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Development of a graded TiCN coating for cemented carbide cutting tools—a design approach
WEAR ELSEVIER
Wear 188 (1995)
123-129
Development of a graded TiCN coating for cemented carbide cutting tools-a design approach Krishnan Narasimhan
*, S. Prasad Boppana, Deepak G. Bhat
Valenite. Inc., I71 I Thunderbird Street, Troy, MI 48084. USA Received 20 September
1994: accepted
10 March 1995
Abstract Cemented carbide cutting tools containing a high percentage of cobalt binder (9-13%) are used for rough machining applications involving interruptions, owing to their high fracture toughness. However, these tools are prone to deformation under such conditions. A design approach can be effectively used to develop a coating scheme which optimizes the critical properties of such coatings for improved tool life. In this paper, we discuss the design and development of chemical-vapor-deposited (CVD) TiCN/TiN multilayer coatings applied to a WC-l 2% Co tool substrate. The performance of the coated tool is compared with a conventional tool coated with a single, monolithic CVD TiN layer, and it is shown that a judicious combination of coating compositions can be tailored to meet the operating requirements. The interactive aspects of the wear resistance of the coating and deformation resistance of the coated tool during machining are highlighted in the turning tests on low alloy steel. Keywords:
Cemented carbide; Cutting tools; Chemical vapour deposition
(CVD)
1. Introduction It has been recognized for some time that the development of a <‘new” class of materials can be carried out by a design approach, i.e. by an a priori determination of the required properties in the final product based on a consideration of the application environment, which then guides the process of design and development. Efforts have been made to improve the properties of cutting tool materials, and especially of the hard coatings, through more efficient process optimization routines, and by refining the overall product through a tailoring of the properties to specific applications. A number of studies in recent years have shown the usefulness of this approach [ l-51. Recently, Eroglu and Gallois [6] reported on a design approach to define and optimize the CVD process for the deposition of a TiCN coating, by systematically varying the C:N ratio in the gas phase in a computer-controlled hot-wall CVD reactor. Of the various hard coatings used for cutting tools, titanium nitride (TIN) and titanium carbonitride (TiCN) have been the mainstays of the recent offerings of multilayered coatings made by CVD, as well as by the physical vapor deposition (PVD) methods [ 4,5,7,8]. Titanium carbonitride coating offers some unique advantages in metal * Corresponding
author.
0043-1648/95/$09.50 0 1995 Elsevier Science S.A. All rights reserved .SSDlOO43-1648(95)06635-7
; TiCN; Gradient coating; Machining
cutting, which have been summarized by a number of researchers [ 7-91. TiCN has excellent hardness and abrasive wear resistance which are superior to the conventional TIN coating, and has a better chemical stability than TIC. Narasimhan [ IO] has shown that this coating can be deposited using a novel CVD approach by which the formation of the brittle eta phase at the interface can be eliminated. Thus, TiCN offers a way to cut abrasive workpiece materials more efficiently than TIN. For certain applications involving interruptions or heavy, roughing cuts, the tool substrate is designed to have a higher percentage of the binder phase. This leads to a higher fracture toughness, but at the expense of some sacrifice in the hardness of the tool. In machining applications where high heat is generated, the strength requirement can be augmented by the addition of cubic carbides (TIC, TaC, NbC) to the substrate. This imparts some deformation resistance to the tool. A coating, typically TIN, provides the required chemical wear resistance to the tool in the machining of steels. In this paper, we describe a coating scheme for a cemented carbide substrate containing a high percentage of cobalt binder. Through a systematic approach which addresses the application requirements by combining the useful properties of TIN and TiCN, we demonstrate a logical improvement in the performance of the coated tools.
124
K. Narusimhun et al. /Wear
2. Experimental The tool substrate used in this study was an IS0 grade K 40, containing nominally 12 wt.% cobalt. The tool geometry used for deposition and machining tests was SNMG 1204 12 (SNMG 433) with a light hone. The inserts were coated with TIN, TiCN and a combination of TiCN + TIN in the conventional CVD process. The TiCN coating process was adjusted in such a way that a gradient TiCN coating, with a varying ratio of carbon to nitrogen was deposited on the substrate. The C:N ratio for different samples was determined from the measurement of lattice parameters, and comparison with a calibration curve developed for the Tic-TiN alloy system. The initial layer of TiCN adjacent to the substrate contained a C:N ratio of approximately 1:2, which was followed by a layer with a C:N ratio of about 1:3. The final TiCN layer was deposited with a C:N ratio of approximately 1:l. The total coating thickness was maintained in the range of about 10 ym. The thickness of individual TiCN layers was about 3 p,m, and the TIN layer was about 1 pm. Microstructural evaluation of the coatings was carried out by a number of techniques, including scanning electron microscopy (SEM), X-ray diffraction, microhardness testing, scratch adhesion testing and optical metallography. The coated inserts were tested in the turning of an AISI 4340 steel, using the following parameters: Workpiece: diameter 152 mm, hardness HB 300; cutting speed = 70 m min ’ (225 sfm) ; feed rate = 0.32 mm rev-’ (0.0125 ipr) , depth
188 (1995) 123-129
of cut = 2.5 mm (0.1 in), lead angle = 15”; no cutting fluid. During the test, flank and nose wear, deformation and cutting forces were measured at a regular interval of 2 min. The results of machining tests were plotted as a function of cutting time.
3. Results The microstructural characterization of the various coatings showed that the morphologies of TiN and TiCN were different. In general, TIN coatings had a larger crystallite size than the TiCN coatings, as shown in Fig. 1. The photograph on top left shows the morphology of TIN, and that on the top right shows the morphology of TiCN coating. The graded TiCN coating layers can also be seen in the optical crosssection photograph (bottom left) where the C:N ratio was changed in a predetermined manner. The coating scheme is schematically depicted in the diagram at bottom right. The microhardness of TiN was found to be HK,, = 2300 kg mm-‘, while the different layers of TiCN exhibited higher values, ranging from HK,,= 2560 kg mm-2 to 2940 kg mm-‘, with an average value of HK,, = 2700 kg mm-‘. The critical adhesive failure loads for TIN and TiCN coatings were 75 + 8 N and 72 &-10 N, respectively. The coefficient of friction was measured simultaneously with the critical adhesion load in the Revetest@ tester, and
C:N 1:l C:N 12 C:N 1:2
Fig. 1. Microaructures of TiN and TiCN coatings: top left, morphology of TiN; top right, morphology of TiCN; bottom left. optical cross-section coating. showing the graded composition; bottom right, schematic representation of the design scheme for graded TiCN coating,
of TiCN
K. Narasimhan et al. /Wear
125
and erratically. It is worth noting that, for the most part, TiCN exhibited a value of friction coefficient which was consistently lower than that for TIN. The results of measurement of tool wear on multiple inserts in turning are presented in Fig. 2, which shows the flank wear, the nose wear, and the deformation of tools coated with TIN, TiCN and a combination TiCN+ TIN coating. The TiNcoated tools showed rapid wear and deformation under the cutting conditions used, whereas the TiCN coating was superior in all respects to TiN. When the two coatings were combined, such that the TiCN coating was followed by a thin flash coating of TiN on the surface, the wear and deformation
generally showed a trend towards an increasing coefficient with increasing normal load. At subcritical loads where the diamond stylus was predicted to be well within the coating (in the load range of 30-60 N), the friction coefficient values for TIN and TiCN were found to be 0.25 and 0.21, respectively. These values remained remarkably constant in this load range. However, there was an initial unsteady response at the beginning of the test, which was attributed to varying degrees of surface roughness of the coatings. Thus, the values were lower initially until the stylus made full contact with the coating. As the stylus approached the interface and broke through the coating, the friction coefficient increased quickly
0.5
188 (1995) 123-129
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The machining
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126
K. Narasimhan et al. /Wear
188 (1995) 123-129
Fig. 3. Photographs showing the deformation of the tool nose due to the tangential cutting force during machining: top, TiN-coated tool-side view (left), top view (right); bottom, TiCN + TiN-coated tool--side view (left), top view (right). The line acrossthe photographshowsthe position of the undeformededge.
behavior of the coating was significantly improved. After the initial run-in period, the duplex coating showed a marked reduction in the wear rate as compared with the single coatings. Similar results were also obtained in the machining of 4150 steel, 1045 steel and nodular cast iron [ 111. The most remarkable difference in behavior, however, was found in the tool deformation. The duplex coating showed that the onset of deformation of the tool nose was significantly delayed as compared with the single coatings. This is attributed to the fact that the duplex coating shows a tremendous improvement in the wear resistance, which provided a greater resistance to deformation. This point is discussed later in more detail. Fig. 3 shows the appearance of the tool nose after the test, for the TiN-coated and TiCN + TIN coated tools. The deformation of the nose of the TiN-coated tool (top photo) can be seen clearly with respect to the reference line in the side view (top left). The bulge produced by this deformation can be seen in the top view of the tool (top right). The tool nose of the TiCN + TIN coated insert is shown in the bottom photo. It can be seen that this insert shows less deformation than the TiN-coated insert. The results of cutting force measurements are shown in Fig. 4. The cutting force at the tool-workpiece contact point can be resolved into three components: the tangential force which acts in the direction of rotation of the workpiece, the radial force which acts along the radial direction of the workpiece and the feed force which acts along the direction of cutting. This is shown schematically in Fig. 4(a). Again, we find a trend between the three coatings which is similar to that observed for wear and deformation resistance. The TiCN coating tends to significantly reduce the cutting forces as
compared with the TIN coating, but the combination of TiCN and TIN shows the best performance. In particular, the tangential cutting force is significantly reduced in the case of the duplex coating, as compared with the single coatings.
4. Discussion It is well known that in the machining of high-strengthhigh-hardness steels, a considerable amount of heat is generated at the cutting point. This leads not only to a rapid wear of the tool, but also causes deformation of the cutting tip. When a high-cobalt grade of cemented carbide is used, the propensity for deformation increases even further. Conventionally, a coating of TIN is often used to impart a higher surface hardness to the tool, as well as to provide a layer of heat barrier and low coefficient of friction to obviate these effects. Another excellent choice in many applications is a coating of aluminum oxide, which is used in combination with TIN or TIC (and TiCN) [ 41. The relatively higher affinity of TIC coating to the cemented carbide substrate during the CVD process, as compared with that of the TIN coating, is well known, and is manifested typically in the greater propensity for the TiCcoated inserts to develop the brittle eta phase at the interface. It has been suggested by many researchers that the formation of a small amount of eta phase is beneficial in that it provides a diffusion interface, leading to improved adhesion [ 12,131. However, it was shown by Cho et al. [ 141 that the presence of eta phase was deleterious, and appropriate process control was essential to obtain consistent tool performance. A method
K. Narasimhan et al. /Wear
188 (1995) 123-129
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Fig. 4. Cutting forces during machining of AI.9 4340 steel as a function of cutting time: (a) schematic diagram of the force components; (c) feed force; (d) radial force. The TiCN + TiN combination provides the most reduction in all the cutting forces.
of carburizing the coating was shown to be highly effective in removing eta phase from the substrate after the CVD coating [15]. Since TIC and TiN are isomorphous, a TiCN coating can have a wide range of compositions, from carbon rich to nitrogen rich. Thus, TiCN lends itself eminently to the tailoring of composition and properties during deposition. The deposition of TiCN can be carried out on a wide range of tool substrates without greatly sacrificing the adhesion of the coating. A gradient of hardness can be achieved by controlling the C:N ratio in the TiCN coating layers. Conventional CVD techniques have been used quite successfully to deposit TiCN coatings on a variety of substrates [ 16-191. In addition, a
14
16
18
20
ClJlTING TIME, minutes
(b) tangential
force;
number of studies of CVD TiCN-coated cutting tools have been reported [ 20-221, in which an organometallic precursor was used to deposit TiCN. The use of these precursors allows the process to be carried out at reduced temperatures (about 800 “C as compared with 1000-l 100 “C for conventional CVD). The so-called medium-temperature CVD (MTCVD) process has a benefit that eta phase formation is reduced significantly, but the need to use an organometallic precursor has certain disadvantages in terms of handling and disposal of the chemical compounds. Nonetheless, coatings of TiCN produced by this method have been shown to have excellent properties for metal cutting. These reports also show that the properties and behavior of the TiCN coating is a
128
K. Narasimhnn et al. /Wear 188 (1995) 123-129
strong function of composition. In our work, we were able to demonstrate excellent control of process and composition of the various TiCN coatings without any of the disadvantages mentioned above. During machining, various forces operate at the cutting point, as mentioned earlier. The primary cutting force is the tangential component which generates the chip. The hardness and work hardening characteristics of the workpiece material play a significant role in determining the level of this force. In the present case, the use of high-hardness steel was dictated by the purpose of studying the deformation behavior of the coated high-cobalt cemented carbide tool. Thus, the cutting parameters were chosen such that the deformation-related effects could be distinguished from those due to wear alone. During the initial screening test on a TiN-coated tool at a speed of 92 m min- I, we found that the excessive deformation of the tool obliterated the pure wear behavior of the coating. Therefore, the speed was reduced to 70 m min- ’ in order to be able to discern the two effects more clearly. One advantage of using the 4340 steel is that the material can be through hardened with a uniform microstructure and properties, so that machining results are not influenced by variations in the workpiece itself as the bar diameter is reduced. The observed machining behavior of various tools can now be examined in this light. The interaction of the three components of the cutting force with the cutting tool essentially determines the local cutting environment. The inter-related factors which come into play include friction, cutting temperature, heat flow, and chemical interaction between the tool and the workpiece. Thermal deformation of the tool is caused by the build up of heat at the contact point, and its propagation into the tool itself. When the heat is transferred into the substrate, it softens the binder phase and leads to the deformation of the tool at the cutting edge. Chip-control geometries of various kinds and appropriate rake angles can circumvent some of these effects, but the coating also plays an important role. The combination of high hardness, high chemical and thermal stability and high wear resistance of the coatings significantly reduces the deleterious effects of the cutting environment. The primary concern in this case is twofold: one, to cut metal without excessive wear, and two, to prevent deformation of the tool. It is very commonly found that deformation of the cutting edge greatly exacerbates the problem since it leads to a rapid wear of the tool. The objective of designing a coating, therefore, is to alleviate the pressure at the cutting point and to reduce the transfer of heat into the tool. This can be accomplished by a variety of means, some of which were mentioned earlier. We suggest that using a hard, abrasion-resistant coating such as TiCN in the place of TIN greatly increases the cutting efficiency, which results in a reduction in cutting forces. Secondly, the ability of the coating to act as a thermal barrier allows the heat at the contact point from being transmitted into the tool. The heat transfer aspect is related to the coefficient of friction of the coating, its thermal conductivity and temperature dependence of hardness. Thus, a combination of coating com-
positions and sequencing pattern can be used effectively to optimize the required properties of the system as a whole. The measured values of the coefficient of friction, taking into account the surface roughness aspects, show that the TiCN coating in this study had a slightly higher “lubricity” as compared with TIN. It is not clear how significant this difference may be, particularly given the fact that the response of TIN coatings to scratching by a diamond stylus sometimes leads to erratic results [ 231. The TiCN coating is smoother (owing to its relatively finer “grain size”) than the TIN coating. Cheng et al. [ 181 showed that the coarseness of the TiCN coating typically increased with increasing nitrogen content of the film. In the present work, the coating sequence was designed to provide a carbon-rich top layer for increased hardness and wear resistance. The beneficial effect of this design was an improvement of surface smoothness of the coating, which appears to have influenced the measured coefficient of friction. Whether or not the difference between TIN and TiCN is significant in the context of an obviously rough application environment, it is clear that the superior hardness of TiCN coating allows the tool to cut the steel more efficiently. In relative terms, this may be compared with cutting with a sharp knife as opposed to a blunt knife, TIN being the example of the latter. It is then not too difficult to consider that a clean and sharp cutting action would not only reduce the cutting forces but also reduce the heat build-up at the cutting edge. This, in turn, should lead to reduced deformation of the tool. The results of Figs. 2 and 4 clearly show this trend. A further improvement in performance is obtained by adding a thin layer of TIN to the top surface of the TiCN coating. TIN possesses some very useful properties, despite its relatively lower hardness. It is chemically more stable, and is generally known to provide excellent resistance to the buildup of workpiece material at the cutting edge. Its superior thermal properties provide an additive effect to the behavior of TiCN coating in the metal cutting process. The cumulative effect is that the coated tool containing a multilayer graded composition of coatings provides an excellent combination of metal cutting properties, and greatly improves the machining efficiency.
5. Summarizing
remarks
It has been shown that the properties of various hard, wearresistant coatings can be judiciously combined in a multilayer scheme to provide a synergistic effect for improving the overall properties of a coated cemented carbide cutting tool. The sacrifice in the strength and hardness of the substrate, resulting from a need to improve its fracture toughness, has been effectively neutralized by selecting a proper combination of a TiCN coating of graded composition and a surface layer of TIN. The results show that a multilayer coating based on this scheme provides excellent resistance against tool deformation and greatly improves the efficiency of metal cutting.
K. Narasimhan et al. /Wear
Acknowledgements We are indebted to many of our colleagues for their assistance in the experimental program. Thanks are due to Sharon Onyskin for the CVD experiments and metallography, and to Tony Lowe for the machining tests. We thank Chris Marushima, Manager, CVD group, and Jim Steigelman, Vice President, Technology, for their continued interest and encouragement during the course of this work. Permission to present and publish the paper is gratefully acknowledged.
188 (1995) 123-129
129
1171 0. Pacher, J. Stamberger and J. Kiefer, Proc. 4th Eur. CVD Conf: Eindhoven, The Netherlands I983, p. 49 1. [ 181 D.J. Cheng, W-P. Sun and M-H. Hon, Thin Solid Films, 146 (1987) 45. [ 191 Z. Wokulski, Cryst. Res. Technol. 26 ( 1991) 1025. [20] R. Chatterjee-Fischerand P. Mayr, Proc. 5th Eur. CVD Cont. Uppsala, Sweden, 1985, p. 395. [21] J. Oakes, paper presented at ASM Materials Week ‘86, Symp. on Coatings for Metal-cutting Applications, Orlando, FL, 1986. [22] R.S. Bonetti, H. Wiprachtiger and E. Mohn, in T.S. Sudarshan and D.G. Bhat (eds.), Surj&e Modification Technologies III, The Metallurgical Society, Warrendale, PA, 1990, p. 291. [231 H.E. Rebenne and D.G. Bhat, Su$ Coat. Technol., 63 (1994) 1.
References Biographies [ 11H. Holleck and H. Schultz, Thin Solid Films, 153 (1987) 11. [2] H. Holleck and H. Schultz, Sue
Coat. Technol., 36 (1988) 707.
[ 31 E. Matsumoto, T. Iida and T. Nomura, Refract. Metals Hard Mater., [4]
[5 ] [6] [7]
[8] [9]
[ 101 [ II] [ 121 [ 13 ]
[ 141 [ 151 [ 161
9 (1990) 26. K.E. Undercoffer, B.K. Downey, F.B. Battaglia and W.A. Bryant, Proc. 7th Eur. CVD Cont. in J. Phys., Colloq. C5, Suppl. au N” 5. 50 (1989) 783. A.T. Santhanam, R.V. Godse, D.T. Quinto, K.E. Undercoffer and P.C. Jindal, U.S. Patent 5 143 488 (September 1, 1992). S. Eroglu and B. Gallois, Surf: Coat. Technol., 49 (1991) 275. E. Bergmann, H. Kaufmann, R. Schmid, J. Vogel and P. Collignon, paper presented at 4th Int. Conf on Sueace Modification Technologies, Paris, France, 1990. 0. Knotek. F. LofIler and G. Kramer, St& Coat. Technol., 54-55 (1992) 241. A.T. Santhanam, D.T. Quint0 and G.P. Grab, in H. Bildstein and R. Eck (eds.), Proc. 13th Int. Plansee Seminar, Metallwerk Plansee, Reutte, Austria, 1993, Vol. 3, p. 31. K. Narasimhan,European PatentApplication EP5307794 (lOMarch 1993). K. Narasimhan, Gradient TiCN Coating, Valenite Internal Research Rep. (unpublished), 1993. W.D. Sproul and M.H. Richman, Met. Technol.. 3 (1976) 489. V.K. Sarin and J.N. Lindstrom, in L.F. Donaghey, P. Rai-Choudhari and R.N. Tauber (eds.), Proc. 6th Int. Conf: on Chemical Vapor Deposition, The Electrochemical Society, Princeton, NJ, 1977, p. 389. T. Cho, D.G. Bhat and P.F. Woemer, Sue Coat. Technol.. 29 (1986) 239. T. Cho, K. Narasimhan and D.G. Bhat, f/.X Patent4 830 886 (16 May 1989). G.F. Waketieldand J.A. Bloom, in F. Glaski (ed.), Proc. 3rdInt. Conf on CVD, American Nuclear Society, 1972, p. 397.
Krishnan Narasimhan: is an Advanced Engineer, New Coatings Development in the Materials Research & Development division of Valenite, Inc., and has developed several new CVD coatings for cutting tools. He holds a Masters degree in materials science. S. Prasad Boppana: is Manager of Machinability Testing group at Valenite, Inc. He has extensive experience in the machining of materials and applications of cutting tools in automotive, aerospace and other industries. He is responsible for testing of new products, development of application data for new cutting tool materials, and conducting machinability studies for customers. He holds a Masters degree in mechanical engineering. Deepak G. Bhat: is a Senior Research Scientist at Valenite, Inc., where he has been involved in the research and development of hard, wear-resistant coating for cutting tools using CVD and PVD methods. Currently he is involved in the development and commercialization of CVD diamond coatings on carbide cutting tools. He is also responsible for establishing liaisons with industry, universities and federal laboratories for the development and evaluation of metal cutting technologies. He holds a Ph.D. degree in materials science.
Wear 188 (1995)
123-129
Development of a graded TiCN coating for cemented carbide cutting tools-a design approach Krishnan Narasimhan
*, S. Prasad Boppana, Deepak G. Bhat
Valenite. Inc., I71 I Thunderbird Street, Troy, MI 48084. USA Received 20 September
1994: accepted
10 March 1995
Abstract Cemented carbide cutting tools containing a high percentage of cobalt binder (9-13%) are used for rough machining applications involving interruptions, owing to their high fracture toughness. However, these tools are prone to deformation under such conditions. A design approach can be effectively used to develop a coating scheme which optimizes the critical properties of such coatings for improved tool life. In this paper, we discuss the design and development of chemical-vapor-deposited (CVD) TiCN/TiN multilayer coatings applied to a WC-l 2% Co tool substrate. The performance of the coated tool is compared with a conventional tool coated with a single, monolithic CVD TiN layer, and it is shown that a judicious combination of coating compositions can be tailored to meet the operating requirements. The interactive aspects of the wear resistance of the coating and deformation resistance of the coated tool during machining are highlighted in the turning tests on low alloy steel. Keywords:
Cemented carbide; Cutting tools; Chemical vapour deposition
(CVD)
1. Introduction It has been recognized for some time that the development of a <‘new” class of materials can be carried out by a design approach, i.e. by an a priori determination of the required properties in the final product based on a consideration of the application environment, which then guides the process of design and development. Efforts have been made to improve the properties of cutting tool materials, and especially of the hard coatings, through more efficient process optimization routines, and by refining the overall product through a tailoring of the properties to specific applications. A number of studies in recent years have shown the usefulness of this approach [ l-51. Recently, Eroglu and Gallois [6] reported on a design approach to define and optimize the CVD process for the deposition of a TiCN coating, by systematically varying the C:N ratio in the gas phase in a computer-controlled hot-wall CVD reactor. Of the various hard coatings used for cutting tools, titanium nitride (TIN) and titanium carbonitride (TiCN) have been the mainstays of the recent offerings of multilayered coatings made by CVD, as well as by the physical vapor deposition (PVD) methods [ 4,5,7,8]. Titanium carbonitride coating offers some unique advantages in metal * Corresponding
author.
0043-1648/95/$09.50 0 1995 Elsevier Science S.A. All rights reserved .SSDlOO43-1648(95)06635-7
; TiCN; Gradient coating; Machining
cutting, which have been summarized by a number of researchers [ 7-91. TiCN has excellent hardness and abrasive wear resistance which are superior to the conventional TIN coating, and has a better chemical stability than TIC. Narasimhan [ IO] has shown that this coating can be deposited using a novel CVD approach by which the formation of the brittle eta phase at the interface can be eliminated. Thus, TiCN offers a way to cut abrasive workpiece materials more efficiently than TIN. For certain applications involving interruptions or heavy, roughing cuts, the tool substrate is designed to have a higher percentage of the binder phase. This leads to a higher fracture toughness, but at the expense of some sacrifice in the hardness of the tool. In machining applications where high heat is generated, the strength requirement can be augmented by the addition of cubic carbides (TIC, TaC, NbC) to the substrate. This imparts some deformation resistance to the tool. A coating, typically TIN, provides the required chemical wear resistance to the tool in the machining of steels. In this paper, we describe a coating scheme for a cemented carbide substrate containing a high percentage of cobalt binder. Through a systematic approach which addresses the application requirements by combining the useful properties of TIN and TiCN, we demonstrate a logical improvement in the performance of the coated tools.
124
K. Narusimhun et al. /Wear
2. Experimental The tool substrate used in this study was an IS0 grade K 40, containing nominally 12 wt.% cobalt. The tool geometry used for deposition and machining tests was SNMG 1204 12 (SNMG 433) with a light hone. The inserts were coated with TIN, TiCN and a combination of TiCN + TIN in the conventional CVD process. The TiCN coating process was adjusted in such a way that a gradient TiCN coating, with a varying ratio of carbon to nitrogen was deposited on the substrate. The C:N ratio for different samples was determined from the measurement of lattice parameters, and comparison with a calibration curve developed for the Tic-TiN alloy system. The initial layer of TiCN adjacent to the substrate contained a C:N ratio of approximately 1:2, which was followed by a layer with a C:N ratio of about 1:3. The final TiCN layer was deposited with a C:N ratio of approximately 1:l. The total coating thickness was maintained in the range of about 10 ym. The thickness of individual TiCN layers was about 3 p,m, and the TIN layer was about 1 pm. Microstructural evaluation of the coatings was carried out by a number of techniques, including scanning electron microscopy (SEM), X-ray diffraction, microhardness testing, scratch adhesion testing and optical metallography. The coated inserts were tested in the turning of an AISI 4340 steel, using the following parameters: Workpiece: diameter 152 mm, hardness HB 300; cutting speed = 70 m min ’ (225 sfm) ; feed rate = 0.32 mm rev-’ (0.0125 ipr) , depth
188 (1995) 123-129
of cut = 2.5 mm (0.1 in), lead angle = 15”; no cutting fluid. During the test, flank and nose wear, deformation and cutting forces were measured at a regular interval of 2 min. The results of machining tests were plotted as a function of cutting time.
3. Results The microstructural characterization of the various coatings showed that the morphologies of TiN and TiCN were different. In general, TIN coatings had a larger crystallite size than the TiCN coatings, as shown in Fig. 1. The photograph on top left shows the morphology of TIN, and that on the top right shows the morphology of TiCN coating. The graded TiCN coating layers can also be seen in the optical crosssection photograph (bottom left) where the C:N ratio was changed in a predetermined manner. The coating scheme is schematically depicted in the diagram at bottom right. The microhardness of TiN was found to be HK,, = 2300 kg mm-‘, while the different layers of TiCN exhibited higher values, ranging from HK,,= 2560 kg mm-2 to 2940 kg mm-‘, with an average value of HK,, = 2700 kg mm-‘. The critical adhesive failure loads for TIN and TiCN coatings were 75 + 8 N and 72 &-10 N, respectively. The coefficient of friction was measured simultaneously with the critical adhesion load in the Revetest@ tester, and
C:N 1:l C:N 12 C:N 1:2
Fig. 1. Microaructures of TiN and TiCN coatings: top left, morphology of TiN; top right, morphology of TiCN; bottom left. optical cross-section coating. showing the graded composition; bottom right, schematic representation of the design scheme for graded TiCN coating,
of TiCN
K. Narasimhan et al. /Wear
125
and erratically. It is worth noting that, for the most part, TiCN exhibited a value of friction coefficient which was consistently lower than that for TIN. The results of measurement of tool wear on multiple inserts in turning are presented in Fig. 2, which shows the flank wear, the nose wear, and the deformation of tools coated with TIN, TiCN and a combination TiCN+ TIN coating. The TiNcoated tools showed rapid wear and deformation under the cutting conditions used, whereas the TiCN coating was superior in all respects to TiN. When the two coatings were combined, such that the TiCN coating was followed by a thin flash coating of TiN on the surface, the wear and deformation
generally showed a trend towards an increasing coefficient with increasing normal load. At subcritical loads where the diamond stylus was predicted to be well within the coating (in the load range of 30-60 N), the friction coefficient values for TIN and TiCN were found to be 0.25 and 0.21, respectively. These values remained remarkably constant in this load range. However, there was an initial unsteady response at the beginning of the test, which was attributed to varying degrees of surface roughness of the coatings. Thus, the values were lower initially until the stylus made full contact with the coating. As the stylus approached the interface and broke through the coating, the friction coefficient increased quickly
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188 (1995) 123-129
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126
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188 (1995) 123-129
Fig. 3. Photographs showing the deformation of the tool nose due to the tangential cutting force during machining: top, TiN-coated tool-side view (left), top view (right); bottom, TiCN + TiN-coated tool--side view (left), top view (right). The line acrossthe photographshowsthe position of the undeformededge.
behavior of the coating was significantly improved. After the initial run-in period, the duplex coating showed a marked reduction in the wear rate as compared with the single coatings. Similar results were also obtained in the machining of 4150 steel, 1045 steel and nodular cast iron [ 111. The most remarkable difference in behavior, however, was found in the tool deformation. The duplex coating showed that the onset of deformation of the tool nose was significantly delayed as compared with the single coatings. This is attributed to the fact that the duplex coating shows a tremendous improvement in the wear resistance, which provided a greater resistance to deformation. This point is discussed later in more detail. Fig. 3 shows the appearance of the tool nose after the test, for the TiN-coated and TiCN + TIN coated tools. The deformation of the nose of the TiN-coated tool (top photo) can be seen clearly with respect to the reference line in the side view (top left). The bulge produced by this deformation can be seen in the top view of the tool (top right). The tool nose of the TiCN + TIN coated insert is shown in the bottom photo. It can be seen that this insert shows less deformation than the TiN-coated insert. The results of cutting force measurements are shown in Fig. 4. The cutting force at the tool-workpiece contact point can be resolved into three components: the tangential force which acts in the direction of rotation of the workpiece, the radial force which acts along the radial direction of the workpiece and the feed force which acts along the direction of cutting. This is shown schematically in Fig. 4(a). Again, we find a trend between the three coatings which is similar to that observed for wear and deformation resistance. The TiCN coating tends to significantly reduce the cutting forces as
compared with the TIN coating, but the combination of TiCN and TIN shows the best performance. In particular, the tangential cutting force is significantly reduced in the case of the duplex coating, as compared with the single coatings.
4. Discussion It is well known that in the machining of high-strengthhigh-hardness steels, a considerable amount of heat is generated at the cutting point. This leads not only to a rapid wear of the tool, but also causes deformation of the cutting tip. When a high-cobalt grade of cemented carbide is used, the propensity for deformation increases even further. Conventionally, a coating of TIN is often used to impart a higher surface hardness to the tool, as well as to provide a layer of heat barrier and low coefficient of friction to obviate these effects. Another excellent choice in many applications is a coating of aluminum oxide, which is used in combination with TIN or TIC (and TiCN) [ 41. The relatively higher affinity of TIC coating to the cemented carbide substrate during the CVD process, as compared with that of the TIN coating, is well known, and is manifested typically in the greater propensity for the TiCcoated inserts to develop the brittle eta phase at the interface. It has been suggested by many researchers that the formation of a small amount of eta phase is beneficial in that it provides a diffusion interface, leading to improved adhesion [ 12,131. However, it was shown by Cho et al. [ 141 that the presence of eta phase was deleterious, and appropriate process control was essential to obtain consistent tool performance. A method
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of carburizing the coating was shown to be highly effective in removing eta phase from the substrate after the CVD coating [15]. Since TIC and TiN are isomorphous, a TiCN coating can have a wide range of compositions, from carbon rich to nitrogen rich. Thus, TiCN lends itself eminently to the tailoring of composition and properties during deposition. The deposition of TiCN can be carried out on a wide range of tool substrates without greatly sacrificing the adhesion of the coating. A gradient of hardness can be achieved by controlling the C:N ratio in the TiCN coating layers. Conventional CVD techniques have been used quite successfully to deposit TiCN coatings on a variety of substrates [ 16-191. In addition, a
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number of studies of CVD TiCN-coated cutting tools have been reported [ 20-221, in which an organometallic precursor was used to deposit TiCN. The use of these precursors allows the process to be carried out at reduced temperatures (about 800 “C as compared with 1000-l 100 “C for conventional CVD). The so-called medium-temperature CVD (MTCVD) process has a benefit that eta phase formation is reduced significantly, but the need to use an organometallic precursor has certain disadvantages in terms of handling and disposal of the chemical compounds. Nonetheless, coatings of TiCN produced by this method have been shown to have excellent properties for metal cutting. These reports also show that the properties and behavior of the TiCN coating is a
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strong function of composition. In our work, we were able to demonstrate excellent control of process and composition of the various TiCN coatings without any of the disadvantages mentioned above. During machining, various forces operate at the cutting point, as mentioned earlier. The primary cutting force is the tangential component which generates the chip. The hardness and work hardening characteristics of the workpiece material play a significant role in determining the level of this force. In the present case, the use of high-hardness steel was dictated by the purpose of studying the deformation behavior of the coated high-cobalt cemented carbide tool. Thus, the cutting parameters were chosen such that the deformation-related effects could be distinguished from those due to wear alone. During the initial screening test on a TiN-coated tool at a speed of 92 m min- I, we found that the excessive deformation of the tool obliterated the pure wear behavior of the coating. Therefore, the speed was reduced to 70 m min- ’ in order to be able to discern the two effects more clearly. One advantage of using the 4340 steel is that the material can be through hardened with a uniform microstructure and properties, so that machining results are not influenced by variations in the workpiece itself as the bar diameter is reduced. The observed machining behavior of various tools can now be examined in this light. The interaction of the three components of the cutting force with the cutting tool essentially determines the local cutting environment. The inter-related factors which come into play include friction, cutting temperature, heat flow, and chemical interaction between the tool and the workpiece. Thermal deformation of the tool is caused by the build up of heat at the contact point, and its propagation into the tool itself. When the heat is transferred into the substrate, it softens the binder phase and leads to the deformation of the tool at the cutting edge. Chip-control geometries of various kinds and appropriate rake angles can circumvent some of these effects, but the coating also plays an important role. The combination of high hardness, high chemical and thermal stability and high wear resistance of the coatings significantly reduces the deleterious effects of the cutting environment. The primary concern in this case is twofold: one, to cut metal without excessive wear, and two, to prevent deformation of the tool. It is very commonly found that deformation of the cutting edge greatly exacerbates the problem since it leads to a rapid wear of the tool. The objective of designing a coating, therefore, is to alleviate the pressure at the cutting point and to reduce the transfer of heat into the tool. This can be accomplished by a variety of means, some of which were mentioned earlier. We suggest that using a hard, abrasion-resistant coating such as TiCN in the place of TIN greatly increases the cutting efficiency, which results in a reduction in cutting forces. Secondly, the ability of the coating to act as a thermal barrier allows the heat at the contact point from being transmitted into the tool. The heat transfer aspect is related to the coefficient of friction of the coating, its thermal conductivity and temperature dependence of hardness. Thus, a combination of coating com-
positions and sequencing pattern can be used effectively to optimize the required properties of the system as a whole. The measured values of the coefficient of friction, taking into account the surface roughness aspects, show that the TiCN coating in this study had a slightly higher “lubricity” as compared with TIN. It is not clear how significant this difference may be, particularly given the fact that the response of TIN coatings to scratching by a diamond stylus sometimes leads to erratic results [ 231. The TiCN coating is smoother (owing to its relatively finer “grain size”) than the TIN coating. Cheng et al. [ 181 showed that the coarseness of the TiCN coating typically increased with increasing nitrogen content of the film. In the present work, the coating sequence was designed to provide a carbon-rich top layer for increased hardness and wear resistance. The beneficial effect of this design was an improvement of surface smoothness of the coating, which appears to have influenced the measured coefficient of friction. Whether or not the difference between TIN and TiCN is significant in the context of an obviously rough application environment, it is clear that the superior hardness of TiCN coating allows the tool to cut the steel more efficiently. In relative terms, this may be compared with cutting with a sharp knife as opposed to a blunt knife, TIN being the example of the latter. It is then not too difficult to consider that a clean and sharp cutting action would not only reduce the cutting forces but also reduce the heat build-up at the cutting edge. This, in turn, should lead to reduced deformation of the tool. The results of Figs. 2 and 4 clearly show this trend. A further improvement in performance is obtained by adding a thin layer of TIN to the top surface of the TiCN coating. TIN possesses some very useful properties, despite its relatively lower hardness. It is chemically more stable, and is generally known to provide excellent resistance to the buildup of workpiece material at the cutting edge. Its superior thermal properties provide an additive effect to the behavior of TiCN coating in the metal cutting process. The cumulative effect is that the coated tool containing a multilayer graded composition of coatings provides an excellent combination of metal cutting properties, and greatly improves the machining efficiency.
5. Summarizing
remarks
It has been shown that the properties of various hard, wearresistant coatings can be judiciously combined in a multilayer scheme to provide a synergistic effect for improving the overall properties of a coated cemented carbide cutting tool. The sacrifice in the strength and hardness of the substrate, resulting from a need to improve its fracture toughness, has been effectively neutralized by selecting a proper combination of a TiCN coating of graded composition and a surface layer of TIN. The results show that a multilayer coating based on this scheme provides excellent resistance against tool deformation and greatly improves the efficiency of metal cutting.
K. Narasimhan et al. /Wear
Acknowledgements We are indebted to many of our colleagues for their assistance in the experimental program. Thanks are due to Sharon Onyskin for the CVD experiments and metallography, and to Tony Lowe for the machining tests. We thank Chris Marushima, Manager, CVD group, and Jim Steigelman, Vice President, Technology, for their continued interest and encouragement during the course of this work. Permission to present and publish the paper is gratefully acknowledged.
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1171 0. Pacher, J. Stamberger and J. Kiefer, Proc. 4th Eur. CVD Conf: Eindhoven, The Netherlands I983, p. 49 1. [ 181 D.J. Cheng, W-P. Sun and M-H. Hon, Thin Solid Films, 146 (1987) 45. [ 191 Z. Wokulski, Cryst. Res. Technol. 26 ( 1991) 1025. [20] R. Chatterjee-Fischerand P. Mayr, Proc. 5th Eur. CVD Cont. Uppsala, Sweden, 1985, p. 395. [21] J. Oakes, paper presented at ASM Materials Week ‘86, Symp. on Coatings for Metal-cutting Applications, Orlando, FL, 1986. [22] R.S. Bonetti, H. Wiprachtiger and E. Mohn, in T.S. Sudarshan and D.G. Bhat (eds.), Surj&e Modification Technologies III, The Metallurgical Society, Warrendale, PA, 1990, p. 291. [231 H.E. Rebenne and D.G. Bhat, Su$ Coat. Technol., 63 (1994) 1.
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Krishnan Narasimhan: is an Advanced Engineer, New Coatings Development in the Materials Research & Development division of Valenite, Inc., and has developed several new CVD coatings for cutting tools. He holds a Masters degree in materials science. S. Prasad Boppana: is Manager of Machinability Testing group at Valenite, Inc. He has extensive experience in the machining of materials and applications of cutting tools in automotive, aerospace and other industries. He is responsible for testing of new products, development of application data for new cutting tool materials, and conducting machinability studies for customers. He holds a Masters degree in mechanical engineering. Deepak G. Bhat: is a Senior Research Scientist at Valenite, Inc., where he has been involved in the research and development of hard, wear-resistant coating for cutting tools using CVD and PVD methods. Currently he is involved in the development and commercialization of CVD diamond coatings on carbide cutting tools. He is also responsible for establishing liaisons with industry, universities and federal laboratories for the development and evaluation of metal cutting technologies. He holds a Ph.D. degree in materials science.