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Using nanoadditives in lubricants is one of the most effective ways to control friction and wear, which is of great significance for energy conservation, emission reduction, and environmental protection. With the scientific and technological development, great advances have been made in nanolubricant additives in the scientific research and industrial applications. This review summarizes the categories of nanolubricant additives and illustrates the tribological properties of these additives. Based on the component elements of nanomaterials, nanolubricant additives can be divided into three types: nanometal-based, nanocarbon-based, and nanocomposite-based additives. The dispersion stabilities of additives in lubricants are also discussed in the review systematically. Various affecting factors and effective dispersion methods have been investigated in detail. Moreover, the review summarizes the lubrication mechanisms of nanolubricant additives including tribofilm formation, micro-bearing effect, self-repair performance, and synergistic effect. In addition, the challenges and prospects of nanolubricant additives are proposed, which guides the design and synthesis of novel additives with significant lubrication and antiwear properties in the future.

Despite numerous experimental and theoretical studies reported in the literature, surface micro-texturing to control friction and wear in lubricated tribo-contacts is still in the trial-and-error phase. The tribological behaviour and advantageous micro-texture geometries and arrangements largely depend on the contact type and the operating conditions. Industrial scale implementation is hampered by the complexity of numerical approaches. This substantiates the urgent need to numerically design and optimize micro-textures for specific conditions. Since these aspects have not been covered by other review articles yet, we aim at summarizing the existing state-of-the art regarding optimization strategies for micro-textures applied in hydrodynamically and elastohydrodynamically lubricated contacts. Our analysis demonstrates the great potential of optimization strategies to further tailor micro-textures with the overall aim to reduce friction and wear, thus contributing toward an improved energy efficiency and sustainability.

The Ni-TiO and Ni-CeO composite coatings with varying hydrophilic/hydrophobic characteristics were fabricated by the electrodeposition method from a tartrate electrolyte at ambient temperature. To meet the requirements of tight regulation by the European Chemicals Agency classifying H BO as a substance of very high concern, Rochelle salt was utilized as a buffer solution instead. The novelty of this study was to implement a simple one-step galvanostatic electrodeposition from the low-temperature electrolyte based on a greener buffer compared to traditionally used, aiming to obtain new types of soft-matrix Ni, Ni-CeO , and Ni-TiO coatings onto steel or copper substrates. The surface characteristics of electrodeposited nickel composites were evaluated by SEM, EDS, surface contact angle measurements, and XPS. Physiochemical properties of pure Ni, Ni-CeO and Ni-TiO composites, namely, wear resistance, microhardness, microroughness, and photocatalytic activity, were studied. Potentiodynamic polarization, EIS, and ICP-MS analyses were employed to study the long-term corrosion behavior of coatings in a 0.5 M NaCl solution. Superior photocatalytic degradation of methylene blue, 96.2% after 6 h of illumination, was achieved in the case of Ni-TiO composite, while no substantial change in the photocatalytic behavior of the Ni-CeO compared to pure Ni was observed. Both composites demonstrated higher hardness and wear resistance than pure Ni. This study investigates the feasibility of utilizing TiO as a photocatalytic hydrophilicity promoter in the fabrication of composite coatings for various applications.

Archard’s wear law encounters challenges in accurately predicting wear damage and volumes, particularly in complex situations like asperity–asperity collisions. A modified model is proposed and validated, showcasing its ability to predict wear in adhesive contacts with better accuracy than the original Archard’s wear law. The model introduces an improved wear coefficient linked to deformation energy, creating a spatially varying relationship between wear volume and load and imparting a non-linear characteristic to the problem. The improved wear model is coupled with the Boundary Element Method (BEM), assuming that the interacting surfaces are semi-infinite and flat. The deformation energy is calculated from the normal contact pressure and displacements, which are the common outputs of BEM. By relying solely on these outputs, the model can efficiently predict the correct shape and volume of the adhesive wear particle, without resorting to large and often slow models. An important observation is that the wear coefficient is expected to increase based on the accumulated deformation energy along the direction of frictional force. This approach enhances the model’s capability to capture complex wear mechanisms, providing a more accurate representation of real-world scenarios.

In this work, the stiffness, i.e., the derivative of the load-separation curve, is studied for self-affine fractal surfaces with non-Gaussian height distribution. In particular, the heights of the surfaces are assumed to follow a Weibull distribution. We find that a linear relation between stiffness and load, well established for Gaussian surfaces, is not obtained in this case. Instead, a power law, which can be motivated by dimensionality analysis, is a better descriptor. Also unlike Gaussian surfaces, we find that the stiffness curve is no longer independent of the Hurst exponent in this case. We carefully asses the possible convergence errors to ensure that our conclusions are not affected by them.

This review aims at introducing an engineering field of lubrication to researchers who are not familiar with tribology, thereby emphasizing the importance of lubricant chemistry in applied science. It provides initial guidance regarding additive chemistry in lubrication systems for researchers with different backgrounds. The readers will be introduced to molecular sciences underlying lubrication engineering. Currently, lubricant chemistry, especially “additive technology”, looks like a very complicated field. It seems that scientific information is not always shared by researchers. The cause of this is that lubrication engineering is based on empirical methods and focuses on market requirements. In this regard, engineering knowhow is held by individuals and is not being disclosed to scientific communities. Under these circumstances, a bird’s-eye view of lubricant chemistry in scientific words is necessary. The novelty of this review is to concisely explain the whole picture of additive technology in chemical terms. The roles and functions of additives as the leading actors in lubrication systems are highlighted within the scope of molecular science. First, I give an overview of the fundamental lubrication model and the role of lubricants in machine operations. The existing additives are categorized by the role and work mechanism in lubrication system. Examples of additives are shown with representative molecular structure. The second half of this review explains the scientific background of the lubrication engineering. It includes interactions of different components in lubrication systems. Finally, this review predicts the technical trends in lubricant chemistry and requirements in molecular science. This review does not aim to be a comprehensive chart or present manufacturing knowhow in lubrication engineering. References were carefully selected and cited to extract “the most common opinion” in lubricant chemistry and therefore many engineering articles were omitted for conciseness.

HighlightsThe as-designed triboelectric nanogenerator (TENG) generates an excellent electric output, which is an order of magnitude higher than that of TENGs made from commercial dielectric materials.The as-designed TENG-based sensor can detect worn debris in oils at least down to 0.01 wt% and water contamination down to 100 ppm, which are much better than other online monitoring methods (particle > 0.1 wt%; water > 1000 ppm).A high-selective monitoring system is successfully developed for distinguishing water contamination from the multi-mixed contaminants in lubricating oils.Triboelectric nanogenerators (TENGs) have potential to achieve energy harvesting and condition monitoring of oils, the “lifeblood” of industry. However, oil absorption on the solid surfaces is a great challenge for oil–solid TENG (O-TENG). Here, oleophobic/superamphiphobic O-TENGs are achieved via engineering of solid surface wetting properties. The designed O-TENG can generate an excellent electricity (with a charge density of 9.1 µC m−2 and a power density of 1.23 mW m−2), which is an order of magnitude higher than other O-TENGs made from polytetrafluoroethylene and polyimide. It also has a significant durability (30,000 cycles) and can power a digital thermometer for self-powered sensor applications. Further, a superhigh-sensitivity O-TENG monitoring system is successfully developed for real-time detecting particle/water contaminants in oils. The O-TENG can detect particle contaminants at least down to 0.01 wt% and water contaminants down to 100 ppm, which are much better than previous online monitoring methods (particle > 0.1 wt%; water > 1000 ppm). More interesting, the developed O-TENG can also distinguish water from other contaminants, which means the developed O-TENG has a highly water-selective performance. This work provides an ideal strategy for enhancing the output and durability of TENGs for oil–solid contact and opens new intelligent pathways for oil–solid energy harvesting and oil condition monitoring.

Nano-graphene oxide (nano-GO) is a new class of carbon based materials being proposed for biomedical applications due to its small size, intrinsic optical properties, large specific surface area and easy to functionalize. To fully exploit nano-GO properties, a reproducible method for its production is of utmost importance. Herein we report, the study of the sequential fracture of GO sheets onto nano-GO with controllable lateral width, by a simple and reproducible method based on a mechanism that we describe as a confined hot spot atomic fragmentation/reduction of GO promoted by ultrasonication. The chemical and structural changes on GO structure during the breakage were monitored by XPS, FTIR, Raman and HRTEM. We found that GO sheets starts breaking from the defects region and in a second phase through the disruption of carbon bonds while still maintaining crystalline carbon domains. The breaking of GO is accompanied by its own reduction, essentially by the elimination of carboxylic and carbonyl functional groups. Photoluminescence and photothermal studies using this nano-GO are also presented highlighting the potential of this nanomaterial as a unique imaging/therapy platform.

Numerous tribological contacts worldwide rely on adequate lubrication quality for proper functionality. Despite this, there is no existing approach to accurately predict the state of lubrication. The default model since introduced in the 1960s—the Λ -ratio, defined as the oil film thickness over the surface roughness height—is unpredictable and may yield erroneous results. Here, we put forward a framework for a new updated film parameter, Λ ∗ , which accounts for the elasto-hydrodynamic lubrication (EHL) effects induced by surface irregularities on the microscopic scale (micro-EHL). This new film parameter was validated in ball-on-disc tribological tests with engineering surfaces comprising isotropic and anisotropic structures. As expected, the new model was found to accurately predict the experimentally measured true mixed and full-film EHL regimes. The ability to accurately predict the mode of lubrication represents a major advance in designing tribological interfaces for optimal efficiency and durability.

The contact between rough metallic bodies almost always involves plastic flow in the area of real contact. We performed indentation experiments on sandblasted aluminum surfaces to explore the plastic deformation of asperities and modeled the contact mechanics using the boundary element method, combined with a simple numerical procedure to take into account the plastic flow. The theory can quantitatively describe the modification of the roughness by the plastic flow. Since the long-wavelength roughness determines the fluid leakage of metallic seals in most cases, we predict that the leakage can be estimated based on the elastoplastic contact mechanics model employed here.