A comparative study of reactive direct current magnetron sputtered CrAlN and CrN coatings

201 (2006) 2193–2201www.elsevier.com/locate/surfcoat

Surface & Coatings Technology

A comparative study of reactive direct current magnetronsputtered CrAlN and CrN coatings

Harish C. Barshilia ⁎, N. Selvakumar, B. Deepthi, K.S. Rajam

Surface Engineering Division, National Aerospace Laboratories, Post Bag No. 1779, Bangalore - 560 017, India

Received 17 February 2006; accepted in revised form 21 March 2006Available online 3 May 2006

Abstract

Approximately 1.5 μm thick CrN and CrAlN coatings were deposited on silicon and mild steel substrates by reactive direct current (DC)magnetron sputtering. The structural and mechanical properties of the coatings were characterized using X-ray diffraction (XRD) andnanoindentation techniques, respectively. The bonding structure of the coatings was characterized by X-ray photoelectron spectroscopy (XPS).The surface morphology of the coatings was studied using scanning electron microscopy (SEM) and atomic force microscopy (AFM). The XRDdata showed that the CrN and CrAlN coatings exhibited B1 NaCl structure. Nanoindentation measurements showed that as-deposited CrN andCrAlN coatings exhibited a hardness of 18 and 33 GPa, respectively. Results of the surface analysis of the as-deposited coatings using SEM andAFM showed a more compact and dense microstructure for CrAlN coatings. The thermal stability of the coatings was studied by heating thecoatings in air from 400 to 900 °C. The structural changes as a result of heating were studied using micro-Raman spectroscopy. The Raman datarevealed that CrN coatings got oxidized at 600 °C, whereas in the case of CrAlN coatings, no detectable oxides were formed even at 800 °C. Afterannealing up to 700 °C, the CrN coatings displayed a hardness of only about 7.5 GPa as compared to CrAlN coatings, which exhibited hardness ashigh as 22.5 GPa. The potentiodynamic polarization measurements in 3.5% NaCl solution indicated that the CrAlN coatings exhibited superiorcorrosion resistance as compared to CrN coatings.© 2006 Elsevier B.V. All rights reserved.

Keywords: CrN and CrAlN coatings; Magnetron sputtering; Structure and mechanical properties; Thermal stability; Surface roughness; Corrosion resistance

1. Introduction

Transition metal nitrides such as TiN have been widely usedas protective hard coatings to increase the lifetime andperformance of cutting and forming tools [1,2]. But the maindrawback of TiN is its limited oxidation resistance (approxi-mately 500 °C). It has been reported that the oxidation of TiN isconsiderably decreased by the presence of elements such as Aland Cr [3,4]. TiAlN coatings have been developed as analternative to TiN, because of their higher oxidation resistance(approximately 750 °C) and higher hardness (approximately30 GPa) [5]. But the wear performance of TiAlN coatings underambient conditions shows insignificant improvement due to its

⁎ Corresponding author. Tel.: +91 80 2508 6494; fax: +91 80 2521 0113.E-mail address: [email protected] (H.C. Barshilia).

0257-8972/$ - see front matter © 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.surfcoat.2006.03.037

high friction coefficient [6]. Similar to TiN, chromium nitride(CrN) has also been used as a hard, protective and wear resistantcoating for cutting tools [7,8]. A great advantage of CrN is thatthe internal stresses are very low, due to which coatings withthicknesses more than 40 μm can be obtained on a variety ofengineering substrates [9]. On the other hand, TiN coatings canbe deposited only with thicknesses less than 10 μm on accountof high internal stresses and consequent poor adhesion. CrNalso exhibits low friction coefficient, high corrosion and wearresistance and high toughness, when compared to TiN [10]. Ithas been reported that the oxidation resistance of CrN is limitedup to about 650 °C [11]. To further improve the oxidationresistance of CrN, ternary compounds consisting of Al, V, Nb,etc. have been explored [12,13]. Among them, CrAlN thin filmswere found to be promising hard coatings [14]. As compared toCrN, CrAlN coatings exhibit high hardness and their propertiesare greatly affected by the aluminum content in the coatings.

Fig. 1. XRD data of CrN and CrAlN coatings deposited at a substratetemperature of 300 °C and a nitrogen flow rate of 2.5 sccm.

2194 H.C. Barshilia et al. / Surface & Coatings Technology 201 (2006) 2193–2201

The aluminum content also controls the structure of CrAlNcoatings (hexagonal and cubic) [15].

For hard coatings, their thermal stability and high temper-ature hardness is very important for various technologicalapplications. The oxidation behavior of TiN, CrN and TiAlNhas been studied by various authors [16–19]. However, thereare a few reports on the thermal stability of CrAlN coatings. Ithas been reported that the activation energies for the oxidationof TiN, CrN and TiAlN are 136, 225 and 471 kJ/mol,respectively [18,19]. Furthermore, the oxidation of CrN iscontrolled by the outward diffusion of Cr ions through theCr2O3 layer formed on each CrN grain and the oxidation ofTiAlN is mainly controlled by the aluminum content [18,19]. Itis therefore expected that the presence of Cr and Al in CrAlNcoatings will greatly improve the oxidation behavior due to theformation of complex aluminum and chromium oxides, whicheventually suppress the oxygen diffusion [20]. Recently,Kawate et al. have reported that CrAlN films were stable upto 900 °C in air and aluminum oxide was formed on top of theCrAlN films at the initial stage of oxidation [21]. They reportedthat CrAlN coatings exhibited higher oxidation resistance thanTiAlN coatings. According to Banakh et al., CrAlN films withhigh Al contents were more resistant to high temperatures ascompared to pure CrN coatings [22].

The aim of the present study is to prepare CrN and CrAlNcoatings using a reactive DC magnetron sputtering system andto compare their structural and mechanical properties, and theoxidation and the corrosion behaviors. We also present themechanical properties of these coatings after heat-treatment inair up to 700 °C. X-ray diffraction (XRD), X-ray photoelectronspectroscopy (XPS), scanning electron microscopy (SEM),nanoindentation tester, atomic force microscopy (AFM) andmicro-Raman spectroscopy techniques have been used tocharacterize the coatings.

2. Experimental details

CrN and CrAlN coatings were deposited on silicon (100) andmild steel (MS) substrates using a multi-target reactive DCmagnetron sputtering system that has been described in detailelsewhere [23]. The sputtering system consisted of 2 sputteringguns (3 in. diameter). Cr (99.95%) and CrAl (99.99%) targetswere sputtered in high purity Ar (99.999%) and N2 (99.999%)plasma. The composition of CrAl target was 50:50. Thecoatings were deposited under a base pressure of 2.0×10−4 Paand a total Ar+N2 gas pressure of 4.0×10

−1 Pa. The flow ratesof N2 (2.5 sccm) and Ar (17 sccm) were controlled separately bymass flow controllers. The substrate to target distance was5.4 cm. A DC substrate bias of −200 V was applied to improvethe mechanical properties of the coatings. The coatings weredeposited at a substrate temperature of 300 °C. For all theexperiments, the power density was approximately 2.20 W/cm2

for both Cr and CrAl targets. Under these conditions the growthrates were approximately 5 Å/s for CrN and approximately 2 Å/sfor CrAlN. Metallographically polished substrates were chem-ically cleaned in an ultrasonic agitator in acetone, absolutealcohol and trichloroethylene. Subsequently, the substrates were

cleaned in situ by Ar+ ion bombardment for 30 min, wherein aDC bias of −850 V was applied to the substrate at an argonpressure of 6.0×10−1 Pa. About 0.5 μm thick Cr interlayer wasdeposited for all the samples to improve the adhesion of thefilms. The process parameters were first optimized to achieve B1NaCl structure of CrN and CrAlN. For this, films were depositedat various nitrogen flow rates. A nitrogen flow rate of 2.5 sccmresulted in CrN and CrAlN coatings with B1 NaCl structure. Thetotal thickness of the coatings was approximately 1.5 μm.

XRD patterns of the coatings in Bragg–Brentano θ–2θgeometry were recorded in a Rigaku D/max 2200 Ultima X-raypowder diffractometer. The X-ray source was a CuKα radiation(λ=1.5418 Å), which was operated at 40 kV and 40 mA. Thebonding structure of the coatings was characterized by X-rayphotoelectron spectroscopy using an ESCA 3000 (V. G.Microtech) system with a monochromatic Al Kα X-ray beam(energy=1486.5 eV and power=150 W). The hardnessmeasurements were performed in a nanoindenter (CSEMInstruments) at a load of 5 mN using a Berkovich diamondindenter. At this load the indentation depth was much less than1/10th of the film thickness, thus minimizing the effect ofsubstrate on the hardness measurements. Ten indentations weremade on each sample and values reported herein represent theaverage of ten values. Root mean square roughness (Ra) of thefilms was measured by atomic force microscopy. The AFM(Surface Imaging Systems) was operated in non-contact mode.

In order to test the thermal stability of the films, CrN andCrAlN coatings were simultaneously heated in air in a resistivefurnace at TA=400, 500, 600, 700, 750, 800, 850 and 900 °C.Annealing involved increasing the temperature of the samplesfrom room temperature to the desired temperature at a slowheating rate of 5 °C/min and maintaining the desiredtemperature for 30 min. Subsequently, the samples were cooleddown at a rate of 5 °C/min. The structural changes and hardnessof the films as a result of heating were measured using micro-Raman spectrometer and nanoindentation, respectively. ADILOR-JOBIN-YVON-SPEX integrated micro-Raman spec-trometer was used for the present study [24]. The corrosionbehavior of the coatings was studied using potentiodynamic

2195H.C. Barshilia et al. / Surface & Coatings Technology 201 (2006) 2193–2201

polarization in 3.5% NaCl solution under free-air conditions atroom temperature. The experimental conditions for thepotentiodynamic polarization measurements are describedelsewhere [25]. The corrosion potential (Ecorr), the corrosioncurrent density (icorr) and the polarization resistance (Rp) werededuced from the Tafel (log i vs. E) plots.

3. Results and discussion

3.1. Structural characterization

The stoichiometric phase of CrN with B1 NaCl structureexists only in a limited range of nitrogen concentration [26].Therefore, the process parameters were first optimized byvarying the nitrogen partial pressure, target power, operatingpressure and substrate bias. The XRD pattern of a typical CrNcoating deposited under the experimental conditions used in thepresent study is shown in Fig. 1. The high intensity peakcentered at 2θ=37.38° is assigned to cubic CrN (111) and thepeak with a lower intensity at 2θ=43.51° is assigned to cubicCrN (200). Other higher angle reflections such as (220) and(311) were very weak, indicating that the films were orientedalong (111).

Binding Energy (eV)

Inte

nsit

y (a

rb. u

nits

)

565 575 585 595

Cr 2p

Cr 2p

3/2 575.3 eV

575.3 eV1/2

577.4 eV

(a)

Binding Energy (eV)

Inte

nsit

y (a

rb. u

nits

)

390 400 405395

N 1s 397.0 eV

398.8 eV

(b)

Fig. 2. XPS core-level spectra of: (a) Cr 2p and (b) N 1s of CrN coatings.

Fig. 3. XPS core-level spectra of: (a) Cr 2p, (b) N 1s and (c) Al 2p of CrAlNcoatings.

After optimizing the deposition conditions for CrN, CrAlNcoatings were also prepared under similar conditions using aCrAl composite target. The XRD data of CrAlN coating is alsopresented in Fig. 1. The XRD pattern revealed a high intensity(200) reflection centered at 2θ=43.91°. The intensities of(111) and other angle reflections were very low. Thus it is clearthat the CrAlN coatings also crystallized in B1 NaCl structure

2196 H.C. Barshilia et al. / Surface & Coatings Technology 201 (2006) 2193–2201

with a (200) preferential orientation. For CrAlN coating, therewas a shift in the 2θ value corresponding to the (200) and (111)reflections. For example, for the (200) peak, the 2θ valueincreased from 43.51° to 43.91°. Similarly, for the (111) peak,it increased from 37.38° to 38.07°. This shift in 2θ value can beattributed to a decrease in lattice parameter of CrAlN(approximately 1.44%) due to the substitution of some of Cratoms by Al atoms in the CrN lattice [15]. The averagecrystallite size of the coatings was calculated using theScherrer formula [27]. Contribution due to micro-strain onthe broadening of XRD profiles has not been considered whilecalculating the crystallite size using the above method,therefore, the crystallite sizes reported herein are approximate.CrN and CrAlN coatings deposited under the above mentionedconditions exhibited an average crystallite size of approxi-mately 118 and 78 Å, respectively. The smaller crystallite sizein the case of CrAlN coating is attributed to higher flux of ioncurrent. The ion currents were 47 and 57 mA for CrN andCrAlN coatings, respectively. It is therefore expected thathigher fluxes of ion current reduce the grain size of CrAlNcoatings [28].

Fig. 2 shows high-resolution XPS core-level spectra of CrNcoatings. The peak associated with Cr metal (Fig. 2(a)) consistsof two peaks centered at 575.3 and 585.4 eV. These peaksoriginate from Cr 2p3/2 and Cr 2p1/2, respectively. Deconvolu-tion of Cr 2p3/2 peak indicated that it consisted of two peakscentered at 575.3 and 577.4 eV. The peak centered at 577.4 eVcould be due to the formation of Cr2O3 [29]. Peaks pertaining tofree chromium (574.3 eV) and Cr2N (574.5 eV) were not

300µm

300µm

(a) (b)

(d) (e)

Fig. 4. SEM images of as-deposited Cr

observed [30], indicating that the bonding state of chromiumwas in the form of CrN with traces of Cr2O3. The N 1s spectrum(Fig. 2(b)) of the CrN coating revealed the presence of a peaktypical of chromium nitride centered at 397.0 eV and a weakpeak associated with chromium oxynitride at a binding energyof 398.8 eV [29]. Fig. 3 shows the XPS spectra of CrAlNcoatings. In Fig. 3(a), the peaks centered at 575.6 and 585.5 eVoriginate from Cr 2p3/2 and Cr 2p1/2, respectively. The peakdeconvolution indicated that the first peak comprised of twopeaks centered at 575.6 and 577.6 eV. Peaks centered at 575.6and 585.5 eV are attributed to CrN. The second weak peakcentered at 577.6 eV can be assigned to Cr2O3 [29]. The N 1sspectrum (Fig. 3(b)) revealed the presence of peaks character-istic of nitrogen in CrN and chromium oxynitride, with bindingenergies at around 397.0 and 399.0 eV, respectively [29].Similarly, the Al 2p spectrum (Fig. 3(c)) of CrAlN coatingsshowed a characteristic peak at a binding energy of 74.3 eV,which corresponds to AlN [31]. Deconvolution of this bandindicated the presence of a second weak peak centered at77.9 eV. The origin of this peak is not clear at present.

The surface morphologies of the as-deposited CrN andCrAlN coatings examined using SEM at three differentlocations are shown in Fig. 4. The SEM image of CrN coating(Fig. 4(a), (b) and (c)) showed the presence of surface defectsunder higher magnifications. The coating also displayedsignificant amount of micro-porosity. On the other hand, onlyminute surface defects were visible for CrAlN coating (Fig.4 (d), (e) and (f)). Results of the SEM surface analysis showed amore compact and dense microstructure for CrAlN coatings.

300µµm 50µm

300µm 50µm

(c)

(f)

N (a–c) and CrAlN (d–f) coatings.

150 300 450 600 750 900 1050

Raman Shift (cm-1)

750°C

700°C

800°C

600°C

500°C

400°C

25°C

61923

8

614

552

348

Inte

nsit

y (a

rb. u

nits

)

CrN

Fig. 6. Composite Raman spectra of as-deposited CrN and CrN coatings heat-treated up to 800 °C in air.

2197H.C. Barshilia et al. / Surface & Coatings Technology 201 (2006) 2193–2201

3.2. Mechanical characterization

For hardness measurements, indentations were made on thesample using a Berkovich diamond indenter. After pressing theindenter on to the sample, the load was increased at apredetermined rate (10 mN/min) to the desired maximum loadand then decreased at the same rate (10 mN/min) to zero. For allthe samples, the maximum applied load was 5 mN. For eachloading and unloading cycle, the load was plotted against thedisplacement of the indenter. Fig. 5 shows typical load vs.displacement curves for CrN, CrAlN and silicon substrate at5 mN load. The maximum indentation depth for CrAlN coatingwas approximately 100 nm, which is less than 1/10th of thecoating thickness. CrN coating and the uncoated siliconsubstrate showed maximum indentation depths of 114 and130 nm, respectively. CrN and CrAlN coatings prepared underidentical conditions displayed hardness values of 18 and33 GPa, respectively. The high hardness can be attributed toseveral factors such as: small inter-atomic distance, covalentnature, solution hardening and small crystallite size of CrAlNcoatings. As mentioned previously a decrease in the latticeparameter of CrAlN (approximately 1.4%) due to thesubstitution of some of the Cr atoms by Al atoms in the CrNlattice was observed. Similar decrease in lattice parameter hasbeen reported in the literature for TiAlN coatings [32,33]. As theinter-atomic distance (d) is related to the covalent band gap (Eh)according to the expression Eh=Kd

−2.5 [32], a decrease in theinter-atomic distance therefore causes an increase in thecovalent energy. Furthermore, addition of Al in CrN increasesthe covalent bonding as CrN is a metallic hard material and AlNis a covalent bonded material [34]. Therefore, an increasingcovalent contribution is expected for CrAlN as compared toCrN. Alloying of solvent with an element having smaller atomicsize also contributes to enhanced hardness, which is commonlyknown as solution hardening [35]. Furthermore, small grain size

0 25 50 75 100 1250.0

0.7

1.4

2.1

2.8

3.5

4.2

4.9

5.6

(a) CrAlN

(c) Silicon

(b)(a)

UnloadingLoading

(b) CrN

Loa

d (m

N)

Indentation Depth (nm)

(c)

Fig. 5. Schematic representations of load vs. displacement curves for (a) CrAlNcoating, (b) CrN coating and (c) silicon substrate at 5 mN load.

of CrAlN as compared to CrN also contributes to enhancedhardness of CrAlN coatings [28].

3.3. Thermal stability of CrN and CrAlN coatings

Thermal stability of hard coatings is very important as theyare frequently used in conditions where high temperature is

Table 1Assignment of various Raman peaks of as-deposited and heat-treated CrN andCrAlN coatings

Sample Peak position (cm−1) Peak assignment

CrN (a) As-deposited 238 (W) Longitudinal andtransverse acoustic

619 (W) Transverse opticmode

(b) Heat-treatedin air (800 °C)

348 (W) Cr2O3

552 (W) Cr2O3

614 (W) Cr2O3

CrAlN (a) As-deposited 296 (W) Longitudinal andtransverse acoustic

640 (W) Transverse opticmode

(b) Heat-treatedin air (850 °C)

348 (W) Cr2O3

552 (W) Cr2O3

605 (B) Cr2O3

(c) Heat-treatedin air (900 °C)

425 (B) Al2O3

605 (B) Cr2O3

713 (B) Not identified

B — Broad; S — Strong; W — Weak.

300 350 400 450 500 550 600 650 700500

1000

1500

2000

2500

3000

3500

CrAlN

Har

dnes

s (k

g/m

m2 )

Annealing Temperature (°C)

As-deposited

As-deposited

CrN

Fig. 8. Variations of hardness values of CrN and CrAlN coatings with annealingtemperature.

2198 H.C. Barshilia et al. / Surface & Coatings Technology 201 (2006) 2193–2201

generated, such as cutting tools and hot forming. The hightemperature applications result in oxidation of the coating,which affects their wear resistance and friction coefficient [10].The thermal stability of CrN and CrAlN coatings was studiedusing micro-Raman spectroscopy. Fig. 6 shows the compositeRaman spectra of as-deposited CrN and CrN coatings heat-treated at different temperatures. The spectrum of CrN coatingshows a weak band at 238 cm−1 and a very weak band at619 cm−1. These bands originate due to acoustic transitions inthe 150–300 cm−1 region (LA and TA) due to vibration of Crions and optic modes in the 400–650 cm−1 region (LO and TO)due to vibration of N ions [36]. After heating the sample up to500 °C, the spectrum did not change significantly. At TA≥600 °C, additional peaks centered at 348, 552 and 614 cm−1 wereobserved. The origin of these peaks is assigned to the formation ofCr2O3 (Table 1) [24]. The Raman spectra of as-deposited CrAlNcoating and coatings heat-treated up to 900 °C are shown in Fig. 7.The Raman spectra of the CrAlN coatings did not changesignificantly up to 800 °C. At 850 °C, peaks centered at 348, 552and 605 cm−1 emerged,which are assigned toCr2O3. However, at900 °C, an additional band centered at 425 cm−1 was observed.This band originates due to the formation of Al2O3 [37]. It is clearfrom Figs. 6 and 7 that substitution of Cr atoms by Al atomsimproves the oxidation resistance of CrAlN coatings considerablyup to 800 °C [22].

As reported earlier, the oxidation of CrN is controlled by theoutward diffusion of Cr ions through the Cr2O3 layer formed on

150 300 450 600 750 900 1050

Raman Shift (cm-1)

750°C

700°C

800°C

850°C

900°C

25°C

348

64029

6

605

55242

5

Inte

nsit

y (a

rb. u

nits

)

713

CrAlN

Fig. 7. Composite Raman spectra of as-deposited CrAlN and CrAlN coatingsheat-treated up to 900 °C in air.

each CrN grain [19]. On the other hand, the oxidation behaviorof CrAlN is mainly controlled by the Al present in the coating[22]. The superior oxidation behavior of CrAlN coatings is dueto the strongly differing values of the Gibbs free energy for theoxide formation. Over a wide temperature range, Al2O3 is muchmore stable than Cr2O3 (i.e., for Al2O3, ΔG°=−378.2 kcal/moland for Cr2O3, ΔG°=−252.9 kcal/mol) [38].

Annealing of physical vapor deposited coatings inducesmicrostructural modifications, which can result in changes inmechanical properties like hardness [39]. Therefore, it isessential to investigate the effect of temperature on themechanical properties of hard coatings. In Fig. 8 we plot thevariations of hardness values of CrN and CrAlN coatings withannealing temperature. Both CrN and CrAlN coatings retainedtheir hardness values up to a temperature of 500 °C. Thehardness of CrN and CrAlN coatings decreased beyond 500 °C.This decrease in hardness can be attributed to the formation ofvery thin, soft and amorphous oxide layer on the surface of thecoatings. These thin amorphous oxides could not be detected bythe Raman spectrometer used in the present study as the incidentpower of the He–Ne laser was very low (approximately10 mW), leading to a very weak Raman signal. However, it isexpected that the nanoindentation measurements are affected bythe soft and amorphous oxides. Even after annealing up to700 °C, the CrAlN coatings exhibited hardness as high as22.5 GPa, whereas CrN coatings displayed a hardness of only

Table 2A comparison of onset of oxidation and ex situ high temperature hardness ofCrN and CrAlN coatings

Sample Onset of oxidation as observed in Ramandata (°C)

High temperature hardness(GPa)

CrN 600 7.5(700 °C) [18]a

CrAlN 800 22.5(700 °C) [33]a

a Hardness of as-deposited films.

2199H.C. Barshilia et al. / Surface & Coatings Technology 201 (2006) 2193–2201

about 7.5 GPa. From the Raman data of CrAlN coatings, it isclear that no detectable oxide formation was observed up to700 °C, which explains the high hardness retained by the CrAlNcoatings. On the other hand, CrN coatings started to getoxidized even at 600 °C. Thus, the ability to resist oxidationattack strongly affects the thermal stability of the coating atelevated temperatures in air. A comparative study of the onset ofoxidation and high temperature hardnesses of CrN and CrAlNcoatings is presented in Table 2.

The surface morphologies of the heat-treated CrN andCrAlN coatings were studied using AFM. The 3-dimensionalAFM images of the as-deposited and heat-treated (400, 500,600, 650 and 700 °C) CrN and CrAlN coatings are shown inFigs. 9 and 10, respectively. The root mean square roughnessvalues of the as-deposited and the heat-treated CrN and CrAlNcoatings are given in Table 3. The as-deposited CrN coating(Fig. 9) showed a root mean square roughness (RMS) value of7.3 nm, whereas in the case of CrAlN coatings (Fig. 10), thesurface roughness was very low (1.13 nm). The roughness valueof heat-treated CrN coating almost doubled at a temperature of500 °C (14 nm). On the other hand, the CrAlN coating retaineda smooth surface up to a temperature of 500 °C (1.59 nm). At600 °C, the surface roughness of the CrAlN coating was 7 nm,which was much less than that of the CrN coating (15 nm). Withfurther increase in temperature to 650 and 700 °C, the RMSroughness of CrN coating increased to 23 and 43 nm,

As-deposited 400

600°C 650°

nm

0

5000

10000

0

0

300

600

nm

5000

10000

nm

0

5000

10000

0

nm

0

5000

10000

0

0

300

600

nm

5000

10000

nm

0

5000

10000

0

Fig. 9. Three-dimensional AFM images of as-deposited CrN coating

respectively. Whereas, for CrAlN coatings heat-treated at 650and 700 °C, the surface roughness increased to 18 and 31 nm,respectively. From the AFM data, it is clear that the RMSroughness value for the as-deposited CrAlN coating is smallerthan that for the CrN coating, which can be attributed to thesmaller crystallite size of the CrAlN coating, arising because ofhigher ion current, as described earlier. However, with anincrease in temperature, the crystallite size of the CrN andCrAlN coatings increases due to increased diffusivity andmobility of grain boundaries, which may lead to variations inthe surface morphology of the coatings in addition to oxidelayer formed on the coating surface.

3.4. Corrosion behavior

The corrosion behavior of CrN and CrAlN coatingsdeposited on mild steel substrates was investigated usingpotentiodynamic polarization in 3.5% NaCl solution under free-air conditions at room temperature. The Tafel plots obtained forthe mild steel substrate, CrN and CrAlN coatings are shown inFig. 11. The corrosion potential, the corrosion current density,the polarization resistance and the Tafel constants (ba and bc)obtained from the measurements are given in Table 4. Thecorrosion potential of the mild steel substrate was found to be−0.62 V. The Ecorr values of the CrN and CrAlN samplesshowed a shift towards the positive side, when compared to the

°C 500°C

C 700°C

0

300

600

nm

5000

10000

nm

0

5000

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nm

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nm

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300

600

nm

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10000

and CrN coatings heat-treated at 400, 500, 600, 650 and 700 °C.

As-deposited 400°C 500°C

600°C 650°C 700°C

nm

0

5000

10000

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0

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600

nm

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nm

5000

10000

nm

0

5000

10000

0

0

300

600

nm

5000

10000

Fig. 10. Three-dimensional AFM images of as-deposited CrAlN coating and CrAlN coatings heat-treated at 400, 500, 600, 650 and 700 °C.

MS

CrN

CrAlN

1×10-5

1×10-7

1×10-3

1×10-1

log

i (A

/cm

2 )

2200 H.C. Barshilia et al. / Surface & Coatings Technology 201 (2006) 2193–2201

mild steel substrate. The positive shift of Ecorr (−0.46 V for CrNand −0.34 V for CrAlN) indicates better corrosion resistance ofthese coatings. In the case of mild steel substrate, the corrosioncurrent is about 16.71 μA/cm2, which decreased to about6.95 μA/cm2 for the CrN coating and 1.03 μA/cm2 for theCrAlN coating. The corrosion rate is normally proportional tothe corrosion current density. The decrease in icorr of the coatedsubstrate thus confirms improvement in the corrosion resis-tance. Furthermore, the corrosion resistance was also found toincrease when compared to that of the uncoated substrate. Rp

increased from 1.30 kΩ cm2 for the mild steel substrate to 5.90and 19.90 kΩ cm2 for CrN and CrAlN, respectively. Eventhough, transition metal nitrides are generally inert to chemicalattacks [40], the coating may suffer from corrosive attack due toinherent coating defects (such as pores, cracks, etc.) andinhomogeneities. The SEM of CrN coatings showed significantamount of pores and surface defects, which open paths for thecorrosive media to enter the substrate, making the CrN coating

Table 3RMS roughness values (in nm) of as-deposited and heat-treated (in air) CrN andCrAlN coatings

Sample As-deposited 400 °C 500 °C 600 °C 650 °C 700 °C

CrN 7.3 8 14 15 23 43CrAlN 1.13 1.6 1.59 7 18 31

less corrosion resistant. On the other hand, CrAlN coatingexhibited dense microstructure, thus preventing diffusion of thecorrosive medium into the substrate. The corrosion behavior ofCrAlN may also have been affected because of the presence ofAl. It has been reported that the addition of Al to the transitionmetal nitrides improves the corrosion resistance [41]. During

1×10-9

-0.850 -0.690 -0.530 -0.370 -0.210 -0.050

E (V vs. Ag/AgCl)

Fig. 11. Potentiodynamic polarization curves of CrN and CrAlN coatingsdeposited on mild steel substrate. Also shown is the polarization curve of mildsteel substrate.

Table 4Potentiodynamic polarization data of CrN and CrAlN coatings deposited onmild steel substrates in 3.5% NaCl solution under free-air conditions at roomtemperature

Sample Ecorr (V) icorr (μA/cm2) bc (V/dec) ba (V/dec) Rp (kΩ cm2)

Mild steel −0.62 16.71 0.28 0.05 1.30CrN −0.46 6.95 0.17 0.09 5.90CrAlN −0.34 1.03 0.09 0.08 19.90

2201H.C. Barshilia et al. / Surface & Coatings Technology 201 (2006) 2193–2201

the chemical attack Al forms an Al2O3 layer on the surface ofthe coating, which passivates the surface and prevents thecoating from further attack [42]. The presence of the passivelayer leads to an additional resistance to the corrosive mediumpassing through the pores. It is clearly evident from thecorrosion data that the CrAlN coatings exhibit superiorcorrosion behavior as compared to the CrN coatings.

4. Conclusions

CrN and CrAlN coatings were prepared using a reactive DCmagnetron sputtering system. At a nitrogen flow rate of2.5 sccm and a substrate bias of −200 V, both CrN andCrAlN coatings exhibited B1 NaCl structure. The XRD data ofCrN coating revealed (111) reflection of cubic CrN phase,whereas CrAlN coatings were oriented along (200). The CrNand CrAlN coatings exhibited an average crystallite size of 118and 78 Å, respectively. The SEM images of CrN coatingsshowed significant amount of porosity and surface defects. Onthe other hand, CrAlN coatings exhibited a dense microstruc-ture with lesser amount of surface defects. The nanoindentationmeasurements showed that the CrAlN and CrN coatingsexhibited hardness values of 33 and 18 GPa, respectively. TheRaman data showed that the CrAlN coatings were stable up to800 °C in air as compared to CrN coatings, which got oxidizedat 600 °C. Measurement of hardness of heat-treated CrN andCrAlN coatings revealed that both the coatings retained theirhardness values up to a temperature of 500 °C. However, afterannealing up to 700 °C, the CrN coatings displayed a hardnessof only about 7.5 GPa compared to CrAlN coatings, whichexhibited hardness as high as 22.5 GPa. The AFM data of as-deposited and heat-treated CrN and CrAlN coatings showed thatthe RMS roughness values for the CrAlN coatings were smallerthan that for the CrN coatings. With increase in the annealingtemperature, the roughness values of CrN and CrAlN coatingsincreased but the increase was lesser for CrAlN coatings. CrNand CrAlN coatings deposited on mild steel substratesdemonstrated good corrosion resistance in 3.5% NaCl solution,and the CrAlN coatings showed better corrosion resistance ascompared to the CrN coatings.

Acknowledgements

The authors thank Director, NAL for giving permission topublish these results. Dr. Anjana Jain is thanked for XRD work.Thanks are due to Ms. E. Selvi for helping in potentiodynamicpolarization measurements. Mr. M. A. Venkataswamy isthanked for SEM measurements. We also thank Mr. Siju and

Mr. N. T. Manikandanath for their help in AFM, nanoindenta-tion and Raman measurements.

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