Tribology Letters最新文献

How the shape of asperities affects the wear behavior is still an open question in contact mechanics. This work developed adhesive wear models including the deformable spherical and sinusoidal asperities in contact with a rigid flat by introducing the ductile failure criterion. For spherical asperity, fractures propagating at the corner and then forming spherical-like wear particles are observed under high normal loads, indicating a new Archard-like wear mode. For sinusoidal asperity, it is almost flattened under high normal loads, yielding lamellar wear particles; the wear rate rises first and then decreases as the normal load increases, and both the peak wear rate and the corresponding normal load greatly depend on its shape.

Graphical Abstract

Analyzing the effect of pressures and normal and shear stresses on chemical reaction rates, especially those occurring in a contact, remains the subject of controversy in the scientific community. This review article aims to clarify the principles that underpin the calculation of reaction rates based on transition-state theory (TST) and of activation volumes using a perturbation method of TST proposed by Evans and Polanyi. The goal is to aid researchers to calculate such tribochemical and mechanochemical parameters. In this paper, the fundamental ideas behind the calculation of reaction rates in chemistry are outlined. This article describes how TST is used to account for the large numbers of molecules involved in chemical reactions. The effects of individual stresses, and combination of normal and shear stresses on tribochemical reaction rates can be understood using a thermodynamics analysis. These concepts are illustrated by two examples of normal-stress-modified processes from results in the literature: homogeneous-phase Diels–Alder reactions and the surface decomposition of adsorbed methyl thiolate species on copper. The paper then reviews how to analyze tribochemical processes, which depend on coupled normal and shear stresses, showing that the effective activation volume is composed of multiple elementary-process activation volumes. Compensation effects, in which the pre-exponential factors and the activation volumes are correlated have been found for tribochemical reactions and this arises naturally from the Evans–Polanyi analysis. Finally, the activation volumes themselves depend on the applied stresses due to molecular distortions and a method for gauging the magnitude of these effects is described.

Graphical abstract

Water-soluble polyalkylene glycols have become a growing subject of research to achieve liquid superlubricity in elastohydrodynamically lubricated contacts. While the influence of various factors, including water content and viscosity, has been extensively studied, the underlying mechanisms responsible for liquid superlubricity under elastohydrodynamic lubrication remain poorly understood. In this study, aqueous polyethylene glycols of varying average chain length with the same viscosity but different water content or average chain length distribution are examined in relation to elastohydrodynamic friction and film thickness. The results indicate, that under fluid film lubrication, the low pressure–viscosity coefficient is the primary factor leading to liquid superlubricity. No running-in period is required, allowing for stable and persistent ultra-low friction level immediately. Additionally, at a constant water content, an optimum average chain length distribution was identified, resulting in reduced friction while the film thickness remained largely unaffected. This enables the targeted design of aqueous lubricants based on polyethylene glycol.

Graphical abstract

Aminoguanidine-based ionic liquid which can reduce the friction coefficient and wear depth has been developed as lubricant additives for the piston ring and cylinder liner within 80–240 °C. The friction coefficient and wear depth reduction become more significant as the temperature increases. At 240 °C, the coefficient of friction decreases by 23.1% compared to fully formulated mineral based engine oil (FFO). The wear depth of the piston ring and cylinder liner decreases by 9.1% and 16.0%, respectively. Addition of aminoguanidine-based ionic liquid (AO-IL) promotes the tribo-chemical reaction of zinc dialkylphosphorodithiloate (ZDDP), distributing S, P and Zn on the cylinder liner honing platform rather than in the honing groove. The plastic deformation causes a significant reduction of flakes on the edges of the cylinder liner honing platforms and less furrow damage along the sliding direction. The worn surface of the diamond-like carbon (DLC) piston ring shows a denser distribution of white bright spots and a higher sp²C=C/sp³C–C ratio.

Modeling mild wear damage mechanisms, such as oxidative wear, is highly complex due to the many mechanical and chemical actors involved. To clarify these mechanisms, the temperature-activated diffusion of oxides through exposed surfaces is used. Results indicate that diffusion kinematics are higher than those determined for the same temperatures without fretting phenomena, an effect that is especially visible when the slip speed is low (< 1 m/s). To understand the mechanism of this damage, the present study examined the evolution of the contact temperature and the dissipated heat, considering temporal non-linearities and roughness effects. This is accomplished by analyzing a case study of an axial bearing in which radial fretting is experimentally induced after applying a variable normal load and by comparing the experimental results to the theoretical calculations in the thermal-activated diffusion model.

This paper describes how research over the last 20 years has advanced our understanding of the mechanisms of action of ZDDP, especially with respect to tribofilm formation. We now know that ZDDP tribofilm formation is promoted by applied shear stress and this explains many of the features of these films. We also now recognise that ZDDP tribofilms evolve during rubbing from relatively soft, long chain polyphosphate films to much more wear-resistant, short chain phosphates. Several disadvantages of ZDDPs as lubricant additives have emerged in recent years, in particular their tendency to increase friction in thin film rubbing conditions, their promotion of micropitting wear, and accelerated wear when present together with soot contaminants in engine oils. Research has revealed the origins of all these effects. Over the last 20 years there have been growing efforts to model ZDDP tribofilm formation, both at a macro- and molecular-scale, so far with limited success. Finally, this paper outlines some aspects of ZDDP behaviour that we still do not fully understand and where further research is needed.

Graphical Abstract

Scuffing, a type of wear found in highly stressed or poorly lubricated contacts, is characterized by a rapid increase in friction and severe plastic deformation of the near-surface material. Scuffing has proven difficult to study because it initiates unpredictably, progresses rapidly, and typically develops within an inaccessible contact interface. Although there have been successful in-situ studies of scuffing in real-time, the transparent counter body needed for these studies changes the interactions between the surfaces and the lubricant, which affects the scuffing process in unknown ways. This paper describes the development of X-ray-compatible tribometry to study the scuffing of self-mated steels in-situ and in real-time. The method uses a crossed cylinders configuration with a thin (500 μm thick) stationary component and a small (≈200 μm) contact width to maximize X-ray interactions with atoms within the stress field generated by the contact. The resulting instrument and method are used to benchmark the scuffing response of self-mated 52,100 steel under tribologically challenging ‘oil-off’ lubrication conditions. The results demonstrate reliable scuffing in this configuration despite the relatively small contact areas and loads used. Following scuffing, gross plastic deformation was observed on both surfaces along with significant subsurface grain refinement and flow only on the stationary surface, which experienced constant contact. Interestingly, high friction initiated at specific locations of the migratory surface, which experienced intermittent contact, and then propagated across the track over time, suggesting that local conditions of the migratory surface dominated friction leading into the failure event.

Graphical Abstract

This study investigates the influence of the crystallinity of MoS₂ nanoparticles on their tribological performance, when used as lubricant additives in presence of a succinimide-based dispersant. Friction tests were conducted at room temperature and 100 °C. Characterization techniques, such as TEM, XPS, TGA, and DLS were used to analyze the structural and chemical properties of the nanoparticles and the tribofilms formed during the friction tests. At room temperature, low crystallinity nanoparticles show superior friction reduction due to their structural defects which facilitate their exfoliation and make easier dispersant adsorption and effective nanoparticle interaction with surfaces. Higher friction coefficients are obtained with the high crystallinity nanoparticles. This is attributed to a less easy exfoliation of the nanoparticles together with greater difficulty for the dispersant to interact with closed-structure nanoparticles. The tribofilm is thicker with high crystallinity particles and made of exfoliated MoS₂ sheets together with intact nanoparticles. At 100 °C, the friction performance of both nanoparticles are similar and very good as the dispersant looks to desorb from the rubbing surfaces, highlighting the significant impact of temperature on dispersant behavior and friction reduction. These findings underline the importance of tailoring lubricant formulations to both the crystallinity of MoS₂ nanoparticles and the operating conditions in order to optimize tribological performance.

A self-developed tube-to-tube contact fretting wear tester was used to conduct fretting wear tests on Zr-4 fuel rods in pressurized water reactors of nuclear power plants under three fretting wear modes (tangential, radial, and dual-motion fretting) with different normal loads [F_n = 15–25 (20), 30–50 (40), and 45–75 (60) N]. The test results show that the normal load and fretting wear mode have a significant effect on the fretting wear behaviors of Zr-4 alloy tube. Comparing the different fretting wear modes, the contact surface is almost coordinated by elastic deformation under the radial fretting wear mode, with the slightest damage; for tangential fretting wear mode, the damage mechanism is mainly fatigue wear and delamination; the most severe damage was observed under the dual-motion fretting wear mode, where the wear mechanism consisted of a combination of delamination, abrasive wear, adhesive wear, and oxidative wear.

For the abrasive wear of hard/soft dynamic seals, simplified uniform hardness research have been difficult to support the technology development at this stage. Therefore, the tribological behavior of non-uniform hardness particles was investigated. The results showed that the particles breakage and non-uniform effect combined to influence wear level. This caused the damage to the interfaces by the particles not to be the result of an average of both the hardness. For a clearer explanation, the influence of non-uniform hardness effect was discussed through a combination of the model and the phenomenon. The significance of this work was to enrich the existing understanding of the hardness effect and thus to provide suggestions for improving the wear resistance of hard/soft dynamic seals.

Graphical abstract