Tribology Letters最新文献

Amorphous carbons can have drastically different physical properties depending on synthetic methods. Among these, hydrogenated diamond-like carbon (HDLC) produced via plasma-enhanced chemical vapor deposition is unique in that it exhibits superlubricity with a coefficient of friction (COF) less than 0.01 in proper environmental conditions. It is known that HDLC undergoes friction-induced graphitization at the shear interface and forms a highly hydrogenated transfer film at the counter-surface sliding against it. In contrast, glassy carbon (GC) produced via pyrolysis of organic precursors rarely exhibits superlubricious behavior even though the graphitic nature probed with Raman spectroscopy is similar to that of the transfer film formed from HDLC. This study addresses this drastic difference in friction of HDLC and GC and identifies key parameters that can be tuned to achieve (nearly) superlubricious behaviors with GC. The factors influencing the superlubricity of amorphous carbon include the composition and structure of the initial carbon coating, which strongly depend on the synthetic method, and the coating failure and transfer film stability, which depend on the surface chemistry of the substrate.

This paper presents a high-resolution characterization of tool–chip interfacial stress distribution in cutting of a ductile metal using full-field photoelasticity. Our findings reveal the complex nature of stress distribution and friction along the contact, influenced by tool geometry. Notably, for negative rake-angle tools, the measurements reveal a distinct zone near the tool tip where the shear stress decreases as one travels toward the tool tip. Stress measurements are complemented with in situ velocimetry of chip flow at the interface to enable correlation between stresses and velocity distribution at the interface. Based on the data, the tool–chip interface is categorized into three distinct zones: (1) a retardation zone near the tool tip, characterized by a high normal stress but small shear stresses, followed by (2) a uniform sliding zone with a relatively constant shear stress and (3) an elastic zone near the contact edge that obeys Coulomb friction. The paper challenges the customary practice of dividing the tool–chip contact into sticking and sliding zones and instead argues for a contact description, in terms of plastic and elastic zones that is more consistent with the experimental observations. The study also highlights the importance of high-resolution in situ characterization techniques for resolving the complex nature of friction in cutting and other similar sliding plastic contacts.

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Understanding the influence of electric current on tribochemical reactions and tribofilm formation at electrified interfaces is crucial for advancing lubrication strategies in electric vehicles (EVs) and other electromechanical systems. In this study, we investigated the formation of polymeric tribofilms from a model precursor (α-pinene) under low and high electric current densities and analyzed their electrical properties using an impedance spectroscopy method implemented using a lock-in amplifier. The results revealed that while electric current does not significantly change the tribopolymerization yield or the chemical properties of tribofilm, it has a substantial impact on the electrical properties of the resulting film. Compared to the tribofilm formed without current, the tribofilm formed at a current density of 0.055 A/cm² exhibited a significant decrease in resistivity while the dielectric constant, film thickness, and elastic modulus remained nearly unchanged. However, at a higher current density of 1.1 A/cm², the tribofilm on the sliding contact was much thinner, more compliant, and less polarizable. These findings highlight the impact of electric current during tribochemical reactions on the resistive and dielectric properties of tribofilms, offering insights for optimizing lubricant formulations in electrified friction interfaces. The impedance measurement method developed here will enable electrical characterization of any tribofilm at electrified surfaces in various lubrication conditions.

Graphical Abstract

This study presents the bio-tribological analysis of Ti₃C₂T_x-reinforced CoCrMo matrix composites fabricated by laser beam powder bed fusion. Raman spectroscopy confirmed the structural and functional integrity during metal matrix composite (MMC) fabrication, while Vickers hardness increased with Ti₃C₂T_x content. Together with roughness and wettability, Ti₃C₂T_x-reinforced CoCrMo composites create a favorable balance between hardness, surface roughness, and hydrophilicity, making them suitable for biomedical applications. Bio-tribological analyses under dry and substitute synovial body fluid (SBF)-lubricated conditions revealed a substantial wear reduction of 78 and 39% compared to reference. These findings underscore Ti₃C₂T_x' ability to mitigate wear through enhanced interfacial interactions and lubrication, promising advancements in biomedical implants.

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The repeated rubbing of a polymer microsphere against a hard, nanorough surface is a paradigmatic and not-so-simple process in tribology. Here, we have investigated this process in the case of poly(methylmethacrylate) (PMMA) colloidal probes (15 µm diameter) elastically driven on a Mo-doped DLC film with RMS roughness of about 3 nm in ambient conditions. The probes are flattened on length scales of few tens of nm, increasing with the applied load (up to few tens of nN). A complementary analysis on a periodic silicon grating enables a quantitative characterization of the early-stage nanowear process. On the Mo-DLC film, two different contrasts are observed, corresponding to the probe forming a multi-contact interface with the rough surface or sliding across the top asperities of the film. The contact area between probe and film, in the multi-contact regime, is also estimated based on the Persson theory. It allows us to confirm that the flattening of the sphere must be limited, with the applied loading force values, to a contact radius of about 750 nm.

Molybdenum Disulfide (MoS₂) is the leading dry film lubricant (DFL) for space applications due to its superior tribological performance in space conditions. However, MoS₂ DFLs are sensitive to the operating environment. In this study, we characterized sputter-deposited MoS₂-based DFLs, either undoped or with Ni, Ti, or Sb₂O₃ + Au dopants. The DFLs were tribotested in dry nitrogen and air conditions at room temperature, as well as in dry nitrogen at 0 °C. Unidirectional sliding tests were run until coating failure to measure friction coefficient and wear life. All four DFLs had lower friction and longer wear life in dry nitrogen than in air and some performed slightly better at room temperature than at 0 °C. Ni doping had a minimal effect on friction and wear life in any of the testing conditions but Ti or Sb₂O₃ + Au dopants led to improved performance in some conditions. Notably, Ti-doped coatings exhibited the least sensitivity to environment and temperature, and the Sb₂O₃ + Au-doped coatings had the best wear life of the DFLs in all testing conditions. Finally, short duration tribotests were performed and the wear tracks were analyzed using scanning electron microscopy to identify possible mechanisms underlying observed trends.

Graphical Abstract

Friction was studied for the human finger pad during the spreading of viscous liquid samples in circular motion on a solid substrate. The samples included both Newtonian and shear-thinning liquids with a range of viscosity between 0.83 mPa(cdot)s and 150 Pa(cdot)s. During active touch, participants applied varying normal forces and sliding speeds depending on the sample and individual behavior. Friction coefficients vary greatly between participants, but fall on one Stribeck curve when shear-thinning effects were accounted for full-film lubrication. A comparison with the measured height variations during spreading demonstrates that the logarithm of the Hersey number is an instantaneous indicator of the film thickness in the full-film lubrication regime. Comparison of the measured friction coefficients with reported values of the perceived slipperiness for the same samples shows a close correspondence along the Stribeck curve.

This paper presents a comprehensive review of wear mechanisms, with a primary focus on rubber wear under sliding conditions. Beginning with classical wear theories, including the Archard and Rabinowicz models, we analyze their applicability to both metals and elastomers and discuss extensions relevant to elastic contact and multiscale surface roughness. Various experimental studies on rubber, such as abrasion by sharp tools, erosion by particle impact, and wear during sliding on rough substrates, are reviewed and interpreted. The effects of environmental factors, such as oxygen and lubrication, are also discussed. In addition, we review a recently proposed wear model based on fatigue crack growth within asperity contact regions, which accounts for energy dissipation and multiscale interactions. This model explains the wide variability in the wear coefficient and predicts wear rates consistent with experimental observations across a broad range of conditions. It also explains the formation mechanism and provides the size distribution of rubber wear particles, from micrometer-scale up to severe cut-chip-chunk (CCC) wear. The results have implications for tire wear and the environmental impact of microscale wear debris (microplastics).

Graphical abstract

Introducing a hardening layer on the component surfaces is a widely used approach to prolong their service life. In cases where the hardening layer exhibits gradient properties in hardness, the lubrication response may be significantly influenced, largely due to the complex elasto-plastic behavior that arises from the gradient. In this study, a systematic set of plasto-elastohydrodynamic lubrication analyzes were conducted to study the evolution of lubricant film with respect to various gradient hardening layers. To validate the developed model, the degenerated results were compared with those from finite element simulations and relevant literature. The effects of layer thickness, hardness level, and hardening behavior on central film thickness were discussed, and the observations were explained by the variations in elasto-plastic deformation and the resulting contact pressure. The findings offer a direction for optimizing the lubrication response by tailoring the thickness, hardness level, and hardening property of hardening layers.

In this study, we employed the Green’s function molecular dynamics (GFMD) to simulate the non-adhesive contact between an elastic half-space and a rough counter face in ((1+1)) dimensions, obtaining the contact stress distribution under varying length scales and Hurst exponents. Subsequently, based on the dataset generated by GFMD and adopting the diffusion equation form from Persson’s theory, we obtained the stress distribution as well as the relative contact area using Physics-informed neural network (PINN). The results demonstrate that in full contact case, the diffusion equation coefficient aligns almost perfectly with Persson’s theoretical prediction. In cases of partial contact, assuming the diffusion coefficient follows a power-law function of the length scale, the stress distribution predicted by PINN exhibits an error of less than (0.5%) compared to GFMD. Furthermore, we verified that PINN can predict contact stress distribution and relative contact area at larger scales based on small-scale data, with predictions closely matching GFMD results.