Friction

  1. Abstract

    In this study, lithium complex grease (LCG) and polyurea grease (PUG) were synthesized using mineral oil (500SN) and polyalphaolefin (PAO40) as base oil, adsorbed onto lithium complex soap and polyurea as thickeners, respectively. The effects of grease formulation (thickener and base oil with different amounts (80, 85, and 90 wt%) on the corrosion resistance and lubrication function were investigated in detail. The results have verified that the as-prepared greases have good anti-corrosion ability, ascribed to good salt-spray resistance and sealing function. Furthermore, the increase in the amount of base oil reduces the friction of the contact interface to some extent, whereas the wear resistance of these greases is not consistent with the friction reduction, because the thickener has a significant influence on the tribological property of greases, especially load-carrying capacity. PUG displays better physicochemical performance and lubrication function than LCG under the same conditions, mainly depending on the component/structure of polyurea thickener. The polyurea grease with 90 wt% PAO displays the best wear resistance owing to the synergistic lubrication of grease-film and tribochemical film, composed of Fe2O3, FeO(OH), and nitrogen oxide.

  2. 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.

  3. Abstract

    The elastic loading behaviour of rough surfaces is derived based on the physical understanding of the contact phenomena, where the pressure distribution is analytically obtained without any negative values or convergence problems, thus the evolution of the contact behaviour is obtained in a semi-analytical manner. Numerical results obtained by the proposed approach facilitate the understanding of the contact behaviour in the following aspects: 1) the ratio of contact area to load decreases with an increase in real contact area; 2) normal approach-load relationship is approximated by an exponential decay under relatively small loads and a linear decay under relatively large loads; and 3) average gap shows an exponential relationship with load only in moderate load range.

  4. Abstract

    Polyimide composites have been extensively used as motion components under extreme conditions for their thermal stability and special self-lubricating performance. In the present study, Ag-Mo hybrids as lubricant fillers were incorporated into thermosetting polyimide to prepare a new type of tribo-materials (TPI-1) at high temperature. Comprehensive investigations at different temperatures reveal that the newly developed TPI-1 exhibits a better reduction in friction and wear rate below 100 °C, but all of them increase significantly when the bulk temperature exceeds 250 °C. The wear mechanisms demonstrated that sandwich-like tribofilms with different layers were established at different temperatures, which was further verified by characterization of scanning electron microscope (SEM), Raman spectroscopy, and transmission electron microscope (TEM). Considering the high-performance TPI coupled with Ag-Mo hybrids, we anticipate that further exploration would provide guidance for designing TPI tribo-materials that would be used at high temperatures.

  5. Abstract

    Carburized gears are widely used in geared machines such as wind turbines. Contact fatigue problems occur in engineering practice, reducing reliabilities of machines. Contact fatigue failures are related to many factors, such as gradients of mechanical properties of the hardening layer. In this work, an elastic-plastic contact model of a carburized gear is developed based on the finite element method to evaluate contact fatigue failure risk, considering variations in hardness and strength. The Dang Van multiaxial equivalent stress is calculated via Python coding within the Abaqus framework. The gradient of yield strength along the depth from case to core is considered. The concept of local material fatigue failure risk is defined to evaluate the probability of pitting failure. The effects of design factors, such as the case hardening depth (CHD), surface hardness, and contact pressure on fatigue failure risk, are studied. As the CHD increases or the surface hardness decreases, the risk of deep spalling failure reduces. The increase in surface hardness leads to a decreased risk of pitting failure, while the variation in CHD hardly affects the pitting failure risk.

  6. Abstract

    Acoustic emission (AE) has been studied for monitoring the condition of mechanical seals by many researchers, however to the best knowledge of the authors, typical fault cases and their effects on tribological behaviour of mechanical seals have not yet been successfully investigated. In this paper, AE signatures from common faults of mechanical seals are studied in association with tribological behaviour of sealing gap to develop more reliable condition monitoring approaches. A purpose-built test rig was employed for recording AE signals from the mechanical seals under healthy and faulty conditions. The collected data was then processed using time domain and frequency domain analysis methods. The study has shown that AE signal parameters: root mean squared (RMS) along with AE spectrum, allows fault conditions including dry running, spring out and defective seal faces to be diagnosed under a wide range of operating conditions. However, when mechanical seals operate around their transition point, conventional signal processing methods may not allow a clear separation of the fault conditions from the healthy baseline. Therefore an auto-regressive (AR) model has been developed on recorded AE signals to classify different fault conditions of mechanical seals and satisfactory results have been perceived.

  7. Abstract

    In this paper, an equation for the calculation of the frictional torque of a dry-lubricated tapered roller bearing (TRB) is provided in which the effect of the roller skewing is emphasized. Calculations were performed to investigate the effect of the roller skewing on the torque of dry-lubricated TRB for two representative preload methods, that is, axial force preload and axial displacement preload. The results show that a proper roller skewing angle under axial force preload benefits the reduction of the TRB torque. However, the roller skewing angle should not exceed a certain critical value; otherwise, it will cause a steep rise in the TRB torque. Finally, the critical value of the roller skewing angle as a function of the friction coefficient and cage pocket clearance is presented. The developed torque model provides a tool for the internal design and torque optimization of dry-lubricated TRBs.

  8. Abstract

    Finite element analysis is used to investigate an elastic-plastic coated spherical contact in full stick contact condition under combined normal and tangential loading. Sliding inception is associated with a loss of tangential stiffness. The effect of coating thickness on the static friction coefficient is intensively investigated for the case of hard coatings. For this case, with the increase in coating thickness, the static friction coefficient first increases to its maximum value at a certain coating thickness, thereafter decreases, and eventually levels off. The effect of the normal load and material properties on this behavior is discussed. Finally, a model for the static friction coefficient as a function of the coating thickness is provided for a wide range of material properties and normal loading.

  9. Abstract

    The deleterious innate attribute of Ti6Al4V, the workhorse material among the alloy series of titanium is its incompetent tribo-behavior. Infinite surface modification techniques, viz., the accretion of adherent appendage layers, diffusion hardening, infusion of residual stresses, microstructural evolution, and phase transformations were attempted to enhance the wear resistance of the alloy. The need lies to establish a bridge between the indigenous material properties and the tribo-characteristics of Ti6Al4V so that the enforced improvement techniques can raise the barriers of its applicability. A critical review of the microstructural transitions, mechanisms governing tribo-behavior and the parametric conditions leading to material removal at dry sliding conditions of Ti6Al4V, falls under the scope of this manuscript. Hence, the prime focus of the approach is to impart a clear-cut perception of the minute variations in mechanical, metallurgical, and tribological characteristics of the alloy at interactive instances with distinct counter-body surfaces.

  10. Abstract

    In thin-film lubrication (TFL), generally, the viscosity of the lubricant and its coefficient of friction (CoF) increase. Finding a method to reduce the CoF in TFL is a significant challenge for tribologists. In the present work, we report a robust superlubricity attained by using polyalkylene glycols (PAGs, polar molecules) and poly-α-olefins (PAOs, nonpolar molecules) as lubricants on steel/steel friction pairs that have been pre-treated by wearing-in with polyethylene glycol aqueous solution (PEG(aq)). A steady superlubricity state with a CoF of 0.0045 for PAG100 and 0.006 for PAO6 could be maintained for at least 1 h. Various affecting factors, including the sliding velocity, normal load, and viscosity of the lubricants, were investigated. Element analysis proved that composite tribochemical layers were deposited on the worn region after the treatment with PEG(aq). These layers were formed by the tribochemical reactions between PEG and steel and composed of various substances including oxides, iron oxides, FeOOH, and Fe(OH)3, which contributed to the superlubricity. In addition to the tribochemical layers, ordered layers and a fluid layer were formed by the PAGs and PAOs during the superlubricity periods. All the three types of layers contributed to the superlubricity, indicating that it was attained in the TFL regime. Accordingly, a mechanism was proposed for the superlubricity of the PAGs and PAOs in the TFL regime in this work. This study will increase the scientific understanding of the superlubricity in the TFL regime and reveal, in the future, the potential for designing superlubricity systems on steel surfaces for industrial applications.

  11. Abstract

    Three-dimensional finite element modeling of the contact between a rigid spherical indenter and a rough surface is presented when considering both the loading and unloading phases. The relationships among the indentation load, displacement, contact area, and mean contact pressure for both loading and unloading are established through a curve fitting using sigmoid logistic and power law functions. The contact load is proportional to the contact area, and the mean contact pressure is related to the characteristic stress, which is dependent on the material properties. The residual displacement is proportional to the maximum indentation displacement. A proportional relationship also exists for plastically dissipated energy and work conducted during loading. The surface roughness results in an effective elastic modulus calculated from an initial unloading stiffness several times larger than the true value of elastic modulus. Nonetheless, the calculated modulus under a shallow spherical indentation can still be applied for a relative comparison.

  12. Abstract

    Lubricant transfer and distribution at the head/disk interface in air-helium gas mixtures is investigated using a developed model that combines an air-bearing model with a molecular dynamics model. The pressure distribution is calculated by the air-bearing model at the head/disk interface with respect to the helium content and the pressure obtained is then input to the molecular dynamics model to understand the lubricant transfer mechanism. Finally, the effects of pressure at the boundary condition and disk velocity on lubricant transfer are discussed in relation to the helium fraction within the air-helium gas mixtures. Results show there is a decrease in the pressure difference with an increase in the helium percentage, which leads to a decrease in the volume of the lubricant transferred. The results also suggest that the lubricant is not easily to transfer in gas mixtures with a high percentage of helium, even when both higher disk velocities and pressure boundary conditions are applied.

  13. Abstract

    Understanding the microstructural and tribo-chemical processes during tribological loading is of utmost importance to further improve the tribological behavior of metals. In this study, the friction, wear and tribo-chemical behavior of Ni with different initial microstructures (nanocrystalline, bi-modal, coarse-grained) is investigated under dry sliding conditions. In particular, the interplay be-tween frictional response, microstructural evolution and tribo-oxidation is considered. Friction tests are carried out using ball-on-disk experiments with alumina balls as counter-bodies, varying the load between 1 and 5 N. The microstructural evolution as well as the chemical reactions beneath the samples’ surface is investigated by means of cross-sections. The samples with finer microstructures show a faster run-in and lower maximum values of the coefficient of friction (COF) which can be attributed to higher oxidation kinetics and a higher hardness. It is observed that with increasing sliding cycles, a stable oxide layer is formed. Furthermore, initially coarse-grained samples show grain refinement, whereas initially finer microstructures undergo grain coarsening converging towards the same superficial grain size after 2,000 sliding cycles. Consequently, the experimental evidence supports that, irrespective of the initial microstructure, after a certain deformation almost identical steady-state COF values for all samples are achieved.

  14. Abstract

    Fretting wear is a common cause of failure of an electrical contact (EC). In this study, we analyzed in detail the failure of EC induced especially by sliding using the representative electrical terminals. Furthermore, combining the friction energy dissipation theory, we proposed a prediction model to evaluate the electrical connector endurance (ECE) based on the contact stress and geometrical changes during the wear process obtained from a numerical model. The study helps establish that the friction energy dissipation theory is a powerful tool to analyze a contact failure due to wear. The proposed model proves to be effective in predicting the ECE for all considered cases such as micro-slip amplitude, contact force, overturning angle, superficial layer thickness, and friction/wear coefficients.

  15. Abstract

    The marvels of the slippery and clean sharkskin have inspired the development of many clinical and engineering products, although the mechanisms of interfacial interaction between the sharkskin and water have yet to be fully understood. In the present research, a methodology was developed to evaluate morphological parameters and to enable studying the effects of scale orientation on the fluidic behavior of water. The scale orientation of a shark skin was defined as the angle between the ridges and fluid flow direction. Textured surfaces with a series orientation of scales were designed and fabricated using 3D printing of acrylonitrile butadiene styrene (ABS). The fluid drag performance was evaluated using a rheometer. Results showed that the shark–skin-like surface with 90 degree orientation of scales exhibited the lowest viscosity drag. Its maximum viscosity reduction was 9%. A viscosity map was constructed based on the principals of fluid dynamic. It revealed that the drag reduction effect of a shark-skin-like surface was attributed to the low velocity gradient. This was further proven using diamond nitrogen-vacancy sensing where florescent diamond particles were distributed evenly when the velocity gradient was at the lowest. This understanding could be used as guidance for future surface design.

  16. Abstract

    A magnetic ionic liquid (abridged as MIL) [C6mim]5[Dy(SCN)8] was prepared and used as the magnetic lubricant of a steel-steel sliding pair. The tribological properties of the as-prepared MIL were evaluated with a commercially obtained magnetic fluid lubricant (abridged as MF; the mixture of dioctyl sebacate and Fe3O4, denoted as DIOS-Fe3O4) as a control. The lubrication mechanisms of the two types of magnetic lubricants were discussed in relation to worn surface analyses by SEM-EDS, XPS, and profilometry, as well as measurement of the electric contact resistance of the rubbed steel surfaces. The results revealed that the MIL exhibits better friction-reducing and antiwear performances than the as-received MF under varying test temperatures and loads. This is because the MIL participates in tribochemical reactions during the sliding process, and forms a boundary lubrication film composed of Dy2O3, FeS, FeSO4, nitrogen-containing organics, and thioether on the rubbed disk surface, thereby reducing the friction and wear of the frictional pair. However, the MF is unable to form a lubricating film on the surface of the rubbed steel at 25 °C, though it can form a boundary film consisting of Fe3O4 and a small amount of organics under high temperature. Furthermore, the excessive Fe3O4 particulates that accumulate in the sliding zone may lead to enhanced abrasive wear of the sliding pair.

  17. Abstract

    Magnetorheological elastomer (MRE) is a type of smart material of which mechanical and electrical properties can be reversibly controlled by the magnetic field. In this study, the influence of the magnetic field on the surface roughness of MRE was studied by the microscopic modeling method, and the influence of controllable characteristics of the MRE surface on its friction properties was analyzed by the macroscopic experimental method. First, on the basis of existing studies, an improved mesoscopic model based on magnetomechanical coupling analysis was proposed. The initial surface morphology of MRE was characterized by the W-M fractal function, and the change process of the surface microstructures of MRE, induced by the magnetic interaction between particles, was studied. Then, after analyzing the simulation results, it is found that with the increase in the magnetic field and decrease in the modulus of rubber matrix, the surface of MRE changes more significantly, and the best particle volume fraction is within 7.5%–9%. Furthermore, through experimental observation, it is found that the height of the convex peak on the surface of MRE decreases significantly with the action of the magnetic field, resulting in a reduction in the surface roughness. Consistent with the simulation results, a particle volume fraction of 10% corresponds to a maximum change of 14%. Finally, the macroscopic friction experiment results show that the friction coefficients of MREs with different particle volume fractions all decrease with the decrease in surface roughness under the magnetic field. When the particle volume fraction is 10%, the friction coefficient can decrease by 24.7% under a magnetic field of 400 mT, which is consistent with the trend of surface roughness changes. This shows that the change in surface morphology with the effect of the magnetic field is an important factor in the control of MRE friction properties by magnetic field.

  18. Abstract

    This study is on the development of drum brake liner for a multi-utility vehicle possessing a hydraulic brake system. The approach adopted was to vary seven weight percent in each friction composite formulation between steel fiber and stainless steel fiber. The friction composites developed were tested for physical, chemical, corrosive, mechanical, and thermal properties as well as tribological characteristics under near-actual conditions using an inertia dynamometer as per industrial standards. ANSYS analysis was performed to obtain the thermal stress distributions of the developed friction composites at maximum temperature rise during brake stops, and an extensive evaluation method was used to rank the composites. The study concludes that the brake factor of stainless steel fiber-based friction composites produce stable performance under all conditions, with a low liner temperature rise of 340°C and low thermal stresses (4.255294 MPa). In contrast, friction composites based on steel fiber initially deliver high performance, which deteriorate after a certain period due to high corrosion levels and a high temperature rise of 361°C, resulting in negative fade (−0.84%) and high thermal stresses (5.619102 MPa). The primary plateaus, secondary plateaus, back transfer of drum wear debris, and distribution of constituents on the worn surface of the developed composites’ resin matrix were identified and studied using a scanning electron microscope (SEM) equipped with energy dispersive spectroscopy (EDS).

  19. Abstract

    To reveal nonlinear dynamic rules of low viscosity fluid-lubricated tilting-pad journal bearings (TPJBs), the effects of design parameters on journal center orbits and dynamic minimum film thicknesses of water-lubricated TPJBs with and without static loads are investigated. The hydrodynamic bearing force used in the nonlinear dynamic analysis is an approximate analytical solution including the turbulence effect. The results reveal the methods for vibration suppression and load capacity improvement and give an optimal pivot offset and clearance ratio that can maximize the minimum film thickness. The results also show that four-pad TPJBs with loads between pads are preferred due to good dynamic performance and load capacity. This study would provide some guidance for nonlinear design of low viscosity fluid-lubricated TPJBs under dynamic loads.

  20. Abstract

    Friction between two contacting surfaces is studied extensively. One of the known friction theories is Amontons’ law which states that the friction force is proportional to the normal force. However, Amontons’ law has been found to be invalid for elastomers. In the present study, the validity of Amontons’ law for short-cut aramid fiber reinforced elastomers is studied. Two types of fillers are used to reinforce the elastomers, namely highly dispersible silica and short-cut aramid fibers. Short-cut aramid fibers with two different surface treatments are used, namely non-reactive fibers with standard oily finish (SF-fibers) and fibers treated with an epoxy coating (EF-fibers). A pin-on-disc tribometer is used to investigate the frictional behavior of the composites in sliding contact with a granite counter surface. The results show that, after the run-in phase, Amontons’ law is valid for those composites that are reinforced by short-cut aramid fibers (without reinforcing filler, i.e., silica) if the contact pressure is below a threshold value. However, once the contact pressure exceeds this threshold value, Amontons’ law will be invalid. The threshold contact pressure of the composites containing EF-fibers is higher than of the composites containing SF-fibers. The composites that are reinforced by silica and short-cut aramid fibers do not follow Amontons’ law.