Lubricant Materials Research Articles

Tribological behavior of lubricant-impregnated porous polyimide

Porous materials impregnated with lubricants can be used in conditions where limited lubricant is desirable. In this work, three porous polyimides (PPI) with different densities were prepared. Polyalphaolefin (PAO) impregnated PPI (iPPI) discs were rubbed against steel and sapphire balls. In operando observations of the iPPI–sapphire contacts show that oil is released under an applied load, forming a meniscus around contacts. Cavitation at the outlet is created at high sliding speeds. The amount of released oil increases with increasing PPI porosity. Contact moduli, E*, estimated based on the actual contact size show that trapped oil in iPPIs contributes to load support. At higher speeds, tribological rehydration of the contact occurs in low density iPPI, with that E* rises with speed. For high density PPIs, high speeds give a constantly high E* which is limited by the viscoelastic properties of the PPI network and possibly the rate of oil exudation. Friction of iPPI–steel contacts is governed by the mechanical properties of the PPI, the flow of the lubricant, and the roughness of the PPI surfaces. For low- and medium- density (highly porous, high roughness) PPIs, large amount of oil is released, and lubrication is mainly via lubricant restricted in the contact in the pores and possibly tribological rehydration. For high density (low porosity) PPI, with lower roughness, hydrodynamic lubrication is achieved which gives the lowest friction. Our results show that polymeric porous materials for effective lubrication require the optimization of its surface roughness, stiffness, oil flow, and oil retentions.

Synthesis of Calcium Fluoride from Discarded Egg Shell - A Potential Solid Lubricant

Solid lubricant is an environment-friendly alternative to liquid lubricants, which can be applied in extreme environmental conditions like high temperature and pressure where liquid lubricants fail. In this work, an attempt was made to synthesize calcium fluoride, a solid lubricant material from scrap eggshell powder by using the ion exchange method. The prepared eggshell powder was treated with 40% concentric hydrofluoric acid and was manually mixed using a mortar and pestle. The synthesized CaF2 powder was characterized by using SEM and XRD. The degree of Crystallinity was calculated from the XRD data by using Origin Pro and Excel software. The Crystallinity of the prepared CaF2 powder was found to be 78.01%. The crystal size of the synthesized CaF2 powder was calculated using Scherer’s formula. The crystal size was found to be between 9–22 nm ranges. The synthesized calcium fluoride XRD intensity peaks were matching exactly with the JCPDS card of the standard CaF2 powder, confirming the presence of CaF2.

Superlubricating UHMWPE composites with functionalized carbon nanotubes reinforcement applicable to artificial joints

Polyelectrolyte-grafted ultra-high-molecular-weight polyethylene (UHMWPE) has been proven efficient in friction and wear reduction. However, owing to the damaged molecular structure and material stability, the modified material experiences compromised mechanical and tribological properties under immersed, aqueous working environment. To achieve enhanced material strength and water resistance, carbon nanotubes (CNTs) were modified and used as reinforcing agent. Nanoindentation tests and macroscopic reciprocating friction tests have shown significant improvements in mechanical and tribological properties: friction coefficient and wear rate down to 0.0086 and 5.6 × 10−8 mm3(Nm)-1 under saline-lubricated conditions, which maintained relatively stable during 120-day saline pre-soaking. This combination of strong hydration lubrication and superior material strength offers a promising design of superlubricating artificial cartilage with high wear resistance and long lifetime.

In situ self-adaptive growth of graphene coatings on hard substrates via competitive NiCo catalysis reaction

Graphene coatings have sprung up as promising solid lubrication materials due to their intriguing mechanical properties. Achieving in situ deposition of a conformally encapsulated, high-quality graphene coating on hard substrates holds immense potential for industrial antifriction applications. However, the challenge persists in ensuring robust adhesion between graphene coatings and hard substrates (WC-Co), while minimizing excessive interfacial bonding to reduce growth energy consumption and friction. To tackle this problem, NiCo solid solution and competitive reaction strategies have been employed to achieve the in situ self-adaptive growth of graphene coating on WC-Co substrates with intimate interfaces. The high-temperature solid solution breaks through the physical diffusion barriers of the Co binding phase within the substrate to the Ni layer, facilitating the strong coupling effect at the NiCo-WC substrates. Experiments and density functional theory demonstrated that the accumulated Co atoms induce competitive reactions to inhibit the overlap of the electron cloud in Ni-C bonds, providing C atoms with sufficient energy to in situ segregate and diffuse laterally. Benefiting from the weakened interfacial covalent interaction and lower sliding energy barrier, graphene coating in situ deposited on substrates with deeper NiCo solid solution demonstrates superior macroscale low-friction and durability. Drawing on this design strategy, it is highly expected that hard substrate can be extended to diverse industrial surfaces composed of competitive metals for engineered graphene anti-friction applications.

Bioinspired multi-crosslinking and solid–liquid composite lubricating MXene/PVA hydrogel based on salting out effect

Due to the unique synergistic lubrication mechanisms, solid–liquid composite lubrication has received widespread attention in tribology. However, the cumbersome preparation process and solid–liquid separated service conditions limit its practical application. Inspired by the phenomenon of cell dehydration in electrolyte solutions, a simple salting out method is employed to develop multi-crosslinked and salted out MXene/polyvinyl alcohol (PVA) (MS-MP) hydrogels that can release water controllably for achieving solid–liquid composite lubrication in a single material. In this work, zinc ions (Zn2+) with both multi-crosslinking effect and salting out effect are incorporated into MXene/PVA hydrogels. These effects not only induce the formation of crystalline domains and the strengthening of network structure of hydrogels, but also release the water in hydrogels to realize the solid–liquid composite lubrication. Furthermore, the structure and tribological properties of MS-MP hydrogels can be regulated by adjusting the salting out degree, resulting in the acquisition of a structurally stable hydrogel with exceptional load-bearing capacity and superior lubrication performance. In brief, this work provides a novel and simple strategy for the fabrication of solid–liquid composite lubricating hydrogels, which shows great potential in fabricating liquid-containing lubrication materials.

A comparison of wear between unidirectional and reciprocating sliding motions under different applied loads and lubricants

Exploring the wear mechanisms of tribo-pairs is essential to reducing friction and wear. Although it has been known that the wear mechanisms between reciprocating sliding motion (RSM) and unidirectional sliding motion (USM) are different, the differences are seldom reported, which causes trouble for many researchers in selecting proper tribo-pairs or wear test methods. Herein, we systematically explore the effect of RSM and USM on the wear mechanism for several typical friction and lubricant materials, such as alloys, liquids, and solid lubricants. It is found that tribo-pairs suffer unstable sliding under RSM, and sliding speed as well as sliding direction change frequently, which makes it difficult to form a transfer layer or tribo-chemical reaction film, leading to high friction and abrasive wear for hard alloys and DLC coating. Additionally, the quasi-static stage under RSM, when lubricated with polyalphaolefin (PAO) 40, does not facilitate elastohydrodynamic (EHD) lubrication, which typically prevents wear. However, stable sliding under USM could lead to adhesive wear, increasing the friction coefficient for soft TC4 alloys. Although wear under RSM is generally higher than that under USM, increasing the applied load could diminish the wear difference for hard metals.

Friction and Lubrication of Eye/Lens/Lid Interface: The Effect of Lubricant and Contact Lens Material

Irritation of the eye in the case of contact lens users may imply dry eye syndrome (DES), commonly treated by eye drops. The paper focuses on measuring the friction coefficient with the simultaneous observation of lubricating film formation of the simulated eye/lens/lid interface. The experiments are performed in a pin-on-plate setup under the linear reciprocal motion. The contact is lubricated by twelve different solutions as the potential ingredients for eye drops, focusing on the role of mucin, buffer type, and molecular weight (MW) of hyaluronic acid (HA). Two commercially available contact lenses are studied, while overall lens performance is evaluated, among others. The results showed that the lens material, the presence of mucin, and the type of buffer have a decisive influence on the contact behavior. Specifically, it is concluded that the selection of eye drops should reflect the type of lens the wearers use and the conditions of their tear film. The appropriate ingredient for Acuvue Oasys and Biofinity contact lenses is suggested based on the score of the biotribological performance of the lubricant, taking both friction and lubrication into account.

Synergistic effect of ceria-zirconia nanoparticles and hBN nanosheets incorporation on the corrosion resistance of MoS2 solid lubricant coatings

Molybdenum disulphide is a conventional solid lubricant material due to their layered crystal structure enabling easy shearing resulting in low coefficient of friction. Even though MoS2 is highly lubricating, the corrosive nature of this dry solid lubricant limits its application, especially in oxidizing and humid environment. The present work focuses on the improvement of corrosion resistance properties of MoS2 nanosheets coatings, while maintaining its excellent anti-friction properties by incorporating hexagonal boron nitride (hBN) nanosheets and ceria-zirconia nanoparticles (CZ NPs). A hybrid structure consisting of MoS2, hBN and CZ NPs was prepared as a composite and multilayer coating. The coatings were characterized by FE-SEM, XRD, Raman spectroscopy and XPS. The inclusion of hBN and CZ NPs did not significantly alter the coefficient of friction (CoF) and wear for the composite and multilayer coatings, as determined by the tribological investigations conducted using high frequency reciprocating rig (HFRR). The MH30-CZ composite coatings showed higher CoF compared to MH30-CZ multilayer coatings, with only 7.75 % increase as compared to pure MoS2 coating. The composite structure and multilayer structure showed high long-term corrosion resistance while the latter had the highest resistance to corrosion and oxidation. The hybrid multilayer structure had the lowest corrosion current density and corrosion rate with 167 % enhancement in total impedance. In the hybrid coating, hBN acts as a physical and insulating barrier to the flow of corrosive ions and charge transfer while CZ NPs forms insoluble precipitates which further impedes the corrosion. The galvanic cell formation of MoS2 with steel was suppressed due to the presence of insulating CZ NPs layers. The synergic effect of hBN and CZ NPs resulted in high long-term corrosion resistance of the coating.

The Biocompatibility Analysis of Artificial Mucin-Like Glycopolymers.

The ocular surface is covered by a tear film consisting of an aqueous/mucin phase and a superficial lipid layer. Mucins, highly O-glycosylated proteins, are responsible for lubrication and ocular surface protection. Due to contact lens wear or eye disorders, lubrication of the ocular surface can be affected. Artificial glycopolymers which mimic natural mucins could be efficient in ophthalmic therapy. Various neutral, positively, and negatively charged mucin-mimicking glycopolymers were synthesized (n = 11), cultured in different concentrations (1%, 0.1%, and 0.01% w/v) with human corneal epithelial cells (HCE), and analyzed by various cytotoxicity/viability, morphology, and immunohistochemistry (IHC) assays. Six of the eleven glycopolymers were selected for further analysis after cytotoxicity/viability assays. We showed that the six selected glycopolymers had no cytotoxic effect on HCE cells in the 0.01% w/v concentration. They did not negatively affect cell viability and displayed both morphology and characteristic markers as untreated control cells. These polymers could be used in the future as mucin-mimicking semi-synthetic materials for lubrication and protection of the ocular surface.

Interfacial adsorption and tribological response of various functional groups on titanium surface: In-depth research conducted on the lubricating mechanism of liquid lubricants

Lubricating materials for highly reactive titanium alloys are not sufficiently abundant, limiting their wide practical applications. In this work, the tribological properties for various lubricants on titanium alloy surfaces are investigated, as well as their physicochemical and adsorption properties in terms of structural composition. The results indicated that high polarity, long alkyl chain length and small spatial site resistance will be positively correlated with the tribological properties. The FIB-TEM analysis further illustrates that the film-forming mechanism of strong polar adsorption and tribo-chemical reaction is vital to high-performance lubricants on titanium alloys. This work shows great instructive significance to investigate the tribological properties and interaction mechanisms on titanium alloy surfaces from the perspective of molecular design.

First-principles Calculations Reveal Frictional Advantage for C2N/C6N6 van der Waals Heterostructures.

Friction at the atomic scale is determined for three different carbon nitride structures namely C2N/C2N, C6N6/C6N6 and C6N6/C2N employing ab-initio density functional theory (DFT). The sliding path along the lowest energy corrugations determines the static frictional forces. Both the homo-layer structures (C2N/C2N and C6N6/C6N6) have higher corrugation energy and correspondingly higher static lateral forces with respect to the hetero-layer structure (C2N/C6N6). The corrugation energy for the C2N/C6N6 heterostructure ([[EQUATION]] = 0.29 meV/atom) is one-order lower than C2N/C2N ([[EQUATION]]= 2.08 meV/atom) and C6N6/C6N6 ([[EQUATION]] = 4.37 meV/atom). Such a significantly lower corrugation energy for the heterostructure arises due to the reduced fluctuation in the interfacial charge density along the sliding pathway. Moreover, the change in the interlayer distance along the sliding pathway is only 0.2 Å for the heterostructure while its 0.3 Å and 0.4 Å for C2N and C6N6 homo-layers respectively. The friction coefficients (FL/FN, FL=static lateral force; FN=normal force) decrease with increasing load for all the systems with the lowest value (0.04) for C2N/C6N6 at 2 GPa. The van der Waals heterostructures are, therefore, predicted to be highly efficient lubricant materials for reducing friction at the atomic scale.

Structurally integrated organosilane modified phosphate-based high-performance atomic oxygen irradiation resistance and self-lubricating coatings

Lubricating materials in harsh space environments demands both good lubricity and excellent properties of the atomic oxygen (AO) irradiation resistance. Herein, an organosilane modified phosphate-based coatings have been fabricated using an air spraying method. The introduction of the organosilane into the phosphate resin at a molecular level extremely improved the flexibility of the coatings which kept the excellent AO resistance performance. It is due to the modified phosphate itself could effectively block diffusion and erosion of the oxygen free radicals during AO irradiation. After AO irradiation, the average coefficient of friction was less than 0.03 and the wear rate reached 10−8 mm3·N·m−1 orders of magnitude. More importantly, the wear life was more than 1.5 million cycles. The coatings have great potential to be applied in lubricating protection for mechanical system that long-term serviced in space environment.

Structural and environmental dependence of superlubricious properties in hydrogenated Graphite-Like carbon Films: Implications in solid state lubrication

A variety of sp2-bonded structures can be formed in carbon materials, and carbon-based lubricants with low shear strength are widely used for harsh working conditions. In this paper, hydrogenated graphite-like carbon (GLC) films with a high content of sp2-bonded structures were prepared by ion beam deposition. Their structures were modified by heat treatment (annealing) and element doping to improve the lubrication performance. The microstructure characterization of tribolayer formed at friction interfaces and the lubricating mechanism were analyzed thoroughly, and an appropriate modification method of sp2-bonded structures was then proposed. Controlling the sp3 cross-linked structures between sp2-bonded carbon could improve the lubrication performance precisely. The results regarding GLC films against tribopairs with different hydrogen contents indicate that a stable friction coefficient (0.006) can be obtained in dry N2 when GLC film and hydrogenated polymer-like carbon (PLC) coated ball were used as tribocouples, and the wear rate is only 6.71 × 10-10 mm3/N·m after 100,000 reciprocating cycles. By utilizing the high sp2-bonded carbon films, a theoretical basis for superlubricity can be established through heat treatment and element doping, which provides the sufficient technical support for the development of solid lubricating materials with low friction coefficients and high wear resistance under harsh working conditions.

A Theoretical and Experimental Study of Oil Wicking Mechanism of Kapok Fibrous Powder Assembly

ABSTRACT Kapok fiber has a large lumen with a natural waxy surface which endows it with distinct hydrophobic-oleophilic properties. Oil-impregnated kapok fibrous powder (KFP) has great potential as a lubricant carrier material. To study the oil wicking mechanism of KFP assemblies with different pore structures (interfibrous pores and the lumens), a dual-scale model was established based on Washburn capillary theory. The theoretical model was verified by capillary rise test of KFP assemblies in various packing densities ranging from 0.08 g/cm3 to 0.14 g/cm3. It was found that the experimental results have good consistency with theoretical values at low packing densities. The deviation between the experimental result and the theoretical value increases with the increase of packing density. This can be largely attributed to compression of the lumens at increased packing density, leading to the reduction of oil sorption capacity within the lumens. The outcomes of this research are believed to provide important references for the usage of fibrous powder materials as lubricant carrier.

Bioinspired ultra-lubrication hydrogels with controllably switchable lubrication

Switchable lubrication materials have great potential in the preparation of intelligent grabbing devices, tissue materials, and intelligent soft robots. In nature, numerous living organisms can dynamically change their frictional behaviors. Inspired by the lubrication to the astringency of the oral cavity, mucin was incorporated with poly(vinyl alcohol) (PVA) to emulate this behavior from an ultra-low friction state to a high friction state. The results show that mucin allows for the reduction in coefficient of friction by 95.8 % relative to mucin-free hydrogels. Furthermore, large-scale switchable lubrication (μ∼0.009 to μ∼0.47) is achieved through the interaction between mucin and tannic acid (TA). The approach may offer a method for preparing intelligent grabbing devices, tissue materials, and intelligent soft robots.

Graphene Enhances the Loading Capacity and Lubrication Performance of Ionic Liquids: A Molecular Dynamics Study.

Ionic liquid (IL) combined with graphene additives have garnered extensive attention in the field of high-performance lubricating materials. However, the ambiguous mechanism of graphene influencing the load-carrying and anti-wear capacity of ILs needs further study. In this work, friction simulation shows that adding graphene causes friction coefficient to reduce by up to 88% compared with pure ILs, but lubrication performance is lost due to the destruction of graphene under high stress. Meanwhile, multilayer graphene has better friction-reducing performance and friction durability as compared to the monolayer structure, which is attributed to the easy-shear property and the reduction in the percentage of high tensile stress sites in multilayer graphene structure. In addition, it was found that excessively thick ILs film would form a three-body abrasive wear structure with graphene, which accelerated the structural destruction of graphene and caused a decline in its tribological properties. It is believed these findings can be valuable for designing of high-performance lubricating oil for practical engineering.

MXene/graphene oxide nanocomposites for friction and wear reduction of rough steel surfaces

Development of solid lubricant materials that render reliable performance in ambient conditions, are amenable to industrial size and design complexities, and work on engineered surfaces is reported. These coatings are composed of Ti3C2Tx-Graphene Oxide blends, spray-coated onto bearing steel surfaces. The tribological assessment was carried out in ambient environmental conditions and high contact pressures in a ball-on-disc experimental set-up. The evaluation yielded that the use of Ti3C2Tx-Graphene-Oxide coatings led to substantial reduction in friction down to 0.065 (at 1 GPa contact pressure and 100 mm/s) in comparison to the uncoated of single-component-coated surfaces, surpassing the state-of-the-art. The coatings also provided excellent protection against wear loss of the substrate and counter-face. The results were explained based on the observations from Raman spectroscopy, scanning electron microscopy, transmission electron microscopy, and nanoindentation measurements. In operando formation of a dense, hard and stiff, dangling-bond-saturated tribolayer was observed to be the reason for the sustained lubricity even at high test loads and sliding speeds. This report presents the holistic exploration and correlation of structure-property-processing pertaining to the advancement of solid lubrication science.

Synthesis and tribological properties of the novel tubular MoS2/GR nanocomposite

This study used a simple one-step hydrothermal method to synthesize the MoS2/GR composites with a new morphology composed of graphene nanotubes and ultra-thin molybdenum disulfide with the help of sodium chloride. The composites were characterized by XRD, XPS, SEM, TEM, and a series of characterization methods. Meanwhile, the tribological properties of the composites were studied. The results show that the addition of 1% MoS2/GR composite nanotubes has excellent tribological properties. In addition, the structure and excellent tribological properties of MoS2-C- nanocomposite lubrication materials will be conducive to designing new nanomaterials with 2D/3D structures, enhancing the anti-friction and anti-wear properties, and expanding their practical applications in industrial and agricultural fields.

Influence of lubricant material in the point contact zone of rolling friction on fatigue life for friction bearing units

Although lubrication is necessary for the satisfactory operation of rolling bearings, the effect of lubricant on the fatigue life of the bearing has not been sufficiently studied. In recent times, the theory of elastohydrodynamic (EHD) lubrication [1] has been used to explain the different effects of lubricants. According to this theory, the thickness of the lubricating layer separating the moving elements of the bearing is determined by the viscosity-pressure dependence of the lubricant. The contact of surface micron irregularities does not occur if it is possible to maintain a sufficient thickness of the lubricating layer - in this case, the long-term durability of the bearing is ensured. If the film thickness is reduced to a level where surface irregularities are encountered, the fatigue life rapidly decreases with increasing contact frequency. In any case, a comprehensive calculation methodology is needed that would allow to take into account the influence of lubricant on the fatigue life of bearing units.

Tribological dissimilarity and solid-liquid interaction of WS2-Al film combining with different space oils in vacuum

Proper solid-liquid lubricating materials can generate synergistic effect, achieving long life under harsh operating condition for space equipment. Herein, the solid-liquid systems were constructed by WS2-Al film and typical space lubricating oils with different molecules (P201 and PFPE). The results indicate that the systems markedly prolong the life and firstly appear obvious tribological dissimilarities. Especially, WS2-Al/P201 system exhibits more longer life exceeding 6 × 105 r. It is attributed that the compact tribolayer consisting of numerous nanosheets lied flat on wear track surface and good transfer film are well formed. Differently, WS2-Al/PFPE system is found to happen more serious tribo-oxidation and only generate numerous floccule-like debris. Eventually, interactions between WS2-Al crystals and oil molecules are essentially revealed during friction process.