fullerenes fight friction - european coatings · orthodontics, hebrew university - faculty of...

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European Coatings Journal

06/2006

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Fullerenes fight friction

Giant inorganic molecules give exceptionalperformance as dry lubricants.Menachem Genut, Niles A. Fleischer, Alla Zak, LevRapoport, Reshef Tenne.A new class of inorganic nanostructures with a closed cagestructure has been discovered. These inorganic fullerenesare multi-walled (i.e. onion-like) spherical particles whichcan also be wrapped to form nanotubes. They are extremelyeffective as solid lubricants, particularly under extremeconditions. The development and initial applications of thesematerials are reviewed.Basic research has led to the discovery of a new class ofinorganic nanostructures with a closed cage structuretermed Inorganic Fullerene (IF)-like nanoparticles andnanotubes [1]. These IF nanoparticles have a similargeometry to hollow C60 carbon fullerenes, but inorganiccompounds such as WS2 and MoS2 have a sphericalmulti-walled, onion-like structure. Their nanosize, shape,chemistry, and structure give them unique properties.One of the most promising applications is a solid lubricantnow commercially available under the name "NanoLub". Itcan be used in the automotive, aerospace, and heavyequipment markets as an additive to enhance fluidlubricants and in coatings. It reduces friction and wearsignificantly better than conventional lubricants, especiallyunder severe contact conditions, humidity and vacuum. Thedevelopment and applications of this product are exploredbelow.

Inorganic innovation: making fullerenes without carbonIn 1992 [1] the Nano-materials Synthesis Group in theWeizmann Institute (Israel) discovered a new class ofinorganic nanostructures. The group had found that certaininorganic compounds such as WS2, MoS2, TiS2 and NbS2that normally occur as large flat platelets can be synthesisedin the form of much smaller nanospheres and nanotubes.They named these inorganic fullerene-like nanostructures orIF for short.As shown in Figure 1, each particle consists of a number ofprogressively smaller concentric forms, sometimes twenty ormore, nested one within another, forming a structure whichhas been compared to that of an onion. The diameter of thenanoparticles is on the order of 100 nm (about a thousandtimes smaller than the diameter of a human hair).Due to their size, shape, chemistry, and structure, theparticles have special properties that cannot be achievedwith conventionally size materials of the same composition,making them attractive for many commercial applications.Comprehensive biocompatibility tests at certified labsaccording to OECD protocols have clearly shown that IF-WS2 (tungsten disulfide) nanoparticles are non-toxic and safe inacute oral administration to rats, dermal sensitisation tests,and acute inhalation evaluations in rats.The Weizmann group succeeded in identifying the particularconditions for inducing layers of the platelet form of certaininorganic compounds to curve and close into multi-wallednanoparticles. The structure of each shell resembles thegeodesic dome design of Buckminster Fuller and is thustermed a 'fullerene'.Until this discovery was made, it was thought that fullerenescould only be made with carbon atoms. The group was thefirst to discover that certain inorganic materials could also beformed into fullerene-like structures, hence the nameinorganic fullerene, or IF.Figure 1 shows the fullerene structure of a typical

nanosphere and beside it a multi-walled nanotube. Themulti-walled structure is a result of the special synthesisconditions used for producing these proprietarynanoparticles.

Potential applications for IF nanoparticlesIF nanoparticles are being developed into advancedcoatings to reduce friction and improve wear resistance inmany applications. In particular, they can be utilised inmedical devices such as orthodontic arch wires and artificialjoints as well as catheters, needles, and other mechanicalitems that are inserted into the human body.A new generation of coatings and surface engineeringsolutions based on the use of IF nanoparticles significantlyreduces friction and extends operational life. This conservesenergy, reduces pollution, and saves money.The coatings can be made by various deposition methodsand techniques that allow accurate control of theincorporation of the IF nanoparticles, their adhesion tosurfaces, their physical properties such as thickness,hardness, and tribological characteristics.

IF loaded coatings can reduce friction of orthodonticwiresIn work [2] carried out in collaboration with Prof. Tenne byDr. Meir Redlich and Alon Katz (both from the Department ofOrthodontics, Hebrew University - Faculty of DentalMedicine) it was shown that the frictional force of orthodonticwires coated with IF-WS2 nanoparticles embedded inelectroless nickel phosphor coatings was reduced by up to54% compared to non-coated wires.Tribological tests using a ball-on-flat configuration performedby Prof. Lev Rapoport of the Holon Institute of Technologyshowed that the IF electroless nickel coating has a frictioncoefficient five times lower than that for uncoated stainlesssteel wire (see Figure 2).Uneven teeth are a problem afflicting large numbers ofyouths and adults, but which can usually be solved byorthodontic therapy. Sliding a tooth along an arch wire is avery common procedure to align a tooth or close a gap.Whenever sliding occurs, a frictional force which resists themovement is encountered. In orthodontic situations, thisforce includes contributions from the motion of the arch wirein its supporting bracket, as well as from the biologicalelements (tooth, root and gum).In practice, about two-thirds of the applied orthodontic forceis required just to overcome the static friction of the wire inthe brackets. However, from a biological viewpoint, justone-third of this overall force is sufficient to move teeth. Thisexcessive application of force causes unwanted movementof aligned teeth and increases the risk of root damage, notto mention causing considerable pain and discomfort to thepatient.Despite numerous efforts, no really satisfactory way hasbeen found to reduce friction. Using highly polished archwires is not sufficiently effective, and the alternatives involvethe use of expensive coatings such as gold or even titaniumdeposited by costly radiofrequency magnetron sputtering.In order to use IF-nanoparticles to reduce friction duringorthodontic treatment it is necessary to coat the orthodonticwires or brackets with the nanoparticles. One method is bycomposite electroless deposition. Of the variety of metalsthat are in use, the most common nickel-phosphorous (Ni-P)films have proven their supremacy in corrosion and wearresistance. An example of such a coating with IFnanoparticles is shown in Figure 3.

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The presence of nickel in orthodontic appliances is knownand they are approved for use (except in people with anallergic sensitivity to nickel). Today various metals and metalalloys are used as orthodontic wires including Ni-Ti alloys,Ni-Co alloy, stainless steel 304 and titanium.

Wear resistance of ceramics can be improvedCeramic materials are increasingly considered for use invarious extreme tribological applications because of theiroutstanding mechanical and physical properties, such asstrength, hardness and chemical inertness.However, the low toughness of this class of materials hasgenerated interest in understanding the relationshipbetween wear and fracture responses and materialmicrostructure. The role of microstructural features iscontradictory. The tensile strength and hardness of aluminaoften decrease with increasing grain size, while fracturetoughness increases with increasing grain size becausevarious toughening mechanisms such as grain bridging andcrack deflection are activated.Therefore, to gain an understanding of the wear-fractureproperties interaction, it is necessary to investigate the roleof fracture processes at the microstructural scale duringcontrolled wear tests. Wear and wear transitions are themajor concerns as regards the tribological application ofceramic materials. Cracking and material loss are the maindamage mechanisms in wear or rubbing of ceramicmaterials.Dry powders of IF nanoparticles are excellent as dry filmlubricants and have been used successfully to reduce thewear of ceramic/ceramic coupled surfaces under severecontact conditions. The nanoparticles appear to reduce thelocal concentration of the shearing force at the point ofcontact and can maintain the coefficient of friction at adesired value for optimal working conditions, as can be seenfrom the examples in Figure 4.

Conventional layered solid lubricants have clearlimitationsSolid lubricants fill a special niche in reducing wear insituations where the use of liquid lubricants is eitherimpractical or inadequate, such as in vacuum, spacetechnology, miniature devices, high loads, maintenance-freeoperations, and clean room type environments [3].Metal dichalcogenides having the general formula MX2(where M is, for instance, Mo or W and X is S or Se) arewidely used as solid lubricants. In their comon, so-called2H-phase they possess a layered structure with weak (vander Waals) inter-layer forces that allow easy, low-strengthshearing for lubrication action. Another type of commonsolid lubricant is graphite, which is also a layeredcompound.Layered compounds are not without their drawbacks,however. The edges of the layers are chemically reactive,causing them to slowly decompose, break apart, and bind tothe metal surface. The relatively large size of the layeredplatelets prevents them from entering the pores of metal andceramic parts and thus they tend to accumulate on thesurface and stick to the components causing themovements to 'stick-slip'.These factors ultimately diminish their lubricating abilitycausing components to grind against each other and weardown. Thus, there is a need for smaller, more stable solidlubricants.

IF particles combine low friction with extended lifetimesWithin the past few years, inorganic fullerene-like (IF)nanoparticles of MX2 with structures similar to those ofnested carbon fullerenes and nanotubes have been

synthesised in bulk quantities.These are now available 'off-the-shelf' from NanoMaterialsand in their fullerene-like nanosphere structure act asextremely effective solid lubricants: IF-WS2 outperforms thelayered forms of 2H-MoS2 and 2H-WS2 as well as graphitein every respect (friction, wear and lifetime of the lubricant)under varied test conditions [3] and especially at high loadsand long operating times.This high performance is attributed to their chemicalinertness, nanosize and the hollow cage structure, whichimparts elasticity and allows the particles to roll rather thanto slide.

REFERENCES[1] R. Tenne, L. Margulis, M. Genut, G. Hodes, Polyhedraland cylindrical structures of tungsten disulphide, Nature,360, 1992, 444-5[2] A. Katz, M. Redlich, L. Rapoport, H. D. Wagner, R.Tenne, Self-lubricating coatings containing fullerene-likeWS2 nanoparticles for orthodontic wires and other possiblemedical applications, Tribology Letters, 2006 (to bepublished).[3] L. Rapoport, Yu. Bilik, Y. Feldman, M. Homyonfer, S. R.Cohen, R. Tenne, Hollow nanoparticles of WS2 as potentialsolid-state lubricants, Nature, 387, No. 6635, 1997, 791-3.

Results at a glance- A new class of inorganic nanostructures with a closed cagestructure termed inorganic fullerene (IF)-like nanoparticlesand nanotubes has been discovered.- The geometry of IF nanoparticles is similar to hollow C60carbon fullerenes but forms such as WS2 and MoS2 have aspherical multi-walled, onion-like structure.- IF has been found to be safe and non-toxic in studiesperformed according to international protocols by certifiedlaboratories worldwide.- The size, shape, chemistry and structure of thesenanoparticles makes them particularly useful for reducingfriction under various difficult or extreme conditions. A solidlubricant based on these spherical inorganic nanoparticles isnow commercially available.- Two applications for this material are discussed, ascoatings and thin filmsin orthodontics and in reducingfrictional damage to ceramic surfaces in mutual contact.

The authors:-> Dr. Menachem Genut is founder, president and CEO ofApNano Materials Inc. (USA) /NanoMaterials Ltd/ (Israel).Dr. Genut was a member of the Nanomaterials SynthesisGroup at the Weizmann Institute of Science that discoveredthe inorganic nanoparticles and was the first to synthesisethem.-> Dr Niles Fleischer is Vice President of businessdevelopment and product development, ApNano MaterialsInc. (USA)/NanoMaterials Ltd. (Israel). He earned his Ph.D.and M.Sc. degrees from the Weizmann Institute of Science,Israel, and his B.A. from The University of Michigan, USA.-> Dr. Alla Zak is Chief Scientist of NanoMaterials Ltd. Shecompleted her doctorate at the Nanomaterials SynthesisGroup at the Weizmann Institute of Science. She has writtena number of scientific papers and has several patentedinventions on inorganic fullerenes.-> Professor Lev Rapoport is Head of the TribologyLaboratory and the Center for Materials Science andEngineering at the Holon Institute of Technology, Israel.-> Professor Reshef Tenne leads the group that discoveredand studied the IF nanospheres and nanotubes at theWeizmann Institute of Science, Israel. There he holds theDrake Family Chair in Nanotechnology, is head of the


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Department of Materials and Interfaces and Director of theHelen and Martin Kimmel Center for Nanoscale Science.This paper was presented at the European CoatingsCopnference "Smart Coatings V", Berlin/Germany, 15/16May 2006


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Figure 1: Highly magnified view of sections through an IF multilayered nanosphere(left) and nanotube (right).


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Figure 2: Friction reduction achieved with IF coating, using a ball-on-flat device (slidingvelocity 0.2mm/s, load 50 g or ca. 1.5 GPa).


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Figure 3: Electron micrograph image of a Ni-P IF coating on steel substrate .


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Figure 4: Wear effects of direct ceramic/ceramic contact: Photographs on left showwear tracks as vertical marks in the upper part of the picture on an alumina flat with anaccumulation of debris at the end of the stroke (top) and on an alumina flat burnished

with a coating of tungsten disulfide IF nanospheres (bottom) where the track isnarrower and without accumulated debris. Photographs on right show the surface ofthe silicon nitride hemispheres after friction tests. The tip which has contacted the IFcoated surface (below) is much less worn. (Test conditions: silicon nitride rod with 2

mm hemispherical tip, maximum contact pressure ca 2 GPa, reciprocal sliding velocity0.2 mm/s).


fullerenes fight friction - european coatings · orthodontics, hebrew university - faculty of...

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