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The role of chemical wear in machining iron based materials by PCD and PCBN super-hard tool materials
Diamond & Related Materials 16 (2007) 435 – 445 www.elsevier.com/locate/diamond
The role of chemical wear in machining iron based materials by PCD and PCBN super-hard tool materials S. Giménez ⁎, O. Van der Biest, J. Vleugels Department of Metallurgy and Materials Engineering, Katholieke Universiteit Leuven, Kasteelpark Arenberg 44, B-3001 Heverlee, Belgium Received 15 March 2006; received in revised form 20 July 2006; accepted 22 August 2006 Available online 29 September 2006
Abstract The chemical compatibility of PCBN and PCD tool materials with iron has been evaluated by means of the static interaction diffusion-couples technique. Experiments were undertaken at different temperatures (700 °C, 900 °C, 1100 °C and 1300 °C) in order to establish the maximum temperature at which the tool and workpiece materials are chemically compatible. Computational equilibrium thermodynamics was used to calculate the chemical solubility of the different tool materials in iron and to identify the interaction phases formed. The agreement between the experimental results and the thermodynamic calculations was excellent. Machining tests were performed in order to assess the relative importance of the chemical wear on the overall rate of tool wear. The results of these machining tests agreed perfectly with the chemical compatibility study, thereby indicating that under certain machining conditions, chemical wear is the main wear mechanism for these materials in machining iron based materials. © 2006 Elsevier B.V. All rights reserved. Keywords: Super-hard materials; Cutting tools; Wear; Thermal stability
1. Introduction The trends in the metal cutting industry are driven by the needs of the manufacturers to continually improve performance, reduce costs and increasingly, comply with the environmental legislation [1]. Additionally, the introduction of new workpiece materials encourages the development of new tool materials and new shaping strategies. Polycrystalline diamond (PCD) and polycrystalline boron nitride (PCBN) represent the two existing most outstanding tool materials due to their combination of extremely high hardness and abrasive wear resistance, high thermal conductivity and thermal stability but with moderate toughness. PCD is normally used for machining of abrasive non-ferrous materials. One example is metal matrix composites (MMCs) reinforced with second-phase particles, whiskers, fibers, wires or filaments. These materials are increasingly used in the automotive and aerospace industry replacing steel components [2]. The use of PCD is not recommended for
⁎ Corresponding author. CEIT, P Manuel Lardizabal 15, 20018 San Sebastián, Spain. Tel.: +34 943 212800; fax: +34 943 213076. E-mail address: [email protected] (S. Giménez). 0925-9635/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2006.08.017
ferrous alloys since the chemical compatibility of the tool and workpiece materials is very poor, leading to high chemical wear —one exception being the machining of certain classes of cast iron with cutting speeds in the range of 200–500 m/min [3]. PCBN tools are primarily used in the machining of hard ferrous materials (case-hardened steels, high speed tool steels, die steels, bearing steels, white cast iron and alloy cast irons) due, it is generally believed to their high chemical compatibility with iron. The scientific literature on the wear mechanisms of PCBN in the machining of hard ferrous alloys shows some contradictory conclusions. While some investigations indicate that PCBN is totally inert with iron during machining of hard ferrous alloys [4], other authors propose that chemical wear is an important mechanism. König and Neises [5] remarked on the importance of the chemical composition of the binder and its percentage in the composite for thermal stability. Zimmermann et al. [6] concluded that during hard turning of case-hardened steel, a tribochemical mechanism (dissolution or diffusion of CBN into the flowing chip) is responsible for wear in the crater region and a chemical mechanism is responsible for the flank wear. More recently, Barry and Byrne [7] and Arsecularatne et al. [8] also identified chemical wear as the dominant wear mechanism during hard machining of steel.
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Table 1 Tool materials and nominal binder contents provided by the manufacturer Reference
Commercial name
Abrasive
Binder phases (vol.%)
1 2A 2B
CTM302 DBC50 DBW85
PCD PCBN PCBN
Co (10) TiC (40), Al2O3 and TiB2 (10) WCoB (15)
A simple procedure to evaluate the possibility of chemical wear is static interaction couple experiments. Despite machining being a dynamic situation where thermodynamic equilibrium does not apply, the information provided by the interaction couple experiments reveals the potential for chemical interaction to take place during a particular machining process. A thermodynamic model for chemical wear was developed by Kramer and Judd [9] and Kramer [10], and successfully applied to the computational design and wear resistance prediction of coatings. Previous work at K.U. Leuven has demonstrated the validity of this approach to understand the wear mechanisms (mainly chemical in nature) of ceramic tools (Si3N4-based and SiAlON-based) during the machining of steel [11–13]. In the present work, the chemical compatibility of WC–Co, PCD and PCBN tool materials with iron has been assessed by means of the static interaction couple method. Computational equilibrium thermodynamics has been used to calculate the chemical solubility of the different cutting tool materials in iron and to identify the interaction phases formed. Finally, machining tests have been carried out to confirm the findings of the theoretical study.
All the tool materials were provided in the form of triangular bi-layer pieces with the super-hard composite layer (approximately 0.5 mm thick) supported on a WC–Co (approximately10 vol.% Co and 0.8 mm thick) substrate. The average grain size of the CBN phase is approximately 2 μm for both PCBN materials. The PCD grade possesses a multimodal distribution of diamond particle size ranging from 2 to 30 μm. The grain size distribution is reported to result in an increased packing density and therefore greater abrasion resistance, chip resistance and higher quality cutting edges [14]. As workpiece materials, heat treated powder compacts of Somaloy 500™ + 0.6 wt.% Kenolube (a soft magnetic composite material) provided by Höganäs AB (Höganäs, Sweden) were
2. Materials and experimental techniques A polycrystalline diamond (PCD) cutting tool material, (grade 1) and two different polycrystalline cubic boron nitride (PCBN) tool materials (grades 2A and 2B respectively), were sourced from Element Six (Table 1). The number used in the commercial code for the PCBN grades (‘50’ and ‘85’ for grade 2A and 2B respectively) refers to the approximate volume fraction of the super-hard phase. For the PCD material, the volume fraction of diamond is approximately 90 vol.%. The binder phases and their nominal volume percentages are indicated in Table 1 for all the tool materials. Table 2 Interaction couples selected for the investigation Identification
Tool material
T (°C)
1 2 3 4 5 6 7 8 9 10 11 12
1 2A 2B 1 2A 2B 1 2A 2B 1 2A 2B
700 700 700 900 900 900 1100 1100 1100 1300 1300 1300
Fig. 1. Tool materials supplied by E6, A) 1, B) 2A and C) 2B.
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used. For the purposes of the present work, the material can be considered as pure iron. The specimens were pressed into prismatic bars 50 × 10 × 5 mm3 with densities up to 7.40 g/cm3 and then heat treated in air at 500 °C for 30 min. From the heat treated bars, 10 × 5 × 5 mm3 specimens were cut for the inter-
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action couples investigation. Microstructural and mechanical characterisation of this material has been reported elsewhere [15]. X-ray diffraction (XRD) was also carried out for crystallographic characterisation of the as-received material; a 3003 T/T
Fig. 2. XRD diffractograms of the as-received tool materials, A) 1, B) 2A and C) 2B.
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X-ray diffractometer (Seifert, Ahrensburg, Germany) was employed using CuKα radiation. The diffraction scans were carried out in the interval 2theta 20–90°, with a step size of 0.02° and a dwell time of 4 s/step. The chemical reactivity of a number of tool–workpiece combinations was studied by assessing the extent of interdiffusion of species between polished slices of the tool and workpiece materials which are pressed together under a low load (2.5 MPa) for 1 h in vacuum (b 0.1 Pa) and at different temperatures (700 °C, 900 °C, 1100 °C and 1300 °C). The heating and cooling rates were 50 K/min. A ‘W100/150-2200-
50LAX’ hot press device, (KCE Sondermaschinen, Rodental, Germany) was used. The polycrystalline super-hard composites (PCD or PCBN) and carbide substrates (WC–Co) were polished and used in the interaction couple investigation. The sample preparation included mounting the specimens on an epoxybased resin (EPOFIX 4000), grinding with diamond (15 and 6 μm oil suspension lubrication) and polishing with diamond paste (3 and 1 μm). Final polishing was carried out with a SiO2 suspension (0.5 μm). For the tests at 1300 °C, the WC–Co substrate was removed as preliminary tests revealed a pronounced influence of the WC–Co on the interaction between
Fig. 3. Interaction couples for material 1, A) 700 °C, B) 900 °C, C) 1100 °C, D) 1100 °C, detail of the interface, E) EPMA compositional profile at the interface.
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the workpiece and super-hard composite layers. After cooling, the interaction couples were cross-sectioned, polished, etched with 2 vol.% Nital and investigated by means of scanning electron microscopy (SEM). An XL30-FEG microscope equipped with an EDAX-Dx-4i system was used for qualitative microchemical analysis (Philips, Eindhoven, The Netherlands). Electron probe microanalysis (EPMA) was used for chemical quantitative analysis; a Superprobe 733 JEOL equipment (Tokyo, Japan) fitted with a Noran EDS. The quantitative algorithm employed was calibrated for the metallic elements. The total amount of B, C and N was indirectly calculated as the difference between the sum of the metallic elements and 100%. The list of interaction couples investigated is given in Table 2. Turning tests were undertaken over a range of cutting speeds with a feed and depth of cut of 0.1 mm and 0.2 mm respectively. For the two PCBN grades, the tool geometry was SNMN 090308 T0220 (a geometry typically for PCBN tools and comprising a 200 μm × 20° chamfer). The PCD inserts were used with a sharp, un-chamfered, edge. The cutting speeds used for the PCBN materials were: 200 m/min, 400 m/min, 600 m/min and 800 m/min. The cutting speeds used for the PCD material were 35 m/min, 45 m/min, 50 m/min and 100 m/min. Flank wear measurements were taken at various intervals over the duration of each test and at least two edges of each grade were tested at each cutting speed. The results are summarized below
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in the form of a Taylor tool life curve, where the tool ‘life’ criteria is taken as a flank wear land of 300 μm. 3. Results The microstructures of the as-received tool materials are shown in Fig. 1. Fig. 1A illustrates the microstructure of the PCD grade (1). The diamond phase constitutes the dark-contrasted areas of the microstructure (∼90 vol.%) indicated as ‘A’. The lighter cavity-like regions (B) are the remnants of the cobalt ‘binder’ which was partially removed during polishing (∼10 vol.%). A bright phase (C) is also observed and was identified as WC and W2C by XRD (Fig. 2A). CoCx is also identified by XRD (Fig. 2A). The microstructure of the 2A PCBN grade is shown in Fig. 1B, (note the change of scale with respect to Fig. 1A). The dark-contrasted CBN grains (A) (approximately 50 vol.%, average size 2 μm) appear embedded in a complex binder system, mainly constituted by TiC (B). Additionally, Al2O3 and TiB2 with very similar contrast have been identified (D), in good agreement with the XRD information (Fig. 2B). Identically as for the PCD, the presence of WC, W2C (C) and Co is also observed and identified by XRD. The 2B PCBN grade (Fig. 1C) is constituted by the dark-contrasted CBN grains (A) embedded in the WCoB binder (B). The presence of WC (C) and CoCx is also observed and identified by XRD (Fig. 2C). It is
Fig. 4. Interaction couples for material 2A, A) 1100 °C, B) 1300 °C overview, C) 1300 °C, detail of the interface, D) EDS compositional profile at 1300 °C.
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reasonable to conclude that the phases WC, W2C, Co and CoCx, appearing in the XRD diffractograms and SEM micrographs are related to the WC–Co substrate. 3.1. Static interaction couples 3.1.1. PCD The interaction couples 1, 4, 7 and 10 were devoted to the study of the chemical interaction of the PCD tool material grade (1) and Fe between 700 °C and 1300 °C (Table 2). At 700 °C, a gap filled with the mounting resin appears in the interface as shown in Fig. 3A. This is an evidence of the absence of interaction or limited reaction since the bond strength is not sufficient to overcome the thermal stresses between the dissimilar materials during cooling. At 900 °C, evidence of interaction is provided by the presence of pearlite at the iron side of the interface (Fig. 3B). This feature is more pronounced after testing at 1100 °C (Fig. 3C). At higher magnification, Fig. 3D, a 20 μm deep layer with similar contrast as iron but without the pearlitic structure is observed at the interface. The compositional profiles revealed this phase to be a Fe–Co rich constituent, (Fig. 3E). Some white contrasted W-rich particles were observed at the interface. A peak for W is observed in the compositional profile accounting for the presence of these particles (Fig. 3E). At 1300 °C, the tool material totally dissolved in the iron. The
morphological integrity of the interaction couple was totally lost and no further characterisation was possible. 3.1.2. PCBN-2A Interaction couples 2, 5, 8 and 11 were devoted to study the chemical interactions between the PCBN-2A tool material and Fe (Table 2). No interaction was detected below 1300 °C. Fig. 4A shows the tool–workpiece interface after testing at 1100 °C, demonstrating the absence of any interaction product. Conversely; a high degree of interaction is observed after the diffusion experiment at 1300 °C. An overview of the interaction couple is shown in Fig. 4B. Evidence of penetration of iron into the tool is provided by the light contrasted region of the tool adjacent to the interface (Fig. 4B). The iron side of the interface appears decorated by a Fe–C rich structure (Fig. 4C and D), which has the appearance of originating from a eutectic reaction. The compositional profile (Fig. 4D) shows diffusion of C into the iron and penetration of Fe into the tool material (to a depth of about150 μm). Al and Ti do not diffuse into Fe, although the Ti content of the tool increases near the interface (Fig. 4D). 3.1.3. PCBN-2B The interaction couples 3, 6, 9 and 12 were used to study the interaction of the PCBN-2B material and Fe (Table 2). As previously observed for the 2A material, no reaction was
Fig. 5. Interaction couples for material 2B, A) evidence of absence of interaction at 1100 °C, B) overview of the interaction couple after testing at 1300 °C, C) Detail of the interface at 1300 °C, D) compositional profile at 1300 °C.
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Fig. 6. Interaction between Fe and WC–Co, A) 700 °C, B) 900 °C, C) 1100 °C, D) compositional profile at 1100 °C.
observed at temperatures below 1300 °C. At 1100 °C, the tool– workpiece interface exhibits a continuous gap as a consequence of its inert behaviour (Fig. 5A). At 1300 °C, evidence of interaction is provided by the rounding of the corners of the tool material, as can be seen in the overview in Fig. 5B. At higher magnification, Fig. 5C, it is observed that the iron side of the interface is decorated with an interaction product containing Fe and C and with a structure similar to pearlite. The compositional Table 3 Summary of the results obtained from the static interaction couples analysed Identification
Abrasive
Temperature (°C)
Tool/workpiece
WC–Co/ workpiece
1 2 3 4 5 6 7 8 9 10 11 12
1 2A 2B 1 2A 2B 1 2A 2B 1 2A 2B
700
0 0 0 • 0 0 •• 0 0 ••• •• •
• • • •• •• •• ••• ••• ••• N/E
900
1100
1300
The symbols “•” indicate the degree of interaction; ‘0’ denotes no interaction; ‘N/E’ not evaluated.
profile (Fig. 5D) does not identify Fe penetration into the tool material or Co or W penetration into iron. 3.1.4. WC–Co In all the interaction couples, the interaction of WC–Co with iron was also assessed. Fig. 6A shows a detail of the interface between WC–Co and Fe at 700 °C. A thin layer X (depth b 1 μm) is formed between the WC grains and iron evidencing that interaction takes place at temperatures ≤700 °C. The qualitative EDS analysis evidenced the presence of Fe and Co in this layer. At 900 °C (Fig. 6B), a phase with intermediate contrast between Fe and WC (Y) is observed surrounding each individual WC
Fig. 7. Taylor curve for the three materials investigated.
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tionship between tool life and cutting speed is clearly observed. It is clear that the ranking of the materials is similar to that concerning the chemical compatibility. At very low cutting speeds (b 25 m/min), it appears as though PCD will outperform both PCBN grades, whereas at more reasonable speeds (N50 m/min), the PCBN grades exhibit considerably greater tool lives. As the temperature at the cutting edge of the tool increases with cutting speed, the ‘cutting speed’ axis in Fig. 7 may be considered representative of cutting temperature. At all reasonable speeds (and therefore, resulting temperatures) employed in the study, the tool materials rank in exactly the same manner as the ‘inertness’ ranking derived from the interaction-couple experiments. It is possible to surmise that below 50 m/min, chemical wear is but one component of the wear process. 4. Discussion 4.1. Identification of the interaction phases In order to better understand the chemical compatibility of the tool and workpiece materials, experimentally characterised using interaction-couples, computer equilibrium thermodynamic calculations were carried out to assist the identification of the interaction phases experimentally observed. The Thermo-Calc software and the SSOL databank [16] were used.
Fig. 8. A) Evolution of the weight fraction of the stable phases of the Fe–Co– W–C system with Fe content (x(W)–x(C) = 0). B) Evolution of the composition of the fcc (Fe,Co) phase with the mole fraction of iron. The substitutional nature of Fe and Co is evident.
grain adjacent to the interface. Additionally, the corners of the WC particles appear rounded suggesting dissolution of WC in this phase. The qualitative EDS analysis revealed the presence of W, Co, Fe and C. At 1100 °C, a similar interaction phase is observed to form a continuous band (about 5 μm in thickness) along the interface and with a gradient of Co and Fe as shown in the compositional profile (Fig. 6D). A thinner band of a Fe–Co rich phase (X) lies between this phase and the iron. This phase X is similar to that observed at 700 °C, Fig. 6A. Table 3 summarizes the experimental observations taken from the interaction couples study. These results clearly indicate that the WC–Co substrate dissolves in “pure” iron at temperatures ≤ 700 °C. The PCD material is not chemically stable in contact with iron from temperatures ≤ 900 °C onwards, whereas the PCBN-composites are more chemically compatible with iron, even up to 1100 °C for 1 h. At 1300 °C, extensive interaction is observed for 2A and clear, but limited, interaction takes place for 2B. Therefore, the chemical compatibility of the tool materials with iron can be ranked as: WC–Co b 1 b 2A b 2B. 3.2. Machining tests The results of the machining tests are summarized in the Taylor tool life chart in Fig. 7 — ‘tool life’ meaning the time to reach a flank wear land of 300 μm. In this figure, the rela-
Fig. 9. Evolution of the weight fraction of the stable phases of the Fe–Co–W–C system with Fe content (x(W)–x(C) = 0.1). B) Evolution of the composition of the M6C phase with the mole fraction of iron. The substitutional nature of Fe and Co is evident.
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Fig. 10. PCBN-2B, micrograph evidencing the absence of WC particles at the interface.
4.2. PCD and WC–Co The results of the interaction studies on PCD/Fe and WC–Co/ Fe can be explained through the thermodynamics of the Fe–W– Co–C system. The Fe–Co rich region adjacent to the PCD–Fe interface (Fig. 3D) and the Fe–Co rich region (X) at the WC–Co/ Fe interface (Fig. 6A and C) exhibit similar features. Consequently, it is believed that it is the same phase. Fig. 8A shows the evolution of the stable phases in the Fe–W–Co–C system as a function of Fe content at 1100 °C. The initial boundary conditions employed in order to satisfy the Gibb's phase rule were T = 1373 K, P = 105 Pa, N = 1, x(Co) = 0.1, x(Fe) = 0.5, x(W)–x (C) = 0, where T, P, N and x refer to absolute temperature, pressure, number of moles of the system and molar fraction respectively. The condition x(W)–x(C) = 0 specifies that the W and C in the system are coming exclusively from the WC phase. At the higher Fe contents in Fig. 8A, the most stable phase is the fcc (Fe,Co) solid solution. The equilibrium composition of this phase with the iron content is shown in Fig. 8B. The compositional profile measured, shown in Fig. 3E, shows a ‘qualitatively’ identical trend evidencing the substitutional role of Fe and Co in this phase. With the atmospheric conditions used in the high temperature interaction-couple tests, a certain decarburisation of the WC phase can take place. This situation has been simulated in the calculations through the boundary condition x(W)–x(C) = 0.1. The evolution of the stable phases with the Fe content are shown in Fig. 9A. In this case, three new stable phases appear
443
Fig. 12. Solubility calculations of all the different tool materials in iron according to Eq. (4).
(M6C, M12C and the μ-phase). Quantitatively, the most important one is M6C; its composition is shown in Fig. 9B. Therefore, it seems reasonable to identify the phase Y in Fig. 6C as M6C. There are multiple examples in the bibliography about the transformation of WC → M6C or (η-phase) when decarburisation takes place [17,18]. The presence of the W-rich particles at the iron side of the interface embedded in the fcc (Fe,Co) phase (Fig. 3D) is not well understood. From a thermodynamic point of view, there is no support for this observation, and it is believed that W-rich particles, originally in the tool material have been pulled out during the grinding and polishing procedure and adhered to the iron. 4.2.1. PCBN The chemical interaction between the PCBN-2A and iron is characterised by the formation of a Fe–C rich band which should originate from a eutectic reaction (Fig. 4C). Diffusion of C from the tool material to the workpiece is evidenced by EDS (Fig. 4D), with a maximum molar fraction of about 20% at 5 μm from the interface. At 1153 °C, a eutectic reaction takes place forming austenite and cementite, accounting for the interaction phase formed in the experiments. Previous work on the wear behaviour of CBN–TiC composites concluded that the superior wear resistance of these materials compared to grades with higher CBN content was based on the higher chemical compatibility of TiC [7]. The experimental results obtained in the present work are in contradiction to this thesis. On the other hand, the interaction phase formed at the interface between the PCBN-2B and Fe cannot be explained by the nominal composition of the tool material. C (which is not a constituent element of any of the nominal phases) was clearly identified by EDS (Fig. 5D) in the interaction region showed in Fig. 5C. The only phase containing C could be WC, which was identified by SEM and XRD (Figs. 1C and 2C respectively). Additionally, reinforcing this argument, no WC particles are observed at the interface as can be seen in Fig. 10. 4.3. Calculation of equilibrium solubility of tool materials in pure iron
Fig. 11. Solubility calculations of all the different phases contained in the tool materials studied in iron.
The chemical compatibility of the tool materials with iron can also be studied from a thermodynamic point of view. The
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equilibrium solubility of a multi-phase material in pure iron can be calculated from the equilibrium solubility of the constituent phases. The equilibrium calculation procedure has been previously explained for the dissolution of a hypothetical AxBy phase in pure iron, [9,10,12,13]. The equilibrium solubility C (mol phase / mol solution) of phase AxBy in iron can then be calculated as: XS CAx By ¼ exp½DðGfAx By −xDGXS A −yDGB −RT
ðxlnx þ ylnyÞÞ=RT ðx þ yÞ
ð1Þ
where ΔGf is the Gibbs energy of formation of the AxBy phase, xs ΔGA , ΔGBxs the relative partial molar excess Gibbs energies of solution of component A and B in Fe respectively, R is the universal gas constant and T, the absolute temperature (K). The formation energy (ΔGf) of the phases was taken from the thermodynamic database of Barin [19], except for the WCoB phase, which was taken from Ref. [20]. The evolution of ΔGf with temperature for this phase was assumed to be similar to that of CoB (available in Ref. [19]). The relative partial molar excess Gibbs energy of solution of the different elements in iron was obtained using the Thermo-Calc software [16] and the SSOL database. The ION database was used for the thermodynamic description of oxygen. At the solubility limit of element A in iron, the solution will be in equilibrium with a second-phase FexAy. Therefore, the Gibbs energy of solution of element A at the solubility limit, NA, must equal the Gibbs energy of formation of the secondphase per mole atom of element A. f XS DGm A ¼ DGA þ RT lnNA ¼ DGFex Ay
ð2Þ
equilibrium solubilities of the multi-phase tool materials studied calculated by Eq. (4). The volume fraction of TiB2 and Al2O3 in PCBN-2A was assumed to be identical (5 vol.%). Only the nominal phases of the tool materials have been considered in the calculations, consequently WC, W2C, Co and CoCx have been ignored for the three super-hard tool materials. The results shown in Fig. 12 demonstrate that the WC–Co and the PCD are less stable in comparison to the PCBN materials, in good agreement with the results of the interaction couples investigation. WC–Co shows to be slightly less stable than PCD, in good agreement with the fact that WC–Co reacts with iron at 700 °C, while PCD shows a clear reactivity from temperatures of 900 °C upwards. With respect to the PCBN materials, 2A shows to be less stable with iron than 2B, in good agreement with the interaction couples investigation. The similar rankings derived from the machining tests and the interaction-couple experiments strongly support the hypothesis that the main wear mechanism during machining is chemical in nature. 5. Conclusions The interaction couples investigation demonstrated that the chemical compatibility of these materials with iron can be ranked as WC–Co b PCD b PCBN-2A b PCBN-2B. The agreement with the equilibrium solubility calculations was excellent. The results of a series of turning tests with the same work and tool materials correlate well with the results of the interaction couples and the solubility calculations. It is therefore concluded that under the machining conditions employed, the main wear mechanism operating is chemical in nature. Acknowledgements
The relative partial molar excess Gibbs energy of solution of element A in iron can be calculated from the solubility limit and the formation energy of the phase in equilibrium. f DGXS A ¼ DGFex Ay −RT lnNA
ð3Þ
The equilibrium solubility of the phases can then be calculated according to Eq. (1). The molar equilibrium solubility (mol/mol solution) of the individual phases can be converted into a volumetric solubility (cm3/mol solution) by means of the molar volume of the phase, calculated from the density and the molar weight. The solubility of a multi-phase material can be calculated from the volume fractions and equilibrium solubility of the constituent phases, with, Vfi, the volume fraction of phase i in the multi-phase material and Ci, the equilibrium solubility of phase i (in cm3 /mol solution). Cmultiphase
material
¼
N X
Vf i Ci
ð4Þ
i¼1
Fig. 11 shows the equilibrium solubility of all the different phases present in the tool materials investigated and calculated using Eq. (1). From this figure, it is clear that the most stable phase is Al2O3 and the least stable is Co. Fig. 12 shows the
The authors wish to thank John Barry from Element Six for the supply of the tool materials, carrying out the machining tests and for his constructive comments and suggestions to improve this manuscript, Mr. Olof Anderson from Höganäs AB for the supply of the workpiece materials and the European Commission for the financial support of this work through the PMMACH Growth project (Contract No G1RD-CT2002-00687). References [1] M.W. Cook, P.K. Bossom, Int. J. Refract. Met. Hard Mater. 18 (2000) 147. [2] P.J. Heath, J. Mater. Process. Technol. 116 (2001) 31. [3] Pretorious, Bowler, Toorney, Proceedings of the 1st International Industrial Diamond Conference, 20–21 October 2005, Barcelona (Spain), 2005, Diamond at Work. [4] N. Narutaki, Y. Yamane, Ann. CORP 28 (1) (1979) 23. [5] W. König, A. Neises, Wear 162–164 (1993) 12. [6] M. Zimmerman, M. Lahres, D.V. Viens, B.L. Laube, Wear 209 (1997) 241. [7] J. Barry, G. Byrne, Wear 247 (2001) 152. [8] J.A. Arsecularatne, L.C. Zhang, C. Montross, Mach. Tools Manufact. xx (2005) 1. [9] B.M. Kramer, P.K. Judd, J. Vac. Sci. Technol. A3 (6) (1985) 2439. [10] B.M. Kramer, J. Vac. Sci. Technol. A4 (6) (1996) 2870. [11] J. Vleugels, O. Van der Biest, Key Eng. Mater. 138–140 (1998) 127. [12] J. Vleugels, O. Van der Biest, Wear 225–229 (April 1999) 285 (Part 1).
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The role of chemical wear in machining iron based materials by PCD and PCBN super-hard tool materials S. Giménez ⁎, O. Van der Biest, J. Vleugels Department of Metallurgy and Materials Engineering, Katholieke Universiteit Leuven, Kasteelpark Arenberg 44, B-3001 Heverlee, Belgium Received 15 March 2006; received in revised form 20 July 2006; accepted 22 August 2006 Available online 29 September 2006
Abstract The chemical compatibility of PCBN and PCD tool materials with iron has been evaluated by means of the static interaction diffusion-couples technique. Experiments were undertaken at different temperatures (700 °C, 900 °C, 1100 °C and 1300 °C) in order to establish the maximum temperature at which the tool and workpiece materials are chemically compatible. Computational equilibrium thermodynamics was used to calculate the chemical solubility of the different tool materials in iron and to identify the interaction phases formed. The agreement between the experimental results and the thermodynamic calculations was excellent. Machining tests were performed in order to assess the relative importance of the chemical wear on the overall rate of tool wear. The results of these machining tests agreed perfectly with the chemical compatibility study, thereby indicating that under certain machining conditions, chemical wear is the main wear mechanism for these materials in machining iron based materials. © 2006 Elsevier B.V. All rights reserved. Keywords: Super-hard materials; Cutting tools; Wear; Thermal stability
1. Introduction The trends in the metal cutting industry are driven by the needs of the manufacturers to continually improve performance, reduce costs and increasingly, comply with the environmental legislation [1]. Additionally, the introduction of new workpiece materials encourages the development of new tool materials and new shaping strategies. Polycrystalline diamond (PCD) and polycrystalline boron nitride (PCBN) represent the two existing most outstanding tool materials due to their combination of extremely high hardness and abrasive wear resistance, high thermal conductivity and thermal stability but with moderate toughness. PCD is normally used for machining of abrasive non-ferrous materials. One example is metal matrix composites (MMCs) reinforced with second-phase particles, whiskers, fibers, wires or filaments. These materials are increasingly used in the automotive and aerospace industry replacing steel components [2]. The use of PCD is not recommended for
⁎ Corresponding author. CEIT, P Manuel Lardizabal 15, 20018 San Sebastián, Spain. Tel.: +34 943 212800; fax: +34 943 213076. E-mail address: [email protected] (S. Giménez). 0925-9635/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2006.08.017
ferrous alloys since the chemical compatibility of the tool and workpiece materials is very poor, leading to high chemical wear —one exception being the machining of certain classes of cast iron with cutting speeds in the range of 200–500 m/min [3]. PCBN tools are primarily used in the machining of hard ferrous materials (case-hardened steels, high speed tool steels, die steels, bearing steels, white cast iron and alloy cast irons) due, it is generally believed to their high chemical compatibility with iron. The scientific literature on the wear mechanisms of PCBN in the machining of hard ferrous alloys shows some contradictory conclusions. While some investigations indicate that PCBN is totally inert with iron during machining of hard ferrous alloys [4], other authors propose that chemical wear is an important mechanism. König and Neises [5] remarked on the importance of the chemical composition of the binder and its percentage in the composite for thermal stability. Zimmermann et al. [6] concluded that during hard turning of case-hardened steel, a tribochemical mechanism (dissolution or diffusion of CBN into the flowing chip) is responsible for wear in the crater region and a chemical mechanism is responsible for the flank wear. More recently, Barry and Byrne [7] and Arsecularatne et al. [8] also identified chemical wear as the dominant wear mechanism during hard machining of steel.
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Table 1 Tool materials and nominal binder contents provided by the manufacturer Reference
Commercial name
Abrasive
Binder phases (vol.%)
1 2A 2B
CTM302 DBC50 DBW85
PCD PCBN PCBN
Co (10) TiC (40), Al2O3 and TiB2 (10) WCoB (15)
A simple procedure to evaluate the possibility of chemical wear is static interaction couple experiments. Despite machining being a dynamic situation where thermodynamic equilibrium does not apply, the information provided by the interaction couple experiments reveals the potential for chemical interaction to take place during a particular machining process. A thermodynamic model for chemical wear was developed by Kramer and Judd [9] and Kramer [10], and successfully applied to the computational design and wear resistance prediction of coatings. Previous work at K.U. Leuven has demonstrated the validity of this approach to understand the wear mechanisms (mainly chemical in nature) of ceramic tools (Si3N4-based and SiAlON-based) during the machining of steel [11–13]. In the present work, the chemical compatibility of WC–Co, PCD and PCBN tool materials with iron has been assessed by means of the static interaction couple method. Computational equilibrium thermodynamics has been used to calculate the chemical solubility of the different cutting tool materials in iron and to identify the interaction phases formed. Finally, machining tests have been carried out to confirm the findings of the theoretical study.
All the tool materials were provided in the form of triangular bi-layer pieces with the super-hard composite layer (approximately 0.5 mm thick) supported on a WC–Co (approximately10 vol.% Co and 0.8 mm thick) substrate. The average grain size of the CBN phase is approximately 2 μm for both PCBN materials. The PCD grade possesses a multimodal distribution of diamond particle size ranging from 2 to 30 μm. The grain size distribution is reported to result in an increased packing density and therefore greater abrasion resistance, chip resistance and higher quality cutting edges [14]. As workpiece materials, heat treated powder compacts of Somaloy 500™ + 0.6 wt.% Kenolube (a soft magnetic composite material) provided by Höganäs AB (Höganäs, Sweden) were
2. Materials and experimental techniques A polycrystalline diamond (PCD) cutting tool material, (grade 1) and two different polycrystalline cubic boron nitride (PCBN) tool materials (grades 2A and 2B respectively), were sourced from Element Six (Table 1). The number used in the commercial code for the PCBN grades (‘50’ and ‘85’ for grade 2A and 2B respectively) refers to the approximate volume fraction of the super-hard phase. For the PCD material, the volume fraction of diamond is approximately 90 vol.%. The binder phases and their nominal volume percentages are indicated in Table 1 for all the tool materials. Table 2 Interaction couples selected for the investigation Identification
Tool material
T (°C)
1 2 3 4 5 6 7 8 9 10 11 12
1 2A 2B 1 2A 2B 1 2A 2B 1 2A 2B
700 700 700 900 900 900 1100 1100 1100 1300 1300 1300
Fig. 1. Tool materials supplied by E6, A) 1, B) 2A and C) 2B.
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used. For the purposes of the present work, the material can be considered as pure iron. The specimens were pressed into prismatic bars 50 × 10 × 5 mm3 with densities up to 7.40 g/cm3 and then heat treated in air at 500 °C for 30 min. From the heat treated bars, 10 × 5 × 5 mm3 specimens were cut for the inter-
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action couples investigation. Microstructural and mechanical characterisation of this material has been reported elsewhere [15]. X-ray diffraction (XRD) was also carried out for crystallographic characterisation of the as-received material; a 3003 T/T
Fig. 2. XRD diffractograms of the as-received tool materials, A) 1, B) 2A and C) 2B.
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X-ray diffractometer (Seifert, Ahrensburg, Germany) was employed using CuKα radiation. The diffraction scans were carried out in the interval 2theta 20–90°, with a step size of 0.02° and a dwell time of 4 s/step. The chemical reactivity of a number of tool–workpiece combinations was studied by assessing the extent of interdiffusion of species between polished slices of the tool and workpiece materials which are pressed together under a low load (2.5 MPa) for 1 h in vacuum (b 0.1 Pa) and at different temperatures (700 °C, 900 °C, 1100 °C and 1300 °C). The heating and cooling rates were 50 K/min. A ‘W100/150-2200-
50LAX’ hot press device, (KCE Sondermaschinen, Rodental, Germany) was used. The polycrystalline super-hard composites (PCD or PCBN) and carbide substrates (WC–Co) were polished and used in the interaction couple investigation. The sample preparation included mounting the specimens on an epoxybased resin (EPOFIX 4000), grinding with diamond (15 and 6 μm oil suspension lubrication) and polishing with diamond paste (3 and 1 μm). Final polishing was carried out with a SiO2 suspension (0.5 μm). For the tests at 1300 °C, the WC–Co substrate was removed as preliminary tests revealed a pronounced influence of the WC–Co on the interaction between
Fig. 3. Interaction couples for material 1, A) 700 °C, B) 900 °C, C) 1100 °C, D) 1100 °C, detail of the interface, E) EPMA compositional profile at the interface.
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the workpiece and super-hard composite layers. After cooling, the interaction couples were cross-sectioned, polished, etched with 2 vol.% Nital and investigated by means of scanning electron microscopy (SEM). An XL30-FEG microscope equipped with an EDAX-Dx-4i system was used for qualitative microchemical analysis (Philips, Eindhoven, The Netherlands). Electron probe microanalysis (EPMA) was used for chemical quantitative analysis; a Superprobe 733 JEOL equipment (Tokyo, Japan) fitted with a Noran EDS. The quantitative algorithm employed was calibrated for the metallic elements. The total amount of B, C and N was indirectly calculated as the difference between the sum of the metallic elements and 100%. The list of interaction couples investigated is given in Table 2. Turning tests were undertaken over a range of cutting speeds with a feed and depth of cut of 0.1 mm and 0.2 mm respectively. For the two PCBN grades, the tool geometry was SNMN 090308 T0220 (a geometry typically for PCBN tools and comprising a 200 μm × 20° chamfer). The PCD inserts were used with a sharp, un-chamfered, edge. The cutting speeds used for the PCBN materials were: 200 m/min, 400 m/min, 600 m/min and 800 m/min. The cutting speeds used for the PCD material were 35 m/min, 45 m/min, 50 m/min and 100 m/min. Flank wear measurements were taken at various intervals over the duration of each test and at least two edges of each grade were tested at each cutting speed. The results are summarized below
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in the form of a Taylor tool life curve, where the tool ‘life’ criteria is taken as a flank wear land of 300 μm. 3. Results The microstructures of the as-received tool materials are shown in Fig. 1. Fig. 1A illustrates the microstructure of the PCD grade (1). The diamond phase constitutes the dark-contrasted areas of the microstructure (∼90 vol.%) indicated as ‘A’. The lighter cavity-like regions (B) are the remnants of the cobalt ‘binder’ which was partially removed during polishing (∼10 vol.%). A bright phase (C) is also observed and was identified as WC and W2C by XRD (Fig. 2A). CoCx is also identified by XRD (Fig. 2A). The microstructure of the 2A PCBN grade is shown in Fig. 1B, (note the change of scale with respect to Fig. 1A). The dark-contrasted CBN grains (A) (approximately 50 vol.%, average size 2 μm) appear embedded in a complex binder system, mainly constituted by TiC (B). Additionally, Al2O3 and TiB2 with very similar contrast have been identified (D), in good agreement with the XRD information (Fig. 2B). Identically as for the PCD, the presence of WC, W2C (C) and Co is also observed and identified by XRD. The 2B PCBN grade (Fig. 1C) is constituted by the dark-contrasted CBN grains (A) embedded in the WCoB binder (B). The presence of WC (C) and CoCx is also observed and identified by XRD (Fig. 2C). It is
Fig. 4. Interaction couples for material 2A, A) 1100 °C, B) 1300 °C overview, C) 1300 °C, detail of the interface, D) EDS compositional profile at 1300 °C.
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reasonable to conclude that the phases WC, W2C, Co and CoCx, appearing in the XRD diffractograms and SEM micrographs are related to the WC–Co substrate. 3.1. Static interaction couples 3.1.1. PCD The interaction couples 1, 4, 7 and 10 were devoted to the study of the chemical interaction of the PCD tool material grade (1) and Fe between 700 °C and 1300 °C (Table 2). At 700 °C, a gap filled with the mounting resin appears in the interface as shown in Fig. 3A. This is an evidence of the absence of interaction or limited reaction since the bond strength is not sufficient to overcome the thermal stresses between the dissimilar materials during cooling. At 900 °C, evidence of interaction is provided by the presence of pearlite at the iron side of the interface (Fig. 3B). This feature is more pronounced after testing at 1100 °C (Fig. 3C). At higher magnification, Fig. 3D, a 20 μm deep layer with similar contrast as iron but without the pearlitic structure is observed at the interface. The compositional profiles revealed this phase to be a Fe–Co rich constituent, (Fig. 3E). Some white contrasted W-rich particles were observed at the interface. A peak for W is observed in the compositional profile accounting for the presence of these particles (Fig. 3E). At 1300 °C, the tool material totally dissolved in the iron. The
morphological integrity of the interaction couple was totally lost and no further characterisation was possible. 3.1.2. PCBN-2A Interaction couples 2, 5, 8 and 11 were devoted to study the chemical interactions between the PCBN-2A tool material and Fe (Table 2). No interaction was detected below 1300 °C. Fig. 4A shows the tool–workpiece interface after testing at 1100 °C, demonstrating the absence of any interaction product. Conversely; a high degree of interaction is observed after the diffusion experiment at 1300 °C. An overview of the interaction couple is shown in Fig. 4B. Evidence of penetration of iron into the tool is provided by the light contrasted region of the tool adjacent to the interface (Fig. 4B). The iron side of the interface appears decorated by a Fe–C rich structure (Fig. 4C and D), which has the appearance of originating from a eutectic reaction. The compositional profile (Fig. 4D) shows diffusion of C into the iron and penetration of Fe into the tool material (to a depth of about150 μm). Al and Ti do not diffuse into Fe, although the Ti content of the tool increases near the interface (Fig. 4D). 3.1.3. PCBN-2B The interaction couples 3, 6, 9 and 12 were used to study the interaction of the PCBN-2B material and Fe (Table 2). As previously observed for the 2A material, no reaction was
Fig. 5. Interaction couples for material 2B, A) evidence of absence of interaction at 1100 °C, B) overview of the interaction couple after testing at 1300 °C, C) Detail of the interface at 1300 °C, D) compositional profile at 1300 °C.
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Fig. 6. Interaction between Fe and WC–Co, A) 700 °C, B) 900 °C, C) 1100 °C, D) compositional profile at 1100 °C.
observed at temperatures below 1300 °C. At 1100 °C, the tool– workpiece interface exhibits a continuous gap as a consequence of its inert behaviour (Fig. 5A). At 1300 °C, evidence of interaction is provided by the rounding of the corners of the tool material, as can be seen in the overview in Fig. 5B. At higher magnification, Fig. 5C, it is observed that the iron side of the interface is decorated with an interaction product containing Fe and C and with a structure similar to pearlite. The compositional Table 3 Summary of the results obtained from the static interaction couples analysed Identification
Abrasive
Temperature (°C)
Tool/workpiece
WC–Co/ workpiece
1 2 3 4 5 6 7 8 9 10 11 12
1 2A 2B 1 2A 2B 1 2A 2B 1 2A 2B
700
0 0 0 • 0 0 •• 0 0 ••• •• •
• • • •• •• •• ••• ••• ••• N/E
900
1100
1300
The symbols “•” indicate the degree of interaction; ‘0’ denotes no interaction; ‘N/E’ not evaluated.
profile (Fig. 5D) does not identify Fe penetration into the tool material or Co or W penetration into iron. 3.1.4. WC–Co In all the interaction couples, the interaction of WC–Co with iron was also assessed. Fig. 6A shows a detail of the interface between WC–Co and Fe at 700 °C. A thin layer X (depth b 1 μm) is formed between the WC grains and iron evidencing that interaction takes place at temperatures ≤700 °C. The qualitative EDS analysis evidenced the presence of Fe and Co in this layer. At 900 °C (Fig. 6B), a phase with intermediate contrast between Fe and WC (Y) is observed surrounding each individual WC
Fig. 7. Taylor curve for the three materials investigated.
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tionship between tool life and cutting speed is clearly observed. It is clear that the ranking of the materials is similar to that concerning the chemical compatibility. At very low cutting speeds (b 25 m/min), it appears as though PCD will outperform both PCBN grades, whereas at more reasonable speeds (N50 m/min), the PCBN grades exhibit considerably greater tool lives. As the temperature at the cutting edge of the tool increases with cutting speed, the ‘cutting speed’ axis in Fig. 7 may be considered representative of cutting temperature. At all reasonable speeds (and therefore, resulting temperatures) employed in the study, the tool materials rank in exactly the same manner as the ‘inertness’ ranking derived from the interaction-couple experiments. It is possible to surmise that below 50 m/min, chemical wear is but one component of the wear process. 4. Discussion 4.1. Identification of the interaction phases In order to better understand the chemical compatibility of the tool and workpiece materials, experimentally characterised using interaction-couples, computer equilibrium thermodynamic calculations were carried out to assist the identification of the interaction phases experimentally observed. The Thermo-Calc software and the SSOL databank [16] were used.
Fig. 8. A) Evolution of the weight fraction of the stable phases of the Fe–Co– W–C system with Fe content (x(W)–x(C) = 0). B) Evolution of the composition of the fcc (Fe,Co) phase with the mole fraction of iron. The substitutional nature of Fe and Co is evident.
grain adjacent to the interface. Additionally, the corners of the WC particles appear rounded suggesting dissolution of WC in this phase. The qualitative EDS analysis revealed the presence of W, Co, Fe and C. At 1100 °C, a similar interaction phase is observed to form a continuous band (about 5 μm in thickness) along the interface and with a gradient of Co and Fe as shown in the compositional profile (Fig. 6D). A thinner band of a Fe–Co rich phase (X) lies between this phase and the iron. This phase X is similar to that observed at 700 °C, Fig. 6A. Table 3 summarizes the experimental observations taken from the interaction couples study. These results clearly indicate that the WC–Co substrate dissolves in “pure” iron at temperatures ≤ 700 °C. The PCD material is not chemically stable in contact with iron from temperatures ≤ 900 °C onwards, whereas the PCBN-composites are more chemically compatible with iron, even up to 1100 °C for 1 h. At 1300 °C, extensive interaction is observed for 2A and clear, but limited, interaction takes place for 2B. Therefore, the chemical compatibility of the tool materials with iron can be ranked as: WC–Co b 1 b 2A b 2B. 3.2. Machining tests The results of the machining tests are summarized in the Taylor tool life chart in Fig. 7 — ‘tool life’ meaning the time to reach a flank wear land of 300 μm. In this figure, the rela-
Fig. 9. Evolution of the weight fraction of the stable phases of the Fe–Co–W–C system with Fe content (x(W)–x(C) = 0.1). B) Evolution of the composition of the M6C phase with the mole fraction of iron. The substitutional nature of Fe and Co is evident.
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Fig. 10. PCBN-2B, micrograph evidencing the absence of WC particles at the interface.
4.2. PCD and WC–Co The results of the interaction studies on PCD/Fe and WC–Co/ Fe can be explained through the thermodynamics of the Fe–W– Co–C system. The Fe–Co rich region adjacent to the PCD–Fe interface (Fig. 3D) and the Fe–Co rich region (X) at the WC–Co/ Fe interface (Fig. 6A and C) exhibit similar features. Consequently, it is believed that it is the same phase. Fig. 8A shows the evolution of the stable phases in the Fe–W–Co–C system as a function of Fe content at 1100 °C. The initial boundary conditions employed in order to satisfy the Gibb's phase rule were T = 1373 K, P = 105 Pa, N = 1, x(Co) = 0.1, x(Fe) = 0.5, x(W)–x (C) = 0, where T, P, N and x refer to absolute temperature, pressure, number of moles of the system and molar fraction respectively. The condition x(W)–x(C) = 0 specifies that the W and C in the system are coming exclusively from the WC phase. At the higher Fe contents in Fig. 8A, the most stable phase is the fcc (Fe,Co) solid solution. The equilibrium composition of this phase with the iron content is shown in Fig. 8B. The compositional profile measured, shown in Fig. 3E, shows a ‘qualitatively’ identical trend evidencing the substitutional role of Fe and Co in this phase. With the atmospheric conditions used in the high temperature interaction-couple tests, a certain decarburisation of the WC phase can take place. This situation has been simulated in the calculations through the boundary condition x(W)–x(C) = 0.1. The evolution of the stable phases with the Fe content are shown in Fig. 9A. In this case, three new stable phases appear
443
Fig. 12. Solubility calculations of all the different tool materials in iron according to Eq. (4).
(M6C, M12C and the μ-phase). Quantitatively, the most important one is M6C; its composition is shown in Fig. 9B. Therefore, it seems reasonable to identify the phase Y in Fig. 6C as M6C. There are multiple examples in the bibliography about the transformation of WC → M6C or (η-phase) when decarburisation takes place [17,18]. The presence of the W-rich particles at the iron side of the interface embedded in the fcc (Fe,Co) phase (Fig. 3D) is not well understood. From a thermodynamic point of view, there is no support for this observation, and it is believed that W-rich particles, originally in the tool material have been pulled out during the grinding and polishing procedure and adhered to the iron. 4.2.1. PCBN The chemical interaction between the PCBN-2A and iron is characterised by the formation of a Fe–C rich band which should originate from a eutectic reaction (Fig. 4C). Diffusion of C from the tool material to the workpiece is evidenced by EDS (Fig. 4D), with a maximum molar fraction of about 20% at 5 μm from the interface. At 1153 °C, a eutectic reaction takes place forming austenite and cementite, accounting for the interaction phase formed in the experiments. Previous work on the wear behaviour of CBN–TiC composites concluded that the superior wear resistance of these materials compared to grades with higher CBN content was based on the higher chemical compatibility of TiC [7]. The experimental results obtained in the present work are in contradiction to this thesis. On the other hand, the interaction phase formed at the interface between the PCBN-2B and Fe cannot be explained by the nominal composition of the tool material. C (which is not a constituent element of any of the nominal phases) was clearly identified by EDS (Fig. 5D) in the interaction region showed in Fig. 5C. The only phase containing C could be WC, which was identified by SEM and XRD (Figs. 1C and 2C respectively). Additionally, reinforcing this argument, no WC particles are observed at the interface as can be seen in Fig. 10. 4.3. Calculation of equilibrium solubility of tool materials in pure iron
Fig. 11. Solubility calculations of all the different phases contained in the tool materials studied in iron.
The chemical compatibility of the tool materials with iron can also be studied from a thermodynamic point of view. The
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equilibrium solubility of a multi-phase material in pure iron can be calculated from the equilibrium solubility of the constituent phases. The equilibrium calculation procedure has been previously explained for the dissolution of a hypothetical AxBy phase in pure iron, [9,10,12,13]. The equilibrium solubility C (mol phase / mol solution) of phase AxBy in iron can then be calculated as: XS CAx By ¼ exp½DðGfAx By −xDGXS A −yDGB −RT
ðxlnx þ ylnyÞÞ=RT ðx þ yÞ
ð1Þ
where ΔGf is the Gibbs energy of formation of the AxBy phase, xs ΔGA , ΔGBxs the relative partial molar excess Gibbs energies of solution of component A and B in Fe respectively, R is the universal gas constant and T, the absolute temperature (K). The formation energy (ΔGf) of the phases was taken from the thermodynamic database of Barin [19], except for the WCoB phase, which was taken from Ref. [20]. The evolution of ΔGf with temperature for this phase was assumed to be similar to that of CoB (available in Ref. [19]). The relative partial molar excess Gibbs energy of solution of the different elements in iron was obtained using the Thermo-Calc software [16] and the SSOL database. The ION database was used for the thermodynamic description of oxygen. At the solubility limit of element A in iron, the solution will be in equilibrium with a second-phase FexAy. Therefore, the Gibbs energy of solution of element A at the solubility limit, NA, must equal the Gibbs energy of formation of the secondphase per mole atom of element A. f XS DGm A ¼ DGA þ RT lnNA ¼ DGFex Ay
ð2Þ
equilibrium solubilities of the multi-phase tool materials studied calculated by Eq. (4). The volume fraction of TiB2 and Al2O3 in PCBN-2A was assumed to be identical (5 vol.%). Only the nominal phases of the tool materials have been considered in the calculations, consequently WC, W2C, Co and CoCx have been ignored for the three super-hard tool materials. The results shown in Fig. 12 demonstrate that the WC–Co and the PCD are less stable in comparison to the PCBN materials, in good agreement with the results of the interaction couples investigation. WC–Co shows to be slightly less stable than PCD, in good agreement with the fact that WC–Co reacts with iron at 700 °C, while PCD shows a clear reactivity from temperatures of 900 °C upwards. With respect to the PCBN materials, 2A shows to be less stable with iron than 2B, in good agreement with the interaction couples investigation. The similar rankings derived from the machining tests and the interaction-couple experiments strongly support the hypothesis that the main wear mechanism during machining is chemical in nature. 5. Conclusions The interaction couples investigation demonstrated that the chemical compatibility of these materials with iron can be ranked as WC–Co b PCD b PCBN-2A b PCBN-2B. The agreement with the equilibrium solubility calculations was excellent. The results of a series of turning tests with the same work and tool materials correlate well with the results of the interaction couples and the solubility calculations. It is therefore concluded that under the machining conditions employed, the main wear mechanism operating is chemical in nature. Acknowledgements
The relative partial molar excess Gibbs energy of solution of element A in iron can be calculated from the solubility limit and the formation energy of the phase in equilibrium. f DGXS A ¼ DGFex Ay −RT lnNA
ð3Þ
The equilibrium solubility of the phases can then be calculated according to Eq. (1). The molar equilibrium solubility (mol/mol solution) of the individual phases can be converted into a volumetric solubility (cm3/mol solution) by means of the molar volume of the phase, calculated from the density and the molar weight. The solubility of a multi-phase material can be calculated from the volume fractions and equilibrium solubility of the constituent phases, with, Vfi, the volume fraction of phase i in the multi-phase material and Ci, the equilibrium solubility of phase i (in cm3 /mol solution). Cmultiphase
material
¼
N X
Vf i Ci
ð4Þ
i¼1
Fig. 11 shows the equilibrium solubility of all the different phases present in the tool materials investigated and calculated using Eq. (1). From this figure, it is clear that the most stable phase is Al2O3 and the least stable is Co. Fig. 12 shows the
The authors wish to thank John Barry from Element Six for the supply of the tool materials, carrying out the machining tests and for his constructive comments and suggestions to improve this manuscript, Mr. Olof Anderson from Höganäs AB for the supply of the workpiece materials and the European Commission for the financial support of this work through the PMMACH Growth project (Contract No G1RD-CT2002-00687). References [1] M.W. Cook, P.K. Bossom, Int. J. Refract. Met. Hard Mater. 18 (2000) 147. [2] P.J. Heath, J. Mater. Process. Technol. 116 (2001) 31. [3] Pretorious, Bowler, Toorney, Proceedings of the 1st International Industrial Diamond Conference, 20–21 October 2005, Barcelona (Spain), 2005, Diamond at Work. [4] N. Narutaki, Y. Yamane, Ann. CORP 28 (1) (1979) 23. [5] W. König, A. Neises, Wear 162–164 (1993) 12. [6] M. Zimmerman, M. Lahres, D.V. Viens, B.L. Laube, Wear 209 (1997) 241. [7] J. Barry, G. Byrne, Wear 247 (2001) 152. [8] J.A. Arsecularatne, L.C. Zhang, C. Montross, Mach. Tools Manufact. xx (2005) 1. [9] B.M. Kramer, P.K. Judd, J. Vac. Sci. Technol. A3 (6) (1985) 2439. [10] B.M. Kramer, J. Vac. Sci. Technol. A4 (6) (1996) 2870. [11] J. Vleugels, O. Van der Biest, Key Eng. Mater. 138–140 (1998) 127. [12] J. Vleugels, O. Van der Biest, Wear 225–229 (April 1999) 285 (Part 1).
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