Comparative studies on the thermal stability and corrosion resistance of CrN, CrSiN,and Cr Si N Al N coatingsGwang Seok Kim, Sung Min Kim, Sang Yul Lee, and Bo Young Lee

Citation: Journal of Vacuum Science & Technology A 27, 873 (2009); doi: 10.1116/1.3155396 View online: http://dx.doi.org/10.1116/1.3155396 View Table of Contents: http://scitation.aip.org/content/avs/journal/jvsta/27/4?ver=pdfcov Published by the AVS: Science & Technology of Materials, Interfaces, and Processing Articles you may be interested in High-corrosion-resistant Al 2 O 3 passivation-film formation by selective oxidation on austenitic stainless steelcontaining Al J. Vac. Sci. Technol. A 29, 021002 (2011); 10.1116/1.3543709 Influence of the bias voltage on the structure and mechanical performance of nanoscale multilayer Cr Al Y N CrN physical vapor deposition coatings J. Vac. Sci. Technol. A 27, 174 (2009); 10.1116/1.3065675 Microstructural analysis and oxidation protection behavior of Cr–Ni surface coatings of 347 stainless steel byrapid mirror scanning of a high power C O 2 laser J. Laser Appl. 21, 16 (2009); 10.2351/1.3071396 Growth and characterization of Ti Al N Cr Al N superlattices prepared by reactive direct current magnetronsputtering J. Vac. Sci. Technol. A 27, 29 (2009); 10.1116/1.3013858 Corrosion behavior of Zr modified CrN coatings using metal vapor vacuum arc ion implantation J. Vac. Sci. Technol. A 25, 110 (2007); 10.1116/1.2400682

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Comparative studies on the thermal stability and corrosion resistanceof CrN, CrSiN, and CrSiN/AlN coatings

Gwang Seok Kim,a� Sung Min Kim, and Sang Yul Leeb�

Department of Materials Engineering, NRL for Cracking Control and Management, Korea AerospaceUniversity, Hangkongdae-Gil 100, KoYang-Si, KyungKi-Do 412-791, South Korea

Bo Young LeeNRL for Cracking Control and Management, Faculty of Mechanical and Aerospace Engineering,Korea Aerospace University, Hangkongdae-Gil 100, KoYang-Si, KyungKi-Do 412-791, South Korea

�Received 1 October 2008; accepted 26 May 2009; published 29 June 2009�

In this work, three kinds of Cr-based nitride coatings such as monolithic CrN, CrSiN coatings, andmultilayered CrSiN /AlN coating with bilayer period of 3.0 nm were deposited on both Si �100�wafer and AISI H13 steel substrates by unbalanced magnetron sputtering. Thermal stability of thesecoatings was evaluated by annealing the coatings at temperatures between 600 and 1000 °C for30 min in air. In addition, the corrosion behaviors of these coatings were investigated bypotentiodynamic polarization tests in a deaerated 3.5 wt. % NaCl solution at 40 °C. Results fromannealing test show the monolithic CrN and CrSiN coatings were completely oxidized afterannealed at 800 and 900 °C, and their cross sectional images and atomic force microscopy showeda loose and very porous morphology due to the oxidation. Also, the hardness values of themonolithic CrN and CrSiN coatings were decreased significantly from 22 and 27 GPa to 8 and14 GPa, respectively. However, the multilayered CrSiN /AlN coating still exhibited a densemicrostructure without visible change after annealed at 1000 °C, and moreover, the relatively highhardness of 25 GPa was maintained. The superior thermal stability of the CrSiN /AlN multilayercoating could be attributed to the formation of the dense and stable oxidation barrier consisted of theAl2O3, Cr2O3, and amorphous SiO2 phases near the surface region, which retard the diffusion ofoxygen into the coating. In the potentiodynamic polarization test results, it was found that thesignificantly improved corrosion resistance of the multilayered CrSiN /AlN coating was observed incomparison with those from the monolithic CrN and CrSiN coatings, and its corrosion currentdensity �icorr� and protective efficiency were measured to be approximately 4.21 �A /cm2 and 95%,

respectively. © 2009 American Vacuum Society. �DOI: 10.1116/1.3155396�

I. INRODUCTION

Binary CrN coatings have been extensively used as a pro-tective hard coating in the field of cutting tools, dies, andmany mechanical parts for the past decades due to their at-tractive properties such as good thermal stability, tribologicalproperty, and corrosion resistance.1,2 However, in spite oftheir excellent properties, the CrN coatings show inadequateproperties for some applications as high speed machining orat high temperature or severely corrosive conditions. Espe-cially, the thermal stability of the CrN coatings is very lim-ited at high temperature above 700 °C because the CrN coat-ings tend to oxidize rapidly.3 Recently, intensive efforts toimprove further thermal stability, corrosion resistance, aswell as mechanical properties of the CrN coatings have beenmade and both multicomponents and multilayer structurecoatings such as Cr–Al–N,4 Cr–Si–N,5 Cr–Al–Si–N,6

CrN /AlN,7 CrN /Si3N48 system have been studied exten-

sively. Particularly, the incorporation of Si leads to a signifi-cant improvement of CrN coatings by changing the coatingstructure. It has been found that the maximum hardness

a�Present address: Cheorwon Plasma Research Institute, Galmaleup,Cherwon-gun, GangWon, South Korea.

b�

Electronic mail: sylee@kau.ac.kr

873 J. Vac. Sci. Technol. A 27„4…, Jul/Aug 2009 0734-2101/2009

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�=24 GPa� is achieved in films with a low �2–6 at. % � Sicontent, suggesting that formation of amorphous Si3N4 at thegrain boundaries of the CrN hinders the propagation ofdislocations.9 Also, CrSiN showed that the highly dense mi-crostructure yields much better corrosion resistance com-pared with CrN coating with a columnar structure.10

However, CrSiN coating with low Si content at high tem-perature showed not only poor hardness but also poor ther-mal stability. Yoo et al.11 indicated that oxidation resistanceof MeN /Si3N4 with high Si content ��25 at. % � could beincreased above 1000 °C. In our previous study, the hard-ness of CrSiN coating with high Si content �=33 at. % �was measured to be approximately 20 GPa. Therefore,the addition of AlN coating layers into the CrSiN with a lowSi content �=5 at. % � to have CrSiN /AlN multilayer coat-ings is expected to be effective in improving thermal stabil-ity, corrosion resistance, as well as mechanical properties.AlN coatings have been reported to have excellent thermalstability and corrosion resistance because of high meltingpoint and strong covalent bonding between Al and Natoms.12,13

In this study, the multilayered CrSiN /AlN coatings withbilayer period of 3.0 nm were synthesized, and their thermal

stability and corrosion behaviors were evaluated by anneal-

873/27„4…/873/7/$25.00 ©2009 American Vacuum Society

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ing treatments and potentiodynamic polarization tests, re-spectively. The results were compared with those of themonolithic CrN and CrSiN coatings

II. EXPERIMENTAL DETAILS

All coatings were deposited on both Si �100� wafer andAISI H13 steel substrates by unbalanced magnetron sputter-ing. Prior to the deposition, the substrates were cleaned withacetone and ethanol in ultrasonic vessel for 10 min, respec-tively. The base pressure of deposition chamber was pumpeddown to less than 1.3�10−3 Pa. During the deposition, Arpressure was set initially at 3.2�10−1 Pa and reactive N2 gaswas subsequently added to obtain desired gas composition,maintaining a total working pressure of 4.4�10−1 Pa. Themultilayer was synthetized by controlling the rotating jigspeed without shutters and the substrate rotating speed waskept at 18 rpm for the CrSiN /AlN multilayer coatings ��=3 nm�. Details for the deposition conditions of the coatingssynthesized in this work are summarized in Table I.

The chemical compositions of the coatings were deter-mined by glow discharge optical emission spectroscopy�GDOES� �LECO GDS 850A� and the crystal phases werecharacterized by x-ray diffraction �XRD� �D/max2200� withCu K� radiation ��=0.154 18 nm�. For the CrSiN /AlNmultilayer coating, the bonding status was analyzed by x-rayphotoelectron spectroscopy �XPS� �VG Multilab ESCA2000� with a monochromatic Al K� radiation �1487 eV�, andthe cross sectional structure was characterized by transmis-sion electron microscopy �TEM� �JEM-3011� operating at300 kV. The hardness of all coatings was measured using aFisherscope H100CXYp instrument with a load of 50 mN.To evaluate the thermal stability, the coatings were heated attemperature from 600 to 1000 °C in air. Annealing was per-formed from room temperature to the desired temperature ata heating rate of 3 °C /min and maintaining at the desiredtemperature for 30 min. After annealing treatment, the el-emental distributions were examined using scanning electronmicroscopy �SEM� �HITACHI S-3500H� equipped withenergy dispersive spectrometer �EDS� �HORIBA EMAX7021-H� and the atomic force microscopy �AFM� �SPI

TABLE I. Deposition conditions of the coatings.

Variable Cr

Base pressure �Pa� 1.3�

Working pressure 4.4�

N2 partial pressure 1.2�

Target Cr ��9

Target power density �W /cm2��pulsed dc: frequency 20 kHz, duty 80%� 8.6

Substrate-target distance �mm� 7Substrate-bias voltage dc −1Deposition temperature 150Substrate rotation speed �rpm� 1Coating thickness ��m: approximately� 3.

3800 N�. The corrosion behaviors of the coatings were

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evaluated using an EG&G PAR model 263A galvanostat/potentiostat system. A three electrode electrochemical cellwas used with highly pure carbon rod counterelectrode andan Ag /AgCl electrode as the reference electrode. The speci-men to be tested was the working electrode. The test solutionwas 3.5 wt. % NaCl, and it was deaerated with pure N2 gasfor 1 h before testing to remove the dissolved oxygen. Alltests were performed at 40 °C and open-air conditions. Thespecimen was placed in such a way that the Luggin capillaryof the reference electrode was close to the working electrodeand this distance was maintained for all the tests. Prior to thepotentiodynamic polarization test, the specimen was kept inthe solution for 60 min in order to establish the open-circuitpotential �EOCP� or the steady state. The polarization curveswere measured at a scan rate of 0.166 mV /s, starting froman initial potential of −250 mV versus the open-circuit po-tential of the tested sample. The Tafel plot was obtained afterthe electrochemical measurements. The corrosion potential�Ecorr� and corrosion current density �icorr� were deducedfrom the Tafel plot �that is, log i versus E plot�.

III. RESULTS AND DISCUSSION

The chemical compositions of the monolithic CrN, CrSiNcoatings, and multilayered CrSiN /AlN coating analyzed byGDOES are measured to be 49.1Cr–50.9N, 44.3Cr–5.3Si–50.4N, and 25.6Cr–2.6Si–20.8Al–51N in at. %, respectively.It appeared that all coatings have a similar N2 content be-tween approximately 50.4 and 51 at. % because the N2 par-tial pressure of 1.2�10−1 Pa was maintained constant duringall deposition process. Also, the Si /Cr atomic ratio, 0.12, ofthe monolithic CrSiN coating was almost equal to that, 0.1,of the multilayered CrSiN /AlN coating because the powerdensity applied to the CrSi target was identical, as shown inTable I.

Figure 1 shows the results of x-ray diffraction analysis onthe coatings. The monolithic CrN and CrSiN coatings haveonly CrN B1-NaCl crystalline structure with preferred orien-tation �200�. In the CrSiN coating, the width of the �200�peak broadened as well as the intensity noticeably decreasedin comparison with the CrN, suggesting the diminution of

CrSiN CrSiN /AlN

1.3�10−3 1.3�10−3

4.4�10−1 4.4�10−1

1.2�10−1 1.2�10−1

� CrSi �Si=10 at. % � CrSi �Si=10 at. % � andAl ��99.99% �

8.64CrSi target: 8.64Al target: 14.81

70 70dc −100 V dc −100 V

150 °C 150 °C12 123.0 3.0

N

10−3

10−1

10−1

9.99%

4

000 V°C

20

the grain size or the residual stress induced in the crystal

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lattice.14 Also, any peaks of crystalline Si3N4 phase were notdetected, suggesting that the Si–N phase in the CrSiN coat-ing could be amorphous phase, i.e., crystalline CrN grainsare embedded in the amorphous SiNx matrix.15 The averagegrain size of CrN and CrSiN was calculated from the fullwidth at half maximum value of the XRD �200� peaks usingthe Scherrer equation,16

Dhkl =K�

��hkl cos hkl�, �1�

where D is the grain size �nm�, K is Scherrer constant �0.89�,� is the x-ray wavelength, � is FWHM, and is the diffrac-tion angle. Compared to the grain size of CrN ��35 nm�, theresults indicate that the average grain size of CrSiN film wasmeasured to be approximately 14 nm. This could be attrib-uted to the broadening of the XRD peak because Si atomssegregate predominantly at the grain boundaries and the Siatom coverage of the possible Cr sites increases,5 giving riseto crystalline refining or amorphous Si3N4 formed. The dif-fraction pattern of the multilayered CrSiN /AlN coating inFig. 1�c� also exhibited B1-NaCl crystalline structure withpreferred �200� orientation, but this peak was shifted moretoward a lower two theta angle than that of the monolithicCrSiN coating due to the presence of the c-AlN. As a result,a diffraction peak between the peaks of the CrSiN �200� andc-AlN �200� appeared and this peak was considered to beformed by mutual diffraction interference from two coatings,CrSiN and AlN. Although the AlN coatings normally have anequilibrium hexagonal structure �wurtzite-type�, the peaks ofthis wurtzite-structured AlN were not detected as shown inFig. 1. However, the Bl NaCl-type peaks of �200� and �220�was observed for the CrSiN /AlN multilayer coating, indicat-ing that the AlN layers in this coating have a metastablecubic structure, which was also confirmed from the results ofTEM analysis.

The high resolution XPS core-level spectra of the multi-

FIG. 1. X-ray diffraction patterns of the as-deposited coatings; �a� CrN, �b�CrSiN, and �c� CrSiN /AlN.

layered CrSiN /AlN coating in the Si2p and N1s energy re-

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gions are shown in Figs. 2�a� and 2�b�. All the XPS spectraare analyzed by Gaussian fit to identify the chemical bondingof the coating. The binding energy of Si2p for Si3N4 revealsthe characteristic peak centered at binding energy of101.5 eV and a higher bonding energy of 102.7 eV is asso-ciated with Si–O bonding. The N1s spectrum also reveals thepresence of three peak characteristic of nitrogen in the AlN,Si3N4, and CrN phases with binding energies at approxi-mately 396.5, 398.1, and 397.1 eV, respectively.17 Therefore,the multilayered CrSiN /AlN coating mainly consists of thecrystalline CrN, AlN, and amorphous Si3N4 phases. Thebright-field cross sectional TEM images of the CrSiN /AlNmultilayer coating are shown in Figs. 2�c� and 2�d�. Thecoating exhibited a dense columnar structure and the inter-faces of component layers are clearly distinguished, whichindicates little interdiffusion between the layers. The CrSiNand AlN layers alternating in growth direction are shown asbright and dark layers, respectively. The lattice fringes for�200� and �220� directions of the coating were measured tobe 2.083 and 1.488 Å �d200=2.068 and d220=1.463 Å inJCPDS data for CrN� as shown in Fig. 2�d�. This resultshows that the crystal structure of AlN layers has grown onCrSiN base layer in the CrSiN /AlN multilayer coating, andit has a metastable cubic lattice structure �c-AlN� instead ofequilibrium hexagonal structure �wurtzite-type: w-AlN� dueto the growth template effect.18 In addition, the interface ofeach layer in the superlattice �nanomultilayer� coatings wasmatched coherently. From the selected area diffraction pat-tern �as shown in the insert in Fig. 2�c��, the spotty ringpattern reveals a typical diffraction pattern of NaCl crystalstructure and no diffractions corresponding to wurtzite-typeAlN was observed. Also, from the high-resolution TEM�HRTEM� image in Fig. 2�d�, the bilayer period ��� of theCrSiN /AlN multilayer coating synthesized in this work wasobserved to be approximately 3.0 nm and the thickness ratioof the CrSiN layer to the AlN layer was approximately 1:1.

Figure 3 presents the hardness of the coatings as a func-tion of annealing temperature from 600 to 1000 °C after an-nealing for 30 min in air. The hardnesses of the as-depositedCrN, CrSiN, and CrSiN /AlN coatings were measured to be22, 27, and 32 GPa, respectively. The hardness of CrSiNcoating is higher than that of CrN coating. This could beexplained by the reduction in grain size due to the increase inthe presence of segregated Si atoms. Therefore, the grainboundary hardening by Hall–Petch relationship would takeplace. The hardness of the CrSiN /AlN multilayer coatingshowed much higher than that of CrSiN coating although thecalculated grain size of 11 nm for the CrSiN /AlN multilayercoating was similar to that of 14 nm for the CrSiN coating.Thus, it is difficult to demonstrate that grain size was notmain factor to hardening in the multilayer system. The sig-nificant hardness enhancement of the CrSiN /AlN multilayercoating could be related to the theoretical model by Koehler,which is based on nanoscale multilayers consisting of twodifferent materials with large differences in shear moduli andsharp interface.19 For CrSiN /AlN multilayers, the shear

moduli of both films were calculated using G=E /2�1+�,

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FIG. 2. XPS core-level spectra �a� Si2p, �b� N1s, and �c� XTEM and �d� HRTEM images of the as-deposited CrSiN /AlN multilayer coating.

0 200 400 600 800 10000

4

8

12

16

20

24

28

32

36

40

Annealing temperature (oC)

Hardness(GPa)

as-deposited

CrNCrSiNCrSiN/AlN with Λ=3.0 nm

CrSiN/AlN

CrSiN

CrN

Cr: 24.3 at.%, N: 37.6 at.%, O: 15.5 at.%Si: 2.7 at.%, Al: 19.9 at.%

Cr: 43.4 at.%, N: 24.4 at.%, O: 27.1 at.%Si: 5.1 at.%

Cr: 44.7 at.%, N: 9.5 at.%, O: 45.8 at.%

0 200 400 600 800 10000

4

8

12

16

20

24

28

32

36

40

Annealing temperature (oC)

Hardness(GPa)

as-deposited

CrNCrSiNCrSiN/AlN with Λ=3.0 nm

CrSiN/AlN

CrSiN

CrN

Cr: 24.3 at.%, N: 37.6 at.%, O: 15.5 at.%Si: 2.7 at.%, Al: 19.9 at.%

Cr: 43.4 at.%, N: 24.4 at.%, O: 27.1 at.%Si: 5.1 at.%

Cr: 44.7 at.%, N: 9.5 at.%, O: 45.8 at.%

FIG. 3. �Color online� Hardness variation and EDS analysis of the coatings after annealing.

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where G is the shear modulus, E is Young’s modulus, and is the Poisson ratio. The calculated GCrSiN and GAlN were74.9 and 97.0 GPa, respectively. As a result, the hardnessenhancement due to the modulus difference across the inter-faces seems to be a significant factor influencing thestrengthening of CrSiN /AlN multilayer coatings. Figure 3indicated that the monolithic CrN and CrSiN coatings werecompletely oxidized at 800 and 900 °C, respectively, andtherefore their hardness values were significantly decreasedfrom approximately 22 and 27 GPa to 8 and 14 GPa, respec-tively. However, for the multilayered CrSiN /AlN coatingwith �=3.0 nm, the hardness of approximately 30 GPa,close to 32 GPa of as-deposited coating, was maintained up

CrN CrSiN CrSiN/AlN

asdeposited

RMS: 4.5 nm (R.T) RMS: 1.4 nm (R.T) RMS: 1.2 nm (R.T)

annealed

22㎛㎛

800 °C 900 °C 1000 °C

RMS: 35.3 nm (800℃) RMS: 20.2 nm (900℃) RMS: 15.03 nm (1000℃)

FIG. 4. �Color online� Cross sectional SEM and surface morphology of thecoatings as-deposited and after annealing.

(a)

35 40 45 50 55 60

500

1000

35 40 45 50 55 600

500

1000

35 40 45 50 55 600

500

1000

35 40 45 50 55 60

500

1000

Intensity

2 Theta

Ad-deposited

0

0

800℃

900℃

1000℃

●

(111)

●

(200)

●

(220)

□

(122)

■

(220)

○

(240)

▼

(111)

■

(200)

35 40 45 50 55 60

500

1000

35 40 45 50 55 600

500

1000

35 40 45 50 55 600

500

1000

35 40 45 50 55 60

500

1000

Intensity

2 Theta

Ad-deposited

0

0

800℃

900℃

1000℃

●

(111)

●

(200)

●

(220)

□

(122)

■

(220)

○

(240)

▼

(111)

■

(200)

35 40 45 50 55 60

500

1000

35 40 45 50 55 600

500

1000

35 40 45 50 55 600

500

1000

35 40 45 50 55 60

500

1000

Intensity

2 Theta

Ad-deposited

0

0

800℃

900℃

1000℃

●

(111)

●

(200)

●

(220)

□

(122)

■

(220)

○

(240)

▼

(111)

■

(200)

35 40 45 50 55 60

500

1000

35 40 45 50 55 600

500

1000

35 40 45 50 55 600

500

1000

35 40 45 50 55 60

500

1000

Intensity

2 Theta

Ad-deposited

0

0

800℃

900℃

1000℃

●

(111)

●

(200)

●

(220)

□

(122)

■

(220)

○

(240)

▼

(111)

■

(200)

●: CrN,▼:Cr2N,○: Cr2O3,■:AlN,□:Al2O3●: CrN,▼:Cr2N,○: Cr2O3,■:AlN,□:Al2O3●: CrN,▼:Cr2N,○: Cr2O3,■:AlN,□:Al2O3●: CrN,▼:Cr2N,○: Cr2O3,■:AlN,□:Al2O3

FIG. 5. �Color online� �a� X-ray diffraction patterns and �b� Si2p XPS

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to 800 °C, and moreover the relatively high hardness of25 GPa was measured even after annealed at 1000 °C al-though a moderate hardness drop was observed. Accordingto EDS analysis on the surface of the coatings after anneal-ing, the oxygen content of the multilayered CrSiN /AlN coat-ing annealed at 1000 °C was measured to be 15.5 at. %, andthis value is approximately 1.8 times lower than that�27.1 at. % � of the monolithic CrSiN coating annealed at800 °C. The cross sectional view for the monolithic CrN andCrSiN coatings annealed 800 and 900 °C, respectively, re-vealed a loose and porous surface due to the oxidation. Cor-responding to the SEM results, the root mean square �rms�values of the monolithic CrN and CrSiN coatings extensivelyvaried from 4.50 and 1.42 nm to 35.3 and 20.2 nm, respec-tively. Thobor-Keck et al.20 reported that the porous oxidephases formed by oxidation become an excellent path foroxygen diffusion into the coating layer. However, the multi-layered CrSiN /AlN coating still exhibited a dense micro-structure without visible change after annealed at 1000 °C,as shown in Fig. 4. Also, according to AFM analysis on thesurface roughness, the rms values of the CrSiN /AlNmultilayer coating were almost constant even after anneal-ing. The results from hardness measurement, EDS, SEM, andAFM analyses for the annealed coatings revealed that themultilayered CrSiN /AlN coating with �=3.0 nm has a con-siderably higher thermal stability than those of the mono-lithic CrN and CrSiN coatings. In addition, it was found thatthe multilayered CrSiN /AlN coating exhibits good thermalstability up to 1000 °C.

Figure 5 shows the x-ray diffraction patterns and Si2pXPS spectra of the multilayered CrSiN /AlN coating afterannealing. In Fig. 5�a�, the diffraction peaks of Al2O3 �122�and Cr2O3 �024� appeared from 900 °C, and the Al2O3 peaksignificantly developed at 1000 °C but there is no obviouschange in the Cr2O3 peak because the Gibbs free energy ofAl2O3 �−884.0 kJ /mole� formation at 1000 °C is more nega-tive than that of Cr2O3 �−401.2 kJ /mole�.21 In addition, any

108 106 104 102 100 98

(b)

(a)

Si3N4

SiO2

101.5eV

103.0eVSi 2p

Intensity(arb.units)

Binding Energy (eV)

ad-deposited

1000 °C

108 106 104 102 100 98

(b)

(a)

Si3N4

SiO2

101.5eV

103.0eVSi 2p

Intensity(arb.units)

Binding Energy (eV)

ad-deposited

1000 °C

as-deposited

after annealedat 1000 °C

b)

65

65

65

65

65

65

65

65

65

65

65

65

65

65

65

65

(

spectra of the multilayered CrSiN /AlN coating after annealing.

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diffraction peaks for the SiO2 phase were not observed inFig. 5�a�, but, in the Si2p XPS spectra in Fig. 5�b� from thesurfaces of the multilayered CrSiN /AlN coating annealed at1000 °C, the SiO2 phase was formed from the amorphousSi3N4 by oxidation, suggesting that the SiO2 exists as anamorphous phase. Therefore, the superior thermal stability ofthe multilayered CrSiN /AlN coating could be attributed tothe formation of dense and stable oxidation barrier consistedof the Al2O3, Cr2O3, and amorphous SiO2 near the surfaceregion at the initial oxidation stage, retarding further oxygendiffusion into the coating.

Potentiodynamic polarization curves for the CrN, CrSiN,and CrSiN /AlN coatings �thickness: approximately 3 �m�deposited on the AISI H13 steel substrate measured in3.5 wt % NaCl solution at 40 °C are presented in Fig. 6. Thepolarization curve of the uncoated substrate is also presentedas a reference. As shown in Fig. 6, the anodic polarizationcurves of all coatings showed relative positive corrosion po-tential and lower current density than those of the substrate.This means that the coatings act as a protective barrieragainst the reaction of the substrate. Table II summarizes thecorrosion parameters of all specimens obtained from the po-larization curves. The corrosion current density �icorr� is animportant parameter to assess the kinetics of corrosion reac-tion, and it is normally proportional to the corrosion rate. Inthe case of the substrate, the corrosion current density isabout 86.8 �A /cm2, which significantly decreased to ap-proximately 35.4 �A /cm2 for the CrN coating. Also, theCrSiN coating exhibited much lower corrosion current den-

1E-8 1E-7 1E-6 1E-5 1E-4 1E-3 0.01-1.0

-0.8

-0.6

-0.4

-0.2

0.0

Current density (A/cm2)

Potential(V)

Substrate

CrNCrSiN

CrSiN/AlN

FIG. 6. �Color online� Polarization curves of the coatings and AISI H13 steelsubstrate in 3.5% NaCl solution.

TABLE II. Potentiodynamic polarization data of the coatings and substrate in3.5% NaCl solution.

Properties Substrate CrN CrSiN CrSiN /AlN

Ecorr �mV� −568.9 −495.4 −443.3 −401.5icorr ��A /cm2� 86.8 35.4 5.65 4.21Protective efficiency �%� ¯ 59 93 95

J. Vac. Sci. Technol. A, Vol. 27, No. 4, Jul/Aug 2009

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sity of 5.65 �A /cm2 than that of the CrN. The multilayeredCrSiN /AlN coating exhibited the lowest corrosion currentdensity of icorr=4.21 �A /cm2. The results implied that theformation of nanomultilayer by the addition of the AlN lay-ers into the CrSiN ternary system improves the chemicalinertness in an aggressive aqueous environment. WilliamGrips et al.22 reported that most of the micropores areblocked in the multilayer coatings as a result of successivedeposition of the two layers, which is responsible for highercorrosion resistance of the multilayer coatings. From polar-ization test results, the protective efficiency Pi�% � of thecoatings can be calculated by Eq. �2�,

Pi� % � = �1 − � icorr

icorr0 � � 100, �2�

where icorr and icorr0 indicate the corrosion current densities of

the coating and substrate, respectively.18 The calculated pro-tective efficiencies for all tested coatings are presented inTable II. The protective ability �93%� of the monolithicCrSiN coating significantly increased in comparison with theCrN �59%�, and the multilayered CrSiN /AlN coatingshowed the highest protective efficiency of 95% caused bythe lowest corrosion current density of 4.21 �A /cm2. Thesuperior corrosion resistance property of the multilayeredCrSiN /AlN coating could be mainly due to the nanomulti-layer structure as well as the formation of much more densemicrostructure in comparison with the single layer coatings,which blocks direct path between a corrosive environmentand the substrate.

IV. CONCLUSIONS

In this work, three kinds of Cr-based nitride coatings suchas monolithic CrN, CrSiN coatings, and multilayeredCrSiN /AlN coating with �=3.0 nm were comparativelystudied on their thermal stability and corrosion behaviors.The results revealed that the ternary CrSiN coating exhibitedbetter thermal stability as compared with the binary CrNcoating, but it was completely oxidized at 900 °C and there-fore the hardness value was significantly decreased from ap-proximately 27 to 14 GPa. However, in the multilayeredCrSiN /AlN coating, the hardness of approximately 30 GPaclosed to that �32 GPa� of as-deposited coating was main-tained up to 800 °C and, moreover, the relatively high hard-ness of 25 GPa was measured even after annealing at1000 °C. The superior thermal stability of the multilayeredCrSiN /AlN coating could be attributed to the formation ofthe dense and stable oxidation barrier consisted of the Al2O3,Cr2O3, and amorphous SiO2 near the surface region at theinitial oxidation stage, retarding further oxygen diffusion intothe coating. In the potentiodynamic test results, the multilay-ered CrSiN /AlN coating showed the lowest current densityof 4.21 �A /cm2 and the highest protective efficiency of 95%in comparison with the monolithic CrN and CrSiN coatings,suggesting that the formation of nanomultilayer by the addi-tion of the AlN layers into the CrSiN ternary system im-proves the chemical inertness in an aggressive aqueous

environment.

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879 Kim et al.: Comparative studies on the thermal stability and corrosion resistance 879

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ACKNOWLEDGMENTS

This research was supported by a grant from the Funda-mental R&D Program for Core Technology of Materialsfunded by the Ministry of Commerce, Industry and Energy,Republic of Korea and by the Korea Science and Engineer-ing Foundation �KOSEF� grant funded by the Korea govern-ment �MOST� �No. M20604005402-06B040040210�.

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