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Coalescence behaviors of telephone cord buckles in SiAlNx films
Surface & Coatings Technology 232 (2013) 884–890
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Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
Coalescence behaviors of telephone cord buckles in SiAlNx films Sen-Jiang Yu a,⁎, Yuan-Chi Shi a, Miao-Gen Chen a, Ping-Zhan Si a, Yun Zhou a, Xiao-Fei Zhang a, Jun Chen b, Hong Zhou a, Zhi-Wei Jiao a a b
Department of Physics, China Jiliang University, Hangzhou 310018, P.R. China Research and Development Department, Zhejiang Zhongli Energy Efficient Glass Manufacturing Co., Ltd, Hangzhou 311228, P.R. China
a r t i c l e
i n f o
Article history: Received 28 April 2013 Accepted in revised form 26 June 2013 Available online 4 July 2013 Keywords: Telephone cord buckle Thin film Coalescence Compressive stress
a b s t r a c t We report on the coalescence behaviors of telephone cord (TC) buckles in SiAlNx films sputtered on 6 mm thick glass substrates. The buckles are found to propagate from the film edges to the central regions gradually after the sample was annealed at 700 °C for 4.5 min and then cooled to ambient temperature. If two buckles meet on their ways during propagation, they may coalesce into one buckle. The buckle morphology after coalescence is strongly dependent on the coincided degree and the phase difference of the original buckles. The coalescence processes and morphological characteristics of the TC buckles with diverse coincided degrees and phase differences have been investigated in detail. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Thin films and coatings fabricated by vapor deposition and sputtering process usually generate an intrinsic residual stress during deposition due to non-equilibrium growth process, lattice mismatch, surface oxidation etc. After deposition, an additional thermal stress may also be introduced in the film during the cooling process due to the thermal expansion mismatch between the film and the substrate. If the residual compressive stress is beyond a critical value, the film is susceptible to buckling and delaminating from its substrate, resulting in many interesting topographical patterns such as straight-sided, varicose and telephone cord (TC) buckles [1–9]. The straight-sided buckle is typically generated under a uniaxial compressive stress [1–3]. The varicose buckle is generally regarded as a post-buckling (or second buckling) phenomenon [4–6]. The TC buckle is the most common buckling mode and can be widely observed in homogeneous films under an equi-biaxial compressive stress [7–9]. The morphological characteristics and growth mechanism of the isolated TC buckle (also including straight and varicose buckles) have been extensively investigated both in experiment and theory, and are now well understood [1–12]. However, the coalescence process and morphological evolution of the meeting TC buckles have not been reported. In this paper, we report on the coalescence behaviors of the TC buckles in SiAlNx films sputtered on 6 mm thick glass substrates. The SiAlNx films are now widely used as medium layers and capping layers in low emissivity windows. For security, the low emissivity coated glass generally needs
⁎ Corresponding author. Tel./fax: +86 571 86835756. E-mail addresses: [email protected], [email protected] (S.-J. Yu). 0257-8972/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.surfcoat.2013.06.117
to be annealed at high temperature before application. During annealing, a high compressive stress may be introduced in the low emissivity film, which makes it convenient to use the annealed SiAlNx film system as a prototype to study the buckling behaviors.
2. Experimental details The SiAlNx films were deposited on a commercial 6 × 600 × 900 mm3 glass by alternating current magnetron sputtering in the coating production lines (Apollon G 3210/7-H). The rotating target was a piece of SiAl alloys composing 90% of Si in atomic ratio. The base pressure was below 1.2 × 10−4 Pa and the working pressure was about 0.8 Pa. The flow of working gas Ar (99.99% purity) was fixed to be 750 sccm, while the flow of reactive gas N2 (99.99% purity) was kept in the range of 450 to 1125 sccm (the ratio of N2 to Ar was ranged from 0.6 to 1.5). During deposition, the sputtering power was kept 70 kW and the moving speed of the sample was tuned in the range of 0.5 to 2 m/min to control the film thickness precisely. After deposition, the initial sample was cut into several small pieces with about 10 × 10 mm2 in size. The small samples were put into a 700 °C muffle furnace for 4.5 min, and then they were removed from the furnace and naturally cooled in the atmosphere condition. The film thickness, from about 100 nm to more than 1 μm, was independently measured by using a spectrometer (Lambda950 UV/VIS) and an atomic force microscopy (XE-100E, PSIA). In fact, the TC buckles in the samples with different film thicknesses and components possess similar propagation and coalescence behaviors. The propagation and coalescence of the TC buckles were in situ measured and investigated by an optical microscope (Leica DMLM) equipped with a charge coupled device camera (Leica DC 300).
S.-J. Yu et al. / Surface & Coatings Technology 232 (2013) 884–890
3. Results and discussion Our previous study showed that the annealed SiAlNx films contain a high compressive stress of about 1 GPa, which is relieved by formation of a large number of TC buckles [13]. Because the stress tends to concentrate at the film imperfections and the interfacial adhesion in the imperfection region is comparatively week [14,15], the TC buckles are found to always nucleate at the film edges or some impurities, and then propagate into the central regions gradually [3,15,16]. The typical propagation process of the TC buckles in the SiAlNx films is shown in Fig. 1 (details see the Supplemental Material 1 for a movie of the buckle propagation). We find that the TC buckles generally have a preferred propagation direction and subsequently they are always parallel with each other in local region. In order to investigate the propagation behaviors of the TC buckles, we have measured the dependence of the buckle length starting from the photo edge, namely L, on the growth time t, and the result is shown in Fig. 2. We find that the length L increases approximately linearly with t. The linear slopes of the buckles 1–5 are about 27.8, 28.2, 30.6, 29.6, and 28.8 μm/hr, respectively, indicating that the TC buckles in the local region have an almost uniform propagation speed. Many previous studies also showed that the propagation speeds of the TC buckles are closely related to many factors such as the film thickness, hydrogen absorption [17], moisture [18], external disturbance [19] etc. According to our experimental observations, the TC buckles always possess completely opposite propagation directions when they start from the opposite film edges. It can be seen clearly from Fig. 1 that the buckles 1 and 3 are moving towards right, while the buckles 2, 4 and 5 are moving towards left. During the propagation process, the TC buckles may meet on their ways and subsequently coalesce (see the buckles 1 and 2 in Fig. 1). Whether or not the buckles coalesce, is mainly determined by the buckle strip width w and the distance between the central lines of the neighboring buckles, namely ξ (see Fig. 1(c)). It is clear that when ξ N w, the buckles will not meet on their ways and no coalescence occurs (see the buckles 3, 4 and 5 in Fig. 1). When ξ b w, however, the meeting and coalescence of the buckles are unavoidable.
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Fig. 3 shows the typical coalescence process of the TC buckles (details see the Supplemental Material 2 for a movie of the buckle propagation and coalescence). It is clear that when the growth time increases, the TC buckles propagating in the opposite directions are approaching to each other gradually, and finally they coalesce into one buckle. In order to further understand the propagation and coalescence characteristics of the TC buckles, we measured the time dependences of the buckle length L, spacing between the propagating tips d, buckle width at the coalescence position 2b and distance between two arbitrary peaks (or troughs) ΔL, respectively, as shown in Fig. 4. We find that the propagation and coalescence process of the TC buckles can be divided into three stages. In the first stage (stage I), the buckle length L increases linearly with the time t, which is quite similar to the phenomenon shown in Fig. 2. Accordingly, the spacing d decays linearly with time. In this stage, the spacing between the two buckles is too large to interact with each other. Therefore, the propagation of the TC buckles is free and independently, just as the case of an isolated TC buckle [13]. When the TC buckles are very close to each other, an interesting phenomenon occurs: the buckles do not coalesce immediately and on the contrary, a restraint process can be clearly observed (see Fig. 3(c,d)). In this stage (stage II), the growth of the TC buckles in the longitudinal direction (along the TC buckle) is almost stopped, and therefore the buckle length remains a constant. The TC buckles keep un-coalesced and a narrow undelaminated gap between the buckle tips can be seen clearly. The spacing d is also a constant of about 4 μm (see the inset of Fig. 4(b)). On the other hand, the TC buckles can grow successively in the transverse direction (perpendicular to the TC buckle), which results in a great deformation of the buckle tips (see Fig. 3(d)). It is clear that a strong interaction exists between the TC buckles when the spacing is less than several micrometers, and therefore the longitudinal growth of the buckles is confined. Finally, the undelaminated gap between the buckle tips starts to detach from the substrate and the coalescence occurs (see Fig. 3(e)). During this process, the buckle morphologies at the tips evolve drastically and the coalescence is finished within a very short time. After coalescence (in stage III), the wave shape of the buckle is no longer changed greatly, but the buckle can grow in the transverse direction successively,
Fig. 1. Typical propagation process of TC buckles in SiAlNx films. The interval between the two neighboring images is 8 h. The time taking the first photo is defined as the time zero point. Each image has a size of 1320 × 800 μm2. The symbols L, w, ξ, λ and ΔL represent the buckle length starting from the photo edge, buckle strip width, spacing between the central lines of the neighboring buckles, intrinsic wavelength and distance between two arbitrary peaks (or troughs), respectively. For buckles 1 and 2, two important factors, which determine the buckle morphology after coalescence, are ξ/w ≈ 0.35 and ΔL/λ ≈ 2.8.
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experimental data can be well fitted by the first order exponential decay equation ΔLðt Þ ¼ ΔLð∞Þ þ ΔLdecay e
Fig. 2. Evolution of the buckle length L with the growth time t. The solid line is a linear fit to the experimental data.
resulting in the increase of the buckle width [13]. Fig. 4(c) shows that the buckle width at the coalescence position increases quickly with time first, then the growth speed slows down gradually and finally it approaches a saturation value. The experimental data can be well fitted by an exponential growth equation −ðt−t 0 Þ=τ
2bðt Þ ¼ 2bð∞Þ−2badd e
;
ð1Þ
where 2b(∞) is the saturation buckle width, 2badd is the added value from t = t0 to ∞, t0 is the time point from which the coalescence completes (t0 = 27 min in this case), τ is a constant, representing the growth rate of the buckle width with time. In Fig. 4(c), 2b(∞), 2badd and τ are about 85.1, 18.7 μm and 10.5 min, respectively. Fig. 4(d) shows that when the time t increases, the distance between two arbitrary peaks ΔL first decreases quickly, then the decay speed slows down and finally it approaches a saturation value. The
−t=τ ′
;
ð2Þ
where ΔL(∞) is the saturation distance, ΔLdecay represents the decreased value from t = 0 to infinitely large, τ′ is a time constant, representing the decay rate of the distance with time. In Fig. 4(d), ΔL(∞), ΔLdecay and τ′ are about 323.7, 35.5 μm and 9.0 min, respectively. The decay behavior of ΔL with time is mainly attributed to the longitudinal evolution of the TC buckles [13]. Fig. 5 shows another coalescence process of the TC buckles (details see the Supplemental Material 3 for a movie of the buckle coalescence and reorganization). We find that after coalescence, the buckle undergoes a reorganization process: two small wave segments are merged into a larger one, which is quite different from the phenomenon shown in Fig. 3. In order to further understand the coalescence and reorganization characteristics, we measured the time dependences of the wavelength λ, amplitude A, their ratio A/λ, buckle width 2b and distance ΔL, respectively, as shown in Fig. 6. We find that the wavelength and amplitude of the left small wave (λ1 and A1) both decrease slowly first and then the decay speeds become large when approaching to the reorganization point (see Fig. 6(a)). After reorganization, this small wave disappears. Similarly, the ratio A1/λ1 also decreases slowly first and then decays drastically (see Fig. 6(b)). A slight increase of the amplitude and A1/λ1 near the coalescence point is mainly due to the transverse growth of the buckle. On the other hand, the wavelength λ2, amplitude A2 and their ratio A2/λ2 of the right wave increase slightly first. In the vicinity of the reorganization point, the wavelength, amplitude and their ratio all increase quickly and then they reach the stable values of about 94, 47 μm and 0.5, respectively (see Fig. 6(a,b)). It should be noted here that these stable values are very close to their intrinsic values for this sample, which are about 100, 43 μm and 0.43, respectively. Fig. 6(c) shows that the curve of the buckle width is composed of two growth stages divided by the reorganization point. After coalescence, the buckle width increases quickly with time first, and then the
Fig. 3. Typical coalescence process of the TC buckles when ξ/w ≈ 0.04 and ΔL/λ ≈ 3.0. The data appeared in the bottom-right corners represent the growth time. The time taking the first photo is defined as the time zero point. Each image has a size of 640 × 180 μm2. The spacing between the propagating tips and the buckle width at the coalescence position are denoted as d and 2b, respectively. Longitudinal and transverse directions represent the directions along and perpendicular to the TC buckle, respectively.
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Fig. 4. Evolution of the buckle length L (a), spacing between the propagating tips d (b), buckle width at the coalescence position 2b (c), distance between two arbitrary peaks (or troughs) ΔL and ratio ΔL/λ (d) with the growth time t. The inset of (b) represents the enlarged view near the coalescence point. The stages I, II and III in (a,b) represent the free propagation, confined growth and coalescence of the buckles, respectively. The solid lines in (c,d) are exponential fits to the experimental data with the equations shown in the black boxes.
growth speed slows down gradually. When approaching to the reorganization point, the growth speed becomes faster. After reorganization, the buckle width increases drastically again, and then it grows slowly until
reaching the saturation value of about 113 μm. Both growth behaviors of these two stages are quite similar to that shown in Fig. 4(c), and can also be well fitted by the exponential growth equations (see Fig. 6(c)).
Fig. 5. Typical coalescence process of the TC buckles when ξ/w ≈ 0.19 and ΔL/λ ≈ 2.4. The data appeared in the bottom-right corners represent the growth time. Each image has a size of 640 × 180 μm2. The wavelengths and amplitudes of the left and right small waves after coalescence are denoted as λ1, A1, λ2 and A2.
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Fig. 6. Evolution of the wavelength λ, amplitude A (a), their ratio A/λ (b), buckle width at the coalescence position 2b (c), distance between two arbitrary peaks ΔL and ratio ΔL/λ (d) with the growth time t. The solid lines in (c) are exponential fits to the experimental data.
Fig. 6(d) shows that before coalescence, the distance ΔL decreases drastically due to the longitudinal evolution of the TC buckles [13]. After coalescence, ΔL decreases approximately linearly but not exponentially as shown in Fig. 4(d) because the wave shape is not stable and it evolves successively in this case. Then the reorganization process occurs and ΔL decreases drastically again. After that, the wave shape tends to be stable and ΔL follows the exponential decay behavior. It can be seen from Fig. 3 that the TC buckle after coalescence possesses a well-defined shape and the coalescence trace can not be detected clearly, just as the case of an isolate TC buckle. But the coalescence phenomenon shown in Fig. 3 is rare in the experiment. In most cases, the TC buckles after coalescence twist and deform obviously, as shown in Figs. 1 and 5. It immediately raises a question: why are the buckle morphologies after coalescence different? In order to answer this question, we have studied more than 40 coalescence processes of the TC buckles. According to the experimental observations and analysis, we conclude that the morphological evolution and final wave shape of the buckle are strongly dependent on two factors: the coincided degree and phase relationship between the two TC buckles before coalescence. The coincided degree can be effectively expressed as the ratio of ξ/w. As we have discussed above, the coalescence only occurs in the range of 0 ≤ ξ/w ≤ 1. It is clear that when ξ/w increases, the twist degree of the buckle will increase. For example, in Fig. 1, the ratio ξ/w ≈ 0.35 for the buckles 1 and 2, and therefore a slight twist of the buckle can be seen clearly. In Figs. 3 and 5, both the ratios ξ/w are comparatively small (about 0.04 in Fig. 3 and 0.19 in Fig. 5), and therefore this factor should not influence the buckle morphology greatly. But the evolution behaviors and final wave shapes in Figs. 3 and 5 are completely different, indicating that the phase relationship determines and controls the buckle
morphology in these two cases. Our experiment shows that the phase relationship can be effectively expressed as a simple parameter ΔL/ λ − N, where λ is the intrinsic wavelength and N is an arbitrary integer. For simplicity, the parameter ΔL/λ − N is in the range of 0–1 by selecting the integer N. The phase difference of the two buckles can also be expressed as. Δϕ ¼ 2π
ΔL −N : λ
ð3Þ
It is clear that when the parameter ΔL/λ − N equals 0 or 1 (Δϕ = 0 or 2π), the phases of the two buckles are same. When ΔL/λ − N equals 0.5 (Δϕ = π), the phases of the two buckles are reversed. In order to further understand the influence of the phase relationship on the buckle morphology after coalescence, the time dependence of the ratio ΔL/λ is also plotted in Figs. 4(d) and 6(d). Because the intrinsic wavelength is a constant, the evolution behavior of the ratio ΔL/λ is same as that of the distance ΔL. We find from Fig. 4(d) that the ratio ΔL/λ is just an integer of 3, i.e., ΔL/λ − N ≈ 0 and the phases of the two buckles are same. In other words, the distance ΔL just contains an integral number of intrinsic wavelengths and the final wave shape is almost distortion-free. It is clear that the buckle morphology without any deformation and twist after coalescence only occurs when ΔL/λ − N = 0 and ξ/w = 0. If the ratio ΔL/λ is not an integer, however, the deformation of the buckle morphology is unavoidable. On one hand, the TC buckle tends to reach the intrinsic wavelength by growth. On the other hand, the limited space within the distance ΔL confines the expansion of the buckle in the longitudinal direction. The final buckle morphology is dependent on the competition of these two factors. Our experiment also shows that when 0.5 b ΔL/λ − N b 1 (π b Δϕ b 2π), the confined effect of the limited space is dominant and the wave shape of the buckle after
S.-J. Yu et al. / Surface & Coatings Technology 232 (2013) 884–890
Fig. 7. Typical coalescence morphologies when ξ/w → 1. (a) Slight coalescence at each closest point of the TC buckles. (b) Closed buckle morphology composed of only one period. (c) Closed buckle morphology composed of multiple periods. (d) Semi-closed buckle morphology.
coalescence is stable by compressing the wavelengths slightly. It is reasonable that in this case, the small wavelengths at the coalescence position are closer to the intrinsic value. The typical morphology of this case
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can be seen in Fig. 1, where ΔL/λ − N ≈ 0.76. It is clear that three stable waves form within the distance ΔL, but their wavelengths are somewhat smaller than the intrinsic value. When 0 b ΔL/λ − N b 0.5 (0 b Δϕ b π), however, the buckle morphology after coalescence is generally unstable and it can undergo a reorganization process by merging of smaller waves. The typical phenomenon can be seen in Fig. 5, where the ratio ΔL/λ is about 2.4 when coalescence (see Fig. 6(d)), below a half-integer and far away from an integer. Therefore, 3 waves with smaller wavelengths form within the distance ΔL after coalescence. Because the wavelengths are much smaller than the intrinsic value, they are unstable. The wave shape evolves successively and finally two smaller waves are merged into a larger one. Via reorganization, the wavelength is closer to the intrinsic value and the ratio ΔL/λ decreases to below 2.2 (see Fig. 6(d)), which is close to the integer 2. After that, the wave is comparatively favorable and its shape is no longer changed greatly. It should be noted that a key question remains unsolved: why do not the buckles coalesce into 2 waves directly in this case since it is favorable in morphology? Our previous study showed that during the propagation process, the TC buckle tends to form smaller wavelength first, and then it evolves in the longitudinal direction to reach the intrinsic value gradually [13]. Therefore, more waves with smaller wavelengths always form within the distance ΔL when coalescence. Our experiment also shows that when ξ/w → 1 (the critical point of coalescence and non-coalescence), various interesting coalescence morphologies can be observed, as shown in Fig. 7. Fig. 7(a) shows that each crest of the lower TC buckle has a slight coalescence with the trough of the upper buckle. The buckle morphology deforms greatly at the coalescence points. It is clear that this phenomenon only occurs when ξ/w ≈ 1 and ΔL/λ − N ≈ 0.5. That is to say, the phases of the two TC buckles are reversed and the crests of one buckle are just coincided with the troughs of the other one. When the TC buckles propagate, each buckle tip does not coalesce with the main body of the other one and they can propagate independently. At the same time, the widths of the TC buckles increase gradually. Finally the closest parts of the buckles touch each other and subsequently coalesce slightly.
Fig. 8. Typical formation process of the closed buckle morphology. The data appeared in the top-right corners represent the growth time. (a–f) Each image has a size of 695 × 220 μm2; (g,h) each image has a size of 800 × 253 μm2.
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Fig. 7(b)–(d) show the closed and semi-closed buckle morphologies. The closed structure can be composed of only one period (Fig. 7(b)) or multiple periods (Fig. 7(c)). Fig. 8 shows the typical formation process of the closed buckle morphology (details see the Supplemental Material 4 for a movie of the buckle propagation and coalescence). We find that the TC buckles propagate independently first. At t = 620 min, the propagating tip of buckle 1 starts to coalesce with one crest of buckle 2 and a semi-closed structure forms. At t = 1000 min, the propagating tip of buckle 2 starts to coalesce with one trough of buckle 1 and finally the closed structure forms. It is clear that the number of periods within the closed structure is determined by the coalescence positions of the buckle tips. 4. Conclusions In summary, the coalescence process and morphological characteristics of TC buckles in SiAlNx films on 6 mm thick glass substrates are described and discussed in detail. After the sample was annealed at 700 °C for 4.5 min and then cooled to ambient temperature, the TC buckles start to propagate from the film edges to the central regions gradually. During the propagation process, two TC buckles may meet on their ways and then coalesce into one buckle. The coalescence process and final buckle morphology are strongly dependent on two factors: the coincided degree (ξ/w) and phase difference ( Δϕ ¼ 2π ΔL λ −N ) of the TC buckles before coalescence. If ξ/w = 0 and Δϕ = 0, the final wave shape has not any twist and deformation. When ξ/w and/or Δϕ increase, the degrees of the twist and deformation of the buckle increase accordingly. When ξ/w → 1, various interesting coalescence morphologies including slight coalescence at each closest point, closed and semi-closed structures can be observed. We anticipate that the results shown in this paper will provide a deep sight on the coalescence and evolution behaviors of the TC buckles and can help to further understand the mechanics of TC buckling mode.
Supplementary data to this article can be found online at http:// dx.doi.org/10.1016/j.surfcoat.2013.06.117. Acknowledgements The authors thank Gen Li, Quan-Lin Ye and Yong-Ju Zhang for useful discussions and technical assistance. This work was supported by the National Natural Science Foundation of China (Grant Nos. 11204283, 11074227, 51271172) and Zhejiang Provincial Natural Science Foundation (No. R6110362, LQ13A040002). References [1] F. Cleymand, C. Coupeau, J. Grilhé, Scripta Mater. 44 (2001) 2623. [2] F. Foucher, C. Coupeau, Surf. Coat. Technol. 202 (2007) 1094. [3] A.A. Abdallah, P.C.P. Bouten, J.M.J. den Toonder, G. de With, Surf. Coat. Technol. 205 (2011) 3103. [4] G. Parry, C. Coupeau, J. Colin, A. Cimetière, J. Grilhé, Acta Mater. 52 (2004) 3959. [5] A.A. Abdallah, P.C.P. Bouten, J.M.J. den Toonder, G. de With, Thin Solid Films 516 (2008) 1063. [6] G. Parry, A. Cimetière, C. Coupeau, J. Colin, J. Grilhé, Phys. Rev. E 74 (2006) 066601. [7] M.W. Moon, H.M. Jensen, J.W. Hutchinson, K.H. Oh, A.G. Evans, J. Mech. Phys. Solids 50 (2002) 2355. [8] M.J. Cordill, N.R. Moody, D.F. Bahr, Acta Mater. 53 (2005) 2555. [9] J.-Y. Faou, G. Parry, S. Grachev, E. Barthel, Phys. Rev. Lett. 108 (2012) 116102. [10] J.W. Hutchinson, M.D. Thouless, E.G. Liniger, Acta Metall. Mater. 40 (1992) 295. [11] B. Audoly, Phys. Rev. Lett. 83 (1999) 4124. [12] M.W. Moon, K.R. Lee, K.H. Oh, J.W. Hutchinson, Acta Mater. 52 (2004) 3151. [13] S.J. Yu, M.G. Chen, J. Chen, H. Zhou, Y.J. Zhang, P.Z. Si, Surf. Coat. Technol. 228 (2013) 258. [14] J.W. Hutchinson, M.Y. He, A.G. Evans, J. Mech. Phys. Solids 48 (2000) 709. [15] M.W. Moon, J.W. Chung, K.R. Lee, K.H. Oh, R. Wang, A.G. Evans, Acta Mater. 50 (2002) 1219. [16] S.J. Yu, M.G. Chen, Y.J. Zhang, H. Zhou, P.Z. Si, Thin Solid Films 519 (2011) 7936. [17] A. Pundt, E. Nikitin, P. Pekarski, R. Kirchheim, Acta Mater. 52 (2004) 1579. [18] J. Moller, D. Reiche, M. Bobeth, W. Pompe, Surf. Coat. Technol. 150 (2002) 8. [19] S.J. Yu, Y.J. Zhang, H. Zhou, P.G. Cai, M.G. Chen, Appl. Surf. Sci. 256 (2009) 909.
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Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
Coalescence behaviors of telephone cord buckles in SiAlNx films Sen-Jiang Yu a,⁎, Yuan-Chi Shi a, Miao-Gen Chen a, Ping-Zhan Si a, Yun Zhou a, Xiao-Fei Zhang a, Jun Chen b, Hong Zhou a, Zhi-Wei Jiao a a b
Department of Physics, China Jiliang University, Hangzhou 310018, P.R. China Research and Development Department, Zhejiang Zhongli Energy Efficient Glass Manufacturing Co., Ltd, Hangzhou 311228, P.R. China
a r t i c l e
i n f o
Article history: Received 28 April 2013 Accepted in revised form 26 June 2013 Available online 4 July 2013 Keywords: Telephone cord buckle Thin film Coalescence Compressive stress
a b s t r a c t We report on the coalescence behaviors of telephone cord (TC) buckles in SiAlNx films sputtered on 6 mm thick glass substrates. The buckles are found to propagate from the film edges to the central regions gradually after the sample was annealed at 700 °C for 4.5 min and then cooled to ambient temperature. If two buckles meet on their ways during propagation, they may coalesce into one buckle. The buckle morphology after coalescence is strongly dependent on the coincided degree and the phase difference of the original buckles. The coalescence processes and morphological characteristics of the TC buckles with diverse coincided degrees and phase differences have been investigated in detail. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Thin films and coatings fabricated by vapor deposition and sputtering process usually generate an intrinsic residual stress during deposition due to non-equilibrium growth process, lattice mismatch, surface oxidation etc. After deposition, an additional thermal stress may also be introduced in the film during the cooling process due to the thermal expansion mismatch between the film and the substrate. If the residual compressive stress is beyond a critical value, the film is susceptible to buckling and delaminating from its substrate, resulting in many interesting topographical patterns such as straight-sided, varicose and telephone cord (TC) buckles [1–9]. The straight-sided buckle is typically generated under a uniaxial compressive stress [1–3]. The varicose buckle is generally regarded as a post-buckling (or second buckling) phenomenon [4–6]. The TC buckle is the most common buckling mode and can be widely observed in homogeneous films under an equi-biaxial compressive stress [7–9]. The morphological characteristics and growth mechanism of the isolated TC buckle (also including straight and varicose buckles) have been extensively investigated both in experiment and theory, and are now well understood [1–12]. However, the coalescence process and morphological evolution of the meeting TC buckles have not been reported. In this paper, we report on the coalescence behaviors of the TC buckles in SiAlNx films sputtered on 6 mm thick glass substrates. The SiAlNx films are now widely used as medium layers and capping layers in low emissivity windows. For security, the low emissivity coated glass generally needs
⁎ Corresponding author. Tel./fax: +86 571 86835756. E-mail addresses: [email protected], [email protected] (S.-J. Yu). 0257-8972/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.surfcoat.2013.06.117
to be annealed at high temperature before application. During annealing, a high compressive stress may be introduced in the low emissivity film, which makes it convenient to use the annealed SiAlNx film system as a prototype to study the buckling behaviors.
2. Experimental details The SiAlNx films were deposited on a commercial 6 × 600 × 900 mm3 glass by alternating current magnetron sputtering in the coating production lines (Apollon G 3210/7-H). The rotating target was a piece of SiAl alloys composing 90% of Si in atomic ratio. The base pressure was below 1.2 × 10−4 Pa and the working pressure was about 0.8 Pa. The flow of working gas Ar (99.99% purity) was fixed to be 750 sccm, while the flow of reactive gas N2 (99.99% purity) was kept in the range of 450 to 1125 sccm (the ratio of N2 to Ar was ranged from 0.6 to 1.5). During deposition, the sputtering power was kept 70 kW and the moving speed of the sample was tuned in the range of 0.5 to 2 m/min to control the film thickness precisely. After deposition, the initial sample was cut into several small pieces with about 10 × 10 mm2 in size. The small samples were put into a 700 °C muffle furnace for 4.5 min, and then they were removed from the furnace and naturally cooled in the atmosphere condition. The film thickness, from about 100 nm to more than 1 μm, was independently measured by using a spectrometer (Lambda950 UV/VIS) and an atomic force microscopy (XE-100E, PSIA). In fact, the TC buckles in the samples with different film thicknesses and components possess similar propagation and coalescence behaviors. The propagation and coalescence of the TC buckles were in situ measured and investigated by an optical microscope (Leica DMLM) equipped with a charge coupled device camera (Leica DC 300).
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3. Results and discussion Our previous study showed that the annealed SiAlNx films contain a high compressive stress of about 1 GPa, which is relieved by formation of a large number of TC buckles [13]. Because the stress tends to concentrate at the film imperfections and the interfacial adhesion in the imperfection region is comparatively week [14,15], the TC buckles are found to always nucleate at the film edges or some impurities, and then propagate into the central regions gradually [3,15,16]. The typical propagation process of the TC buckles in the SiAlNx films is shown in Fig. 1 (details see the Supplemental Material 1 for a movie of the buckle propagation). We find that the TC buckles generally have a preferred propagation direction and subsequently they are always parallel with each other in local region. In order to investigate the propagation behaviors of the TC buckles, we have measured the dependence of the buckle length starting from the photo edge, namely L, on the growth time t, and the result is shown in Fig. 2. We find that the length L increases approximately linearly with t. The linear slopes of the buckles 1–5 are about 27.8, 28.2, 30.6, 29.6, and 28.8 μm/hr, respectively, indicating that the TC buckles in the local region have an almost uniform propagation speed. Many previous studies also showed that the propagation speeds of the TC buckles are closely related to many factors such as the film thickness, hydrogen absorption [17], moisture [18], external disturbance [19] etc. According to our experimental observations, the TC buckles always possess completely opposite propagation directions when they start from the opposite film edges. It can be seen clearly from Fig. 1 that the buckles 1 and 3 are moving towards right, while the buckles 2, 4 and 5 are moving towards left. During the propagation process, the TC buckles may meet on their ways and subsequently coalesce (see the buckles 1 and 2 in Fig. 1). Whether or not the buckles coalesce, is mainly determined by the buckle strip width w and the distance between the central lines of the neighboring buckles, namely ξ (see Fig. 1(c)). It is clear that when ξ N w, the buckles will not meet on their ways and no coalescence occurs (see the buckles 3, 4 and 5 in Fig. 1). When ξ b w, however, the meeting and coalescence of the buckles are unavoidable.
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Fig. 3 shows the typical coalescence process of the TC buckles (details see the Supplemental Material 2 for a movie of the buckle propagation and coalescence). It is clear that when the growth time increases, the TC buckles propagating in the opposite directions are approaching to each other gradually, and finally they coalesce into one buckle. In order to further understand the propagation and coalescence characteristics of the TC buckles, we measured the time dependences of the buckle length L, spacing between the propagating tips d, buckle width at the coalescence position 2b and distance between two arbitrary peaks (or troughs) ΔL, respectively, as shown in Fig. 4. We find that the propagation and coalescence process of the TC buckles can be divided into three stages. In the first stage (stage I), the buckle length L increases linearly with the time t, which is quite similar to the phenomenon shown in Fig. 2. Accordingly, the spacing d decays linearly with time. In this stage, the spacing between the two buckles is too large to interact with each other. Therefore, the propagation of the TC buckles is free and independently, just as the case of an isolated TC buckle [13]. When the TC buckles are very close to each other, an interesting phenomenon occurs: the buckles do not coalesce immediately and on the contrary, a restraint process can be clearly observed (see Fig. 3(c,d)). In this stage (stage II), the growth of the TC buckles in the longitudinal direction (along the TC buckle) is almost stopped, and therefore the buckle length remains a constant. The TC buckles keep un-coalesced and a narrow undelaminated gap between the buckle tips can be seen clearly. The spacing d is also a constant of about 4 μm (see the inset of Fig. 4(b)). On the other hand, the TC buckles can grow successively in the transverse direction (perpendicular to the TC buckle), which results in a great deformation of the buckle tips (see Fig. 3(d)). It is clear that a strong interaction exists between the TC buckles when the spacing is less than several micrometers, and therefore the longitudinal growth of the buckles is confined. Finally, the undelaminated gap between the buckle tips starts to detach from the substrate and the coalescence occurs (see Fig. 3(e)). During this process, the buckle morphologies at the tips evolve drastically and the coalescence is finished within a very short time. After coalescence (in stage III), the wave shape of the buckle is no longer changed greatly, but the buckle can grow in the transverse direction successively,
Fig. 1. Typical propagation process of TC buckles in SiAlNx films. The interval between the two neighboring images is 8 h. The time taking the first photo is defined as the time zero point. Each image has a size of 1320 × 800 μm2. The symbols L, w, ξ, λ and ΔL represent the buckle length starting from the photo edge, buckle strip width, spacing between the central lines of the neighboring buckles, intrinsic wavelength and distance between two arbitrary peaks (or troughs), respectively. For buckles 1 and 2, two important factors, which determine the buckle morphology after coalescence, are ξ/w ≈ 0.35 and ΔL/λ ≈ 2.8.
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experimental data can be well fitted by the first order exponential decay equation ΔLðt Þ ¼ ΔLð∞Þ þ ΔLdecay e
Fig. 2. Evolution of the buckle length L with the growth time t. The solid line is a linear fit to the experimental data.
resulting in the increase of the buckle width [13]. Fig. 4(c) shows that the buckle width at the coalescence position increases quickly with time first, then the growth speed slows down gradually and finally it approaches a saturation value. The experimental data can be well fitted by an exponential growth equation −ðt−t 0 Þ=τ
2bðt Þ ¼ 2bð∞Þ−2badd e
;
ð1Þ
where 2b(∞) is the saturation buckle width, 2badd is the added value from t = t0 to ∞, t0 is the time point from which the coalescence completes (t0 = 27 min in this case), τ is a constant, representing the growth rate of the buckle width with time. In Fig. 4(c), 2b(∞), 2badd and τ are about 85.1, 18.7 μm and 10.5 min, respectively. Fig. 4(d) shows that when the time t increases, the distance between two arbitrary peaks ΔL first decreases quickly, then the decay speed slows down and finally it approaches a saturation value. The
−t=τ ′
;
ð2Þ
where ΔL(∞) is the saturation distance, ΔLdecay represents the decreased value from t = 0 to infinitely large, τ′ is a time constant, representing the decay rate of the distance with time. In Fig. 4(d), ΔL(∞), ΔLdecay and τ′ are about 323.7, 35.5 μm and 9.0 min, respectively. The decay behavior of ΔL with time is mainly attributed to the longitudinal evolution of the TC buckles [13]. Fig. 5 shows another coalescence process of the TC buckles (details see the Supplemental Material 3 for a movie of the buckle coalescence and reorganization). We find that after coalescence, the buckle undergoes a reorganization process: two small wave segments are merged into a larger one, which is quite different from the phenomenon shown in Fig. 3. In order to further understand the coalescence and reorganization characteristics, we measured the time dependences of the wavelength λ, amplitude A, their ratio A/λ, buckle width 2b and distance ΔL, respectively, as shown in Fig. 6. We find that the wavelength and amplitude of the left small wave (λ1 and A1) both decrease slowly first and then the decay speeds become large when approaching to the reorganization point (see Fig. 6(a)). After reorganization, this small wave disappears. Similarly, the ratio A1/λ1 also decreases slowly first and then decays drastically (see Fig. 6(b)). A slight increase of the amplitude and A1/λ1 near the coalescence point is mainly due to the transverse growth of the buckle. On the other hand, the wavelength λ2, amplitude A2 and their ratio A2/λ2 of the right wave increase slightly first. In the vicinity of the reorganization point, the wavelength, amplitude and their ratio all increase quickly and then they reach the stable values of about 94, 47 μm and 0.5, respectively (see Fig. 6(a,b)). It should be noted here that these stable values are very close to their intrinsic values for this sample, which are about 100, 43 μm and 0.43, respectively. Fig. 6(c) shows that the curve of the buckle width is composed of two growth stages divided by the reorganization point. After coalescence, the buckle width increases quickly with time first, and then the
Fig. 3. Typical coalescence process of the TC buckles when ξ/w ≈ 0.04 and ΔL/λ ≈ 3.0. The data appeared in the bottom-right corners represent the growth time. The time taking the first photo is defined as the time zero point. Each image has a size of 640 × 180 μm2. The spacing between the propagating tips and the buckle width at the coalescence position are denoted as d and 2b, respectively. Longitudinal and transverse directions represent the directions along and perpendicular to the TC buckle, respectively.
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Fig. 4. Evolution of the buckle length L (a), spacing between the propagating tips d (b), buckle width at the coalescence position 2b (c), distance between two arbitrary peaks (or troughs) ΔL and ratio ΔL/λ (d) with the growth time t. The inset of (b) represents the enlarged view near the coalescence point. The stages I, II and III in (a,b) represent the free propagation, confined growth and coalescence of the buckles, respectively. The solid lines in (c,d) are exponential fits to the experimental data with the equations shown in the black boxes.
growth speed slows down gradually. When approaching to the reorganization point, the growth speed becomes faster. After reorganization, the buckle width increases drastically again, and then it grows slowly until
reaching the saturation value of about 113 μm. Both growth behaviors of these two stages are quite similar to that shown in Fig. 4(c), and can also be well fitted by the exponential growth equations (see Fig. 6(c)).
Fig. 5. Typical coalescence process of the TC buckles when ξ/w ≈ 0.19 and ΔL/λ ≈ 2.4. The data appeared in the bottom-right corners represent the growth time. Each image has a size of 640 × 180 μm2. The wavelengths and amplitudes of the left and right small waves after coalescence are denoted as λ1, A1, λ2 and A2.
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Fig. 6. Evolution of the wavelength λ, amplitude A (a), their ratio A/λ (b), buckle width at the coalescence position 2b (c), distance between two arbitrary peaks ΔL and ratio ΔL/λ (d) with the growth time t. The solid lines in (c) are exponential fits to the experimental data.
Fig. 6(d) shows that before coalescence, the distance ΔL decreases drastically due to the longitudinal evolution of the TC buckles [13]. After coalescence, ΔL decreases approximately linearly but not exponentially as shown in Fig. 4(d) because the wave shape is not stable and it evolves successively in this case. Then the reorganization process occurs and ΔL decreases drastically again. After that, the wave shape tends to be stable and ΔL follows the exponential decay behavior. It can be seen from Fig. 3 that the TC buckle after coalescence possesses a well-defined shape and the coalescence trace can not be detected clearly, just as the case of an isolate TC buckle. But the coalescence phenomenon shown in Fig. 3 is rare in the experiment. In most cases, the TC buckles after coalescence twist and deform obviously, as shown in Figs. 1 and 5. It immediately raises a question: why are the buckle morphologies after coalescence different? In order to answer this question, we have studied more than 40 coalescence processes of the TC buckles. According to the experimental observations and analysis, we conclude that the morphological evolution and final wave shape of the buckle are strongly dependent on two factors: the coincided degree and phase relationship between the two TC buckles before coalescence. The coincided degree can be effectively expressed as the ratio of ξ/w. As we have discussed above, the coalescence only occurs in the range of 0 ≤ ξ/w ≤ 1. It is clear that when ξ/w increases, the twist degree of the buckle will increase. For example, in Fig. 1, the ratio ξ/w ≈ 0.35 for the buckles 1 and 2, and therefore a slight twist of the buckle can be seen clearly. In Figs. 3 and 5, both the ratios ξ/w are comparatively small (about 0.04 in Fig. 3 and 0.19 in Fig. 5), and therefore this factor should not influence the buckle morphology greatly. But the evolution behaviors and final wave shapes in Figs. 3 and 5 are completely different, indicating that the phase relationship determines and controls the buckle
morphology in these two cases. Our experiment shows that the phase relationship can be effectively expressed as a simple parameter ΔL/ λ − N, where λ is the intrinsic wavelength and N is an arbitrary integer. For simplicity, the parameter ΔL/λ − N is in the range of 0–1 by selecting the integer N. The phase difference of the two buckles can also be expressed as. Δϕ ¼ 2π
ΔL −N : λ
ð3Þ
It is clear that when the parameter ΔL/λ − N equals 0 or 1 (Δϕ = 0 or 2π), the phases of the two buckles are same. When ΔL/λ − N equals 0.5 (Δϕ = π), the phases of the two buckles are reversed. In order to further understand the influence of the phase relationship on the buckle morphology after coalescence, the time dependence of the ratio ΔL/λ is also plotted in Figs. 4(d) and 6(d). Because the intrinsic wavelength is a constant, the evolution behavior of the ratio ΔL/λ is same as that of the distance ΔL. We find from Fig. 4(d) that the ratio ΔL/λ is just an integer of 3, i.e., ΔL/λ − N ≈ 0 and the phases of the two buckles are same. In other words, the distance ΔL just contains an integral number of intrinsic wavelengths and the final wave shape is almost distortion-free. It is clear that the buckle morphology without any deformation and twist after coalescence only occurs when ΔL/λ − N = 0 and ξ/w = 0. If the ratio ΔL/λ is not an integer, however, the deformation of the buckle morphology is unavoidable. On one hand, the TC buckle tends to reach the intrinsic wavelength by growth. On the other hand, the limited space within the distance ΔL confines the expansion of the buckle in the longitudinal direction. The final buckle morphology is dependent on the competition of these two factors. Our experiment also shows that when 0.5 b ΔL/λ − N b 1 (π b Δϕ b 2π), the confined effect of the limited space is dominant and the wave shape of the buckle after
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Fig. 7. Typical coalescence morphologies when ξ/w → 1. (a) Slight coalescence at each closest point of the TC buckles. (b) Closed buckle morphology composed of only one period. (c) Closed buckle morphology composed of multiple periods. (d) Semi-closed buckle morphology.
coalescence is stable by compressing the wavelengths slightly. It is reasonable that in this case, the small wavelengths at the coalescence position are closer to the intrinsic value. The typical morphology of this case
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can be seen in Fig. 1, where ΔL/λ − N ≈ 0.76. It is clear that three stable waves form within the distance ΔL, but their wavelengths are somewhat smaller than the intrinsic value. When 0 b ΔL/λ − N b 0.5 (0 b Δϕ b π), however, the buckle morphology after coalescence is generally unstable and it can undergo a reorganization process by merging of smaller waves. The typical phenomenon can be seen in Fig. 5, where the ratio ΔL/λ is about 2.4 when coalescence (see Fig. 6(d)), below a half-integer and far away from an integer. Therefore, 3 waves with smaller wavelengths form within the distance ΔL after coalescence. Because the wavelengths are much smaller than the intrinsic value, they are unstable. The wave shape evolves successively and finally two smaller waves are merged into a larger one. Via reorganization, the wavelength is closer to the intrinsic value and the ratio ΔL/λ decreases to below 2.2 (see Fig. 6(d)), which is close to the integer 2. After that, the wave is comparatively favorable and its shape is no longer changed greatly. It should be noted that a key question remains unsolved: why do not the buckles coalesce into 2 waves directly in this case since it is favorable in morphology? Our previous study showed that during the propagation process, the TC buckle tends to form smaller wavelength first, and then it evolves in the longitudinal direction to reach the intrinsic value gradually [13]. Therefore, more waves with smaller wavelengths always form within the distance ΔL when coalescence. Our experiment also shows that when ξ/w → 1 (the critical point of coalescence and non-coalescence), various interesting coalescence morphologies can be observed, as shown in Fig. 7. Fig. 7(a) shows that each crest of the lower TC buckle has a slight coalescence with the trough of the upper buckle. The buckle morphology deforms greatly at the coalescence points. It is clear that this phenomenon only occurs when ξ/w ≈ 1 and ΔL/λ − N ≈ 0.5. That is to say, the phases of the two TC buckles are reversed and the crests of one buckle are just coincided with the troughs of the other one. When the TC buckles propagate, each buckle tip does not coalesce with the main body of the other one and they can propagate independently. At the same time, the widths of the TC buckles increase gradually. Finally the closest parts of the buckles touch each other and subsequently coalesce slightly.
Fig. 8. Typical formation process of the closed buckle morphology. The data appeared in the top-right corners represent the growth time. (a–f) Each image has a size of 695 × 220 μm2; (g,h) each image has a size of 800 × 253 μm2.
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Fig. 7(b)–(d) show the closed and semi-closed buckle morphologies. The closed structure can be composed of only one period (Fig. 7(b)) or multiple periods (Fig. 7(c)). Fig. 8 shows the typical formation process of the closed buckle morphology (details see the Supplemental Material 4 for a movie of the buckle propagation and coalescence). We find that the TC buckles propagate independently first. At t = 620 min, the propagating tip of buckle 1 starts to coalesce with one crest of buckle 2 and a semi-closed structure forms. At t = 1000 min, the propagating tip of buckle 2 starts to coalesce with one trough of buckle 1 and finally the closed structure forms. It is clear that the number of periods within the closed structure is determined by the coalescence positions of the buckle tips. 4. Conclusions In summary, the coalescence process and morphological characteristics of TC buckles in SiAlNx films on 6 mm thick glass substrates are described and discussed in detail. After the sample was annealed at 700 °C for 4.5 min and then cooled to ambient temperature, the TC buckles start to propagate from the film edges to the central regions gradually. During the propagation process, two TC buckles may meet on their ways and then coalesce into one buckle. The coalescence process and final buckle morphology are strongly dependent on two factors: the coincided degree (ξ/w) and phase difference ( Δϕ ¼ 2π ΔL λ −N ) of the TC buckles before coalescence. If ξ/w = 0 and Δϕ = 0, the final wave shape has not any twist and deformation. When ξ/w and/or Δϕ increase, the degrees of the twist and deformation of the buckle increase accordingly. When ξ/w → 1, various interesting coalescence morphologies including slight coalescence at each closest point, closed and semi-closed structures can be observed. We anticipate that the results shown in this paper will provide a deep sight on the coalescence and evolution behaviors of the TC buckles and can help to further understand the mechanics of TC buckling mode.
Supplementary data to this article can be found online at http:// dx.doi.org/10.1016/j.surfcoat.2013.06.117. Acknowledgements The authors thank Gen Li, Quan-Lin Ye and Yong-Ju Zhang for useful discussions and technical assistance. This work was supported by the National Natural Science Foundation of China (Grant Nos. 11204283, 11074227, 51271172) and Zhejiang Provincial Natural Science Foundation (No. R6110362, LQ13A040002). References [1] F. Cleymand, C. Coupeau, J. Grilhé, Scripta Mater. 44 (2001) 2623. [2] F. Foucher, C. Coupeau, Surf. Coat. Technol. 202 (2007) 1094. [3] A.A. Abdallah, P.C.P. Bouten, J.M.J. den Toonder, G. de With, Surf. Coat. Technol. 205 (2011) 3103. [4] G. Parry, C. Coupeau, J. Colin, A. Cimetière, J. Grilhé, Acta Mater. 52 (2004) 3959. [5] A.A. Abdallah, P.C.P. Bouten, J.M.J. den Toonder, G. de With, Thin Solid Films 516 (2008) 1063. [6] G. Parry, A. Cimetière, C. Coupeau, J. Colin, J. Grilhé, Phys. Rev. E 74 (2006) 066601. [7] M.W. Moon, H.M. Jensen, J.W. Hutchinson, K.H. Oh, A.G. Evans, J. Mech. Phys. Solids 50 (2002) 2355. [8] M.J. Cordill, N.R. Moody, D.F. Bahr, Acta Mater. 53 (2005) 2555. [9] J.-Y. Faou, G. Parry, S. Grachev, E. Barthel, Phys. Rev. Lett. 108 (2012) 116102. [10] J.W. Hutchinson, M.D. Thouless, E.G. Liniger, Acta Metall. Mater. 40 (1992) 295. [11] B. Audoly, Phys. Rev. Lett. 83 (1999) 4124. [12] M.W. Moon, K.R. Lee, K.H. Oh, J.W. Hutchinson, Acta Mater. 52 (2004) 3151. [13] S.J. Yu, M.G. Chen, J. Chen, H. Zhou, Y.J. Zhang, P.Z. Si, Surf. Coat. Technol. 228 (2013) 258. [14] J.W. Hutchinson, M.Y. He, A.G. Evans, J. Mech. Phys. Solids 48 (2000) 709. [15] M.W. Moon, J.W. Chung, K.R. Lee, K.H. Oh, R. Wang, A.G. Evans, Acta Mater. 50 (2002) 1219. [16] S.J. Yu, M.G. Chen, Y.J. Zhang, H. Zhou, P.Z. Si, Thin Solid Films 519 (2011) 7936. [17] A. Pundt, E. Nikitin, P. Pekarski, R. Kirchheim, Acta Mater. 52 (2004) 1579. [18] J. Moller, D. Reiche, M. Bobeth, W. Pompe, Surf. Coat. Technol. 150 (2002) 8. [19] S.J. Yu, Y.J. Zhang, H. Zhou, P.G. Cai, M.G. Chen, Appl. Surf. Sci. 256 (2009) 909.