Showing papers in "Tribology Letters in 2011"
TL;DR: In this paper, the Raman vibrational modes were investigated for excitation wavelengths at 632.8 and 488 nm using both micro-crystalline MoS2 powder and natural MoS 2 crystals.
Abstract: Molybdenum disulfide (MoS2) and molybdenum trioxide are investigated using Raman spectroscopy with emphasis on the application to tribological systems. The Raman vibrational modes were investigated for excitation wavelengths at 632.8 and 488 nm using both micro-crystalline MoS2 powder and natural MoS2 crystals. Differences are noted in the Raman spectra for these two different wavelengths, which are attributed to resonance effects due to overlap of the 632.8 nm source with electronic absorption bands. In addition, significant laser intensity effects are found that result in laser-induced transformation of MoS2 to MoO3. Finally, the transformation to molybdenum trioxide is explored as a function of temperature and atmosphere, revealing an apparent transformation at 375 K in the presence of oxygen. Overall, Raman spectroscopy is an useful tool for tribological study of MoS2 coatings, including the role of molybdenum trioxide transformations, although careful attention must be given to the laser excitation parameters (both wavelength and intensity) when interpreting Raman spectra.
556 citations
TL;DR: Graphene platelets were chemically modified in a reflux reaction with stearic and oleic acids as mentioned in this paper, which led to an improvement in the dispersion of graphene platelets in base oil.
Abstract: Graphene platelets were chemically modified in a reflux reaction with stearic and oleic acids. Examination of the surface features of the graphene platelets before and after modification by infrared spectroscopy and ultraviolet–visible spectrophotometer revealed that the modification led to an improvement in the dispersion of graphene platelets in base oil. The tribological behavior of the lubricating oil containing modified graphene platelets (MGP) was further investigated using a four-ball machine. The results indicated that the oil containing only 0.075 wt% of MGP clearly improved the wear resistance and load-carrying capacity of the machine. Scanning electron microscopy and energy dispersive spectrometer performed to analyze the wear scar surfaces after friction confirmed that the outstanding lubrication performance of MGP could be attributed to their small size and extremely thin laminated structure, which allow the MGP to easily enter the contact area, thereby preventing the rough surfaces from coming into direct contact.
425 citations
TL;DR: In this paper, a series of friction test experiments on different rubbing surfaces (steel, alumina, diamond-like carbon) were performed with Inorganic fullerene-(IF)-like nanoparticles made of metal dichalcogenides (IF-MoS2, IF-WS2).
Abstract: Inorganic fullerene-(IF)-like nanoparticles made of metal dichalcogenides (IF-MoS2, IF-WS2) have been known to be effective as anti-wear and friction modifier additives under boundary lubrication. The lubrication mechanism of these nanoparticles has been widely investigated in the past and it is now admitted that their lubrication properties are attributed to a gradual exfoliation of the external sheets of the particles during the friction process leading to their transfer onto the asperities of the reciprocating surfaces. However, the chemical interaction between these molecular sheets and the rubbing surfaces has so far never been investigated in detail. In this study, the tribochemistry of the IF nanoparticles was carefully investigated. A series of friction test experiments on different rubbing surfaces (Steel, Alumina, Diamond-Like Carbon) were performed with IF-MoS2 nanoparticles. High-resolution transmission electron microscopy, scanning electron microscopy, Auger electron spectroscopy, and X-ray photoelectron spectroscopy were used to characterize the tribostressed areas on rubbing surfaces. A tribofilm composed of hexagonal 2H-MoS2 nanosheets was only observed on the steel surface. This transfer film was found to be incorporated into an iron oxide layer. A tribochemical reaction between the 2H-MoS2 nanolayers and the iron/iron oxide has been proposed as an explanation for the adhesion of this tribofilm. The tribochemical mechanism of the IF-MoS2 nanoparticles is discussed in this article.
162 citations
TL;DR: In this paper, the influence of graphite content on the dry sliding wear characteristics of Al6061/Gr composites along with Al 6061/30SiC/Gr hybrid composites has been assessed using a pin-on-disc wear test.
Abstract: The influence of graphite content on the dry sliding wear characteristics of Al6061/Gr composites along with Al6061/30SiC/Gr hybrid composites has been assessed using a pin-on-disc wear test. The composites with different volume fraction of graphite particles up to 13% were processed by in situ powder metallurgy (IPM) technique. The porosity and hardness of the resultant composites were also examined. It was found that an increase in the graphite content reduced the porosity, hardness, and friction coefficient of both types of composites. The hybrid composites were more porous and exhibited higher hardness and lower coefficient of friction at identical graphite contents. The increased graphite content in the range of 0–13 vol.% resulted in increased wear rate of Al/Gr composites. The Al/30SiC composite exhibited a lower wear rate as compared with the base alloy and graphite addition up to 9 vol.% improved the wear resistance of these hybrid composites. However, more graphite particles addition resulted in increased wear rate. SEM micrographs revealed that the wear mechanism was changed from mostly adhesive in the base alloy sample (Al/0Gr) to the prominently abrasive and delamination wear for Al/Gr and Al/SiC/Gr/composites.
151 citations
TL;DR: In this article, the role of sweat secretion and contact occlusion in producing wide-ranging values for the coefficient of friction that are particularly sensitive to the tribological configuration, sliding velocity, surface roughness and porosity of the counterbody.
Abstract: The current paper describes an experimental study of the friction of the human finger pad. The data highlight the role of sweat secretion and contact occlusion in producing wide-ranging values for the coefficient of friction that are particularly sensitive to the tribological configuration, sliding velocity, surface roughness and porosity of the counterbody. In particular, the large coefficients of friction typically observed on dry smooth surfaces are associated with a relatively damp interface, and can be considerably reduced by either decreasing or increasing the interfacial moisture content or by surface roughening. It is concluded that the very large range in the values of the coefficient of friction reported in the literature mainly result from differences in occlusion time associated with different tribological configurations, as well as from variations in surface roughness and finger pad sweat rates.
144 citations
TL;DR: In this article, the tribological properties of the CaCO3 nanoparticles as an additive in lithium grease were evaluated with a four-ball tester, and the results show that these CaCO 3 nanoparticles exhibit good performance in anti-wear and friction-reduction, load-carrying capacity, and extreme pressure properties.
Abstract: CaCO3 nanoparticles with an average size of 45 nm were synthesized via the carbonation method. The tribological properties of the CaCO3 nanoparticles as an additive in lithium grease were evaluated with a four-ball tester. The results show that these CaCO3 nanoparticles exhibit good performance in anti-wear and friction-reduction, load-carrying capacity, and extreme pressure properties. The action mechanism was estimated through analysis of the worn surface with X-ray photoelectron spectroscopy (XPS) and scanning electron microscope (SEM). The results indicate that a boundary film mainly composed of CaCO3, CaO, iron oxide, and other organic compounds was formed on the worn surface during the friction process.
131 citations
TL;DR: In this paper, in situ Transmission Electron Microscopy (TEM) observations of the behavior of single IF-MoS2 nanoparticles were conducted using a sample holder that combines TEM and atomic force microscopy (AFM) which simultaneously can apply normal and shear loads.
Abstract: Inorganic fullerene-(IF)-like nanoparticles made of metal dichalcogenides (IF-MoS2, IF-WS2) have been known to be effective as anti-wear and friction modifier additives under boundary lubrication. The lubrication mechanism of these nanoparticles has been widely investigated in the past and even if the exfoliation and third body transfer of molecular sheets onto the asperities constitute the prevalent mechanism for the improved tribological behavior of IF nanoparticles, it has also been suggested that a rolling friction process could also play a role for well crystallized and spherical particles. In this study, in situ Transmission Electron Microscopy (TEM) observations of the behavior of single IF-MoS2 nanoparticles were conducted using a sample holder that combines TEM and Atomic Force Microscopy (AFM) which simultaneously can apply normal and shear loads. It was shown that depending on the test conditions, either a rolling process or a sliding of the fullerenes could be possible. These in situ TEM observations are the first carried out with IF nanoparticles.
108 citations
TL;DR: In this article, the effect of sliding speed on friction and wear performance of a copper-graphite composite was investigated over a wide range of speeds with a pin-on-disc configuration.
Abstract: In practice, the sliding speed is an important parameter for materials applied in sliding condition. We have conducted an experimental study to explore the effect of sliding speed on friction and wear performance of a copper–graphite composite. The sliding tests were carried out over a wide range of speeds with a pin-on-disc configuration. The results show that there is a critical speed at which there is a transition of the friction and wear regimes of the composite. In addition, the formation of a lubricant layer on the contact surface (surface modification) determines the actual tribological performance of the composite. The wear mechanisms in different wear regimes are also discussed.
108 citations
TL;DR: In this paper, a survey of fingertip moisture was conducted to investigate the effect of moisture on friction, and it was found that water absorption is the principle mechanism responsible for the increase in friction, followed by capillary adhesion.
Abstract: Human hands sweat in different circumstances and the presence of sweat can alter the friction between the hand and contacting surface. It is, therefore, important to understand how hand moisture varies between people, during different activities and the effect of this on friction. In this study, a survey of fingertip moisture was done. Friction tests were then carried out to investigate the effect of moisture. Moisture was added to the surface of the finger, the finger was soaked in water, and water was added to the counter-surface; the friction of the contact was then measured. It was found that the friction increased, up until a certain level of moisture and then decreased. The increase in friction has previously been explained by viscous shearing, water absorption and capillary adhesion. The results from the experiments enabled the mechanisms to be investigated analytically. This study found that water absorption is the principle mechanism responsible for the increase in friction, followed by capillary adhesion, although it was not conclusively proved that this contributes significantly. Both these mechanisms increase friction by increasing the area of contact and therefore adhesion. Viscous shearing in the liquid bridges has negligible effect. There are, however, many limitations in the modelling that need further exploration.
108 citations
TL;DR: In this paper, the tribological properties of the HVOF-sprayed coatings WC 17%Co, WC 10%Co4%Cr, WC 15% NiMoCrFeCo, Cr3C2−25%NiCr, (Ti,Mo)(C,N)−37%NiCo, NiCrSiB, and AISI 316L were compared with the properties of electrolytic hard chrome and surface-hardened steel.
Abstract: The tribological properties of part surfaces, namely their wear resistance and friction properties, are decisive in many cases for their proper function. To improve surface properties, it is possible to create hard, wear-resistant coatings by thermal spray technologies. With these versatile coating preparation technologies, part lifetime, reliability, and safety can be improved. In this study, the tribological properties of the HVOF-sprayed coatings WC–17%Co, WC–10%Co4%Cr, WC–15% NiMoCrFeCo, Cr3C2–25%NiCr, (Ti,Mo)(C,N)–37%NiCo, NiCrSiB, and AISI 316L and the plasma-sprayed Cr2O3 coating were compared with the properties of electrolytic hard chrome and surface-hardened steel. Four different wear behavior tests were performed; the abrasive wear performance of the coatings was assessed using a dry sand/rubber wheel test according to ASTM G-65 and a wet slurry abrasion test according to ASTM G-75, the sliding wear behavior was evaluated by pin-on-disk testing according to ASTM G-99, and the erosion wear resistance was measured for three impact angles. In all tests, the HVOF-sprayed hardmetal coatings exhibited superior properties and can be recommended as a replacement for traditional surface treatments. Due to its tendency to exhibit brittle cracking, the plasma-sprayed ceramic coating Cr2O3 can only be recommended for purely abrasive wear conditions. The tested HVOF-sprayed metallic coatings, NiCrSiB and AISI 316L, did not have sufficient wear resistance compared with that of traditional surface treatment and should not be used under more demanding conditions. Based on the obtained data, the application possibilities and limitations of the reported coatings were determined.
100 citations
TL;DR: In this paper, the friction and wear behaviors of polyether ether ketone (PEEK), poly(phenyl p-hydroxybenzoate) (PHBA), polyimide (PI), and perfluoroethylene propylene copolymer (FEP) sliding against GCr15 and 316 steel rings under the lubrication of sea water were studied and compared with that under pure water.
Abstract: Ocean tribology, a new research field of tribology, is currently being established and developed. The tribological behaviors of polyether ether ketone (PEEK), poly(phenyl p-hydroxybenzoate) (PHBA), polyimide (PI), and perfluoroethylene propylene copolymer (FEP) sliding against GCr15 and 316 steel rings under the lubrication of sea water were studied and compared with that under the lubrication of pure water. The results show that the friction and wear behaviors of a polymer under the lubrication of aqueous medium are not only related to the properties of polymer itself, but also to the corrosive effect and lubricating effect of the medium. When a polymer slid against GCr15 steel under sea water lubrication, the friction coefficient and wear rate of polymer were much larger than that under pure water lubrication because of indirect corrosive wear. However, when sliding against corrosion-resistant 316 steel, polymers PEEK, FEP, and PI exhibited lower coefficients of friction and wear rates under sea water lubrication, this was attributed to better lubricating effect of sea water as a result of the deposition of CaCO3 and Mg(OH)2 on the counterface. On the contrary, the friction coefficient and wear rate of PHBA sliding against 316 steel under sea water lubrication were larger than that under pure water lubrication, which may be related to the properties of PHBA itself.
TL;DR: By optimizing parameters such as the composition of the friction counter surface, the lubricant solution, the normal load and the velocity, an ideal protocol and setup for microtribological testing could be established and used to perform a comparative study of various commercially available soft contact lenses.
Abstract: An important issue concerning the use of soft contact lenses is comfort, which, among other factors, has been related to the level of friction between the anterior side of the lens and the inner eyelid. Although several studies have been carried out to investigate the frictional properties of contact lenses, these have not taken the physiological environment of the eye into account. In use, lenses are in contact with proteins present in tears, with corneal cells and with the palpebral conjunctiva (clear membrane on inner eyelid). The focus of this study was to establish a biologically relevant measurement protocol for the investigation of friction of contact lenses that would mimic the eye’s physiological environment. By optimizing parameters such as the composition of the friction counter surface, the lubricant solution, the normal load and the velocity, an ideal protocol and setup for microtribological testing could be established and used to perform a comparative study of various commercially available soft contact lenses.
TL;DR: In this article, anatase (TiO2) nanoparticles with an average diameter of 10 nm were synthesized by solvothermal method followed by surface modification with stearic acid (SA).
Abstract: Anatase (TiO2) nanoparticles with an average diameter of 10 nm were synthesized by solvothermal method followed by surface modification with stearic acid (SA). As-prepared, the nanoparticles (SA-TiO2) were characterized by X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), Fourier transform infrared spectroscopy (FT-IR), and thermogravimetry (TGA) and differential scanning calorimetry (DSC). The tribological properties of SA-TiO2 as an additive of liquid paraffin (LP) were evaluated by a four-ball tester. The results show that LP with SA-TiO2 exhibited good anti-wear and friction-reduction properties under the all applied loads. Comparing LP with LP-containing SA, LP-containing TiO2, and LP-containing SA-TiO2, the LP-containing SA-TiO2 had the best load-carrying capacity. It was deduced that the boundary lubricating film was mainly composed of TiO2 deposits and an adsorbing film of SA which contribute to the excellent lubricating effect of SA-TiO2 in LP.
TL;DR: Where the severity of the sliding conditions dominates the wear and degradation of typical engineering tribomaterials, the results suggest that joint motion is actually beneficial for maintaining low matrix stresses, low contact areas, and effective lubrication for the fluid-saturated porous cartilage tissue.
Abstract: The progression of local cartilage surface damage toward early stage osteoarthritis (OA) likely depends on the severity of the damage and its impact on the local lubrication and stress distribution in the surrounding tissue. It is difficult to study the local responses using traditional methods; in situ microtribological methods are being pursued here as a means to elucidate the mechanical aspects of OA progression. While decades of research have been dedicated to the macrotribological properties of articular cartilage, the microscale response is unclear. An experimental study of healthy cartilage microtribology was undertaken to assess the physiological relevance of a microscale friction probe. Normal forces were on the order of 50 mN. Sliding speed varied from 0 to 5 mm/s, and two probes radii, 0.8 and 3.2 mm, were used in the study. In situ measurements of the indentation depth into the cartilage enabled calculations of contact area, effective elastic modulus, elastic and fluid normal force contributions, and the interfacial friction coefficient. This work resulted in the following findings: (1) at high sliding speed (V = 1–5 mm/s), the friction coefficient was low (μ = 0.025) and insensitive to probe radius (0.8–3.2 mm) despite the fourfold difference in the resulting contact areas; (2) the contact area was a strong function of the probe radius and sliding speed; (3) the friction coefficient was proportional to contact area when sliding speed varied from 0.05 to 5 mm/s; (4) the fluid load support was greater than 85% for all sliding conditions (0% fluid support when V = 0) and was insensitive to both probe radius and sliding speed. The findings were consistent with the adhesive theory of friction; as speed increased, increased effective hardness reduced the area of solid–solid contact which subsequently reduced the friction force. Where the severity of the sliding conditions dominates the wear and degradation of typical engineering tribomaterials, the results suggest that joint motion is actually beneficial for maintaining low matrix stresses, low contact areas, and effective lubrication for the fluid-saturated porous cartilage tissue. Further, the results demonstrated effective pressurization and lubrication beneath single asperity microscale contacts. With carefully designed experimental conditions, local friction probes can facilitate more fundamental studies of cartilage lubrication, friction and wear, and potentially add important insights into the mechanical mechanisms of OA.
TL;DR: In this article, the authors describe application of Prandtl-tomlinson models and their extensions to interpret dry atomic-scale friction, and provide a practical overview of how to use these models to study frictional phenomena.
Abstract: In this methods article, we describe application of Prandtl–Tomlinson models and their extensions to interpret dry atomic-scale friction. The goal is to provide a practical overview of how to use these models to study frictional phenomena. We begin with the fundamental equations and build on them step-by-step—from the simple quasistatic one-spring, one-mass model for predicting transitions between friction regimes to the two-dimensional and multi-atom models for describing the effect of contact area. The intention is to bridge the gap between theoretical analysis, numerical implementation, and predicted physical phenomena. In the process, we provide an introductory manual with example computer programs for newcomers to the field, and an illustration of the significant potential for this approach to yield new fundamental understanding of atomic-scale friction.
TL;DR: In this paper, the ploughing friction coefficient and the adhesion friction coefficient are distinguished for the first time using MD simulations, and the contribution of chip to friction coefficient is also evaluated.
Abstract: In this study, three-dimensional MD simulations are carried out to study the nanometric scratching process. The ploughing friction coefficient and the adhesion friction coefficient are distinguished for the first time using MD simulations. The contribution of chip to friction coefficient is also evaluated. The simulation results show that the macroscale theory can qualitatively evaluate the ploughing friction coefficient, but it slightly overestimates the ploughing friction coefficient on the nanoscale for the scratching depths studied. It is found that the adhesion friction coefficient is independent of the scratching depth as predicted by macroscale theory. It is also found that the contribution of chip to friction coefficient is independent of the scratching depth and cannot be neglected on the nanoscale.
TL;DR: In this article, the texturing effects on tribological performance of textured diamond-like carbon (DLC) films in water-lubricated condition were investigated and it was shown that introducing specific patterns into DLC film not only retains the low friction coefficients, but also dramatically extends coating lifetime through affecting the coating delamination behavior and graphitization process during friction.
Abstract: Textured diamond-like carbon (DLC) films with the pattern of parallel grooves were developed by depositing DLC on textured stainless substrates in a PVD system. The texturing effects on tribological performance of DLC in water-lubricated condition were investigated. Results show that introducing specific patterns into DLC film not only retains the low friction coefficients, but also dramatically extends coating lifetime through affecting the coating delamination behavior and graphitization process during friction. Besides the adherence difference induced by surface texturing which could influence the delamination, another possible mechanism, “buffer stripes”, which is characteristic of the lateral soft/hard periodical structure, was proposed by us based on the Micro-Raman spectroscopy and nanoindentation analysis. Additionally, a much lower graphitization for textured DLC during friction may also be responsible for the improved wear resistance.
TL;DR: In this paper, the effect of surface texture on the friction was attributed to the variation of the plowing component of friction for different surfaces, and it was also observed that the variations of plowing friction as a function of hardness depends on surface textures.
Abstract: In the present investigation, experiments were conducted by unidirectional sliding of pins made of FCC metals (Pb, Al, and Cu) with significantly different hardness values against the steel plates of various surface textures and roughness using an inclined pin-on-plate sliding apparatus in ambient conditions under both the dry and lubricated conditions. For a given material pair, it was observed that transfer layer formation and the coefficient of friction along with its two components, namely adhesion and plowing, are controlled by the surface texture of the harder mating surfaces and are less dependent of surface roughness (R (a)) of the harder mating surfaces. The effect of surface texture on the friction was attributed to the variation of the plowing component of friction for different surfaces. It was also observed that the variation of plowing friction as a function of hardness depends on surface textures. More specifically, the plowing friction varies with hardness of the soft materials for a given type of surface texture and it is independent of hardness of soft materials for other type of surface texture. These variations could be attributed to the extent of plane strain conditions taking place at the asperity level during sliding. It was also observed that among the surface roughness parameters, the mean slope of the profile, Delta (a), correlated best with the friction. Furthermore, dimensionless quantifiable roughness parameters were formulated to describe the degree of plowing taking place at the asperity level.
TL;DR: In this article, non-perturbed microstructures of several commercial and model lubricating greases, differing in nature and concentration of the thickener agent, were examined using the atomic force microscopy (AFM) technique.
Abstract: In this work, non-perturbed microstructures of several commercial and model lubricating greases, differing in nature and concentration of the thickener agent, were examined using the atomic force microscopy (AFM) technique. Grease microstructure mainly depends on the nature of the thickener employed and, also, on thickener concentration and viscosity of the base oil. Thermal-induced changes in the viscoelastic response of lubricating greases have been investigated by using different rheological techniques in a temperature range of 0–175 °C. Small-amplitude oscillatory shear (SAOS) measurements were carried out to determine the mechanical spectra of the different samples studied. Lubricating grease rheological thermal susceptibility was analysed by following the evolution of the plateau modulus with temperature. SAOS functions dramatically decrease, in most cases, above a characteristic temperature, which depends on nature and/or concentration of the thickener used and therefore on grease microstructure. The thermo-mechanical reversibility of grease microstructure has been studied by carrying out triple-step-shear stress tests (shear stresses inside and outside of the linear viscoelasticity range), at different temperatures. The degree of lubricating grease non-reversible structural breakdown, which increases with temperature, depends on the shear stress applied above the linear viscoelasticity limits.
TL;DR: In this article, the authors measured the film thickness of ionic liquids and silicone oils at high pressure up to 3 GPa using the relative optical interference intensity (RII) method.
Abstract: Ionic liquids are generally considered as environmentally friendly material. The film thicknesses of ionic liquids and silicone oils at high pressures up to 3 GPa are measured employing the relative optical interference intensity method. The results show that for the three ionic liquids the relative order of film thickness is 1-octyl-3-methylimidazolium hexafluorophosphate ([OMIM]PF6) > 1-hexyl-3-methylimidazolium hexafluorophosphate ([HMIM]PF6) > 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM]PF6). In elastohydrodynamic lubrication the order of viscosity can simply account for this fact. In thin film lubrication condition the length of alkyl side chain and arrangement manner of cation are used to explain the experimental results. Another remarkable phenomenon is that even though the viscosities of silicone oils are close to those of ionic liquids, the measured film thicknesses of silicone oils are quite less than those of the ionic liquids. The results show that long alkyl chain ionic liquid can form rather thick films at high pressure.
TL;DR: In this article, the effects of resin type and fiber length on mechanical properties and friction characteristics of automotive brake materials were studied, where three types of resin, viz. straight phenolic resin (SR), cashew nut shell liquid modified resin (CR), and melamine resin (MR) were used as matrix material.
Abstract: In this study, the effects of resin type and fiber length on mechanical properties and friction characteristics of automotive brake materials were studied. Three types of resin, viz. straight phenolic resin (SR), cashew nut shell liquid modified resin (CR), and melamine resin (MR) were used as matrix material. Lapinus with different lengths was used as inorganic fiber. Three series of friction composites composed of nine composites in the form of brake materials were manufactured. Physical, mechanical, and tribological properties of all composites were investigated. The friction tests were performed using a Chase type friction tester. The results showed that both resin type and fiber length played an important role on the mechanical and tribological properties of the friction materials. The highest and the lowest friction coefficient for resin types were recorded for SR and MR composites, respectively, while MR and CR composites showed the highest and the lowest wear resistance, respectively. For the fiber length considered, increasing the fiber length increased the wear resistance of the composites. The coefficient of friction, in general, showed a good correlation with the wear resistance of the composites. But, there was no clear correlation with the mechanical and tribological properties of the composites. The morphological features of worn surfaces and wear debris of the composites were analyzed in order to understand the friction and wear mechanisms of this tribosystem.
TL;DR: In this paper, a laser partially textured thrust bearing is theoretically and experimentally analyzed using an adiabatic model in order to theoretically investigate the performances of the bearing, and a comparison between a laser textured bearing and a bearing textured using the photolithographic method is presented.
Abstract: In this article, a laser partially textured thrust bearing is theoretically and experimentally analyzed. An adiabatic model is developed in order to theoretically investigate the performances of the bearing. The bearing sample is partially textured both in radial and circumferential direction using the laser texturing process. The performance of the bearing (fluid film thickness and friction torque) is evaluated on a specially adapted test rig and the experimental results are compared with the theoretical model. A good agreement is found between the theoretical model and the experimental data. Also a comparison between a laser textured bearing and a bearing textured using the photolithographic method is presented.
TL;DR: In this paper, the authors use molecular dynamics simulations to model the friction of two rough deformable surfaces, while changing the surface roughness, the sliding speed, and the applied normal load.
Abstract: Although, a lot is known about the factors contributing to friction, a complete physical understanding of the origins of friction is still lacking. At the macroscale several laws have long since described the relation between load (Amontons, Coulomb), apparent and real area of contact (Bowden and Tabor), and frictional forces. But it is not yet completely understood if these laws of friction extend all the way down to the atomistic level. Some current research suggests that a linear dependence of friction on the real contact area is observed at the atomistic level, but only for specific cases (indentors and rigid substrates). Because continuum models are not applicable at the atomic scale, other modeling techniques (such as molecular dynamics simulations) are necessary to elucidate the physics of friction at the small scale. We use molecular dynamics simulations to model the friction of two rough deformable surfaces, while changing the surface roughness, the sliding speed, and the applied normal load. We find that friction increases with roughness. Also all sliding cases show considerable surface flattening, reducing the friction close to zero after repetitive sliding. This questions the current view of (static) roughness at the atomistic scale, and possibly indicates that the macroscopic laws of friction break down several orders of magnitude before reaching the atomic scale.
TL;DR: In this paper, a 1 wt% additive to a polyalphaolefin (PAO 6) was used to improve the anti-friction and anti-wear performance of CrN PVD coating.
Abstract: This paper studies ethyl-dimethyl-2-methoxyethylammonium tris(pentafluoroethyl)trifluorophosphate ionic liquid [(NEMM)MOE][FAP] as 1 wt% additive to a polyalphaolefin (PAO 6) in the lubrication of CrN PVD coating. The tribological behavior of this mixture has also been compared with a traditional oil additive, such as zinc dialkyldithiophosphate (ZDDP). Friction and wear tests were performed by means of a ball-on-plate reciprocating tribometer, and XPS was used to analyze wear surfaces. The experimental results showed that both additives substantially improve the anti-friction and anti-wear performance of the base oil. However, the tribological behavior of the ionic liquid as oil additive does not reach that of ZDDP. The interactions of each additive with the surface and tribofilm formation contributed to improve the tribological behavior of the lubricants.
TL;DR: The aim of this article is to propose an analytical approximate squeeze-film lubrication model of the human ankle joint for a quick assessment of the synovial pressure field and the load carrying due to the squeeze motion.
Abstract: The aim of this article is to propose an analytical approximate squeeze-film lubrication model of the human ankle joint for a quick assessment of the synovial pressure field and the load carrying due to the squeeze motion. The model starts from the theory of boosted lubrication for the human articular joints lubrication (Walker et al., Rheum Dis 27:512–520, 1968; Maroudas, Lubrication and wear in joints. Sector, London, 1969) and takes into account the fluid transport across the articular cartilage using Darcy’s equation to depict the synovial fluid motion through a porous cartilage matrix. The human ankle joint is assumed to be cylindrical enabling motion in the sagittal plane only. The proposed model is based on a modified Reynolds equation; its integration allows to obtain a quick assessment on the synovial pressure field showing a good agreement with those obtained numerically (Hlavacek, J Biomech 33:1415–1422, 2000). The analytical integration allows the closed form description of the synovial fluid film force and the calculation of the unsteady gap thickness.
TL;DR: In this article, multiwalled carbon nanotubes (MWCNTs) were functionalized and used as additives in paraffin oil to improve its lubrication effect for bismaleimide resin.
Abstract: Multiwalled carbon nanotubes (MWCNTs) were functionalized and were used as additives in paraffin oil to improve its lubrication effect for bismaleimide resin. The tribological behavior of bismaleimide resin lubricated by the paraffin oil filled with the functionalized carbon nanotubes was investigated by friction and wear tester. The wear surface of the resin with steel ball as tribopair was analyzed by means of scanning electron microscopy (SEM). It was found that the addition of this kind of functionalized MWCNTs effectively reduced the friction coefficient. An optimal additive concentration existed in the system and was found to be 0.025 wt%. A lubrication model for the resin and steel ball system was postulated and it was the isolating effect and bearing structure of f-MWCNTs that played a key role in friction and wear reduction.
TL;DR: In this paper, the magnetic field intensity distributions on the rubbing surface have a significant influence on the tribological properties of Ferrofluids and the experimental results indicate that FFs have a good friction reduction performance in the presence of an external magnetic field compared with the carrier liquid and that its lifetime of friction can be greatly improved.
Abstract: Ferrofluids (FFs) are stable colloidal systems consisting of single-domain magnetic particles with a diameter of approximately 10 nm coated with surfactants and dispersed in a carrier liquid. By applying an external magnetic field, these fluids can be confined, positioned, shaped and, controlled at desired places. The load capacity of a lubricant film of FF can also be increased with an appropriate magnetic field. In this paper, Fe3O4-based FFs with different saturation magnetizations (M
s) were prepared by the co-precipitation technique. The tribological experiments of FFs under different magnets distributions were conducted on a ring-on-cylinder tribometer. The results show that the magnetic field intensity distributions on the rubbing surface have a significant influence on the tribological properties of FFs. The experimental results also indicate that FFs have a good friction-reduction performance in the presence of an external magnetic field compared with the carrier liquid and that its lifetime of friction can be greatly improved.
TL;DR: In this article, a multi-technique three-dimensional approach was used to characterize the boundary films on ionic liquids (ILs) lubricated metallic surfaces, and complementary characterizations at the top surface, cross-section, and different layers of the boundary film provided direct measurement of the film thickness.
Abstract: Since the idea of using ionic liquids (ILs) as lubricants was raised in 2001, many studies have been conducted in this area and results have demonstrated superior lubricating performance for a variety of ILs. It is widely believed that a protective tribo-boundary film is formed on the contact area by tribochemical reactions between the metal surface and the IL during the wear process and, as a result, reduces friction and wear. However, the study of this critical boundary film in the literature has been limited to two-dimensional topography examination and chemical analysis from the top surface. This study demonstrates a multi-technique three-dimensional approach to characterize the boundary films on IL-lubricated metallic surfaces. The complementary characterizations at the top surface, cross-section, and different layers of the boundary film provide direct measurement of the film thickness, visualization of the nanostructure, and analysis of the composition change. The boundary films observed on different alloys are substantially distinct from both physical and chemical perspectives. The measured mean film thicknesses for cast iron, steel, and aluminum worn surfaces are 300, 60, and 200 nm, respectively. The boundary films on ferrous alloys are dominated by amorphous phase mixing with well-dispersed very fine (a few nm) nanocrystals, while the film on aluminum contains many larger size (tens of nm) metallic particles in a less organized manner.
TL;DR: In this article, a sliding wear test was performed for H13 steel and a cast steel under atmospheric conditions of 25, 200, and 400°C, and the results indicated that the former oxidative wear could be classified into mild wear; the latter one fell beyond mild wear.
Abstract: Sliding wear tests were performed for H13 steel and a cast steel under atmospheric conditions of 25, 200, and 400°C. XRD results identify that oxidative wear prevailed for the steels during sliding at 200–400°C. However, the oxidative wear at 200°C presented entirely different wear behaviors from the one at 400°C. With an increase of load, the oxidative wear at 200°C exhibited slowly increased and lower wear rates, despite relatively less tribo-oxides. On the contrary, although there were more tribo-oxides, the oxidative wear at 400°C presented rapidly increased and higher wear rates. The former oxidative wear could be classified into mild wear; the latter one fell beyond mild wear. The two types of oxidative wear universally existed; their discrepancies were mainly attributed not to the tribo-oxides, but to the extent of softening and deformation of substrate. Hence, we suggested that the two types of oxidative wear should be distinguished in the coming research, and were termed oxidative mild wear and oxidative wear, respectively.
TL;DR: In this paper, single-walled carbon nanotubes (NTs) were added in a 0.5 wt% proportion to the room temperature ionic liquid (IL) 1-octyl, 3-methylimidazolium chloride ([OMIM]Cl).
Abstract: Single-walled carbon nanotubes (NTs) were added in a 0.5 wt% proportion to the room temperature ionic liquid (IL) 1-octyl, 3-methylimidazolium chloride ([OMIM]Cl). The [OMIM]Cl + NT mixtures obtained by mechanical grinding in an agate mortar ([OMIM]Cl + NT(g)) or by mechanical grinding and ultrasound dispersion ([OMIM]Cl + NT(g + us)) were used as lubricants of the polycarbonate (PC) disc/AISI 316L stainless steel pin contact. When the [OMIM]Cl + NT(g + us) dispersion is used, a negligible wear rate and a friction coefficient reduction of a 66% with respect to neat IL, and of a 50% with respect to [OMIM]Cl + NT(g), is achieved. Results are discussed on the basis of DSC, TGA, Raman spectroscopy, TEM microscopy, XRD, XPS and surface topography analysis.