Nanolayered multilayer coatings of CrN/CrAlN prepared by reactive DC magnetron sputtering

www.elsevier.com/locate/apsusc

Applied Surface Science 253 (2007) 5076–5083

Nanolayered multilayer coatings of CrN/CrAlN prepared

by reactive DC magnetron sputtering

Harish C. Barshilia a,*, B. Deepthi a, N. Selvakumar a, Anjana Jain b, K.S. Rajam a

a Surface Engineering Division, National Aerospace Laboratories, Post Bag No. 1779, Bangalore 560017, Indiab Materials Science Division, National Aerospace Laboratories, Bangalore 560017, India

Received 14 June 2006; received in revised form 9 November 2006; accepted 13 November 2006

Available online 8 December 2006

Abstract

Single-phase CrN and CrAlN coatings were deposited on silicon and mild steel substrates using a reactive DC magnetron sputtering system. The

structural characterization of the coatings was done using X-ray diffraction (XRD). The XRD data showed that both the CrN and CrAlN coatings

exhibited B1 NaCl structure with a prominent reflection along (2 0 0) plane. The bonding structure of the coatings was characterized by X-ray

photoelectron spectroscopy and the surface morphology of the coatings was studied using atomic force microscopy. Subsequently, nanolayered

CrN/CrAlN multilayer coatings with a total thickness of approximately 1 mm were deposited on silicon substrates at different modulation

wavelengths (L). The XRD data showed that all the multilayer coatings were textured along {2 0 0}. The CrN/CrAlN multilayer coatings exhibited

a maximum nanoindentation hardness of 3125 kg/mm2 at a modulation wavelength of 72 A, whereas single layer CrN and CrAlN deposited under

similar conditions exhibited hardness values of 2375 and 2800 kg/mm2, respectively. Structural changes as a result of heating of the multilayer

coatings in air (400–800 8C) were characterized using XRD and micro-Raman spectroscopy. The XRD data showed that the multilayer coatings

were stable up to a temperature of 650 8C and peaks pertaining to Cr2O3 started appearing at 700 8C. These results were confirmed by micro-Raman

spectroscopy. Nanoindentation measurements performed on the heat-treated coatings revealed that the multilayer coatings retained hardness as

high as 2250 kg/mm2 after annealing up to a temperature of 600 8C.

# 2006 Elsevier B.V. All rights reserved.

Keywords: CrN and CrAlN films; CrN/CrAlN nanolayered multilayers; Magnetron sputtering; Structural and mechanical properties; Thermal stability

1. Introduction

Thin films based on carbides and nitrides of transition metals

have been very successful in improving the performance of

cutting tools. Although TiN is the most widely used in

industrial applications, CrN has been gaining importance in

recent years due to its superior corrosion and wear resistances,

friction behavior and low internal stress [1–3]. However, there

are an increasing number of applications where properties of

single layer coatings are not sufficient. Therefore, recent

research is focused on the development of nanolayered

multilayer coatings in order to meet the requirements in terms

of hardness, thermal stability, coefficient of friction, corrosion

resistance and internal stress [4–11]. Nanolayered multilayer

* Corresponding author. Tel.: +91 80 2508 6494; fax: +91 80 2521 0113.

E-mail address: [email protected] (H.C. Barshilia).

0169-4332/$ – see front matter # 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.apsusc.2006.11.021

coatings are composed of two different alternating layers with

nanometer scale dimensions. These nanostructured multilayer

coatings are also called superlattices. The thickness of each

successive pair of layers is called modulation wavelength or

bilayer thickness (L), and it greatly affects the properties of the

multilayer coatings. The bilayers of the multilayers can be

metals, carbides, nitrides or oxides of different materials or a

combination of one layer made of nitride, carbide or oxide of

one metal and the second layer made of another metal.

A variety of multilayer systems such as TiN/CrN [4,5],

TiAlN/CrN [6,7], TiN/NbN [8–11], etc., have been studied

extensively. However, there are very few reports on the

multilayer coatings based on CrN and CrAlN [12]. CrN films

have been used for cutting tools, molding dies and machine

parts. But the oxidation resistance of CrN is limited up to

600 8C [13]. For hard protective coatings, thermal stability is

important as they are exposed to high temperatures during the

cutting process. The addition of Al to CrN raises the

Fig. 1. Typical cross section of a CrN coating.

Table 1

Summary of the deposition parameters

Target to substrate distance (cm) 5.4

Cr and CrAl targets diameter (cm) 7.5

Initial vacuum (Pa) 2.0 � 10�4

Working pressure (Pa) 4.0 � 10�1

Nitrogen flow rate (sccm) 3.5

Argon flow rate (sccm) 17

Power density to the Cr target (W/cm2) 2.20

Power density to the Cr–Al target (W/cm2) 2.20

Substrate bias (V) �200

Ion current density on CrN film (mA/cm2) 3.13

Ion current density on CrAlN film (mA/cm2) 3.8

Growth rate for CrN (m/h) 1.2

Growth rate for CrAlN (m/h) 0.8

Substrate temperature (8C) 300

Total thickness of the coating (mm) 1

H.C. Barshilia et al. / Applied Surface Science 253 (2007) 5076–5083 5077

temperature at which onset of oxidation occurs [14]. CrAlN

coatings have been reported to be stable up to a temperature of

900 8C depending upon the Al content in the coatings [15].

CrAlN coatings also exhibit higher hardness and a lower

friction co-efficient when compared to CrN coatings [16,17].

Even though CrAlN coatings have not been able to replace the

harder TiAlN coatings in tool coating applications, it is still an

alternative for some component applications due to its good

corrosion protection properties [18]. The addition of a third

element in the transition metal nitride coating generally

increases the residual stress [19]. Therefore, the CrAlN

coatings are expected to have higher residual stresses than

CrN coatings. High internal stress critically affects the adhesion

of the coatings, due to which thick coatings of CrAlN cannot be

obtained. These factors led to the exploration of the multilayer

coatings of CrN/CrAlN, wherein the properties of CrN and

CrAlN can be combined. In the present study, nanolayered

multilayer coatings of CrN/CrAlN have been deposited using

DC reactive magnetron sputtering. The structural and

mechanical properties have been studied using X-ray diffrac-

tion (XRD) and nanoindentation, respectively. The bonding

structure was characterized using X-ray photoelectron spectro-

scopy (XPS) and the surface morphology was studied using an

atomic force microscope (AFM). The structural changes as a

result of heating of the CrN/CrAlN multilayer coatings in air

were characterized using XRD and micro-Raman spectroscopy

to study their thermal stability.

2. Experimental details

Alternate layers of CrN and CrAlN of varying thicknesses

were deposited on silicon and steel substrates using a multi-

target reactive DC magnetron sputtering system that has been

described in detail elsewhere [20]. High purity Cr (99.95%)

CrAl (99.9%) targets were sputtered in high purity Ar

(99.999%) and N2 (99.999%) plasma. The composition of

CrAl target was approximately 50:50. In order to get varying

thicknesses of the CrN and CrAlN layers, the Cr and CrAl

targets were sputtered for different durations. The multilayer

coatings were deposited under a base pressure of 2.0 � 10�4 Pa

and a total Ar + N2 gas pressure of 4.0 � 10�1 Pa. The flow

rates of N2 and Ar were controlled separately by mass flow

controllers. The substrate to target distance was 5.4 cm. A DC

substrate bias of �200 V was applied to improve the

mechanical properties of the coatings. This resulted in ion

current densities of the order of 3.13 and 3.8 mA/cm2,

respectively, for CrN and CrAlN sputtered layers. The coatings

were deposited at a substrate temperature of 300 8C. For all the

experiments, the power density was approximately 2.20 W/cm2

for both Cr and CrAl targets. Under these conditions the growth

rates were approximately 3.5 A/s for CrN and approximately

2 A/s for CrAlN. The substrates were cleaned in an ultrasonic

agitator in acetone, isopropyl alcohol and trichloroethylene.

Subsequently, the substrates were cleaned in situ by Ar+ ion

bombardment for 45 min, wherein a DC bias of �850 V was

applied to the substrate at an argon pressure of 6.0 � 10�1 Pa.

Prior to the deposition, the targets were sputter cleaned for

5 min. A Cr interlayer of approximately 0.5 mm thickness was

deposited on the substrates for improved adhesion of the

coatings. The CrN and CrAlN layers had approximately equal

thickness. The total thickness of the coating was approximately

1 mm. The thickness of the coatings was determined from the

cross-section of the coating. Fig. 1 shows the typical cross-

section of a CrN coating deposited for 4 h. The coating

exhibited a thickness of approximately 4.6 mm, indicating a

growth rate of 1 mm/h. Multilayer coatings with controlled

layer thicknesses and repeatability were deposited using a PC-

based dwell time controller. In this system, a stepper motor was

connected to the substrate holder through a rotary feed-through.

The stepper motor was controlled by a driver circuit. The

process conditions used for the deposition of CrN/CrAlN

multilayer coatings are summarized in Table 1.

The XRD patterns of the coatings in Bragg–Brentano u–2u

geometry were recorded in a Rigaku D/max 2200 Ultima X-ray

powder diffractometer. The X-ray source was a Cu Ka radiation

(l = 0.15418 nm), which was operated at 40 kV and 40 mA.

The bonding structure of the coatings was characterized by XPS

using an ESCA 3000 (V. G. Microtech) system with a

monochromatic Al Ka X-ray beam (energy = 1486.5 eV and

power = 150 W). The mechanical properties (hardness and

elastic modulus) of the coatings were measured in a

H.C. Barshilia et al. / Applied Surface Science 253 (2007) 5076–50835078

nanoindenter (CSEM Instruments) at a load of 5 mN using a

Berkovich diamond indenter. At this load the indentation depth

was much less than 1/10th of the film thickness, thus

minimizing the effect of substrate on the hardness measure-

ments. Ten indentations were made on each sample and the

values reported herein represent the average of 10 values.

Surface imaging of the coatings was carried out using AFM.

The AFM (Surface Imaging Systems) was operated in non-

contact mode.

In order to test the thermal stability of the films, the CrN/

CrAlN multilayer coatings were heated in air in a resistive

furnace at TA = 400, 500, 550, 600, 650, 700, 750 and 800 8C.

Annealing involved increasing the temperature of the samples

from room temperature to the desired temperature at a slow

heating rate of 3 8C/min and maintaining the desired

temperature for 30 min. Subsequently, the samples were cooled

down at a rate of 3 8C/min. The structural changes as a result of

heating were measured using XRD and micro-Raman spectro-

scopy. A DILOR-JOBIN-YVON-SPEX integrated micro-

Raman spectrometer was used for the present study [21].

3. Results and discussion

3.1. Structural properties

The structure of CrN and CrAlN coatings depends strongly

on the nitrogen partial pressure and the substrate bias. The

stoichiometric phase of CrN with B1 NaCl structure exists in a

limited range of nitrogen concentration [22]. Therefore it

becomes necessary to first optimize the process parameters for

the deposition of cubic CrN and CrAlN phases. At a nitrogen

flow rate of less than 2.0 sccm, the XRD data of CrN coatings

showed a mixed structure of cubic (CrN) and hexagonal (b-

Cr2N) phases (results not shown). However, at higher flow rates

cubic phase of CrN was observed. Fig. 2 shows the XRD

patterns of CrN coatings at nitrogen flow rates in the range of

2.5–4.0 sccm. The XRD pattern showed the formation of cubic

phase of CrN for all the nitrogen flow rates. For a nitrogen flow

rate of 3.5 sccm, the CrN coatings exhibited a (2 0 0) prominent

Fig. 2. XRD patterns of CrN coatings deposited at different nitrogen flow rates.

The substrate bias was �200 V and the power density was 2.20 W/cm2.

reflection, accompanied by (1 1 1) reflection. The Cr (1 1 0)

peak observed in Fig. 2 originates from the Cr interlayer, which

was used for improved adhesion of the coatings. For all

subsequent depositions, a nitrogen flow rate of 3.5 sccm was

used.

CrN coatings were also deposited at different bias voltages,

which critically affected the mechanical properties of the

coatings. Load versus displacement curves obtained for the CrN

coatings deposited at bias voltages of �40 and �200 V are

shown in Fig. 3. The maximum indentation depth for the

coating deposited at �40 V was 210 nm, whereas at �200 V

the coating exhibited a maximum indentation depth of 100 nm.

CrN coatings exhibited hardness values of 900 and 2375 kg/

mm2 at bias voltages of �40 and �200 V, respectively. The ion

current densities were 1.8 and 3.1 mA/cm2 at �40 and �200 V

bias voltages, respectively. Higher hardness for the coating

deposited at higher substrate bias is attributed to ion current

density, leading to small crystallite size [23]. This was

supported by the AFM data, which showed a fine-grained

morphology for CrN coatings deposited at the higher bias

voltage. The three-dimensional surface morphologies of CrN

coatings deposited at two different bias voltages are shown in

Fig. 4. In the case of CrN coating deposited at a bias voltage of

�40 V (Fig. 4(a)), the film was rough with a root mean square

(RMS) roughness value of 14.3 nm. On the other hand, CrN

coating deposited at a substrate bias of �200 V (Fig. 4(b))

exhibited a surface roughness of 4.54 nm.

After optimizing the deposition conditions for CrN coating,

CrAlN coatings were also deposited. The CrAlN coatings were

deposited at a nitrogen flow rate of 3.5 sccm, a substrate bias of

�200 V and a power density of 2.20 W/cm2. The XRD data of

CrAlN coating is shown in Fig. 5 along with CrN coating.

Fig. 3. Schematic representations of load vs. displacement curves for CrN

coatings deposited at two different substrate bias voltages. The nitrogen flow

rate was 3.5 sccm.

Fig. 4. Three-dimensional AFM images of CrN coatings deposited at a bias

voltage of: (a) �40 and (b) �200 V. The nitrogen flow rate was 3.5 sccm.

Fig. 6. XPS core-level spectra of: (a) Cr 2p, (b) N 1s of CrN coatings.

H.C. Barshilia et al. / Applied Surface Science 253 (2007) 5076–5083 5079

CrAlN coating displayed a prominent reflection along the

(2 0 0) plane and a low intensity reflection along (1 1 1). The

intensities of other higher angle reflections were very low.

Fig. 6 shows high-resolution XPS core level spectra of CrN

coatings. The peak associated with Cr metal (Fig. 6(a)) consists

of two peaks centered at 575.3 and 585.4 eV. These peaks

originate from Cr 2p3/2 and Cr 2p1/2, respectively. Deconvolu-

tion of the Cr 2p3/2 peak indicated that it consisted of two peaks

centered at 575.3 and 577.4 eV. The peak centered at 577.4 eV

could be due to the formation of Cr2O3 [24]. Peaks pertaining to

free chromium (574.3 eV) and Cr2N (574.5 eV) were not

Fig. 5. XRD data of CrN and CrAlN coatings deposited at a nitrogen flow rate

of 3.5 sccm and a substrate bias of �200 V.

observed [25], indicating that the bonding state of chromium

was in the form of CrN and some amount of Cr2O3. The

formation of Cr2O3 on the surface of the coatings was also

confirmed qualitatively by energy dispersive X-ray analysis

(EDAX) measurements in the scanning electron microscope.

For example, the CrN coating showed the presence of about

1 at.% O in the EDAX data. The N 1s spectrum (Fig. 6(b)) of

the CrN coating revealed the presence of a peak typical of

chromium nitride centered at 397.0 eV [24]. A broad and weak

peak centered at a binding energy of 399.0 eV was also

observed in the deconvoluted data. The intensity of this peak

was approximately 1/8th of the intensity of the main peak

associated with nitrogen attached to chromium. The origin of

this peak is not clear and may be attributed to impurities such as

nitrogen attached to carbon or free N2 present in the coating

[26,27]. Fig. 7 shows the XPS spectra of CrAlN coatings. In

Fig. 7(a), the peaks centered at 575.6 and 585.5 eV originate

from Cr 2p3/2 and Cr 2p1/2, respectively. The peak deconvolu-

tion indicated that the first peak comprised of two peaks

centered at 575.6 and 577.6 eV. Peaks centered at 575.6 and

585.5 eVare attributed to CrN. The second weak peak centered

at 577.6 eV can be assigned to Cr2O3 [24]. The N 1s spectrum

(Fig. 7(b)) revealed the presence of a high intensity peak

characteristic of nitrogen in CrN with a binding energy of

397.0 eValong with a weak and broad peak at a binding energy

of 399.0 eV. Similarly, the Al 2p spectrum (Fig. 7(c)) of CrAlN

Fig. 7. XPS core-level spectra of: (a) Cr 2p, (b) N 1s and (c) Al 2p of CrAlN

coatings.

Fig. 8. XRD data of CrN/CrAlN multilayer coatings deposited at different

modulation wavelengths. The nitrogen flow rate was 3.5 sccm and the substrate

bias voltage was �200 V.

Fig. 9. Variations of nanoindentation hardness and elastic modulus of CrN/

CrAlN multilayer coatings with modulation wavelength. The nitrogen flow rate

was 3.5 sccm and the substrate bias voltage was �200 V.

H.C. Barshilia et al. / Applied Surface Science 253 (2007) 5076–50835080

coatings showed a characteristic peak at a binding energy of

74.3 eV, which corresponds to AlN [28]. Deconvolution of this

band indicated the presence of a second weak peak centered at

77.9 eV; this peak may be attributed aluminum oxy-nitride.

The XRD data of CrN/CrAlN multilayer coatings deposited at

different modulation wavelengths is shown in Fig. 8. For all the

coatings, intensity of (1 1 1) principal reflection (PR) was low as

compared to (2 0 0) principal reflection, indicating {2 0 0}

texture of the coatings. The multilayer coatings exhibited broader

XRD peaks than CrN and CrAlN coatings, indicating that the

multilayers had smaller grain sizes than single layer CrN and

CrAlN films. However, the peak positions remained unaffected.

Superlattice structure was not observed for the multilayer

coatings, due to the absence of sharp interfaces between the

alternate CrN and CrAlN layers [29]. The broad interfaces could

be attributed to miscible nature of CrN and CrAlN layers.

3.2. Mechanical properties

The variations of nanoindentation hardness and elastic

modulus of CrN/CrAlN multilayers with modulation wave-

length are shown in Fig. 9. The hardness and the elastic

modulus of the multilayer coatings increased initially with an

increase in the modulation wavelength. A maximum hardness

of 3125 kg/mm2 and an elastic modulus of 324 GPa were

obtained at L of 72 A. Single layer CrN and CrAlN coatings

exhibited a hardness of 2375 and 2800 kg/mm2, respectively.

The maximum hardness was much higher than the value of rule-

of-mixture (approximately 2600 kg/mm2 for CrN–CrAlN).

With further increase in the modulation wavelength the

hardness decreased. The elastic modulus also followed the

H.C. Barshilia et al. / Applied Surface Science 253 (2007) 5076–5083 5081

same trend. At a modulation wavelength of 112 A, the hardness

and elastic modulus decreased to 2250 kg/mm2 and 280 GPa,

respectively. A variety of mechanisms (namely, effect of elastic

anomalies, coherency strains, elastic modulus differences,

Hall–Petch strengthening, etc.) have been used to explain the

hardness enhancement in nanolayered multilayer coatings [30].

It has been shown that the elastic anomalies are too small to

explain the hardness enhancements and the coherency strain

effects also appear to be relatively small [30]. Barnett and Shinn

[31] have shown that the elastic moduli differences are a critical

factor in determining the hardness enhancement. The enhance-

ment in hardness results from the resistance to dislocation glide

across the interfaces, which is proportional to the difference in

layer shear moduli. In other words, there will be a large

difference in the dislocation line energy in the alternating

layers. Further enhancement in the hardness of the nanolayered

multilayer coatings arises from the fact that the thicknesses of

the component layers are very small and dislocation generation

mechanisms such as Frank-Read sources cannot operate inside

a given layer [32]. According to Hall–Petch model, the

strengthening of polycrystalline materials result from the fact

that grain boundaries are obstacles to the passage of slip across

a boundary. Hence slip cannot propagate freely from grain to

grain. Since the slip cannot propagate freely across a boundary

in a polycrystal, a slip band there can sustain a higher stress than

in a single crystal [33]. Therefore, an increase in the hardness of

a coating is expected with a decrease in the grain size.

3.3. Thermal stability

In order to test the thermal stability, the CrN/CrAlN

multilayer coatings deposited at a L of 72 A, were annealed

in air at temperatures in the range 400–750 8C for 30 min. The

composite XRD plots obtained for the as-deposited and heat-

treated multilayer coatings are shown in Fig. 10. As it is evident

from the figure, all the plots exhibited a prominent reflection

along (2 0 0). Therewas no significant change in the XRD pattern

up to a temperature of 650 8C. However, the position of (1 1 1)

and (2 0 0) reflections shifted to higher 2u values with an increase

Fig. 10. XRD data of as-deposited CrN/CrAlN multilayer coating (L = 72 A)

and coatings annealed at 400, 500, 550, 600, 700 and 750 8C.

in the annealing temperature. This shift can be attributed to stress

relaxation as a result of annealing temperature. Compressive

stresses are commonly observed in bias-sputtered transition

metal nitride films due to Ar+ ion bombardment-induced point

defects [34]. Further increase in temperature to 700 8C showed

evidence of slight oxidation, as peaks associated with Cr2O3

started appearing. For example, a Cr2O3 (1 0 4) peak was

observed at a 2u value of 33.388 and a weak peak associated with

Cr2O3 (0 0 6) appeared at a 2u value of 40.148. The intensity of

the peaks increased when the annealing temperature was

increased to 750 8C, showing an increase in the oxide growth.

Also, at 750 8C, other chromium oxide peaks like Cr2O3 (1 1 0)

were observed. No reflections of aluminum oxide were detected

in the XRD data of heat treated CrN/CrAlN multilayers,

suggesting amorphous nature of the aluminum oxide formed.

In Fig. 11, we plot the composite Raman spectra of the as-

deposited CrN/CrAlN multilayer coating and coatings heat-

treated at different temperatures. Annealing of CrN/CrAlN

multilayer coatings up to a temperature of 650 8C did not

change the nature of the Raman spectra. At 700 8C, weak peaks

centered at 350 and 553 cm�1 emerged, which are assigned to

Cr2O3 [21]. The intensity of these peaks increased with an

increase in the annealing temperature. With further increase in

temperature up to 750 8C, additional peaks centered at 307 and

613 cm�1 were observed. The origin of these peaks is also

attributed to the formation of Cr2O3. A weak peak at 521 cm�1

also emerged which originates from the silicon substrate.

Fig. 11. Composite Raman spectra of as-deposited and heat-treated CrN/

CrAlN multilayer coatings (L = 72 A).

Fig. 12. Variations of hardness and elastic modulus values of CrN/CrAlN

multilayer coatings (L = 72 A) with annealing temperature.

H.C. Barshilia et al. / Applied Surface Science 253 (2007) 5076–50835082

In order to test the suitability of the coatings for high-

temperature applications, the effect of temperature on the

hardness and the elastic modulus of the CrN/CrAlN multilayer

coatings was studied. Fig. 12 shows the variations in the

hardness and the elastic modulus of the coatings as a function of

the annealing temperature. The hardness and the elastic

modulus of the CrN/CrAlN multilayer coatings decreased

gradually with an increase in the annealing temperature. At

550 8C, the hardness and elastic modulus decreased to 2400 kg/

mm2 and 270 GPa, respectively. This can be attributed to the

formation of very thin oxide layer on the surface of the

coatings, which were not detected by the XRD and Raman

spectroscopy measurements. However, it is expected that

nanoindentation measurements are affected by the formation of

soft and amorphous oxides. The multilayer coatings retained

hardness as high as 2250 kg/mm2 even after heating up to

600 8C. The hardness and the elastic modulus of the CrN/

CrAlN multilayer coatings dropped drastically with further

increase in temperature to 650 8C. Apart from oxide layer

formation, the drastic fall in the hardness is also due to the

interdiffusion between the CrN and CrAlN layers.

The oxidation mechanisms of CrN and CrAlN coatings have

been discussed in the literature. The oxidation of CrN is

controlled by the outward diffusion of Cr ions through the

Cr2O3 layer formed on each CrN grain and the oxidation of

CrAlN is mainly controlled by the aluminum content [35,36]. It

has been reported that Al forms an amorphous aluminum oxide

layer on the surface of the CrAlN coating, which makes the

diffusion of oxygen into the coating difficult. The superior

oxidation behavior of CrAlN coatings is also due to the strongly

differing values of the Gibbs free energy for the oxide

formation. Over a wide temperature range, Al2O3 is much more

stable than Cr2O3 (i.e., for Al2O3, DG8 = �378.2 kcal/mol and

for Cr2O3, DG8 = �252.9 kcal/mol) [37]. In general, the grain

size and the re-crystallization also affect the oxidation

mechanism of thin coatings [38]. In the case of multilayers,

annealing can result in microstructural changes like interdiffu-

sion, coarsening of the layers, reactions between the layers to

produce a new phase and transformation within one or both

layers. Interdiffusion is one of the important factors deciding

the mechanical properties of the multilayer coatings at higher

temperatures as the interdiffusion critically affects the interface

width [39]. The different diffusion properties of the constituent

metals in transition metal nitrides based multilayers, also affect

the oxidation behavior [40,41]. The oxidation of CrN/CrAlN is

thus complicated by the interdiffusion between the layers and

different diffusion properties of Cr, Al, N and O in CrN and

CrAlN. There are no reports on the thermal stability of CrN/

CrAlN nanolayered multilayer coatings. The XRD data of the

heat-treated CrN/CrAlN multilayer coatings in the present

study did not change up to 650 8C (Fig. 10). Similarly, no

change in the nature of the Raman spectra of CrN/CrAlN

multilayers was observed up to 650 8C (Fig. 11). These results

indicate that the annealing of CrN/CrAlN multilayer coatings

caused almost no intermixing or oxide formation up to 650 8C.

Hence, these coatings can be used for machining applications,

wherein the working temperature is less than 650 8C.

4. Conclusions

CrN and CrAlN coatings deposited on silicon substrates at a

nitrogen flow rate of 3.5 sccm and a substrate bias of �200 V,

exhibited B1 NaCl structure with (2 0 0) reflection of cubic

phase. Approximately 1 mm thick CrN/CrAlN multilayer

coatings prepared with modulation wavelengths 102 A � L� 28 A, also exhibited a prominent reflection along (2 0 0)

plane. The multilayer coatings exhibited a maximum hardness

of 3125 kg/mm2 at L = 72 A, whereas CrN and CrAlN coatings

exhibited hardness values of 2375 and 2800 kg/mm2, respec-

tively. The XRD data of the heat-treated coatings showed that

the CrN/CrAlN multilayer coatings were stable in air up to

650 8C. Various oxide phases of chromium were observed at a

temperature of 700 8C and above. The Raman data also showed

that the CrN/CrAlN multilayer coating started to oxidize at

700 8C. Measurement of the hardness of the heat-treated CrN/

CrAlN multilayer coatings showed that the coatings retained

hardness as high as 2250 kg/mm2 even after heating up to

600 8C. These results indicate that nanolayered multilayer

coatings of CrN/CrAlN can be used as hard coatings for high

temperature applications.

Acknowledgements

The authors thank Director, NAL for giving permission to

publish these results. We thank Mr. Siju and Mr. N.T.

Manikandanath for their help in AFM, nanoindentation and

Raman measurements.

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