The surfaces of two engineering polymers including polyamide 66 (PA66) and polytetrafluoroethylene (PTFE) were treated by diffuse coplanar surface barrier discharges in atmospheric air. We found that plasma treatment improved the adhesion of PA66 for either polymer/polymer or polymer/steel joints, however, it was selective for the investigated adhesive agents. For PTFE the adhesion was unaltered for plasma treatment regardless the type of used adhesive. Tribological properties were slightly improved for PA66, too. Both the friction coefficient and wear decreased. Significant changes, again, could not be detected for PTFE. The occurred variation in the adhesion and tribology was discussed on the basis of the occurred changes in surface chemistry, wettability and topography of the polymer surface.

In this article, we focus on utilising electrical impedance spectroscopy (EIS) for the assessment of global and contact impedances in roller bearings. Our primary objective is to establish a quantitative prediction of lubricant film thickness in elasto-hydrodynamic lubrication (EHL) and investigate the impedance transition from ohmic to capacitive behaviour as the system shifts from boundary lubrication to EHL. To achieve this, we conduct measurements of electrical impedance, bearing and oil temperature, and frictional torque in a cylindrical roller thrust bearing (CRTB) subjected to pure axial loading across various rotational speeds and supply oil temperatures. The measured impedance data is analysed and translated into a quantitative measure of lubricant film thickness within the contacts using the impedance-based and capacitance-based methods. For EHL, we observe that the measured capacitance of the EHL contact deviates from the theoretical value based on a Hertzian contact shape by a factor ranging from 3 to 11, depending on rotational speed, load, and temperature. The translation of complex impedance values to film thickness, employing the impedance and capacitance method, is then compared with the analytically estimated film thickness using the Moes correlation, corrected for inlet shear heating effects. This comparison demonstrates a robust agreement within 2% for EHL film thickness measurement. Monitoring the bearing resistance and capacitance via EIS across rotational speeds clearly shows the transition from boundary to mixed lubrication as well as the transition from mixed lubrication to EHL. Finally, we have observed that monitoring the electrical impedance appears to have the potential to perform the run-in of bearings in a controlled way.

When simulating elastohydrodynamic lubrication, two main approaches are usually followed to predict the pressure and fluid film thickness distribution throughout the contact. The conventional approach relies on the Reynolds equation to describe the thin lubricant film, which is coupled to a Boussinesq description of the linear elastic deformation of the solids. A more accurate, yet a time-consuming method is the use of computational fluid dynamics in which the Navier–Stokes equations describe the flow of the thin lubricant film, coupled to a finite element solver for the description of the local contact deformation. This investigation aims at assessing both methods for different lubrication conditions in different elastohydrodynamic lubrication (EHL) regimes and quantify their differences to understand advantages and limitations of both methods. This investigation shows how the results from both approaches deviate for three scenarios: (1) inertial contributions (Re > 1), i.e., thick films, high speed, and low viscosity; (2) high shear stresses leading to secondary flows; and (3) large deformations of the solids leading to inaccuracies of the Boussinesq equation.

Viscosity is a crucial property of liquid lubricants, and it is theoretically a well-defined quantity in molecular dynamics (MD) simulations. However, no standardized protocol has been defined for calculating this property from equilibrium MD simulations. While best practices do exist, the actual calculation depends on several ad hoc decisions during the post-processing of the raw MD data. A common protocol for calculating the viscosity with equilibrium MD simulations is called the time decomposition method (TDM). Although the TDM attempts to standardize the viscosity calculation using the Green–Kubo method, it still relies on certain empirical rules and subjective user observations, e.g., the plateau region of the Green–Kubo integral or the integration cut-off time. It is known that the TDM works reasonably well for low-viscosity fluids, e.g., at high temperatures. However, modified heuristics have been proposed at high pressures, indicating that no single set of rules works well for all circumstances. This study examines the effect of heuristics and ad hoc decisions on the predicted viscosity of a short, branched lubricant molecule, 2,2,4-trimethylhexane. Equilibrium molecular dynamics simulations were performed at various operating conditions (high pressures and temperatures), followed by post-processing with three levels of uncertainty quantification. A new approach, “Enhanced Bootstrapping”, is introduced to assess the effects of individual ad hoc parameters on the viscosity. The results show a strong linear correlation (with a Pearson correlation coefficient of up to 36%) between the calculated viscosity and an ad hoc TDM parameter, which determines the integration cut-off time, under realistic lubrication conditions, particularly at high pressures. This study reveals that ad hoc decisions can lead to potentially misleading conclusions when the post-processing is performed ambiguously.