Coated cutting tool and method of manufacturing the same

Abstract

A coated cutting tool includes a substrate and a surface coating, wherein the coating is a Ti(C,N,O) layer comprising at least one columnar fine-grained MTCVD Ti(C,N) layer with an average grain width of 0.05-0.4 m and an atomic ratio of carbon to the sum of carbon and nitrogen (C/(C+N)) contained in the MTCVD Ti(C,N) layer being on average 0.50-0.65. A method for manufacturing the coated cutting tool includes depositing the MTCVD Ti(C,N) layer.

Claims

1. A coated cutting tool comprising; a substrate; a surface coating disposed on the substrate, said surface coating having a Ti(C,N,O) layer including at least one columnar MTCVD Ti(C,N) layer with an average grain width of 0.05-0.2 m, measured on a cross section with a surface normal perpendicular to a surface normal of the substrate, on a rake face of said coated cutting tool, along a straight line in a direction parallel to a surface of the substrate, at a centered position between a lowermost and an uppermost interface of said columnar MTCVD Ti(C,N) layer, an average thicknesses of said columnar MTCVD Ti(C,N) layer being 8-15 m, wherein an atomic ratio of carbon to the sum of carbon and nitrogen (C/(C+N)) contained in said MTCVD Ti(C,N) layer is in average 0.50-0.65 measured with electron microprobe analysis at 10 positions spaced 50 m along said straight line, the Ti(C,N,O) layer including an outermost Ti(C, O) layer; and an Al.sub.2O.sub.3 layer deposited on said Ti(C, O) layer.

2. The coated cutting tool according to claim 1, wherein the average grain width is 0.1-0.2 m.

3. The coated cutting tool according to claim 1, wherein the C/(C+N) ratio is 0.56-0.60.

4. The coated cutting tool according to claim 1, wherein said MTCVD Ti(C,N) layer has an X-ray diffraction pattern measurable by CuK radiation, wherein texture coefficients TC(hkl) are defined as: $\mathrm{TC}\left(\mathrm{hkl}\right)={\frac{I\left(\mathrm{hkl}\right)}{I_0\left(\mathrm{hkl}\right)}\left[\frac{1}{n}\underset{n=1}{\overset{n}{.Math.}}\frac{I\left(\mathrm{hkl}\right)}{I_0\left(\mathrm{hkl}\right)}\right]}^{-1}$ wherein I(hkl) is the measured intensity of the (hkl) reflection; I0(hkl) is the standard intensity according to ICDD's PDF-card No. 42-1489; n is the number of reflections used in the calculation; (hkl) reflections used are (111),(200),(220),(311),(331),(420),(422) and (511); and a sum of TC(422) and TC(311) is >5.5.

5. The coated cutting tool according to claim 1, wherein the Al.sub.2O.sub.3 layer is an -Al.sub.2O.sub.3 layer with an average thickness of 2-6 m.

6. The coated cutting tool according to claim 1, wherein the Ti(C,N,O) layer further comprises a Ti(C,O) layer adjacent to the Al.sub.2O.sub.3 layer.

7. The coated cutting tool according to claim 1, wherein the columnar MTCVD Ti(C,N) layer has an X-ray diffraction pattern having a 2 position of 123.15-123.25 degrees.

8. A method for producing a coated cutting tool using a CVD process, comprising the steps of: providing a substrate in a vacuum chamber; providing precursors to said vacuum chamber; and depositing a Ti(C,N,O) layer having at least one columnar MTCVD Ti(C,N) layer on said substrate, wherein the columnar MTCVD Ti(C,N) layer is deposited at a temperature of 800-850 C., using precursors comprising at least TiCl.sub.4, CH.sub.3CN or other nitrile, and H.sub.2, wherein a gas flow of said CH.sub.3CN or other nitrile is from 0.2 up to 0.5 vol-%, a gas flow of TiCl.sub.4 is about 2-4 vol-%, and a flow of N.sub.2 gas is less than 10 vol-%, of a total precursors gas flow during deposition of said MTCVD Ti(C,N) layer, and with a Ti/CN ratio, based on a volume percent of TiCl.sub.4 and CH.sub.3CN or other nitrile provided to the vacuum chamber, of 4-8.

9. The method according to claim 8, wherein the Ti/CN ratio is 6-7.

10. The method according to claim 8, wherein the precursors consist of TiCl.sub.4, CH.sub.3CN and H.sub.2.

11. The method according to claim 8, further comprising the step of depositing a Ti(C,N,O) layer of TiN, MTCVD Ti(C,N), HTCVD Ti(C,N) and Ti(C,O) on the substrate.

12. The method according to claim 8, further comprising the step of depositing an -Al2O3 layer.

13. The method according to claim 8, wherein the flow of N.sub.2 gas is less than 5 vol-%, of the total precursor gas flow.

14. The method according to claim 8, wherein the columnar MTCVD Ti(C,N) layer is deposited at a temperature of 820-850 C.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) Embodiments of the invention will now be described with reference to the accompanying drawings, wherein:

(2) FIG. 1a is a cross-sectional view of a coating in accordance with one embodiment of the invention,

(3) FIG. 1b is a magnified part of FIG. 1a schematically illustrating measurement of grain width in the MTCVD Ti(C,N) layer of the coating,

(4) FIG. 2 is a histogram representing the grain width distribution in the MTCVD Ti(C,N) layer of the coating of FIG. 1a,

(5) FIG. 3 is a cross-sectional view of a coating in accordance with one embodiment of the invention where the coated cutting tool has been subjected to heat treatment to diffuse heavy elements of the substrate into the coating, and

(6) FIG. 4 is a cross-sectional view of a coating in accordance with prior art.

DETAILED DESCRIPTION

EXAMPLE 1

(7) Coated cutting tools in accordance with one embodiment of the invention were manufactured. First, cemented carbide CNMG120412-KM substrates with a composition of 6.0 wt-% Co and balance WC, a Hc value of 17.52 kA/m (using a Foerster Koerzimat CS1.096 according to DIN IEC 60404-7) and a hardness of HV3=1.6 GPa were manufactured by pressing powder and sintering the pressed bodies. Prior to coating deposition the substrates were edge rounded to about 35 m by wet blasting. A coating consisting of a Ti(C,N,O) layer with a total thickness of about 10.3 m, which consists of the layer sequence 0.4 m TiN, 9.1 m MTCVD Ti(C,N), 0.2 m HTCVD Ti(C,N) and 0.6 m Ti(C,O), an -Al.sub.2O.sub.3 layer with a (012) texture and a thickness of about 3.8 m and a 0.7 m TiC/TiN color layer was deposited by CVD on the substrates. The coating was deposited in a CVD reactor having radial gas flow using deposition conditions for growth of the MTCVD Ti(C,N) layers and the -Al.sub.2O.sub.3 layer as described in Table 1. Oxidation and nucleation steps were performed prior to growth of the -alumina layer. After deposition the coated cutting tools were subjected to wet blasting to remove the color layer on the rake faces.

(8) FIG. 1a shows a cross-sectional SEM image of the coating and the outermost part of the substrate on the rake face of one of the coated cutting tools at a magnification of 15 000. The MTCVD Ti(C,N) layer has a columnar structure with fine columnar grains. In order to evaluate the grain size of the MTCVD Ti(C,N) layer the grain width was measured in the SEM image as schematically shown in FIG. 1b and further explained in the following. Minimum grain width was 26 nm, maximum grain width was 474 nm, average grain width was 140 nm and median grain width was 118 nm. Referring to FIG. 2, a histogram representing the grain width distribution of the MTCVD Ti(C,N) layer was made based on this measurement. The measured grain widths are distributed into discrete intervals (bins) with a width of 40 nm from 30 to 470 nm and 20 nm from 470 to 570 nm. The maximal frequency of measured grain widths are within the interval 70 to 110 nm.

(9) Referring to FIG. 3, the coated cutting tool used for the grain width determination was subjected to a heat treatment in a gas flow of H.sub.2 at 55 mbar and 1100 C. a in 1.5 hours in order to diffuse heavy elements of the substrate, i.e. W and/or Co, into the grain boundaries of the MTCVD Ti(C,N) layer to give contrast in a SEM image. At a magnification of 30 000 the in-diffusion can be observed as bright lines between the grains and the grain width is determined as the distance between these bright lines, see FIG. 3. The grain width was measured along a 10 m line parallel with the substrate at a position about 4-5 m from the surface of the substrate. Minimum grain width was 73 nm, maximum grain width was 390 nm, average grain width was 162 nm and median grain width was 146 nm. The maximal frequency of measured grain widths are within the interval 110 to 150 nm.

(10) Texture coefficients TC (hkl) indicating preferential growth directions of the columnar grains of the MTCVD Ti(C,N) layer, see Table 2, and the -Al.sub.2O.sub.3 layer were determined by X-ray diffraction on coated cutting tools manufactured according to Example 1 as explained in the following. The MTCVD Ti(C,N) has a strong (422) texture with large value also for (311). The -Al.sub.2O.sub.3 layer has a (012) texture.

(11) The MTCVD Ti(C,N) layer exhibits an X-ray diffraction pattern having the peak of the (422) reflection at 2=123.22, which has been determined as explained in the following. This peak position corresponds to a C/(C+N) ratio in the MTCVD Ti(C,N) layer of 0.57. A second method used to determine the carbon content by X-Ray diffraction is by using Rietveld refinement. The result from this approach is the same as the result from peak position. The FWHM of the peak of the (422) reflection is 0.44. Elemental analysis was also performed on the coated cutting tool used for the grain width determination by electron micro probe analysis as explained in the following, which demonstrated a C/(C+N) ratio in the MTCVD Ti(C,N) layer of 0.58.

EXAMPLE 2

(12) Coated cutting tool in accordance with prior art were manufactured to serve as reference when testing the coated cutting tool of Example 1. First, cemented carbide CNMG120412-KM substrates with a composition of 5.2 wt-% Co, 0.23 wt-% Cr carbides and balance WC, Hc value of 22.91 kA/m (using a Foerster Koerzimat CS1.096 according to DIN IEC 60404-7) and a hardness of HV3=1.8 GPa were manufactured by pressing powder and sintering the pressed bodies. Prior to coating deposition the substrates were edge rounded to about 35 m by wet blasting. A coating consisting of a Ti(C,N,O) layer, which consists of the layer sequence 0.4 m TiN, 9.8 m MTCVD Ti(C,N), 0.2 m HTCVD Ti(C,N), 0.6 m Ti(C,O), with a total thickness of about 10.3 m, an -Al.sub.2O.sub.3 layer with a (012) texture and a thickness of about 4.0 m and a 1.2 TiN/TiC color layer was deposited by CVD on the substrates. The deposition conditions for growth of the MTCVD Ti(C,N) layer are described in Table 1. After deposition the coated cutting tools were subjected to a wet blasting to remove the color layer on the rake face.

(13) Texture coefficients TC (hkl) indicating preferential growth directions of the columnar grains of the MTCVD Ti(C,N) layer, see Table 2, and the -Al.sub.2O.sub.3 layer were determined as explained in the following. The MTCVD Ti(C,N) layer has a strong (422) texture with large value also for (311). The -Al.sub.2O.sub.3 layer has a (012) texture.

(14) The MTCVD Ti(C,N) layer exhibits an X-ray diffraction pattern having the peak of the (422) reflection at 2=123.47, which has been determined as explained in the following. This peak position corresponds to a C/(C+N) ratio in the MTCVD Ti(C,N) layer of 0.52. The FWHM of the peak of the (422) reflection is 0.27. Elemental analysis was also performed by micro probe analysis as explained in the following, which demonstrated a C/(C+N) ratio in the MTCVD Ti(C,N) layer of 0.56.

(15) FIG. 4 shows a cross-sectional SEM image of the coating of the reference and the outermost part of the substrate on the rake face of the coated cutting tool. The MTCVD Ti(C,N) layer has a columnar structure with coarse columnar grains that extends through the MTCVD Ti(C,N) layer.

EXAMPLE 3

(16) Coated cutting tools were manufactured in accordance with Example 1 with the same Ti(C,N,O) layer but with a different -Al.sub.2O.sub.3 layer with a layer thickness of 4.2 mm and using a different -Al.sub.2O.sub.3 process giving a higher TC(006) than in the -Al.sub.2O.sub.3 layer of Example 1 as measured by X-ray diffraction.

EXAMPLE 4

(17) Coated cutting tools were manufactured in accordance with Example 1 with the same Ti(C,N,O) layer and -Al.sub.2O.sub.3 layer as in Example 3, but where the MTCVD Ti(C,N) layer was deposited at 870 C. instead of at 830 C. The higher deposition temperature resulted in much more fine-grained MTCVD Ti(C,N) layer than in Example 1 and Example 3 as observed in corr-sectional SEM images.

(18) TABLE-US-00001 TABLE 1 MTCVD MTCVD TiN/TiC/TiN/ Ti(C, N) Ti(C, N) HTCVD TiC/TiN TiN Ex. 1 Ex. 2 Ti(C, N) Ti(C, O) -Al.sub.2O.sub.3 TiN TiC Precursors Vol-% Vol-% Vol-% Vol-% Vol-% Vol-% Vol-% Vol-% H.sub.2 60.2 96.6 82.25 76.9 90.9 83.0 49.1 93.3 N.sub.2 38.3 7.83 15.4 49.1 CH.sub.4 5.1 HCl 7.83 5.5 CO 6.1 TiCl.sub.4 1.5 2.95 1.44 2.6 3.0 1.7 2.5 CH.sub.3CN 0.45 0.65 CO.sub.2 8.8 H.sub.2S 0.55 AlCl.sub.3 2.2 Total gas flow (l/h) 3655 5600 7660 1950 3300 9100 7020 4770 Temperature ( C.) 930 830 885 1010 1010 1010 1010 1010 Pressure (mbar) 160 80 55 55 55 55 atm atm Ti/CN ratio 6.6 2.2 Layer thickness (m) 0.3 0.15 0.4 9.1 9.8 0.2 0.6 3.8 1.2

(19) TABLE-US-00002 TABLE 2 MTCVD Ti(C, N) TC(111) TC(200) TC(220) TC(311) TC(331) TC(420) TC(422) TC(511) Example 1 0.22 0.42 0.18 1.47 0.41 0.10 4.62 0.58 Example 2 0.37 1.07 0.35 2.61 1.10 0.24 1.94 0.32

EXAMPLE 5

(20) Coated cutting tools of Example 1 and 2 were tested in turning of nodular cast iron 09.2 GGG60 without coolant including intermittent external axial and facing cutting operations under the following conditions.

(21) TABLE-US-00003 Cutting speed, V.sub.c 350 m/min Feed, f.sub.n 0.3 mm/rev Depth of cut, a.sub.p 4 mm Time/component, T.sub.c 1.25 min/piece

(22) Tool life criterion for the tested tools was deviation from dimensional tolerances of the work piece. The coated cutting tool of Example 2 representing state-of-the-art managed to cut 12 pieces. The coated cutting tool of Example 1 representing one example of an embodiment of the present invention managed to cut 18 pieces. The intermittent dry cutting of nodular cast iron is a demanding cutting operation and flaking and other discontinuous wear mechanisms, as well as insufficient oxidation resistance, often limit the tool life. In this test both tool variants exhibit a good oxidation resistance, but the tool of Example 1 outperforms the tool of Example 2 due to superior flaking resistance. Coated cutting tools of Example 3, differing from the coated cutting tools essentially only in the texture of the -Al.sub.2O.sub.3 layer, exhibited the same advantageous performance as the coated cutting tools of Example 1 in this performance test.

EXAMPLE 6

(23) Coated cutting tools of Example 1 and 2 were tested in turning of nodular cast iron (09.2 GS500 HB220) with coolant including continuous internal axial roughing cutting operations under the following conditions

(24) TABLE-US-00004 Cutting speed, V.sub.c 160 m/min Feed, f.sub.n 0.35 mm/rev Depth of cut, a.sub.p 3 mm Time component, T.sub.c 1.5 min/piece

(25) Tool life criterion for the tested tools was deviation from dimensional tolerances of the work piece. The coated cutting tool of Example 2 representing state-of-the-art managed to cut 15 pieces. The coated cutting tool of Example 1 representing one example of an embodiment of the present invention managed to cut 22 pieces. In contrast to the wear mechanism in Example 3 the tool life in this test is limited by flank wear resistance, which is superior in the coated cutting tool of Example 1. Coated cutting tools of Example 3, differing from the coated cutting tools essentially only in the texture of the -Al.sub.2O.sub.3 layer, exhibited the same advantageous performance as the coated cutting tools of Example 1 in this performance test.

EXAMPLE 7

(26) Coated cutting tools of Example 3 and 4 were tested in longitudinal turning of nodular cast iron SS0717 including intermittent cutting operations with coolant under the following conditions.

(27) TABLE-US-00005 Cutting speed, V.sub.c 250 m/min Feed, f.sub.n 0.2 mm/rev Depth of cut, a.sub.p 2.5-2 mm

(28) The cutting tool of Example 3 was superior over the cutting tool of Example 4 in flaking resistance.

(29) For the purpose of the present application, and in particular for the above examples, methods for determining properties of the coating are defined in the following.

(30) In order to evaluate the thicknesses and grain size of individual layers of the coating the coated cutting tool is cut, ground and polished to obtain a polished cross section with a surface normal perpendicular to a surface normal of the substrate on the rake face of the coated cutting tool.

(31) The layer thicknesses are measured using a light optical microscope.

(32) In order to enable grain width measurement it is necessary to obtain a smooth surface that gives sufficient contrast between grains of different orientation in the MTCVD Ti(C,N) layer by electron channelling. Thus for the purpose of grain width measurement the polishing of the cross section comprises the steps of: rough polishing on paper using an oil-based diamond suspension (from Microdiamant AG) with an average diamond particle size of 9 m and 0.7 g of diamond particles per 2 dl oil (Mobil Velocite no. 3), fine polishing on paper using an oil-based diamond suspension (from Microdiamant AG) with an average diamond particle size of 1 m and 0.7 g of diamond particles per 2 dl oil (Mobil Velocite no. 3), and oxide polishing using a soft cloth and under dripping of a suspension comprising a mixture of SiO.sub.2 (10-30%) and Al.sub.2O.sub.3 particles (1-20%) with average particle size of 0.06 m (Masterpolish 2, Buehler) at 150 rev/min and pressure 35 N for 220 s.

(33) The grain width is measured from a SEM micrograph of the polished cross section at a magnification of 15 000 in the SEM obtained at 5.0 kV and working distance 5 mm as schematically shown in FIG. 1b. The grain boundaries are identified by differences in contrast between adjacent grains and grain widths are measured as the distance between the identified adjacent grain boundaries along a 10 m straight line in a direction parallel to a surface of the substrate, at a centered position between a lowermost and an uppermost interfacial surface of the MTCVD Ti(C,N) layer. Grain widths smaller than 20 nm are not readily identified in the SEM image and are not considered.

(34) The columnar MTCVD Ti(C,N) layer comprises twinned columnar grains and may comprise even other intergranular defects or dislocations, which are not intended to be counted as grain boundaries in this method. Twin boundaries may be identified and excluded since the symmetry and orientation of the twin grains may not generate any substantial difference in contrast when passing the twin boundaries. Hence, the twinned columnar grain is intended to be treated as one grain when determining the grain width. However, sometimes it may be difficult to verify this and counting of a twin boundary as an intergranular boundary will decrease the average grain width value. To overcome this difficulty in grain width measurement, a method comprising diffusion of heavy elements of the substrate into the grain boundaries can be used, by way of example in accordance with the method used in Example 1. This is advantageous due to that the heavy elements cannot diffuse into the above mentioned defects or dislocations. In order to prepare the cross section for viewing the in-diffused binder the cross sections are subjected to only the rough polishing step and the fine polishing step and without the oxide polishing step. This gives a larger surface roughness than obtained by the oxide polishing and hence the contrast will be completely different and composition mode backscatter contrast is used to visualize the grain boundaries with in-diffused heavier elements therein.

(35) In order to investigate the texture of the MTCVD Ti(C,N) layer X-Ray diffraction is conducted on the flank face using a PANalytical CubiX.sup.3 diffractometer equipped with a PIXcel detector. The coated cutting tools are mounted in sample holders that ensure that the flank face of the samples are parallel to the reference surface of the sample holder and also that the flank face is at appropriate height. CuK.sub. X-rays are used for the measurements, with a voltage of 45 kV and a current of 40 mA. Anti-scatter and slits of degree and divergence slit of degree are used. The diffracted intensity from the coated cutting tool is measured around 2 angles were TiCN peaks occur, ranging from approximately 20 to 140, i.e. over an incident angle range from 10 to 70.

(36) Data analysis, including background subtraction and CuK.sub. stripping, is performed using PANalytical's X'Pert HighScore software, and integrated peak areas emanating from this are used to calculate the texture coefficients TC (hkl) of the MTCVD Ti(C,N) layer using X'Pert Industry software by comparing the ratio of the measured intensity data to standard intensity data according to

(37) $\mathrm{TC}\left(\mathrm{hkl}\right)={\frac{I\left(\mathrm{hkl}\right)}{I_0\left(\mathrm{hkl}\right)}\left[\frac{1}{n}\underset{n=1}{\overset{n}{.Math.}}\frac{I\left(\mathrm{hkl}\right)}{I_0\left(\mathrm{hkl}\right)}\right]}^{-1}$

(38) where I(hkl)=measured area intensity of the (hkl) reflection, I.sub.0(hkl)=standard intensity according to ICDD's PDF-card no 42-1489, n=number of reflections used in the calculation, (hkl) reflections used are: (111), (200), (220), (311), (331), (420), (422) and (511).

(39) Since the MTCVD Ti(C,N) layer is a finitely thick film the relative intensities of a pair of peaks of the same compound are different than they are for bulk samples, due to the differences in path length through the Ti(C,N) layer. Therefore, thin film correction is applied to the integrated peak area intensities, taken into account also the linear absorption coefficient of Ti(C,N), when calculating the TC values. Since the substrates used in the examples were WC a further correction is applied to correct for the overlap of the TiCN (311) by the WC (111) peak. This is made by deducting 25% of the area intensity of another WC peak, namely WC(101) from the TiCN (311) area intensity. Since possible further layers above the MTCVD Ti(C,N) layer will affect the X-ray intensities entering the MTCVD Ti(C,N) layer and exiting the whole coating, corrections need to be made for these as well, taken into account the linear absorption coefficient for the respective compound in a layer.

(40) In order to estimate the carbon content the diffraction angle 2 of the (422) reflection in the X-ray diffraction pattern obtained using CuK.sub. radiation is determined. The position of the (422) reflection relates to the carbon content in the coating such that a higher carbon content correlates to a lower angle of the (422) reflection. The C/N ratio, in the interval from TiC.sub.0N.sub.1 to TiC.sub.1N.sub.0, shows a linear dependence to the diffraction angle 2, making it possible to extract information about the C/N ratio by measuring the position of the (422) reflection.

(41) A second method used to determine the carbon content is by using Rietveld refinement to the complete diffraction pattern collected as discussed above. From the refinement it is possible to extract data on lattice parameters for the TiCN phase. The lattice parameter also varies linearly with the C/N ratio as discussed above. The result from this approach correlates well with the results where the diffraction angle was the parameter used to probe the carbon content.

(42) The (422) reflection is also used to estimate the grain width. This is accomplished by determining the FWHM of the peak in the diffractogram. The FWHM is related to the grain size such that a higher value of the width correlates to smaller grains.

(43) Elemental analysis is performed by electron microprobe analysis using a JEOL electron microprobe JXA-8900R equipped with wavelength dispersive spectrometers (WDS) in order to determine the C/(C+N) ratio of the MTCVD Ti(C,N) layer. The analysis of the MTCVD Ti(C,N) layer average composition is conducted on a polished cross section on the flank face within the MTCVD Ti(C,N) layer in 10 points with spacing of 50 m along a straight line in a direction parallel to a surface of the substrate, at a centered position between a lowermost and an uppermost interfacial surface of the MTCVD Ti(C,N) layer using 10 kV, 29 nA, a TiCN standard, and with corrections for atomic number, absorption and fluorescence. In Example 1 the points were placed within the MTCVD Ti(C,N) coating at a distance of 4-6 m from the interface between the substrate and the MTCVD Ti(C,N) layer.

(44) 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.