Surface and Coatings Technology 167(2003) 113–119

0257-8972/03/$ - see front matter� 2002 Elsevier Science B.V. All rights reserved.doi:10.1016/S0257-8972(02)00903-9

Review

Recent advances of superhard nanocomposite coatings: a review

Sam Zhang*, Deen Sun, Yongqing Fu, Hejun Du

School of Mechanical and Production Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore

Abstract

In this paper, a review of the present status of the research and technological development in the field of superhardnanocomposite coatings is attempted. Various deposition techniques have been used to prepare nanocomposite coatings. Amongthem, reactive magnetron sputtering is most commonly used. Nanocomposite coating design methodology and synthesis aredescribed with emphasis on the magnetron sputtering deposition technique. Also discussed are the hardness and fracture toughnessmeasurements of the coatings and the size effect. Superhard nanocomposite thin films are obtainable through optimal design ofmicrostructure. So far, much attention is paid to increasing hardness, but not enough to toughness. The development of nextgeneration superhard coatings should base on appropriate material design to achieve high hardness and at the same time hightoughness.� 2002 Elsevier Science B.V. All rights reserved.

Keywords: Superhard; Nanocomposite; Coatings; Fracture toughness

1. Introduction

Nanostructured coatings have recently attractedincreasing interest because of the possibilities of synthe-sizing materials with unique physical–chemical pro-perties w1–4x. Highly sophisticated surface relatedproperties, such as optical, magnetic, electronic, catalyt-ic, mechanical, chemical and tribological properties canbe obtained by advanced nanostructured coatings, mak-ing them attractive for industrial applications in high-speed machiningw5,6x, tooling w6,7x optical applicationsw8,9x and magnetic storage devicesw10–13x because oftheir special mechanical, electronic, magnetic and opticalproperties due to size effect. There are many types ofdesign models for nanostructured coatings, such asnanocomposite coatings, nano-scale multilayer coating,superlattice coating, nano-graded coatings, etc. Design-ing of nanostructured coatings needs consideration ofmany factors, e.g. the interface volume, crystallite size,single layer thickness, surface and interfacial energy,texture, epitaxial stress and strain, etc., all of whichdepend significantly on materials selection, depositionmethods and process parametersw14–16x.

*Corresponding author. Tel.:q65-6790-4400; fax:q65-6791-1859.

E-mail address: msyzhang@ntu.edu.sg(S. Zhang).

A nanocomposite coating comprises of at least twophases: a nanocrystalline phase and an amorphous phase,or two nanocrystalline phases. Nanocomposite coatingsexhibit hardness significantly exceeding that given bythe rule of mixture. Hard materials usually refer tomaterials with hardness greater than 20 GPa. Materialswith hardness above 40 GPa are classified as superhard,and those with hardness above 80 GPa are often calledultra-hard materials. Nanocomposite coatings can behard, superhard or even ultra-hard, depending on coatingdesign and application. Extensive theoretical and exper-imental efforts have been made to synthesize and studythese nanocomposite coatings with superhardness andhigh toughness.This paper reviews the present status of the research

and technological development in the field of superhardnanocomposite coatings covering design concepts andpreparation methods with emphasis on magnetron sput-tering. Coating hardness and fracture toughness meas-urements are also discussed.

2. Design methodology for nanocomposite coating

In bulk materials, grain boundary hardening is one ofthe possibilities for hardness improvement. The same istrue for films or coatings. With a decrease in grain size,

114 S. Zhang et al. / Surface and Coatings Technology 167 (2003) 113–119

Fig. 1. Hardness of a material as a function of the grain sizew17x.

the multiplication and mobility of the dislocations arehindered, and the hardness of materials increases accord-ing to the ‘Hall–Petch’ relationship:H(d)sH q0Kd . This effect is especially prominent for grainy1y2

size down to tens of nanometers(cf., Fig. 1). However,dislocation movement, which determines the hardnessand strength in bulk materials, has little effect when thegrain size is less than approximately 10 nm. At thisgrain size, further reduction in grain size brings about adecrease in strength because of grain boundary sliding.Softening caused by grain boundary sliding is mainlyattributed to large amount of defects in grain boundaries,which allow fast diffusion of atoms and vacancies understress w17,18x. As such, further increase in hardnessrequires hindering of grain boundary sliding. This canbe realized through proper microstructural design, i.e.by increasing the complexity and strength of grainboundariesw19x. Since different crystalline phases oftenexhibit different sliding systems and provide complexboundary to accommodate a coherent strain thus pre-venting formation of voids or flawsw20x, multiphasestructures are expected to have interfaces with highcohesive strength. Apart from hardness, good mechani-cal properties also include high fracture toughness. It isquite straightforward to think that a high fracture tough-ness can be obtained in the nanocomposite coatings dueto the nano-size grain structure as well as deflection,meandering and termination of nanocracksw21x. How-ever, in order to obtain superhardness, usually plasticdeformation is strongly prohibited, and dislocationmovement and grain boundary sliding are prevented,thus probably causing a loss in ductility. Today, moreand more researchers realize that a certain degree ofgrain boundary diffusion and grain boundary sliding arenecessary in order to improve toughness of nanocom-posite coatings. The following few types of designmethods have been put forth for superhard high tough-ness nanocomposite.

First, a combination of two or more nanocrystallinephases provides complex boundaries to accommodatecoherent strain, which result in the increase of coatinghardness. In this case, the phases involved must show awide miscibility in solid state, display thermodynami-cally driven segregation during deposition, and havecertain chemical affinity to each other to strengthen thegrain boundariesw19,20x. Successful examples includeTiN–TiB , Ti–B–N, (Ti, Si, Al)N, as well as other2

metal–nitride and carbideyboride systemsw15,22–24x.Second, segregation of nanocrystalline phases to grainboundaries of the first phase to generate the grainboundaries strengthening effect, and stops grain growthw1,25,26x. Though this composite design could signifi-cantly increases hardness and elastic modulus, it maynot be able to improve toughnessw27x because disloca-tion movement is prohibited and crack opening becomesthe main mechanism of relieving strain. If the cohesivestrength of the interface is not sufficient to withstandthe local tensile stress at the crack tip, unstable crackpropagation and debonding of the nanocrystals occur.To address this problem, Miterer et al.w24x formed

hard nanocrystalline phases within a metal matrix, suchas TiN in Ni w28x, ZrN in Ni w14x, Zr–Y–N w29x, ZrNin Cu w30,31x, CrN in Cu w32x. The hardness of thesecoating systems varied from 35 GPa to approximately60 GPa. In these coatings, one metal may be convertedinto nitride in the form of hard nanocrystalline phaseand the other transported into the growing film un-reacted. High hardness could be obtained for compoundsshowing a wide miscibility gap in the solid state but acertain chemical affinity to each other to form highstrength grain boundaries. In this case, both the dislo-cation mechanism and the grain boundary mechanismcontribute to the hardnessw33–35x, while the existenceof a metal matrix improves toughness. However, therecould be poor toughness and thermal stability problemsw27x. Dislocation movement initiated from grain bound-aries may be prohibited because of the small separationof grains in the metal matrix. This gives rise to poortoughness. Diamond-like carbon(DLC) based or metalmatrix nanocomposite coatings may undergo structuralchange at 300–5008C. This is the thermal stabilityissue. Thermal stability of nanocomposite coatings isyet another important property in mechanical applica-tions. Hardness decreases upon annealing due to relax-ation of compressive stress built up during deposition,and due to rapid diffusion at higher temperaturesw36x.Grain boundary sliding depends on temperature, thusthe mechanisms of preventing grain boundary sliding inroom temperature may not be useful at elevated temper-atures. One way of improving thermal stability of thecoating is to include elements of high thermal stability,such as yttrium, in the coating. Another way is tomodify the interface complexity, such as using a ternary

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Fig. 2. Hard materials for nanocomposite coatings in the bond triangleand changes in properties with the change in chemical bondingw39x.

Fig. 3. TEM micrograph of TiCrCN nanocomposite coating showingnano-sized TiCrCN crystals embedded into the amorphous DLCmatrix w37x.

system that displays immiscibility and undergo spinodaldecomposition and segregation at high temperaturesw5x.Another way is to embed nanocrystalline phases in

an amorphous phase matrixw15,37,38x. DLC, amorphouscarbon nitride or other hard amorphous materials(withhigh hardness and elastic modulus) have been recog-nized as the primary candidates for the amorphousmatrix while nano-sized refractory nitrides, such as TiN,Si N , AlN, BN, etc., used as strengthening phases. A3 4

variety of hard materials can be used in nanocompositecoating design. Fig. 2 shows potential hard materialsw39x. Nanocrystalline TiC has been embedded in DLCmatrix to produce a nanocomposite of hardness of 32GPa w40,41x. Veprek et al.w42,43x imbedded 4–11 nmTiN crystals in amorphous Si N matrix and obtained a3 4

coating hardness of 50–70 GPa. Recently, Zhang et al.w37x prepared TiCrCN nanocomposite coatings withhardness of 40 GPa, in which 8–15 nm TiCrCN crystalswere embedded in an amorphous DLC matrix as shownin Fig. 3. In this design, the size, volume percentageand distribution of the nanocrystals need to be optimizedin order to obtain a compromise between superhardnessand toughness. The distance between two nanocrystalsshould be within a few nanometers. Too large a distancewill easily cause a crack to propagate in matrix, whiletoo close to each other will cause the interaction ofatomic planes in the adjacent nanocrystalline grains. Thenanocrystalline grains should have random orientation(i.e. high angle grain boundaries) to minimize incoher-ent strain and facilitate many nanocrystalline grains toslide in amorphous matrix to release strain and obtainhigh toughness. The amorphous phase must possess highstructural flexibility in order to accommodate coherentstrains without forming dangling bonds, voids, or otherdefects. The presence of amorphous phase on the bound-aries helps to deflect and terminate nanocracks in addi-

tion to the enhancement of grain boundary sliding, thusimproving coating toughness.To design a nanocomposite coating with both high

hardness and high toughness, one must take all theabove into consideration. Probably the best way is touse ternary, quaternary or even more complex systems,with high strength amorphous phase as matrix(such asa-SiN , a-BN, a-C, etc.) and hard transition metal-nitridex

nanocrystals(such as TiN, W N, BN, etc.) as nanocrys-2

talline phase to increase grain boundary complexity andstrength. These nanocrystalline phases should be refrac-tory and immiscible with each other, and could result incompositional modulation, segregation and high thermalstability of the nanostructure. The aim is to maximizeinterfaces and form well-defined spinodal structure atinterfaces. The structure should be stable at a tempera-ture of 10008C or above. Of course, there is tediouswork needed for the selection of different materials,optimization of grain size, volume percentage and dis-tribution of these nanocrystals in the amorphous struc-tures thus obtaining both superhardness and hightoughnessw27x. Following this idea, Veprekw25,38xproduced nc-TiNya-Si N ya- and nc-TiSi nanocompos-3 4 2

ite coating with unbelievable hardness of 105 GPaw16x,and the coating retains the hardness after annealing to900–10008C w44x.

3. Synthesis methods

Different techniques are now available for preparationof nanocomposite coatings. The most promising methodsare magnetron sputtering and chemical vapor deposition(CVD) w5,38,44,45x, although other methods, such aslaser ablationw46x, thermal evaporationw47x, ion beamdepositionw48x and ion implantationw49x, are also used

116 S. Zhang et al. / Surface and Coatings Technology 167 (2003) 113–119

by various researchers. High deposition rate and uniformdeposition for complicated geometries are the advantag-es of CVD method compared to sputtering. However,the main concern for CVD method is that the precursorgases TiCl , SiCl or SiH may pose problems in4 4 4

production because of their corrosive nature and dangerof fire hazard. Moreover, the incorporation of chloridein protective films deposited from plasma CVD mayinduce interface corrosion problems during exposure toelevated temperatures under working condition. Formost application, a low deposition temperature isrequired to prevent substrate distortion and loss ofmechanical properties. This is difficult to realize in CVDprocesses.Magnetron sputtering and pulsed-laser deposition

were combined to fabricate superhard and tough nano-composite coatingsw40x. In general, the choice ofspecific deposition technique is made on considerationof coating application, coating thickness, desired coatingproperties, cost and production output available fromthe process, and temperature limitation of the substrate,etc. At present, significant effort is devoted to preparenanocomposite coatings using reactive magnetron sput-tering since this technology is a low-temperature and afar less dangerous method compared with CVD. Also,it can easily scale up for industrial application. Co-sputtering of single element targets allows independentregulation of each source by changing the power densitythus enables adjustment of chemical stoichiometry ofthe resultant compound. A wide range of hardysuperhardnanocomposite coatings has been synthesized usingsputtering methodw50–53x.Precision control and determination of grain size is

important for nanocomposite coatings. In magnetronsputtering process, many basic process parameters affectthe grain size of the coatings including substrate tem-perature, substrate ion current density, bias voltage,partial pressure of reactive gas(e.g. nitrogen for nitrides)and post-annealing temperature. Magnetron sputteringcan operate at low temperatures to deposit films withcontrolled texture and crystallite size. A minimum tem-perature is required to promote growth of crystallinephase to the required diameter andyor to allow asufficient diffusion within the segregating system. Atemperature ceiling exists, however, to prevent signifi-cant grain growth and grain boundary segregation. Opti-mized target power density and bias voltage are alsoneeded. Low-energy ion bombardment is preferred inthe synthesis of nanocomposite coatings. The kineticenergy of bombarding atomsyions is transferred intosmall areas of atomic dimensions and quickly convertedinto their close vicinity, causing extremely fast coolingprocess and produce dense films. The crystallite sizegenerally increases with increase in substrate biasw32x.In the transition metal-nitride coatings, such as Ni–Cr–N and Zr–Y–N systems, partial pressure of nitrogen

has a great effect on crystallite size in the coatingw29,53x. The concentration of the ‘alloying element’,such as the Cu concentration in TiN–Cu, Si concentra-tion in nc-TiNya-Si N , Si N fraction in the nc-TiBya-3 4 3 4

Si N and nc-W NySi N also has significant effect on3 4 2 3 4

the size of the crystallites in the coatingsw21,54,55x.

4. Evaluation of mechanical properties

Good mechanical properties of a coating require highhardness, high toughness, low friction, high adhesionstrength on substrate, good load support capability andchemical and thermal stability, etc. Of all these, hardnessis probably of number one importance for an industrialcoatings especially in tribological applications. At pres-ent, nanoindentation is regarded as a good method inhardness determination of thin films and coatings. Innanoindentation test, a diamond indenter is forced intothe coating surface. The load and depth of penetration(the indentation profile) is recorded, from which thehardness and elastic properties are calculated. However,calculation methodology varies: the Oliver and Pharrmethodw56–58x; the deformation energy methodw59–61x; the force indentation function methodw36,62,63x,the Joslin–Oliver methodw64,65x and the energy densitymethod w66x, etc. Usually, the hardness of the samecoating varies depending on evaluation method. There-fore, it is important that the calculation method ispresented when presenting hardness data. Hardness of acoating is also affected by residual stress built up duringdeposition, usually a few GPa for superhard coatings.Such a large stress could cause significant exaggerationof hardness readingw42,67x. Veprek and Argonw6xsuggested that the real hardness values are either meas-ured with a low stress(-1 GPa) in coating or measuredafter stress-relief annealing above 400–5008C. Forcoatings based on DLC, however, annealing at such atemperature produces structural changes, thus is notsuitable.Nanoindentation is now a routine in hardness testing,

but there is not yet a standard method for quantitativeevaluation of fracture toughness of a coating. Fracturetoughness measures the ability of a material to resist thegrowth of a pre-existing crack or flaw. At present, mostresearchers use ultra-low load indentation to evaluatefracture toughnessw58,63,68,69x. After the indentation,when no cracking occurs, the coating is said to havegood toughness. However, quantitative descriptionrequires measurement of crack length, which is relativelyeasier in thick coatings(a few micron meters) butextremely difficult in thin films even under scanningelectron microscopew70x. This method also depends onthe type of the indenter used. If the indenter is not sharpenough(like the Vickers Indenter widely used), crackingdoes not occur even under relatively high load. But thatdoes not mean the coating is really tough because the

117S. Zhang et al. / Surface and Coatings Technology 167 (2003) 113–119

Fig. 4. Schematic of various stages in nanoindentation fracture for the coatingsysubstrate systemsw72x.

indentation stress resulted does not reach the crackingthreshold. This happens in most brittle materialsw71x.Another method is based on energy release in through-

thickness cracking in a coating obtained from a stepobserved in the indentation profilew72x. The fractureprocess progresses can be viewed as occurs three stagesdepicted in Fig. 4: in stage 1, the first ring-like through-thickness crack form in the contact area around theindenter; in stage 2, delamination and buckling occuraround the contact area at the coating–substrate interfacedue to high lateral pressure; in stage 3, the second ring-like through-thickness crack and spallation are generatedby high bending stresses at the edges of the buckledcoating. The area under the indentation profile is thework done by the indenter during elastic–plastic defor-mation of the coatingysubstrate system. The strain ener-gy release in the firstysecond ring-like cracking andspallation can be calculated from the correspondingsteps in the loading curve. Fig. 5 illustrates the inden-tation profile in such a process. In Fig. 5, OACD is the

loading curve and DE is the unloading curve. Theenergy difference before and after the crack generationis the area ABC. This energy will be released as strainenergy to create the ring-like through-thickness crack.The fracture toughness of an ultra-thin film can bewritten as:

1y2w zE UK s (1)x |IC 21yn 2pC ty ~Ž . R

whereE is the elastic modulus,n is the Poisson’s ratio,2pC is the crack length in the coating plane,t is theR

coating thickness, andU the strain energy differencebefore and after cracking. This method was used toassess the fracture toughness of thin amorphous carbonthin films w70,71x.Adhesion of coating to the substrate is always another

big concern for industrial coatings. Scratch adhesion testis commonly used to evaluate the coating adhesionstrength. However, actually this only reveals load bear-

118 S. Zhang et al. / Surface and Coatings Technology 167 (2003) 113–119

Fig. 5. Schematic diagram of a load–displacement curve, showing astep during the loading cycle and associated energy releasew72x.

ing capacity of the coating rather than the true adhesionbetween the coating and the substrate, since there aremany factors that influence the results, including sub-strate hardness, coating surface roughness, coating hard-ness, coating thickness, loading rate, indenter radius,and friction between the coating and the indenter.Adding a functionally graded layerw73x or a bondinglayer w74x in between the coating and the substrate aretwo important ways to improve coating adhesion. Thefunctions of this interlayer include(1) facilitating goodbonding, (2) relaxation and modification of the stressdistribution, (3) providing a supporting layer,(4)imparting better chemical stability to substrate,(5)increasing the hardening depth.

5. Summary

Superhard nanocomposite thin films are at the begin-ning of their development, and appear to have a widerange of applications. Various deposition techniqueshave been used to prepare nanocomposite coatings.Among them, reactive magnetron sputtering is mostcommonly used. So far, much attention is paid toincreasing hardness, but not enough on toughness.Superhard nanocomposite thin films are obtainablethrough optimal design of microstructure. It is clear thatthe development of next generation superhard coatingsshould base on appropriate material design for both highhardness and high toughness. In this nanocompositefield, many issues exist that need vigorous theoreticaltreatment and experimental verification. These include,but not limited to, effect of the change of the latticeparameter, the role of the crystallite size, the nature ofthe grain boundaries, the effects of impurities, multipha-

se and nanostructure, the formation mechanism and thereasons behind thermal stability. Until then the strength-ening mechanism of nanocomposite coatings will bebetter understood and excellent coatings be engineeredand fabricated.

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