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Si3N4 nano-multilayers synthesized by reactive magnetron sputtering
Journal of Alloys and Compounds 481 (2009) 710–713
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Microstructure and mechanical properties of TiAlN/Si3 N4 nano-multilayers synthesized by reactive magnetron sputtering Jianling Yue a,∗ , Geyang Li b a b
Institute of Marine Materials Science and Engineering, Shanghai Maritime University, Shanghai 201306, China State Key Laboratory of Metal Matrix Composites, Shanghai Jiaotong University, Shanghai 200030, China
a r t i c l e
i n f o
Article history: Received 3 November 2008 Received in revised form 16 March 2009 Accepted 17 March 2009 Available online 25 March 2009 Keywords: Thin films Crystal growth Microstructure Vapour deposition
a b s t r a c t TiAlN/Si3 N4 nano-multilayers with various Si3 N4 layer thicknesses were synthesized by reactive magnetron sputtering. The composition, microstructure, and mechanical properties of the films were studied by energy dispersive X-ray spectroscopy, X-ray diffraction, scanning electron microscopy, and nanoindentation. It reveals that under the template effect of TiAlN layers in multilayers, as-deposited amorphous Si3 N4 is crystallized and grows coherently with TiAlN layers when Si3 N4 layer thickness is below 0.9 nm. Correspondingly, the hardness and elastic modulus of the multilayers increase abnormally and reach the maximum values of 52 and 588 GPa, respectively. With further increase in the layer thickness, Si3 N4 transforms into amorphous and blocks the coherent growth of multilayers, resulting in a decrease of hardness and modulus. The remarkable hardness enhancement of multilayers is correlated to the crystallization of Si3 N4 , and the coherent strain in multilayers that could change the modulus of two constituents. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Hard films have been widely applied to the protection of cutting tools, molds and mechanical components. It was found in 1987 that multilayers deposited alternately by two kinds of materials at nanometer-scale thickness could exhibit an anomalous increase of hardness as compared with the individual single layer films [1]. It supplied an alternative way to obtain the film with high hardness. In particular, since the mechanical properties of the nano-multilayers can be enhanced by the microstructure design, their strengthening mechanism is worthy of further study. TiAlN films are attracting more attentions owing to their superior hardness and oxidation resistance compared with the conventional binary transition-metal nitride films, such as TiN, ZrN or CrN, etc. [2–4]. Silicon nitride films are useful for structural applications due to their attractive properties such as high hardness and chemical inertness. It was reported that the films consisting of Si3 N4 could present a higher oxidation resistance, since the compact silicon oxide formed on the surface of films can prevent the inward diffusion of oxygen at high temperatures [5]. Therefore, many kinds of nano-multilayer systems consisting of Si3 N4 have been studied [6–11]. Among them, the hardness and anti-oxidation temperature of TiN/Si3 N4 nano-multilayers are significantly increased to 35–45 GPa and 1000 ◦ C, respectively, which are much higher than
∗ Corresponding author. Tel.: +86 21 38284812; fax: +86 21 54742268. E-mail address: [email protected] (J. Yue). 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.03.103
22 GPa and 600 ◦ C of the TiN monolithic film [6,7]. ZrN/Si3 N4 nanomultilayers can present the hardness of 33 GPa compared with 24 GPa of ZrN [8]. However, the significant hardness increase has not been obtained in the nano-multilayer systems of Cr2 N/Si3 N4 [9], HfN/Si3 N4 [10] and NbN/Si3 N4 [11]. This indicates that the crystal growth and the strengthening mechanism existing in these nanomultilayer systems still need investigation. In this study, the TiAlN/Si3 N4 nano-multilayers with various Si3 N4 layer thicknesses have been synthesized by reactive magnetron sputtering. The crystallization of silicon nitride and its effects on the crystal growth and mechanical properties of the TiAlN/Si3 N4 nano-multilayers were investigated. 2. Experimental details TiAlN/Si3 N4 nano-multilayers and TiAlN, Si3 N4 monolithic films were prepared by an ANELVA SPC-350 magnetron sputtering system. A Si (99.99%) target and a TiAl (99.99%) composite target with a diameter of 75 mm were controlled by two RF cathodes, respectively. The composition of TiAl target was approximately 50:50. Mirror polished stainless steel substrates were ultrasonically cleaned in acetone and absolute alcohol before being mounted on a rotatable substrate holder in the vacuum chamber. The distance between the substrate and cathode was 50 mm. Before deposition, the system was evacuated to a base pressure of 4 × 10−4 Pa, and then sputtering gas Ar (99.999%) and reactive gas N2 (99.999%), at pressures of 2.4 × 10−1 and 0.8 × 10−1 Pa, were introduced. Individual layer thickness of multilayers was controlled by the target power and the resident time that the substrates were exposed to the target. In this study, the thickness of individual TiAlN layer (lTiAlN ) is 4 nm for all multilayers, while Si3 N4 layer thickness (lSi3 N4 ) is in the range of 0.3–1.8 nm. The TiAlN and Si3 N4 monolithic film were deposited under the same conditions as that of TiAlN/Si3 N4 multilayers. No additional bias voltage or heating was applied to the substrate during the deposition. The overall thickness of each specimen was about 2 m.
J. Yue, G. Li / Journal of Alloys and Compounds 481 (2009) 710–713
Fig. 1. Low-angle XRD patterns of TiAlN/Si3 N4 multilayers with various Si3 N4 layer thickness.
The chemical composition of the TiAlN and Si3 N4 monolithic film was determined by energy dispersive X-ray spectroscopy (EDS) using an EDAX DX-4 energy dispersive analyzer. The microstructure of all films was characterized by a BRUKER AXS D8X X-ray diffractometer (XRD) using Cu K␣ radiation and a Philips XL30 FEG scanning electron microscope (SEM). The hardness and modulus of the films were measured by using an MTS Nanoindenter XP equipped with a Berkovich indenter.
3. Experimental results EDS shows that both monolithic TiAlN and Si3 N4 films are stoichiometric. This indicates the multilayers deposited under the same conditions are composed of TiAlN and Si3 N4 layers. Fig. 1 shows the low-angle XRD patterns of the TiAlN/Si3 N4 nano-multilayers with various Si3 N4 layer thicknesses. Low-angle reflections are clearly observed for all multilayers, indicating that the multilayers have the composition modulated structure. The modulation period of the multilayers can be calculated using the modified Bragg formula [12]. Since lTiAlN is kept unchanged for all the samples, their lTiAlN and lSi3 N4 can be evaluated, as captioned in Fig. 1. XRD pattern indicates the amorphous character of the Si3 N4 monolithic film (not shown here). Fig. 2 shows the XRD patterns of the TiAlN monolithic film and TiAlN/Si3 N4 multilayers with different Si3 N4 layer thicknesses. The monolithic TiAlN film presents
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a B1-NaCl structure with (2 0 0) preferred orientation. For multilayers with low Si3 N4 layer thickness (lSi3 N4 = 0.3 nm), the intensity of TiAlN (2 0 0) peak significantly increases, accompanied by the emergence of TiAlN (1 1 1) peak. At larger lSi3 N4 , however, the intensity of the (2 0 0) peak of multilayers gradually decreases, and its broadening is observed at lSi3 N4 of 0.9 nm. When lSi3 N4 is further increased to 2.2 nm, the multilayer exhibits the amorphous character. With the combination of the studies from other researchers [7,8,13], it may be deduced that Si3 N4 layers are likely to be crystallized and grow coherently with TiAlN layers when lSi3 N4 is below 0.9 nm. Exceeding this critical thickness (0.9 nm), however, Si3 N4 layers transform into an amorphous structure, which impedes the coherent growth of the multilayers. Compared with the TiAlN monolithic film, the TiAlN/Si3 N4 multilayer with lSi3 N4 of 0.3 nm shows a shift towards lower angle in the (2 0 0) diffraction peak. This shift corresponds to the change in the associated (2 0 0) interplanar distances. The (2 0 0) interplanar distance of multilayers increases with lSi3 N4 and reaches a maximum value when lSi3 N4 is 0.3 nm. Then the further increase of lSi3 N4 leads to the reversion of interplanar distance. This phenomenon indicates that when the Si3 N4 layers grow coherently with the TiAlN layers, TiAlN is subjected to compressive strain in the (2 0 0) plane, while Si3 N4 is under tensile strain. The difference is ascribed to the lattice mismatch between the two layers. Thus an alternating-stress field forms in multilayers. The largest tensile strain along the [2 0 0] direction in TiAlN layers was calculated to be 0.5% at lSi3 N4 of 0.3 nm. Owing to a Poisson effect, a corresponding compressive strain of ∼2% is induced in the (2 0 0) plane of TiAlN layers. The cross-sectional SEM images of TiAlN monolithic film, TiAlN (4 nm)/Si3 N4 (0.3 nm) and TiAlN (4 nm)/Si3 N4 (1.8 nm) multilayers are shown in Fig. 3, respectively. The TiAlN monolithic film shows a columnar structure (Fig. 3a), and the multilayer with lSi3 N4 of 0.3 nm (Fig. 3b) also presents the columnar structure, which indicates the insertion of Si3 N4 does not block the crystal growth of TiAlN layers in multilayers. For the multilayer with larger lSi3 N4 (Fig. 3c), however, the fracture is fairly smooth, exhibiting the nanocrystal or amorphous character. This suggests that Si3 N4 layers at this thickness (1.8 nm) transform into amorphous and the coherent structure of the multilayer is hence damaged. Fig. 4 shows the dependence of hardness (HV ) and elastic modulus (E) of the multilayers on the thickness of Si3 N4 layers. The HV and E of corresponding TiAlN and Si3 N4 monolithic films deposited under the same conditions are also labeled in Fig. 4. At the initial increase of lSi3 N4 , both HV and E of the multilayers increase sharply and reach a maximum value of 52 and 588 GPa at lSi3 N4 of 0.3 nm, respectively. The HV and E rapidly decrease by further increase of lSi3 N4 , and finally close to that of the TiAlN monolithic film when lSi3 N4 is 1.8 nm. It is obvious that there is a close correlation between the hardness enhancement and the coherent structure of the multilayers, reflected by the XRD results.
4. Discussion 4.1. The growth of the multilayers
Fig. 2. XRD patterns of TiAlN monolithic film and TiAlN/Si3 N4 multilayers with various Si3 N4 layer thickness.
The XRD and SEM results clearly reveal that under the template effect of B1-NaCl TiAlN layers, as-deposited amorphous Si3 N4 below a critical layer thickness (∼0.9 nm) can be crystallized and grow coherently with the TiAlN layers in multilayers. Similar phenomena have also been observed in some nano-multilayer systems such as TiN/Si3 N4 [7], ZrN/Si3 N4 [8], and VN/AlON [13], etc. Such crystallization of amorphous Si3 N4 can be understood by using a simple thermodynamic energy-balance model involving strainfree bulk energy, coherency strain energy, and interfacial energy [14].
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J. Yue, G. Li / Journal of Alloys and Compounds 481 (2009) 710–713
Fig. 3. Cross-sectional SEM images of: (a) TiAlN monolithic film, (b) TiAlN (4 nm)/Si3 N4 (0.3 nm) and (c) TiAlN (4 nm)/Si3 N4 (1.8 nm) multilayers.
In addition, all the multilayers exhibit a better performance in crystal perfection when their Si3 N4 layers are crystallized. Similar phenomena have been observed in some other material systems as well [7,8,13]. It is well known that the diffusion of adatoms on the growth front is by far the most important kinetic process in film growth [15]. If there is sufficient diffusion mobility, more adatoms will overcome the potential-energy barrier and hop from metastable sites to steady sites, and therefore it is more possible to form perfect crystal. In TiAlN/Si3 N4 nano-multilayers, the diffusion mobility of TiAlN particles deposited on the surface of crystalline Si3 N4 layers is higher than that on the homogeneous material, TiAlN; therefore the crystal perfection is markedly improved for multilayers.
Si3 N4 [18]. R = GB − GA /GB + GA , GA and GB are shear modulus of two layers, respectively, and GB > GA , G = E/2(1 + ) (E is the elastic modulus and is the Poisson’s ratio). Using the data obtained from the monolithic films (ETiAlN = 405 GPa, ESi3 N4 = 225 GPa), the Poisson’s ratio is 0.25 for TiAlN and Si3 N4 [18], and is 45◦ for multilayers, the shear modulus can be calculated, yielding GTiAlN = 162 GPa and GSi = 90 GPa. From these values, one should expect a hardness increment of 7.2 GPa compared with Si3 N4 monolithic film, which is much lower than the measured value of about 34 GPa.
4.2. The strengthening of the multilayers So far, two main theories, Koehler’s modulus-difference strengthening theory [16] and the alternating-stress strengthening theory [17] have been proposed to explain the superhardness effects of nano-multilayers. Both of these theories are based on the model of dislocations being blocked at interfaces. According to Koehler’s modulus-difference strengthening theory [16], the maximum hardness increase of multilayers compared with lower hardness of Si3 N4 could be calculated as Eq. (1): Hmax =
3RGA sin 8m
(1)
where is the smallest angle between the interface and the glide plane of low modulus layer, m, the Taylor factor, is 0.3 for TiAlN and
Fig. 4. Variation of hardness and elastic modulus of multilayers with Si3 N4 layer thicknesses.
J. Yue, G. Li / Journal of Alloys and Compounds 481 (2009) 710–713
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Obviously, the hardness enhancement of TiAlN/Si3 N4 multilayers could not be merely explained by Koehler’s strengthening theory. In reference to the alternating-stress field strengthening theory [17], it is known that when the dislocations go across the interfaces in multilayers, they are not only resisted by the potential barriers coming from the change of line energy of dislocations due to two different modulus layers (Koehler’s strengthening theory), but also by that coming from the alternating-stress field. Thus the maximum hardness increase from the alternating-stress field is
From the present TiAlN/Si3 N4 multilayers, it can be deduced that the formation of coherent interfaces is one of the important characteristics for multilayers with high hardness enhancement. On the other hand, since the coherent strains can change the modulus of two constituents of multilayers, they are the essential conditions for the enforcement of multilayers as well. This means that when the coherent strains increase the modulus difference between the two constituents of multilayers, the hardness enhancement of multilayers can be further increased as compared with the strain-free multilayers.
H ∼ = 10max .
5. Conclusion
(2) √ 6 AE is the maximum shear stress on the interfaces, 6
where max = A is the modulation amplifying factor that is closely correlated to structural parameters of multilayers such as modulation period, modulation ratio, and roughness and width of interfaces. E is the weighted average elastic modulus of two constituents of multilayers, and is the lattice mismatch between two layers of multilayers. According to the studies from Mirkarimi et al. [19,20], A takes the value of 0.5 for calculation in this study. E, the weighted average value of the elastic modulus of TiAlN and Si3 N4 , is 315 GPa. Because of pseudocrystal structure of Si3 N4 , it is difficult to calculate the lattice mismatch between two layers. If it is presumed that the lattice mismatch is between 2% and 3%, the hardness enhancement is about 12.9–19.3 GPa according to Eq. (2). Here, it can be found that even the sum of the hardness increase from both the alternating-stress field model and modulus-difference model, 20.1–26.5 GPa, is still lower than the measured value (34 GPa). The main reason is that both Koehler’s theory and alternating-stress field strengthening theory do not take into account any changes in the modulus of each individual layer under coherent strains in multilayers. Some studies [21,22] indicate that an applied elastic strain can significantly alter the in-plane biaxial modulus of thin films, that is, the biaxial modulus increases with increasing compressive strain, and reduces with tensile strain. Thus, under the coherent strain in the present TiAlN/Si3 N4 nano-multilayers, the biaxial modulus in the (2 0 0) plane of TiAlN layers with relatively higher modulus should increase, while the corresponding biaxial modulus of Si3 N4 layers should decrease. This leads to the fact that the modulus difference of two constitutional layers, TiAlN and Si3 N4 , increase under the alternating-stress fields in multilayers. Therefore, the hardness enhancement in TiAlN/Si3 N4 multilayers should increase according to Koehler’s strengthening theory [16] as well as the alternating-stress field theory [17]. In addition, the calculations from Jankowski et al. [21,22] reveal that 3% compressive strain can lead to a 100% increase in biaxial modulus of film materials, while 3% tensile strain can cause biaxial modulus decrease by ∼50%. If it’s presumed that the modulus of TiAlN layers increases by 50% under ∼2% compressive strain in this study, and the modulus of Si3 N4 decrease by 25% (actually it is difficult to measure the exact tensile strain in Si3 N4 layers due to the pseudocrystal structure), we can accordingly obtain a hardness increment of 26.6–34.5 GPa from Koehler’s theory and alternating-stress field theory, which is in agreement with the measurements.
Under the template effect of TiAlN layers in the TiAlN/Si3 N4 nano-multilayers synthesized by reactive magnetron sputtering, as-deposited amorphous Si3 N4 is crystallized when its layer thickness is lower than 0.9 nm, and grows coherently with TiAlN layers. Correspondingly, the hardness and modulus of multilayers abnormally increase and reach the maximum value of 52 and 588 GPa, respectively. Further increasing the layer thickness, Si3 N4 gradually transforms into amorphous and blocks the coherent growth between two layers, resulting in a decrease of hardness. The significant enhancement of hardness is correlated to the crystallization of Si3 N4 and the coherent strains in multilayers that could increase the modulus difference between two constituents. Acknowledgments The authors gratefully acknowledge financial support from the Science & Technology Program of Shanghai Maritime University and National Natural Chinese Foundation of China, under Grant No. U0774001. References [1] U. Helmersson, S. Todorova, S.A. Barnett, J. Appl. Phys. 62 (1987) 48l–485. [2] J.T. Chen, J. Wang, F. Zhang, G.A. Zhang, X.Y. Fan, et al., J. Alloys Compd. 472 (2009) 91–96. [3] E.K. Tentardini, C. Kwietniewski, F. Perini, E. Blando, R. Hübler, et al., Surf. Coat. Technol. 203 (2009) 1176–1181. [4] X.Z. Ding, A.L.K. Tan, X.T. Zeng, C. Wang, T. Yue, C.Q. Sun, Thin Solid Films 516 (2008) 5716–5720. [5] S. Vepˇrek, S. Reiprich, L. Shizhi, Appl. Phys. Lett. 66 (1995) 2640–2642. [6] L. Hultman, J. Bareno, A. Flink, H. Soderberg, K. Larsson, et al., Phys. Rev. B 75 (2007) 5437–5440. [7] M. Kong, W.J. Zhao, L. Wei, G.Y. Li, J. Phys. D: Appl. Phys. 40 (2007) 2853–2858. [8] Y.S. Dong, W.J. Zhao, J.L. Yue, G.Y. Li, Appl. Phys. Lett. 89 (2006) 121916. [9] J.H. Xu, K. Hattori, Y. Seino, I. Kojima, Thin Solid Films 414 (2002) 239–245. [10] J.J. Jeong, S.K. Hwang, C.M. Lee, Mater. Chem. Phys. 77 (2002) 27–33. [11] J.J. Jeong, C.M. Lee, Appl. Surf. Sci. 214 (2003) 11–16. [12] C. Kim, S.B. Qadri, M.R. Scanlon, R.C. Cammarata, Thin Solid Films 240 (1994) 52–55. [13] J.L. Yue, Y. Liu, W.J. Zhao, G.Y. Li, Scripta Mater. 55 (2006) 895–898. [14] A. Madan, I.W. Kim, S.C. Cheng, P. Yashar, Phys. Rev. Lett. 78 (1997) 1743–1746. [15] Y. Zhang, M.G. Lagally, Science 276 (1997) 377–379. [16] J.S. Koehler, Phys. Rev. B 2 (1970) 547–551. [17] M. Kato, T. Mori, L.H. Schwartz, Acta Metall. 28 (1980) 285–291. [18] X. Chu, S.A. Barnett, J. Appl. Phys. 77 (1995) 4403–4407. [19] P.B. Mirkarimi, S.A. Barnett, K.M. Hubbard, T.R. Jervis, L. Hultman, J. Mater. Res. 9 (1994) 1456–1463. [20] M. Shinn, S.A. Barnett, Appl. Phys. Lett. 64 (1994) 61–63. [21] A.F. Jankowski, T. Tsakalakos, J. Phys. F: Met. Phys. 15 (1985) 1279–1292. [22] R.C. Cammarata, K. Sieradzki, Phys. Rev. Lett. 62 (1989) 2005–2008.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Microstructure and mechanical properties of TiAlN/Si3 N4 nano-multilayers synthesized by reactive magnetron sputtering Jianling Yue a,∗ , Geyang Li b a b
Institute of Marine Materials Science and Engineering, Shanghai Maritime University, Shanghai 201306, China State Key Laboratory of Metal Matrix Composites, Shanghai Jiaotong University, Shanghai 200030, China
a r t i c l e
i n f o
Article history: Received 3 November 2008 Received in revised form 16 March 2009 Accepted 17 March 2009 Available online 25 March 2009 Keywords: Thin films Crystal growth Microstructure Vapour deposition
a b s t r a c t TiAlN/Si3 N4 nano-multilayers with various Si3 N4 layer thicknesses were synthesized by reactive magnetron sputtering. The composition, microstructure, and mechanical properties of the films were studied by energy dispersive X-ray spectroscopy, X-ray diffraction, scanning electron microscopy, and nanoindentation. It reveals that under the template effect of TiAlN layers in multilayers, as-deposited amorphous Si3 N4 is crystallized and grows coherently with TiAlN layers when Si3 N4 layer thickness is below 0.9 nm. Correspondingly, the hardness and elastic modulus of the multilayers increase abnormally and reach the maximum values of 52 and 588 GPa, respectively. With further increase in the layer thickness, Si3 N4 transforms into amorphous and blocks the coherent growth of multilayers, resulting in a decrease of hardness and modulus. The remarkable hardness enhancement of multilayers is correlated to the crystallization of Si3 N4 , and the coherent strain in multilayers that could change the modulus of two constituents. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Hard films have been widely applied to the protection of cutting tools, molds and mechanical components. It was found in 1987 that multilayers deposited alternately by two kinds of materials at nanometer-scale thickness could exhibit an anomalous increase of hardness as compared with the individual single layer films [1]. It supplied an alternative way to obtain the film with high hardness. In particular, since the mechanical properties of the nano-multilayers can be enhanced by the microstructure design, their strengthening mechanism is worthy of further study. TiAlN films are attracting more attentions owing to their superior hardness and oxidation resistance compared with the conventional binary transition-metal nitride films, such as TiN, ZrN or CrN, etc. [2–4]. Silicon nitride films are useful for structural applications due to their attractive properties such as high hardness and chemical inertness. It was reported that the films consisting of Si3 N4 could present a higher oxidation resistance, since the compact silicon oxide formed on the surface of films can prevent the inward diffusion of oxygen at high temperatures [5]. Therefore, many kinds of nano-multilayer systems consisting of Si3 N4 have been studied [6–11]. Among them, the hardness and anti-oxidation temperature of TiN/Si3 N4 nano-multilayers are significantly increased to 35–45 GPa and 1000 ◦ C, respectively, which are much higher than
∗ Corresponding author. Tel.: +86 21 38284812; fax: +86 21 54742268. E-mail address: [email protected] (J. Yue). 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.03.103
22 GPa and 600 ◦ C of the TiN monolithic film [6,7]. ZrN/Si3 N4 nanomultilayers can present the hardness of 33 GPa compared with 24 GPa of ZrN [8]. However, the significant hardness increase has not been obtained in the nano-multilayer systems of Cr2 N/Si3 N4 [9], HfN/Si3 N4 [10] and NbN/Si3 N4 [11]. This indicates that the crystal growth and the strengthening mechanism existing in these nanomultilayer systems still need investigation. In this study, the TiAlN/Si3 N4 nano-multilayers with various Si3 N4 layer thicknesses have been synthesized by reactive magnetron sputtering. The crystallization of silicon nitride and its effects on the crystal growth and mechanical properties of the TiAlN/Si3 N4 nano-multilayers were investigated. 2. Experimental details TiAlN/Si3 N4 nano-multilayers and TiAlN, Si3 N4 monolithic films were prepared by an ANELVA SPC-350 magnetron sputtering system. A Si (99.99%) target and a TiAl (99.99%) composite target with a diameter of 75 mm were controlled by two RF cathodes, respectively. The composition of TiAl target was approximately 50:50. Mirror polished stainless steel substrates were ultrasonically cleaned in acetone and absolute alcohol before being mounted on a rotatable substrate holder in the vacuum chamber. The distance between the substrate and cathode was 50 mm. Before deposition, the system was evacuated to a base pressure of 4 × 10−4 Pa, and then sputtering gas Ar (99.999%) and reactive gas N2 (99.999%), at pressures of 2.4 × 10−1 and 0.8 × 10−1 Pa, were introduced. Individual layer thickness of multilayers was controlled by the target power and the resident time that the substrates were exposed to the target. In this study, the thickness of individual TiAlN layer (lTiAlN ) is 4 nm for all multilayers, while Si3 N4 layer thickness (lSi3 N4 ) is in the range of 0.3–1.8 nm. The TiAlN and Si3 N4 monolithic film were deposited under the same conditions as that of TiAlN/Si3 N4 multilayers. No additional bias voltage or heating was applied to the substrate during the deposition. The overall thickness of each specimen was about 2 m.
J. Yue, G. Li / Journal of Alloys and Compounds 481 (2009) 710–713
Fig. 1. Low-angle XRD patterns of TiAlN/Si3 N4 multilayers with various Si3 N4 layer thickness.
The chemical composition of the TiAlN and Si3 N4 monolithic film was determined by energy dispersive X-ray spectroscopy (EDS) using an EDAX DX-4 energy dispersive analyzer. The microstructure of all films was characterized by a BRUKER AXS D8X X-ray diffractometer (XRD) using Cu K␣ radiation and a Philips XL30 FEG scanning electron microscope (SEM). The hardness and modulus of the films were measured by using an MTS Nanoindenter XP equipped with a Berkovich indenter.
3. Experimental results EDS shows that both monolithic TiAlN and Si3 N4 films are stoichiometric. This indicates the multilayers deposited under the same conditions are composed of TiAlN and Si3 N4 layers. Fig. 1 shows the low-angle XRD patterns of the TiAlN/Si3 N4 nano-multilayers with various Si3 N4 layer thicknesses. Low-angle reflections are clearly observed for all multilayers, indicating that the multilayers have the composition modulated structure. The modulation period of the multilayers can be calculated using the modified Bragg formula [12]. Since lTiAlN is kept unchanged for all the samples, their lTiAlN and lSi3 N4 can be evaluated, as captioned in Fig. 1. XRD pattern indicates the amorphous character of the Si3 N4 monolithic film (not shown here). Fig. 2 shows the XRD patterns of the TiAlN monolithic film and TiAlN/Si3 N4 multilayers with different Si3 N4 layer thicknesses. The monolithic TiAlN film presents
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a B1-NaCl structure with (2 0 0) preferred orientation. For multilayers with low Si3 N4 layer thickness (lSi3 N4 = 0.3 nm), the intensity of TiAlN (2 0 0) peak significantly increases, accompanied by the emergence of TiAlN (1 1 1) peak. At larger lSi3 N4 , however, the intensity of the (2 0 0) peak of multilayers gradually decreases, and its broadening is observed at lSi3 N4 of 0.9 nm. When lSi3 N4 is further increased to 2.2 nm, the multilayer exhibits the amorphous character. With the combination of the studies from other researchers [7,8,13], it may be deduced that Si3 N4 layers are likely to be crystallized and grow coherently with TiAlN layers when lSi3 N4 is below 0.9 nm. Exceeding this critical thickness (0.9 nm), however, Si3 N4 layers transform into an amorphous structure, which impedes the coherent growth of the multilayers. Compared with the TiAlN monolithic film, the TiAlN/Si3 N4 multilayer with lSi3 N4 of 0.3 nm shows a shift towards lower angle in the (2 0 0) diffraction peak. This shift corresponds to the change in the associated (2 0 0) interplanar distances. The (2 0 0) interplanar distance of multilayers increases with lSi3 N4 and reaches a maximum value when lSi3 N4 is 0.3 nm. Then the further increase of lSi3 N4 leads to the reversion of interplanar distance. This phenomenon indicates that when the Si3 N4 layers grow coherently with the TiAlN layers, TiAlN is subjected to compressive strain in the (2 0 0) plane, while Si3 N4 is under tensile strain. The difference is ascribed to the lattice mismatch between the two layers. Thus an alternating-stress field forms in multilayers. The largest tensile strain along the [2 0 0] direction in TiAlN layers was calculated to be 0.5% at lSi3 N4 of 0.3 nm. Owing to a Poisson effect, a corresponding compressive strain of ∼2% is induced in the (2 0 0) plane of TiAlN layers. The cross-sectional SEM images of TiAlN monolithic film, TiAlN (4 nm)/Si3 N4 (0.3 nm) and TiAlN (4 nm)/Si3 N4 (1.8 nm) multilayers are shown in Fig. 3, respectively. The TiAlN monolithic film shows a columnar structure (Fig. 3a), and the multilayer with lSi3 N4 of 0.3 nm (Fig. 3b) also presents the columnar structure, which indicates the insertion of Si3 N4 does not block the crystal growth of TiAlN layers in multilayers. For the multilayer with larger lSi3 N4 (Fig. 3c), however, the fracture is fairly smooth, exhibiting the nanocrystal or amorphous character. This suggests that Si3 N4 layers at this thickness (1.8 nm) transform into amorphous and the coherent structure of the multilayer is hence damaged. Fig. 4 shows the dependence of hardness (HV ) and elastic modulus (E) of the multilayers on the thickness of Si3 N4 layers. The HV and E of corresponding TiAlN and Si3 N4 monolithic films deposited under the same conditions are also labeled in Fig. 4. At the initial increase of lSi3 N4 , both HV and E of the multilayers increase sharply and reach a maximum value of 52 and 588 GPa at lSi3 N4 of 0.3 nm, respectively. The HV and E rapidly decrease by further increase of lSi3 N4 , and finally close to that of the TiAlN monolithic film when lSi3 N4 is 1.8 nm. It is obvious that there is a close correlation between the hardness enhancement and the coherent structure of the multilayers, reflected by the XRD results.
4. Discussion 4.1. The growth of the multilayers
Fig. 2. XRD patterns of TiAlN monolithic film and TiAlN/Si3 N4 multilayers with various Si3 N4 layer thickness.
The XRD and SEM results clearly reveal that under the template effect of B1-NaCl TiAlN layers, as-deposited amorphous Si3 N4 below a critical layer thickness (∼0.9 nm) can be crystallized and grow coherently with the TiAlN layers in multilayers. Similar phenomena have also been observed in some nano-multilayer systems such as TiN/Si3 N4 [7], ZrN/Si3 N4 [8], and VN/AlON [13], etc. Such crystallization of amorphous Si3 N4 can be understood by using a simple thermodynamic energy-balance model involving strainfree bulk energy, coherency strain energy, and interfacial energy [14].
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Fig. 3. Cross-sectional SEM images of: (a) TiAlN monolithic film, (b) TiAlN (4 nm)/Si3 N4 (0.3 nm) and (c) TiAlN (4 nm)/Si3 N4 (1.8 nm) multilayers.
In addition, all the multilayers exhibit a better performance in crystal perfection when their Si3 N4 layers are crystallized. Similar phenomena have been observed in some other material systems as well [7,8,13]. It is well known that the diffusion of adatoms on the growth front is by far the most important kinetic process in film growth [15]. If there is sufficient diffusion mobility, more adatoms will overcome the potential-energy barrier and hop from metastable sites to steady sites, and therefore it is more possible to form perfect crystal. In TiAlN/Si3 N4 nano-multilayers, the diffusion mobility of TiAlN particles deposited on the surface of crystalline Si3 N4 layers is higher than that on the homogeneous material, TiAlN; therefore the crystal perfection is markedly improved for multilayers.
Si3 N4 [18]. R = GB − GA /GB + GA , GA and GB are shear modulus of two layers, respectively, and GB > GA , G = E/2(1 + ) (E is the elastic modulus and is the Poisson’s ratio). Using the data obtained from the monolithic films (ETiAlN = 405 GPa, ESi3 N4 = 225 GPa), the Poisson’s ratio is 0.25 for TiAlN and Si3 N4 [18], and is 45◦ for multilayers, the shear modulus can be calculated, yielding GTiAlN = 162 GPa and GSi = 90 GPa. From these values, one should expect a hardness increment of 7.2 GPa compared with Si3 N4 monolithic film, which is much lower than the measured value of about 34 GPa.
4.2. The strengthening of the multilayers So far, two main theories, Koehler’s modulus-difference strengthening theory [16] and the alternating-stress strengthening theory [17] have been proposed to explain the superhardness effects of nano-multilayers. Both of these theories are based on the model of dislocations being blocked at interfaces. According to Koehler’s modulus-difference strengthening theory [16], the maximum hardness increase of multilayers compared with lower hardness of Si3 N4 could be calculated as Eq. (1): Hmax =
3RGA sin 8m
(1)
where is the smallest angle between the interface and the glide plane of low modulus layer, m, the Taylor factor, is 0.3 for TiAlN and
Fig. 4. Variation of hardness and elastic modulus of multilayers with Si3 N4 layer thicknesses.
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Obviously, the hardness enhancement of TiAlN/Si3 N4 multilayers could not be merely explained by Koehler’s strengthening theory. In reference to the alternating-stress field strengthening theory [17], it is known that when the dislocations go across the interfaces in multilayers, they are not only resisted by the potential barriers coming from the change of line energy of dislocations due to two different modulus layers (Koehler’s strengthening theory), but also by that coming from the alternating-stress field. Thus the maximum hardness increase from the alternating-stress field is
From the present TiAlN/Si3 N4 multilayers, it can be deduced that the formation of coherent interfaces is one of the important characteristics for multilayers with high hardness enhancement. On the other hand, since the coherent strains can change the modulus of two constituents of multilayers, they are the essential conditions for the enforcement of multilayers as well. This means that when the coherent strains increase the modulus difference between the two constituents of multilayers, the hardness enhancement of multilayers can be further increased as compared with the strain-free multilayers.
H ∼ = 10max .
5. Conclusion
(2) √ 6 AE is the maximum shear stress on the interfaces, 6
where max = A is the modulation amplifying factor that is closely correlated to structural parameters of multilayers such as modulation period, modulation ratio, and roughness and width of interfaces. E is the weighted average elastic modulus of two constituents of multilayers, and is the lattice mismatch between two layers of multilayers. According to the studies from Mirkarimi et al. [19,20], A takes the value of 0.5 for calculation in this study. E, the weighted average value of the elastic modulus of TiAlN and Si3 N4 , is 315 GPa. Because of pseudocrystal structure of Si3 N4 , it is difficult to calculate the lattice mismatch between two layers. If it is presumed that the lattice mismatch is between 2% and 3%, the hardness enhancement is about 12.9–19.3 GPa according to Eq. (2). Here, it can be found that even the sum of the hardness increase from both the alternating-stress field model and modulus-difference model, 20.1–26.5 GPa, is still lower than the measured value (34 GPa). The main reason is that both Koehler’s theory and alternating-stress field strengthening theory do not take into account any changes in the modulus of each individual layer under coherent strains in multilayers. Some studies [21,22] indicate that an applied elastic strain can significantly alter the in-plane biaxial modulus of thin films, that is, the biaxial modulus increases with increasing compressive strain, and reduces with tensile strain. Thus, under the coherent strain in the present TiAlN/Si3 N4 nano-multilayers, the biaxial modulus in the (2 0 0) plane of TiAlN layers with relatively higher modulus should increase, while the corresponding biaxial modulus of Si3 N4 layers should decrease. This leads to the fact that the modulus difference of two constitutional layers, TiAlN and Si3 N4 , increase under the alternating-stress fields in multilayers. Therefore, the hardness enhancement in TiAlN/Si3 N4 multilayers should increase according to Koehler’s strengthening theory [16] as well as the alternating-stress field theory [17]. In addition, the calculations from Jankowski et al. [21,22] reveal that 3% compressive strain can lead to a 100% increase in biaxial modulus of film materials, while 3% tensile strain can cause biaxial modulus decrease by ∼50%. If it’s presumed that the modulus of TiAlN layers increases by 50% under ∼2% compressive strain in this study, and the modulus of Si3 N4 decrease by 25% (actually it is difficult to measure the exact tensile strain in Si3 N4 layers due to the pseudocrystal structure), we can accordingly obtain a hardness increment of 26.6–34.5 GPa from Koehler’s theory and alternating-stress field theory, which is in agreement with the measurements.
Under the template effect of TiAlN layers in the TiAlN/Si3 N4 nano-multilayers synthesized by reactive magnetron sputtering, as-deposited amorphous Si3 N4 is crystallized when its layer thickness is lower than 0.9 nm, and grows coherently with TiAlN layers. Correspondingly, the hardness and modulus of multilayers abnormally increase and reach the maximum value of 52 and 588 GPa, respectively. Further increasing the layer thickness, Si3 N4 gradually transforms into amorphous and blocks the coherent growth between two layers, resulting in a decrease of hardness. The significant enhancement of hardness is correlated to the crystallization of Si3 N4 and the coherent strains in multilayers that could increase the modulus difference between two constituents. Acknowledgments The authors gratefully acknowledge financial support from the Science & Technology Program of Shanghai Maritime University and National Natural Chinese Foundation of China, under Grant No. U0774001. References [1] U. Helmersson, S. Todorova, S.A. Barnett, J. Appl. Phys. 62 (1987) 48l–485. [2] J.T. Chen, J. Wang, F. Zhang, G.A. Zhang, X.Y. Fan, et al., J. Alloys Compd. 472 (2009) 91–96. [3] E.K. Tentardini, C. Kwietniewski, F. Perini, E. Blando, R. Hübler, et al., Surf. Coat. Technol. 203 (2009) 1176–1181. [4] X.Z. Ding, A.L.K. Tan, X.T. Zeng, C. Wang, T. Yue, C.Q. Sun, Thin Solid Films 516 (2008) 5716–5720. [5] S. Vepˇrek, S. Reiprich, L. Shizhi, Appl. Phys. Lett. 66 (1995) 2640–2642. [6] L. Hultman, J. Bareno, A. Flink, H. Soderberg, K. Larsson, et al., Phys. Rev. B 75 (2007) 5437–5440. [7] M. Kong, W.J. Zhao, L. Wei, G.Y. Li, J. Phys. D: Appl. Phys. 40 (2007) 2853–2858. [8] Y.S. Dong, W.J. Zhao, J.L. Yue, G.Y. Li, Appl. Phys. Lett. 89 (2006) 121916. [9] J.H. Xu, K. Hattori, Y. Seino, I. Kojima, Thin Solid Films 414 (2002) 239–245. [10] J.J. Jeong, S.K. Hwang, C.M. Lee, Mater. Chem. Phys. 77 (2002) 27–33. [11] J.J. Jeong, C.M. Lee, Appl. Surf. Sci. 214 (2003) 11–16. [12] C. Kim, S.B. Qadri, M.R. Scanlon, R.C. Cammarata, Thin Solid Films 240 (1994) 52–55. [13] J.L. Yue, Y. Liu, W.J. Zhao, G.Y. Li, Scripta Mater. 55 (2006) 895–898. [14] A. Madan, I.W. Kim, S.C. Cheng, P. Yashar, Phys. Rev. Lett. 78 (1997) 1743–1746. [15] Y. Zhang, M.G. Lagally, Science 276 (1997) 377–379. [16] J.S. Koehler, Phys. Rev. B 2 (1970) 547–551. [17] M. Kato, T. Mori, L.H. Schwartz, Acta Metall. 28 (1980) 285–291. [18] X. Chu, S.A. Barnett, J. Appl. Phys. 77 (1995) 4403–4407. [19] P.B. Mirkarimi, S.A. Barnett, K.M. Hubbard, T.R. Jervis, L. Hultman, J. Mater. Res. 9 (1994) 1456–1463. [20] M. Shinn, S.A. Barnett, Appl. Phys. Lett. 64 (1994) 61–63. [21] A.F. Jankowski, T. Tsakalakos, J. Phys. F: Met. Phys. 15 (1985) 1279–1292. [22] R.C. Cammarata, K. Sieradzki, Phys. Rev. Lett. 62 (1989) 2005–2008.