Tribology Letters最新文献

Droplet flows, termed misting, are significant lubrication flow mechanisms to, in and around the piston assembly. Therefore, these are important in piston assembly tribology and engine performance. Crankcase lubricant degradation rate has been hypothesised to be influenced by lubricant droplet flows through the piston assembly and crankcase, but not previously confirmed. Lubricant was sampled from the sump, top ring zone (TRZ), and mist and aerosol from the crankcase during an extended run. The physical and chemical degradation of these samples was characterised. Droplet flows were intermediate in degradation and fuel dilution between TRZ and sump. Flows with smaller droplet sizes were more degraded that those with larger droplets. The degradation of polymers was dependent on their molecular architecture.

Coupled corrosion and wear damage in marine atmospheric environment seriously restricts the development of aerospace bearings, the aim of present work is to study the influence of various corrosion and wear conditions on the corrosion–wear interaction behavior of 8Cr4Mo4V-bearing steel. The influence of corrosion on wear and wear on corrosion for 8Cr4Mo4V-bearing steel under five corrosion temperatures, NaCl concentrations, rotational speeds, and normal loads was investigated and discussed, respectively, and the quantitative characterization of corrosion–wear interaction (CWI) effect was established. The corroded steels were prepared using salt spray corrosion, a ball-on-disc friction testing machine was used to obtain the coefficient of friction (COF) of corroded steels under dry wear. The corrosion weight losses, wear mass losses, and worn surface morphologies of corroded steels were characterized. The results show that both synergistic and antagonistic effects existed in CWI of 8Cr4Mo4V-bearing steel due to coupling action of corrosion pits and rust layers, which depends on operating conditions. The corrosion temperature and NaCl concentration directly affect the friction stability and corrosion contribution for tribological process via initial corrosion surface, while rotational speed and normal load influence the wear contribution for corrosion process through surface wear degree. This paper gives an idea for analysis of tribocorrosion properties of bearing steel under various alternating corrosion and wear operating conditions.

According to random process theory, the existing autocorrelation function (ACF) expression that characterizes the spatial features of the shot peening (SP) surface topography makes it difficult to constrain the 3D roughness spatial parameters defined in ISO 25178, which restricts the correlation studies between surface topography and service performance. This paper introduces a new ACF expression for reconstructing the SP surface topography with specified spatial parameters. Based on the new expression, a numerical simulation method for stratified surface topography applicable to SP after finishing is introduced. The main idea is to perform feature extraction and feature modeling on the measured surface with the help of machine learning. The new method is applied to the numerical simulation of the SP and grinding-shot peening (Gr-SP) surface topography with a coverage of 200%. The relative error in height distribution and spatial parameters between the measured and simulated surface topography are less than 5%. The new method of height distribution and spatial parameters active design is provided to study the correlation between surface topography and service performance of shot peening parts.

Graphical Abstract

Polytetrafluoroethylene (PTFE) is renowned for its remarkably low friction coefficient (µ ~ 0.1) yet exhibits notably high wear rates (K ~ 10⁴) in dry sliding applications. To mitigate this, various metallic and non-metallic fillers have been explored, consistently demonstrating a reduction in wear rates of unfilled PTFE between 10 and 10⁴ times. Among these fillers, α-Al₂O₃ is one of the most extensively studied materials. 5 wt% of α-Al₂O₃ filler into PTFE yields a composite material, PTFE- α-Al₂O₃, characterized by a wear rate a staggering 10⁴ times lower than unfilled PTFE. This reduction in wear has been attributed to the formation of tribofilms on the PTFE composite and metal counterbody material. These tribofilms emerge due to the interaction between broken fluropolymer chains and environmental water and oxygen. This interaction results in the creation of carboxylate salt groups, which subsequently react with metal/metal oxide particles (both from the counterbody and the metal filler) to form tribofilms. Despite numerous studies scrutinizing the chemical composition of the tribofilms pre- and post-test, the chemical development of these films has remained largely unexplored. In this study, the authors utilize attenuated total reflection infrared spectroscopy (ATR-IR), transmission infrared (IR) spectroscopy, optical microscopy, and stylus profilometry to observe tribofilm development. A thorough topographical and chemical description of the tribofilm is provided via these techniques. The ratio of carboxylate salt groups directly corresponds with improved wear performance and these changes are very local to the worn polymer surface. This discovery contributes to a deeper understanding of the tribological behavior of PTFE-α-Al₂O₃ composites.

Graphical Abstract

Recent investigations have pointed to physical entanglements that greatly outnumber chemical crosslinks as key sources of energy dissipation and low friction in hydrogel networks. Slide-ring gels are an emerging class of hydrogels described by their mobile crosslinks, which are formed by rings topologically constrained to slide along linear polymer chains within the network. These materials have enjoyed decades of study by polymer chemists but have been underexplored by the tribology community. In this work, we synthesized a pseudo-rotaxane crosslinker from poly(ethylene glycol) diacrylate (PEG-diacrylate) and α-cyclodextrin-acrylate followed by hydrogel networks by connecting the sliding crosslinks with polyacrylamide chains. The mechanical and tribological properties of slide-ring hydrogels were investigated using a custom-built microtribometer. Slide-ring hydrogels exhibit unique behavior compared to conventional covalently crosslinked polyacrylamide hydrogels and offer a vast design space for future investigations.

Graphical Abstract

Graphene (GP), when used as a lubricant additive, not only reduces the friction coefficient but also enhances wear resistance by forming a protective lubrication film. However, there are still several challenges in practical applications related to graphene preparation. Therefore, this study employs a novel type of ionic liquid deep eutectic solvent as an interlayer agent for graphene preparation and investigates its tribological properties when used as an additive. We used choline chloride/ethylene glycol deep eutectic solvent as the intercalation agent and successfully prepared graphene samples using liquid-phase exfoliation. The resulting graphene samples had a thickness of 4–5 layers. The peeling mechanism is analyzed through molecular dynamics simulations and characterization techniques such as Raman spectroscopy, XRD, SEM, and AFM. In friction experiments conducted with different mass fractions of 1.5 wt% DES and 0.05 wt% GP as lubricant additives, it is observed that the mixture exhibits optimal lubrication performance compared to base oil alone; specifically reducing average friction coefficient by 56.8% and depth of wear marks by 59.8%. This enhancement in friction performance can be attributed to both high wettability and synergistic effects between composite lubricants. Considering the wide range of available DESs and two-dimensional materials, these newly developed functional two-dimensional materials based on DES hold significant research potential.

The influence of roughness on adhesion has been studied since the time of Fuller and Tabor, but recently there has been debate about how roughness exactly seems to kill (but sometimes enhance!) adhesion, particularly with reference to the accepted model of fractal roughness. We show that the Persson–Tosatti criterion does not depend on anisotropy of the surface for a typical power law PSD if written in terms of rms roughness and magnification. Instead, a very simple extension of the Bearing Area Model (BAM) of Ciavarella to anisotropic fractal surface shows a weak but clear dependence on the anisotropy, with higher adhesion in the 1D case, showing better agreement than the Persson–Tosatti criterion to actual numerical results of Afferrante Violano and Dini. However, neither of the two models permit to capture the strong hysteresis found in experiments between loading and unloading, which is very likely to enhance adhesion more as we move from the isotropic to the full 1D case. This suggests the mechanism of load amplification along contact lines and the associated elastic instabilities, is not captured by either the Persson–Tosatti or the BAM model applied to anisotropic surfaces.

Recent intriguing experimental observations of atomic scale friction enhancement, that takes place at scanning velocities correspondent with the cantilever frequency and/or its fractions (1/n, n = 1, 2, 3, …), can be explained as the manifestation of an autoparametric resonance. Taking explicitly into account high flexibility of AFM tips, the developed theory reveals the autoparametric resonance to be a natural consequence of the rich dynamics of the combined tip–cantilever system. Besides the explanation of the observed friction force peaks, the theory predicts a dense multiplicity of smaller peaks to appear when the washboard frequency coincides with a rational part (m/n, with integer m and n) of the cantilever frequency. An important conclusion is made that the resonance enhancement of friction is independent of frequency of excited phonons, and it should manifest itself for any possible mechanism of frictional energy dissipation in the substrate, phononic, electronic, or any other.

Graphical Abstract

Friction coefficient depends on various factors or surface characteristics during tribological testing, and this friction coefficient can be modified by altering the properties of one of the two contacting surfaces. It is crucial to monitor the friction coefficient continuously, not only at the conclusion of the test. This research examined the evolution of friction coefficient of silicon nitride (Si₃N₄) coating and H13 steel over different sliding distances (250, 500, 750, 1000 m). The study assessed surface wear and oxidation through three-dimensional profilometry and SEM/EDX. The findings indicated a reduction in friction coefficient by 22%, a decrease in wear rate by 88%, and a reduction in wear volume by 87% when comparing the silicon nitride coated steel to the uncoated steel. Furthermore, the changes in friction coefficient provided insights into the timing of the complete fracture of the hard coating.

Graphical abstract