Applied Physics A (2018) 124:328

https://doi.org/10.1007/s00339-018-1746-7

Effect of crystallographic orientation on structural and mechanical

behaviors of Ni–Ti thin films irradiated by ­Ag7+ ions
Veeresh Kumar1 · Rahul Singhal1

Received: 17 November 2017 / Accepted: 14 March 2018

© Springer-Verlag GmbH Germany, part of Springer Nature 2018

Abstract
In the present study, thin films of Ni–Ti shape memory alloy have been grown on Si substrate by dc magnetron co-sputtering
technique using separate sputter targets Ni and Ti. The prepared thin films have been irradiated by 100 MeV ­Ag7+ ions at
three different fluences, which are 1 × 1012, 5 × 1012, and 1 × 1013 ions/cm2. The elemental composition and depth profile of
pristine film have been investigated by Rutherford backscattering spectrometry. The changes in crystal orientation, surface
morphology, and mechanical properties of Ni–Ti thin films before and after irradiation have been studied by X-ray diffrac-
tion, atomic force microscopy, field-emission scanning electron microscopy, and nanoindentation techniques, respectively.
X-ray diffraction measurement has revealed the existence of both austenite and martensite phases in pristine film and the
formation of precipitate on the surface of the film after irradiation at an optimized fluence of 1 × 1013 ions/cm2. Nanoindenta-
tion measurement has revealed improvement in mechanical properties of Ni–Ti thin films after ion irradiation via increasing
hardness and Young modulus due to the formation of precipitate and ductile phase. The improvement in mechanical behavior
could be explained in terms of precipitation hardening and structural change of Ni–Ti thin film after irradiation by Swift
heavy ion irradiation.

1 Introduction community due to their excellent properties for MEMS

applications, such as high power density, high power-to-
The shape memory behavior and superelasticity of Ni–Ti weight ratio, outstanding chemical resistance, pseudoelas-
make it a promising material for micro-electro-mechanical ticity (or superelasticity), and excellent biocompatibility
system (MEMS) and biomedical applications [1, 2]. These [2, 8–10]. The work output per unit volume of SMA thin
unique properties are mainly due to the reversible phase film is quite large in comparison to other micro-actuation
transformations between low-temperature martensite and mechanisms. Phase transformation in Ni–Ti SMA thin film
high-temperature austenite crystal structures, which can is accompanied by significant changes in physical, electri-
be achieved by variation in temperature and applied load cal, chemical, and mechanical properties, such as shape
[3, 4]. Nowadays, shape memory behavior of Ni–Ti alloy recovery, thermal expansion coefficient, surface roughness,
is commercially used for different applications such as cou- electrical resistivity, dielectric constant, yield stress, Young
pling, sensors, actuators, and cellular phones antennas [5, 6]. modulus, hardness, and damping [11, 12]. These changes
Recently, Ni–Ti has attracted wide interest as a biomaterial, can be fully utilized in design and fabrication of pumps and
due to shape recovery behavior after deformation beyond grippers for MEMS. Hence, it is essential to explore the
its elastic limit either by heating or by removing the applied mechanical properties of the SMA films.
load [7]. Numerous attempts have been made to enhance the prop-
In the form of thin film, Ni–Ti shape memory alloy erties of Ni–Ti through different surface modification tech-
(SMA) have received significant interest of the scientific niques, such as gas nitriding [13], ion nitriding [14], heat
oxidation [15], laser surface melting [16], and ion irradiation
[17–19]. The critical challenge for ion or gas nitriding is the
* Veeresh Kumar
vks8361@gmail.com formation of the non-superelastic T ­ i2Ni layer that limits its
application in dentistry as reported by several authors [20,
1
Department of Physics, Malaviya National Institute 21]. However, equilibrium precipitate phase of nickel and
of Technology Jaipur, JLN Marg, Malviya Nagar, titanium ­Ni3X, ­Ni2X, ­Ni4X3, TiX, ­Ti3X4, and ­TiX2 (where
Jaipur 302017, India

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Vol.:(0123456789)
328 Page 2 of 10 V. Kumar, R. Singhal

X = Si, Ti, Sn, Al, V, Nb) are potential stable phases which to martensite phase via R-phase transformation because of
contribute to enhancing the mechanical strength of Ni–Ti the formation of the T ­ i3Ni4 precipitates [32]. Moreover, it is
SMA [22]. Swift heavy ion (SHI) irradiation has unique reported that Ni-rich composition of the Ni–Ti films exhibits
capabilities for modification of surface layers on metals and stable superelasticity effect in addition to an excellent shape
alloys including their crystal structure, chemical composi- memory behavior [28]. Therefore, investigations on surface
tion, and physical and mechanical properties. SHI-induced characteristic and mechanical behavior of 56.7 at.%Ni thin
damaging effects can benefit the shape memorial behavior films, irradiated by SHI irradiation are not adequate and
and bias the shape recovery if the detrimental effects are need to be explored in detail.
more controlled [23]. It is reported that mechanical proper- In the present study, we have used an innovative material
ties of the materials strongly depend upon the precipitation, modification technique namely SHI irradiation to engineer
defect density and grain refinement, which significantly the surface structure and mechanical properties of Ni–Ti
increases the resistance of materials against plastic defor- thin films [33]. The effect of energetic ions on material
mation. However, these treatments reduce the martensite depends on the electronic energy, ion fluence, ion species,
transformation temperature by changing the internal stress and temperature. It is well known that the energy loss of
and surface composition of the matrix [19, 24]. incident ions play a major role in the material modification
There are various studies of the effect of precipitation on that leads to the formation of lattice defects and excitation/
structural, mechanical, and phase transformation behavior ionization [33–35]. If ­Se exceeds a threshold value 46 keV/
of SMAs. Nunomura et al. have thoroughly investigated the nm, then there will be formation of track or amorphization
phase relation and microstructure of pseudo-ternary alloy of Ni–Ti bulk alloy, whereas if Se = 32 keV/nm, no indi-
based on ­Ni3Ti, ­Ni3Nb, and ­Ni3Al [25]. The effect of anneal- vidual tracks formation can be observed at low fluence, only
ing temperature on microstructure and mechanical properties the monoclinic phase to cubic austenite phase transforma-
of intermetallic Ni enriched NiTi films is reported by Reddy tion is observed at high fluences [36]. If Se ≤ 17 keV/ nm,
et al. [26]. They have observed the higher values of hardness swift heavy ions are unable to induce any visible structural
due to the nucleation, growth process of grains, and segre- modifications in Ni–Ti bulk alloy by electronic excitations
gation of N ­ i3Ti precipitation phase at the higher annealing [36]. Silver (Ag) is a potential metallic element to enhance
temperature. Recently, the influence of ­Ti2Cu precipitate in the properties of SMA due to its exceptional mechanical
NiTiCu-based ultralow-fatigue SMA films has been reported properties of high strength, excellent thermal-driven ability,
by Chluba et al. for 10 million transformation cycles for high mechanical damping and good superelasticity, etc. [3,
artificial heart valve or elastocaloric cooling [27]. Zu et al. 37–39]. Therefore, Ag ions with 120 MeV energy are used
have investigated the effect of 1.7 MeV electron irradia- to observe the modification in Ni–Ti thin films. The value of
tion on martensite phase transformation of NiTi alloy. The Se for Ag ions is 25 keV/nm, which is a moderate value, and
martensite temperature decreases with irradiation fluence therefore, it is enabled to modifications in Ni–Ti thin films.
due to relaxation of elastic stress around the N­ i4Ti3 precipi- The Ni–Ti thin films irradiated at three optimized fluence of
tate phase [17]. In addition, Ishida et al. have reported that 1 × 1012, 5 × 1012 and 1 × 1013 ions/cm2 to observe structural
Ni-51 at. pct and Ti-49 at. pct thin films exhibit the stable and mechanical modifications.
super elasticity and excellent shape memory effect [28]. In The primary objective of the present study is to deposit
bulk NiTi alloy, the higher composition of Ni (55 to 60 at. the films with two different phases, ordered austenite and
pct) shows excellent properties such as high yield strength, low-symmetry allotrope martensite phase with the higher
non-magnetization, and high corrosion resistance at promi- composition of Ni. The films are irradiated at three different
nent temperature [29]. Moreover, formation of ­Ni4Ti3 and fluences by using SHI irradiation to promote the growth of
­Ni3Ti phases in NiTi alloy makes them chemically and struc- hard ­Ni3Ti precipitate which would enhance the mechanical
turally stable at higher annealing temperature [30]. Afzal behavior of Ni–Ti thin films. The results presented here to
et al. have studied the microstructure and mechanical behav- bring forth the understanding of the strengthening mecha-
ior of 2 MeV proton beam irradiated Ni-rich-nitinol alloy nisms and deformation behavior of Ni–Ti films by SHI irra-
[18]. The effect of different types of perturbations such as diation. This study is also essential to investigate the effects
the electron, proton, and ion irradiation on microstructure, of SHI irradiation on the shape memory alloy for future
phase transformation, and mechanical properties have been application of these materials in harsh radiation zones such
reported in the literature [17, 18, 31], The purpose of the as space or nuclear reactor. Figure 1 shows the schematic
present study is improved the mechanical behavior of Ni–Ti diagram of ion–solid interaction. A previous study on Ni–Ti
films by Ag ion irradiation. The Ni composition (55 to 60 at. thin films with composition (Ti-56.7 at.%Ni) has been done
pct) is the best range to increase the mechanical behavior by the same author using 120 MeV Au ions irradiation to
of alloy with higher mechanical strength and ductility [29]. investigate the critical value of fluence and acceptable radia-
The Ni-rich Ni–Ti films also undergo two-stage austenite tion limit for this composition [40]. Furthermore, the present

13
Effect of crystallographic orientation on structural and mechanical behaviors of Ni–Ti thin… Page 3 of 10 328

(IUAC), New Delhi, at room temperature using 15 UD

Pelletron Tandem accelerator facility. During irradia-
tion, a high vacuum of the order of ~ 6 × 10 −7 torr was
maintained inside the irradiation chamber. The values of
nuclear energy loss (S n) and electronic energy loss (Se)
were calculated by SRIM 2008 code [41] and found to
be 0.01 × 103, 2.5 × 103 eV/Å, respectively. Moreover, the
range of 100 MeV Ag ions in Ni–Ti films (density 6.14 g/
cm3) was also calculated by SRIM 2008 code and deter-
mined as 8.23 µm. The modifications in Ni–Ti films can
be understood by Se and Sn. When the SHI interact with
the target of Ni–Ti film, it loses their energy into mate-
rial by two different ways; (a) direct transfer of energy to
target lattice by elastic collision, term is known as nuclear
energy loss (S n) and (b) transfer of energy of the inci-
Fig. 1  Schematics diagram for ion–solid interaction for SHI
dent ions to electrons of target atoms, term is called elec-
tronic energy loss (Se). In high-energy regime (≈ MeV/u),
study has been performed to investigate the effect of SHI energy transfer by incident ions causes the displacement
irradiation on structural and mechanical properties. of atoms in cylindrical zones around the ion path in the
target material. In the present study, the calculated value
of Se is too high than that of Sn; that’s why Se dominates
2 Experimental details over S n; therefore, modifications are mainly attributed
due to Se effect. The ion beam was scanned over an area
In the present study, we have deposited Ni–Ti thin film on of 1 × 1 cm2 to achieve the uniform irradiation of Ni–Ti
2-inch diameter Si (100) substrate (Matsurf Tech. Inc, USA) films and the beam current was kept constant at ~ 2 pnA
and after films deposition; substrate was divided into several (particle nanoampere).
1 × 1 cm2 small pieces for irradiation. The dimension of each The composition and thickness of the pristine film
film during the irradiation was 1 × 1 cm2 in the irradiation were measured by Rutherford backscattering spectrom-
chamber. Raster scanning (1 × 1 cm2) of ion beam was done etry (RBS) technique. The crystal structure of the pristine
over the surface during irradiation to cover the complete area and irradiated Ni–Ti thin films has been studied by X-ray
of the film. The Si wafer was attached to the substrate holder diffractometer (Bruker D8 Advance). The XRD measure-
with the help of clips and rotated (60 rpm) in the horizontal ment is performed at an incident angle of 1° with Cu-Kα1
plane of targets during deposition to achieve the uniform radiation source of wavelength 1.54 Å in Bragg–Brentano
films growth. Before deposition, the Si substrate was first (θ/2θ) geometry at a scan rate of 0.6°/min at UGC-DAE-
cleaned in a mixture of Trichloroethylene and distilled water CSR Indore. Furthermore, the surface morphology of the
(1:4) ratio in an ultrasonic bath and then washed with boiled films was investigated by AFM (Bruker Nanoscope V sys-
acetone. High-purity Ti (99.9%) and Ni (99.9%) metal tar- tem) with a ­Si3N4 cantilever in tapping mode and field-
gets of 3 mm thick and 50 mm diameter (Neyco supplier, emission scanning electron microscope (FESEM) with
France) were used for deposition. The vacuum chamber was (Model:-TESCAN, MIRA II LMH) at IUAC, New Delhi.
repeatedly flushed with argon gas for 10 min to minimize the The mechanical properties of pristine and irradiated
possible contamination. Before deposition, a base pressure Ni–Ti thin films have been investigated using the nanoin-
of 2 × 10 −7 torr was achieved inside the vacuum chamber dentation tester equipped (CSM Instruments) with a dia-
using a turbo molecular pump, while, during deposition, the mond Berkovich-type indenter tip. The test was performed
pressure was kept at 3 × 10 −3 torr using a dynamic throttling in air at room temperature at different positions on films
valve. The deposition was performed for 1 h 40 min using surface, and average hardness and Young modulus were
two different powers: 50 W for Ni and 100 W for Ti tar- calculated. Three nanoindentation tests are performed on
get respectively. All Ni–Ti thin films have been prepared in pristine and irradiated films at different locations to calcu-
argon (~ 99.9%) atmosphere at 550 °C by an AJA Int. make late the average hardness and Young modulus values. Each
ATC Orion-8 series sputtering system. The substrate holder indenter test consisted of 8-s linear load segment to a peak
to target distance was fixed about 16 cm. load, 10-s holding, and an 8-s linear unloading segments.
The prepared Ni–Ti films were irradiated by 100 MeV The hold periods are used to reduce the time-dependent
­Ag 7+ ion beam at Inter University Accelerator Centre effects (creep effects) generate in the specimen.

13
328 Page 4 of 10 V. Kumar, R. Singhal

3 Results and discussion

3.1 Rutherford backscattering spectrometry

RBS is a non-destructive technique and capable to determin-

ing both elemental composition and depth of a thin layer.
Figure 2 shows simulated RBS spectrum along with depth
profile of pristine Ni–Ti film deposited at 550 °C. The ele-
mental stoichiometry and film thickness was estimated using
the SIMNRA simulation of RBS data [42]. The composition
of Ni and Ti present in the film was determined by H ­ e2+
ions with a beam of 2 MeV energy and at normal incidence
with the detector positioned at 165° scattering angle. The
calculated near surface concentration of Ti was found to be
~ 43.3 at% and Ni concentration was found to be ~ 56.7 at%.
The thickness of the pristine film was found to be ~ 270 nm.
­ g7+ ion irradiated Ni–
Fig. 3  XRD spectra of pristine and 100 MeV A
The elemental composition versus depth profile for Ni and Ti Ti thin films at different fluences
is shown in Fig. 2b and it is showing the uniform distribution
of Ni and Ti content through the film thickness.
Acetone) may be left its residues in some cases which can
affect the surface of Si [43]. After cleaning, the residues of
3.2 Structural properties Acetone may reside on the surface of the substrate, which
could directly affect the oxygen concentration at the sur-
3.2.1 X‑ray diffraction face and responsible for the intensity variation of the peaks.
The film irradiated at a fluence of 1 × 1012 ions/cm2 shows
Figure 3 shows the room temperature XRD pattern of the decrease in intensity of both phases due to the phase trans-
pristine and irradiated Ni–Ti thin films at different fluences formation from austenite (110) and martensite (002) phase
in the 2θ range of 35°–54°. The XRD pattern of pristine film into ­Ni3Ti phase on Ag ion bombardment. Furthermore, the
deposited at 550 °C exhibits three phases: martensite (mono- film irradiated at a fluence of 5 × 1012 ions/cm2 shows the
clinic; JCPDS file no. 77-2170), austenite (cubic; JCPDS file dominance of ­Ni3Ti phase with concurrent decrease in the
no. 65-5537), and the small amount of ­Ni3Ti phase (hexago- intensity of austenite phase. The film irradiated at (5 × 1012
nal; JCPD file no. 65-2038). In addition to substrate peaks, ions /cm 2) fluence shows the ordering of ­Ni3Ti phase,
XRD pattern of the pristine film shows the strong austenite which corresponds to hexagonal crystal structure of films.
peak at 2θ = 42.5˚corresponding to (110) fundamental reflec- It is observed that film irradiated at this fluence releases
tion, martensite peak at 2θ = 43.9° corresponding to (002) strain energy which is primarily responsible for the order-
fundamental reflection, and a small amount of Ni enriched ing of (004) plane, because (004) plane of Ni–Ti possesses
buried ­Ni3Ti precipitate peak at 2θ = 43.5° corresponding to the minimum surface energy in accordance of basic crystal
(004) plane, respectively. Two other peaks observed in pris- growth theory [44]. Furthermore, the film irradiated at a
tine films at 46° and 47.7° are corresponding to oxidized Si higher fluence of 1 × 1013 ions/cm2 shows the dominance
substrate [40]. During the cleaning process, solvent (boiled of ­Ni3Ti phase and partial amorphization of austenite and

Fig. 2  RBS spectra (2 MeV

­He2+) along with SIMNRA
simulation (a) and depth profile
(b) of Ni–Ti pristine film depos-
ited at 550 °C

13
Effect of crystallographic orientation on structural and mechanical behaviors of Ni–Ti thin… Page 5 of 10 328

martensite phase. The decrease in intensity of austenite and ranging from 1 × 1012 to 1 × 1013 ions/cm2 is studied with
martensite phase depends upon the amount of energy depos- AFM in tapping mode. Figure 4a–d shows two-dimensional
ited in Ni–Ti films by the A ­ g7+ ions. When the SHI irradia- AFM micrographs of pristine and irradiated Ni–Ti thin films
tion passes through the Ni–Ti film, it causes ionization and at scan area of 1 µm2. AFM micrograph of the pristine film
excitation of Ni–Ti atoms, which leads to the modifications (Fig. 4a) shows the very fine and dense grains of Ni–Ti with
in properties such as structure and phase. Such electronic well-defined boundary. The film irradiated at a fluence of
excitations can also cause local heating followed by a rapid 1 × 1012 ions/cm2 shows the change in surface morphology
quenching (thermal spikes) which subsequently produces with fluence. Moreover, grains are non-uniformly distrib-
lattice distortions. These lattice distortions are so drastic and uted over the film surface due to electronic energy bombard-
they relax into an amorphous state [31]. The modifications ment on Ni–Ti film. The modification of film surface can be
in materials structure or phases depend upon irradiation con- understood on the reference of electronic energy deposited
dition such as ion fluence, irradiation temperature, and the into the Ni–Ti thin film. When SHI pass through the Ni–Ti
nature of ions as reported by several authors [3, 44]. The film, it loses its energy into the material followed by ioniza-
precipitation of phases and recrystallization of monoclinic tion and excitation of the atoms. Electronic energy deposited
into cubic phase by 2 MeV proton beam irradiation has also by SHI irradiation beyond a certain threshold (Se ~ 32 keV/
been observed by Afzal et al. [18] in Ni–Ti alloy at room nm for Ni–Ti) can cause the significant movement of the
temperature. The irradiation of Nitinol by proton beam pro- atoms resulting into change in the crystal structure and sur-
duces small displacement cascade and thus generates vacan- face roughness [36]. Furthermore, it is found that irradiation
cies, interstitials, or precipitates. Irradiation-induced defects at higher fluence of 1 × 1013 ions/cm2 decreases the grain
in the material increase the shear movement of atoms and size and promotes the agglomeration of the grains. The
lead to increase in anti-phase boundary which decreases the agglomeration of the grains is attributed to multiply ions
martensite transformation. impact turning on Ni–Ti film by SHI irradiation [45].
The average roughness (Ravg) and root-mean-square sur-
3.2.2 Atomic force microscopy face roughness (Rrms) of the pristine and irradiated Ni–Ti
films are obtained from the AFM images of 1 µm × 1 µm
The surface morphology (grain size and roughness) of pris- scan area of films surface, three times at different posi-
tine and ­Ag7+ irradiated Ni–Ti thin films at different fluences tion for each film, and it has been taken as an average

Fig. 4  AFM 2 D surface micro-

graphs of pristine and 100 MeV
­Ag7+ ion irradiated Ni–Ti films
at different fluences over a scan
area of 1 × 1 µm2

13
328 Page 6 of 10 V. Kumar, R. Singhal

roughness estimated from AFM micrograph. The Ravg and 3.2.3 Field‑emission scanning electron microscopy
Rrms are defined by the following equations [46]:
[ ] Apart from AFM, the surface morphology of pristine and
N
1∑ irradiated Ni–Ti thin films is also investigated by FESEM.
Ravg = |Z − Z| ,
N i=1 i Figure 6a–d shows the FESEM images of pristine and ­Ag7+
irradiated Ni–Ti thin film at different fluences ranging from
1 × 1012 to 1 × 1013 ions/cm2. The FESEM micrographs have
[ ]1∕2
1∑
N confirmed the change in surface morphology of Ni–Ti films
Rrms = |Z − Z|2 , with ion irradiation at different fluences. Figure 6a shows
N i=1 i
the dense granular morphology with nearly same granule
sized in the pristine film deposited at 550 °C. In Fig. 6b,
where Z is mean height distance and N is the number of
FESEM image of the irradiated film at a fluence of 1 × 1012
surface height data. The Ravg and Rrms of the pristine and
shows that the density of granular size grains decreases as
irradiated films are observed to increase with increasing
fluence increases. These SHI-induced structural modifica-
the ion fluences. This could be due to agglomeration of
tions are also supported by AFM analysis. Furthermore, the
grains and N ­ i3Ti precipitate formation at higher fluences.
film irradiated at fluence of 5 × 1012 ions/cm2 shows the dif-
The Ravg value of the pristine film is ~ 2.06 nm and for the
fused grains morphology of the film; however, it is possible
films irradiated at different fluences found to be ~ 2.82,
to observe the boundary of grains. The grains of the films
~ 3.73 and ~ 4.10 nm, respectively. The Rrms values found
are seemed to be agglomerated at this fluence. With further
to be ~ 1.61 nm for the pristine film and ~ 2.32, ~ 3.02, and
increase in the fluence (1 × 1013 ions/cm2), the granular mor-
~ 3.35 nm for the films irradiated at three different fluences
phology of film is completely disappeared and it becomes
respectively. AFM results indicate that the surface morphol-
difficult to observe the boundary of grains. The agglomera-
ogy of the Ni–Ti films strongly depends upon the ion flu-
tion of the grains can be ascribed to the electronic energy
ences, as shown in Fig. 5.
deposited by the incoming ions in Ni–Ti regime [48, 49].
In the present study, the Ni–Ti films with same thick-
The surface features of the pristine and the films irradiated
ness are irradiated using different fluences. Different flu-
at different fluences of Ag ions have no evident defaults such
ences have different effects on the surface of the grown
as cracks and holes etc.
films such as clustering of grains and formation of cra-
ters, etc. [47]. The surface morphology is changed with
3.3 Nanoindentation
fluences with the simultaneous change in the Z-height of
the sample. The Z-height of the sample depends upon the
The change in crystal structure in nm-sized crystallites by
arrangement of grains and roughness of the films. The var-
SHI irradiation may play an important role to enhance the
iation in roughness value of film also changes the Z-height
mechanical properties of nanocrystalline materials. Materi-
of the film. In the present study, the Z- height of the four
als under the lattice contraction resist the large force, thus,
samples is different due to the difference in roughness val-
decreasing the penetration depth of indentation and inhibit-
ues of the films.
ing the crack propagation [50]. This phenomenon leads to
increase in toughness and hardness of materials; however,
lattice expansion leads to opposite phenomena. N ­ i3Ti pre-
cipitate increases the mechanical strength by promoting the
interaction between grain boundaries and dislocation which
subsequently increase the average hardness and Young mod-
ulus values.
The CSM nanoindentation has been used to characterize
the mechanical properties of pristine and irradiated Ni–Ti
films. Figure 7 shows the load versus displacement curves
of pristine and the films irradiated at different fluences of
100 MeV ­Ag7+ ions. These curves are used to calculate
the fundamental mechanical properties such as hardness
(H), Young modulus (Eeff), and plastic resistance param-
eter (H/Eeff), etc. All results obtained by nanoindentation
are analyzed using the Oliver and Pharr method [51] and
listed in Table 1. The load–displacement curves (Fig. 7)
Fig. 5  Variation of Ravg and Rrms with different ion fluences show that the behavior of each film is consistent from test

13
Effect of crystallographic orientation on structural and mechanical behaviors of Ni–Ti thin… Page 7 of 10 328

Fig. 6  FESEM images of

pristine and 100 MeV ­Ag7+ ion
irradiated Ni–Ti films at differ-
ent fluences

to test, indicating the homogeneous surface of the films

over the area tested. The Eeff is a significant parameter of
materials related to stiffness: the larger the value of E eff,
the stiffer the material. Furthermore, the H/Eeff is also an
important parameter to differentiate in elastic and elas-
tic–plastic nature of the materials. The higher value of
H/Eeff shows higher elastic nature of material or higher
resistance to plastic deformation, while the lower value
of H/Eeff corresponds to its elastic–plastic behavior [52].
The Young modulus is calculated using the following
relation:
√
𝜋 S
Eeff = √ ,
2 A

Fig. 7  Load versus depth profile of pristine and 100 MeV ­Ag7+ ion where S is unloading stiffness at maximum load ( S = dP )
irradiated Ni–Ti films at different fluences dh
and A is the projected contact area. The Young modulus
of the material is related to the modulus of elasticity by the
following relation:

13
328 Page 8 of 10 V. Kumar, R. Singhal

Table 1  Comparison of hardness and Young modulus values of pristine and irradiated Ni–Ti thin films
Fluence (ions/cm2) Hardness H Young modulus H/Eeff Average H (GPa) Average Eeff (GPa) Average (H/Eeff)
(GPa) Eeff (GPa)

Pristine 11.11 172.5 0.064 11.62 ± 1.07 172.62 ± 1.04 0.067 ± 0.005
12.86 173.73 0.074
10.89 171.65 0.063
1 × 1012 11.47 171.36 0.066 12.81 ± 1.18 171.83 ± 3.32 0.074 ± 0.007
13.71 168.77 0.081
13.24 175.37 0.075
5 × 1012 13.89 183.37 0.075 13.58 ± 0.27 181.81 ± 1.61 0.074 ± 0.001
13.48 180.15 0.074
13.38 181.93 0.073
1 × 1013 14.89 176.94 0.084 16.12 ± 1.64 176.93 ± 1.96 0.090 ± 0.008
17.99 178.90 0.100
15.48 174.97 0.88

(1 − v2i ) (1 − v2s ) metallic thin films increases vividly when the thickness of
1
= + , the deposited material falls below ~ 300 nm; and the hall
Eeff Ei ES
Petch relation, obeyed by thin films, is also dependent on
where subscript i represents the indenter material, subscript film thickness [53]. The formation of defects such as precipi-
s corresponds to sample material, and ν is the Poisson’s tation, dislocation, and irradiation-induced phase in material
ratio. The hardness (H) of the deposited material is calcu- up to a certain limit contributes to increasing the resistivity
lated by the following equation: and barrier strength. However, these treatments suppress
the martensite transformation temperature by changing the
Pmax internal stress and chemical composition of the Ni–Ti matrix
H= ,
A [19, 54]. The hardness of pristine film is 11.62 ± 1.07 GPa
where Pmax is the maximum indenter load and A is the and for the films irradiated at different fluences is found to
projected contact area at that load. Figure 8a, b shows the be 12.81 ± 1.18, 13.58 ± 0.27, and 16.12 ± 1.64 GPa, respec-
variation of hardness and Young modulus values for pristine tively, as reported in Table 1. The Young modulus of the
and for the film irradiated at different fluences ranging from pristine film is 172.62 ± 1.04 GPa and for the films irradi-
1 × 1012 to 1 × 1013 ions/cm2. From the figure, it depicts that ated at different fluences are 171.83 ± 3.32, 181.81 ± 1.61,
hardness of Ni–Ti films is increase with the increase in ion and 176.93 ± 1.96 GPa, respectively. The H/Eeff ratio is an
fluence and the film irradiated at a fluence of 1 × 1013 ions/ important parameter to measure the material elastic and elas-
cm2 shows the higher hardness value. The improvement in tic–plastic behaviors. The higher value of H/Eeff ratio shows
hardness of Ni–Ti films at different fluences is attributed to the excellent wear resistance and better films quality, while
the formation of lattice disorder and the associated change a lower value of H/Eeff reveals the large fraction of work is
in crystal structure. It is also reported that hardness of the consumed in plastic deformation, and large strain energy
is required while contacting a material [22]. A higher (H/

Fig. 8  Variation of hardness (a)

and elastic modulus (b) of pris-
tine and irradiated Ni–Ti films
at different fluences of 100 MeV
­Ag7+ ions

13
Effect of crystallographic orientation on structural and mechanical behaviors of Ni–Ti thin… Page 9 of 10 328

Fig. 9  SRIM simulation of nuclear (Sn) and electronic stopping (Se) Fig. 10  SRIM simulation, the variation of electronic (Se), and nuclear
power versus energy for ­Ag7+ ions in Ni–Ti matrix energy (Sn) loss of 100 MeV ­Ag7+ ions with thickness in Ni–Ti
matrix

Eeff) value (0.090 ± 0.008) is obtained for the film irradiated 4 Conclusions
at a fluence 1 × 1013 ions /cm2. The higher value of H/Eeff
shows the better wear resistance and small strain energy for In the present study, effects of 100 MeV A ­ g7+ ion irradia-
deformation. tion on structural, morphology, and mechanical behavior
Figure 9 shows the variation of electronic (S e) and of Ni–Ti thin films have been investigated. XRD meas-
nuclear stopping (Sn) power versus energy for 100 MeV urement reveals the change in crystal structure after irra-
­Ag7+ ions calculated by SRIM program in Ni–Ti matrix. diation and formation of buried N ­ i3Ti precipitate phases
The strength of interaction between incident ions and at a fluence of 1 × 1013 ions/cm2. XRD measurement also
target atom depends on charge, mass, and energy of the evidences that irradiation of Ni–Ti film at a higher fluence
incident ions. The energy deposited by bombardment of suppresses the austenite and martensite phase by introduc-
ions can modify the structural and phase transformation ing an intermediate ­Ni3Ti precipitation phase. AFM and
properties of films. The electronic excitation and ioniza- FESEM measurements show the change in surface mor-
tion of materials by SHI irradiation causes the significant phology with fluence. The Ni–Ti films surface roughness
displacement of atoms. When energetic ions pass through increases with increase in ion fluence, which may be due to
the Ni–Ti films, loses their energy via two independent the ion irradiation-induced sputtering effects as confirmed
processes; (a) nuclear stopping (Sn) loss: incident ions by AFM. Nanoindentation measurement reveals that hard-
transfer energy to the target lattice by elastic collision ness and Young modulus of Ni–Ti thin films are improved
and cause significant atomic displacements, which fur- as the ion fluences increases. The obtained results demon-
ther results in creation of Frenkel defects (vacancies or strate that the ion irradiation of Ni–Ti films by 100 MeV
interstitial), (b) electronic stopping (Se) loss: incident ions ­Ag7+ ions leads to the formation of nickel enrich hard pre-
transfer energy to the electrons of target atoms by inelas- cipitate phase. Formation of precipitation phase reduces
tic collision known as electronic stopping loss. In the Ni concentration in the surface which contributes to poten-
high-energy region (energy range of the order of ~ MeV), tial mechanical applications.
electronic stopping dominates over nuclear stopping and
modifications are mainly due to the electronic stopping. Acknowledgements V. Kumar is very much thankful to Technical
In the present study, Ag ions are chosen due to its higher Education Quality Improvement Program (TEQUIP), MNIT Jaipur for
the Ph.D. scholarship. R. Singhal acknowledges the financial supports
mass and energy regime. Figure 10 shows the variation provided by Department of Science & Technology, New Delhi in terms
of Se and Sn with projected depth of ~ 800 nm at 100 MeV of DST FAST Young Scientist project (SR/FTP/PS-081/2011). The
for ­Ag7+ ions, which is much larger than ~ 270 nm, and author would like to acknowledge Mr. Sunil Ojha and Mr. S.A. Khan
the film thickness of Ni–Ti films. Therefore, the effect from IUAC, New Delhi for their help and support in RBS and FESEM
characterizations. The author is also acknowledging UGC-DAE CSR
of electronic stopping power on structural and mechani- Indore, for synthesis and characterization of Ni–Ti thin films. The crew
cal properties of Ni–Ti films is anticipated to be uniform of pelletron accelerator IUAC, New Delhi is also highly acknowledged
irradiation. for providing the stable beam of 100 MeV Ag ions.

13
328 Page 10 of 10 V. Kumar, R. Singhal

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