Tribology Letters最新文献

Developing and utilizing high-extreme pressure (EP) performance solid lubricants has become imperative to meet the lubrication requirements for harsh operating conditions. Based on the action mechanism of rare earth on alloys, the influence of material corrugated layered crystal structure, and sulfur content on lubrication properties at the atomic and molecular levels, layered rare earth sulfates have become a new type of high EP performance solid lubricants. Herein, we prepared layered rare earth sulfate materials (RE₂(OH)₄SO₄·nH₂O, RE = La/Sm/Ce, i.e., RES, LaS, SmS, CeS, respectively) using a mild hydrothermal method for controlled particle size and morphology. To evaluate its applicability, we tested the new solid lubricants against high load, high temperature, long-term, different friction pair materials, and fretting. Overall, the RES samples exhibited excellent lubricating behavior when added to polyurea grease, better than MoS₂. By adding layered lanthanum sulfate, samarium sulfate, and cerium sulfate, the seizure loads can reach up to 1400, 1300, and 1300 N, respectively. Microscopic analysis of the friction pairs confirmed that the lubricating films formed on the worn surface during friction. Excitingly, the RES materials exhibited excellent synergistic effects on EP performance and corrosion resistance. Thus, these materials have broad application prospects as lubricants.

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In this study, Fe₃O₄/WS₂ and CuO/WS₂ composites were synthesized via a one-step hydrothermal method, and conductive lubricating greases were formulated using polyurea grease (PUG) as the base. The tribological performance and conductivity of the prepared greases were systematically investigated. The experimental results demonstrate that compared to WS₂ grease, Fe₃O₄/WS₂ and CuO/WS₂ greases exhibit superior tribological performance and enhanced conductivity. Fe₃O₄/WS₂ and CuO/WS₂ composites can form physically adsorbed films and tribochemical reaction films on frictional surfaces, reducing the COF and wear. Furthermore, they enhance the carrier concentration at the friction interface, thereby improving conductivity and significantly mitigating triboelectric effects. These properties underscore their promising potential for applications in conductive lubrication and electrostatic elimination.

This work investigates the quantitative effect of the hydrogen concentration of 100Cr6 bearing steel on the surface-initiated damage induced during lubricated rolling/sliding tribotesting. Hydrogen was introduced to the samples prior to tribotesting by electrochemical pre-charging, and hydrogen concentration was measured using thermal desorption analysis. The surface-initiated damage was quantified by optical profilometry and scanning electron microscopy. An upper limit for the critical hydrogen concentration was determined to be 1.4–2 wppm diffusible hydrogen. At this concentration, the area fraction covered by damage features was found to double compared to uncharged samples. As both charged and uncharged samples exhibited the same type of surface damage (early-stage micropitting), it was concluded that hydrogen did not change the wear mechanism but decreased the number of contact cycles necessary for the initiation of surface defects.

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The contact of rough surfaces is a fundamental issue in analyzing friction, wear, and assembly of two parts. This study developed a multi-scale contact model for rough surfaces. Shoulder-shoulder elastoplastic contact, substrate deformation, and asperity interaction were considered in the asperity scale. A fractal representation was introduced to the contact model in the rough-surface scale, where the critical length scale and critical contact area were determined in the asperity scale. The proposed contact model was validated through comparisons with experimental data, simulation results, and other analytical models. Then, the effects of the surface topography, material properties, and largest length scale on rough-surface contact were examined, and the difference between the shoulder-shoulder and tip-tip contact modes was clarified.

Until recently, it was believed that both central and minimum film thickness in elastohydrodynamic lubricated conjunctions are governed by the lubricant low-pressure rheology, in the contact inlet. A recent study by the authors has shown that minimum film thickness in circular contacts is also affected by the high-pressure rheology. It was shown that higher lubricant viscosity in the high-pressure central zone of the contact leads to lower minimum film thicknesses. This was linked to a reduction in lubricant outflow from the contact through the sides. That is why such effects would not be observed in infinitely long line contacts. It was then posited that minimum-film-thickness sensitivity to high-pressure rheology should be higher in circular contacts than wide elliptical ones and even higher in slender contacts. The current work aims to verify this assumption and elucidate the intricate relation between contact aspect ratio, high-pressure rheology, and minimum film thickness in elastohydrodynamic conjunctions. This is done through finite element simulations of elliptical contacts of variable ellipticity, lubricated with two hypothetical fluids having the exact same low-pressure viscosity response, but a different high-pressure one.

The ZDDP-derived tribofilm was recently reported to be viscoelastic based on a creep experiment, where a Burgers material model mathematically represents its creep compliance. This study develops a contact model for layered materials by extending a previously established viscoelastic half-space contact model. The approach involves converting analytical frequency response functions into influence coefficients, enabling the investigation of the viscoelastic behaviour of ZDDP-derived tribofilms. The results reveal that the tribofilm exhibits a highly fluid-like response when modelled as a half-space body being in contact with a carbon steel ball during indentation or sliding. When bonded to an elastic substrate in its typical thin-film form (on the nanometre scale), the contact behaviour can still exhibit time-dependent characteristics, depending on the operating conditions. Creep and stress relaxation are observed during indentation, particularly under low loads, while high loads result in a more pronounced viscoelastic response in extremely slow-speed contacts. However, under moderate sliding speeds ranging from millimetres to meters per second, time-dependent effects become negligible, regardless of the applied load. These findings indicate that although ZDDP-derived tribofilms exhibit significant viscoelasticity, their behaviour in practical applications generally resembles that of a soft elastic layer, as typical sliding speeds fall outside the range where pronounced time-dependent effects occur.

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The present study investigates the tribological performance of 3D printed Ti6Al4V total hip replacements (THR) compared to conventionally produced THRs from CoCrMo and FeNiCr alloys. The objective was to evaluate the suitability of 3D printed titanium alloy, with and without DLC coating, for THR rubbing surfaces and to investigate the potential benefits of 3D printing technology for friction and lubrication. A pendulum hip joint simulator was employed to replicate the swinging motion of a hip joint, thereby enabling the measurements of coefficient of friction (COF) and the observation of lubricant film formation under realistic conditions between the metal femoral head and acetabular cup. The experiments demonstrated that additive manufacturing enables the creation of specific surface topographies that can enhance protein adsorption, but also introduce surface imperfections negatively affecting tribological properties. The elevated surface roughness of additively manufactured femoral heads did not inevitably result in an increase in COF and was comparable to that of conventionally manufactured femoral heads. The additively manufactured Ti6Al4V head without DLC coating also exhibited a more rapid increase in lubricant film thickness during dynamic motion. In conclusion, the findings indicate that while 3D printing offers promising advancements in implant customization and material properties, its application requires careful consideration of surface finishing and coating methods to achieve optimal tribological performance.

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Polyether ether ketone (PEEK) is being investigated as a promising alternative to conventional dental implant materials such as Titanium and Zirconia, which have certain limitations. The friction and wear caused by mastication can result in significant damage to both teeth and implants due to the tribological interactions involved. In this study, both extruded and 3D-printed PEEK specimens were fabricated, and their tribological behavior was evaluated against Vita enamel-coated Cobalt–Chromium (Co–Cr) plates under both dry and artificial saliva conditions to simulate the oral environment. The analysis focused on friction coefficients, specific wear rates, and wear mechanisms using linear reciprocating sliding tests. Extruded and 3D-printed PEEK exhibited comparable tribological performance. Under dry conditions, their friction coefficients were 0.306 and 0.318, respectively. In saliva, the coefficients decreased to 0.109 for extruded PEEK and 0.114 for 3D-printed PEEK. Specific wear rates were also closely matched, with extruded and 3D-printed PEEK showing rates of 5.989 × 10^–5 and 6.151 × 10^–5 mm³/Nm under dry conditions, and 1.745 × 10^–5 and 1.878 × 10^–5 mm³/Nm under saliva conditions. Notably, the specific wear rates of PEEK under saliva conditions were comparable to those of natural human tooth enamel when tested against the enamel-coated Co–Cr plates. This suggests that PEEK, with its proven tribological properties, is suitable for use in dental crown implant applications. These findings highlight PEEK’s potential as a strong and suitable alternative material than conventional for dental implants, demonstrating consistent performance in conditions that closely mimic the oral environment.

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With the increasing modernization of machinery, more and more friction parts are exposed to alternating electromagnetic fields. To investigate the impact of alternating electromagnetic fields on the tribological behavior of key components, a Helmholtz coil was incorporated into the ball/disc reciprocating friction tester as the source of the alternating electromagnetic fields. Then, the effect of alternating electromagnetic fields on the tribological behavior of the self-mating pair of GCr15 bearing steel was investigated in detail. The results showed both the coefficient of friction and wear volume increased significantly in the presence of an alternating electromagnetic fields. The underlying mechanism was explored by analyzing the changes in surface temperature, the morphology and chemical composition of the wear surface and wear debris, combined with electromagnetic fields simulations of wear debris.

Aqueous lubricants are increasingly used in industry due to their excellent cooling capability and to environmental considerations. However, the film forming mechanisms in high-pressure contacts, such as in EHL regime, differ from those, well-known, of classical piezoviscous oil lubricants. This work investigated the role of the molecular architecture of polyalkylene glycol (PAG) molecules dispersed in water, with co-solvent monopropylene glycol, on the film forming capability in elastohydrodynamic lubrication regime by means of in situ in-operando chemical distribution mapping of the contact. These mappings were based on infrared hyperspectral images. A specific focus was made on the analysis of the area absorbance ratio of the ether stretching band, characteristic of PAG polymer, and the O–H stretching band of continuous water media, in order to identify the relative contribution of the solvent and of the polymer additives. Linear and branched PAG molecules were used as well as mixtures of different molecular weight PAGs. The penetration capability of the polymers in the high-pressure zone of the contact under pure rolling conditions, was discussed in terms of molar mass, architecture and polarity.

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