Contrib. Plasma Phys. 52, No. 7, 601 – 606 (2012) / DOI 10.1002/ctpp.201210056 Comparison of (Cr0.75 Al0.25 )N Coatings Deposited by Conventional and High Power Pulsed Magnetron Sputtering N. Bagcivan, K. Bobzin, and S. Theiß∗ Surface Engineering Institute, RWTH Aachen University, Kackertstr. 15, D-52072 Aachen Received 19 July 2011, accepted 18 November 2011 Published online 16 August 2012 Key words HPPMS, HIPIMS, CrAlN, texturing, DC, MF, PVD. Pulsed high power plasma discharges offer high potential by means of developing new coating systems with outstanding characteristics and enhanced application possibilities. To get high-performance materials the plasma physics of sputtering, transport and deposition during a High Power Pulse Magnetron Sputtering (HPPMS) process have to be investigated and correlated to coating characteristics as well as compound and system characteristics. Every coating has to be adapted to the specific application which it addresses. In the frame of this research project the HPPMS technology is used to investigate coating systems regarding their possibility to be used in injection molding processes. This manuscript shows a first comparison of conventional sputtered ternary Cr-Al-N by direct current (DC) magnetron sputtering (MS), pulsed middle frequency (MF) MS and Cr-Al-N deposited by HPPMS. The Cr:Al ratio was kept constant at about 75 : 25 (at-%). All three coatings were analyzed regarding thickness, morphology, chemical composition, texturing and mechanical properties like hardness and Young’s modulus. Furthermore, all three coatings were deposited on the complex structure of a gear wheel to obtain the uniformity of the coating. The results show that HPPMS-(Cr0.75 Al0.25 )N coatings cannot provide outstanding advantages regarding mechanical properties or advantages in morphology compared to conventionally deposited coatings. However, HPPMS shows a completely different phase texturing measured by X-ray diffraction and a better uniformity on complex components. 1 Introduction In 1980 Gühring introduced the first TiN coating deposited by physical vapor deposition (PVD) to the market. In subsequent years it was shown that this technology offers a wide range of possibilities to produce ternary metastabile material compounds [1]. High power pulse magnetron sputtering (HPPMS) is a modern and promising technology for PVD. It is an advancement of pulsed magnetron sputter ion plating (MSIP) where, however, a plasma density is achieved which is higher by three orders of magnitude in comparison to conventional (DC) sputtering [2]. Due to its high power pulses, HPPMS technology offers outstanding advantages with respect to adhesion, hardness and dense morphology [3, 4]. Furthermore, using this technology, complex-shaped tools can be coated with high thickness uniformity and with high deposition rates on surfaces oriented non-parallel to the target [5]. First investigations of sputtering of Cr and Ti targets with optical emission spectroscopy (OES) have shown a significant enhancement of ion/atom ratio by highly ionized plasmas compared to conventional DC sputtering [6]. Other investigations show an advantage of HPPMS regarding thickness uniformity compared to DC processes by sputtering of Ta and Ti/Al [5,7]. Furthermore, working with modulated power pulses (MPP) enables a deposition of dense coatings with outstanding deposition rates up to 15 µm/h and high hardness [8, 9]. By the way most investigations show an empiric design of coatings. A first way to control the microstructure of CrNx films was shown by Greczynski et al. in [10]. However, only the microstructure and deposition rate were correlated to ion energy distribution functions (IEDF) and the ratio of ions and neutrals. ∗ Corresponding author. E-mail: theiss@iot.rwth-aachen.de, Phone: +49 241 80 96282, Fax: +49 241 80 92941 c 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim  602 N. Bagcivan et al.: Comparison of DC, MF and HPPMS (Cr0.75 Al0.25 ) N This paper shows first results gained by using HPPMS for deposition of low-Al containing Cr-Al-N in comparison to Cr-Al-N coatings with the same Cr:Al ratio deposited by DC and pulsed middle frequency (MF) discharges. All coatings are analyzed regarding phase structure, chemical composition and mechanical properties. Additionally, the analyzed processes were used to deposit Cr-Al-N on complex components. As example component a gear wheel was used to evaluate the thickness uniformity on the cogs of the gear wheel. 2 Experimental setup 2.1 Coating deposition The MSIP PVD technology has been applied for the deposition of the coatings. The coating unit CC800/9 HPPMS by CemeCon AG, Würselen, is equipped with two HPPMS power supplies Sinex 3.0 made by Chemfilt Ionsputtering AB, and also with two DC sources (figure 1). For the DC and HPPMS processes only two cathodes were used. An equal industrial coating unit CC800/9 SinOx was used for depositing the Cr-Al-N coatings by bipolar pulsed MF sputtering. Fig. 1 Schema of the used coating setup The used pulsed power supplies generate a Kouznetsov pulse shape as described by Theiß et al. in [11]. The actual coating processes were preceded by two plasma cleaning processes which provide for the cleaning and the preparation of the substrates. For deposition of each Cr-Al-N on cemented carbide two cathodes were used. To get the aimed chemical composition Cr targets with 20 pieces of diameter 15 mm Al plugs [12, 13] (CrAl20) (purity: Cr 99.9 % and Al 99.5 %) and Al targets with 20 pieces of diameter 15 mm Cr plugs (AlCr20) (purity: Al: 99.5 % and Cr: 99.9 %) were used. The deposition parameters of both systems are listed in table 1. Table 1 Deposition parameters of the used coating systems Process parameter Substrate Targets Time t Temperature at heater T Argon flow F(Ar) Krypton flow F(Kr) Nitrogen flow F(N2 ) Pressure p Bias voltage UBias Cathode mean power P̄ Cathode peak power P̂ Peak power density Frequency f Pulse duration τ Unit min ◦ C sccm sccm sccm mPa V kW kW Wmm−1 Hz µs DC MF HPPMS Cemented Carbide (Ra 0.02 µm) CrAl20/AlCr20 CrAl20/AlCr20 CrAl20/CrAl20 90 100 130 560 600 560 120 120 120 80 80 80 108 105 70 500 570 500 −120 −120 −120 2 · 5.0 2 · 6.0 2 · 6.9 6 15 490 0.14 0.33 11.14 18.51 · 103 500 7 200 For deposition of the DC and MF coatings one CrAl20 and one AlCr20 target was used. To keep the chemical composition constant two CrAl20 targets were used in the HPPMS process. As shown in [11] the deposition c 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim  www.cpp-journal.org Contrib. Plasma Phys. 52, No. 7 (2012) / www.cpp-journal.org 603 rate of aluminum in a HPPMS process is about 100 times higher compared to chromium. Hence, the chromium content of the targets had to be increased. 2.2 Coating characterization In order to evaluate the topography, morphology and the thickness of the coatings, scanning electron microscope (SEM) micrographs of fractured cross sections were taken (ZEISS DSM 982 Gemini) using SE (secondary electrons) mode. Within this SEM an energy dispersive X-ray spectroscopy (EDS) was used to determine the chemical composition of the surfaces. Hardness and Youngs modulus were, moreover, determined using the method of nano indentation. A Nanoindenter XP, by MTS Nano Instruments, was applied for this purpose. The indentation depth did not exceed 1/10 of coating thickness. The evaluation of the measured results was based on the equations according to Oliver and Pharr [14]. A constant Poissons ratio of v=0.25 was assumed. The phase analysis was carried out via grazing incidence X-ray diffractrometry for measuring thin coatings. An Xray diffractometer XRD 3003 provided by General Electric, Munich, was used. All measurements were carried out using the following parameters: CuK(α), 40 kV, 40 mA; Grazing incidence: 5◦ ; Diffraction angle: 15◦ to 90◦ ; Step width: 0.05◦ , 5 s; 1st Slit: 0.3 mm; 2nd Slit: 0.2 mm. The thickness of the coatings on the gear wheels were measured by using a calo tester. Roughness of the different coatings were measured by means of 3D laser scanning microscopy (Keyence VK-9710). 3 Results and discussion The results of the mechanical and chemical analyses as well as the thickness measurements are shown in table 2. The chemical composition as well as the thickness parallel to the target of the different coatings are nearly the same. Due to the different depositing times the deposition rates can be calculated to 2.1 µm/h, 2.0 µm/h and 1.5 µm/h for the DC, MF and HPPMS processes, respectively. It is known that the deposition rate parallel to the target surface is lower when using HPPMS compared to DC and pulsed MF. This is for example shown by Konstantinidis et al. in [15]. Both MF-(Cr0.73 Al0.27 )N and HPPMS-(Cr0.75 Al0.25 )N show a hardness of nearly 29 GPa. Compared to nearly 26 GPa reached by DC-(Cr0.72 Al0.28 )N this is a light increase. With respect to the standard deviation of the Young’s modulus all coatings are in the same range. Table 2 Coating properties Coating properties Chemical composition Thickness d Deposition rate R Hardness H Young’s modulus E Arithmetic average roughness Ra Root mean squared roughness Rq Unit µm µm/h GPa GPa µm µm DC (Cr0.72 Al0.28 )N 3.2 2.1 25.9 ± 2.4 380 ± 30 0.070 0.09 MF (Cr0.73 Al0.27 )N 3.3 2.0 28.6 ± 1.7 393 ± 19 0.025 0.03 HPPMS (Cr0.75 Al0.25 )N 3.3 1.5 28.9 ± 2.1 379 ± 27 0.045 0.06 The deposited coatings were analyzed regarding topography and morphology by means of SEM. Figure 2 shows the surfaces of the three deposited coatings. It is obvious that the DC-(Cr0.72 Al0.28 )N coating has the roughest surface with large grains on it. Moreover, the HPPMS-(Cr0.75 Al0.25 )N coating has large grains on the surface, which look a little bit flattened compared to DC. Only the MF-(Cr0.73 Al0.27 )N coating shows a smooth surface without any grain structure. This results can be proven by determination of the arithmetic average roughness Ra and the root mean squared roughness Rq. While the DC coating reaches Ra 0.070 µm and Rq 0.09 µm the HPPMS and MF coatings reach values for Ra of 0.045 µm and 0.025 µm and Rq values of 0.06 µm and 0.03 µm, respectively (table 2). By taking a look at the cross section fracture micrographs in figure 3 the large grains can be explained by the columnar structure of the DC coating. The HPPMS coating has a slightly columnar structure as well. Only the MF-(Cr0.73 Al0.27 )N shows a fine grained morphology with a very slight columnar structure. The morphology and the density are functions of the energy induced to the substrate [16]. This energy can be introduced thermally or kinetically. By using higher energetic species the adatom mobility is increased and denser, fine grained micro www.cpp-journal.org c 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim  604 N. Bagcivan et al.: Comparison of DC, MF and HPPMS (Cr0.75 Al0.25 ) N structure can be formed. By using MF instead of DC the ionization especially of noble gas atoms is increased. By increasing the pulse power using HPPMS an additional ionization of the metal ions happens. Nevertheless, the ionization rates of all species have to be measured to proof these assumptions. For the current experiment this reasoning leads to the assumption that a special ratio of noble gas ions and metal gas ion will result in the best morphology. a) b) c) Fig. 2 SEM images of the surfaces of the coatings: a) DC-(Cr0.72 Al0.28 )N, b) MF-(Cr0.73 Al0.27 )N and c) HPPMS(Cr0.75 Al0.25 )N a) b) c) Fig. 3 SEM cross section fracture micrographs of the coatings: a) DC-(Cr0.72 Al0.28 )N, b) MF-(Cr0.73 Al0.27 )N and c) HPPMS-(Cr0.75 Al0.25 )N Fig. 4 XRD phase analyses of the different Cr-Al-N coating system deposited by means of DC, MF, HPPMS and a hybrid process of DC and HPPMS c 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim  www.cpp-journal.org Contrib. Plasma Phys. 52, No. 7 (2012) / www.cpp-journal.org 605 To observe the phase composition all three coatings were analyzed by means of X-ray diffraction. Figure 4 shows the results of the phase analyses. After deposition, the phase analysis of the DC-(Cr0.72 Al0.28 )N and MF-(Cr0.73 Al0.27 )N coating systems shows a distinct formation of solid solution crystals made of c-CrN and c-AlN with an preferred orientation in 111 direction. By using the HPPMS technology this preferred orientation switches to a significant 200 orientation. As described by Gall et al. in [17] this can be dedicated to the energy impact of the incoming ions and atoms on the substrate. This energy can either be induced by thermal heat or by increasing the kinetic energy of the incoming species. As shown by Paulitsch et al. in [19] and Petrov et al. in [18] the 200 orientation can be stabilized by a high ion to neutral ratio. Adapted to this special case this means that a higher ion to atom ratio during the HPPMS process compared to the DC as well as the MF processes is present. That is why the 200 orientation is still stable despite that the coating thickness is 3.3 µm. This has to be proven in a next step by using retarding field analyzer (RFA) and mass spectroscopy. An additional experiment was carried out by combining the DC and the HPPMS processes to a hybrid DC/HPPMS process with four running cathodes. As shown in figure 4 this leads to a loss of texturing and gains an anisotropic microstructure. This can be attributed to the still running weakly ionized plasma during the off-time of the HPPMS pulse. Within these time periods only the DC cathodes are working and the ionization rate is decreased. This leads to the preferred 111 orientation. During the on-time of the HPPMS pulse the ionization rate of the target material is increased the texturing is changing to a predominant 200 orientation. Thus, the crystallographic microstructure can be influenced by the impact ratio of neutrals and ions and their energy distribution. This leads to the assumption that a specific texturing can be reached for example by the combination of DC and HPPMS processes and modulated substrate bias. A modulated bias is able to form a specific energy distribution of ionized species. The fundamentals are for example shown by Baloniak et al. in [20]. Hence, a control of the energy distribution of ionized species is possible by using a modulated bias. This leads to new possibilities regarding the architecture of a coating. The applicability of this method in an industrial coating unit will be part of further research. a) b) Fig. 5 Uniformity of the coating thickness of the three discussed coating systems: a) Measurement positions, b) Normalized results of the coating thicknesses at the different positions of the gear wheel To take a look at the uniformity of the thicknesses of the coatings gear wheels were coated and analyzed regarding the thicknesses at the face, the flank and the bottom of a cog (figure 5a). The measured thicknesses were normalized for each coating type on the maximum of the coating thickness at the face of the cog. Figure 5b shows the results of the measurements. DC-(Cr0.72 Al0.28 )N and MF-(Cr0.73 Al0.27 )N show the same behavior. At the flank the thickness of both coatings decrease to about 96 % and then drops to about 67 % at the bottom. In contrast HPPMS-(Cr0.75 Al0.25 )N can reach a thickness of about 82 % at the bottom of the cog. This shows the advantage of HPPMS by coating complex geometries compared to DC or pulsed MF. 4 Conclusions and outlook Three Cr-Al-N coatings were deposited by DC, pulsed MF and HPPMS sputtering by leaving the chemical composition constant at a Cr:Al ratio of about 75:25 (at-%). All three coatings show a high hardness between www.cpp-journal.org c 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim  606 N. Bagcivan et al.: Comparison of DC, MF and HPPMS (Cr0.75 Al0.25 ) N 25.9 GPa and 28.9 GPa. Only the MF deposited coating shows a fine grained morphology while the DC and HPPMS coatings show columnar structures. Both mechanical properties and morphology do not show any advantage of HPPMS in this low-Al containing compound. Nevertheless, the phase composition can be switched from a 111 preferred orientation to a 200 preferred orientation by using HPPMS instead of DC or pulsed MF. Moreover, it is possible to get an anisotropic phase composition by combining DC and HPPMS. Thus, the energy distributions of the ions and neutrals in combination with the substrate temperature lead to different orientations of the grains due to high energetic species from the HPPMS discharge and lower energetic species by DC sputtering. Furthermore, it was shown that the uniformity of the thickness on complex geometries like gear wheels can be increased by using HPPMS. In a next step further chemical compositions will be deposited using DC, MF and HPPMS to get an overview of the whole phase diagrams. Additionally, measurements of the production processes and the movement of atoms and ions at the target by means of phase resolved optical emission spectroscopy (PROES), absolute calibrated optical emission spectroscopy (OES) and Langmuir-probe will be carried out to get a correlation of the process parameters and the plasma parameters. Furthermore, the energy distribution functions of ions and neutrals at the target and the chemical composition of the fluxes to the substrate will be obtained by means of retarding field analyzer (RFA) and mass spectroscopy to correlate the plasma parameters to the coating properties. Finally, the coating properties will be correlated to the results gained in application tests. All results will be used to evaluate a new, innovative methodology for coating design with respect to a specific application. Acknowledgements The authors gratefully acknowledge the financial support of the German Research Association (DFG) within the collaborative research center SFB-TR 87 “Pulsed high power plasmas for the synthesis of nanostructured functional layers” (www.sfbtr87.de). References [1] H. Holleck and V. Schier, Surf. Coat. Tech. 76-77, 328-336 (1995). [2] J. Alami, P.O.A. Persson, D. Music, J.T. Gudmundsson, J. Bhlmark, and U. Helmersson, Am. Vac. Soc. 23, 278-280 (2005). [3] K. Bobzin, N. 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