important that the lubricant’s wetting behaviour and viscosity are sufficient to provide enough film in the contact and prevent starvation, thus ensuring lower friction, less wear, and a longer lifetime of the contact.
This study investigates the use of 1,3-dimethylimidazolium dimethylphosphate ionic liquid (IL) additive in glycerol base oil for improving the lubrication-properties of tungsten-doped diamond-like ...carbon (WDLC) coatings. Tribological tests were conducted at various applied loads (5 N, 10 N, 20 N) and elevated temperature (100 °C), and the addition of 1 wt% of IL resulted in a significant reduction (∼50%) in friction-coefficient. In-situ neutron-reflectometry analysis revealed a multi-step tribofilm formation mechanism involving IL and W-clusters, resulting in the formation of a 1.24-nm thick tribofilm on the WDLC surface within the first 30-minutes of the friction experiment. The reaction kinetics for tribofilm formation are discussed involving dissociation of dimethylphosphate into dioxo-phosphate, which interacts with W-clusters to form tungsten phosphates and cations adsorbed on their surface.
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Molecular recognition of a lipophilic deoxyguanosine derivative at the air-water interface was investigated by film balance experiments and Brewster Angle Microscopy. Results showed that guanosine, ...despite strong tendency towards self-assembly, interacts with both complementary and noncomplementary liponucleosides (lipophilic derivatives of deoxycytidine and deoxythymidine). At surface pressures below 17mN/m, attractive and repulsive interactions were present in case of both mixed monolayers and were the strongest at guanosine molar fractions of 0.5 and 0.75. At higher values of surface pressure, deoxyguanosine-deoxycytidine interactions were strictly attractive and were present only for monolayers with guanosine molar fraction of 0.75. On the contrary, attractive and repulsive interactions remained present in case of deoxyguanosine-deoxythymidine mixed monolayers. This indicates that interactions between guanosine and cytidine are much stronger than guanosine-thymidine interactions. Interactions for none of the nucleoside pairs, however, are specific and π–stacking interactions between the aromatic planes of liponucleoside derivatives probably dominate over hydrogen bonding interactions. This article is part of a Special Issue entitled "G-quadruplex" Guest Editor: Dr. Concetta Giancola and Dr. Daniela Montesarchio.
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•Guanosine at the air–water interface interacts with complementary as well as noncomplementary liponucleosides.•Film balance experiments and BAM Microscopy indicate that interactions between none of these nucleoside pairs are specific.•At larger surfaces pressures, π–π stacking interactions dominate over the hydrogen bonding interactions.
We investigated the effect of various ions on the surface assembly of a guanosine derivative with one hexadecanoyl chain at the air–water interface. The ions were added to the water subphase prior to ...spreading of the surface film. Like in bulk water, also at the air–water interface, K+ ions exhibit the strongest influence on the assembly features as they induce structural transformation from lamellar to mosaic-like assembly. In contrast, Li+ and Na+ ions only slightly modify the properties of the assembled film with respect to those observed on pure water. The nature of anions plays an important role in the surface self-assembly as well. We found that (Pic–) is 2 orders of magnitude more effective for assembly regulation than Cl–. All surface assemblies observed in our study are very stable and robust, and consequently they remain practically unperturbed after Langmuir–Blodgett transfer onto a solid support.
Neutron reflectometry (NR) has proven to be an important analytical tool for investigations of the adsorption in boundary lubrication for both simple and complex additives by using either deuterated ...additives or the base carrier lubricant. NR enables in-situ measurements of the adsorption during surface exposure to additivated lubricants, and at the same time provides quantitative parameters for the adsorbed layer with sub-nanometer precision. In this work, NR was successfully used to determine the thickness and the density of the adsorbed layers of various lubricating additives (oiliness additives and ZDDP) on different DLC coatings. First, the influence of the coating structure and the presence of the doping element on the adsorption of two simple oiliness additives, i.e., hexadecanol and hexadecanoic acid, are presented. Second, the interaction ability of the ZDDP additive on a hydrogenated DLC (a-C:H) coating was measured as a function of the temperature and exposure time. Finally, an in-house-developed tribological rig was implemented in the neutron beam reflectometer to investigate the ZDDP film's growth and removal. The results show the unique ability of NR to provide direct evidence of the adsorption of additives on DLC coatings as a function of several relevant engineering parameters, such as time, temperature, and rubbing.
We performed a comparative study on the self-assembling properties of four guanosine derivatives with one and two lipophilic chains of two different lengths at the air–water interface and after ...Langmuir–Blodgett (LB) deposition onto various solid supports (mica, silicon wafer, graphite). At the air–water interface the derivatives with one lipophilic chain exhibit surface compression behaviour with a profound first order phase transition from the liquid-expanded to the liquid-condensed phase. They assemble into lamellar surface formations, whose structural characteristics remain practically unmodified after their transfer onto the solid substrates. Domain regions with orientationally aligned lamellar formations of sizes up to 150μm2 can be obtained. The compression behaviour of double-chain derivatives is more diverse. While the derivative with two decanoyl chains exhibits the liquid-expanded as well as the liquid-condensed phase, the derivative with two hexadecanoyl chains reveals only the condensed-analogous phase with a relatively high collapse pressure. LB films of double chain derivatives show formation of very homogeneous and compact surface structures with high surface coverage.
We demonstrate the control of wettability of non-structured and microstructured magnetoactive elastomers (MAEs) by magnetic field. The synthesized composite materials have a concentration of carbonyl ...iron particles of 75 wt.% (≈27 vol.%) and three different stiffnesses of the elastomer matrix. A new method of fabrication of MAE coatings on plastic substrates is presented, which allows one to enhance the response of the apparent contact angle to the magnetic field by exposing the particle-enriched side of MAEs to water. A magnetic field is not applied during crosslinking. The highest variation of the contact angle from (113 ± 1)° in zero field up to (156 ± 2)° at about 400 mT is achieved in the MAE sample with the softest matrix. Several lamellar and pillared MAE structures are fabricated by laser micromachining. The lateral dimension of surface structures is about 50 µm and the depth varies between 3 µm and 60 µm. A systematic investigation of the effects of parameters of laser processing (laser power and the number of passages of the laser beam) on the wetting behavior of these structures in the absence and presence of a magnetic field is performed. In particular, strong anisotropy of the wetting behavior of lamellar structures is observed. The results are qualitatively discussed in the framework of the Wenzel and Cassie–Baxter models. Finally, directions of further research on magnetically controlled wettability of microstructured MAE surfaces are outlined. The obtained results may be useful for the development of magnetically controlled smart surfaces for droplet-based microfluidics.
In this work, we evaluated the effect of the counter-body material (the same or dissimilar) and contact configuration (moving or stationary body), at similar contact tribological conditions, on the ...tribochemical and nanotopography characteristics of adsorbed surface films. Zinc dialkyldithiophosphate (ZDDP), the best performing anti-wear additive, was used in self-mated steel/steel and DLC/DLC contacts, which were compared with mixed steel/DLC and DLC/steel contacts in 1-h and 6-h sliding tests. The macroscale (tribometer) and nanoscale (atomic force microscopy) friction, thickness, topography, and chemical (attenuated total reflection-Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy) properties of the tribofilms were studied. The results revealed unexpectedly large differences in all the studied tribofilm parameters; this is because all the tribofilms are completely different; this includes the chemical composition, which is known to have a crucial effect on the nano- and macro-scale tribological properties. These results clearly demonstrate that the surface material, additives, and common contact operating parameters, that is, pressure, velocity, and temperature, crucially affect the ZDDP tribofilm as well as the position of the moving or stationary surface within the contact, and the material of the moving/stationary bodies.
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