Formation Of Transfer Film Research Articles

Self-organized transfer film-induced ultralow friction of Graphene/MoWS4 heterostructure nanocomposite

We report the tribological performance evaluation of Graphene/MoWS4 heterostructure nanocomposite prepared via simple spin-coating to lubricate the silicon-based MEMS devices. The lubrication mechanism of the nanocomposite was studied by a ball-on-disk tribometer, a three-dimensional surface profilometer, and high-resolution characterization techniques. The results show that the coefficient of friction was reduced from 0.4 to an ultralow friction level (μ ∼ 0.02), and the specific wear rate of the silicon substrate was reduced by 3 orders of magnitude due to the lubricating effect and formation of transfer film of the nanocomposite. High-resolution characterization confirmed the formation of beneficial transfer film, indicating that the nanostructured graphene and MoWS4 were completely mixed in the transfer film. The transfer film enables the formation of low-friction contact by transferring the direct interaction of steel/silicon into an easy sliding steel/transfer film/silicon contact. Furthermore, the particular stacking and orientation of the nanosheets revealed the structural mechanism of ultralow friction.

Fabrication of permanent self-lubricating 2D material-reinforced nickel mould tools using electroforming

In the replication of polymeric micro/nano structures, adhesion and friction between mould tool and polymer cause significant feature failure. For the first time, this work developed a novel strategy for the fabrication of high-hardness and self-lubricating 2D material-reinforced nickel mould tools using electroforming to achieve high precision replication of polymeric micro structures. In this study, layered 2D material, including graphene oxide (GO), molybdenum disulfate (MoS2), and tungsten disulfide (WS2), for the fabrication of self-lubricating mould tools, were systematically studied. Our results demonstrated that nickel/WS2 mould tools, followed by nickel/GO and nickel/MoS2 mould tools, presented the most significant microhardness improvement. A maximum microhardness of ∼660 HV together with a minimum crystallite size of 12 nm was achieved from 0.5 g/L WS2, indicating a 3.67 times microhardness increase and 3 times crystallite size reduction relative to the pure nickel mould tool. The enhanced microhardness can be attributed to the 2D material-induced crystal refinement, and inherent hardness and incorporation content of 2D material. Additionally, friction and wear tests revealed that a low concentration of WS2 at 0.14 g/L achieved the lowest coefficient of friction (COF) and superior wear resistance. The COFs in the initial stage and steady state stage were 0.08 and 0.18, respectively, implying decreases of 42.8% and 72.3%, respectively, and a 27-fold increase in lifetime compared with those of the pure nickel mould tool. Such a significant improvement in tribological properties was due to the formation of self-lubricating transfer film by the interlayer shear effect of few-layered 2D material nanosheets. Finally, defect-free polymeric microfluidic chips were micro hot embossed using an optimal self-lubricating nickel/WS2 mould tool for validation. This work provides significant insight into the fabrication of potential self-lubricating micro/nano mould tools for microfluidics applications.

Tribological behavior and wear mechanism of Ti/MoS2 films deposited on plasma nitrided CF170 steel sliding against different mating materials

To improve the load-bearing capacity and tribological properties of CF170 steel, a duplex treatment including plasma nitriding and Ti/MoS2 films was carried out on CF170 steel. The morphology, structure and composition of the films were characterized by SEM, XRD, AFM and XPS. The mechanical and tribological behaviors of the films sliding against different mating materials were investigated by hardness tester, scratch tester and ball-on-disc tribometer, respectively. The results indicated that the Ti/MoS2 film deposited on plasma nitrided substrate was composed of MoS2, γ-Fe4N, ε-Fe2,3N and α-Fe phases, and the MoS2 phase had a strong (002) preferred orientation. The load-bearing capacity and critical adhesion load of the duplex-treated samples were significantly improved compared to the single Ti/MoS2 film. The duplex-treated samples exhibited different wear behaviors against different mating materials, the contact pressure and formation of the valid MoS2 transfer film on the friction pairs were notable in determining the tribological behavior. The duplex-treated sample sliding against the SiC ball under air showed a lower friction coefficient and wear rate than that of others, indicated an excellent wear resistance.

A Combined Experimental and Atomistic Investigation of PTFE Double Transfer Film Formation and Lubrication in Rolling Point Contacts

Solid lubricants such as polytetrafluoroethylene (PTFE) are used in rolling-element bearings (REBs) when conventional lubrication (i.e. by fluids or greases) cannot be applied owing to extreme operating conditions (e.g. high temperatures or vacuum). Often a double transfer film mechanism is used with a cage acting as a lubricant reservoir resupplying the REB with solid lubricant by cage wear. An increase in service life of such bearings requires a better understanding of the transfer processes in the sliding and rolling contacts. Here, we investigate the effect of PTFE resupply on friction and lubricant film formation in a steel/steel and steel/glass rolling contact by tribometry and classical molecular dynamics (MD). A ball-on-disk tribometer is enhanced by a pin-on-disk sliding contact that transfers PTFE to the disk. The experiment allows simultaneous in situ measurement of friction and film thickness by white light interferometry in the rolling point contact. Increasing the pin load results in an increased PTFE film thickness in the rolling contact accompanied by a significant decrease in friction. To elucidate the observed film transfer and friction mechanism, sliding MD simulations with a newly developed density-functional-based, non-reactive force field for PTFE-lubricated iron oxide surfaces are performed. A strong adhesion of PTFE chains to iron oxide drives transfer film formation, whilst shear-induced chain alignment within PTFE results in reduced friction. The simulations reveal an anti-correlation between PTFE film thickness and friction coefficient—in agreement with the experiments. These investigations are a first step towards methods to control PTFE transfer film formation in REBs.Graphic

Tribological properties of bismaleimide-based self-lubricating composite enhanced by MoS2 quantum dots/graphene hybrid

In order to promote the formation of high-quality graphene-based self-lubricating transfer film. The reduced graphene oxide (RGO) deposited with molybdenum disulfide quantum dots (MoS2QDs) were successfully prepared. The as-prepared MoS2QDs/RGO hybrid was used for the first time to improve the tribological properties of polymer composites. Compared with the neat bismaleimide (BMI) and RGO filled composites, the MoS2QDs/RGO/BMI showed the best tribological properties, and the frictional coefficient and volume wear rate could be decreased to 0.15 and 8.9 × 10−6 mm3/(N·m), respectively. This is mainly due to the MoS2QDs loaded on the graphene surface are beneficial to prevent the graphene nanosheet stacking, and the introduction of MoS2QDs can make the self-lubricating transfer film formed more uniform, smooth and robust during the friction process.

Tribological properties of PTFE fiber filled polyoxymethylene composites: The influence of fiber orientation

Inspired by the structure of traditional Chinese bamboo rafts, the influence of orientation of solid fiber lubricants on the tribological properties of self-lubricating polymers was systematically investigated. Polytetrafluoroethylene (PTFE) fibers were adopted as solid lubricants to improve the tribological properties of polyoxymethylene (POM) composites. Results indicated that the tribological behavior of POM was greatly improved with the presence of PTFE fibers. Both the friction coefficient and specific wear rate decreased with increasing content of PTFE fibers. Moreover, the orientation of PTFE fibers is beneficial to the reduction of both the friction coefficient and specific wear rate, which is attributed to the ease formation of transfer film on the countersurface of rolling steels.

Effect of Nanofillers on Tribological Properties of Polymer Nanocomposites: A Review on Recent Development.

Polymer nanocomposites with enhanced performances are becoming a trend in the current research field, overcoming the limitations of bulk polymer and meeting the demands of market and society in tribological applications. Polytetrafluoroethylene, poly(ether ether ketone) and ultrahigh molecular weight polyethylene are the most popular polymers in recent research on tribology. Current work comprehensively reviews recent advancements of polymer nanocomposites in tribology. The influence of different types of nanofiller, such as carbon-based nanofiller, silicon-based nanofiller, metal oxide nanofiller and hybrid nanofiller, on the tribological performance of thermoplastic and thermoset nanocomposites is discussed. Since the tribological properties of polymer nanocomposites are not intrinsic but are dependent on sliding conditions, direct comparison between different types of nanofiller or the same nanofiller of different morphologies and structures is not feasible. Friction and wear rate are normalized to indicate relative improvement by different fillers. Emphasis is given to the effect of nanofiller content and surface modification of nanofillers on friction, wear resistance, wear mechanism and transfer film formation of its nanocomposites. Limitations from the previous works are addressed and future research on tribology of polymer nanocomposites is proposed.

Experimental-FEM Study on Effect of Tribological Load Conditions on Wear Resistance of Three-Component High-Strength Solid-Lubricant PI-Based Composites.

The structure, mechanical and tribological properties of the polyimide-based composites reinforced with chopped carbon fibers (CCF) and loaded with solid-lubricant commercially available fillers of various natures were investigated. The metal- and ceramic counterparts were employed for tribological testing. Micron sized powders of PTFE, colloidal graphite and molybdenum disulfide were used for solid lubrication. It was shown that elastic modulus was enhanced by up to 2.5 times, while ultimate tensile strength was increased by up 1.5 times. The scheme and tribological loading conditions exerted the great effect on wear resistance of the composites. In the tribological tests by the ‘pin-on-disk’ scheme, wear rate decreased down to ~290 times for the metal-polymer tribological contact and to ~285 times for the ceramic-polymer one (compared to those for neat PI). In the tribological tests against the rougher counterpart (Ra~0.2 μm, the ‘block-on-ring’ scheme) three-component composites with both graphite and MoS2 exhibited high wear resistance. Under the “block-on-ring” scheme, the possibility of the transfer film formation was minimized, since the large-area counterpart slid against the ‘non-renewable’ surface of the polymer composite (at a ‘shortage’ of solid lubricant particles). On the other hand, graphite and MoS2 particles served as reinforcing inclusions. Finally, numerical simulation of the tribological test according to the ‘block-on-ring’ scheme was carried out. Within the framework of the implemented model, the counterpart roughness level exerted the significantly greater effect on wear rate in contrast to the porosity.

Tribological performances of hybrid composites of Epoxy, UHMWPE and MoS2 with in situ liquid lubrication against steel and itself

In this paper, hybrid polymeric composites of epoxy with two solids (UHMWPE and MoS2) and one liquid (base oil SN150) fillers were prepared to improve the thermal, mechanical and tribological properties for load bearing applications. Due to relatively high friction of solid lubricants compared to liquid lubricants, base oil was incorporated into the epoxy composite as liquid filler to achieve enhanced tribological properties. All the experiments were conducted with two interfacial conditions: polymer composite against steel (EN31) and polymer composite against itself. The base oil-filled epoxy composite showed excellent tribological performance with specific wear rate in the range of 10−7 mm3/Nm and low coefficient of friction of 0.08–0.1 at a constant nominal contact pressure of 0.611 MPa and speed 1 m/s. Durable transfer film formation was identified as the main wear reduction mechanism. These fillers also made the surface of epoxy hydrophobic in nature which reduced adhesion and friction. By adding fillers, thermal degradation temperature of the composite was increased by 22.8%. The lubrication mechanism was identified as mixed lubrication with three different sub-regimes.

Morphological Characteristics and Wear Mechanism of Recycled Carbon Fibre Prepreg reinforced Polypropylene Composites

The polypropylene (PP) reinforced with recycled carbon fibres (rCF) was successfully produced using a Haake internal mixer via melt compounding. The compounding was performed at 180°C, rotor speed of 50 rpm and compounding period of 10 minutes. The standard samples for the pin on disc testing were prepared using injection moulding. The effect of rCF filler loadings of 0.5, 1, 3, 5, 7, 10, 13, 15 and 20 wt% was studied for the tribological properties. The results were compared with 100% PP. The morphological behaviours for the effect of low and high fibre loadings were observed using scanning electron microscopy analyses. The composites with low carbon fibre loading of up to 3 wt% imposed higher resistance to dry sliding friction. In contrast, the increment of fibre loading at 5 wt% to 20 wt% decreased the wear rate of the composites due to patch film and transfer film formation. The wear mechanism of the composites for different fibre loading was graphically sketched from morphological observation. As the conclusions, the composites showed promising self-lubricating properties, capable of wear reduction with significant physical and mechanical properties.

Chemisorption mechanism of defluorinated fluorine on bcc Fe surface during formation of PTFE transfer film

Since defluorination is most likely to occur during the formation of polytetrafluoroethylene (PTFE) transfer film, understanding the adsorption mechanism of defluorinated fluorine can help describe the interfacial tribochemical reaction. XPS analysis showed that the defluorinated fluorine was chemisorbed on the Fe surface to generate iron fluoride. Density functional theory (DFT) calculations revealed that the adsorption of fluorine on the Fe (100) surface at the bridge site with 0.25 monolayer coverage exhibited the lowest adsorption energy (−4.71 eV), indicating a preferable adsorption configuration. At an identical adsorption site, the adsorption strength decreases with increasing coverage, which is attributed to the repulsive interaction of adsorbed fluorine. The chemisorption of fluorine on the Fe surface induces a considerable redistribution of charges, i.e. electrons are transferred from the Fe surface and accumulate around the adsorbed fluorine. The adsorbed fluorine mainly interacted with the topmost Fe layer. The two Fe atoms located at the bridge sites of the topmost layer play significant roles in the formation of Fe–F bonds that have high degrees of ionic character. The electronic states of Fe–F bond are derived from the hybridisation of Fe 4s, Fe 4p, Fe 3d, F 2s, and F 2p orbitals.

An investigation on the tribological and mechanical properties of silicone rubber/graphite composites

In this study, tribological properties and surface characteristics of silicone rubber/graphite (QMG) composites were investigated as a function of applied load, sliding velocity (1–5 m/s) and temperature. This study discusses a novel approach to improve the tribological properties of silicone rubber (QM). Inclusion of graphite significantly reduces the friction coefficient (̴40%) and specific wear rate of QM (50%). Wear mechanism involves formation of transfer film at the interphase between composite and steel counter surface. Tribological performance of QMG composites were found to be in agreement with its morphological, mechanical and dielectric properties.

Achieving Ultra-Low Friction with Diamond/Metal Systems in Extreme Environments.

In the search for achieving ultra-low friction for applications in extreme environments, we evaluate the interfacial processes of diamond/tungsten sliding contacts using an on-line macro-tribometer and a micro-tribometer in an ultra-high vacuum. The coefficient of friction for the tests with the on-line tribometer remained considerably low for unlubricated sliding of tungsten, which correlated well with the relatively low wear rates and low roughness on the wear track throughout the sliding. Ex situ analysis was performed by means of XPS and SEM-FIB in order to better understand the underlying mechanisms of low friction and low-wear sliding. The analysis did not reveal any evidence of tribofilm or transferfilm formation on the counterface, indicating the absence of significant bonding between the diamond and tungsten surfaces, which correlated well with the low-friction values. The minimal adhesive interaction and material transfer can possibly be explained by the low initial roughness values as well as high cohesive bonding energies of the two materials. The appearance of the wear track as well as the relatively higher roughness perpendicular to the sliding indicated that abrasion was the main wear mechanism. In order to elucidate the low friction of this tribocouple, we performed micro-tribological experiments in ultra-high vacuum conditions. The results show that the friction coefficient was reduced significantly in UHV. In addition, subsequently to baking the chamber, the coefficient of friction approached ultra-low values. Based on the results obtained in this study, the diamond/tungsten tribocouple seems promising for tribological interfaces in spacecraft systems, which can improve the durability of the components.

Sliding Wear Characteristics of Carbon Filled Polytetrafluoroethylene (PTFE) Composite against AISI 304 Stainless Steel Counterface

In the present work, dry sliding wear characteristics of polytetrafluoroethylene (PTFE) composite reinforced with 35% by weight carbon fiber against AISI 304 stainless steel counterface is investigated with a view to consider PTFE composite as an alternative material for automotive applications. Dry sliding experiments were performed on pin-on-disk wear testing machine varying the normal load on pin, disk rotation (rpm) and temperature correlating with the range of pressure, sliding velocity and temperature variation at reciprocating conditions of compressor. A mathematical model to predict specific wear rate in terms of pressure and temperature was developed to understand parametric effect on wear rate. Specific wear rate has been observed to decrease with increase in pressure and temperature. Although, pressure (normal load) has been observed as more significant in lowering specific wear rate than temperature, no significant benefit was observed at higher ranges of pressure. Decrease in specific wear rate with increase in normal load (pressure) can be attributed to formation of transfer film at sliding interfaces. However, it is necessary to characterize transition of formation of transfer film varying with normal load and temperature respectively.

Influence of Bronze Filler upon The Friction and Wear of Polytetrafluoroethylene and Polyoxymethylene.(Dept.M)

The present paper deals with an experimental investigation carried out to elucidate the role played by polymeric and metallic fillers upon the coefficient of friction and wear rate of Polyoxymethylene (POM) and Polytetrafluoroethylene (PTFE).Lead bronze powder and PTFE powder were added by different percentages of volume to the base material to form a Composite. The study was performed on a pin-on-disk machine under constant load of 50 N and constant sliding speed of 1.0 m/s under dry sliding conditions. The results indicate that the addition of bronze or PTFE fillers always reduces the coefficients of friction and wear rates of tested materials and that reduction increases with the increase of filler content up to about 40%. The minimum valves of friction and wear were experienced by POM containing 36% PTFE. The paper attributes the reduction in friction and wear to the formation of polymeric transfer film adhering to the steel counterface which facilitates sliding and decreases adhesion.

Synergy of core-shell Cu@rGO hybrids for significantly improved thermal and tribological properties of polyimide composites

A facile electrostatic self-assembly way was developed to prepare Cu nanoparticles @ rGO (reduced graphene oxide) nanosheets to enhance the properties of PI (polyimide) composites. Through the electrostatic self-assembly between Cu and rGO, interface compatibility of Cu and PI matrix was improved. Meanwhile, the zero-dimensional Cu and two-dimensional rGO in core-shell Cu@rGO hybrids could integrate the multiple advantages and provide synergistic enhancement for PI composites. As a result, the maximum thermal decomposition temperature of PI/Cu@rGO composites recorded an increase of 12 °C in comparision with that of pure PI. In addition, the Cu@rGO hybrids demonstrated the optimal lubrication properties for PI matrix compared to individual Cu, rGO and Cu/rGO blend. For PI/Cu@rGO-0.5 wt% composites, the specific wear rate and average friction coefficient were decreased by 44.1% and 11.6%. The outstanding tribological performance of PI/Cu@rGO composites can be ascribed to the synergistic enhancement between Cu nanoparticles and rGO nanosheets as well as the formation of high-quality transfer film. Furthermore, the inherent abrasion mechanism of PI/Cu@rGO composites was explored systematically.

Mono-dispersed Ag nanoparticles decorated graphitic carbon nitride: An excellent lubricating additive as PPESK composite film

Studies show that two dimensional (2D) nanomaterial and its hybrid have a great promise in tribology for the special laminar microstructure. However, the majority of performed investigations about 2D graphitic carbon nitride (g-C3N4) nanosheets are most focused on energy storage, catalysis, adsorption, rarely tribology. In the present study, g-C3N4 supporting mono-dispersed Ag nanoparticle hybrid (g-C3N4/Ag) is prepared, and its microstructure and chemical composition are determined. More specifically, the tribological performance as the lubricating additive of poly phthalazinone ether sulfone ketone (PPESK) composite is investigated using the ball-on-disc friction tester. Moreover, the corresponding enhancement mechanism is well proposed based on the experimental analysis and theoretical simulation. Obtained results show that Ag nanoparticles with a size of about 10–20 nm are homogeneously anchored on g-C3N4 nanosheets, favoring for good compatibility between g-C3N4/Ag and PPESK. It is found that when 0.3 wt% of g-C3N4/Ag is added to PPESK, the friction coefficient and wear rate of PPESK decrease by 68.9% and 97.1%, respectively. These reductions are mainly attributed to the synergistic self-lubricating effect of Ag nanoparticles and g-C3N4 nanosheet, the formation of transfer film, as well as the limited effect of g-C3N4/Ag on the shear deformation of PPESK composite film. Furthermore, it is found that the proposed g-C3N4/Ag-PPESK composite still keeps reasonable friction-reducing and wear-resistant properties under heavy loads and high rotating speeds. The present study demonstrates that the proposed g-C3N4/Ag hybrid is an excellent lubricating additive for polymer composites.

Effects of water molecules on the formation of transfer films and the occurrence of superlow friction

The establishment of superlow friction in moist air is very important for the engineering application of hydrogenated diamond-like carbon (H-DLC) films. Nevertheless, water molecules in the surrounding atmosphere always result in the failure of the near-frictionless state. This work aims to explore the effects of water molecules in the environment and the material of the counterparts on the tribological performance of a composite structure prepared by depositing MoS2 on a H-DLC film. The results indicated that the existence of water molecules in the atmosphere is beneficial for achieving stable superlubricity for the material system because it helps retain the in-situ formed MoS2 transfer film on the counterpart. In the presence of water molecules, the wear interface was replaced by a robust and incommensurate MoS2 tribolayer/H-DLC sliding interface, which was responsible for the superlow friction achieved in this work. The results also revealed that the ZrO2 counterpart was capable of retaining the as-formed MoS2 transfer film and establishing long-lasting superlow friction even in dry air. The mechanisms behind this phenomenon are also discussed in this paper.

Improving mechanical properties and fretting wear resistance of ultra‐high‐molecular‐weight‐polyethylene via incorporation of zeolitic imidazolate frameworks‐carbon nano‐fiber

Zeolitic imidazolate frameworks (denoted as ZIF8) were combined with carbon nano‐fiber (denoted as CNF) by a facile liquid‐phase chemical route to afford ZIF8‐CNF nano‐hybrid as the filler of ultra‐high‐molecular‐weight‐polyethylene (abridged as UHMWPE). The as‐prepared ZIF8‐CNF nano‐filler was incorporated into UHMWPE matrix via hot pressing to obtain the UHMWPE‐matrix composite. The effects of CNF, ZIF8, and ZIF8‐CNF on the thermal stability, mechanical properties, and fretting wear behavior of UHMWPE were comparatively investigated based on thermogravimetric analysis‐differential scanning calorimetry, scanning electron microscopy, X‐ray diffraction, and stress–strain testing. Findings indicate that ZIF8‐CNF can significantly promote the crystallization of UHMWPE and improve its thermal stability and mechanical properties. Besides, the high interfacial strength between the ZIF8‐CNF nano‐filler and UHMWPE matrix is favorable for facilitating the transfer of load from the polymer matrix to the inorganic filler and for the formation of transfer film on worn steel surface, thereby alleviating adhesion, abrasion, and plastic deformation. Therefore, the ZIF8‐CNF/UHMWPE composite exhibits much better fretting wear resistance than neat UHMWPE, CNF/UHMWPE composite, and ZIF8/UHMWPE composite.

The pivotal role of oxygen in establishing superlow friction by inducing the in situ formation of a robust MoS2 transfer film

The achievement of superlow friction is vital for the engineering application of hydrogenated diamond-like carbon (H-DLC), but it always fails in an oxygen atmosphere. In this paper, robust superlow friction was achieved by MoS2 flakes and H-DLC composite films in a large range of atmospheres, especially in oxygen. The results showed that the composite structure could only retain the superlow friction for an short time in pure argon, nitrogen and carbon dioxide; surprisingly, oxygen was capable of remaining in the near frictionless state with a friction coefficient as low as 0.002, and the duration was prolonged significantly by the introduction of oxygen in those other gases. The stability of the transfer film that induced the near frictionless state was also studied comprehensively. The experimental results and first-principle calculations demonstrated that oxygen could bond with the molybdenum, sulfur and aluminum atoms to form bridge bonds that fixed the MoS2 transfer film on the counterface; this led to the formation of incommensurate contact between the MoS2 tribo-layer and H-DLC film, which enabled robust superlow friction. This finding supports a simple strategy to resolve the challenge of superlubric failure and opens a path for the actual application of H-DLC in oxygen-rich environments.