y 201 (2006) 313–319www.elsevier.com/locate/surfcoat

Surface & Coatings Technolog

Multi-technique study of corrosion resistant CrN/Cr/CrNand CrN:C coatings

S. Kaciulis a, A. Mezzi a,⁎, G. Montesperelli d, F. Lamastra c,f, M. Rapone c,F. Casadei g, T. Valente b,e, G. Gusmano c,f

a Institute for the Study of Nanostructured Materials, CNR, P.O. Box 10, 00016 Monterotondo Stazione, Roma, Italyb Dept. of ICMMPM, University of Rome “La Sapienza”, Via Eudossiana 18, 00184 Roma, Italy

c Dept. of STC, University of Rome “Tor Vergata”, Via della Ricerca Scientifica, 00133 Roma, Italyd Dept. of PME, Technical University of Ancona, Via Brecce Bianche, 60131 Ancona, Italy

e INSTM Operative Unit “Roma La Sapienza”, Italyf INSTM Operative Unit “Roma Tor Vergata”, Italy

g Centro Sviluppo Materiali, Castel Romano, Roma, Italy

Received 10 February 2005; accepted in revised form 18 November 2005Available online 18 January 2006

Abstract

The three-layer CrN/Cr/CrN coatings were deposited on three types of steel substrates by means of PVD. The second series of these coatingswas implanted with nanoclusters of carbon by using a pulsed microplasma cluster source. The physical and chemical properties of the coatingswere investigated by using a multi-technique approach. The chemical composition of the samples was studied by XPS combined with cyclic Arion sputtering. Multi-point AES measurements and Auger chemical imaging were used to characterize the oblique cross-section of the coatings.Surface investigations revealed that CrN layers were characterized by the ratio of Cr :N=1 with a low concentration of oxynitride compounds andthat the nanoclusters of carbon were localized in the topmost sublayer of the coatings. XRD measurements were performed to study the residualstresses present in the CrN multiple-layers and in the coatings doped with carbon nanoclusters. It was found that the compressive residual stressesare present in the external layer of all multiple-layer coatings, independently from the type of substrate. The corrosion tests have been carried outby the techniques of open circuit potential (OCP) and electrochemical impedance spectroscopy (EIS) in electrochemical cell with NaCl solution.Obtained experimental results demonstrated that the corrosion process begins at the surface defects of the coatings and propagates towards thesubstrate.© 2005 Elsevier B.V. All rights reserved.

PACS: 61.10.Kw; 61.16.Ms; 81.05.Je; 81.65.Kn; 82.80.PyKeywords: Chromium nitride; C-implantation; Impedance spectroscopy; Auger electron spectroscopy; Photoelectron spectroscopy; X-ray diffraction

1. Introduction

The deposition of thin films by means of vapour basedtechniques, such as Physical (PVD) and chemical (CVD)Vapour Deposition, plays an important role in many applica-tions (electronics and optical devices, decorative tools, etc.), butthis technique is widely used also for tribological purposes[1,2]. Hard coatings based on transition metal nitrides orcarbides have been successfully used for materials protection

⁎ Corresponding author. Tel.: +39 06 90672 474; fax: +39 06 90672839.E-mail address: mezzi@mlib.cnr.it (A. Mezzi).

0257-8972/$ - see front matter © 2005 Elsevier B.V. All rights reserved.doi:10.1016/j.surfcoat.2005.11.128

and particularly to improve cutting tools life since almost30 years, but both the production process and their properties,i.e. microhardness, wear and oxidation resistance, are con-tinuously being improved. In recent years also superhardnanostructured films became promising materials due to theirimproved properties, as in the form of superlattice coatings or inthe form of nanostructured coatings obtained by means ofproper dispersion of nanoclusters [3–7].

Among various possibilities, a key role is played by multi-layered coatings which have a number of advantages over singlecomponent systems. They are claimed to possess improvedtoughness and hardness, better adhesion and lower stresses,

Table 1Main parameters of the pulsed microplasma cluster source PMCS

Sample He pressure(mbar)

Voltage(V)

Frequency(Hz)

Pulse(μs)

Delay(μs)

Gas supply(μs)

NANO1 12 500 7 70 1200 750NANO2 12 700 7 70 1200 750NANO3 12 900 7 70 1200 750

Table 2XPS surface quantitative analysis (at.%) of CrN/Cr/CrN and C-doped coatings

Sample Cr 2p3/2 N 1s O 1s C 1s

Atomic % Atomic % Atomic % Atomic %

CRN1 11.0 14.4 17.4 57.2CRN2 15.0 15.6 16.3 53.1CRN3 9.8 11.2 27.0 52.0NANO1 8.0 10.0 17.6 64.4NANO2 12.5 16.2 15.9 55.4NANO3 8.0 10.3 16.8 65.0

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depending on the combination of different layers. They cancombine positive properties of different materials and, at thesame time, can exhibit novel properties, if compared with singlelayer coatings [8]. As an example, an increased number ofinterfaces can act as obstacles for the dislocations and it cancontrol crack propagation directions, thus increasing coatingcapacity of energy dissipation before loosing substrate protec-tion. Coatings with hard/soft (or high/low elastic modulus)layers can offer improved tribological properties and can helppartially to avoid the hard and brittle material to be subjected tohigh bending stresses and strains [9]. Important factors whichcan limit coatings performance under service conditions are stillresidual stress values induced by the fabrication process, andcorrosion resistance properties.

In the present work, X-ray Photoelectron Spectroscopy(XPS) and Auger Electron Spectroscopy (AES) have been usedto study the surface and the cross-section composition of thethree-layer CrN/Cr/CrN coatings and CrN coatings with Cnanoclusters, prepared by PVD. X-ray Diffraction (XRD)spectra have been used to perform the phase analysis and toevaluate the residual stresses in the coatings. Finally, thecorrosion behaviour has been studied by ElectrochemicalImpedance Spectroscopy (EIS) and the corrosion productshave been analysed by XPS and AES.

2. Experimental

2.1. Sample preparation

The first series of coatings consisted of CrN/Cr/CrNmultiple-layer, produced by cathodic arc evaporation on AISI H13, AISI1040 and K 340 steel substrates with the size of 60×60×3 mm.Three different systems, named CrN1 (1,6 μm CrN/0,6 μm Cr/1,6 μm CrN), CrN2 (1,6 μm CrN/1,1 μm Cr/1,6 μm CrN) and

Fig. 1. Typical photoelectron spectra of the CrN coatings: (a) Cr 2p with two peB—398.6 eV.

CrN3 (1,6 μm CrN/1,6 μm Cr/1,6 μm CrN), were deposited.The CrN layers were deposited by using Cr cathode and N2

reactive gas flow in the following experimental conditions:P=10−2 mbar, U=−100 V (bias), I=55 A. The Cr layer wasdeposited at P=10−3 mbar, U=−100 V (bias), I=50 A. Thesecond set of the coatings, named NANO1, NANO2 andNANO3, was deposited on the same steel substrates by usingthe same equipment, but combined with a pulsed microplasmacluster source (PMCS), in order to dope the top CrN layer withC nanoclusters. Carbon is vaporized applying an electricaldischarge of 700÷1000 V between two graphite electrodes in aceramic cavity. Mixing the carbon vapour with a pulse of Hegas, the cluster condensation was promoted. The cluster sourcewas connected by a nozzle to the deposition chamber. Thesamples consisted of the first CrN layer (0.5 μm thick) and ofthe second CrN:C layer (1.0 μm thick), with increasing amountof C nanoclusters from NANO1 to NANO3. The mainparameters used for the CrN deposition were the same as inthe first series of the samples. Main parameters of the pulsedmicroplasma source, adopted for the deposition of CrN:C, arereported in Table 1. All the samples have been produced at theCentro Sviluppo Materiali Spa (Rome, Italy).

2.2. Characterization

Photoemission spectra have been collected by using a VGESCALAB MkII spectrometer, equipped with a standard Al Kα

excitation source and a 5-channeltron detection system. Theexperiments have been performed at a pressure of about1×10−10 mbar that has been increased up to 1×10−7 mbarduring the depth profiling. The energy of the ion gun (Ar+) has

aks: A—574.2 eV; B—576.0 eV; (b) N 1s with two peaks: A—397.1 eV;

Fig. 2. XPS depth profiles of the samples CrN1 on AISI H13 substrate (a) and NANO1 on K 340 substrate (b).

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been set to 2.0 keVand the sample current density— to 2×10−3

mA/cm2, that corresponds to the average sputtering rate ofabout 0.2 nm/min. The binding energy (BE) scale wascalibrated by measuring the C 1s peak (BE=285.0 eV) fromthe surface contamination. The accuracy of the measured BEwas ±0.1 eV.

AES/SAM experiments have been performed by using thesame spectrometer. The submicron-spot electron gun, a LEG200, was operated at 10 keV and 1–10 nA current. Augerspectra were registered in a constant retard ratio (1 :2) analysermode. Surface chemical images were obtained as (P−B) :Peakheight minus Background height.

Oblique section of the coatings has been prepared by using alapping tool, composed of two parallelepipeds cut at 45° angleand connected by two screws [10,11].

The measurements of X-ray diffraction were carried out byusing an X-Pert Philips equipped with copper anode(Kα=1.54056 Å), in a 2θ range between 20 and 100° withthe step of 0.02°.

Electrochemical tests were performed by fixing on thesample surface a Pyrex glass cylinder of 4 cm in inner diameterby means of silicone sealant. Thus the sample surface acted as abase of the electrochemical cell, that was equipped with an Ag/

Fig. 3. SEM image of the multiple-layer coating CRN3 on AISI 1040 substrate.

AgCl reference electrode and a platinum counter electrode. EISwas performed in 0.5% NaCl solution by using a Solartron SI1287 Electrochemical Interface and a Solartron 1260 FrequencyResponse Analyser at the OCP, in the frequency range from5 mHz to 50 kHz imposing an AC signal of 10 mV. EIS spectrawere analyzed by Zview software (Scribner Associates), usingNon Linear Least Squares (NLLS) method.

3. Results and discussion

3.1. XPS and AES characterization

The chemical composition of the coatings has beeninvestigated by means of surface-sensitive XPS and AEStechniques. A typical photoelectron spectrum of Cr 2p region,acquired for CrN sample after a light Ar+ sputtering (10 min at2.0 keV), is shown in Fig. 1a. The synthetic components of Cr2p3 / 2 peak, obtained by using the peak-fitting routine, are alsopresented in this figure. As it is known from the literature[12,13], for correct interpretation of Cr 2p spectra of chromiumcompounds it is necessary to take into account the effect ofmultiplet splitting. The main photoemission peak was fixed atBE=574.2 eV, whereas a large synthetic component related tothe contributions of multiplet splitting was centred at 576.0 eV.Only the main photoemission peak was used for the quantifica-tion of Cr concentration. This component (BE and FWHM

N KLL Cr LMM(a) (b)

Fig. 4. SAM images of the multiple-layer coating CRN3 on AISI 1040 substrate:(a) N KLL; (b) Cr L3M23M45.

Table 3Values of residual stress of the CrN and Cr in CrN3 and NANO samples on AISIH 13 substrate

Sample Plane ε σ (Gpa)

CrN CrN3 (AISI H 13) (111) 0.00465 0.3721(200) 0.00245 0.19593

Cr CrN3 (AISI H 13) (110) −0.00168 0.33545(200) −0.0014 0.28018

CrN NANO3 (AISI H 13) (111) −0.00315 0.25167(200) −0.00769 0.6153(200) −0.01896 1.51655

Fig. 5. Auger spectra acquired in the different points (marked on the SEM imagein Fig. 3) of the cross-section of the sample CRN3 on AISI 1040 substrate.

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values of the peak) was determined by using the analysis of Cr2p3 / 2 signal from the Cr2O3 reference powder. Obtained BEvalues of both synthetic peaks (the main photoemission andmultiplet splitting) are in good agreement with literature data[14,15].

N 1s spectrum of the same sample is presented in Fig. 1b.The peak fitting revealed two chemical species of nitrogen:chromium nitride at BE=397.1 eV and a component of oxy-nitrides at BE=398.6 eV. The second contribution is probablydue to the presence of low amount of oxygen in the depositionchamber [16,17].

The high amount of C and O, detected in all the coatings, isattributed to the surface contamination. The coatings NANOshowed a higher amount of carbon, consequently to thepresence of carbon nanoclusters and atmospheric surfacecontamination. However, it is practically impossible to discernthese two contributions only from the analysis of C 1s spectra.

The surface chemical composition of the coatings, deter-mined from XPS data, is shown in Table 2, where theconcentration of all elements is reported in atomic percent.All the contaminants were rapidly removed after a short time ofAr sputtering. The Fig. 2a,b illustrate the decrease of theconcentrations of carbon and oxygen after the removal of the

Fig. 6. XRD spectra of the sample CrN2 on AISI H13 substra

first few nanometers of the coating. From Fig. 2b, it is possiblenote that the ion sputtering removes efficiently both species ofcarbon (nanoclusters and contaminants). Therefore, thenanoclusters of carbon were formed only in the topmost surfacesublayer of the coatings.

The high roughness of the samples obstructs the completeremoval of the environmental contamination. Namely for thisreason, some traces of these elements were still present evenafter a long time of ion sputtering.

The total thickness of the coatings was too high to reach thesubstrate by using the ion sputtering. Therefore, the XPS depthprofiling, reported in Fig. 2, was carried out only for the analysisof the outermost CrN layers. All investigated samples (multiple-layered CrN and coatings doped with nanoclusters of C) werecharacterized by the constant composition in depth, where theratio of Cr /N was near to 1.0.

The cross-section of some selected samples were studied byAES/SAM. The physical (SEM) and chemical images (SAM) ofmultiple-layer coating CrN/Cr/CrN on AISI 1040 substrate areshown in Figs. 3 and 4. The SAM images (Fig. 4) were acquiredby using (P−B) of the Cr L3M23M45 peak and N KLL peak. Inthe SEM physical image, it is possible to distinguish fourdifferent regions (Fig. 3), while in the chemical image isevidenced the metallic layer of Cr (Fig. 4). Each region of theSEM image was investigated by the AES. The substrate (region1) is characterized by the presence of Fe LMM peaks(KE=594.0, 652.0 and 705.7 eV) and low intensity KLLpeaks of C and O (see Fig. 5). The Auger spectra of the regions

te: (a) wide spectrum; (b) a zoom of the same spectrum.

Fig. 7. EIS experimental data (squares) and the simulation of equivalent circuit (curves) as a function of immersion time for the sample CrN1 on K340 substrate.

102

103

104

105

0 50 100 150 200

CrN1

CrN2

CrN3

Rct

(O

hm)

Time (hours)

Fig. 8. Charge transfer resistance of CrN samples on AISI 1040 substrate as afunction of immersion time.

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2 and 4 reveal the presence of Cr L3M23M45 (KE=530.6 eV)and N KLL (KE=385.4 eV) peaks, whereas in the spectrum ofthe region 3 is present only a peak of Cr L3M23M45 [18,19].

Multi-point AES spectra confirm that the region 3 iscomposed from a layer of metallic Cr located between twolayers of CrN, (regions 2 and 4 in Figs. 3 and 4). The brightnarrow strip, visible between regions 1 and 2, is a thin metallicsublayer of Cr deposited on the substrate.

Total thickness of this coating, estimated from the SEM/SAM images, is about 9.0 μm, what is in good agreement withthe nominal thickness, considering all the experimental errors(deposition process and characterization).

3.2. XRD investigations

A typical XRD spectrum, acquired for the sample CrN2 onAISI H13 steel substrate, is presented in Fig. 6a. CharacteristicXRD peaks of CrN were detected in all the samples. Moreover,the diffraction pattern of metallic Cr deposited under the layer ofCrN was revealed in multiple-layer samples. Fig. 6b shows azoom of the XRD spectrum between 2θ=60 and 67, evidencinga strong shift of the reflections with respect to the standardpattern. This behaviour is attributed to residual stress in CrNfilms. Taking into account the elastic modulus and the Poissoncoefficient for CrN film, the numerical estimation of theresidual stresses permitted to conclude for all the samples thatthe external layer of CrN is in compressive state (Table 3).

3.3. Impedance measures

EIS data were analysed by using the equivalent circuitreported in literature for the coated metals [20,21] in whichthe electrolyte resistance (Rel), the charge transfer resistance(Rct), the double layer capacitance (Cdl), the coating resistance(Rco) and capacitance (Cco) are taken in consideration. Theuse of a Constant Phase Element (CPE), i.e. non-idealcapacitance was required due to sample roughness [22]. As anexample of EIS data fitting, Fig. 7 reports three EIS spectraacquired for CrN1-K340 sample after 24, 48 and 96 h ofimmersion in NaCl solution. The results of the NLLSsimulation are shown in the same figure. It is worth pointingout that the calculated data are in a good agreement with the

experimental ones, as it is also confirmed by the χ2 test thatassumed the values lower than 5·10−4. As a consequence ofthis, the error associated to parameter calculation was lowerthan 10%.

As a general trend, the spectra acquired after few hours ofimmersion showed only single time constant, attributed to thecoating characteristics, since the film is still protectivetowards the solution and the Faradaic contribution (Rct andCdl) to the impedance characteristic is negligible. After avariable period of time, EIS revealed the second time constantdue to corrosion reaction. It can be assumed that the solutionpermeated the coating through its defects and reached themetal surface thus starting a corrosion attack. The onset ofcorrosion phenomena is followed by a general decrease of theimpedance of the samples.

The trend of Rct for CrN multiple-layer sample depositedon 1040 carbon steel is reported in Fig. 8. It can generally beseen that a noticeable decrease of Rct can be detected after thefirst hours of immersion for all the samples. After 50 h, theRct assumed a constant value of around 450 Ω. As expected,the values of Rct were higher for thicker films. The values,recorded at the end of the test (last 50 h of immersion), areinfluenced by the corrosion resistance of the substrate, sincethe solution permeated the film and it gave no protection atall. For this reason, the coatings deposited on H13 and K340steels, showed the same trend as in Fig. 8, but with theresistance higher than the films deposited on 1040 steel. TheRct final values for the coatings on H13 and K340 steels were

Fig. 9. Photoelectron spectra of the corroded sample CrN3 on K 340 substrate: (a) Fe 2p with three peaks: A—709.7 eV; B—712.2 eV; C—715.6 eV; (b) Cr 2p withtwo peaks: A—574.6 eV; B—576.8 eV.

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about 400–900 Ω. The samples with carbon clusters gavesimilar Rct trends, but with lower resistance values, as aconsequence of the thinner coating. Their Rct values rangedbetween 230 and 450 Ω.

At the end of the corrosion tests, the samples have beenanalysed by using XPS and AES techniques. In Fig. 9, Fe 2pand Cr 2p spectra of the sample CrN3 on K340 substrate arepresented. The peak fitting of the Fe 2p3 / 2 spectrum was doneby using three synthetic components: the component A at709.7 eV was assigned to FeO, the component B at 712.2 eV toFeOOH, whereas the component C to shake-up satellite. Thepeak fitting of the Cr 2p3 / 2 region has been realized with onlytwo components: A is the nitride signal at 574.6 eVand B is theoxide (Cr III) signal at 576.8 eV. The XPS analysis revealed alsothe presence of N and O on the sample surface.

Fig. 10 shows the AES spectra of the same sample. Thepeaks of Fe, Cr, O and N are well identified and the SAM image(Fig. 11) shows the lateral distribution of Cr and O in thecorroded surface area. The oxygen was present over all thecorroded surface (Fig. 10), in particular with a concentrationhigher in the zones containing iron. As it was proved by XPSanalysis, Cr was present in two chemical compounds: CrN andCr2O3. Moreover, it is possible to note that the corrosionprocess was not uniform and this is probably due to the porosityof the coating, which was proved also by the EIS measurements.The presence of Cr2O3 can be explained by the oxidation of thesecond layer (metallic C), that is oxidized after the removal ofthe first layer of CrN.

Fig. 10. Auger spectra acquired in two different points of the corroded sampleCrN3 on K 340 substrate.

4. Conclusions

In order to improve the tribological properties, the coatingsof multiple-layer CrN/Cr and CrN, doped with C nanoclus-ters, were produced by using PVD. The samples werecharacterized by using multi-technique surface analysis, thatrevealed the stoichiometry and uniformity in-depth of all thecoatings. A low amount of oxy-nitrides due to the presence ofO in the deposition chamber was also observed. A method ofoblique cross-section was successfully applied for the AES/SAM characterization of the multiple-layer coatings. XRDresults demonstrated that the external layer of the multiple-layer coatings is always characterised by the presence ofcompressive residual stresses, which have similar values,independently from the type of substrate. The corrosion testsdemonstrated that the samples, covered with CrN/Cr multiple-layer coatings, are more resistant to corrosion than the ones,covered with the coatings doped with clusters of C. Finally,the results of EIS and AES/SAM measurements revealed thatthe corrosion process begins at the surface defects andpropagates towards the substrate.

Fig. 11. Overlay of two SAM images (Cr L3M23M45 and O KLL) of the corrodedsample CrN3 on K 340 substrate: dark regions correspond to Cr; clear regionscorrespond to O.

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Acknowledgement

This research was supported by the Italian Ministry ofUniversities and Research (MIUR) under the framework of theNational Research Program L. 449/97.

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