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Cutting temperature effect on PCBN and CVD coated carbide tools in hard turning of D2 tool steel
Cutting temperature effect on PCBN and CVD coated carbide tools in hard turning of D2 tool steel G.K. Dosbaeva, M.A. El Hakim, M.A. Shalaby, J.E. Krzanowski, S.C. Veldhuis PII: DOI: Reference:
S0263-4368(14)00267-4 doi: 10.1016/j.ijrmhm.2014.11.001 RMHM 3953
To appear in:
International Journal of Refractory Metals and Hard Materials
Received date: Revised date: Accepted date:
3 June 2014 14 October 2014 2 November 2014
Please cite this article as: Dosbaeva GK, El Hakim MA, Shalaby MA, Krzanowski JE, Veldhuis SC, Cutting temperature effect on PCBN and CVD coated carbide tools in hard turning of D2 tool steel, International Journal of Refractory Metals and Hard Materials (2014), doi: 10.1016/j.ijrmhm.2014.11.001
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ACCEPTED MANUSCRIPT Cutting temperature effect on PCBN and CVD coated carbide tools in hard turning of D2 tool steel G. K. Dosbaeva(1) , M. A. El Hakim(2), M. A. Shalaby(3), J. E. Krzanowski (4) , S. C. Veldhuis(1) (1)
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Department of Mechanical Engineering, McMaster University, Hamilton, Canada (2) Faculty of Engineering, Ain Shams University, Cairo, Egypt (3) Technical Research Center, Cairo, Egypt (3) Department of Mechanical Engineering, University of New Hampshire, Durham, USA
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Abstract
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This study compares the performance of CVD coated tungsten-carbide tools having an intermediate Al2O3 layer to low content PCBN tools in hard turning of D2 tool steel (52 HRC). Results revealed that the coated carbide tool can outperform PCBN in machining the selected workpiece material within a certain range of cutting speeds (cutting temperature range). This is associated with tribo-film formation on the coated carbide tool surface. SEM and EDS were used to study the the wear patterns of the used cutting tools. The tribo-films were investigated using X-ray photoelectron method (XPS). The cutting temperature was measured using tool-workpiece thermocouple technique. The formation of the tribo-films was found to be highly-dependent on the cutting temperature and friction kinetics control at the tool-chip interface. Lubricious Cr-O tribo-films formed on the surface of the coated carbide tool, improving tool life at cutting speeds up to 100 m/min, which relates to an average toolchip interface temperature up to 930 0C.
1. Introduction
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Keywords: Hard turning, Cutting temperature. PCBN, CVD coated carbide
Grinding is used to produce hardened steel parts with high accuracy and surface quality.
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Due to the relatively high cost of the grinding process, hard machining has been suggested to partially replace grinding. Flexibility and positive ecological effects are other advantages of dry hard machining [1]. As well, the hard turned surfaces may have a longer fatigue life when compared with ground ones [2]. The tool cutting edge in hard turning is subjected to significant mechanical and thermal loading, not to mention aggressive chemical reactions, all of which cause the tool to wear out. Cutting tool wear plays a major role during finish hard turning due to its effects on surface integrity and dimensional accuracy. The most reported wear patterns are flank wear and crater wear [3]. Flank wear is the predominant pattern, tool life is also mainly defined by flank wear due to its effect on surface finish and dimensional accuracy. Capability to predict 1
ACCEPTED MANUSCRIPT tool wear during hard turning is necessary to avoid catastrophic tool failure, which affects the machining process performance.
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Abrasive wear has been frequently reported as a main wear mechanism in hard turning [4,
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5]. Due to the high temperature and high stresses in hard turning, diffusion wear may also
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occur. Chemical reactions, including oxidation at high speeds due to high cutting temperatures, have also been reported [4]. Chemical properties may be very important at the high cutting speeds in which the cutting temperature could accelerate any chemical reaction
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between the tool and workpiece [6, 7]. Severe chemical wear occurs due to the high affinity
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of ferrous materials to carbon in diamond tools, especially at high cutting speeds. For this reason, diamond is no longer used to machine hardened steel parts [8].
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Al2O3 in a bulk ceramic form has been used in machining; however, its brittleness is a
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significant limiting factor to its more widespread use as a cutting tool material. The TiC/ Al2O3 coated carbide insert was first developed in 1975. The first thick Al2O3 coating was
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developed in 1980 using the CVD technique. The coating thickness was in the range of 8-10 μm, which enabled more wear resistance. TiCN and Al2O3 sub-layers have complementary
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effects. At the rake face where the cutting temperature is expected to be the highest, chemical wear dominates, while the flank wear is abrasive in nature [9]. Grzesik and Nieslony [10], as well as Grzesik [11] carried out experimental and analytical investigations into the thermodynamically activated effects influencing the behavior of multilayered coated tools. Results have shown that coated tools with an intermediate ceramic layer can modify the tool-chip interface behavior in terms of friction and dissipation of heat generated during metal cutting. The intermediate Al2O3 layer could improve the heat flow into the chip compared with other types of coating. It was also recognized that the reduction of the friction coefficient reduces the cutting temperature. CVD Al2O3 hard coatings have also shown the best results in term of chemical stability in machining ferrous materials [7,
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ACCEPTED MANUSCRIPT 12]. The Al2O3 coated carbide tool provides a higher crater wear limit while the substrate retains its fracture limit [13]. In the case of the multilayer coating, changes of the ranking of
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microhardness of different layers can occur with temperature rise. While Al2O3 has lower
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room temperature hardness than TiC and TiN, it is the hardest at 1000 0C. By combining
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chemical inertness and high hot hardness, Al2O3 can be the most effective coating for high speed machining of ferrous materials [7, 12, 14]. A TiN layer on top of alumina provides dry lubrication, further which reduces the adherence of chip on the cutting edge and reduces built
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up edge (BUE) formation. It is also believed that TiN film fills the irregularities in the Al2O3,
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smoothening the insert surface.
Thermal properties of coatings have important influences on the behavior of the tribo-
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contact at the tool-chip interface. The amount of heat transferred to the chip greatly depends
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on the thermal conductivities of the outer films. The thermal conductivity of Al2O3 decreases with increasing temperature, which may contribute in dissipating a large fraction of heat
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induced during chip formation, thus protecting the tool core [12]. Polycrystalline cubic boron nitride (PCBN) is considered a suitable choice for hard turning
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applications which require high accuracy and good surface finish [15]. CBN tools can be classified into two categories; high CBN contents (around 90%), and low CBN content (around 60%) with a ceramic binder added (usually TiC or TiN). The high CBN content of these tools can make them harder than those with low amount of CBN. The CBN grade in which part of the CBN content is replaced by ceramic phase loses some of its hardness, but allegedly gains chemical stability. Generally in finish hard turning of structural-ball bearing and case hardened steels, the low content CBN grades were found to offer superior wear resistance when compared to high content grades [16]. Many explanations have been presented for this phenomenon, such as: the higher toughness of low CBN content grades; the lower thermal conductivity of low
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ACCEPTED MANUSCRIPT rather than high CBN content tools, which may increase the cutting temperature and consequently can aid in chip removal due to the thermal softening effect; reduced adhesion
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between chip and tool for low CBN content grades; high bonding strength between CBN
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particles and ceramic binders; lower dislocation density in low CBN content inserts; and the
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high chemical stability of low CBN content tools compared to the high content ones [17]. Kramer and Suh [18] developed a quantitative model to calculate relative wear rates of various potential tool materials. In their study, the solubility of different tool materials in iron
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had been estimated from reported thermodynamic properties and phase diagrams. It was
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mentioned that the developed theory explains the relatively high wear rate of cubic boron nitride in machining of steel. It also showed that the wear of aluminum oxide is not controlled
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by its solubility in iron.
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Angseryd and Andrén [19] investigated the effect of the cutting speed on the degradation of low content PCBN using advanced microscopy techniques. A higher cutting speed was
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found to drastically affect the wear of the cutting edge. Chemical wear was shown to be the dominant wear mechanism and accelerates with increasing cutting speed; the cBN phase is
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more affected than the major matrix phase, Ti(C, N). Angseryd et al [20] characterized an adherent layer consisting of many elements of workpiece material on the worn edge of the low content PCBN tool. They concluded that the chemical interaction between the PCBN tool, the oxidizing atmosphere and the workpiece material at high cutting temperatures and speeds in the cutting edge plays an important role in the cutting tool’s wear. Heat during machining is generated in three deformation zones: the primary deformation zone due to shearing action along the shear plane, in which the workpiece material undergoes severe plastic deformation; the secondary deformation zone as a result of chip deformation and sliding friction of the chip against the tool rake face; and the tertiary deformation zone due to the rubbing effect of the workpiece surface against the tool flank [21]. Cutting
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ACCEPTED MANUSCRIPT temperature is defined as the temperature measured at the tool-chip interface. Cutting speed has a significant effect on the cutting temperature. Increasing the cutting speed increases the
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rate at which energy is consumed through plastic deformation and friction, and thus the rate
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of the heat generated in the cutting zone. Many methods have been used to measure the
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cutting temperature, such as: tool-workpiece thermocouple, inserted thermocouples, implanted thermocouple, radiation methods, and changes in hardness and microstructure in tool steels [22]. The tool-workpiece thermocouple method for measuring the cutting
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temperature was first introduced independently by Shore in 1924 in the USA and in 1925 by
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Gottwein in Europe [23]. The e.m.f. can be captured and recorded during cutting. In order to convert the measured signal (volt) to temperature, each tool-workpiece material combination
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must be calibrated against a standard thermocouple. This cutting temperature measuring
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method gives the average tool-chip interface temperature, not the maximum. The tool and workpiece must be electrically conductive for the measurement to take place [22].
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AISI D2 cold work high carbon, high chromium tool steel is characterized by its high wear resistance. It can be used in the medium-hardened state (52-56 HRC) as a tool for deep
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drawing, rolling, punching, and extrusion dies. Referring to literature, it is believed that the CVD coated carbide tool with an intermediate Al2O3 layer needs more investigation from the tribological point of view to have a deeper understanding of its observed high performance. As the cutting temperature can affect friction kinetics at the tool-chip interface, and consequently the tool wear; this work investigates the effect of the cutting temperature on the frictional behavior of multilayer TiCN/Al2O3 coated by TiN over a carbide tool, and low content PCBN (60%) with TiN as a binder (40%) in machining medium hardened D2 tool steel.
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ACCEPTED MANUSCRIPT 2. Experimental work AISI D2 high carbon, high chromium tool steel hardened to 52 HRC was used in this
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work. Considering the workpiece material’s ability to harden, the machined depth of cut did
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not exceed 4 mm after all the machining passes, maintaining uniform workpiece properties
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throughout the machining tests. The cutting tools used are low content PCBN (60%) with TiN as a binder (40%), and multilayer TiCN/Al2O3 coated by TiN over a carbide tool. The details of the cutting tool materials and tool holders are presented in Table 1. After each
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cutting test, the workpieces were machined using a coated carbide tool at a relatively low
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cutting speed of 30 m/min to minimize the probable effect of tool wear on the machined surface during the previous pass.
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The tool nose radius (r) was kept constant at 1.2 mm. The depth of cut (a) and feed (f)
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were selected to be in the range of precision turning, 0.06 mm and 0.1 mm/rev, respectively. Machining tests were carried out on a 3 kW digitally controlled general
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purpose centre lathe.
The tool-workpiece thermocouple technique was chosen for measuring cutting
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temperature (θc) in the present study. The general description of this method mentions that the tool and workpiece must be electrically insulated from the machine tool to avoid any secondary junctions. The reduction of Machine-Fixture-Tool-Workpiece [MFTW] system stiffness as a result of electrical insulation should be avoided. Since only potential difference was measured, it would be enough to insulate only one component of the circuit. The tool holder was chosen to be insulated due to the difficulty of workpiece insulation as shown in Figure 1-a. The preliminary tests showed that MFTW system rigidity was not affected by insulation the tool holder. One of the lead wires was connected to the tool holder and the other was connected directly to the tailstock. The direct connection to the machine (instead of slip rings or mercury path) would not affect
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ACCEPTED MANUSCRIPT the emf reading remarkably [22]. Several tests were carried out in the present work to identify the probable effects of connecting the lead wires directly to the machine, instead
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of the workpiece; no differences were found, especially when a prolonged cut was
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avoided.
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When using PCBN, the presence of another secondary junction between the CBN tip and the tungsten carbide substrate should be considered. However, EMF measurements between the CBN tip and tungsten carbide during cutting were conducted. It was found
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that the emf at this junction starts to generate after the ninth second at the used cutting
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speeds. Accordingly, the emf signal was captured within 8 seconds. The insert was completely insulated by the ceramic-based composite coating with high thermal
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resistance with the exception of the CBN tip to prevent any contact between the chip and
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the tungsten carbide as illustrated in Figure 1-b.
presented in [24].
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The calibration procedures for the used tool-workpiece thermocouple arrangement are
A tool maker’s microscope was used to measure the tool flank wear land width VBB
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indicated in Figure 1-c. Progressive flank wear has been plotted against the machining time (t), in minutes. The scatter of tool wear measurements was around 7%. Three cutting speeds (v) of 60, 100 and 175 m/min were used to study the tool wear. Scanning electron microscopy (SEM) was used to study the wear patterns of the cutting tools at v=100 m/min and EDS analysis was used to analyze the different elements deposited on the worn tools. Micrographs of the underside of chips generated at cutting speeds of 100 and 175 m/min were captured using SEM for the sake of assessing the wear mechanisms. The characteristics of the tribo-films formed on the worn surfaces were studied using Xray photoelectron spectroscopy (XPS) on a Kratos-HS XPS system. A MgKα X-ray source was used, running at 15 kV and 10 mA. For the detailed scans on individual elements (as
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ACCEPTED MANUSCRIPT shown in this work for Cr), the scan was run at a pass energy of 160 eV, a step size of 50 meV and a dwell time of 100 ms with 10 passes. To reduce the impact of surface
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impurities on the final result, the sample was etched in the XPS system before collection of
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the scan using 4 keV Ar+ ions for 5 min. The deconvolution analysis was carried out using
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the system-supplied software and database. 3. Results 3.1. Cutting temperature
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Figure 2 shows the effect of the cutting speed (v) on the cutting temperature (θc). At v=60
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m/min, θc=779 0C for PCBN and 824 0C when the coated carbide tool is used. It is noticed that the coated carbide tool experiences a relatively higher cutting temperature until reaching
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a cutting speed of 100 m/min (930 0C), after which cutting temperature becomes almost the
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same as the PCBN tool. This is thought to be due its relatively lower thermal conductivity in this temperature range [24]. At a cutting speed of 175 min, the cutting temperature reaches
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1130 0C for the coated carbide tool and somewhat higher (11700C) for PCBN. 3.2. Wear curves and tool life
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Figure 3 shows the wear curves for PCBN and coated carbide tools at the three cutting speeds. At the lowest cutting speed (v= 60 m/min), θc=779 0C for PCBN and 824 0C for coated carbide tool. It can be noticed that before reaching a flank wear land width (VBB) of 0.12 mm, the coated carbide tool experiences the higher wear rate, after which its wear rate decreases to below that of the PCBN tool. Increasing the cutting speed to 100 m/min and the cutting temperature to 930 oC for both tool materials, the coated carbide tool shows the same behavior before and after a higher flank wear of (VBB) of 0.16 mm. The
same
experiments
were
carried
out
at
a
higher
cutting
speed
of
175 m/min, which saw cutting temperature in excess of 1100 0C for both tool materials. The coated carbide shows the higher wear rate until reaching the end of tool life (VBB =0.2 mm).
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ACCEPTED MANUSCRIPT Distinct running-in and gradual wear stages can be easily identified with the coated carbide tool, especially at v=60, 100 m/min; in contrast, the PCBN tool wears steadily
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throughout the duration of its life, at all cutting speeds, until reaching flank wear land width
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(VBB) of 0.2 mm.
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Figure 4 compares the machining time (in minutes) required to wear both tool materials to flank land widths (VBB) of 0.1 and 0.2 mm. Generally, it can be deduced that PCBN performs better than the coated carbide tool at all cutting speeds up to a flank land width of VBB =0.1
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mm, lasting in the range of 280-400% longer.
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Up to a tool flank wear land width of VBB = 0.2 mm, however, the coated carbide tool lasts longer than the PCBN tool at lower speeds (330% longer at 60 m/min, 140% at 100
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3.3. Tool wear mechanisms
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m/min), while PCBN tool lasts longer (by about 240%) when v=175 m/min.
SEM and EDS analyses have been carried out to capture the necessary micrographs of the
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worn out tools (VBB =0.2 mm) and to make the chemical analysis of the cutting tool edge. Comparison has been performed on the worn inserts run at the same cutting speed and cutting
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temperature, v=100 m/min and θc= 930 oC, respectively. Figure 5-a presents the SEM micrographs and EDS analysis of the PCBN insert. Crater wear is observed and (Fe) as an element coming from the workpiece material is found on the tool cutting edge. The abrasive effect of the workpiece material is also observed. The (Fe) rich areas on the worn PCBN cutting edge were also found by Angseryd et al [20] and are considered an indication of chemical wear. Figure 5-b shows the SEM micrographs and EDS analysis of the worn coated carbide tool. Crater wear can also be observed. The appearance of (W) on the worn flank and rake, which is one of the elements that comes from the coated carbide tool substrate, indicates the complete removal of the coating layers of the cutting tool. Elements from the workpiece
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ACCEPTED MANUSCRIPT material can be observed on the tool flank and rake face (Fe, Cr, Si, Mn), which is considered an indication of adhesive and/or chemical wear. An (O) peak is also found in the EDS
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analysis as well as (Mg), the latter of which could be an addition to alumina as a sintering
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additive.
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3.4. Chip formation
Figure 6 illustrates the SEM micrographs of chip underside formed at cutting speeds of 100 and 175 m/min for the two used cutting tool materials. The examination of the
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smoothness of the chip underside can therefore reveal the intensity of adhesion of the chip
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underside to the tool face.
Generally, it can be noticed that that the intensity of the adhesion marks are increased in proportion to the cutting speed for both tool materials. At v=100 m/min, the coated carbide
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chip underside surface is somewhat smoother than PCBN (θc=923 0C for both tools), whereas at 175 m/min (θc>1100 0C for both tool materials) PCBN gives a smoother surface. The SEM
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micrograph of the chip underside produced by the coated carbide tool at a cutting speed of 175 m/min (Figure 6-d) shows the highest intensity of adhesion marks.
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3.5. Characterization of the surface tribo-films on the worn surface To understand the cause of the improved tool life of the coated carbide insert at cutting speeds of up to 100 m/min and cutting temperatures up to 923 0C, an investigation of its worn surface was made. Two kinds of tribo-films could be identified on the worn rake surface of the tool using X-ray Photoelectron Spectroscopy (XPS) analysis. It was only possible to identify tribo-films on the surface of the coated carbide tool. PCBN did not show formation of tribo-films. Minor amounts of Ti-O films were found (Figure 7, a) that correspond to our previously obtained results [25, 26]. Significant amounts of lubricating polyvalent chromium oxides are generated. A corresponding spectrum is shown in Figure 7, b. The XPS spectra of the Cr 2p region (Figure
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ACCEPTED MANUSCRIPT 7, b) indicate the presence of two Cr states on the surface of the worn ceramic insert. The fitted spectrum has peaks at 576.2 (Cr2O3). This interpretation of chromium bonds is obtained
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in accordance with the data published elsewhere [25, 26]. The position of the photoelectron
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lines is shifted from the binding energy for standard elements, which indicates their partial
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oxidation with the formation of non-equilibrium phases. The positions of the photoelectron lines are shifted from the binding energy values for pure elements, indicating their partial oxidation with the formation of non-equilibrium phases close to the Cr+3-O phase.
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4. Discussion
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Referring to Figure 3, the coated carbide tool exhibits the highest tool wear at the running-in zone at v=60 and 100 m/min. After reaching the end of the running-in zones, the
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coated carbide shows a lower wear rate than PCBN until the end of tool life (VBB =0.2 mm).
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At v=175 m/min, a clear difference in wear curves can be noticed between PCBN and coated carbide, with PCBN performs better over the duration of its life.
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Tool life, represented in Figure 4 (at VBB =0.2 mm), is longer for coated carbide tools at v=60, 100 m/min, at v=175 m/min PCBN tool life is longer. At VBB =0.1 mm, PCBN tool life
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is longer at all the used cutting speeds, which illustrates the advantage of using PCBN in high precision turning when high dimensional accuracy is required. The presence of (Fe) on the worn PCBN insert, and (Fe, Cr) on the worn coated carbide tool, in the EDS analyses presented in Figure 5 can be used as an indication of adhesive and/or chemical wear mechanisms for both tool materials. Adhesion marks observed on the chip underside in Figure 6 support this conclusion. It can be noticed that the adhesive wear mechanism increases with increasing the cutting speed for both tool materials. The highest adhesive effect is noticed when using the coated carbide tool at v=175 m/min (Figure 6-d). Crater wear observed in SEM micrographs for both tool materials (Figure 5) can be evidence of the occurrence of the diffusion/chemical wear mechanism. Abrasion marks are observed
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ACCEPTED MANUSCRIPT clearly on the worn PCBN tool but are not easily noticed on the worn coated carbide cutting edge.
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In addition to the adhesion and diffusion/chemical wear mechanisms, the nature of wear
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of PCBN can be attributed to the abrasive effect of hard carbides such as chromium carbide
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in the workpiece material. The binder material in PCBN (TiN) at the start of cutting could have been worn out. When the binder material is abraded, CBN particle bonding strength becomes weaker and may not be present at some points. Therefore, CBN particles will not be
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bonded together and start to lose their advantage. Consequently the wear rate of CBN tools
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becomes higher than the coated carbide. The diffusion/chemical wear mechanism indicated by crater wear may also be another reason for increasing the wear rate of PCBN as cBN has a
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higher solubility in iron than alumina as mentioned in [18]. The nature of the chemical wear
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mechanism of PCBN is discussed deeply in [19, 20] Two types of tribo-films are forming on the surface of the coated carbide tool. The first is
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due to the chemical modification of the tool material, based on the Ti-O phase caused by the chemical modification of the TiN top layer. However, the machining conditions used here
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are very hard. It is reasoned that any Ti-O tribo-films formed could not efficiently sustain such high temperature/heavy load conditions and consequently were rapidly worn out [26]. This would support why the XPS analysis performed in this study (Figure 7, a) found only small amounts of these Ti-O based tribo-films. Referring to Figure 2, θc= 930 0C at v= 100 m/min, the Ti-O tribo-films formed cannot sustain harsh operating conditions of dry hard turning. Accordingly, it can be deduced that in this case, the adaptive behavior of the coated carbide tool was developed by means of the ability of the tool to generate a protective tribo-film on the frictional surfaces due to the presence of certain elements in this tool material [26], which reduce the tool wear under severe operating conditions. As was expected, the second type of tribo-films forms due to the
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ACCEPTED MANUSCRIPT tribo-oxidation of the workpiece material, which is sticking to the rake surface as a result of the friction at the chip/tool interface.
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The XPS spectra of the Cr 2p region (Figure 7, b) indicate the presence of a Cr-O phase
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on the surface of the worn coated carbide insert. The fitted spectrum has a peak at 576.2
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(Cr2O3). This interpretation of chromium bonds is obtained in accordance with the data published elsewhere [27, 28]. The positions of the photoelectron lines are shifted from the binding energy for standard elements, which attributes their partial oxidation to the formation
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of non- equilibrium phases. Formation of high temperature lubricious Cr-O tribofilms that are
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identified by XPS (Figure 7, b) results in improvement of lubricating properties at the tool/chip interface. The long-term oxidation of chromium can help dissolve this oxide in
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alumina as an intermediate layer in the coated carbide tool, and reduce the inter-diffusion
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between the workpiece and tool materials [29]. Similar results were previously obtained for the TiAlCrN PVD coating with high Cr content [30], and the same tribo-film was obtained
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using a mixed alumina ceramic tool to machine the same workpiece material [31]. These tribo-films were formed at the end of the running-in stages as observed in the wear curve in
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Figure 3-a, b. For the coated carbide tool, it can be noticed that at a cutting speed of 60 m/min and cutting temperature of 824 0C, which is considered an adequate temperature for Cr-O tribo-films to sustain their lubricious properties [26], the cutting tool life can reach 330% when compared to PCBN. At a cutting speed of 175 m/min, and due to the high cutting temperature associated with the increase of the cutting speed (>1100
o
C, as indicated by cutting temperature
measurements in Figure 2), both Ti-O and Cr-O tribo-films would lose their lubricous properties [26]. The effect of the coating on increasing tool life will be not remarkable. Intense adhesion marks observed on the chip underside of the coated carbide tool used at this cutting speed (cutting temperature) shown in Figure 6-d supports this result. At this speed,
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ACCEPTED MANUSCRIPT PCBN can outperform this type of coated carbide tool in its hot hardness, which is estimated to be 2-3 times harder than the coated carbide tool at this elevated temperature. [32].
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5. Conclusion
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This study demonstrates the effect of the cutting temperature measured by the tool-
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workpiece thermocouple on the wear characteristics of carbide tools with the CVD multilayer TiCN/Al2O3 coating, and low content Polycrystalline Cubic Boron Nitride tools in turning hardened D2 tool steel. Friction kinetics can controls the wear mechanisms during harsh
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machining conditions. XPS analysis showed the formation of Ti-O and Cr-O tribo-films on
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the surface of the worn CVD coated carbide tool exposed to cutting temperatures up to 923 C (cutting speeds up to 100 m/min). These tribo-films were investigated at the end of the
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running-in period as indicated by the wear curves. The increases of coated carbide tool life
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when compared to PCBN can reach 330% at a cutting speed of 60 m/min and corresponding cutting temperature of 824 0C. The latter temperature can be considered a suitable
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temperature for Cr-O tribo-films to exhibit their lubricious properties. The long-term oxidation of chromium can help dissolve this oxide in alumina as an intermediate layer in the
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CVD coated carbide tool and improve its wear resistance. When increasing the cutting speed to 175 m/min, and consequently the cutting temperature to over 1100 OC, these tribo-films become ineffective, and PCBN has the longer tool life due to its higher hot hardness. This was supported by the higher intensity of adhesion marks on the chip underside produced by the coated carbide tool at this cutting speed (cutting temperature). Adhesive and chemical wear can be considered the main wear mechanisms in the used PCBN cutting tool. In high precision turning, when high dimensional accuracy is required, it is recommended to use PCBN for its lower wear rate in the early stages of tool wear (tool flank land width not exceeding 0.1 mm).
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11.Grzesik, W.: Friction behavior of heat isolating coating in machining: mechanical, thermal and energy-based considerations. International Journal of Machine Tools &
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Manufacture 43, 145–150(2003).
12.Grzesik, W.: Experimental investigation of the cutting temperature when turning with
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coated indexable inserts. International Journal of Machine Tools & Manufacture 39,
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355–369 (1999).
13.Quinto, D. T.: Technology prospective on CVD and PVD coated metal-cutting tools.
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Int. J. of Refractory Metals and hard materials 14, 7-20 (1996).
14. Grzesik, W.: The role of coating in controlling the cutting process when turning with
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coated indexable inserts. Journal of Material Processing Technology 79, 133-143(1998).
15.Poulachon, G., Moisan, A., Jawahir, I. S.: Tool-wear mechanism in hard turning with polycrystalline cubic boron nitride tool. Wear 250, 576-586 (2001).
16. Chou, Y. K., Evans, C. J., Barash, M. M.: Experimental investigation on CBN turning of hardened AISI 52100 steel. Journal of Materials Processing Technology 124, 274– 283 (2002).
17.Diniz, A. E., Adilson, de Oliveira, A. J.: Hard turning of interrupted surfaces using CBN tools. Journal of materials processing technology 195, 275–281(2008).
18.Kramer, B. M., Suh, N. P.: Tool wear by solution: a quantitative understanding. Transactions of the ASME, Journal of Engineering for Industry102, 303-309 (1980). 16
ACCEPTED MANUSCRIPT 19.Angseryd, J., Andrén, H.-O.: An in-depth investigation of the cutting speed impact of the degraded microstructure of worn PCBN cutting tools. Wear 271, 2610-2618 (2011).
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20.Angseryd, J., Coronel, E., Elfwing, M., Olsson, E., Andrén, H.-O.: The microstructure
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of the affected zone of a worn PCBN cutting tool characterized with SEM and TEM.
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Wear 267, 1031-1040 (2009).
21.Ghani, M. U., Abukhshim, N. A., Sheikh, M. A.: An investigation of heat partition and
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tool wear in hard turning of H13 tool steel with CBN cutting tool. Int J Adv Manuf Technol 170, 1282-1287 (2007).
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22.Stephenson, D. A.: Tool-work thermocouple temperature measurements-theory and implementation issue. Transactions of the ASME, Journal of Engineering for Industry
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115, 432-437 (1993).
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23.Lezanski, P., Shaw, M. C.: Tool face temperatures in high speed milling, Transactions of the ASME, Journal of Engineering for Manufacture 112, 132-135 (1990).
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24.Shalaby, M. A.: An investigation into high precision turning of some alloy steels. PhD thesis, Ain Shams University, Cairo, Egypt (2011).
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25.Kovalev, A. I., Wainstein, D. L., Mishina, V. P., Fox-Rabinovich, G. S.: Investigation of atomic and electronic structure of films generated on a cutting tool surface. Journal of Electron Spectroscopy and Related Phenomena, 63-75 (1999).
26.Fox-Rabinovich, G. S., Totten, G. E.: Self-organization during friction. Boca Raton NY Taylor & Francis (2007).
27.Tsitsumi, T., Ikemoto, I., Namikawa, T.: Bull Chem Soc Jpn 54/3:913 (1981). 28. Romand, H., Robin, M.: Analyses 4, 7:308 (1974). 29.Fox-Rabinovich, G. S., Weatherly, G. C., Wilkinson, D. S., Kovalev, A.I., Wainstein, D. L.: The role of chromium in protective alumina scale formation during the oxidation of ternary TiAlCr alloys in air. Intermetallics 12, 165-180 (2004). 17
ACCEPTED MANUSCRIPT 30.Fox-Rabinovich, G. S., Yamamoto, K.,Veldhuis, S. C., Kovalev, A. I., Dosbaeva, G. K.: Tribological adaptability of TiAlCrN PVD coatings under high performance dry
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machining conditions. Surf Coat Technol 200, 1804–10 (2005).
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31.Shalaby, M. A., El Hakim, M. A., Abdelhameed, M. M. , Krzanowski, J. E., Veldhuis,
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S.C. , Dosbaeva, G. K.: Wear mechanisms of several cutting tool materials in hard turning of high carbon–chromium tool steel. Tribology International 70, 148–154 (2014).
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32.Harris, T.K., Brookes, E.J., Taylor, C.J.: The effect of temperature on the hardness of
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polycrystalline cubic boron nitride cutting tool materials. International Journal of Refractory Metals & Hard Materials 22, 105–110, (2004).
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Figures captions:
Figure 1. (a) Tool-workpiece thermocouple arrangement for measuring the cutting
width VBB ;
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temperature (b) using PCBN (BNX20) and (c) the drawing shows tool flank wear land
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Figure 2. Effect of cutting speed on the cutting temperature when using coated carbideand PCBN tool materials;
Figure 3. Wear curves of coated carbide and PCBN cutting tool materials in hard turning of D2 tool steel: (a) v=60 m/min, (b) v=100 m/min, (c) v= 175 m/min; Figure 4. Tool life at different cutting speeds for different tool flank wear land widths (VBB): (a) VBB =0.1 mm, (b) VBB =0.2 mm;
Figure 5. SEM micrographs and EDS analysis of the worn inserts at 100 m/min: (a)PCBN, (b) Coated carbide; 18
ACCEPTED MANUSCRIPT Figure 6. SEM micrographs of chip underside for PCBN and coated carbide tools at different cutting speeds: (a) PCBN at v=100 m/min, (b) coated carbide at v= 100 m/min (c)
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PCBN at v=175 m/min, (d) coated carbide at v= 175 m/min.
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Figure 7. Photoelectron spectra on the (a) Ti 2p and (b) Cr 2p regions taken from the
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worn rake surface of the coated carbide tool.
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Tailstock
Insert
Tool holder
Lead wire
+
Sensitive voltmeter
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Chuck
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Insulation
-
Lead wire
Carbide substrate
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PCBN tip
Lead wire
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Thermal resistant coating
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a)
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Tool holder
b)
VBB
c) Fig. 1
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Fig. 2
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(b)
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a)
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(c) Fig. 3
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(a)
(b) Fig. 4
23
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Spectrum 1
Ti
W
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Crater wear
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Al Ti N Ti
Abrasion marks
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Fe Fe
0 1 2 3 Full Scale 321 cts Cursor: 0.000
Fe 4
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200 m
Ti
5
6
Fe 7
8
9
10 keV
(a)
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Fe Mn Ti Mn Cr Cr O
C
Crater wear
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W Si
Fe
Fe Mn
Ti W
Ti W
0 1 2 3 Full Scale 652 cts Cursor: 0.000
200 m
(b)
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Fig. 5
Spectrum 2
Ti
24
4
5
Cr Cr Mn 6
Fe 7
W W
W 8
W W 9
10 keV
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10 m
10 m
(b)
Fig. 6
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(b)
10 m
10 m
(d)
(c)
25
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Ti-O (458. 09)
(a)
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Ti-O (465.86)
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Cr203 (576.2)
(b)
Fig. 7
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ACCEPTED MANUSCRIPT Table 1. Specifications of tool materials and tool holder Tool Chemical composition Hardne
60%CBN+TiN binder
ss
standard
specifications
3100–
SNMA120
3300
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(BNX20)
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HV
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CVD
TiCN (4µm) /Al2O3(6 µm )
carbide
/TiN (2µm), coated over
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coated
– 25o Chamfer
clearance
SNMA120
angle, 75
Honed
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edge
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-5 rake angle,5
angle
cutting
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carbide substrate
412
412 _
holder
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PCBN
Tool
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Material
ISO
setting angle
S0263-4368(14)00267-4 doi: 10.1016/j.ijrmhm.2014.11.001 RMHM 3953
To appear in:
International Journal of Refractory Metals and Hard Materials
Received date: Revised date: Accepted date:
3 June 2014 14 October 2014 2 November 2014
Please cite this article as: Dosbaeva GK, El Hakim MA, Shalaby MA, Krzanowski JE, Veldhuis SC, Cutting temperature effect on PCBN and CVD coated carbide tools in hard turning of D2 tool steel, International Journal of Refractory Metals and Hard Materials (2014), doi: 10.1016/j.ijrmhm.2014.11.001
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Cutting temperature effect on PCBN and CVD coated carbide tools in hard turning of D2 tool steel G. K. Dosbaeva(1) , M. A. El Hakim(2), M. A. Shalaby(3), J. E. Krzanowski (4) , S. C. Veldhuis(1) (1)
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Department of Mechanical Engineering, McMaster University, Hamilton, Canada (2) Faculty of Engineering, Ain Shams University, Cairo, Egypt (3) Technical Research Center, Cairo, Egypt (3) Department of Mechanical Engineering, University of New Hampshire, Durham, USA
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Abstract
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This study compares the performance of CVD coated tungsten-carbide tools having an intermediate Al2O3 layer to low content PCBN tools in hard turning of D2 tool steel (52 HRC). Results revealed that the coated carbide tool can outperform PCBN in machining the selected workpiece material within a certain range of cutting speeds (cutting temperature range). This is associated with tribo-film formation on the coated carbide tool surface. SEM and EDS were used to study the the wear patterns of the used cutting tools. The tribo-films were investigated using X-ray photoelectron method (XPS). The cutting temperature was measured using tool-workpiece thermocouple technique. The formation of the tribo-films was found to be highly-dependent on the cutting temperature and friction kinetics control at the tool-chip interface. Lubricious Cr-O tribo-films formed on the surface of the coated carbide tool, improving tool life at cutting speeds up to 100 m/min, which relates to an average toolchip interface temperature up to 930 0C.
1. Introduction
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Keywords: Hard turning, Cutting temperature. PCBN, CVD coated carbide
Grinding is used to produce hardened steel parts with high accuracy and surface quality.
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Due to the relatively high cost of the grinding process, hard machining has been suggested to partially replace grinding. Flexibility and positive ecological effects are other advantages of dry hard machining [1]. As well, the hard turned surfaces may have a longer fatigue life when compared with ground ones [2]. The tool cutting edge in hard turning is subjected to significant mechanical and thermal loading, not to mention aggressive chemical reactions, all of which cause the tool to wear out. Cutting tool wear plays a major role during finish hard turning due to its effects on surface integrity and dimensional accuracy. The most reported wear patterns are flank wear and crater wear [3]. Flank wear is the predominant pattern, tool life is also mainly defined by flank wear due to its effect on surface finish and dimensional accuracy. Capability to predict 1
ACCEPTED MANUSCRIPT tool wear during hard turning is necessary to avoid catastrophic tool failure, which affects the machining process performance.
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Abrasive wear has been frequently reported as a main wear mechanism in hard turning [4,
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5]. Due to the high temperature and high stresses in hard turning, diffusion wear may also
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occur. Chemical reactions, including oxidation at high speeds due to high cutting temperatures, have also been reported [4]. Chemical properties may be very important at the high cutting speeds in which the cutting temperature could accelerate any chemical reaction
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between the tool and workpiece [6, 7]. Severe chemical wear occurs due to the high affinity
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of ferrous materials to carbon in diamond tools, especially at high cutting speeds. For this reason, diamond is no longer used to machine hardened steel parts [8].
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Al2O3 in a bulk ceramic form has been used in machining; however, its brittleness is a
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significant limiting factor to its more widespread use as a cutting tool material. The TiC/ Al2O3 coated carbide insert was first developed in 1975. The first thick Al2O3 coating was
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developed in 1980 using the CVD technique. The coating thickness was in the range of 8-10 μm, which enabled more wear resistance. TiCN and Al2O3 sub-layers have complementary
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effects. At the rake face where the cutting temperature is expected to be the highest, chemical wear dominates, while the flank wear is abrasive in nature [9]. Grzesik and Nieslony [10], as well as Grzesik [11] carried out experimental and analytical investigations into the thermodynamically activated effects influencing the behavior of multilayered coated tools. Results have shown that coated tools with an intermediate ceramic layer can modify the tool-chip interface behavior in terms of friction and dissipation of heat generated during metal cutting. The intermediate Al2O3 layer could improve the heat flow into the chip compared with other types of coating. It was also recognized that the reduction of the friction coefficient reduces the cutting temperature. CVD Al2O3 hard coatings have also shown the best results in term of chemical stability in machining ferrous materials [7,
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ACCEPTED MANUSCRIPT 12]. The Al2O3 coated carbide tool provides a higher crater wear limit while the substrate retains its fracture limit [13]. In the case of the multilayer coating, changes of the ranking of
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microhardness of different layers can occur with temperature rise. While Al2O3 has lower
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room temperature hardness than TiC and TiN, it is the hardest at 1000 0C. By combining
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chemical inertness and high hot hardness, Al2O3 can be the most effective coating for high speed machining of ferrous materials [7, 12, 14]. A TiN layer on top of alumina provides dry lubrication, further which reduces the adherence of chip on the cutting edge and reduces built
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up edge (BUE) formation. It is also believed that TiN film fills the irregularities in the Al2O3,
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smoothening the insert surface.
Thermal properties of coatings have important influences on the behavior of the tribo-
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contact at the tool-chip interface. The amount of heat transferred to the chip greatly depends
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on the thermal conductivities of the outer films. The thermal conductivity of Al2O3 decreases with increasing temperature, which may contribute in dissipating a large fraction of heat
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induced during chip formation, thus protecting the tool core [12]. Polycrystalline cubic boron nitride (PCBN) is considered a suitable choice for hard turning
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applications which require high accuracy and good surface finish [15]. CBN tools can be classified into two categories; high CBN contents (around 90%), and low CBN content (around 60%) with a ceramic binder added (usually TiC or TiN). The high CBN content of these tools can make them harder than those with low amount of CBN. The CBN grade in which part of the CBN content is replaced by ceramic phase loses some of its hardness, but allegedly gains chemical stability. Generally in finish hard turning of structural-ball bearing and case hardened steels, the low content CBN grades were found to offer superior wear resistance when compared to high content grades [16]. Many explanations have been presented for this phenomenon, such as: the higher toughness of low CBN content grades; the lower thermal conductivity of low
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ACCEPTED MANUSCRIPT rather than high CBN content tools, which may increase the cutting temperature and consequently can aid in chip removal due to the thermal softening effect; reduced adhesion
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between chip and tool for low CBN content grades; high bonding strength between CBN
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particles and ceramic binders; lower dislocation density in low CBN content inserts; and the
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high chemical stability of low CBN content tools compared to the high content ones [17]. Kramer and Suh [18] developed a quantitative model to calculate relative wear rates of various potential tool materials. In their study, the solubility of different tool materials in iron
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had been estimated from reported thermodynamic properties and phase diagrams. It was
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mentioned that the developed theory explains the relatively high wear rate of cubic boron nitride in machining of steel. It also showed that the wear of aluminum oxide is not controlled
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by its solubility in iron.
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Angseryd and Andrén [19] investigated the effect of the cutting speed on the degradation of low content PCBN using advanced microscopy techniques. A higher cutting speed was
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found to drastically affect the wear of the cutting edge. Chemical wear was shown to be the dominant wear mechanism and accelerates with increasing cutting speed; the cBN phase is
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more affected than the major matrix phase, Ti(C, N). Angseryd et al [20] characterized an adherent layer consisting of many elements of workpiece material on the worn edge of the low content PCBN tool. They concluded that the chemical interaction between the PCBN tool, the oxidizing atmosphere and the workpiece material at high cutting temperatures and speeds in the cutting edge plays an important role in the cutting tool’s wear. Heat during machining is generated in three deformation zones: the primary deformation zone due to shearing action along the shear plane, in which the workpiece material undergoes severe plastic deformation; the secondary deformation zone as a result of chip deformation and sliding friction of the chip against the tool rake face; and the tertiary deformation zone due to the rubbing effect of the workpiece surface against the tool flank [21]. Cutting
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ACCEPTED MANUSCRIPT temperature is defined as the temperature measured at the tool-chip interface. Cutting speed has a significant effect on the cutting temperature. Increasing the cutting speed increases the
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rate at which energy is consumed through plastic deformation and friction, and thus the rate
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of the heat generated in the cutting zone. Many methods have been used to measure the
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cutting temperature, such as: tool-workpiece thermocouple, inserted thermocouples, implanted thermocouple, radiation methods, and changes in hardness and microstructure in tool steels [22]. The tool-workpiece thermocouple method for measuring the cutting
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temperature was first introduced independently by Shore in 1924 in the USA and in 1925 by
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Gottwein in Europe [23]. The e.m.f. can be captured and recorded during cutting. In order to convert the measured signal (volt) to temperature, each tool-workpiece material combination
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must be calibrated against a standard thermocouple. This cutting temperature measuring
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method gives the average tool-chip interface temperature, not the maximum. The tool and workpiece must be electrically conductive for the measurement to take place [22].
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AISI D2 cold work high carbon, high chromium tool steel is characterized by its high wear resistance. It can be used in the medium-hardened state (52-56 HRC) as a tool for deep
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drawing, rolling, punching, and extrusion dies. Referring to literature, it is believed that the CVD coated carbide tool with an intermediate Al2O3 layer needs more investigation from the tribological point of view to have a deeper understanding of its observed high performance. As the cutting temperature can affect friction kinetics at the tool-chip interface, and consequently the tool wear; this work investigates the effect of the cutting temperature on the frictional behavior of multilayer TiCN/Al2O3 coated by TiN over a carbide tool, and low content PCBN (60%) with TiN as a binder (40%) in machining medium hardened D2 tool steel.
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ACCEPTED MANUSCRIPT 2. Experimental work AISI D2 high carbon, high chromium tool steel hardened to 52 HRC was used in this
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work. Considering the workpiece material’s ability to harden, the machined depth of cut did
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not exceed 4 mm after all the machining passes, maintaining uniform workpiece properties
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throughout the machining tests. The cutting tools used are low content PCBN (60%) with TiN as a binder (40%), and multilayer TiCN/Al2O3 coated by TiN over a carbide tool. The details of the cutting tool materials and tool holders are presented in Table 1. After each
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cutting test, the workpieces were machined using a coated carbide tool at a relatively low
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cutting speed of 30 m/min to minimize the probable effect of tool wear on the machined surface during the previous pass.
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The tool nose radius (r) was kept constant at 1.2 mm. The depth of cut (a) and feed (f)
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were selected to be in the range of precision turning, 0.06 mm and 0.1 mm/rev, respectively. Machining tests were carried out on a 3 kW digitally controlled general
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purpose centre lathe.
The tool-workpiece thermocouple technique was chosen for measuring cutting
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temperature (θc) in the present study. The general description of this method mentions that the tool and workpiece must be electrically insulated from the machine tool to avoid any secondary junctions. The reduction of Machine-Fixture-Tool-Workpiece [MFTW] system stiffness as a result of electrical insulation should be avoided. Since only potential difference was measured, it would be enough to insulate only one component of the circuit. The tool holder was chosen to be insulated due to the difficulty of workpiece insulation as shown in Figure 1-a. The preliminary tests showed that MFTW system rigidity was not affected by insulation the tool holder. One of the lead wires was connected to the tool holder and the other was connected directly to the tailstock. The direct connection to the machine (instead of slip rings or mercury path) would not affect
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ACCEPTED MANUSCRIPT the emf reading remarkably [22]. Several tests were carried out in the present work to identify the probable effects of connecting the lead wires directly to the machine, instead
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of the workpiece; no differences were found, especially when a prolonged cut was
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avoided.
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When using PCBN, the presence of another secondary junction between the CBN tip and the tungsten carbide substrate should be considered. However, EMF measurements between the CBN tip and tungsten carbide during cutting were conducted. It was found
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that the emf at this junction starts to generate after the ninth second at the used cutting
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speeds. Accordingly, the emf signal was captured within 8 seconds. The insert was completely insulated by the ceramic-based composite coating with high thermal
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resistance with the exception of the CBN tip to prevent any contact between the chip and
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the tungsten carbide as illustrated in Figure 1-b.
presented in [24].
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The calibration procedures for the used tool-workpiece thermocouple arrangement are
A tool maker’s microscope was used to measure the tool flank wear land width VBB
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indicated in Figure 1-c. Progressive flank wear has been plotted against the machining time (t), in minutes. The scatter of tool wear measurements was around 7%. Three cutting speeds (v) of 60, 100 and 175 m/min were used to study the tool wear. Scanning electron microscopy (SEM) was used to study the wear patterns of the cutting tools at v=100 m/min and EDS analysis was used to analyze the different elements deposited on the worn tools. Micrographs of the underside of chips generated at cutting speeds of 100 and 175 m/min were captured using SEM for the sake of assessing the wear mechanisms. The characteristics of the tribo-films formed on the worn surfaces were studied using Xray photoelectron spectroscopy (XPS) on a Kratos-HS XPS system. A MgKα X-ray source was used, running at 15 kV and 10 mA. For the detailed scans on individual elements (as
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ACCEPTED MANUSCRIPT shown in this work for Cr), the scan was run at a pass energy of 160 eV, a step size of 50 meV and a dwell time of 100 ms with 10 passes. To reduce the impact of surface
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impurities on the final result, the sample was etched in the XPS system before collection of
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the scan using 4 keV Ar+ ions for 5 min. The deconvolution analysis was carried out using
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the system-supplied software and database. 3. Results 3.1. Cutting temperature
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Figure 2 shows the effect of the cutting speed (v) on the cutting temperature (θc). At v=60
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m/min, θc=779 0C for PCBN and 824 0C when the coated carbide tool is used. It is noticed that the coated carbide tool experiences a relatively higher cutting temperature until reaching
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a cutting speed of 100 m/min (930 0C), after which cutting temperature becomes almost the
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same as the PCBN tool. This is thought to be due its relatively lower thermal conductivity in this temperature range [24]. At a cutting speed of 175 min, the cutting temperature reaches
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1130 0C for the coated carbide tool and somewhat higher (11700C) for PCBN. 3.2. Wear curves and tool life
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Figure 3 shows the wear curves for PCBN and coated carbide tools at the three cutting speeds. At the lowest cutting speed (v= 60 m/min), θc=779 0C for PCBN and 824 0C for coated carbide tool. It can be noticed that before reaching a flank wear land width (VBB) of 0.12 mm, the coated carbide tool experiences the higher wear rate, after which its wear rate decreases to below that of the PCBN tool. Increasing the cutting speed to 100 m/min and the cutting temperature to 930 oC for both tool materials, the coated carbide tool shows the same behavior before and after a higher flank wear of (VBB) of 0.16 mm. The
same
experiments
were
carried
out
at
a
higher
cutting
speed
of
175 m/min, which saw cutting temperature in excess of 1100 0C for both tool materials. The coated carbide shows the higher wear rate until reaching the end of tool life (VBB =0.2 mm).
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ACCEPTED MANUSCRIPT Distinct running-in and gradual wear stages can be easily identified with the coated carbide tool, especially at v=60, 100 m/min; in contrast, the PCBN tool wears steadily
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throughout the duration of its life, at all cutting speeds, until reaching flank wear land width
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(VBB) of 0.2 mm.
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Figure 4 compares the machining time (in minutes) required to wear both tool materials to flank land widths (VBB) of 0.1 and 0.2 mm. Generally, it can be deduced that PCBN performs better than the coated carbide tool at all cutting speeds up to a flank land width of VBB =0.1
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mm, lasting in the range of 280-400% longer.
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Up to a tool flank wear land width of VBB = 0.2 mm, however, the coated carbide tool lasts longer than the PCBN tool at lower speeds (330% longer at 60 m/min, 140% at 100
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3.3. Tool wear mechanisms
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m/min), while PCBN tool lasts longer (by about 240%) when v=175 m/min.
SEM and EDS analyses have been carried out to capture the necessary micrographs of the
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worn out tools (VBB =0.2 mm) and to make the chemical analysis of the cutting tool edge. Comparison has been performed on the worn inserts run at the same cutting speed and cutting
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temperature, v=100 m/min and θc= 930 oC, respectively. Figure 5-a presents the SEM micrographs and EDS analysis of the PCBN insert. Crater wear is observed and (Fe) as an element coming from the workpiece material is found on the tool cutting edge. The abrasive effect of the workpiece material is also observed. The (Fe) rich areas on the worn PCBN cutting edge were also found by Angseryd et al [20] and are considered an indication of chemical wear. Figure 5-b shows the SEM micrographs and EDS analysis of the worn coated carbide tool. Crater wear can also be observed. The appearance of (W) on the worn flank and rake, which is one of the elements that comes from the coated carbide tool substrate, indicates the complete removal of the coating layers of the cutting tool. Elements from the workpiece
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ACCEPTED MANUSCRIPT material can be observed on the tool flank and rake face (Fe, Cr, Si, Mn), which is considered an indication of adhesive and/or chemical wear. An (O) peak is also found in the EDS
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analysis as well as (Mg), the latter of which could be an addition to alumina as a sintering
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additive.
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3.4. Chip formation
Figure 6 illustrates the SEM micrographs of chip underside formed at cutting speeds of 100 and 175 m/min for the two used cutting tool materials. The examination of the
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smoothness of the chip underside can therefore reveal the intensity of adhesion of the chip
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underside to the tool face.
Generally, it can be noticed that that the intensity of the adhesion marks are increased in proportion to the cutting speed for both tool materials. At v=100 m/min, the coated carbide
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chip underside surface is somewhat smoother than PCBN (θc=923 0C for both tools), whereas at 175 m/min (θc>1100 0C for both tool materials) PCBN gives a smoother surface. The SEM
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micrograph of the chip underside produced by the coated carbide tool at a cutting speed of 175 m/min (Figure 6-d) shows the highest intensity of adhesion marks.
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3.5. Characterization of the surface tribo-films on the worn surface To understand the cause of the improved tool life of the coated carbide insert at cutting speeds of up to 100 m/min and cutting temperatures up to 923 0C, an investigation of its worn surface was made. Two kinds of tribo-films could be identified on the worn rake surface of the tool using X-ray Photoelectron Spectroscopy (XPS) analysis. It was only possible to identify tribo-films on the surface of the coated carbide tool. PCBN did not show formation of tribo-films. Minor amounts of Ti-O films were found (Figure 7, a) that correspond to our previously obtained results [25, 26]. Significant amounts of lubricating polyvalent chromium oxides are generated. A corresponding spectrum is shown in Figure 7, b. The XPS spectra of the Cr 2p region (Figure
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ACCEPTED MANUSCRIPT 7, b) indicate the presence of two Cr states on the surface of the worn ceramic insert. The fitted spectrum has peaks at 576.2 (Cr2O3). This interpretation of chromium bonds is obtained
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in accordance with the data published elsewhere [25, 26]. The position of the photoelectron
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lines is shifted from the binding energy for standard elements, which indicates their partial
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oxidation with the formation of non-equilibrium phases. The positions of the photoelectron lines are shifted from the binding energy values for pure elements, indicating their partial oxidation with the formation of non-equilibrium phases close to the Cr+3-O phase.
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4. Discussion
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Referring to Figure 3, the coated carbide tool exhibits the highest tool wear at the running-in zone at v=60 and 100 m/min. After reaching the end of the running-in zones, the
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coated carbide shows a lower wear rate than PCBN until the end of tool life (VBB =0.2 mm).
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At v=175 m/min, a clear difference in wear curves can be noticed between PCBN and coated carbide, with PCBN performs better over the duration of its life.
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Tool life, represented in Figure 4 (at VBB =0.2 mm), is longer for coated carbide tools at v=60, 100 m/min, at v=175 m/min PCBN tool life is longer. At VBB =0.1 mm, PCBN tool life
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is longer at all the used cutting speeds, which illustrates the advantage of using PCBN in high precision turning when high dimensional accuracy is required. The presence of (Fe) on the worn PCBN insert, and (Fe, Cr) on the worn coated carbide tool, in the EDS analyses presented in Figure 5 can be used as an indication of adhesive and/or chemical wear mechanisms for both tool materials. Adhesion marks observed on the chip underside in Figure 6 support this conclusion. It can be noticed that the adhesive wear mechanism increases with increasing the cutting speed for both tool materials. The highest adhesive effect is noticed when using the coated carbide tool at v=175 m/min (Figure 6-d). Crater wear observed in SEM micrographs for both tool materials (Figure 5) can be evidence of the occurrence of the diffusion/chemical wear mechanism. Abrasion marks are observed
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ACCEPTED MANUSCRIPT clearly on the worn PCBN tool but are not easily noticed on the worn coated carbide cutting edge.
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In addition to the adhesion and diffusion/chemical wear mechanisms, the nature of wear
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of PCBN can be attributed to the abrasive effect of hard carbides such as chromium carbide
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in the workpiece material. The binder material in PCBN (TiN) at the start of cutting could have been worn out. When the binder material is abraded, CBN particle bonding strength becomes weaker and may not be present at some points. Therefore, CBN particles will not be
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bonded together and start to lose their advantage. Consequently the wear rate of CBN tools
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becomes higher than the coated carbide. The diffusion/chemical wear mechanism indicated by crater wear may also be another reason for increasing the wear rate of PCBN as cBN has a
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higher solubility in iron than alumina as mentioned in [18]. The nature of the chemical wear
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mechanism of PCBN is discussed deeply in [19, 20] Two types of tribo-films are forming on the surface of the coated carbide tool. The first is
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due to the chemical modification of the tool material, based on the Ti-O phase caused by the chemical modification of the TiN top layer. However, the machining conditions used here
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are very hard. It is reasoned that any Ti-O tribo-films formed could not efficiently sustain such high temperature/heavy load conditions and consequently were rapidly worn out [26]. This would support why the XPS analysis performed in this study (Figure 7, a) found only small amounts of these Ti-O based tribo-films. Referring to Figure 2, θc= 930 0C at v= 100 m/min, the Ti-O tribo-films formed cannot sustain harsh operating conditions of dry hard turning. Accordingly, it can be deduced that in this case, the adaptive behavior of the coated carbide tool was developed by means of the ability of the tool to generate a protective tribo-film on the frictional surfaces due to the presence of certain elements in this tool material [26], which reduce the tool wear under severe operating conditions. As was expected, the second type of tribo-films forms due to the
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ACCEPTED MANUSCRIPT tribo-oxidation of the workpiece material, which is sticking to the rake surface as a result of the friction at the chip/tool interface.
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The XPS spectra of the Cr 2p region (Figure 7, b) indicate the presence of a Cr-O phase
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on the surface of the worn coated carbide insert. The fitted spectrum has a peak at 576.2
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(Cr2O3). This interpretation of chromium bonds is obtained in accordance with the data published elsewhere [27, 28]. The positions of the photoelectron lines are shifted from the binding energy for standard elements, which attributes their partial oxidation to the formation
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of non- equilibrium phases. Formation of high temperature lubricious Cr-O tribofilms that are
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identified by XPS (Figure 7, b) results in improvement of lubricating properties at the tool/chip interface. The long-term oxidation of chromium can help dissolve this oxide in
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alumina as an intermediate layer in the coated carbide tool, and reduce the inter-diffusion
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between the workpiece and tool materials [29]. Similar results were previously obtained for the TiAlCrN PVD coating with high Cr content [30], and the same tribo-film was obtained
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using a mixed alumina ceramic tool to machine the same workpiece material [31]. These tribo-films were formed at the end of the running-in stages as observed in the wear curve in
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Figure 3-a, b. For the coated carbide tool, it can be noticed that at a cutting speed of 60 m/min and cutting temperature of 824 0C, which is considered an adequate temperature for Cr-O tribo-films to sustain their lubricious properties [26], the cutting tool life can reach 330% when compared to PCBN. At a cutting speed of 175 m/min, and due to the high cutting temperature associated with the increase of the cutting speed (>1100
o
C, as indicated by cutting temperature
measurements in Figure 2), both Ti-O and Cr-O tribo-films would lose their lubricous properties [26]. The effect of the coating on increasing tool life will be not remarkable. Intense adhesion marks observed on the chip underside of the coated carbide tool used at this cutting speed (cutting temperature) shown in Figure 6-d supports this result. At this speed,
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ACCEPTED MANUSCRIPT PCBN can outperform this type of coated carbide tool in its hot hardness, which is estimated to be 2-3 times harder than the coated carbide tool at this elevated temperature. [32].
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5. Conclusion
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This study demonstrates the effect of the cutting temperature measured by the tool-
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workpiece thermocouple on the wear characteristics of carbide tools with the CVD multilayer TiCN/Al2O3 coating, and low content Polycrystalline Cubic Boron Nitride tools in turning hardened D2 tool steel. Friction kinetics can controls the wear mechanisms during harsh
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machining conditions. XPS analysis showed the formation of Ti-O and Cr-O tribo-films on
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the surface of the worn CVD coated carbide tool exposed to cutting temperatures up to 923 C (cutting speeds up to 100 m/min). These tribo-films were investigated at the end of the
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running-in period as indicated by the wear curves. The increases of coated carbide tool life
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when compared to PCBN can reach 330% at a cutting speed of 60 m/min and corresponding cutting temperature of 824 0C. The latter temperature can be considered a suitable
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temperature for Cr-O tribo-films to exhibit their lubricious properties. The long-term oxidation of chromium can help dissolve this oxide in alumina as an intermediate layer in the
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CVD coated carbide tool and improve its wear resistance. When increasing the cutting speed to 175 m/min, and consequently the cutting temperature to over 1100 OC, these tribo-films become ineffective, and PCBN has the longer tool life due to its higher hot hardness. This was supported by the higher intensity of adhesion marks on the chip underside produced by the coated carbide tool at this cutting speed (cutting temperature). Adhesive and chemical wear can be considered the main wear mechanisms in the used PCBN cutting tool. In high precision turning, when high dimensional accuracy is required, it is recommended to use PCBN for its lower wear rate in the early stages of tool wear (tool flank land width not exceeding 0.1 mm).
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ACCEPTED MANUSCRIPT References
1. Tonshoff, H. K., Arend, C., Ben Amor, R.: Cutting of Hardened Steel. Annals of the
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ClRP 49/2, 547-566 (2000).
2. Hashimoto, F., Guo, Y. B., Warren, A. W: Surface integrity difference between hard
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turned and ground surfaces and its impact on fatigue life. Annals of the CIRP 55/1, 8184 (2006).
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3. Poulachon, G., Alberta, A., Schluraff, M., Jawahir, I. S.: An experimental investigation of work material microstructure effects on white layer formation in PCBN hard turning.
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International Journal of Machine Tools & Manufacture 45, 211–218 (2005).
4. Lahiff Cora, Gordon Seamus, Phelan Pat: PCBN tool wear modes and mechanisms in
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finish hard turning. Robotics and Computer-Integrated Manufacturing 23, 638–644
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(2007).
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5. Arsecularatne, J. A., Zhang, L. C., Montross, C.: Wear and tool life of tungsten carbide, PCBN, PCD cutting tools. International Journal of Machine Tools & Manufacture 46,
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482–491(2006).
6. Biest, O. Van Der, Vleugels, J.: Prospective on the development of ceramic composites for cutting tool applications. Key Engineering Materials Vols. 206-213, 955-960 (2002).
7. Cook, M.W., Bossom, P.K., Trends and recent developments in the material manufacture and cutting tool application of polycrystalline diamond and polycrystalline cubic boron nitride, International Journal of Refractory Metals & Hard Materials 18,147-152 (2000).
8. Dogra. M, Sharma, V.S., Sachdeva A, Suri, N. M., Durejam, J. S.: Tool wear, chip formation and workpiece surface issues in CBN hard turning: a review. Int J Precis Eng Manuf 11, 341–58 (2010).
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ACCEPTED MANUSCRIPT 9. Söderberg Staffan, Sjöstrand Mats, Ljungberg Björn: Advances in coating technology for
metal
cutting
tools.
Metal
Powder
Report
56/4,
24-30
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(2001).
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10.Grzesik, W., Nieslony, P.: Prediction of friction and heat flow in machining
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incorporating thermophysical properties of the coating-chip interface. Wear 256, 108117 (2004).
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11.Grzesik, W.: Friction behavior of heat isolating coating in machining: mechanical, thermal and energy-based considerations. International Journal of Machine Tools &
MA
Manufacture 43, 145–150(2003).
12.Grzesik, W.: Experimental investigation of the cutting temperature when turning with
D
coated indexable inserts. International Journal of Machine Tools & Manufacture 39,
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355–369 (1999).
13.Quinto, D. T.: Technology prospective on CVD and PVD coated metal-cutting tools.
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Int. J. of Refractory Metals and hard materials 14, 7-20 (1996).
14. Grzesik, W.: The role of coating in controlling the cutting process when turning with
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coated indexable inserts. Journal of Material Processing Technology 79, 133-143(1998).
15.Poulachon, G., Moisan, A., Jawahir, I. S.: Tool-wear mechanism in hard turning with polycrystalline cubic boron nitride tool. Wear 250, 576-586 (2001).
16. Chou, Y. K., Evans, C. J., Barash, M. M.: Experimental investigation on CBN turning of hardened AISI 52100 steel. Journal of Materials Processing Technology 124, 274– 283 (2002).
17.Diniz, A. E., Adilson, de Oliveira, A. J.: Hard turning of interrupted surfaces using CBN tools. Journal of materials processing technology 195, 275–281(2008).
18.Kramer, B. M., Suh, N. P.: Tool wear by solution: a quantitative understanding. Transactions of the ASME, Journal of Engineering for Industry102, 303-309 (1980). 16
ACCEPTED MANUSCRIPT 19.Angseryd, J., Andrén, H.-O.: An in-depth investigation of the cutting speed impact of the degraded microstructure of worn PCBN cutting tools. Wear 271, 2610-2618 (2011).
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20.Angseryd, J., Coronel, E., Elfwing, M., Olsson, E., Andrén, H.-O.: The microstructure
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of the affected zone of a worn PCBN cutting tool characterized with SEM and TEM.
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Wear 267, 1031-1040 (2009).
21.Ghani, M. U., Abukhshim, N. A., Sheikh, M. A.: An investigation of heat partition and
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tool wear in hard turning of H13 tool steel with CBN cutting tool. Int J Adv Manuf Technol 170, 1282-1287 (2007).
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22.Stephenson, D. A.: Tool-work thermocouple temperature measurements-theory and implementation issue. Transactions of the ASME, Journal of Engineering for Industry
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115, 432-437 (1993).
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23.Lezanski, P., Shaw, M. C.: Tool face temperatures in high speed milling, Transactions of the ASME, Journal of Engineering for Manufacture 112, 132-135 (1990).
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24.Shalaby, M. A.: An investigation into high precision turning of some alloy steels. PhD thesis, Ain Shams University, Cairo, Egypt (2011).
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25.Kovalev, A. I., Wainstein, D. L., Mishina, V. P., Fox-Rabinovich, G. S.: Investigation of atomic and electronic structure of films generated on a cutting tool surface. Journal of Electron Spectroscopy and Related Phenomena, 63-75 (1999).
26.Fox-Rabinovich, G. S., Totten, G. E.: Self-organization during friction. Boca Raton NY Taylor & Francis (2007).
27.Tsitsumi, T., Ikemoto, I., Namikawa, T.: Bull Chem Soc Jpn 54/3:913 (1981). 28. Romand, H., Robin, M.: Analyses 4, 7:308 (1974). 29.Fox-Rabinovich, G. S., Weatherly, G. C., Wilkinson, D. S., Kovalev, A.I., Wainstein, D. L.: The role of chromium in protective alumina scale formation during the oxidation of ternary TiAlCr alloys in air. Intermetallics 12, 165-180 (2004). 17
ACCEPTED MANUSCRIPT 30.Fox-Rabinovich, G. S., Yamamoto, K.,Veldhuis, S. C., Kovalev, A. I., Dosbaeva, G. K.: Tribological adaptability of TiAlCrN PVD coatings under high performance dry
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machining conditions. Surf Coat Technol 200, 1804–10 (2005).
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31.Shalaby, M. A., El Hakim, M. A., Abdelhameed, M. M. , Krzanowski, J. E., Veldhuis,
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S.C. , Dosbaeva, G. K.: Wear mechanisms of several cutting tool materials in hard turning of high carbon–chromium tool steel. Tribology International 70, 148–154 (2014).
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32.Harris, T.K., Brookes, E.J., Taylor, C.J.: The effect of temperature on the hardness of
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polycrystalline cubic boron nitride cutting tool materials. International Journal of Refractory Metals & Hard Materials 22, 105–110, (2004).
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Figures captions:
Figure 1. (a) Tool-workpiece thermocouple arrangement for measuring the cutting
width VBB ;
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temperature (b) using PCBN (BNX20) and (c) the drawing shows tool flank wear land
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Figure 2. Effect of cutting speed on the cutting temperature when using coated carbideand PCBN tool materials;
Figure 3. Wear curves of coated carbide and PCBN cutting tool materials in hard turning of D2 tool steel: (a) v=60 m/min, (b) v=100 m/min, (c) v= 175 m/min; Figure 4. Tool life at different cutting speeds for different tool flank wear land widths (VBB): (a) VBB =0.1 mm, (b) VBB =0.2 mm;
Figure 5. SEM micrographs and EDS analysis of the worn inserts at 100 m/min: (a)PCBN, (b) Coated carbide; 18
ACCEPTED MANUSCRIPT Figure 6. SEM micrographs of chip underside for PCBN and coated carbide tools at different cutting speeds: (a) PCBN at v=100 m/min, (b) coated carbide at v= 100 m/min (c)
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PCBN at v=175 m/min, (d) coated carbide at v= 175 m/min.
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Figure 7. Photoelectron spectra on the (a) Ti 2p and (b) Cr 2p regions taken from the
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worn rake surface of the coated carbide tool.
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Tailstock
Insert
Tool holder
Lead wire
+
Sensitive voltmeter
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Chuck
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Insulation
-
Lead wire
Carbide substrate
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PCBN tip
Lead wire
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Thermal resistant coating
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a)
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Tool holder
b)
VBB
c) Fig. 1
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Fig. 2
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(b)
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CE P
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a)
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(c) Fig. 3
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(a)
(b) Fig. 4
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Spectrum 1
Ti
W
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Crater wear
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Al Ti N Ti
Abrasion marks
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Fe Fe
0 1 2 3 Full Scale 321 cts Cursor: 0.000
Fe 4
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200 m
Ti
5
6
Fe 7
8
9
10 keV
(a)
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Fe Mn Ti Mn Cr Cr O
C
Crater wear
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W Si
Fe
Fe Mn
Ti W
Ti W
0 1 2 3 Full Scale 652 cts Cursor: 0.000
200 m
(b)
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Fig. 5
Spectrum 2
Ti
24
4
5
Cr Cr Mn 6
Fe 7
W W
W 8
W W 9
10 keV
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10 m
10 m
(b)
Fig. 6
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(b)
10 m
10 m
(d)
(c)
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Ti-O (458. 09)
(a)
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Ti-O (465.86)
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Cr203 (576.2)
(b)
Fig. 7
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ACCEPTED MANUSCRIPT Table 1. Specifications of tool materials and tool holder Tool Chemical composition Hardne
60%CBN+TiN binder
ss
standard
specifications
3100–
SNMA120
3300
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(BNX20)
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HV
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CVD
TiCN (4µm) /Al2O3(6 µm )
carbide
/TiN (2µm), coated over
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coated
– 25o Chamfer
clearance
SNMA120
angle, 75
Honed
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edge
27
-5 rake angle,5
angle
cutting
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carbide substrate
412
412 _
holder
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PCBN
Tool
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Material
ISO
setting angle