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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]

Determination of wear rate in spur gears requires treatment of the mixed elastohydrodynamic lubrication problem along the gear's line of action (LoA). For this purpose, a procedure is developed to predict the film thickness, traction coefficient and the wear rate with provision for the interactions between the surface asperities of the gear's teeth at each point on LoA. The results reveal that the surface finish has a more pronounced influence on the wear rate compared to the traction coefficient. The wear results are also compared to those obtained by an engineering approach that utilizes an estimate of the contact flash temperature with reasonable accuracy. Also investigated is the effect of starvation on the wear rate for applications where the inlet is not fully flooded. The results show that wear rate can be enormously large depending on the degree of starvation. [ABSTRACT FROM AUTHOR]

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]

Pitch bearings in wind turbines are affected by reciprocating motion due to pitch control. The combination of oscillating operation, high loads, and mixed lubrication often leads to wear. Grease lubricants in wind turbine pitch bearings should be designed to avoid such wear. Due to different available grease lubricants, the anti-wear properties are investigated under downscaled wind turbine pitch bearing conditions. The downscaling is accomplished by load simulation for a pitch bearing and analysis of the pitch movements for a 7.5 MW reference turbine. The operational conditions of the four point contact ball bearing with 5 m outer diameter are scaled to angular contact ball bearings of the size 7208. The investigations are concluded with contact model experiments. Six Industrial grease lubricants for wind turbine pitch bearings are tested that follow very different compositions. The investigations reveal, that none of the tested grease lubricants was able to reduce wear for all tested conditions. After 250 000 cycles certain conditions lead to severe wear. Therefore, the pitch controller should avoid such conditions if possible. Nevertheless, grease lubricants with low base oil viscosities and high bleeding rates are best on average for the tested conditions. Furthermore, the results of the bearing experiments are comparable to the model experiments. • Operating behaviour considered of pitch bearings under individual pitch control. • Method for scaling the operating behaviour to test rig conditions. • Experimental investigations on six commercial greases used in pitch bearings. • None of the greases was able to show satisfactory wear protection for all conditions. • Greases with low base oil viscosities and high bleeding rates showed the least wear. [ABSTRACT FROM AUTHOR]