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Wear mechanisms of ultra-hard cutting tools materials
Journal of Materials Processing Technology 115 (2001) 402±412
Wear mechanisms of ultra-hard cutting tools materials Farhad Nabhani* School of Science and Technology, University of Teesside, Middlesborough TS1 3BA, UK Accepted 28 June 2001
Abstract Two experimental techniques are used in the investigation of cubic boron nitride (CBN) and polycrystalline diamond (PCD) as cutting tool materials for titanium alloy workpieces, in comparison with the currently used coated tungsten carbide speci®cations. One employs a quasi-static contact method to establish the temperature above which marked adhesion and welding occurs between the tool and the workpiece materials. After separation, the mode of failure of the welded junctions is studied to establish the path of crack propagation. The critical temperatures are shown to be 740, 760 and 9008C for the carbide, PCD and CBN tools, respectively, and in all cases failure of the junctions occurs in the bulk of the tool material. The other method used is the `quick-stop' technique, under otherwise normal cutting conditions, to study chip formation and tool wear. The predominant wear mechanisms are identi®ed and discussed for each of the tool materials and reasons advanced for observed differences in performance when removing the material from a titanium alloy workpiece. The wear resistance and quality of the machined surface is observed to be consistently better with the ultra-hard materials than with the carbide, and in particular, the PCD tool produces exceptionally good surface ®nish. In the case of the carbide tool, the rapid removal of the coated layers, leaving the substrate vulnerable to reaction with the workpiece material, is seen as contributing to its relatively poor performance. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Wear mechanism; Cubic boron nitride; Polycrystalline diamond; Carbide tools; Titanium alloys
1. Introduction Titanium is an attractive material for aerospace designers due to its unique combination of strength and lightness. However, it poses considerable problems in manufacturing because of its poor machinability [1±6]. Traditionally, high speed steel and solid carbide monolithic cutting tools have been employed and a relatively short lifetime or the need for frequent cutter re-grinding have been accepted. More recently, there has been a move towards the adoption of insert tooling based upon coated carbide systems [7±19]. With the evolution of a number of new cutting tool materials, such as those based on polycrystalline diamond (PCD) or cubic boron nitride (CBN) [20±45], may be advantageous in the machining of titanium alloys. However, the cost of both the workpiece material and the tool is not trivial in these cases and, therefore, there is a need to develop techniques for evaluating new materials for cutting that are simpler, quicker and less expensive than machining trials. Tools fail most commonly, under otherwise optimum cutting conditions, by mechanisms based on adhesion and * Tel.: 44-1642-342482; fax: 44-1642-342401. E-mail address: [email protected] (F. Nabhani).
diffusion [46±54] and these aspects that form the basis of this paper. The experimental work can be considered in two parts. The ®rst is based on a technique capable of measuring the critical resolved shear stress of hard ceramic crystals [55,56]. This technique uses point loading due to pressing cones, made from softer materials than the ceramic, on to the surface of the crystal to produce a contact pressure suf®cient to exceed its critical resolved shear stress but insuf®cient to cause cracking. Thus a range of contact pressures can be developed, essentially determined by the hardness of the material chosen for the cone, and the occurrence of plastic deformation can be detected through such techniques as dislocation etching [57]. In this particular study, the method has been adapted to establish the critical temperature above which a welded junction is formed between the workpiece and the tool. Observations on the subsequent breaking of that junction, once formed, gives further information concerning the relative strength of the workpiece, its interface with the tool, and the tool material in bulk. In addition, the integrity of intermediate interfaces can be assessed as, for example, when coated cutting tools are investigated. In the second part of the experimental work, the classical `quick-stop' technique pioneered by Williams et al. [58] is used, in conjunction
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with machining tests, to compare and contrast the performance of a coated carbide tool (KC850) Ð typical of those being used currently to cut titanium alloys Ð and tools based on CBN (Amborite) and PCD (Syndite). 2. Experimental details and observations 2.1. Material specifications The workpiece material used throughout all these experiments was an as rolled and annealed titanium alloy with a nominal composition speci®cation (in wt.%) given as Al 5; Mo 4; Sn 2±2.5; Si 6±7; Fe 2.0 max.; H 0.015; O 0.025; N 0.05; Ti remainder. It had a Knoop hardness (1.0 kg load) of 425 kg/mm2 (4.17 GPa) and the micro-structure consisted of elongated alpha phase in a ®ne dark-etching beta matrix. The coated carbide tool (Kennometal KC 850) had a substrate consisting of WC 85.4; TiC 2.5; Ta(Nb)C 6.1; Co 6 as the binder phase for particles in the size range 2±8 mm. The coating consisted of TiC/TiC±N/TiN Ð with the nitride as the outermost surface layer. The Knoop hardness of the surface coating was HK0.05 2614 (25.61 GPa) and the substrate tungsten carbide was HK1.0 1576 (15.45 GPa). The aggregate of CBN (Amborite) consisted of particles of about 1±2 mm in size with a ®ller or matrix phase comprising aluminium compounds (boride and nitride). The HK1.0 hardness was approximately 3100 kg/mm2 (30.1 GPa). The aggregate of PCD (Syndite) consisted of randomly orientated synthetic diamond crystals of 10 mm in size bonded to a tungsten carbide substrate. The hardness was typically in excess of 5000 HK1.0 (49 GPa). 2.2. Quasi-static adhesion experiments Fig. 1 is a schematic representation of the equipment used in these experiments. The cutting tool material was positioned on a graphite susceptor which was directly heated by a radio-frequency coil within an enclosed chamber evacuated to a pressure of approximately 10 5 mbar. A cone, with an included angle of 1208 (Fig. 2), was machined from the titanium alloy workpiece material for each experiment. This cone was mounted in a turret which, when not restrained such as during point loading between cone and specimen, compressed the ¯exible bellows due to the difference between atmospheric pressure and the vacuum. Then, under these conditions, a normal force of 115 N was transmitted to the tool via the cone. Naturally, during the application of this load, the titanium alloy cone ¯attened to form a contact area suf®cient to support the applied load elastically. Subsequently, after measuring the area of ¯attened tip, the nominal contact was calculated. The time of contact was 10 min in all cases and a new cone was used for each experiment. The lowest temperature at which adhesion and welding developed was measured for each of the three cutting tool materials.
Fig. 1. (a) Schematic cross-section of high temperature apparatus. (b) Apparatus used for adhesion tests at elevated temperatures.
Fig. 2. Titanium alloy cone with an included angle of 1208 used in quasistatic adhesion test.
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Fig. 3. (a) Crater formed in CBN tool after separation of welded junction shown in (b). (b) Titanium alloy indentor with strongly adherent CBN after fracture within bulk tool material.
The relevant critical temperatures were 740, 760 and 9008C for the carbide, PCD and CBN tools, respectively, and the nominal contact pressures developed were approximately 0.23, 0.142 and 0.146 GPa in those particular cases. It should be noted that this measured contact pressure is not simply related to the hardness of the metal cone but that it will also be in¯uenced by the friction at the metal/ceramic interface. Once formed, the separation of the welded junction can be made, and the nature of failure established, at any given temperature. Here, the separation was always carried out in a normal atmosphere at room temperature. Fig. 3b shows the result of separating a junction formed between the titanium alloy and CBN such that a pit in the original surface was formed as a hemispherical fragment which was removed with the titanium cone (Fig. 3a). Similarly, Fig. 4a shows the effect of separating a previously formed junction between a titanium indentor and PCD. It can be clearly seen that the failure was again within the bulk of the tool material (Fig. 4b). Finally, Fig. 5 shows the failure occurring in the tungsten carbide substrate with formation of a pit in the surface of the coated carbide tool and, as can be seen in the longitudinal section of the junction, all the layers were removed with the indentor cone (Fig. 5c).
Fig. 4. (a) Titanium alloy indentor with strongly adherent PCD after fracture within bulk tool material. (b) Crater formed in PCD tool after separation of welded junction shown in (a).
2.3. Machining tests A series of single point (SNGN 120408) turning tests were conducted using a Churchill Computurn 290 CNC lathe, with all three tool materials, at a surface speed of 75 m/min; a feed rate of 0.25 mm/rev; a depth of cut of 1.0 mm and without a cutting ¯uid. These conditions were chosen to create a rake face temperature as high as possible without causing tool failure [56] and to conserve the limited workpiece material. They were also representative, apart from the absence of a cutting ¯uid, of those used by aerospace manufacturers. The results of the tests are summarised in Figs. 6±8. From Fig. 7 it can be seen that the average ¯ank face wear of the CBN and PCD tools was consistently less than that of the coated carbide tool. This was particularly evident in the case of PCD where the rate of wear was lower than the carbide at least by a factor of 2±5 min cutting time and thereafter much lower still. In fact, when the rate of wear of the carbide tool accelerated rapidly, the PCD wear rate was effectively unchanged.
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Fig. 6. Tool wear performance when cutting titanium alloy.
Fig. 7. Average flank wear vs. cutting time (min).
Fig. 8. Average surface roughness vs. cutting time (min).
Fig. 5. (a) Titanium alloy indentor with strongly adherent coated carbide after fracture within bulk tool material. (b) Crater formed in coated carbide tool after separation of welded junction shown in (a). (c) Longitudinal section through titanium alloy/tool welded junction showing the removal of coated layers and substrate from carbide tool during adhesion test.
Furthermore, the surface ®nish of the workpiece achieved with the ultra-hard materials was always better than that with coated carbide (Fig. 8), and in the case of PCD remained at a very low level (2 mm Ra max.) throughout the cutting trials. The performance of the PCD material was not at the expense of tool life, which, as shown in Fig. 10, considerably exceeded that of both CBN and the coated
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Fig. 9. (a) The quick-stop device utilising a humane killer to disengage the tool from the workpiece rapidly leaving the flow of metal in the direction of chip flow. (b) Quick-stop test showing chip/workpiece interface.
carbide. Indeed, the tool did not fail Ð according to the normal test criteria Ð even after 26 min, when cutting was stopped to conserve workpiece material. 2.4. Quick-stop tests Following the above tests, which were conducted to examine wear rate, a number of `quick-stop' tests were carried out to enable detailed investigation of tool/workpiece interaction. The `quick-stop' device employs a pivotted tool holder supported by a shear pin. When the desired cutting conditions have been achieved, a captive bolt gun is used to break the shear pin and thereby accelerate the tool in its holder away from the workpiece (Fig. 9a and b). By this means, the material removal process is effectively frozen in time, and the chip/workpiece interface can be sectioned for detailed examination. A feature of the quick-stop tests, with both the coated carbide and the CBN material, was the fact that fracture obviously occurred within the bulk of the insert Ð well away from the rake face/workpiece interface Ð leaving part of the cutting edge bonded to the underside of the chip (Figs. 10±12). This clearly demonstrates the strength of the chip/tool interface bonding with titanium alloys, and is comparable with the failure of the corresponding junction as described in Section 2.2. In contrast, bulk fracture was less apparent in the case of the PCD insert (Fig. 13a). However, a closer examination of the underside of the chip being formed at the moment of detachment, due to the action of the quick-stop device, reveals that the normally smooth surface is now very rough
Fig. 10. Section through `quick-stop' specimen showing part of coated carbide tool adhering to underside of chip (100).
Fig. 11. Section through `quick-stop' specimen showing part of CBN tool adhering to underside of chip (100).
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Fig. 12. Tracing of a showing essential features, f, primary shear plane angle.
(Fig. 13b). The appearance of this region is consistent with the tensile fracture of a strongly adherent interface as the tool is accelerated away from the workpiece. Further testimony to the strength of this bonding is evident from energy
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dispersive X-ray (EDX) analysis in an electron microscope, which con®rmed that transfer of the cutting tool material from the rake face had occurred. All these observations imply that the temperature developed at the rake face/chip interface is above the critical level determined in the relevant quasi-static contact experiments. The maximum chip thickness (0.31 mm) is small [56,59] compared with workpiece materials such as plain carbon steel, re¯ecting the large shear plane angle, f, and the small chip/tool contact length observed [59]. As a result, the maximum rake face temperature occurs close to the cutting edge. The general chip form was segmented [60±63] with narrow bands of intense shear, separated by relatively undeformed regions, in which the alpha phase is elongated in the direction of chip travel. Thus it would appear that the primary shear is not continuous but rather proceeds in discrete `bursts' of catastrophic shear [64]. Freeman [65] working with commercially pure titanium observed submicron alpha grains in the shear bands indicating that suf®cient heat is generated during the intense primary shear to promote dynamic recrystallisation. During the short periods of intense catastrophic shear, the chip is displaced across the surface of the tool by plastic deformation within the ¯ow (or secondary shear) zone. For a chip thickness of 0.31 mm at a surface speed of 75 m/min, the effective chip velocity is 64 m/min [59], if it is assumed that the bottom surface of the chip is welded to the upper surface of the stationary tool, in the order of 27 10 6 and 5:35 10 6 s 1 , respectively. These conditions are quite suf®cient to raise local temperatures above 9008C and give rise to dynamic recrystallisation as indicated by the presence of ®ne grains within the ¯ow zone. The sequence of events leading to cyclic chip formation when machining titanium has been described by Komanduri and von Turkovich [66] based on their detailed study of video tapes of low speed machining under workshop conditions, and microscopic examination of sections through chips. Insofar as the actual mechanism of chip formation is concerned, cutting speed appears to have no signi®cant effect. However, it is an important factor in determining tool temperature, tool wear and secondary chip generation [60±63]. 3. Discussion of failure and wear modes 3.1. Welded junctions
Fig. 13. (a) Fragments of PCD tool adhering to underside of chip (100). (b) Close-up view of (a) (400).
The preliminary results from the use of workpiece/tool adhesion tests to evaluate candidate cutting tool materials appeared promising in that both the critical welding temperature and the failure of the bulk or substrate material were identi®ed. When the junctions between the workpiece and CBN or PCD are broken, there are three possible routes for crack propagation Ð through the workpiece, the interface, or the bulk of the ultra-hard material. However, there are a greater
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Fig. 14. SEM micrograph of exposed carbide substrate after fracture of welded junction (refer Fig. 5).
number of possibilities for the coated carbide tool where failure might also take place within the coating or at its interface with the substrate. In the event, in this work, failure was always through the bulk of the CBN, the PCD or tungsten carbide, respectively (Figs. 3±5). At higher magni®cations (Fig. 14), it can be seen that the cracks in the tungsten carbide had propagated around the hard particles Ð presumably through the cobalt phase. In other work in this laboratory, primarily designed to measure the critical resolved shear stress of ceramic crystals, the incidence of sub-surface lateral cracking (sometimes referred to as lateral venting) in the bulk of the harder crystal is quite common Ð even at temperatures in the region of 10008C. In normal indentation studies, such cracks are thought to be formed at the elastic/plastic boundary as a result of tensile stresses developed by the recovery of compressive plastic strain as the load is removed. On the assumption that the critical resolved shear stress of tungsten carbide is comparable to that of titanium carbide at 8008C, i.e. 0.25 GPa [67], the contact pressure applied by the titanium alloy cone might have been suf®cient to produce a small degree of plastic deformation, but this is unlikely to be suf®cient to develop venting. A more likely crack initiation mechanism is now thought to be associated with the differential thermal contraction of the titanium and the cutting tool materials. When the weld cools from critical temperature, tensile stresses will be induced in the tool materials around the periphery of the weld contact area due to the greater rate of contraction of the titanium. This situation is currently being modelled to determine whether these tensile stresses could be of suf®cient magnitude to account for the observed fracture. 3.2. Wear performance of coated carbide, CBN and PCD A feature of the wear test conditions was the absence of a well-de®ned built-up edge on the tool inserts. Nevertheless,
the strongly adherent workpiece maintains an intimate and sustained contact with the rake face through an interfacial layer. The ¯ow zone, where shear takes place to form the base of the chip, exists at or within this layer. Ideally, separation of material should occur in the chip side of this interfacial layer so as to provide protection to the cutting tool. When this layer is detached from the chip, as will inevitably happen from time to time, the high adhesive forces are likely to result in the plucking out of hard particles from the tool causing its surface to become grooved and crater depth to increase. It is during this part of the process that resistance to plastic deformation, at elevated temperatures, will be an important intrinsic property of the tool material. A similar process of attrition and grooving wear will be developing on the ¯ank face leading to deterioration in the machined surface ®nish. Ultimately, the combination of this crater and ¯ank wear will undermine the integrity of the cutting edge and, unsupported, it will then break away. In this ®nal stage, the fracture toughness of the cutting tool material will be important. When cutting the titanium alloy, the chemically vapour deposited coating was rapidly removed from the carbide tools. In most cases, discrete fragments of the coating were removed by a process of adhesive wear similar to that reported with tools coated with hafnium nitride [59]. However, as the cutting continued, a stage of rapid but smoothly progressive coating wear was entered leading to the exposure of the substrate. Examination of the rake face of the tool shows a crater within which are the remains of some of the metallic surface layer (Fig. 15a). In other places, where this layer has been removed, the surface is smoothly grooved as though by plastic deformation. The pitch of the ridges on these grooves is approximately 50 mm Ð too large for them to have been due to scoring by individual carbide particles. Within these grooves, a close examination revealed (Fig. 15b) the occurrence of progressive wear of the carbide substrate on a ®ne scale as indicated by the parallel scoring in the direction of chip ¯ow. These ®ner score marks are most likely to have been caused by plastic deformation as carbide particles are detached and removed from the chip/tool interface [68]. However, it should be noted that an alternative explanation is offered by Trent [69] who refers to the likelihood of increased chemical activity leading to etching of the tool material under conditions of high strain such as occur in the ¯ow zone. Further machining/etching of the tool will result in diffusion wear mechanism. This process of wear eventually will lead to catastrophic failure of the tool due formation of large crater on the rake face (Fig. 15c). Turning now to the tests with CBN cutting tools, Fig. 16 shows the condition of the rake face of a CBN insert after cutting the titanium alloy workpiece for 3 min at the standard conditions. Although gross wear of the insert has occurred due to fracture, it is still possible to see clearly parts of a smoothly worn crater on the rake face. Such wear
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Fig. 16. (a) Worn surface of flank and rake faces of CBN showing smooth crater wear. (b) Close-up views of rake face crater wear of CBN tool.
As in the case of the other two materials, a strongly adherent interfacial layer is formed on the top of the rake face of the PCD tool (Fig. 17). However, in contrast, a signi®cant cratering was not developed. This may simply re¯ect a difference in the rate of wear rather than the
Fig. 15. (a) View of coated carbide tool with smooth rake face crater wear and remains of adherent metal layer. (b) Close-up views of rake face crater wear showing smooth ridges with fine scoring in direction of chip flow. (c) Smoothly worn crater surface evidence of diffusion/dissolution wear.
was also observable on the ¯ank face. It is considered likely that, subsequently, the combined effects of the ¯ank and rake face wear gives rise to an unsupported section of the cutting edge which breaks away.
Fig. 17. Formation of strongly adherent layer on the rake face of a PCD tool.
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Fig. 18. Comparison of machining swarf produced with PCD, CBN and coated carbide tools.
mechanism Ð leading to failure of the carbide and CBN tools in 9 and 11 min, respectively, whilst the PCD tool had not failed even after 30 min. It is possible that diffusion across the interface results in the formation of a titanium carbide layer which would then protect the tool in forming a barrier to further diffusion and loss of tool material in the chip. Some support from this contention can be obtained by using scanning electron microscope (SEM) with microanalysis facility [56] and observation that graphite crucibles are able to withstand attack by liquid titanium metal crucible [70] by forming a stable layer of titanium carbide on the surface. However, this assumption requires further investigation. Nevertheless, it should be emphasised that the low wear rate of PCD is not at the expense of other machining parameters since the workpiece surface ®nish was always signi®cantly better than with the other tool materials (Fig. 8). The nature of the swarf produced is also markedly different as illustrated in Figs. 18 and 19. Fig. 19 shows formation of smooth surface ®nish generated by PCD tool while machining aerospace titanium alloys. The process of wear leading to the formation of craters is thought to involve dissolution of material from the tool by
Fig. 19. Turning aerospace alloys using PCD tool.
diffusion into the adjacent zones of the chip and the workpiece [71]. A simple model relating the factors involved in the dissolution±diffusion process has been described by Dearnley [72]. The most important of these factors is the solubility limit of the tool material in the workpiece, which determines the magnitude of the concentration gradient in the shear zone, and hence the diffusion ¯ux. The intimate contact between the tool and the chip/workpiece at temperatures above 7008C provides an ideal environment for diffusion of tool material atoms across the tool/chip and the tool/workpiece interfaces [59]. The better wear resistance of CBN when cutting titanium at high speed may be attributable in part to the relatively low solubility of boron. From these and other cutting tests [59,61,73±76], it has been possible to show that, when conditions for dissolution±diffusion wear predominate, wear with CBN is less than that with coated carbide. 4. Summary and conclusions This has been a preliminary investigation and it is appreciated that subtle changes in machining conditions can have signi®cant effects on both the quality of the machined ®nish and the wear of the tool. Nevertheless, under the conditions prevailing in this investigation, a number of conclusions may be justi®ed. The simple quasi-static contact method can be used to identify the workpiece/tool interfacial temperature above which strongly adherent surface layers may be formed on the rake face. This temperature is 740, 760 and 9008C for coated carbide, PCD and CBN tools, respectively, when cutting a titanium alloy. Furthermore, the method can be used to study the integrity of the bulk material, and/or individual coatings on substrates, when welded junctions are subsequently separated. Generally, fracture is initiated in the bulk of the harder tool material Ð rather than in the workpiece or at the welded junction interface. The presence of coated layers on the carbide tool appears to have had no bene®cial effect on their performance since these layers are rapidly removed leaving the tungsten carbide substrate vulnerable to cratering. The results of this investigation illustrate the complexity of the wear mechanisms in metal cutting and, whilst the resistance to plastic deformation and fracture of the cutting tool material is predictable, they underline the importance of chemical reactions which determine the nature of the interface formed between the tool and the chip. For this reason, we should not anticipate the development of an ultimate, ubiquitous, cutting tool material but rather select an optimum candidate for a given workpiece. Certainly, we may conclude that it is desirable to adjust conditions so as to form a stable interfacial layer which both protects the rake face and is easily sheared to form a smooth chip. In any event, the ®nal selection of cutting tool material will always be based on a compromise between several
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[45] T. Le Huu, M. Schmitt, D. Paulmier, A.G. Mamalis, A. Grabchenko, Tribological properties of smooth diamond coatings for cutting tools, Wear 225±229 (2) (1999) 843±847. [46] S. Silva, V.P. Mammana, M.C. Salvadori, O.R. Monteiro, I.G. Brown, WC±Co cutting tool inserts with diamond coatings, Diamond Relat. Mater. 8 (10) (1999) 1913±1918. [47] H.K. ToÈnshoff, A. Mohlfeld, C. Gey, J. Winkler, Mechanical pretreatment for improved adhesion of diamond coatings, Surf. Coat. Technol. 116±119 (1999) 440±446. [48] H.K. ToÈnshoff, B. Karpuschewski, A. Mohlfeld, H. Seegers, Influence of subsurface properties on the adhesion strength of sputtered hard coatings, Surf. Coat. Technol. 116±119 (1999) 524±529. [49] H. Itoh, S.-S. Lee, K. Sugiyama, H. Iwahara, T. Tsutsumoto, Adhesion improvement of diamond coating on silicon nitride substrate, Surf. Coat. Technol. 112 (1±3) (1999) 199±203. [50] N. Sato, Adhesion of aluminium to tungsten carbide±cobalt tools, Met. Powder Rep. 53 (11) (1998) 39. [51] N. Dilawar, R. Kapil, Brahamprakash, V.D. Vankar, D.K. Avasthi, D. Kabiraj, G.K. Mehta, Adhesion enhancement of diamond coatings on WC tools by high energy ion irradiation, Thin Solid Films 323 (1±2) (1998) 163±169. [52] F. Sun, Z. Li, B. Chen, D. Jiang, Adhering wear mechanism of cemented carbide cutter in the intervallic cutting of stainless steel, Wear 214 (1) (1998) 79±82. [53] J. Mroz, R.H. Dauskardt, U. Schleinkofer, New adhesion measurement technique for coated cutting tool materials, Int. J. Refract. Met. Hard Mater. 16 (4±6) (1998) 395±402. [54] M. Hakovirta, Adhesion of amorphous diamond-like film on sputtered hardmetal (WC±Co) cutting tools, Diamond Relat. Mater. 5 (2) (1996) 186±189. [55] C.A. Brooks, V.R. Howes, A.R. Parry, Nature 332 (1988) 139±141. [56] F. Nabhani, Ph.D. Thesis, University of Hull, 1991.
[57] C.A. Brooks, A.R. Parry, Proceedings of the ICSHM3, Nassau, 1987. Mater. Sci. Eng. A 105±106 (1988) 143±150. [58] J.E. Williams, E.F. Smart, D.R. Miller, Metallurgia 81 (1970) 3. [59] A.N. Greason, P.A. Dearnley, Mater. Sci. Technol. 2 (1986). [60] J.-D. Kim, V.R. Marinov, D.-S. Kim, Built-up edge analysis of orthogonal cutting by the visco-plastic finite-element method, J. Mater. Process. Technol. 71 (3) (1997) 367±372. [61] J.A. Arsecularatne, R.F. Fowle, P. Mathew, Prediction of chip flow direction, cutting forces and surface roughness in finish turning, J. Manuf. Sci. Eng., Trans. ASME 120 (1) (1998) 1±12. [62] K. Oishi, Built-up edge elimination in mirror cutting of hardened steel, J. Eng. Ind., Trans. ASME 117 (1) (1995) 62±66. [63] J.Y. Wang, C.R. Liu, New concept for decoupling the cutting forces due to tool flank wear and chip formation in hard turning, Mach. Sci. Technol. 2 (1) (1998) 77±90. [64] R. Komanduri, Wear 76 (1982). [65] R.M. Freeman, Ph.D. Thesis, University of Birmingham, 1974. [66] R. Komanduri, B.F. von Turkovich, Wear 69 (1981). [67] W.S. Williams, J. Appl. Phys. 35 (1965). [68] R. Milovic, E.F. Smart, M.L.H. Wise, Mater. Sci. Technol. 2 (1986). [69] E.M. Trent, Metal Cutting, 2nd Edition, Butterworths, London, 1984. [70] P.A. Dearnley, Wear 101 (1985) 1. [71] P.A. Dearnley, Surf. Eng. 1 (1985) 1. [72] A.D. McQuillan, M.K. McQuillan, Titanium, Butterworth Scientific Publications, London, 1956. [73] Z.Y. Wang, K.P. Rajurkar, Wear of CBN tool in turning of silicon nitride with cryogenic cooling, Int. J. Mach. Tools Manuf. 37 (3) (1997) 319±326. [74] Z.Y. Wang, K.P. Rajurkar, M. Murugappan, Cryogenic PCBN turning of ceramic (Si3N4), Wear 195 (1±2) (1996) 1±6. [75] Z.-C. Lin, D.-Y. Chen, Study of cutting with a CBN tool, J. Mater. Process. Technol. 49 (1±2) (1995) 149±164. [76] I.A. Petrusha, Cubic boron nitride for cutting nickel base materials, Met. Powder Rep. 53 (2) (1998) 41.
Wear mechanisms of ultra-hard cutting tools materials Farhad Nabhani* School of Science and Technology, University of Teesside, Middlesborough TS1 3BA, UK Accepted 28 June 2001
Abstract Two experimental techniques are used in the investigation of cubic boron nitride (CBN) and polycrystalline diamond (PCD) as cutting tool materials for titanium alloy workpieces, in comparison with the currently used coated tungsten carbide speci®cations. One employs a quasi-static contact method to establish the temperature above which marked adhesion and welding occurs between the tool and the workpiece materials. After separation, the mode of failure of the welded junctions is studied to establish the path of crack propagation. The critical temperatures are shown to be 740, 760 and 9008C for the carbide, PCD and CBN tools, respectively, and in all cases failure of the junctions occurs in the bulk of the tool material. The other method used is the `quick-stop' technique, under otherwise normal cutting conditions, to study chip formation and tool wear. The predominant wear mechanisms are identi®ed and discussed for each of the tool materials and reasons advanced for observed differences in performance when removing the material from a titanium alloy workpiece. The wear resistance and quality of the machined surface is observed to be consistently better with the ultra-hard materials than with the carbide, and in particular, the PCD tool produces exceptionally good surface ®nish. In the case of the carbide tool, the rapid removal of the coated layers, leaving the substrate vulnerable to reaction with the workpiece material, is seen as contributing to its relatively poor performance. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Wear mechanism; Cubic boron nitride; Polycrystalline diamond; Carbide tools; Titanium alloys
1. Introduction Titanium is an attractive material for aerospace designers due to its unique combination of strength and lightness. However, it poses considerable problems in manufacturing because of its poor machinability [1±6]. Traditionally, high speed steel and solid carbide monolithic cutting tools have been employed and a relatively short lifetime or the need for frequent cutter re-grinding have been accepted. More recently, there has been a move towards the adoption of insert tooling based upon coated carbide systems [7±19]. With the evolution of a number of new cutting tool materials, such as those based on polycrystalline diamond (PCD) or cubic boron nitride (CBN) [20±45], may be advantageous in the machining of titanium alloys. However, the cost of both the workpiece material and the tool is not trivial in these cases and, therefore, there is a need to develop techniques for evaluating new materials for cutting that are simpler, quicker and less expensive than machining trials. Tools fail most commonly, under otherwise optimum cutting conditions, by mechanisms based on adhesion and * Tel.: 44-1642-342482; fax: 44-1642-342401. E-mail address: [email protected] (F. Nabhani).
diffusion [46±54] and these aspects that form the basis of this paper. The experimental work can be considered in two parts. The ®rst is based on a technique capable of measuring the critical resolved shear stress of hard ceramic crystals [55,56]. This technique uses point loading due to pressing cones, made from softer materials than the ceramic, on to the surface of the crystal to produce a contact pressure suf®cient to exceed its critical resolved shear stress but insuf®cient to cause cracking. Thus a range of contact pressures can be developed, essentially determined by the hardness of the material chosen for the cone, and the occurrence of plastic deformation can be detected through such techniques as dislocation etching [57]. In this particular study, the method has been adapted to establish the critical temperature above which a welded junction is formed between the workpiece and the tool. Observations on the subsequent breaking of that junction, once formed, gives further information concerning the relative strength of the workpiece, its interface with the tool, and the tool material in bulk. In addition, the integrity of intermediate interfaces can be assessed as, for example, when coated cutting tools are investigated. In the second part of the experimental work, the classical `quick-stop' technique pioneered by Williams et al. [58] is used, in conjunction
0924-0136/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 0 1 3 6 ( 0 1 ) 0 0 8 5 1 - 2
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with machining tests, to compare and contrast the performance of a coated carbide tool (KC850) Ð typical of those being used currently to cut titanium alloys Ð and tools based on CBN (Amborite) and PCD (Syndite). 2. Experimental details and observations 2.1. Material specifications The workpiece material used throughout all these experiments was an as rolled and annealed titanium alloy with a nominal composition speci®cation (in wt.%) given as Al 5; Mo 4; Sn 2±2.5; Si 6±7; Fe 2.0 max.; H 0.015; O 0.025; N 0.05; Ti remainder. It had a Knoop hardness (1.0 kg load) of 425 kg/mm2 (4.17 GPa) and the micro-structure consisted of elongated alpha phase in a ®ne dark-etching beta matrix. The coated carbide tool (Kennometal KC 850) had a substrate consisting of WC 85.4; TiC 2.5; Ta(Nb)C 6.1; Co 6 as the binder phase for particles in the size range 2±8 mm. The coating consisted of TiC/TiC±N/TiN Ð with the nitride as the outermost surface layer. The Knoop hardness of the surface coating was HK0.05 2614 (25.61 GPa) and the substrate tungsten carbide was HK1.0 1576 (15.45 GPa). The aggregate of CBN (Amborite) consisted of particles of about 1±2 mm in size with a ®ller or matrix phase comprising aluminium compounds (boride and nitride). The HK1.0 hardness was approximately 3100 kg/mm2 (30.1 GPa). The aggregate of PCD (Syndite) consisted of randomly orientated synthetic diamond crystals of 10 mm in size bonded to a tungsten carbide substrate. The hardness was typically in excess of 5000 HK1.0 (49 GPa). 2.2. Quasi-static adhesion experiments Fig. 1 is a schematic representation of the equipment used in these experiments. The cutting tool material was positioned on a graphite susceptor which was directly heated by a radio-frequency coil within an enclosed chamber evacuated to a pressure of approximately 10 5 mbar. A cone, with an included angle of 1208 (Fig. 2), was machined from the titanium alloy workpiece material for each experiment. This cone was mounted in a turret which, when not restrained such as during point loading between cone and specimen, compressed the ¯exible bellows due to the difference between atmospheric pressure and the vacuum. Then, under these conditions, a normal force of 115 N was transmitted to the tool via the cone. Naturally, during the application of this load, the titanium alloy cone ¯attened to form a contact area suf®cient to support the applied load elastically. Subsequently, after measuring the area of ¯attened tip, the nominal contact was calculated. The time of contact was 10 min in all cases and a new cone was used for each experiment. The lowest temperature at which adhesion and welding developed was measured for each of the three cutting tool materials.
Fig. 1. (a) Schematic cross-section of high temperature apparatus. (b) Apparatus used for adhesion tests at elevated temperatures.
Fig. 2. Titanium alloy cone with an included angle of 1208 used in quasistatic adhesion test.
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Fig. 3. (a) Crater formed in CBN tool after separation of welded junction shown in (b). (b) Titanium alloy indentor with strongly adherent CBN after fracture within bulk tool material.
The relevant critical temperatures were 740, 760 and 9008C for the carbide, PCD and CBN tools, respectively, and the nominal contact pressures developed were approximately 0.23, 0.142 and 0.146 GPa in those particular cases. It should be noted that this measured contact pressure is not simply related to the hardness of the metal cone but that it will also be in¯uenced by the friction at the metal/ceramic interface. Once formed, the separation of the welded junction can be made, and the nature of failure established, at any given temperature. Here, the separation was always carried out in a normal atmosphere at room temperature. Fig. 3b shows the result of separating a junction formed between the titanium alloy and CBN such that a pit in the original surface was formed as a hemispherical fragment which was removed with the titanium cone (Fig. 3a). Similarly, Fig. 4a shows the effect of separating a previously formed junction between a titanium indentor and PCD. It can be clearly seen that the failure was again within the bulk of the tool material (Fig. 4b). Finally, Fig. 5 shows the failure occurring in the tungsten carbide substrate with formation of a pit in the surface of the coated carbide tool and, as can be seen in the longitudinal section of the junction, all the layers were removed with the indentor cone (Fig. 5c).
Fig. 4. (a) Titanium alloy indentor with strongly adherent PCD after fracture within bulk tool material. (b) Crater formed in PCD tool after separation of welded junction shown in (a).
2.3. Machining tests A series of single point (SNGN 120408) turning tests were conducted using a Churchill Computurn 290 CNC lathe, with all three tool materials, at a surface speed of 75 m/min; a feed rate of 0.25 mm/rev; a depth of cut of 1.0 mm and without a cutting ¯uid. These conditions were chosen to create a rake face temperature as high as possible without causing tool failure [56] and to conserve the limited workpiece material. They were also representative, apart from the absence of a cutting ¯uid, of those used by aerospace manufacturers. The results of the tests are summarised in Figs. 6±8. From Fig. 7 it can be seen that the average ¯ank face wear of the CBN and PCD tools was consistently less than that of the coated carbide tool. This was particularly evident in the case of PCD where the rate of wear was lower than the carbide at least by a factor of 2±5 min cutting time and thereafter much lower still. In fact, when the rate of wear of the carbide tool accelerated rapidly, the PCD wear rate was effectively unchanged.
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Fig. 6. Tool wear performance when cutting titanium alloy.
Fig. 7. Average flank wear vs. cutting time (min).
Fig. 8. Average surface roughness vs. cutting time (min).
Fig. 5. (a) Titanium alloy indentor with strongly adherent coated carbide after fracture within bulk tool material. (b) Crater formed in coated carbide tool after separation of welded junction shown in (a). (c) Longitudinal section through titanium alloy/tool welded junction showing the removal of coated layers and substrate from carbide tool during adhesion test.
Furthermore, the surface ®nish of the workpiece achieved with the ultra-hard materials was always better than that with coated carbide (Fig. 8), and in the case of PCD remained at a very low level (2 mm Ra max.) throughout the cutting trials. The performance of the PCD material was not at the expense of tool life, which, as shown in Fig. 10, considerably exceeded that of both CBN and the coated
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Fig. 9. (a) The quick-stop device utilising a humane killer to disengage the tool from the workpiece rapidly leaving the flow of metal in the direction of chip flow. (b) Quick-stop test showing chip/workpiece interface.
carbide. Indeed, the tool did not fail Ð according to the normal test criteria Ð even after 26 min, when cutting was stopped to conserve workpiece material. 2.4. Quick-stop tests Following the above tests, which were conducted to examine wear rate, a number of `quick-stop' tests were carried out to enable detailed investigation of tool/workpiece interaction. The `quick-stop' device employs a pivotted tool holder supported by a shear pin. When the desired cutting conditions have been achieved, a captive bolt gun is used to break the shear pin and thereby accelerate the tool in its holder away from the workpiece (Fig. 9a and b). By this means, the material removal process is effectively frozen in time, and the chip/workpiece interface can be sectioned for detailed examination. A feature of the quick-stop tests, with both the coated carbide and the CBN material, was the fact that fracture obviously occurred within the bulk of the insert Ð well away from the rake face/workpiece interface Ð leaving part of the cutting edge bonded to the underside of the chip (Figs. 10±12). This clearly demonstrates the strength of the chip/tool interface bonding with titanium alloys, and is comparable with the failure of the corresponding junction as described in Section 2.2. In contrast, bulk fracture was less apparent in the case of the PCD insert (Fig. 13a). However, a closer examination of the underside of the chip being formed at the moment of detachment, due to the action of the quick-stop device, reveals that the normally smooth surface is now very rough
Fig. 10. Section through `quick-stop' specimen showing part of coated carbide tool adhering to underside of chip (100).
Fig. 11. Section through `quick-stop' specimen showing part of CBN tool adhering to underside of chip (100).
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Fig. 12. Tracing of a showing essential features, f, primary shear plane angle.
(Fig. 13b). The appearance of this region is consistent with the tensile fracture of a strongly adherent interface as the tool is accelerated away from the workpiece. Further testimony to the strength of this bonding is evident from energy
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dispersive X-ray (EDX) analysis in an electron microscope, which con®rmed that transfer of the cutting tool material from the rake face had occurred. All these observations imply that the temperature developed at the rake face/chip interface is above the critical level determined in the relevant quasi-static contact experiments. The maximum chip thickness (0.31 mm) is small [56,59] compared with workpiece materials such as plain carbon steel, re¯ecting the large shear plane angle, f, and the small chip/tool contact length observed [59]. As a result, the maximum rake face temperature occurs close to the cutting edge. The general chip form was segmented [60±63] with narrow bands of intense shear, separated by relatively undeformed regions, in which the alpha phase is elongated in the direction of chip travel. Thus it would appear that the primary shear is not continuous but rather proceeds in discrete `bursts' of catastrophic shear [64]. Freeman [65] working with commercially pure titanium observed submicron alpha grains in the shear bands indicating that suf®cient heat is generated during the intense primary shear to promote dynamic recrystallisation. During the short periods of intense catastrophic shear, the chip is displaced across the surface of the tool by plastic deformation within the ¯ow (or secondary shear) zone. For a chip thickness of 0.31 mm at a surface speed of 75 m/min, the effective chip velocity is 64 m/min [59], if it is assumed that the bottom surface of the chip is welded to the upper surface of the stationary tool, in the order of 27 10 6 and 5:35 10 6 s 1 , respectively. These conditions are quite suf®cient to raise local temperatures above 9008C and give rise to dynamic recrystallisation as indicated by the presence of ®ne grains within the ¯ow zone. The sequence of events leading to cyclic chip formation when machining titanium has been described by Komanduri and von Turkovich [66] based on their detailed study of video tapes of low speed machining under workshop conditions, and microscopic examination of sections through chips. Insofar as the actual mechanism of chip formation is concerned, cutting speed appears to have no signi®cant effect. However, it is an important factor in determining tool temperature, tool wear and secondary chip generation [60±63]. 3. Discussion of failure and wear modes 3.1. Welded junctions
Fig. 13. (a) Fragments of PCD tool adhering to underside of chip (100). (b) Close-up view of (a) (400).
The preliminary results from the use of workpiece/tool adhesion tests to evaluate candidate cutting tool materials appeared promising in that both the critical welding temperature and the failure of the bulk or substrate material were identi®ed. When the junctions between the workpiece and CBN or PCD are broken, there are three possible routes for crack propagation Ð through the workpiece, the interface, or the bulk of the ultra-hard material. However, there are a greater
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Fig. 14. SEM micrograph of exposed carbide substrate after fracture of welded junction (refer Fig. 5).
number of possibilities for the coated carbide tool where failure might also take place within the coating or at its interface with the substrate. In the event, in this work, failure was always through the bulk of the CBN, the PCD or tungsten carbide, respectively (Figs. 3±5). At higher magni®cations (Fig. 14), it can be seen that the cracks in the tungsten carbide had propagated around the hard particles Ð presumably through the cobalt phase. In other work in this laboratory, primarily designed to measure the critical resolved shear stress of ceramic crystals, the incidence of sub-surface lateral cracking (sometimes referred to as lateral venting) in the bulk of the harder crystal is quite common Ð even at temperatures in the region of 10008C. In normal indentation studies, such cracks are thought to be formed at the elastic/plastic boundary as a result of tensile stresses developed by the recovery of compressive plastic strain as the load is removed. On the assumption that the critical resolved shear stress of tungsten carbide is comparable to that of titanium carbide at 8008C, i.e. 0.25 GPa [67], the contact pressure applied by the titanium alloy cone might have been suf®cient to produce a small degree of plastic deformation, but this is unlikely to be suf®cient to develop venting. A more likely crack initiation mechanism is now thought to be associated with the differential thermal contraction of the titanium and the cutting tool materials. When the weld cools from critical temperature, tensile stresses will be induced in the tool materials around the periphery of the weld contact area due to the greater rate of contraction of the titanium. This situation is currently being modelled to determine whether these tensile stresses could be of suf®cient magnitude to account for the observed fracture. 3.2. Wear performance of coated carbide, CBN and PCD A feature of the wear test conditions was the absence of a well-de®ned built-up edge on the tool inserts. Nevertheless,
the strongly adherent workpiece maintains an intimate and sustained contact with the rake face through an interfacial layer. The ¯ow zone, where shear takes place to form the base of the chip, exists at or within this layer. Ideally, separation of material should occur in the chip side of this interfacial layer so as to provide protection to the cutting tool. When this layer is detached from the chip, as will inevitably happen from time to time, the high adhesive forces are likely to result in the plucking out of hard particles from the tool causing its surface to become grooved and crater depth to increase. It is during this part of the process that resistance to plastic deformation, at elevated temperatures, will be an important intrinsic property of the tool material. A similar process of attrition and grooving wear will be developing on the ¯ank face leading to deterioration in the machined surface ®nish. Ultimately, the combination of this crater and ¯ank wear will undermine the integrity of the cutting edge and, unsupported, it will then break away. In this ®nal stage, the fracture toughness of the cutting tool material will be important. When cutting the titanium alloy, the chemically vapour deposited coating was rapidly removed from the carbide tools. In most cases, discrete fragments of the coating were removed by a process of adhesive wear similar to that reported with tools coated with hafnium nitride [59]. However, as the cutting continued, a stage of rapid but smoothly progressive coating wear was entered leading to the exposure of the substrate. Examination of the rake face of the tool shows a crater within which are the remains of some of the metallic surface layer (Fig. 15a). In other places, where this layer has been removed, the surface is smoothly grooved as though by plastic deformation. The pitch of the ridges on these grooves is approximately 50 mm Ð too large for them to have been due to scoring by individual carbide particles. Within these grooves, a close examination revealed (Fig. 15b) the occurrence of progressive wear of the carbide substrate on a ®ne scale as indicated by the parallel scoring in the direction of chip ¯ow. These ®ner score marks are most likely to have been caused by plastic deformation as carbide particles are detached and removed from the chip/tool interface [68]. However, it should be noted that an alternative explanation is offered by Trent [69] who refers to the likelihood of increased chemical activity leading to etching of the tool material under conditions of high strain such as occur in the ¯ow zone. Further machining/etching of the tool will result in diffusion wear mechanism. This process of wear eventually will lead to catastrophic failure of the tool due formation of large crater on the rake face (Fig. 15c). Turning now to the tests with CBN cutting tools, Fig. 16 shows the condition of the rake face of a CBN insert after cutting the titanium alloy workpiece for 3 min at the standard conditions. Although gross wear of the insert has occurred due to fracture, it is still possible to see clearly parts of a smoothly worn crater on the rake face. Such wear
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409
Fig. 16. (a) Worn surface of flank and rake faces of CBN showing smooth crater wear. (b) Close-up views of rake face crater wear of CBN tool.
As in the case of the other two materials, a strongly adherent interfacial layer is formed on the top of the rake face of the PCD tool (Fig. 17). However, in contrast, a signi®cant cratering was not developed. This may simply re¯ect a difference in the rate of wear rather than the
Fig. 15. (a) View of coated carbide tool with smooth rake face crater wear and remains of adherent metal layer. (b) Close-up views of rake face crater wear showing smooth ridges with fine scoring in direction of chip flow. (c) Smoothly worn crater surface evidence of diffusion/dissolution wear.
was also observable on the ¯ank face. It is considered likely that, subsequently, the combined effects of the ¯ank and rake face wear gives rise to an unsupported section of the cutting edge which breaks away.
Fig. 17. Formation of strongly adherent layer on the rake face of a PCD tool.
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Fig. 18. Comparison of machining swarf produced with PCD, CBN and coated carbide tools.
mechanism Ð leading to failure of the carbide and CBN tools in 9 and 11 min, respectively, whilst the PCD tool had not failed even after 30 min. It is possible that diffusion across the interface results in the formation of a titanium carbide layer which would then protect the tool in forming a barrier to further diffusion and loss of tool material in the chip. Some support from this contention can be obtained by using scanning electron microscope (SEM) with microanalysis facility [56] and observation that graphite crucibles are able to withstand attack by liquid titanium metal crucible [70] by forming a stable layer of titanium carbide on the surface. However, this assumption requires further investigation. Nevertheless, it should be emphasised that the low wear rate of PCD is not at the expense of other machining parameters since the workpiece surface ®nish was always signi®cantly better than with the other tool materials (Fig. 8). The nature of the swarf produced is also markedly different as illustrated in Figs. 18 and 19. Fig. 19 shows formation of smooth surface ®nish generated by PCD tool while machining aerospace titanium alloys. The process of wear leading to the formation of craters is thought to involve dissolution of material from the tool by
Fig. 19. Turning aerospace alloys using PCD tool.
diffusion into the adjacent zones of the chip and the workpiece [71]. A simple model relating the factors involved in the dissolution±diffusion process has been described by Dearnley [72]. The most important of these factors is the solubility limit of the tool material in the workpiece, which determines the magnitude of the concentration gradient in the shear zone, and hence the diffusion ¯ux. The intimate contact between the tool and the chip/workpiece at temperatures above 7008C provides an ideal environment for diffusion of tool material atoms across the tool/chip and the tool/workpiece interfaces [59]. The better wear resistance of CBN when cutting titanium at high speed may be attributable in part to the relatively low solubility of boron. From these and other cutting tests [59,61,73±76], it has been possible to show that, when conditions for dissolution±diffusion wear predominate, wear with CBN is less than that with coated carbide. 4. Summary and conclusions This has been a preliminary investigation and it is appreciated that subtle changes in machining conditions can have signi®cant effects on both the quality of the machined ®nish and the wear of the tool. Nevertheless, under the conditions prevailing in this investigation, a number of conclusions may be justi®ed. The simple quasi-static contact method can be used to identify the workpiece/tool interfacial temperature above which strongly adherent surface layers may be formed on the rake face. This temperature is 740, 760 and 9008C for coated carbide, PCD and CBN tools, respectively, when cutting a titanium alloy. Furthermore, the method can be used to study the integrity of the bulk material, and/or individual coatings on substrates, when welded junctions are subsequently separated. Generally, fracture is initiated in the bulk of the harder tool material Ð rather than in the workpiece or at the welded junction interface. The presence of coated layers on the carbide tool appears to have had no bene®cial effect on their performance since these layers are rapidly removed leaving the tungsten carbide substrate vulnerable to cratering. The results of this investigation illustrate the complexity of the wear mechanisms in metal cutting and, whilst the resistance to plastic deformation and fracture of the cutting tool material is predictable, they underline the importance of chemical reactions which determine the nature of the interface formed between the tool and the chip. For this reason, we should not anticipate the development of an ultimate, ubiquitous, cutting tool material but rather select an optimum candidate for a given workpiece. Certainly, we may conclude that it is desirable to adjust conditions so as to form a stable interfacial layer which both protects the rake face and is easily sheared to form a smooth chip. In any event, the ®nal selection of cutting tool material will always be based on a compromise between several
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