Microstructure and mechanical properties of AlCrN ...

Surface & Coatings Technology 201 (2006) 4361 – 4366 www.elsevier.com/locate/surfcoat

Microstructure and mechanical properties of AlCrN films deposited by CFUBMS Gwang Seok Kim, Sang Yul Lee ⁎ Center for Advanced Plasma Surface Technology, Department of Materials Engineering, HanKuk Aviation University, 200-1, KoYangSi, KyungKi-Do, 412-791, South Korea Available online 20 September 2006

Abstract In this paper, Al1−xCrxN films were synthesized by closed field unbalanced magnetron sputtering (CFUBMS) with vertical magnetron sources for high temperature applications and their chemical composition, crystalline structure, morphology and mechanical properties were characterized by Auger electron spectroscopy (AES), X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), nanoindentation, scratch and wear tests. Also, the thermal stability of the films was evaluated by hardness measurements after annealing treatments at temperatures between 600 and 1000 °C in air for 30 min. Al1−xCrxN films with 0.29 ≤ X ≤ 0.69 formed a ternary solid solution and exhibited the crystalline phases of fcc B1 type structure with strong (111) preferential orientation. Depending on the Cr content (X value), their hardness values were ranging from 31 to 41 GPa and the residual stresses were in the range of − 4.5 to −5.6 GPa. In comparison to other films, the enhanced mechanical properties and thermal stability were observed from the Al rich Al1−xCrxN film (X = 0.29). © 2006 Elsevier B.V. All rights reserved. Keywords: Al–Cr–N; Hardness; Wear property; Thermal stability; CFUBMS

1. Introduction For many years, binary transition metal nitride films have been used widely as protective and wear resistant hard coatings for cutting and forming tools due to their superior mechanical and tribological properties [1,2]. However, in spite of their outstanding properties, binary systems are still inadequate for high temperature applications. During operation at high temperatures (above 700 °C [3]), their mechanical properties degrade rapidly by the formation of porous oxides at the film surface. In order to overcome these problems, a ternary nitride coating system was explored and until now, various ternary nitride films such as Ti–Zr–N [4], Cr–Al–N [5], Zr–Al–N [6] and Ci–Si–N [7] were developed. Among these ternary coatings, the Cr–Al–N films are probably the most promising coatings for high temperature applications, due to their excellent oxidation resistance compared to other coatings. Sanjines et al. reported that a significant oxidation of Ti0.5Al0.5N occurs only at temperatures above 970 K while Cr0.5Al0.5N remains stable up to 1170 K [8]. ⁎ Corresponding author. Tel.: +82 2 300 0166; fax: +82 2 3158 3770. E-mail address: [email protected] (S.Y. Lee). 0257-8972/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2006.08.076

Fig. 1. Schematic diagram of the closed field unbalanced magnetron sputtering system with vertical magnetron sources.

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Table 1 Relative ratios of X = Cr/(Al + Cr) and N/(Al + Cr) atomic percent in the Al1−xCrxN films as a function of the discharge current of Cr target Discharge current of Cr target (A)

Chemical composition (at.%)

X = Cr/ (Al + Cr)

N/(Al + Cr)

0.0 0.8 1.2 1.6 2.0 2.4 3.0

43.5 Al–56.5 N 32.1 Al–13.2 Cr–54.7 N 27.0 Al–18.9 Cr–54.1 N 22.6 Al–22.8 Cr–54.6 N 20.0 Al–25.5 Cr–54.5 N 17.6 Al–28.6 Cr–53.8 N 15.0 Al–32.9 Cr–52.1 N

0.0 0.29 0.41 0.50 0.56 0.62 0.69

1.30 1.21 1.18 1.20 1.20 1.16 1.10

In the Cr–Al–N system, it was reported that the Al-rich film [9] has a higher oxidation resistance than the Cr-rich film. However, the mechanical properties of Al-rich film were rarely published. In addition, Musil et al. reported in their recent work that reactively sputtered Me–Si–N coating (Me = Zr, Ta) with high content of Si (N40 at.% Si) showed a significant increase in oxidation resistance up to 1350 °C in flowing air environment [10,11]. In the present work, the Al1−xCrxN films with 0 ≤ X ≤ 0.69 were synthesized by closed field unbalanced magnetron sputtering with vertical magnetron sources for high temperature applications such as die casting mould and semi-solid forming tools. 2. Experimental details Al1−xCrxN films were deposited on the silicon (100) and nitrided AISI H13 steel by closed field unbalanced magnetron sputtering with vertical magnetron sources. The deposition apparatus used in this work is shown in Fig. 1. In this setup, two magnetron sources with 99.99% metallic Al and Cr targets (diameter: 100 mm) were mounted in the upper part of the deposition chamber. The polarity of the magnets between the

two magnetron sources was arranged oppositely in order to form a closed magnetic field. This device has the advantage of highlevel ion bombardments being continuously provided during the deposition process as well as a dense plasma is maintained near the substrate by the closed trap of magnetic field lines between the two magnetron sources. Prior to film deposition, the sputtering chamber was evacuated to less than 1.34 × 10− 3 Pa and a pre-sputtering process at an Ar pressure of 0.4 Pa was carried out by placing a shutter between the targets and the substrate for 10 min to clean the target surface. After target cleaning, the film deposition was performed in an Ar–N2 atmosphere (a constant total pressure of pT = 0.493 Pa with a pN2 = 0.093 Pa). For film deposition with Al target a pulse DC discharge with duty cycle of 80%, a frequency of 25 kHz and a discharge current of 3.0 A was used. For the Cr target, the DC current was adjusted between 0 and 3.0 A in order to control the Cr content in the films. Other conditions such as substrate bias voltage, distance of target-tosubstrate, deposition temperature and substrate rotation speed were fixed to − 100 V, 90 mm, 200 °C and 15 rpm, respectively. The chemical composition of the deposited films was determined by Auger electron spectroscopy (AES: PHI 670) and their crystal phase and texture were characterized by X-ray diffraction (XRD: Rigaku) with θ–2θ method using CuKα radiation. The surface and cross sectional morphologies were investigated using atomic force microscopy (AFM: Digital Instruments) and scanning electron microscopy (SEM: HITHACH S-3500H). To obtain the intrinsic hardness and elastic modulus of films without the substrate effect, a nanoindenter with continuous stiffness measurement (CSM) technique (Nano Indenter XP developed by MTS) was applied. On each sample, until the depth of 200 nm, ten indentations were performed using a Berkovich diamond tip. The loading was controlled such that the loading rate divided by the load was held constant at 0.05/s. The

Fig. 2. X-ray diffraction patterns of the Al1−xCrxN films for various Cr contents (X values). C-AlN represents cubic AlN.


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Fig. 3. Cross-sectional and surface morphologies of the Al1−xCrxN films for various Cr contents (X values).

residual stress in the films was determined by wafer-curvature measurements using a laser reflectance system and the critical load for the chipping of films (thickness: approximately 3.5 μm) on the pre-nitrided hot working tool steel having a Cr interlayer (thickness: approximately 1.0 μm) was evaluated by using a revetest scratch tester (CSEM) with a spherical diamond tip radius of 200 μm. The wear property of films was evaluated using a ballon-disk type tribometer with a 9.2 mm diameter Al2O3 ball as a counterpart material. The test was performed at 500 °C without lubrication under an applied normal load of 5.0 N. The sliding velocity was 0.24 m/s with a wear track diameter of 25 mm and a total sliding distance was 1000 m. After wear test, the morphologies of wear tracts developed on the film surface were examined by scanning electron microscopy. The thermal stability of films as a function of chemical composition was evaluated by hardness measurement after annealing experiments at temperatures between 600 and 1000 °C in ambient air for 30 min.

structure. Any wurtzite-type (B4) phase, such as w-Al(Cr)N was not observed in the investigated composition range (0.29 ≤ X ≤ 0.69). According to several authors [12,13], the phase transition

3. Results and discussion 3.1. Chemical composition and crystalline structure In Table 1 the atomic concentrations of Al, Cr and N as determined by AES on the Al1−xCrxN films are listed as a function of the discharge current of Cr target. In all films, impurities, such as carbon and oxygen, were rarely found and the Cr content, defined as X = Cr/(Al + Cr), increased monotonically from 0 to 0.69 with increasing Cr target current, whereas the nitrogen-to-metal atomic ratio of N/(Cr + Zr) maintained at a constant value of approximately 1.19 (from 1.1 to 1.3). Fig. 2 shows the X-ray diffraction patterns of Al1−xCrxN films deposited on the Si (100) substrate. For the AlN film (X =0), diffraction peaks corresponding to the (100), (102) and (103) crystal planes of wurtzite-type (B4) phase were observed, while the Al1−xCrxN films with 0.29 ≤X ≤ 0.69 exhibited the crystalline phase of fcc B1 type structure with strong (111) preferential orientation, which is the lowest strain energy plane of the B1 type

Fig. 4. Hardness and elastic modulus of the Al1−xCrxN films as a function of the Cr content (X value).

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At X = 0, the film had a typical columnar structure of about 150–200 nm in width, which corresponds to the Zone T in the Thornton's structure zone model (SZM). The addition of Cr during the deposition process tends to reduce the columnar size and surface roughness of AlN film. These observations are basically consistent with the results from multi-component coatings reported by other researchers [14,15]. In this study, for the film with X = 0.29, the most dense and fine structure was observed with columns of about 50–100 nm and the root-meansquare roughness (Rms = 1.95 nm) was decreased by a factor of three in comparison with that (Rms = 5.81 nm) achieved in the binary AlN film (X = 0). 3.3. Mechanical properties and thermal stability Fig. 5. Dependence of the hardness and residual stress on the Cr content (X value) in Al1−xCrxN films.

from B1-type to B4-type structure occurs in Cr–Al–N system at an AlN content of approximately 77 mol%. In addition, for films with 0.29≤X ≤ 0.69, the position of the X-ray diffraction peaks gradually shifts towards lower diffraction angles with increasing Cr content. This result indicates the lattice expansion by the incorporated Cr atoms, resulting in the formation of a ternary solid solution. The larger lattice parameter of CrN (0.414 nm) compared to that of cubic-AlN (0.412 nm) could explaine the lattice parameter increase of Al1−xCrxN films with increasing Cr content (X value). 3.2. Cross sectional and surface morphology The morphologies of films with X = 0, 0.29, 0.50 and 0.69 estimated by observing the AFM and SEM are shown in Fig. 3.

Fig. 4a shows the hardness curves of films measured using a nanoindenter with continuous stiffness measurement (CSM) technique. In the curve of the hardness over the indentation depth, the intrinsic hardness of the thin film could be obtained from a plateau region, if the substrate effect could be ignored. The average values for the hardness (H) and elastic modulus (E* = E / (1 − ν2)) of Al1−xCrxN films were determined at the indentation depths between 75 and 125 nm. These results are summarized in Fig. 4b. In the Al1−xCrxN films with 0.29 ≤ X ≤ 0.69, H and E* were measured to be in a range from 31 to 41 GPa and from 415 to 480 GPa, respectively. The maximum hardness of 41 GPa was observed for the film with X = 0.29 and this value is approximately 1.8 times higher than that of the binary AlN film (H = 23 GPa). This enhancement could be attributed to the solid solution hardening, in that with Cr (for low values of X) or Al (for high values of X) insertion, the lattice distortion in the film is developed and this distortion inhibits the

Fig. 6. Critical load for the chipping of films on the pre-nitrided hot working tool steel having a Cr interlayer: (a) X = 0.29, (b) X = 0.69, (c) CrN.


Fig. 7. SEM micrographs and profiles of wear tracks formed on the Al1−xCrxN films after wear tests.

Fig. 8. Hardness variation of Al1−xCrxN and CrN films after annealing treatments for various Cr content: (a) X = 0.29, (b) X = 0.50, (c) X = 0.69, (d) CrN.

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mobility of the dislocations, inducing a hardness enhancement. Also, the residual compressive stress of films could be considered as a factor on the hardness enhancement since the variation of residual stress shows a similar trend like the hardness of the films, as shown in Fig. 5. Fig. 6 shows the critical load for the chipping of Al1−xCrxN and CrN films deposited on the pre-nitrided hot working tool steel having a Cr interlayer (thickness = 0.7 μm). The CrN film (thickness = 3.0 μm) was deposited at a discharge current of 3 A DC and other conditions were the same as those used in deposition of the Al1−xCrxN films. Generally, the high residual stress causes the low adhesion strength. In this study, the critical load of films gradually increased with decreasing residual stress. However, their differences were insignificant because the Cr interlayer relax the internal stress effectively. Profile images after wear tests against counterpart Al2O3 ball and micrographs of wear tracks produced on Al1−xCrxN films with different Cr content (X value) are presented in Fig. 7. During the wear test, for films with X = 0.50 and 0.69, the surface of films were gradually worn and the macro-failures of films were observed in several places of the wear track. However the film with X = 0.29 was hardly worn by the counterpart Al2O3 ball during the wear test. The film failure was not observed inside the wear track. For this film, however, the counterpart Al2O3 ball had suffered some wear, so that some debris of Al2O3 ball could be observed as shown in Fig. 7. Due to the increase of contact area with a counterpart, it was found that the width of wear track was developed much wider than that of other films. This result could be attributed to the high hardness (H = 41 GPa) and dense microstructure. Fig. 8 shows the annealing effect on the hardness of Al1−xCrxN films (X = 0.29, 0.50 and 0.69) and binary CrN film. In case of CrN film, its hardness decreased markedly to 7.5 GPa after annealed at 800 °C. This indicates that the CrN was completely oxidized at this temperature. In contradiction to that, Al1−xCrxN films with X = 0.29, 0.50 and 0.69 did not show any sharp change of the hardness due to their superior thermal stability. The best thermal stability was observed for an Al1−xCrxN film with X = 0.29. In this film, the relatively high hardness of 30 GPa was maintained even after it was annealed at 950 °C although a moderate hardness drop was observed in this film as well. 4. Conclusions The main results obtained in this study could be summarized as follows: 1. The Al1−xCrxN films with 0.29 ≤ X ≤ 0.69 synthesized in this work formed a ternary solid solution and exhibited the

crystalline phases of fcc B1 type structure with a strong (111) preferential orientation. 2. Depending on the Cr content (X value), the hardness values of Al1−xCrxN films (excluding X = 0) ranged from 31 to 41 GPa and the residual stresses were in the range of − 4.5 to − 5.6 GPa. The maximum hardness of 41 GPa was observed at the film with X = 0.29 and this value is approximately 1.8 times higher than that (H = 23 GPa) of the binary AlN film. 3. During the wear tests, for the films with X = 0.50 and 0.69, the film surface was gradually worn out and macro-failures of films were observed in several places of wear track, while the film with X = 0.29 was scarcely worn by the counterpart Al2O3 ball and film failure was hardly observed inside the wear track. 4. Annealing treatments revealed that the Al1−xCrxN film with X = 0.29 has a higher thermal stability compared to other films. For this film, it was shown that the relatively high hardness of 30 GPa was maintained even after being annealed at 950 °C. Acknowledgements This work was supported by a grant (code No. R-11-2000086-0000-0) from the Center of Excellency Research program of the Korea Science and Engineering Foundation. References [1] V.R. Parameswaran, J.P. Immarigeon, D. Nagy, Surf. Coat. Technol. 52 (1992) 251. [2] G.S. Kim, S.Y. Lee, J.H. Hahn, et al., Surf. Coat. Technol. 171 (2003) 83. [3] X.T. Zeng, S. Zhang, C.Q. Sun, Y.C. Liu, Thin Solid Films 424 (2003) 99. [4] J.V. Ramana, Sanjiv Kumar, Christopher David, V.S. Raju, Mater. Lett. 58 (2004) 2553. [5] O. Banakh, P.E. Schmid, R. Sanjines, F. Levy, Surf. Coat. Technol. 163 (2003) 57. [6] R. Lamni, R. Sanjines, F. Lévy, Thin Solid Films 478 (2005) 170. [7] S.Y. Lee, S.D. Kim, G.S. Kim, Y.S. Hong, Mater. Sci. Forum 486–487 (2005) 444. [8] R. Sanjines, O. Banakh, C. Rojas, P.E. Schmid, F. Levy, Thin Solid Films 420–421 (2002) 312. [9] J. Vetter, E. Lugscheider, S.S. Guerreiro, Surf. Coat. Technol. 98 (1998) 1233. [10] H. Zeman, J. Musil, P. Zeman, J. Vac. Sci. Technol., A 22 (3) (2004) 646. [11] J. Musil, R. Daniel, P. Zeman, O. Takai, Thin Solid Films 478 (2005) 238. [12] Y. Makino, K. Nogi, Surf. Coat. Technol. 98 (1998) 1008. [13] A. Sugishima, H. Kajioka, Y. Makino, Surf. Coat. Technol. 97 (1997) 590. [14] J. Musil, J. Vlcek, Surf. Coat. Technol. 142–144 (2001) 557. [15] P.B. Barna, M. Adamik, Surf. Coat. Technol. 317 (1998) 27.