Structure, Mechanical and Electrical Properties of CN Coatings … · 2006-09-28 · Modification...

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Modification of material properties 318 Abstract – CN x films 1–3 μm thick were deposited on metal substrates by pulsed arc sputtering of graphite in a nitrogen containing atmosphere at P=10 –2 –5 Pa. The nitrogen con centration of the coatings was determined by the method of nuclear reactions 14 N(d, α 0 ) 12 C and 14 N(d, α 1 ) 12 C. The coating microhardness H V and internal stresses σ 0 were measured us ing methods proposed by the authors. The initial (f 0 ) and sta tionary (f st ) friction coefficients were determined during the reciprocal motion of a WCCo ball. The surface morphology was examined by methods of scanning and tunnel electron microscopy (SEM and STM). It was found that the hardness and internal stresses sharply decreased in CN x coatings at x~(0.25–0.35). Coatings with these compositions were suf ficiently hard (H V ~50–60 GPa), but they were stressed less than nitrogenfree coatings ( σ 0 ~10–12 GPa at x = 0). Given the same compositions, f st ~f 0 ~0.10–0.05. The STM exami nation showed that nitrogencontaining coatings had a mar ked columnar structure. The dependence of the mechanical properties on the nitrogen concentration and the electrical resistance minimum, which was revealed at x~0.3, were in terpreted well in terms of the percolation theory if the CN x condensate was conceived of as a mixture of diamondlike (sp 3 ) CN x0.1 domains and C 2 N domains with sp 2 bonds. At x~0.3 the composition corresponded to equal fractions of these components, i.e. had an extremely developed interface between unlike structural elements and was characterized by a continuous cluster of each of the elements. 1 Introduction Hard diamondlike (~80 % sp 3 bonds) coatings (DLCs) possess an excellent set of properties for mo dification of the surface of tools and friction parts so as to extend their lifetime and expand the fields of their practical application. DLCs have the hardness of about 100 GPa and the stationary friction coeffici ent of not over 0.1 [1]. However, large internal stres ses (~10 GPa) weaken their adhesion to the substra te. This factor limits the lifetime of DLCs in real operating conditions primarily because of peeling. Furthermore, surface roughness of DLCs is respon sible for large (f 0 ~0.5) initial friction coefficients [2]. Since the hardness and the wear resistance of DLCs are high, the initial runningin time is longer, the surface of parts in mechanical contact with DLCs deteriorates, unwanted heat is released at the contact point, etc. These circumstances have stimulated the search for methods providing modification of DLCs and eliminating the factors that limit their applications. In doing so, hardness may be somewhat sacrificed to clear other faults. It was this practical task that we posed ourselves in the present study. We also inten ded to clarify how the mechanical and tribological properties of condensates, which were prepared by arc sputtering of graphite in a nitrogendoped vacu um, depended on the condensate structure. 2. Methods CN x films 1–3 μm thick were deposited on metal substrates by pulsed arc sputtering of graphite in a ni trogencontaining atmosphere at P=10 –2 –5 Pa in a UVNIIPA001 installation. The microhardness of the coating material H f and internal stresses σ 0 were measured using methods proposed by the authors [3]. The initial (f 0 ) and stationary (f st ) friction coeffi cients were determined during the reciprocal motion of a WCCo ball under a load of 2 N. The surface morphology was examined in a Philips 515 scanning electron microscope at the Institute of Metal Physics and a SMM2000T scanning tunnel microscope. The nitrogen concentration was measured by the method of nuclear reactions 14 N(d,α 0 ) 12 C and 14 N(d,α 1 ) 12 C. 3. Results We failed, as in [4, 5], to prepare CN x condensa tes with the nitrogen concentration of over (33–35) at. %. That is, the limiting composition of the coatings corresponded to the formula C 2 N. It was shown [6] that the structure of CN x coa tings was formed mainly by an amorphous matrix, which included homogeneous amorphous regions and regions decorated with crystalline precipitates at 0.15x0.4. The STM analysis of the surface morphology of the condensates demonstrated that, unlike diamondlike carbon films, the CN x conden sates had a columnar structure with irregularities oc Structure, Mechanical and Electrical Properties of CN x Coatings (0x0.5) Prepared by Pulsed Arc Sputtering of Graphite Target A.P . Rubshtein , I.Sh. Trakhtenberg, E.G. Volkova, V.B. Vykhodets, T.E. Kurennykh, A.B. Vladimirov, V.A. Yugov *Institute of Metal Physics, Ural Branch RAS, 18 SKovalevskaya St., 620044, Ekaterinburg, Russia, Phone +7(343) 3783866, Fax +7(343)3745244, [email protected]

Transcript of Structure, Mechanical and Electrical Properties of CN Coatings … · 2006-09-28 · Modification...

Modification of material properties

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Abstract – CNx films 1–3 μm thick were deposited on metalsubstrates by pulsed arc sputtering of graphite in a nitrogen�containing atmosphere at P=10–2–5 Pa. The nitrogen con�centration of the coatings was determined by the method ofnuclear reactions 14N(d,α0)12C and 14N(d,α1)12C. The coatingmicrohardness HV and internal stresses σ0 were measured us�ing methods proposed by the authors. The initial (f0) and sta�tionary (fst) friction coefficients were determined during thereciprocal motion of a WC�Co ball. The surface morphologywas examined by methods of scanning and tunnel electronmicroscopy (SEM and STM). It was found that the hardnessand internal stresses sharply decreased in CNx coatings atx~(0.25–0.35). Coatings with these compositions were suf�ficiently hard (HV~50–60 GPa), but they were stressed lessthan nitrogen�free coatings (σ0~10–12 GPa at x = 0). Giventhe same compositions, fst~f0~0.10–0.05. The STM exami�nation showed that nitrogen�containing coatings had a mar�ked columnar structure. The dependence of the mechanicalproperties on the nitrogen concentration and the electricalresistance minimum, which was revealed at x~0.3, were in�terpreted well in terms of the percolation theory if the CNx

condensate was conceived of as a mixture of diamond�like(sp 3) CNx≤≤0.1 domains and C2N domains with sp 2 bonds. Atx~0.3 the composition corresponded to equal fractions ofthese components, i.e. had an extremely developed interfacebetween unlike structural elements and was characterized bya continuous cluster of each of the elements.

1 Introduction

Hard diamond�like (~80 % sp3 bonds) coatings(DLCs) possess an excellent set of properties for mo�dification of the surface of tools and friction parts soas to extend their lifetime and expand the fields oftheir practical application. DLCs have the hardnessof about 100 GPa and the stationary friction coeffici�ent of not over 0.1 [1]. However, large internal stres�ses (~10 GPa) weaken their adhesion to the substra�te. This factor limits the lifetime of DLCs in realoperating conditions primarily because of peeling.Furthermore, surface roughness of DLCs is respon�sible for large (f0~0.5) initial friction coefficients [2].Since the hardness and the wear resistance of DLCsare high, the initial running�in time is longer, thesurface of parts in mechanical contact with DLCs

deteriorates, unwanted heat is released at the contactpoint, etc.

These circumstances have stimulated the searchfor methods providing modification of DLCs andeliminating the factors that limit their applications.In doing so, hardness may be somewhat sacrificed toclear other faults. It was this practical task that weposed ourselves in the present study. We also inten�ded to clarify how the mechanical and tribologicalproperties of condensates, which were prepared byarc sputtering of graphite in a nitrogen�doped vacu�um, depended on the condensate structure.

2. Methods

CNx films 1–3 μm thick were deposited on metalsubstrates by pulsed arc sputtering of graphite in a ni�trogen�containing atmosphere at P=10–2–5 Pa in aUVNIIPA�001 installation. The microhardness ofthe coating material Hf and internal stresses σ0 weremeasured using methods proposed by the authors[3]. The initial (f0) and stationary (fst) friction coeffi�cients were determined during the reciprocal motionof a WC�Co ball under a load of 2 N. The surfacemorphology was examined in a Philips 515 scanningelectron microscope at the Institute of Metal Physicsand a SMM�2000T scanning tunnel microscope.The nitrogen concentration was measured by themethod of nuclear reactions 14N(d,α0)

12C and14N(d,α1)

12C.

3. Results

We failed, as in [4, 5], to prepare CNx condensa�tes with the nitrogen concentration of over(33–35) at. %. That is, the limiting composition ofthe coatings corresponded to the formula C2N.

It was shown [6] that the structure of CNx coa�tings was formed mainly by an amorphous matrix,which included homogeneous amorphous regionsand regions decorated with crystalline precipitatesat 0.15≤x≤0.4. The STM analysis of the surfacemorphology of the condensates demonstrated that,unlike diamond�like carbon films, the CNx conden�sates had a columnar structure with irregularities oc�

Structure, Mechanical and Electrical Properties of CNx Coatings

(0≤x≤0.5) Prepared by Pulsed Arc Sputtering of Graphite Target

A.P. Rubshtein, I.Sh. Trakhtenberg, E.G. Volkova, V.B. Vykhodets,

T.E. Kurennykh, A.B. Vladimirov, V.A. Yugov

*Institute of Metal Physics, Ural Branch RAS, 18 SKovalevskaya St., 620044, Ekaterinburg, Russia,

Phone +7(343) 3783866, Fax +7(343)3745244, [email protected]

Poster session

319

curring in 4 to 7μm steps on the average (Fig. 1, a, b).A detailed examination of the structure of "columns"revealed that they in turn consisted of bunches0.1–1.0 μm across (Fig. 1, c). Furthermore, accor�ding to the SEM examination, the surface had spar�se spherical formations whose quantity increasedwith growing concentration of nitrogen in the film(Fig. 2, a). The electrical resistance of the sphericalformations was higher than the electrical resistanceof the basic amorphous structure.

Fig. 1. STM images of DLC (a) and СN0.4 (b, c)

In addition, the amorphous structure itself wasinhomogeneous at 0.15≤x≤0.4. As seen in reflectedelectrons, the film surface looked like having twophases since light and dark areas were observed(Fig. 2, b). Light areas had a higher electrical resi�stance. This was confirmed by measurements of the

spreading resistance using an electron probe and thecharge accumulation in these areas. Their size was0.1 to several micrometers, which was in agreementwith TEM [6] and STM (Fig. 1) data.

Fig. 2. SEM images of CN0.5 (a) and CN0.35 (b)

The main implication of the structural analysiswas that at 0≤x≤0.5 the CNx condensate representeda mixture of two types of amorphous domains, thatis, presumably diamond�like domains with the nitro�gen concentration as large as possible for preserva�tion of sp3 bonds (not over 10 at.% according to [4])and C2N domains with sp2 bonds [5].

The mechanical and tribological properties of theCNx coatings as a function of the nitrogen concen�tration are given in Fig. 3.

These dependences can be used for selection ofCNx deposition conditions in different practical ap�plications.

The following points are noteworthy: – At x≤0.15 the mechanical characteristics σ0 and

HV remained nearly stable, while the initial fric�tion coefficient sharply decreased (probably be�cause rough peaks were etched with nitrogenions). Therefore, CNx≤0.15 coatings are preferablein practical terms to nitrogen�free DLCs.

– At 0.25≤x≤0.35 the hardness of the coatings wasstill sufficiently high (Hf~40 GPa) and they pos�sessed the best functional properties (f0~fst~0.1). Considering the observed regularities, we tested

CNx coatings (including those with different compo�sitions) on metal cutting tools. The lifetime of the to�

a

b

10 μ

10 μ

0.5 μ

a

b

c

ols increased at least 2 times. Taps for cutting of thre�ad in blind holes performed especially well.

One more result presenting practical significancewas that not only DLCs, but also nitrogen�rich CN0.5

coatings had the resistance R>107 Ohm. Therefore,unstressed C2N coatings (σ0~1.5 GPa, Fig. 3) showpromise as insulators that readily adhere to varioussubstrates.

Fig. 3. Dependences of σ0 (a), Нf (b), f0 (c) and fst (d)on nitrogen concentration for CNx coatings

4. Discussion

The analysis of the structure of the amorphousCN0<x<0.5 matrix showed that it can be presented as amixture of CNx1 and CNx2 domains. In this case, theaverage nitrogen concentration x is related to thefractionγ of the nitrogen�rich component x1 by theexpression x=γ(x2–x1)+x1.

It is known from the percolation theory that inthe case of a random distribution of two structuralelements of the volume, an infinite cluster (the pos�sibility to pass the whole volume along the trajectorythat goes through particles of one component only)appears when its volume concentration becomes~20 % [7]. If we assume x1=0.05 [4] and x2=0.5 [5],the infinite cluster from γ=0.2 to γ=0.8 will exist atthe average value of x=0.14 to x=0.41 respectively.The concentration x=0.275 corresponds to equalfractions of both components (γ=0.5).

These characteristic values of x are marked in thedependence σ0=f(x) (Fig. 3). It is seen that the CNx

properties remained relatively stable until particles ofone of the components were isolated and changedwhen one of them formed a three�dimensional "fra�mework". The properties changed most dramaticallywhen these components were equal.

The idea that the CNx matrix represents a mixtu�re of DLC and C2N domains is well illustrated by thedependence of the electrical resistance R on fractionγ of the nitrogen�rich component (Fig. 4).

Fig. 4. Dependence of CNx electrical resistance R onthe fraction of the nitrogen�rich component

It was observed that R0<x<0.5<R0; 0.5. Since DLCs andC2N undoubtedly had a domain structure, we assu�med that the conductivity 1/R of the CNx coatingswas due primarily to boundaries between unlike do�mains. Different models, which are based on the per�colation theory [7], give one and the same result: themaximum of the relative fraction of boundariesbetween unlike components (in our case, the mini�mum of the electrical resistance) hits the region whe�re the fractions of the components in the structure areequal. We measured R (the coating – the back side ofthe metal substrate) of samples used for determina�tion of internal stresses σ0 [3]. These measurementsconfirmed the expected regularity (Figs. 3 and 4).

5. Conclusion

The study provided experimental data, which canbe used to select conditions for deposition of CNx

coatings by sputtering of graphite in a nitrogen�con�taining atmosphere taking into account their desig�nated purpose.

Considering the obtained results and the data in[4, 5], the structure of CN0≤x≤0.5 coatings was related totheir properties by recognizing general regularitiesinherent in mixtures of two different objects.

The authors hope that this approach to interpre�tation of properties of coatings may be useful forother inhomogeneous films too.

0,0 0,2 0,4 0,6 0,8 1,0100

101

102

103

104

105

106

107

R, o

hm

0

0,1

0,2

0,3

0,4

0,5

0,6

0 0,1 0,2 0,3 0,4 0,5

x(CN/Cc)

fo

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0 0,1 0,2 0,3 0,4 0,5

x (CN/Cc)

fst

c d

0123456789

1011

0,0 0,1 0,2 0,3 0,4 0,5 0,6x (CN/Cc)

0,0 0,1 0,2 0,3 0,4 0,5 0,60

20

40

60

80

100

120

Hf,

GPa

x (CN / CC)

0, G

Pa

=0.5

a b

=0.2 =0.8

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Poster session

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References

[1] Y. Lifshitz, Diamond and Related Materials, 8,1659 (1999).

[2] A.B. Vladimirov, I.Sh. Trakhtenberg,A.P. Rubshtein et al, Diamond and Related Ma�terials, 9, 838 (2000).

[3] I.Sh. Trakhtenberg, A.B. Vladimirov, A.P. Rub�stein et al, Diamond and Related Materials, 12,1788 (2003).

[4] B. Kleinsorge, A.C. Ferrari, J. Robertson et al,Diamond and Related Materials, 9, 643 (2000).

[5] P. Merel, M. Tobbal, M. Chaker, Diamond andRelated Materials, 12, 1075 (2003).

[6] A.P. Rubshtein, I.Sh. Trakhtenberg, E.G. Volko�va, A.B. Vladimirov, A.G. Gontar, K. Uemura,Diamond and Related Materials, 2005, v. 14,p. 1820–1823.

[7] J.M. Ziman, Models of Disorder, Mir, Moscow,1982, 436.