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In-situ annealing of NiTi thin films at different temperatures
Sensors and Actuators A 221 (2015) 9–14
Contents lists available at ScienceDirect
Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna
In-situ annealing of NiTi thin films at different temperatures Wolfgang Tillmann ∗ , Soroush Momeni ∗∗ Institute of Materials Engineering, Technische Universität Dortmund,Leonhard-Euler-Str 2, 44227 Dortmund, Germany
a r t i c l e
i n f o
Article history: Received 26 May 2014 Received in revised form 27 October 2014 Accepted 28 October 2014 Available online 5 November 2014 Keywords: Thin films Sputtering Annealing Shape memory alloys Precipitation
a b s t r a c t Magnetron sputtered NiTi thin films are usually sputtered at ambient temperature and need a postannealing treatment to promote crystallization and obtain shape memory effect. However, this treatment could adversely affect the microstructure as well as the morphology of the film. Within this study, NiTi thin films were generated by annealing during the sputtering process. The effect of the sputtering temperature on the morphology of the film, the composition, and shape memory behavior was studied using X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDX), field emission scanning electron microscopy (FESEM), and differential scanning calorimetry (DSC). © 2014 Elsevier B.V. All rights reserved.
1. Introduction NiTi shape memory alloy (SMA) thin films have recently attracted great interest for the development of microelectromechanical systems (MEMS), such as micro valves [1], micro fluid pumps [2], micro wrappers [3], and micro grippers [4]. The main benefits of MEMS applications for NiTi thin films are for example a large displacement, high damping capacity, large actuation force, and a low operating voltage. In addition, the shape memory effect (SME) and the superelasticity (SE) of NiTi thin films can be employed to engineer surfaces with excellent tribological properties. This is even possible when the material is only present as a surface coating [5,6]. Despite these excellent properties, there are some difficulties to fully integrate NiTi shape memory thin films into MEMS and tribological coating systems as their high stoichiometry sensitivity restricts the manufacturability and versatility in MEMS [7]. In addition, the as-deposited sputtered NiTi thin films are amorphous without any shape memory effect. In order to promote the shape memory effect, they need to be crystallized during a post-annealing process (using temperatures of 600 ◦ C and higher) [8]. The crystallization process is a direct consequence of the grain nucleation and growth at elevated temperatures. As a result, the annealing
∗ Corresponding author. Tel.: +49 231 755 2581. ∗∗ Corresponding author. Tel.: +49 231 755 6113. E-mail addresses: [email protected] (W. Tillmann), [email protected] (S. Momeni). http://dx.doi.org/10.1016/j.sna.2014.10.034 0924-4247/© 2014 Elsevier B.V. All rights reserved.
temperature and time will influence the structure as well as property of NiTi thin films and their shape memory behavior. A connection between the annealing temperature and phase transformation temperatures of NiTi thin films has previously been reported by Surbled et al. [9]. The effect of annealing parameters on mechanical and shape memory properties of NiTi thin films was investigated by Satoh et al. [10]. Nevertheless, it was noticed that the effects of the annealing parameters during the sputtering of NiTi thin films have not been investigated so far. The in-situ annealing technique could be more reliable than a post-annealing of NiTi thin films, sputtered at room temperature. The most important advantage of this technique is to prevent surface oxidation, polymorphic crystallization, and a filmsubstrate reaction which can occur during the post-annealing process. By employing in-situ annealing treatment, lower temperatures (300–450 ◦ C) are required to obtain crystallized NiTi thin films with good shape memory effects [11]. Thus, it is a cost-efficient treatment because these required lower crystallization temperatures can further be beneficial in terms of conservation of thermal processing budgets. Last but not the least, annealing during sputtering makes it possible to deposit another coating layer on NiTi thin films (protective layers) without breaking the vacuum inside of the sputtering chamber. As an example, the authors of the present paper have already reported the deposition of composite NiTi/TiCN films by employing this technique [12]. In spite of the benefits of the in-situ annealing method, only a few researchers worked on in-situ annealed NiTi SMA thin films [13,14], and the role of sputtering temperature during deposition has not been precisely reported.
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Table 1 Description of sputtering parameters.
Table 3 The compositions of the deposited NiTi films recorded by EDX.
Amount of target used Gas Ar flow (ml/min) Chamber pressure (mPa) Substrate bias voltage (Kv) Sputtering power (W) Substrate rotation speed (rpm)
2 Ar 320 350 −0.075 1400 5
The main aim of the present research work is to investigate the effect of the sputtering temperature on properties of NiTi thin films as well as their shape memory behaviors. The term of sputtering temperature in this study refers to the temperature of the sample holder in the coating chamber in which the substrates were installed. Moreover, it was observed that employing Ti-rich NiTi alloy targets can ensure a reproducible production of nearly equiatomic NiTi thin films and solve the stoichiometry sensitivity problem. 2. Experimental NiTi thin films were deposited on a silicon (1 0 0) substrate by means of a DC magnetron sputtering device (CC800sinox TM, CemeCon AG, Germany). Ti-rich NiTi alloy targets (51.8 at% Ti–Ni) were employed to sputter of the NiTi SMA thin films. In order to achieve a uniform film composition, the sample holder was rotated on a horizontal table during sputtering. The target to substrate distance was fixed at approximately 9.5 mm. NiTi thin films were deposited at four different sputtering temperatures of 525 ◦ C, 425 ◦ C, 305 ◦ C, and 80 ◦ C by adjusting the heating powers to 25,000 W, 10,000 W, 5000 W, and 0 W, respectively, during the coating processes. Since the maximum applicable heating power was 10,000 W, an extra heating system was installed inside the coating chamber which supplied an additional 15,000 W of heating power. By employing this extra heating system, it was possible to reach the sputtering temperature of 525 ◦ C. The deposition at the heating power of 0 W was performed without an intentional heating of the substrates. However, using the thermocouple, some substrate heating was detected around 80 ◦ C during the deposition process. The temperature of the targets did not exceed 60 ◦ C because of the water circulating cooling system at the backside of the targets. Except of the heating power, all of the sputtering parameters were kept constant during the deposition of NiTi thin films. These parameters are summarized in Table 1.No postannealing process was conducted after the deposition of these films. The detailed description of the deposited thin films is presented in Table 2. The microstructure of the thin films was analyzed by employing X-ray diffraction using Cu K␣ radiation and a 9◦ incident angle (D8 Advance, BRUKER AXS, Germany). The thin film morphology and thickness results were analyzed on a fracture cross-section of coated samples by means of a field emission scanning electron microscope (FESEM) (Jeol JXA840, JSM 35, Japan), while the composition of coatings was determined using energydispersive X-ray spectrometry (EDX) with an electron acceleration voltage of 20 kV and a beam current of 15 nA. All sampling was done by analyzing in areas but not point measurements to investigate the chemical compositions. Phase transformations of free-standing Table 2 Description of specimens. Sample ID
Sputtering time (min)
Sputtering temperature (◦ C)
A1 A2 A3 A4
180 180 180 180
80 305 425 525
Sample ID
Sputtering time (◦ C)
Composition (at%) Ni/Ti
A1 A2 A3 A4
80 305 425 525
50.65/49.35 50.40/49.60 50.66/49.34 50.69/49.31
NiTi coatings were characterized using a differential scanning calorimeter (DSC 2920 CE from TA Instruments). DSC specimens with a mass of 20 mg were heated and cooled at the rate of 10 K/min over a temperature range of −150 to 150 ◦ C. 3. Result 3.1. Morphology and composition of thin films The compositions of the deposited films, measured by EDX, are shown in Table 3. Since the atomic ratios of Ni to Ti in the samples are all close to 1, it can be concluded that the sputtering temperature do not significantly affect the film compositions in this case study. It has been previously reported that the sputtering profiles of Ni and Ti atoms from cold and hot targets are different [15]. Nevertheless, this result introduces a new approach to compensate the dependency of the sputtering profile on the target temperature by employing Ti-rich NiTi alloy targets. In fact, this finding can lead to reproducible depositions of NiTi SMA thin films with a nearly equiatomic composition ratio. NiTi thin films sputtered at 80 ◦ C showed ductile fractures, while the other films showed brittle fractures during the sample crosssection preparation. Such ductile fractions can be ascribed to the lack of crystallization in NiTi thin films deposited at a low temperature (80 ◦ C). The fracture analysis by means of FESEM reveals various microstructures of NiTi thin films sputtered at different temperatures. Fig. 1 shows the FESEM images of the fractured cross-section of the deposited thin films. As it can be seen, the NiTi thin film deposited at 80 ◦ C shows a thick fibrous microstructure. Thin films deposited at 305 ◦ C and 425 ◦ C exhibit a finely packed fibrous or bamboo-like structure. This structure changes to a densely packed columnar structure for the film deposited at 525 ◦ C. This difference in microstructures is a direct consequence of the relation between the crystallization temperature, nucleation, and growth. Increasing the sputtering temperature to 525 ◦ C can enhance the activity of absorbed atoms and promote the migration of atoms to the favorable energy positions that support the formation of densely packed columnar structures. Interestingly, this finding corresponds with the results obtained in the research work of Zhang et al. [16]. They reported that the crystallization of as-deposited NiTi thin films was the process of generation and development of columnar structures during post-annealing operation. As a result, during in-situ annealing of the NiTi thin films, the increasing of annealing temperature leads to the increase of columnar structures (lateral growth of grains) and surface roughness. One important point regarding the sputtering temperature is its effect on the thickness of the film. Fig. 2 shows the thickness of NiTi thin films deposited at different temperatures. Generally speaking, the thickness of a film during the magnetron sputtering can be dominantly affected by the sputtering power and time. Since the processing parameters for the deposition of these thin films are the same, the observed variation of the film thickness could be a consequence of employing different sputtering temperatures. The NiTi thin films deposited at 80 ◦ C possess the lowest thickness (2.73 m). This can be attributed to the low temperature of the substrate which cannot adequately provide active spots for
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Fig. 1. A series of FESEM micrographs of the final microstructure of NiTi thin films deposited at sputtering temperatures of (a) 80 ◦ C (b) 425 ◦ C (c) 305 ◦ C, and (d) 525 ◦ C.
the nucleation and growth of NiTi thin films. Increasing the sputtering temperature up to 305 ◦ C results in a 20% increment of the film thickness. As it was mentioned, the increased thickness was a consequence of more nucleation and an increased growth of NiTi crystalline materials. The thickness of the thin film deposited at sputtering temperatures of 425 ◦ C does not show any significant difference compared to those deposited at 305 ◦ C. However, the thickness of the film deposited at 525 ◦ C shows a 12% decrease compared to the films deposited at 305 ◦ C and 425 ◦ C. This is probably due to the higher volume diffusion of the deposited atoms, which leads to a more compacted and even denser microstructure. 3.2. Crystalline structure of thin films Fig. 3(a) and (b) shows XRD spectra of deposited NiTi thin films at low sputtering temperatures. The film deposited at 80 ◦ C shows a typical amorphous spectrum with no recognizable peaks. The XRD pattern of the film deposited at 305◦ shows that it has not been fully crystallized during the sputtering process. The broad peak at 2Â within a range of 36–51◦ is characteristic for an amorphous matrix containing tiny NiTi nanocrystallites. However, there is a sharp peak at 2Â of 67◦ which shows that the film was partially crystallized. The position of this peak correlates with (2 0 0) peak
Thickness (µm) 4
3,43
3,5 3
3,4 3,04
2,73
2,5 2 1,5 1
of the austenite phase. This is perhaps due to the initial stacking of the austenite B2 NiTi phase onto the (h 0 0) planes. Fig. 4(a) and (b) shows XRD profiles of NiTi thin films deposited at high sputtering temperatures. The XRD pattern of the film deposited at 425 ◦ C shows three obvious diffraction peaks at 2Â = 42.3◦ , 62◦ , and 78.2◦ , respectively, which correspond with the (1 1 0), (2 0 0), and (2 1 1) lattice orientations of austenite B2 structure of NiTi thin films. The existence of a weak and broad Ni4 Ti3 peak at 2Â = 37.8◦ implies slight non-equilibrium precipitation reactions during sputtering. The presence of other Ni4 Ti3 peaks such as (2 0 2) at 2Â = 41◦ and (1 2 2) at 2Â = 43◦ was not confirmed because of an intense B2 (1 1 0) peak at 42.3◦ and the absence of fully crystalline and large precipitates. The NiTi thin film deposited at 525◦ shows diffraction patterns similar to that of the films deposited at 425 ◦ C with 2Â = 42.3◦ , 62◦ , and 78.2◦ , indexed as (1 1 0), (2 0 0), and (2 1 1), respectively. However, an extra peak in the XRD pattern of this film appears at 2Â = 38.0◦ . There are two hypotheses for this phenomenon: the first hypothesis is a silicide formation close to the film–substrate interface as a result of an increasing sputtering temperature. The second hypothesis is the formation of Ni4 Ti3 precipitations. The small Ni4 Ti3 peak observed for the film deposited at 425 ◦ C appears also with an increased intensity and width for the film deposited at 525 ◦ C. By increasing the sputtering temperature, diffusion becomes more active, which causes the precipitates to form and grow at a higher rate. This leads to the formation of larger and more crystalline precipitations and consequently increases the intensity and width of the Ni4 Ti3 peak. Based on this hypothesis, it can be assumed that the small peak at 2Â = 55◦ belongs to the (2 3 2) diffraction pattern of these precipitates. Furthermore, the broadening of the austenite NiTi peaks at 2Â of 62◦ and 78◦ could be a consequence of their overlapping with the (4 2 2) and (5 3 2) peaks of the Ni4 Ti3 precipitates. This was also reported by Ho et al. [17] that the depositions of NiTi thin films from a cold target to a hot Si substrate with temperatures of 500 ◦ C and 600 ◦ C could intensively promote the growth of precipitates.
0,5
0
Sample A1
Sample A2
Sample A3
Sample A4
Fig. 2. Film thickness of NiTi SMA thin films deposited at the sputtering temperatures of 80 ◦ C (sample A1), 305 ◦ C (sample A2), 425 ◦ C (sample A3), and 525 ◦ C (sample A4).
3.3. Phase transformation analysis of thin films Fig. 5(a) and (b) shows DSC curves of the NiTi thin films deposited at 80 ◦ C and 305 ◦ C. It can be clearly seen that no thermal events happen when cooling the samples from
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Fig. 3. A series of XRD patterns of NiTi thin films deposited at sputtering temperatures of (a) 80 ◦ C and (b) 305 ◦ C.
Fig. 4. A series of XRD patterns of NiTi thin films deposited at sputtering temperatures of (a) 425 ◦ C and (b) 525 ◦ C.
120 ◦ C to −60 ◦ C. However, a weak endothermic peak started at 9 ◦ C during the heating process and stopped at 48 ◦ C. This might be attributed to the non-uniform partial crystallization of the films, either through the thickness or radially
which is in agreement with the XRD results in Fig. 3(a) and (b). The DSC curve of the film deposited at a sputtering temperature of 425 ◦ C is shown in Fig. 6(a) In this measurement, exothermic
Fig. 5. A series of DSC thermographs of NiTi thin films deposited at sputtering temperatures of (a) 80 ◦ C and (b) 305 ◦ C.
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Fig. 6. A series of DSC thermographs of NiTi thin films deposited at sputtering temperature of (a) 425 ◦ C and (b) 525 ◦ C.
and endothermic peaks can be clearly observed at the martensitic and reverse martensitic transformation on cooling and heating, respectively. It was shown that the transformation into the parent austenitic phase starts (As ) at −2.2 ◦ C and completes transforming into austenite (Af ) at 55.7 ◦ C. As the temperature decreases, a transformation back into the martensite phase starts (Ms ) at 53.7 ◦ C. The phase transformation into martensite (Mf ) is completed at 25.8 ◦ C while cooling down the coating. The DSC plot uncovered that the transformation to austenite is a two-step transformation. The first step begins at the austenite start (As ) temperature of −2.2 ◦ C and the second step at 28 ◦ C. In order to simplify labeling, (As ) and (Af ) are used to indicate the temperature of the first appearance of austenite and the completion of the transformation. The two arguments for a two-step transition are a type of R-phase and the presence of precipitates [12]. The first hypothesis concerning the R-phase formation corresponds with a DSC measurement that was conducted by Miyazaki et al. [18]. They found such a two-step curve for a Ti-51.9 at% Ni alloy thin film that was sputtered at room temperature and subsequently aged at 500 ◦ C for 1 h. They called the first and second peaks A* and RA*, representing the reverse transformation from a martensite to a R-phase and from a R-phase to austenite, respectively. However, the R-phase is more commonly found before the transition from austenite to martensite. Based on the second hypothesis, the elongated transition could be due to the Ni4 Ti3 precipitates. These precipitates can elongate the transition temperatures and cause a two-peak transition. Fan et al. [19] demonstrated that polycrystalline alloys with a low Ni supersaturation (50.6 at%) showed an abnormal two-stage transformation. In these alloys, Ni4 Ti3 preferentially precipitated only at the grain boundary (GB) regions, where nucleation barriers were low compared to those of grain interiors (GIs), resulting in a localized two-stage B2-R-B19 transformation near the GBs and a one-stage B2–B19 transformation in the GIs. This could be the reason for the observed DSC curve of the film deposited at 425 ◦ C, as the EDX result shows its slightly Ni-rich (50.6 at%) composition. Nevertheless, more investigations are needed to exactly clarify the reason of the two-step martensitic transformations. The DCS curve of the film deposited at 525 ◦ C is shown in Fig. 6(b). In the cooling cycle, the forward transformation into the martensite phase started at a temperature of 36.1 ◦ C and was completed at 14.8 ◦ C. In the reverse transformation on heating, austenite start and finish temperatures were determined as 33.2 ◦ C (As ) and 63 ◦ C (Af ), respectively. It can be obviously seen that there
are not two-peak transitions in the thermal transformation behavior of the NiTi thin film. This phenomenon is probably due to the coherency strains associated with the Ni4 Ti3 precipitate formation within NiTi grains. It is well known that coherency strain fields around precipitates can spread the transformation over a range of temperatures or cause multiple-stage transformations [20,21]. Although the initial Ni4 Ti3 precipitates are coherent with the NiTi matrix, increasing the sputtering temperature to up to 525 ◦ C causes the precipitates to grow in size and to lose their coherency. Consequently, the inhomogeneity of the stress distribution between these precipitates and the surrounding NiTi matrix is removed. This, in turn, can prevent two-stage transformations from martensite to austenite. The formation of larger Ni4 Ti3 precipitates is also in agreement with the XRD pattern of this film in Fig. 4(b). The existence of a small transformation temperature hysteresis for the films deposited at 425 ◦ C and 525 ◦ C is an excellent property for applications in microelectromechanical systems (MEMS). It is particularly beneficial when a short response time is required for a fast cyclic microactuation. However, it will be very interesting to complement the present results with further investigations concerning a mechanical and tribological analysis of these thin films. Such an investigation is in progress by the authors of the present work to clarify the effect of the sputtering temperature on mechanical as well as tribological properties of NiTi thin films.
4. Conclusion In this study, nearly equiatomic NiTi thin films were deposited by means of magnetron sputtering of a Ti-rich NiTi alloy. The effect of the sputtering temperature (temperature inside the coating chamber) on the microstructure and shape memory behavior of NiTi films was investigated. The thickness of the deposited films was about 3 m and shows a variation of 12–20% upon changing the sputtering temperature. In addition, the following points are highlighted:
1. Employing a Ti-rich NiTi target can compensate the discrepancy in the angular flux distribution of Ni and Ti atoms from cold and hot targets. 2. NiTi thin films deposited for 180 min at various temperatures possess different microstructures. The films deposited at 80 ◦ C are fully amorphous, while films deposited at 305 ◦ C are partially
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crystallized. A deposition at a sputtering temperature above 425◦ leads to a fully crystallized formation of NiTi thin films. 3. NiTi thin films deposited at 425 ◦ C show two-stage transformation behavior upon heating due to the formation of a R-phase or Ni4 Ti3 precipitates, while the films deposited at 525 ◦ C show one-stage transformation upon heating and cooling. 4. NiTi thin films deposited at 425 ◦ C and 525 ◦ C possess very small transformation temperature hysteresis which can enable them to show fast frequency response. References [1] K. Otsuka, T.K. Sawamura, Shimizu crystal structure and internal defects of equiatomic TiNi martensite, Phys. Sat. Sol. A 5 (1971) 457–470. [2] D.Y. Li, X.F. Wu, T. Ko, The effect of stress on soft modes for phase transition in Ti–Ni alloy, Philos. Mag. A 63 (1991) 585–603. [3] J.J. Gill, D.T. Chang, L.A. Momoda, G.P. Carman, Manufacturing issues of thin film NiTi micro wrapper, Sens. Actuators. A 93 (2001) 148–156. [4] M. Nishida, T. Honma, Electron microscopy studies of the all-round shape memory effect in a Ti-51.0 atm% Ni alloy, Scr. Mater. 18 (1984) 1293–1296. [5] D.S. Grummon, S. Nam, L. Chang, Effect of super elastically deforming NiTi surface microalloys on fatigue crack nucleation in copper, Proc. Mater. Res. Soc. 246 (1992) 259–264. [6] Li. Hou, D.S. Grummon, Transformational superelasticity in sputtered titanium–nickel thin films, Scr. Mater. 33 (1995) 989–995. [7] A. Ishida, V. Martiniv, Sputter-deposited shape-memory alloy thin films: properties and applications, MRS Bull. 27 (2) (2002) 111–114. [8] J.J. Kim, P. Moine, D.A. Stevenson, Crystallization behavior of amorphous NiTi alloys prepared by sputter deposition, Scr. Metall. 20 (1986) 243–248. [9] P. Surbled, C. Clerc, B.L. Piofle, M. Ataka, F. Fujita, Effect of composition and thermal annealing on the transformation temperatures of sputtered TiNi shape memory alloy thin films, Thin Solid Films 401 (2001) 52–59. [10] G. Satoh, A. Birnbaum, Y.L. Yao, Effect of annealing parameters on the shape memory properties of NiTi thin films, in: ICALEO 2008 Congress Proceedings, Poster presentation gallery, 100–167. [11] Y.Q. Fu, H.J. Du, Effects of film composition and annealing on residual stress evolution for shape memory TiNi film, Mater. Sci. Eng. A 342 (2003) 236. [12] W. Tillmann, S. Momeni, Deposition of superelastic composite NiTi based films, Vacuum 104 (2014) 41–46. [13] K. Gisser, G.D. Busch, A.D. Johnson, A.B. Ellis, Oriented nickel–titanium shape memory alloy films prepared by annealing during deposition, Appl. Phys. Lett. 61 (14) (1992). [14] A. Kumar, S.K. Sharma, S. Bysakh, S.V. Kamat, S. Mohan, Effect of substrate and annealing temperatures on mechanical properties of Ti-rich NiTi films, J. Mater. Sci. Technol. 26 (11) (2010) 961–966. [15] K. Ho, K.K. Mohanchandra, G.P. Garmen, Examination of the sputtering profile of NiTi under target heating conditions, Thin Solid Films 413 (1–2) (2002) 1–7.
[16] L. Zhang, C. Xie, J.W., Effect of annealing temperature on surface morphology and mechanical properties of sputter-deposited Ti–Ni thin films, J. Alloys Compd. 424 (2007) 238–243. [17] K.K. Ho, G.P. Carman, Sputter deposition of NiTi thin film shape memory alloy using a heated target, Thin Solid Films 370 (2000) 18–29. [18] S. Miyazaki, A. Ishida, Martensitic transformation and shape memory behavior in sputter-deposited TiNi-base thin films, Mater. Sci. Eng. A 273–275 (1999) 106–133. [19] G. Fan, W. Chen, S. Yang, J. Zhu, X. Ren, K. Otsuka, Origin of abnormal multistage martensitic transformation behavior in aged Ni-rich Ti–Ni shape memory alloys, Acta Mater. 52 (2004) 4351. [20] W. Tiry, D. Schryvers, Quantitative determination of strain fields around Ni4 Ti3 precipitates in NiTi, Acta Mater. 53 (4) (2005) 1041–1049. [21] D. Schryvers, W. Tirry, Z.Q. Yang, Measuring strain fields and concentration gradients around Ni4 Ti3 precipitates, Mater. Sci. Eng. A 438–440 (2006) 485–488.
Biographies
Soroush Momeni received his B.Sc. in Textile Engineering (Textile Chemistry and Fiber Science) from the Azad University of Tehran, Iran. He gained his M.Sc. in Advanced Materials (Nanomaterials) in February 2012 from the University of Ulm, Germany. Since April 2012, he is pursuing his PhD degree under the direction of Professor Wolfgang Tillmann at the Institute of Materials Engineering, Technical University of Dortmund, Germany. His research interest currently focuses on magnetron sputtering physical vapor deposition techniques, binary and ternary NiTi shape memory alloy thin films, self-healing coating systems, antibacterial magnetron sputtered coatings, wear resistance hard coatings as well as cavitation erosion resistance coatings. Wolfgang Tillmann got his PhD degree in 1992 in the field of joining engineering ceramics from the Material Science Institute (MSI), Aachen University of Technology, Germany where he was a chief engineer subsequently after his graduation until 1996. After that he started working as the head of Materials and Mechanics department at technical center Hilti Corp, Principality of Liechtenstein from 1997 to 2000. From 2001 to 2002, he was a managing director of the business unit diamond tools at Hilti Germany ltd. Since November 2002, he is a full Professor at the Institute of Materials Engineering, Technical University of Dortmund. His research interests include brazing technology, powder metallurgy, thermal Spraying, and magnetron sputtering physical vapor deposition techniques.
Contents lists available at ScienceDirect
Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna
In-situ annealing of NiTi thin films at different temperatures Wolfgang Tillmann ∗ , Soroush Momeni ∗∗ Institute of Materials Engineering, Technische Universität Dortmund,Leonhard-Euler-Str 2, 44227 Dortmund, Germany
a r t i c l e
i n f o
Article history: Received 26 May 2014 Received in revised form 27 October 2014 Accepted 28 October 2014 Available online 5 November 2014 Keywords: Thin films Sputtering Annealing Shape memory alloys Precipitation
a b s t r a c t Magnetron sputtered NiTi thin films are usually sputtered at ambient temperature and need a postannealing treatment to promote crystallization and obtain shape memory effect. However, this treatment could adversely affect the microstructure as well as the morphology of the film. Within this study, NiTi thin films were generated by annealing during the sputtering process. The effect of the sputtering temperature on the morphology of the film, the composition, and shape memory behavior was studied using X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDX), field emission scanning electron microscopy (FESEM), and differential scanning calorimetry (DSC). © 2014 Elsevier B.V. All rights reserved.
1. Introduction NiTi shape memory alloy (SMA) thin films have recently attracted great interest for the development of microelectromechanical systems (MEMS), such as micro valves [1], micro fluid pumps [2], micro wrappers [3], and micro grippers [4]. The main benefits of MEMS applications for NiTi thin films are for example a large displacement, high damping capacity, large actuation force, and a low operating voltage. In addition, the shape memory effect (SME) and the superelasticity (SE) of NiTi thin films can be employed to engineer surfaces with excellent tribological properties. This is even possible when the material is only present as a surface coating [5,6]. Despite these excellent properties, there are some difficulties to fully integrate NiTi shape memory thin films into MEMS and tribological coating systems as their high stoichiometry sensitivity restricts the manufacturability and versatility in MEMS [7]. In addition, the as-deposited sputtered NiTi thin films are amorphous without any shape memory effect. In order to promote the shape memory effect, they need to be crystallized during a post-annealing process (using temperatures of 600 ◦ C and higher) [8]. The crystallization process is a direct consequence of the grain nucleation and growth at elevated temperatures. As a result, the annealing
∗ Corresponding author. Tel.: +49 231 755 2581. ∗∗ Corresponding author. Tel.: +49 231 755 6113. E-mail addresses: [email protected] (W. Tillmann), [email protected] (S. Momeni). http://dx.doi.org/10.1016/j.sna.2014.10.034 0924-4247/© 2014 Elsevier B.V. All rights reserved.
temperature and time will influence the structure as well as property of NiTi thin films and their shape memory behavior. A connection between the annealing temperature and phase transformation temperatures of NiTi thin films has previously been reported by Surbled et al. [9]. The effect of annealing parameters on mechanical and shape memory properties of NiTi thin films was investigated by Satoh et al. [10]. Nevertheless, it was noticed that the effects of the annealing parameters during the sputtering of NiTi thin films have not been investigated so far. The in-situ annealing technique could be more reliable than a post-annealing of NiTi thin films, sputtered at room temperature. The most important advantage of this technique is to prevent surface oxidation, polymorphic crystallization, and a filmsubstrate reaction which can occur during the post-annealing process. By employing in-situ annealing treatment, lower temperatures (300–450 ◦ C) are required to obtain crystallized NiTi thin films with good shape memory effects [11]. Thus, it is a cost-efficient treatment because these required lower crystallization temperatures can further be beneficial in terms of conservation of thermal processing budgets. Last but not the least, annealing during sputtering makes it possible to deposit another coating layer on NiTi thin films (protective layers) without breaking the vacuum inside of the sputtering chamber. As an example, the authors of the present paper have already reported the deposition of composite NiTi/TiCN films by employing this technique [12]. In spite of the benefits of the in-situ annealing method, only a few researchers worked on in-situ annealed NiTi SMA thin films [13,14], and the role of sputtering temperature during deposition has not been precisely reported.
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W. Tillmann, S. Momeni / Sensors and Actuators A 221 (2015) 9–14
Table 1 Description of sputtering parameters.
Table 3 The compositions of the deposited NiTi films recorded by EDX.
Amount of target used Gas Ar flow (ml/min) Chamber pressure (mPa) Substrate bias voltage (Kv) Sputtering power (W) Substrate rotation speed (rpm)
2 Ar 320 350 −0.075 1400 5
The main aim of the present research work is to investigate the effect of the sputtering temperature on properties of NiTi thin films as well as their shape memory behaviors. The term of sputtering temperature in this study refers to the temperature of the sample holder in the coating chamber in which the substrates were installed. Moreover, it was observed that employing Ti-rich NiTi alloy targets can ensure a reproducible production of nearly equiatomic NiTi thin films and solve the stoichiometry sensitivity problem. 2. Experimental NiTi thin films were deposited on a silicon (1 0 0) substrate by means of a DC magnetron sputtering device (CC800sinox TM, CemeCon AG, Germany). Ti-rich NiTi alloy targets (51.8 at% Ti–Ni) were employed to sputter of the NiTi SMA thin films. In order to achieve a uniform film composition, the sample holder was rotated on a horizontal table during sputtering. The target to substrate distance was fixed at approximately 9.5 mm. NiTi thin films were deposited at four different sputtering temperatures of 525 ◦ C, 425 ◦ C, 305 ◦ C, and 80 ◦ C by adjusting the heating powers to 25,000 W, 10,000 W, 5000 W, and 0 W, respectively, during the coating processes. Since the maximum applicable heating power was 10,000 W, an extra heating system was installed inside the coating chamber which supplied an additional 15,000 W of heating power. By employing this extra heating system, it was possible to reach the sputtering temperature of 525 ◦ C. The deposition at the heating power of 0 W was performed without an intentional heating of the substrates. However, using the thermocouple, some substrate heating was detected around 80 ◦ C during the deposition process. The temperature of the targets did not exceed 60 ◦ C because of the water circulating cooling system at the backside of the targets. Except of the heating power, all of the sputtering parameters were kept constant during the deposition of NiTi thin films. These parameters are summarized in Table 1.No postannealing process was conducted after the deposition of these films. The detailed description of the deposited thin films is presented in Table 2. The microstructure of the thin films was analyzed by employing X-ray diffraction using Cu K␣ radiation and a 9◦ incident angle (D8 Advance, BRUKER AXS, Germany). The thin film morphology and thickness results were analyzed on a fracture cross-section of coated samples by means of a field emission scanning electron microscope (FESEM) (Jeol JXA840, JSM 35, Japan), while the composition of coatings was determined using energydispersive X-ray spectrometry (EDX) with an electron acceleration voltage of 20 kV and a beam current of 15 nA. All sampling was done by analyzing in areas but not point measurements to investigate the chemical compositions. Phase transformations of free-standing Table 2 Description of specimens. Sample ID
Sputtering time (min)
Sputtering temperature (◦ C)
A1 A2 A3 A4
180 180 180 180
80 305 425 525
Sample ID
Sputtering time (◦ C)
Composition (at%) Ni/Ti
A1 A2 A3 A4
80 305 425 525
50.65/49.35 50.40/49.60 50.66/49.34 50.69/49.31
NiTi coatings were characterized using a differential scanning calorimeter (DSC 2920 CE from TA Instruments). DSC specimens with a mass of 20 mg were heated and cooled at the rate of 10 K/min over a temperature range of −150 to 150 ◦ C. 3. Result 3.1. Morphology and composition of thin films The compositions of the deposited films, measured by EDX, are shown in Table 3. Since the atomic ratios of Ni to Ti in the samples are all close to 1, it can be concluded that the sputtering temperature do not significantly affect the film compositions in this case study. It has been previously reported that the sputtering profiles of Ni and Ti atoms from cold and hot targets are different [15]. Nevertheless, this result introduces a new approach to compensate the dependency of the sputtering profile on the target temperature by employing Ti-rich NiTi alloy targets. In fact, this finding can lead to reproducible depositions of NiTi SMA thin films with a nearly equiatomic composition ratio. NiTi thin films sputtered at 80 ◦ C showed ductile fractures, while the other films showed brittle fractures during the sample crosssection preparation. Such ductile fractions can be ascribed to the lack of crystallization in NiTi thin films deposited at a low temperature (80 ◦ C). The fracture analysis by means of FESEM reveals various microstructures of NiTi thin films sputtered at different temperatures. Fig. 1 shows the FESEM images of the fractured cross-section of the deposited thin films. As it can be seen, the NiTi thin film deposited at 80 ◦ C shows a thick fibrous microstructure. Thin films deposited at 305 ◦ C and 425 ◦ C exhibit a finely packed fibrous or bamboo-like structure. This structure changes to a densely packed columnar structure for the film deposited at 525 ◦ C. This difference in microstructures is a direct consequence of the relation between the crystallization temperature, nucleation, and growth. Increasing the sputtering temperature to 525 ◦ C can enhance the activity of absorbed atoms and promote the migration of atoms to the favorable energy positions that support the formation of densely packed columnar structures. Interestingly, this finding corresponds with the results obtained in the research work of Zhang et al. [16]. They reported that the crystallization of as-deposited NiTi thin films was the process of generation and development of columnar structures during post-annealing operation. As a result, during in-situ annealing of the NiTi thin films, the increasing of annealing temperature leads to the increase of columnar structures (lateral growth of grains) and surface roughness. One important point regarding the sputtering temperature is its effect on the thickness of the film. Fig. 2 shows the thickness of NiTi thin films deposited at different temperatures. Generally speaking, the thickness of a film during the magnetron sputtering can be dominantly affected by the sputtering power and time. Since the processing parameters for the deposition of these thin films are the same, the observed variation of the film thickness could be a consequence of employing different sputtering temperatures. The NiTi thin films deposited at 80 ◦ C possess the lowest thickness (2.73 m). This can be attributed to the low temperature of the substrate which cannot adequately provide active spots for
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Fig. 1. A series of FESEM micrographs of the final microstructure of NiTi thin films deposited at sputtering temperatures of (a) 80 ◦ C (b) 425 ◦ C (c) 305 ◦ C, and (d) 525 ◦ C.
the nucleation and growth of NiTi thin films. Increasing the sputtering temperature up to 305 ◦ C results in a 20% increment of the film thickness. As it was mentioned, the increased thickness was a consequence of more nucleation and an increased growth of NiTi crystalline materials. The thickness of the thin film deposited at sputtering temperatures of 425 ◦ C does not show any significant difference compared to those deposited at 305 ◦ C. However, the thickness of the film deposited at 525 ◦ C shows a 12% decrease compared to the films deposited at 305 ◦ C and 425 ◦ C. This is probably due to the higher volume diffusion of the deposited atoms, which leads to a more compacted and even denser microstructure. 3.2. Crystalline structure of thin films Fig. 3(a) and (b) shows XRD spectra of deposited NiTi thin films at low sputtering temperatures. The film deposited at 80 ◦ C shows a typical amorphous spectrum with no recognizable peaks. The XRD pattern of the film deposited at 305◦ shows that it has not been fully crystallized during the sputtering process. The broad peak at 2Â within a range of 36–51◦ is characteristic for an amorphous matrix containing tiny NiTi nanocrystallites. However, there is a sharp peak at 2Â of 67◦ which shows that the film was partially crystallized. The position of this peak correlates with (2 0 0) peak
Thickness (µm) 4
3,43
3,5 3
3,4 3,04
2,73
2,5 2 1,5 1
of the austenite phase. This is perhaps due to the initial stacking of the austenite B2 NiTi phase onto the (h 0 0) planes. Fig. 4(a) and (b) shows XRD profiles of NiTi thin films deposited at high sputtering temperatures. The XRD pattern of the film deposited at 425 ◦ C shows three obvious diffraction peaks at 2Â = 42.3◦ , 62◦ , and 78.2◦ , respectively, which correspond with the (1 1 0), (2 0 0), and (2 1 1) lattice orientations of austenite B2 structure of NiTi thin films. The existence of a weak and broad Ni4 Ti3 peak at 2Â = 37.8◦ implies slight non-equilibrium precipitation reactions during sputtering. The presence of other Ni4 Ti3 peaks such as (2 0 2) at 2Â = 41◦ and (1 2 2) at 2Â = 43◦ was not confirmed because of an intense B2 (1 1 0) peak at 42.3◦ and the absence of fully crystalline and large precipitates. The NiTi thin film deposited at 525◦ shows diffraction patterns similar to that of the films deposited at 425 ◦ C with 2Â = 42.3◦ , 62◦ , and 78.2◦ , indexed as (1 1 0), (2 0 0), and (2 1 1), respectively. However, an extra peak in the XRD pattern of this film appears at 2Â = 38.0◦ . There are two hypotheses for this phenomenon: the first hypothesis is a silicide formation close to the film–substrate interface as a result of an increasing sputtering temperature. The second hypothesis is the formation of Ni4 Ti3 precipitations. The small Ni4 Ti3 peak observed for the film deposited at 425 ◦ C appears also with an increased intensity and width for the film deposited at 525 ◦ C. By increasing the sputtering temperature, diffusion becomes more active, which causes the precipitates to form and grow at a higher rate. This leads to the formation of larger and more crystalline precipitations and consequently increases the intensity and width of the Ni4 Ti3 peak. Based on this hypothesis, it can be assumed that the small peak at 2Â = 55◦ belongs to the (2 3 2) diffraction pattern of these precipitates. Furthermore, the broadening of the austenite NiTi peaks at 2Â of 62◦ and 78◦ could be a consequence of their overlapping with the (4 2 2) and (5 3 2) peaks of the Ni4 Ti3 precipitates. This was also reported by Ho et al. [17] that the depositions of NiTi thin films from a cold target to a hot Si substrate with temperatures of 500 ◦ C and 600 ◦ C could intensively promote the growth of precipitates.
0,5
0
Sample A1
Sample A2
Sample A3
Sample A4
Fig. 2. Film thickness of NiTi SMA thin films deposited at the sputtering temperatures of 80 ◦ C (sample A1), 305 ◦ C (sample A2), 425 ◦ C (sample A3), and 525 ◦ C (sample A4).
3.3. Phase transformation analysis of thin films Fig. 5(a) and (b) shows DSC curves of the NiTi thin films deposited at 80 ◦ C and 305 ◦ C. It can be clearly seen that no thermal events happen when cooling the samples from
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Fig. 3. A series of XRD patterns of NiTi thin films deposited at sputtering temperatures of (a) 80 ◦ C and (b) 305 ◦ C.
Fig. 4. A series of XRD patterns of NiTi thin films deposited at sputtering temperatures of (a) 425 ◦ C and (b) 525 ◦ C.
120 ◦ C to −60 ◦ C. However, a weak endothermic peak started at 9 ◦ C during the heating process and stopped at 48 ◦ C. This might be attributed to the non-uniform partial crystallization of the films, either through the thickness or radially
which is in agreement with the XRD results in Fig. 3(a) and (b). The DSC curve of the film deposited at a sputtering temperature of 425 ◦ C is shown in Fig. 6(a) In this measurement, exothermic
Fig. 5. A series of DSC thermographs of NiTi thin films deposited at sputtering temperatures of (a) 80 ◦ C and (b) 305 ◦ C.
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Fig. 6. A series of DSC thermographs of NiTi thin films deposited at sputtering temperature of (a) 425 ◦ C and (b) 525 ◦ C.
and endothermic peaks can be clearly observed at the martensitic and reverse martensitic transformation on cooling and heating, respectively. It was shown that the transformation into the parent austenitic phase starts (As ) at −2.2 ◦ C and completes transforming into austenite (Af ) at 55.7 ◦ C. As the temperature decreases, a transformation back into the martensite phase starts (Ms ) at 53.7 ◦ C. The phase transformation into martensite (Mf ) is completed at 25.8 ◦ C while cooling down the coating. The DSC plot uncovered that the transformation to austenite is a two-step transformation. The first step begins at the austenite start (As ) temperature of −2.2 ◦ C and the second step at 28 ◦ C. In order to simplify labeling, (As ) and (Af ) are used to indicate the temperature of the first appearance of austenite and the completion of the transformation. The two arguments for a two-step transition are a type of R-phase and the presence of precipitates [12]. The first hypothesis concerning the R-phase formation corresponds with a DSC measurement that was conducted by Miyazaki et al. [18]. They found such a two-step curve for a Ti-51.9 at% Ni alloy thin film that was sputtered at room temperature and subsequently aged at 500 ◦ C for 1 h. They called the first and second peaks A* and RA*, representing the reverse transformation from a martensite to a R-phase and from a R-phase to austenite, respectively. However, the R-phase is more commonly found before the transition from austenite to martensite. Based on the second hypothesis, the elongated transition could be due to the Ni4 Ti3 precipitates. These precipitates can elongate the transition temperatures and cause a two-peak transition. Fan et al. [19] demonstrated that polycrystalline alloys with a low Ni supersaturation (50.6 at%) showed an abnormal two-stage transformation. In these alloys, Ni4 Ti3 preferentially precipitated only at the grain boundary (GB) regions, where nucleation barriers were low compared to those of grain interiors (GIs), resulting in a localized two-stage B2-R-B19 transformation near the GBs and a one-stage B2–B19 transformation in the GIs. This could be the reason for the observed DSC curve of the film deposited at 425 ◦ C, as the EDX result shows its slightly Ni-rich (50.6 at%) composition. Nevertheless, more investigations are needed to exactly clarify the reason of the two-step martensitic transformations. The DCS curve of the film deposited at 525 ◦ C is shown in Fig. 6(b). In the cooling cycle, the forward transformation into the martensite phase started at a temperature of 36.1 ◦ C and was completed at 14.8 ◦ C. In the reverse transformation on heating, austenite start and finish temperatures were determined as 33.2 ◦ C (As ) and 63 ◦ C (Af ), respectively. It can be obviously seen that there
are not two-peak transitions in the thermal transformation behavior of the NiTi thin film. This phenomenon is probably due to the coherency strains associated with the Ni4 Ti3 precipitate formation within NiTi grains. It is well known that coherency strain fields around precipitates can spread the transformation over a range of temperatures or cause multiple-stage transformations [20,21]. Although the initial Ni4 Ti3 precipitates are coherent with the NiTi matrix, increasing the sputtering temperature to up to 525 ◦ C causes the precipitates to grow in size and to lose their coherency. Consequently, the inhomogeneity of the stress distribution between these precipitates and the surrounding NiTi matrix is removed. This, in turn, can prevent two-stage transformations from martensite to austenite. The formation of larger Ni4 Ti3 precipitates is also in agreement with the XRD pattern of this film in Fig. 4(b). The existence of a small transformation temperature hysteresis for the films deposited at 425 ◦ C and 525 ◦ C is an excellent property for applications in microelectromechanical systems (MEMS). It is particularly beneficial when a short response time is required for a fast cyclic microactuation. However, it will be very interesting to complement the present results with further investigations concerning a mechanical and tribological analysis of these thin films. Such an investigation is in progress by the authors of the present work to clarify the effect of the sputtering temperature on mechanical as well as tribological properties of NiTi thin films.
4. Conclusion In this study, nearly equiatomic NiTi thin films were deposited by means of magnetron sputtering of a Ti-rich NiTi alloy. The effect of the sputtering temperature (temperature inside the coating chamber) on the microstructure and shape memory behavior of NiTi films was investigated. The thickness of the deposited films was about 3 m and shows a variation of 12–20% upon changing the sputtering temperature. In addition, the following points are highlighted:
1. Employing a Ti-rich NiTi target can compensate the discrepancy in the angular flux distribution of Ni and Ti atoms from cold and hot targets. 2. NiTi thin films deposited for 180 min at various temperatures possess different microstructures. The films deposited at 80 ◦ C are fully amorphous, while films deposited at 305 ◦ C are partially
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crystallized. A deposition at a sputtering temperature above 425◦ leads to a fully crystallized formation of NiTi thin films. 3. NiTi thin films deposited at 425 ◦ C show two-stage transformation behavior upon heating due to the formation of a R-phase or Ni4 Ti3 precipitates, while the films deposited at 525 ◦ C show one-stage transformation upon heating and cooling. 4. NiTi thin films deposited at 425 ◦ C and 525 ◦ C possess very small transformation temperature hysteresis which can enable them to show fast frequency response. References [1] K. Otsuka, T.K. Sawamura, Shimizu crystal structure and internal defects of equiatomic TiNi martensite, Phys. Sat. Sol. A 5 (1971) 457–470. [2] D.Y. Li, X.F. Wu, T. Ko, The effect of stress on soft modes for phase transition in Ti–Ni alloy, Philos. Mag. A 63 (1991) 585–603. [3] J.J. Gill, D.T. Chang, L.A. Momoda, G.P. Carman, Manufacturing issues of thin film NiTi micro wrapper, Sens. Actuators. A 93 (2001) 148–156. [4] M. Nishida, T. Honma, Electron microscopy studies of the all-round shape memory effect in a Ti-51.0 atm% Ni alloy, Scr. Mater. 18 (1984) 1293–1296. [5] D.S. Grummon, S. Nam, L. Chang, Effect of super elastically deforming NiTi surface microalloys on fatigue crack nucleation in copper, Proc. Mater. Res. Soc. 246 (1992) 259–264. [6] Li. Hou, D.S. Grummon, Transformational superelasticity in sputtered titanium–nickel thin films, Scr. Mater. 33 (1995) 989–995. [7] A. Ishida, V. Martiniv, Sputter-deposited shape-memory alloy thin films: properties and applications, MRS Bull. 27 (2) (2002) 111–114. [8] J.J. Kim, P. Moine, D.A. Stevenson, Crystallization behavior of amorphous NiTi alloys prepared by sputter deposition, Scr. Metall. 20 (1986) 243–248. [9] P. Surbled, C. Clerc, B.L. Piofle, M. Ataka, F. Fujita, Effect of composition and thermal annealing on the transformation temperatures of sputtered TiNi shape memory alloy thin films, Thin Solid Films 401 (2001) 52–59. [10] G. Satoh, A. Birnbaum, Y.L. Yao, Effect of annealing parameters on the shape memory properties of NiTi thin films, in: ICALEO 2008 Congress Proceedings, Poster presentation gallery, 100–167. [11] Y.Q. Fu, H.J. Du, Effects of film composition and annealing on residual stress evolution for shape memory TiNi film, Mater. Sci. Eng. A 342 (2003) 236. [12] W. Tillmann, S. Momeni, Deposition of superelastic composite NiTi based films, Vacuum 104 (2014) 41–46. [13] K. Gisser, G.D. Busch, A.D. Johnson, A.B. Ellis, Oriented nickel–titanium shape memory alloy films prepared by annealing during deposition, Appl. Phys. Lett. 61 (14) (1992). [14] A. Kumar, S.K. Sharma, S. Bysakh, S.V. Kamat, S. Mohan, Effect of substrate and annealing temperatures on mechanical properties of Ti-rich NiTi films, J. Mater. Sci. Technol. 26 (11) (2010) 961–966. [15] K. Ho, K.K. Mohanchandra, G.P. Garmen, Examination of the sputtering profile of NiTi under target heating conditions, Thin Solid Films 413 (1–2) (2002) 1–7.
[16] L. Zhang, C. Xie, J.W., Effect of annealing temperature on surface morphology and mechanical properties of sputter-deposited Ti–Ni thin films, J. Alloys Compd. 424 (2007) 238–243. [17] K.K. Ho, G.P. Carman, Sputter deposition of NiTi thin film shape memory alloy using a heated target, Thin Solid Films 370 (2000) 18–29. [18] S. Miyazaki, A. Ishida, Martensitic transformation and shape memory behavior in sputter-deposited TiNi-base thin films, Mater. Sci. Eng. A 273–275 (1999) 106–133. [19] G. Fan, W. Chen, S. Yang, J. Zhu, X. Ren, K. Otsuka, Origin of abnormal multistage martensitic transformation behavior in aged Ni-rich Ti–Ni shape memory alloys, Acta Mater. 52 (2004) 4351. [20] W. Tiry, D. Schryvers, Quantitative determination of strain fields around Ni4 Ti3 precipitates in NiTi, Acta Mater. 53 (4) (2005) 1041–1049. [21] D. Schryvers, W. Tirry, Z.Q. Yang, Measuring strain fields and concentration gradients around Ni4 Ti3 precipitates, Mater. Sci. Eng. A 438–440 (2006) 485–488.
Biographies
Soroush Momeni received his B.Sc. in Textile Engineering (Textile Chemistry and Fiber Science) from the Azad University of Tehran, Iran. He gained his M.Sc. in Advanced Materials (Nanomaterials) in February 2012 from the University of Ulm, Germany. Since April 2012, he is pursuing his PhD degree under the direction of Professor Wolfgang Tillmann at the Institute of Materials Engineering, Technical University of Dortmund, Germany. His research interest currently focuses on magnetron sputtering physical vapor deposition techniques, binary and ternary NiTi shape memory alloy thin films, self-healing coating systems, antibacterial magnetron sputtered coatings, wear resistance hard coatings as well as cavitation erosion resistance coatings. Wolfgang Tillmann got his PhD degree in 1992 in the field of joining engineering ceramics from the Material Science Institute (MSI), Aachen University of Technology, Germany where he was a chief engineer subsequently after his graduation until 1996. After that he started working as the head of Materials and Mechanics department at technical center Hilti Corp, Principality of Liechtenstein from 1997 to 2000. From 2001 to 2002, he was a managing director of the business unit diamond tools at Hilti Germany ltd. Since November 2002, he is a full Professor at the Institute of Materials Engineering, Technical University of Dortmund. His research interests include brazing technology, powder metallurgy, thermal Spraying, and magnetron sputtering physical vapor deposition techniques.