Tribofilm Formation Research Articles

Friction and wear properties of wide-velocity range high-energy plasma sprayed CuSn–NiCr solid self-lubricating coatings under heavy load

As one of important copper-based solid self-lubricating coatings, the low wear resistance of Cu–Sn coatings greatly limits its application in some heavy-duty cases. In the present work, two types of CuSn–NiCr composite powders designed by electroless plating (E-CN) and mechanical mixing (M − CN) methods, respectively. The friction and wear properties of wide-velocity range high-energy plasma sprayed CuSn–NiCr coatings were studied. The results suggested that, the fracture toughness of E-CN and M − CN coatings increased by 1.38 times and 1.55 times compared with as-sprayed Cu–Sn coatings, and the wear rate decreased by 29 % and 50 % correspondingly. Due to the tribofilm formation and micro-pores storage effect, the M − CN coating with 10 wt% addition ratio of Ni–Cr phase exhibited the lowest friction coefficient and the smallest wear rate. With the increase of Ni–Cr phase, the wear mechanism was transformed from adhesive to abrasive, and accompanied by fatigue step-shaped spallation. Friction and wear model of the CuSn–NiCr coatings was divided into rapid, stable and declining stages.

Tribological characteristics and vibration response of grease lubricated contacts under environmental particles and water impact

In practical applications, the lubricating performance of grease gradually deteriorates owing to the inclusion of foreign contaminants. This study investigates the impact of environmental contaminants, including silicon dioxide (SiO2) particles and water content, on the degradation of lithium-base grease and its subsequent effect on the surface damage of bearing steel material. The experimental results reveal an interactive effect between water content and particles in grease. The presence of particles and water contamination in separated or mixed conditions leads to increased friction, wear, and pitting compared to the base grease. The wear mechanisms of the two contaminant factors are different as follows: the particles predominantly function as third bodies, which enhance the oil film thickness between contacting surfaces while inhibiting the formation and growth of a protective tribofilm from the base grease, resulting in elevated friction and wear, and severe surface damage. Additionally, the addition of suitable water content in grease can absorb friction heat produced by the particles, mitigating friction and wear. Vibration exhibits behavior similar to friction, where both friction and vibration increase proportionally in the absence of water contamination. This observation suggests that vibration can be used as a valuable indicator for monitoring friction in contaminated conditions.

Tunable amorphous carbon films formed on ultralow wear, Pt–Au alloys

The mechanocatalytic formation of carbonaceous films at the interface between sliding metallic contacts is simultaneously advantageous for reducing friction and adhesion in several tribological applications and detrimental for electrical contacts as they can induce device failure by increasing the contact resistance. Yet, remarkably little is still known about the chemistry, structural and mechanical properties, and tunability of these interfacial layers. In this study, we performed contact pressure-dependent tribological experiments in dry nitrogen containing trace organics on four, nanocrystalline Pt–Au alloys ([Au] from 0 at.% to 10 at.%) – a promising class of alloys for ultralow wear and electrical contact applications. The ex-situ, multi-technique characterization results did not only provide insights into the chemical nature and mechanical behavior of the mechanocatalytic, carbon-rich films formed on Pt–Au surfaces, but also revealed the interplay between catalytic and mechanochemical tribofilm formation controlled by the composition-dependent electronic structure of the Pt–Au substrate and the applied contact pressure. The results of this work provide guidelines for tailoring nanocrystalline alloys to control their mechanocatalytic activity on the basis of variations of the alloy mechanical properties and element's electronic structure with the alloy stoichiometry.

Effect of additive combinations on tribofilm formation in fully formulated 0W‐8 oil

Reducing engine oil viscosity is an effective way to meet automotive fuel efficiency requirements. Clarifying the effects of multiple coexisting additives in low-viscosity oil is important for the development of low-viscosity engine oil. In this study, six kinds of 0W‐8 engine oil, including those without a Ca-derived detergent, a dispersant or molybdenum dialkyldithiocarbamate (MoDTC), were selected to study the effects of these additives on the tribofilm formation and the friction characteristics of fully formulated low-viscosity oil. A custom-made rolling and sliding tribometer was used to evaluate the tribological properties and form the tribofilms. The elemental distribution of the tribofilms was characterized by electron probe microanalysis (EPMA), and the chemical compositions of the films were determined using X-ray photoelectron spectroscopy (XPS). The Ca-derived detergent hindered both the ZnDTP-derived and MoDTC-derived tribofilm formation in the fully formulated low-viscosity oil due to adsorption competition. The effect of the ashless dispersants on the ZnDTP-derived tribofilm formation in the fully formulated 0W‐8 oil showed opposite results in the presence or absence of MoDTC. The addition of MoDTC had different effects on the ZnDTP-derived tribofilm formation under different additive combinations.

Ab initio informed machine learning potential for tribochemistry and mechanochemistry: Application for eco–friendly gallate lubricant additive

Mechanochemistry and tribochemistry processes involve multiple physical/chemical interactions induced by extreme conditions including molecular confinement, high temperatures and mechanical stress applied. Simulating these processes by molecular dynamics is very challenging. While force fields fall short reproducing the enhanced reactivity arising by quantum effects, ab initio molecular dynamics is severely limited by the complexity of the systems of interest, their sizes, and the long-time scale on which relevant events take place. In this work using an active learning approach, a landmark deep neural network potential has been developed which reproduces the accuracy of ab initio interactions at the classical molecular dynamics computational cost and permits to successfully simulate the tribochemical processes occurring at the interface between butyl gallate molecules and iron substrates under tribological conditions. The simulations of the dynamics of the Fe-gallate system when sliding under an imposed external load reveal the key atomistic mechanisms underlying the formation of the friction reducing lubricant tribofilm and permit to characterize the tribological properties of the explored systems, clearly exposing the shortcoming of reactive force field based approaches. The successful development of neural network potentials, making it possible to push the limits of molecular dynamics marrying accuracy with system sizes and long time scales, paves the route toward a new area in computational tribochemistry.

In-situ tribo-induced formation of superb lubricious carbon-based tribofilms on the catalytical active MoN-Ag film surfaces

The establishment of environmentally friendly lubrication systems is of significant importance to the sustainable development. Here, we demonstrated an environmentally friendly approach, where superb lubricious carbon-based tribofilms formed in situ originating from base oil through tribochemical reactions, on the surfaces of catalytically active MoN-Ag film. We focused on the study of the tribological behavior of the MoN-Ag/oil solid-liquid system under varying loads. Through detailed characterization analysis of the carbon-based tribofilms, we elucidated the mechanism of friction reduction and wear resistance of the MoN-Ag film in the base oil, as well as the formation mechanism of carbon-based tribofilms. Furthermore, we provided a thermodynamic perspective to clarify the influence of load on the tribo-induced tribochemical reactions within the MoN-Ag/oil solid-liquid system.

Effect of Nickel Acetyl Acetonate as Lubricant Additive in Base Oils with Different Molecular Structure on In-Situ Formation and Tribomechanism of Carbon-Based Tribofilms of Steel-Steel Sliding Pair

Effect of Nickel Acetyl Acetonate as Lubricant Additive in Base Oils with Different Molecular Structure on In-Situ Formation and Tribomechanism of Carbon-Based Tribofilms of Steel-Steel Sliding Pair

Investigating the Roles of Protein on the Cobalt Alloy Surface Degradation for Biomedical Implant Through Tribocorrosion Mechanisms

Previous investigation has established the formation of tribofilm is influenced by tribochemical reactions between the electrolyte and the articulating surface of cobalt alloy through sliding tests in various simulated fluids. Although it has successfully characterized the film composition via spectroscopy analysis and indicated to have impact on material loss, a comprehensive understanding of the material degradation mechanism in tribocorrosion condition was still lacking. Therefore, this study aims to investigate the role of protein in the tribocorrosive degradation of cobalt-chromium-molybdenum (CoCrMo) alloy in different simulated physiological electrolytes. Using a similar testing protocol, tribocorrosion tests were conducted with reciprocating ceramic ball against CoCrMo samples immersed in saline and culture medium, compared to both electrolytes diluted with 25% fetal bovine serum (FBS). Synergistic and mechanistic approaches were employed to model the tribocorrosive degradation. Results reveal that protein plays a beneficial role in reducing corrosive (electrochemical) surface degradation under tribocorrosion condition, whilst increasing mechanical wear degradation in the process. Despite studies have shown that tribocorrosion behavior in metal alloys is highly influenced by the presence of organic matter, this study provides a more clarity of the roles played by protein in tribocorrosive degradation on CoCrMo surface as its novel finding.

Tribofilm Formation of Zinc Dialkyldithiophosphate as Oil Additives Impregnated in Porous Polyimide under Different Contacts.

To improve the tribological properties of porous polyimide (PPI), ZDDP-mixed PAO4 was impregnated in PPI (denoted as ZPPI), and the tribological properties of ZPPI under single- and double-contacts were investigated. In the single-contact of ZPPI-steel, a rough and thick tribofilm was formed on the steel ball, which could protect the steel surface but resulted in large fluctuations in the friction coefficient. In the double-contact of ZPPI-steel-steel, ZDDP formed a uniform and thinner tribofilm on steel surfaces, leading to a lower friction. ZDDP could inhibit the formation of iron oxides significantly in the double-contact, while the antioxidant effect of ZDDP in the single-contact of ZPPI-steel was not obvious. ZnS and ZnO generated from ZDDP were adsorbed in the ZPPI pores, which aggravated the blackening of the ZPPI worn surface.

Tribological behavior of sulfonitrided tribofilm generated from non-corrosive dimercaptobenzothiadiazole-based additives as effective boundary lubrication layer

Lubricant additives are essential to protect mechanical devices. Although sulfur (S)-based dialkylpentasulfide (RC2540) exhibits acceptable extreme pressure (EP) performance, a high amount of S causes severe corrosion and poorer anti-wear (AW) performance. Herein, dimercaptobenzothiadiazole-based additives (DMTDs) consist of four and five S atoms, namely 2-(benzyldisulfanyl)-5-(benzylsulfanyl)-1,3,4-thiadiazole (4SBZ), 2-(octyldisulfanyl)-5-(octylsulfanyl)-1,3,4-thiadiazole (4SC8), 2,5-bis(benzyldisulfanyl)-1,3,4-thiadiazole (5SBZ), and 2,5-bis(octyldisulfanyl)-1,3,4-thiadiazole (5SC8) were synthesized. The synthesized DMTDs are non-corrosive (Grade 1a-1b) compared to RC2540 (Grade 4c). The 10 wt% 5SBZ demonstrated exceptional EP and AW performance than RC2540. The 10 wt% 5SBZ has improved weld load and reduced wear scar area by 5.0 folds and 94.1% compared to naphthenic base oil. The exceptional tribological properties of 5SBZ are attributed to the strong non-covalent interaction between the benzene ring and metal surfaces, high S content, and synergistic effect between S and N. Alternatively, the DMTDs exhibit better AW performance compared to RC2540. The XPS depth profiling demonstrated the formation of sulfonitrided tribofilm after lubrication with synthesized DMTDs. This tribofilm contributed to an excellent boundary lubrication layer for wear protection. The lubrication mechanism was discussed and proposed. This work provides promising DMTDs as EP and AW additives for industrial applications.

Terahertz time-domain spectroscopy of nanoadditive macroscale superlubricity: Quantitative hydration investigation and deeper insight into the solid-liquid synergistic lubrication

As a promising approach, terahertz time-domain spectroscopy (THz-TDs) provides the powerful assistance for quantitatively characterizing the solid-liquid synergistic lubrication. The hydration ratio in nanoadditive solutions was calculated by terahertz dielectric spectrum, and the exact threshold for superlubricity was found as 40%. The high hydration ratio also promoted the formation of high-quality tribofilm. Besides, not all hydrogen bonds, but the solid-liquid hydrogen bond was the true answer to the nanoadditive superlubricity. Finally, a synergistic lubrication model was proposed and it gave a detailed description of the molecular states with different lubrication. This work exerts the sensitivity of THz-TDs on hydration and liquid lubrication, providing avenues for the quick assessment of superlubricity and optimal design of nanoadditive superlubricity in the future.

Tailoring Tribological Characteristics in Titanium Alloys by Laser Surface Texturing and 2D Ti3C2Tx MXene Nanocoating

AbstractThe inferior tribological properties of titanium (Ti) and its alloys, due to their limited work hardening capability and the mechanical instability of their oxide scale, pose substantive challenges in practical applications. Surface texturing and the application of 2D material coatings are acknowledged as efficacious strategies for enhancing these properties. Nevertheless, the rapid loss of effectiveness when employing these methods in isolation highlights a significant limitation. Herein, a 50‐nm‐thick MXene nanocoating is deposited onto groove‐textured pure Ti surface, effectuating a reduction in the coefficient of friction (COF) from ≈0.57 to 0.17, thereby substantially mitigating wear on the Ti substrate. The grooved surface architecture plays a vital role in storing MXenes and facilitating lubricant replenishment through the stirring effect of frictional stress during subsequent friction process. Further, the continuous decomposition and reorganization of MXene coating results in the formation of a robust tribofilm that resists oxidation and plastic deformation in the subsurface layer, markedly enhancing the friction and wear resistance. These findings are crucial for improving tribological performance in Ti and its alloys and provide a straightforward yet effective strategy for tailoring their tribological properties by integrating LST with 2D material nanocoating.

Solid lubrication performance of hybrid Ti3C2Tx/MoS2 coatings

MXenes have gained notable attention in tribology due to their excellent wear resistance based on the formation of beneficial tribofilms. However, studies using MXenes as solid lubricants have mainly focused on multi-layer Ti3C2Tx coatings, while little is known about the tribological performance of MXene composites. Therefore, our study aims at scrutinizing and understanding the tribological behavior of MXenes and MXene composites as solid lubricants under reciprocating sliding conditions. Theoretical predictions regarding the resulting interlayer adhesion and coating-substrate adhesion helped to design the hybrid coatings. Multi-layer Ti3C2Tx, molybdenum disulfide (MoS2) and two hybrid coatings using Ti3C2Tx and MoS2 (random mixture and sandwich-like) were spray-coated onto steel substrates with a coating thickness of about 800 nm. Dry sliding tests using a steel ball as counter-body were carried out at room temperature. The coatings’ morphology and formed tribofilms were holistically characterized by scanning and transmission electron microscopy (SEM, TEM) as well as X-ray photoelectron spectroscopy (XPS). Our results demonstrate that both hybrid coatings notably reduce friction and wear, outperforming their respective pure coatings (Ti3C2Tx and MoS2). This is attributed to synergistic effects between Ti3C2Tx and MoS2, with adhesion forces appearing to be the governing mechanism in enhancing the formation of stable tribofilms. Numerical calculations validate our experimental results, verifying that hybrid coatings exhibit low interlayer friction and high adhesion to ferrous substrates. Consequently, our work reveals the potential of Ti3C2Tx/MoS2 hybrid coatings to further optimize friction and wear.

Evaluation of Temperature-Dependent Boundary Lubrication Behavior of Stearic Acid Using a Force-Controlled Tribometer.

A quick evaluation of the effectiveness of additives is important in lubricant formulation. In this study, we employed a friction-force measurement approach using a newly designed lateral force-controlled tribometer. This tribometer evaluates the lubricant properties under boundary lubrication. In this lateral force-controlled tribometer, the absence of a stiffness-altering sensor enables the modeling of the actual contact conditions without altering the contact stiffness. Indirect friction force measurement ensures precise measurements of friction properties while avoiding common measurement errors encountered in conventional tribometers, such as sensor misalignment and changes in the stiffness of the machine due to the sensor. The tribometer designed and built consists of a pendulum that measures the rate of dissipation of the oscillation amplitude as a function of time. The unique characteristics of the machine are the possibility of changing the energy input into the tribosystem without altering the tribo-contact conditions and the capability to do experiments at higher temperatures. To evaluate the capabilities of this tribometer, the impact of temperature on the frictional properties of a base oil and a blend of base oil and stearic acid (SA) (a prominent Organic Friction Modifier) is investigated. The test result shows that frictional energy dissipation decreases when stearic acid is present in the lubricant. And, as the temperature of the oil increases, the energy dissipation increases for pure base oil but reduces for the blend. The observed frictional trends are attributed to the decrease in the viscosity of the base oil with an increase in the temperature. The decrease in friction for the SA blend is attributed to tribofilm formation. Fourier transform infrared (FTIR) and X-ray Photoelectron Spectroscopy (XPS) analyses confirm the presence of the SA tribofilm on the surface. XPS indicates an increase in tribofilm quantity with rising temperatures. The kinetics of film formation and thickness increase with temperature, consequently reducing the friction in the SA blend.

Tribological and Mechanochemical Properties of Nanoparticle-Filled Polytetrafluoroethylene Composites under Different Loads.

For the tribological properties of nanoparticle-modified PTFE, a more comprehensive study has been conducted, but there is still some room for research on tribology behavior, tribofilm formation and structure evolution of polytetrafluoroethylene (PTFE) filled with α-Al2O3 and SiO2 nanoparticles during sliding against steel counterparts under different loads. At the same time, it establishes the linkage and mechanism between the maintenance of mechanical strength and the tribological application of polymers in service and provides corresponding scientific data and theoretical guidance for the long-lasting application of polymer lubrication materials. It is found that both composites exhibit good wear resistance across the pressure of 1 MPa to 10 MPa, with the α-Al2O3/PTFE composite demonstrating better performance stability compared to the SiO2/PTFE composite. The high wear resistance is attributed to the formation of tribofilms at the friction interface. For the α-Al2O3/PTFE, an island-like tribofilm is formed with a thickness ranging from 100 to 200 nm, while the tribofilm of the SiO2/PTFE composite is thinner, measuring approximately 50 to 100 nm, and manifests a striped pattern. The chemical composition, both at the surface and subsurface levels, as well as the morphology of the tribofilms, were studied using FTIR spectrometry, X-ray photoelectron spectroscopy (XPS), and FIB-TEM. It is found that the difference in thickness and microstructure of the tribofilms for the two composites is mainly due to the tribochemistry of the nanoparticles. The α-Al2O3 nanoparticle plays a "cohesion" role during the formation of the tribofilm, which facilitates the formation of a thicker, more uniform, and stronger adhered tribofilm on the metallic counterpart, making it more robust against higher shear stress.

Novel Anti-Wear Performance of Nano Magnesium Silicate Hydroxide in Boundary Lubrication Under High Temperature

Abstract In the operation of the machine, the tribological properties of the lubricant are apt to deteriorate with temperature elevation. Therefore, it is of great significance to seek a lubricant additive with good high-temperature tribological properties. In this study, the tribological performance of nano magnesium silicate hydroxide (MSH) as a lubricant additive were investigated at 40–200 ℃ and the results show that the wear of the sliding surface lubricated by PAO10 with MSH as the additive is very slight. The worn surface was characterized by the scanning electron microscope (SEM), Raman spectrometer, and X-ray photoelectron spectrometer (XPS). The experimental results display that the electrostatic adsorption between [Si4O10]4− released from MSH and steel surface contributes to the formation of SiO2 film; whereas ion exchange reaction between the phase transformation product of MSH in the contact zone and steel surface results in the chemical bonding Si–O–Fe, and the existence of the bonding facilitate the formation of the protective tribofilm with strong adhesion at high temperature.

Unraveling Water-Based Lubrication with Carbon Dots of Asphaltene Origin.

Despite the lower toxicity of water-based lubricants over nonrenewable petroleum-based analogues, they face challenges in achieving widespread adoption due to low stability and inadequate friction-reduction performance. To address this, a cost-effective nanoadditive is synthesized by expansive oxidation of asphaltenes to create biocompatible asphaltene-derived carbon dots [(ACDs); 5 nm]. These ACDs exhibit excellent water redispersibility, promoting long-term friction reduction and marking the first use of an asphaltene-based system for friction reduction in water or oil. Even at low loadings (0.2-4.0 wt %), ACDs significantly reduce friction on steel surfaces (>54%) with tribofilm stability surpassing pristine carbon dots, typical carbon-based graphene quantum dots, and inorganic nanomaterials (commercial 5 and 20 nm silica). The ACDs' attributes include high negative zeta potential, considerable water uptake, varied functional groups, biocompatibility, and a nanodisc shape conducive to stable tribofilm formation through effective particle stacking. The scalable synthesis, high yield, and impressive water redispersibility of ACDs position them favorably for commercial water-based lubrication.

Lubricious yet tough fabric composites at cryogenic temperature by sulfonated polyether-ether-ketone reinforcement

A novel PTFE/Nomex fiber fabric composite reinforced by sulfonated polyether-ether-ketone (PN-SPEEK) was designed and fabricated, which has comprehensive properties with self-lubricity, high strength and toughness at wide cryogenic temperature range. The tensile strength and toughness were increased by 16.9 % and 64.3 % at −160 °C compared to conventional phenolic-formaldehyde-reinforced fabric (PN-PF), and the wear rate dramatically reduced to 2.7 × 10-16m3/(N•m). Within the temperature range of −160 °C to 25 °C, the variation of friction coefficient for PN-SPEEK was flattened between 0.086 and 0.126, which increased dramatically to approximately 0.2 with decreasing temperatures for PN-PF. The transfer film was analyzed comprehensively and thoroughly, demonstrating that the improved stable lubricity was primarily attributed to favorable formation of tribofilm composed of plenty of PTFE and strong chelation between SPEEK and iron base in transfer film. The developed PN-SPEEK is greatly expected for engineering applications.

Dynamics of Tribofilm Formation in Boundary Lubrication Investigated Using In Situ Measurements of the Friction Force and Contact Voltage.

The complex dynamics of tribofilm formation on boundary-lubricated steel surfaces were investigated in real time by combining in situ measurements of the temporal variation of the coefficient of friction and contact voltage. Sliding experiments were performed with various blends consisting of base oil, zinc dialkyl dithiophosphate (ZDDP) additive, and two different dispersants at an elevated oil temperature for a wide range of normal load and fixed sliding speed. The evolution of the transient and steady-state coefficient of friction, contact voltage, and critical sliding distance (time) for stable tribofilm formation were used to evaluate the tribological performance of the tribofilms. The blend composition affected the load dependence of the critical sliding distance for stable tribofilm formation. Tribofilm friction was influenced by competing effects between the additive and the dispersants. Among various formulations examined, the tribofilm with the best friction characteristics was found to be the blend consisting of base oil, a small amount of ZDDP, and a bis-succinimide dispersant treated with ethylene carbonate. The results of this study demonstrate the effectiveness of the present experimental approach to track the formation and removal of protective tribofilms under boundary lubrication conditions in real time.

Tribofilm Formation and Friction Reduction Performance on Laser-Textured Surface with Micro-Grooved Structures

Tribofilm Formation and Friction Reduction Performance on Laser-Textured Surface with Micro-Grooved Structures