Boundary Lubrication Research Articles

Evaluation of surface texturing on chrome-coated cylinder liners via deterministic mixed lubrication simulation

This study investigates the mixed lubrication performance of various surface texture configurations in the piston ring/cylinder liner conjunction of a two-stroke internal combustion engine using a deterministic mixed lubrication model. The numerical model simultaneously solves the Reynolds equation with mass-conserving cavitation to calculate inter-asperity hydrodynamic pressures and an elastic, perfectly plastic, rough contact model to determine contact pressures at each asperity interaction. Gaussian Mixture Model clustering was employed to enhance surface characterization. The deterministic simulation approach considers the full-scale representation of the cylinder liner topography to accurately capture the influence of surface features on the hydrodynamic support and friction under mixed lubrication conditions. The investigated cylinder liners were initially hard-chrome-coated and honed, resulting in a stochastic arrangement of surface pores, and then deterministic patterns of surface pockets were created by micro electrodischarge machining (EDM). Surface measurements were performed using laser interferometry, providing input for the mixed lubrication simulations. The study also explored the virtual removal of ridges formed around the pockets by the EDM technique. Key findings indicate that the stochastic texture outperformed the hybrid texture (stochastic + deterministic) in the boundary and mixed lubrication regimes, showing higher hydrodynamic support at low separations but increased hydrodynamic shear stresses at higher speeds. Conversely, deterministic textures exhibited a significant decrease in average hydrodynamic shear stress at high velocities. These results highlight the critical role of surface texture in tribological behavior and suggest that localized textures on cylinder liners can potentially optimize engine performance. The study recommends further exploration of a broader range of texture geometries, densities, and distribution patterns to enhance engine design strategies.

Synergistic effects of nitrogen-containing functionalized copolymer and silicon-doped DLC for friction and wear reduction

We present a strategy to enhance the tribological performances of diamond-like carbon (DLC) films based on synergistic chemical modifications in the polymer lubricant additives and in the carbon film. Our experiments revealed that the polymer functionalization with nitrogen-containing groups and the DLC doping with silicon atoms result in significant friction and wear reduction under severe boundary lubrication conditions. Atomic Force Microscopy revealed the formation of a lubricious tribofilm. Ab initio calculations highlighted the key role of N-Si interactions in promoting the polymer chemisorption on the film, which represents the first, essential step for the tribofilm formation. Our findings suggest that targeted chemical modifications in lubricants and films can substantially advance the tribological performances of DLC films for various industrial applications.

Effect of phosphide additives on the tribological properties of polyethylsiloxane (PES) in Si3N4 and M50 system

PurposeThe incorporation of phosphide additives is regarded as a highly effective strategy for enhancing the lubricative qualities of base oils. This study aims to assess the lubrication behavior and efficacy of various phosphide additives in polyethylsiloxane (PES) through the employment of the Schwingum Reibung Verschleiss test methodology, across a temperature range from ambient to 300°C.Design/methodology/approachPES demonstrated commendable lubrication capabilities within the Si3N4/M50 system, primarily attributable to the Si-O frictional reaction film at the interface. This film undergoes disintegration as the temperature escalates, leading to heightened wear. Moreover, the phosphide additives were found to ameliorate the issues encountered by PES in the Si3N4/M50 system, characterized by numerous boundary lubrication failure instances. A chemical film comprising P-Fe-O was observed to form at the interface; however, at elevated temperatures, disintegration of some phosphide films precipitated lubrication failures, as evidenced by a precipitous rise in the coefficient of friction.FindingsThe results show that a phosphide reactive film can be formed and a reduction in wear rate is achieved, which is reduced by 64.7% from 2.98 (for pure PES at 300°C) to 1.05 × 10–9 μm3/N m (for triphenyl phosphite at 300°C).Originality/valueThe data derived from this investigation offer critical insights for the selection and deployment of phosphide additives within high-temperature lubrication environments pertinent to PES.Peer reviewThe peer review history for this article is available at: https://publons.com/publon/10.1108/ILT-04-2024-0139/

Tribological behavior and mechanism of Ti3SiC2/Cu composites in a wide temperature range under lubrication condition

Preparation of Ti3SiC2/Cu composites using Spark Plasma Sintering (SPS) process. The friction and wear behavior of Ti3SiC2/Cu-45# steel tribo-pair under lubrication conditions was investigated in a wide temperatures range. The results demonstrated that the influence of temperature on the tribological properties of the Ti3SiC2/Cu-45# steel tribo-pair under lubrication condition was characterized by a continuous transition from a lubricant to an oxide film. At 200 ℃, mobility of the lubricant increased and the friction interface was in the boundary lubrication state. As the temperature increased, the base oil in the lubricant rapidly evaporated, causing to solidification of the lubricant, and the friction interface formed the friction oxide film. The mechanisms of friction and wear were primarily abrasive wear and tribo-oxidation wear.

Competition Between Growth and Removal in Zirconia Nanocrystal-Derived Tribofilms: The Role of Co-additives

Antiwear additives permit energy-efficient lubrication of gearboxes, bearings, and other tribological interfaces. We study zirconia (ZrO2) nanocrystal additives, which readily form protective tribofilms in tribological contacts. Our prior work demonstrated cooperative antiwear performance between ZrO2 and the S- and P-based co-additives in fully formulated hydrocarbon gear oils. Here, we extend that work by examining the growth kinetics of the ZrO2 tribofilms, including the influence of the co-additives. In the boundary lubrication regime for mixed rolling-sliding contacts, the initial phase of ZrO2 tribofilm growth is soon overtaken by removal processes, phenomena whose importance has gone unnoticed in prior work. Tribofilm removal affects the steady-state thickness and morphology of the tribofilm as well as its growth kinetics. The S- and P-based co-additives are incorporated into the ZrO2 tribofilm, and alter the competition between the growth and removal processes, increasing initial net growth rates per contact cycle and contributing to a more polished final interface. This work highlights the significance of removal processes in determining tribofilm antiwear performance, and suggests several routes for improving tribofilm growth kinetics using co-additives.Graphical abstract

Optimum texture shape under different lubrication conditions applied to the start-up phase of journal bearings

This paper presents a parameterized model to optimize the texture units using an improved particle swarm optimization. The texture units are optimized under three lubrication states: boundary lubrication, mixed lubrication, and hydrodynamic lubrication, by utilizing the Stribeck curve. The optimal texture units can be applied to journal bearings and thrust bearings in different operating conditions to achieve maximum load-carrying capacity. The paper applies the optimal textures under the three lubrication states to the start-up stage of journal bearings to examine the lubrication performance and dynamic characteristics of the textured journal bearing system. The results demonstrate that compared to the untextured journal-bearing system, the introduction of optimal textures improves the tribological performance of the journal-bearing system, and the optimal textured units under hydrodynamic lubrication have the best friction reduction effect. Furthermore, it is found that the introduction of textures has no significant effect on the dynamic characteristics of the journal bearings.

Dual neural network for contact temperature and cooling state monitoring in water-lubricated bearings

Harsh operating conditions subject the water-lubricated stern bearings of a ship to a non-uniform stress field, leading to frequent boundary lubrication between the bearing and shaft. Poor lubrication results in local temperature rise due to friction heat, leading to heat aging or even carbonization of the bearings. This study proposes a novel universal monitoring approach based on the internal temperature of water-lubricated bearings (WLBs). Radial basis function neural networks (RBFNN) were established to evaluate the surface temperature and cooling statuses of the bearing surface during rubbing, in which simulation results obtained from a temperature model were used as a training set. The accuracy of the simulation results and prediction model was verified through experimental data. The results showed that the maximum error of predicted contact surface temperatures is only 0.84 °C, 0.02 °C, and 1.00 °C in MAE, MAPE, and RMSE, respectively. The accuracy of cooling conditions identification can be achieved at 91.7 % with 600 s of continuous temperature data. The scheme proposed in this study can effectively predict thermal failures of water-lubricated bearings without requiring extensive experimentation, which promotes the development of intelligent water-lubricated bearings.

Simulation-based research on enhancing lubricity: investigating wettability and textured surfaces in alkane lubricants under boundary lubrication conditions.

Changing the wettability and surface texturing have a significant impact on lubrication. In this study, the researchers used the molecular dynamics (MD) method to investigate how adjusting the interaction between alkanes and the wall affects oil film morphology and frictional properties under boundary lubrication. The findings revealed that the bearing capacity was influenced by both the morphology of the oil film and the strength of solid-liquid adsorption. In cases where the walls had weak wettability, the alkanes formed clusters to effectively separate the walls, while in cases where the walls had strong wettability, the oil film spread and formed a strong adsorption film. The super oleophilic textured surface could enhance the oil film adsorption capacity and replenish the oil film to the friction area in time, and the super oleophobic smooth surface could further reduce the friction coefficient. Therefore, a composite surface consisting of a super oleophilic textured surface and a super oleophobic smooth surface can be designed to enhance the bearing capacity of the oil film and reduce friction.

The effects of medium and friction pair on the tribological behavior of Mo-doped DLC films

Diamond-like carbon (DLC) films are widely used to improve the tribological properties of key components in internal combustion engines. However, their performance can be significantly affected by the service environment, which may impact engine reliability. This study investigates the impact of various media and friction pairs on the tribological behavior of molybdenum-doped DLC (Mo-DLC) films. The results demonstrate that Mo-DLC films exhibit excellent tribological properties across different media, showcasing their adaptability to various conditions. The lubrication mechanism varies with media viscosity: in methanol, Mo-DLC films operate under boundary lubrication conditions, leading to a relatively high wear rate of approximately 5.2 × 10−8 mm3/N·m. Conversely, in diesel and polyalphaolefin-based oil (PAO), where fluid dynamic lubrication occurs, wear rates are significantly lower, at 4.4 × 10−8 mm3/N·m and 3.1 × 10−8 mm3/N·m, respectively. In addition, the friction pair significantly influences the tribological performance of the Mo-DLC film. When paired with D-GCr15 in methanol, Mo-DLC films exhibit a low friction coefficient of 0.09 and a wear rate of 1.6 × 10−8 mm3/N·m, respectively. However, when coupled with N-GCr15, the increased surface roughness extends the running-in period and raises the friction coefficient in methanol. These findings offer valuable theoretical insights and practical guidance for optimizing DLC films in internal combustion engines.

Development of precipitation lubrication with palmitic acid and its friction mechanism

Conventional friction modifier MoDTC becomes ineffective at lower temperatures. This paper investigates the concept of "precipitation lubrication," where saturated fatty acid precipitation enhances load-bearing capacity and solid lubrication at contact points to overcome this issue. PAO8 with 4.2 % palmitic acid lubricant was prepared, precipitating below 35 °C, reducing friction by approximately 14 % at 30 °C for both boundary and mixed lubrication. In situ reflectance spectroscopy demonstrated palmitic acid forming a solid layer on the disk specimen, preventing direct contact between asperities and significantly reducing friction. These findings underscore the effectiveness of palmitic acid precipitation in enhancing lubrication performance at lower temperatures.

Dynamical behavior of lubricant molecules under boundary lubrication explored via molecular dynamics simulations

Dynamical behavior of lubricant molecules under boundary lubrication explored via molecular dynamics simulations

Nonlinear Dynamics and Meshing State Transition Analysis of Spalling Defect Gears with Multi-Lubrication Conditions

The purpose of this paper is to analyze the dynamic characteristics and the meshing state transition characteristics of spalling defect gears under various lubrication conditions. The time-varying geometric parameters, motion parameters and load distribution of the gears in the state of positive meshing and reverse impact are analyzed. The time-varying meshing stiffness considering the time-varying friction coefficient under elastohydrodynamic lubrication (EHL), mixed lubrication and boundary lubrication are calculated, and the spalling defect is introduced in the stiffness calculation process. The equivalent lubrication model of the gear is established, the switching condition of the lubrication state is derived, and the distribution of the lubrication state in the torque-speed plane is discussed. The multimeshing state dynamic model of the spalling defect gear is established, and the evolution characteristics of the coexistence response and the transition of the meshing state under different lubrication conditions are simulated by using the attraction domain, phase trajectory, meshing force and bifurcation diagram. The results show that the phase trajectory of the gear with the spalling defect is deformed compared with that of the healthy gear. The change of lubrication state has little effect on the phase trajectory and attraction domain of the gear with spalling defect, but has a great influence on the meshing force. At the spalling boundary, the meshing force is changed abruptly, and the worse the lubrication state, the more obvious the mutation. Due to the influence of spalling on tooth stiffness, it will also affect the meshing force in the nonspalling area.

Study on the influence of tire polishing on surface texture durability and skid resistance deterioration of asphalt pavement

As an inherent attribute of asphalt pavement, texture durability is the essential reason for the change in skid resistance. However, different scale texture deterioration behavior remains unclear by tire wearing at present. To explore the influence of the durability of multi-scaled textures on skid resistance, nine pavement specimens were tested using the Harbin polishing and dynamic friction integrated experimental machine (HPFM). The pavement anti-skid texture tester (PATT-II) was used for high-precision texture scanning and reconstruction. The results show that the texture richness and skid resistance of the three types of pavements in 300 min polishing are ranked as Open-graded Friction Coarse (OGFC) > Stone Mastic Asphalt (SMA) > Asphalt Concrete (AC), while SMA presents the best durability, determined by the material skeleton structure of asphalt mixture. The macro-texture deterioration occurs slightly at first and then tends to stabilize in 300 min polishing. The micro-texture deterioration behavior is the same as the μHPFM, increasing rapidly at the beginning, then slowly decreasing, and finally remaining constant. This deterioration is caused by the asphalt damage and continuously wearing aggregate surface. The 2D-PSD deterioration behavior is more significant with a smaller texture scale in 300min polishing, and the most considerable contribution wavelength in the microscopic scale range is 0.15 mm–0.3 mm, which is determined by the adhesion characteristics and molecular mechanism between a mineral mixture and asphalt material. The influence of pavement texture on the dynamic friction coefficient is significantly different under different water film thicknesses, the micro-texture and macro-texture play the main roles in boundary and mixed lubrication, respectively.

Electrochemical Investigation of the Stability of Poly-Phosphocholinated Liposomes.

Poly[2-(methacryloyloxy)ethyl phosphorylcholine] liposomes (pMPC liposomes) gained attention during the last few years because of their potential use in treating osteoarthritis. pMPC liposomes that serve as boundary lubricants are intended to restore the natural lubrication properties of articular cartilage. For this purpose, it is important that the liposomes remain intact and do not fuse and spread as a lipid film on the cartilage surface. Here, we investigate the stability of the liposomes and their interaction with two types of solid surfaces, gold and carbon, by using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). With the aid of a hydrophilic species used as an electroactive probe in the solution, the charge transfer characteristics of the electrode surfaces are obtained. Additionally, from EIS, the capacitance characteristics of the surfaces are derived. No decrease of the peak currents and no displacement of the peak potentials to greater overpotentials are observed in the CV experiments. No decrease in the apparent capacitance and increase in the charge transfer resistance is observed in the EIS experiments. On the contrary, all parameters in both CV and EIS do change in the opposite direction. The obtained results confirm that there is only physical adsorption without fusion and spreading of the pMPC liposomes and without the formation of lipid films on the surfaces of both gold and carbon electrodes.

MXene decorated hierarchical phase change microcapsules to strengthen the tribological properties of PPS/PTFE fabric composites

The reduction of the wear rate and friction coefficient of composites simultaneously has a great challenge. Herein, a unique phase change microcapsule is designed with n-docosane as the core, TiO2 coated with sodium alginate as the shell, as well as surface-decorated Ti3C2Tx nanosheets. In this microcapsule system, n-docosane absorbs part of the frictional heat via solid-liquid phase change and releases it continuously to achieve self-lubrication. The TiO2 shell possesses excellent load-bearing capacity and Ti3C2Tx is susceptible to interlayer slip, which reduces the friction and wear of fabric composites. The results indicate that an average wear rate and friction coefficient of fabric composites are decreased by 60.85 % and 11.83 % with the addition of microcapsules. A strong and continuous tribofilm is formed on the steel surface, and the tribofilm enhances the boundary lubrication of the steel counterpart. This paper is expected to provide a novel design thought for the application of core-shell structure in tribology.

Effects of sodium ions on the tribological and antioxidation properties of borates

A borate ester (BMBS) and its sodium salts (BMBSNa) were synthesized and used as multifunctional lubricant additives in oleic acid. Results showed that BMBSNa possessed better anti-oxidation and tribological properties than BMBS. Worn surfaces analysis revealed that BMBSNa was more favorable to forming boundary film containing h-BN and FeS. Theoretical calculation suggested that BMBSNa had a higher chemical reactivity and better metal adsorption ability than BMBS. Fukui indices showed that sodium ions favor free radical reaction. Radial distribution functions (RDF) indicated that sodium ions help the adsorption of BMBSNa on the iron surface. It can be concluded that sodium ions benefit the oxidation resistance of base oil, and contribute to forming good boundary lubrication film to achieve good tribological properties.

Macroscopic liquid superlubric triboelectric nanogenerator: An in-depth understanding of solid-liquid interfacial charge behavior

Triboelectric nanogenerator (TENG) has been seen as one of the most promising energy harvesting technologies. However, there are an irreconcilable contradiction between low friction and high electrical output performance of the TENG. Here, we have devised a macroscale liquid superlubric triboelectric nanogenerator based on solid-liquid-solid structure, leveraging the concept of liquid superlubricity technology, resulting in a remarkable increase in both the open circuit voltage and short-circuit current by 53.0 % and 58.4 %, respectively. This huge increase in electrical output performance is accompanied by a 99.1 % reduction in friction coefficient (≈ 0.0025) and 99.993 % reduction in wear rate (≈ 0.76×10−7 mm3/N·m), when compared to those of a lubricant-free triboelectric nanogenerator. This work delves into the lubrication and charge transfer mechanisms. The liquid superlubricity technology achieves friction reduction through a hybrid mechanism combining boundary and hydrodynamic lubrication. And the high outputs arise from the charges transfer at solid-liquid interface. The triboelectric charges generated by the friction pair are transferred from solid to the lubricating liquid. Meanwhile, the lubricating liquid promotes electron transfer and contributes additional electrons. Our work provides profound insights into the lubrication and charge transfer mechanisms at solid-liquid interfaces, and addresses the paradox of high output and low friction in TENGs.

Zwitterionic Poly-Carboxybetaine Polymers Restore Lubrication of Inflamed Articular Cartilage.

Osteoarthritis is a degenerative joint disease that is associated with decreased synovial fluid viscosity and increased cartilage friction. Though viscosupplements are available for decades, their clinical efficacy is limited and there is ample need for more effective joint lubricants. This study first evaluates the tribological and biochemical properties of bovine articular cartilage explants after stimulation with the inflammatory cytokine interleukin-1β. This model is then used to investigate the tribological potential of carboxybetaine (CBAA)-based zwitterionic polymers of linear and bottlebrush architecture. Due to their affinity for cartilage tissue, these polymers form a highly hydrated surface layer that decreases friction under high load in the boundary lubrication regime. For linear pCBAA, these benefits are retained over several weeks and the relaxation time of cartilage explants under compression is furthermore decreased, thereby potentially boosting the weeping lubrication mechanism. Bottlebrush bb-pCBAA shows smaller benefits under boundary lubrication but is more viscous than linear pCBAA, therefore providing better lubrication under low load in the fluid-film regime and enabling a longer residence time to bind to the cartilage surface. Showing how CBAA-based polymers restore the lost lubrication mechanisms during inflammation can inspire the next steps toward more effective joint lubricants in the future.

Nano-innovations in lubrication: Unveiling the synergistic boundary lubrication potential of zinc titanate with phosphonium ionic liquid

This study investigates the potential of Zinctitanate (ZnTiO3) nanoparticles (APS <100 nm) as nano-additives in lubricant formulations, in conjunction with phosphonium phosphate ionic liquids. Comparative analysis against well-established nano-additives highlights ZnTiO3's exceptional dispersion stability, enduring beyond 225 days. Tribological assessments unveil significant enhancements in antiwear, antifriction, and extreme pressure properties, with 1 wt% ZnTiO3 outperforming the base oil. The ZnTiO3 hybrid nanolubricants exhibited notable synergy, achieving a 49 % reduction in wear compared to hexagonal boron nitride (hBN) and Boehmite NPs. Surface characterization confirms the formation of a protective transfer film (≥100 nm thick), attributed to ZnTiO3's disintegration under tribostress. This results in the development of a mixed metallic metal oxide interfacial film composed of (FeTiO3, FePO4, and ZnO) domains on exposed nascent steel surfaces through tribochemical reactions. These findings suggest ZnTiO3 as a promising nano-additive for advanced lubricants, offering substantial improvements in dispersion stability and tribological performance.

Influence of ZDDP tribofilm on micropitting formation and progression

This paper presents insights into how Zinc Dialkyl Dithiophosphate (ZDDP) influences the formation and progression of micropitting. Experimental investigations were conducted using a twin-disc tribometer in rolling-sliding contacts under mixed or boundary lubrication conditions, focusing on the impact of ZDDP on micropitting in bearing steel samples. Results show that ZDDP reduces wear and increases surface friction by forming a tribofilm. This facilitates micropitting formation, but also retards the progression of micropitting. It highlights that understanding the chemical and mechanical interactions at the tribological interface is essential for designing effective mitigation strategies for managing micropitting and avoiding related issues in bearing applications.