- Email: [email protected]
Effects of substrate temperature on the structure and mechanical properties of (TiVCrZrHf)N coatings
Applied Surface Science 257 (2011) 7709–7713
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Effects of substrate temperature on the structure and mechanical properties of (TiVCrZrHf)N coatings Shih-Chang Liang a , Zue-Chin Chang b , Du-Cheng Tsai a , Yi-Chen Lin b , Huan-Shin Sung b , Min-Jen Deng a,c , Fuh-Sheng Shieu a,∗ a
Department of Materials Science and Engineering, National Chung Hsing University, Taichung 402, Taiwan Department of Mechanical Engineering, National Chin-Yi University of Technology, Taichung 411, Taiwan c Department of Optometry, Jen-Teh Junior College of Medicine, Nursing and Management, Miaoli County 356, Taiwan b
a r t i c l e
i n f o
Article history: Received 9 November 2010 Received in revised form 2 April 2011 Accepted 2 April 2011 Available online 12 April 2011 Keywords: Coating materials Nitride materials Thin films Vapor deposition Crystal structure Microstructure
a b s t r a c t The present paper reports the influence of growth conditions on the characteristics of (TiVCrZrHf)N films prepared by rf reactive magnetron sputtering at various substrate temperatures. The nitrogen content is observed to decrease with increasing substrate temperature. The X-ray diffraction results indicate that all (TiVCrZrHf)N films are simple face centered cubic (FCC) structures. Initially, there is an obvious decrease followed by an increase in grain size with the increase in substrate temperature. The lower part of the microstructure has an amorphous structure. A nano grain structure (size ∼1 nm) with a random orientation is also observed above the amorphous structure. The fully dense columnar structure with an fcc crystal phase then starts to develop. Extreme hardness of around 48 GPa is obtained in the present alloy design. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Interesting alloy systems added with multi-principal elements (more than five elements; 5 at.%≤ each element content ≤35 at.%) called high entropy alloys (HEAs) have been proposed and investigated by Yeh et al. [1]. Based on previous research, HEAs have been found to form simple phases with nanocrystalline and even amorphous structures. Aside from the high-entropy effect, these structural characteristics are ascribed to the large lattice distortion and sluggish diffusionof such multielemental mixtures. Following Boltzmann’s hypothesis on the relationship between entropy and system complexity [1], the configurational entropy change, Sconf , during the formation of a solid solution from n elements with equimolar fractions can be calculated using the following equation:
S = −R In
1 n
= R In(n)
where R is the ideal gas constant, and is n the number of mixed elements. When n = 5, Sconf = 1.61R, which approaches the size of the melting entropy of most intermetallic compounds (about R–2R).
∗ Corresponding author. Tel.: +886 4 2284 0500; fax: +886 4 2285 7017. E-mail address: [email protected] (F.-S. Shieu). 0169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2011.04.014
This indicates that a single solid solution, i.e., face-centered cubic (FCC), body-centered cubic (BCC), or FCC + BCC, is formed preferentially for HEAs [1–3]. This makes these alloys easier to analyze and understand as to control and develop. HEAs have high lattice distortions because they comprise multi-principal elements with different atomic sizes. This hinders atomic movement and consequently limits the effective diffusion rate in HEAs. As a result, HEAs tend to form nanosized and even amorphous structures [1–3]. Based on the high mixing entropy effect and the formation of amorphous or nanocrystallized (FCC and/or BCC phases) structure, some interesting properties are expected, such as high stiffness, strength, toughness, hardness, thermal stability, and corrosion resistance [4–9]. To extend the alloy concept to coating materials, HEA nitride coatings were synthesized by sputtering an alloyed target formed by the arc melting process. In these studies, TiVCrZrY, AlCrTaTiZr, AlCrNbSiTiV, and AlMoNbSiTaTiVZr show simple solid solution structures and outstanding mechanical properties [10–13]. The TiVCrZrY and AlCrTaTiZr nitride coatings exhibit hardness at a range of 14–32 GPa when deposited without substrate bias and/or heating [10,11]. The AlCrTaTiZr, AlCrNbSiTiV, and AlMoNbSiTaTiVZrnitrides coatings exhibit high hardness value of 35, 41, and 37 GPa under biased and/or heated conditions, respectively [12–14]. These findings confirm that HEAs have great potential for use as protective coatings. Thus, in the current study, (TiVCrZrHf)Nx
7710
S.-C. Liang et al. / Applied Surface Science 257 (2011) 7709–7713
multi-component coatings with the addition of quinary metallic elements were prepared by a reactive sputtering process. The effects of substrate temperature on structure and properties of deposited coatings were investigated. 2. Experiment The (TiVCrZrHf)N coatings were deposited on p-Si (100) wafers by an rf magnetron sputtering system using equi-molar TiVCrZrHf targets 75 mm in diameter. Before deposition, the Si substrates were cleaned and rinsed with ethanol and distilled water in an ultrasonic bath. The sputtering chamber was pumped down to 2.67 × 10−2 Pa using a turbo pump. The deposition of the (TiVCrZrHf)N coatings was carried out in an Ar + N2 mixed atmosphere under a dc power of 350 W and a working pressure of 6.67 × 10−1 Pa.During deposition, the Ar flow and N2 flow are 100 and 4 SCCM, respectively. The substrate temperature was chosen as the controlling parameter, which varied from room temperature (RT) to 450 ◦ C. The work distance between the substrate and target was 90 mm. The deposition time was set to 60 min. The bias of the substrate was fixed at −100 V during the deposition process. The targets were presputtered by Ar to remove their surface oxide layers before deposition. The chemical compositions of the (TiVCrZrHf)N coatings were determined by field emission electron probe micro-analyses (FEEPMA JOEL JXA-8800 M). At least three tests were performed for each sample. The crystal structures were analyzed by a glancingincidence (1◦ ) X-ray diffractometer (XRD, BRUKER D8 Discover) using Cu K␣ radiation at a scanning speed of 1 deg/min. The scanning step was 0.01◦ , and the scanning range was 20–80◦ . Further, from the full width at half-maximum, the average grain sizes of the coatings were calculated by Scherrer’s formula [15]. The morphology studies and thickness measurements were carried out using field emission scanning electron microscopy (SEM, JEOL JSM6700F). The deposition rate was obtained by dividing the thickness with the deposition time. Microstructural examinations were conducted by an analytical transmission electron microscope (TEM, JEM-2100). The microhardness and elastic modulus of the coatings were measured under a load of 3000 N using a TriboLabnanoindenter (Hysitron). A Berkovich diamond indenter tip was used to reduce the influence of the substrate on the microhardness and elastic modulus of the coatings. At least five tests were performed for each sample. 3. Results and discussion
Fig. 1. EPMA element contents in (TiVCrZrHf)N coatings deposited at various substrate temperature.
Fig. 2 shows the XRD patterns of (TiVCrZrHf)N coatings deposited at various substrate temperatures. (TiVCrZrHf)N coatings exhibit a single FCC solid solution structure without significant phase separation. (TiVCrZrHf)N shows a single FCC solid solution structure because TiN, VN, CrN, ZrN, and HfN are all FCC structures. The same phenomenon has also been reported for as-deposited mixtures of FCC forming multi-principal element nitrides, such as TiVCrZrY, AlCrTaTiZr, AlCrNbSiTiV, and AlMoNbSiTaTiVZr [10–14]. This demonstrates that the high entropy effect in multi-component nitrides can stabilize the solid-solution phase. Fig. 3 shows the grain size of (TiVCrZrHf)N coatings deposited at various substrate temperatures. In the current study, the substrate temperature alters the crystallinity and grain size. The (TiVCrZrHf)N coatings deposited at RT have a strong (1 1 1) orientation and lesser orientation from others. When the substrate temperature increases to 250 ◦ C, the overall crystallinity becomes poorer and the grain size becomes smaller. Generally, due to the enhanced adatom mobility at a higher substrate temperature, grain growth occurs. However, in the current study, an opposite trend was observed, which could be considered to result from stoichiometric deviation due to N desorption. The lack of N atoms can result in the generation of defects and consequently lattice distortions, thus hindering grain growth. Sundgren and Wang et al. also proposed that the increase in deviation from stoichometry causes the reduction in grain size; however,
3.1. Crystal structure Fig. 1 presents the composition of (TiVCrZrHf)N coatings deposited at various substrate temperatures. The N content was observed to be larger than 50 at.% when the substrate temperatures was RT. This indicates that the N atoms interstitially incorporated in the coatings are regarded as over-stoichiometric nitride coatings. With the increasing substrate temperature, the N content greatly decreased and reached about 40 at.% as the substrate temperature reached 450 ◦ C; it was regarded as an understoichiometric nitride coating. Under low substrate temperature deposition, extra entrapped nitrogen atoms at non-equilibrium sites may exist. Once the substrate temperature is increased during the deposition process, the entrapped atoms can obtain more energy, leading to their escape from non-equilibrium sites; this is called the desorption process [16,17]. The decrease in N content obtained at an elevated substrate temperature may be due to its higher desorption rate from the film surface at a higher substrate temperature.
Fig. 2. X-ray diffraction pattern of the (TiVCrZrHf)N coatings deposited at substrate temperature.
S.-C. Liang et al. / Applied Surface Science 257 (2011) 7709–7713
7711
Fig. 3. Grain size of the (TiVCrZrHf)N coatings deposited at various substrate temperature.
in their study, the specimens were over-stoichiometric TiN, ZrN, and (AlCrTaTiZr)N [12,18,19]. The grain size gradually increases as the substrate temperature increases further. We considered the grain growth effect at an elevated substrate temperature to dominate the structure evolution in this situation. In short, this is a result of the competition between the two different factors aforementioned, whereas the grain size varies with the substrate temperature. 3.2. Microstructure Fig. 4 shows the SEM micrographs of (TiVCrZrHf)N coatings deposited at various substrate temperatures. The surface of the coating deposited at RT exhibits a grain-like structure. The SEM reveals the surface of the coating to be very smooth as the substrate temperature increases. A similar behavior has been recently reported by Ghosh and Yang et al. in the case of sputtered Cu–N and Mo–Al–N films, respectively [20,21]. This is considered to be related to the enhancement of the diffusivity of the atomic species on the surface, which helps in filling in the pores as well as voids and thus determines a smooth growth. Moreover, it is observed that the structure becomes denser as the substrate temperature increases; this can be also seen clearly in the cross-sectional SEM images. Fig. 5 shows the TEM micrographs with the selected area diffraction (SAD) patterns of (TiVCrZrHf)N coatings deposited at a substrate temperature is 450 ◦ C. Based on the SAD patterns, zones A and B correspond to the amorphous phase and FCC phase, respectively. Zone A shows an amorphous structure with a thickness of approximately 600 nm. A nano grain structure (size ∼1 nm) with a random orientation can also be seen above the amorphous structure. According to the XRD results, the lattice parameter of the (TiVCrZrHf)N deposited at a substrate temperature of 450 ◦ C is calculated as 0.4403 nm. The lattice parameter of the Sicrystal is 0.5431 nm. The initial deposition was under tremendous stress because (TiVCrZrHf)N and the substrate had a significant lattice mismatch of 19% in the lattice. Associated with the severe lattice distortion of multi-principal elements, the formation of the crystalline phase was prevented. Due to the relaxation of these stresses away from the interface between (TiVCrZrHf)N and the substrate, the lattice order started to form nano grains. The fully dense columnar structure with an FCC crystal phase began to develop. A similar interfacial microstructure evolution was also obtained by Song and Tsaiet al. [22,23]. The (B, Al)N films showed a continuous variation in microstructure as a function of film thickness, from amorphous to crystal columns. Tsai et al. also
Fig. 4. SEM micrographs of the (TiVCrZrHf)N coatings deposited at various substrate temperature: (a) RT; (b) 250 ◦ C; (c) 350 ◦ C, and (d) 450 ◦ C.
observed that there is an amorphous (TiVCr)N thin layer between crystal (TiVCr)N and Si substrate. In the current study, the coating thickness decreased to 1250, 1200, 1066, and 1030 nm at a substrate temperature at RT, 250, 350, and 450 ◦ C, respectively, as shown in Fig. 4. The deposited rate decreased from 20.8 to 17.1 nm nm/min as the substrate temperature increased. As explained above, the deposition at a higher substrate temperature leads to nitrogen desorption and the elimination of pores and voids, which are helpful in decreasing the coating thickness [24]. 3.3. Hardness Fig. 6 shows the hardness and elastic modulus of (TiVCrZrHf)N deposited at various substrate temperatures. The coatings exhibit high hardness and modulus values of 30–48 GPa and 290–316 GPa, respectively. Surprisingly, the maximum hardness of 48 GPa in the as-deposited state of the present films is the highest among all the high entropy alloy coatings, as listed in Table 1. This demonstrates the present alloy design and process parameter to be effective in achieving high hardness. Fig. 6 also shows that hardness significantly increases when the substrate temperature also increases. In the current study, the grain size varied with the substrate temperature. However, the variation
7712
S.-C. Liang et al. / Applied Surface Science 257 (2011) 7709–7713
Fig. 5. TEM micrographs with SAD patterns of the (TiVCrZrHf)N coatings deposited at substrate temperature is 450 ◦ C.
is quite small and does not conform with the trend of the hardness with substrate temperature. This suggests that another factor for the hardness of the present coatings needs to be considered. As discussed previously, because the substrate temperature increases, the higher mobility of the deposited atoms on the surface should be expected. This results in the reduction of growth voids and consequently the formation of a denser structure, as shown in Fig. 4. Accordingly, the densification of structure may account for the enhanced hardness [10,28]. Note that there a significant increase in hardness by around 36% when substrate temperature increases Table 1 The maximum hardness of other HEA coatings research reported.
Fig. 6. Hardness and elastic modulus of the (TiVCrZrHf)N coatings deposited at various substrate temperature.
Composition
Maximum hardness (GPa)
Reference
Al–Cr–Ni–Si–Ti–N Ti–V–Cr–Zr–Y–N Al–Cr–Mo–Si–Ti–N Ti–Al–Cr–Si–V–N Al–Cr–Ta–Ti–Zr–N Al–Mo–Nb–Si–Ta–Ti–V–Zr–N Al–Cr–Nb–Si–Ti–V–N
15 18 25 31 35 37 41
[25] [10] [26] [27] [12] [14] [13]
S.-C. Liang et al. / Applied Surface Science 257 (2011) 7709–7713
from RT to 250 ◦ C. The poorer crystallinity indicates higher density of grain boundaries. An increase in the density of the grain boundaries diminishes dislocation activity, resulting in grain boundary hardening. 4. Conclusions The crystal structure, microstructure, and mechanical properties of (TiVCrZrHf)N alloy coatings prepared by rf magnetron sputtering at different substrate temperatures ranging from RT to 450 ◦ C were investigated. The (TiVCrZrHf)N has a simple FCC solid solution structure. The decrease in N content obtained at an elevated substrate temperature may be due to its higher desorption rate from the film surface at a higher substrate temperature. The increase in deviation from the stoichometry results in a reduction in grain size. However, the grain size gradually increases as the substrate temperature increases further due to the enhanced adatom mobility. The lower part of the microstructure has an amorphous structure, which results from the lattice distortion of the multi-principal elements and the mismatch in the lattice between (TiVCrZrHf)N and the substrate. With the relaxation of these stresses away from the interface between (TiVCrZrHf)N and the substrate, the lattice order begins to form nano grains. The fully dense columnar structure with an FCC crystal phase then starts to develop. Surprisingly, the coatings reach the highest hardness of 48 GPa, higher than that of all current HEA coatings. This confirms the effectiveness of the present alloy design obtaining a high hardness. Acknowledgements The authors gratefully acknowledge the financial support for this research by the National Science Council of Taiwan under grant no. NSC96-2628-E-005-003-MY3. References [1] J.W. Yeh, S.K. Chen, S.J. Lin, J.Y. Gan, T.S. Chin, T.T. Shun, C.H. Tsau, S.Y. Chang, Adv. Eng. Mater. 6 (2004) 299–303.
7713
[2] J.W. Yeh, S.K. Chen, J.Y. Gan, S.J. Lin, T.S. Chin, T.T. Shun, C.H. Tsau, S.Y. Chang, Metall. Mater. Trans. A 35 (2004) 2533–2536. [3] P.K. Huang, J.W. Yeh, T.T. Shun, S.K. Chen, Adv. Eng. Mater. 6 (2004) 74–78. [4] Y.S. Huang, L. Chen, H.W. Lui, M.H. Cai, J.W. Yeh, Mater. Sci. Eng. A 457 (2007) 77–83. [5] C.C. Tung, J.W. Yeh, T.T. Shun, S.K. Chen, Y.S. Huang, H.C. Chen, Mater. Lett. 61 (2007) 1–5. [6] S.W. Kao, J.W. Yeh, T.S. Chin, J. Phys. C 20 (2008), 145214- 1- 145214- 7. [7] Y.J. Zhou, Y. Zhang, T.N. Kim, G.L. Chen, Mater. Lett. 62 (2008) 2673–2676. [8] S. Varalakshmi, M. Kamaraj, B.S. Murty, J. Alloys Compd. 460 (2008) 253–257. [9] H.W. Chang, P.K. Huang, J.W. Yeh, A. Davison, C.H. Tsau, C.C. Yang, Surf. Coat. Technol. 202 (2008) 3360–3366. [10] D.C. Tsai, Y.L. Huang, S.R. Lin, S.C. Liang, F.S. Shieu, Appl. Surf. Sci. 257 (2010) 1361–1367. [11] C.H. Lai, S.J. Lin, J.W. Yeh, S.Y. Chang, Surf. Coat. Technol. 201 (2006) 3275–3280. [12] C.H. Lai, M.H. Tsai, S.J. Lin, J.W. Yeh, Surf. Coat. Technol. 201 (2007) 6993–6998. [13] P.K. Huang, J.W. Yeh, Surf. Coat. Technol. 203 (2009) 1891–1896. [14] M.H. Tsai, C.H. Lai, J.W. Yeh, J.Y. Gan, J. Phys. D 41 (2008), 235402- 1- 2354027. [15] H.P. Klug, L.E. Alexander, X-ray Diffraction Procedures for Polycrystalline and Amorphous Materials, Wiley & Sons, New York, 1974. [16] C.W. Ong, X.A. Zhao, Y.C. Tsang, C.L. Choy, P.W. Chan, Thin Solid Films 280 (1996) 1–4. [17] J. Musiland, R. Daniel, Surf. Coat. Technol. 166 (2003) 243–253. [18] J.E. Sundgren, B.O. Johansson, S.E. Karlsson, H.T.G. Hentzell, Thin Solid Films 105 (1983) 367–384. [19] S.H. Wang, C.C. Chang, J.S. Chen, J. Vac. Sci. Technol. A 22 (2004) 2145–2151. [20] S. Ghosh, F. Singh, D. Choudhary, D.K. Avasthi, V. Ganesan, P. Shah, A. Gupta, Surf. Coat. Technol. 142 (2001) 1034–1039. [21] J.F. Yang, Z.G. Yuan, Q. Liu, X.P. Wang, Q.F. Fang, Mater. Res. Bull. 44 (2009) 86–90. [22] J.H. Song, S.C. Wang, J.C. Sung, J.L. Huang, D.F. Lii, Thin Solid Films 517 (2009) 4753–4757. [23] D.C. Tsai, Y.L. Huang, S.R. Lin, S.C. Liang, F.S. Shieu, Nucl. Instrum. Methods Phys. Res., Sect. B 269 (2011) 685–691. [24] H.N. Shah, R. Jayaganthan, D. Kaur, R. Chandra, Thin Solid Films 518 (2010) 5762–5768. [25] T.K. Chen, M.S. Wong, T.T. Shun, J.W. Yeh, Surf. Coat. Technol. 200 (2005) 1361–1365. [26] H.W. Chang, P.K. Huang, A. Davison, J.W. Yeh, C.H. Tsau, C.C. Yang, Thin Solid Films 516 (2008) 6402–6408. [27] C.H. Lin, J.G. Duh, J.W. Yeh, Surf. Coat. Technol. 201 (2007) 6304–6308. [28] D.C. Tsai, Y.L. Huang, S.R. Lin, D.R. Jung, S.Y. Chang, F.S. Shieu, J. Alloys Compd. 509 (2011) 3141–3147.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Effects of substrate temperature on the structure and mechanical properties of (TiVCrZrHf)N coatings Shih-Chang Liang a , Zue-Chin Chang b , Du-Cheng Tsai a , Yi-Chen Lin b , Huan-Shin Sung b , Min-Jen Deng a,c , Fuh-Sheng Shieu a,∗ a
Department of Materials Science and Engineering, National Chung Hsing University, Taichung 402, Taiwan Department of Mechanical Engineering, National Chin-Yi University of Technology, Taichung 411, Taiwan c Department of Optometry, Jen-Teh Junior College of Medicine, Nursing and Management, Miaoli County 356, Taiwan b
a r t i c l e
i n f o
Article history: Received 9 November 2010 Received in revised form 2 April 2011 Accepted 2 April 2011 Available online 12 April 2011 Keywords: Coating materials Nitride materials Thin films Vapor deposition Crystal structure Microstructure
a b s t r a c t The present paper reports the influence of growth conditions on the characteristics of (TiVCrZrHf)N films prepared by rf reactive magnetron sputtering at various substrate temperatures. The nitrogen content is observed to decrease with increasing substrate temperature. The X-ray diffraction results indicate that all (TiVCrZrHf)N films are simple face centered cubic (FCC) structures. Initially, there is an obvious decrease followed by an increase in grain size with the increase in substrate temperature. The lower part of the microstructure has an amorphous structure. A nano grain structure (size ∼1 nm) with a random orientation is also observed above the amorphous structure. The fully dense columnar structure with an fcc crystal phase then starts to develop. Extreme hardness of around 48 GPa is obtained in the present alloy design. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Interesting alloy systems added with multi-principal elements (more than five elements; 5 at.%≤ each element content ≤35 at.%) called high entropy alloys (HEAs) have been proposed and investigated by Yeh et al. [1]. Based on previous research, HEAs have been found to form simple phases with nanocrystalline and even amorphous structures. Aside from the high-entropy effect, these structural characteristics are ascribed to the large lattice distortion and sluggish diffusionof such multielemental mixtures. Following Boltzmann’s hypothesis on the relationship between entropy and system complexity [1], the configurational entropy change, Sconf , during the formation of a solid solution from n elements with equimolar fractions can be calculated using the following equation:
S = −R In
1 n
= R In(n)
where R is the ideal gas constant, and is n the number of mixed elements. When n = 5, Sconf = 1.61R, which approaches the size of the melting entropy of most intermetallic compounds (about R–2R).
∗ Corresponding author. Tel.: +886 4 2284 0500; fax: +886 4 2285 7017. E-mail address: [email protected] (F.-S. Shieu). 0169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2011.04.014
This indicates that a single solid solution, i.e., face-centered cubic (FCC), body-centered cubic (BCC), or FCC + BCC, is formed preferentially for HEAs [1–3]. This makes these alloys easier to analyze and understand as to control and develop. HEAs have high lattice distortions because they comprise multi-principal elements with different atomic sizes. This hinders atomic movement and consequently limits the effective diffusion rate in HEAs. As a result, HEAs tend to form nanosized and even amorphous structures [1–3]. Based on the high mixing entropy effect and the formation of amorphous or nanocrystallized (FCC and/or BCC phases) structure, some interesting properties are expected, such as high stiffness, strength, toughness, hardness, thermal stability, and corrosion resistance [4–9]. To extend the alloy concept to coating materials, HEA nitride coatings were synthesized by sputtering an alloyed target formed by the arc melting process. In these studies, TiVCrZrY, AlCrTaTiZr, AlCrNbSiTiV, and AlMoNbSiTaTiVZr show simple solid solution structures and outstanding mechanical properties [10–13]. The TiVCrZrY and AlCrTaTiZr nitride coatings exhibit hardness at a range of 14–32 GPa when deposited without substrate bias and/or heating [10,11]. The AlCrTaTiZr, AlCrNbSiTiV, and AlMoNbSiTaTiVZrnitrides coatings exhibit high hardness value of 35, 41, and 37 GPa under biased and/or heated conditions, respectively [12–14]. These findings confirm that HEAs have great potential for use as protective coatings. Thus, in the current study, (TiVCrZrHf)Nx
7710
S.-C. Liang et al. / Applied Surface Science 257 (2011) 7709–7713
multi-component coatings with the addition of quinary metallic elements were prepared by a reactive sputtering process. The effects of substrate temperature on structure and properties of deposited coatings were investigated. 2. Experiment The (TiVCrZrHf)N coatings were deposited on p-Si (100) wafers by an rf magnetron sputtering system using equi-molar TiVCrZrHf targets 75 mm in diameter. Before deposition, the Si substrates were cleaned and rinsed with ethanol and distilled water in an ultrasonic bath. The sputtering chamber was pumped down to 2.67 × 10−2 Pa using a turbo pump. The deposition of the (TiVCrZrHf)N coatings was carried out in an Ar + N2 mixed atmosphere under a dc power of 350 W and a working pressure of 6.67 × 10−1 Pa.During deposition, the Ar flow and N2 flow are 100 and 4 SCCM, respectively. The substrate temperature was chosen as the controlling parameter, which varied from room temperature (RT) to 450 ◦ C. The work distance between the substrate and target was 90 mm. The deposition time was set to 60 min. The bias of the substrate was fixed at −100 V during the deposition process. The targets were presputtered by Ar to remove their surface oxide layers before deposition. The chemical compositions of the (TiVCrZrHf)N coatings were determined by field emission electron probe micro-analyses (FEEPMA JOEL JXA-8800 M). At least three tests were performed for each sample. The crystal structures were analyzed by a glancingincidence (1◦ ) X-ray diffractometer (XRD, BRUKER D8 Discover) using Cu K␣ radiation at a scanning speed of 1 deg/min. The scanning step was 0.01◦ , and the scanning range was 20–80◦ . Further, from the full width at half-maximum, the average grain sizes of the coatings were calculated by Scherrer’s formula [15]. The morphology studies and thickness measurements were carried out using field emission scanning electron microscopy (SEM, JEOL JSM6700F). The deposition rate was obtained by dividing the thickness with the deposition time. Microstructural examinations were conducted by an analytical transmission electron microscope (TEM, JEM-2100). The microhardness and elastic modulus of the coatings were measured under a load of 3000 N using a TriboLabnanoindenter (Hysitron). A Berkovich diamond indenter tip was used to reduce the influence of the substrate on the microhardness and elastic modulus of the coatings. At least five tests were performed for each sample. 3. Results and discussion
Fig. 1. EPMA element contents in (TiVCrZrHf)N coatings deposited at various substrate temperature.
Fig. 2 shows the XRD patterns of (TiVCrZrHf)N coatings deposited at various substrate temperatures. (TiVCrZrHf)N coatings exhibit a single FCC solid solution structure without significant phase separation. (TiVCrZrHf)N shows a single FCC solid solution structure because TiN, VN, CrN, ZrN, and HfN are all FCC structures. The same phenomenon has also been reported for as-deposited mixtures of FCC forming multi-principal element nitrides, such as TiVCrZrY, AlCrTaTiZr, AlCrNbSiTiV, and AlMoNbSiTaTiVZr [10–14]. This demonstrates that the high entropy effect in multi-component nitrides can stabilize the solid-solution phase. Fig. 3 shows the grain size of (TiVCrZrHf)N coatings deposited at various substrate temperatures. In the current study, the substrate temperature alters the crystallinity and grain size. The (TiVCrZrHf)N coatings deposited at RT have a strong (1 1 1) orientation and lesser orientation from others. When the substrate temperature increases to 250 ◦ C, the overall crystallinity becomes poorer and the grain size becomes smaller. Generally, due to the enhanced adatom mobility at a higher substrate temperature, grain growth occurs. However, in the current study, an opposite trend was observed, which could be considered to result from stoichiometric deviation due to N desorption. The lack of N atoms can result in the generation of defects and consequently lattice distortions, thus hindering grain growth. Sundgren and Wang et al. also proposed that the increase in deviation from stoichometry causes the reduction in grain size; however,
3.1. Crystal structure Fig. 1 presents the composition of (TiVCrZrHf)N coatings deposited at various substrate temperatures. The N content was observed to be larger than 50 at.% when the substrate temperatures was RT. This indicates that the N atoms interstitially incorporated in the coatings are regarded as over-stoichiometric nitride coatings. With the increasing substrate temperature, the N content greatly decreased and reached about 40 at.% as the substrate temperature reached 450 ◦ C; it was regarded as an understoichiometric nitride coating. Under low substrate temperature deposition, extra entrapped nitrogen atoms at non-equilibrium sites may exist. Once the substrate temperature is increased during the deposition process, the entrapped atoms can obtain more energy, leading to their escape from non-equilibrium sites; this is called the desorption process [16,17]. The decrease in N content obtained at an elevated substrate temperature may be due to its higher desorption rate from the film surface at a higher substrate temperature.
Fig. 2. X-ray diffraction pattern of the (TiVCrZrHf)N coatings deposited at substrate temperature.
S.-C. Liang et al. / Applied Surface Science 257 (2011) 7709–7713
7711
Fig. 3. Grain size of the (TiVCrZrHf)N coatings deposited at various substrate temperature.
in their study, the specimens were over-stoichiometric TiN, ZrN, and (AlCrTaTiZr)N [12,18,19]. The grain size gradually increases as the substrate temperature increases further. We considered the grain growth effect at an elevated substrate temperature to dominate the structure evolution in this situation. In short, this is a result of the competition between the two different factors aforementioned, whereas the grain size varies with the substrate temperature. 3.2. Microstructure Fig. 4 shows the SEM micrographs of (TiVCrZrHf)N coatings deposited at various substrate temperatures. The surface of the coating deposited at RT exhibits a grain-like structure. The SEM reveals the surface of the coating to be very smooth as the substrate temperature increases. A similar behavior has been recently reported by Ghosh and Yang et al. in the case of sputtered Cu–N and Mo–Al–N films, respectively [20,21]. This is considered to be related to the enhancement of the diffusivity of the atomic species on the surface, which helps in filling in the pores as well as voids and thus determines a smooth growth. Moreover, it is observed that the structure becomes denser as the substrate temperature increases; this can be also seen clearly in the cross-sectional SEM images. Fig. 5 shows the TEM micrographs with the selected area diffraction (SAD) patterns of (TiVCrZrHf)N coatings deposited at a substrate temperature is 450 ◦ C. Based on the SAD patterns, zones A and B correspond to the amorphous phase and FCC phase, respectively. Zone A shows an amorphous structure with a thickness of approximately 600 nm. A nano grain structure (size ∼1 nm) with a random orientation can also be seen above the amorphous structure. According to the XRD results, the lattice parameter of the (TiVCrZrHf)N deposited at a substrate temperature of 450 ◦ C is calculated as 0.4403 nm. The lattice parameter of the Sicrystal is 0.5431 nm. The initial deposition was under tremendous stress because (TiVCrZrHf)N and the substrate had a significant lattice mismatch of 19% in the lattice. Associated with the severe lattice distortion of multi-principal elements, the formation of the crystalline phase was prevented. Due to the relaxation of these stresses away from the interface between (TiVCrZrHf)N and the substrate, the lattice order started to form nano grains. The fully dense columnar structure with an FCC crystal phase began to develop. A similar interfacial microstructure evolution was also obtained by Song and Tsaiet al. [22,23]. The (B, Al)N films showed a continuous variation in microstructure as a function of film thickness, from amorphous to crystal columns. Tsai et al. also
Fig. 4. SEM micrographs of the (TiVCrZrHf)N coatings deposited at various substrate temperature: (a) RT; (b) 250 ◦ C; (c) 350 ◦ C, and (d) 450 ◦ C.
observed that there is an amorphous (TiVCr)N thin layer between crystal (TiVCr)N and Si substrate. In the current study, the coating thickness decreased to 1250, 1200, 1066, and 1030 nm at a substrate temperature at RT, 250, 350, and 450 ◦ C, respectively, as shown in Fig. 4. The deposited rate decreased from 20.8 to 17.1 nm nm/min as the substrate temperature increased. As explained above, the deposition at a higher substrate temperature leads to nitrogen desorption and the elimination of pores and voids, which are helpful in decreasing the coating thickness [24]. 3.3. Hardness Fig. 6 shows the hardness and elastic modulus of (TiVCrZrHf)N deposited at various substrate temperatures. The coatings exhibit high hardness and modulus values of 30–48 GPa and 290–316 GPa, respectively. Surprisingly, the maximum hardness of 48 GPa in the as-deposited state of the present films is the highest among all the high entropy alloy coatings, as listed in Table 1. This demonstrates the present alloy design and process parameter to be effective in achieving high hardness. Fig. 6 also shows that hardness significantly increases when the substrate temperature also increases. In the current study, the grain size varied with the substrate temperature. However, the variation
7712
S.-C. Liang et al. / Applied Surface Science 257 (2011) 7709–7713
Fig. 5. TEM micrographs with SAD patterns of the (TiVCrZrHf)N coatings deposited at substrate temperature is 450 ◦ C.
is quite small and does not conform with the trend of the hardness with substrate temperature. This suggests that another factor for the hardness of the present coatings needs to be considered. As discussed previously, because the substrate temperature increases, the higher mobility of the deposited atoms on the surface should be expected. This results in the reduction of growth voids and consequently the formation of a denser structure, as shown in Fig. 4. Accordingly, the densification of structure may account for the enhanced hardness [10,28]. Note that there a significant increase in hardness by around 36% when substrate temperature increases Table 1 The maximum hardness of other HEA coatings research reported.
Fig. 6. Hardness and elastic modulus of the (TiVCrZrHf)N coatings deposited at various substrate temperature.
Composition
Maximum hardness (GPa)
Reference
Al–Cr–Ni–Si–Ti–N Ti–V–Cr–Zr–Y–N Al–Cr–Mo–Si–Ti–N Ti–Al–Cr–Si–V–N Al–Cr–Ta–Ti–Zr–N Al–Mo–Nb–Si–Ta–Ti–V–Zr–N Al–Cr–Nb–Si–Ti–V–N
15 18 25 31 35 37 41
[25] [10] [26] [27] [12] [14] [13]
S.-C. Liang et al. / Applied Surface Science 257 (2011) 7709–7713
from RT to 250 ◦ C. The poorer crystallinity indicates higher density of grain boundaries. An increase in the density of the grain boundaries diminishes dislocation activity, resulting in grain boundary hardening. 4. Conclusions The crystal structure, microstructure, and mechanical properties of (TiVCrZrHf)N alloy coatings prepared by rf magnetron sputtering at different substrate temperatures ranging from RT to 450 ◦ C were investigated. The (TiVCrZrHf)N has a simple FCC solid solution structure. The decrease in N content obtained at an elevated substrate temperature may be due to its higher desorption rate from the film surface at a higher substrate temperature. The increase in deviation from the stoichometry results in a reduction in grain size. However, the grain size gradually increases as the substrate temperature increases further due to the enhanced adatom mobility. The lower part of the microstructure has an amorphous structure, which results from the lattice distortion of the multi-principal elements and the mismatch in the lattice between (TiVCrZrHf)N and the substrate. With the relaxation of these stresses away from the interface between (TiVCrZrHf)N and the substrate, the lattice order begins to form nano grains. The fully dense columnar structure with an FCC crystal phase then starts to develop. Surprisingly, the coatings reach the highest hardness of 48 GPa, higher than that of all current HEA coatings. This confirms the effectiveness of the present alloy design obtaining a high hardness. Acknowledgements The authors gratefully acknowledge the financial support for this research by the National Science Council of Taiwan under grant no. NSC96-2628-E-005-003-MY3. References [1] J.W. Yeh, S.K. Chen, S.J. Lin, J.Y. Gan, T.S. Chin, T.T. Shun, C.H. Tsau, S.Y. Chang, Adv. Eng. Mater. 6 (2004) 299–303.
7713
[2] J.W. Yeh, S.K. Chen, J.Y. Gan, S.J. Lin, T.S. Chin, T.T. Shun, C.H. Tsau, S.Y. Chang, Metall. Mater. Trans. A 35 (2004) 2533–2536. [3] P.K. Huang, J.W. Yeh, T.T. Shun, S.K. Chen, Adv. Eng. Mater. 6 (2004) 74–78. [4] Y.S. Huang, L. Chen, H.W. Lui, M.H. Cai, J.W. Yeh, Mater. Sci. Eng. A 457 (2007) 77–83. [5] C.C. Tung, J.W. Yeh, T.T. Shun, S.K. Chen, Y.S. Huang, H.C. Chen, Mater. Lett. 61 (2007) 1–5. [6] S.W. Kao, J.W. Yeh, T.S. Chin, J. Phys. C 20 (2008), 145214- 1- 145214- 7. [7] Y.J. Zhou, Y. Zhang, T.N. Kim, G.L. Chen, Mater. Lett. 62 (2008) 2673–2676. [8] S. Varalakshmi, M. Kamaraj, B.S. Murty, J. Alloys Compd. 460 (2008) 253–257. [9] H.W. Chang, P.K. Huang, J.W. Yeh, A. Davison, C.H. Tsau, C.C. Yang, Surf. Coat. Technol. 202 (2008) 3360–3366. [10] D.C. Tsai, Y.L. Huang, S.R. Lin, S.C. Liang, F.S. Shieu, Appl. Surf. Sci. 257 (2010) 1361–1367. [11] C.H. Lai, S.J. Lin, J.W. Yeh, S.Y. Chang, Surf. Coat. Technol. 201 (2006) 3275–3280. [12] C.H. Lai, M.H. Tsai, S.J. Lin, J.W. Yeh, Surf. Coat. Technol. 201 (2007) 6993–6998. [13] P.K. Huang, J.W. Yeh, Surf. Coat. Technol. 203 (2009) 1891–1896. [14] M.H. Tsai, C.H. Lai, J.W. Yeh, J.Y. Gan, J. Phys. D 41 (2008), 235402- 1- 2354027. [15] H.P. Klug, L.E. Alexander, X-ray Diffraction Procedures for Polycrystalline and Amorphous Materials, Wiley & Sons, New York, 1974. [16] C.W. Ong, X.A. Zhao, Y.C. Tsang, C.L. Choy, P.W. Chan, Thin Solid Films 280 (1996) 1–4. [17] J. Musiland, R. Daniel, Surf. Coat. Technol. 166 (2003) 243–253. [18] J.E. Sundgren, B.O. Johansson, S.E. Karlsson, H.T.G. Hentzell, Thin Solid Films 105 (1983) 367–384. [19] S.H. Wang, C.C. Chang, J.S. Chen, J. Vac. Sci. Technol. A 22 (2004) 2145–2151. [20] S. Ghosh, F. Singh, D. Choudhary, D.K. Avasthi, V. Ganesan, P. Shah, A. Gupta, Surf. Coat. Technol. 142 (2001) 1034–1039. [21] J.F. Yang, Z.G. Yuan, Q. Liu, X.P. Wang, Q.F. Fang, Mater. Res. Bull. 44 (2009) 86–90. [22] J.H. Song, S.C. Wang, J.C. Sung, J.L. Huang, D.F. Lii, Thin Solid Films 517 (2009) 4753–4757. [23] D.C. Tsai, Y.L. Huang, S.R. Lin, S.C. Liang, F.S. Shieu, Nucl. Instrum. Methods Phys. Res., Sect. B 269 (2011) 685–691. [24] H.N. Shah, R. Jayaganthan, D. Kaur, R. Chandra, Thin Solid Films 518 (2010) 5762–5768. [25] T.K. Chen, M.S. Wong, T.T. Shun, J.W. Yeh, Surf. Coat. Technol. 200 (2005) 1361–1365. [26] H.W. Chang, P.K. Huang, A. Davison, J.W. Yeh, C.H. Tsau, C.C. Yang, Thin Solid Films 516 (2008) 6402–6408. [27] C.H. Lin, J.G. Duh, J.W. Yeh, Surf. Coat. Technol. 201 (2007) 6304–6308. [28] D.C. Tsai, Y.L. Huang, S.R. Lin, D.R. Jung, S.Y. Chang, F.S. Shieu, J. Alloys Compd. 509 (2011) 3141–3147.