Technical field

The present disclosure relates to a coated cutting tool for chip forming machining of metals, more precisely a coated cutting tool comprising a substrate coated with a multi layered wear resistant coating comprising a layer of alumium oxide and a layer of titanium carbonitride. The coated cutting tool in accordance with the present disclosure is particularly useful in applications with high demands of abrasive wear resistance in for example turning, milling or drilling of a metallic material such as alloyed steel, carbon steel or tough hardened steel.

Background

Depositing thin, refractory coatings on cutting tools have been widely used in the machining industry for several decades. Coatings such as TiCN and AI2O3 have shown to improve the wear resistance on cutting inserts in cutting of many different materials. A combination on an inner layer of TiCN and an outer layer of a-Al2O3 can be found on many commercial cutting tools designed for turning or milling of, for example, steel. However, as technology develops higher demands are set on the cutting tools. Thus, there exists a need for coated cutting tools having an improved wear resistance in metal cutting operations.

Disclosure of the invention

The present disclosure provides a coated cutting tool having improved performance in cutting operations, particularly a coated cutting tool having improved wear resistance, for example a higher resistance to crater wear and flank wear. The present disclosure further provides a method for producing a coated cutting tool having the above mentioned properties.

According to aspects illustrated herein, there is provided a coated cutting tool comprising a substrate coated with a multi-layered wear resistant coating comprising a layer of a-Al2O3 and a layer of titanium carbonitride TixCyNi-y, with 0.85 < x < 1 .3, preferably 1 .1 < x < 1 .3, and 0.4 < y < 0.85, deposited on the a-Al2O3 layer, wherein the TixC _y Ni _-y exhibits a texture coefficient TC(hkl), as measured by X-ray diffraction using CuKa radiation and Θ-2Θ scan, the TC(hkl) being defined according to Harris formula:

wherein

l(hkl) is the measured intensity (integrated area) of the (hkl) reflection;

lo(hkl) is standard intensity of the standard powder diffraction data according to JCPDS card no. 42-1489;

n is the number of reflections used in the calculation, and where the (hkl) reflections used are (1 1 1 ), (2 0 0), (2 2 0), (3 1 1 ), (3 3 1 ), (4 2 0) and (4 2 2); and wherein TC(1 1 1 )≥ 3.

It has surprisingly been shown that the TixC _y Ni _-y layer of a cutting tool according the present disclosure exhibits an unexpectedly high hardness. An increased hardness of a coating layer is typically associated with an improved wear resistance, such as crater wear and flank wear resistance. As used herein, the term cutting tool includes, but is not limited to, replaceable cutting tool inserts, indexable cutting tool inserts, but also solid cutting tools.

The present disclosure is based on the realization that by coating a cutting tool with a coating comprising a layer of 0AI2O3 and a layer of titanium carbonitride Ti _x C _y Ni _-y deposited on top of the 0AI2O3 layer, and where the TixC _y Ni _-y is having a specifically preferred orientation, a cutting tool having a titanium carbonitride layer with an improved hardness, and thus an improved wear resistance in machining applications, can be achieved. More

specifically, such properties can be achieved by a cutting tool with a coating comprising a layer of a-Al2O3 and a layer of titanium carbonitride Ti _x C _y Ni _-y wherein the geometrically equivalent crystallographic planes {1 1 1 } of the TixC _y Ni _-y are found to be preferentially oriented parallel to the substrate, expressed herein as the texture coefficient TC (1 1 1 )≥ 3. The TixCyNi-y layer is typically deposited with moderate temperature chemical vapor deposition (MTCVD) at a temperature of 600-900 °C. The a- AI2O3 is typically deposited by chemical vapor deposition (CVD) at a temperature of 800-1200 °C. The TixC _y Ni _-y layer is typically deposited immediately on top of the AI2O3 layer without an intermediate layer. However, the scope of the disclosure also includes embodiments comprising a thin intermediate layer present between the TixC _y Ni _-y layer and the 0AI2O3 layer. The grains of deposited Ti _x C _y Ni _-y and 0AI2O3 are preferably columnar.

The coating according to the present disclosure furthermore provides and an excellent adhesion between the Ti _x C _y Ni _-y layer and underlying layers.

The multi-layer coating covers at least the area of the cutting tool that is engaged in cutting in a cutting operation, and at least the areas exposed for crater wear and/or flank wear. Alternatively, the whole cutting tool can be coated with the multi-layer coating of the present disclosure.

In some embodiments of the present disclosure the a-Al2O3 layer exhibits a texture coefficient TC(hkl), as measured by X-ray diffraction using CuKa radiation an Θ-2Θ scan, defined according to Harris formula wherein l(hkl) is the measured intensity (integrated area) of the (hkl) reflection, lo(hkl) is standard intensity of the standard powder diffraction data according to JCPDS card no. 00-010-0173, n is the number of reflections used in the calculation, and where the (hkl) reflections used are (1 0 4), (1 1 0), (1 1 3), (0 2 4), (1 1 6), (2 1 4), (3 0 0) and (0 0 12); and TC(0 0 12) > 7, preferably TC(0 0 12) > 7.2. A high intensity from the (0 0 12) reflection has shown to be advantageous in that it is one way to promote a strong <1 1 1 > texture of the subsequent Ti _x C _y Ni _-y layer.

In some embodiments the thickness of the Ti _x C _y Ni _-y layer is 1 -10 μιτι, preferably 1 -5 μιτι, more preferably 1 -3 μιτι, most preferably 1 -2 μιτι. The thickness of the 0AI2O3 layer is 0.1 -7 μιτι, preferably 0.1 -5 μιτι or 0.1 -2 μιτι or 0.3-1 μηη.

In some embodiments the coating comprises a further layer of titanium carbonitride Ti _u CvNi _-v , with 0.85 < u < 1 .3, preferably 1 .1 < u < 1 .3, and 0.4 < v < 0.85, located between the substrate and the a-Al2O3 layer. The Ti _u CvNi _-v layer can be deposited immediately on the substrate. However, the scope of this disclosure also includes embodiments comprising a thin intermediate layer between the substrate and the Ti _u CvNi _-v layer, such as a layer of TiN. Preferably, the Ti _u CvNi _-v is deposited by MTCVD at a temperature of 600-900 °C. The thickness of the Ti _u CvNi _-v layer is typically 3-20 μιτι, preferably 3-10 μιτι or 3-7 μιτι or 3-5 μιτι.

In some embodiments, the Ti _u CvNi _-v layer located between the a-Al2O3 layer and the substrate exhibits a texture coefficient TC(hkl), as measured by X-ray diffraction using CuKa radiation and Θ-2Θ scan, defined according to Harris formula where l(hkl) is the measured intensity (integrated area) of the (hkl) reflection, lo(hkl) is the standard intensity according to JCPDS card No. 42-1489, n is the number of reflections, the reflections used in the calculation are (1 1 1 ), (2 0 0), (2 2 0), (3 1 1 ), (3 3 1 ), (4 2 0) and (4 2 2); and

wherein TC(4 2 2) > 3, preferably TC(4 2 2) > 3.5. In one embodiment, the TiuCvNi-v layer exhibits a TC(3 1 1 ) + TC(4 2 2) > 4; > 5; > 6; or > 7. A high intensity from the (4 2 2) reflection of the Ti _u CvNi _-v has shown to be

advantageous in that it is one way to promote a strong <0 0 1 > texture of the subsequent a-Al2O3 layer.

In some embodiments, the TixC _y Ni _-y layer exhibits a higher mean hardness than the Ti _u CvNi _-v layer. The hardness is preferably measured by nano indentation using a Berkovich indenter, the hardness H being defined as H=(P/24.5hc ² ), wherein P is the maximum contact pressure exhibited by the indenter on the coating layer and h _c is the is the depth of the indentation made by the indenter. The hardness measurement is made at a flat surface of the layer with an indentation in a direction perpendicular to the outer surface of the layer. The indentations are preferably made at a constant load of 3000 μΝ/min to a depth of h _c =1 10 nm.

In some embodiments the TixC _y Ni _-y layer exhibits a mean hardness of more than 25 GPa, preferably more than 26 GPa, more preferably more than 27 GPa, even more preferably more than 30 GPa. The hardness is preferably measured by nano indentation using a Berkovich indenter, the hardness H being defined as H=(P/24.5h _c ² ), wherein P is the maximum contact pressure exhibited by the indenter on the coating layer and h _c is the is the depth of the indentation made by the indenter. The indentations are preferably made at a constant load of 3000 μΝ/min to a depth of h _c =1 10 nm. Other indenters known in the art may also be contemplated. A high hardness of the TixC _y Ni _-y may be advantageous in that it provides the coated cutting tool with an improved wear resistance.

In some embodiments, the coating has a total thickness of 4-32 μιτι, preferably 4.5-20 μιτι or 5-15 μιτι.

In some embodiments, the substrate is selected from cemented carbide, cermet, ceramics, steel or cubic boron nitride. These substrates have hardnesses and toughnesses that suit the coating of the present disclosure.

In some embodiments, the substrate of the coated cutting tool consists of cemented carbide comprising 4-12 wt% Co, preferably 6-8 wt% Co, optionally 0.1 -10 wt% cubic carbides, nitrides or carbonitrides of metals from groups IVb, Vb and Vlb of the periodic table, preferably Ti, Nb, Ta or combinations thereof, and balance WC.

In some embodiments, the substrate is cemented carbide with a binder phase enriched surface zone. The thickness of the binder phase enriched surface zone is preferably 5-35 μιτι as measured from the surface of the substrate and towards the core of the substrate. The binder phase enriched zone has in average a binder phase content at least 50% higher than the binder phase content in the core of the substrate. A binder phase enriched surface zone enhances the toughness of the substrate. A substrate with a high toughness is preferred in cutting operations such as turning of steel.

In some embodiments, the substrate is cemented carbide

with a surface zone essentially free from cubic carbides. The thickness of the surface zone essentially free from cubic carbides is preferably 5-35 μιτι as measured from the surface of the substrate and towards the core of the substrate. By "essentially free" means that no cubic carbides are visible in an ocular analysis of a cross section in a light optical microscope. In some embodiments, the substrate is a cemented carbide with a binder phase enriched surface zone, as disclosed above, in combination with a surface zone essentially free from cubic carbides as disclosed above.

According to other aspects illustrated herein, there is also provided a method for producing a coated cutting tool having a substrate, the method comprising the steps of

a) depositing a layer of Ti _u CvNi _-v by MTCVD at a temperature

600-900 °C on the substrate

b) depositing a layer of a-Al2O3 by CVD at a temperature of 800-1200 °C on top of the TiuCvNi-v layer;

c) depositing a layer of titanium carbonitride TixC _y Ni _-y layer on top of the AI2O3 layer by MTCVD at a temperature 600-900 ⁰ C in an atmosphere comprising TiCU, ChbCN, N2 and hb at a partial pressure of 3-13 vol% H2 and 83-94 vol% N ₂ , preferably 3-10 vol% H ₂ and 85-93 vol% N ₂ ,

wherein the TixC _y Ni _-y exhibits a texture coefficient TC(hkl), as measured by X- ray diffraction using CuKa radiation and Θ-2Θ scan, the TC(hkl) being defined according to Harris formula:

wherein

l(hkl) is the measured intensity (integrated area) of the (hkl) reflection; lo(hkl) is standard intensity of the standard powder diffraction data according to JCPDS card no. 42-1489;

n is the number of reflections used in the calculation, and where the (hkl) reflections used are (1 1 1 ), (2 0 0), (2 2 0), (3 1 1 ), (3 3 1 ), (4 2 0) and (4 2 2); and wherein TC(1 1 1 )≥ 3.

It has surprisingly been found that by using a low amount of H2 in the CVD reactor a Ti _x C _y Ni _-y having a texture according to the present disclosure can be obtained. According to the present disclosure, a low amount of H2 is supposed to denote an amount in the range 3-13 vol%, preferably 3-10 vol%. Furthermore, a high amount of N2, such as in the range of 83-94 vol%, preferably 85-93 vol%, may be advantageous. The total gas pressure in the reactor is preferably around 80 mbar.

The coated cutting tool produced in accordance with the method may be further defined as set out above with reference to the inventive coated cutting tool. Particularly, the thickness of the TixC _y Ni _-y coating layer may be 1 -

10 μιτι, preferably 1 -5 μιτι, more preferably 1 -3 μιτι, most preferably 1 -2 μιτι.

In some embodiments the a-Al2O3 layer of the method preferably exhibits a texture coefficient TC(hkl), as measured by X-ray diffraction using

CuKa radiation an Θ-2Θ scan, defined according to Harris formula wherein l(hkl) is the measured intensity (integrated area) of the (hkl) reflection, lo(hkl) is standard intensity of the standard powder diffraction data according to

JCPDS card no. 00-010-0173, n is the number of reflections used in the calculation, and where the (hkl) reflections used are (1 0 4), (1 1 0), (1 1 3), (0

2 4), (1 1 6), (2 1 4),

(3 0 0) and (0 0 12); and TC(0 0 12) > 7, preferably TC(0 0 12) > 7.2.

In some embodiments of the method, the thickness of the a-Al2O3 layer is preferably 0.1 -7 μιτι, preferably 0.3-5 μιτι or 0.3-2 μιτι or 0.3-1 μιτι.

In some embodiments of the method, the thickness of the Ti _u CvNi _-v layer is 3-20 μιτι, preferably 3-10 μιτι or 3-7 μιτι or 3-5 μιτι.

In some embodiments of the method, the Ti _u CvNi _-v layer located between the a-Al2O3 layer and the substrate exhibits a texture coefficient

TC(hkl), as measured by X-ray diffraction using CuKa radiation and Θ-2Θ scan, defined according to Harris formula where l(hkl) is the measured intensity (integrated area) of the (hkl) reflection, lo(hkl) is the standard intensity according to JCPDS card No. 42-1489, n is the number of

reflections, the reflections used in the calculation are (1 1 1 ), (2 0 0), (2 2 0),

(3 1 1 ), (3 3 1 ), (4 2 0) and (4 2 2); and wherein

TC(4 2 2) > 3.

In some embodiments of the method, the TixC _y Ni _-y layer exhibits a higher mean hardness than the Ti _u CvNi _-v layer. In some embodiments of the method, the TixC _y Ni _-y layer exhibits a mean hardness of more than 25 GPa, preferably more than 26 GPa or more than 27 GPa.

In some embodiments of the method, the coating has a total thickness of 4-32 μιτι, preferably 4.5-20 μιτι or 5-15 μιτι.

In some embodiments of the method, the substrate is selected from cemented carbide, cermet, ceramics, steel or cubic boron nitride.

Examples

In the following, the coated cutting tool and method according to the present disclosure will be described more in detail by way of non-limiting examples.

DEFINITIONS and METHODS

CVD coatings

The CVD coatings were prepared in a radial flow reactor, type Bernex BPX 325S, having 1250 mm height and 325 mm diameter.

Texture Coefficient, TC

The crystallographic plane of a crystal is defined by the Miller indices, h, k, I. A means to express preferred growth, i. e. that one set of geometrically equivalent crystallographic planes {h k I} is found to be preferentially oriented parallel to the substrate, is the texture coefficient TC (h k I) calculated using the Harris formula on the basis of a defined set of XRD reflections measured on the respective sample. The intensities of the XRD reflections are standardized using a JCPDS card indicating the intensities of the XRD reflections of the same material, e. g. TiCN, but with random orientation, such as in a powder of the material. A texture coefficient TC (h k I) > 1 of a layer of crystalline material is an indication that the grains of the crystalline material are oriented with their {h k I} crystallographic plane parallel to the substrate surface more frequently than in a random distribution, at least compared to the XRD reflections used in the Harris formula to determine the texture coefficient TC.

The term "columnar" grains is herein intended to denote crystal grains that grow from the bottom of the layer towards the outer surface of the layer and that typically are extended in this direction. Columnar grains differ from equiaxed grains in that equiaxed grains continuously re-nucleates during growth of the layer.

X-ray diffraction (XRD) measurements

The crystallography of the thin films, phase compositions and the out- of-plane orientations were evaluated by Θ-2Θ X-ray diffraction using a Philips MRD-XPERT diffractometer equipped with a primary hybrid monochromator and a secondary x-ray mirror. Cu-Ka radiation was used for the

measurements, with a voltage of 45 kV and a current of 40 mA. Anti-scatter slit of ½ degree and a receiving slit of 0.3 mm were used. The diffracted intensity from the coated cutting tool was measured in the range 30° to 140° 2Θ, i.e. over an incident angle Θ range from 15 to 70°.

The data analysis, including background subtraction and profile fitting of the data, was done using PANalytical's X'Pert HighScore Plus software. The output (integrated peak areas for the profile fitted curve) from this program was then used to calculate the texture coefficients of the layer by comparing the ratio of the measured intensity data to the standard intensity data according to a JCPDS card of the specific layer (such as a layer of TiCN or a-A Os), using the Harris formula as disclosed above.

Since the layer was a finitely thick film the relative intensities of a pair of peaks at different 2Θ angles are different than they are for bulk samples, due to the differences in path length through the layer. Therefore, thin film correction was applied to the extracted integrated peak area intensities for the profile fitted curve, taken into account also the linear absorption coefficient of layer, when calculating the TC values. Since possible further layers above for example the TixC _y Ni _-y layer will affect the X-ray intensities entering the TixCyNi-y 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. The same applies for X-ray diffraction measurements of a a-Al2O3 layer if the a-Al2O3 layer is located below for example an TixC _y Ni _-y layer. Alternatively, a further layer, such as TiN, above a TixCyNi-y layer can be removed by a method that does not substantially influence the XRD measurement results, e.g. chemical etching or mechanical polishing. In embodiments comprising a lower Ti _u CvNi _-v layer located between the 0AI2O3 layer and the substrate the outer TixC _y Ni _-y layer needs to be removed before making X-ray diffraction measurements of the lower Ti _u CvNi _-v layer.

Hardness measurements

The hardness of the titanium carbonitride layer were measured using nanoindentation. The nanoindentation was performed using a CSM UNHT nanoindenter with a Berkovich tip diamond indenter. The indentations were made at a constant load of 3000 μΝ/min to a depth of h _c =1 10 nm. Hardness was measured at a flat outer surface or the layer after gentle surface polish (with 6μηη diamond slurry) to decrease the surface roughness. Equipment reference measurements were performed on fused silica to ensure optimal indenter performance. The hardness H being defined as H=(P/24.5h _c ² ), wherein P is the maximum contact pressure exhibited by the indenter on the coating layer and h _c is the is the depth of the indentation made by the indenter. The indentation was made in a direction perpendicular to the surface of the layer. Any outer layers need to be removed with for example chemical etching or mechanical polishing before making the hardness measurement.

EXAMPLE 1 - TixCvNi-v on sapphire

Sample preparation and analysis

TixCyNi-y were grown on polished single crystal c-sapphire (001 ) substrates in a Bernex 325 hot wall CVD reactor, having 1250 mm height and 325 mm diameter at a temperature of 830 °C. The experimental conditions for the deposition of the coatings according to the present disclosure (sample 1 and 2) and for the comparative example (sample 3) are shown in table 1 . The coatings were grown to a thickness of about 1 .5 μιτι.

Table 1 . Experimental conditions in CVD chamber.

X-ray diffraction (XRD measurements) and texture coefficients

The TixCyNi-y layers of the coatings were analysed by XRD, and the texture coeffiencts of the (h k I) reflections (1 1 1 ), (2 0 0), (2 2 0), (3 1 1 ), (3 3 1 ), (4 2 0) and (4 2 2) of TiCN were determined as described herein. A thin film correction was applied to the XRD raw data. The results are shown in table 2.

Table 2. Texture coefficients of Ti _x C _y Ni

Hardness measurements

The hardness of the TixC _y Ni _-y layers was measured nanoindentation was performed using a CSM UNHT nanoindenter with a Berkovich tip diamond indenter and calculated as described herein above. The average hardness after 36 indentations were considered the coating hardness. The results are shown in table 3.

Table 3. Hardness measurements.

EXAMPLE 2 - Ti _x CvNi- _v on cemented carbide

Sample preparation and analysis

Cemented carbide substrate of ISO-type CNMG120408 for turning was manufactured from 7.2 wt% Co, 2.7 wt% Ta, 1 .8 wt% Ti, 0.4 wt% Nb,

0.1 wt% N and balance WC, comprising a Co enriched surface zone of about 25 μιτι from the substrate surface and to a depth into the body being essentially free from cubic carbides. The composition of the cemented carbide is thus about 7.2 wt% Co, 2.9 wt% TaC, 1 .9 wt% TiC, 0.4 wt% TiN, 0.4 wt% NbC and 86.9 wt% WC.

The insert, sample 4, was first coated with a thin approximately 0.4 μιτι

TiN-layer then with an approximately 12 μιτι Ti _u CvNi _-v layer by employing the well-known MTCVD technique using TiCU, CH3CN, N2, HCI and H2 at 885 °C. The volume ratio of T1CI4/CH3CN in an initial part of the MTCVD deposition of the TiuCvNi-v layer was 6.6, followed by a period using a ratio of T1CI4/CH3CN of 3.7. The details of the TiN and the Ti _u CvNi _-v deposition are shown in Table 4. Table 4. MTCVD of TiN and Ti _u CvNi _-v deposition sample 4

On top of the MTCVD Ti _u CvNi _-v layer a 1 -2 m thick bonding layer was deposited at 1000 °C. by a process consisting of four separate reaction steps. First, a HTCVD Ti _u CvNi _-v step using TiCU, CHU, N2, HCI and H2 at 400 mbar, then a second step (TiCNO-1 ) using TiCU, CH3CN, CO, N ₂ and H2 at 70 mbar, then a third step (TiCNO-2) using TiCU, CH3CN, CO, N2 and H2 at 70 mbar and finally a fourth step (TiCNO-3) using TiCU, CO, N2 and H2 at 70 mbar. During the third and fourth deposition steps some of the gases were continuously changed as indicated by a first start level and a second stop level presented in Table 5. Prior to the start of the subsequent AI2O3 nucleation, the bonding layer was oxidized at 55 mbar for 4 minutes in a mixture of CO2, CO, N2 and H2. The details of the bonding layer deposition are shown in Table 5. Table 5. Bonding layer deposition sample 4

On top of the bonding layer an a-Al2O3 layer was deposited using CVD. 5 All the 0AI2O3 were deposited at 1000 °C and 55 mbar in two steps. The first step using 1 .2 vol% AlC , 4.7 vol% CO2, 1 .8 vol% HCI and balance H2 giving about 0.1 μιτι a-Al2O3 and a second step as disclosed below giving a total a- AI2O3 layer thickness of about 10 μιτι.

The second step of the 0AI2O3 layer was deposited using 1 .2% AlCb, 0 4.7% CO2, 2.9% HCI, 0.58% H2S and balance H _2> see table 6.

Table 6. Second a-Al2O3 deposition step sample 4 (balance H2)

On top of the 0AI2O3 layer a 1 .7 μιτι thick TixC _y Ni _-y layer was 5 deposited using MTCVD. The TixC _y Ni _-y layer was deposited at 830 °C and 80 mbar using 3.3 vol% TiCU, 0.5 vol% CH3CN, 8.75 vol% H2 and balance N2, see table 7. 0 Table 7. Outermost TiCN layer deposition

X-ray diffraction (XRD measurements) and texture coefficients

The outermost TixC _y Ni _-y layer, the inner Ti _u CvNi _-v layer and the a-Al2O3 layer of the coating was analyzed by XRD, and the texture coefficients of the (h k I) reflections were determined as described herein. A thin film correction was applied to the XRD raw data. The results are shown in tables 8-10.

Table 8. Texture coefficients of the outermost TiCN layer of Sample 4

(hkl) TC

(111) 3.41

(200) 0.00

(220) 0.04

(311) 0.37

(331 0.16

(420) 0.23

(422) 2.81

Table 9. Texture coefficients of the inner TiCN layer of Sample 4

(hkl) TC

(111) 0.03

(200) 0.00

(220) 0.02

(311) 1.08

(331 0.16

(420) 0.04

(422) 5.67 Table 10. Texture coefficients of the a-Al2O3 layer of Sample 4

Hardness measurements

The hardness of the outermost TixC _y Ni _-y layer was measured by nanoindentation was performed using a CSM UNHT nanoindenter with a Berkovich tip diamond indenter and calculated as described herein above. The average hardness after 15 indentations were considered the hardness of the outermost TixC _y Ni _-y layer. The average hardness of the outermost TixC _y Ni _-y layer was measured to 26.7 GPa.