Coated cutting tool

Abstract

A coated cutting tool includes a substrate and a coating. The coating has an inner layer of 4-14 μm thick Ti.sub.1-xAl.sub.xN, an intermediate layer of 0.05-1 μm TiCN and at least one outer layer of 1-9 μm α-Al.sub.2O.sub.3. The α-Al.sub.2O.sub.3 layer exhibits an X-ray diffraction pattern, as measured using CuKα radiation and theta-2theta scan. A texture coefficient TC(hkl) is defined according to Harris formula, wherein the (hkl) reflections used are (0 2 4), (1 1 6), (3 0 0) and (0 0 12), I(hkl)=measured intensity (peak intensity) of the (hkl) reflection, I0(hkl)=standard intensity according to ICDD's PDF-card No. 00-042-1468, n=number of reflections used in the calculation, and 3<TC(0 0 12)<4.

Claims

1. A coated cutting tool comprising: a substrate; and a coating, wherein the coating has an inner layer of 4-14 μm thick Ti.sub.1-xAl.sub.xN, an intermediate layer of 0.05-1 μm TiCN and at least one outer layer of 1-9 μm α-Al.sub.2O.sub.3, wherein the TiCN layer is in epitaxial relation to the Ti.sub.1-xAl.sub.xN layer, and wherein said α-Al.sub.2O.sub.3 layer exhibits an X-ray diffraction pattern, as measured using CuKα radiation and theta-2theta scan, and a texture coefficient TC(hkl) being defined according to Harris formula $\mathrm{TC}\left(\mathrm{hkl}\right)={\frac{I\left(\mathrm{hkl}\right)}{I0\left(\mathrm{hkl}\right)}\left[\frac{1}{n}\underset{i=0}{\overset{n}{.Math.}}\frac{I\left(\mathrm{hkl}\right)}{I0\left(\mathrm{hkl}\right)}\right]}^{-1}$ wherein the (hkl) reflections used are (0 2 4), (1 1 6), (3 0 0) and (0 0 12), I(hkl)=measured intensity (peak intensity) of the (hkl) reflection, 10(hkl)=standard intensity according to ICDD's PDF-card No. 00-042-1468, n=number of reflections used in the calculation, wherein 3<TC(0 0 12)<4.

2. The tool according to claim 1, wherein the thickness of the intermediate layer of TiCN is 0.05-0.3 μm.

3. The tool according to claim 1, wherein the TC(111) of the intermediate layer of TiCN is >3, and wherein the texture coefficient TC (111) is defined as: $\mathrm{TC}\left(111\right)={\frac{I\left(111\right)}{I_0\left(111\right)}\left[\frac{1}{n}\underset{n=1}{\overset{n}{.Math.}}\frac{I\left(\mathrm{hkl}\right)}{I_0\left(\mathrm{hkl}\right)}\right]}^{-1}$ wherein I(hkl) are the intensities of the diffraction reflections measured by X-ray diffraction, I.sub.0(hkl) are the standard intensities of the diffraction reflections according to the pdf card 00-042-1489 n is the number of reflections used in the calculation, and for the calculation of TC(111) the reflections (111), (200), (220) and (311) are used.

4. The tool according to claim 1, wherein the Σ3 grain boundary length to the total grain boundary length in the α-Al.sub.2O.sub.3 layer is >30%.

5. The tool according to claim 1, wherein the Ti.sub.1-xAl.sub.xN layer has at least 90 vol-% face-centered cubic (fcc) crystal structure.

6. The tool according to claim 1, wherein the Ti.sub.1-xAl.sub.xN layer has a columnar microstructure.

7. The tool according to claim 1, wherein precipitations of Ti.sub.1-yAl.sub.yN are present at grain boundaries of the Ti.sub.1-xAl.sub.xN crystallites, said precipitations having a higher Al content than inside crystallites and comprise AN with hexagonal crystal structure (hcp) wherein y>x.

8. The tool according to claim 1, wherein the Ti.sub.1-xAl.sub.xN layer has stoichiometric coefficients of 0.60≤x≤0.90.

9. The tool according to claim 1, wherein the Ti.sub.1-xAl.sub.xN layer has a orientation of the crystal growth with respect to a crystallographic {111} plane, characterized by a texture coefficient TC (111) >3, wherein the texture coefficient TC (111) is defined as follows: $\mathrm{TC}\left(111\right)={\frac{I\left(111\right)}{I_0\left(111\right)}\left[\frac{1}{n}\underset{n=1}{\overset{n}{.Math.}}\frac{I\left(\mathrm{hkl}\right)}{I_0\left(\mathrm{hkl}\right)}\right]}^{-1}$ wherein I(hkl) are the intensities of the diffraction reflections measured by X-ray diffraction, I.sub.0(hkl) are the standard intensities of the diffraction reflections according to the pdf card 00-046-1200, n is the number of reflections used in the calculation, and for the calculation of TC(111) the reflections (111), (200), (220) and (311) are used.

10. The tool according to claim 1, wherein the thickness of the TiCN layer is 0.1-0.3 μm.

11. The tool according to claim 1, wherein an average aspect ratio of the TiCN grains of the TiCN layer is ≤1.

12. The tool according to claim 1, wherein a coating thickness relation of the Ti.sub.1-xAl.sub.xN layer, t(Ti.sub.1-xAl.sub.xN), in relation to the thickness of the α-Al.sub.2O.sub.3 layer, t(α), is t(Ti.sub.1-xAl.sub.xN):t(α) from 2:1 to 3:1.

13. The tool according to claim 1, wherein the coating includes an innermost layer of TiN, a thickness of the innermost layer of TiN being 0.1-2 μm.

14. The tool according to claim 1, wherein the substrate is cemented carbide.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 shows a scanning electron microscope (SEM) image of through cut of the sample A1 (invention).

(2) FIG. 2 shows a scanning electron microscope (SEM) image of a through cut of sample A1 (invention).

EXAMPLES

(3) Deposition Process

(4) In the present examples substrates of cemented carbide with the cutting inserts geometry CNMA120408 were used. The cemented carbide composition was 86.1 wt % WC, 5.5 wt % Co, 8.0 wt % (NbC, TaC and TiC) and 0.4 wt % other carbides. The substrates have a binder phase enriched surface zone of about 20 μm.

(5) The substrates were CVD coated in a CVD coating chamber of type Bernex BPX325S with a reactor height of 1250 mm, a reactor diameter of 325 mm and a volume of charge of 40 litres.

(6) The example coatings according to the invention and the reference have been obtained using the herein described equipment and the process conditions as given in the following tables 1a, 1b, 2a and 2b. However, it is well known in the art that the process conditions to produce CVD coatings may to a certain degree vary depending on the equipment used. It is therefore within the purview of the person skilled in the art to modify the deposition conditions and/or the equipment used to achieve the coating properties of the present invention.

(7) Sample A1 (Invention):

(8) The substrates were deposited with a coating of the following layers starting from the surface of the substrate: 0.25 μm TiN, 9.0 μm T.sub.1-xAl.sub.xN, 0.075 μm TiN, 0.3 μm TiCN, 3.2 μm α-Al.sub.2O.sub.3. The deposition conditions are shown in Table 1a1 and 1b1.

(9) The basic TiN layer was deposited at a temperature of 850° C. on the substrate. The reaction gases comprise TiCl.sub.4, N.sub.2, H.sub.2.

(10) The T.sub.1-xAl.sub.xN layer was deposited at a temperature of 725° C. The reaction gases comprise TiCl.sub.4, AlCl.sub.3, NH.sub.3, H.sub.2.

(11) The intermediated TiN layer is then deposited directly on top of the T.sub.1-xAl.sub.xN layer at a temperature of 725° C. The reaction gases comprise TiCl.sub.4, NH.sub.3, H.sub.2.

(12) The TiCN layer is then deposited directly on top of the intermediate TiN layer at a temperature of 850° C. The reaction gases comprise TiCl.sub.4, CH.sub.3CN, N.sub.2, H.sub.2.

(13) As a bonding layer the TiCN layer was oxidized in an oxidation step using CO, CO.sub.2 and H.sub.2.

(14) The α-Al.sub.2O.sub.3 layer is then deposited on top of the bonding layer at a temperature of 850° C. The reaction gases comprise AlCl.sub.3, CO.sub.2, H.sub.2, HCl, H.sub.2S, CO. The deposition of the α-Al.sub.2O.sub.3 layer was carried out in two deposition steps, wherein in the first step a nucleation layer is grown, and in the second step the α-Al.sub.2O.sub.3 layer is grown to the desired thickness.

(15) TABLE-US-00001 TABLE 1a1 Process parameters during deposition on Sample A1 Layer Temp [° C.] Pressure [mbar] Time [min] TiN 850 150 60 Ti.sub.1−xAl.sub.xN 725 8 90 TiN 725 8 3 TiCN 850 75 5 Oxidation 850 75 5 α-Al2O3 850 75 20 α-Al2O3 850 75 210

(16) TABLE-US-00002 TABLE 1b1 Reaction gases during deposition on Sample A1 TiCl.sub.4 N.sub.2 H.sub.2 CH.sub.3CN NH.sub.3 AlCl.sub.3 CO.sub.2 CO H.sub.2S HCl Layer [vol %] TiN 1.03 44.0 55.0 0 0 0 0 0 0 0 Ti.sub.1−xAl.sub.xN 0.019 0 99.4 0 0.41 0.17 0 0 0 0 TiN 0.15 0 99.1 0 0.72 0 0 0 0 0 TiCN 1.60 9.8 88.2 0.42 0 0 0 0 0 0 Oxidation 0 0 87.7 0 0 0 8.8 3.5 0 0 α-Al.sub.2O.sub.3 0 0 93.5 0 0 1.47 2.06 0 1.47 1.47 α-Al.sub.2O.sub.3 0 0 93.1 0 0 1.12 2.68 0.40 1.34 1.34
Sample A2 (Invention):

(17) The substrates were deposited with a coating of the following layers starting from the surface of the substrate: 0.5 μm TiN, 4.7 μm T.sub.1-xAl.sub.xN, 0.25 μm TiN, 0.6 μm TiCN, 3.0 μm α-Al.sub.2O.sub.3. The deposition conditions are shown in Table 1a2 and 1b2.

(18) The basic TiN layer was deposited at a temperature of 850° C. on the substrate. The reaction gases comprise TiCl.sub.4, N.sub.2, H.sub.2.

(19) The T.sub.1-xAl.sub.xN layer was deposited at a temperature of 700° C. The reaction gases comprise TiCl.sub.4, AlCl.sub.3, NH.sub.3, H.sub.2.

(20) The intermediated TiN layer is then deposited directly on top of the T.sub.1-xAl.sub.xN layer at a temperature of 700° C. The reaction gases comprise TiCl.sub.4, NH.sub.3, H.sub.2.

(21) The TiCN layer is then deposited directly on top of the intermediate TiN layer at a temperature of 850° C. The reaction gases comprise TiCl.sub.4, CH.sub.3CN, N.sub.2, H.sub.2.

(22) As a bonding layer the TiCN layer was oxidized in an oxidation step using CO, CO.sub.2 and H.sub.2.

(23) The α-Al.sub.2O.sub.3 layer is then deposited on top of the bonding layer at a temperature of 850° C. The reaction gases comprise AlCl.sub.3, CO.sub.2, H.sub.2, HCl, H.sub.2S, CO. The deposition of the α-Al.sub.2O.sub.3 layer was carried out in two deposition steps, wherein in the first step a nucleation layer is grown, and in the second step the α-Al.sub.2O.sub.3 layer is grown to the desired thickness.

(24) TABLE-US-00003 TABLE 1a2 Process parameters during deposition on Sample A2 Layer Temp [° C.] Pressure [mbar] Time [min] TiN 850 150 90 Ti.sub.1−xAl.sub.xN 700 7 75 TiN 700 7 10 TiCN 850 75 10 Oxidation 850 75 5 α-Al2O3 850 75 30 α-Al2O3 850 100 240

(25) TABLE-US-00004 TABLE 1b2 Reaction gases during deposition on Sample A2 TiCl.sub.4 N.sub.2 H.sub.2 CH.sub.3CN NH.sub.3 AlCl.sub.3 CO.sub.2 CO H.sub.2S HCl Layer [vol %] TiN 1.03 44.0 55.0 0 0 0 0 0 0 0 Ti.sub.1−xAl.sub.xN 0.019 0 99.5 0 0.35 0.17 0 0 0 0 TiN 0.20 0 98.8 0 1.0 0 0 0 0 0 TiCN 1.60 9.8 88.2 0.42 0 0 0 0 0 0 Oxidation 0 0 87.7 0 0 0 8.8 3.5 0 0 α-Al.sub.2O.sub.3 0 0 93.5 0 0 1.47 2.06 0 1.47 1.47 α-Al.sub.2O.sub.3 0 0 93.1 0 0 1.12 2.68 0.40 1.34 1.34
Sample B1 (Reference):

(26) Cemented carbide substrates were deposited with coating of the following layers starting from the surface of the substrate: 0.4 μm TiN, 7.8 μm TiCN, 1.3 μm TiAlCNO, 3.4 μm α-Al.sub.2O.sub.3. Both the deposition of the TiCN and the α-Al.sub.2O.sub.3 layer was carried out in two deposition steps, wherein in the first step a nucleation layer is grown, and in the second step the layer is grown to the desired thickness. The deposition conditions are shown in Table 2a1 and 2b1.

(27) TABLE-US-00005 TABLE 2a1 Process parameters during deposition on Sample B1 Layer Temp [° C.] Pressure [mbar] Time [min] TiN 920 160 45 TiCN 900 60 44 TiCN 900 90 110 TiAlCNO 1010 120 35 Oxidation 1010 65 5 α-Al.sub.2O.sub.3 1015 65 30 α-Al.sub.2O.sub.3 1015 65 180

(28) TABLE-US-00006 TABLE 2b1 Reaction gases during deposition on Sample B1 TiCl.sub.4 N.sub.2 H.sub.2 CH.sub.3CN AlCl.sub.3 CO.sub.2 CO H.sub.2S HCl CH.sub.4 Layer [vol %] TiN 1.08 39.1 59.8 0 0 0 0 0 0 0 TiCN 2.68 41.4 55.2 0.68 0 0 0 0 0 0 TiCN 1.65 12.8 85.5 0.55 0 0 0 0 0 0 TiAlCNO 0.84 2.76 91.8 0 0.74 0 1.11 0 0 2.76 Oxidation 0 0 98.5 0 0 0.71 0.79 0 0 0 α-Al.sub.2O.sub.3 0 0 90.9 0 2.57 2.97 1.58 0 1.98 0 α-Al.sub.2O.sub.3 0 0 91.0 0 2.38 3.97 0 0.32 2.38 0
Sample B2 (Reference):

(29) Cemented carbide substrates were deposited with coating of the following layers starting from the surface of the substrate: 0.3 μm TiN, 4.6 μm TiCN, <0.1 μm TiAlCNO, 2.4 μm α-Al.sub.2O.sub.3. The deposition conditions are shown in Table 2a2 and 2b2.

(30) TABLE-US-00007 TABLE 2a2 Process parameters during deposition on Sample B2 Layer Temp [° C.] Pressure [mbar] Time [min] TiN 800 600 90 TiCN 800 80 120 TiAlCNO 800 70 20 Oxidation 800 75 4 α-Al.sub.2O.sub.3 800 55 30 α-Al.sub.2O.sub.3 800 200 240

(31) TABLE-US-00008 TABLE 2b2 Reaction gases during deposition on Sample B2 TiCl.sub.4 N.sub.2 H.sub.2 CH.sub.3CN AlCl.sub.3 CO.sub.2 CO H.sub.2S HCl CH.sub.4 Layer [vol %] TiN 1.80 29.5 68.7 0 0 0 0 0 0 0 TiCN 2.63 32.2 64.4 0.83 0 0 0 0 0 0 TiAlCNO 2.59 26.4 64.5 0.57 0.66 0 3.97 0 1.32 0 Oxidation 0 31.8 57.1 0 0 3.17 7.94 0 0 0 α-Al.sub.2O.sub.3 0 14.6 80.7 0 1.17 2.05 0 0 1.46 0 α-Al.sub.2O.sub.3 0 0 95.7 0 1.02 2.16 0 0.38 0.76 0

(32) Cross-section SEM microphotographs were prepared from the coatings and cross sections of Sample A1 are shown in FIGS. 1 and 2. The Ti.sub.1-xAl.sub.xN layers of Sample A1 and Sample A2 are Ti.sub.0.15Al.sub.0.85N layers, i.e. x=0.85.

(33) Coating Analyzes and Results

(34) The coatings were analyzed in X-ray diffraction measurements.

(35) X-ray diffraction measurements were performed in a XRD3003 PTS diffractometer of GE Sensing and Inspection Technologies using CuKα-radiation. The X-ray tube was run in point focus at 40 kV and 40 mA. A parallel beam optic using a polycapillary collimating lens with a measuring aperture of fixed size was used on the primary side whereby the irradiated area of the sample was defined in such manner that a spill over of the X-ray beam over the coated face of the sample was avoided. On the secondary side a Soller slit with a divergence of 0.4° and a 25 μm thick Ni K.sub.β filter was used. Thin film correction and absorption correction were made and the linear absorption coefficient used was μ.sup.α-Al2O3=0.01258 μm.sup.−1 and μ.sup.TiCN=0.08048 μm.sup.−1 and μ.sup.TiAlN=0.02979 μm.sup.−1. Absorption of the overlaying layers was considered and corrected for.

(36) TABLE-US-00009 TABLE 3 TC values Ti.sub.1−xAl.sub.xN layer TC(1 1 1) TC(2 0 0) TC(2 2 0) TC(3 1 1) Sample A1 3.77 0.02 0.06 0.15 Sample A2 3.04 0 0.07 0.89 TiCN layer TC(1 1 1) TC(2 0 0) TC(2 2 0) TC(3 1 1) Sample A1 3.43 0.02 0.06 0.15 Sample A2 3.51 0.19 0.10 0.20 Sample B1 1.34 0.11 0.35 2.21 Sample B2 2.63 0.12 0.62 0.63 α-Al.sub.2O.sub.3 layer TC(0 2 4) TC(1 1 6) TC(3 0 0) TC(0 0 12) Sample A1 0.05 0.05 0 3.90 Sample A2 0.01 0.04 0 3.95 Sample B1 1.64 0.55 0.16 1.65 Sample B2 0.01 0.01 0 3.98

(37) Furthermore, texture and grain boundary orientation of the coatings were analyzed by electron backscatter diffraction (EBSD). For this purpose the coating surfaces (samples) were first polished subsequently using slurries of diamond having average grain sizes of 6 μm, 3 μm and 1 μm, respectively. Then, the samples were polished using colloidal silica having an average grain size of 0.04 μm. The last polishing step was done manually, and polishing time was increased stepwise until the sample quality was good enough to perform the EBSD maps, i.e. indexing of EBSD patterns would be accomplished with an average confidence index (CI)>0.2 at typical scan rates of 15-35 frames per second. The precise preparation conditions will depend on the individual sample and equipment, and can easily be determined by a person skilled in the art. Care was taken to ensure that the polished surfaces were smooth and parallel to the original coating surface. Finally, the samples were ultrasonically cleaned before EBSD examination.

(38) Planar surfaces on the rake face were used for the polishing, and the EBSD measurements were performed at distances <250 μm from the cutting edge of the insert. Due to the roughness of the cemented carbide substrate, areas of the lower TiCN or T.sub.1-xAl.sub.xN layers of the coatings according to the examples above were exposed as well. EBSD maps were acquired with an EDAX system (Digiview IV camera) using 8×8 or lower binning and typical scan rates of 15-35 frames per second on a Zeiss Supra 40 VP field emission scanning electron microscope using a 60 μm aperture, 15 kV acceleration voltage, high current mode and working distances between 6 mm and 15 mm. Areas for EBSD measurement of the upper α-Al.sub.2O.sub.3 layers, TiCN layers or T.sub.1-xAl.sub.xN layers were chosen by inspection of the surface in the SEM according to atomic number contrast. The maps had sizes of at least 10×10 μm, and step size ≤0.04 μm. Measurement of the upper α-Al.sub.2O.sub.3 layers was thus performed at a remaining layer thickness of 30% to 70% of the grown thickness. This was checked after polishing and EBSD measurement by preparing a cross section through the area where EBSD map had been acquired using a focused ion beam (FIB), and taking an average of >5 thickness measurements evenly distributed over the width of the EBSD map. The information depth of the electron diffraction patterns is small (on the order of a few tens of nanometers) compared to the layer thicknesses. The measurement depth of the TiCN layers in samples B1 and B2 was 0.5-1 μm below the interface between TiCN and α-Al.sub.2O.sub.3. For sample A2, EBSD maps of the 0.6 μm thick TiCN layer could be acquired and processed. The texture information thus obtained represents an approximate average over the TiCN layer thickness. Since the texture of the TiCN layer in sample A2 is given by epitaxy to the underlying T.sub.1-xAl.sub.xN layer, there is no significant texture gradient in the TiCN layer. The texture and epitaxial relations were checked by EBDS analysis in cross section. For sample A1, no EBSD map of the TiCN layer could be acquired due to too low thickness. Also in this case, texture and grain boundary orientation of the TiCN layer are defined by the underlying T.sub.1-xAl.sub.xN layer, as indicated by EBSD cross sectional analysis and by the XRD results given in table 3-1.

(39) For data acquisition and analysis, the software packages OIM Data Collection 7.3.1 and OIM Analysis 7.3.1 from EDAX were used, respectively. From the data orientation distribution functions were calculated and plotted as 001 inverse pole figures to evaluate the preferential crystallographic orientation. The α-Al.sub.2O.sub.3 layers of samples A1, A2 and B2 had strong {0001} fiber textures, whereas sample B1 had a weak bimodal texture showing intensity maxima around {0001} and {01-12} in the inverse pole figure. TiCN layers of the coatings according to the invention had strong {111} texture, whereas TiCN-layers of the comparative samples had {211} texture. The strength and sharpness of the fiber texture is characterized by the maximum intensity in the 001 inverse pole figure appearing at the direction of preferential crystallographic orientation.

(40) For the evaluation of grain boundary orientation, ΣN grain boundaries were identified. The Brandon criterion (ΔΘ<Θ.sub.0 (Σ).sup.−0.5, where Θ.sub.0=15°) was used to account for the allowed deviations ΔΘ of experimental values from the theoretical values (D. Brandon, Acta metall. 14 (1966) 1479-1484.). The amount of Σ3 grain boundaries in the layers was calculated as fraction of Σ3/ΣN (N≤9) for α-Al.sub.2O.sub.3 layers. The ΣN (N≤49) boundaries used in the calculation for α-Al.sub.2O.sub.3 correspond to the common lattice rotations in rhombohedral lattice materials given in H. Grimmer, Acta Crystallographica A (1989), A45, 505-523. In addition, the amount of Σ3 grain boundaries was calculated as the fraction of Σ3 grain boundary length to the total grain boundary length.

(41) For the present invention and the definitions herein, the ODF calculation and texture calculation, as well as Σ-values for the calculation of Σ-type grain boundaries are based on EBSD data without noise reduction or other cleanup procedures which alter the indexing and orientation data. Care should be taken that specimen preparation is carried out as described here with sufficient smoothness.

(42) The results of EBSD measurements are given in table 4. It is notable that in the coatings according to the invention, the α-Al.sub.2O.sub.3 layers have a much higher fraction of Σ3 grain boundaries with respect to the total grain boundary length.

(43) TABLE-US-00010 TABLE 4 EBSD data Max. intensity in 001 inverse Σ3/total pole figure grain boundary Sam- Tex- [times Σ3/ΣN length ple Layer ture average] [%] [%] A1 α-Al.sub.2O.sub.3 001 28.0 98 37 TiCN 111 not not not measurable measurable measurable A2 α-Al.sub.2O.sub.3 001 35.0 97 41 TiCN 111 6.53 12 3 B1 α-Al.sub.2O.sub.3 001 2.4 78 9 TiCN 211 2.60 49 20 B2 α-Al.sub.2O.sub.3 001 30.8 91 14 TiCN 211 4.51 51 22
Wear Test

(44) Cemented carbide inserts of the geometry CNMA120408 were coated as disclosed above. The inserts were tested in a turning test against 56NiCrMo steel without coolant. The following cutting parameters were used:

(45) Cutting speed, v.sub.c: 150 m/min

(46) Feed, f: 0.32 mm

(47) Depth of cut, a.sub.p: 2.5 mm

(48) The samples A1 and B1 were tested in longitudinal turning. One and the same cutting edge was run for 3 minutes, removed and analyzed in a LOM, remounted and run again for 3 minutes. This was repeated 4 times such that a total cutting time of 12 minutes was reached. The flank wear of the cutting edges (VB.sub.max) as an average of two repeats are shown in table 5.

(49) TABLE-US-00011 TABLE 5 Flank wear, Tool VB.sub.max [mm] A1 (invention) 0.16 B1 (reference) 0.25

(50) While the invention has been described in connection with various exemplary embodiments, it is to be understood that the invention is not to be limited to the disclosed exemplary embodiments, on the contrary, it is intended to cover various modifications and equivalent arrangements within the appended claims. Furthermore, it should be recognized that any disclosed form or embodiment of the invention may be incorporated in any other disclosed or described or suggested form or embodiment as a general matter of design choice. It is the intention, therefore, to be limited only as indicated by the scope of the appended claims appended hereto.