1. The original version of this article unfortunately contained incorrect author’s name and address. On the first page of this article, instead ofSuprakash SAMANTA1,2, Santosh SINGH1, Rashmi R. SAHOO1,2,*1Surface Engineering & Tribology Division, CSIR-Central Mechanical Engineering Research Institute, Durgapur 713209, India2Academy of Scientific and Innovative Research (AcSIR), Ghaziabad 201002, India

    It should readSuprakash SAMANTA1,2, Santosh Kumar SINGH1, Rashmi R. SAHOO1,2,*1Enviromental Engineering Group, CSIR-Central Mechanical Engineering Research Institute, Durgapur 713209, India2Academy of Scientific and Innovative Research (AcSIR), Ghaziabad 201002, India

  2. Abstract

    Understanding the mechanism of precision sliding contacts with thin, adherent solid nano lubricating particle films is important to improve friction and wear behavior and ensure mechanical devices have long service lifetimes. Herein, a facile and multistep approach for the preparation of graphene oxide (GO) is presented. Subsequently, surface modification of as-synthesized GO with octadecyl amine (ODA) is performed to prepare hydrophobic GO-ODA and with 6-amino-4-hydroxy-2-naphthalenesulfonic acid (ANS) to prepare amphoteric GO-ANS through a nucleophilic addition reaction. X-ray diffraction and ultraviolet-visible, Fourier transform infrared, and Raman spectroscopy provide significant information about the reduction of oxygen functionalities on GO and the introduction of new functionalities in GO-ODA and GO-ANS. The effects of particle functionalization for the improved control of particle adhesion to the tribocontact have been studied. Wettability and thermal stability were determined using the water contact angle, and atomic force microscopy and differential scanning calorimetry (DSC) were used to characterize particle adhesion to the tribocontact. The tribological performances of the particles have been investigated using macro- and micro-tribometry using pin/ball-on-disc contact geometries. The influence of particle functionalization on the contact pressure and sliding velocity was also studied under rotating and reciprocating tribo-contact in ambient conditions. With an increase in the contact pressure, the functionalized particles are pushed down into the contact, and they adhere to the substrate to form a continuous film that eventually reduces friction. Amphoteric GO-ANS provides the lowest and most steady coefficient of friction (COF) under all tested conditions along with low wear depth and minimal plastic deformation. This is because particles with superior wetting and thermal properties can have better adherence to and stability on the surface. GO-ANS has a superior ability to adhere on the track to form a thicker and more continuous film at the interface, which is investigated by field emission scanning electron microscopy, energy dispersive spectroscopy, and Raman analysis.

  3. Abstract

    In this paper, a square textured parallel slider is considered for a study to improve the hydrodynamic performance of moving parts. The numerical method is employed for the analysis of a square texture with different bottom profiles: flat, triangle T1, triangle T2, and curved. The governing Reynolds equation is solved using a finite difference numerical discretization technique with the Gauss—Seidel iterative scheme. To obtain optimized process parameters, the response surface methodology-based central composite design along with grey relational analysis multi-objective optimization is used. The multi-objective responses are the load capacity and friction coefficient. The triangle T2 bottom profile yields the highest load capacity and the lowest friction coefficient compared to flat, triangle T1, and curved bottom profiles, of which the triangle T1 bottom profile yields the worst results. For the triangle T2 bottom profile, the flow speed is found to be the most significant process parameter, followed by the aspect ratio. Texture density is found to be the least significant parameter based on increasing the load capacity and decreasing the friction coefficient.

  4. Abstract

    Despite excellent tribological behaviors of ionic liquids (ILs) as lubricating oils, their friction-reducing and anti-wear properties must be improved when they are used under severe conditions. There are only a few reports exploring additives for ILs. Here, MoS2 and WS2 quantum dots (QDs, with particle size less than 10 nm) are prepared via a facile green technique, and they are dispersed in 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIm]PF6), forming homogeneous dispersions exhibiting long-term stabilities. Tribological test results indicate that the addition of MoS2 and WS2 QDs in the IL can significantly enhance the friction-reducing and anti-wear abilities of the neat IL under a constant load of 500 N and a temperature of 150 °C The exceptional tribological properties of these additives in the IL are ascribed to the formation of protective films, which are produced not only by the physical absorption of MoS2 and WS2 QDs at the steel/steel contact surfaces, but also by the tribochemical reaction between MoS2 or WS2 and the iron atoms/iron oxide species.

  5. Abstract

    Although graphene is well known for super-lubricity on its basal plane, friction at its step edge is not well understood and contradictory friction behaviors have been reported. In this study, friction of mono-layer thick graphene step edges was studied using atomic force microscopy (AFM) with a Si tip in dry nitrogen atmosphere. It is found that, when the tip slides over a ‘buried’ graphene step edge, there is a resistive force during the step-up motion and an assistive force during the step-down motion due to the topographic height change. The magnitude of these two forces is small and the same in both step-up and step-down motions. As for the ‘exposed’ graphene step edge, friction increases in magnitude and exhibits more complicated behaviors. During the step-down motion of the tip over the exposed step edge, both resistive and assistive components can be detected in the lateral force signal of AFM if the scan resolution is sufficiently high. The resistive component is attributed to chemical interactions between the functional groups at the tip and step-edge surfaces, and the assistive component is due to the topographic effect, same as the case of buried step edge. If a blunt tip is used, the distinct effects of these two components become more prominent. In the step-up scan direction, the blunt tip appears to have two separate topographic effects elastic deformation of the contact region at the bottom of the tip due to the substrate height change at the step edge and tilting of the tip while the vertical position of the cantilever (the end of the tip) ascends from the lower terrace to the upper terrace. The high-resolution measurement of friction behaviors at graphene step edges will further enrich understanding of interfacial friction behaviors on graphene-covered surfaces.

  6. Abstract

    In this study, expanded graphite and natural graphite were introduced into resin-based friction materials, and the tribological behavior of the composites was investigated. The tribo-performance of the two friction composites was evaluated using a constant speed friction tester. The results showed that the expanded graphite composite (EGC) displayed better lubricity in both the fading and the recovery processes. The wear rate of the EGC decreased by 22.43% more than that of the natural graphite composite (NGC). In the fading process, and the EGC enhanced the stability of the coefficient of friction. The recovery maintenance rate of the NGC was 4.66% higher than that of the EGC. It can be concluded that expanded graphite plays an important role in the formation of a stable contact plateau and can effectively reduce the wear.

  7. Abstract

    Abrasive wear is a common failure phenomenon that often limits the service life of sealing elements. Evaluation and comparison of the abrasion resistance of polytetrafluoroethylene (PTFE) were conducted using Al2O3 particles with sizes in the range 5 to 200 µm on a pin-on-flat tribo-tester under dry reciprocating sliding conditions at room temperature. Based on the examined worn surface characteristics of both PTFE and 316L stainless steel (as a counterpart) and the analyzed coefficient of friction (COF) evolutions, the wear mechanism and particle size effect have been explored in detail. The results demonstrate that the abrasive size is the main contributing factor, which can drastically impact the wear mechanism and tribological properties of tribo-pairs. The COF exhibits different evolution characteristics (trends) for different abrasive sizes. For moderate particle sizes, the COF trends become more complicated and the most evident wear of the metallic counterpart is evident. The activity behaviors of abrasives are dominated by the particle size. Particles can becomes embedded in one of the tribo-pair materials to plough-cut the counterpart, thus causing two-body abrasive wear. The abrasives can also behave as free rolling bodies, which play the role of third body to realize three-body “PTFE-abrasive-316L” abrasion. When abrasives are involved in the wear process, both the wear rate and COF of the metallic counterpart increase, but the material removal rate of the PTFE is reduced. The results obtained can offer guidelines regarding the design and protection of seals.

  8. Abstract

    Superlubricity has been developing very rapidly in recent years as a new and important area in tribology. Many new phenomena and materials, as well as some new mechanisms in both liquid and solid superlubricity have been obtained. In liquid superlubricity, tens of new kinds of liquids with superlubricity have been found (e.g., water-based liquids, oil-based lubricants, and liquids combined with additives of two-dimensional (2D) materials that exhibit very good superlubricity properties under high pressure). In the field of solid superlubricity, more materials with superlubricity have been observed, including graphene-to-graphene surfaces, highly oriented pyrolytic graphite to graphene surfaces, and heterostructure surfaces where a friction coefficient as low as 0.00004 has been obtained. However, superlubricity is still under laboratory research. What is the future of superlubricity? What is the barrier restricting superlubricity from industrial applications? How do we transfer superlubricity from scientific research to industrial application? These questions and application fields of superlubricity in near future have been analyzed, and the concept of “superlubricitive engineering” has been proposed in the present work.

  9. Abstract

    Inorganic nanomaterials exhibit superior friction-reduction and anti-wear properties in oils. In this study, 2D layered α-zirconium phosphate (α-ZrP) nanosheets intercalated with different amines have been synthesized to study their dispersion stabilities in lubricating oil and tribological applications. The intercalated amines should be sufficiently long and lipophilic to provide stabilization to α-ZrP nanosheets in mineral oil. The results of tribological tests illustrate that with the addition of well-dispersed nanosheets, the coefficient of friction (COF) and pin volume loss reduce by ~47% and 75%, respectively. The excellent dispersion stability enables the nanosheets to flow into the contact area at the beginning, and thereby protect the rubbing surface. A reduction in the van der Waals forces between the adjacent layers induced by the intercalated amines transforms the friction between adjacent layers from pin disk to sliding, leading to a decrease in the COF under hydrodynamic lubrication. The study provides a new method to enhance the tribological properties via tuning the dispersion stabilities of nanomaterials in oils.

  10. Abstract

    We use Langevin simulations to study the sliding friction of two-dimensional colloidal particles on a substrate with randomly distributed point-like pinning centers. The colloidal particles are modeled to interact with each other through repulsive magnetic dipole and attractive Lennard–Jones potentials. The subsequent occurrence of superlubricity, wherein the average friction force equals to zero, is accompanied by the appearance of islands with clear boundaries in the microscopic colloidal structures for weak pinning substrates. Friction arises for strong pinning substrates, and the average friction force increases with the substrate pinning intensity, and further, the islands disperse into disordered plastic structures. Moreover, the average friction force decreases with the repulsion intensity between the colloidal particles, and superlubricity finally results when the repulsion becomes sufficiently strong. Superlubricity also occurs for sufficiently weak attraction between colloidal particles, with an increase in the attraction intensity between colloidal particles leading to a nonlinear increase in the average friction force. With increasing temperature, the average friction force firstly increases and subsequently decreases rapidly. The above results can provide a theoretical framework for biological self-organization via utilization of the friction properties of microscopic or mesoscopic colloidal systems.

  11. Abstract

    Adhesion is one of essences with respect to rubber friction because the magnitude of the friction force is closely related to the magnitude of adhesion on a real contact area. However, the real contact area during sliding depends on the state and history of the contact surface. Therefore, the friction force occasionally exhibits rate-, state-, and pressure dependency. In this study, to rationally describe friction and simulate boundary value problems, a rate-, state-, and pressure-dependent friction model based on the elastoplastic theory was formulated. First, the evolution law for the friction coefficient was prescribed. Next, a nonlinear sliding surface (frictional criterion) was adopted, and several other evolution laws for internal state variables were prescribed. Subsequently, the typical response characteristics of the proposed friction model were demonstrated, and its validity was verified by comparing the obtained results with those of experiments conducted considering the contact surface between a rough rubber hemisphere and smooth acrylic plate.

  12. Abstract

    Cd coating is used in aerospace industries from last five decades due to its sacrificial protection and lubrication properties. Although Cd coating is primarily used due to its sacrificial corrosion protection when applied on steel substrate, the added benefit of modifying the tribological behavior by acting as a lubricious layer gives it a leading-edge than other coatings. Often the measurement of friction coefficient (CoF) is reported as a value generated after full sliding cycle. This measurement of average CoF generally limits the study of local variation in CoF occurring within one sliding cycle, which can be significantly different with change in spatial position due to change in third body morphology. In this study, a linearly reciprocating sliding test is used to measure the CoF at a sampling rate of 800 Hz along the track length to generate triboscopic image with steel countersphere. The instantaneous CoF obtained with triboscopy is correlated with the wear track morphology using scanning electron microscope (SEM) and optical profilometer for variation in contact conditions. Tribological test performed in dry atmosphere shows an average CoF of 0.4 till the end of the test whereas with increase in relative humidity to 60%, the average CoF changes from 0.4 to 0.8 at the end of the test due to change in contact conditions. Soft Cd coating on low carbon steel substrate is used to study these variations in third body morphology.

  13. Abstract

    Formations of clots were found inside the hydrodynamic bearings of the left ventricular assisted devices (LVADs) and caused tremendous risks to the long-term use of these devices. For the hydrodynamic bearings used in the LVAD, not only the lubrication status but also the motion of the blood cells in the bearing will take great effect on the performance of the device. Based on the analysis of the hydrodynamic pressures distribution and the flowing trajectory of red blood cells in the lubrication film, the bearing is designed in a region where enough hydrodynamic pressure is generated to float the rotor to reduce the wear, and the entrainment of red blood cells in the gap of the bearing can be prevented to avoid the formation of clots.

  14. Abstract

    The processes of degradation of engine oils operated in passenger cars of a uniform fleet of 25 vehicles were analyzed for oxidation content using infrared (IR) spectroscopy. As part of the experiment, the changes in engine oils occurring during actual operation (under conditions which can be described as “harsh”, i.e., short distance driving, frequent starting of the engine, and extended engine idling) have been observed. An evaluation of the Fourier transform infrared spectroscopy (FTIR) spectrum of an engine oil sample was presented. The infrared spectra of both fresh and used oils were recorded with the Thermo Nicolett IS5. The tests were conducted according to the Appendix A2 of ASTM 2412. For the used engine oil differentiation process, FTIR spectra were analyzed in the regions of 1,700–2,000 cm−1 and 3,600-3,700 cm−1. The FTIR spectrometry is demonstrated to be effective for the analysis and monitoring of processes of oxidation and shown to provide rapid and accurate information relating to the aging process of engine oils. The results may facilitate decision-making regarding the service life of engine oils. The achieved dependencies can make it possible to upgrade the sensor assembly consisting of an FTIR source.

  15. Abstract

    Anthracite sheets were coated by sorbitol fatty acid ester (span80) through ball-milling process. The tribological properties of the span80-coated anthracite sheets as the additive in polyalpha olefin were evaluated through a series of friction tests using a four ball machine. The results revealed that the span80-coated anthracite sheets exhibited excellent dispersion stability in base oil. In addition, compared with base oil, the average coefficient of friction, wear scar diameter, and wear volume of modified oil at a mass fraction of 0.03% span80-coated anthracite sheets decreased by 45.39%, 60.13%, and 95.95%, respectively. The oil containing span80-coated anthracite sheets achieved good friction-reducing and anti-wear effects over a wide range of applied loads, temperatures, or rotating speeds. Control experiments were performed as well. The results obtained using span80-coated anthracite sheets were superior to those obtained using pure anthracite. The lubrication mechanism was attributed to the synergistic action of the crystalline and amorphous carbon in anthracite sheets as they formed a protective film and played a mitigative role on the surface of friction pair, which mitigated the wear extent of the friction pair.

  16. Abstract

    In recent years, attempts to improve the mechanical properties of composites have increased remarkably owing to the inadequate utilization of matrices in demanding technological systems where efficiency, durability, and environmental compatibility are the key requirements. The search for novel materials that can potentially have enhanced mechanical properties continues. Recent studies have demonstrated that two-dimensional (2D) nanomaterials can act as excellent reinforcements because they possess high modulus of elasticity, high strength, and ultralow friction. By incorporating 2D nanomaterials in a composite, 2D nanomaterial-based composites (2DNBCs) have been developed. In view of this, a critical review of recent mechanical and tribological studies based on 2DNBCs has been undertaken. Matrices such as polymers, ceramics, and metals, as well as most of the representative 2D nanomaterial reinforcements such as graphene, boron nitride (BN), molybdenum disulfide (MoS2), and transition metal carbides and nitrides (MXenes) have been included in this review. Their preparation strategies, intrinsic mechanical properties, friction and lubrication performances, strengthening mechanisms, influencing factors, and potential applications have been comprehensively discussed. A brief summary and prospects are given in the final part, which would be useful in designing and fabricating advanced 2D nanocomposites in the future.

  17. Abstract

    Fluorographene, a new alternative to graphene, it not only inherits the 2-dimensional (2D) layered structure and outstanding mechanical properties, but also possesses controllable C–F bonds. It is meaningful to reveal the evolution processes of the tribological behaviors from graphene to fluorographene. In this work, fluorinated reduced graphene oxide nanosheets (F–rGO) with different degree of fluorination were prepared using direct gas-fluorination and they were added into gas to liquid-8 (GTL-8) base oil as lubricant additive to improve the tribological performance. According to the results, the coefficient of friction (COF) reduced by 21%, notably, the wear rate reduced by 87% with the addition of highly fluorinated reduced graphene oxide (HF–rGO) compared with rGO. It was confirmed that more covalent C–F bonds which improved the chemical stability of HF–rGO resisted the detachment of fluorine so the HF–rGO nanosheets showed less damage, as demonstrated via X-ray photoelectron spectroscopy (XPS), Raman spectra, and transmission electron microscopy (TEM). Meanwhile, the ionic liquid (IL) adsorbed on HF–rGO successfully improved the dispersibility of F–rGO in GTL-8 base oil. The investigation of tribofilm by TEM and focused ion beam (FIB) illustrated that IL displayed a synergy to participate in the tribochemical reaction and increased the thickness of tribofilm during the friction process.

  18. Abstract

    Tailoring a material’s properties for low friction and little wear in a strategic fashion is a long-standing goal of materials tribology. Plastic deformation plays a major role when metals are employed in a sliding contact; therefore, the effects of stacking fault energy and mode of dislocation glide need to be elucidated. Here, we investigated how a decrease in the stacking fault energy affects friction, wear, and the ensuing sub-surface microstructure evolution. Brass samples with increasing zinc concentrations of 5, 15, and 36 wt% were tested in non-lubricated sphere-on-plate contacts with a reciprocating linear tribometer against Si3N4 spheres. Increasing the sliding distance from 0.5 (single trace) to 5,000 reciprocating cycles covered different stages in the lifetime of a sliding contact. Comparing the results among the three alloys revealed a profound effect of the zinc concentration on the tribological behavior. CuZn15 and CuZn36 showed similar friction and wear results, whereas CuZn5 had a roughly 60% higher friction coefficient (COF) than the other two alloys. CuZn15 and CuZn36 had a much smaller wear rate than CuZn5. Wavy dislocation motion in CuZn5 and CuZn15 allowed for dislocation self-organization into a horizontal line about 150 nm beneath the contact after a single trace of the sphere. This feature was absent in CuZn36 where owing to planar dislocation slip band-like features under a 45° angle to the surface were identified. These results hold the promise to help guide the future development of alloys tailored for specific tribological applications.

  19. Abstract

    The mechanism of adhesive friction between viscoelastic materials is a key question. In this study, the friction process of the adhesive interface between a friction lining and a wire rope is dynamically observed in real time to analyze the adhesion hysteresis friction intuitively and quantitatively. The adhesion is determined by the state of motion, while the relative displacement of the wire rope and lining is used to find the magnitude of the adhesive friction. The hysteresis friction is reflected by the internal deformation of the lining. The magnitude of the hysteresis friction is determined by the displacement difference (Δχ) in the sliding direction of two marked points at different distances from the contact surface. The results show that the adhesion friction is proportional to the loss modulus and the hysteresis friction is proportional to the ratio of the loss modulus to the square of the storage modulus (E″/(E2)). The frictional vibration first decreases and then increases with the increase in pressure. The K25 lining has the highest adhesion hysteresis friction and minimal frictional vibration. The result provides a simple and intuitive method for research into the friction transmission and vibration of viscoelastic materials.

  20. Abstract

    This study focuses on microstructural and micromechanical modeling of abrasive sliding contacts of wear-resistant Hadfield steel. 3D finite element representation of the microstructure was employed with a crystal plasticity model including dislocation slip, deformation twinning, and their interactions. The results showed that deformation twinning interacting with dislocations had a key role in the surface hardening of the material, and it was also important for the early hardening process of the sub-surface grains beyond the heavily distorted surface grains. The effects of grain orientation and microstructural features were discussed and analyzed according to the micromechanical model to give a perspective to the anisotropy of the material and the feasibility of using micromechanics in virtual material design.