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Surface roughness is significant to friction and wear. However, the influence remains elusive, especially at the nanoscale. This study aimed to explore the effect of the nanoscale surface roughness on sliding friction and wear. For bearing steel tribo-pairs, the surface roughness can affect the lubrication state transition. The lower the surface roughness is, the easier it is to enter mixed lubrication and full film lubrication. Moreover, reducing the surface roughness can decrease friction and wear in mixed lubrication. At 94 mm/s sliding speed, as the surface roughness S q decreases from 769.7 nm to 21.1 nm, the average friction coefficient first remains unchanged and then decreases by 20%, while the wear rate decreases by 85%. However, further reducing the surface roughness may result in friction failure, raising friction and wear. At 10 mm sliding radius, about 20∼40 nm S q can lead to the minimal friction and wear. For the mechanism, as the surface roughness decreases, the asperity contact diminishes. Therefore, friction and wear decrease. However, when the surface roughness passes the critical point, asperities cannot fully accommodate the interfacial wear debris. Consequently, friction failure occurs. The findings may provide an enhanced understanding of the role of surface roughness in tribology. [Display omitted] • The surface roughness can affect the lubrication state transition. • Reducing the surface roughness can decrease friction and wear in mixed lubrication. • Too low surface roughness may result in friction failure, raising friction and wear. [ABSTRACT FROM AUTHOR]

Lubrication is often employed in fretting contacts to reduce wear and stresses associated with high friction. Owing to the very small displacements associated with fretting, penetration of lubricating oils into the contact may not be effective. The efficacy of the penetration of the lubricant into the contact is very difficult to observe experimentally, and accordingly, this paper presents a numerical simulation of a lubricated fretting contact using a Coupled–Eulerian–Lagrangian (CEL) finite element method. Meso-scale CEL finite element models are developed to simulate the cylinder-on-flat arrangement used experimentally at the University of Nottingham in which the roughness of contact surfaces is characterized as fractal geometry by the Weierstrass-Mandelbrot (W-M) function. The fluid–solid and solid–solid contact in the lubricated fretting contact are simulated, and from these, wear and friction coefficients are determined. The effects of contact geometry on lubricated fretting contacts and lubricant on fretting wear are modelled and compared with experimental observations. Results indicate that oil lubrication reduces fretting wear and friction effectively in the less-conforming contacts but has little effect in the more-conforming contacts. • A novel approach of modeling lubricated contacts using a CEL finite element method. • The efficacy of the penetration of lubricant into the fretting contacts is simulated. • The wear and friction coefficients in a lubricated fretting contact are determined. • The effects of contact geometry on lubricated fretting contacts are simulated. [ABSTRACT FROM AUTHOR]

Frictional vibration is a common but not entirely understood tribological phenomenon in water-lubricated rubber stern bearings. This paper proposes a transparent glass cylinder and rubber block model which reveals the frictional contact area under a high-speed camera. The local displacements of different grid points on the contact area can be measured by machine vision. A conjoint analysis method for friction coefficient, vibration, noise, and image information is used to determine the mechanisms of frictional vibration. The frequency of noise accompanying the vibrations can be exploited to divide frictional vibration phenomena in the bearing into "squeal" and "chatter" categories, each with different vibration modes. The frictional force and image information can be compared to determine the causes of frictional vibrations in different frequency bands. And a physical model which reflects frictional vibration effects on the stern tube bearing is created by images captured by high speed camera. The results presented here may provide a theoretical foundation for the manufacturing and optimization of water-lubricated rubber stern bearings. • A transparent glass cylinder was used to observe the contact area under high-speed camera. • A conjoint analysis method is used to determine the mechanisms of frictional vibration. • Frictional vibration phenomena was divided into "squeal" and "chatter" categories. • A physical model is created by images captured by high speed camera. [ABSTRACT FROM AUTHOR]

A 3D time-domain modelling approach is developed with the explicit finite element method to simulate transient meshing contact of gear pairs and the resulting wear. Taking a helical gear pair used in a locomotive as an example, the dynamic contact states of gear teeth are predicted for different speeds in consideration of actual geometries (including those of damaged teeth), time dependent torques, and structural and continuum vibrations of the gear system. The friction between mating teeth is taken into account by a non-Newtonian EHL friction model. A tiny time step of 4.5 × 10−8 s determined by conditional stability of the explicit time integration ensures the capture of transient effects. Based on detailed contact solutions during meshing, the dynamic wear on teeth is further calculated using an Archard wear model to predict tooth profile evolvement. Taking initial involute profiles and a speed of 120 km/h, the model is first validated for dynamic contact solutions. More results show that structural vibrations and worn profiles influence the dynamic meshing contact significantly, tooth wear increases as lubrication fails, and tooth pitting's influence is localized. This approach provides key load boundaries for detailed studies on mating gears, and may be employed for studies on tooth damages and as a reference for lumped parameter dynamic models of gear pairs. • A 3D time-domain approach developed for transient meshing contact of worn gears. • Normal and tangent contact solutions obtained in dynamics for teeth at any instant. • Tooth wear is calculated from transient contact solutions during gear meshing. • Influence of vibrations, worn profiles, pitting and lubrication failures identified. [ABSTRACT FROM AUTHOR]

The textured surface is one of the hotspots in the field of tribology research on reducing friction and wear. By calculating and optimizing the micro-texture structure, researchers designed and prepared textures with excellent tribological properties on the mechanical part surfaces. In this work, the tribological performance of 42CrMo matrix composite textured surfaces (CTSs) with different textured densities under grease lubrication was studied. The CTS was composed of 42CrMo alloys and SnAgCu-MXenes-Ti 3 C 2 (SACM). The results showed that when the textured density ρ of CTS was 36%, the friction coefficient (0.090) and wear rate (5.4 × 10−9 mm3N−1m−1) of counterpart ball could be respectively reduced by 75.7% and 38.6% compared with those of counterpart ball against the untextured sample. CTS could enhance the stability of friction system. Mxenes-Ti 3 C 2 wrapped by SnAgCu could form the lubricating layer containing sub-micron rollers on the worn surface, effectively reducing the friction and wear of samples under grease conditions. [Display omitted] • The composite textured surfaces (CTSs) were designed and prepared. • MXenes wrapped by SnAgCu could form sub-micrometer rollers in wear scar. • CTS exerted tribological enhancement function under starvation. • CTS could produce excellent synergistic effect with grease. [ABSTRACT FROM AUTHOR]

Nitrile butadiene rubber (NBR) is a water-lubricated bearing material that demonstrates excellent vibrational absorption and impact resistance; however, rapid wear under abnormal conditions is still a serious problem. Graphene (Gra) and carbon nanotubes (CNTs) are two types of carbon nanomaterials with different dimensions and excellent mechanical and friction properties. In this work, Gra and CNTs were used to fill the NBR, and a multifunctional friction and wear testing machine was used to study their impacts on the friction between NBR and ZCuSn10Zn2 under water lubrication conditions. The results indicate that both Gra and CNTs can effectively reduce the adhesion and the friction coefficient of NBR during the friction process. The two-dimensional lamellar structure of Gra is more likely to precipitate from the NBR matrix to generate a surface lubrication film, which can effectively reduce the friction coefficient. Both Gra and CNTs can prevent crack initiation and propagation in NBR during the wear process, thereby effectively reducing the wear of NBR. Compared with the two-dimensional structure of Gra, the one-dimensional linear structure of carbon nanotubes can "trap" the NBR molecular chains more effectively during the friction process, thus reducing the initiation and propagation of wear cracks and further improving the wear performance of NBR. • Effect of different dimensions of carbon nanomaterials on friction behavior of NBR. • Effect mechanism of two-dimensional graphene and one-dimensional carbon nanotubes on friction behavior of NBR. • NBR wear mechanism enhanced by two-dimensional graphene and one-dimensional carbon nanotubes. [ABSTRACT FROM AUTHOR]

Due to the operation in mixed and boundary friction, minimization of friction losses in automobile powertrains offers high potential for lowering of greenhouse gas emissions and saving fossil fuels. In cooperation between Surface Engineering Institute (IOT) and Gear Research Center (FZG), surface modifications in terms of stochastic and deterministic structures, diamond-like carbon (DLC) and nitride hard physical vapor deposition (PVD) coatings were tested in lubricated rolling-sliding contacts in twin-disk and gear efficiency test-rig. It was found that deterministic structured surfaces provide for reduction of friction losses by up to 7% under extreme boundary friction conditions at high Hertzian stresses and small relative lubricant film thicknesses. Application-oriented tests using DLC coated gear wheels in gear efficiency test-rig exhibited a reduction of friction losses between 10% and 21% compared to uncoated gear wheels. The DLC coating ZrC g developed at IOT provided for friction reduction of 15% for small circumferential speeds in boundary friction. Friction losses were reduced by up to 39% at higher circumferential speeds in spite of fully separated surfaces in the fluid friction regime. Due to their thermophysical properties, it is assumed that DLC coatings have an impact on viscosity and shear resistance of the lubricant. [ABSTRACT FROM AUTHOR]

Friction, wear and plastic deformation of copper (Cu) and α/β brass were studied under lubrication conditions. Hardness and microstructure of surface layers were evaluated. The friction properties were presented by Stribeck curves. The friction coefficient and wear of brass were significantly larger in comparison to Cu. The critical load of transition from the elasto-hydrodynamic lubrication (EHL) to boundary lubrication (BL) was two times lower for brass. Superplastic deformation of α-phase grains in brass is appeared under friction in BL, whereas plastic deformation localized at thin surface layers is observed for Cu. Intragranular sliding in α-phase is accompanied with the formation of shear bands and many pores coalesced to cracks. Thickness of the wear particles of brass is close to the width of shear bands. The deformation hardness of brass is significantly higher in comparison to Cu rubbed in the BL region. [ABSTRACT FROM AUTHOR]

Abstract: This paper describes a method of utilizing oil-based nanolubricants containing abrasive nanoparticles for in-situ nanopolishing (ISN) of surfaces of rolling elements. Nanopolishing reduces the composite surface roughness of bearing elements and results in an increased film thickness ratio. Using a four-ball tester, the test and material conditions such as normal load, rotational speed, and initial surface roughness of balls were selected so that the initial film thickness ratio λ varied in the 1–3 range, i.e. within the partial (mixed) elastohydrodynamic (EHL) regime. The lubricant was either engine oil or engine oil-based nanolubricants containing diamond nanoparticles. The rolling ring track on the top ball was examined by a non-contact profilometer. The root mean square (RMS) roughness of the wear track using the nanolubricant was decreased. In-situ nanopolishing by the nanolubricant resulted in a significant increase in the film thickness ratio and a lower frictional torque. [Copyright &y& Elsevier]

Two dimensional (2d) lubricant materials further explore their application in oil and gas field with the research direction of reducing friction and wear. In this study, we added multilayer graphene into water-based sand cleaning liquid to reduce friction and wear of the CT80 coiled tubing. The results showed that the addition of multilayer graphene significantly improved the anti-friction and wear resistance. Adding 0.5% multilayer graphene effectively reduced COF and wear by 75%. Additionally, the higher multilayer graphene concentration, the greater CT80 material withstood the load of severe wear. Multilayer graphene can be adsorbed on the wear debris or form a lubrication film on both worn surfaces, which facilitated the transportation of the debris. Moreover, the wear analysis indicated the main wear form of the CT80 material was abrasive wear and fatigue wear. • Multilayer graphene reduces friction and wear of CT80 coiled tubing in water-based sand cleaning liquid. • The higher multilayer graphene concentration, the greater CT80 material withstand the load. • Abrasive and fatigue wear are the main form of the CT80 material. • Multilayer graphene can be adsorbed on the debris and contact surfaces to form lubrication films. [ABSTRACT FROM AUTHOR]

This paper describes the evolution of friction-induced microstructures underneath the wear surfaces of nickel single crystals, and their role in influencing the steady state coefficient of friction. Using a custom built tribometer, friction measurements were made on {001} and {011} crystal faces along several crystallographic directions in unidirectional linear mode with a Si 3 N 4 ball. Cross-sections of wear scars were prepared by focused ion beam (FIB) microscopy, and electron backscattered diffraction (EBSD) and transmission electron microscopy (TEM) were used to analyze the evolution of crystallographic texture and recrystallization. The formation of a thin (10–15 nm) ultra-nanocrystalline layer in the {011}<211> orientation resulted in significant reductions in the coefficient of friction for that configuration, from the initial maximum of 0.64 to a steady state value of 0.32, after approximately 500 cycles of linear, unidirectional sliding. Crystal plasticity theory was used to describe the evolution of dislocation content in the worn material, which in turn provides an estimate of the characteristic grain size as a function of the imposed strain. An analysis of grain boundary sliding in ultra-fine-grained material suggests a mechanism for friction reduction. Model predictions of the contribution of grain boundary sliding (relative to plastic deformation) to friction provide a phenomenological description of the lubrication due to ultra-nanocrystalline surface microstructures. • A novel combination of electron microscopy and tribotesting techniques were utilized. • Friction measurements on single crystal surfaces along specific crystallographic directions was demonstrated. • Subsurface characterization provided insights into the relationships between microstructure and friction. • A model was proposed to help understand the relationships between plasicity, microstructure, and friction. • The model predictions agree well with the available observations and measurements. [ABSTRACT FROM AUTHOR]

Experimental and numerical analyses were conducted to explore the influence of DLC/TiAlN-coated die surfaces in sheet-metal forming under dry and oil-lubricated conditions. In this study, ironing and deep-drawing experiments were performed to determine the potential of the DLC/TiAlN coating in the sheet forming of stainless steels under different tribological conditions. The performance and physical properties of the DLC/TiAlN-coated die surface were obtained through load, surface roughness, and wear measurements as well as hardness and microstructure examination. The experimental results indicated that the DLC/TiAlN coating strongly resists galling under dry friction and thin film lubrication conditions that reduces the friction and forming load. The presence of a thin oil film reduces the sliding-originated surface tensile stresses of the DLC/TiAlN coating, improving the wear resistance of the die surface even at high temperatures and high contact pressures. Thermomechanical numerical analysis supported the experimental results, which confirmed that the lubricant discharged the heat generated in the die–workpiece contact region to reduce the friction and forming load. With the DLC/TiAlN coating, the plain mineral oil with no extreme pressure additives can function as effective as chlorinated paraffin oil for protecting the die surface, thus extending the die service life. Image 1 • Ironing and deep drawing tests performed under different tribological conditions. • DLC/TiAlN coating resists galling under dry friction and thin film lubrication. • DLC/TiAlN utilized more environment-friendly oils than chlorinated paraffin oil. • TiAlN interlayer coating enhanced DLC performance. [ABSTRACT FROM AUTHOR]