Metallurgical Engineering, Serdivan, Sakarya, Turkey. Available online: 22 May 2009

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1 This article was downloaded by: [Kocaeli Universitesi Rektorlugu] On: 17 November 2011, At: 04:48 Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: Registered office: Mortimer House, Mortimer Street, London W1T 3JH, UK Materials and Manufacturing Processes Publication details, including instructions for authors and subscription information: Correlation Between Sputtering Conditions and Growth Properties of (TiAl)N/AlN Multilayer Coatings Ekrem Altuncu a & Fatih Üstel b a Kocaeli University, Borusan Campus, Vocational School of Asim Kocabiyik, Hereke, Kocaeli, Turkey b Sakarya University, Esentepe Campus, Engineering Faculty, Department of Materials and Metallurgical Engineering, Serdivan, Sakarya, Turkey Available online: 22 May 2009 To cite this article: Ekrem Altuncu & Fatih Üstel (2009): Correlation Between Sputtering Conditions and Growth Properties of (TiAl)N/AlN Multilayer Coatings, Materials and Manufacturing Processes, 24:7-8, To link to this article: PLEASE SCROLL DOWN FOR ARTICLE Full terms and conditions of use: This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. The publisher does not give any warranty express or implied or make any representation that the contents will be complete or accurate or up to date. The accuracy of any instructions, formulae, and drug doses should be independently verified with primary sources. The publisher shall not be liable for any loss, actions, claims, proceedings, demand, or costs or damages whatsoever or howsoever caused arising directly or indirectly in connection with or arising out of the use of this material.

2 Materials and Manufacturing Processes, 24: , 2009 Copyright Taylor & Francis Group, LLC ISSN: print/ online DOI: / Correlation Between Sputtering Conditions and Growth Properties of (TiAl)N/AlN Multilayer Coatings Ekrem Altuncu 1 and Fatih Üstel 2 1 Kocaeli University, Borusan Campus, Vocational School of Asim Kocabiyik, Hereke, Kocaeli, Turkey 2 Sakarya University, Esentepe Campus, Engineering Faculty, Department of Materials and Metallurgical Engineering, Serdivan, Sakarya, Turkey Multilayered coatings show better thermo/mechanical properties than monolayered coatings in cutting tool applications. In this study, TiAlN/AlN multilayer coatings were produced using diffirent magnetron sputtering physical vapor deposition (PVD) process parameters. Magnetron discharge power, bias voltages, and N 2 flow rate will change growth morphology and surface quality of the coatings. The microstructure and growth morphologies correlated to sputtering conditions in a range of TiAlN/AlN multilayer coatings with nanoscale bilayer period thickness investigated carefully in order to identify the excellent properties of the coatings. Surface and fracture surfaces were characterized by scanning electron microscopy (SEM). Keywords Cutting tool coatings; Electron microscopy; Growth morpholgy; Magnetron sputtering PVD; Multilayer thin films; Oxidation behavior; Thermal stability. 1. Introduction Hard coatings deposited by physical vapor deposition (PVD) have been evolved from monolayer coatings (e.g., TiN, CrN, etc.) to multilayered coatings (e.g., TiAlN, TiCN, TiN/CrN, CrN/NbN, TiN/ZrN, TiAlN/CrN, TiAlN/VN, etc.) for improving performance in the field of cutting tools and molds [1 10]. Magnetron sputtering PVD techniques allowed production of multilayer coatings with a sequence of layers of desired thickness and structure [8]. TiAlN coatings have been successfully developed as a promising alternative to TiN for cutting and forming tools since the 1980s [2, 11, 12]. The addition of Al to TiN, thus forming TiAlN improves the oxidation behavior and the thermal stability of the coating, by forming a stable oxide layer on the surface of the film during oxidation [4, 13]. The Al forms an amorphous aluminum oxide surface layer, which makes oxygen diffusion through the film difficult [13, 14]. TiAlN coatings exhibit superior oxidation resistance and better cutting performance in the use of higher cutting speeds compared with TiN coatings. It has been reported that the oxidation behavior of TiAlN is mainly controlled by Al content [4, 15 17]. The lattice parameter decreases with increasing Al content. The phase structure of coatings was siginificantly influenced by Al doping. When the Al content exceeds at 26%, the hexagonal phase structure occurs in addition to the cubic phase. Consequently, mechanical properties of the coatings are negatively affected [2, 11, 17, 18]. Composition, texture, and grain size of deposited coatings are highly dependent on Received February 27, 2008; Accepted August 13, 2008 Address correspondence to Ekrem Altuncu, Kocaeli University, Borusan Campus, Vocational School of Asim Kocabiyik, Hereke, Kocaeli 41800, Turkey; Fax: ; altuncu@kou.edu.tr 796 the sputtering parameters [16]. The control of the reactive gas flow, magnetron discharge power, and bias voltages especially criticial for the optimization of the coating morphology and properties. Lately, monolayer TiAlN coating applications are replaced by multilayer TiAlN/TiN, TiAlN/VN, TiAlN/CrN coatings. Multilayer coatings exhibit higher mechanical and tribological properties than monolayers [13, 19 21]. However, multilayer coatings have diffirent growth morphology of the coatings. Therefore, the coating phase structure changed from columnar to equiaxed [22, 23]. The TiAlN/AlN multilayer coatings have different phase structure: TiAlN has a face centered cubic (fcc) crystal, and AlN has a hexagonal crystal structure. For this reason, the TiAlN/AlN multilayer system exhibits nonisostructural characteristics. Properties of the multilayer coatings are dependent on bilayer thickness [13, 24]. The application of negative bias voltage results in effective ion bombardment, which influences the surface quality and the composition of coatings [25, 26]. In this study, the correlation of sputtering parameters and growth properties of (TiAl)N/AlN multilayer coatings were characterized. 2. Experimental details TiAlN/AlN multilayer coatings were deposited by the pulsed DC reactive magnetron sputtering system (Tip L 560 Leybold). Two substrates were used: high-speed steel (AISI Co-HSS-M35; wt% 0.92 C, 4.15 Cr, 5 Mo, 1.80 V, 6.25 W, 4.75 Co, 0.30 Mn, 0.28 Si, balance Fe) and hardmetal (HM; P4M, wt% 86 WC, 8(TiTaNbW)C, 6 Co). Prior to insertion into the vacuum chamber, the HSS substrates were metallographically polished. After polishing the samples were ultrasonically cleaned in acetone and alcohol and then dried. HM substrates were only ultrasonically cleaned. For investigation of the microstructure of the multilayer coating

3 SPUTTERING CONDITIONS AND GROWTH PROPERTIES 797 sputtering different bias voltages (self bias, 30 V, 60 V) were applied. The flow rates of nitrogen (N 2 were changed as 25 sccm and 50 sccm. The power densities were used 0.08 W/cm 2 and 0.16 W/cm 2 for Al target. TiAl target power density was 0.48 W/cm 2. The coating thickness change in TiAlN/AlN multilayer coatings by different sputtering parameters were summarized in Table 1. The thickness of the coatings was determined by ball cratering (Calotest); a steel ball of 10 mm diameter was used. The multilayer microstructure and fracture surfaces were investigated by scanning electron microscopy (SEM). Figure 1. Schematic representation of magnetron sputtering system. by cross-section SEM, samples were notched (wide: 2 mm, dept: 4 mm) on the unpolished side of HSS samples. The prepared samples were inserted into the sputtering system by clamping on to the substrate holder. Figure 1 shows a schematic drawing of the magnetron sputtering system. Prior to sputtering, the chamber was evacuated to less than mbar. Once the desired vacuum was reached, the chamber was back filled with argon to mbar, and the substrates were sputter cleaned for 30 min using 200 W RF discharge (DC-120 V). TiAl (at %20Al) and pure Al targets were used for deposition of the TiAlN/AlN multilayer coatings. By the rotating sample holder respectively over and over TiAlN and AlN bilayers were deposited on substrates as 24 times and last formed multilayer coatings. The process constants were: partial pressure of Ar: mbar, deposition time: 30 min, rotation speed of the sample holder: 0.8 rpm. During Table 1. Sputtering parameters of TiAlN/AlN multilayer coatings and coating thickness. Power Power N 2 density density flow Sputt. for Al for TiAl Deposition Bilayer DC-bias rate power target target temperature Thickness thickness (V) (sccm) W (W/cm 2 (W/cm 2 ) ( C) ( m) (nm) Self bias V Not meas V Results and discussions 3.1. Deposition Rate and Thickness Totally deposited layer and bilayer thickness of the multilayer coatings are dependant on rotation speed of substrate holder and deposition time. Substrate rotation achieved multilayer and facilitate homogenous deposition. The results show that increased aluminium magnetron discharge power increases deposition rate. Increase of reactive gas flow rate from 25 sscm to 50 sscm leads to a decrease in thickness of the coating layer. The bilayer thickness of TiAlN/AlN coating produced as a multilayer is observed to be between 65 nm and 206 nm, depending on N 2 flow rate, magnetron discharge power and bias voltage (Fig. 2). The bilayer film thickness increased with increasing sputtering power Growth Morphology and Structure of Multilayer Coatings SEM micrographs which demonstrate surface morphology of the TiAlN/AlN multilayer coatings deposited at different aluminum magnetron discharge (sputtering) powers, are shown in Figs. 3(a) and (b). Very fine grain structures appeared due to the increasing aluminium magnetron discharge (sputtering) power. Increase in magnetron discharge power promotes the flux Figure 2. Effect of aluminium magnetron discharge power, bias voltage, and N 2 flow rate on the deposition rate of TiAlN/AlN multilayer coatings.

4 798 E. ALTUNCU AND F. ÜSTEL Figure 3. Surface SEM micrograph of TiAlN/AlN multilayer coatings. (a) 0.08 W/cm 2, 0 V; (b) 0.16 W/cm 2, 0 V; and (c) 0.16 W/cm 2, 60 V. of substrate bombarding particles. However, increasing the magnetron discharge power significantly increased the surface quality in the growing coatings. Increasing the substrate bias voltage from self bias voltage to 60 V, changed the structure as shown in Fig. 3(c). The correlation between the growth morphology and process parameters of the multilayer films can be understood on the basis of the fundamental structure forming phenomena. This correlation completed the structure zone models by relating the growth textures to the specific zones. Cross-sectional SEM were taken of multilayer films deposited with various sputtering power. In Fig. 4(a), microstructure of the film is evidently columnar, and surface is quite rough. However, by increasing sputtering power columnar structure is seen in Fig. 4(b). It is also evident Figure 4. The fracture surface of TiAlN/AlN multilayer coatings (self bias, 25 sccm). (a) 0.08 W/cm 2 and (b) 0.16 W/cm 2. Figure 5. Fracture surface SEM micrographs of TiAlN/AlN multilayer coatings, ( 60 V, 25 sccm, 0.16 W/cm 2 ).

5 SPUTTERING CONDITIONS AND GROWTH PROPERTIES 799 from Fig. 4 that the film with a higher sputtering power has a much denser and smooth surface morphology. Figure 5 shows that the columnar morphology transforms into an equiaxed structure by increasing bias voltage. 4. Conclusions Control of the reactive gas flow rate, sputtering power, and bias voltage is especially critical for the optimization of multilayer coating growth morphology. In this work, TiAlN/AlN multilayer coatings can be deposited in terms of bilayer thickness by rotating the substrate holder in the sputtering process. The thickness of the bilayer was varied between 65 and 206 nm. Bilayer thickness was varied by various rotation speeds of the substrate holder in order to produce different nanoscale multi-layered period thickness. Effect of the bias voltage and sputtering power on bilayer thickness is negligible, but nitrogen flow rate siginificantly dominates thickness. 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