Beyond - C 3 N 4 —Fullerene-like carbon nitride: A promising coating materialJörg Neidhardt and Lars Hultman

Citation: Journal of Vacuum Science & Technology A 25, 633 (2007); doi: 10.1116/1.2738505 View online: http://dx.doi.org/10.1116/1.2738505 View Table of Contents: http://scitation.aip.org/content/avs/journal/jvsta/25/4?ver=pdfcov Published by the AVS: Science & Technology of Materials, Interfaces, and Processing Articles you may be interested in Stabilization of tetragonal and cubic phases of ZrO 2 in pulsed sputter deposited ZrO 2 / Al 2 O 3 and ZrO 2 / Y 2O 3 nanolayered thin films J. Appl. Phys. 104, 113532 (2008); 10.1063/1.3040720 Nanoindentation-induced plastic deformation and fracture behavior difference between a - and c -domains ofBaTiO 3 single crystal J. Appl. Phys. 104, 103506 (2008); 10.1063/1.3021097 Nanoscale deformation in Ti C a - C multilayered nanocomposite coatings Appl. Phys. Lett. 92, 241913 (2008); 10.1063/1.2945891 Nanoindentation of the a and c domains in a tetragonal Ba Ti O 3 single crystal J. Appl. Phys. 102, 083507 (2007); 10.1063/1.2795664 Mechanical response of KD 2x H 2(1x) PO 4 crystals during nanoindentation Appl. Phys. Lett. 84, 2274 (2004); 10.1063/1.1690867

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REVIEW ARTICLE

Beyond �-C3N4—Fullerene-like carbon nitride: A promising coatingmaterial

Jörg Neidhardta�

Christian Doppler Laboratory for Advanced Hard Coatings, Department of Physical Metallurgyand Materials Testing, University of Leoben, 8700 Leoben, Austriaand Thin Film Physics Division, Department of Physics (IFM), Linköping University,58183 Linköping, Sweden

Lars HultmanThin Film Physics Division, Department of Physics (IFM), Linköping University,58183 Linköping, Sweden

�Received 17 April 2007; accepted 18 April 2007; published 11 May 2007�

Even though the synthesis of super-hard-crystalline �-C3N4 remains elusive, noncrystalline CNx

compounds are of increasing importance owing to their competitive properties. Especially thefullerene-like allotrope of CNx exhibits outstanding elasticity in combination with low work ofindentation. This new class of thin solid film materials is characterized by a microstructure of bentand intersecting basal planes. Substitutional incorporation of nitrogen into the predominantly sp2

hybridized graphitic layer triggers the formation of curvature-inducing pentagons and interplanarcross-links at a much lower energy cost as compared to carbon-only materials. The term“fullerene-like” was coined to reflect the nanometer scale of curved structural units. Thus,fullerene-like CNx deforms by bond angle deflection and compression of the graphitic interplanarlattice spacing, whereas the superior strength of the sp2 bonds inhibits plastic deformation giving thematerial an extremely resilient character. The orientation, radius of curvature of basal planes, anddensity of cross-linking can be adjusted by the synthesis conditions. Here, the existence ofsignificant numbers of precursor molecules is a determining factor. The inherent resiliency of thematerial in combination with the carbon-based beneficial friction promises to give rise to numeroustribological applications. © 2007 American Vacuum Society. �DOI: 10.1116/1.2738505�

I. HISTORICAL PERSPECTIVE

The appearance of reports on carbon nitrides can be back-dated to the beginning of the 19th century. In 1816 Guy-Lussac observed that cyanogen C2N2 �NwC–CwN�,which is a gas at room temperature, can polymerize to formsolid paracyanogen–�CN�n–.1 The first reference in the litera-ture to the compound C3N4 was given by Franklin in 1921.2

The author suggested that graphitic C3N4 could be the finalproduct after a series of deammonation of ammono carbonicacids. A voluminous solid residue close in composition toC3N4 was reported, though containing from 0.6% to 2.4%hydrogen. The author called this product “carbonic nitride”but did not study its structure and properties. Later, Paulingand Sturdivant did the first quantum mechanical calculationsand suggested the possible structure of new C–N and C–N–Hcompounds.3 On the basis that the graphitic C3N4 proposedby Franklin might be converted to a hard form, analogous tothe graphite-diamond relationship, a substantial effort in the1970s was devoted without success at GE Research Labora-tory to the synthesis of possible new C–N phases.4

a�

Electronic mail: joerg.neidhardt@unileoben.ac.at

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The efforts for an ultrahard material made from carbonand nitrogen were revived by Cohen following his work re-lating the bulk modulus of diamond and zinc blende. In 1985Cohen published an empirical formula relating the bulkmodulus of tetrahedrally coordinated materials to the lengthand ionicity of their chemical bonds.5 From this relationship,he predicted that a material made of carbon and nitrogenshould exhibit a bulk modulus higher than diamond due tothe short length and high covalency of the C–N bond. Sub-sequently, Liu and Cohen predicted that �-C3N4, a materialisostructural to �-Si3N4, would possess a bulk modulus ap-proaching, but not surpassing, that of diamond. The newstructure contains 14 atoms �6 carbon and 8 nitrogen� in ahexagonal unit cell, where CN4 tetrahedra are linked at thecorners. By theoretically fitting the total energy as a functionof the lattice parameter they found a bulk modulus of427 GPa with a corresponding cohesive energy of5.8 eV/at., suggesting that the � structure is at leastmetastable.6,7 Yet more crystalline CxNy structures weretheoretically explored, including zinc-blende and rhombohe-dral forms as well as different stoichiometries.8–12 These the-oretical works, especially the first two papers by Liu and

6,7

Cohen, triggered an immense interest in the CxNy system

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as the possible source of a material “harder” than diamond.This excitement within the scientific community was alsoreflected by articles in the general press and popular sciencereports.13–17

The first attempt to produce carbon nitride in the form ofa thin film was reported by Cuomo et al. using a rf diodesputtering system in 1979,18 which they identified to be aparacyanogen-like material. After the publications by Liuand Cohen in 1989 and 1990 numerous deposition methodsand techniques have been explored in the quest for the super-hard-crystalline phases of C3N4 as described in more detailin the review by Wang,19 while Muhl and Mendez1 as well asBadzian et al.,20 might give a more balanced view. Despitenumerous claims of the successful synthesis of �-C3N4, noconvincing evidence has been presented yet for its synthesis.Ambiguities arise primarily from incomplete diffractionpatterns21 as well as nitrogen deficient stoichiometries as al-ready mentioned relatively early by DeVries,22 while Hugh-banks and Tian23 as well as Malkow24 and Teter25 questionedthe validity of some aspects of the original predictions byLiu and Cohen.

As a matter of fact, mostly amorphous allotropes are ob-served for the wide variety of synthesis methods used. Equi-librium growth conditions �e.g., pyrolysis� result in a varietyof polymeric structures, which lack the high degree of cross-linking required for hard materials. Thus, energetic nonequi-librium growth methods, such as physical vapor deposition�PVD� �e.g., ion beam assisted deposition, arc evaporation,magnetron sputtering, and laser ablation� as well as plasmaassisted chemical vapor deposition, were favored to incorpo-rate the required 57 at. % nitrogen in the film and to providethe energetics for a dense and cross-linked structure. How-ever, most films rarely surpass a nitrogen concentration of�30 at. % due to the inherent drive of nitrogen to desorb asN2. Despite the discrepancy in stoichiometry, the amorphousallotropes of CNx exhibit a wide field of complex structuresand useful properties, which analogous to amorphouscarbon,29 vary widely with growth conditions �substrate tem-perature, ion energies, etc.�. However, the addition of nitro-gen to a carbon structure adds significant complexity, due tothe variety of potential bonding configuration between thehybridized constituents �sp, sp2, sp3�.

An attempt to categorize the synthesis/structure relationsin the form of phase diagrams was published by Kleinsorgeet al.27 as well as Hellgren et al.28 and later on refined formagnetron sputtering by Neidhardt et al.29 The commontenor is that the incorporation of a significant fraction ofnitrogen generally results in a more sp2-coordinated network,as observed for the same growth condition without nitrogen.The same effect is obtained by a higher growth temperature.Contrary to most of the pure carbon allotropes, however,deposition of CNx at elevated temperatures ��250 °C�yields denser films. A moderate ion energy has in generalalso the effect of a denser film structure postulating a higherdegree of cross-linking by, e.g., tetrahedrally coordinatedcarbon, while any nitrogen concentration surpassing

�30 at. % seems to be due to interstitial N2 dimers. On the

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other hand, the use of lower ion energies during reactivemagnetron sputtering of carbon in a nitrogen-containing at-mosphere enabled the evolution of structured films, whichlead to the discovery of the fullerene-like allotrope of CNx

by Sjöström et al. in 1995.30

CNx films exhibit a variety of beneficial properties froman electronic31–36 as well as mechanical viewpoint.37,38 As tothe latter, CNx films are very damage, tolerant, and elastic,while retaining the wear resistance and beneficial tribologicalproperties of pure carbon films and as such have replacedthem as overcoats on read and write as well as hard drives inmagnetic recording.39 Especially the nanostructuredfullerene-like allotrope consisting of bent and intersectinggraphene planes amplifies the beneficial properties. It exhib-its an extreme elasticity combined with a high resistanceagainst plastic deformation making it comparable to a “su-perhard rubber.”40 As such it has sparked the interest of thescientific community over the last years, while the generalinterest in crystalline CNx has faded, especially after the pub-lication of the critical reviews in the late 1990s.1,20–25 Thisarticle is dedicated to the synthesis-structure-performance re-lations of the remarkable fullerene-like �FL� CNx thin filmcompounds.

II. FULLERENE-LIKE MATERIALS ANDMICROSTRUCTURE

Fullerene-like materials are characterized as compoundmaterials with, by the effect of the structural incorporation ofodd-membered rings, heavily bent and intersecting hexago-nal basal planes. Besides FL-CNx these structures are ob-served in a number of materials, e.g., C,41 BN:C,42 BN,43

MoS2,44 TiO,45 NbS2,46 and WS2,47 for a recent review oninorganic fullerene-like nanoparticles, see Ref. 48. Out ofthese material systems only C, CNx, and BC:N exhibit cross-linking. These qualities enable the material to extend thestrength of the covalently bonded two-dimensional hexago-nal network into three dimensions.

FL-CNx consists of substitutionally bonded N in bent, in-tersecting, and cross-linked graphite sheets, as seen in thehigh-resolution transmission electron microscopy plan-viewimage in Fig. 1. This implies that the majority of carbon isincorporated in a three-coordinated planar sp2-hybridizedstate. The double bond between sp2-hybridized carbon as ingraphite is with a bond energy of 6.3 eV compared to3.6 eV, actually stronger than the single bond sp3-hybridebond of diamond.50 The overall strength of graphite is, how-ever, limited by its two-dimensional structure of hexagonalplanes and, thus, the rather weak interplanar van der Waals-type bonding. Hence, graphite deforms easily by gliding ofbasal planes. If on the other hand nitrogen is incorporatedduring growth the energy required to form pentagons is con-siderably lowered and thus the graphite sheet curve andbuckle �inset of Fig. 1�.51 This enables the material to extendthe extraordinary strength of a planar sp2-coordinated carbonnetwork into three dimensions.

The cross-linkage between the sheets, which is assumed

to be induced by the structural nitrogen incorporation, con-

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tributes considerably to the strength of fullerene-like CNx bypreventing interplanar slip �see Sec. II B for a more detaileddescription�. This results in an extremely fracture tough,elastic, and compliant material, which deforms by reversiblebond rotation and bond angle deflection rather than slip andbond breaking.40 Diamondlike carbon �DLC� films, for in-stance, also exhibit properties beneficial for many applica-tions, such as high hardness due to a high fraction of sp3

carbon bonding and favorable tribological behavior. How-ever, contrary to FL-CNx, the application of DLC films suf-fers mostly from the inherently higher brittleness of thematerial.

Depending on the growth conditions, FL-CNx thin filmsexhibit a large variation in basal plane curvature and, hence,extension and alignment of sheets. Thus, due to the variationin microstructure it denotes a material with a large spread inproperties, e.g., elastic modulus, elasticity, and electricalresistance,52–54 all having in common an extraordinary me-chanical strength. This provides for tailoring of the proper-ties to given applications as, e.g., wear protective coatings.

A. Bonding configuration

Carbon is an excellent constituent in the synthesis of hardcompounds, since it fulfills both requirements of a smallatomic size and the ability to form strong bonds. The fourvalence electrons of carbon ��He�2s22p2� are apt to formthree hybridization states enabling three major bond configu-rations; the sp3 tetrahedral bonds like in diamond, and thesp2 planar graphitelike bonds and the sp bonds like inalkynes. For sp3-hybridized carbon, all four electrons takepart in forming � bonds pointing towards the corners of atetrahedron yielding a strong three-dimensional network. In asp2-bonded network, only three electrons form three planarbonds, while the fourth forms a �-orbital perpendicular tothe plane of the three � bonds. The electron in the � orbital

FIG. 1. Plan-view high-resolution transmission electron micrograph ofcurved and intersecting fullerene-like sheets in a CNx film grown at 450 °Cwith a N2 fraction of 0.5 and a bias voltage of −25 V. The inset illustratesthe buckling of the graphene sheets by pentagon incorporation obtainedfrom ab initio calculations �Ref. 49�.

can be delocalized to strengthen the bonds by forming a

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resonance structure of single and double bonds in an appro-priate matrix. sp-hybridized carbon, in turn, forms two lin-ear, but very strong bonds due to overlap of two orthogonal� orbitals. The strength of carbon compounds is easily envi-sioned by the example of diamond, still the hardest materialknown, due to the strong carbon-carbon bond, the high three-dimensional coordination, and the small atomic size. How-ever, the overlap of the � orbitals for the sp and sp2 carbon-carbon bonds, commonly referred to as double and triplebonds, results in bond energies of 6.3 and 8.7 eV, which aremuch higher bond energy compared to 3.6 eV for the singlebond as in the sp3 hybrid. Similar to graphite and polymersthis bonding situation renders an extraordinary in-plane/chain strength to FL-CNx.

Nitrogen, like carbon, forms predominantly covalentbonds. Its valence electrons ��He�2s22p3� exhibit, despite theadditional electron, bond hybridizations similar to carbon.However, the extra valence electron implies certain differ-ences. In case of sp3-hybridized nitrogen, four hybrid orbit-als are formed just like in carbon, while the extra electronforms a nonbonding lone pair with an electron in a hybridorbital leaving only three �-bonding orbitals. The bond angleis slightly deflected to 109° instead of 107° compared todiamond due to the effect of the more spacious lone pair.There are two possibilities for the sp2-hybridized bond con-figuration. First, the extra electron is able to form a nonbond-ing lone pair with a sp-hybrid orbital leaving the atom withtwo nonlinear � bonds and the possibility to participate in�-resonance structures with the electron in the remaining 2porbital. For the second case, the extra electron forms the lonepair with the remaining 2p orbital leaving the atom withthree planar � bonds and no possibility to contribute to a�-resonance structure, due to the filled, nonbonding 2p or-bital. sp hybridization of nitrogen is also common, where theatom can form one � and two � bonds, while the fifth va-lence electron forms a lone pair with the second sp-hybridorbital.

The effect of impurities such as hydrogen might also be ofimportance, as it is known to improve the properties of, e.g.,DLC films.26 However, the very low concentrations of impu-rities such as hydrogen and oxygen in the magnetron sput-tered CNx films should not have any significant effect uponthe film structure. On the other hand, Hellgren et al. studiedthe effect of intentional hydrogen addition to the dischargeduring CNx deposition.55 It was shown that hydrogen termi-nates potential bonding sites for CN precursors and, thus, theformation of a fullerene-like structure is hindered. Addition-ally, hydrogen is enhancing the etching process by formationof volatile C2N2 �see next section�, NH3, and CxHy

molecules.Due to the inherently disordered structure of CNx com-

pounds, it has been difficult to determine the exact bondingstructure of carbon and nitrogen in the material. Thus,complementary spectroscopic techniques have to be used todetermine the local bonding environment of FL-CNx. It hasbeen shown that the bonding structure is most conveniently

56,57

investigated by x-ray photoelectron spectroscopy �XPS�.

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The N1s core level peak has typically two strong compo-nents �Fig. 2� P1 at �400.7 eV and P2 at 398–399 eV, andat least two smaller ones P3 positioned at �402 eV and P4at �399.5 eV. The assignment of these peaks has beenwidely divergent in literature �see Ref. 58 for a comprehen-sive review�. However, strong support can be found for thefollowing peak interpretation by comparing the XPS resultsof a wide range of films with other spectroscopic techniques,such as x-ray absorption near-edge structure/x-ray emissionspectroscopy,59,60 nuclear magnetic resonance �NMR�spectroscopy,61 electron energy loss spectroscopy,62 Ramanspectroscopy,63 and theoretical calculations.60

1. P1 �400.7 eV� corresponds to threefold coordinatedsp2-hybridized nitrogen substitutionally bonded to threecarbon atoms in a predominantly sp2-coordinated graphitenetwork �see inset in Fig. 2�, while a sp3-hybridized car-bon as neighbor cannot be ruled out.60 P1 is also affectedby exposure to atmospheric moisture.

2. P2 �398.0–399.0 eV� was initially assigned to nitrogenbonded to an at least partially sp3-hybridized carbon ma-trix, but has rather to be seen as nitrogen in a pyridinelikestructure �see inset in Fig. 2�, where a twofold coordi-nated sp2-hybridized nitrogen forms two bonds to an oth-erwise graphitic structure. This bonding configurationmay occur along edges of planes or next to defects and isessential for eventual cross-linking.

3. P3 �402.5 eV� gives a minor contribution and is assignedto N–O bonds formed after air exposure.60

FIG. 2. XPS N1s photoelectron spectra from CNx films synthesized at vari-ous N2 fractions, a bias voltage of −25 V, and substrate temperatures from600 down to −100 °C. P1– P4 indicate different spectral contributions. Theinsets illustrate the associated major bonding configurations �figure takenfrom Ref. 57�.

4. P4 �399.5 eV� has also a minor contribution to the XPS

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spectra. It most likely corresponds to sp-hybridized nitro-gen in a nitrile structure. Raman spectroscopy is a moresuited technique for the detection of residual nitriles63,64

and has indicated that a minor fraction is present inFL-CNx films.

The ratio of the two major contributions P1 and P2 canbe used to assess the structure of CNx films, where a higherfraction indicates the formation of extended fullerene-likedomains.57 However, it has to be mentioned at this point thatexposure of the as deposited films to ambient moisture leadsto a general decrease in intensity, while P1 is most affected.The intensity of P4, on the other hand, strongly increases,indicating a contribution of N–H bonds in this region.

B. Role of nitrogen

1. Evolution of curvature

The primary role of nitrogen in FL-CNx is that its incor-poration induces curvature to graphene. This is evident fromthe fingerprintlike appearance of basal planes of a fullerene-like CNx film in Fig. 1. The highly corrugated and curvedbasal planes imply a radius of curvature in the nanometerrange. This curvature is induced as soon as nitrogen ispresent during magnetron sputtering, while its degree can becontrolled by the N2 fraction.52 Thus, nitrogen plays an es-sential role for inducing spherical curvature. For pure carbonstructures, curvature is known to be due to the incorporationof odd-membered rings in an otherwise hexagonal structure.As, e.g., 12 pentagons provide for the spherical appearanceof closed caged carbon fullerenes, such as C60. Experimentalresults as well as calculations actually suggest that isostruc-tures to C60 might exist in FL-CNx thin solid films �Fig. 3�.These so called nano-onions form under certain depositionconditions, such as elevated temperatures �Ts=450 °C� andlow N2 fractions �0.1–0.16�. Total energy calculations veri-

FIG. 3. Spatially resolved electron energy loss spectroscopy showing thecharge-coupled device counts and relative nitrogen concentration scannedacross a CNx nano-onion as indicated in the respective high-resolution elec-tron transmission microscopy plan-view image. The inset shows the struc-ture of C48N12 believed to exist in the center of the nano-onion �Ref. 65�.

fied, furthermore, the stabilizing effect of nitrogen for penta-

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gons in aromatic carbon structures,51 whereas the structuresare shown to be less stable if more than one carbon is sub-stituted in a ring and/or nitrogen is incorporated next to eachother, which from geometrical conditions explains the appar-ent saturation in maximum nitrogen concentration in FL-CNx

of approximately 20–25 at. %.66 Thus, experiments and the-oretical calculations suggest that nitrogen triggers the evolu-tion of curvature by stabilizing the required pentagons atmuch less energetic deposition conditions as compared topure carbon structures, while the incorporation of more thanone nitrogen atom in a carbon ring structure seems to beenergetically less favorable.

2. Formation of interplanar cross-linking

The mechanical properties, especially toughness and elas-ticity, of FL-CNx surpass crystalline graphite by far, whichimplies the existence of strong interplanar cross-links or in-terlocking inhibiting interplanar slip of the predominantlysp2-coordinated graphene sheets. Here, nitrogen plays a sec-ond, but crucial role, since pure carbon thin film materialsgrown under similar conditions have a sootlike appearanceleading to deficient cohesion and strength.

The precise nature of the cross-linking mechanisms is,however, still under discussion. A first explanation for cross-linking mechanisms was established by total energy calcula-tions performed in Ref. 52. The presented results suggestthat, upon nitrogen incorporation in a hexagon, it is energeti-cally more favorable for the adjacent carbon atom to changefrom a sp2 to a sp3-hybridized state providing for the re-quired out-of-plane bond. For that reason, nitrogen-dopedhetero fullerenes also known as aza fullerenes, e.g., C59N,instantaneously dimerize upon synthesis via a carbon-carbonbond next to the substituted nitrogen atom.67 Experimentalresults, however, indicate that FL-CNx is a predominantlysp2 bonded and the existence of significant numbers ofsp3-hybridized carbon remains to be affirmed. On the otherhand, only a few interplanar cross-links are needed to in-crease the stiffness significantly and any low fraction ofsp3-hybridized carbon might pass undetected due to experi-mental limitations of, e.g., NMR.68 Cross-linking at the ni-trogen site on the other hand is rather unlikely. Firstly, it wasshown by total energy calculations in Ref. 52 that the struc-ture is more stable if two incorporated nitrogen atoms in agraphite fragment are separated by at least two carbon atoms.Secondly, chemical synthesis of, e.g., aza-fullerene dimers,indicates that the cross-linking is initiated at the carbon nextto an incorporated nitrogen atom and not at the nitrogen siteitself.67

While the high elasticity and toughness of FL-CNx couldalso be explained by the three-dimensional interlocking ofcurved meandering fullerene-like sheets, purely sp2-basedcross-linking mechanisms might also be present, such asbranching of sheets and/or carbon-carbon bond rotation assuggested in Refs. 41, 61, 66, and 69. For the latter, theelectron lone pair in the 2p orbital of substitutional incorpo-rated nitrogen or the formation of pentagons disturbs locally

the �-resonance structure. Therefore, the double bond reso-

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nance structure between two neighboring carbon atoms isdisrupted and, thus, the carbon-carbon bond is free to rotatedue to a missing 2p-2p orbital coupling. This might result inan out-of-plane rotation of the subsequently growing struc-tures and hence enables cross-linking leading to a multitudeof possible configurations. This is a likely outcome if oneconsiders also the disruption of the continuous sheet struc-ture by the incorporation of two-coordinated, pyridinelikebonded nitrogen that is present in significant numbers �seeprevious chapter�. Most recently, computational studies cor-roborated the existence of out-of-plane bond deflection androtation obtained by simulated �synthetic� growth of multi-atomic precursors70 �Fig. 4�.

3. Formation of CxNy „x ,yÏ2… species

For the third and last role of N in the formation of thefullerene-like structure, we note that FL-CNx thin solid filmsare typically grown by plasma-activated deposition tech-niques, where carbon is prone to react with activated nitro-gen originating from the discharge. This reactivity of nitro-gen is considered to be the dominant compound formingreaction on surfaces for most PVD nitride films, due to therather low probability of gas phase reactions under the sub-atmospheric pressures. However, the experimentally estab-lished existence of a low-mass gaseous compound in thecarbon-nitride system, namely, C2N2,7 adds numerous de-grees of freedom to the formation mechanisms of FL-CNx

films as compared to other nitrides.As a consequence the deposition flux from a carbon target

sputtered in a N2-containing atmosphere consists to a largeextent of multiatomic species. They form via reaction be-tween the carbon atoms in the target and the vast number ofnitrogen ions implanted into the top surface layers duringsputtering. In turn C2N2 and fragments thereof are emittedwhereas their significant number scales with the nitrogenfraction in the discharge.71,72

The presence of nitrogen-containing species in the depo-sition flux leads to various effects also on the substrate site.One important process is that they can react upon adsorptionto form volatile gases such as N2 and C2N2, which subse-quently desorb, resulting in a film-surface etching commonlyreferred to as chemical sputtering.73 This process is observed

FIG. 4. Stable cross-linked structures �a� with cross-linking bridge—enlarged in the inset �b�—obtained from synthetic growth simulations ofmultiatomic precursors in Ref. 70.

for various CNx deposition techniques especially those uti-

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lizing hyperthermal species to assist the growth.74–77 It wasfirst established for ion beam assisted deposition, where adeficiency of carbon and nitrogen in the film was detectedcompared to the calibrated fluxes of both elements.78 Theextent of the chemical desorption is determined by the reac-tion probability to form a volatile gas and, thus, depends onthe type and mobility of adsorbed species.79 The overall pro-cess can therefore be seen as a preferential etching of lessfavorably bonded adsorbates and, thus, results in dense andsmooth films.75 Prevailing CN precursors bonded in analigned manner along edges of sheets are more resilient toconcurrent etching and therefore play an important role forthe evolution of fullerene-like planes �see next chapter�. Thechemical desorption process also accounts for the apparentmaximum of incorporated nitrogen at �25 at. %, due to itshigher desorption probability �see, e.g., Ref. 72�.

C. Structure evolution mechanisms

For magnetron sputtering, the presence of nitrogen in thedischarge gas leads to complex chemical reactions at the tar-get and substrate surface. The intense high-energy ion bom-bardment at the target results in sputtered target constituents,where most of the nitrogen ions are implanted during theprocess. The implanted nitrogen radicals react readily withthe carbon atoms in the target. Thus, the composition of thetarget surface is altered substantially, especially if one con-siders the initially rather low sputter yield of pure C. Hence,predominantly CxNy �x ,y�2� molecules are emitted fromsuch a dynamically self-modified target surface, due to therelatively high strength of the CwN bond, alongside withdesorbing gases such as N2 and C2N2. The extent of theseprocesses scales with the availability of N2 in the dischargeand therefore leads to a strong increase in flux of molecularCN species at higher N2 fractions, while the carbon fluxseems to play only a minor role.71

Although low ion assist energies ��20–50 eV� are gen-erally beneficial for film growth by enhancing adatom mo-bility at relatively low substrate temperatures, it has beenshown that they are not the dominant structure-determiningfactor in the case of FL-CNx films. Instead the flux and typeof preformed CN species carrying predominantly only ther-mal energy are crucial for the evolution of fullerene-likestructures. These molecules become stabilized against de-sorption in an evolving sheet upon aligned incorporation bymultiple bonding to C and N terminating the film surface.Thus, they promote the evolution of planar structures as cor-roborated by step-by-step ab initio approaches in Ref. 80.The same reference also indicates that, upon incorporation ofmolecular CN species, the energy barrier for the formation ofcurvature-inducing pentagonal defects is significantly re-duced, highlighting the importance of preformed CN speciesfor the evolution of planar and curved CNx structures. Fur-thermore, the formation of C2N2 as a volatile gas is alsoactive on the substrate site during selective etching of mis-aligned molecules by reaction of adsorbed CN moleculeswith subsequent desorption �see previous section�. This pro-

cess is a contributing factor for the apparent saturation in

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structural nitrogen uptake in the films to �25 at. %, which isin congruence with the theoretical established most stablestructures.65,70 The incorporated nitrogen seems to originatepredominantly from the already bonded nitrogen in the pre-formed molecules arriving at the substrate, while the exten-sive low energetic nitrogen ion bombardment from the dis-charge plasma readily reacts upon adsorption to form volatileN2. Thus, the evolution of the fullerene-like microstructurehas to be seen as a delicate interplay of adsorbing preformedmolecules, which are either incorporated in a stabilized and,thus, aligned manner or desorb via the formation of C2N2 ifthey are less favorably bonded.79

These requirements of low to moderate energy input dur-ing growth via ion bombardment or substrate heating and, inparticular, the existence of a predominant number of pre-formed molecules in the film forming flux are not only in-herent to reactive magnetron sputtering, but can also befound for pulsed laser deposition.77,81 Both techniques resultin FL-CNx films, where the structured domains are muchlarger for magnetron sputtered films. On the other hand, if nopreformed species are present in the deposition flux no struc-ture evolution is observed as it is the case for low-energy ionbeam assisted deposition.72,82 For magnetron sputtering, thegrowth parameters such as substrate temperature, N2 frac-tion, and bias voltage strongly affect the fullerene-like struc-ture evolution in terms of extension, alignment, and cross-linking. All of the mentioned parameters primarily affect themagnitude of the chemical desorption and, thus, the selectiv-ity incorporation sites either by influencing the adatom �mol-ecule� mobility �bias and temperature� or by changing thenumber and type of film forming species �N2 fraction�.

The general trends in terms of deposition parameters andresulting structures for the deposition setup described in de-tail in Ref. 52 are summarized in Fig. 5. According to high-resolution transmission electron microscopy �HRTEM� andXPS,57 films grown at higher temperatures �TS�300 °C�and intermediate bias �Vs�−25 V� are all fullerene-like,since both parameters provide sufficient selectivity to incor-porate most of the arriving species in an aligned fashion,regardless of the N2 fraction. The availability of N2, how-ever, determines the extension and alignment of the sheets,since the dominance of preformed species at higher N2 frac-tions promotes the growth of extended, well-aligned sheetsresulting in an almost graphitelike appearance. At lower N2

fractions the flux of carbon atoms becomes instead relevantand the complexity and total number of the preformed spe-cies are decreasing, which in turn leads to shorter heavilybent fullerene-like fragments. This is an effect of a lowerselectivity of incorporation sites via more complex reactionpathways for the formation of C2N2.

If the temperature is decreased �T�300 °C� the adatom�molecule� mobility is reduced and an increasing number ofpreformed molecules are incorporated in an out-of-plane ori-entation due to a decreased efficiency of the chemical de-sorption process. This results in the amorphization of theotherwise graphitic nature of the films grown at higher N2

fractions. The smaller number of species arriving at lower N2

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fractions is, however, still effectively incorporated in analigned fashion into short fragments down to a depositiontemperature of 150 °C �Fig. 5�.

A higher bias or ion energy, exceeding the displacementenergy of carbon �E�30 eV�, on the other hand, induces anincreasing number of defects into the growing sheets and,thus, decreases the fullerene-like fragment size. This is mostpronounced at higher N2 fractions, which is likely due to ahigh sensitivity of the aligned incorporation process towards

FIG. 5. Structure zone diagrams of structures and properties observed inmagnetron sputtered fullerene-like CNx thin films depending on the sub-strate temperature, the degree of ion bombardment, comprising the ion toneutral ratio and ion energy, and the N2 fraction in the discharge. The HR-TEM plan-view inserts should be seen as representatives only �taken fromRef. 29�.

energy input �Fig. 5�.

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III. MECHANICAL AND TRIBOLOGICALPROPERTIES

FL-CNx denotes a new class of superelastic materials, dueto its unique microstructure of bent and cross-linked basalplanes, which extends the extraordinary strength of thesp2-hybridized planar network into three dimensions. Themechanical strength of graphite, for instance, is limited by itseasily activated slip system along the graphite basal planes.On the other hand, this weakening by interplanar slip alsoholds the key for understanding the very low friction ingraphite. Carbon fullerene cages, instead, are known to beextremely resilient and to withstand severe deformationwithout any plastic yield via broken carbon-carbon bonds.83

However, the mechanical properties of crystalline com-pounds formed from fullerenes—so-called fullerites—stillsuffer from the rather weak intermolecular bonding.84,85 Theother extreme of hard carbon-only materials is representedby diamond, where the highest degree of three-dimensionalcross-linking is provided by 100% sp3 hybridization. This ismimicked by diamondlike carbon thin films synthesized byvarious techniques resulting in materials with an outstandinghardness, while still retaining favorable low frictionproperties.26 However, these superhard materials only have alimited tolerance to deformation and overloading results inimminent brittle fracture and, thus, mechanical failure of thesubstrate-coating system, whereas FL-CNx is extremelydamage tolerant �see next section and Ref. 40�.

A. Indentation hardness and modulus

The mechanical properties, such as hardness and modu-lus, of macroscopic materials are easily determined by, e.g.,tensile testing and purely plastic indentation methods.86

These properties are also of vital importance for thin filmmaterials. Their evaluation is, however, not straightforwardmainly due to limitations in lateral dimensions and the rathersmall volume of the analyzed thin film material. The problemis partially overcome by depth sensing indentation tech-niques, where load and displacement of a sharp indenter pen-etrating a thin film on a substrate can be recorded on micron-ewton and nanometer scale.87 In the past, however, theextreme elasticity of FL-CNx made the straightforward ap-plication of this technique in terms of quantification ratherambiguous.88 In order to overcome these problems, theelastic-plastic behavior of the material was investigated indetail in Ref. 40. The so gained understanding allowed forthe numerical evaluation the mechanical properties ofFL-CNx by novel adaptation of existing techniques.52 How-ever, these methods could not be applied to the extremelythin films and, thus, purely elastic indents accompanied byHertzian modeling had to be employed.54

Figure 6 shows two typical examples of load-displacement curves for CNx films grown at a N2 fraction of0.5, but for two different bias voltages. CN0.18 grown at−40 V bias has a higher degree of cross-linkage and there-fore exhibits a higher resistance to penetration �smaller maxi-mum displacement� while the more graphiticlike appearance

of CN0.13 with its extended, well-aligned sheets has a fairly

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low resistance to penetration, due to the weak interplanarinteractions via cross-links �see Sec. II C�. These differencesalso manifest itself in lower modulus and hardness numbers.Remarkably, both films, despite their different compliance,show no or only very little tendency to plastic deformationand most of the indentation energy is stored elastically andreleased upon unload. This high elasticity is readily appreci-ated by comparing the residual displacement of the load-displacement curves of FL-CNx to depicted reference mate-rials such as TiN and fused silica �a-SiO2�.89

Similar tendencies in the dependence of H and E as forthe ion energy are observed during substrate temperaturevariation �Table I�. Both growth parameters basically influ-ence the number of defects and, thus, the number of potentialcross-linking sites. A lower substrate temperature increasesthe H and E significantly, presumably by increasing the num-ber of cross-linking sites via incorporation of out-of-planealigned preformed species �see Sec. II C�. However, this in-crease in cross-linking sites goes hand in hand with a de-crease in extension of fullerence-like planes. This impliesrestrictions to elastic buckling and compression of interpla-

FIG. 6. Load-displacement curves from two CNx films grown at −25 V bias�CN0.13�, −40 V bias �CN0.18�, and at a N2 fraction of 0.5 in comparison toTiN and fused silica �a-SiO2� as reference materials �taken from Ref. 90�.

TABLE I. Hardness and modulus values determined bcube corner geometry for various growth conditions.

Ts �°C�N2

fraction

Hardness �G

−10 V −25 V

0 7150 0.16 15

1 10

0 0.3450 0.16 10

1 5 6

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nar spacing in the material. Thus, the more resistant filmsgrown at higher bias and lower temperatures commonly ex-hibit a reduced relative elastic recovery.52

Furthermore, the microstructure and, thus, the propertiesof FL-CNx are also influenced by the N2 fraction, see Fig. 5and Ref. 52. A pronounced fullerene-like structure as synthe-sized at lower N2 fraction �0.16� and a moderate bias�−25 V� consists of heavily bent sheets �Fig. 1�, which be-comes restricted in extension by curling as compared to moreelongated sheet structures grown at higher N2 fractions. Thisimplies a higher cross-linking density and thus a higher re-sistance to indentation, but lower relative elastic recovery.

The detailed analysis of the mechanical response ofFL-CNx characterizes a new class of extremely resilient ma-terials, where deformation energy is predominantly storedelastically. Figure 7 shows a typical TEM micrograph from across section of a film containing an indent made to a dis-placement of �80% of the film thickness. Remarkably, noplastic deformation can be detected in the film material eventhough the amorphization of the Si substrate indicates hydro-static pressures exceeding 13 GPa. This elastic recovery canbe understood by the cross-linked planar structure of

stic-plastic indentations with a diamond indenter ofrene-like films are in boldface.

Modulus �GPa�

−40 V −10 V −25 V −40 V

11516691

1118 75 12012 43 50 135

FIG. 7. Cross-sectional TEM micrograph from a fully recovered CNx filmindented by a diamond indenter of a Berkovich geometry under a maximumload of 40 mN. The dimensions include the tip roundness ��200 nm� andtotal displacement is indicated alongside with the maximum displacement�dotted line�. The right picture shows the hydrostatic pressure distribution�GPa� over the film and the Si substrate assuming an ideal elastic contactunder a load of 20 mN.

y elafulle

Pa�

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FL-CNx, where the sheetlike domains wrap and buckle underload, while the presumed strong C–C cross-links preventgliding. Thus, plastic deformation by mechanisms similar tographite is inhibited. Additionally, the interplanar strength ofa predominantly sp2-hybridized material is considerable,which in turn makes plastic yielding by cracking or bondbreaking less likely. On the other hand, the rather weak in-traplanar van der Waals forces and the large lattice spacingprovides for a high compressibility, which explains the ob-served low to moderate resistance to penetration. Therefore,FL-CNx deforms by bond angle deflection rather than break-ing providing for the compression of the intraplanar latticespacing, much like in a “superhard rubber.” A similar behav-ior is also inherent to C60 fullerenes, which are known towithstand extreme deformation up to 80% of their originalsize without yielding.48,83 However, the strength in fullerenesolids �fullerites� is limited due to the weak cross-linkingbetween the molecules. This is not the case for FL-CNx

where the curved sheets make the material deform elasticallyon the macroscopic scale.

B. Friction and wear

Because of their remarkable mechanical properties carbonnitrides have been discussed as a potential material for manytribological applications since the beginning of the1990s.90–92 For that reasons CNx films of amorphous orfullerene-like structure are already industrially applied asprotective overcoats, e.g., on hard disks90,93 and many pat-ents of products using CNx films have been filed.94

There are several studies on the wear rate and frictioncoefficient of CNx films, as reviewed by Broitman et al.95

Although many researchers have accounted for improvedproperties in terms of wear resistance in comparison to DLCfilms, there is an apparent contradiction in the results pub-lished by different authors regarding an increase, no change,or decrease of the friction coefficient with the incorporationof nitrogen in the films. Furthermore, a large spread of fric-tion values for CNx films has been reported, from 0.08 up to0.45 or more �see Ref. 95 and references therein�. This con-

TABLE II. Moduli and hardness numbers of some re25 eV �CN0.13� and 40 eV �CN0.18� alongside with telastic behavior.

Hard and brittle

Diamonda Al2O3b

TiNthin filmb

H�GPa�

80 23 25

E�GPa�

1050 320 350

H /E 0.076 0.072 0.071

aTaken from Ref. 96.bOriginal data.cHigh speed steel.

fusion in findings seems to arise from the difficulty to corre-

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late the tribological behavior of CNx films with their respec-tive micro structure �in many cases the structural andbonding characterization of the materials is lacking� and thelack of standardization in tribological testing.

The outstanding mechanical properties, in particular, thehigh compliance, make FL-CNx to be a promising candidatefor tribological applications.95 Its high hardness-to-modulusratio is descriptive for superelasticity �Table II� and is assuch the delimiting factor in the so-called plasticity indexdescribing the compliance of a material.97 The extremelyhigh H /E ratio suggests that asperities in a tribological con-tact are likely to behave elastic.40 This is of crucial impor-tance, since microscopic asperities are present for all techno-logical surfaces and as such bear the load in a tribologicalcontact. Their total area, however, is only a fraction of thenominal contact area, which results locally in extreme pres-sures. Under these conditions, commonly used hard thin filmmaterials are likely to behave in a brittle manner, whichmight result in abrasive wear debris. FL-CNx on the otherhand is highly compliant �see previous section� and, thus, thecontact pressure is dissipated. Furthermore, the rather lowmodulus �E�200 GPa� of FL-CNx suggests that the macro-scopic contact stresses are dissipated over larger volumescompared to hard coatings with higher moduli �250 GPa�E�450 GPa�. This in turn reduces stress discontinuitiesalong the film-substrate interface and, thus, prevents sub-strate deformation. Additionally, the shear stresses along theinterface plane for a strained substrate-coating system arealso reduced, which might be beneficial for the adhesion ofthe film under load.98

The tribological performance of FL-CNx was tested inRefs. 29 and 94 by reciprocal and ball-on-disk sliding con-ditions, respectively, and benchmarked to the behavior ofamorphous CNx as well as graphitic and diamondlike carbonfilms. Broitman et al. also investigated the tribology offullerene-like CNx on orthopedic relevant substrates in vari-ous aqueous solutions in Ref. 99.

The coefficient of friction for FL-CNx for the above-described setups and under an average contact pressure of

ce materials compared to two CNx films grown at/E ratio. A ratio exceeding 0.15 indicates a super-

Plastic Elastic Superelastic

HSSa,cDLC

thin filmb CN0.18b CN0.13

b

9 10−20

18 7

250 100−200

120 37

0.036 �0.1 0.15 0.19

ferenheir H

0.7 GPa is with approximately 0.2±0.05 slightly higher than

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the values for pure carbon films �0.15±0.02�, while no dif-ference is observed for amorphous CNx.

29 These values canbe considered low in comparison to the friction coefficientof, e.g., Si �0.6� and TiN �0.7� under comparable conditions.A low coefficient of friction is desired for a tribological con-tact not only because it increases the system efficiency, butmost importantly reduces the thermal energy dissipated inthe asperities, which prevents them from, e.g., oxidation.100

Furthermore, a dependence on the humidity of the surround-ing atmosphere is observed, while no correlation in betweenthe friction coefficient and hardness �resistance to penetra-tion� can be detected.29 This alongside with the commonlyunsteady friction traces indicates that of third bodies �tribolayers� form in the tribological contact.101

The wear rate of FL-CNx is significantly lower comparedto amorphous CNx �100�150�25� �%N2�temperature�bias�and carbon films �0�150�25, 0�450�25� �Fig. 8�. Most re-markably the wear rates of FL-CNx in ambient air are almostconstant despite the dramatic difference in hardness rangingfrom 5 to 15 GPa depending on the synthesis conditions.29

For dry conditions, the wear rates are increasing and scalewith the hardness of the films �Fig. 8�. This indicates that forambient sliding either thin transfer layers form or that thereactive surface sites of CNx are passivated by reactions withambient moisture. Both effects result in the separation of thetwo bulk counterparts and, thus, the wear rate is not solelybased on the mechanical properties of the film. During drysliding, however, transfer layers break up and the initiallylow coefficient of friction increases and wear accelerates asseen by nonsteady friction traces in Ref. 29.

IV. CONCLUDING REMARKS

Fullerene-like CNx is shown to be an inherently nano-structured material consisting of bent and cross-linked pre-dominantly sp2-hybridized graphene planes. The incorpora-tion of sp2-hybridized nitrogen, either substitutional forcarbon in a graphene network or in a pyridinelike manner,induces bending by formation of pentagons and initiates

3

FIG. 8. Wear rates of various fullerene-like �FL� and amorphous CNx as wellas carbon films for sliding in dry and ambient air. In the right graph therelation to the hardness is plotted for the three groups of films. The hardnessindependent wear rate for ambient sliding of fullerene-like CNx is indicatedby the horizontal line �guide to the eyes only� �taken from Ref. 102�.

cross-linking, while a cross-linking mechanism on the C sp

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site still cannot be ruled out. Although low ion assist energiesare generally beneficial for film growth by enhancing adatommobility at relatively low substrate temperatures, it has beenshown that it is not the structure-determining factor inFL-CNx films. Instead, the flux of preformed CxNy�x ,y�2�species, originating from an under N-ion bombardment self-modified C target surface is an important factor in the evo-lution of fullerene-like structures. That is because these spe-cies are stabilized by multiple bonding on the C and N sidesupon aligned incorporation during growth and, thus, supportthe evolution of sheet structures. The required curvature isinitiated by a concurrent growth process, where pentagonaldefects in graphene are also stabilized by the incorporationof preformed CN molecules. In addition, the formation ofvolatile C2N2 species is also active on the substrate side andprovides selective etching of misaligned molecules by reac-tion with adsorbed CN and subsequent desorption. Thus, theevolution of the fullerene-like microstructure has to be seenas a delicate interplay between the aligned incorporation ofadsorbed preformed molecules and desorption of less favor-ably bonded ones as C2N2. At the same time the extension ofthe so formed sheet structures is determined by their curva-ture and/or defect density of, e.g., pyridine type. This inhibitsthe growth of well-ordered and extended flexible structures,but at the same time promotes the materials mechanicalstrength via an increased number of interplanar cross-links.

The mechanical deformation of FL-CNx thin films is de-termined by the rather weak van der Waals interactions be-tween the planes, the high in-plane strength, as well as stronginterplanar cross-links. The fairly large �0.35 nm� interplanarspacing is easily compressed under stress, whereas interpla-nar slip, as it is limiting the strength of graphite, is hindereddue to the strong alleged C–C cross-linking and mechanicalinterlocking. Hence, FL-CNx is an extremely resilient mate-rial where deformational stresses are distributed over largevolumes. In the absence of bond breaking and slip, fullerene-like structures recover elastically after unload. Therefore, thedeformation mechanism can be seen to correspond to thebehavior of a nanostructured polymeric sponge of great in-traplanar strength upon compression, where the walls bendwithout rupturing or breaking. It is concluded that on a mac-roscopic scale, FL-CNx exhibits a fairly low resistance toindentation �high compliance� combined with a compara-tively low modulus. It also shows very little tendency toplastic deformation.

This superior resiliency to deformation in FL-CNx makesit more likely that surface asperities in a tribological contactwill deform elastically. Also, brittle fracture is suppressedgiving rise to possible applications in especially abrasive en-vironments. Furthermore, the low modulus found in FL-CNx

films is a valuable property for thin film applications, sincecontact stresses become dissipated over large volumes mak-ing adhesive failures of the film-substrate system less likely,due to reduced stress gradients at the interface. The above-mentioned conditions lead to a reduced wear compared toamorphous CNx or diamondlike carbon �DLC� films. This in

combination with the rather low coefficient of friction makes

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FL-CNx a promising candidate for tribological applications.Also for components on a more macroscopic scale, as on anatomic scale it has already for the most part replaced pure Covercoats for the magnetic recording devices. In addition,with the introduction of fullerene-like CNx thin solid films,one can access a new process-property window compared tothat of DLC, which provides for tailoring of mechanicalproperties in difficult applications involving high contactstresses.

ACKNOWLEDGMENTS

The authors acknowledge valuable discussion by J.-E.Sundgren, H. Sjöström, N. Hellgren, R. Gago, B. Holloway,S. J. Bull, E. Broitman, J. M. Molina-Aldereguia, and I. L.Singer as well as the financial support from the EuropeanCommission, the Swedish Foundation for Strategic Research�SSF�, and SKF ERC. Additionally the support of E. Broit-man regarding the “history” of CNx is highly acknowledged.

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