TiN multilayers

Surfaceand CoatingsTechnology89(1997)114-120

Corrosion resistance of steel coated with Ti/TiN multilayers L.A.S. Ries a, D.S. Azambuja a,*, I. J.R. Baumvol b a Instituto de Quimica, Universidade Federal do Rio Grande do Sul, Av. Bento Gonplves 9500, Caixa Postal 15003, CEP 91501-970, Porto Alegre, RS, Brazil b hstituto de Fisica, Urkersidade Federal do Rio Grande do SUE,Caixa Postal 15003, CEP 91501-970, Porto Alegre, RS, Brazil

Received 11 December 1995; acceptedin hai form 25 July 1996

Abstract The corrosion resistance behaviour of steel coated with TiN thin films deposited by physical vapour deposition (PVD) has been studied by electrochemical techniques in de-aerated 1 M sodium acetate solution at pH 5.6. Two different types of coatings deposited on carbon steel sampleshave been studied: (1) multilayered coatings of Ti/TiN in which the interfaces are compositionally abrupt, and (2) multilayered coatings of Ti/TiN in which there is a mixture between the two materials at the interfaces leading to gradual composition interfaces. The electrochemical results obtained have been correlated to structural defects studied by scanning electron microscopy. It has been observed that the corrosion resistance of coated steel was higher than the substrate resistance. The graded composition interface coatings showed a more protective character than the sharp interface. The corrosion resistance is mainly controlled by the occurrence of coatings defects and their presence can be easily detected by electrochemical measurements. Keywords: Physical vapour deposition; Ti/TiN multilayers; Corrosion resistance

1. Introduction The excellent properties of thin films of TIN such as high hardness, good wear and corrosion resistance, high electrical conductivity, chemical stability and good adhesion have led to many useful applications [ 11. TIN hard coatings obtained by physical vapour deposition (PVD) techniques are used to increase the wear and corrosion resistance of steel components [2]. However, these coatings are rarely completely protective because of the presence of defects in the coating that allow the access of corrodents to the substrate [3]. The use of multilayered thin film composite coatings permits to overcome several of the difi?culties in producing an adherent, stress free and fully dense coating at a reduced cost with total thickness above 0.5 pm [4]. The nature of the interfaces between the individual layers and their relative thickness plays a decisive role in the performance of the multilayered coatings. Since the corrosion reaction starts from a defective part of the film, measurements of the porosity are very important in order to evaluate the corrosion resistance of a nitride-coated substrate [S]. *Corresponding author. Tel: +51 316 6321; fax: +51 336 3699; e-mail:[email protected]

This paper aims at studying the corrosion behaviour of a medium-carbon steel coated with two different types of coatings: (i) multilayered coatings of Ti/TiN with a graded composition interface and (ii) multilayered coatings of Ti/TiN with a sharp composition interface. In both cases the Ti is the layer in contact with the steel substrate. The porosity of each coating has been calculated by using electrochemical techniques. To get more information about the coatings, scanning electron microscopy @EM) studies have been used as a support for electrochemical measurements.

2. Experimental details 2.1. Coating characterization In Table 1 characteristics of the Ti/TiN multilayers are listed. Fig. 1 shows the Ti signal in the RBS spectrum (Rutherford backscattering spectrometry) of a Ti/TiN multilayered structure deposited on silica substrate by reactive magnetron sputtering. The TiN layers have been deposited in a routine of gradually increasing the Nz partial pressure, keeping it at a constant value that corresponded to a N : Ti ratio of 1 : 1 for some time,

0257-8972/97/$17.00 Copyright0 1997ElsevierScienceS.A.All rights reserved PII SO257-8972(96)03092-7

L.A.S. Table 1 Composition Interfaces

Sharp

Graded

and structure

of multilayered

Sample

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115

coatings

Layers

A40 A41 A42 A43 A44 A45

Technology

Thickness

(A)

Time (s) of increasing/decreasing of N, partial pressure

First

Last

Ti

TiN

Total

Ti Ti Ti Ti Ti Ti

TiN TiN TiN TiN TiN TiN

1000 500 1000 1000 1000 1000

1000 1000 500 1000 1000 1000

8000 6000 6000 8000 8000 8000

2s ,

0.6 I

0.8 I

Energy @&V) 1.o 1.2 I

I

1A I

1.6 I

120/30 180/45 240/60

1.8 I

600

Channel Fig.

1. RBS spectrum

of a Ti/TiN

multilayer

and finally gradually decreasing the N, partial pressure. This resulted in Ti/TiN multilayered with interfaces that, instead of a sharp transition between the Ti and the TiN individual layers, have composition gradients at the interface. The magnitude of this gradient can be controlled by the speed of increasing and decreasing the partial pressure of Nz, which has been expressed as a time function (Table 1). In the example given in Fig. 1 the RBS spectrum has been measured with 2.5 MeV incident u-particles and tilting the samples such that the beam is incident at 50” with the normal to the samples. The RUMP simulation has been used to determine the composition gradient at the Ti/TiN interfaces. A bad fitting to the experimental points is obtained if sharp interfaces are assumed between the different Ti/TiN interfaces. If gradient composition interfaces are assumed in the simulations, a much better fit to the experimental points is obtained. 2.2. Electrochemical testing

The samples used as substrates for the corrosion experiment were rods of a CK 45 steel (0.42 wt% C)

with

graded

interfaces

deposited

on Si substrate.

polished with diamond paste down to 1 urn plus 0.05 urn SiOz suspension to a mirror-like surface. The samples have been degreased with 2-propanol by means of an ultrasonic generator. In the case of coated steel, the deposition process has been made after this polish procedure. The Ti/TiN multilayers have been prepared by reactive magnetron sputtering, using as cathode a pure Ti target and alternating pure Ar or a mixture of Ar and N, in the plasma. The partial pressures were, respectively, 3 x 10m3 mbar of Ar and 3 x 10m4 mbar of N,. More details have been published elsewhere [ 61. Working electrodes have been made with the uncoated steel and with each one of the multilayered coatings described in Table 1. They have been axially embedded in Teflon holders to offer a flat disc shaped surface of 0.5 cm2 geometric area which could be used either still or under rotation (o = 1000 rpm). The potential of the working electrode has been measured against a saturated calomel electrode (SCE). All the potentials given here refer to this electrode. A large area platinum counter-electrode has been employed.

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Solutions from analytical grade reagents and twice distilled water have been prepared. Experiments have been performed in a 1 M acetate solution at pH 5.6. The experiments have been carried out under purified nitrogen gas saturation at 298 K. Electrochemical measurements have been performed with a EG&G PAR 366. Several tests have been done with each sample, including: (i) measurement of the corrosion potential; (ii) determination of the polarization resistance; and (iii) multisweep cyclic voltammetry. X-ray energy dispersive analysis (EDX) and scanning electron microscopy (SEM) have been used to characterize the microstructure of the coated substrates and to correlate them with the electrochemical data obtained. 3. Results and discussion

Fig. 2 shows the voltammograms of the uncoated steel and of the nitride-coated steels for the first cycle. The voltammogram of the steel runs between - 1.2 and 1.2 V at 0.03 V s-l and at 1000 rpm exhibits an anodic peak at -0.34 V (peak I), and the reverse scan presents a reactivation peak at - 0.33 V (peak II). This behaviour is typical of iron dissolution in acetate media and at this potential region only Fe(I1) species are expected as reaction products [ 71.According to thermodynamic data [8], the formation of Fe(II1) speciesis expected when the applied potential is higher than - 0.1 V. The voltam10

Fig. 3. Scanning electron micrographs showing the appearence of the coated steel after the first cycle in 1 M sodium acetate solution, pH 5.6, ~l=O.O30V s-l at two electrode rotation: (a) o=O rpm and (b) o=lOOO rpm.

‘peak1

1

E J VSCE Fig. 2. Voltammograms of uncoated steel (s-l and w= 1000rpm.

), coated steel (- - -) in 1 M sodium acetate solution, pH= 5.6, de-arated solution. u=O.O30V

L.A.S.

Ries et al. 1 Surjace

and Coatings

Table 2 Average values of anodic peak current densities under stationary and dynamic conditions Anodic peak current density

Uncoated steel Coated steel

co=0 rpm

w = 1000 r-pm

10 mA cmV2 20 uA cmmz

10 rd. cm-’ 100 fi cm+

mogram of the coated steel shows an anodic peak at -0.34 V but the reactivation peak is absent. A slight cathodic reduction process is present at -0.30 V, suggesting that a protective state is achieved at the coated samples. The experiments in the present work have been carried out under dynamic conditions (o = 1000 rpm) to investigate the role of mass transport on the corrosion process [9]. This is very important in technological applications of steel coated engineering components. Table 2 shows the anodic peak current values obtained for the lirst cycle at the same conditions cited above. It can be seen that the rate of steel corrosion is independent on electrode rotation, as has been found in previous work [lo]. However the dissolution rate of the coated steels increases with electrode rotation.

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A comparison by scanning electron microscopy (SEM) of the steel coated surfaces after repetitive cycles at zero velocity and under 1000 rpm reveals the presence of a more intense attack on the electrode surface under stirring (Fig. 3). These results denote that electrode rotation favours the partial removal of the coating material which are poorly adhered to the steel surface. Consequently, a higher substrate area becomes exposed to the corrosive attack. For each coated sample as well as for the steel, the corrosion potential (E,,,) has been followed for 1 h, under electrode rotation (Fig. 4). The values for EC,, of the coatings are more anodic than the substrate ~(E,,,= -0.69 V), except sample A42 that reaches this value after 30 min of immersion (E,,, = - 0.66 V). This behaviour is thought to be due to structural defects present on the sample that extend from the solution/coating interface to the steel base. The graded interfaces have more positive values for the corrosion potential after 60 min, indicating a lower porosity. Repetitive voltammograms run at the same conditions of Fig. 2 permit to obtain some information about the electrochemical behaviour of the samples. The height of peak I after 100 cycles represents the critical current density and reflects the performance of the coatings

Time (min.) 0 O,oO

10

20

30

40

50

60

-0,05 -0,lO -0,15 -020 -025 E

-0,30

k 8 w

-0,35 -0,40 -0,45 -0,50 -0,55 4,60

Fig. 4. Corrosion potential (I&,) r-pm, de-arated solution.

variation during 1 h for the uncoated steel and coated steels in 1 M sodium acetate solution, pH = 5.6, w = 1000

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[ 111. Fig. 5 shows the critical current densities as a function of the number of sweepcycles for the uncoated steel and all multilayered samples.The TiN is chemically inert in sodium acetate at this pH, and on the other hand the Ti becomes passivated by a protective oxide layer which does not allow the electrolyte to come into contact with the metallic surface. Therefore icrit represents, for the coated samples, the steel dissolution through defects originated during film deposition (micropores, fatigue crevices, pinholes, etc.). Through these defects the substrate becomes exposed to the aggressive medium and allows the corrosive attack on the interface and the consequent loss of adhesion by the film. The dissolution rate decreasesat the graded interface coatings revealing a higher corrosion resistance, which can be attributed to lower interfaces stressesoriginated when couples that develop tensile and compressive stresses are involved, as in the case of TiN and Ti couples [ 6,121. X-ray energy dispersive analysis (EDX) carried out on the coated samples after the multisweep measurements, shown in Fig. 6, indicates the existence of two types of porosity: (i) through porosity (point l), which presents segregatedmaterial around the edge and whose

40

50

analysis has shown iron in its interior; (ii) no through porosity (point 2) which does not present segregated material around the edge and whose analysis showed titanium in its interior. The difference between them is that only the first type of porosity exposesthe steel base allowing the free accessof the corrodents to the substrate, thus causing an increase in the corrosion current. The polarization resistance I?, has been calculated from the experimental current-polarization curve [ 131 as the slope of the curve at the corrosion potential (EC,,+ 10 mV). Table 3 shows that the graded interface coatings present a very high polarization resistance, if compared to the sharpened interface coatings. By using electrochemical parameters, the total coating porosity (F) can be calculated [5,14]

where R,,, is the polarization resistance of the base material, R, the measured polarization resistance of the coated sample, AE,,,, is the difference between the corrosion potentials and b, the anodic Tafel slope of the base material. The steel base material CK 45 presents a corrosion potential EC,,, = -0.69 V, an anodic Tafel slope

60

Number of Cycles Fig. 5. Critical current densities for the anodic dissolution for the uncoated steel and coated steels as a function of the number of cycles in 1 M acetate solution, pH = 5.6, v=O.O30 V s-l and w= 1000 rpm, de-aerated solution.

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89 (1997)

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119

(b) Point

1

6.0 keV

Point

-I-

2

TI

G.C

Fig. 6. Image obtained different points (point

by backscattering (a) showing 1 and point 2) of this surface.

the coated

steel

surface

after

100 cycles

and X-ray

energy

dispersive

analysis

(b)

at two

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Table 3 Results of electrochemical measurementsfor multilayered coatings Sample

EC,, WI

R, (kQ cm-‘)

Calculated porosity (%)

Tafel slope (mV per decade)

A40 A41 A42 A43 A44 A45 Substrate

-84 -95 -660 -110 -90 -20 -690

3.9 3.8 2.3 62 87 90 2.5

2x1o-Q 4x 10-g 8x10° 4 x lo-r0 1 x lo-lo 8 x 10-12

107 98 90 102 108 118 58

V per decade and a polarization resistance kR cm-’ in 1 M acetate solution at pH 5.6. The very low calculated porosity of the multilayered coatings, shown in Table 3, can be explained in terms of the electrochemical parameters: a high polarization resistance associatedwith a noble open circuit potential. The calculated porosity values are correlated with the kinetics parameters obtained from the polarization curves, in a region of potential where the current depends exponentially on potential, i.e., close to equilibrium potential [ 5,151. Examination of the porosity values and the SEM micrographs of the graded interface coatings can explain the higher corrosion resistance observed as compared to the sharpened interface coatings. b,=O.O%

R,,,=2.5

4.

Conclusion

24 18 106 88 126 32 10000

(5) EDX analysis permits to confirm the presence of through porosity, which promotes initiation of sites of corrosion.

Acknowledgement

The authors acknowledge the support of CNPq, CAPES and FAF’ERGS (Brazil).

References [I] B. Matthes, E. Broszeit and K.H. Kloos, Surf Coat. Technol., 57 (1993) 97. [2] A. Matthews and A.R. Lefkow, Thin Solid Films, 126 (1985) 283. [3] J. Munemasa and T. Kumakiri, Surf: Coat. Technol., 49 (1991) 496.

The following conclusions can be drawn regarding the electrochemical behaviour of steel coated with Ti/TiN multilayers: (1) The steel coated with Ti/TiN multilayers shows a better corrosion resistance than the uncoated steel. (2) The corrosion resistance of the coated steel is affected by the dynamic conditions. A more intense corrosion is observed on a rotated coated steel. (3) The corrosion resistance of coated steel can be more effectively increased by using graded interface coatings instead of a sharp interface coatings. The better performance of the graded interface coatings is related to the nature of the interfaces between the individual layers, which promotes a reduction of the interface stresses. (4) The lower porosity values obtained with the graded interface coatings permit an improvement of their corrosion resistance, by hindering the accessof the electrolyte to the steel base.

[4] I.J.R. Baumvol, Nucl. Instrum. Meth. B, 85 (1994) 230. [5] B. Matthes, E. Broszeit, J. Aromaa, H. Ronkainen, S.P. Hannula, A. Leyland and A. Matthews, Surf Coat. Technol., 49 (1991) 489. [6] R. Hubler, A. Schroer, W. Ensinger, G.K. Wolf, F.C. Stedile, W.H. Schreiner and I.J.R. Baumvol, I. Vat. Sci. Technol. A, ll(2) (1993) 451. [7] D.S. Azambuja and I.L. Muller, Corros. Ski., 36 (1994) 1854. [8] M. Pourbaix, Atlas of Electrochemical Equilibria in Aqueous Solutions, Pergamon Press, Oxford, 1966. [9] M.A. Castro and B.E. Wilde, Corros. Sci., 19 (1979) 923. [lo] L.A.S. Ries, Thesis, Federal University of Rio Grande do Sul, UFRGS-Brazil, 1996. [ll] R. Hiibler, A. Schroer, W. Ensinger, G.K. Wolf, W.H. Schreiner and I.J. Baumvol, Surf: Coat. Technol., 60 (1993) 561. [12] L. van Leaven, M.N. Alias and R. Brown, Surf Coat. Technol., 53 (1992) 25. [ 131F. Mansfeld, Electrochemical Techniques for Corrosion - A Symposium Sponsored by NACE Technical Committee T-3L, 2nd. edn., 1978, p. 18. 1141J. Aromaa, H. Ronkainen, A. Mahiout and S.P. Hannula, Surf Coat. Technol., 49 (1991) 353. [ 151C.M.A. Brett and A.M.O. Brett, Electrochemistry, Oxford Press, New York, 1993,p. 112.