Tribology Letters最新文献

The development of environmentally acceptable lubricants and lubricant additives has become a focal point within tribology due to increasing regulatory and sustainability demands. In this context, low-viscosity lubricants are gaining attention for their potential to reduce energy losses. However, their performance under a boundary lubrication regime, where thinner oil film build-up is present, requires more efficient boundary additives. This work evaluates a polyoxometalate-ionic liquid (POM-IL) as a multifunctional boundary additive in a low-viscosity polyalphaolefin-based lubricant, comparing its performance to zinc dialkyldithiophosphate (ZDDP) and a halogen-containing ionic liquid (IL). Tribological tests on AISI 316L stainless steel and AISI 52100 bearing steel revealed that while ZDDP showed substrate-independent adsorption and tribological performance, the IL-based additives had substrate-dependent behaviour. Strong chemisorption was consistent for both IL-based additives, yet their anti-wear and friction-reducing properties differed, showing evidence for the presence of a combined mechanism that includes both strong adsorption and tribochemical reactions. Additionally, the interaction between POM-ILs’ negatively charged surfaces, W atoms, and Cr(III) in 316L was identified as a key factor in their performance. Notably, significant work-hardening was observed in 316L lubricated with POM-IL-containing blends, further enhancing its anti-wear properties. These findings emphasize the role of substrate chemistry in boundary lubricant additive performance in low-viscosity lubricants, offering insights for the development of more efficient multifunctional boundary lubrication solutions.

Pressure boundary conditions are required to solve the Reynolds equation for hydrodynamic lubrication. Several boundary conditions have been proposed for the outlet end of the pressure profile while a limited choice is available for the inlet zone. In order not to impose a priori conditions, here we apply the smoothed particle hydrodynamics method (SPH) to the hydrodynamic lubrication problem solved with the Navier–Stokes equations. First, surface tension calculation which is based on the CSF method is developed to consider the deformation of the liquid–air interface. The action of surface tension is verified by comparing the theoretical values of the Laplace pressure and the period of the surface tension oscillation of a circular droplet. The SPH analysis is then used to simulate the hydrodynamic lubrication problem with a limited amount of fluid. The pressure profiles obtained by the SPH analysis show a good agreement with reported FEM and experimental results. Especially in the outlet zone, the minimum pressure and the location of the outlet meniscus boundary agree with the experimental results over a wide range of capillary numbers. Film profiles in the inlet zone are affected by the direction of the gravitational force. In addition, the approach developed here allows the visualization of a vortex flow in the inlet zone and shows that only a limited part of the bottom flow driven by the moving surface is passing through the minimum gap toward the outlet side. This approach opens the way to simulate accurately and without a priori assumptions the hydrodynamic lubrication problem with a free surface flow as found in all starved lubricated contacts.

Polytetrafluoroethylene (PTFE) improves friction and wear performance by forming a transfer film on counterfaces. However, its high wear rates reduce the fitting accuracy and limit its service life. Adding fillers to PTFE can significantly lower its wear rate. Early theory suggests that fillers reduce wear by providing preferential load support and preventing the development and propagation of subsurface cracks. However, this theory cannot explain the ultra-low wear behavior of some nano fillers and lamellar fillers, and it is also found that the model ignored the effect of the transfer film. With the in-depth study of the wear reduction mechanism, it has been revealed that the ultra-low wear behavior of these fillers is closely related to tribochemistry and the formation of the transfer film. Moreover, research has shown that the type of filler affects the tribological properties of composites. Therefore, it is essential to investigate the wear-reduction mechanisms of different fillers. In this study, we investigated the wear reduction mechanisms of four representative filler-filled PTFE composites, which included carbon-based materials, metals, polar polymers, and non-polar polymers. The results show that (1) the accumulation and preferential load support of fillers on the polymer surface determine its wear resistance, and (2) filler-induced PTFE chain breakage promotes tribochemistry and facilitates the formation of adherent transfer films. Based on these findings, recommendations are provided for the design of low-wear PTFE friction systems.

Graphical Abstract

Nano-sized calcium carbonate and micro-sized graphite are commonly used additives in calcium sulfonate grease. It is worth studying how the two additives together affect the tribological properties in the complex oil-soap structure of calcium sulfonate grease. Regarding this issue, nano-sized CaCO₃ and micro-sized graphite blends in different proportions were selected to investigate the lubrication performance. A combination of 1 wt% micro-sized graphite and 5 wt% nano-sized CaCO₃ contributed the excellence friction-reducing and anti-wear compared to the calcium sulfonate grease without additives or the calcium sulfonate grease with above additions alone. The additive blends could be synergistic to take part in chemical reactions on the Hertz contact area and form films to maintain a low friction state during whole friction process.

Additives made of nitrogen-functionalized copolymers were paired with diamond-like carbon (DLC) coatings doped with different amounts of oxygen and silver to form systems capable of improving tribological performance against undoped-DLC/additive systems. Initial characterisation indicated that silver doping reduced hardness and wettability in the surface, contrary to oxygen doping. Adhesion improved with higher doping levels. Tribological testing was done in boundary conditions, with silver-doped DLC coatings achieving a reduction in wear, but not friction. Oxygen-doped DLC coatings showed similar behaviour. Micrographs identified the wear mechanism as pure polishing and proved the protective effect of doped-DLC/additive systems. The findings suggest an across-scales effect of properties in the performance of the system and promising use in applications requiring wear resistance and friction reduction.

Graphical abstract

Amorphous carbons can have drastically different physical properties depending on synthetic methods. Among these, hydrogenated diamond-like carbon (HDLC) produced via plasma-enhanced chemical vapor deposition is unique in that it exhibits superlubricity with a coefficient of friction (COF) less than 0.01 in proper environmental conditions. It is known that HDLC undergoes friction-induced graphitization at the shear interface and forms a highly hydrogenated transfer film at the counter-surface sliding against it. In contrast, glassy carbon (GC) produced via pyrolysis of organic precursors rarely exhibits superlubricious behavior even though the graphitic nature probed with Raman spectroscopy is similar to that of the transfer film formed from HDLC. This study addresses this drastic difference in friction of HDLC and GC and identifies key parameters that can be tuned to achieve (nearly) superlubricious behaviors with GC. The factors influencing the superlubricity of amorphous carbon include the composition and structure of the initial carbon coating, which strongly depend on the synthetic method, and the coating failure and transfer film stability, which depend on the surface chemistry of the substrate.

This paper presents a high-resolution characterization of tool–chip interfacial stress distribution in cutting of a ductile metal using full-field photoelasticity. Our findings reveal the complex nature of stress distribution and friction along the contact, influenced by tool geometry. Notably, for negative rake-angle tools, the measurements reveal a distinct zone near the tool tip where the shear stress decreases as one travels toward the tool tip. Stress measurements are complemented with in situ velocimetry of chip flow at the interface to enable correlation between stresses and velocity distribution at the interface. Based on the data, the tool–chip interface is categorized into three distinct zones: (1) a retardation zone near the tool tip, characterized by a high normal stress but small shear stresses, followed by (2) a uniform sliding zone with a relatively constant shear stress and (3) an elastic zone near the contact edge that obeys Coulomb friction. The paper challenges the customary practice of dividing the tool–chip contact into sticking and sliding zones and instead argues for a contact description, in terms of plastic and elastic zones that is more consistent with the experimental observations. The study also highlights the importance of high-resolution in situ characterization techniques for resolving the complex nature of friction in cutting and other similar sliding plastic contacts.

Graphical abstract

Understanding the influence of electric current on tribochemical reactions and tribofilm formation at electrified interfaces is crucial for advancing lubrication strategies in electric vehicles (EVs) and other electromechanical systems. In this study, we investigated the formation of polymeric tribofilms from a model precursor (α-pinene) under low and high electric current densities and analyzed their electrical properties using an impedance spectroscopy method implemented using a lock-in amplifier. The results revealed that while electric current does not significantly change the tribopolymerization yield or the chemical properties of tribofilm, it has a substantial impact on the electrical properties of the resulting film. Compared to the tribofilm formed without current, the tribofilm formed at a current density of 0.055 A/cm² exhibited a significant decrease in resistivity while the dielectric constant, film thickness, and elastic modulus remained nearly unchanged. However, at a higher current density of 1.1 A/cm², the tribofilm on the sliding contact was much thinner, more compliant, and less polarizable. These findings highlight the impact of electric current during tribochemical reactions on the resistive and dielectric properties of tribofilms, offering insights for optimizing lubricant formulations in electrified friction interfaces. The impedance measurement method developed here will enable electrical characterization of any tribofilm at electrified surfaces in various lubrication conditions.

Graphical Abstract

This study presents the bio-tribological analysis of Ti₃C₂T_x-reinforced CoCrMo matrix composites fabricated by laser beam powder bed fusion. Raman spectroscopy confirmed the structural and functional integrity during metal matrix composite (MMC) fabrication, while Vickers hardness increased with Ti₃C₂T_x content. Together with roughness and wettability, Ti₃C₂T_x-reinforced CoCrMo composites create a favorable balance between hardness, surface roughness, and hydrophilicity, making them suitable for biomedical applications. Bio-tribological analyses under dry and substitute synovial body fluid (SBF)-lubricated conditions revealed a substantial wear reduction of 78 and 39% compared to reference. These findings underscore Ti₃C₂T_x' ability to mitigate wear through enhanced interfacial interactions and lubrication, promising advancements in biomedical implants.

Graphical abstract

The repeated rubbing of a polymer microsphere against a hard, nanorough surface is a paradigmatic and not-so-simple process in tribology. Here, we have investigated this process in the case of poly(methylmethacrylate) (PMMA) colloidal probes (15 µm diameter) elastically driven on a Mo-doped DLC film with RMS roughness of about 3 nm in ambient conditions. The probes are flattened on length scales of few tens of nm, increasing with the applied load (up to few tens of nN). A complementary analysis on a periodic silicon grating enables a quantitative characterization of the early-stage nanowear process. On the Mo-DLC film, two different contrasts are observed, corresponding to the probe forming a multi-contact interface with the rough surface or sliding across the top asperities of the film. The contact area between probe and film, in the multi-contact regime, is also estimated based on the Persson theory. It allows us to confirm that the flattening of the sphere must be limited, with the applied loading force values, to a contact radius of about 750 nm.