Surface & Coatings Technology 204 (2010) 3517–3524

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Surface & Coatings Technology

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Fabrication of CrAlN nanocomposite films with high hardness and excellentanti-wear performance for gear application

Liping Wang a,b,⁎, Guangan Zhang a, R.J.K. Wood b, S.C. Wang b, Qunji Xue a

a State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, PR Chinab National Centre for Advanced Tribology at Southampton (nCATS), School of Engineering Sciences, University of Southampton, SO17 1BJ, UK

⁎ Corresponding author. State Key Laboratory of Solid LChemical Physics, Chinese Academy of Sciences, Lanzhou 74968080; fax: +86 931 4968163.

E-mail address: lpwang@licp.cas.cn (L. Wang).

0257-8972/$ – see front matter © 2010 Elsevier B.V. Aldoi:10.1016/j.surfcoat.2010.04.014

a b s t r a c t

a r t i c l e i n f o

Article history:Received 21 September 2009Accepted in revised form 6 April 2010Available online 14 April 2010

Keywords:CrAlN nanocomposite filmsMagnetron sputteringMicro-structureMechanical propertiesTribological performance

CrAlN nanocomposite films with various Al contents were prepared by a reactive magnetron sputteringtechnique using Cr and Al targets in the reactive gas mixture. The micro-structure, mechanical properties andtribological behaviors of traditional CrN and newly-fabricated CrAlN nanocomposite films were compared.XRD results showed that single-phase fcc (face-centered cubic) CrAlN films formed a solid solution wherebyCr atoms were substituted by Al atoms. High resolution TEM results demonstrate that the CrAlN filmpossesses a unique amorphous/crystalline nanocomposite micro-structure with ∼5 nm crystalline uniformlyembedded into an amorphous matrix. In addition, CrAlN nanocomposite films exhibited relatively smallergrain size and denser structure when compared with CrN films. All the CrAlN nanocomposite films with thehighest hardness being approximately 33.4 GPa exhibited much higher hardness and better wear resistancethan traditional CrN films The excellent anti-wear performance of CrAlN nanocomposite films can beattributed to an amorphous/crystalline nanocomposite micro-structure and their high hardness. CrAlNnanocomposite films are considered as potential protective surfaces for such kinds of moving parts as gears,cutting tools and shafts used in engine and other mechanical mechanisms.

ubrication, Lanzhou Institute of30000, PR China. Tel.: +86 931

l rights reserved.

© 2010 Elsevier B.V. All rights reserved.

1. Introduction

Transition metal nitride films exhibited a great interest for variousapplications due to the high hardness, high melting point and highchemical stability [1–3]. However, there are increasing numbers ofapplications where the mechanical and tribological properties of thebinary transition metal nitride films are not sufficient. Therefore,much work has been focused on the design and development of newhard films to further improve the mechanical and tribologicalproperties. One way for such improvement is the deposition ofmore complex film architecture such as multilayer films or chemicalgraded films, e. g. Cr/CrN multilayer [4], Ti/Ti-DLC/DLC graded films[5]. An alternative route is the creation of composition-complex hardfilms, by alloying the binary nitride with another element, such asCrTiN, CrAlN, CrTiAlN etc. [6–8], or by the formation of nanocompositemicro-structures consisting of nanocrystalline grains embedded in anamorphous matrix, e. g. nc-CrN/a-Si3N4 [9].

Chromium nitride (CrN) films, with good anti-oxidation, anti-corrosive and anti-adhesive properties, find wide industrial applica-tions in metal forming and plastic molding operations. However, lowhardness and relatively poor abrasive wear resistance of traditional

CrN greatly restricted their wide applications to other engineeringfields such as engine parts, gears and shafts. To further improve thehardness and anti-wear performance of CrN films, alloying withanother metal to form a ternary hard composite film has beenexplored for its advantage. The new ternary film structure broughtabout significant advances in coating designs, such as the decrease ofthe grain size and the formation of grain boundaries between the twophases. Of them, it is reported that CrAlN films exhibited excellentmechanical properties and oxidation resistance owing to their solidsolution structure [7,10–15]. Furthermore, it is well known that mostproperties of these solid solution composite films are influenced bycertain factors such as crystalline structure and micro-structure. Theknowledge and characterization of these parameters are of majorimportance in order to understand both the process involved in thepreparation and the future behavior of such composite films.

In this paper, comparative experiments were carried out ontraditional CrN and newly-fabricated CrAlN coatings to reveal theirmicro-structure, mechanical properties and tribological behaviors as afunction of Al content. Thus, the purpose of this study is to fabricateCrAlNnanocompositefilmswith better anti-wearperformance andhighhardness for the wide applications in potential engineering fields.

2. Experimental process

The CrN films with various Al contents were deposited on silicon p(111) wafers using medium frequency magnetron sputtering. The

Fig. 1. XRD patterns of CrAlN nanocomposite films and CrN film.

3518 L. Wang et al. / Surface & Coatings Technology 204 (2010) 3517–3524

frequency of the power supply was fixed at 20 kHz for all thedeposition process [16]. A pair of magnetron planar Cr (99.8 wt.%in purity) and Al (99.9 wt.% in purity) targets with size of280 mm×80 mm×8 mm were set in cylindrical vacuum chamberwall. The sputtering chamber was evacuated to a pressure of4.0×10 −3 Pa by a turbomolecular pump and then the sputteringgas was introduced. The Si substrates were cleaned ultrasonically inacetone followed by de-ionized water. Then the Si substrates wereglowing cleaned for 10 min at 1 Pa argon pressure at the substratebias of−700 V. The film deposition process was carried out for 2 h at a40 sccm Ar flow rate and a 160 sccm N2 flow rate with the substratebias at −100 V power at 1.1 kW (465–470 V×2.4 A). In order toacquire uniform films, the substrate was positioned 100 mm awayfrom the target. To obtain the different Al/(Cr+Al) ratio of the CrAlNfilms, the specimens were placed at varying intervals between the Crand Al targets. CrN films without Al were also deposited for thereference. In all deposition process, no external heat was provided tothe substrates. And the measured deposition temperature was alwaysno more than 200 °C after ion bombardment.

Film crystallinity and phase structure were characterized usinggrazing incidence X-ray diffraction (GIXRD). A Philips X'perts X-raydiffractometer with Cu Ka radiationwas also employed to test the thinfilms. The scanning was performed from 20° to 90° at an incidentangle 1°. The Al/(Cr+Al) ratio of the CrAlN films were determined byenergy dispersive X-ray spectroscopy (EDS) analysis in a JSM-5600 Lvscanning electron microscopy (SEM). Field emission-SEM (Hitachi,S-4800) was utilized to observe the cross-sectional micro-structure.High resolution transmission electron microscopy (HRTEM) imagesand selected area electron diffraction (SAED) patterns were obtainedusing a JEOL 3010 TEM operated at 300 kV. The hardness of the filmswas determined by a nano-indenter (MTS Systems Corporation) usinga Berkovich diamond tip and continuous stiffness option, with themaximum indentation depth within 100 nm (less than 10% of totalfilm thickness to minimize the substrate contribution). Five replicateindentations were made for each film sample and the hardness wascalculated from the load–unloading curves.

Wear tests were performed using a micro-tribometer (UMT-2MT,CETR Co., US). Reciprocating wear tests were performed using 100Cr6steel ball under an applied normal load of 1 N, reciprocated with afrequency of 2 Hz, and amplitude of 5 mm. The tests were performed inambient atmosphere with temperature of 18 °C and humidity of 40%without lubrication. The friction coefficient was determined by theaverage of continuously examining data. And the morphology of weartracks was examined by scanning electron microscopy (SEM) and 3Dnon-contacting surface mapping microscope (MicroXam, USA).

3. Results and discussions

3.1. Composition and micro-structure of CrAlN nanocomposite films

The CrAlN filmswith five different Al/(Cr+Al) ratios from 23.6% to59.8% and the traditional CrN films were successfully deposited. The

Table 1The composition, (200) peak position, hardness and friction coefficient of the CrAlNnanocomposite films and traditional CrN film.

Samplenumber

Crcontent(at.%)

Alcontent(at.%)

(200) peakposition(°)

Hardness(GPa)

Reducedelasticmodulus(GPa)

Frictioncoefficient

A 100 0 42.77 13.9 196.7 0.816B 76.4 23.6 42.94 23.2 335.9 0.771C 69.2 30.8 43.16 27.8 334.7 0.772D 62.0 38.0 43.30 26.0 340.8 0.757E 51.6 48.4 43.22 33.4 359.6 0.757F 40.2 59.8 43.54 32.4 343.1 0.776

relative composition of CrAlN films determined by EDS was shown inTable 1. Fig. 1 shows the XRD patterns of CrAlN films deposited onsilicon wafer with various Al contents as well as the XRD pattern ofCrN film for comparison. The XRD peaks in the CrN film wereconsistent to the diffraction peaks of cubic NaCl-type structure.However, the diffraction peaks [for the (111) and (200) peaks was36.80° and 42.77°, respectively from the as-deposited CrN film] werelower than the powder diffraction peaks [the (111) and (200) peakswas 37.60° and 43.69°, respectively from the PDF 76-2494, PCPDFWINVersion 2.3, JCPDS-ICDD (2002)], which meant the CrN film sufferhigh stress. For the CrAlN films, the XRD peaks were similar to that ofthe CrN film and can be well indexed using the cubic NaCl-typestructure. The crystallography structure of only single-phase fcc (face-centered cubic) CrAlN solid solution was detected with the absence ofother separated phase structure of CrxN and AlN. However, the B4structure phase was not observed as the AlN content in the films wasbelow the critical composition for the phase transition from B1-NaCl-type to B4-Wurtzite-type structure. The theoretical maximumsolubility of fcc-AlN in fcc-CrN is approximately 77 at.% [17]. It hasbeen reported that the crystal structure of CrAlN films also changedfrom the B1-NaCl-type into B4-wurtzite-type between 57 at.% and75 at.% Al [17–19]. In this study, the maximum Al/Al+Cr ratio of theas-deposited films was 59.8%, below the critical composition for thephase transition from B1-NaCl-type to B4-wurtzite-type structure. Inaddition, as the Al content in the films increased, the position of thesediffraction peaks gradually shifted towards the higher diffractionangles (Fig. 2). Above results suggested that the lattice structure ofCrAlN films fabricated in this work formed a solid solution whereby Cr

Fig. 2. Evolution of the (200) peak position of CrAlN films as a function of Al content.

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atoms were substituted by Al atoms since the covalent radius of Al(0.121 nm) is smaller than that of Cr (0.139 nm). This also caused theshift of the diffraction peaks of the films. In addition, the peakbroadening phenomenon and the decrease of the diffraction peaksintensitywas also observedwith the incorporation of Al in CrNmatrix,which could be attributed to the fact that the crystallinity of filmcorrespondingly decreased.

In order to investigate the micro-structural changes of CrAlN films,high resolution TEM observations were performed on the CrN andCrAlN films, respectively. Fig. 3 shows the plane view TEM images andelectron diffraction patterns for CrN and CrAlN films. From the TEMimage of the CrN films (Fig. 3a), the film layer was found to have awell-grown crystalline phase with irregular grain shapes and sizefrom ten to several ten nanometers. From the TEM images for theCrAlN films (Fig. 3b), it was observed that the coating layer was well-grown crystalline phase with relatively regular grain shapes and grainsize about 20 nm. In addition, it was also found that fine grains aboutseveral nanometers were observed in the big grains of this film, and atthe higher magnification small crystalline grains about 5 nm (blackarea distinguished by the lattice fringe contrast) were embedded inthe amorphous (white area) matrix (Fig. 3c). The SAD pattern of thefilm (see the inset) clearly shows the (111), (200), and (220)diffraction rings for the face-centered-cubic phase structure, whichcould be attributed to nanocrystalline Cr–Al–N solid solution

Fig. 3. Plan-view TEM image and the selected area electron diffraction (SAD) pattern of CrN afilm, which clearly shows an amorphous/crystalline nanocomposite micro-structure.

embedded in an amorphous matrix. While the further amplifiedimage (Fig. 3c) displayed some nanocrystalline particles with theinterplanar crystal spacing of 0.206 nm, whichwell in agreement withthe typical CrN (111) crystal plane. The Al mainly appeared in theamorphous phase. The HRTEM image in Fig. 3 confirmed that thedoping of Al in CrN matrix greatly promote the formation of anamorphous/crystalline nanocomposite micro-structure.

Fig. 4 showed XPS core-level spectra of CrN and CrAlN (48.4% Alcontent) films. The peak associated with Cr (Fig. 4a and c) consisted oftwo peaks centered at 575.3 eV and 585.4 eV, which originated fromCr 2p3/2 and Cr 2p1/2, respectively. For the CrN films, deconvolutionof Cr 2p3/2 peak indicated that it consisted of two peaks centered at575.1 eV and 577.4 eV (Fig. 4a). The peak centered at 577.4 eV couldbe due to the oxidation forms of chromium [20]. Peaks correspondingto metallic chromium (574.3 eV) and Cr2N (574.5 eV) were notobserved [21], indicating that the bonding state of chromium was inthe form of CrN with traces of chromium oxide. The N 1s spectrum(Fig. 4b) of the CrN film revealed the presence of a peak typical ofchromium nitride centered at 396.4 eV and a weak peak associatedwith chromium oxynitride at a binding energy of 398.7 eV [21]. Whilefor the CrAlN nanocomposite film, similarly, the peak deconvolutionindicated that the Cr 2p3/2 peak comprised of two peaks centered at575.2 and 577.5 eV (Fig. 4c), which was corresponding to CrN andchromium oxide, respectively. The N 1s spectrum (Fig. 4d) revealed

nd CrAlN films: (a) traditional CrN film; (b) CrAlN film; and (c) HRTEM picture of CrAlN

Fig. 4. XPS core-level spectra of CrN and CrAlN (48.4% Al content) films. (a) Cr2p, (b) N1s of CrN film; (c) Cr2p, (d) N1s, and (e) Al2p of CrAlN films.

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the presence of peaks characteristic of nitrogen in metal nitride (CrNand AlN) and oxidation forms, with binding energies at around 396.5and 399.3 eV, respectively. The Al 2p spectrum (Fig. 4e) of CrAlN filmshowed a characteristic peak at a binding energy of 74.1 eV, whichcorresponds to AlN [22]. Deconvolution of this band also indicated thepresence of a second weak peak centered at 76.8 eV. The origin of thispeak may be the presence of oxidation of aluminum.

In the case of the nanocomposites formed by spinodal decompo-sition, an isotropic nanostructure with sharp interfaces is formedautomatically if the necessary thermodynamic (high chemicalactivity) and kinetic (sufficiently fast diffusion) conditions are met[23]. In this experiment, the CrAlN films were prepared by mediumfrequency magnetron sputtering. When using medium frequencymode, a very high ion current density, self-bias voltage and thermal

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substrate load were observed as the high peak current and voltage ofthe power supplies, indicating a strong bombardment of the substratewith electrons and ions from the plasma [24]. Consequently a veryhigh plasma density and electron temperature in the substrate regionoccur in medium frequency mode. Obviously, the first choice for thepreparation of the CrAlN nanocomposites are physical vapor deposi-tion (PVD) techniques, because they provide a high chemical activityof the gaseous reactants as the application of medium frequencymagnetron sputtering (nitrogen in the present case). Although thedeposition temperature was lower than the temperature of thethermodynamic driving force (typical 550 °C), the non-equilibriumphysical vapor deposition process provides the thermodynamicdriving force to form spinodal phase segregation, which assures asufficiently fast diffusion. The local temperature is sufficiently high toassure fast diffusion within the growing film, which is needed for theformation of the stable nanostructure during the growth but still lowenough to avoid intermixing and a concomitant roughening of theinterface. In this process, the formation of spinodal decompositionwas satisfaction due to the necessary thermodynamic (high chemicalactivity) and kinetic (sufficiently fast diffusion) conditions, and anisotropic nanostructure with sharp interfaces is formed automatically.XRD, XPS and HRTEM investigations detailed above have proved thatthe incorporation of Al into the CrN matrix led to the formation of aunique nanocompositemicro-structures composed of nanocrystallitesuniformly embedded in an amorphous matrix. Such formation isbased on a strong, thermodynamically driven and diffusion-rate-controlled (spinodal) phase segregation that leads to the formation ofthe stable nanostructures by self-organization [23,25]. Liu and Shen[26] have suggested that the energy difference between the grainboundary and crystallite/amorphous phase interface can give rise tothe amorphous phase located at the boundary as seen in Figs. 3 and 4.Based on the literature, this amorphous/crystalline nanocomposite

Fig. 5. Cross-sectional SEM images of (a) CrN and (b) CrAlN (59.8% Al content).

structure may be favorable to the high performance of mechanicalproperties owing to the following concepts [28]: (i) combinecrystalline and amorphous CrAlN matrix to achieve high hardnesswith good toughness; and (ii) maintain nanocrystalline size at severalnm level to restrict crack and create a large volume of grainboundaries.

Fig. 5 showed the cross-sectional morphologies of the traditionalCrN and nanocomposite CrAlN (59.8% Al content) films. It was foundthat the pure CrN film exhibited strong columnar structure (Fig. 5a),and no significant difference was observed after alloying with Al(Fig. 5b). However, the CrAlN film had much smaller grain size anddenser structure when compared with CrN film deposited under thesame conditions. The dissolving columnar agglomerates were con-sidered resulted from reduced crystallinity due to Al alloying and theformation of an amorphous/crystalline nanocomposite structure.

3.2. Mechanical and anti-wear properties of CrAlN nanocomposite films

The hardness of the as-deposited films was summarized in Table 1.All CrAlN films showed significantly higher hardness as compared tothe CrN film deposited under similar conditions. The maximumhardness was measured to be approximately 33.4 GPa with 48.4% Alcontent (Table 1 and Fig. 6), which is more than 2 times higher thanthat of traditional CrN films (13.9 GPa). Also revealed by the load–displacement curves is a higher degree of elastic recover (shown inFig. 6 about 25% plasticity) generated during the indentation of theCrAlN film, which is consistent with its higher hardness and elasticmodulus.

The greatly enhanced mechanical properties of CrAlN films can beexplained by solid solution hardening, complex chemical bondingstrengthening, and the novel nanocomposites micro-structure thatenhances material strength by a dislocation blocking effect. Anexplanation of the high hardness of the CrAlN films was the localbond strengthening and dislocation motion hindered by latticedistortion. Taking into account the bonding characteristics andassuming that the bulk modulus increases when the interatomicdistance in AB compounds reduces [28,29], one can suppose that theincrease of hardness in CrAlN films also originates from the decreaseof the nearest neighbor distance. The fact that the lattice parameterand elastic modulus correlates with the hardness values also indicatesthat the nature of bonding in the CrN host lattice is modified by theintroduction of Al atoms with a smaller atomic radius and differentelectronic structure. And the hardness of the CrAlN films is partiallydetermined by solid solution hardening and complex chemicalbonding strengthening. the dislocation mobility depends on chemicalbonding. However, the CrAlN films possesses a unique amorphous/

Fig. 6. Typical load–displacement curves for the CrN film and CrAlN film with 48.4% Alcontent (the hardness is 32.4 GPa).

3522 L. Wang et al. / Surface & Coatings Technology 204 (2010) 3517–3524

crystalline nanocomposite micro-structure as evident by XRD, XPSand HRTEM. According to the amorphous/crystalline nanocompositedesign concept [27,30,31], the high hardness of the composite filmwas based on the combination of the absence of the dislocationactivity in the small nanocrystals and the blocking of grain boundarysliding by the formation of a strong interface between the two phases.And the formation of superhard nanocomposites requires 3–5 nmcrystallite sizes with less than 1 nm thick separation in an amorphousmatrix. The films investigated here had crystallite sizes andamorphous layer thickness similar to the superhard nanocomposites,which were sufficient for formation of nanosized dislocations.Therefore, another phenomenological explanation to the observedcombination of hardness can be deduced from results of structuralanalyses and their comparison with existing concepts on nanocom-posite mechanisms.

Fig. 7. SEMmicrographs of the worn surfaces of CrN and CrAlN films with different Al conten(f) 59.8 Al content CrAlN films.

The tribological properties of the as-deposited CrAlN nanocompo-site films were determined by reciprocating wear test. From the weartest against 100Cr6 steel ball under dry wear conditions, the averagefriction coefficient of the as-deposited films was also list in Table 1. Itwas found that the coefficient of friction were all found to be high(∼0.8), and there was no obviously variations with the change of Alcontents. Figs. 7 and 8 showed the micrographs of wear tracks on thefilms after reciprocating wear tests. Although the as-deposited filmshad similar friction coefficients in the dry sliding tests, themorphologies of the wear tracks on the as-deposited films werequite different. Usually, stresses existing in a coating may promotecrack propagation and, thus, together with the plastic deformation bythe local coating fracture (brittle cracking) can occur, increasing thewear rate of the coating [32]. For the high stress coatings delamina-tions and cracking appear within the wear track. The wear surface of

t sliding against GCr15 steel ball. (a) CrN, (b) 23.6%, (c) 30.8%, (d) 38.0%, (e) 48.4%, and

Fig.

8.Optical

3Dmorph

olog

iesof

wea

rtrackforCrN

andCrAlN

film

swithdifferen

tAlc

ontent.(a)

CrN,(b)

23.6%,

(c)30

.8%,

(d)38

.0%,

(e)48

.4%,

and(f)59

.8Alc

ontent

CrAlN

film

s.

3523L. Wang et al. / Surface & Coatings Technology 204 (2010) 3517–3524

3524 L. Wang et al. / Surface & Coatings Technology 204 (2010) 3517–3524

the traditional CrN filmwas heavily damaged as the CrN films sufferedhigh stress. The SEM picture on wear track showed delamination-likeasperities, which indicated that the film had locally detached from thesubstrate surface. The depth of thewear trackwas about 1 µm, and thefilm was nearly worn out and penetrated in the wear process. Thispoor wear resistance of the CrN film was attributed to a lowerhardness, relatively looser structure, high stress and brittle nature.However, the wear tracks of the CrAlN nanocomposite films werequite different from the CrN film. In the wear track of CrAlN film,relatively smooth wear track was observed (Fig. 7b–f). Almost nodelamination or film crack was observed besides the wear tracks. Itwas noted that the wear track mainly consists of some slight groovesand nodules, and the depth of the wear track ( from 80 nm to 150 nm)for CrAlN nanocomposite films with different Al contents decreaseddramatically (Fig. 8b–f). The bulges on the film surface wereattributed to the large material transfer from the steel surface.These indicated that the CrAlN nanocomposite filmwas advantageousto suffer the adhesive scuffing/galling and show excellent dry wearresistance. The CrAlN films had excellent anti-wear performance ascompared to traditional CrN film and this enhancement could beattributed to the small grain size of film, an amorphous/crystallinenanocomposite micro-structure, high hardness and the ability toreduce the brittle fracture of the films. When the Al content of theCrAlN nanocomposite films reached 58.9%, local spallation phenom-enon was observed on the worn surface (Fig. 7f). The large contentbrittle nature of AlN dominant amorphous phase in the CrAlN filmswith an Al content of 58.9% led to the brittle delamination of localizedzones within the wear track (Fig. 7f), but the whole wear depth is stilllow (∼110 nm in depth).

Such nanocomposite films are considered as potential protectivesurfaces for moving parts such as gears, cutting tools and shafts used inengines or other industries to replace the traditional CrN films. Suchkinds of nanocomposite films have now been successfully deposited onlarge-scale gears (nearly 300 mm in diameter, stainless steel AISI 304)used in polishing machines. Fig. 9 shows the whole picture of CrAlN-coated gears with a film thickness of approximately 15 µm. It can beclearly observed that the coated tooth surface of gears is very uniform,smooth and compact. In addition, the CrAlN films exhibited veryexcellent bondingwith the gears. The servicing life and failure process ofsuchnanocomposite CrAlN-coatedgears are nowunder investigation onreal polishing machine used in a Chinese magnetic recording company.

4. Conclusions

Hard CrAlN nanocomposite films with different Al content havebeen fabricated by reactive magnetron sputtering technique. The

Fig. 9. Nanocomposite CrAlN-coated gears with a film thickness of approximately10 µm.

effect of the Al contents on the micro-structure, mechanical andtribological properties of CrAlN films have been investigated. XRD,XPS and HRTEM result showed that the CrAlN film has a uniqueamorphous/crystalline nanocomposite micro-structure with ∼5 nmcrystalline uniformly embedded in an amorphous matrix. The CrAlNnanocomposite films exhibited relatively smaller grain size anddenser structure when compared with CrN films. All the CrAlNnanocomposite films possessed significantly higher hardness andwear resistance than traditional CrN films and the highest hardnessfor CrAlN film was approximately 33.4 GPa. The excellent anti-wearperformance of CrAlN nanocomposite films can be attributed to anamorphous/crystalline nanocomposite micro-structure and highhardness. Such nanocomposite films are considered as potentialprotective surfaces for such kinds of moving parts as gears, cuttingtools and shafts used in engines.

Acknowledgments

The authors are grateful to the National Natural Science Founda-tion of China (nos. 50772115 and 50823008) and the 863 Program ofChinese Ministry of Science and Technology (no. 2009AA03Z105) forfinancial support of this research work.

References

[1] C. Rebholz, H. Ziegele, A. Leyland, A. Matthew, Surf. Coat. Technol. 115 (1999) 222.[2] G.A. Zhang, P.X. Yan, P. Wang, Y.M. Chen, J.Y. Zhang, Mater. Sci. Eng. A 460–461

(2007) 301.[3] Y.J. Zhang, P.X. Yan, Z.G. Wu, J.W. Xu, W.W. Zhang, X. Li, W.M. Liu, Q.J. Xue, J. Vac.

Sci. Technol. A 22 (2004) 2419.[4] E. Martınez, J. Romero, A. Lousa, J. Esteve, Surf. Coat. Technol. 163–164 (2003) 571.[5] A.A. Voevodin, S.D. Walck, J.S. Zabinski, Wear 203–204 (1997) 516.[6] G.A. Zhang, P.X. Yan, P. Wang, Y.M. Chen, J.Y. Zhang, Appl. Surf. Sci. 253 (2007)

7353.[7] M. Uchida, N. Nihira, A. Mitsuo, K. Toyoda, K. Kubota, T. Aizawa, Surf. Coat.

Technol. 177–178 (2004) 627.[8] P.L. Tam, Z.F. Zhou, P.W. Shum, K.Y. Li, Thin Solid Films 516 (2008) 5725.[9] K. Yamamoto, T. Sato, M. Takeda, Surf. Coat. Technol. 193 (2005) 167.

[10] J. Vetter, E. Lugscheider, S.S. Guerreiro, Surf. Coat. Technol. 98 (1998) 1233.[11] H. Hasegawa, M. Kawate, T. Suzuki, Surf. Coat. Technol. 200 (2005) 2409.[12] J. Romero, M.A. Gómez, J. Esteve, F. Montalà, L. Carreras, M. Grifol, A. Lousa, Thin

Solid Films 515 (2006) 113.[13] H. Scheerer, T.H. Hoche, E. Broszeit, B. Schramm, E. Abele, C. Berger, Surf. Coat.

Technol. 200 (2005) 203.[14] O. Banakh, P.E. Schmid, R. Sanjines, F. Levy, Surf. Coat. Technol. 163–164 (2003)

57.[15] A.E. Reiter, T.V.H. Derflinger, B. Hanselmann, T. Bachmann, B. Sartory, Surf. Coat.

Technol. 200 (2005) 2114.[16] P. Wang, X. Wang, T. Xu, W. Liu, J. Zhang, Thin Solid Films 515 (2007) 6899.[17] A. Sugishima, H. Kajioka, Y. Makino, Surf. Coat. Technol. 97 (1997) 590.[18] Y. Makino, K. Nogi, Surf. Coat. Technol. 98 (1998) 1008.[19] J. Lin, B. Mishra, J.J. Moore, W.D. Sproul, Surf. Coat. Technol. 201 (2006) 4329.[20] A. Lippitz, T. Hubert, Surf. Coat. Technol. 200 (2005) 250.[21] C. Emery, A.R. Chourasia, P. Yashar, J. Electron Spectrosc. Relat. Phenom. 104

(1999) 91.[22] I. Bertoti, Surf. Coat. Technol. 151–152 (2002) 194.[23] R.F. Zhang, S. Veprek, Mater. Sci. Eng. A. 424 (2006) 128.[24] H. Bartzsch, P. Frach, K. Goedicke, Surf. Coat. Technol. 132 (2000) 244.[25] S.L. Ma, D.Y. Ma, Paul K. Chu, Acta Mater. 55 (2007) 6350.[26] Z.J. Liu, Y.G. Shen, Acta Mater. 52 (2004) 729.[27] A.A. Voevdin, J.S. Zabinski, Thin Solid Films 370 (2000) 223.[28] M.L. Cohen, Phys. Rev. B 32 (1985) 7988.[29] Min Zhou, Y. Makino, M. Nose, K. Nogi, Thin Solid Films 339 (1999) 203.[30] S. Veprek, S. Reiprich, Thin Solid Films 268 (1995) 64.[31] S. Veprek, J. Vac. Sci. Technol. A 17 (1999) 2401.[32] A.A. Voevodin, C. Rebholz, J.M. Schneider, P. Stevenson, A. Matthews, Surf. Coat.

Technol. 73 (1995) 185.