ISSN 1068798X, Russian Engineering Research, 2010, Vol. 30, No. 11, pp. 1116–1123. © Allerton Press, Inc., 2010. Original Russian Text © V.V. Kuzin, V.N. Anikin, S.Yu. Fedorov, M.Yu. Fedorov, 2010, published in Vestnik Mashinostroeniya, 2010, No. 11, pp. 50–56.
Wear and Failure of Ceramic Cutting Plates V. V. Kuzina, V. N. Anikinb, S. Yu. Fedorova, and M. Yu. Fedorova a
Stankin Moscow State Technical University b Russian Research Institute of Hard Alloys email:
[email protected] Received September 6, 2010
Abstract—The wear and failure of ceramic cutting plates depend on the properties and structure of the ceramics, the tool geometry, and the operational loads. Under their action, specific defects accumulate in the ceramic and serve as points of crack nucleation. Their development in the ceramic may lead to wear and fail ure of the ceramic cutting plates. DOI: 10.3103/S1068798X10110109
It is difficult for ceramic cutting plates to meet cur rent performance requirements. The most serious deficiency of ceramic tools is their unsatisfactory reli ability. The basic principles that govern ceramic failure remain unknown for lack of data regarding the behav ior of ceramic cutting plates under the extremal loads typical of their applications and regarding specific properties of ceramics. The common view that the unsatisfactory performance of ceramic cutting plates is primarily due to lack of strength does not fully reflect the situation. In the present work, we study the wear and failure of various ceramic cutting plates. Industrial operation of tools with various ceramic cutting plates shows that they are effective within a rel atively narrow range of conditions [1]. Slight change in the cutting conditions sharply impairs tool perfor mance and increases the frequency of unpredictable tool failure. This sensitivity to the operating condi tions cannot be explained solely by the inadequate strength of the ceramic plates. The presumed relation ship between tool reliability and perturbing factors is also oversimplified. In fact, the state of the technolog ical system and the instability in the properties of the cut materials significantly affect the performance of tools with ceramic cutting plates, but these are hardly the determining factors. A more complex set of factors determines the performance and failure of ceramic cutting plates. In particular, we should note the factors characteriz ing the singular behavior of ceramics under the action of a thermal load. First, the large number of ionic–cova lent bonds, their directionality, and the complexity of the elementary cell in oxide ceramics result in limited mobility of their structural components [2]. This accounts for the increased brittleness of the ceramic at temperatures T < 0.5Tm, where Tm is the melting point. The brittleness is exacerbated by structural defects,
which act as stress concentrators. At T ≥ 0.5Tm, the ceramic is capable of some plastic deformation, and crack development is retarded; however, the strength of the ceramic is reduced. Second, the ceramic success fully withstands high temperatures but cannot effec tively endure the thermal stress arising under the action of temperature gradients [3]. Third, the microstress formed in ceramic components under the action of mechanical loads (especially impact loads) and thermal loads has an extremely complex influence on the stress–strain state of the cutting plates. Another important factor is that, in highspeed cutting, for which ceramic tools are intended, the reaction of the ceramic cutting plate with the cut material and the atmosphere plays a major role. As a result of reactions at high temperature, complex films may form on the ceramic surface, with various effects on the performance of the cutting plates. On the one hand, these films protect the cutting plate from dam age and improve the frictional conditions; on the other, surface defects appear and the wear of the con tact areas is intensified. Taking account of these factors, we consider the cutting system, so as to determine the relation between the operating conditions and the failure of ceramic cutting plates (Fig. 1). In this system, the operating conditions of the tools (the cutting conditions, the properties of the cut material and the coolant) deter mine the load conditions of the ceramic cutting plates. These conditions are characterized by the mechanical loads (P) and the thermal loads (Q) at the contact areas of the plate and the heat transfer to the surround ings (h). Under the action of the loads and thermal loads, the ceramic cutting plates are heated (T) and deformed (ε); in addition, stress is formed within the plates (σ). The failure of ceramic cutting plates is determined by their stress state, together with the physicochemical processes in the contact zone of the cutting plate with the part and the chip.
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Details of relations in system Operating conditions of the tools
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Micro and macrostress Physicochemical processes Fig. 1. Cutting system with ceramic cutting plates.
The relations in this system may be analyzed by studying the wear and failure of ceramic cutting plates. In the experiments, we use SNGN 120408 multifac eted nonsharpening cutting plates (strengthening facet 0.2 mm × 20°). The configuration of these plates in the holder is such that γ = –7°, α = 7°, and ϕ = ϕ1 = 45°. In some experiments, we employ cutting plates of spe cial geometry. Using a Kalibr 202 profilometer and optical and scanning electron microscopes, we inves tigate the state of the working surfaces, after removing the buildup of cut material on the plates. Special attention is paid to physical aspects of the wear and failure of ceramic cutting plates at the macro and micro levels. In Fig. 2, we plot the results of comparative labora tory tests of various ceramic cutting plates in continu ous turning of SCh32 cast iron (v = 300–1000 m/min; S = 0.075–0.3 mm/turn; t = 1–3 mm); the materials are oxide ceramic (VO13 plates), oxide–carbide ceramic (VOK71 plates), and nitride–carbide ceramic (RKS22 plates). We find that, in comparison with the other plates, the life and reliability of VO13 plates are most dependent on the operational loads. This influ ence is moderate for VOK71 plates and least for RKS22 plates. Therefore, the benefit of RKS22 tools is most evident in intense cutting: for example, the life RUSSIAN ENGINEERING RESEARCH
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of RKS22 plates is 1.3–2.1 times that of VOK71 plates in turning with v = 1000 m/min, for different S. Of the plates considered, only the RKS22 plates permit machining of SCh32 cast iron with S = 0.8 mm/turn up to v = 1000 m/min, without failure. In these conditions, failure of all the ceramic cut ting plates is mainly due to wear at the rear surface and chipping of the blade. Adhesion of the cut material at the contact areas of the oxide and nitrideceramic cutting plates indicates that the ceramic interacts with the practically molten material of the blank during cutting. However, cutting plates made from different ceramics have specific features in wear, which are most evident in the surface relief of the wear sources. For example, at active cutter sections of VO13 plates, flaws as large as 0.2 mm are seen; their incidence increases with increase in the cutting speed. Such large flaws are extremely rare in VOK71 cutting plates, but the lower boundary of the wear facet at the rear surface is extremely unstable, with many defects. Considerably fewer chipping flaws are seen at the cutting edge of the RKS22 plates than for oxideceramic plates, while the wear facet at the rear surface is relatively stable (Fig. 3). The better thermophysical properties for nitride ceramic than for oxide ceramic result in less
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Fig. 3. Wear kinetics of RKS22 cutting plates in turning SCh32 cast iron: v = 500 m/min; S = 0.5 mm/turn; t = 2 mm. RUSSIAN ENGINEERING RESEARCH
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thermal stress and hence less crack nucleation and growth. RKS22 cutting plates also more effectively with stand impact loads than do VOK71 plates. Investigation of the operational characteristics of such plates in dis continuous turning (v = 400 m/min; S = 0.4 mm/turn; t = 1 mm) of special SCh32 castiron blanks (220– 240 HB) indicate that the mean life of VOK71 and RKS22 cutting plates (hr = 0.3 mm) in continuous turn ing in the given conditions is 34–39 min (Fig. 4). In machining blanks with a single slot, RKS22 cutting plates have the best performance. In machining blanks with two or four slots, the advantages of RKSS plates are even more evident. In machining blanks with a casting crust, the wear area of the rear surface in the peripheral section of the primary cutting edge (the contact point with the cast ing crust) is greater. Such wear of the RKS22 cutting plates is evidently associated, first, with chipping of the tool as a result of considerable impacts from the hardened layer of the blank and, second, with intense abrasive wear. Failure is local and does not signifi cantly affect the performance of the RKS22 plates. It follows from the wear kinetics of VOK71 cutting plates in turning ШX15 steel (56–60 HRC) that the contact areas at the plates’ front and rear surfaces exhibit microchipping (Fig. 5). This is associated with the very local and intense heat sources, which lead to nonuniform heating of the cutting plates. In these con ditions, the behavior of the cutting plates is mainly determined by the temperature gradients, rather than the temperature. Moreover, intense heat fluxes may change the ceramic structure, as a result of grain growth and extension along the temperature gradient [4]. Data regarding the wear sources in RKS22 cutting plates when turning У10A steel (56–60 HRC) indicate that the surfacelayer structure at the wear sources is somewhat different from the initial structure of the Vol. 30
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Fig. 5. Wear kinetics of VOK71 cutting plates in turning ШX15 steel: v = 90 m/min; S = 0.075 mm/turn; t = 0.35 mm.
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Fig. 4. Influence of the conditions of discontinuous turn ing of SCh32 cast iron on the mean life of VOK71 (1) and RKS22 (2) cutting plates: N, number of slots in the blank.
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ceramic on account of the considerable heating of the plates’ contact areas and the high temperature coeffi cient of linear expansion of the ceramic. These changes result in greater development of the relief, the formation of a layer with looser structure at the ceramic surface, and microchipping (Fig. 6). Such microchipping is the result of critical thermal stress (Fig. 7). Hence, we may conclude that the wear of ceramic cutting plates is due to periodic microchipping (the separation and removal of granular conglomerates) in the surface layers at the contact areas, with gradual change in the relief. As a rule, analogous processes occur at the micro level and are controlled by the microstress [5, 6]. In this process, physical and chem ical phenomena are closely related, mutually depen dent, and mutually reinforcing. For example, at high temperatures, the following physicochemical pro cesses occur in ceramics, according to [7]: adsorp tion–desorption, evaporation–condensation, aggre gation–solution, and surface and bulk diffusion through the grain and crystal boundaries. Study of sudden cuttingplate failure reveals the basic aspects of their disintegration for various ceramic cutting plates. Oxide and nitride ceramics disintegrate by the same mechanisms. It is found that ceramic cut ting plates disintegrate as a result of instantaneous macroscopic discontinuity under the action of exter nal loads, in a brittle process. As a rule, the disintegra tion of cutting plates actively involves various struc tural defects of the ceramic, which serve as stress con centrators. We now consider the most characteristic instance of cuttingplate disintegration. As an example, micro photographs of the contact area of a VOK71 cutting plate prior to its failure and the chipped surface after disintegration are shown in Fig. 8. It is evident that disintegration of the cutting plates involves three cracks (A, B, and C), whose active role is evident after its complete disintegration (Fig. 8a). At the lateral sur face of the disintegrated plate, relief with a character istic folded configuration is formed (Fig. 8b). This
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Fig. 6. Wear kinetics of RKS22 cutting plates in turning У10A steel (56–60 HRC): v = 96 m/min; S = 0.075 mm/turn; t = 0.15 mm.
indicates disintegration of the cutting plate as a result of simultaneous growth of several cracks separated by some distance. At first, these cracks have different growth trajectories, but they eventually coalesce into a major crack. In the given example, the major crack is formed as a result of adding cracks A and B to crack C, as indicated by the sharp step in the relief (denoted by arrow C) on the grown trajectory of crack C (Fig. 8b). This may be associated with high stress in a local region of the cutting zone. The disintegration of the cutting plates is mainly determined by the mechanical loads and thermal loads, which depend on the type of treatment, the cutting con ditions, and the properties of the cut material. In dis continuous turning or milling, thermocyclic stress gen erates numerous surface cracks in the cutting plates. We may assume that the conversion of local cracks to a major crack does not exhaust the energy generated dur ing a thermal cycle. Therefore, the balance of the energy is consumed in new crack nucleation.
Fig. 7. Relief of the wear section at the rear surface of a VOK71 ceramic cutting plate in turning SCh32 cast iron after the removal of cutmetal buildup: v = 750 m/min; S = 0.5 mm/turn; t = 1 mm.
The shape and geometric parameters of the ceramic cutting plates significantly affect their disintegration. In Fig. 9, as an example, we show microphotographs of the disintegration sources in VOK71 cutting plates with dif ferent geometry, in turning SCh32 cast iron (v = 500 m/min; S = 0.05 mm/turn; t = 1 mm). It is evident that chipping of the cutting plates may be due to the positive front angle γ (Fig. 9a), the small rounding radii ρ and ζ of the blade and the tip (Figs. 9b and 9c), and the wear facet hr at the plate’s rear surface (Fig. 9d). Data regarding the influence of the geometric parameters on the failure of the cutting plates show that the probability of brittle failure of the ceramic cut ting plates increases with decrease in ρ and ζ and increase in γ. This may be explained in that the ceramic cutting plates are strengthened with increase in ρ from 25 to 50 μm and in ζ from 0.8 to 2.4 mm and with negative γ. The influence of the strengthening facet at the cut ting blade on the probability of cuttingplate disinte gration is complex. On the hand, the facet strengthens the cutting blade; on the other hand, it creates unfa vorable cutting conditions and correspondingly desta bilizes plate operation. The optimal size of the strengthening facet is largely determined by the supply in cutting. With increase in S, the strengthening facet plays a greater role in determining cuttingplate per formance. When the width of the strengthening facet ff = 0.2 mm, the stability of VO13, VOK71, and RKS22 plates in turning SCh32 cast iron (v = 500 m/min; S = 0.05 mm/turn; t = 1 mm) is greatest. Further increase in width of the strengthening facet is inexpe dient on account of the increased dynamic component of the cutting force. The properties and structure of the ceramic have considerable influence on the disintegration of the cutting plates. The high failure probability of oxide ceramic plates is due to their inadequate thermal sta bility. RKS22 plates, which surpass oxide ceramic in
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terms of thermostability, are better able to resist ther mal stress. The failure of ceramics is also affected by the pres ence of cracks, pores, defects, phase boundaries, dis locational (smallangle) boundaries, dislocations, and vacancies [8]. Current technology is unable to produce defectfree ceramic blanks. Therefore, structural RUSSIAN ENGINEERING RESEARCH
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defects of technological origin pose the biggest risk for ceramic cutting plates. The appearance of technological defects depends on the physical processes at different stages in the manufacture of ceramic cutting plates. The most com mon defects are large grains (Fig. 10a), nonuniformly distributed phases (Fig. 10b), cracks (Fig. 10c), and pores (Fig. 10d). Often, these technological defects
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Fig. 11. Operational defects in VOK71 ceramic cutting plates when turning ШX15 steel: v = 90 m/min; S = 0.075 mm/turn; t = 0.35 mm.
are comparable in size with the cutting plates and serve as stress concentrators. An important aspect of the wear and failure of ceramic cutting plates is the appearance of structural defects (discontinuities) within regions of high local stress. The accumulation of such defects was consid ered in some detail in [9]. Such structural defects are observed in the surface layers of ceramic cutting plates. (They will be referred to as operational defects, since
they are induced by operational loads.) In Fig. 11, as an example, we show operational defects at the worn section of the rear surface of a VOK71 cutting plate. As a rule, these defects (0.05–0.3 μm) first appear at the junctions of several large grains (shown by arrows A, B, C, and D)—that is, in the regions of max imum local stress. These defects tend to increase the local stress, which creates more structural defects. They accumulate at the grain boundaries. (Their tra
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jectories are shown by arrows 1 and 2.) Under the action of loads, operational defects appear at some distance from the blank’s surface, as shown in [10]. Under thermal loads, defects are most likely to appear at the surface of solids. The accumulation of operational defects implies the nucleation of operational cracks, which are com parable in size with grains. This considerably weakens the ceramic, with decrease in its Young’s modulus and strength [11]. Once formed, these cracks develop from the region with maximum concentration of opera tional microdefects along the trajectory of high local stress. For a crack formed within the cutting plate, its appearance at the surface depends on the presence of a continuous system of high local stress. The develop ment of operational cracks in the immediate vicinity of the surface leads to the separation of wear particles from the surface of the ceramic cutting plates or to microchipping at their contact areas. When the crack mouth is deeper in the material, its stable development gives way to fast and unstable growth. This leads ulti mately to macrochipping in the cutting plate or its complete disintegration as a result of the coalescence of several similar cracks CONCLUSIONS (1) The wear of ceramic cutting plates is a complex process that occurs in the presence of the practically molten cut material. Several mechanisms are at work here. Physical and chemical phenomena are closely related, mutually dependent, and mutually reinforc ing. Microchipping in the surface layers is fundamen tal to the wear of ceramic cutting plates. Such micro chipping is controlled by microstress and leads to gradual change in the contactarea relief. (2) The failure of ceramic cutting plates is a brittle process. Initially, several cracks form at once, prima rily as a result of the stress in the cutting plates, and give rise to technological defects in the ceramic. The disintegration of ceramic cutting plates will depend on their geometric parameters, the properties of the ceramic, and the operating conditions. (3) An important aspect of the wear and failure of ceramic cutting plates is the accumulation of opera tional defects in the ceramic; such defects are sources
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of crack nucleation. Regions where operational defects accumulate are formed in the surface layer of the cutting plates, under the action of operational loads. The development of operational cracks may lead to microchipping at the plate surface (wear), or else these cracks may coalesce to form a major crack (disintegration). The development of the operational cracks is determined by the stress–strain state of the cutting plates. REFERENCES 1. Kuzin, V.V., Instrumenty s keramicheskimi rezhushchimi plastinami (Tools with Ceramic Cutting Plates), Mos cow: YanusK, 2006. 2. Fizikokhimicheskie svoistva okislov (Physicochemical Properties of Oxides), Samsonov, G.V., Ed., Moscow: Metallurgiya, 1978. 3. Evans, A.G. and Langdon, T.G., Structural Ceramics, New York: Pergamon, 1980. 4. Vel’skaya, E.A. and Peletskii, V.E., Experiments Regarding the Thermal Conductivity of Corundum Ceramic as a Function of the Density, Teplo i Masso perenos, 1972, vol. 7, pp. 307–311. 5. Mekhanika kontaktnykh vzaimodeistvii (Mechanics of Contact Interaction), Moscow: Fizmatlit, 2001. 6. Lawn, B.R. and Wilshaw, T.R., Indentation Fracture: Principles and Application, J. Mater. Sci., 1975, vol. 10, no. 6, pp. 1049–1081. 7. Polezhaev, Yu.V., Kitikh, V.E., and Narozhnyi, Yu.G., Unsteady Heating of Thermoprotective Materials, Inzh.Fiz. Zh., 1975, vol. 29, no. 1, pp. 39–54. 8. Bakunov, V.S. and Belyakov, A.V., Strength and Struc ture of Ceramics, Ogneupory Tekhn. Keram., 1998, no. 3, pp. 11–15. 9. Regel’, R.V., Slutsker, A.I., and Tamashevskii, E.E., Kineticheskaya priroda prochnosti tverdykh tel (Kinetic Nature of the Strength of Solids), Moscow: Nauka, 1974. 10. Chekina, O.G., Simulation of SurfaceLayer Failure in the Contact of Rough Bodies, Tr. 9i konf. po prochnosti i plastichnosti (Proceedings of the Ninth Conference on Strength and Plasticity), Moscow: 1996, vol. 1, pp. 186–191. 11. Cocks, A.C. and Ashby, M.F., The Growth of Domi nant Crack in a Creeping Material, Scr. Metall., 1982, vol. 16, pp. 109–114.
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