EP4627133A1

EP4627133A1 - A coated cutting tool

Info

Publication number: EP4627133A1
Application number: EP23809246.4A
Authority: EP
Inventors: Lars Johnson; Isabella SCHRAMM
Original assignee: Sandvik Coromant AB
Current assignee: Sandvik Coromant AB
Priority date: 2022-11-28
Filing date: 2023-11-21
Publication date: 2025-10-08
Also published as: KR20250114499A; CN120187891A; WO2024115196A1; JP2025537916A

Abstract

The present invention relates to a coated cutting tool for metal machining comprising a rake face and a flank face and a cutting edge in between, the coated cutting tool further comprises a substrate body and a coating thereon, wherein the coating comprises a from 0.2 to 10 μm thick Ti_1-xAI_xN layer on both the rake face and the flank face, 0.60≤x≤0.77, the ratio of the average grain size of crystal grains in the upper 50% part of the Ti_1-xAI_xN layer on the flank face to the average grain size of the crystal grains in the upper 50% part of the Ti_1-xAI_xN layer on the rake face is from 0.15 to 0.90.

Description

A coated cutting tool

The present invention relates to a coated cutting tool for metal machining wherein the coating comprises a Tii-_XAI_XN layer on both a rake face and a flank face, 0.60<x<0.77, the Tii-_XAI_XN layer has an average crystal grain size being larger on the rake face than on the flank face.

Introduction

Cutting tools, such as inserts, are used in metal machining operations. A cutting tool generally has at least one rake face and at least one flank face. A cutting edge is present where a rake face and flank face meet. Metal machining operations include, for example, turning, milling, and drilling.

In order to provide a long tool life, a cutting tool should have high resistance against different types of wear. In order to increase wear resistance of a cutting tool various types of wear resistant coatings are known in the art.

(Ti,AI)N coatings made by physical vapour deposition (PVD) are commonly used in the area of cutting tools for metal machining.

The crystal structure of (Ti,AI)N in PVD coatings can be cubic (NaCI (=B1 )) structure or hexagonal (wurzite) structure. Generally a lower Al content, such as < 65 at% Al, out of Al+Ti, in (Ti,AI)N gives a single phase cubic structure and at increasingly higher Al content, such as > 70%, first gives a mixture of cubic and hexagonal structure and then finally a single phase hexagonal structure. However, the limit of the level of Al content in a (Ti,AI)N coating when for giving either a single phase cubic structure or a mixed structure comprising both cubic and hexagonal structure vary to some extent depending on, for example, the deposition conditions.

A cubic structure of (Ti, Al) N is beneficial due to its better mechanical properties, such as hardness and elastic modulus, as compared with a hexagonal structure. A (Ti, AI)N coating comprising a comparatively high amount of Al, i.e. , >60 at, or even >67 at%, of the metal elements Ti and Al, is generally beneficial in order to provide a high oxidation stability. Thus, in the field of metal machining there is a desire to provide single phase cubic high-AI content (Ti , AI)N coatings for cutting tools. Cutting tools, such as cutting tool inserts, are subjected to different types of wear during use. One type is flank wear which takes place on a flank face of the cutting edge, mainly from an abrasive wear mechanism. The flank face is subjected to workpiece movement and too much flank wear will lead to poor surface texture of the workpiece, inaccuracy in the cutting process and increased friction in the cutting process. Another type of wear is crater wear which is localised to the rake face of a cutting tool. It originates from a chemical reaction between the workpiece material and the cutting tool and is amplified by cutting speed. Excessive crater wear weakens the cutting edge and may lead to fracture.

Different metal machining operations affect a coated cutting tool in different ways. Turning, for example, is a continuous metal machining operation while milling is more intermittent in nature. In milling the thermal and mechanical load will vary over time. Thermal load induces thermal tensions which may lead to so-called thermal cracks, herein referred to as "comb cracks", in a coating, while mechanical load may cause fatigue in the cutting edge leading to chipping, i.e., small fragments of the cutting edge loosening from the rest of the substrate. Thus, common wear types of a coated cutting tool in milling are cracking and chipping. A high comb crack resistance is thus of importance for tool lifetime in a milling operation.

There is a continuing demand for wear resistant coated cutting tools in which the coating has a high wear resistance such as high flank wear resistance and/or high crater wear resistance, high comb crack resistance etc..

Object of the invention

There is an object of the present invention to provide a coated cutting tool which, at least, shows high flank wear resistance or high crater wear resistance in turning operations, preferably a combination of both. Most preferably also high comb crack resistance in milling operations is provided. The invention

It has now been provided a coated cutting tool for metal machining showing a surprising level of high flank wear resistance and/or high crater wear resistance in metal cutting operations. Excellent comb crack resistance is also provided.

The present invention relates to a coated cutting tool for metal machining comprising a rake face and a flank face and a cutting edge inbetween, the coated cutting tool further comprises a substrate body and a coating thereon, wherein the coating comprises a from 0.2 to 10 pm thick Tii-_XAI_XN layer on both the rake face and the flank face, 0.60<x<0.77, the Tii-_XAI_XN layer on the rake face comprises crystal grains having an average grain size in the upper 50% part of the Tii-_XAI_XN layer, as measured in parallel to the surface plane of the substrate body, of from 40 to 200 nm, the Tii-_XAI_XN layer on the flank face comprises crystal grains having an average grain size in the upper 50% part of the Tii-_XAI_XN layer, as measured in parallel to the surface plane of the substrate body, of from 10 to 100 nm, the ratio of said average grain size in the upper 50% part of the Tii-_XAI_XN layer on the flank face to said average grain size in the upper 50% part of the Tii-_XAI_XN layer on the rake face is from 0.15 to 0.90.

In the Tii-_XAI_XN layer on both the rake face and the flank face suitably 0.63<x<0.75, preferably 0.66<x<0.75, most preferably 0.68<x<0.73.

In one embodiment, in the Tii-_XAI_XN layer on both the rake face and the flank face 0.66<x<0.77, preferably 0.68<x<0.77, most preferably 0.68<x<0.75.

The Tii-_XAI_XN layer on the rake face comprises crystal grains having an average grain size in the upper 50% part of the Tii-_XAI_XN layer, as measured in parallel to the surface plane of the substrate body, of preferably from 50 to 150 nm.

The Tii-_XAI_XN layer on the flank face comprises crystal grains having an average grain size in the upper 50% part of the Tii-_XAI_XN layer, as measured in parallel to the surface plane of the substrate body, of preferably from 20 to 80 nm.

The ratio of said average grain size in the upper 50% part of the Tii-_XAI_XN layer on the flank face to said average grain size in the upper 50% part of the Tii-xAlxN layer on the rake face is preferably from 0.20 to 0.75, most preferably from 0.25 to 0.60.

In one embodiment, the ratio of said average grain size in the upper 50% part of the Tii-_XAI_XN layer on the flank face to said average grain size in the upper 50% part of the Tii-_XAI_XN layer on the rake face is from 0.15 to 0.75, preferably from 0.20 to 0.60.

In one embodiment, in the Tii-_XAI_XN layer on the rake face the crystal grains are of a columnar microstructure throughout the Tii-_XAI_XN layer.

In one embodiment, the upper 50% part of the Tii-_XAI_XN layer on the flank face, comprises predominantely a nanocrystalline microstructure of crystal grains, or consists of a nanocrystalline microstructure of crystal grains. By "predominantely" is herein meant that more than 50 area% of an STEM image as made herein of the Tii-_XAI_XN layer shows a nanocrystalline microstructure.

In one embodiment, the crystal grains in the Tii-_XAI_XN layer on the rake face is of cubic structure.

By "is of cubic structure" is herein meant that there are no hexagonal peaks seen in an XRD theta-2theta analysis of the Tii-_XAI_XN layer, using the method as described herein, but only cubic (NaCI (=B1 )) peaks.

In one embodiment, the Tii-_XAI_XN layer on the rake face has a peak area intensity ratio of 1(111 ) to l(200) in a theta-2theta XRD analysis of from 0.2 to 3.0, suitably from 0.35 to 2.5, preferably from 0.5 to 2.0, most preferably from 0.8 to 1.5.

In one embodiment, the Tii -_XAI_XN layer on the flank face has a peak area intensity ratio of 1(111 ) to l(200) in a theta-2theta XRD analysis of < 0.05, or < 0.01.

In one embodiment the coating comprises one or more further layers of metal nitride(s) below the Tii-_XAI_XN layer. The metal nitride(s) is/are suitably a nitride/ nitrides of one or more metal elements belonging to group 4 to 6 in the IIIPAC periodic table of elements, optionally together with Al and/or Si. Preferably, a nitride/ nitrides of one or more metal elements selected from the group of Ti, V, Cr and Zr, optionally together with Al and/or Si. Examples of such metal nitride(s) are TiN, (Ti,AI)N, (Cr,AI)N, (Ti,Si)N, (Ti,AI,Si)N and (Zr,AI)N. The one or more further layers of metal nitride(s) below the Tii-_XAI_XN layer may be present as monolithic single layers or as sub-layers in a multilayer structure. The total thickness of these one or more metal n itride(s) layers can be from about 0.1 to about 8 pm, or from about 0.2 to about 6 pm, or from about 0.5 to about 4 pm.

In one embodiment, the Tii -_XAI_XN layer is an outermost layer of the coating and has a thickness of preferably from 0.2 to 1 .5 pm.

In one embodiment the Tii-_XAI_XN layer is situated directly onto the substrate body.

In one embodiment, the thickness of the Tii-_XAI_XN layer on both the rake face and the flank face is from 0.5 to 8 pm, or from 1 to 6 pm.

In one embodiment, the thickness of the Tii-_XAI_XN layer on the rake face is thinner than the thickness of the Tii-_XAI_XN layer on the flank face, the ratio thickness of the Tii-_XAI_XN layer on the rake face to thickness of the Tii-_XAI_XN layer on the flank face suitably being from 0.20 to 0.95, or from 0.35 to 0.90, or from 0.50 to 0.80.

In one embodiment, the Tii-_XAI_XN layer is a PVD layer, preferably a cathodic arc evaporation deposited layer.

The substrate body of the coated cutting tool can be made of cemented carbide, cermet, ceramic, cubic boron nitride or high speed steel. In one embodiment the substrate body is made of a cemented carbide comprising from 5 to 18 wt% Co.

The coated cutting tool can be a cutting tool insert for metal machining. The cutting tool insert is suitably a milling, drilling or turning insert.

The Tii-_XAI_XN layer according to this disclosure can be made in a physical vapour deposition (PVD) process, preferably in a cathodic arc evaporation process. A combination of a comparatively high level of bias voltage, such as 300 V, with a comparatively high N2 pressure level, such as 10 Pa, is used. Further details of a suitable process is disclosed in the Examples section of this disclosure. Brief description of the drawings

Figure 1 shows a schematic view of one embodiment of a cutting tool being a milling insert.

Figure 2 shows a schematic view of one embodiment of a cutting tool being a turning insert.

Figure 3 shows a schematic view of a cross section of an embodiment of the coated cutting tool of the present invention showing a substrate body and a coating being a Tii-_XAI_XN layer.

Figure 4 shows a schematic indication of the position of measurement of microstructure on a rake face of a cutting insert.

Figure 5 shows a schematic indication of the position of measurement of microstructure on a flank face of a cutting insert.

Detailed description of embodiments in drawings

Figure 1 shows a schematic view of one embodiment of a cutting tool 1 having a rake face 2 and flank faces 3 and a cutting edge 4. The cutting tool 1 is in this embodiment a milling insert.

Figure 2 shows a schematic view of one embodiment of a cutting tool 1 having a rake face 2 and flank face 3 and a cutting edge 4. The cutting tool 1 is in this embodiment a turning insert.

Figure 3 shows a schematic view of a cross section of an embodiment of the coated cutting tool of the present invention having a substrate body 5 and a coating 6 consisting of Tii-_XAI_XN layer.

Figure 4 shows a schematic view of a rake face 2 of a cutting tool 1 wherein the position for the microstructure analysis is indicated. Figure 5 shows a schematic view of a flank face 3 of a cutting tool 1 wherein the position for the microstructure analysis is indicated. Methods

Elemental analysis:

In order to confirm the actual elemental content of Ti, Al in the Tii -_XAI_XN layer the average composition was analysed by using energy-dispersive X-ray spectroscopy (EDX) for some samples. The EDX measurements were made on a cross-section of the Tii-_XAI_XN layer. The measurements were performed using an energy-dispersive X-ray spectrometer (Oxford Instruments X-Max 80 mm2 Silicon Drift Detector) at an accelerating voltage of 10 kV on a Supra 40 (Carl Zeiss AG) scanning electron microscope.

Microstructure analysis:

The microstructure analysis of the TiAIN layer on the flank face and rake face should be made on the respective face at a distance from the cutting edge but still in a region close to the cutting edge which are of relevance during metal machining.

Thus, the microstructure analysis on the flank face is made at a position of about 150 to 300 pm, preferably at about 200 pm, distance from the surface plane of a rake face. Also, there should be a distance of at least 1 mm away from any other flank face. Correspondingly, the microstructure analysis on the rake face is made at a position of about 150 to 300 pm, preferably at about 200 pm, distance from the surface plane of one flank face. Also, there should be a distance of at least 1 mm away from any other flank face. See a schematic visualisation of the position of analysis in Fig. 4 for a rake face and in fig. 5 for a flank face.

The TEM lamella preparation was done by cutting a section of the TiAIN coating in a direction perpendicular to the surface plane of the substrate body at the positions defined above on the flank face and rake face.

The grain size quantifications were obtained by using transmission electron microscope (TEM) obtaining STEM images of about 28000X magnification. The size in pixels of the images being 4096 x 4096. The microscope used for taking the STEM images was Thermo Fisher Scientific Titan G2 operated at an acceleration voltage of 300 kV. The camera length was set to 91 mm and three detectors were used to acquire images: STEM BF (bright field), ADF (annular dark field) and HAADF (high-angle annular dark field).

The grain size of the crystal grains of the Tii -_XAI_XN layer was measured along a total length so that at least 100 grains are measured. The total length is drawn as lines of at least 1 pm length each at different distances from the cutting tool substrate body surface within the upper 50% part of the Tii-_XAI_XN layer on a TEM image.

The lines should be drawn in parallel to the surface plane of the substrate body.

For columnar grains the growth direction is substantially perpendicular to the surface plane of the substrate body so the columnar width of the crystal grains are considered. For nanocrystalline grains the width of the grains are considered as well even if the nanocrystalline grains are more equiaxial than columnar grains.

A suitable software for image processing can be used for determining the number of crystal grains along the drawn lines. The software for image processing for grain size quantification used herein was Gatan DigitalMicrograph. The contrast, brightness and gamma factor were adjusted for each image to get the best grain size contrast. For the STEM images, an ADF (annular dark field) image was used since the grains become well defined. A line of at least 1 pm length was drawn in parallel to the surface plane of the substrate body and the intensity change along the line was determined when crossing crystal grains and this intensity change data was used to count the number of grains along the whole drawn line. The STEM image with the drawn lines is suitably considered along with the intensity change data to assist in identifying individual crystal grains. The procedure was repeated until at least a total of 100 grains had been measured along the total length of drawn lines. Using the total length of the lines and the total number of grains along the lines, an average grain size is obtained by dividing the total length by the total number of grains. X-Ray Diffraction:

As for the microstructure analysis, the XRD analysis of the TiAIN layer on the flank face and rake face should be made on the respective face at a distance from the cutting edge but still in a region close to the cutting edge which are of relevance during metal machining. Thus, all XRD analysis were made at a position on the flank face and at a position on the rake face as described under "Microstructure analysis" above.

The X-ray diffraction (XRD) analysis was conducted on the rake face and the flank face of coated inserts using a Bruker D8 Discover diffractometer equipped with a 2D detector (VANTEC-500) and a IpS X-ray source (Cu-Ka, 50.0 kV, 1 .0 mA) with an integrated parallel beam Montel mirror. The XRD measurements included the same position on the rake face and flank face of the coated cutting tool as considered for the microstructure analysis described above. The coated cutting tool inserts were mounted in sample holders that ensure that the measured face of the samples was parallel to the reference surface of the sample holder and also that the measured face was at appropriate height. The diffracted intensity from the coated cutting tool was measured around 20 angles where relevant peaks occur. Data analysis, including background subtraction and Cu-Ka2 stripping, was performed using PANalytical’s X’Pert HighScore Plus software. A Pseudo-Voigt-Fit function was used for peak analysis. No thin film correction was applied to the obtained peak intensities. Possible peak overlap of a (200) peak with any diffraction peak not belonging to the PVD layer, e.g., a substrate reflection like WC and/or Co, was compensated for by the software (deconvolution of combined peaks) when determining the peak intensities and peak widths. Examples

Example 1 :

Different coatings of (Ti, AI)N, both according to the present invention and outside the present invention, were deposited on sintered cemented carbide cutting tool insert blanks of the geometries SNMA 120408 (flat insert for analysis), CNMG120804-MM (turning insert) and R390-11T308M-PM (milling insert).

The composition of the cemented carbide was 10 wt% Co, 0.4 wt% Cr and rest WC. The cemented carbide blanks were coated by cathodic arc evaporation in a PVD vacuum chamber comprising four arc flanges, each flange comprising several cathode evaporators.

Targets of Ti-AI of the same composition were mounted in the evaporators in all of the flanges. The targets were circular and planar with a diameter of 100 mm available on the open market. Suitable target technology packages for arc evaporation are available from suppliers on the market such as IHI Hauzer Techno Coating B.V., Kobelco (Kobe Steel Ltd.) and Oerlikon Metco.

In the different coating deposition runs targets of compositions Tio.33Alo.67, Tio.29Alo.71 and Tio.25Alo.75 and Ti0.20AI0.80 were used, respectively.

The PVD chamber comprises a circular rotatable substrate table and the uncoated cutting tool insert blanks, which each has a hole like the inserts in the schematic figures 1 and 2, were mounted on pins located at the circumference of the substrate table. The table diameter was 0.82 m.

The distance between the circumference of the substrate table and the targets was about 27 cm.

The mounting of the inserts was such that the flank faces of the inserts would substantially face the cathode evaporators during rotation in the PVD chamber during deposition of the (Ti, AI)N layer. The rake faces were consequently not directly facing the cathode evaporators.

In one process used the chamber was pumped down to high vacuum (less than 10’² Pa) and heated to about 450°C by heaters located inside the chamber. The blanks were then etched for 60 minutes in an Ar plasma. The chamber pressure (reaction pressure) was then set to 10 Pa of N2 gas, and a unipolar DC bias voltage of -300 V (relative to the chamber walls) was applied to the blank assembly. The cathodes were run in an arc discharge mode at a current of 150 A (each).

The cutting tool insert blanks underwent a three-fold rotation in the PVD chamber during deposition of the coating.

For the target compositions Tio.33Alo.67 and Tio.25Alo.75 one deposition run was made using a low table rotational speed of 1 .2 rpm, and one deposition run was made using a high table rotational speed of 5 rpm. Assuming that the mounted inserts during deposition are positioned at the circumference of the substrate table a velocity of an insert passing a cathode evporator can be calculated. A rotational speed of 5 rpm and a table diameter of 0.82 m then corresponds to a velocity of 0.215 m/s. A rotational speed of 1 .2 rpm then corresponds to a velocity of 0.051 m/s.

For the target compositions Tio.29Alo.71 and Ti0.20AI0.80 there were only one deposition run made using a high table rotational speed of 5 rpm.

(Ti,AI)N layers of about 3 pm thickness were deposited on the blanks. The thickness was determined by light optical microscopy in a cross sectional cut.

A reference sample to be included in the performance testing a coated cutting tool was made by depositing an about 3 pm layer of (Ti , Al )N using a target composition of Ti0.40AI0.60.

The (Ti,AI)N layer was deposited on sintered cemented carbide cutting tool insert blanks of the geometries SNMA 120408, CNMG120804-MM and R390-11T308M-PM. By using this reference TiAIN layer, any kind of possible differences between performance testing runs were compensated for and results from different test runs could be compared. The composition of the cemented carbide was 10 wt% Co, 0.4 wt% Cr and rest WC. The cemented carbide blanks were coated by cathodic arc evaporation in a PVD vacuum chamber comprising four arc flanges, each flange comprising several cathode evaporators. Targets of Ti0.40AI0.60 were mounted in the evaporators in all of the flanges. The targets were circular and planar with a diameter of 100 mm available on the open market. Suitable target technology packages for arc evaporation are available from suppliers on the market such as IHI Hauzer Techno Coating B.V., Kobelco (Kobe Steel Ltd.) and Oerlikon Metco.

The PVD chamber comprises a circular substrate table and the uncoated cutting tool insert blanks were mounted on pins. The mounting of the inserts was such that the flank faces of the inserts would directly face the targets during rotation.

In a process for making the reference sample the chamber was pumped down to high vacuum (less than 10’² Pa) and heated to about 450°C by heaters located inside the chamber. The blanks were then etched for 60 minutes in an Ar plasma. The chamber pressure (reaction pressure) was set to 4 Pa of N2 gas, and a DC bias voltage of -70 V (relative to the chamber walls) was applied to the blank assembly. The cathodes were run in an arc discharge mode at a current of 150 A (each). The table rotational speed was 2.4 rpm.

When depositing (Ti,AI)N there may be a small difference between the Ti-AI composition in the target used and the Ti-AI composition in the deposited (Ti,AI)N layer.

In order to determine the actual elemental composition in the (Ti,AI)N layers the average composition was analysed by using energy-dispersive X-ray spectroscopy (EDX). The EDX measurements were made in SEM on a crosssection of the coating.

The result was that the actual Al content out of Ti+AI in the (Ti, Al) N layers deviates by -3 to -7 at% units from the Al content in the targets. The samples 1-7 made are listed in Table 1.

Table 1 .

The average grain size of (Ti,AI)N crystal grains was determined by using TEM as defined herein obtaining STEM images.

Table 2 shows the average crystal grain sizes on the rake face and flank face of samples, as well as their ratios of average grain size flank face to rake face. Table 2.

Samples 1 and 3 were of a columnar microstructure throughout the whole Tii-xAlxN layer on the rake face as seen on the STEM image. Sample 4 comprised a nanocrystalline microstructure of crystal grains on the flank face in an extent of about 95 area% as seen on the STEM image.

XRD analysis was made on the deposited (Ti,AI)N layer of the samples. The X-ray diffraction (XRD) theta 2theta analysis was conducted on both the rake face and the flank face of the coated inserts. There were no signs of any hexagonal peaks for the layers of samples 1 to 5 and 7 but only cubic (NaCI (=B1 )) peaks. Thus, the XRD analysis conclude that the (Ti,AI)N layer present in the samples 1 to 5 and 7 is of cubic structure on the rake face and on the flank face. However, sample 6 was concluded to contain a substantial amount of hexagonal phase on both its rake face and flank face.

Table 3 shows the peak area intensities 1(111 ) and l(222) on the rake face for the samples.

Table 3.

Table 4 shows the peak area intensities 1(111 ) and l(222) on the flank face for the samples.

Table 4. Example 2:

Cutting tests were made in order to determine the performance of the samples made.

Explanations to terms used:

The following expressions/terms are commonly used in metal cutting, but nevertheless explained in the table below:

Vc (m/min): cutting speed in meters per minute fz (mm/tooth): feed rate in millimeter per tooth (in milling) fn (mm/rev) feed rate in millimeter per revolution (in turning) z: (number) number of teeth in the cutter a_e (mm): radial depth of cut in millimeter a_P (mm): axial depth of cut in millimeter Flank wear test:

Longitudinal turning

Work piece material: Sverker 21 (tool steel), Hardness ~210HB, D=180, L=700 mm,

V_c=125 m/min fn=0.072 mm/rev a_P=2 mm without cutting fluid

The cut-off criteria for tool life (in minutes) is a flank wear VB of 0.15 mm.

Crater wear test:

Longitudinal turning

Work piece material: Ovako 825B, ball bearing steel. Hot rolled and annealed,

Hardness ~200HB, D=160, L=700 mm,

Vc=160 m/min fn=0.3 mm/rev a_P=2 mm with cutting fluid

The criteria for end of tool life (in minutes) is a crater area of 0.8 mm².

Comb crack resistance:

Operation: Shoulder milling

Tool holder: C5-391 .20-25 080

Work piece material: Toolox 33 (tool steel), L=600 mm, l=200 mm, h=100 mm,

Insert type: R390-11T308M-PM

Cutting speed V_c=250 m/min

Feed rate f_z=0.2 mm/rev

Depth of cut a_P=3 mm

Radial engagement a_e= 12.5 mm with cutting fluid The criteria for end of tool life (in minutes) is a max. chipped height VB>0.3 mm.

Table 5 summarises the cutting test results. Sample 7 (reference) was used in order to be able to compare data of samples tested in different test runs. The reference, Sample 7, was included in all tests and its results were set as "100%". The results of all other samples tested along with the reference in a test run were then related to the reference by calculating the percentage of the result of the reference.

Table 5.

The reference, Sample 7, has overall quite decent performance in all tests but its TiAIN coating has a quite low aluminium content. The present invention is directed to TiAIN coatings of higher aluminium content than the reference and the cutting performance generally tends to be more difficult to maintain at higher aluminium contents of the TiAIN.

It is concluded that sample 1 , within the invention, have a decent level of flank wear resistance at the same time the crater wear resistance is very good.

It is concluded that sample 3, within the invention, have a decent level of flank wear resistance at the same time the crater wear resistance is very good.

It is concluded that sample 4, within the invention, have an exceptionally high flank wear resistance at the same time the crater wear resistance is at a decent level. The comb crack resistance of around 80% of the reference level is also actually quite decent.

The comparative samples show worse results in one or both of the flank wear resistance and crater wear resistance tests.

It can also be noted that sample 3, which exemplifies a most preferred embodiment of the invention, shows the best comb crack resistance of all samples.

Claims

1 . A coated cutting tool (1 ) for metal machining comprising a rake face (2) and a flank face (3) and a cutting edge (4) inbetween, the coated cutting tool further comprises a substrate body (5) and a coating (6) thereon, wherein the coating (6) comprises a from 0.2 to 10 pm thick Tii-_XAI_XN layer on both the rake face (2) and the flank face (3), 0.60<x<0.77,

- the Tii-_XAI_XN layer on the rake face (2) comprises crystal grains having an average grain size in the upper 50% part of the Th -_XAI_XN layer, as measured in parallel to the surface plane of the substrate body (5), of from 40 to 200 nm, preferably from 50 to 150 nm,

- the Tii-_XAI_XN layer on the flank face (3) comprises crystal grains having an average grain size in the upper 50% part of the Th -_XAI_XN layer, as measured in parallel to the surface plane of the substrate body (5), of from 10 to 100 nm, preferably from 20 to 80 nm,

- the ratio of said average grain size in the upper 50% part of the Th -_XAI_XN layer on the flank face (3) to said average grain size in the upper 50% part of the Th - xAl_xN layer on the rake face (2) is from 0.15 to 0.90.

2. A coated cutting tool (1 ) according to claim 1 , wherein in the Th -_XAI_XN layer on both the rake face (2) and the flank face (3), 0.63<x<0.75, preferably 0.66<x<0.75, most preferably 0.68<x<0.73.

3. A coated cutting tool (1 ) according to claim 1 , wherein in the Th -_XAI_XN layer on both the rake face (2) and the flank face (3) 0.66<x<0.77, preferably 0.68<x<0.77, most preferably 0.68<x<0.75.

4. A coated cutting tool (1 ) according to any one of claims 1 -3, wherein the ratio of said average grain size in the upper 50% part of the Th -_XAI_XN layer on the flank face (3) to said average grain size in the upper 50% part of the Tii - xAlxN layer on the rake face (2) is from 0.15 to 0.75, preferably from 0.20 to 0.60.

5. A coated cutting tool (1 ) according to any one of claims 1 -3, wherein the ratio of said average grain size in the upper 50% part of the Tii -_XAI_XN layer on the flank face (3) to said average grain size in the upper 50% part of the Tii - xAlxN layer on the rake face (2) is from 0.20 to 0.75, preferably from 0.25 to 0.60.

6. A coated cutting tool (1 ) according to any one of claims 1 -5, wherein the crystal grains in the Tii-_XAI_XN layer on the rake face (2) and flank face (3) is of cubic structure.

7. A coated cutting tool (1 ) according to any one of claims 1 -6, wherein the Tii-xAlxN layer on the rake face (2) has a peak area intensity ratio of 1(111 ) to l(200) in a theta-2theta XRD analysis of from 0.2 to 3.0.

8. A coated cutting tool (1 ) according to any one of claims 1-7 wherein in the Tii-xAlxN layer on the rake face (2) the crystal grains are of a columnar microstructure throughout the Tii -_XAI_XN layer.

9. A coated cutting tool (1 ) according to any one of claims 1 -8, wherein the upper 50% part of the Tii-_XAI_XN layer on the flank face (3), in an area of a cross sectional cut perpendicular to the surface plane of the substrate body (5), comprises predominantely a nanocrystalline microstructure of crystal grains.

10. A coated cutting tool (1 ) according to any one of claims 1 -9, wherein the coating comprises one or more further layers of metal nitride(s) below the Tii- xAlxN layer.

11. A coated cutting tool (1 ) according to any one of claims 1 -10, wherein the Tii-xAlxN layer is an outermost layer of the coating and has a thickness of from 0.2 to 1.5 pm.

12. A coated cutting tool (1 ) according to any one of claims 1 -10, wherein the thickness of the Tii-_XAI_XN layer on both the rake face (2) and the flank face (3) is from 0.5 to 8 pm, or from 1 to 6 pm.

13. A coated cutting tool (1 ) according to any one of claims 1 -12, wherein the ratio thickness of the Tii-_XAI_XN layer on the rake face to thickness of the Tii-_XAI_XN layer on the flank face is from 0.20 to 0.95.

14. A coated cutting tool (1 ) according to any one of claims 1 -13, wherein the Tii-xAlxN layer is a cathodic arc evaporation deposited layer.

15. A coated cutting tool (1 ) according to any one of claims 1 -14, wherein the substrate body (5) of the coated cutting tool (1 ) is made of cemented carbide, cermet, ceramic, cubic boron nitride or high speed steel.

16. A coated cutting tool (1 ) according to any one of claims 1 -15, wherein the coated cutting tool (1 ) is a cutting tool insert for metal machining, preferably a milling, drilling or turning insert.