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NbN superlattice coated cemented carbide tools in stainless steel machining
Surface and Coatings Technology 105 (1998) 51–55
Performance of PVD TiN/TaN and TiN/NbN superlattice coated cemented carbide tools in stainless steel machining ˚.O ¨ stlund a, S. Hogmark b T.I. Selinder a,*, M.E. Sjo¨strand a, M. Nordin b, M. Larsson b, A a AB Sandvik Coromant, SE-126 80 Stockholm, Sweden b Department of Technology, Uppsala University, Box 534, SE-751 21 Uppsala, Sweden Received 1 May 1997; accepted 5 January 1998
Abstract Cemented carbide cutting tools coated with superlattice TiN/TaN and TiN/NbN lamellae thin films were evaluated in face milling machining tests of austenitic stainless steel (AISI/SAE 303/304). The coatings were grown by PVD on cemented carbide inserts by a combination of ion plating of TiN and reactive magnetron sputtering of NbN and TaN. The lamellae coatings were macroscopically disordered as a consequence of the relatively rough surface of the carbide substrate, but the coatings did exhibit a microscopic superlattice structure. For both TiN/TaN and TiN/NbN, the two constituent layers of the superlattices crystallized in the cubic B1 structure. The TiN/NbN lamellea coating showed a Vickers microhardness (HV ) of 32 GPa and the TiN/TaN 0.5 N lamellea coating exhibited a HV value of 39 GPa. The results from the machining tests indicate a superior performance of 0.5 N tools coated with the harder lamellae coatings as compared to tools coated with single layer PVD or CVD coatings. © 1998 Elsevier Science S.A. Keywords: Cemented carbide; PVD; TiN/NbN; TiN/TaN; Stainless steel machining
1. Introduction Physical vapor deposition (PVD) techniques have opened up a wide range of opportunities for the deposition of ‘‘tailor-made’’ refractory coatings on cutting tools for metal machining. Methods like reactive magnetron sputtering, ion plating and cathodic arc deposition, or reactive hybrid processes, e.g. by combining electron beam evaporation with reactive magnetron sputtering [1], are all established thin film deposition technologies. Further improvements in these techniques, for instance by introducing unbalanced magnetrons in reactive sputtering [2] or utilizing a steered [3] or filtered [4] arc in cathodic arc deposition, have resulted in better control of the coating processes. Improved intrinsic properties of the coating material follow suit, such as higher hardness, enhanced coating fracture resistance, relaxed residual state of stress, etc. An improvement in the wear resistance or the edge integrity of a PVD coated cutting tool used in a specific machining operation can thus be accomplished by optimizing one or several of the abovementioned properties. * Corresponding author. Tel: +46 8 7266371; Fax: +46 8 186705; e-mail: [email protected] 0257-8972/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved. PII S 02 5 7 -8 9 7 2 ( 9 8 ) 0 0 44 6 - 0
In addition to single-layer coatings, i.e. homogeneous layers of TiN, TiCN, TiAlN, etc., a new class of coatings with multilayered structures has started to emerge. These coatings are obtained by alternately depositing at least two chemically different materials from separate sources, either on a micrometer-thickness range scale [5] or on a nanometer scale. Films deposited with different materials on a nanometer range scale are called superlattice coatings [6–8]. In particular, such films of alternating metal nitrides, e.g. TiN/NbN and TiN/VN, have shown interesting properties when compared to single layers of TiN, NbN and VN in the form of an enhanced hardness when deposited on single crystal substrates. When applying nanoscaled lamellae coatings on carbide cutting inserts that do not possess the surface smoothness of single crystals, the coatings will be ‘‘superlattice-like’’, but the structure may, nevertheless, be interesting as far as enhanced hardness and fatigue strength are concerned. Furthermore, the interfaces between alternating layers of materials with different elastic/plastic properties may function as an impediment to crack propagation [9]. Sufficiently thin individual layers in the lamellae structure may also result in a more favorable crystal structure, as has been reported for the TiN/AlN system [10].
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T.I. Selinder et al. / Surface and Coatings Technology 105 (1998) 51–55
Drilling tests in cast iron with superlattice-structured TiAlN/ZrN coated carbide drills were recently reported by Smith et al [11]. In the present investigation, multilayered TiN/TaN and TiN/NbN films were deposited on cemented carbide cutting inserts, and the tools were tested in face milling of austenitic stainless steel.
2. Deposition and properties of the coatings The lamellae coatings were deposited in a Balzers BAI 640R [1] coating unit (Fig. 1) on cemented carbide ( WC/9 wt%Co) cutting inserts. These inserts were of ISO style SEKN 1204 designed for a face milling cutter, and had an edge radius of 30 mm. The substrates had a Vickers hardness of HV =1370 kgf cm−2 and a coercive 3 force of Hc=12.6 kA m−1. Prior to deposition, the inserts were degreased in an ultrasonic bath and thereafter blown dry with nitrogen. The inserts were then mounted in a sample holder with one of the four cutting edges oriented towards the sources. In this arrangement, the resulting coating thicknesses on the tool flank and rake faces were approximately equal. After being resistively heated by an electron beam to 450 °C at an argon pressure of 0.2 Pa, the inserts were plasma-etched in argon at a substrate bias of −200 V. For both types of multilayered coatings in this investigation, the deposition sequence was initiated with the electron beam evaporation of a 30-nm thick Ti layer. Subsequently, nitrogen was added, and an approximately 250-nm thick TiN layer was grown. Thereafter, the lamellae coatings were deposited by applying a continuous rotation (10 rpm) of the table of the substrate holder during the simultaneous deposition of TiN by using the e-gun and NbN or TaN by using the planar magnetron. This resulted in lamellae thicknesses of 7 nm TiN and 5 nm NbN [11] for the TiN/NbN system and 7 nm TiN and 4 nm TaN for the
TiN/TaN system. The coatings were deposited with a total thickness of approximately 4 mm. The hardness of the TiN/NbN lamellea coating was 32 GPa, and the (compressive) residual stress was −1.2 GPa. The TiN/TaN lamellae coating exhibited a hardness of 39 GPa and a residual stress of −3.1 GPa. Pure TaN and NbN have residual stresses of approximately −6 GPa, which is enough to cause spontaneous flaking from the carbide substrate. The residual stress in the coatings were determined by a substrate deflection technique [12]. The particular multilayer systems used in this investigation were chosen on the basis of their fairly low coating residual stresses in combination with a relatively high hardness as compared to single layers of NbN and TaN coatings. Single layers of pure NbN or TaN are hard but far too brittle to be applied on cutting tools for metal machining. This is, at least partly, due to the high levels of stress in these coatings. X-ray diffraction ( XRD) and cross-section transmission electron microscopy ( TEM ) techniques were employed to determine the structure of the lamellae coatings. The results can be summarized as follows: XRD showed that the overall crystal structures of the coatings were fcc-textured with the (001) planes essentially parallel to the substrate surface. TEM inferred that the coatings exhibited a columnar growth structure with no porosity at the grain boundaries. The layered structure showed a substantial waviness originating from the roughness of the substrate surface. At high TEM magnification, however, it was evident that the local structure was ordered with sharp lamellae interfaces. This was further confirmed by low-angle XRD measurements that showed clear superlattice reflections. Regardless of the superlattice reflections in the X-ray diffractograms, these coatings on cemented carbide substrates are by no means single crystals and should consequently be regarded as multilayers or ‘‘lamellae coatings’’ rather than superlattices due to the relatively high degree of disorder in the local structure as compared to similar coatings on single crystal substrates.
3. Machining tests and results
Fig. 1. Schematic view of the coating unit including a 270° e-gun and a planar magnetron. A thermionic arc between the ionization source and the anode ensures operation under conditions of high plasma density.
The cutting performance of the coated tools was evaluated in the machining of austenitic stainless steel (AISI/SAE 303/304). Two tests, according to Tables 1 and 2, were carried out under dry cutting conditions in a one-tooth face milling operation. A 100 mm diameter milling cutter centered on the workpiece was used, and the workpiece material was in the form of 600 mm long and 60 mm wide bars. In Test round 1, the TiN/TaN (#1) lamellae coating was tested against two commercial PVD coatings of TiCN (#2) and TiAlN (#4). In addition, a CVD trilayer
53
T.I. Selinder et al. / Surface and Coatings Technology 105 (1998) 51–55 Table 1 Machining data and coated cemented carbide tools evaluated in Test round 1 Tool number
Substrate/grade
Coating type
Coating thickness (mm)
1 2 3 4 5
WC/9 wt%Co WC/9 wt%Co WC/9 wt%Co WC/9 wt%Co Commercial WC/Co+PVD TiCN, milling grade
PVD TiN/TaN lamellae coating PVD TiCN, single layer CVD Trilayer, steel milling grade PVD TiAlN, single layer
3.3 4.8 5.0 4.0 5.0
Workpiece material: stainless steel 303/304; cutting speed, 172 m min−1; feed, 0.12 mm tooth−1; depth of cut, 4.0 mm. Table 2 Machining data and coated cemented carbide tools evaluated in Test round 2 Tool number
Substrate/grade
Coating
Coating thickness (mm)
1 2 3 4 5
WC/9 wt%Co WC/9 wt%Co WC/9 wt%Co Commercial WC/Co+CVD TiCN, all-round grade Commercial WC/Co+PVD TiCN, milling grade
PVD TiN/NbN lamellae coating PVD TiCN, single layer CVD Trilayer, steel milling grade
4.5 4.5 5.0 4.5 5.0
Material: stainless steel 303/304; cutting speed, 166 m min−1; feed, 0.12 mm tooth−1; depth of cut, 4.0 mm.
coating of TiCN/Al O /TiN (#3) was also included in 2 3 this round. The cemented carbide substrates for the tools #1–4 originated from the same manufacturing order; hence, any substrate or microgeometry effects on the machining results were ruled out. Finally, still another commercial PVD TiCN coated P30/M25 milling grade (#5) was included in this test. The coating on this latter tool represents the state-of-the-art of the PVD technology, but the substrate of tool #5 had a higher binder phase content ( WC/11 wt%Co) than that of the tools #1–4 rendering tool #5 somewhat tougher than the other tools. The milling operation was carried on until the measured average flank wear either exceeded approximately 0.50 mm or until a cutting edge breakage occurred. Fig. 2 shows the milled lengths for the five coated tools according to the above-defined tool-life criteria. Except
Fig. 2. Diagram showing the results from Test round 1. The bars indicate milled length until tool failure. The numbers refer to the tools described in Table 1.
for the commercial PVD grade #5, all the tools suffered an edge breakage. The fracture was caused by a segment of the cutting edge being sheared off between two adjacent comb cracks. Tool #5, however, displayed a dominant flank wear that possibly concealed any comb cracks. In Test round 2, which is summarized in Table 2, a TiN/NbN lamellae coated tool (#1) was tested against inserts coated with a single layer PVD TiCN (#2) and a CVD trilayer coating (#3). Two commercial cutting tools (#4–5) were also included in this round in order to obtain a more complete view of the product performance of the novel PVD lamellae coated tools in this specific machining application. Tool #4 was an all-round ISO P30/M25 CVD-TiCN coated milling grade, and tool #5 was identical with the PVD P30/M25 milling grade tested in round 1. The cemented carbide substrates for the tools #1–3 originated from the same manufacturing order, but the substrates of the tools #4 and #5 had a slightly different WC/Co composition rendering tool #4 ( WC/10 wt%Co/6% cubic carbides) somewhat harder and tool #5, as mentioned above, somewhat tougher than tools #1–3. The machining performance of the tools in Test round 2 was evaluated by measuring the average flank wear of the cutting edge after each passage of 600 mm. For each tool, the wear was averaged over four points along the cutting edge. The diagrams in Fig. 3 show the average flank wear of the coated milling inserts as a function of milled length. The TiN/NbN lamellae coated tool outperformed the other coated tools in this particular test. The flank wear of this tool was a mere 0.15 mm after a milled length of 3000 mm at which point the test was aborted. All the PVD grades developed a more or less
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T.I. Selinder et al. / Surface and Coatings Technology 105 (1998) 51–55
Fig. 3. Diagram showing the average flank wear as a function of milled length in Test round 2. The numbers refer to the tools described in Table 2.
continuous flank wear including a few minor comb cracks, but these cracks never grew to the extent that they caused any chipping of the cutting edge. However, both of the CVD coated tools failed due to chipping in the depth-of-cut region after a milled length of only 1800 mm. The worn tools are shown in the SEM micrographs in Fig. 4.
4. Discussion The most significant difference between a turning and a milling operation is the intermittent character or ‘‘interrupted cutting mode’’ of the latter operation. The wear picture in milling is, in many aspects, more complicated than, for example, in continuous turning. In many cases, the tool life is not determined solely by wear resistance of a particular substrate/coating combination. The interrupted cutting process generates both thermal and mechanical cycling, which require a high fatigue
strength of the tool material. The thermal fatigue usually results in the formation of comb cracks perpendicular to the cutting edge. A shortened tool life may occur as a result of a chip of the edge being sheared off between two adjacent comb cracks as was the case for all tools except tool #5 in Test round 1. The substrate of this tool was somewhat tougher than the substrates of the other tools, and tool #5 was therefore able to withstand thermal fatigue better at the expense of the flank wear resistance. The lamellae-coated tools retained their cutting edge integrity longer than the other tools and, furthermore, the high hardness of the lamellae coatings ensured a good wear resistance while simultaneously preserving the toughness of the cutting edge. The complexity in the machining of stainless steels is usually greater as compared to the machining of low alloy steels. One feature characteristic of machining stainless steel is that the chip being formed has a strong tendency to weld to the rake face of the tool. Furthermore, the work hardening of the machined surface of austenitic steels makes the depth-of-cut region of the tool particularly sensitive to chipping. In the present study, the PVD coated tools outperformed the CVD-coated tools, and the lamellae-coated tools were superior to the other PVD-coated tools. The intercalation of TiN lamellae on a nanoscale range results in significantly less residual stress in the multilayer coating. The high flank wear resistances of the lamellae coated tools are likely due to the high hardness in combination with moderate levels of intrinsic compressive stresses. Compressive stress generally has beneficial effects on the edge toughness because the coating will resist crack initiation. Excessive levels of stress, however, cause poor adhesion and brittle behavior, which was observed for single-layer NbN and TaN coatings Much work remains to be done in the field of multilayer PVD coatings on a nanoscale range. Nevertheless, this investigation has shown that this new class of coatings may eventually open up new avenues for the development of high-performance stainless steel cutting tools.
References
Fig. 4. Scanning electron micrographs showing worn edges of tools at the completion of Test round 2. The numbers refer to the tools described in Table 2.
[1] E. Bergmann, Surf. Coat. Technol. 57 (1993) 133. [2] S. Kadlec, J. Musil, W.-D. Mu¨nz, J. Vac. Sci. Technol. A 8 (1990) 1318. [3] H. Curtins, Surf. Coat. Technol. 7677 (1995) 632. [4] K. Akari, H. Tamagaki, T. Kumakiri, E.S. Koh, C.N. Tai, Surf. Coat. Technol. 4344 (1990) 312. [5] M. Larsson, M. Bromark, P. Hedenquist, S. Hogmark, Surf. Coat. Technol. 7677 (1995) 202. [6 ] U. Helmersson, S. Todorova, S.A. Barnett, J.-E. Sundgren, J. Appl. Phys. 62 (1987) 481.
T.I. Selinder et al. / Surface and Coatings Technology 105 (1998) 51–55 [7] X. Chu, M.S. Wong, W.D. Sproul, S.L. Rhode, S.A. Barnett, J. Vac. Sci. Technol. A 10 (1992) 1604. [8] S.A. Barnett, in: M.H. Francombe, J.L. Vossen ( Eds.), Physics of Thin Films, Academic Press, New York, 1993. [9] S. Suresh, Internal Report, Brown University, Providence, RI, 1992.
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[10] M. Setoyama, A. Nakayama, M. Tanaka, N. Kitagawa, T. Nomura, Surf. Coat. Technol. 8687 (1996) 225. [11] I.J. Smith, D.B. Lewis, D. Gillibrand, J.S. Brooks, W.-D. Mu¨nz, Fifth International Conference on Plasma Surface Engineering, Garmisch-Partenkirchen, 9–13 September 1996, in press. [12] M. Larsson, P. Hedenqvist, S. Hogmark, Surf. Eng. 12 (1996) 43.
Performance of PVD TiN/TaN and TiN/NbN superlattice coated cemented carbide tools in stainless steel machining ˚.O ¨ stlund a, S. Hogmark b T.I. Selinder a,*, M.E. Sjo¨strand a, M. Nordin b, M. Larsson b, A a AB Sandvik Coromant, SE-126 80 Stockholm, Sweden b Department of Technology, Uppsala University, Box 534, SE-751 21 Uppsala, Sweden Received 1 May 1997; accepted 5 January 1998
Abstract Cemented carbide cutting tools coated with superlattice TiN/TaN and TiN/NbN lamellae thin films were evaluated in face milling machining tests of austenitic stainless steel (AISI/SAE 303/304). The coatings were grown by PVD on cemented carbide inserts by a combination of ion plating of TiN and reactive magnetron sputtering of NbN and TaN. The lamellae coatings were macroscopically disordered as a consequence of the relatively rough surface of the carbide substrate, but the coatings did exhibit a microscopic superlattice structure. For both TiN/TaN and TiN/NbN, the two constituent layers of the superlattices crystallized in the cubic B1 structure. The TiN/NbN lamellea coating showed a Vickers microhardness (HV ) of 32 GPa and the TiN/TaN 0.5 N lamellea coating exhibited a HV value of 39 GPa. The results from the machining tests indicate a superior performance of 0.5 N tools coated with the harder lamellae coatings as compared to tools coated with single layer PVD or CVD coatings. © 1998 Elsevier Science S.A. Keywords: Cemented carbide; PVD; TiN/NbN; TiN/TaN; Stainless steel machining
1. Introduction Physical vapor deposition (PVD) techniques have opened up a wide range of opportunities for the deposition of ‘‘tailor-made’’ refractory coatings on cutting tools for metal machining. Methods like reactive magnetron sputtering, ion plating and cathodic arc deposition, or reactive hybrid processes, e.g. by combining electron beam evaporation with reactive magnetron sputtering [1], are all established thin film deposition technologies. Further improvements in these techniques, for instance by introducing unbalanced magnetrons in reactive sputtering [2] or utilizing a steered [3] or filtered [4] arc in cathodic arc deposition, have resulted in better control of the coating processes. Improved intrinsic properties of the coating material follow suit, such as higher hardness, enhanced coating fracture resistance, relaxed residual state of stress, etc. An improvement in the wear resistance or the edge integrity of a PVD coated cutting tool used in a specific machining operation can thus be accomplished by optimizing one or several of the abovementioned properties. * Corresponding author. Tel: +46 8 7266371; Fax: +46 8 186705; e-mail: [email protected] 0257-8972/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved. PII S 02 5 7 -8 9 7 2 ( 9 8 ) 0 0 44 6 - 0
In addition to single-layer coatings, i.e. homogeneous layers of TiN, TiCN, TiAlN, etc., a new class of coatings with multilayered structures has started to emerge. These coatings are obtained by alternately depositing at least two chemically different materials from separate sources, either on a micrometer-thickness range scale [5] or on a nanometer scale. Films deposited with different materials on a nanometer range scale are called superlattice coatings [6–8]. In particular, such films of alternating metal nitrides, e.g. TiN/NbN and TiN/VN, have shown interesting properties when compared to single layers of TiN, NbN and VN in the form of an enhanced hardness when deposited on single crystal substrates. When applying nanoscaled lamellae coatings on carbide cutting inserts that do not possess the surface smoothness of single crystals, the coatings will be ‘‘superlattice-like’’, but the structure may, nevertheless, be interesting as far as enhanced hardness and fatigue strength are concerned. Furthermore, the interfaces between alternating layers of materials with different elastic/plastic properties may function as an impediment to crack propagation [9]. Sufficiently thin individual layers in the lamellae structure may also result in a more favorable crystal structure, as has been reported for the TiN/AlN system [10].
52
T.I. Selinder et al. / Surface and Coatings Technology 105 (1998) 51–55
Drilling tests in cast iron with superlattice-structured TiAlN/ZrN coated carbide drills were recently reported by Smith et al [11]. In the present investigation, multilayered TiN/TaN and TiN/NbN films were deposited on cemented carbide cutting inserts, and the tools were tested in face milling of austenitic stainless steel.
2. Deposition and properties of the coatings The lamellae coatings were deposited in a Balzers BAI 640R [1] coating unit (Fig. 1) on cemented carbide ( WC/9 wt%Co) cutting inserts. These inserts were of ISO style SEKN 1204 designed for a face milling cutter, and had an edge radius of 30 mm. The substrates had a Vickers hardness of HV =1370 kgf cm−2 and a coercive 3 force of Hc=12.6 kA m−1. Prior to deposition, the inserts were degreased in an ultrasonic bath and thereafter blown dry with nitrogen. The inserts were then mounted in a sample holder with one of the four cutting edges oriented towards the sources. In this arrangement, the resulting coating thicknesses on the tool flank and rake faces were approximately equal. After being resistively heated by an electron beam to 450 °C at an argon pressure of 0.2 Pa, the inserts were plasma-etched in argon at a substrate bias of −200 V. For both types of multilayered coatings in this investigation, the deposition sequence was initiated with the electron beam evaporation of a 30-nm thick Ti layer. Subsequently, nitrogen was added, and an approximately 250-nm thick TiN layer was grown. Thereafter, the lamellae coatings were deposited by applying a continuous rotation (10 rpm) of the table of the substrate holder during the simultaneous deposition of TiN by using the e-gun and NbN or TaN by using the planar magnetron. This resulted in lamellae thicknesses of 7 nm TiN and 5 nm NbN [11] for the TiN/NbN system and 7 nm TiN and 4 nm TaN for the
TiN/TaN system. The coatings were deposited with a total thickness of approximately 4 mm. The hardness of the TiN/NbN lamellea coating was 32 GPa, and the (compressive) residual stress was −1.2 GPa. The TiN/TaN lamellae coating exhibited a hardness of 39 GPa and a residual stress of −3.1 GPa. Pure TaN and NbN have residual stresses of approximately −6 GPa, which is enough to cause spontaneous flaking from the carbide substrate. The residual stress in the coatings were determined by a substrate deflection technique [12]. The particular multilayer systems used in this investigation were chosen on the basis of their fairly low coating residual stresses in combination with a relatively high hardness as compared to single layers of NbN and TaN coatings. Single layers of pure NbN or TaN are hard but far too brittle to be applied on cutting tools for metal machining. This is, at least partly, due to the high levels of stress in these coatings. X-ray diffraction ( XRD) and cross-section transmission electron microscopy ( TEM ) techniques were employed to determine the structure of the lamellae coatings. The results can be summarized as follows: XRD showed that the overall crystal structures of the coatings were fcc-textured with the (001) planes essentially parallel to the substrate surface. TEM inferred that the coatings exhibited a columnar growth structure with no porosity at the grain boundaries. The layered structure showed a substantial waviness originating from the roughness of the substrate surface. At high TEM magnification, however, it was evident that the local structure was ordered with sharp lamellae interfaces. This was further confirmed by low-angle XRD measurements that showed clear superlattice reflections. Regardless of the superlattice reflections in the X-ray diffractograms, these coatings on cemented carbide substrates are by no means single crystals and should consequently be regarded as multilayers or ‘‘lamellae coatings’’ rather than superlattices due to the relatively high degree of disorder in the local structure as compared to similar coatings on single crystal substrates.
3. Machining tests and results
Fig. 1. Schematic view of the coating unit including a 270° e-gun and a planar magnetron. A thermionic arc between the ionization source and the anode ensures operation under conditions of high plasma density.
The cutting performance of the coated tools was evaluated in the machining of austenitic stainless steel (AISI/SAE 303/304). Two tests, according to Tables 1 and 2, were carried out under dry cutting conditions in a one-tooth face milling operation. A 100 mm diameter milling cutter centered on the workpiece was used, and the workpiece material was in the form of 600 mm long and 60 mm wide bars. In Test round 1, the TiN/TaN (#1) lamellae coating was tested against two commercial PVD coatings of TiCN (#2) and TiAlN (#4). In addition, a CVD trilayer
53
T.I. Selinder et al. / Surface and Coatings Technology 105 (1998) 51–55 Table 1 Machining data and coated cemented carbide tools evaluated in Test round 1 Tool number
Substrate/grade
Coating type
Coating thickness (mm)
1 2 3 4 5
WC/9 wt%Co WC/9 wt%Co WC/9 wt%Co WC/9 wt%Co Commercial WC/Co+PVD TiCN, milling grade
PVD TiN/TaN lamellae coating PVD TiCN, single layer CVD Trilayer, steel milling grade PVD TiAlN, single layer
3.3 4.8 5.0 4.0 5.0
Workpiece material: stainless steel 303/304; cutting speed, 172 m min−1; feed, 0.12 mm tooth−1; depth of cut, 4.0 mm. Table 2 Machining data and coated cemented carbide tools evaluated in Test round 2 Tool number
Substrate/grade
Coating
Coating thickness (mm)
1 2 3 4 5
WC/9 wt%Co WC/9 wt%Co WC/9 wt%Co Commercial WC/Co+CVD TiCN, all-round grade Commercial WC/Co+PVD TiCN, milling grade
PVD TiN/NbN lamellae coating PVD TiCN, single layer CVD Trilayer, steel milling grade
4.5 4.5 5.0 4.5 5.0
Material: stainless steel 303/304; cutting speed, 166 m min−1; feed, 0.12 mm tooth−1; depth of cut, 4.0 mm.
coating of TiCN/Al O /TiN (#3) was also included in 2 3 this round. The cemented carbide substrates for the tools #1–4 originated from the same manufacturing order; hence, any substrate or microgeometry effects on the machining results were ruled out. Finally, still another commercial PVD TiCN coated P30/M25 milling grade (#5) was included in this test. The coating on this latter tool represents the state-of-the-art of the PVD technology, but the substrate of tool #5 had a higher binder phase content ( WC/11 wt%Co) than that of the tools #1–4 rendering tool #5 somewhat tougher than the other tools. The milling operation was carried on until the measured average flank wear either exceeded approximately 0.50 mm or until a cutting edge breakage occurred. Fig. 2 shows the milled lengths for the five coated tools according to the above-defined tool-life criteria. Except
Fig. 2. Diagram showing the results from Test round 1. The bars indicate milled length until tool failure. The numbers refer to the tools described in Table 1.
for the commercial PVD grade #5, all the tools suffered an edge breakage. The fracture was caused by a segment of the cutting edge being sheared off between two adjacent comb cracks. Tool #5, however, displayed a dominant flank wear that possibly concealed any comb cracks. In Test round 2, which is summarized in Table 2, a TiN/NbN lamellae coated tool (#1) was tested against inserts coated with a single layer PVD TiCN (#2) and a CVD trilayer coating (#3). Two commercial cutting tools (#4–5) were also included in this round in order to obtain a more complete view of the product performance of the novel PVD lamellae coated tools in this specific machining application. Tool #4 was an all-round ISO P30/M25 CVD-TiCN coated milling grade, and tool #5 was identical with the PVD P30/M25 milling grade tested in round 1. The cemented carbide substrates for the tools #1–3 originated from the same manufacturing order, but the substrates of the tools #4 and #5 had a slightly different WC/Co composition rendering tool #4 ( WC/10 wt%Co/6% cubic carbides) somewhat harder and tool #5, as mentioned above, somewhat tougher than tools #1–3. The machining performance of the tools in Test round 2 was evaluated by measuring the average flank wear of the cutting edge after each passage of 600 mm. For each tool, the wear was averaged over four points along the cutting edge. The diagrams in Fig. 3 show the average flank wear of the coated milling inserts as a function of milled length. The TiN/NbN lamellae coated tool outperformed the other coated tools in this particular test. The flank wear of this tool was a mere 0.15 mm after a milled length of 3000 mm at which point the test was aborted. All the PVD grades developed a more or less
54
T.I. Selinder et al. / Surface and Coatings Technology 105 (1998) 51–55
Fig. 3. Diagram showing the average flank wear as a function of milled length in Test round 2. The numbers refer to the tools described in Table 2.
continuous flank wear including a few minor comb cracks, but these cracks never grew to the extent that they caused any chipping of the cutting edge. However, both of the CVD coated tools failed due to chipping in the depth-of-cut region after a milled length of only 1800 mm. The worn tools are shown in the SEM micrographs in Fig. 4.
4. Discussion The most significant difference between a turning and a milling operation is the intermittent character or ‘‘interrupted cutting mode’’ of the latter operation. The wear picture in milling is, in many aspects, more complicated than, for example, in continuous turning. In many cases, the tool life is not determined solely by wear resistance of a particular substrate/coating combination. The interrupted cutting process generates both thermal and mechanical cycling, which require a high fatigue
strength of the tool material. The thermal fatigue usually results in the formation of comb cracks perpendicular to the cutting edge. A shortened tool life may occur as a result of a chip of the edge being sheared off between two adjacent comb cracks as was the case for all tools except tool #5 in Test round 1. The substrate of this tool was somewhat tougher than the substrates of the other tools, and tool #5 was therefore able to withstand thermal fatigue better at the expense of the flank wear resistance. The lamellae-coated tools retained their cutting edge integrity longer than the other tools and, furthermore, the high hardness of the lamellae coatings ensured a good wear resistance while simultaneously preserving the toughness of the cutting edge. The complexity in the machining of stainless steels is usually greater as compared to the machining of low alloy steels. One feature characteristic of machining stainless steel is that the chip being formed has a strong tendency to weld to the rake face of the tool. Furthermore, the work hardening of the machined surface of austenitic steels makes the depth-of-cut region of the tool particularly sensitive to chipping. In the present study, the PVD coated tools outperformed the CVD-coated tools, and the lamellae-coated tools were superior to the other PVD-coated tools. The intercalation of TiN lamellae on a nanoscale range results in significantly less residual stress in the multilayer coating. The high flank wear resistances of the lamellae coated tools are likely due to the high hardness in combination with moderate levels of intrinsic compressive stresses. Compressive stress generally has beneficial effects on the edge toughness because the coating will resist crack initiation. Excessive levels of stress, however, cause poor adhesion and brittle behavior, which was observed for single-layer NbN and TaN coatings Much work remains to be done in the field of multilayer PVD coatings on a nanoscale range. Nevertheless, this investigation has shown that this new class of coatings may eventually open up new avenues for the development of high-performance stainless steel cutting tools.
References
Fig. 4. Scanning electron micrographs showing worn edges of tools at the completion of Test round 2. The numbers refer to the tools described in Table 2.
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