Tribology Letters最新文献

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

Skin friction underpins tactile perception of formulated products, such as cosmetics, fabric softeners, and surface coatings, that are designed to deliver satisfactory tactile sensory properties. Establishing the correlation between the tribological properties of skin contacts and tactile perception is therefore critical. We have developed a novel approach to acquire the acoustic emission (AE) signal generated by human finger sliding against fabric and non-fabric substrates, whilst capturing its frictional characteristics with a force plate. Principal Component Analysis was deployed to construct clusters formed for each material based on the sensory evaluation, and to establish the correlation between frictional and acoustic emission data. Our results show that the planar solid substrates could be discriminated solely based on the friction results, whilst the fabric materials were not significantly discriminated by the Coefficient of Friction values. However, AE was able to differentiate the fabric substrates successfully, and therefore evidences a promising potential to provide complementary information on tactile perception whilst being versatile enough as a standalone method.

During bearing operation, lubricating grease is mechanically displaced toward track peripheries by rolling elements. With limited replenishment capacity from flanking grease bands, lubrication starvation conditions readily develop within the contact zone. To address this problem, a strategy for improving the lubrication state through external micro base oil supply is proposed in this study. A dedicated oil supply system has been constructed to achieve scheduled-metered-bearing-specific delivery of trace base oil with precision. The effects of single and periodic multiple base oil supplies on the evolution of the lubricating film in the contact area under pure rolling conditions were investigated using a ball-on-disc optical interference point contact measurement apparatus. The time required for the oil film to reach dynamic equilibrium under different oil supply rates and intervals was compared. And the mechanism of extending the service life of grease by external oil supply was analyzed. The results demonstrated that the film thickness in the contact area can be instantaneously enhanced by introducing a small amount of base oil during operation. The duration of the entire dynamic equilibrium process from film thickness increase to natural recession back to the initial thickness was influenced by different oil supply rates and periodic supply intervals.

The development of eco-friendly lubricating additives attracts more and more research attention. However, their tribological performance remains a challenge compared to conventional metal thiophosphate additives in the industry. This study investigated the use of cellulose nanocrystals (CNCs) as an eco-friendly additive to partially replace conventional metal thiophosphate additives in a commercial lithium grease. The friction tests showed that compared to the Full grease containing the standard content of molybdenum dialkyl dithiophosphate (MoDTP) and zinc dialkyl dithiophosphate (ZDDP), the Half grease (i.e., the content of MoDTP and ZDDP was halved based on the Full grease) performed superior anti-friction and anti-wear ability by incorporating 0.5 wt% CNC nanoparticles. Surface analysis of the friction pairs revealed that CNC nanoparticles facilitated lubrication through a mending effect on the worn surfaces. Moreover, the function of CNCs became even more effective under high loads, resulting in an excellent extreme-pressure capacity. This study suggests that CNCs are a promising bio-based additive that can simultaneously improve the eco-friendliness and tribological performance of lubricating greases.

Graphical Abstract

The tribological behavior of barrel steel paired with brass, a critical interface in artillery systems, remains underexplored despite its direct impact on barrel lifespan. This study systematically investigates the tribological behavior and underlying mechanisms of this specific pairing across a temperature spectrum (RT, 200 °C, 400 °C, and 600 °C). The two materials exhibited distinctly different wear rate trends: the barrel steel's wear rate peaked at 200 °C before declining, contrary to the typical thermal softening trend. While the brass counterpart exhibited a high wear rate at RT, which then decreased significantly at elevated temperatures. A key finding is the discovery of a nanocrystalline-structured tribolayer that forms at 600 °C. This tribolayer is characterized by a distinct stratified architecture, composed of a Cu-rich top layer, a fine-grained intermediate zone, and a coarse-grained substrate, demonstrates exceptional wear resistance. This work reveals the transition in dominant wear mechanisms from adhesive or abrasive to oxidative wear, providing novel insights into the 'copper fouling' phenomenon and a scientific basis for developing long-life barrel materials.