Tribology Letters最新文献

This paper advances a contact model for rough elastic solids in the presence of adhesion. Based on the JKR energy balance method and an incremental contact model, the adhesive contact of rough surfaces is modeled as a combination of loading in the absence of adhesion and unloading keeping a constant real contact area due to adhesion. The relations between the normal load, the real contact area, and the average interfacial separation are presented, and the contact processes of Gaussian rough surfaces are thoroughly discussed. For solids with large work of adhesion and relatively smooth surfaces, full stick might happen spontaneously. While for solids with lower work of adhesion and rougher surfaces, initial contact generates only limited contact area, and external compression is required to increase the contact area further. When a critical area is achieved, the contact area will increase spontaneously again even without compression. Once the separation shrinks to a critical value, instability will be triggered with a sharp increment of contact area and an abrupt fall in normal load. The current study presents a simple method to determine the bonding strength of macroscopic rough solids.

Fretting wear is a critical failure mechanism in grease-lubricated oscillating bearings, such as the pitch bearings in wind turbines, with lubrication starvation being a primary root cause. While tribochemical solutions remain underexplored, this study demonstrates the efficacy of antiwear (AW) additives in preventing fretting via tribofilm formation in oscillatory contacts. Experiments employed Falex (thrust bearings), optical ball-on-disc interferometry (in-situ film/onset observation), and SRV-V (pure sliding) testers across varying oscillation amplitudes and rolling/sliding conditions. Results reveal that four distinct AW additives significantly reduced wear through tribofilm generation. Performance was stroke-dependent: butylated triphenyl phosphorothionate (B-TPPT) and tris(4-methylphenyl) phosphate (TCP) exhibited superior fretting resistance, when A < 1 (the value of A is the ratio of the oscillation amplitude to Hertzian contact size). High-sulfur additive (DDPE) was effective when A < 1 but induced corrosion and tribofilm depletion when A > 1. Crucially, this work establishes that conventional AW additives can form functional tribofilms under oscillatory conditions, despite lower energy input, thereby validating tribochemistry as a viable approach for fretting failure prevention in practical engineering systems.

Hydrogen and ammonia have been considered as prominent fossil-free energy source candidates. While their combustion characteristics and emission profiles are well-documented, the implications for engine lubrication systems remain underexplored. This study aims to bridge this knowledge gap by investigating the effect of gaseous green fuels, specifically hydrogen and ammonia, and their influence on lubricants and the tribological performance of the lubricants. Applying a rapid and cost-effective lab-scale ageing process, the ageing effects of gas and gas admixtures were simulated and differentiated in a controlled environment. Detailed chemical, physical, and tribological analyses provide valuable insights into the different degradation outcomes of the lubricants with different exposing gases. The results revealed degradation of lubricant performance after ageing with the gases, due to changes in the lubricant chemistry and, in some cases, viscosity at 100 °C. In all cases, lubricant ageing introduced increase in friction coefficient between steel surfaces and impairment of the lubricant load-carrying capacity. The results obtained from this work will benefit the appropriate selection of lubricant alternatives for future engines utilizing green fuels and facilitate the development of more effective and durable lubrication solutions to support the broader adoption of hydrogen and ammonia as sustainable energy sources in internal combustion engines.

Surface modification treatments are widely employed to enhance the performance of materials, often resulting in layered mechanical properties along the material’s height. The majority of research into friction and wear of materials relies on finite element methods (FEM). However, when accounting for topography of very thin surface layers, FEM encounters specific constraints. This paper introduces a novel approach for predicting the wear morphology evolution of layered materials subsequent to surface modification, specifically under ball-on-disk contact conditions. This methodology discretizes the surface into cells, considering the wear process of the cells from a microscopic perspective. The stress distribution within the contacting area is computed based on a balance of forces between the ball and the discretized surface, with the equivalent elastic modulus serving as a proxy for the substrate’s elastic modulus. Additionally, the model is made more realistic by incorporating the effects of boundary lubrication via a load-sharing approach and plastic deformation of surface asperities. Leveraging the Archard’s model, a wear equation for discrete surface cells is formulated to ascertain the wear volume. The availability of this method is substantiated by comparing simulation outcomes with experimental data for carburized 16Cr3, carburized followed by shot-peened 16Cr3, and carburized, shot peened, and subsequently coated 16Cr3 materials subjected to different temperature conditions, revealing a maximum discrepancy of 17.1% between predicted and experimental wear rates. This methodology enables swift predictions of material wear performance under varying conditions, thus aiding in layered material selection, design, and optimization processes.

Astringency is a property of food and beverages that results largely from a friction-enhancing effect in the mouth. It can be conveniently assessed by tribological measurements, which have typically been carried out by investigating the effect of the astringent on the lubricating properties of whole-mouth saliva (WMS). Saliva is notoriously variable as a reagent, however, differing in its properties from person to person, day to day, and changing its behavior rapidly after removal from the mouth. We describe a far more convenient, reproducible, and potentially useful alternative to the use of WMS for ex vivo determination of the friction-enhancing effects of an astringent substance. Key to this development is the use of a nitrene-generating adhesion promoter to immobilize a crucial lubricating mucin, MUC5B, onto the tribopair—in this case silicone rubber (PDMS) and glass. Upon sliding in saline solution, this mucin-modified tribopair exhibits friction coefficients in the range of 0.002–0.006. Addition of 1 wt% potassium alum solution (a known astringent) leads to an order-of-magnitude increase in friction coefficient, while addition of samples of greater relevance to the food and beverage industry (grapeskin extract, red wine) shows comparable effects. Interestingly, the astringents’ effect on the MUC5B-modified surfaces appears to be essentially reversible upon washing with saline solution and continued sliding, suggesting that the effects of astringents in the mouth are not necessarily associated with removal of the MUC5B from the oral mucosa.

Graphical Abstract

We study the fluid-mediated impact of a deformable axisymmetric object against a rigid substrate, focusing on how its shape influences contact formation. For low approach velocities and large Stokes numbers, we show that sharper profiles (e.g., conical) maximize contact at the center and avoid fluid entrapment, while blunter ones form central dimples that trap bubbles. We also find that the resulting pressure distributions in the presence of thin viscous films can be predicted remarkably well by classical (dry) contact mechanics. These findings reveal shape as a design parameter for contact optimization in soft matter, adhesion, and elastohydrodynamics. Finally, we also theorize the possibility of a mechanical equivalence between shape and approach velocity.

Mucins, a family of glycoproteins, provide highly efficient lubrication on the eyes, in the mouth, and in the gastrointestinal tract of mammals. Although two key mechanisms responsible for the outstanding lubricity of mucin solutions, i.e., sacrificial layer formation and hydration lubrication, have been identified, recent research indicates that the complex structure of mucins, e.g., the presence of folded domains in the termini of the glycoproteins, might also be relevant for the tribological performance of mucin solutions. Here, we manipulate the molecular configuration of three different mucins (i.e., porcine gastric mucin MUC5AC, bovine submaxillary mucin MUC5B, and porcine intestinal mucin MUC2) by treating the glycoproteins either with the crosslinking agent glutaraldehyde (GA) or with the denaturing agent guanidine hydrochloride (Gua) and assess the ensuing consequences of those treatments. Although we observe a clear reduction in the lubricity of all mucin solutions upon treatment, neither mucin aggregation, nor a reduction of mucin hydration, nor an altered adsorption behavior of mucins to hydrophobic PDMS surfaces can account for this effect. Instead, we suggest that the treatments prevent the stretching of mucins and/or the unfolding of the mucin termini, and both processes may play an important role in retarding the force-induced desorption of mucins from surfaces as required for efficient hydration lubrication. Our results pinpoint molecular mechanisms which—to date—were not sufficiently considered for macromolecular lubricants. A better characterization of these molecular mechanisms will not only deepen our understanding of mucin-based lubricants but will also open the path for the development of more efficient, bio-inspired lubricants.

Graphical abstract

The stretching of the mucin glycoproteins and the unfolding of their hydrophobic termini contribute to the lubricity of mucin solutions. Suppressing this stretching/unfolding results in reduced lubricity