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Cubic boron nitride based coating systems with different interlayers for cutting inserts
Surface & Coatings Technology 205 (2011) S103–S106
Contents lists available at ScienceDirect
Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Cubic boron nitride based coating systems with different interlayers for cutting inserts Christian Stein a,⁎, Martin Keunecke a, Klaus Bewilogua a, Thomas Chudoba b, Werner Kölker c, Henk van den Berg d a
Fraunhofer IST, Bienroder Weg 54E, 38108 Braunschweig, Germany ASMEC GmbH, Bautzner Landstraße 45, 01454 Radeberg OT Rossendorf, Germany CemeCon AG, Adenauerstr. 20 A4, 52146 Würselen, Germany d Kennametal Technologies GmbH, Münchener Straße 125–127, 45145 Essen, Germany b c
a r t i c l e
i n f o
Available online 12 March 2011 Keywords: Cubic boron nitride (c-BN) PVD Sputtering Interlayer Cutting insert coating
a b s t r a c t Due to its outstanding properties, cubic boron nitride (c-BN) has a high application potential, in particular as a super hard cutting tool coating. Unfortunately until now there have been no c-BN tool coatings available at an industrial scale. The preparation of c-BN coatings with a film-thickness in the μm range is challenging and succeeded only by few research groups in the last few years. In the Fraunhofer IST a PVD sputtering process was used employing a boron carbide target in an argon/nitrogen atmosphere. By this PVD process c-BN films with a thickness between 1 and 2 μm were deposited on pre-coated cemented carbide cutting inserts. The pre-coating is an important part of the interlayer system design which is used to improve the adhesion of the c-BN film and therefore is crucial for a successful application as a cutting tool coating. Several TiAlN based coatings were deposited on cemented carbide cutting inserts and tested as interlayers for the c-BN layer system. High temperature experiments at ambient air show a high oxidation resistance of c-BN with a very high remaining hardness after heat treatment. Furthermore investigations of adhesion and wear will be presented as well as morphology and composition analysis. © 2011 Elsevier B.V. All rights reserved.
1. Introduction The continuously increasing requirements of tools in production engineering like high speed cutting, dry cutting or hard machining lead to a strong demand for new super hard tool coatings (hardness N40 GPa). Cubic boron nitride (c-BN) is the second hardest of all known materials and simultaneously it shows a high-temperature oxidation resistance and the ability to machine ferrous-based metals like steel, in contrast to diamond. The machining of steel has unambiguously the largest market volume in the field of metal cutting operations. Today sintered polycrystalline bulk c-BN (PCBN) cutting inserts with grains that are synthesised in an expensive high-pressure/hightemperature (HPHT) process [1] are in industrial use, allowing only simple shaped inserts. This kind of PCBN (c-BN + binder) has a c-BN content of more than 90%. Another form of PCBN used for cutting inserts is a composite of c-BN and TiC with typical c-BN content around 80% or lower. c-BN coated cemented carbide cutting inserts are able to combine the high toughness of the substrate and the very high hardness of the coating. In addition, the application of a coating is more flexible with respect to various tool geometries in contrast to
⁎ Corresponding author. Tel.: + 49531 2155 647. E-mail address: [email protected] (C. Stein). 0257-8972/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2011.03.016
PCBN. Until today, no industrial scale process to deposit such coatings on cutting inserts could be found to establish. Various physical vapor deposition (PVD) techniques as well as plasma-assisted chemical vapor deposition (PACVD) processes succeeded in c-BN film deposition in the last two decades. Due to an enormous residual intrinsic compressive stress (up to 20 GPa), poor adhesion and a lack of long-term stability under ambient humidity conditions, the films were limited to a thickness range less than 0.5 μm in most cases. Such thin c-BN films deposited on tools were found to be insufficient for cutting applications [2]. Besides, in the majority of publications on c-BN films only silicon substrates are treated [3]. During the last decade a few research groups succeeded in the preparation of thick (N1 μm) stable coatings deposited on technical relevant substrates like metals [4] or cemented carbides [5–9] which are typical materials for cutting inserts. One way to increase the capability of tool coatings is the implementation of a coating system consisting of different layers with different materials and properties or phases. Interesting for such combinations are conventional hard coatings like TiN or TiAlN, widely used in tool industry, and on top a layer system consisting of a boron carbide layer (B4C), a B–C–N gradient layer and the super hard c-BN as the outer layer [10,11]. In the present paper efforts were made to replace the conventional hard coating (TiAlN) by modified TiAlN coatings containing additionally chromium or chromium and silicon as interlayers for the c-BN layer system. It is known that these modified TiAlN coatings show
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substrate cleaning (HF-power: 2 kW, Ar flux: 120 sccm), and (iii) deposition of hard coating layer. The most important deposition parameters are summarised below: ▪ ▪ ▪ ▪ ▪ ▪ ▪ ▪ ▪ Fig. 1. SEM cross section image of the c-BN layer system on top of a conventional TiAlN interlayer coated on cemented carbide.
enhanced properties like better oxidation behaviour compared to conventional TiAlN. Hence, their usage as an interlayer should lead to a better thermal stability of the entire coating system. Furthermore high temperature experiments are performed to prove the excellent characteristics of c-BN coated inserts at typical temperatures reached in challenging cutting operations. 2. Material and methods On fine and ultra-fine grain cemented carbide cutting inserts (Co, 6%) three different types of hard coatings (TiAlN, CrTiAlN and CrTiAlSiN) were deposited followed by the B4C + B − C − N + c-BN layer system on top. The TiAlN based hard coatings were prepared in a commercial magnetron sputter unit. After this coating application, a transfer of the samples to another sputtering device in laboratory scale for the c-BN layer system took place.
Targets: TiAl, Cr, and Si Target power: TiAl (5 kW), Cr (3 kW), and Si (1 kW) Reactive gas: nitrogen (N2) Total gas pressure: ≈0.4 Pa Substrate potential (bias voltage Ub): floating potential up to −120 V (d.c.) Substrate holder temperature: in the range of 350 °C Substrate materials: cemented carbide cutting inserts, high speed steel, and silicon wafer pieces Substrate rotation: 2 rpm of planetary; twofold rotation Deposition rate: ≈1 μm h− 1.
2.2. Deposition of the c-BN layer system onto the pre-coated cemented carbide cutting inserts The deposition of the c-BN coating system was performed in an r.f. (13.56 MHz) diode sputtering device equipped with a boron carbide target (B4C: purity 99.5%) using an Ar/N2 mixture sputter plasma. Following Ar+ ion etching of the substrates, the deposition started with a B4C layer (sputtering with pure Ar (pressure p = 0.3 Pa, bias voltage Ub = 0 V)). After the B4C layer had been deposited, the process gas was continuously replaced by N2 until 10% of N2 fraction was reached while the substrate bias Ub was kept constant at − 200 V. At the end of this replacement process, the nucleation of the cubic BN phase took place. After the nucleation step, the gas composition was changed to 100% of N2 and the bias was reduced (to the range between −150 and −170 V) to lower the ion impact into the growing film [13]. Independent of the gas composition, in all process steps, the total gas flow was kept constant at 50 sccm. Caused by ion bombardment, the substrate temperature increased up to 600 °C. More details of the sputtering system as well as the deposition procedure were reported earlier [10,11,13].
2.1. Deposition of the TiAlN based pre coatings 2.3. Coating characterisation TiAlN based pre-coatings were prepared by reactive d.c. pulsed magnetron sputtering in the unbalanced mode using a CemeCon CC800/9 sputtering system (CemeCon AG, Würselen, Germany). This coating machine provides 4 magnetron sources, which can be equipped with various targets. For the deposition of conventional TiAlN coatings four TiAl mosaic targets were employed. Chromium modified coatings were prepared by replacing one of these TiAl targets by a Cr target (CrTiAlN). In a similar way Cr and Si modified coatings were fabricated by replacing two TiAl targets by one Cr target and one Si target respectively (CrTiAlSiN). More details on these TiAlN based coatings are published elsewhere [12].The process chamber has a volume of about 0.8 m3 and a 10 kW heating system. The residual pressure before starting the process was less than 10− 3 Pa. The pre-coating process of the cemented carbide consisted of three main steps: (i) heating for about 1 h to 350 °C, (ii) 30 min Ar ion etching for
The morphology of the films was investigated by Scanning Electron Microscopy (SEM) cross section images. Energy Disperse X-ray Analysis (EDX) measurements on calottes ground into the coatings were performed to analyse the depth distribution of the coating elements. In addition, the hardness and Young's modulus of the coatings were measured by a Fischerscope H100 device with a Vickers indenter. The hardness and Young's Modulus were derived from load vs. indentation–depth curves according to the model by Oliver and Pharr [14] (indentation depth around 200 nm) The resistance to abrasive wear was measured by a ball cratering tester operating with an alumina glycerin suspension (25 wt.% Al2O3, particle size 1 μm) as the abrasive medium. The characterisation of the coating adhesion was accomplished by measuring critical loads using scratch test equipment from CSEM.
Table 1 Summary of mechanical and tribological properties of different TiAlN based coatings alone and in combination with a c-BN layer system on top. (B4C thickness about 800 nm, B–C–N thickness about 200 nm for all c-BN coating systems).
Coating thicknesses (μm) Microhardness (GPa) Young's modulus (GPa) Abrasive wear rate (× 10−15 m3 N−1 m−1) Critical load (N)
TiAIN
CrTiAIN
CrTiASiN
c-BN/TiAIN
c-BN/CrTiAIN
c-BN/CrTiAISiN
c-BN two stacks
3.4 24.2 345 5.3
2.8 30.0 352 5.1
2.4 31.5 340 4.6
c-BN:1.1 71.8 339 0.6
c-BN:1.1 71.0 329 0.6
c-BN:1.1 75.2 339 0.5
c-BN:2.2 74.8 341 0.5
N50
N 50
N50
N50
C. Stein et al. / Surface & Coatings Technology 205 (2011) S103–S106
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Fig. 2. SEM cross section image of the c-BN layer system on top of a Cr and Si modified TiAlN interlayer coated on cemented carbide. Fig. 3. EDX analysis on a calotte, ground through the c-BN layer system down to the TiAlN interlayer. Measured elements: Boron (red), Carbon (blue), Nitrogen (yellow), Aluminium (green) and Oxygen (violet).
2.4. High temperature experiments A temperature controlled commercial furnace (Carbolite, GPC 1300) was employed for high temperature tests on coated cutting inserts. The tests were performed in ambient air (humidity at room temperature 48%) and the samples were exposed to heat for 30 min up to 1000 °C to get as close as possible to conditions in a typical high performance cutting application. The furnace was pre-heated to the selected test temperature before the samples were put in. When the set time had expired the specimens were taken out from the furnace to cool down to room temperature. After the high temperature experiments, the coatings characteristics were measured again. 3. Results and discussion 3.1. c-BN coating system with different interlayers The morphology and the layer sequence of the c-BN layer system on a TiAlN pre-coated cemented carbide cutting insert can be observed in the SEM cross sectional images in Fig. 1. The different layers are clearly distinguished. The TiAlN layer shows a pronounced columnar structure. According to earlier investigations of the structural and compositional features using Transmission Electron Microscopy TEM and X-ray diffraction XRD [10,15], it can be assumed that the boron carbide layer is amorphous, the B–C–N gradient layer is mostly hexagonally (h-BN) structured and the c-BN layer is nanocrystalline with a grain size of approximately 10–20 nm. A special feature of thick c-BN films deposited via B–C–N gradient layers is the rather wavy transition region to the cubic phase [16]. A summary of basic mechanical and tribological properties of the produced TiAlN based coatings, and with a c-BN coating on top is given in Table 1. The measurements reveal the successful adaption of the c-BN layer system to the modified interlayers with Cr and Cr + Si. Especially the hardness values are close to those known for bulk c-BN [1] for all tested interlayers. The corresponding Young's modules are also in the same region for all samples, but they are significantly lower
Table 2 Mechanical and tribological properties of c-BN coatings after heat treatment.
Coating thicknesses (μm) Microhardness (GPa) Young's modulus (GPa) Abrasive wear rate (× 10− 15 m3 N− 1 m− 1)
c-BN after 900 °C
c-BN after 1000 °C
1.3 67 340 0.8
1.7 59 326 0.9
than it is known for bulk material (N800 GPa). This phenomenon was reported by Richter et al. [17], who could solve this problem by a special measurement procedure and thus found values N800 GPa for c-BN films too. In addition, some of our samples were measured with the Universal Nanomechanical Tester (UNAT) from ASMEC GmbH. The measurements confirm that the c-BN coatings have the mechanical properties of pure c-BN, with a Young's modulus of about 800 GPa, a hardness of 60–70 GPa and an indentation hardness to Young's modules ratio of about 12%. Compared to the TiAlN based interlayers whose mechanical properties are all similar, c-BN exhibits twice as high hardness and a much lower abrasive wear rate. Furthermore, in scratch tests all interlayer c-BN combinations can sustain a critical load Lc for first chipping to more than 50 N. SEM images show similar coating morphologies independent of the interlayer system. An example of this is shown in Fig. 2. These findings therefore deduce that two alternative hard coating types with a higher thermal [18,19] and similar mechanical properties compared to conventional TiAlN are at disposal now as an interlayer for the c-BN layer system. 3.2. High temperature experiments The capability of c-BN coatings to withstand high temperatures in ambient air was investigated in the way described in Section 2.4 at 900 and 1000 °C. The experiments were performed on a c-BN coating system using a conventional TiAlN interlayer. Table 2 shows mechanical and tribological properties of c-BN coated samples measured after high temperature exposure. Even after 1000 °C in ambient air the samples are still super hard and show a very low wear resistance. Whilst the film hardness decreases slightly after heat treatment, the wear resistance is unaffected. Possible reasons for these findings could be: stress relaxation as well as growth processes at high temperatures that lead to lower residual hardness measurements, while the wear resistance properties seem to be unaffected. The Young's module is in the range of the as-deposited films. SEM cross section images indicate no significant morphology change. To get information on the distribution of the coating elements along the surface normal, EDX analyses on calottes were made. It should be noted here that the measurement system was not calibrated to determine the absolute quantity of a certain element in the sample, thus the relation between two signals of different species doesn't represent the real film composition. Instead, this method was used to detect possible diffusion or oxidation, which might have taken place
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done. After heat treatment at 1000 °C in ambient air, c-BN is still super hard and there are neither indications for major morphology changes, nor for immoderate oxidation or element diffusion across the layer system. The presented results confirm the high potential of c-BN coated cemented cutting inserts for challenging cutting operations. Acknowledgement The authors would like to thank the German Federal Ministry of Education and Research (BMBF) for funding the research work presented here (grant number 02PU2462). References
Fig. 4. Twofold stacked c-BN multilayer system with sequence B4C–cBN–B4C–cBN coated on a TiAlN pre-coated cemented carbide sample.
during heat treatment. Therefore the relative development of the each signal along the surface normal was analysed. After 1000 °C there is no indication of element diffusion across the layer system (Fig. 3). Oxidation seems to take place mainly at the surface region. The sharp drop of the oxygen signal indicates that almost no oxidation penetrates the coating. 3.3. Multilayer coating design A possible way to deposit well adherent c-BN films with an even higher thickness than achieved with the single layer method presented here (N2 μm on cemented carbide) are multilayer systems consisting of layer sequences B4C–cBN–B4C–cBN and so on. Fig. 4 shows an example with two stacks [20]. 4. Summary and outlook c-BN films with outstanding properties could be successfully deposited on top of advanced TiAlN based interlayer coatings (Cr and Cr/Si modified) to further improve the properties of the whole film system, especially with respect to high temperature applications. Further investigations on these new coating combinations must be
[1] G. Demazeau, Diamond Relat. Mater. 4 (1995) 284. [2] M. Murakawa, S. Watanabe, S. Miyake, in: A. Feldman, et al., (Eds.), Applications of Diamond Films and Related Materials: Third Conference (8th edn.), 885, NIST Special Publication, Washington, 1995, pp. 823–830. [3] S. Ulrich, J. Ye, M. Stüber, C. Ziebert, Thin Solid Films 518 (2009) 1443. [4] P.B. Mirkarimi, K.F. McCarty, G.F. Cardinale, D.L. Medlin, D.K. Ottesen, H.A. Johnsen, J. Vac. Sci. Technol., A 14 (1996) 251. [5] W. Kulisch, R. Freudenstein, Thin Solid Films 516 (2007) 216. [6] I. Bello, C.Y. Chan, W.J. Zhang, Y.M. Chong, K.M. Leung, S.T. Lee, Y. Lifshitz, Diamond Relat. Mater. 14 (2005) 1154. [7] M. Keunecke, E. Wiemann, K. Weigel, S.T. Park, K. Bewilogua, Thin Solid Films 515 (2006) 967. [8] G. Bejarano Gaitan, J.C. Caicedo, P. Prieto, A.G. Balogh, Phys. Status Solidi C 4 (2007) 4282. [9] S. Weissmantel, G. Reisse, Appl. Surf. Sci. 197–198 (2002) 331. [10] K. Yamamoto, M. Keunecke, K. Bewilogua, Thin Solid Films 377–378 (2000) 331. [11] K. Yamamoto, M. Keunecke, K. Bewilogua, Z. Czigany, L. Hultmann, Surf. Coat. Technol. 142-144 (2001) 881. [12] M. Keunecke, C. Stein, K. Bewilogua, W. Koelker, D. Kassel, H. van den Berg, Surf. Coat. Technol. 205 (2010) 1273. [13] A. Schütze, K. Bewilogua, H. Lüthje, S. Kouptsidis, M. Gaertner, Surf. Coat. Technol. 97 (1997) 33. [14] W.C. Oliver, G.M. Pharr, J. Mater. Res. 7 (1992) 1564. [15] M. Keunecke, K. Yamamoto, K. Bewilogua, Thin Solid Films 398–399 (2001) 142. [16] K. Bewilogua, M. Keunecke, K. Weigel, E. Wiemann, Thin Solid Films 469–470 (2004) 86. [17] F. Richter, M. Herrmann, F. Molnar, T. Chudoba, N. Schwarzer, M. Keunecke, K. Bewilogua, X.W. Zhang, H.-J. Boyen, P. Ziemann, Surf. Coat. Technol. 201 (2006) 3577. [18] H. Ezura, K. Ichijo, H. Hasegawa, K. Yamamoto, A. Hotta, T. Suzuki, Vacuum 82 (2008) 476. [19] A. Raveh, I. Zukerman, R. Shneck, R. Avni, I. Fried, Surf. Coat. Technol. 201 (2007) 6136. [20] G. Bejarano, J.M. Caicedo, E. Baca, P. Prieto, A.G. Balogh, S. Enders, Thin Solid Films 494 (2006) 53.
Contents lists available at ScienceDirect
Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Cubic boron nitride based coating systems with different interlayers for cutting inserts Christian Stein a,⁎, Martin Keunecke a, Klaus Bewilogua a, Thomas Chudoba b, Werner Kölker c, Henk van den Berg d a
Fraunhofer IST, Bienroder Weg 54E, 38108 Braunschweig, Germany ASMEC GmbH, Bautzner Landstraße 45, 01454 Radeberg OT Rossendorf, Germany CemeCon AG, Adenauerstr. 20 A4, 52146 Würselen, Germany d Kennametal Technologies GmbH, Münchener Straße 125–127, 45145 Essen, Germany b c
a r t i c l e
i n f o
Available online 12 March 2011 Keywords: Cubic boron nitride (c-BN) PVD Sputtering Interlayer Cutting insert coating
a b s t r a c t Due to its outstanding properties, cubic boron nitride (c-BN) has a high application potential, in particular as a super hard cutting tool coating. Unfortunately until now there have been no c-BN tool coatings available at an industrial scale. The preparation of c-BN coatings with a film-thickness in the μm range is challenging and succeeded only by few research groups in the last few years. In the Fraunhofer IST a PVD sputtering process was used employing a boron carbide target in an argon/nitrogen atmosphere. By this PVD process c-BN films with a thickness between 1 and 2 μm were deposited on pre-coated cemented carbide cutting inserts. The pre-coating is an important part of the interlayer system design which is used to improve the adhesion of the c-BN film and therefore is crucial for a successful application as a cutting tool coating. Several TiAlN based coatings were deposited on cemented carbide cutting inserts and tested as interlayers for the c-BN layer system. High temperature experiments at ambient air show a high oxidation resistance of c-BN with a very high remaining hardness after heat treatment. Furthermore investigations of adhesion and wear will be presented as well as morphology and composition analysis. © 2011 Elsevier B.V. All rights reserved.
1. Introduction The continuously increasing requirements of tools in production engineering like high speed cutting, dry cutting or hard machining lead to a strong demand for new super hard tool coatings (hardness N40 GPa). Cubic boron nitride (c-BN) is the second hardest of all known materials and simultaneously it shows a high-temperature oxidation resistance and the ability to machine ferrous-based metals like steel, in contrast to diamond. The machining of steel has unambiguously the largest market volume in the field of metal cutting operations. Today sintered polycrystalline bulk c-BN (PCBN) cutting inserts with grains that are synthesised in an expensive high-pressure/hightemperature (HPHT) process [1] are in industrial use, allowing only simple shaped inserts. This kind of PCBN (c-BN + binder) has a c-BN content of more than 90%. Another form of PCBN used for cutting inserts is a composite of c-BN and TiC with typical c-BN content around 80% or lower. c-BN coated cemented carbide cutting inserts are able to combine the high toughness of the substrate and the very high hardness of the coating. In addition, the application of a coating is more flexible with respect to various tool geometries in contrast to
⁎ Corresponding author. Tel.: + 49531 2155 647. E-mail address: [email protected] (C. Stein). 0257-8972/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2011.03.016
PCBN. Until today, no industrial scale process to deposit such coatings on cutting inserts could be found to establish. Various physical vapor deposition (PVD) techniques as well as plasma-assisted chemical vapor deposition (PACVD) processes succeeded in c-BN film deposition in the last two decades. Due to an enormous residual intrinsic compressive stress (up to 20 GPa), poor adhesion and a lack of long-term stability under ambient humidity conditions, the films were limited to a thickness range less than 0.5 μm in most cases. Such thin c-BN films deposited on tools were found to be insufficient for cutting applications [2]. Besides, in the majority of publications on c-BN films only silicon substrates are treated [3]. During the last decade a few research groups succeeded in the preparation of thick (N1 μm) stable coatings deposited on technical relevant substrates like metals [4] or cemented carbides [5–9] which are typical materials for cutting inserts. One way to increase the capability of tool coatings is the implementation of a coating system consisting of different layers with different materials and properties or phases. Interesting for such combinations are conventional hard coatings like TiN or TiAlN, widely used in tool industry, and on top a layer system consisting of a boron carbide layer (B4C), a B–C–N gradient layer and the super hard c-BN as the outer layer [10,11]. In the present paper efforts were made to replace the conventional hard coating (TiAlN) by modified TiAlN coatings containing additionally chromium or chromium and silicon as interlayers for the c-BN layer system. It is known that these modified TiAlN coatings show
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substrate cleaning (HF-power: 2 kW, Ar flux: 120 sccm), and (iii) deposition of hard coating layer. The most important deposition parameters are summarised below: ▪ ▪ ▪ ▪ ▪ ▪ ▪ ▪ ▪ Fig. 1. SEM cross section image of the c-BN layer system on top of a conventional TiAlN interlayer coated on cemented carbide.
enhanced properties like better oxidation behaviour compared to conventional TiAlN. Hence, their usage as an interlayer should lead to a better thermal stability of the entire coating system. Furthermore high temperature experiments are performed to prove the excellent characteristics of c-BN coated inserts at typical temperatures reached in challenging cutting operations. 2. Material and methods On fine and ultra-fine grain cemented carbide cutting inserts (Co, 6%) three different types of hard coatings (TiAlN, CrTiAlN and CrTiAlSiN) were deposited followed by the B4C + B − C − N + c-BN layer system on top. The TiAlN based hard coatings were prepared in a commercial magnetron sputter unit. After this coating application, a transfer of the samples to another sputtering device in laboratory scale for the c-BN layer system took place.
Targets: TiAl, Cr, and Si Target power: TiAl (5 kW), Cr (3 kW), and Si (1 kW) Reactive gas: nitrogen (N2) Total gas pressure: ≈0.4 Pa Substrate potential (bias voltage Ub): floating potential up to −120 V (d.c.) Substrate holder temperature: in the range of 350 °C Substrate materials: cemented carbide cutting inserts, high speed steel, and silicon wafer pieces Substrate rotation: 2 rpm of planetary; twofold rotation Deposition rate: ≈1 μm h− 1.
2.2. Deposition of the c-BN layer system onto the pre-coated cemented carbide cutting inserts The deposition of the c-BN coating system was performed in an r.f. (13.56 MHz) diode sputtering device equipped with a boron carbide target (B4C: purity 99.5%) using an Ar/N2 mixture sputter plasma. Following Ar+ ion etching of the substrates, the deposition started with a B4C layer (sputtering with pure Ar (pressure p = 0.3 Pa, bias voltage Ub = 0 V)). After the B4C layer had been deposited, the process gas was continuously replaced by N2 until 10% of N2 fraction was reached while the substrate bias Ub was kept constant at − 200 V. At the end of this replacement process, the nucleation of the cubic BN phase took place. After the nucleation step, the gas composition was changed to 100% of N2 and the bias was reduced (to the range between −150 and −170 V) to lower the ion impact into the growing film [13]. Independent of the gas composition, in all process steps, the total gas flow was kept constant at 50 sccm. Caused by ion bombardment, the substrate temperature increased up to 600 °C. More details of the sputtering system as well as the deposition procedure were reported earlier [10,11,13].
2.1. Deposition of the TiAlN based pre coatings 2.3. Coating characterisation TiAlN based pre-coatings were prepared by reactive d.c. pulsed magnetron sputtering in the unbalanced mode using a CemeCon CC800/9 sputtering system (CemeCon AG, Würselen, Germany). This coating machine provides 4 magnetron sources, which can be equipped with various targets. For the deposition of conventional TiAlN coatings four TiAl mosaic targets were employed. Chromium modified coatings were prepared by replacing one of these TiAl targets by a Cr target (CrTiAlN). In a similar way Cr and Si modified coatings were fabricated by replacing two TiAl targets by one Cr target and one Si target respectively (CrTiAlSiN). More details on these TiAlN based coatings are published elsewhere [12].The process chamber has a volume of about 0.8 m3 and a 10 kW heating system. The residual pressure before starting the process was less than 10− 3 Pa. The pre-coating process of the cemented carbide consisted of three main steps: (i) heating for about 1 h to 350 °C, (ii) 30 min Ar ion etching for
The morphology of the films was investigated by Scanning Electron Microscopy (SEM) cross section images. Energy Disperse X-ray Analysis (EDX) measurements on calottes ground into the coatings were performed to analyse the depth distribution of the coating elements. In addition, the hardness and Young's modulus of the coatings were measured by a Fischerscope H100 device with a Vickers indenter. The hardness and Young's Modulus were derived from load vs. indentation–depth curves according to the model by Oliver and Pharr [14] (indentation depth around 200 nm) The resistance to abrasive wear was measured by a ball cratering tester operating with an alumina glycerin suspension (25 wt.% Al2O3, particle size 1 μm) as the abrasive medium. The characterisation of the coating adhesion was accomplished by measuring critical loads using scratch test equipment from CSEM.
Table 1 Summary of mechanical and tribological properties of different TiAlN based coatings alone and in combination with a c-BN layer system on top. (B4C thickness about 800 nm, B–C–N thickness about 200 nm for all c-BN coating systems).
Coating thicknesses (μm) Microhardness (GPa) Young's modulus (GPa) Abrasive wear rate (× 10−15 m3 N−1 m−1) Critical load (N)
TiAIN
CrTiAIN
CrTiASiN
c-BN/TiAIN
c-BN/CrTiAIN
c-BN/CrTiAISiN
c-BN two stacks
3.4 24.2 345 5.3
2.8 30.0 352 5.1
2.4 31.5 340 4.6
c-BN:1.1 71.8 339 0.6
c-BN:1.1 71.0 329 0.6
c-BN:1.1 75.2 339 0.5
c-BN:2.2 74.8 341 0.5
N50
N 50
N50
N50
C. Stein et al. / Surface & Coatings Technology 205 (2011) S103–S106
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Fig. 2. SEM cross section image of the c-BN layer system on top of a Cr and Si modified TiAlN interlayer coated on cemented carbide. Fig. 3. EDX analysis on a calotte, ground through the c-BN layer system down to the TiAlN interlayer. Measured elements: Boron (red), Carbon (blue), Nitrogen (yellow), Aluminium (green) and Oxygen (violet).
2.4. High temperature experiments A temperature controlled commercial furnace (Carbolite, GPC 1300) was employed for high temperature tests on coated cutting inserts. The tests were performed in ambient air (humidity at room temperature 48%) and the samples were exposed to heat for 30 min up to 1000 °C to get as close as possible to conditions in a typical high performance cutting application. The furnace was pre-heated to the selected test temperature before the samples were put in. When the set time had expired the specimens were taken out from the furnace to cool down to room temperature. After the high temperature experiments, the coatings characteristics were measured again. 3. Results and discussion 3.1. c-BN coating system with different interlayers The morphology and the layer sequence of the c-BN layer system on a TiAlN pre-coated cemented carbide cutting insert can be observed in the SEM cross sectional images in Fig. 1. The different layers are clearly distinguished. The TiAlN layer shows a pronounced columnar structure. According to earlier investigations of the structural and compositional features using Transmission Electron Microscopy TEM and X-ray diffraction XRD [10,15], it can be assumed that the boron carbide layer is amorphous, the B–C–N gradient layer is mostly hexagonally (h-BN) structured and the c-BN layer is nanocrystalline with a grain size of approximately 10–20 nm. A special feature of thick c-BN films deposited via B–C–N gradient layers is the rather wavy transition region to the cubic phase [16]. A summary of basic mechanical and tribological properties of the produced TiAlN based coatings, and with a c-BN coating on top is given in Table 1. The measurements reveal the successful adaption of the c-BN layer system to the modified interlayers with Cr and Cr + Si. Especially the hardness values are close to those known for bulk c-BN [1] for all tested interlayers. The corresponding Young's modules are also in the same region for all samples, but they are significantly lower
Table 2 Mechanical and tribological properties of c-BN coatings after heat treatment.
Coating thicknesses (μm) Microhardness (GPa) Young's modulus (GPa) Abrasive wear rate (× 10− 15 m3 N− 1 m− 1)
c-BN after 900 °C
c-BN after 1000 °C
1.3 67 340 0.8
1.7 59 326 0.9
than it is known for bulk material (N800 GPa). This phenomenon was reported by Richter et al. [17], who could solve this problem by a special measurement procedure and thus found values N800 GPa for c-BN films too. In addition, some of our samples were measured with the Universal Nanomechanical Tester (UNAT) from ASMEC GmbH. The measurements confirm that the c-BN coatings have the mechanical properties of pure c-BN, with a Young's modulus of about 800 GPa, a hardness of 60–70 GPa and an indentation hardness to Young's modules ratio of about 12%. Compared to the TiAlN based interlayers whose mechanical properties are all similar, c-BN exhibits twice as high hardness and a much lower abrasive wear rate. Furthermore, in scratch tests all interlayer c-BN combinations can sustain a critical load Lc for first chipping to more than 50 N. SEM images show similar coating morphologies independent of the interlayer system. An example of this is shown in Fig. 2. These findings therefore deduce that two alternative hard coating types with a higher thermal [18,19] and similar mechanical properties compared to conventional TiAlN are at disposal now as an interlayer for the c-BN layer system. 3.2. High temperature experiments The capability of c-BN coatings to withstand high temperatures in ambient air was investigated in the way described in Section 2.4 at 900 and 1000 °C. The experiments were performed on a c-BN coating system using a conventional TiAlN interlayer. Table 2 shows mechanical and tribological properties of c-BN coated samples measured after high temperature exposure. Even after 1000 °C in ambient air the samples are still super hard and show a very low wear resistance. Whilst the film hardness decreases slightly after heat treatment, the wear resistance is unaffected. Possible reasons for these findings could be: stress relaxation as well as growth processes at high temperatures that lead to lower residual hardness measurements, while the wear resistance properties seem to be unaffected. The Young's module is in the range of the as-deposited films. SEM cross section images indicate no significant morphology change. To get information on the distribution of the coating elements along the surface normal, EDX analyses on calottes were made. It should be noted here that the measurement system was not calibrated to determine the absolute quantity of a certain element in the sample, thus the relation between two signals of different species doesn't represent the real film composition. Instead, this method was used to detect possible diffusion or oxidation, which might have taken place
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done. After heat treatment at 1000 °C in ambient air, c-BN is still super hard and there are neither indications for major morphology changes, nor for immoderate oxidation or element diffusion across the layer system. The presented results confirm the high potential of c-BN coated cemented cutting inserts for challenging cutting operations. Acknowledgement The authors would like to thank the German Federal Ministry of Education and Research (BMBF) for funding the research work presented here (grant number 02PU2462). References
Fig. 4. Twofold stacked c-BN multilayer system with sequence B4C–cBN–B4C–cBN coated on a TiAlN pre-coated cemented carbide sample.
during heat treatment. Therefore the relative development of the each signal along the surface normal was analysed. After 1000 °C there is no indication of element diffusion across the layer system (Fig. 3). Oxidation seems to take place mainly at the surface region. The sharp drop of the oxygen signal indicates that almost no oxidation penetrates the coating. 3.3. Multilayer coating design A possible way to deposit well adherent c-BN films with an even higher thickness than achieved with the single layer method presented here (N2 μm on cemented carbide) are multilayer systems consisting of layer sequences B4C–cBN–B4C–cBN and so on. Fig. 4 shows an example with two stacks [20]. 4. Summary and outlook c-BN films with outstanding properties could be successfully deposited on top of advanced TiAlN based interlayer coatings (Cr and Cr/Si modified) to further improve the properties of the whole film system, especially with respect to high temperature applications. Further investigations on these new coating combinations must be
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