Friction

  1. Abstract

    In this study, piezoelectric elements were added to a reciprocating friction test bench to harvest friction-induced vibration energy. Parameters such as vibration acceleration, noise, and voltage signals of the system were measured and analyzed. The results show that the piezoelectric elements can not only collect vibration energy but also suppress friction-induced vibration noise (FIVN). Additionally, the wear of the friction interface was examined via optical microscopy (OM), scanning electron microscopy (SEM), and white-light interferometry (WLI). The results show that the surface wear state improved because of the reduction of FIVN. In order to analyze the experimental results in detail and explain them reasonably, the experimental phenomena were simulated numerically. Moreover, a simplified two-degree-of-freedom numerical model including the original system and the piezoelectric system was established to qualitatively describe the effects, dynamics, and tribological behaviors of the added piezoelectric elements to the original system.

  2. Abstract

    Using nanoadditives in lubricants is one of the most effective ways to control friction and wear, which is of great significance for energy conservation, emission reduction, and environmental protection. With the scientific and technological development, great advances have been made in nanolubricant additives in the scientific research and industrial applications. This review summarizes the categories of nanolubricant additives and illustrates the tribological properties of these additives. Based on the component elements of nanomaterials, nanolubricant additives can be divided into three types: nanometal-based, nanocarbon-based, and nanocomposite-based additives. The dispersion stabilities of additives in lubricants are also discussed in the review systematically. Various affecting factors and effective dispersion methods have been investigated in detail. Moreover, the review summarizes the lubrication mechanisms of nanolubricant additives including tribofilm formation, micro-bearing effect, self-repair performance, and synergistic effect. In addition, the challenges and prospects of nanolubricant additives are proposed, which guides the design and synthesis of novel additives with significant lubrication and antiwear properties in the future.

  3. Abstract

    sp2 nanocrystallited carbon films with large nanocrystallite sizes, smooth surfaces, and relative high hardness were prepared with different ion irradiation densities regulated with the substrate magnetic coil current in an electron cyclotron resonance plasma sputtering system. Their multiscale frictional behaviors were investigated with macro pin-on-disk tribo-tests and micro nanoscratch tests. The results revealed that, at an ion irradiation density of 16 mA/cm2, sp2 nanocrystallited carbon film exhibits the lowest friction coefficient and good wear resistant properties at both the macroscale and microscale. The film sliding against a Si3N4 ball under a contact pressure of 0.57 GPa exhibited a low friction coefficient of 0.09 and a long wear life at the macroscale. Furthermore, the film sliding against a diamond tip under a contact pressure of 4.9 GPa exhibited a stable low friction coefficient of 0.08 with a shallow scratch depth at the microscale. It is suggested that sp2 nanocrystallites affect the frictional behaviors in the cases described differently. At the macroscale, the contact interface via the small real contact area and the sp2 nanocrystallited transfer layer dominated the frictional behavior, while the sp2 nanocrystallited structure in the film with low shear strength and high plastic resistivity, as well as the smooth surface morphology, decided the steady low nanoscratch properties at the microscale. These findings expand multiscale tribological applications of sp2 nanocrystallited carbon films.

  4. Abstract

    With the rapid development of industry, the inconsistency between the rapid increase in energy consumption and the shortage of resources is becoming significant. Friction is one of the main causes of energy consumption; thus, the emergence of superlubricity technology can substantially improve the energy efficiency in motion systems. In this study, an efficient method to control superlubricity at the atomic-scale is proposed. The method employs vibrational excitation, which is called vibration induced superlubricity (VIS). The VIS can be easily and steadily achieved by employing external vibration in three directions. The simple method does not depend on the type of sample and conductivity. Dependence of oscillation amplitude, frequency, scanning speed, and normal force (FN)on friction were investigated. Experimental and simulated explorations verified the practical approach for reducing energy dissipation and achieving superlubricity at the atomic-scale.

  5. Abstract

    The high specific-strength of glass fibers and exceptional self-lubrication of polytetrafluoroethylene (PTFE) fibers promote the potential application of hybrid PTFE/glass fabric composites in the tribological field, but their weak interfacial adhesion and inferior thermal properties significantly inhibit their tribological performance and reliability. Herein, a hybrid of polydopamine/silicon carbide/polyethyleneimine (PDA/SiC/PEI) functional coating was co-deposited onto the hybrid PTFE/glass fabric surface through a one-step impregnation method, leading to increased surface roughness and abundant amine groups. Tensile and peeling tests showed that this functional coating offered 47.8% enhancement in the fabric/matrix interfacial adhesion without compromising the strength of the pristine fabric. Moreover, the additional incorporation of WS2, and aluminum nitride (AlN) micro-fillers contributed to the development of a high-quality tribofilm and improved the thermal properties of fabric composites. The results of wear tests proved that the hybrid-fabric composites, after the introduction of functional coating and micro-fillers, exhibited outstanding tribological performance, which was attributed to the superior interfacial adhesion as well as the synergistic enhancement effects between WS2 and AlN micro-fillers.

  6. Abstract

    The motion of droplets on a super-hydrophobic surface, whether by sliding or rolling, is a hot research topic. It affects the performance of super-hydrophobic materials in many industrial applications. In this study, a super-hydrophobic surface with a varied roughness is prepared by chemical-etching. The adhesive force of the advancing and receding contact angles for a droplet on a super-hydrophobic surface is characterized. The adhesive force increases with a decreased contact angle, and the minimum value is 0.0169 mN when the contact angle is 151.47°. At the same time, the motion of a droplet on the super-hydrophobic surface is investigated by using a high-speed camera and fluid software. The results show that the droplet rolls instead of sliding and the angular acceleration increases with an increased contact angle. The maximum value of the angular acceleration is 1,203.19 rad/s2 and this occurs when the contact angle is 151.47°. The relationship between the etching time, roughness, angular acceleration, and the adhesion force of the forward and backward contact angle are discussed.

  7. Abstract

    Thickener formulation plays a significant role in the performance characteristics of grease. The polyurea greases (PUGs) were synthesized using mineral oil (500SN) as the base oil, and by regulating the reaction of diphenylmethane diisocyanate (MDI) and different organic amines. The as-prepared PUGs from the reaction of MDI and cyclohexylamine/p-toluidine exhibit the optimum physicochemical and friction-wear properties, confirming that the regulation of thickener formulation can improve the performance characteristics of grease, including friction reduction, wear, corrosion resistance, and load-carrying capacity. The anti-corrosion and lubrication properties of as-prepared PUGs depend on good sealing functions and a boundary lubrication film (synergy of grease-film and tribo-chemical reaction film), as well as their chemical components and structure.

  8. Abstract

    The tribological characteristics of cotton fibers play an important role in engineering and materials science, and real contact behavior is a significant aspect in the friction behavior of cotton fibers. In this study, the tribological characteristics of cotton fibers and their relationship with the real contact behavior are investigated through reciprocating linear tribotesting and real contact analysis. Results show that the friction coefficient decreases with a general increase in load or velocity, and the load and velocity exhibit a co-influence on the friction coefficient. The dynamic change in the real contact area is recorded clearly during the experiments and corresponds to the fluctuations observed in the friction coefficient. Moreover, the friction coefficient is positively correlated with the real contact area based on a quantitative analysis of the evolution of friction behavior and the real contact area at different loads and velocities. This correlation is evident at low velocities and medium load.

  9. Abstract

    The layered double hydroxide (LDH) is a kind of natural mineral, which can also be manually prepared. It has been practically applied in various fields due to its unique crystal structure and diversity of composition, size, and morphology. In this work, LDHs with different chemical compositions (Co2+, Mg2+, Zn2+, and Ni2+) and topographical features (flower-like, spherical, and plate-like) were successfully prepared by controlling the reaction conditions. Then, they were mechanically dispersed into base grease and their tribological properties were evaluated by a ball-on-disk tester under a contact pressure of 2.47 GPa. It was found that the variation of morphology, instead of chemical composition, had great influence on the tribological performance. The “flower-like” LDH sample with high specific surface area (139 m2/g) was demonstrated to show the best performance. With 1 wt% additive, the wear volume was only about 0.2% of that lubricated by base grease. The tribofilm with unique microscopic structure and uniform composition was derived from tribochemical reaction between LDH additives and sliding solid surfaces, effectively improving tribological properties of the lubrication system. This work provided the guidance for optimizing lubricant additives and held great potential in future applications.

  10. Abstract

    Tribocorrosion denotes an irreversible material degradation for several metallic components used in corrosive environments, and it arises from the interplay between chemical, mechanical, and electrochemical processes. In this study, some investigation has been performed to compare the tribocorrosion behavior of AISI 1045 steel and AISI 2205 duplex stainless steel sliding against an alumina pin in seawater. The lowering in the open circuit potential (OCP) of AISI 2205 during the tribocorrosion demonstrates that its protective passive film was damaged by wear and resulted in a wear-accelerated corrosion in the wear track. However, sliding was found to accelerate the corrosion of the unworn areas for AISI 1045, leading to an anodic shift of the OCP. Moreover, the total material loss increased with an increase in the applied potential for both materials. It was revealed that AISI 1045 was more sensitive to corrosion under sliding than AISI 2205. Therefore, pure corrosion loss and corrosion-induced wear constituted the primary reasons for the degradation of AISI 1045 at applied anodic potentials.

  11. Abstract

    The dry sliding behavior of three commercial friction materials (codenamed FM1, FM2, and FM3) tested against a Co-free cermet coating produced by high-velocity oxy-fuel (HVOF) on gray cast-iron discs is investigated. FM1 is a conventional low-metallic friction material, FM2 is developed for using against HVOF-coated discs, and FM3 is a Cu-free friction material with a low content of abrasives and a relatively high concentration of steel fibers. For the tribological evaluation, they are tested on a pin-on-disc (PoD) test rig against Co-free HVOF-coated discs, with particular attention to the running-in stage, which is fundamental for the establishment of a friction layer between the two mating surfaces, i.e., the pin and disc. The PoD tests are performed at room temperature (RT) and a high temperature (HT) of 300 °C. At RT, all materials exhibit a long running-in stage. At HT, no running-in is observed in FM1 and FM2, whereas a shorter running-in period, with respect to the RT case, is observed in FM3 followed by the attainment of a comparatively high coefficient of friction. At RT, the pin wear is mild in all cases but severe at HT. FM3 shows the lowest wear rate at both temperatures. Moreover, the coated disc shows no wear when sliding against the FM3 friction material. All the results are interpreted considering the microstructural characteristics of the friction layers formed on the sliding surfaces. The findings of the present study provide insights into reducing wear in braking system components and hence reducing environmental particulate matter emissions from their wear, through the use of disc coatings.

  12. Abstract

    Herein, a series of Ag coatings with different micro-dimples were fabricated on copper surfaces by laser surface texturing (LST) and magnetron sputtering. Multilayer graphene lubricating grease (MGLG) was prepared using multilayer graphene as an additive. The textured Ag coatings and MGLG were characterized. Moreover, the tribological and electrical performances of the textured Ag coatings under MGLG lubrication were investigated in detail. Results demonstrated that the textured Ag coating with an appropriate dimple diameter could exhibit improved tribological and electrical properties when compared to the non-textured Ag coating under MGLG lubrication. The characterization and analysis of the worn surfaces suggest that the synergetic effect of LST and MGLG contributes to these excellent tribological and electrical properties.

  13. Abstract

    The effects of impacting particles from a jet of liquid on the removal of a surface material (on the impacted workpiece) were investigated. Experimental observations show that the cross section of the material removed changed from ‘W’-shaped to ‘U’-shaped as the size of abrasive particles was increased. Comparisons between removed material profiles and particle collision distributions indicate that the particle-surface collisions are the main reason for the material removal. The deduced number of atoms removed by a single collision implies that a transition occurs in the removal mode. For nanoscale particles, the polished surface is likely to be removed in an atom-by-atom manner, possibly due to the chemisorption of the impacting particles on the impacted surface. Contrarily, for the case of microscale particles, bulk material removal produced by particle bombardment is more likely to occur. The present mechanism of material removal for particle-surface collisions is further corroborated experimentally.

  14. Abstract

    The tribological behaviors of Ti-Ni51.5 at% alloy strengthened by finely dispersed Ni4Ti3 particles in reciprocating sliding against GCr15, Al2O3, and ZrO2 at room temperature were studied. Interestingly, the coefficient of friction (COF) suffered a sheer drop (from 0.9 to 0.2) when the aged alloy slid against GCr15 at a frequency of 20 Hz under a 20 N load without lubrication. However, severe-mild wear transition disappeared when a solutionized alloy was used. Moreover, the COF stabilized at a relatively high level when Al2O3 and ZrO2 were used as counterparts, although their wear mechanisms showed signs of oxidation. Scanning electron microscopy (SEM) and X-ray element mappings of the wear scars of the counterparts clearly indicate that the formation of well-distributed tribo-layer and material transfer between the ball and disk are pivotal to the severe-to-mild wear transition in the aged Ti-Ni51.5 at% alloy/GCr15 friction pair. The higher microhardness and superelasticity of the aged alloy significantly accelerate the material transfer from GCr15 to the disk, forming a glazed protective tribo-layer containing Fe-rich oxides.

  15. Abstract

    The movement pattern of ellipsoidal nanoparticles confined between copper surfaces was examined using a theoretical model and molecular dynamics simulation. Initially, we developed a theoretical model of movement patterns for hard ellipsoidal nanoparticles. Subsequently, the simulation indicated that there are critical values for increasing the axial ratio, driving velocity of the contact surface, and lowering normal loads (i.e., 0.83, 15 m/s, and 100 nN under the respective conditions), which in turn change the movement pattern of nanoparticles from sliding to rolling. Based on the comparison between the ratio of arm of force (e/h) and coefficient of friction (μ) the theoretical model was in good agreement with the simulations and accurately predicted the movement pattern of ellipsoidal nanoparticles. The sliding of the ellipsoidal nanoparticles led to severe surface damage. However, rolling separated the contact surfaces and thereby reduced friction and wear.

  16. Abstract

    The effect of grain boundary (GB) defects on the tribological properties of MoS2 has been investigated by molecular dynamics (MD) simulations. The GB defects-containing MoS2 during scratching process shows a lower critical breaking load than that of indentation process, owing to the combined effect of pushing and interlocking actions between the tip and MoS2 atoms. The wear resistance of MoS2 with GB defects is relevant to the misorientation angle due to the accumulation of long Mo-S bonds around the GBs. Weakening the adhesion strength between the MoS2 and substrate is an efficient way to improve the wear resistance of MoS2 with low-angle GBs.

  17. Abstract

    To enhance the interface bonding of polyimide (PI)/carbon fiber (CF) composites, CFs were functionalized by introducing a polydopamine (PDA) transition layer, whose active groups provide absorption sites for the growth of molybdenum disulfide (MoS2) nanosheets and improve the bonding strength with PI. Uniform and dense MoS2 nanosheets with thicknesses of 30–40 nm on the surface of the PDA@CF were obtained via a subsequent hydrothermal method. As a result, the interface between the CF and the PI matrix becomes more compact with the help of the PDA transition layer and MoS2 nanosheets. This is beneficial in forming PI/CF-MoS2 composites with better thermal stability, higher tensile strength, and enhanced tribological properties. The lubricating and reinforcing effects of the hybrid CF-MoS2 in the PI composite are discussed in detail. The tensile strength of the PI/CF-MoS2 composite increases by 43%, and the friction coefficient and the wear rate reduce by 57% and 77%, respectively, compared to those of the pure PI. These values are higher than those of the PI/CF composites without MoS2 nanosheets. These results indicate that the CF-MoS2 hybrid material can be used as an additive to improve the mechanical and tribological properties of polymers.

  18. Abstract

    The aim of this study is to fabricate the nanocomposite with low friction and high wear resistance using binary solid lubricant particles. The microstructure and tribological performance of the nanocomposite are evaluated, and the composition and film thickness of the lubricating film are observed and analyzed by scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The nanocomposite exhibited improved tribological properties with a friction coefficient as low as 0.12 and a low wear rate of 2.17 × 10−6 mm3/(N·m) in high-purity nitrogen atmosphere. Decreasing sliding speed can increase lubricating film thickness, and the thickest lubricating film is approximately 125 nm. As the film thickness of the lubricating film exceeded 90 nm, the friction coefficient curves became smooth. In compared with WS2, MoS2 can be more effective in forming the transfer layer on the worn surfaces at the initial stage of the tribological process.

  19. Abstract

    Abrasives, such as oxides of alumina (Al), silica (Si), zirconia (Zr), chromium (Cr) etc., are added to raise the friction level and also to remove the glaze on the disc so that surface will be rejuvenated continuously during braking and will contribute to maintain the desired friction level. However, these inorganic particles have less adhesion with the resin/binder and hence are easily dug out during wearing process contributing to higher wear. If efforts are made to enhance the filler-matrix adhesion, not only the wear of friction material (FM) should reduce, the particles may stay for a longer time on the tribo-surface of the pads to contribute fully towards controlling the coefficient of friction (μ). In the present study, alumina particles were selected for siloxane treatment to improve the filler-matrix adhesion. Two types of eco-friendly (free from asbestos and Cu) brake-pads were developed using alumina as a theme ingredient (treated and untreated) keeping all the parent formulation identical. An additional type of brake-pads without alumina particles was also developed to observe the effect of abrasive particles on the tribo-performance. The performance properties (physical, mechanical, and tribological) of brake-pads were compared when evaluated in identical conditions. The tribo-testing was done on full-scale brake inertia dynamometer following the procedure in Japanese automobile standard (JASO C 406). It was observed that siloxane treatment affected both friction and wear of brake-pads in a beneficial way. Wear resistance got increased 35% for siloxane treated pads. Worn surfaces were analysed using scanning electron microscopy (SEM) and energy dispersive X-ray (EDAX) technique.

  20. Abstract

    Alloys used as bearings in aircraft landing gear are required to reduce friction and wear as well as improve the load-carrying capability due to the increased aircraft weights. Cu-15Ni-8Sn-0.8Nb alloy is well known for possessing good mechanical and wear properties that satisfy such requirements. In this study, the microstructure, mechanical properties, and grease-lubricated sliding wear behavior of Cu-15Ni-8Sn-0.8Nb alloy with 0.8 wt% Nb are investigated. The nanoscale NbNi3 and NbNi2Sn compounds can strengthen the alloy through the Orowan strengthening mechanism. A Stribeck-like curve is plotted to illustrate the relationship among friction coefficient, normal load, and sliding velocity and to analyze the grease-lubricated mechanism. The wear rate increases with normal load and decreases with sliding velocity, except at 2.58 m/s. A wear mechanism map has been developed to exhibit the dominant wear mechanisms under various friction conditions. When the normal load is 700 N and the sliding velocity is 2.58 m/s, a chemical reaction between the lubricating grease and friction pairs occurs, resulting in the failure of lubricating grease and an increase in wear.