Understanding elastohydrodynamic (EHL) film formation and the pressure-viscosity response of lubricants is necessary for designing rolling/sliding tribological contacts. This article investigates the EHL behaviour of four formulated water-based lubricants (glycerol-water, glycol-water, and ionic liquid-water) and one reference oil under moderately high pressures, typical in gears and bearings applications. A ball-on-disc tribometer with optical interferometry was employed to measure the film thickness of the water-based lubricants. The results highlight the sensitivity of film formation to entrainment speed, slide-to-roll ratio (SRR), temperature, and lubricant composition. Water loss due to evaporation significantly impacts film formation at high temperatures. Additionally, an unusual increase in film thickness was observed for the glycol-water solution, likely due to complex tribological conditions. The limitations of the classical Hamrock-Dowson film thickness equation for water-based lubricants are also discussed. Furthermore, pressure-viscosity coefficients of the water-based lubricants were estimated using both optical interferometry and high-pressure viscometer methods. The effect of water content on the pressure-viscosity coefficient was also examined, revealing that higher water content leads to reduced pressure and temperature dependence of viscosity.

The long-term intelligent machine condition monitoring system is essential in improving maintenance costs and decision-making. Triboelectric nanogenerator (TENG) has a great advantage in developing self-powered machine condition monitoring. The main issues preventing TENG for such applications are poor integration to machine components, limited operational range, and weak durability. In this work, durable non-contact TENG energy harvester adaptive is designed to mechanical shafts for harvesting rotational energy. The harvester is designed to automatically switch to the non-contact mode by using centrifugal force to avoid manual switching while operating over a wide range of speeds of 0–2000 rpm. The designed TENG generates a high output of up to 25 mW with excellent stability for >20 days of continuous operation and exhibits a high-power density of 286 W m−3. Moreover, a self-powered long-term continuous condition monitoring system is developed from a TENG sensor, energy harvester, and wireless module. The developed system successfully sends possible machine fault frequency every 74 min to the cloud and accessible anywhere. This is the only TENG design reported in the literature that can fully power a Wi-Fi module to send data. Hence, the result promises the practical application of the system in developing internet of things (IoT) in the Industry.

Greenwood and Williamson (GW) and Greenwood and Tripp (GT) elastic contact models have extensively been used previously for the mixed-lubrication analysis of water-lubricated journal bearings (WLJBs). The approximate expressions of a parabolic cylinder function involved in these contact models are available in the literature which makes these models easy to implement in the mixed-lubrication analysis. However, approximate expressions available so far are valid only for the Gaussian distribution of asperity heights. Moreover, elastic-plastic contact models with few exceptions have rarely been used in the mixed-lubrication analysis of WLJBs. The present work demonstrates a simple numerical procedure to predict the mixed-lubrication parameters for WLJBs. The average Reynolds equation considering non-Gaussian flow factors is solved along with film thickness and load balance equations in a coupled manner. A routine is developed to predict the pressure curve (a plot between average asperity pressure and rigid body displacement) for different contact models and non-Gaussian surface topography, and the asperity pressure for a particular surface topography is predicted utilizing the concept of a macro-micro approach. The effect of non-Gaussian surface topography on the tribological performance of WLJBs is discussed in detail. The influence of different contact models on the performance of WLJBs is also presented. It is shown that the GT elastic contact model fails to simulate the effect of kurtosis and skewness properly. However, the Kogut and Etsion (KE) elastic-plastic contact model which is based on finite element method (FEM)-based analysis excellently simulates the non-Gaussian effect on the performance of the WLJB.

Surface performance is critically influenced by topography in virtually all real-world applications. The current standard practice is to describe topography using one of a few industry-standard parameters. The most commonly reported number is a, the average absolute deviation of the height from the mean line (at some, not necessarily known or specified, lateral length scale). However, other parameters, particularly those that are scale-dependent, influence surface and interfacial properties; for example the local surface slope is critical for visual appearance, friction, and wear. The present Surface-Topography Challenge was launched to raise awareness for the need of a multi-scale description, but also to assess the reliability of different metrology techniques. In the resulting international collaborative effort, 153 scientists and engineers from 64 research groups and companies across 20 countries characterized statistically equivalent samples from two different surfaces: a “rough” and a “smooth” surface. The results of the 2088 measurements constitute the most comprehensive surface description ever compiled. We find wide disagreement across measurements and techniques when the lateral scale of the measurement is ignored. Consensus is established through scale-dependent parameters while removing data that violates an established resolution criterion and deviates from the majority measurements at each length scale. Our findings suggest best practices for characterizing and specifying topography. The public release of the accumulated data and presented analyses enables global reuse for further scientific investigation and benchmarking.

In this study, non-equilibrium molecular dynamics (NEMD) simulations with a reactive force field were used to investigate the tribochemical properties of glycerol, with and without water, confined between two ferrous surfaces. The results demonstrated that glycerol significantly reduced friction on α-Fe slabs more effectively than on functionalized amorphous magnetite. A numerical method was introduced to identify the interface region and evaluate the dissociated surface atoms. It was found that the dissociation rate of glycerol molecules increased with applied normal pressure and shear stress. Additionally, the production rate of water molecules from glycerol dissociation was consistently positive for all solutions above 80 % wt. The assumption of linear velocity distribution across the film thickness was validated for all systems studied.

Many mechanical systems including rolling/sliding parts, require traction data across a spectrum of operating conditions to predict their motion effectively. Numerous studies have examined the thermal effects and shear-thinning concerning the traction curve, but only a few have focused on the traction coefficient in the linear isothermal regime for low SRR. In this work, we investigate traction coefficient characteristics of EHL point contacts in the linear isothermal regime, over a wide range of operational conditions. To this end, we conduct numerical simulations utilizing a fully-coupled finite element-based model, resulting in a prediction formula for the traction coefficient slope. With this formula, the traction coefficient slope could be predicted for the operating conditions considered.

Determining an accurate state of lubrication is of utmost importance for the precise functionality of machine elements and to achieve elongated life and durability. In this work, a homogenized mixed-lubrication model is developed to study the effect of surface topographies on the coefficient of friction. Various measured real surface topographies are integrated in the model using the roughness homogenization method. The shear-thinning behavior of the lubricant is incorporated by employing the Eyring constitutive relation. Several Stribeck curves are generated to analyze the effect of roughness lays and root mean square (RMS) roughness on the coefficient of friction. The homogenized mixed lubrication model is validated against experimental rolling/sliding ball-on-disc results, and a good agreement between simulated and experimental coefficient of friction is found.

This paper introduces an optimized computational domain for fully flooded circular elastohydrodynamic lubrication (EHL) contacts, enhancing the accuracy of numerical calculations of pressure and oil film thickness. First, the computational domain was configured based on Kapitza's analytical solution. Then, a resolution sensitivity study for the mesh of the 2D computational domain for the Reynolds equation was conducted to investigate the effect of mesh resolution on the accuracy of the numerical solution. Subsequently, the impact of the size of the full 3D computational domain on the simulation's accuracy and computational efficiency was analyzed. The main result is the 3D computational domain, which automatically adapts to operating conditions within the piezoviscous rigid, the isoviscous rigid, the piezoviscous elastic, and the isoviscous elastic regions, as well as in the transition regions between them. This results in a model which provides accurate predictions across a wide range of operational conditions. Another outcome is a new approximate expression for the central oil film thickness, showing a maximum relative difference of less than 4.6% compared to the numerical model.

Unwanted ice build-up is a ubiquitous phenomenon in nature, which creates a series of catastrophic impacts on a wide range of human activities. Various anti/de-icing materials have been proposed for dealing with icing issues. Superhydrophobic anti/de-icing coatings have been widely reported since it has high efficiency and can be achieved in different ways. The surface functional groups have a significant influence on surface energy which is related to surface wettability. However, the influence of the coating surfaces functional groups on the anti-/de-icing properties is still not well studied. To investigate this influence, different groups with different hydrophilicity have been introduced to 3,3,4,4,5,5,6,6,7,7,8,8,8-Tridecafluorooctyl acrylate (TFOA) to fabricate several branch copolymer ice-phobic coatings. The anti-icing performance and the influence of group radius and interaction were studied. The acrylic acid TFOA showed a great superhydrophobic property (over 150° water contact angle), lower ice adhesion strength (<50 kPa), and lower wear depth compared with other copolymer coatings. The mechanism was studied via the molecular dynamic calculation carried out in ChemDraw software. The interaction between hydrophobic and hydrophilic groups and the steric length of the hydrophilic groups influence the surface structure and surface element distribution, further influencing the ice adhesion strength.