Surface & Coatings Technology 206 (2011) 1735–1743

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Surface & Coatings Technology

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Surface modification of NiTi/PZT heterostructure thin films using various protectivelayers for potential MEMS applications

Nitin Choudhary a, D.K. Kharat b, Davinder Kaur a,⁎a Functional Nanomaterials Research Lab, Department of Physics and Centre of Nanotechnology, Indian Institute of Technology Roorkee, Roorkee-247667, Uttarakhand, Indiab PZT Centre, Armament Research and Development Establishment, Pune-411021, India

⁎ Corresponding author. Tel.: +91 1332 285407; fax:E-mail address: dkaurfph@iitr.ernet.in (D. Kaur).

0257-8972/$ – see front matter © 2011 Elsevier B.V. Alldoi:10.1016/j.surfcoat.2011.08.056

a b s t r a c t

a r t i c l e i n f o

Available online 12 September 2011

Keywords:NiTi/PZTMagnetron sputteringShape memoryNanoindentationHeterostructures

The present study explored the in-situ deposition of hard and adherent nanocrystalline protective coatingson NiTi/PZT/TiOx thin film heterostructure prepared by dc/rf magnetron sputtering. Protective layers (AlN,CrN and TiCrN) of approximate thickness (~200 nm) were used to improve the surface, mechanical and cor-rosion properties of NiTi/PZT/TiOx heterostructure without sacrificing the shape memory effect and ferroelec-tricity of the NiTi and PZT layers, respectively. The influence of the protective layer on structural, electricaland mechanical properties of NiTi/PZT/TiOx heterostructure was systematically investigated and the resultswere compared. Nanoindentation studies were performed at room temperature to determine the hardnessand reduced modulus. The surface modified NiTi/PZT/TiOx heterostructures were found to exhibit high hard-ness, high elastic modulus and thereby better wear resistance as compared to pure NiTi/PZT/TiOx films. Fromthe results of potentiodynamic polarization test conducted in 1 M NaCl solution, the CrTiN coated NiTi/PZT/TiOx heterostructure showed the best corrosion resistance with the lowest corrosion current density(1.52×10−8 A cm−2) and the highest protective efficiency (96.8%). The results presented here prove the po-tential of a surface modified NiTi/PZT/TiOx heterostructure to be used in various microelectromechanical(MEMS) applications.

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1. Introduction

Certain combinations of thermoelastic and ferroelectric materialsprovide “smart” sensor-actuator structures which can be advanta-geous for suppression of stress and acoustic waves resulting fromair blasts and explosives [1,2]. These heterostructures have becomeincreasingly important for the protection of strategic buildings, ordi-nance storage and airplane security. Ferroelectric materials are verysensitive to applied stresses and can produce an electric field throughthe piezoelectric effect. The phenomenon is reversible with fast re-sponse times but very small displacements, which is called inversepiezoelectric effect. Shape memory alloys (SMA) exhibit large actua-tion displacement during the martensite-to-austenite transformationbut much slower cycling times, because to get the large actuation dis-placement, heating above and cooling below the transformation tem-perature are required and cooling is a relatively slow process.Coupling SMAs with ferroelectric materials produces a technically in-triguing hybrid structure that combines the thermo-mechanicalproperties of SMAs with the electro-mechanical relationship associ-ated with ferroelectric materials, which can utilize the different ac-tive and adaptive properties of the individual materials and may

have effective applications for smart systems such as active dampingof mechanical vibration [3].

The logic of vibration damping by thermoelastic–ferroelectric struc-tures (as shown in Fig. 1) can be explained when a blast wave (stresswave) hits the SMA layer, a martensitic–austenitic transformationoccurs, the effect of which is to convert some of the elastic shock energyinto heat. When the blast wave is removed, the material reverts to itsoriginal position with the dissipation of heat. As the wave continuesto propagate, it is further attenuated by the ferroelectric layer byusing its piezoelectric properties [4,5]. Traditional vibration dampingmaterials, e.g., viscoelastics, have a very highmaterial dispersion coeffi-cient, tan δ and very low material stiffness, which limits the amount ofenergy that can be damped [6]. A successful vibration damping deviceused to mitigate the effect of spurious vibrations can be achieved bythe development of a heterostructure composite bilayer configurationcomposed of piezoelectric and shape memory alloy thin films [7].

NiTi as shape memory alloy and PZT ferroelectric ceramics arechosen for the preparation of such kind of heterostructure due totheir superior shape memory behavior and ferroelectric properties,respectively [8–10]. However, in our previous report [11], we suc-ceeded in fabricating NiTi/PZT/TiOx thin film heterostructures, exhi-biting excellent structural and electrical properties. Apart fromgenerating the highly oriented crystalline phases of individual mate-rial without pronounced interdiffusion at their interface, there arestill some concerns for the wide application of NiTi/PZT thin film

Protects from corrosion and provides mechanical stability

Protective layer (AlN, CrN, CrAlN)

NiTi PZT

TiO

xla

yer

Pt

coat

ed s

ilico

n

Fig. 1. Schematic diagram of protective layers deposited on a NiTi/PZT/TiOx thin filmheterostructure.

1736 N. Choudhary et al. / Surface & Coatings Technology 206 (2011) 1735–1743

heterostructures because of their unsatisfactory mechanical andchemical resistance reliability, pronounced interdiffusions and chem-ical reactions at the interface [12]. The limited hardness and wear re-sistance of the top NiTi layer make it difficult to be used in effectivedamping applications for long period of time. Prolonged thermaland/or mechanical cycling will both influence the lattice defect struc-ture leading to changes in hysteresis, transformation temperaturesand functional properties thereby degrading the performance of theNiTi/PZT system. If corrosion and wear resistance of the NiTi/PZT het-erostructure can be increased, their usage will become widespread.Therefore, there is a need to search for hard and corrosion resistantprotective coatings for damping device applications of NiTi/PZTbased thin film heterostructures with improved operating character-istics (high mechanical stability and corrosion resistance). Great pro-gress has been made in the last decade on high-performance hard-coatings applied to ceramics, cermets and tool steels. Nitrides havebeen most commonly used as hard protective coatings owing totheir superior chemical stability, excellent strength, wear, and corro-sion resistance. The hard protective coatings for tribological applica-tions include CrN, TiN, TiAlN, CrSiN, TiC, carbonitrides, and boridessynthesized by a variety of techniques such as high-temperaturechemical vapor deposition (CVD), low-temperature reactive sputter-ing (PVD), activated reactive evaporation and plasma CVD [13–15].Among them, magnetron sputtering has been recognized as a

Table 1Deposition conditions for AlN, CrN and CrTiN protective coatings inmagnetron sputtering.

Deposition process Magnetron sputteringSputtering targets Aluminum (99.99% purity): for AlN coatings

Chromium (99.99%): CrN coatingsChromium and titanium (99.99%): CrTiN coatings

Substrate NiTi/PZT/TiOx/Pt/Ti/SiO2/SiSputtering power (dc) Al: 100 W, Cr: 100 W, Ti: 40 WSubstrate bias 0 VReactive gas Ar+N2

Deposition pressure 1.5 PaSubstrate temperature 573 KTarget-substrate distance 4 cmProtective coating thickness ~200 nm

promising versatile technique for the deposition of stoichiometricfilms of metal, metal oxides and nitrides. Its main advantage is largescale deposition of high quality films at high deposition rates at rela-tively low deposition temperature. Co-sputtering of different metaltargets in a mixture of argon, nitrogen or oxygen plasmamakes it ver-satile for the deposition of high quality binary and ternary com-pounds [16,17].

It is, therefore, an object of the present study to improve the per-formance of NiTi/PZT heterostructures by employing hard and adher-ent protective layers which give them mechanical and corrosionstability so as to increase their life time and reliability. In order toachieve this, we propose to modify an existing NiTi/PZT heterostruc-ture by depositing thin protective coatings of AlN, CrN and CrTiNfilms by using magnetron sputtering, which ensures long-lasting pro-tection and enhanced properties of NiTi/PZT heterostructure. The in-vestigation revealed better surface, mechanical and corrosionproperties of surface modified NiTi/PZT heterostructures by hardcoatings as compared to pure NiTi/PZT heterostructure thin films.

2. Experimental details

NiTi/PZT/TiOx, AlN/NiTi/PZT/TiOx, CrN/NiTi/PZT/TiOx, and CrTiN/NiTi/PZT/TiOx heterostructures were deposited on Pt/Ti/SiO2/Si sub-strates by using dc/rf magnetron sputtering. High purity (99.99%)nickel titanium (Ni0.50Ti0.50), titaniummetal targets of 50 mm diame-ter, 3 mm thickness and Pb(Zr0.52Ti0.48)O3 ceramic target prepared bysolid state reaction were used for the fabrication of NiTi/PZT/TiOx het-erostructures. In case of the PZT ceramic target, powders of PbO, TiO2

and ZrO2 of 99.95% purity were mixed in ethanol at the desired com-position; a 10% excess of PbO was added to the powders to compen-sate for the Pb loss in the sintering process and deposition. Thepowder mixture was calcinated at 800 °C for 2 h. The calcinated pow-der was pressed at 18 MPa to form pellets with 25 mm diameter, andthen sintered at 1150 °C for 3 h. Before every sputtering run, the tar-gets have been pre-sputtered for 5 min in order to ascertain thesame state of the targets in every run. A thin TiOx layer (0.25 μm)was deposited on Pt/Ti/SiO2/Si substrates prior to PZT film depositionby rf sputtering (52 W) of a pure Ti target in a mixture of argon (70%)and oxygen (30%) gas plasma. The rf sputtering ambient for PZT thinfilms was a mixture of 90% Ar and 10% O2 at a total pressure of2.6 Pa. For NiTi films, the deposition was carried out in pure argon at-mosphere at a fixed dc sputtering power of 114 W. The target to sub-strate distancewas fixed at approximately 5 cm for NiTi and TiOx filmsand 4.5 cm for PZT thin films. The thickness of PZT and NiTi films inNiTi/PZT/TiOx/Pt/Ti/SiO2/Si heterostructures was 1 μm and 1.7 μm, re-spectively. All filmswere deposited at a fixed substrate temperature of823 K and no post annealing treatment was performed after the depo-sition. The details of the deposition parameters of TiOx, PZT and NiTithin films are also discussed in detail in our previous report [11]. AlNand CrN protective coatings were subsequently deposited on NiTi/PZT/TiOx heterostructures by dcmagnetron sputtering of single alumi-num (purity 99.9%) and chromium (purity 99.9%) targets, respective-ly. On the other hand, CrTiN coatings were deposited by dc magnetronco-sputtering from a chromium 99.95% pure disk (50.8 mm diameterand 5 mm thickness) used simultaneouslywith a titaniumdisk (purity99.95%). The base pressurewas less than 2.66×10−4 Pa and the depo-sitions were carried out in Ar+N2 environment at a fixed substratetemperature of 573 K. The flow rates of the reactive gas (N2, 99.99%)and the inert gas (Ar, 99.99%) were controlled by electronic massflowmeters. The deposition parameters for AlN, CrN and CrTiN protec-tive coatings are summarized in Table 1. The thicknesses of all the pro-tective layers were kept constant at ~200 nm.

The orientation and crystallinity of the films were studied by X-raydiffraction using a Bruker advanced diffractometer with CuKα(1.54 Å) radiation in θ–2θ geometry at a scan speed of 1°/min. The mi-crostructure was studied using atomic force microscope (AFM) and

1737N. Choudhary et al. / Surface & Coatings Technology 206 (2011) 1735–1743

field emission scanning electron microscope (FESEM). The chemicalanalysis of the films was performed using energy dispersive X-ray(EDX) spectrometry attached with FESEM and the film thicknesswas measured using a surface profilometer and cross-sectionalFESEM. The electrical resistance (ER) of the films was measured bythe four probe resistivity method using a cryohead with a heliumcompressor interfaced with Keithley instruments over a temperaturerange from 100 to 400 K. The contacts over the samples were made bysilver paint. The hysteresis loop of polarization (P) as a function of ap-plied electric field (E) (P–E curve) was evaluated using the RadiantPrecision Ferrroelectric measurement system. To measure the me-chanical properties of NiTi/PZT heterostructures, depth-sensingnanoindentation tests were used. Tests were performed on a Nano In-denter device (Micromaterials, UK) using a Berkovich indenter. Six-teen nanoindentation tests were performed on each sample todetermine the hardness and reduced modulus. All measurementswere carried out at room temperature. In order to evaluate the corro-sion behavior of different protective coatings, an electrochemical ex-periment was performed with BAS (Bioanalytical Systems, WestLafayette, IN, USA) CV-50 W Voltammetric analyzer. The corrosionbehavior of pure NiTi/PZT/TiOx, AlN/NiTi/PZT/TiOx, CrN/NiTi/PZT/TiOx, and CrTiN/NiTi/PZT/TiOx heterostructures was recorded in 1 MNaCl solution. Before measurement, each sample was immersed inthe electrolyte for 20 min. The sample area exposed to the electrolytewas 0.0707 cm2 (3 mm diameter). Fig. 1 shows the schematic dia-gram of surface modified NiTi/PZT heterostructure.

20 30 40 50 60

NiTi(110)

Pt(111)PZT(100)

H-AlN(100)

NiTi/PZT/TiO x

AlN/NiTi/PZT/TiOx

CrN/NiTi/PZT/TiOx

PZT(100)

CrTiN/NiTi/PZT/TiOx

Inte

nsit

y (a

.u.)

CrN

-(11

1)

C- A

lN(2

00)

2θ (Degree)

(a)

(d)

(c)

(b)

TiN (111)

CrN (111)

Pt

(111

)

NiT

i (11

1)

Fig. 2. XRD patterns of (a) NiTi/PZT/TiOx (b) AlN/NiTi/PZT/TiOx (c) CrN/NiTi/PZT/TiOx

and (d) CrTiN/NiTi/PZT/TiOx heterostructures.

3. Results and discussion

3.1. Structural properties

Fig. 2 shows the X-ray diffraction (XRD) patterns of NiTi/PZT/TiOx,AlN/NiTi/PZT/TiOx, CrN/NiTi/PZT/TiOx, and CrTiN/NiTi/PZT/TiOx thinfilm heterostructures. XRD patterns of pure NiTi/PZT/TiOx (Fig. 2(a))reveal that the Pb(Zr0.52Ti0.48)O3 thin film deposited on the Pt/Ti/SiO2/Si substrate possesses a single perovskite tetragonal phase witha preferred (100) orientation, showing a significant improvement inthe quality of the film compared to previously reported results [18].A thin TiOx layer deposited prior to PZT promotes the growth of thePZT film by increasing the number of active sites for PZT nucleationand exhibits lower activation energy for nucleation. In addition, TiOx

plays an important role in controlling the capacitance and resistanceof the NiTi/PZT heterostructure system when it is used as a smart ac-tive damping structure [19]. The top NiTi layer exhibits a dominant(110) reflection of the austenite phase of NiTi at 2θ=42.6°. No tracesof other phases like Ti2Ni and Ni4Ti3 were observed in the pattern.After the NiTi/PZT/TiOx heterostructure was fabricated with appropri-ate crystalline phases of NiTi and PZT, the ferroelectric measurementswere performed in order to confirm the individual properties of eachcomponent. The Polarization versus electric field [P–E] loop of a NiTi/PZT/TiOx sample as shown in Fig. 3(a) confirms the ferroelectric be-havior. The values obtained for the remnant polarization

0.00

0.05

0.10

0.15

0.20

0.25

200

300

400

500

600

700

Frequency (Hz)

Die

lect

ric

cons

tant

(ε)

Die

lect

ric

loss

(ta

n δ)

0.0 2.0×106 4.0×106 6.0×106 2.0×106 1.0×106

(b)

(a)

Pol

ariz

atio

n (

µC/c

m2 )

Applied Field (kV/cm)

-40

-30

-20

-10

0

10

20

30

40

-300 -200 -100 0 100 200 300

NiTi/PZT/TiOx

Fig. 3. (a) P-E hysteresis loop (b) dielectric constant and dissipation factor (tan δ) of aNiTi/PZT/TiOx heterostructure on Pt/Ti/SiO2/Si substrate.

(d)

200 nm

(b)

200 nm

200 nm

(c)

200 nm

(a)

Fig. 4. FESEM images of (a) NiTi/PZT/TiOx (b) AlN/NiTi/PZT/TiOx (c) CrN/NiTi/PZT/TiOx and (d) CrTiN/NiTi/PZT/TiOx heterostructures.

1738 N. Choudhary et al. / Surface & Coatings Technology 206 (2011) 1735–1743

(Pr=17.1 μC/cm2) and coercive field (Ec=69.6 kV/cm) were in goodagreement with previously reported values of PZT single films. Fig. 3(b) shows the dielectric constant (ε) and dielectric loss (tan δ) of aNiTi/PZT/TiOx heterostructure as a function of frequency with an oscil-lation level at 500 mV measured at room temperature. The values ofthe dielectric constant (ε) and the dielectric loss (tan δ) at a frequencyof 500 kHz were found to be 545 and 0.038, respectively.

Protective coatings of AlN, CrN and CrTiN were subsequently de-posited on the existing NiTi/PZT/TiOx heterostructure. In the case ofAlN/NiTi/PZT/TiOx (with AlN deposited under Ar+N2 atmosphere),we note that besides the peak of the (100) orientation of the hexago-nal AlN structure, there is another small peak at 2θ=43.1° (Fig. 2(b)).This is identified as the (200) orientation of cubic AlN. The coexis-tence of hexagonal and cubic structures for the AlN top layer couldbe due to stacking faults as both phases of AlN differ only in the stack-ing sequence of nitrogen and metal atoms (polytypes) [20]. Besidesthis, the theoretical investigations of the thermal conductivity haveshown that the energy difference between two kinds of phase struc-ture of AlN is so small that both hexagonal and cubic phases areable to coexist in the current material [21]. The results of the XRDanalysis for CrN and CrTiN protective coatings on a NiTi/PZT/TiOx het-erostructure are shown in Fig. 2(c) and (d), respectively. Fig. 2(c)clearly indicates that dc magnetron sputtered CrN thin films crystal-lize in a strong (111) texture. Other diffraction planes such as (200)and (311) were completely absent in the diffraction pattern. For theCrTiN films (Fig. 2(d)), the XRD pattern was similar to that of CrN

film but with a relatively higher intensity of the (111) reflection, indi-cating that the (111) preferred orientation has become more promi-nent. It has also been observed from the XRD pattern of CrTiN/NiTi/PZT/TiOx that the peak obtained from CrTiN along the (111) planewas broadened and shifted to lower diffraction angle as comparedto that of the pure CrN protective layer. The broadening of the CrTiN(111) peak could be due to the reduction of grain size from 12 nmfor CrN to 8.9 nm for CrTiN and the shifting of the CrN(111) peak to-wards lower angle could be attributed to the formation of TiN phaseor the expansion of the lattice. Referring to the PCPDFWIN-ICDD,the dominant preferred orientation in Fig. 2(d) corresponds to thediffraction from the (111) planes of fcc-CrN (PCPDFWIN 76-2494)and fcc-TiN (PCPDFWIN 74-1214). There is no evidence for the forma-tion of other phases like Cr2N phase in the deposited Cr–Ti–N coat-ings. Therefore, Cr–Ti–N coatings show the B1-NaCl structure, andthese results are also in well agreement with the work by other re-search groups [22–24].

The surface morphology of the AlN, CrN and CrTiN thin filmsgrown on top of the NiTi/PZT/TiOx heterostructure was examined byfield emission scanning electron microscopy (FESEM) and atomicforce microscopy (AFM). The FE-SEM and AFM images of the coatingsare shown in Fig. 4 and Fig. 5, respectively. A NiTi film deposited onthe PZT/TiOx/Pt/Ti/SiO2/Si substrate shows a uniform, fine and ho-mogenous microstructure with a grain size of 77.6 nm. Fig. 4(b)–(d)clearly shows the change of the surface morphology from sphericaltype grains to a triangular needle shaped pyramid like structure

(c)

200 nm

(b)

200 nm

200 nm

(d)

200 nm

(a)

Fig. 5. AFM images of (a) NiTi/PZT/TiOx (b) AlN/NiTi/PZT/TiOx (c) CrN/NiTi/PZT/TiOx and (d) CrTiN/NiTi/PZT/TiOx heterostructures.

1739N. Choudhary et al. / Surface & Coatings Technology 206 (2011) 1735–1743

with change of the protective layer from AlN to CrTiN. It is evidentthat the CrTiN top layer was comparatively smoother and denserwith smaller grain size (27.2 nm) as compared to AlN and CrN toplayers. It was also observed that the grain sizes, determined by AFMand FESEM are quite similar. But the overall particle size shown byFESEM and AFM was much larger as compared with that calculatedfrom the XRD results (Table 2). This was due to the fact that AFMand FESEM show agglomeration of the particles whereas XRD givesan average mean crystallite size. The XRD and FESEM/AFM data canbe reconciled by the fact that smaller primary particles have a largesurface free energy and would therefore, tend to agglomerate fasterand grow into larger grains. For the quantitative evaluation of the sur-face topography, the average roughness (Ravg) of the surface of allsamples was obtained from AFM scans over substrate areas of1 μm×1 μm, three times at a different spot for each sample by using

Table 2Various parameters of the NiTi/PZT/TiOx thin films with different protective layers.

Sample name Grain size (nm) Average roughnessAFM (nm)

XRD FESEM AFM

NiTi/PZT/TiOx 19.8 77.6 80.5 13.8AlN/NiTi/PZT/TiOx 15.2 61.1 70.3 11.0CrN/NiTi/PZT/TiOx 12.0 48.4 59.6 7.5CrTiN/NiTi/PZT/TiOx 8.9 27.2 33.0 4.2

the following relationship [25];

Ravg ¼ 1N∑N

i¼1 Zi−Z�� �� ð1Þ

where N is the number of surface height data and Z is the mean-height distance. AFM micrographs reveal that the Ravg of the filmswas different for different protective layers and minimum surfaceroughness (4.2 nm) was obtained for CrTiN coated NiTi/PZT/TiOx het-erostructures. A comparison between different protective layers withrespect to their grain size and average roughness as calculated byAFM analysis is given in Table 2. In addition, the elemental composi-tions of the coatings were determined by EDX using the ZAF (Z: atom-ic number effect, A: absorption effect, F: fluorescence excitationeffect) correction method. The EDX analyses of all samples alongwith their elemental composition are shown in Fig. 6. For the pureNiTi/PZT/TiOx heterostructure (Fig. 6(a)), the EDX analysis of thetop NiTi layer shows the presence of nearly stoichiometric NiTi filmswith (49.71 at.%, 54.78 wt.%) Ni and (50.29 at.%, 45.22 wt.%) Ti as itsmain constituents. In case of the AlN protective layer, the EDX analy-sis shows the presence of Al (52.45 at.%, 69.39 wt.%) along with(44.55 at.% 30.61 wt.%) nitrogen (Fig. 6(b)). Further, Chromium(54.34 T.%, 81.54 wt.%), nitrogen (45.66 at.%, 18.46 wt.%) and Chromi-um (44.29 at.%, 65.04 wt.%), Titanium (13.50 at.%, 18.24 wt.%), Nitro-gen (42.21 at.%, 16.72 wt.%) are the main constituents of CrN andCrTiN protective coatings as indicated by Fig. 6(c) and (d), respectively.

KCnt

Element

At%

Wt%

KCnt

Element At% Wt%

Element At% Wt%

KCnt

KCnt

Element At% Wt%

NK

CrK

2.00 4.001.00 3.00 5.00 6.00 7.00 8.00 9.00 10.00.0

0.3

0.6

0.9

1.3

1.6

CrK

(c)CrK 54.34 81.54

NK 45.66 18.46

Matrix corr. ZAF

AlK

NK

1.8

0.0

0.4

0.7

1.1

1.4

Energy-KeV

Energy-KeVEnergy-KeV

Energy-KeV2.00 6.000.00 4.00 8.00 10.00 12.00 14.00

2.00 6.000.00 4.00 8.00 10.00 12.00 14.00

(b)AlK 52.45 69.39

NK 44.55 30.61

Matrix Correction ZAF

(d) CrK 44.29 65.04

TiK 13.50 18.24

NK 42.21 16.72

Matrix correction ZAF

0.0

0.4

0.7

1.1

1.4

1.8

CrK

CrK

TiK

NK

(a)

NiL

TiK

NiK

NiK

TiK

0.6

1.1

0.0

0.2

0.4

0.9

4.00 8.002.00 6.00 10.00 12.00 14.00 16.00

NiK 49.71 54.78

TiK 50.29 45.22

Matrix Correction ZAF

Fig. 6. EDX analysis along with EDX spectra for (a) NiTi/PZT/TiOx (b) AlN/NiTi/PZT/TiOx (c) CrN/NiTi/PZT/TiOx and (d) CrTiN/NiTi/PZT/TiOx heterostructures.

1740 N. Choudhary et al. / Surface & Coatings Technology 206 (2011) 1735–1743

3.2. Shape memory properties

Plotting the electrical resistance versus temperature (R–T) curve isthe most common technique for the direct observation of phasetransformation in shape memory materials [26]. Fig. 7 shows the R–T plots of pure and surface modified NiTi/PZT/TiOx thin film hetero-structures deposited on Pt/Ti/SiO2/Si substrate. The curves give aclear picture of the phase transformation behavior of the NiTi toplayer from martensite (low temperature phase) to austenite (hightemperature phase), during subsequent cooling and heating cycles.The presence of cap shape with a significant change in the resistancevalue during both, heating and cooling cycles confirms the presenceof an intermediate rhombohedral R-phase [27,28]. Thus, the phasetransformation of the NiTi/PZT/TiOx (Fig. 7(a)), AlN/NiTi/PZT/TiOx

(Fig. 7(b)), CrN/NiTi/PZT/TiOx (Fig. 7(c)) and CrTiN/NiTi/PZT/TiOx

(Fig. 7(d)) heterostructures takes place in two steps:

M� phase→R � phase→B2� phase during heatingð ÞB2� phase→R � phase→M� phase during coolingð Þ:

The values of transformation temperatures As, Af, Rs, Rf, Ms, Mf, A:austenite (B2); R: martensite (R-phase); M: martensite (B19′); s:start temperature; f: finish temperature and thermal hysteresiswidth of all the samples were calculated and summarized inTable 3. It was observed that a NiTi/PZT/TiOx heterostructure coated

with protective layers exhibits a larger hysteresis as compared topure NiTi/PZT/TiOx films, which could be due to additional strain inNiTi/PZT films due to top protective layers. The values of hysteresiswidth were 32, 57, 48 and 40 K, for NiTi/PZT/TiOx, AlN/NiTi/PZT/TiOx, CrN/NiTi/PZT/TiOx and CrTiN/NiTi/PZT/TiOx heterostructures, re-spectively. The shifting in the austenite and martensite transforma-tion temperatures and also the hysteresis loop was not closed evenat low temperatures in AlN, CrN and CrTiN coated NiTi/PZT/TiOx het-erostructures which could be due to large residual stress in the de-posited films due to some intrinsic stresses formed during filmgrowth, lattice mismatching and due to the small grain size of thetop nanocrystalline protective layers [29–31].

3.3. Mechanical properties

The Nanoindentation test involves indenting a specimen with avery small load using a high precision instrument, which recordsthe load and displacement continuously as shown in Fig. 8(a). It isshown in Fig. 8(a) that, during indentation, hf is the final unloadingdepth, hmax is the maximum loading depth, Pmax is the maximumload, and S is the slope of the tangent line to the unloading curve atthe maximum loading point (hmax, Pmax), which is termed as the sys-tem contact stiffness. hc is the intercept value of the above mentionedtangent line down to P=0 and is termed as the contact depth. Thearea covered under O–A–hf–O represents plastic deformation while

Fig. 7. Electrical resistance versus temperature curve of NiTi/PZT/TiOx heterostructures with various protective layers (a) NiTi/PZT/TiOx (b) AlN/NiTi/PZT/TiOx (c) CrN/NiTi/PZT/TiOx

and (d) CrTiN/NiTi/PZT/TiOx.

1741N. Choudhary et al. / Surface & Coatings Technology 206 (2011) 1735–1743

the area covered under hf–A–hmax–hf represents elastic recovery dur-ing indentation process. The mechanical properties such as hardness(H), reduced elastic modulus (Er) and plasticity index (H/Er) of NiTi/PZT/TiOx heterostructures (B1–B4) were calculated by directly mea-suring the physical dimensions of the indentation using the standardOliver and Phar method [32]. The relation between penetration depthh and load P for a given indenter geometry can be represented as:

P ¼ k h−hfð Þm ð2Þ

where ‘k’ is a fitting parameter, which contains geometric constants,

Table 3Details of transformation temperatures obtained from electrical resistance versus tem-perature curves of pure and surface modified NiTi/PZT/TiOx heterostructures.

Sample name Transformation temperature (K) Hysteresis width(ΔT=Af−R′s)Heating Cooling

Rs Rf=As Af R′s R′f=Ms Mf

NiTi/PZT/TiOx 183 325 345 313 283 200 32AlN/NiTi/PZT/TiOx 175 314 356 299 267 162 57CrN/NiTi/PZT/TiOx 241 345 364 316 263 191 48CrTiN/NiTi/PZT/TiOx 195 309 349 309 251 159 40

elastic modulus and Poisson's ratio of the specimen and the indenter,‘m’ is an exponent which depends on the geometry of the indenter.The reduced elastic modulus (Er) is related to the modulus of elastic-ity (E) through the equation:

1Er

¼1−υi

2� �

Eiþ

1−υs2

� �Es

ð3Þ

where the subscript i corresponds to the indenter material, the sub-script s refers to the indented sample material, and υ is the Poisson'sratio. The hardness (H) is defined by the ratio of the maximum load(Pmax) to the projected contact area (Ac),

H ¼ Pmax

Ac: ð4Þ

The hardness of thin films can be influenced by the substrate.Therefore, to avoid the influence of substrate, its thickness shouldbe at least 10 times higher than the film thickness so that the film isdeformed by the indenter only up to the elastic limit, due to whichthe deformation is not influenced by the interface or upper free sur-face. The indentations were carried out at a fixed indentation depthof 300 nm for all coatings. The indentation was made on sixteen

hc for ε = 0.72hc for ε = 1

(a)

Loading

Unloading

S

hf

hmax

Pmax

For

ce (

P)

Displacement (h)

A

O

Ran

ge o

f h c

geom

etry

dep

ende

nt

(b)

Har

dnes

s (G

Pa)

NiTi/PZT/TiOx

AlN/NiTi/PZT/TiOx

CrN/NiTi/PZT/TiOx

CrTiN/NiTi/PZT/TiOxHardnessH/Er

Plasticity Index (H

/Er )

0.06

0.07

0.08

0.09

8

10

12

14

16

18

Fig. 8. (a) Typical indentation loading–unloading curve and (b) Hardness (H) and plas-ticity index (H/E) ratio curves for NiTi/PZT/TiOx, AlN/NiTi/PZT/TiOx, CrN/NiTi/PZT/TiOx

and CrTiN/NiTi/PZT/TiOx heterostructures.

Table 4Results of potentiodynamic polarization tests for NiTi/PZT/TiOx films with differentprotective coatings.

Sample Protectivelayer

Corrosionpotential,Vcorr (volts)

Corrosion currentdensity, Jcorr(A cm−2)

ProtectiveefficiencyPi (%)

NiTi/PZT/TiOx – −0.553 4.78×10−7 –

AlN/NiTi/PZT/TiOx AlN −0.351 6.63×10−8 86.1CrN/NiTi/PZT/TiOx CrN −0.206 4.45×10−8 90.7CrTiN/NiTi/PZT/TiOx CrTiN −0.128 1.52×10−8 96.8

1742 N. Choudhary et al. / Surface & Coatings Technology 206 (2011) 1735–1743

different points in each sample to obtain its average hardness value.Fig. 8(b) presents the hardness (H) and plasticity index (H/Er) of allsamples measured at room temperature. The hardness of the samples

Fig. 9. Potentiodynamic polarization curves of NiTi/PZT/TiOx, AlN/NiTi/PZT/TiOx, CrN/NiTi/PZT/TiOx and CrTiN/NiTi/PZT/TiOx heterostructures.

was found to be 7.8 GPa (min) for the pure NiTi/PZT/TiOx heterostruc-ture (without protective coatings) and 18.1 GPa (max) for CrTiN/NiTi/PZT/TiOx, while its reduced elastic modulus values were 139.28 GPa(min) and 198.9 GPa (max), respectively. The hardness and reducedelastic modulus values for AlN and CrN coated NiTi/PZT/TiOx filmswere evaluated to be 10.2 GPa, 161.9 GPa and 13.7 GPa, and187.6 GPa, respectively. A significant increment in the value of thehardness for CrTiN/NiTi/PZT/TiOx could be attributed to the veryfine composite microstructure of the top CrTiN protective layer con-sisting of small crystallites, which leads to the development of com-pact films. Apart from grain size, the hardness and reduced elasticmodulus of the coatings, measured by Nanoindentation are also af-fected by the surface roughness of the coatings and generally de-crease with increase in surface roughness of the coatings [33, 34]. Inthe present study, the surface roughness of the protective coatingswas found to be varying from 11 nm (AlN) to 4.2 nm (CrTiN), whichare comparatively lower than that of the pure NiTi/PZT/TiOx hetero-structure. Hardness (H) to reduced elastic modulus (Er) ratio hasbeen proposed as the key factor to measure the behavior of wear re-sistance of bilayer coatings [35, 36]. The effect of H/Er on wear can beunderstood by assuming that gradual material removal or gradualwear is caused by plastic deformation and not by elastic deformation[37]. The ratio of reversible work, We, to total work, Wtot, under con-ical, pyramidal and spherical indentation has recently been shown tobe proportional to H/Er [38];

We

Wtot∝ H

Er: ð5Þ

Consequently, a larger fraction of the work is consumed in plasticdeformation when H/Er is smaller for the same degree of deformation.Second, the unrecoverable strain, measured by the ratio of finaldepth, hf, to maximum indentation depth, hmax, is found to be relatedto H/Er through a relationship that [39];

hf

hmax∝Wtot−We

Wtot: ð6Þ

Thus, large plastic strain is expected when contracting a materialwith smaller H/Er. Assuming similar relationships hold for multi-asperity contact during sliding, materials with higher H/Er areexpected to have smaller accumulative strain, smaller accumulativeenergy, and thus better wear resistance. In the present study, a rela-tive low value of the H/Er ratio (0.056) for a pure NiTi/PZT/TiOx het-erostructure indicates that more fraction of work is consumed inplastic deformation and large plastic strain is expected when contact-ing a material. In case of the CrTiN/NiTi/PZT/TiOx film, the H/Er ratio(0.091) was found to be higher as compared to NiTi/PZT, AlN/NiTi/PZT and CrN/NiTi/PZT heterostructures, which indicates that theCrTiN passivated NiTi/PZT/TiOx heterostructure could exhibit excel-lent wear resistance.

1743N. Choudhary et al. / Surface & Coatings Technology 206 (2011) 1735–1743

3.4. Corrosion properties

The potentiodynamic polarization curves of NiTi/PZT/TiOx hetero-structures without and with protective coatings are shown in Fig. 9 andthe corrosion parameters in Table 4. The corrosion resistance of surfacemodified NiTi/PZT/TiOx heterostructures was found to be improved,which could be observed by shifting of the whole polarization curve to-wards the region of lower current density and higher potential [40,41].The values of corrosion potential and corrosion current density werefound to be −0.553 V and 4.78×10−7A cm−2 for a pure NiTi/PZT/TiOx

heterostructure; −0.351 V and 6.63×10−8A cm−2 for AlN/NiTi/PZT/TiOx; −0.206 V and 4.45×10−8A cm−2 CrN/NiTi/PZT/TiOx; 0.128 Vand 1.52×10−8A cm−2 for CrTiN/NiTi/PZT/TiOx, respectively. Highercorrosion potential and lowest corrosion current density of the CrTiNcoated NiTi/PZT/TiOx heterostructures suggest that these films exhibit alow corrosion rate and better corrosion resistance as compared to otherprotective coatings, which could be attributed to the fact that CrTiN hasinherently a better corrosion resistance. From polarization test results,the protective efficiency, Pi (%) of the films can be calculated by Eq. (7):

Pi %ð Þ ¼ 1−JfilmJsub

� �� �×100 ð7Þ

where Jfilm and Jsub are the corrosion current density of the film and sub-strate, respectively [42]. The calculated protective efficiencies are pre-sented in Table 4. The CrTiN film showed the highest protectiveefficiency of 96.8% caused by lowest corrosion current density of1.52×10−8A cm−2.

4. Conclusions

In summary NiTi/PZT/TiOx, AlN/NiTi/PZT/TiOx, CrN/NiTi/PZT/TiOx

and CrTiN/NiTi/PZT/TiOx heterostructures were successfully grownon Pt/Ti/SiO2/Si substrate using dc/rf magnetron sputtering. The ob-servation shows that phase transformation takes place in the NiTi/PZT/TiOx heterostructure even after coating with protective layers.Thin protective layers did not deteriorate the characteristic propertiesof the constituent active thin films. Nanoindentation studies revealrelatively low surface roughness, high hardness, high elastic modulusand thereby better wear behavior for (AlN, CrN, CrTiN)/NiTi/PZT filmsas compared to pure NiTi/PZT/TiOx films. Among them, preeminentproperties were shown by CrTiN passivated NiTi/PZT/TiOx hetero-structure. The corrosion current density of protective coatings wasfound to be much lower than that of pure NiTi/PZT/TiOx films. TheCrTiN coating has performed very well and showed best corrosion re-sistance on the basis of corrosion current density and protective effi-ciency. It was concluded that the use of nanocrystalline protectivecoatings on NiTi/PZT/TiOx heterostructure significantly improvetheir surface, corrosion and mechanical properties.

Acknowledgment

The financial support provided by Ministry of Communicationsand Information Technology (MIT), India, under Nanotechnology Ini-tiative Program with Reference no. 20(11)/2007-VCND and DRDOARMREB/MAA/2008/91, NewDelhi, is highly acknowledged. The authorNitin Choudhary is thankful to University Grants Commission, India foraward of Senior Research Fellowship.

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