Perfluorinated tetradecanoic acid was added as an additive to a base oil and tested under full film elastohydrodynamic (EHD) contact conditions between a steel ball and a steel disc. By varying key performance parameters, we aimed to assess the feasibility and limitations of perfluorinated carboxylic acids in reducing friction in lubricated contacts. The results demonstrate that the tested perfluorinated additive is effective in reducing friction when blended with a non-polar synthetic poly-alpha-olefin oil. However, no significant friction reduction was observed when the same additive was used in a slightly polar synthetic ester. The carboxylic acid functional group plays a crucial role in the observed friction-reducing effect. Adjusting the additive concentration further plays an important role in reducing friction. A concentration of at least 0.35 wt.% is required to achieve a notable friction reduction of approximately 10%. Increasing the concentration beyond this threshold continues to improve the friction-reducing effect. Conversely, increasing the contact pressure has a detrimental impact on friction reduction. The greatest reduction in friction—over 20% compared to the base oil—was achieved at the lowest contact pressure tested (0.69 GPa).

Bone diseases lead to an increasing demand for implants to treat long bone defects and for load-bearing applications. Osteoporosis care and accidental injuries are major contributors to this rising need. Our research aims to demonstrate innovative material systems and methods for preparing implants that can be used in regenerative medicine. We hypothesize that by combining titanium alloys (Ti6Al4V) with hydroxyapatite (Hap), we can enhance biocompatibility and tribo-mechanical performance, which are critical for the longevity of Ti-based surgical implants. Additionally, we investigate the application of laser surface treatments to expose the underlying porosity, thereby enhancing cell transport and promoting cell growth. In this study, we investigate the effects of two fabrication techniques—Spark Plasma Sintering (SPS) and powder metallurgy (PM)—on the properties of laser-textured Ti64/Hap biocomposites. Our findings demonstrate that the selected processing route significantly influences the microstructure, tribological performance, and surface properties of these materials. An X-ray diffraction (XRD) analysis corroborates our results from incubation studies in simulated body fluids, highlighting the impact of phase transformations during sintering on the chemical properties of Ti-Hap composites. Additionally, while laser surface texturing was found to slightly increase the friction coefficient, it markedly enhanced the wear resistance, particularly for the PM and SPS Ti + 5%Hap composites.

In elastohydrodynamic lubrication (EHL) contacts, lubricant viscosity plays an important role in friction and film formation. Previous studies have established that shear thinning and thermal softening influence both friction and film thickness. However, the distinct effects of temperature- and shear-rate-dependent viscosity on local cross-film lubricant flow and their relationship to changes in film thickness at high slide-to-roll ratios (SRRs) remain insufficiently resolved. A validated full Navier–Stokes thermo-EHL line-contact model with three viscosity formulations was employed to distinguish the effects of shear thinning and thermal softening on friction, film thickness, and local lubricant flow: (i) a thermal shear-thinning (non-Newtonian) formulation, (ii) a thermal Newtonian formulation, and (iii) a thermally uncoupled shear-thinning (isothermal non-Newtonian) formulation. Two parameter sweeps were performed: the slide-to-roll ratio (SRR) was varied at a fixed entrainment velocity, and the entrainment velocity was varied at a fixed SRR. With increasing sliding speed, thermal softening becomes more important for friction behaviour relative to shear thinning, resulting in the transition from shear-thinning-dominated to thermal-softening-dominated friction behaviour. Similarly, thermal softening has a greater influence on film thickness than shear thinning. Shear thinning primarily reduces viscosity and wall shear stress while maintaining a nearly linear Couette-type velocity profile, with only a minor effect on film thickness. In contrast, an important new phenomenon is that thermal softening has a distinct effect on film thickness. When the contacting surfaces move at different velocities, thermal softening produces a non-uniform cross-film viscosity distribution, which redistributes the velocity gradient and increases the mean lubricant velocity. For a given mass flow, this increase in mean velocity reduces film thickness, consistent with the conservation of mass.

In this study, we investigated the tribological properties of various additives (lubricity, friction modifiers, anti-wear and extreme pressure) in a highly volatile paraffinic base oil formulated for stamping applications, using a newly developed methodology for tribological testing. The investigation focused on the short-term (10 cycles) and long-term (10,000 cycles) effects of the different additive mixtures on friction and wear behaviour. It was found that the performance of the additive mixtures evolves with sliding time, which is due to changes in contact conditions: the transfer of the Fe film from the steel sheet to the WC-Co surface increases the contact area, which in turn leads to a significant reduction in contact pressure and changes the activation of tribofilm formation. The presence of tribofilms influences the amount and size of the contact area and reduces the adhesion between the contact surfaces. Among the conventional additives, sulphurised additive mixtures show stable performance under both short and long-term conditions, while more aggressive chlorinated additive mixtures perform well in the short term, but their performance decreases with prolonged sliding. Importantly, the additives with a decreasing environmental impact outperformed the conventional additives under long-term conditions: the less harmful phosphorus-based mixture outperformed the sulphurised mixtures in terms of wear properties, while the performance of environmentally acceptable polyol ester was particularly encouraging, exhibiting the lowest friction coefficient (~0.11, compared with ~0.12 for S-oil and 0.14 for S-ester) and the second lowest wear coefficient (~1.1 × 10⁻¹ mm³/Nm compared with ~1.5 × 10⁻¹ mm³/Nm for S-ester). Overall, the polyol ester reduced the coefficient of friction by approximately 8 to 21% compared to sulphurised additive mixtures, and its wear coefficient was also about 27% lower.

In this study, the frictional behaviours of three different guide elements—guide rings, labyrinth seals, and hydrostatic bearings—in hydraulic cylinders is investigated experimentally. A modular, double-acting hydraulic cylinder was designed to compare these three different design elements under different pressures (0 bar, 120 bar, and 240 bar), velocities, and radial loads. The results show that the guide rings exhibit the highest friction, especially at high pressures. Labyrinth seals exhibit significantly lower friction and extend the service life of the components. Hydrostatic bearings allow low friction but require precise control of the fluid, which limits their use. The results provide practical guidelines for selecting guide elements and optimising the friction performance, durability, and efficiency of hydraulic systems. We found that the best solution from the points of view of design, friction, and economics is to use labyrinth seals as guiding elements for the fast reciprocal moving rods of hydraulic cylinders.