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Journal of Alloys and Compounds 483 (2009) 321–333

Contents lists available at ScienceDirect

Journal of Alloys and Compounds

journa l homepage: www.e lsev ier .com/ locate / ja l l com

oncepts for the design of advanced nanoscale PVD multilayer protectivehin films

. Stueber ∗, H. Holleck, H. Leiste, K. Seemann, S. Ulrich, C. Ziebertorschungszentrum Karlsruhe, Institute for Materials Research I (IMF-I), Hermann-von-Helmholtz-Platz 1, D-76344 Eggenstein-Leopoldshafen, Germany

r t i c l e i n f o

rticle history:eceived 30 August 2007eceived in revised form 21 July 2008ccepted 29 August 2008vailable online 17 November 2008

eywords:hin filmsurfaces and interfacesapour deposition

a b s t r a c t

Technological challenges in future surface engineering applications demand continuously new materialsolutions offering superior properties and performance. Concepts for the design of such advanced multi-functional materials can be systematically evolved and verified by means of physical vapour deposition.The classical multilayer coating concept today is well established and widely used for the design of protec-tive thin films for wear and tribological applications. It has proven great potential for the development ofnovel thin film materials with tailored properties. In the past decade, the emerging new class of nanoscalecoatings has offered to the material scientists an even more powerful toolbox for the engineering thin filmdesign through a combination of the multilayer concept with new nano-coatings. Some examples are theuse and integration of low friction carbon-based nanocomposites in advanced multilayer structures or

icrostructure the stabilization of a specific coating in another structure in a nanolaminated multilayer composite. Thispaper reviews the latest developments in hard, wear-resistant thin films based on the multilayer coat-ing concept. It describes the integration of nanocrystalline, amorphous and nanocrystalline/amorphouscomposite materials in multilayers and covers various phenomena such as the superlattice effect, stabi-lization of materials in another, foreign structure, and effects related to coherent and epitaxial growth.Innovative concepts for future, smart multilayer designs based on an extremely fine structural ordering

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at the nanoscale are prese

. Multilayer coatings for wear protection: pioneering worknd fundamental knowledge

The realization of future technical solutions for tribology-relatedngineering applications in the tool industry and highly-stressedomponents for instance in the automotive or aircraft industriesequires the development of new multifunctional thin film mate-ials that meet the challenges of providing superior mechanical,ribological, chemical and high-temperature properties and per-ormance. Concepts for the design of such advanced coatings cane systematically evolved and verified by means of physical vapoureposition (PVD). The classical multilayer coating concept today

s well established and widely used for the design of protectivehin films for wear and tribological applications. It has proven

reat potential for the development of novel thin film materialsith tailored properties. In the past decade, the emerging new

lass of nano-coatings has offered to the material scientists anven more powerful toolbox for the engineering thin film design

∗ Corresponding author. Tel.: +49 7247 82 3889; fax: +49 7247 82 4567.E-mail address: michael.stueber@imf.fzk.de (M. Stueber).

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925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved.oi:10.1016/j.jallcom.2008.08.133

as well.© 2008 Elsevier B.V. All rights reserved.

hrough a combination of the multilayer concept and new coatingsith extremely fine structural ordering at the nanoscale leading to

ignificantly enhanced properties and functionalities.This paper presents recent developments in hard, wear-resistant

hin films based on the multilayer coating concept. It brieflyddresses the pioneering work on multilayers and focuses on aescription of the state-of-the-art in multilayer thin films based onanoscale approaches. Innovative concepts for future, smart mul-ilayer designs based on an extremely fine structural ordering athe nanoscale are discussed as well. The paper is limited to mul-ilayers composed of hard materials with metallic and/or covalentonding characteristics only and does not refer to multilayers withetal layers or composed of alternating metal and ceramic hard

ayers. Excellent reviews on various aspects of multilayer thin filmevelopment are available [1–14].

Since the first reports on the deposition of hard and even super-ard multilayer thin films and their corresponding microstructure

ormation mechanisms such as superlattice growth came up after

he mid 1980s [15–20], an enormous run on both experimentallynd theoretically driven research on the development of tailor-ade multilayer coatings was initiated. (Please note: Timely parallel

evelopments on metal/metal multilayer coatings and the so-calledupermodulus effect are not referred to here; for an introduction

322 M. Stueber et al. / Journal of Alloys and C

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ig. 1. Change in properties of nanoscale multilayer coatings as a function of thenterface volume (schematically, disregarding superlattice structures), re-designedrom a review article of Holleck [8].

nto this topic see for example the early papers of Schuller ando-workers [21,22], Wolf and Lutsko [23], or the review articlesf Barnett and Shinn [4] and of Abadias et al. [14] and referencesherein.) Based on early results obtained for classical multilayeroatings, which in the easiest model can be described as an alter-ating layer-by-layer arrangement of two single-phase materialsith remarkable individual layer thickness (in the range of some

en or hundred nanometers), it was soon recognized, that a signifi-ant reduction both of the thickness of the individual layers and ofhe crystallite sizes of the layer materials (down to a few nanome-ers) can result in a drastic enhancement of the thin film properties.esides the properties of the individual layer materials, the grainoundaries and interfaces play an important role for the overalloating properties with regard to the nanoscale architecture of theoatings. Fig. 1, re-designed from a review article of Holleck [8],isplays some general conclusions on the structure–property rela-ionships of hard PVD multilayer coatings composed of single layer

aterials with similar bonding characteristics (disregarding com-ositionally modulated superlattice structures). The properties anderformance of such coatings were found to vary systematicallyith the amount and area of interfaces (denoted as interface vol-

sasF

Fig. 2. Toughening and strengthening mechanisms in ceramic multilay

ompounds 483 (2009) 321–333

me in [8], taking into account that interfaces mostly are extendedver a few nm). It was demonstrated that optimum properties anderformance could be achieved at specific values of the interfaceolume. Not all properties reached their optimum levels at theame interface volume or at the same number of layers. For spe-ific multilayer films, in example combinations of TiN, TiC, andiB2, a range has been identified where optimum properties coulde achieved: this range included some 100–200 layers for a 5 �mhick coating. Considering the bilayer modulation period�, intro-uced as a characteristic of superlattice coatings (see Section 2),ptimum properties were obtained for such coatings for valuesf � between 50 and 100 nm. Holleck classified multilayer coat-

ngs by the type, bonding and microstructural characteristics of thendividual layer materials as well as by the specific nano-designchieved by interface modeling [8], a systematic described in moreetail in [24]. Many of the design concepts for multilayer thinlms realized perfectly later on by other research groups werenticipated by his early, trendsetting papers [1,25]. With regardo the covalent bonding characteristics of most nitrides, carbidesnd borides used in multilayer coatings, Holleck suggested to seri-usly consider strengthening and toughening mechanisms knownor brittle bulk ceramics when discussing mechanical, elastic andlastic properties of multilayers [5]. These include crack initia-ion, crack propagation and crack energy dissipation, for exampley crack deflection. When a multilayer surface is exposed to aechanical load, various effects may play together, covering surface

ffects (crack initiation), in-layer, interface, phase or grain bound-ry and through-coating effects. A crack starting from the coatingurface may be split and deflected at the grain or phase bound-ries within a layer (grain boundary toughening or hardening) ort the interface zone between the layers (interface toughening orardening). On the other hand, local delamination can occur at

nterfaces through the opening of nano-voids which can result inocal stress relaxation and even plasticity at the nanoscale. Fur-

train–stress fields across the interfaces in nanoscale multilayers,nd also an interaction of through going cracks with the sub-trate material. These mechanisms are shown schematically inig. 2. Obviously, internal interfaces can have a beneficial effect

er coatings (schematically) according to Holleck and Schier [5].

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osNiptHogsomh1620 kg/mm , respectively). This effect since then was referred to assuperlattice effect, and the corresponding structure as superlatticestructure. Superlattice structures with superlattice effect (signif-icant hardness enhancement) were observed for various othersingle-crystalline nitride multilayer coatings as well: Mirkarimi et

M. Stueber et al. / Journal of Alloy

n the microscopic and macroscopic coating properties when theirmount is carefully adjusted to the overall volume of the coating.he modeling and understanding of elastic and plastic properties ofultilayers is however more complex as many coatings also display

arge amounts of metal bonds and grow in metal-like lattice struc-ures. Hence, dislocation motion and interaction of dislocationsith microstructural features such as grain boundaries, interfaces

nd column boundaries in PVD coatings as well as various defectsnd coherency strains between the individual layers have to beonsidered as well [9].

Further progress in multilayer thin film research was stimulatedy the first successful epitaxial stabilization of a layer material

n a different microstructure as shown by Setoyama et al. [26]nd Madan et al. [27] for the TiN–AlN system (see Section 3). Thencreasing knowledge on existing PVD methods and emerging neweposition technologies supported the development of multilayeroatings as well [2,28]. Considering the period from the mid 1990sntil today, a large variety of highly sophisticated nanoscale mul-ilayer coatings with promising properties has been developed,owever only a few coatings have been introduced already intohe market in engineering applications. In fact, there is a con-rast between very tricky multilayer growth processes available inhe laboratories and the demand for easy to scale-up depositionrocesses for industrial use [11]. On the other hand, a fairly unde-eloped database of detailed experimental information on variousypes of wear-resistant, hard multilayer coatings (covering nearlyll combinations of nitrides, carbides, carbonitrides, oxides andther hard phases such as boron carbide, boron nitride or diamond-

ike carbon) has been collected until today, which should give riseo the expectation of similar breakthroughs through new coating

aterials in the future as was related to the first reports on hardultilayers some decades ago.

To provide a complete overview of this field of materials sciences almost impossible. Thus only a few, but important highlightsf the state-of-the-art in multilayer development with the focusn principal design concepts are described in this paper. Thiseview is exemplarily amended by selected results obtained byhe authors. According to Yashar and Sproul [7] multilayer coat-ngs can be classified in two categories: iso-structural (individualayers have the same structure) and non-iso-structural (individ-al layers have different structures). With regard to the increasingumber of publications dealing with new types of multilayeroatings (integration of amorphous layer materials or of novel mul-iphase, nanocrystalline/amorphous composite materials, or evenompletely amorphous multilayer systems), both the classificationsf Yashar and Sproul and of Holleck [7,8] today might be updatedo cover all these different materials. Multilayer design by epitax-al stabilization and superlattice structure formation has become a

ajor driver in coating development today and are therefore useds a main characteristic to classify the coatings. Section 2 describesultilayer coatings without epitaxial stabilization effects, while in

ection 3 multilayer coatings using epitaxial stabilization effectsre presented. Selected new concepts for multilayer design withne-scale structural ordering are suggested in Section 4.

. Nanoscale multilayer coatings without epitaxialtabilization effects

The transition metal nitrides, carbides and carbonitrides (espe-ially those of the elements of the groups IV B, V B, and, VI B of the

eriodical system) belong to the most relevant coating materials

or wear applications. The majority of these nanocrystalline hardVD coatings are usually synthesized by various magnetron sput-ering or arc evaporation processes (or hybrid deposition processesombining sputtering and arc methods). All these coatings grow

Ft

ompounds 483 (2009) 321–333 323

ore or less, and dependent on the growth conditions, with a poly-rystalline, columnar or nanocolumnar structure according to thehornton model [29], while under certain conditions the growth ofingle-crystalline coatings is possible. A significant part of the workone so far on hard PVD multilayer coatings can be attributed tohe combination of such materials. One should note, that the same

ultilayer material system can exist in different structures show-ng different properties, which depends on the material selection,he deposition processes and kinetics of growth, and on the spe-ial nanoscale thin film architecture (i.e. one may design the sameultilayer coating in the form of a superlattice showing signifi-

ant hardness enhancement or as a nanoscale multilayer coatingithout superlattice structure and properties). In this paper, a more

henomenological description of growth effects in multilayer coat-ngs is provided, while a materials science based consideration isnder preparation.

.1. Single-crystalline superlattice thin films

Numerous papers are available today on multilayers composedf two nitride or carbide hard materials with cubic face-centeredtructure and metallic bonding character (i.e. TiN, VN, ZrN, CrN,bN, TiC, and VC). The periodic arrangement of such materials

n a nanoscale multilayer structure can result in the growth of aerfect compositionally modulated structure. A breakthrough inhe development of superhard coatings was achieved in 1987 byelmersson et al. [16], who demonstrated an enormous increasef the hardness of single-crystalline TiN/VN multilayers epitaxiallyrown on MgO substrates by reactive magnetron sputtering at sub-trate temperature of 750 ◦C (Fig. 3). The hardness enhancementccurred at a bilayer modulation period � of 5.2 nm, and maxi-um hardness values of 5560 kg/mm2 were determined (while the

ardness for pure single-crystal TiN and VN films were 2200 and2

ig. 3. Microindentation hardness of TiN/VN superlattice structure as a function ofhe superlattice period. Diagram originally presented by Helmersson et al. [16].

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24 M. Stueber et al. / Journal of Alloy

l. [19] reported enhanced hardness in TiN/V0.6Nb0.4N superlatticesrown by reactive magnetron sputtering on MgO substrates at aubstrate temperature of 700 ◦C; the materials for the multilayeresign were selected carefully by providing layer materials show-

ng no lattice mismatch. Further systematic studies of Mirkarimit al. [30] addressed the impact of lattice mismatch and coherencytrains at layer interfaces on possible hardness changes. Shinn etl. also reported significant hardness enhancements for TiN/NbNuperlattices [31] (deposited at temperatures around 700 ◦C ongO substrates again). A more detailed insight into this field ofultilayer development is given by the review articles of Barnett

nd Shinn [4], Yashar and Sproul [7], and Hultman [13].

.2. Poly-crystalline superlattice thin films with superlattice effect

The above-described basic experiments on the epitaxial growthf superlattice structures on single-crystalline substrates at ele-ated temperatures were useful for the identification of generalardening mechanisms. Using however different substrate mate-ials (i.e. poly-crystalline ones) or applying less severe depositionnd growth conditions (such as significantly lower substrate tem-eratures or increased substrate bias) results in the growth ofoly-crystalline multilayer structures which quite often show aardness increase at bilayer modulation periods in the range of–10 nm (similar to the behaviour shown in Fig. 3). These coat-

ngs are therefore in the literature also referred to as superlattices.any examples of poly-crystalline superlattices showing a super-

attice effect (significant hardness increase as a function of theultilayer design) are reported for various combinations of iso-

tructural nitride layers (and for combinations of nitride layers ofifferent crystalline structure as well) [7].

Chu et al. [32,33] reported a pronounced superlattice effect ineactively magnetron-sputtered TiN/NbN coatings grown on steelubstrates at substrate temperatures below 500 ◦C. They claimedor a strong control of the bilayer modulation period�, of the sto-chiometry of the individual nitride layers and for the design ofigh-density multilayer structures in order to obtain high hard-ess values in poly-crystalline superlattices. A moderate change

n the deposition parameters (i.e. of the nitrogen partial pres-ure) could result in failure for obtaining the superlattice effect.urthermore, ion bombardment during film growth could affecthe composition modulation, defect densities and porosity of theuperlattice structure [32,33]. Significant hardness enhancementn TiN/NbN superlattice films was also reported by Li et al. [34],

ho observed maximum hardness values of 39 GPa at a modulationeriod of 8.3 nm in films grown on single-crystal silicon wafers. Theuperlattice effect was assigned to be due to the alternating stresseld in the TiN/NbN superlattice, which resulted from the latticeismatch between fcc TiN and fcc NbN. Barshilia and Rajam [35]

dentified a specific range of the modulation period � where theormation of superlattice structures in the TiN/NbN system (withegard of the deposition conditions applied, such as a substrateemperature of 400 ◦C) and the superlattice effect were observed.hese coatings exhibited a thermal stability up to 700 ◦C. A detailednalysis of TiN/NbN coatings with a modulation period of about.7 nm deposited on silicon (1 0 0) at a substrate temperature of00 ◦C [36] indicated a poly-crystalline, (1 1 1) textured, columnaricrostructure (with columns along the growth direction with an

verage column size of about 100 nm). HRTEM studies showed aon-planar nature of the lattice planes which was attributed to

he presence of a sub-grain structure with lattice misorientationsaused by the lattice mismatch between TiN and NbN. Superlatticetructures and superlattice effects with a variation of the coat-ng hardness values have also been reported for TiN/CrN coatings37–42]. Technologically important material combinations based

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ompounds 483 (2009) 321–333

n the integration of AlN layers, for example TiN/AlN superlatticesshowing enhanced hardness as well) are described in Section 3s they make use of the stabilization of hexagonal AlN in a cubictructure. Other crystalline nitride-based multilayer systems withuperlattice effects not referred to here in detail include TiN/TaNnd other TaN, NbN or MoN based coatings [43–45]. A review ofusil [46] mentions superhard TiC/VC and TiC/NbC superlattice

oatings. Further approaches towards the development of innova-ive high performance coatings address the design of ternary anduaternary (or even multi-elemental) multilayer and superlatticetructures. While the integration of metastable layer materials fromhe (Ti, Al)N and (Cr, Al)N families is well established today, lessapers are published in this field in comparison to superlatticesomposed of crystalline binary nitrides. Superlattice structuresith more or less pronounced hardness modification were reported

or material combinations like TiAlN/ZrN [47], TiAlN/CrN [48–50],iAlN/VN [48,51–53], TiAlYN/VN [49], TiAlCrN/TiAlYN ([50,51]; auperlattice effect was not found in this Y-containing system;hese superlattices are listed here however with regard to theirignificant improvement of tribological properties), TiAlN/TiAlCrN54], CrAlYN/CrN [55], TiCN/ZrCN [56], and TiHfN/CrN [57]. Singleeports have been published also on superlattice structures of highardness or with hardness enhancement for combinations of crys-alline carbide and nitride hard materials, for example in WC/TiNr WC/CrAlN [58,59], and for combinations of crystalline nitrider carbide with boride materials, for example in TiC/TiB2, ZrC/ZrB2nd ZrAlN/ZrB2 [60–62].

Hardness enhancement in crystalline superlattice structures isxplained by the widely accepted concept of blocking of dislocationotion at the layer interfaces due to differences in the shear moduli

f the individual layer materials, and by coherency strain caus-ng periodical strain–stress fields in the case of lattice-mismatched

ultilayer films [13]. In many material combinations, the evolutionf the hardness as a function of the bilayer modulation period �hows a characteristic behaviour for all these coatings with maxi-um hardness values typically obtained for values of� between 2

nd 10 nm [13,14]. The high interface density of nanoscale multilay-rs or superlattice structures contributes to impeding dislocationotion and dislocation glide across the layer interfaces, whichould require a critical yield stress being related to the difference

n the elastic shear modulus of the individual layer materials [63].etailed studies of Barnett and co-workers of mechanical prop-rties of transition metal nitride, metal/nitride and metal/metaluperlattices taking into consideration more realistic assumptionsn a superlattice architecture (for example: different shape of inter-aces instead of abrupt, smooth interfaces; allowing an interface

idth; dislocation motion within layers as another mechanism oflastic deformation additionally to dislocation glide across layer

nterfaces) resulted in a theoretical description of the strength andardness evolution of superlattice systems which gave good agree-ent with experimental work (with regard to the conditions set

or this model: miscibility of the individual layer materials andeferring only to materials with the same dislocation slip systems)64]. According to this model, the strength or hardness of a super-attice structure should (for a wide range of values of the bilayer

odulation period�) be limited by dislocation glide within layers,ut not across layers. The potential for hardness enhancement inuch mulitlayers is directly related to the difference in the shearoduli of the layer materials. The interface width effect would

ecome important only for relatively small values of �. Chu and

arnett further mentioned the Hall–Petch relation not to be a suit-ble model for the type of superlattices described in their model.he Hall–Petch relation was considered to be applicable for mul-ilayer systems composed of materials having different dislocationlip systems, where stress concentrations at dislocation pileups in

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M. Stueber et al. / Journal of Alloy

ne layer would cause dislocation nucleation in another, neighbour-ng layer [64]. Considering the ceramic character (due to substantialmounts of covalent bonds) of many of the superlattice materials,he overall columnar growth structure resulting from most PVDrocesses, the specific kinetic conditions for thin film nucleationnd growth, nano- and microscale growth defects occurring in mosteposition processes, as well as defects in the coating microstruc-ure, further mechanisms contributing to hardening or softening ofhe coatings should be regarded. These include initiation and prop-gation cracks at various length scales, starting or ending possiblyt grain boundaries or column boundaries [8]. Other features beingelevant for the coating properties are the overall stress state ofhe coating, the stress profile of the individual layers of the mul-ilayer, and their interaction with each other, or with interfaces,islocations and microcracks.

.3. Poly-crystalline superlattice thin films without superlatticeffect

A variety of hard nanoscale multilayer coatings can be grownith a superlattice structure (as indicated for example by the

orresponding low-angle XRD reflections of the coatings) but nouperlattice effect with regard to a significant variation of theardness is observed. Such behaviour was reported for examplen technologically relevant CrN/NbN superlattice coatings devel-ped by Münz, Hovsepian and co-workers [65–67] and otherroups [68,69], but also for TiN/NbN superlattice coatings [70]. Theon-existence of a superlattice hardening effect in the TiN/NbNoatings (which is in contrast to the results described in Sec-ions 2.1 and 2.2) was explained to be due to microcrackingt grain boundaries under the indenter tip during nanoinden-ation measurements. Ducros et al. [71] showed a continuousransition from nanoscale poly-crystalline multilayer coatings tooly-crystalline superlattice films in the TiN/AlTiN and CrN/AlTiNystems. Superlattice structures without superlattice effect havelso been reported in carbide systems, for example in TiC/NbC andiC/VC [72,73].

Another interesting material system in this category is theiN/ZrN system ([74–78], see also the review of Ziebert and Ulrich79], and references therein). Both TiN and ZrN exist in the sameubic face-centered crystal structure with a lattice mismatch of.1% (aTiN = 0.424 nm, and, aZrN = 0.457 nm, according to JCPDS 38-

420, and, JCPDS 35-0735, respectively). Moreover, TiN and ZrNxhibit similar properties such as high hardness and high meltingoint. The shear moduli of TiN and ZrN are 1.96 × 1011 N/m2 and.51 × 1011 N/m2, respectively [80], which gives a difference in thehear moduli of 28%.

ae8mm

ig. 4. Transmission electron microscopy analysis of a TiN/ZrN multilayer coating of 20attern and HRTEM image of an interface region between TiN and ZrN layers. Right side:

ompounds 483 (2009) 321–333 325

In the following, results obtained by the authors for the epitax-al growth of nanoscale TiN/ZrN multilayer coatings on Si wafernd cemented carbide by reactive dc magnetron sputtering areescribed (for detailed information see [76,79]). The coatings wereeposited with an industrial sputter equipment in a stop-and-goode, which means, that the substrate samples were coated in front

f one cathode and then moved to the other cathode for depositionf the next layer. By this method, well-pronounced sharp inter-aces between the TiN and ZrN layers and layer numbers between

and 1000 at an overall coating thickness of 4 �m (correspondingo bilayer modulation periods down to 8 nm) could be realised. Thendividual TiN and ZrN layers were stoichiometric, and the coatinghowed a fine columnar morphology. High-angle XRD analysis inBragg–Brentano experiment showed a poly-crystalline structuref the TiN and ZrN layers and indicated a heteroepitaxial growth ofrN on TiN: reflections of the (1 1 1), (2 0 0), (2 2 0), (3 1 1) and (2 2 2)rientations of the fcc ZrN structure and (1 1 1), (2 0 0) and (2 2 0)eflections of the fcc TiN structure were identified. Fig. 4 exem-larily shows results of the TEM analysis of a TiN/ZrN multilayerf 20 individual layers (bilayer period 400 nm): The selected areaiffraction (SAD) patterns confirm the (locally) epitaxial growthf ZrN on TiN. The lattice constant derived from the TEM analysisesulted in values of aTiN = 0.41352 nm and aZrN = 0.4568 nm, whichre in good agreement with the above mentioned values, indicat-ng nearly stress-free fcc TiN and ZrN lattices. The HRTEM image inig. 4 shows a fine microstructure with pronounced column bound-ries (with columns diameters between 450 and 600 nm). From thenlarged picture of an interface between a TiN and ZrN layer it cane clearly seen that the lattice fringes grow through the TiN lay-rs, ZrN layers and across their interfaces, which indicates that theiN/ZrN coatings grew epitaxially. The interface width between theiN and ZrN layers is in the range of 2 nm, and, the interface is notully sharp. Fig. 5 shows both the Vickers microhardness and theeduced Young’s modulus determined by nanoindentation for thelms as a function of the number of individual layers. A superlat-

ice effect is not observed for these coatings; the hardness increasesith increasing number of layers (corresponding with a decreas-

ng bilayer period) and then remains nearly unchanged between00 and 1000 layers (corresponding to bilayer periods between 80nd 8 nm). The Young’s modulus increases with increasing num-er of layers, reaches a maximum value around 50–100 individual

ayers (corresponding to bilayer periods between 160 and 80 nm)

nd then decreases drastically with increasing the number of lay-rs from 100 to 1000 (corresponding to a bilayer periods between0 and 8 nm). At high numbers of individual layers (low bilayerodulation period) the TiN/ZrN coatings exhibit a reduced Young’sodulus and a less ductile behaviour at the nanoscale which could

individual layers at a total thickness of 4 �m. Left side: Selected area diffractionHRTEM image of a column structure and column boundary.

326 M. Stueber et al. / Journal of Alloys and C

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ig. 5. Vickers microhardness and Young’s modulus of TiN/ZrN multilayer coatingss a function of the layer number (total film thickness: 4 �m).

ndicate a more brittle character probably caused by microcrack-ng at the pronounced column boundaries. Soe and Yamamoto [74]bserved a minimum in hardness and elastic modulus of (2 0 0) ori-nted TiN/ZrN superlattices at a bilayer modulation period of 3 nm.he hardness of their films decreased significantly if the bilayereriod was less than 8 nm (bilayer periods below 8 nm in our modelould require a higher number of layers which is difficult to real-

ze with the experimental stop-and-go deposition procedure). Theyuggested elastic anomalies in terms of the coherency strain causedy interfaces as explanation for this behaviour. In contrast to thesendings, Tavares et al. [75] found a moderate hardness increase

n TiN/ZrN multilayers at a bilayer period of 7.5 nm (for coatingseposited in a rotation mode); however not enough information onhe coating microstructure is available in this case. Rizzo et al. [77]nalyzed their TiN/ZrN superlattices by nanoindentation and foundminimum value for the indentation depth at a bilayer period ofnm. Xu et al. [78] found a minimum in hardness and elastic mod-lus in TiN/ZrN superlattices at a bilayer period around 8 nm; foralues of the bilayer period between 15 and 30 nm the hardnessnd elastic modulus were nearly unchanged. They discuss the for-ation of an interlayer of a solid solution of TixZr1−xN under the

eposition conditions applied in their experiments; this interlayerould form new interfaces in the multilayer and clearly impact theechanical properties of the coatings.

.4. Multiphase multilayer thin films

This emerging new approach to hard multilayer coatings byntegration of an amorphous layer material is briefly overlookedere only. An important factor for the development of multiphaserystalline/amorphous multilayers refers to the periodical inter-uption or even complete suppression of the columnar growthn PVD coatings. The columnar grain boundaries often act asites for crack initiation resulting in failure of the coatings. Byntroducing a few nm thin amorphous layers of boron carbide

a-B4C) in TiC/TiN multilayer coatings, Holleck and Schier [5]ould improve the coating properties and performance signifi-antly by forcing the crystalline layers to periodically re-nucleaten the amorphous interface layer. It has to be mentioned that theesearch on nanocrystalline/amorphous multilayer coatings was

3

T

ompounds 483 (2009) 321–333

nd is intensively stimulated by the work of Veprek et al. on super-ard nanocomposite structures in the Ti–Si–N and other systems81,82].

Chen et al. [83] reported on the integration of amorphousiNx layers in TiN/a-SiNx multilayers. Under certain conditions theolumnar growth of the coatings was completely suppressed. Aemarkable hardness enhancement and residual stress reductioncompared to pure TiN coatings) was observed for a thickness of.5 nm of the a-SiNx layer (while the thickness of the TiN layeras fixed at 2.0 nm). Yau et al. [84] observed an increase in theardness of nanocrystalline TiAlN/amorphous Si3N4 multilayerst bilayer periods around 20 nm. A similar behaviour on TiAlN/a-i3N4 nanolaminated coatings was reported recently by Park etl. [85] who identified the thickness of the amorphous layer ashe most important factor to control the coating properties. Max-mum hardness values were found for a 0.3 nm thick amorphousayer at bilayer periods of 3.5 nm. Other material combinationsnot referred to here) intended for the development of hard, wear-esistant multilayer coatings for tribological applications includehe incorporation of amorphous or diamond-like carbon layers orven completely amorphous multilayer films.

. Nanoscale multilayer coatings with epitaxialtabilization effects

Nanoscale multilayer structures composed of materials havingifferent crystal lattice structures (under thermodynamic equi-

ibrium conditions) quite often tend to form coherent interfacesetween the layers through epitaxial stabilization of one material

n the lattice structure of the other one. This means, the thermody-amically unfavourable state of incoherent interfaces is balancedy the growth of a metastable structure for one of the layer mate-ials that is able to build a coherent interface with the other layer.onsequently, the minimisation of the interfacial energy is the driv-

ng force behind the epitaxial stabilization effect. Such metastabletructures are usually observed in extremely fine-scale superlat-ice structures at very low layer thickness and have been reportedor various nitride, carbide and mixed nitride/carbide multilayerystems. Recent developments address the stabilization inducedrystalline growth of layer materials that (under the usually appliedeposition and growth conditions of most PVD processes) wouldrow in an amorphous structure. It has to be mentioned that, apartf the stabilization of a crystal structure in some material systems,he stabilization of a textured orientation can occur as well (i.e. theew layer material would grow in a texture of the other, templateaterial or in another texture of its own structure in order to min-

mise the interfacial energy). Texture stabilization was observedor example in nanoscale TiC/TiB2 multilayers where TiC layersrew in a (1 1 1) texture on the (0 0 1) oriented TiB2 layers [86].n the following the focus is on the structure stabilization in hard

ultilayer coatings. (Note: In a strong physical sense the word-ng epitaxial stabilization might not always be used fully correct;ften poly-crystalline substrate materials are used or the coatingsre poly-crystalline ones with a columnar morphology, but withocal epitaxy within the columns. For the latter reason, the word-ng “epitaxial growth” is well accepted for such coatings in thehin film community as well.) In this section we continue the phe-omenological description of growth effects in multilayer thin filmsccording to the comments made in the introductory paragraph ofhe previous section.

.1. Stabilization effects in nitride multilayer systems

The most prominent example of this class of materials is theiN/AlN system. It has found considerable scientific and technical

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nterest since the first reports on the formation of cubic AlN iniN/AlN superlattice structures were published nearly simulta-eously by Setoyama et al. [26] and Madan et al. [27] one decadego. AlN under thermodynamic equilibrium conditions forms a

urtzite-type structure (hexagonal), while TiN forms a NaCl-typetructure (cubic).

Setoyama et al. [26] reported on the growth of AlN in aetastable NaCl-type structure on Si wafer and cemented carbide

ubstrates for arc ion-plated TiN/AlN superlattice films at modula-ion periods below 3 nm. The coatings showed a strong superlatticeffect, i.e. a significant increase of the hardness with decreasingodulation period. A maximum in the hardness values (about 1.6

imes that of TiN single layer coatings) was obtained at a modula-ion period of 2.5 nm. The microstructural analysis of a film withmodulation period of 3 nm revealed a poly-crystalline superlat-

ice structure with a columnar morphology. The individual columnsere shown to be single crystals with an ordered periodic struc-

ure. In the superlattice with low modulation periods (�= 2.5,nm) TiN and AlN exhibited a single-phase cubic structure. In

uperlattice films with larger modulation period (�= 35 nm) TiNhowed a cubic structure, while AlN grew in a hexagonal structure.ig. 6 displays some of the original results of Setoyama. Madan etl. [27] reported on the epitaxial stabilization of cubic NaCl-typelN in AlN/TiN superlattices grown on MgO (0 0 1) substrates at50 ◦C by magnetron sputtering. Epitaxial stabilization of AlN inhe metastable cubic NaCl-type structure was demonstrated withlN layer thickness below 2 nm; for AlN layer thickness >2 nm,

he thermodynamically stable hexagonal phase was observed forlN.

Since these pioneering works large efforts to understand thetabilization effect [10,87,88] and to further optimize the TiN/AlNuperlattice architecture and properties [89–91] have been madentil today. It is state-of-the-art knowledge that a superlattice effecti.e. hardness enhancement) is related to the stabilization effect inhe TiN/AlN system, and, that the stabilization of AlN in a metastableubic lattice is successful only for low thickness of the AlN layer.ncreasing this thickness of the AlN layer above a critical valueabout 2 nm) results in a loss of the stabilization and superlattice

ffect as the AlN layer transforms to the stable hexagonal phase10]. The critical thickness of the AlN layer is dependent on theum of the Gibbs free energy�GI of the interface of AlN to the tem-late layer (being directly related to the lattice mismatch between

0ine

ig. 6. Epitaxial stabilization of fcc AlN in TiN/AlN multilayer coatings. Original results prmages from a TiN/AlN multilayer with a bilayer period of 30 nm. Dark layers are TiN, ligop, while in the picture on the right is from left to right.

ompounds 483 (2009) 321–333 327

lN and the template layer) and of the Gibbs free energy�GAlN ofhe AlN lattice (which gives a relation between the �G values ofhe cubic NaCl-type or hexagonal Wurtzite-type structure in cor-elation to the thermodynamic conditions such as pressure andemperature, weighted with the AlN layer thickness). For stabi-ization of AlN in the cubic structure �GI,fcc/fcc (interface energyor coherent interfaces between two fcc lattices) is lower than

GI,fcc/hex (interface energy for incoherent interfaces between ancc and an hexagonal lattice). Simultaneously, �GAlN,hex is lowerhan �GAlN,fcc according to the thermodynamic equilibrium con-itions. Similar considerations are described in a model of Barnettnd co-workers [92]. Karimi et al. [93] addressed the correlationetween relative crystallographic orientations of the AlN and TiN

ayers and the formation of coherent structures and their impactn the superlattice effect. For a special architecture of the TiN/AlNuperlattice (i.e. for values of the layer thickness below 10 nm) andell-defined growth conditions, the formation of a strong (1 1 1)

exture of the superlattice could be adjusted, being a prerequisiteor the stabilisation of AlN in the cubic structure and the formationf epitaxially coherent structures.

AlN has also been stabilized in the cubic NaCl-type structure inther multilayer systems like AlN/VN [94,95], AlN/NbN [10], andrN/AlN [96]. In AlN/VN superlattice structures the phase trans-

ormation of AlN from the epitaxially stabilized cubic NaCl-typetructure to the hexagonal Wurtzite-type structure occurred at crit-cal values of the AlN layer thickness above 4 nm [94]. Accordingo Li and co-workers [95], this superlattice system shows both auperlattice effect in hardness variation and a supermodulus effectincrease of the elastic modulus with decreasing bilayer modula-ion period) with maximum values both for the hardness and thelastic modulus achieved at a modulation period of 1 nm. Epitax-al stabilization, sometimes accompanied by superlattice effects,

as observed in a variety of other nitride multilayer systems,or example in TiN/TaN [97–99], TiN/MoN [7], NbN/TaN [100], orn combination with CNx [7] or even oxynitride materials [101].öderberg et al. [102,103] reported the successful epitaxial sta-ilization of cubic silicon nitride in TiN/SiNx superlattice filmst a SiNx layer thickness of 0.3 nm. Increasing this value up to

.8 nm resulted in the transformation of the cubic SiNx phase

nto an amorphous (equilibrium) phase. The superlattices withanostabilization showed a pronounced superlattice hardeningffect.

esented by Setoyama et al. [26]. Bright field (left) and high-resolution (right) TEMht layers are AlN. The growth direction in the picture on the left is from bottom to

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.2. Stabilization effects in carbide, mixed nitride/carbide andther multilayer systems

In contrast to the intensive work carried out on epitaxial growthnd superlattice effects in nitride multilayer systems, only poornformation is available on similar growth effects occurring inarbide (or other non-nitride) multilayer material combinations.his is quite as surprising as materials like SiC or oxides due toheir excellent properties are very interesting candidates for theesign of advanced hard and wear-resistant coatings. In magnetronputtering and other relevant PVD methods these materials underonventionally applied deposition conditions tend to grow in anmorphous structure (there is however evidence, that under severeonditions such as high substrate temperature and extremelytrong ion bombardment applied in parallel in magnetron sputter-ng, crystalline SiC might form [104,105]). The epitaxial stabilizationf SiC in a cubic NaCl-type structure is however a challenging ques-ion in thin film development.

Our group addressed this question as early as in 1999 withultilayer coatings in the TiC/SiC system [106,107]. In the case

f non-reactive magnetron sputtering from ceramic TiC and SiCargets, the SiC layers for all deposition conditions applied (i.e. sub-trate temperature between 300 ◦C and 550 ◦C) grew amorphous,nd, consequently, nanocrystalline TiC/amorphous SiCx multilayerlms were obtained. Decreasing however the modulation periodf this multilayer system down to 4 nm and below resulted in aomplete intermixing of both TiC and SiC and in the formation of aanocrystalline metastable (Ti1−xSix)C coating. Changing the depo-ition method to reactive magnetron sputtering from elementaryi and Si targets at a substrate temperature of 400 ◦C, the growth

f the coatings changed significantly also. Single-layer SiCx thinlms grew in a hexagonal structure when the carbon concentra-

ion X exceeded a critical value, otherwise they grew amorphous.pplying the deposition conditions of crystalline SiC growth while

orming a multilayer coating with TiC resulted at least in three dif-

cvsr[

ig. 7. Nanostabilization and growth effects in reactively magnetron-sputtered TiC/SiC mtrengthened nanocrystalline (Ti1−xSix)C film is formed, while for modulation periods becc TiC-like layers, (Ti1−xSix)C, occurred.

ompounds 483 (2009) 321–333

erent microstructures: For large values of the bilayer modulationeriod (i.e. >23 nm) the multilayer was composed of nanocrys-alline fcc TiC and hexagonal SiCx. For bilayer modulation periodselow 23 nm (and above 4 nm) the hexagonal structure of the pureiCx layer disappeared, and, due to intermixing effects between SiCnd TiC, a Ti-containing solid solution (Si1−yTiy)C in a nanocrys-alline fcc structure grew. The template layer changed as well to

Si-containing solid solution of (Ti1−xSix)C in the fcc structuref TiC. Consequently, this multilayer architecture was consideredo have a epitaxially stabilized structure of a SiC-type layer on aiC-type layer (see Fig. 7). For bilayer modulation periods belownm, a complete intermixing of both these layers was observed,nd the coating microstructure again was characterized to be aanocrystalline metastable solid solution of (Ti1−xSix)C (see Fig. 7).hese results clearly imply that besides thermodynamic consider-tions the kinetic aspects of thin film growth necessarily have toe considered for the design of new PVD coatings with improvedroperties.

Recently, Li and co-workers [108,109] demonstrated a truepitaxial stabilization of SiC in a cubic NaCl-type structure iniN/SiC superlattice coatings. Stoichiometric TiN and SiC filmsere deposited by magnetron sputtering from ceramic compound

argets without additional substrate heating or bias-induced ionombardment in order to avoid interdiffusion between the lay-rs. Depending on the nanoscale thin film architecture (i.e. for ahickness of the SiC layer below 0.8 nm and a thickness of the TiNayer of 4.3 nm) a well-defined compositionally modulated struc-ure and a poly-crystalline fcc microstructure were identified. Theattice fringes grew across the individual TiN and SiC layers support-ng the mutual epitaxial growth of SiC and TiN (see Fig. 8). For these

oatings a remarkable superlattice hardening effect with maximumalues of the hardness in the range of 60 GPa was observed. Similartructure stabilization including a superlattice hardening effect waseported by the same group to occur in TiN/SiO2 multilayer coatings110]. To complete this picture, it should be mentioned, that already

ultilayer coatings. For a bilayer modulation period below 4 nm, a solid-solutiontween 4 nm and 23 nm, epitaxial stabilization of fcc SiC-like layers, (Si1−yTiy)C, on

M. Stueber et al. / Journal of Alloys and C

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108,109].

n 1996 Sproul had claimed for the formation of hard oxide superlat-ice coatings [111], a subject which may find much more attentionn the coming years due to significant progress made in the PVDeposition technology (but being already an intensively develop-

ng research field in microelectronics: just take the developmentf perovskite-type ABO3 oxides with A, B being metals such as Sr,i, La, and their superlattice structures as an impressive example112,113]).

. New approaches in the design of hard and wear-resistantanoscale multilayer films

In this section we will briefly describe some new approachesowards the engineering design of advanced hard and wear-esistant nanoscale multilayer coatings. The concepts presented aref course selectively chosen and do not provide for an evaluation ofheir expected value and applicability. Generally, two philosophiesill be regarded: the integration of new materials with advanced

tructural and functional properties into a multilayer compound,nd, the objective of achieving benefits in the multilayer prop-rties from extremely fine-scale ordering processes occurring inuch novel multilayer systems. Two examples of innovative mul-ilayer designs, based on the integration of advanced metastableayer materials and new nanocomposite thin films are suggested.

.1. Integration of advanced hard metastable materials in

ultilayer coatings

The issue of multilayer coatings containing metastable solidolutions was already discussed in the previous sections of thisaper. The most prominent examples having achieved industrial

cidsA

ompounds 483 (2009) 321–333 329

elevance are related to the incorporation of a metastable fccTi1−xAlx)N layer in a multilayer system. With regard to thexceptional properties of these coatings it is quite reasonable tonvestigate if similar coatings showing even superior propertiesan be designed.

Here results of our group on the formation of new metastablehases with promising mechanical properties in the material sys-em V–Al–C–N are presented. The rationale behind the formation of

etastable phases in PVD thin films was described by Holleck [114].hermodynamic considerations and calculations following thisationale led to the conclusion that metastable phases should formetween VC and AlN, and, that a material with a covalent bondingharacter (AlN) should be able to grow in a metal-like structure (asf VC) if the kinetics of the deposition would be adjusted properly.ne key parameter determining the growth of PVD coatings is theiffusion length of atoms within the forming film. This parameteran be tuned by controlling for example the substrate tempera-ure, the substrate bias induced ion energy deposited to the coating,nd the fluxes of the film forming particles. For a given composi-ion in the V–Al–C–N system different coating microstructures cane designed in dependence of the diffusion length, ranging fromompletely amorphous structures to metastable mixed crystals andwo-phase nanocomposites. V–Al–C–N coatings were deposited byon-reactive r.f. magnetron sputtering from an isostatically hot-ressed compound target of a previously calculated compositionf 60 mol% VC and 40 mol% AlN with a diameter of 75 mm in aeybold Z 550 unit (in the following referred to as VC/AlN 60/40).ubstrate materials used were Silicon (1 0 0) and cemented car-ide plates. The r.f. target power was varied between 5.65 and1.3 W/cm2, the substrate temperature was 150 ◦C or 220 ◦C, respec-ively, and the substrate bias was varied between −25 and −200 V.nder certain deposition and growth conditions (i.e. substrate

emperature: 220 ◦C, substrate bias: −175 V), fcc metastable solid-olution strengthened (V0.3Al0.2)(C0.3N0.2) coatings were depositedthe elemental composition of the films was determined by electron

icroprobe analyses). Fig. 9 displays the results of the transmis-ion electron microscopy. The diffraction pattern is clearly relatedo a cubic face-centered poly-crystalline microstructure, while theright field and dark field images show a dense, fine-scale columnarorphology of the coatings. These metastable coatings showed a

trong solid-solution strengthening effect: their Vickers microhard-ess was measured to be about 3200 HV0.05 which is significantlybove the values determined for pure VC and AlN thin films whichave been deposited under the same conditions as well (Vickersicrohardness of VC: 2300 HV0.05, and of AlN: 1200 HV0.05). These

oatings have not yet been integrated into multilayer arrange-ents, but seem to have some potential for the design of multilayer

oatings with superior properties if compared to their constituentquilibrium phases.

The formation of such metastable phases can occur in manyaterial systems. Again, the most intensively investigated sys-

em in this regard is the Ti–Al–N system due to the formationf the (Ti1−xAlx)N thin film materials with outstanding mechan-

cal properties and thermal stability. While these propertiesere early recognized and are widely technically exploited today

115–118], only recently Mayrhofer et al. explained the basicechanism behind the high hardness and high thermal stabil-

ty of these coatings [119–121]. For an AlN mole fraction X below0.7, the (Ti1−xAlx)N coatings crystallize in a supersaturated cubicaCl-type structure where Al substitutes for Ti. The metastable

ubic (Ti1−xAlx)N structure decomposes during annealing form-ng extremely fine-scale precipitates of cubic TiN and cubic AlNomains, before phase transforming into the thermodynamicallytable constituents (i.e. cubic NaCl-type TiN and ZnS-Wurtzite-typelN). The formation of the cubic TiN and AlN precipitates, building

330 M. Stueber et al. / Journal of Alloys and Compounds 483 (2009) 321–333

F le fcca bic N

chdthhstmpsfi

4m

ssn

Fidao(

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ig. 9. Transmission electron microscopy analysis of magnetron-sputtered metastabfine-scale columnar morphology. The diffraction patterns are characteristic of a cu

oherent interfaces with the cubic (Ti1−xAlx)N matrix, results in aardness enhancement by providing additional obstacles for theislocation movement (see Fig. 10). Mayrhofer et al. found suchhermally induced self-organisation effects resulting in temporaryardness increase as well in metastable Ti–B–N thin films, basedere on segregation processes [122,123]. These examples clearlyhow the potential for optimization of properties in thin films dueo nanoscale ordering effects. The thermal stability of metastable

aterials and the effective design of new nanolaminated com-osite coatings (i.e. multilayer coatings with extremely fine-scaletructural ordering) are crucial issues of future research in thiseld.

.2. Integration of multiphase nanocomposite materials inultilayer coatings

While nanocomposite materials are in the focus of materialscience in nearly all scientific and engineering disciplines [124],pecific multiphase nanocomposite materials composed both ofanocrystalline and amorphous phases still are an emerging but

ig. 10. Hardening effects in metastable thin film materials due to structural order-ng through the formation of temporary coherent nanoscale precipitation domainsuring thermally induced decomposition of supersaturated fcc (Ti1−xAlx)N coatingsccording to Mayrhofer et al. [119]. The diagram shows the hardness H as a functionf the isothermal annealing temperature Ta and dynamical DSC measurements ofTi0.34Al0.66)N and TiN films.

tanoamccibtciaiarnirCwClmvit

(V0.3Al0.2)(C0.3N0.2) coatings. Bright field (left) and dark field (right) images indicateaCl-type structure.

romising issue. We have developed carbon based (Ti, Al)(N, C)/a-Canocomposite thin films combining nanocrystalline fcc titaniumluminium carbonitride and amorphous carbon phases in one layertructure. These coatings were synthesized for example by reac-ive magnetron sputtering from a TiAl 50:50 alloy target [125–127].eeping all deposition parameters fixed at constant values (i.e.he cathode power, the substrate temperature, the substrate bias,nd the total gas pressure; the nitrogen gas flow was fixed first athe optimum condition required for the deposition of the fcc (Ti,l)N phase) and varying only the methane gas flow, we observed

he following thin film microstructures in dependence of the car-on concentration: For a low carbon concentration (below 8 at.%),single-phase fcc metastable solid solution of (Ti, Al)(N, C) was

dentified. At medium C content (8 at.% < C < 16.5 at.%), isolated car-on nanoclusters at grain boundaries of the (Ti, Al)(N, C) phaseere observed. At higher carbon concentration (above 16.5 at.%)

he (Ti, Al)(N, C) nanocrystals were found to be completely sep-rated from each other by an amorphous carbon grain boundaryetwork or matrix phase (see Fig. 11). A five-step growth modelf these coatings was suggested [127]. Disregarding that issuesddressing the growth of such nanocomposite coatings requireore detailed experimental and theoretical research work, one can

learly state that the specific design of such coatings by completelyovering nanocrystals with an amorphous phase (which means byntroducing new phase boundaries) might be a tool to optimizeoth hardness and toughness in a coating by generating an elas-ic coupling between hard nanocrystals via sp2- or sp3-hybridizedarbon atoms. It was shown that both the mechanical and tribolog-cal properties (i.e. the Vickers microhardness, the elastic modulus,nd the wear rates and friction coefficients in unlubricated slid-ng pin-on-disk experiments against steel counterpart materials)re a function of the carbon concentration, and could clearly beelated to the microstructure of the coatings. Integration of suchanocomposite materials into multilayer systems can result in very

nteresting material properties, as was demonstrated by our groupecently for nanolaminated composite coatings of TiN and (Ti, Al)(N,)/a-C layers. Coatings with various bilayer modulation periods,ith a modified ratio of the thickness of both the TiN and (Ti, Al)(N,

) layer, and with optimized carbon concentration showed an excel-

ent performance in wear studies and cutting tests on ramp copy

illing of hardened A2 cold work steel, comparable to that of con-entional (Ti, Al)N-based benchmark coatings [128]. These resultsndicate the technological potential of nanocomposite-based mul-ilayer thin films.

M. Stueber et al. / Journal of Alloys and Compounds 483 (2009) 321–333 331

F tingse

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ig. 11. Microstructure of magnetron-sputtered (Ti, Al)(N, C)/a-C nanocomposite coalectron microscopy analyses).

. Concluding remarks

The intensive work on nanoscale multilayer coatings showingutstanding properties has been reviewed and new suggestions forhe development of advanced coatings are made in this paper. Itas shown that an extremely fine-scale structural ordering at theanoscale is a prerequisite for the engineering design of new mul-ifunctional hard and tough coatings. These new nanoscale designrinciples include the formation of superlattice structures, epitax-

al growth, and stabilization effects [7], age hardening, recovery andecrystallization processes [129] as well as interfacial engineering5] or residual stress management [14].

Considering the great variety of experimental results availableoday on hard multilayer coatings, a systematic approach towardshe understanding of the role of special materials, growth condi-ions, microstructure and resulting properties is required. While aood database exists for nitride multilayer systems, only selectivenformation is found for other material combinations. This databasehould not only address parameters being relevant for the nucle-tion and growth of the materials (i.e. plasma conditions, kineticsf the deposition process) but should also refer to thermodynamicspects (i.e. phase relations, energetic states of interfaces, grain andhase boundaries).

The elastic and plastic properties of multilayer coatings (orheir hardness and toughness) today are critically discussed ver-us dislocation motion and various effects promoting or impedingislocation motion [4,14]. It is generally accepted that a nanoscalerain growth and coherent layer interfaces (as well as a reduc-ion of a columnar morphology on the microscale) can result in

significant hardness enhancement. Taking into account how-ver the suggested limitation of the applicability of the Hall–Petchelation [130,131] and the fact, that many hard materials inte-rated in multilayer films have strong covalent bonds, aspectselated to the deformation of ceramic materials (i.e. crack initiation,rack propagation, reduction of crack energies) and mechanisms ofoughening of ceramics should be considered seriously in theoret-

cal modeling [8,132,133]. Correlations between elastic properties,racture toughness, residual stress and macroscopic thin film prop-rties (i.e. interactions between stress fields and dislocations orracks) require more detailed research work in order to under-tand the deformation mechanisms in nanostructured coatings.

fimirs

as a function of the carbon concentration (schematically and results of transmission

he identification and modeling of stress states and stress dis-ribution profiles in hard multilayer coatings and their impactn the micro- and macroscopic properties are of seminal impor-ance in this field. Stress generation and interaction should beonsidered across various scales, considering especially grain andhase boundaries as well as artificial layer interfaces. Thus, models

ike the periodic stress field distribution suggested by Mendidibet al. [134] for nanoscale TiN/CrN multilayer coatings or the tri-xial stress field found by Abadias and Tse [135] for texturediN films could be derived, verified and refined. These objec-ives demand substantial progress on the analytical side as well,nd, advanced characterization methods mostly combined withomplex evaluation and interpretation of the results are needed.nalytical methods that should provide detailed insight into theomposition/structure–property relationships of nanoscale thinlms include advanced highly sophisticated transmission elec-

ron microscopy (i.e. energy-filtered TEM), X-ray diffraction (i.e.he sin2 -method) and further high-resolution X-ray scattering

ethods. Nanoindentation analysis also needs special attentionith regard of their interpretation (i.e. modeling of the inter-

ction of stress fields induced under the indenter tip and thexisting stress field of the coating) and of improving of the exper-mental procedure. Recently, Ziebert et al. [136] proposed themall-angle-cross-section (SACS) method offering a new tool forhigh-resolution depth profiling of the mechanical properties on

he nanoscale in multilayer coatings in order to obtain informationspecially on the interface regions.

Material concepts for the development of multilayer coatingsith advanced multifunctional properties address for example

he integration of new nanocrystalline, metastable, amorphous oranocomposite layers and combinations of those. Superimposinganoscale gradients in the chemical composition, growth of theoatings with gradients in the substrate temperature, or inter-acial engineering by modulation of the substrate bias inducedntermixing or stress relaxation at layer interfaces may result as

ell in an enhancement of physical properties of multilayer thin

lms. Future work may address phase-transformation-reinforcedultilayer systems (i.e. obtaining beneficial effects on the mechan-

cal properties by applying high-temperature or stress activatedeversible phase transformations in one layer) or the design ofmart nanolaminated composite systems, i.e. by incorporating

3 s and C

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[

32 M. Stueber et al. / Journal of Alloy

unctional layer materials such as ferromagnetic Fe–Co–Ta–N films,hich could be used as contactless sensing elements indicating

n-situ wear-related effects on the coating microstructure.

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