y 201 (2006) 329–337www.elsevier.com/locate/surfcoat

Surface & Coatings Technolog

Superhard nanocomposite coatings of TiN/Si3N4 prepared by reactive directcurrent unbalanced magnetron sputtering

Harish C. Barshilia ⁎, B. Deepthi, A.S. Arun Prabhu, K.S. Rajam

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

Received 3 November 2005; accepted in revised form 23 November 2005Available online 18 January 2006

Abstract

Approximately 1.5-μm-thick superhard nanocomposite coatings of TiN/Si3N4 with varying silicon contents were deposited on silicon andstainless steel (SS 304) substrates by reactive direct current (DC) unbalanced magnetron sputtering. An asymmetric bipolar-pulsed DC powersupply and a DC power supply were used to sputter Si and Ti targets, respectively in Ar+N2 plasma. Structural characterization of the coatingswas done using X-ray diffraction (XRD). The bonding structure of the nanocomposite coatings was characterized by X-ray photoelectronspectroscopy (XPS). The elemental composition of the TiN/Si3N4 nanocomposite coatings was determined using energy dispersive X-ray analysis.The mechanical properties of the coatings were measured using a nanoindentation hardness tester. The surface morphology of the coatings wasstudied using atomic force microscopy. The nanocomposite coatings exhibited a broad (200) reflection of cubic TiN phase in the XRD data. Therewas a significant broadening of the (200) reflection with an increase in the silicon content in the nanocomposite coatings, which suggested adecrease in the average crystallite size. Nanoindentation data showed that about 1.5-μm-thick TiN/Si3N4 nanocomposite coatings exhibited amaximum hardness of 5200 kg/mm2 and an elastic modulus of 340 GPa at a silicon concentration of approximately 12 at.%. The hardness and theelastic modulus of the nanocomposite coatings decreased with further increase in the silicon content. Structural changes as a result of heating(400–850 °C) of the nanocomposite coatings in air were characterized using micro-Raman spectroscopy. The Raman data showed that thenanocomposite coatings started to oxidize at 800 °C as compared to TiN, which have been oxidized even at 500 °C. To isolate the oxidation-induced spectral changes as a result of heating of the coatings in air, the samples were also annealed in vacuum at 850 °C under similar conditions.The Raman data of the vacuum-annealed coatings showed no phase transformation even after annealing up to 850 °C. The corrosion behaviors ofsingle-phase TiN and TiN/Si3N4 nanocomposite coatings deposited on stainless steel substrates were investigated using potentiodynamicpolarization in 3.5% NaCl solution. The results indicated that the nanocomposite coatings exhibited superior corrosion resistance as compared tothe uncoated substrate. The wear data showed that the nanocomposite coatings also exhibited better wear resistance as compared to the uncoatedsubstrate.© 2005 Elsevier B.V. All rights reserved.

Keywords: TiN/Si3N4 superhard nanocomposites; Unbalanced magnetron sputtering; Structure and mechanical properties; Thermal stability; Wear and corrosionresistance

1. Introduction

In recent years, there have been considerable advances in thedevelopment of hard and wear-resistant coatings for applica-tions in the field of cutting tools and other moving parts toimprove durability. Among the more significant developmentsin the hard coatings has been the synthesis of superhard(hardness ≥4000 kg/mm2) nanocomposite coatings [1–3].

⁎ Corresponding author. Tel.: +91 80 2508 6494; fax: +91 80 2521 0113.E-mail address: harish@css.nal.res.in (H.C. Barshilia).

0257-8972/$ - see front matter © 2005 Elsevier B.V. All rights reserved.doi:10.1016/j.surfcoat.2005.11.124

Nanocomposite coatings consist of two phases—either twonanocrystalline phases or a nanocrystalline phase and anamorphous phase. These coatings exhibit superior mechanicalproperties. One of the most well known examples is thedeposition of TiN-based nanocomposites, which have foundwidespread applications for tools and other hard surfaces [4,5].At present, the origin of the enhanced mechanical properties ofthe nanocomposite coatings is not very clear. It is believed thatthe enhanced mechanical properties of the nanocompositecoatings are due to the fact that, at very small grain sizes (≤10 nm),the number of atoms in a boundary region, which surrounds the

330 H.C. Barshilia et al. / Surface & Coatings Technology 201 (2006) 329–337

grain, is comparable to that in the grain. This hinders thegeneration of dislocations and prevents crack propagation due tograin boundary enhancement and suppression of grain boundarysliding [6].

Interest in Ti–X–N-based materials (where X=B, C, Al, Si)as superhard coatings has risen sharply in the last few yearsespecially because of their high hardness [7–9]. In particular,TiN–Si3N4 nanocomposite coatings have been studied widelybecause of very high hardness and high elastic modulus,improved wear resistance and high oxidation resistance [10–14]. In these coatings, TiN nanocrystals are embedded in anamorphous Si3N4 matrix. These coatings have been synthesizedby different techniques such as plasma-induced chemical vapordeposition [15,16] and reactive DC/RF magnetron sputtering[17–20]. It has been reported that the hardness of the TiN/Si3N4

nanocomposite coatings depends on silicon content. Rebouta etal. [21] have reported that the TiN/Si3N4 nanocompositecoatings showed hardness as high as 45 GPa (approximately4500 kg/mm2) for silicon content in the range of 4–10 at.%. Inaddition, the preferential growth as a function of silicon contentchanged from a strong (111) orientation at the lowest siliconcontent to a weak (200) orientation at the highest silicon content.Diserens et al. have reported a hardness of about 27 GPa for TiNand 38 GPa for TiN–SiNx nanocomposite films containing 5 at.% silicon, which decreased to the value of amorphous SiNx atsilicon concentration above 15 at.% [22].

Among the different magnetron sputtering techniques,unbalanced magnetron sputtering is preferred for the depositionof hard coatings. The use of unbalanced magnetic field results inthe formation of a secondary plasma near the substrate, resultingin increased ion bombardment on the substrate [23]. Ion

Sputtetar

He

Subs

Water

Pumping port

Gas m

Ar+N2

Fig. 1. Schematic diagram of the reactive unb

bombardment of the substrate is essential for enhanced growthrate and dense microstructure of the deposited films. DCreactive sputtering of non-conducting targets (such as silicon)results in target poisoning, thus causing an arc [24]. Thisarcing is particularly violent for reactive DC sputtering ofhighly insulating oxides and nitrides and it can damage thepower supply, leading to poor quality films. Radio frequencysputtering enables the use of non-conducting materials astargets and allows reactive deposition of dielectric films. Butin this case, deposition rates are low since sputtering takesplace only during one half of the cycle. Asymmetric bipolar-pulsed DC power supply, in which the polarity of the targetvoltage periodically shifts from negative to positive, is idealfor reactive sputtering of silicon target [25]. In the presentwork, superhard TiN/Si3N4 nanocomposite coatings have beensynthesized using a reactive unbalanced DC magnetronsputtering of titanium and silicon targets in Ar+N2 plasma.The nanocomposite coatings have been deposited at varioussilicon concentrations. Structural and mechanical properties ofthe coatings have been measured using X-ray diffraction(XRD), X-ray photoelectron spectroscopy (XPS), energydispersive X-ray analysis (EDAX), nanoindentation, atomicforce microscopy (AFM) and micro-Raman spectroscopytechniques.

2. Experimental details

Superhard nanocomposite coatings of TiN/Si3N4 wereprepared on stainless steel (SS 304) and silicon substratesusing a reactive DC unbalanced magnetron sputtering system.Fig. 1 shows the schematic diagram of the unbalanced

End-Hall Kaufman ionsource

Cooling jacket

Shutter

Cathode

Substrate bias

ringget

ater

trate

anifold

Ar

alanced DC magnetron sputtering system.

Table 1Parameters for the deposition of TiN/Si3N4 nanocomposite coatings

Step Process Parameters

1. Substrate cleaning withion gun

Ion current=2.00 A, ion energy=70 eV,pressure=1.0×10−1 Pa, substratetemperature=300 °C, duration=10 min

2. Substrate cleaning withion bombardment

Substrate bias=−850 V,Ar flow rate=17 sccm,pressure=4.0×10−1 Pa,duration=30 min

3. Target cleaning Ti: Power=30 W, duration=5 minSi: Power=40 W, duration=5 min

4. Ti interlayer Power=200 W, substrate bias=−100 V,Ar flow rate=17 sccm,pressure=1.0×10−1 Pa, substratetemperature=300 °C, depositiontime=30 min, thickness=0.5 μm

5. TiN/Si3N4

nanocompositedeposition

Nitrogen flow rate=3.5 sccm, argon flowrate=17 sccm, pressure=1.0×10−1 Pa,substrate bias=−100 V, substratetemperature=300 °CTi: Power=225 WSi: Power=30–90 WDeposition time=150 minThickness of the coating=1.5 μm

6. Cooling down and venting 45 min

331H.C. Barshilia et al. / Surface & Coatings Technology 201 (2006) 329–337

magnetron sputtering system used in the present study. Thesputtering system consists of two sputtering guns (dia-meter=3 in.), which are inclined at an angle to the normal.The use of these guns allows independent sputtering of thetwo materials. High purity titanium (99.95%) and silicon(99.999%) targets were used for the deposition of thenanocomposite coatings, wherein a DC power supply wasused to sputter the titanium target and an asymmetric bipolar-pulsed DC power supply (frequency=100 kHz, pulsewidth=2976 ns) was used to sputter the silicon target. Thesystem is also equipped with an End–Hall Kaufman ionsource. Metallographically polished substrates were cleaned inan ultrasonic agitator using acetone, isopropyl alcohol andtrichloroethylene. The vacuum chamber was evacuated to abase pressure of 4.0×10−4 Pa using a turbomolecular pumpbacked by a rotary pump. The substrates were degassed at300 °C for 10 min and were cleaned in situ using an ion gunto remove any organic impurities. Finally, the substrates weresubjected to an Ar+ ion bombardment at a substrate bias of−800 V and a pressure of 4.0×10−1 Pa for 30 min to removeoxide impurities. The flow rates of argon and nitrogen werecontrolled separately by mass flow controllers. Prior to thedeposition, the targets were sputter cleaned for 5 min. Atitanium interlayer of approximately 0.5 μm thickness wasdeposited on the substrates for improved adhesion of thecoatings. During the deposition, a DC substrate bias of −100 Vwas applied to improve the mechanical properties of thecoatings. The ion current density was of the order of 1 mA/cm2 under these conditions. The nanocomposite coatings weredeposited at a substrate temperature of 300 °C and at an Ar+N2 gas pressure of 1.0×10

−1 Pa. For all the experiments, thepower density for the titanium target was 4.93 W/cm2. Inorder to obtain nanocomposite coatings with different siliconcontents, the power density for the silicon target was variedfrom 0.66 to 1.97 W/cm2. The thickness of the nanocompositecoating was approximately 1.5 μm. The process conditions forthe deposition of TiN/Si3N4 nanocomposite coatings aresummarized in Table 1.

X-ray diffraction data of the films were recorded in a RigakuD/max 2200 Ultima X-ray powder diffractometer using a Cu Kα

radiation (λ=1.5418 Å). The average crystallite size of thecoatings was determined using Debye–Scherrer formula [26].The elemental compositions of the coatings were determinedusing energy dispersive X-ray analysis in a scanning electronmicroscope (Leo 440I). The bonding structure of the nano-composite coatings was characterized by X-ray photoelectronspectroscopy using an ESCA 3000 (V. G. Microtech) systemwith a monochromatic Al Kα X-ray beam (energy=1486.5 eVand power=150 W). The mechanical properties of the coatings(hardness and elastic modulus) were measured in a nanoinden-ter (CSEM Instruments). The details of nanoindentationmeasurements are described elsewhere [27]. Surface imagingof the as-deposited samples was carried out using atomic forcemicroscopy. The maximum scan ranges for AFM in X, Y and Z-axes were 40, 40 and 4 μm, respectively. The AFM resolutionalong the X, Yand Z-axes was b1 nm. The AFMwas operated innon-contact mode.

In order to test the thermal stability of the coatings, TiNand TiN/Si3N4 nanocomposite coatings were simultaneouslyheated in air in a resistive furnace at TA=500, 600, 700, 800and 850 °C. Annealing involved increasing the temperature ofthe samples from room temperature to the desired temperatureat a slow heating rate of 3 °C/min and maintaining the desiredtemperature for 30 min. Subsequently, the samples werecooled down at a rate of 3 °C/min. Structural changes as aresult of the heating were measured using micro-Ramanspectroscopy. Coatings were also annealed in vacuum toseparate the effect of oxidation-induced spectral changes in theRaman data. For this, the samples were vacuum-sealed in aquartz tube at approximately 3.0×10−3 Pa pressure. Thequartz tube was then placed in the furnace, and the sampleswere annealed at 850 °C for 30 min under identical heatingand cooling rates. A DILOR–JOBIN–YVON–SPEX inte-grated micro-Raman spectrometer was used for the presentstudy [28].

The corrosion behavior of the coatings was studied usingpotentiodynamic polarization in 3.5% NaCl solution underfree-air conditions at room temperature. The experimentalconditions for the polarization measurements are describedelsewhere [29]. The corrosion potential (Ecorr), the corrosioncurrent density (icorr) and the polarization resistance (Rp) werededuced from the Tafel (log i vs. E) plots. Wear experimentswere carried out using a pin-on-disc friction and wear monitor(Ducom). The disc was heat-treated EN31 having a hardnessof 750 VHN. The disc revolution was 200 rpm and the weartrack radius was 35 mm. The coatings were deposited on theflat side of the stainless steel pins (diameter=6mm, height=8mm)and loaded against the rotating disc at 9.8 N load. The frictionalforce and the height loss were recorded against the slidingdistance.

9 12 15 18 21 24 27

40

60

80

100

120TiN

Ave

rage

Cry

stal

lite

Size

(Å

)Silicon Content (at.%)

Fig. 3. Variation of average crystallite size of TiN/Si3N4 nanocomposite coatingswith silicon content, as determined using the broadening of (200) principalreflection.

332 H.C. Barshilia et al. / Surface & Coatings Technology 201 (2006) 329–337

3. Results and discussion

3.1. Structural characterization

The process parameters (such as nitrogen flow rate, powerdensity, bias voltage and substrate temperature) were firstoptimized to achieve single-phase TiN coatings. In particular,the substrate bias and the nitrogen flow rate affected themechanical properties of the TiN coatings significantly. For anitrogen flow rate of 3.5 sccm, a substrate bias of −100 V, asubstrate temperature of 300 °C and a power density of 4.93 W/cm2, best quality TiN coatings with a nanoindentation hardnessof approximately 2800 kg/mm2 were obtained. XRD pattern ofa typical TiN coating deposited under the above conditions isshown in Fig. 2. The high intensity peak centered at 2θ=42.26°is assigned to TiN (200) and the peak with a lower intensity at2θ=36.30° is assigned to TiN (111). Other higher anglereflections such as (220), (311), (222), (400) were very weak,indicating that majority of the TiN grains were oriented along(200). Subsequently, nanocomposite coatings of TiN/Si3N4

were prepared at different silicon concentrations. The relativesilicon content of the nanocomposite coatings was determinedusing EDAX. The XRD patterns of TiN/Si3N4 nanocompositecoatings deposited at various silicon contents are also shown inFig. 2. The XRD data of single-phase Si3N4 coating showed nopeaks, indicating its amorphous nature. For the nanocompositecoatings, the (200) reflection broadened with an increase in thesilicon content. This broadening is attributed to a decrease in thecrystallize size of the coatings presumably because ofincorporation of amorphous Si3N4 in the TiN matrix. A lowintensity (111) reflection was also observed at low siliconcontents, which disappeared completely at higher siliconcontents in the nanocomposite coatings. The average crystallitesize of the coatings was calculated from the broadening of the(200) peak in the XRD data. For TiN/Si3N4 nanocomposites, the

30 35 40 45

2θθ (deg.)

Inte

nsit

y (a

rb. u

nits

)

(a)

(b)

(c)

(111

)

(200

)

TiN

TiN/Si3N4

Fig. 2. XRD patterns of TiN/Si3N4 nanocomposite coatings deposited at varioussilicon contents: (a) 15 at.%; (b) 21 at.%; (c) 27 at.%. Also shown is the XRDdata of TiN coating. These coatings were deposited at a substrate temperature of300 °C, a substrate bias of −100 V and a nitrogen flow rate of 3.5 sccm.

crystallite size, as shown in Fig. 3, decreased from approxi-mately 73 Å for a silicon content of 9 at.% to 32 Å for a siliconcontent of 27 at.%. Similar results have been reported in theliterature [16]. Single-phase TiN deposited under similarconditions exhibited a crystallite size of approximately 120 Å.The decrease in the crystallite size of the nanocompositecoatings with silicon content is due to the formation of stablenanostructure, composed of TiN and Si3N4 [30].

Fig. 4 shows typical high-resolution XPS core level spectraof TiN/Si3N4 nanocomposite coating with a silicon content of15 at.%. The N 1s spectrum in Fig. 4(a) reveals the presence oftwo peaks characteristic of nitrogen in TiN and Si3N4 withbinding energies at around 397.0 and 399.3 eV, respectively[20,31]. The intensity of the two peaks indicated that most of thenitrogen present in the TiN/Si3N4 nanocomposite coating was inthe form of TiN with relatively small amounts of Si3N4. The Si2p spectrum in Fig. 4(b) shows a characteristic peak at a bindingenergy of 102.0 eV corresponding to the Si 2p electrons inSi3N4 [4]. Peaks pertaining to free silicon (99.0 eV) and othersilicon compounds were not observed, indicating that thebonding state of silicon was in the form of Si3N4. The XRD andthe XPS data of the nanocomposite coatings thus showed thatthe coatings consisted of TiN nanocrystals and amorphousphase of stoichiometric Si3N4.

3.2. Mechanical characterization

Fig. 5 shows typical load vs. displacement curves for TiN,Si3N4 and TiN/Si3N4 nanocomposite coatings at 5 mN load. Forthe TiN/Si3N4 nanocomposite coating, the maximum indenta-tion depth was about 85 nm, which is less than one-tenths of thecoating thickness. For the TiN and the Si3N4 coatings, the

Binding Energy (eV)

Inte

nsit

y (a

rb. u

nits

)

92.5 97.5 102.5 107.5

Si 2p 102.0 eV

Binding Energy (eV)

Inte

nsit

y (a

rb. u

nits

)

385

a

b

390 395 400 405 410

N 1s 399.3 eV

N 1s 397.0 eV

Fig. 4. XPS spectra of (a) N 1s peak and (b) Si 2p peak for the TiN/Si3N4

nanocomposite coating with a silicon content of 15 at.%.

0 15 30 45 60 75 90 105 1200

1

2

3

4

5 (a)(b)(c)

(c) TiN/Si3N4

Loa

d (m

N)

Indendation Depth (nm)

(b) TiN

Unl

oadi

ngLoadi

ng

(a) Si3N4

Fig. 5. Typical load vs. displacement curves for TiN, Si3N4 and TiN/Si3N4

nanocomposite coatings at 5 mN load.

333H.C. Barshilia et al. / Surface & Coatings Technology 201 (2006) 329–337

maximum indentation depths were about 100 and 115 nm,respectively. For the nanoindentation measurements, afterinitial contact of the indenter on the surface, the load wasincreased at a predetermined rate (10 mN/min) to the desiredmaximum load (5 mN) and then decreased at the same rate(10 mN/min) to zero. From the load vs. displacement plot, thehardness was calculated using the method of Oliver and Pharr[32]. In general, the loading data are influenced more by theplastic properties of the material and the unloading data by theelastic properties. The hardness values were calculated fromthe loading portion of the curves. The variation of hardness ofTiN/Si3N4 nanocomposite coatings with silicon content isshown in Fig. 6. TiN and Si3N4 coatings prepared underidentical conditions showed hardness values of 2800 and2600 kg/mm2, respectively. For the nanocomposite coatings, atlower silicon contents (b12 at.%) the hardness was low. Itincreased with an increase in the silicon content and thenanocomposite coatings exhibited a maximum hardness of5200 kg/mm2 for a silicon concentration of about 12 at.%. Themaximum hardness of the nanocomposite coatings is very high

as compared to the rule-of-mixtures value. Similar results havebeen reported in the literature [18,19]. The hardness of thenanocomposite coatings decreased significantly with furtherincrease in the silicon content. For example, for a siliconconcentration of approximately 27 at.%, the nanocompositecoating showed a hardness of 3200 kg/mm2. This decrease inthe hardness is attributed to an increase in the amorphous Si3N4

content in the TiN matrix. Also shown in Fig. 6 is the variationof elastic modulus with silicon content. It is interesting to notethat nanocomposite coatings exhibited both high hardness andhigh elastic modulus.

In general, increase in the hardness of the nanocompositecoatings is attributed to the small crystallite size. The XRDdata clearly showed that the crystallite size of the TiN/Si3N4

nanocomposite coatings decreased with incorporation ofsilicon in the coating. It is known that the small crystallitesize of the nanocomposite coating (i) hinders the generation ofdislocations, (ii) prevents crack propagation, and (iii) sup-presses grain boundary sliding. These three factors result inenhanced mechanical properties of the nanocomposite coat-ings. However, at very small crystallite size (a case when theamorphous Si3N4 content is very high in the nanocompositecoating), the hardness of the nanocomposite coating decreasesbecause of the presence of a large amount of amorphous Si3N4

phase. Therefore, an optimum volume fraction of amorphousSi3N4 is required to obtain high hardness in the TiN/Si3N4

nanocomposite coatings.Fig. 7(a) and (b) show three-dimensional surface morphol-

ogies of a TiN coating and a typical TiN/Si3N4 nanocompositecoating, respectively. It is clear that in the case of TiN, the grainsize was large and the film was rough (root mean squareroughness=3.73 nm). However, the grain size decreased in thecase of nanocomposite coating and the roughness was also low(approximately 1.22 nm).

9 12 15 18 21 24 27

2500

3000

3500

4000

4500

5000

Si3N4

TiN

Si3N4

TiN

H

Har

dnes

s (k

g/m

m2 )

Silicon Content (at.%)

200

220

240

260

280

300

320

340

Ela

stic

Mod

ulus

(G

Pa)

Y

Fig. 6. Variations of nanoindentation hardness and elastic modulus of TiN/Si3N4

nanocomposite coatings with silicon content.

611

334 H.C. Barshilia et al. / Surface & Coatings Technology 201 (2006) 329–337

3.3. Oxidation behavior

For the hard coatings, apart from high hardness the thermalstability with respect to oxidation at higher temperatures is veryimportant, as these coatings are potential candidates for wearresistance applications in cutting tools. We used micro-Raman

nm0

5000

10000

0

5000

10000

0

40

80

nm(a)

nm0

5000

10000

0

5000

10000

0

40

80

nm

(b)

Fig. 7. Three-dimensional AFM images of 1.5-μm-thick (a) TiN coating and (b)TiN/Si3N4 nanocomposite coating. For the nanocomposite coating, the siliconcontent was 15 at.%.

spectroscopy to study the thermal stability of the coatingsstudied in the present work, as it is very sensitive to the surfaceoxide formation. In Fig. 8, we plot the composite Raman spectraof as-deposited TiN and TiN coatings heat treated at differenttemperatures. The spectrum of TiN shows two broad bandscentered at 299 and 556 cm−1. These bands originate due toacoustic transitions in the 150–300 cm−1 region (LA and TA)and optic modes in the 400–650 cm−1 region (LO and TO). Forsputtered TiN coatings, scattering in the acoustic range isprimarily determined by the vibrations of the Ti ions and that inthe optic range by vibrations of the N ions [28]. After heatingthe sample at 400 °C, the spectrum (data not shown) did notchange significantly, however, at TA=500 °C additional peakscentered at 449 and 611 cm−1 were observed. The origin ofthese peaks is assigned to rutile TiO2 phase. The intensities ofthese peaks increase with an increase in the annealingtemperature. For TA≥850 °C, an additional peak centered at521 cm−1 emerges— this peak is assigned to anatase TiO2 phase(Table 2). The intensity of the acoustic mode band decreaseswith TA, whereas, the optic mode band disappears completely atTA≥500 °C. This indicates removal of nitrogen atoms fromTiN matrix and oxidation of the coating at TA≥500 °C.Raman spectra of as-deposited TiN/Si3N4 nanocomposite

200 300 400 500 600 700 800

299

600°C

500°C

25°C

Inte

nsit

y (a

rb. u

nits

)

Raman Shift (cm-1)

556

700°C

800°C

521

449

850°C

TiN

Fig. 8. Composite Raman spectra of as-deposited TiN and TiN coatings heat-treated up to 850 °C in air.

Table 2Assignment of various Raman peaks of as-deposited and heat-treated TiN andTiN/Si3N4 nanocomposite coatings

Sample Peak position(cm−1)

Peak assignment

TiN (a) As-deposited 299 (B) Longitudinal andtransverse acoustic

556 (B) Transverse opticmode

(b) Heat treated inair (850 °C)

449 (S) TiO2 (rutile)521 (S) TiO2 (anatase)611 (S) TiO2 (rutile)

TiN/Si3N4

nanocomposite(15 at.% Si)

(a) As-deposited 306 (B) Longitudinal andtransverse acoustic

582 (B) Transverse opticmode

(b) Heat treated inair (850 °C)

449 (W) TiO2 (rutile)611 (W) TiO2 (rutile)

(c) Heat treated invacuum (850 °C)

296 (S) Longitudinal andtransverse acoustic

554 (S) Transverse opticmode

B–broad; S–strong; W–weak.

200 300 400 500 600 700 800

500°C

25°C

Inte

nsit

y (a

rb. u

nits

)

Raman Shift (cm-1)

306

582

600°C

700°C

800°C

TiN/Si3N4 449

611

850°C

Fig. 9. Composite Raman spectra of as-deposited TiN/Si3N4 nanocompositecoating and coatings heat-treated up to 850 °C in air.

200 300 400 500 600 700 800

Raman Shift (cm-1)

TiN/Si3N4; As-deposited at 300°C

Inte

nsit

y (a

rb. u

nits

)

TiN/Si3N4; TA = 850°C

Fig. 10. Representative Raman spectra of as-deposited and vacuum-annealedcoatings of TiN/Si3N4 nanocomposites at 850 °C.

335H.C. Barshilia et al. / Surface & Coatings Technology 201 (2006) 329–337

coating and coatings heat treated up to 850 °C are shown inFig. 9. As it is shown in the figure, the Raman spectra of thenanocomposite coatings do not change up to 800 °C and atTA=850 °C very weak bands associated with TiO2 (rutile) startappearing. Thus, it is clear that the oxidation resistance of TiNcoatings can be improved considerably by forming a two-phasenanocomposite structure in which nanocrystalline TiN grainsare embedded in an amorphous Si3N4 matrix. In single-phaseTiN coatings, the boundaries between the columns and thegrain boundaries between the TiN crystallites offer a short cutfor diffusion of oxygen [33]. As a result of this, the oxidationenergy of TiN is low (136 kJ/mol) [33]. However, in the TiN/Si3N4 nanocomposite coatings, the Si3N4 phase boundaryaround the TiN crystallite acts as a diffusion barrier. Thisinhibits the diffusion of oxygen and thus retards the oxidationrate of the TiN/Si3N4 nanocomposite coatings.

Heating of the nanocomposite coatings in air can also lead tophase transformation in addition to the oxide formation. Boththe phase transformation and the presence of oxides affect thenature of the Raman spectra. In order to study whether thecoatings were structurally stable at higher annealing tempera-tures, they were heated in high vacuum for 30 min at 850 °C. InFig. 10, we plot the representative Raman spectra of as-deposited and vacuum annealed coatings of TiN/Si3N4 nano-composites. It is quite clear from the spectra that annealing of thecoatings in vacuum did not change the nature of the Ramanspectra considerably, compared to coatings annealed in air,which showed the presence of various oxide phases at higherannealing temperatures. TiN coatings heated under high vacuumconditions at 850 °C, also showed similar behavior. Theseresults demonstrate that the coatings studied in the current workshow no phase transformation up to 850 °C as a result of heattreatment in vacuum. Although the optical phonon band is verybroad, it shifts to lower frequencies after heat treatment. Theshift in the Raman spectra can be attributed to relaxation ofcompressive stress in the coatings. Compressive stresses are

commonly observed in sputter-deposited physical vapor-depos-ited (PVD) hard coatings. The compressive stress in the PVDhard coatings has been related to Ar+ ion bombardment-inducedpoint defects [34]. The ion bombardment is required to achieve

Table 3Potentiodynamic polarization data of TiN, Si3N4 and TiN/Si3N4 nanocompositecoatings deposited on stainless steel substrates in 3.5% NaCl solution underfree-air conditions at room temperature

Sample Ecorr (V) icorr (A/cm2) Rp (Ω cm2)

Stainless steel −0.220 8.23×10−8 1.12×105

TiN −0.185 6.94×10−8 1.75×105

Si3N4 −0.102 1.56×10−8 1.37×106

TiN/Si3N4 (12 at.% Si) −0.172 6.29×10−8 2.63×105

TiN/Si3N4 (15 at.% Si) −0.150 5.89×10−8 2.99×105

TiN/Si3N4 (21 at.% Si) −0.176 2.07×10−8 4.29×105

TiN/Si3N4 (27 at.% Si) −0.131 1.26×10−8 4.92×105

336 H.C. Barshilia et al. / Surface & Coatings Technology 201 (2006) 329–337

noncolumnar and dense microstructure, reduced crystallite sizeand improved adhesion of the coatings.

3.4. Corrosion behavior

Generally, physical vapor deposited coatings are inert tochemical attacks due to their higher position in the electro-chemical series. However, in practice, these coatings exhibitgalvanic and crevice corrosion between the coating and thesubstrate because of the presence of defects such as micro-pores. The electrolyte penetrates to the substrate via thesedefects. If the porosity is very low, the sample behaves totallylike the coating. If the porosity is high, a significant galvaniccorrosion with substrate being anode and the coating being thecathode will make the coated sample behave like the substratematerial. So the corrosion current is a good measure of thecoating porosity. The corrosion behavior of TiN, Si3N4 and TiN/Si3N4 nanocomposite coatings deposited on stainless steelsubstrates were investigated using potentiodynamic polarizationin 3.5% NaCl solution under free-air conditions at roomtemperature. The Tafel plots obtained for stainless steelsubstrate, single-phase TiN, single-phase Si3N4 and TiN/Si3N4

nanocomposite coatings are shown in Fig. 11. The corrosionpotential, the corrosion current density and the polarizationresistance obtained from the measurements are given in Table 3.After coating the substrate with approximately 1.5-μm-thickTiN/Si3N4 nanocomposite coatings, Ecorr increased from−0.220 to −0.131 V, whereas, icorr decreased from 8.23×10−8 A/cm2 to 1.26×10−8 A/cm2. Single-phase TiN and Si3N4

coatings exhibited corrosion currents of 6.94×10−8 and1.56×10−8 A/cm2, respectively. This increase in the corrosionpotential indicates an improvement in the corrosion resistanceof the stainless steel substrate with the nanocomposite coating.The corrosion current density is commonly utilized as animportant parameter to evaluate the kinetics of corrosionreactions. The corrosion rate is normally proportional to the

1.0×10-3

1.0×10-5

1.0×10-7

1.0×10-9

1.0×10-11

-0.450 -0.320 -0.190 -0.060 0.070 0.200

E (V vs. Ag/AgCl)

log

i (A

/cm

2 )

SS

TiN

(c)

(a)(b) (d)

Si3N4

Fig. 11. Potentiodynamic polarization curves of TiN/Si3N4 nanocompositecoatings at different silicon concentrations: (a) 12 at.%; (b) 15 at.%; (c) 21 at.%;(d) 27 at.%. Also shown are the polarization curves of the stainless steelsubstrate, Si3N4 and TiN coatings.

corrosion current density. The decrease in icorr of coatedsubstrate thus confirms improvement in the corrosion resis-tance. Furthermore, the corrosion resistance was also found toincrease with an increase in the silicon concentration in the TiN/Si3N4 nanocomposite coatings. Rp increased from 2.63×105 Ωcm2 for a silicon concentration of 12 at.% to 4.92×105 Ω cm2

for a silicon concentration of 27 at.%. Single-phase TiN andSi3N4 coatings exhibited a corrosion resistance of 1.75×105 Ωcm2 and 1.37×106 Ω cm2, respectively. Good corrosionbehavior of TiN/Si3N4 nanocomposite coatings is believed tobe due to a dense microstructure with low through-coatingporosity. The dense microstructure of the coatings can beattributed to ion bombardment during deposition, low growthrates and moderately higher deposition temperature (300 °C).These parameters resulted in a fine-grained morphology of thecoatings, as indicated by AFM studies. The corrosion datademonstrates better corrosion resistance for TiN/Si3N4 nano-composite coatings as compared to single-phase TiN coatings.

3.5. Wear measurements

Fig. 12 shows preliminary wear data for the uncoatedstainless steel pin and pins coated with TiN and TiN/Si3N4

0 1000 2000 3000 4000 50000

25

50

75

100

125

Hei

ght

Los

s (μ

m)

Sliding Distance (m)

3

3 TiN/Si3N4 (27 at.% Si)

2

2 TiN/Si3N4 (21 at.% Si)

1

1 Stainless Steel0 200 400 600 800

0

20

40

60

80

100

120TiN

Fig. 12. Wear data of the uncoated stainless steel pin and pins coated with TiNand TiN/Si3N4 nanocomposites.

337H.C. Barshilia et al. / Surface & Coatings Technology 201 (2006) 329–337

nanocomposites. The uncoated pin showed very high wear lossfrom the beginning, with a wear loss of about 75–100 μm. Onthe other hand, TiN-coated pin did not show greater wear lossfor a sliding distance of 300 m, as displayed in the inset.However, a significant improvement in the wear resistance wasobserved for the TiN/Si3N4 nanocomposites. It has beenobserved that if the concentration of Si3N4 was increased inthe nanocomposite coatings the improvement in the wearresistance was very high. This improvement in the wearresistance for the TiN/Si3N4 nanocomposites could be due toincreased hardness of the nanocomposite coatings and also dueto lower coefficient of friction of Si3N4. Detailed work iscurrently underway to further study the wear mechanism forthese coatings.

4. Conclusions

Superhard nanocomposite coatings of TiN/Si3N4 wereprepared by reactive DC unbalanced magnetron sputtering atdifferent silicon contents. The XRD data of the nanocompositecoatings exhibited a broad (200) reflection of cubic TiN phase,which broadened significantly at very high silicon content. Theaverage crystallite size of the nanocomposite coatings, asdetermined from the broadening of the (200) peak in the XRDdata, decreased with an increase in the silicon content. Forexample, the nanocomposite coatings prepared at a siliconcontent of 9 at.% showed an average crystallite size of 73 Å,whereas those prepared at 27 at.% showed an average crystallitesize of 32 Å. The XPS data indicated that nitrogen present in thenanocomposite coatings was predominantly in the form of TiNwith small amount of stoichiometric Si3N4. The TiN/Si3N4

nanocomposite coatings exhibited a maximum nanoindentationhardness of 5200 kg/mm2 at a silicon content of 12 at.%. Thenanocomposite coatings also exhibited high elastic modulus.TiN/Si3N4 nanocomposite coatings were stable up to 800 °C inair as compared to TiN coatings, which have been oxidized evenat 500 °C. The Raman data of the vacuum-annealed coatings at850 °C showed no phase transformation, indicating that thecoatings were thermally stable up to 850 °C. About 1.5-μm-thick TiN, Si3N4 and TiN/Si3N4 nanocomposite coatingsdeposited on stainless steel substrates demonstrated goodcorrosion resistance in 3.5% NaCl solution, and the nanocom-posite coatings showed better corrosion resistance as comparedto the single-phase TiN coatings. The wear data showed thatTiN/Si3N4 nanocomposite coatings exhibited better wearresistance as compared to TiN coatings.

Acknowledgements

The authors thank Director, NAL for giving permission topublish these results. Dr. Anjana Jain is thanked for XRD work.Thanks are also due to Mr. V. K. W. Grips for help in

potentiodynamic polarization and wear resistance measure-ments. This work was supported by the Council of Scientificand Industrial Research, New Delhi, India (Grant No. CMM0022).

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