Tribology Letters最新文献

The rising application of diamond-like carbon (DLC) coatings on engine components requires a thorough examination of the interaction processes between DLC and fully formulated engine oil. This study demonstrates the critical role of overbased detergent additives, both in fully formulated and model lubricants, in reducing friction as a function of contact pressure in the boundary lubrication regime for hydrogen-containing DLC a-C:H involved tribo-pairs. Transmission Electron Microscope (TEM) reveals that the calcite formed during friction from overbased detergents plays a significant role in decreasing friction under high contact loads. Furthermore, the interaction between pressure conditions and the nature of the counterfaces dictates the type of calcium carbonate polymorph formed in the sliding contact, which significantly affects the tribological performance of the lubricant.

Biofilms produce and maintain extracellular polymeric substances (EPS) essential for their form and function. While biofilms are commonly lamellar and frequently targets of removal, granular biofilms are increasingly incorporated into water treatment strategies. In both cases, the EPS (mainly consisting of proteins, polysaccharides, and extracellular DNA) is largely responsible for their persistence. Unlike many granular biofilms, which are formed in engineered industrial bioreactors, the “pink berry” consortia is a naturally occurring and robust granular biofilm of photosynthetic bacteria, found only in intertidal pools of salt marshes around Woods Hole, Massachusetts (USA). The pink berry biofilm’s unique ecological niche has sparked over three decades of study, yet their mechanical properties are completely unknown. Here, we characterized the structural and mechanical landscape of pink berry granules to determine the extent to which microscale heterogeneity influences macroscale material properties. We performed microindentation measurements on intact granules and nanoindentation measurements on thin sections. We report that intact pink berry granules exhibited low reduced elastic moduli (E*_{pink berry} ≈ 0.5–10 kPa) and fast stress relaxation times (τ_1/2 ≈ seconds), consistent with previous investigations of soft and viscoelastic biofilms. Nanomechanical measurements of thin pink berry sections revealed two mechanically distinct domains: a very soft extracellular polymeric substance (EPS) matrix surrounding stiffer microcolonies of purple sulfur bacteria (PSB). Light sheet fluorescence microscopy revealed the spatial organization and distribution of cell-dense PSB microcolonies (34 vol%) within EPS matrix (66 vol%), suggesting the nanomechanical behavior of EPS dominates macroscale pink berry mechanics. Our multiscale experimental approach combining mechanics and imaging may be broadly applicable to investigations of complex soft materials, from synthetic hydrogel composites to biologically heterogeneous spheroids, organoids, and tissues.

This study investigates the influence of substrate surface finish on the morphological, mechanical, and microstructural properties of AlTiN/AlTiCrN coatings on AISI A8 tool steel, and how this cascade of property changes collectively dictates the wear and corrosion behaviour of the coated tool steels. The wear resistance was evaluated as per the ASTM G65 test, while the corrosion performance was assessed using electrochemical impedance spectroscopy and potentiodynamic polarization techniques. Surface roughness was measured using profilometry, while the hardness (H) and elastic modulus (E) were determined via nanoindentation. Microstructural characterization was carried out using scanning electron microscopy, energy dispersive spectroscopy, and X-ray diffraction techniques. The results showed that increasing the average substrate roughness by about 500-fold (from 0.003 to 1.7 µm) led to a relatively modest rise of about 22% in the wear rate (from 1.8 to 2.2 × 10⁻⁴ mm³/Nm). This reduction in the wear performance of the coatings on the rougher substrates was attributed to lower hardness, a reduced H/E ratio, and an elevated surface roughness of the coatings. Similarly, the coating’s corrosion resistance deteriorated with increasing surface roughness, as reflected by a decline in protective efficiency (from 77 to 58%) and an increase in porosity (from 0.27 to 0.49%). This observation was linked to the cumulative effect of the reduction in (111) crystallographic orientation, the presence of larger macroparticles, and an elevated surface roughness of the coatings on the rougher substrate finish. Overall, these findings underscore the potential of a smoother AISI A8 substrate surface finish to simultaneously improve the wear and corrosion performance of AlTiN/AlTiCrN coatings by promoting better coating morphology, mechanical strength, and electrochemical stability.

Graphical Abstract

The reduction of elastohydrodynamic (EHD) friction has become increasingly important to improve the efficiency of mechanical components. This study investigated the effect of friction modifiers (FMs) on full-film EHD friction using a ball-on-disc tribometer to elucidate the underlying mechanisms by which FMs affect full-film EHD friction. It was found that glycerol monooleate (GMO) lowered full-film EHD friction more than glycerol tris(2-ethylhexanoate) (GTE). This effect became more pronounced with increasing concentration of FMs. Neutron reflectometry (NR) and in situ reflectance spectroscopy suggested that GMO formed a thicker and denser adsorption film on surfaces than GTE, correlating directly with full-film EHD friction reduction. One of the possible mechanisms is that FMs, particularly GMO, adsorb on surfaces and may form oleophobic planes, where methyl groups are aligned in the outermost region of the adsorption film. A bulk base oil may flow more easily on such planes due to an interfacial slip than on steel surfaces. This reduces full-film EHD friction. The insights obtained in this study can be used to the design of lubricants that reduce full-film EHD friction.

Graphical Abstract

In rotating machinery, water contamination in lubricants can lead to oil film failure and exacerbate equipment wear. The acoustic emission (AE) technology demonstrates superior performance over traditional detection methods for early fault detection due to its sensitivity to weak signals. Regarding the specific features of AE signals during oil film shearing, the traditional Bayesian method lacks comprehensive diagnosis and is constrained by single-fidelity optimization, dedicating most evaluations to high-cost configurations. Accordingly, this paper proposes an adaptive Multi-fidelity Bayesian Optimization (MFBO) method for detecting water contamination in lubricants. The approach denoises acoustic emission (AE) signals by combining the detection index (DI) and the interquartile range (IQR) with wavelet packet transform (WPT), which markedly improves the signal-to-noise ratio. Within MFBO, an adaptive penalty parameter α dynamically adjusts to reduce costly evaluations. This framework is used to optimize CNN hyperparameters, including the learning rate, mini-batch size, and number of training epochs. Experiments on twelve groups at different shear rates yield a recognition accuracy of 99.15% for water contamination, representing an 18.48% improvement over the non-optimized baseline; introducing α further reduces computation time by 7.35%. Overall, the method achieves an effective balance between exploration and exploitation, maintaining reliable identification across varying moisture levels.

Adhesion plays a pivotal role in computer chip manufacturing, directly affecting the precision and durability of positioning components, such as wafer stages. Electrical biasing is widely employed to eliminate floating potential and to enable electrostatic clamping. However, upon electrical grounding adhesion can persist and there is limited knowledge about the nature of this adhesion hysteresis. Here, we investigate potential causes underlying electric field-induced adhesion hysteresis at the interface between an n-type AFM tip and a p-type silicon sample using atomic force microscopy. Our findings reveal that neither charge trapping nor siloxane bond formation significantly impacts the measured adhesion. Surprisingly, we show that adhesion can be tuned through electric field-induced water adsorption under low relative humidity (RH (< 10%)). Our results provide new insights into adhesion hysteresis and opportunities for adhesion control.

Graphical Abstract

Accurate, traceable force calibration remains a challenge for microtribometers because of lateral/normal cross-talk and operating-point dependence bias in conventional procedures. We present a traceable force calibration (TFC) that solves the full 2 × 2 calibration matrix (C₁-C₄) from angle-resolved loading using a diamagnetic-levitation spring with a microbalance as an SI-traceable force standard. A known load is applied while the cantilever is tilted through multiple angles; linear slopes ΔV_x,z/ΔF₁ at each angle provide sufficient equations to recover C without neglecting cross-talk. Using 18 angles, TFC yields stable coefficients C₁ =  − 746.98 ± 0.01 μN/V and C₄ =  − 737.17 ± 0.45 μN/V, with small cross-terms C₂ = 3.372 ± 0.003 μN/V and C₃ =  − 45.14 ± 0.63 μN/V. Subsampling shows convergence with only 3–4 angles, and a ± 1° reference-angle bias changes C₁ and C₄ by ≤ 1.01% and ≤ 0.49%, respectively. Compared head-to-head with the diamagnetic-levitation force-calibration (DLFC) route, TFC produces C₁ values invariant to angle choice, whereas DLFC yields set-point (V_z)-dependent results and larger scatter (e.g., C₁ =  − 758 ± 41 μN/V). TFC thus offers a compact, low-uncertainty, SI-traceable workflow that quantifies cross-talk and delivers reliable calibration over micro- to milli-newton forces, enabling high-precision, reproducible microtribometry.

Leveraging advantageous properties including low density, high specific strength, and corrosion resistance, titanium alloys have become essential structural joining components in aerospace systems. Nevertheless, their inherently low hardness and poor wear resistance constrain reliability and service life in demanding in-service conditions. This study aims to enhance their surface properties through a combination of ultrasonic shot peening (USP) and thermal oxidation (TO). The results indicate that USP pretreatment refines surface grains, increases surface roughness, and optimizes oxygen atoms diffusion pathways, resulting in the formation of a thicker oxygen diffusion layer. Following the USP/TO composite treatment, a composite strengthening layer is formed on the Ti-6Al-4V alloy, consisting of Rutile TiO₂, Brookite-TiO₂, and other phases, which enhances both the microhardness of the surface and subsurface. Notably, the sample treated with USP/TO for 5 min exhibits a 98% reduction in wear volume and mitigates abrasive wear.

Graphical Abstract

A simple and nonsynthetic way of bicomponent supramolecular gel lubricants was prepared by adding carboxylic acid and sodium organosulfur oxy-salts with varying chain lengths and saturation degrees into white oil (WO). The gelling properties have shown that the bicomponent supramolecular gelator with longer alkyl chains and higher saturation, such as stearic acid and sodium dodecyl sulfate (SA-SDS), possessed better gelling properties. SA self-assembles to helical fibers, and SDS enhances fiber cross-linking via the noncovalent interaction with SA, leading to the formation of three-dimensional networks with significantly increased rigidity. Tribological data show that SA-SDS gel reduces wear scar diameter and friction coefficient by approximately 25 and 24% compared to WO. And the gel system of SA-SDS in WO has better anti-friction and anti-wear performances than the sol system. This is because the multilayer tribofilm consisting of oxide layers, carbonaceous species, and organic films is formed at the interface, significantly improving lubrication. These findings provide insights into the design of supramolecular gel-based lubricants for enhanced friction and wear control.

Evaluation of boundary lubricants is crucial for enhancing the energy efficiency of a system, as the boundary lubrication (BL) regime involves direct asperity contact and exhibits the highest friction. Fatty acids are widely used as friction modifiers in BL regimes, forming tribofilms that reduce direct asperity contact and lower friction. This study examines the effect of molecular unsaturation on the frictional behaviour of oleic acids across temperatures. A Force-Controlled Pendulum Tribometer (FCPeT), which operates on energy dissipation methodology, was used to evaluate the frictional performance of a blend of oleic acid with base oil at temperatures of 27 °C, 60 °C, and 100 °C. The FCPeT quantifies friction by measuring energy dissipation during sliding. Results indicate that the coefficient of friction increases with temperature, rising from 0.10 at 27 °C to ~ 0.12 (steady state) at 100 °C. FTIR and XPS analyses of the wear track confirmed tribofilm formation. The XPS results reveal that at higher temperatures, tribofilm degrades, contributing to increased frictional dissipation.

Graphical Abstract