Tribology Letters最新文献

Recent studies suggest that the joints of beetles and other insects comprise micro-structured surfaces in combination with lubricants. Here, we utilize friction force microscopy (FFM) to analyse the tribological properties of the femoro-tibial leg joints by the example of Coelorrhina aurata (metallic green flower beetle) and Otiorhynchus sulcatus (black vine weevil). To preserve the original state of the lubricant as well as the microstructures, the FFM measurements were conducted in silicone oil, which satisfies our requirements of transparency, customizable viscosity, absent health risks and lower density compared to the expected density of the lubricant. Microscopic friction was measured on fresh and air-dried samples to stress the change of the lubricant properties over time. Despite the similarity of the two beetle joints, the FFM measurements reveal different frictional properties of the respective lubricants.

In sediment environments, water-lubricated rubber bearings are inevitably subjected to particle abrasion, especially during frictional vibration. However, the invasion-migration-wear mechanism of hard particles under frictional vibration excitation remains unclear. This study analyzes the contact strain at the friction interface and the dynamic response of the friction system by constructing a visualized friction pair at the interface and employing digital image processing technology. The results reveal that the friction-induced vibration in the water-lubricated rubber bearing-rotor system primarily manifests as chatter and squeal. Chatter represents a more intense stick–slip behavior, during which larger sediment particles are allowed to invade. These invading particles tend to sink deeper into the friction pair during the stick phase and migrate with the water flow during the slip phase, leading to combined wear in the form of scratches and pits. During squeal, the amplitude of stick–slip behavior is relatively small, allowing only small sediment particles to invade, which result in scratches on the bearing surface. When the system does not experience friction vibration, sediment particles are unlikely to enter the friction interface, even in a sediment-rich environment, and therefore, no significant wear occurs.

Graphical abstract

This study investigates a method combining a porous bush and herringbone groove to extend the effective working speed range of conventional fluid film journal bearings for machine tool spindles. A novel bearing, termed the porous herringbone groove journal bearing (PHGJB), is proposed. However, existing literature lacks a model that accurately describes the lubrication behavior of the PHGJB. To address this gap, this paper presents a lubrication model for the PHGJB that incorporates velocity slip, angular misalignment, and turbulence effects. This model employs the boundary-fitted coordinate system and finite control volume methods to address the challenges posed by the herringbone grooves and three-dimensional flow within the porous bush. The performances of the PHGJB, a typical hydrodynamic herringbone groove journal bearing (HGJB), and a hydrostatic porous plain journal bearing (PPJB) are compared. Differences between the calculated results from the one- and three-dimensional flow models for the flow within the porous bush are also analyzed. Results show that the PHGJB significantly improves stiffness at low speeds, while enhancing stability and controlling temperature rise at high speeds compared to the HGJB and PPJB. Consequently, it offers a broader operating speed range. The proposed model offers an effective tool for structural design and performance analysis of PHGJBs.

The Weierstrass-Mandelbrot (W-M) function method is commonly used in surface reconstruction, which integrates parameters such as fractal dimension (D) and fractal roughness (G). The parameter G is typically calculated using the power spectral density (PSD) method. This study identifies deviations in the G calculations. These deviations arise from simplifications made during the derivation process when applying the PSD method. To address this issue, this paper introduces a novel approach based on statistical methods to define the scale interval. This approach involves calculating the G post-alignment of the generated target profile within this interval, followed by surface generation using the W-M function. A comparison between the scale interval method (SI) and the PSD method reveals that the SI method demonstrates better accuracy. A comparative analysis between the generated and actual surfaces substantiates the accuracy of the G calculations using the SI method. This method provides a theoretical foundation for subsequent fractal surface measurements, reconstructions, contact calculations, and dynamic characterizations.

The development of green and environmentally friendly lubricant additives is crucial in the field of friction. A green modifier, Cardanol polyoxyethylene ether xanthate (CPEOCS₂K), was synthesized from natural cashew nut shells derived from Cardanol. Cu nanoparticles were prepared using a simple one-step reduction method, which involved modifying metallic Cu with CPEOCS₂K. The long chains of the modifier on the surface of the nanocore interconnected to form a nano-network, and the resulting Cu nanochains exhibited excellent lubrication performance when applied to a water-ethylene glycol (EG) system as a lubricant additive. This encapsulated nanoparticle structure not only enhances the dispersion stability of the nanoparticles but also generates a chemically reactive film that adsorbs onto the wear surface during friction, thereby improving the lubrication effectiveness of the water-based lubricant. The use of natural organic compounds to synthesize modifiers enhances the environmental friendliness of metal nanoparticles as lubricant additives, and the outstanding lubricating performance demonstrates the potential of green lubrication technology.

Graphical Abstract

Physical vapor deposited (PVD) molybdenum disulfide (MoS₂) solid lubricant coatings are an exemplar material system for machine learning methods due to small changes in process variables often causing large variations in microstructure and mechanical/tribological properties. In this work, a gradient boosted regression tree machine learning method is applied to an existing experimental data set containing process, microstructure, and property information to create deeper insights into the process-structure–property relationships for molybdenum disulfide (MoS₂) solid lubricant coatings. The optimized and cross-validated models show good predictive capabilities for density, reduced modulus, hardness, wear rate, and initial coefficients of friction. The contribution of individual deposition variables (i.e., argon pressure, deposition power, target conditioning) on coating properties is highlighted through feature importance. The process-property relationships established herein show linear and non-linear relationships and highlight the influence of uncontrolled deposition variables (i.e., target conditioning) on the tribological performance.

This study investigates the reciprocating motion of a rigid Hertzian indenter on a viscoelastic substrate with adhesion, using a finite element-based numerical model. An innovative methodology is employed to transform the sliding contact problem into an equivalent normal contact problem, enabling the accurate simulation of adhesion effects at the contact interface. The results reveal that system behaviour is governed by the interplay between viscoelasticity and adhesion, leading to notable changes in contact pressure distribution, contact area, and energy dissipation during reciprocating motion. Specifically, viscous dissipation within the substrate material dominates at intermediate sliding speeds, where the interaction between adhesion and viscoelastic relaxation processes results in pronounced hysteresis cycles. In contrast, at low and high sliding speeds (corresponding to the rubbery and glassy regions, respectively), the material behaviour is predominantly elastic, and no hysteresis is observed. Adhesion influences contact pressure distribution and contact size, particularly in the transition regime, where its effects on viscous dissipation are measurable. Moreover, the study clarifies that adhesion alone does not induce hysteresis in elastic regimes, distinguishing reciprocating contact from normal contact, where adhesive hysteresis is typically observed. New insights are also provided into how adhesion and viscoelasticity jointly impact tribological performance, offering a deeper understanding of energy dissipation mechanisms and contact mechanics during motion reversal. Interestingly, our results also show that there is a lag period after motion reversal, where friction aligns with motion direction before eventually changing direction as pressure redistribution occurs within the system. This phenomenon highlights how changes in contact mechanics affect local tribological interactions and can lead to variations in overall system response.

Graphical abstract

The application study of sliding bearings in planetary gearboxes of wind turbines has drawn much attention in recent years. However, the heavy-load and low-velocity working conditions will lead to serious wear of bearings that will affect service performance and life. CuSn12Ni2, a competitive material for such situations, is evaluated to reveal the time-varying wear behavior in this paper on a typical ball-on-disk testing rig. Furthermore, a wear model was proposed by the dimensional analysis theory. The results show that CuSn12Ni2 represents a lower friction coefficient and wear rate during the varying test duration under 90 ℃ than at room temperature, and 10 to 30 min can be considered to be the transition zone, marking the shift from a severe running-in stage to a relatively stable, rapid running-in stage. The wear surface of CuSn12Ni2 is dominated by abrasive wear and adhesive wear, accompanied by fatigue wear and tribochemical reaction at high temperature. However, the evolution model of the wear width and depth changing with wear duration is given in explicit form, and the adjusted R² is no less than 0.99. These time-varying wear data and models are meaningful for constructing the time-varying life evaluation model of sliding planet gear bearings.

Graphical abstract

The strains in α-Fe (bainite) and retained austenite (RA) exhibit an apparent discrepancy during deformation, namely strain partitioning, which can determine the tensile behavior of bainitic steels. Due to the involvement of plastic deformation and strain accumulation in the wear process, the strain partitioning phenomenon must be considered in the study of wear mechanisms. The impact wear tests results indicated that the wear behavior was strongly affected by strain partitioning between α-Fe and RA, and the initial hardness and work-hardening rate were not the main factors determining the wear resistance. The strain partitioning is closely related to the RA morphology tailored by isothermal processes, which was proved by in situ tensile test. The strain discrepancy between the continuous thin-film RA and α-Fe was shown to be less significant. A smaller strain discrepancy alleviated stress concentration and minimized the occurrence of cracks and material spalling during wear. The strain discrepancy between blocky RA and α-Fe was shown to be greater. The high-strain in blocky RA promoted the strain-induced transformation (SIT) effect and increased the work-hardening rate; however, it led to strain concentration on the worn surface and accelerated surface spalling.

The characterization of wear response in crystalline materials poses some challenges due to the presence of the size effect at small scales. In this study, we systematically conducted spherical nano-scratch simulations on (101)-oriented copper, using the mechanism-based strain gradient crystal plasticity theory, to explore the indenter size effect in the scratch hardness. The developed nano-scratch models are validated experimentally by comparing scratch depths and topographies. By examining the results obtained from conventional crystal plasticity and mechanism-based strain gradient crystal plasticity simulations, an indenter size effect in scratch hardness was identified. Furthermore, the mechanism of the indenter size effect in scratch hardness was quantitatively analyzed, by discussing the proportion of geometrically necessary dislocation lengths in the cumulative increments of dislocations.