Tribofilm Formation Research Articles

Ti3C2Tx and Mo2TiC2Tx MXenes as additives in synovial fluids - towards an enhanced biotribological performance of 3D-printed implants

Synovial joints, critical for limb biomechanics, rely on sophisticated lubrication systems to minimize wear. Disruptions, whether from injury or disease, often necessitate joint replacements. While additive manufacturing offers personalized implants, ensuring wear resistance remains a challenge. This study delves into the potential of Ti3C2Tx and Mo2TiC2Tx nanosheets in mitigating wear of additively manufactured cobalt-chromium tungsten alloy substrates when incorporated as additives into synovial fluid. The colloidal solutions demonstrate an excellent stability, a crucial factor for reproducible assays and potential clinical applicability. Analysis of contact angles and surface tensions reveals MXene-induced alterations in substrate wettability, while maintaining their general hydrophilic character. Viscosity analysis indicates that MXene addition reduces the dynamic viscosity, particularly at higher concentrations above 5 mg/mL, thus enhancing dispersion and lubrication properties. Friction and wear tests demonstrate a dependency on the MXene concentration, while Ti3C2Tx exhibits stable friction coefficients and up to 77 % wear reduction at 5 mg/mL, which was attributed to the formation of a wear-protecting tribo-film (amorphous carbon and MXene nano-sheets). Our findings suggest that Ti3C2Tx, when supplied in favorable concentrations, holds promise for reducing wear in biotribological applications, offering avenues for future research into optimizing MXene utilization in load-bearing joint replacements and other biomedical devices.

Preparation of Diisooctyl Phosphate‐Modified Ultra‐Small Zinc Oxide Nanoparticle and Investigation of Its Tribological Properties as Additive in Alkylnaphthalene

ABSTRACTZnO nanoparticle surface‐modified by diisooctyl phosphate (P204) was prepared by liquid‐phase in situ surface modification technique with zinc acetate as the raw material, P204 as the modifier and anhydrous ethanol as the solvent. The morphology and microstructure of the P204‐ZnO nanoparticle were characterised by transmission electron microscopy, X‐ray diffraction and Fourier transform infrared spectrometry. Its thermal stability was evaluated by thermogravimetric analysis; and its tribological properties as the additive in alkylnaphthalene were evaluated with an SRV‐5 friction and wear tester in reciprocal sliding mode. The results show that the as‐prepared P204‐ZnO nanoparticle has an average particle size of about 4 nm and a P204 content of about 42% (mass fraction); and the surface modifier is grafted onto the surface of ZnO nanoparticle by physical adsorption. With a mass fraction of 0.5% in alkylnaphthalene base oil, P204‐ZnO nano‐additive can mildly reduce the friction coefficient and drastically reduce the wear rate of the steel–steel sliding pair. This is due to the formation of the composite tribofilm via the adsorption and deposition of the nano‐additive on rubbed steel surfaces as well as the tribochemical reactions of the modifier P204 yielding phosphate and of steel substrate yielding iron oxides. The as‐formed composite tribofilm with a thickness of about 20 nm, consisting of phosphate and iron oxides as the binder as well as deposited ZnO nanoparticle as the filling phase, is responsible for the excellent friction‐reducing and antiwear abilities of P204‐ZnO nano‐additive for the steel sliding contact.

One-step ultrafast laser-induced graphitization on PS-SiC surfaces for superior friction performance

Pressureless sintered silicon carbide (PS-SiC) ceramics are widely used as friction materials in the aerospace industry, and enhancing the self-lubricating properties of PS-SiC ceramics under dry friction is highly significant. In this study, an infrared femtosecond laser was used to treat the surface of PS-SiC ceramics, and the effects of various processing parameters on surface microstructure, chemical composition, and graphitization degree were investigated. More importantly, SiC decomposes into amorphous carbon and stays on the surface of PS-SiC ceramics under the photothermal effect, and the amorphous carbon realizes the transition to the ordered graphite structure by controlling the laser energy. The highly graphitized, carbon-containing micro/nanostructures on the surface of laser-treated PS-SiC ceramics promote the formation of stable carbon-based tribofilms during sliding, which significantly enhances the tribological properties of PS-SiC ceramics under dry friction. This study proposes a method for inducing graphitization on the surface of PS-SiC ceramics using an infrared femtosecond laser, providing a manufacturing approach and theoretical support for the development of high-performance PS-SiC ceramic friction materials.

Enhancing Mechanical and Tribological Properties of Epoxy Composites with Ultrasonication Exfoliated MoS2: Impact of Low Filler Loading on Wear Performance and Tribofilm Formation.

This study highlights the impact of low amounts of MoS2 quantities on composite performance by examining the effects of ultrasonication exfoliated MoS2 at different loadings (0.1-0.5 wt%) on the mechanical and tribological parameters of epoxy composites. Even at low concentrations, the ultrasonication and exfoliation procedures greatly improve the dispersion of MoS2 in the epoxy matrix, enabling its efficient incorporation into the tribofilm during sliding. Optimum mechanical properties were demonstrated by the MoS2/epoxy composite at 0.3 wt%, including a modulus of elasticity of 0.86 GPa, an ultimate tensile strength of 61.88 MPa, and a hardness of 88.0 Shore D, representing improvements of 61.5%, 35.45%, and 16.21%, respectively. Corresponding tribological tests revealed that high sliding velocity (10 N load, 0.2 m/s) resulted in a 44.07% reduction in the coefficient of friction and an 86.29% reduction in wear rate compared to neat epoxy. The enhanced tribological performance is attributed to the efficient removal and incorporation of MoS2 into the tribofilm, where it acts as a solid lubricant that significantly reduces friction and wear. Even though an ultra-low amount of filler concentration was added to the composite, a unique finding was the high MoS2 content in the tribofilm at higher sliding speeds, enhancing lubrication and wear protection. This study establishes that even ultralow MoS2 content, when uniformly dispersed, can profoundly improve the mechanical and tribological properties of epoxy composites, offering a novel approach to enhancing wear resistance.

Synergistic Tribological Effects of Ti3C2Tx MXene and ZDDP under Simple Grafting Strategies for Efficient Lubrication.

MXenes, a novel class of two-dimensional materials, possess exceptional physical and chemical properties, positioning them as promising candidates for lubricant additives. However, their potential is constrained by challenges in dispersion and stability, coupled with a paucity of research on interactions with additives in full-formula oils. In this study, hexadecylphosphonic acid (HDPA) is grafted onto Ti3C2Tx to formulate a polyalkylene glycol dispersion system. The findings reveal that the HDPA-modified Ti3C2Tx (HDPA-Ti3C2) is successfully synthesized, demonstrating superior dispersion stability and notable friction-reduction and antiwear properties. Notably, when combined with zinc dialkyl dithiophosphate (ZDDP), the HDPA-Ti3C2/ZDDP composite additive outperforms single additives in tribological performance, suggesting synergistic effects between them. This enhanced performance may be attributed to the formation of an amorphous polyphosphate tribofilm offering wear resistance, followed by the generation of a TiO2 tribofilm that further safeguards and repairs the worn surface, thereby enhancing the load-bearing capacity. Concurrently, the interlayer sliding mechanism of nanosheets, which substitutes the relative motion of the friction pair, reduces friction under boundary lubrication, ensuring prolonged effective lubrication. This work broadens the application prospects of Ti3C2Tx MXene for the design and development of commercial lubricating additives.

Preparation of molybdenum disulfide/bentonite nanohybrid and its tribological properties as lubricant for water-based drilling fluids

In recent years, with the increase in energy demand and gradual scarcity of medium and shallow oil and gas resources, oil and gas exploration has shifted to special wells such as deep wells, ultra-deep wells, and extended reach wells. These complex wellbore structures inevitably increase the torque and friction between the casing and the drill pipe during drilling, which aggravates friction and wear, and leads to accidents such as drill sticking and drill breakage in severe cases. In this study, molybdenum disulfide/bentonite (MoS2/Bent) nanohybrids as drilling fluid lubricant were synthesized by hydrothermal method using sodium molybdate (Na2MoO4) and thiourea (CH4N2S) as raw materials and bentonite as carrier. The as-synthesized MoS2/Bent nanohybrid was characterized by X-ray powder diffractometer, transmission electron microscopy, Fourier transform infrared spectroscopy, and the high temperature resistance and tribological properties of MoS2/Bent nanohybrid were evaluated in base slurry. The results show that the friction coefficient of MoS2/Bent nanohybrid drilling fluid before aging is reduced by 74 % and the wear rate is reduced by 97 % compared with the base slurry. After high-temperature aging at 240 °C, the friction coefficient is reduced by 77 % and the wear rate is reduced by 90 %. The excellent friction reducing and antiwear performance is attributed to the formation of low shear strength MoS2 deposition film and oxide tribofilm on the surface of the friction pair during the rubbing process.

A study on mechanisms of saw-tooth chip formation in hard turning under hybrid nanofluid-assisted MQL environment

Saw-tooth chip generated from hard turning is one of the most distinguishing features with conventional turning process. The presented work aims to contribute an extensive understanding the formation mechanism and main parameters of saw-tooth chips in oblique hard turning of 90CrSi hardened tool steel (60–62 HRC). The effects of dry, Al2O3 nanofluid MQL, Al2O3/MoS2 hybrid nanofluid MQL conditions on the serrated chip formation were experimentally investigated in terms of chip morphology, chip color, segment spacing, serrated segmentation frequency, and shear angle. The obtained experimental results indicated that, due to the better cooling and lubricating performance of nanofluid and hybrid nanofluid, the shear angles ϕ increase about 91.73% and 94.88% respectively when compared to dry cutting, which makes the formation of saw-tooth chip more favorable. Moreover, the analysis of microstructure image of the back side of chip proves the better lubrication from ball roller and tribo-film formation of Al2O3 and MoS2 nanoparticles. Based on the chip morphology analysis, the shear angle ϕ, the degree of chip segmentation on the top side in the chip’s transverse section K2, and the chip deformation coefficient in the transverse section K3 should be used as the additional criterions to evaluate the frictional improvement in cutting zone.

Physical and Chemical Evolution of PTFE-α-Al2O3 Composites Versus 304 SS Tribofilms During Dry Sliding

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

Enhancing lubrication of electrified interfaces by inert gas atmosphere

Abstract Considering the growing interest in increasing performance and efficiency of driveline components of modern electric vehicles, this work aims to analyze and report the wear mechanisms and notable enhancement of the lubrication of electrified contact interfaces by inert gas atmospheres. Systematic tribological studies were conducted on AISI 52100 steel test pairs using driveline lubricants under unelectrified and electrified conditions in ambient air and dry N2. Test results showed that in ambient air and electrification, the formation of iron oxides (in particular hematite) was most dominant and gave rise to severe abrasive wear regardless of the lubricant type being used. In dry N2, however, the tribo-oxidation was suppressed but the formation of a carbon-rich tribofilm was favored (especially under electrified conditions). Such a shift from surface oxidation to carbonaceous film formation resulted in dramatic reductions (by factors of 8 to 10) in the wear of test pairs.

Plasma Spraying NiCoCrAlY-Cr2O3-AgMo Coatings: Fabrication and Tribological Mechanisms

The increasing demand for high-performance aircraft engines has led to a greater emphasis being placed on advanced sealing coating technologies. Developing long-life, self-lubricating, and wear-resistant coatings is of significant research value. This study focuses on the fabrication of a novel self-lubricating and wear-resistant NiCoCrAlY-Cr2O3-AgMo composite coating. This coating was deposited onto a GH4169 substrate utilizing plasma spraying. Scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDS) and X-ray diffraction (XRD) methods were employed to characterize the elemental composition and microstructure of the fabricated NiCoCrAlY-Cr2O3-AgMo composite coating. Microhardness measurements across the coating cross-section indicated a gradual increase in hardness from the GH4169 substrate to the NiCoCrAlY-Cr2O3-AgMo coating. The average hardness of the GH4169 substrate was 413.92 HV0.2, while the CoNiCrAlY bonding layer region exhibited an average hardness of 467.60 HV0.2. The NiCoCrAlY-Cr2O3-AgMo coating itself demonstrated an average microhardness of 643.22 HV0.2. Room temperature friction tests indicated that the average coefficient of friction (COF) of the GH4169 substrate was 0.665. In contrast, the NiCoCrAlY-Cr2O3-AgMo coating exhibited a significantly lower average COF of 0.16, representing a 75.94% reduction compared to the uncoated GH4169 substrate. High-temperature friction tests were conducted at 400 °C, 500 °C, and 600 °C, indicating average COF values of 0.438, 0.410, and 0.268, respectively, for the NiCoCrAlY-Cr2O3-AgMo coating. Specifically, at 600 °C, the formation of a lubricious NiMoO4 tribofilm on the coating surface was observed. This tribofilm effectively reduced the wear rate of the GH605 pin to 2.78 × 10−6 mm3/N·m, highlighting the potential of the NiCoCrAlY-Cr2O3-AgMo coating to reduce wear in high-temperature sliding contact applications.

Synergistic impact of scan strategy and scan angles in laser powder bed fusion process on mechanical, tribological and cell viability properties of porous 316L SS structure

Poor load bearing ability and concentrated stress within cross-linked porous structures was found to be the major cause for the mechanical and wear failure of SS (Stainless Steel) 316L implants. Therefore, the present study introduces a novel scan pattern to enhance the mechanical and wear properties. Importantly, the role of porous structures on the constituents and formation of Tribo-film regimes under the simulated body fluid (SBF) condition were assessed using the X-ray photoelectron spectroscopy (XPS). Furthermore, the impact of the surface roughness of porous structures on the cell adhesion and cytotoxicity were explored. Various cross-linked porous structures are produced at three different scanning rotation angles (0°, 45°, and 90°) using the Laser powder bed fusion (LPBF) process. Morphology analysis reveals balling and stair-case effects at 0°, controlled for structures built at 45° and 90°. Porous-90° structure exhibits negative skewness and a platykurtic surface with low peaks and more valleys. It demonstrates higher strength with low density due to better particle fusion, net-like structure, and uniform element distribution. X-ray photoelectron spectroscopy (XPS) evaluates the impact of porous structures on Tribo-film regimes in simulated body fluid. Surface roughness effects on cell adhesion and cytotoxicity are also investigated. Cross linked porous structures presented two major compression failure modes include wrinkling and diagonal shearing. The densified structure of Porous-45° reduces contamination and cell death, facilitating stable cell growth. Tribo-film formation and load bearing ability of Porous-90° structure was better than the other structures, due to the cross-linked netlike structure and further more detailed in the XPS study.

Magnetic-Manipulation of tribofilm for Si3N4/1045 steel contact toward sustainable reduction in friction and wear

Conventional anti-wear approaches lead to energy inefficiency and excessive waste, posing significant challenges to sustainability and cleaner production. Herein, we investigate the influence of eco-friendly magnetic fields on the tribological behavior of interfaces, with an emphasis on the formation and characteristics of the protective tribofilm. The experimental design combined finite element analysis and friction tests to systematically reveal the relationship between magnetic field strength, oxidation of wear debris, and tribological properties. The results demonstrated that the introduction of a magnetic field led to a 22% reduction in the friction coefficient and a 28% decrease in wear volume. High-intensity magnetic fields refined the wear debris by up to 42.6% and also promoted the oxidation of wear debris, leading to a predominance of Fe3O4 in the tribofilm. The structured arrangement and reorganization of Fe3O4 particles within the tribofilm enhance its density and thickness (from 2 μm to 3 μm). Simultaneously, the rotational motion of Fe2O3 particles at the interface modifies the friction contact state, thereby refining friction performance. The findings underscore the significance of considering magnetic fields in the design of tribological systems for enhanced performance and sustainability.

Tribofilm distribution and tribological analysis of piston ring-cylinder liner interfaces under realistic engine conditions

The study investigates tribofilm distribution and tribology performance on piston ring-cylinder liner surfaces under realistic internal combustion (IC) engine conditions. By examining the effects of load, temperature, and oil film thickness ratio, the research reveals that the tribofilm is significantly greater on the cylinder liner surface compared to the piston ring surface. The findings highlight that a specific excitation pressure is necessary for tribofilm formation. An oil film thickness ratio of 1.8041 is evaluated as an accurate boundary for tribofilm distribution on the cylinder liner surface. These insights provide valuable data for designing piston ring-cylinder liner interfaces and improving IC engine performance.

ZDDP Tribofilm Formation and Removal

While ZDDP tribofilm formation has been widely studied, the mechanism of ZDDP tribofilm removal during rubbing is still unclear. The study employs a ball on disc tribometer to monitor ZDDP tribofilm development in rolling-sliding, mixed lubrication conditions. It is found that when ZDDP tribofilms are formed very rapidly, as is the case with short alkyl chain, secondary ZDDPs, a large proportion of the initially-formed tribofilm is suddenly lost during rubbing. By contrast, the tribofilms that form more slowly from primary ZDDPs and longer chain secondaries are not partially lost during rubbing. XPS analysis showed that a rapidly-formed tribofilm before its partial removal has a very small Zn/O ratio, and a high BO/NBO. This suggests that such tribofilm contains a significant proportion of ultraphosphate, which is likely to have a relatively weak structure due to lack of stabilising cations. This may result in the tribofilm being partially removed when it reaches a certain thickness. By comparison, the remaining tribofilm, and also tribofilms that form slowly, have high Zn/O and low BO/NBO. This suggests that they consist of short chain polyphosphates and are thus stronger and more durable.Graphical

Improving the lubricating performance of poly-alpha-olefin base oil using SnO2 nanosized-additives

The majority of previous studies have been focused on the thermal properties of SnO2 nanofluids. In order to understand the lubricating performance of SnO2 nanoparticles as additives, the current study investigates the effects of the addition of SnO2 nanoparticles on the tribological properties of poly-alpha-olefin 6 base oil. The dual-step method is utilized to disperse the SnO2 nanoparticles in base oil with oleic acid as surfactant. The shape and size of SnO2 nanoparticles are confirmed by transmission electron microscopy, and the dispersion stability of SnO2 nanoparticles is examined by dynamic light scattering tests. The lubricating properties of SnO2 nanofluids are explored on a universal mechanical tribometer with a ball-on-plate reciprocating sliding configuration. It is found that the SnO2 nanofluids show good stability and dispersibility. The addition of SnO2 nanoparticles decreases the friction and wear for steel-steel tribo-pairs. The positive effects on friction and wear reductions become more significant with increasing concentrations of SnO2 nanoparticles. In this work, nanofluids containing the 5wt% SnO2 nanoparticles and 5wt% oleic acid is considered as the optimum composition, which shows the best lubricating performance with the reductions of 13.8% in coefficient of friction and 41.4% in wear volume loss. After observing the wear tracks by scanning electron microscopy, energy dispersive spectrometer and a white-light interferometer, it is shown that the wear mechanisms are dominated by abrasive wear and adhesive wear. The enhancement in tribological properties of base oil is attributed to the formation of SnO2 tribo-film and oleic acid tribo-layer which reduces the shearing resistance, separates the mating areas and withstands the loads. The findings obtained in this study can be used as references in the development of high-performance nanofluids.

Synergistic effects of nitrogen-containing functionalized copolymer and silicon-doped DLC for friction and wear reduction

We present a strategy to enhance the tribological performances of diamond-like carbon (DLC) films based on synergistic chemical modifications in the polymer lubricant additives and in the carbon film. Our experiments revealed that the polymer functionalization with nitrogen-containing groups and the DLC doping with silicon atoms result in significant friction and wear reduction under severe boundary lubrication conditions. Atomic Force Microscopy revealed the formation of a lubricious tribofilm. Ab initio calculations highlighted the key role of N-Si interactions in promoting the polymer chemisorption on the film, which represents the first, essential step for the tribofilm formation. Our findings suggest that targeted chemical modifications in lubricants and films can substantially advance the tribological performances of DLC films for various industrial applications.

The Lubricity of Gases

A sealed reciprocating tribometer has been used to study the influence of different gaseous environments on the friction and wear properties of AISI52100 bearing steel at atmospheric pressure and 25 °C. Helium, argon, hydrogen, carbon dioxide and nitrogen all give high friction and wear, suggestive of very little, if any tribofilm formation under the conditions studied. Dry air and oxygen also give high friction, slightly lower than the inert gases, but produce extremely high wear, much higher than the inert gases. This is characteristic of the phenomenon of “oxidational wear”. The two gases ammonia and carbon monoxide give relatively low friction and wear, and XPS analysis indicates that this is due to the formation of adsorbed ammonia/nitride and carbonate films respectively. For the hydrocarbon gases studied, two factors appear to control friction and wear, degree of unsaturation and molecular weight. For the saturated hydrocarbons, methane and ethane give high friction and wear but propane and butane give low friction after a period of rubbing that decreases with molecular weight. The unsaturated hydrocarbons all give an immediate reduction in friction with correspondingly low wear. Raman analysis shows that all the hydrocarbons that reduce friction and wear form a carbonaceous tribofilm on the rubbed surfaces.Graphical

Tribological properties of diamond-like carbon films lubricated with water-emulsified engine oil

In this work, the tribological properties of two diamond-like carbon (DLC) films were investigated at different temperatures using water-engine oil emulsions as lubricants, considering emerging fuel systems such as hydrogen and ammonia. The results revealed that the friction coefficient of a-C film raised with the increasing water content at 80 °C. The friction coefficient of a-C:H film initially raised and then decreased with varying water fractions at different temperatures. Elevated temperature exacerbated the wear of both DLC films at high water contents. The most significant wear was observed for a-C film at 80 °C and a-C:H film at 50 °C. The a-C:H film exhibited more sensitive wear characteristics to water content than the a-C film. Surface analysis indicated that water-engine oil emulsions prevented the formation of phosphate tribo-film on steel friction pair for two DLC films but promoted the tribo-oxidation of steel pair sliding contact with a-C film, especially at higher temperatures. Distinct reactivities of two DLC films towards water molecules could affect the adsorption of lubricants on DLC surfaces, leading to their different friction and wear behavior. It suggests that the water-caused emulsification of lubricating oil will be a pivotal consideration in tribological applications of DLC films in future internal combustion engines.

The friction behavior and wear mechanism of RV reducer gear steel (20CrMo) subjected to three different heat treatment processes from −20 °C to 100 °C

The gear friction pairs in RV reducer always withstand a broad range of temperature, sliding velocity and heavy load. In the research, three types of heat treatment methods (carburizing + quenching, nitrocarburizing and high concentration nitrocarburizing) are applied on the 20CrMo (cycloidal gear material) disc to study the tribological behavior and wear mechanism. The friction coefficient, wear depth and wear rate of the three types of friction pairs under various temperature (−20 °C, 25 °C, 50 °C, 100 °C) are measured and the microstructure characteristics are analyzed by SEM and EDS. The results reveal that high temperature can dramatically decrease the wear scar dimensions and wear rate due to the more active lubricant. Although the lubricant lost its activity at −20 °C, the friction coefficient and maximum wear depth for GCr15/20CrMo (carburizing + quenching) and GCr15/20CrMo (high concentration nitrocarburizing) are relatively less than 0.1 and 4 μm. The wear rate of GCr15/20CrMo (carburizing + quenching) and GCr15/20CrMo (high concentration nitrocarburizing) are 14.49 μm3/N·m and 17.20 μm3/N·m, which are only 28.7% and 34.1% of GCr15/20CrMo (nitrocarburizing). With the temperature increases to 25 °C, tribofilm is more likely to form on the 20CrMo specimen after nitrocarburizing, leading to better wear resistance. At high temperature (50 °C and 100 °C), GCr15/20CrMo (high concentration nitrocarburizing) system presents excellent tribological behavior by the combination of good lubricant activity, formation of tribofilm and refined nitrides. The wear rate even reaches 0.45 μm3/N·m and 0.43 μm3/N·m at the sliding velocity of 300 mm/s and 562 mm/s under 100 °C. Overall, specimen with high concentration nitrocarburizing process exhibits exceptional wear performance under full operating conditions of RV reducer.

Tribological mechanism based on tribo-chemistry and molecular dynamics of polyurea/polyimide shape memory copolymers at high temperature

Exploring the tribological mechanism of the shape memory polymers plays an important role in designing intelligent lubricating materials. Here, a series of shape memory polyurea (PUA)/polyimide (PI) were prepared and their tribological performance were investigated. The result showed that the friction and wear of PUA/PI were higher at shape memory transition temperature (Ttrans) than at 200 ℃ and room temperature (RT). However, introducing PUA units into PI enhanced its shape memory stability and tribological performance. SEM and XPS analysis revealed that introducing PUA promoted the formation of polymer-based tribofilm. Molecular dynamics simulations further illustrated that the high relative atomic concentration of PUA/6PI at the sliding interface improves the bonding between the tribofilm and the counterpart, thereby improving overall tribological performance.