influence of lamellae thickness on the corrosion behaviour of multilayered pvd tin/crn coatings

In¯uence of lamellae thickness on the corrosion behaviour of multilayeredPVD TiN/CrN coatings

Maria Nordina,*, Merja Herranenb, Sture Hogmarka

aDepartment of Materials Science, The AÊ ngstroÈm laboratory, Uppsala University, Box 534, 75121 Uppsala, SwedenbDepartment of Inorganic Chemistry, The AÊ ngstroÈm laboratory, Uppsala University, Box 538, 75121 Uppsala, Sweden

Received 20 October 1998; accepted 20 January 1999

Abstract

The corrosion behaviour of two multilayered TiN/CrN coatings with different chemical modulation period (10 and 125 nm, respectively)

have been investigated in a 0.1 M H2SO4 solution. Single layered TiN and CrN have been included in the investigation and used as reference

coatings. All coatings have been deposited on single crystalline Si in commercial PVD equipment.The multilayered coating with thin

lamellae showed superior corrosion resistance as compared to the coating with thicker lamellae. This is concluded from the following; it was

observed that TiN oxidised to a solid passivating Ti(N,O), whereas CrN was oxidised and dissolved. Furthermore, it has been shown that the

oxidation rate of the multilayered coatings is determined by the rate of CrN dissolution. The rate is directly related to the number of lamellae

since the dissolution of CrN lamellae propagates laterally, rather than the vertically. From this it can be concluded that the higher the number

of lamellae in the multilayered coating, the lower the oxidation rate, in the present H2SO4 solution. q 1999 Elsevier Science S.A. All rights

reserved.

Keywords: Corrosion behavior; Multilayer; Physical vapor deposition; TiN/CrN

1. Introduction

Increased automation together with higher cutting speeds

demand reliable performance and increased lifetime of the

cutting tool. For this reason cutting inserts made of cemen-

ted carbide are usually coated with thin (3±6 mm), hard,

mechanically and thermally stable coatings. Much research

has been performed on CVD (chemical vapour deposition)

coatings but during the last decades more and more interest

has been subjected to PVD (physical vapour deposition)

coatings. There are two main reasons for this; the possibility

to deposit PVD coatings at a lower temperature (100±

5008C) which may result in a possibility to use a sharper

cutting edge, and the presence of compressive residual

stress which may prohibit thermo-mechanical cracking.

However, the corrosion protection of a PVD coating on a

steel substrate is generally worse than the same coating

deposited using CVD. This is partly due to a high defect

density and a columnar microstructure often present in a

PVD coating [1]. A columnar structure can allow pores/

pinholes to run all through a coating. This can be detrimen-

tal to the lifetime of the insert, since when a coated material

is subjected to a corrosive medium, a galvanic cell can be

formed and the current can be constricted to very small

areas, i.e. almost entirely through the pores [2]. In order to

increase the corrosion resistance of a PVD coating it is

therefore of great interest to inhibit the columnar growth.

It has been shown that by layering a nitride with a metal,

i.e. Ti/TiN [3,4], the columnar structure of the nitride is

inferred at regular intervals during deposition and the corro-

sion resistance is improved. Knotek et al. have shown that

the corrosion protection from a ceramic coating on a steel

substrate can be improved by using a metallic starting layer

under the ceramic coating [5]. Furthermore, it has been

found that, by testing in a 0.5 M Na2SO4 solution with pH �4 and scan rate of 1 mV/s, that the corrosion properties of a

steel substrate (AISI 304) coated with a (Ti,Cr)N coating is

superior to the same substrate coated with single layered

TiN [6]. The positive effect of Cr addition to a TiN coating,

up to a potential of 0.5 V vs. SSE (0.9 V vs. SCE) in a 0.5 M

sulfuric acid, has also been observed by Massiani et al. [7].

Also (Ti,Al)N has shown superior corrosion properties as

compared to single layered TiN in a 3.5% NaCl solution [2].

Furthermore, multilayered coatings have shown other inter-

esting properties such as increased hardness together with

Thin Solid Films 348 (1999) 202±209

0040-6090/99/$ - see front matter q 1999 Elsevier Science S.A. All rights reserved.

PII: S0040-6090(99)00192-3

* Corresponding author. Tel.: 1 46-18-471-7266; fax: 1 46-18-471-

3572.

E-mail address: [email protected] (M. Nordin)

high fracture toughness resulting in an enhanced wear resis-

tance as compared to singe layered coatings [8].

TiN and CrN are well established PVD coatings and have

been used for several years by the industry. Both have been

thoroughly investigated with respect to high temperature

and electrochemical oxidation. The mechanism of electro-

chemical oxidation has been the subject of several investi-

gations performed on TiN, and only a few are presented here

[5,7,9±13]. The mechanism of electrochemical oxidation of

CrN has not been as extensively investigated [7,10,12,14±

16]. However the high temperature oxidation of CrN has

been more thoroughly investigated (see e.g. [4,17±19]).

When it comes to multilayered coatings, many authors

have studied the mechanical properties of multilayered coat-

ings in recent years [8,20±23], but limited information about

oxidation mechanisms in such structures can be found in the

literature. High-temperature oxidation of a TiN/CrN multi-

layered coating was recently reported [19]. Furthermore,

Jehn et al. and Massiani et al. have shown results from

electrochemical oxidation of multicomponent (Ti,Cr)N

coatings [6,7].

In this work, two multilayered PVD TiN/CrN coatings

have been evaluated with respect to the corrosion properties

in a 0.1 M sulfuric solution and compared to single layered

TiN and CrN. The in¯uence of lamellae thickness has been

evaluated. All coatings were deposited on silicon substrates

which remain passive in the sulfuric acid electrolyte used

for the electrochemical measurements. This is done to

ensure that only the inherent corrosion properties of the

coatings are investigated without any contribution from

the substrate, in order to evaluate the effect of lamellae

thickness in a multilayered coating, on the corrosion resis-

tance.

2. Experimental

2.1. Coating deposition

Smooth single crystalline (100)-Si was used as substrate

material.

All coatings were deposited in a commercial Balzers BAI

640R apparatus with high plasma density ®tted with an

electron beam evaporation source (Ti) and a planar magne-

tron sputtering source (Cr). Prior to coating deposition, the

substrates were heated to 4508 C for 60 min. They were then

Ar-ion cleaned for 1 min. with a negative substrate bias of

200 V. The depositions, performed at a pressure of 1.7 £1023 mbar for TiN, 3 £ 1023 mbar for CrN and 2 £ 1023

mbar for the multilayered coatings on substrates on ¯oating

potential, started with growth of a thin metallic adhesion

layer (30 nm Ti for the TiN and the multilayered coatings

and 30 nm Cr for the CrN coating). Prior to the multilayer

depositions a 150-nm thick TiN layer was deposited with

the substrates held stationary above the e-gun simulta-

neously as the Cr target was sputter cleaned. The multi-

layered coatings were deposited by alternately expose the

substrates to the Ti-source and the Cr-source, respectively,

and to admit reactive nitrogen gas in the chamber. The

chemical modulation period, i.e. the thickness of one TiN

lamella together with one CrN lamella, was varied by chan-

ging the rotational speed of the substrate holder. Two coat-

ings with approx. 10 nm (coating denoted TiN/CrN(L10))

and 125 nm (coating denoted TiN/CrN(L125)) were depos-

ited. A total coating thickness of 1 mm was aimed at in all

cases. After the deposition the substrates were cooled in He

at a pressure of 2 £ 1022 mbar.

2.2. Coatingcharacterisation

The ®lm thickness was determined using FEG-SEM (®eld

emission gun scanning electron microscopy). The chemical

modulation period, i.e. the thickness of one lamella of TiN

together with one of CrN, in TiN/CrN(L125) was deter-

mined using FEG-SEM and in TiN/CrN(L10) using trans-

mission electron microscopy (TEM).

The surface morphology, prior to and after the corrosion

tests, was studied using SEM and AFM (atomic force micro-

scopy).The surface roughness, Ra-value, was determined

using AFM on a representative area of 100 mm2.

The coating composition was determined using AES

(Auger electron spectroscopy) and an electron beam energy

of 10 keV after sputtering with Ar1-ions to an approximate

depth of 200 nm.For the Ti-containing coatings the techni-

que of Dawson and Tzatzov to subdivide the intensity from

the overlapping signals of Ti and N at about 382 eV into

separate components [24] was assessed. The sensitivity

factors used for quanti®cation were determined from nitride

reference powders. AES has also been used for depth pro®l-

ing using 10 keV electron beam energy and no sample rota-

tion. Between each measurement of the peak intensities,

contributing to the depth pro®les, 2 nm was sputtered

away using Ar1-ions.

The phase composition was investigated by XRD (X-ray

diffraction) using Cu Ka radiation. The total texture coef®-

cients, TChkl, of the (hkl)-planes are determined using Eq. 1.

TChkl � I 0hkl=Ihkl

1=nXn

i�1

�I 0hikili=Ihikili

��1�

where Ihkl0 is measured intensity, Ihkl is standard intensity

and n the number of re¯ections.

2.3. Corrosion tests

The sample preparation procedure for the electrochemi-

cal measurements has been reported elsewhere [3]. The

experimental set-up consisted of a conventional three-elec-

trode cell. The working electrode was a disc rotating at 1000

rpm. A saturated calomel electrode (SCE) was used as refer-

ence electrode and a platinum spiral served as counter elec-

trode. The coatings were corroded by potentiodynamic

M. Nordin et al. / Thin Solid Films 348 (1999) 202±209 203

scans in 0.1 M H2SO4 electrolyte prepared from reagent

grade chemicals and Millipore water. The electrolyte was

deaerated by a 30-min nitrogen purge, immediately before

the sample was immersed. All experiments were performed

at 25 ^ 18C. Before recording the anodic polarisation curve,

the corrosion potential was registered for about 3 h or until it

had reached a stable value. The anodic polarisation curves

were recorded up to 3 V, with a scan rate of 1 mV/s. To

prevent the access of air into the solution a nitrogen purge

was used during the experiments.

3. Results

3.1. Coating characterisation

The total coating thickness, chemical modulation period

and the chemical composition of the coatings prior to corro-

sion tests are presented in Table 1.

The b -Ti adhesion layer displays a (200) preferred

growth orientation for the single layered TiN and the multi-

layered coatings. The TiN single layered coating has a cubic

NaCl-structure and displays a 100% (111) preferred growth

orientation.

The TiN/CrN(L10) displays low intensity peaks indicat-

ing a smaller grain size as compared to the other coatings. It

consists of a mixed (200) and (111) preferred growth orien-

tation with a majority of (200), see Table 1.

The TiN/CrN(L125) also displays a (200) and (111)

mixture of preferred orientation but with a majority of

(111), see Table 1. This indicates that the lamella thickness

of the TiN layers in this coating is above the critical thick-

ness for transformation from a preferred growth orientation

of (200) to (111).

The single layered CrN displays a (200) peak from the Cr

adhesion layer and a (200) peak from the CrN; however, it is

dif®cult to distinguish whether it is the hexagonal Cr2N or

the cubic CrN phase due to a peak overlap. It is likely a

mixture of these since the chemical composition indicates a

slightly understoichiometric CrN or an overstoichiometric

Cr2N.

3.2. Corrosion properties

Typical anodic polarisation curves of the different coat-

ings are presented in Fig. 1. The TiN coating displays a

passive behaviour up to about 1.2 V, where an anodic peak

is found. The CrN and both the multilayered coatings show

the typical behaviour of a passive material, with a passivation

current (,1 mA/cm2) up to the transpassive potential (above

0.9 V). The current density above 1.1 V remains high (.10

mA/cm2) when increasing the potential for the CrN and

TiN/CrN(L125). In contrast, TiN/CrN(L10) shows an

anodic peak at 1.1 V and thereafter a decrease in current

density. The current density for this coating above 1.1 V is

about two orders of magnitude lower than the current densi-

ties for the CrN and the TiN/CrN(L125). As the potential is

even further increased, at 1.2±1.3 V, a current density rise is

observed for TiN/CrN(L10) which is probably associated to

oxidation of TiN to oxide and/or oxynitride.

The electrolyte turned yellow as electrochemical oxida-

tion was performed for CrN and TiN/CrN(L125), indicating

that Cr(VI)-ions are present in the solution. During corro-

sion of TiN/CrN(L125) golden coloured ¯akes, likely

lamellae of TiN, were observed in the electrolyte.

3.3. Coating characteristics prior to and after corrosion

tests

All coatings exhibited a mirror-like ®nish and did not

reveal any topographical features as deposited, whereas

the surface roughness increased as a result of the corrosion

for all coatings, see Table 2.

The CrN coating corroded uniformly above 1.1 V, see

M. Nordin et al. / Thin Solid Films 348 (1999) 202±209204

Table 1

Coating thickness, t, chemical modulation period, L, the chemical compo-

sition (measurement error about 10%) and the texture coef®cient for the

111, TC111, and the 200, TC200, direction

TiN CrN TiN/CrN(L10) TiN/CrN(L125)a

T (nm) 800 ^ 40 1400 ^ 70 1130 ^ 50 1140 ^ 50

L (nm) ± ± 9 ^ 1 125 ^ 10

at% Ti 51 ± 45 57TiN

at% Cr ± 59 15 52CrN

at% N 49 41 40 43TiN/48CrN

TC111 1 0 0.89b 1.11

TC200 0 1 1.77b 0.23

a TiNin the TiN lamellae, CrNin the CrN lamellae.b Part of the intensity from the (111) peaks originates from the 90 nm TiN

adhesion layer, why the value of TC111 is lower if only the multilayered

structure contributed to the value.

Fig. 1. Anodic polarisation curves of the different coatings.

Fig. 2a, while the TiN displayed a much smoother surface

with a number of pits, uniformly distributed on the surface,

see Fig. 2b.

During corrosion testing of the TiN/CrN(L125) a lot of TiN

lamellae coating was found as ¯akes in the electrolyte, and

parts of the TiN lamellae were still found on the surface after

corrosion attached to the substrate by uncorroded CrN lamel-

lae, see Fig. 2c. The surface roughness increased for TiN/

CrN(L10) as a result of corrosion, see Table 2 and Fig. 2d.

During the corrosion tests the TiN single layered coating

was oxidised to a passivating Ti(N,O) to approx. 10 nm

depth, see Figs. 3a and b. The CrN single layered coating

did not form any passivating oxide and its surface composi-

tion did not change as a result of the corrosion and therefore

the corresponding depth pro®les will not be shown here. The

multilayer with thicker lamellae did not contain any Cr at

all, after corrosion, indicating that all the CrN had oxidised

and dissolved. In fact, only the starting layer of Ti and TiN

(30 and 150 nm, respectively) was present at the surface

after corrosion, see Fig. 3c,d. The coating had lost its origi-

nal mixed silver-golden colour and exhibited a pure golden

colour, typical for stoichiometric TiN. The multilayer with

thinner lamellae, on the other hand, still possessed the origi-

nal silver-golden colour. The composition of this coating

was unchanged, except for the near surface region to a

depth of approximately 15 nm, see Fig. 3e,f (observe that

the AES depth pro®ling technique without sample rotation

can only resolve the top few individual TiN and CrN lamel-

lae of TiN/CrN(L10) due to the degrading depth resolution

with analysis depth).

4. Discussion

It has been shown previously that columnar growth is


Table 2

Surface roughness, Ra, prior to and after the corrosion tests

Coating Ra-value (nm)

Prior to corrosion After corrosion

TiN 0.8 ^ 0.1 4.3 ^ 0.4

CrN 4.6 ^ 0.5 44.8 ^ 5

TiN/CrN(L10) 1.2 ^ 0.1 10.9 ^ 1

TiN/CrN(L125) 1.7 ^ 0.2 Flakes

Fig. 2. Typical features of the corroded surfaces. (a) Corroded CrN surface, (b) corroded TiN surface, (c) ¯aking behaviour of the corroded TiN/CrN(L125)

and (d) corroded surface of the TiN/CrN(L10).

detrimental to coatings used in severe corrosion applications

[2,3]. Introduction of a multilayered structure is therefore of

great interest since it is known to provide a more three

dimensional structure and therefore a reduced amount of

pores running through the coating. In this work two multi-

layered coatings consisting of TiN and CrN have been eval-

uated. To ensure that only the inherent corrosion properties

of the coatings are investigated, without any contribution

from the substrate material, all coatings were deposited on

silicon substrates which remain passive in the sulfuric acid

solution used for the electrochemical tests.

Anodic oxidation of TiN leads to the formation of stable,


Fig. 3. Composition vs.depth as obtained by AES depth pro®ling for (a) uncorroded TiN, (b) corroded TiN, (c) uncorroded TiN/CrN(L125), (d) corroded TiN/

CrN(L125), (e) uncorroded TiN/CrN(L10) and (f) corroded TiN/CrN(L10).

solid, oxides or oxynitrides. This has also been observed in

several other work [7,9±11,13]. Milosev et al. [10] found

that TiN, corroded in a phthalate buffer, pH 5.0, exhibits an

anodic peak centred at 1.3 V vs. standard hydrogen elec-

trode (SHE). They have also shown using electrochemical

and X-ray photoelectron spectroscopy (XPS) that the oxida-

tion of TiN starts with the formation of a mixed oxide/

oxynitride layer in the lower potential range (,1 V vs.

SHE). As the oxidation potential is increased, oxidation to

TiO2 takes place. This is also characterised by an anodic

peak at about 1.6 vs. SHE [10].

Also, the electrochemical oxidation of CrN leads to the

formation of a thin mixed oxynitride/oxide layer. Electro-

chemical measurements in phthalate buffer, pH � 5:0, and

subsequent characterisation of the surfaces using XPS

revealed the formation of a very thin (#2 nm) mixed oxyni-

tride/oxide layer. The formation takes place above 0.456 V

vs. SCE [10]. The corrosion behaviour of CrN is similar to

that of Cr in acid solutions [10,25], except from the fact that

the CrN in our experiments is in the passive state and does

not exhibit an active peak. The formation of a mixed oxyni-

tride/oxide layer could not be detected from this work since

the potential was increased to 3 V before surface character-

isation was performed. However, from the polarisation

curves it was observed that at potentials above 1.0 V vs.

SCE the Cr(III) species present in the passive layer undergo

transpassive corrosion. Furthermore, the electrolyte turned

yellow during corrosion testing indicating that the CrN had

oxidised to Cr(VI) which was dissolved. The CrN corrodes

uniformly over the surface which was also found by Milosev

et al. [10].

The transpassive dissolution of passivated Cr is known to

be a two-stage process [14] and takes place by a sequence of

a permanent Cr(III)-oxide formation, oxidation to Cr(VI),

and dissolution as chromate. To describe the transpassive

dissolution of CrN the following reaction can be considered:

2CrN 1 7H2O � Cr2O227 1 N2 1 14H1 1 12e2 �2�

A rough estimate of the amount of CrN in the coatings that

was oxidised, above the transpassive potential, can be

obtained from the charge transferred. From reaction 2 and

applying Faraday's law (Eq. (3)), using a density of 6.179 g/

cm3 for the cubic CrN and MCrN � 66 g/mol, a theoretical


Fig. 4. A schematic representation of the corrosion mechanism of multilayered TiN/CrN. Prior to corrosion (a), the coating may contain some pores running all

the way through the TiN lamellae (and the CrN lamellae; however, this is of minor importance since the CrN is dissolved). When the coating is exposed to the

corrosive solution and the current is applied, new pores are formed in the TiN, at which CrN lamellae are exposed to the corrosive solution and dissolved, see b.

This will continue and will result in coating degradation (c) until only the bottom TiN lamella is left, see d. Some undetached ¯akes of TiN were also found on

the surface as also shown in d.

®lm thickness of the dissolved CrN can be calculated. For

the TiN/CrN(L10), a potential scan up to about 1.2 V, i.e. to

the ®rst passive potential, corresponds to 0.018±0.019 C/

cm2 of charge transferred. Assuming a 12-electron oxida-

tion according to reaction 2, this charge corresponds to

oxidation and dissolution of a 3±4-nm thick CrN ®lm.

This is about the same thickness as one CrN lamella in

TiN/CrN(L10).

Faraday's law used for calculation of removed CrN can

be written as:

l � Q £ M

Z £ F £ r�3�

where l is the ®lm thickness, Q is the current density, M is

the molar mass (66 g/mol for CrN), Z is the number of

electrons, F is Faraday's constant and r is the density.

The passive potential is somewhat higher for TiN/

CrN(L125), registered to 1.5 V. The charge transferred

(appr. 0.18 C/cm2) to the passive potential for this multi-

layer corresponds to a CrN thickness of about 30±35 nm, i.e.

also in this coating approx. the same thickness as one CrN

lamella.

An estimate of the oxidised and dissolved CrN ®lm thick-

ness at 3 V, i.e. after the whole corrosion experiment

showed that the dissolved CrN ®lm thickness is approxi-

mately ten times as large at 3 V than at the ®rst passive

potential for the multilayered coatings. This means that

ten CrN lamellae have been dissolved at 3 V, for both

TiN/CrN(L10) and TiN/CrN(L125). This corresponds to

all CrN in TiN/CrN(L125) but only approximately 10% of

the CrN in TiN/CrN(L10).

An estimate of the CrN dissolution for the single layered

CrN corroded up to 3 V, is in good agreement with the

thickness measured on the corroded sample from a FEG-

SEM cross-section. Up to 3 V the charge transferred was

approx. 3.65 C/cm2 which corresponds to a dissolved CrN

®lm thickness of 670 nm. Since the total coating thickness

prior to the experiments is 1400 nm the calculation suggests

a remaining coating thickness of 730 nm. This is in very

good agreement with the coating thickness measured after

corrosion, i.e. 690 nm.

The higher corrosion resistance of TiN/CrN(L10) coating

as compared to TiN/CrN(L125) is due to the larger number

of lamellae in this coating. Likely, the pits formed in the

TiN lamellae are also smaller in this coating, allowing less

electrolyte to penetrate the TiN lamellae to new CrN lamel-

lae and dissolve them. However, for simplicity we will

assume that the pits formed in the TiN lamellae in the two

multilayered coatings during corrosion are of the same size

and at the same distance as the pits formed in the TiN single

layered coating. The latter has been con®rmed from surface

studies using FEG-SEM. The pits in single layered TiN are

at an approximate distance of 5±40 mm, i.e. at an average

distance of 25 mm (see Fig. 2b). The thickness of the CrN

lamellae in TiN/CrN(L125) and TiN/CrN(L10) coating are

approx. 35 nm and 3 nm, respectively, i.e. much thinner than

the distance between two neighbouring pits in both coatings.

This means that it is the lateral dissolution of the CrN lamel-

lae that will determine the removal time of one lamella, see

Fig. 4. The number of CrN lamellae in the multilayered

coating consequently determines the dissolution time. The

number of lamellae is approximately ten times as high in the

TiN/CrN(L10) than in TiN/CrN(L125), which means that

TiN/CrN(L10) is more corrosion resistant in a solution

where one of the lamella materials (here CrN) is dissolved.

Also in this case, where one of the lamella materials

forms a passive oxynitride, as in the case of TiN, it is likely

to be advantageous to have a large number of lamellae since

the more frequent a new fresh TiN surface is exposed to the

solution, the longer is the time for the coating to degrade.

5. Conclusion

The multilayered coating with thin lamellae showed

superior corrosion resistance as compared to the coating

with thicker lamellae. This can be concluded from the

following:

² It was observed that TiN oxidised to a solid passivating

Ti(N,O), whereas CrN was oxidised and dissolved.

² It was shown that the oxidation rate of the multilayered

coatings is determined by the rate of CrN dissolution.

² Since the oxidation proceeds by lateral dissolution of

CrN, rather than vertical, the oxidation rate is approxi-

mately inversely proportional to the number of lamellae

in the multilayered coating.

Acknowledgements

The ®nancial support from Sandvik Coromant AB, the

National Swedish Board for Technical and Industrial Devel-

opment (NUTEK) and the Swedish Research Council for

Engineering Sciences (TFR) is gratefully acknowledged

by the authors.

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influence of lamellae thickness on the corrosion behaviour of multilayered pvd tin/crn coatings

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