Key Engineering Materials Vol. 438 (2010) pp 35-40 © (2010) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/KEM.438.35
Coating development for gear cutting tools Jörn Kohlscheen1, a, Hans-Joachim Knoche, Martin Hipke2, b, Andreas Lümkemann3, c 1
Fette GmbH, Schwarzenbek, Germany
2
O.v.G. Universität, IFQ, Magdeburg, Germany 3
Platit AG, Selzach, Switzerland
a
[email protected],
[email protected],
[email protected], c
[email protected]
Keywords: Gear cutting, tailored coating, wear mechanisms
Abstract. Pre-machining of gear wheels is often done by hobbing using a soft work-piece material. Today, almost all of the hobs are used with a hard coating to enhance their life-time considerably. This paper describes the results obtained with a tailored coating in a single tooth cutting test (fly cutting). We show that a nano-structured multi-layer coating based on AlCrN is able to protect a tool better than conventional single-layer coatings. The specific mechanical and structural properties of the coating are explained in detail. Introduction Gear cutting is a highly productive process for manufacturing gear wheels in automotive and other industries. The gear wheels made of alloyed steel are mainly machined in the soft state. After carburization and heat treatment final grinding operation is performed [1]. The gear cutting tools (hobs) are made of high speed steel (HSS) and also increasingly of solid carbide (SC) [2]. A large fraction of tools is coated with a wear resistant coating. In fact, dry gear cutting is only possible with coated tools due to the prevention of chip welding. Hob coating is mainly performed by PVD (physical vapor deposition) as the last production step in the tool factory. Up to now mainly off-theshelf coatings like TiAlN are applied to hobs by either arc PVD or sputtering [3, 4]. The ternary aluminium containing coatings utilize the cubic TiN structure in a single layer design (up to 70 % aluminium in the metallic fraction). They offer an increase in oxidation resistance and enable gear cutting without lubricant. The highly oxidation resistant AlCrN coating may outperform titanium based coatings in certain dry cutting applications [5]. In fact, arc coating with AlCrN is used widely today as wear protection for hobs in industrial production. However, tailored coatings taking into account the specifics of gear cutting like interrupted cut are only beginning to emerge. Advanced coating types utilizing further chemical elements and/or complex structures have been reported but are much less in use [6]. A good performance has been claimed by using nano-composite coatings (based on AlCrSiN) for a defined hobbing application [7]. Hobbing experiments with a lubricating top layer on a conventional TiAlN coating were also reported [8]. Here, we show that a nano-structured multi-layer coating design based on the ternary system Al-CrN has the potential to outperform state-of-the-art single layer coatings. It should be mentioned that apart from using advanced coatings, edge preparation is also significant to improve tool life-time [9]. Experimental The test of coated teeth was done in a model cutting test using two single teeth in a special holder (fly cutting test). Fig. 1 shows this holder together with a typical gear cut by this method. Cutting was done on a Pfauter gear cutting machine type P200. The teeth were obtained from an original hob by wire erosion. The hob had a modulus of 2.7 mm and was made either of HSS or SC. By All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of the publisher: Trans Tech Publications Ltd, Switzerland, www.ttp.net. (ID: 62.2.135.162-02/03/10,13:24:27)
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using only two teeth maintaining the original kinematics of hobbing with a complete tool, real parts can be produced but with greatly shortened time until wear occurs at the cutting edge. Case hardening steel 20MnCr5 (AISI 5120) was used as work-piece material. Only dry cutting was tested at cutting speeds of 220 m/min for HSS and 380 m/min for SC, respectively. Climb hobbing was performed. The wear on the cutting edge was determined periodically during a cutting test using light microscopy and a micro-optical 3D measurement system.
20 mm
Fig. 1: Experimental setup in single-tooth gear cutting: single tooth holder (left) and part manufactured (right), modulus 2.7 mm. Coating of the teeth was done in a specially prepared hob. A row of teeth was cut out so that the teeth to be coated could be inserted in that row with distance pieces. Using this holder has the advantage that a realistic coating thickness distribution across the tooth surface is maintained which implies a decrease in coating thickness down to the root section. Two different commercial PVD coating systems were used (PVD = physical vapor deposition) for arc coating in-house. One with planar cathodes from Balzers and the other with rotating cathodes [10]. Whereas for the planar cathodes AlCr alloy with a fixed ratio was used, the round cathodes consisted of pure aluminium and chromium giving high flexibility in coating composition. Sputter deposition was done externally on CC800 coating units. The teeth were pre- and post-treated by dry and wet blasting for edge and surface preparation. Typical cutting edge radii were determined to be around 10 to 20 µm after coating. Coating thickness at the tooth tip was 3 to 4 µm. Further details regarding coating and edge preparation will be described elsewhere. A PVD coating batch normally lasts around 6 h including heating and cooling. Results and Discussion Coating specifics The newly developed multi-layer coating received the official designation Nanosphere. Table 1 contains some of its characteristics compared to a conventional coating often used in gear cutting. One can see that the main difference is a layered structure with about 50 single layers each being almost 0.1 µm thick. The aluminum concentration in these layers was varied periodically around an approximate ratio of 2:1 (Al:Cr). The layered design in combination with a nano-structure resulting from the usage of spatially separate metallic cathodes leads to an increased coating flexibility (reduced E-modulus) though still maintaining a high hardness of approx. 35 GPa. Table 1: Characteristics of a conventional and newly developed arc PVD coating for gear cutting. Coating Nanosphere AlCrN
Color blueish grey blueish grey
Layers Layer thickness [µm] > 50 0.05 - 0.1 1 3
E-modulus [GPa] approx. 550 approx. 600
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Results for HSS In the first series of experiments, single teeth made of powder-metallurgical HSS were used. The teeth were prepared and coated according to the procedure mentioned above in an industrial environment. Fig. 2 shows the resulting flank wear for teeth used in fly cutting with three different coatings: a commercial sputtered TiAlN coating, a conventional single layer AlCrN arc coating and the new multi-layered AlCrN coating. The test was stopped when a flank wear of more than 130 µm was determined by microscopy. As one can see, during the first 7 m of cutting, the flank wear proceeds more or less comparable in all cases. However, above approx. 7 to 10 m of cutting length the wear proceeds much slower for the multi-layer coating. A final cutting length of almost 19 m is achieved in that case corresponding to 28 work-pieces machined. Cutting length per tooth / m 0
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160
max. flank wear [ µm ]
max.wear = 130 µm
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AlCrN arc mono-layer
TiAlN sputtered mono-layer
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Tool PM-HSS no. of starts: 2 only 2 teeth Work-piece material: 20MnCr5 modulus: 2.7 mm Cutting climb hobbing / dry speed = 220 m/min feed = 3.6 mm/rev.
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Fig. 2: Evolution of flank wear in HSS single tooth cutting using different coatings. The ultimate wear at the flank can only be understood by closer inspection of the rake face. Fig. 3 shows the resulting crater wear on the rake face after test end. The crater occurs where the removed chips rub on the tool suface. The wear crater ultimately breaks through to the flank resulting in the final wear value shown in Fig. 2 (above 130 µm). Obviously, the two aluminum-chromium based coatings resist cratering better than the TiAlN coating, which will be explained shortly. Interestingly, even though the multi-layer coating could produce about 30 % more work-pieces than the single layer AlCrN arc coating, its crater wear is still less.
life-time:
13 pieces
TiAlN (sputtered)
22 pieces
28 pieces
AlCrN (arc monolayer)
AlCrN (arc multilayer)
Fig. 3: Resulting crater wear at the rake face in single tooth cutting at end of life-time.
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The Coatings in Manufacturing Engineering
We attribute the observed difference in crater wear mainly to the ability of the coatings to protect the underlying steel substrate against thermally induced softening. As dry cutting is performed the area where the chips rub on the rake face heats up considerably [11]. Thereby the martensitic microstructure of the underlying HSS starts to decompose roughly above 550°C. As one can see in Fig. 4 the AlCrN ternary system (with a elemental ratio of Al:Cr ~ 2:1) has lower thermal conductivity than TiAlN [12]. Therefore, AlCrN should be a better heat “shield” to protect the underlying steel against softening. According to Fig. 4, the heat conductivity even decreases when the temperature (here: in dry cutting at the cutting edge) increases. As the multi-layered AlCrN coating is 1 µm thicker than the mono-layer coating better heat shielding and consequently smaller crater can be explained. The significant reduction in crater wear proved to be reproducible and was also confirmed in gear cutting tests with customers.
Fig. 4: Thermal conductivity of selected aluminum rich coatings depending on temperature [12]. Results for SC The superior wear protection of the newly developed coating was also observed for solid carbide single teeth. In this case, a thermal softening is not expected due to the different micro-structure of the tool material. Carbide gets softer at very high temperatures around 1000°C. A higher cutting speed of 380 m/min was used. As expected, the resulting wear occurs only as flank wear without cratering on the rake face. The results are shown in Fig. 5. In this case only aluminum-chromium based arc PVD coatings were compared. The test was stopped when 100 µm of maximum wear was observed at the flanks by microscopy. The wear values determined at the cutting edge of single teeth are shown. Flank wear values follow a similar pattern but were observed to be lower for the multi-layer coating. The life-time of the teeth is almost doubled. Also the wear proceeds more rapidly in case of the conventionally coated teeth right from the beginning. Taking into account that the hardness and roughness of both arc coatings are comparable and that the cutting edges are prepared identically, the higher performance of the multi-layer coating is most likely due to the difference in layering. In fact, Karimi et al. showed by indentation experiments that a multi-layered coating structure is able to hinder crack propagation effectively [13]. This is especially useful in interrupted cutting, e.g. hobbing, where stresses and associated cracking is likely to occur. One can conclude that whereas the wear mechanism in HSS is mainly thermal softening at the rake face, for SC it is mainly mechanical abrasion giving a somewhat smoother wear curve (Fig.4). In total, the multi-layered AlCrN coating acts against both wear mechanisms more effectively (as heat and abrasive “shield”) than a conventional single-layer coating.
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Summary Gear cutting is a specific manufacturing process that can be made even more productive by using tailored tool coatings. We have shown that by using a multi-layered structure based on AlCrN a better tool protection is obtained in single tooth dry cutting tests. The crater wear that occurs in dry hobbing with HSS single teeth is significantly reduced. This is attributed to the excellent thermal shielding and oxidation resistance of AlCrN combined with a high resistance to mechanical disruption. The life-time of solid carbide single teeth was also drastically enhanced with the new coating. Thus, the superior mechanical resistance of the newly developed multi-layer coating is even more efficient when only abrasive wear prevails. max. wear = 100 µm
max. edge wear [ µm ]
100
AlCrN arc multi-layer
AlCrN arc mono-layer
80
Tool solid carbide ISO K30 no. of starts: 2 only 2 teeth Work-piece material: 20MnCr5 modulus: 2.7 mm Cutting climb hobbing / dry speed: 380 m/min feed: 3.0 mm/rev
60
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Fig. 5: Evolution of cutting edge wear in carbide single tooth cutting using conventional and the newly developed coating.
References [1] K. Felten: Verzahntechnik (expert-Verlag, 1999). [2] F. Klocke and O. Winkel: Gear Technol. 1-2 (2004), p. 42. [3] J. Vetter, O. Kayser, H.W. Bieler: Vakuum 17 (2005) p. 131. [4] G. Erkens: Surf. Coat. Technol. 201 (2007) p. 4806. [5] W. Kalss, A. Reiter, V. Derflinger, C. Gey, J.L. Endrino: Int. J. Refr. Met. Hard Mat. 24 (2006), p. 399. [6] G. Erkens: JOT 49 (2009) p. 68. [7] T. Cselle: Swiss Qual. Prod. (2008) p. 28. [8] J. Rech, M. A. Djouadi, J. Picot: Wear 250 (2001) p. 45. [9] J. Rech: Wear 261 (2006) p. 505. [10] M. Jilek, T. Cselle, P. Holubar, P. Blösch, M. Morstein: Arc-coating process with rotating cathodes, European Patent EP1357577, 2003. [11] J. Gerth, M. Larsson, U. Wiklund, F. Riddar, S. Hogmark: Wear 266 (2009), p. 444.
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[12] D.G. Cahill, W.K. Ford, K.E. Goodson, G.D. Mahan, A. Majumdar, H.J. Maris, R. Merlin, S. R. Phillpot: J. Appl. Phys. 93 (2003), p. 793. [13] A. Karimi, Y. Wang, T. Cselle, M. Morstein: Thin Sol. Films 420-421 (2002) p. 275.
