Fullerene-like WS2 Nanoparticles Research Articles

Nanoparticle based and sputtered WS2 low-friction coatings — Differences and similarities with respect to friction mechanisms and tribofilm formation

MoS2 and WS2 are widely known intrinsic low-friction materials that have been extensively used and thoroughly investigated in literature. They are commonly produced in the form of sputtered coatings and show extremely low friction coefficients in non-humid environments, but rapidly degrade in humid conditions. Close nested fullerene-like nanoparticles of these materials have been proposed to have better oxidation resistance due to their closed form with the absence of dangling bonds. In the present study, an electrochemically deposited coating consisting of fullerene-like nanoparticles of WS2 embedded in a Ni–P matrix is tested under various loads and humidity conditions and compared with a sputtered WS2 coating with respect to their tribological behavior. The formation of a tribofilm on both surfaces is known to be crucial for the low-friction mechanism of WS2 and the different mechanisms behind this formation for the two types of coatings are investigated. It is shown that despite the completely different transformation processes, the resulting tribofilms are very similar. This is analyzed thoroughly using SEM, AES and TEM. The friction coefficient is known to be lower at higher normal loads for these materials and in the present study the mechanical and chemical responses of the tribofilm to higher normal loads during sliding are investigated. It was observed that the basal planes become aligned more parallel to the surface at higher loads, and that the tribofilm is less oxidized. It is suggested that these mechanisms are connected and are crucial keys to the wear life of these materials.

Tribological performance of the epoxy-based composite reinforced by WS2 fullerene-like nanoparticles and nanotubes

Recently large amounts of inorganic nanotubes (INT) and inorganic fullerene-like (IF) nanoparticles of WS2 became available and methods for their dispersion in different media were developed. In the present work the tribological properties of epoxy composite compounded with tungsten disulfide particles of different sizes and morphologies, including quasi-spherical IF nanoparticles, one-dimensional INT as well as micron-size platelets (2H) were investigated. The coefficient of friction and wear loss were measured under dry contact conditions using different tribological rigs. Remarkable reduction in wear and also friction (under high load) was demonstrated for the IF/INT epoxy nanocomposite. The reduced wear is attributed in general to the reinforcement of the polymer matrix by nanoparticles and the simultaneous reduction of the epoxy brittleness. Contrarily, the friction of the neat epoxy sample and epoxy mixed with platelets was accompanied with strong wear and transfer of a polymer film onto the rubbed surfaces. These results are consistent with the recently reported improvements in the fracture toughness, peel and shear strength of the epoxy-nanoparticles (IF/INT) composites.

Codeposition of inorganic fullerene-like WS2 nanoparticles in an electrodeposited nickel matrix under the influence of ultrasonic agitation

Abstract The effects of ultrasound (US) agitation (at different intensities of ultrasound) on the codeposition of inorganic fullerene-like (IF) WS 2 in a metallic Ni matrix are investigated. The wt.% of IF-WS 2 particles in the nanocomposite coatings increases from 4.5 wt.% (in films grown under mechanical stirring but without US agitation) up to about 7 wt.% (in films prepared under US agitation at the optimum intensity of ultrasound). While composite coatings obtained only under mechanical stirring exhibit a rather irregular surface morphology, with pronounced 3D multinodular protrusions, a more uniform and compact coating is attained under application of US agitation. In turn, the introduction of WS 2 particles significantly modifies the growth of Ni, inducing grain size refinement, increasing the stacking fault probability and causing a drastic change of crystallographic texture. These microstructural features act in a synergistic manner to simultaneously increase the hardness of the composite coatings while causing the expected decrease in the friction coefficient (due to the presence of WS 2 solid lubricant particles), therefore enhancing the wear resistance, particularly when the coatings are deposited under the presence of US.

Insights into the Structure of MoS2/WS2 Nanomaterial Catalysts as Revealed by Aberration Corrected STEM

Molybdenum disulfide/Tungsten disulphide (MoS2/WS2) is a compound very useful for its properties; it is used as lubricant, catalyst in hydrodesulfuration, in hydrogen fuel storage, etc. As part of the 2nd Joint Congress of the Portuguese and Spanish Microscopy Societies the present work reports about the different types of MoS2/WS2 nanomaterials which have been investigated by using aberration corrected STEM namely: (1) MoS2 nanotubes (2) MoS2 hexagonal nanoplates, (3) rippled or helical MoS2 nanowires, (4) Co-doped MoS2/WS2 nanowires and (5) fullerene-like WS2 nanoparticles.

Alleviating fatigue and failure of NiTi endodontic files by a coating containing inorganic fullerene-like WS2 nanoparticles

Abstract

New hybrid nanocomposites containing carbon nanotubes, inorganic fullerene-like WS2 nanoparticles and poly(ether ether ketone) (PEEK)

Single-walled carbon nanotubes (SWCNTs) and inorganic fullerene-like WS2 nanoparticles were successfully dispersed in poly(ether ether ketone) (PEEK) by advantageously traditional melt processing techniques. This strategy offers an attractive way to combine the merits of organic and inorganic materials into novel hybrid systems that challenge emerging polymer/CNT nanocomposites in terms of performance, cost and processability. In particular, the incorporation of IF-WS2 has shown to be efficient to produce well-dispersed PEEK/SWCNT composites influenced in turn by the processing method. The morphology, thermal stability, crystallization behaviour, thermal conductivity as well as mechanical and electrical properties of PEEK/SWCNT composites were tuneable by the introduction of small amounts of IF-WS2. The overall structural, thermal, mechanical and electrical performances confirm the potential use of SWCNTs and IF-WS2 for the preparation of advanced hybrid nanocomposites as lightweight alternatives for use in critical industrial applications of high-performance and high-temperature thermoplastics like PEEK.

Tribological behaviour of MoS2 and inorganic fullerene-like WS2 nanoparticles under boundary and mixed lubrication regimes

For several years different types of nanoparticles have been considered and studied as potential friction modifying lubricant additives. Some nanoparticles can reduce the friction coefficient by 30–70%, depending on the base oil and the experimental conditions. In the present study, an experimental analysis on tribological properties of inorganic fullerene-like metal dichalcogenides was performed in comparison with MoS2 2H layered structures. Tribological tests were carried out on a pin on disc tribometer in ambient air. Several contact conditions are analysed in order to realise boundary and mixed lubrication regimes. The experimental study was performed on a mineral base oil, and particle concentration effects were analysed. Antifriction properties were evaluated by measuring the friction coefficient and are presented as generalised Stribeck diagrams. Inorganic fullerene-like WS2 and MoS2 nanoparticles present interesting friction reduction properties when tested in boundary and mixed lubrication.

Towards medical applications of self-lubricating coatings with fullerene-like (IF) WS<SUB align=right>2 nanoparticles

Medicine's prime concern is first not to harm, thereafter elongate and benefit lifespan. The vast technological advances of the last decade permit implementation of minimal invasive medicine in the different medical disciplines. Laparoscopic procedures and intubations, stents insertion and joint replacement had become commonplace in the operating rooms. Overcoming wear and frictional forces in order to elevate treatment efficiency and diminish unwanted side effects is now a rising challenge in designing new medical appliances. Here, two kinds of self-lubricating metal coatings with impregnated fullerene-like (IF) nanoparticles of WS2 are presented: cobalt films deposited on NiTi substrate (foils and orthodontic wires) and nickel films deposited on orthodontic wires made of stainless steel. Tribological and mechanical tests provide strong evidence for the efficacy of the coatings in reducing the static and kinetic friction. It is concluded that the use of these self-lubricating coatings may have important impact on ubiquitous medical procedures.

Synthesis and Tribological Performance of Novel Mo x W1−x S2 (0 ≤ x ≤ 1) Inorganic Fullerenes

Inorganic fullerene-like (IF) MoS2 and WS2 nanoparticles were found to be good friction modifiers and anti-wear additives when dispersed in a lubricant. Their tribological performance seems to be related to the structure, size, and shape of these nanomaterials. The present study describes the tribological properties of a new inorganic fullerene IF-MoxW1−xS2 containing both molybdenum and tungsten disulfide under boundary lubrication. MoxW1−xS2 amorphous inorganic fullerene nanostructures were synthesized by means of MOCVD using an induction furnace setup. The average diameters range from 25 to 45 nm. Upon variation of the amounts of precursors and S, various solid solutions of IF-MoxW1−xS2 were obtained. In addition, a morphological, chemical, and structural analysis of the samples was performed using high resolution scanning electron microscopy (HRSEM), transmission electron microscopy (TEM), and X-ray powder diffraction (XRD). Friction experiments were carried out with a ball-on-flat contact using an environmental tribometer. The results show interesting friction reducing and wear properties of these nanomaterials. The change in the molecule stoichiometry, which led to a variation of particles size but also to a variation of the crystallinity of the particles, affects the tribological performance.

Synthesis of WS2 and MoS2 fullerene-like nanoparticles from solid precursors

Inorganic fullerene-like WS2 and MoS2 nanoparticles have been synthesized using exclusively solid precursors, by reaction of the corresponding metal oxide nanopowder, sulfur and a hydrogen-releasing agent (NaBH4 or LiAlH4), achieved either by conventional furnace heating up to ∼900 °C or by photothermal ablation at far higher temperatures driven by highly concentrated white light. In contrast to the established syntheses that require toxic and hazardous gases, working solely with solid precursors permits relatively safer reactor conditions conducive to industrial scale-up.

Fullerene-like WS2 nanoparticles and nanotubes by the vapor-phase synthesis ofWCln and H2S

Inorganic fullerene-like (IF) nanoparticles and nanotubes ofWS2 were synthesized by a gas phase reaction starting fromWCln (n = 4, 5,6) and H2S. The effect of the various metal chloride precursors on the formation ofthe products was investigated during the course of the study. Variousparameters have been studied to understand the growth and formation of theIF-WS2 nanoparticles and nanotubes. The parameters that have been studiedinclude flow rates of the various carrier gases, heating of the precursor metalchlorides and the temperature at which the reactions were carried out. Thebest set of conditions wherein maximum yields of the high quality pure-phaseIF-WS2 nanoparticles and nanotubes are obtained have been identified. A detailed growth mechanismhas been outlined to understand the course of formation of the various products ofWS2.

On the Efficacy of IF–WS2 Nanoparticles as Solid Lubricant: The Effect of the Loading Scheme

Fullerene-like WS2 (MoS2) nanoparticles (IF) have been studied in the past as solid lubricants. Using the tribological ball-on-flat experiments, it was shown that the size of the aggregates and their distribution determine the penetration and entrapping of the IF nanoparticles at the interface. It is expected that the wedge clearance at the inlet of the contact, i.e., the oblique-angle entrance to the contact zone between the two mating tribological surfaces, as well as the average surface roughness, can limit the supply of the lubricant into the interface in, e.g., the block-on-ring experiment. In the present series of experiments, the Stribeck curve was designed first using a linear loading scheme and pure oil. It was concluded that a wedge clearance (oblique-angle) in the inlet of the contact zone leads to entrapment of the IF nanoparticles and their compaction, which hamper the supply of the fluid lubricant into the interface. A ball-on-flat and flat block-on-ring friction devices with wedge clearance in the inlet of the contact can distort the efficacy of IF. Procedures for improving the supply of the IF nanoparticles to the contact zone and improving thereby their efficacy are considered.

Inorganic Nanotubes and Fullerene‐like Nanoparticles

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Fabrication of self-lubricating cobalt coatings on metal surfaces

Composite coatings of Co + fullerene-like WS2 nanoparticles on stainless steel substrate were obtained through electroless deposition, usingDMAB (dimethyl borane complex, 97%) as the reducing agent, and by electroplating in acidicsolution. Phase analysis results show that the coatings consist of Co and the fullerene-likeWS2 nanoparticles alone. Tribological measurements show reduced wear and friction of thecomposite coatings as compared with the pure cobalt film or the stainless steel substrate.

Friction of fullerene-like WS2 nanoparticles: effect of agglomeration

Inorganic fullerene-like (IF) solid lubricant nanoparticles with extremely useful tribological characteristics have been realized, offering a plethora of new applications for these nanomaterials. The IF nanoparticles were found to be in the aggregated state. The main goal of this work is to elucidate the effect of the mixing time of IF-WS2 nanomaterial in the oil on the size of the IF aggregates and their influence on the friction and wear. The fraction of small aggregate increases and that of the large aggregates decreases with longer mixing time. Consequently, the spread of the tribological results diminishes with the lengthening of the mixing time. The reproducibility of the friction results for the pairs lubricated with oil +IF nanoparticles is determined by distribution of the IF aggregates in the lubricant and the size of the solid lubricant aggregates.

Inorganic nanotubes and fullerene-like nanoparticles

We have proposed in 1992 that nanoparticles of layered compounds will be unstable against folding and close into fullerene-like structures and nanotubes (IF). Nanotubes and fullerene-like structures were prepared from numerous compounds with layered and recently also non-layered structure by various groups. Much progress has been achieved in the synthesis of inorganic nanotubes and fullerene-like nanoparticles of WS2 and MoS2 and many other metal dichalcogenides over the last few years. Substantial progress has been accomplished in the use of such nanoparticles for tribological applications and lately for impact resilient nanocomposites. These tests indicated that IF-MoS2 and IF-WS2 are heading for large scale applications in the automotive, machining, aerospace, electronics, defense, medical and numerous other kinds of industries. A few products based on these nanoparticles have been recently commercialized by “ApNano Materials, Inc”. Novel applications of inorganic nanotubes and fullerene-like nanoparticles in the fields of catalysis; microelectronics; Li rechargeable batteries; medical and opto-electronics will be discussed.

Nuclear Magnetic Resonance Study of Fullerene-Like WS<SUB>2</SUB>

Inorganic fullerene-like nanoparticles of WS2 (IF-WS2), are synthesized by a reaction of tungsten oxide with molecular hydrogen and hydrogen sulfide. The synthesized nanoparticles appear as large agglomerates (>40 microns), each one counting thousands of IF nanoparticles. 1H nuclear magnetic resonance study of these nanoparticles is reported. The measurements show that the prepared product contains water (and possibly some hydrogen) molecules that occupy the voids in the central part of the fullerene-like nanoparticles and the nanopores between the adhering IF-WS2 particles. Defects in the IF-WS2 structure, arising due to the strain release during the folding of the layers, may result in additional sites for the absorbed water. Vacuum annealing of the powder leads to substantial reduction in the amount of absorbed water molecules.

Observation of Current Reversal in the Scanning Tunneling Spectra of Fullerene-like WS2 Nanoparticles

Current-voltage characteristics measured using STM on fullerene-like WS2 nanoparticles show zero-bias current and contain segments in which the tunneling current flows opposite to the applied bias voltage. In addition, negative differential conductance peaks emerge in these reversed current segments, and the characteristics are hysteretic with respect to the change in the voltage sweep direction. Such unusual features resemble those appearing in cyclic voltammograms, but are uniquely observed here in tunneling spectra measured in vacuum, as well as in ambient and dry atmosphere conditions. This behavior is attributed to tunneling-driven electrochemical processes.

Synthesis of inorganic fullerene-likeWS2 nanoparticles and their lubricating performance

A facile, large-scale and low-cost route was used to synthesize inorganic fullerene-like (IF)WS2 nanoparticles by the reaction of sulfur powder (S) and as-preparedWO3 nanoparticles in a hydrogen atmosphere at a heating temperature of500–650 °C. Theas-synthesized IF-WS2 nanoparticles are of a closed hollow cage structure with an average size of about 50 nm. Thecomposition, morphology and structure of the products were characterized by XRD,TEM, FE-SEM, and HRTEM. The influences of the main reaction conditionswere investigated and the possible growth mechanism are proposed. It is worthnoting that through changing the amount of as-prepared composite powder(WO3 and S) and the content of hydrogen, yields ofIF-WS2 in larger scales can accordingly be easily obtained, and such a synthetic route may also beused in the synthesis of other transition IF metal dichalcogenides. Furthermore,tribological experiments emphasized the important role played by these as-synthesizedIF-WS2 nanoparticles in providing excellent lubricating performance, which may bring a muchbrighter future for their applications in the lubricating field, or even arouse great interest ofboth scientists and industrialists for their many other important applications.

Self-lubricating coatings containing fullerene-like WS2 nanoparticles for orthodontic wires and other possible medical applications

Uneven malaligned teeth are a problem afflicting large numbers of people, having significant economic and societal repercussions. Sliding a tooth along an archwire during orthodontic treatment involves a frictional type of force which resists this movement, causing a number of adverse effects. First, using excessive orthodontic force, leads to unwanted movements of the anchor teeth and increasing the risk of damage to the roots of the teeth. Furthermore, the frictional force is distributed unevenly between the archwire and the brackets interface, leading to strong adhesion between the wire and the bracket’s corner. This force-asymmetry causes lengthening of treatment and frequent visits for fine-tuning of the orthodontic appliances. Despite numerous efforts to lower the friction, no satisfactory solution to this issue has been obtained. In the present work a self-lubricating metal coating containing fullerene-like WS2 (IF) nanoparticles is demonstrated. Such coatings significantly reduce archwire friction, and may alleviate the adverse complications. Moreover, a number of other medical applications of the self-lubricating coatings are foreseen.