Sulfur Containing Alpha-Alumina Coated Cutting Tool

Info

Publication number: 20140173996
Type: Application
Filed: Sep 17, 2012
Publication Date: Jun 26, 2014
Applicant: WALTER AG (Tubingen)
Inventors: Dirk Stiens (Reutlingen), Sakari Ruppi (Tubingen)
Application Number: 14/131,716

Abstract

Cutting tool insert has a substrate and a coating of one or more refractory layers of which at least one layer is an α-Al2O3 layer having thickness of 1 to 25 μm, sulphur content of more than 100 ppm analysed by Secondary Ion Mass Spectroscopy (SIMS), and texture coefficient TC (0 0 12)>4 for the (0 0 12) growth direction. The at least one α-Al2O3 layer is deposited by chemical vapour deposition (CVD) using reaction gases comprising H2, CO2, AlCl3 and X, with X being H2S, SO2, SF6, or combinations thereof, and optional additions of N2 and Ar. The amount of X is at least 1.0 vol-% of the total volume of gases in the reaction chamber. The volume ratio of CO2 and X in the reaction chamber lies within the range of 1≦CO2/X≦7 during deposition of the at least one α-Al2O3 layer.

Description

Description

FIELD OF THE INVENTION

The present invention relates to a cutting tool insert consisting of a substrate of cemented carbide, cermet, ceramics, steel or a superhard material such as cubic boron nitride (CBN) and a hard coating consisting of one or more refractory layers of which at least one layer is an α-Al₂O₃layer containing sulphur and having a specified growth orientation defined by the texture coefficient, and a method of manufacturing the cutting tool insert.

BACKGROUND OF THE INVENTION

The early approaches to deposit Al₂O₃on a substrate surface in a CVD process based on a AlCl₃/CO₂/H₂reaction gas mixture had a very low deposition rates in the order of about 0.2 μm/h on flat surfaces. Besides the fact that it was not possible to control the phase content, this early Al₂O₃deposition process also suffered from a pronounced dog-bone-effect, i.e. the deposition rate was higher on the edges than on the flat surfaces of the substrate. U.S. Pat. No. 4,619,866 by Smith and Lindstöm discloses that dopants, such as H₂S, could be used as a catalyst both to enhance the overall deposition rate but also to suppress the dog-bone effect. With the introduction of H₂S as a catalyst for the α-Al₂O₃deposition process the deposition rate increased by a factor of about five coinciding with a more or less complete elimination of the dog-bone effect as compared to the process without any H₂S present

Several attempts have been made to deposit industrial alpha and gamma alumina coatings onto cutting tools by CVD or PVD using sulphur containing dopants. In EP-A-0 045 291 the addition of 0.02-0.3 vol-% of sulphur, selenium or tellurium containing gas, preferably H₂S, to the deposition gas in the CVD process has been found to increase the growth rate and to improve the uniformity of alumina coatings. EP-A-1 788 124 describes the deposition of alpha alumina coatings having a defined crystal grain boundary orientation wherein the deposition of the alumina is performed by CVD adding from 0.25-0.6 vol-% of H₂S to the deposition gas. EP-A-1 683 893 describes the deposition of alpha alumina coatings having a defined amount of Σ3 type grain boundary length wherein the deposition of the alumina is performed by CVD adding from 1.5-5 vol-% HCl and from 0.05-0.2 vol-% of H₂S to the deposition gas. The prior art literature does not describe the actual sulphur content in the alpha alumina coatings. Since H₂S has only been used to enhance the growth rate and prevent the dog-bone effect, there have been no attempts to use higher amounts of sulfur-containing dopants or consider the sulfur content in the α-Al₂O₃coatings in general. One reason for this is that, as disclosed in U.S. Pat. No. 4,619,866, the effect of H₂S on the growth rate on alumina was found to be at maximum at a H₂S concentration of 0.25 to 0.3 vol %. Larger amounts of H₂S than about 0.3 vol % were found to result in strongly reduced growth rates”

OBJECT OF THE INVENTION

It is an object of the present invention is to provide a coated cutting tool having an α-Al₂O₃layer that exhibits improved cutting properties, improved chipping resistance and improved crater wear resistance as well as lower friction in contact with the workpiece over the prior-art.

DESCRIPTION OF THE INVENTION

The present invention relates to a cutting tool insert consisting of a substrate of cemented carbide, cermet, ceramics, steel or a superhard material such as cubic boron nitride (CBN) and a coating with a total thickness of 5 to 40 μm, the coating consisting of one or more refractory layers of which at least one layer is an α-Al₂O₃layer having a thickness of 1 to 25 μm, wherein the at least one α-Al₂O₃layer having a sulphur content of more than 100 ppm analysed by Secondary Ion Mass Spectroscopy (SIMS) and the at least one α-Al₂O₃layer having a texture coefficient TC (0 0 12)>4 for the (0 0 12) growth direction, the TC (0 0 12) being defined as follows:

$TC (0 0 12) = {\frac{I (0 0 12)}{I_{0} (0 0 12)} [\frac{1}{n} \sum_{n - 1}^{n} \frac{I (hkl)}{I_{0} (hkl)}]}^{- 1}$

(hkl)=measured intensity of the (hkl) reflection
I₀(hkl)=standard intensity of the standard powder diffraction data according to JCPDF-card no. 42-1468
n=number of reflections used in the calculation, whereby the (hkl) reflections used are: (012), (104), (110), (113), (116), (300) and (0 0 12).

It has surprisingly been found that improved cutting properties, improved chipping resistance and improved crater wear resistance of the cutting tool insert as well as lower friction in contact with the workpiece can be achieved if the α-Al₂O₃layer has a high sulphur content of more than 100 ppm analysed by Secondary Ion Mass Spectroscopy (SIMS) and a texture coefficient TC (0 0 12)>4 for the (0 0 12) growth direction.

In a preferred embodiment of the present invention the at least one α-Al₂O₃layer of the cutting tool insert has a sulphur content of more than 120 ppm, preferably more than 150 ppm analysed by SIMS. It has been found that the cutting properties of the inventive cutting tool can be further improved by a higher sulphur content.

However, the sulphur content of the α-Al₂O₃layer should not exceed 2000 ppm, since a larger sulphur content may impair the properties of the cutting tool, such as grain boundary strength, and, in addition, cause porosity.

In another preferred embodiment of the cutting tool insert of the present invention the coating comprises, in addition to the at least one α-Al₂O₃layer, one or more refractory layers consisting of carbide, nitride, carbonitride, oxycarbonitride or borocarbonitride of one or more of Ti, Zr, V and Hf, or combinations thereof deposited using CVD or MT-CVD, having a thickness of from 0.5 to 20 μm, preferably from 1 to 10 μm.

Preferably, the coating comprises a first layer adjacent to the substrate body of CVD deposited Ti(C,N), TiN, TiC or HfN, or MT-CVD deposited Ti(C,N), Zr(C,N), Ti(B,C,N), or combinations thereof. Most preferably, the first layer is if Ti(C,N).

In yet another preferred embodiment of the cutting tool insert of the present invention

a) the uppermost layer of the coating is the α-Al₂O₃layer or
b) the uppermost layer of the coating is a layer of carbide, nitride, carbonitride or oxycarbnitride of one or more of Ti, Zr, V and Hf, or combinations thereof (herein called Ti top coating), having a thickness of from 0.5 to 3 μm, preferably 0.5 to 1.5 μm, being deposited atop of the α-Al₂O₃layer or
c) surface areas of the cutting tool insert, preferably the rake face of the cutting tool insert, comprise the α-Al₂O₃layer a) as the uppermost layer whereas the remaining surface areas of the cutting tool insert comprise as the uppermost layer a layer b) of carbide, nitride, carbonitride or oxycarbnitride of one or more of Ti, Zr, V and Hf, or combinations thereof, having a thickness of from 0.5 to 3 μm, preferably 0.5 to 1.5 μm, being deposited atop of the α-Al₂O₃layer.

The Ti top coating layer atop the α-Al₂O₃layer can be provided as a wear indicator or as a layer of other functions. Embodiments, where only parts of the surface areas of the cutting tool insert, preferably the rake face of the cutting tool insert, comprise the α-Al₂O₃layer as the uppermost layer whereas the remaining surface areas are covered with the Ti top coating as the outermost layer, can be produced by removing the deposited Ti top coating by way of blasting or any other well known method.

In another preferred embodiment of the cutting tool insert of the present invention the substrate consists of cemented carbide, preferably of cemented carbide consisting of 4 to 12 wt-% Co, optionally 0.3-10 wt-% cubic carbides of the metals from groups IVb, Vb and VIb of the periodic table, preferably Ti, Nb, Ta or combinations thereof, and balance WC.

For steel machining applications the cemented carbide substrate preferably contains 7.0 to 9.0 wt-% cubic carbides of the metals from groups IVb, Vb and VIb of the periodic table, preferably Ti, Nb and Ta, and for cast iron machining applications the cemented carbide substrate preferably contains 0.3 to 3.0 wt-% cubic carbides of the metals from groups IVb, Vb and VIb of the periodic table, preferably Ti, Nb and Ta.

In another preferred embodiment of the cutting tool insert of the present invention the substrate consists of cemented carbide comprising a binder phase enriched surface zone having a thickness of 5 to 30 μm, preferably 10 to 25 μm, from the substrate surface, the binder phase enriched surface zone having a Co content that is at least 1.5 times higher than in the core of the substrate and having a content of cubic carbides that is less than 0.5 times the content of cubic carbides in the core of the substrate. The thickness of the α-Al₂O₃layer in this embodiment is preferably about 4 to 12 μm, most preferably 4 to 8 μm.

Preferably, the binder phase enriched surface zone of the cemented carbide body is essentially free from cubic carbides. The binder enriched surface zone enhances toughness of the substrate and widens the application range of the tool. Subtrates having a binder enriched surface zone are particularly preferred for cutting tool inserts for metal cutting operations in steel, whereas cutting tool inserts for metal cutting operations in cast iron are preferably produced without binder enriched surface zone.

In another preferred embodiment of the cutting tool insert of the present invention the at least one α-Al₂O₃layer has a texture coefficient TC (0 0 12)>5, more preferably a texture coefficient TC (0 0 12)>6 for the (0 0 12) growth direction.

The present invention further provides a method of manufacturing a cutting tool insert as defined herein wherein said at least one α-Al₂O₃layer is deposited by chemical vapour deposition (CVD) the reaction gas of the CVD process comprising H₂, CO₂, AlCl₃and X, with X being H₂S, SO₂, SF₆, or combinations thereof, and optional additions of N₂and Ar, wherein the X is present in the reaction gas mixture in an amount of at least 1.0 vol-% of the total volume of gases in the CVD reaction chamber and wherein the volume ratio of CO₂and X in the CVD reaction chamber lies within the range of 1≦CO₂/X≦7 during deposition of the at least one α-Al₂O₃layer.

It has surprisingly been found that the inventive α-Al₂O₃coating can be controlled by particular deposition conditions. The inventive kind of high sulphur content and the texture coefficient TC(0 0 12)>4 of the α-Al₂O₃coating can be achieved by the control of the volume portion of the sulfur containing dopant X in the reaction gas mixture of the total volume of gases in the CVD reaction chamber in an amount of at least 1.0 vol-%, preferably at least 1.2 vol %, and, at the same time, by control of the volume ratio of CO₂and X in the CVD deposition reaction. Cutting tests and friction tests have clearly confirmed the beneficial effects of high sulfur content in the α-Al₂O₃layer.

If the amount X is less than 1.0 vol-% of the total volume of gases in the CVD reaction chamber the sulphur content and the texture coefficient TC(0 0 12) that can be achieved in the α-Al₂O₃coating will not be sufficiently high.

It has been found that the introduction of a high amount of sulphur containing dopant X alone will not lead to a high sulphur content and the desired texture coefficient TC(0 0 12) in the coating. The inventors have found that the ratio of sulfur containing dopant X to CO₂during CVD strongly affects the sulfur content and the texture coefficient TC(0 0 12) in the deposited α-Al₂O₃layer. Studies by the inventors have confirmed that deposition of α-Al₂O₃with a high sulfur content and the texture coefficient TC(0 0 12) is difficult if too high CO₂/X ratios during deposition are used. It was surprising that the control of the CO₂/X ratio in the CVD deposition process of α-Al₂O₃is the most important factor to obtain a high sulfur content and the desired texture coefficient TC(0 0 12) in the α-Al₂O₃layer and, surprisingly and most importantly, that certain ratios resulted exclusively in high amounts of sulfur with good reproducibly. Thus, the present invention provides for a new a method to control the sulfur content and the texture coefficient TC(0 0 12) of α-Al₂O₃deposited by CVD.

In a preferred embodiment of the method of the present invention the volume proportion of the component X or the combination of components X is present in the reaction gas mixture during deposition of the at least one α-Al₂O₃layer in an amount of at least 1.2 vol-%, preferably at least 1.5 vol-% of the total volume of gases in the CVD reaction chamber. In another embodiment the volume proportion of the component X or the combination of components X lies within the range of 2.0 to 3.0 vol-%.

It has surprisingly been found that the sulphur content and the texture coefficient TC(0 0 12) of the α-Al₂O₃layer can be further improved by higher X content in the reaction gas mixture during deposition of the α-Al₂O₃layer resulting in improved cutting properties, improved chipping resistance and improved crater wear resistance of the cutting tool insert. However, a too high content of X, for example above 5.0 vol-%, should be avoided due to the danger of handling the sulphur sources. For example, the preferred sulphur source, H₂S, is a flammable and extremely hazardous gas.

In another preferred embodiment of the method of the present invention the volume ratio of CO₂and X in the CVD reaction chamber lies within the range of 1≦CO₂/X≦6 during deposition of the at least one α-Al₂O₃layer. When the deposition is carried out within this range of CO₂/X, both a sufficient amount of sulphur of >100 ppm in the alumina layer together with a strong preferred growth of alumina along the (0 0 12) direction, resulting in a relatively high texture coefficient TC(0 0 12) for the α-Al₂O₃layer, can be obtained.

In yet another preferred embodiment of the method of the present invention the volume ratio of CO₂/AlCl₃in the CVD reaction chamber is equal or smaller than 1.5 and/or the volume ratio of AlCl₃/HCl in the CVD reaction chamber is equal or smaller than 1, during deposition of the at least one α-Al₂O₃layer. If the ratio of CO₂/AlCl₃is too high (>1.5) and/or if the ratio of AlCl₃/HCl is too high (>1.0), corresponding to too low amounts of HCl, this will enhance growth along the (0 1 2) direction and, consequently, will lead to a lower TC(0 0 12) in the resuiting alumina coating.

The CVD process of the present invention during deposition of the at least one α-Al₂O₃layer is suitably conducted at a temperature in the range of 850 to 1050° C., preferably 980 to 1050° C., most preferably 1000 to 1020° C. If the temperature of the CVD process is too low, the growth rate would be too low, and f the temperature of the CVD process is too high, gas-phase nucleation and non-uniform growth will occur.

The reaction gas pressure range where the CVD process of the present invention is conducted during deposition of the at least one α-Al₂O₃layer is preferably from 50 to 120 mbar, more preferably from 50 to 100 mbar.

In yet another preferred embodiment of the method of the present invention the component X in the CVD process is H₂S or SO₂or a combination of H₂S and SO₂, whereby, if the component X in the CVD process is a combination of H₂S and SO₂, the volume proportion of SO₂does not exceed 20% of the volume amount of H₂S. If too much SO₂is used the coating uniformity can be reduced due to the so-called dog-bone effect.

In yet another preferred embodiment of the method of the present invention the reaction gas of the CVD process comprises additions of N₂and/or Ar in a volume amount in the range of 4 to 20 vol %, preferably 10-15 vol %, of the total volume of gases in the CVD reaction chamber.

As will be shown in the examples below, the coatings of the invention exhibit an excellent chipping resistance in a high-speed intermittent cutting and enhanced crater wear resistance in continuous turning over the prior-art coatings.

Methods Secondary Ion Mass Spectroscopy (SIMS)

The measurement of sulphur in the alumina coatings has been done by Secondary Ion Mass Spectroscopy (SIMS) on a Cameca ims3f spectrometer. The quantitative determination of the sulphur concentration in a sample was done relative to the known aluminum concentration in the sample. For the determination of the sensitivity (relative ion yield) for sulphur relative to aluminum the reference glas SRM 610 of the National Institute of Standards and Technology (NIST) was used. The sulphur concentration in SRM 610 is 575 μg/g, and the relative accuracy of the measurements is about ±20%.

The sample surface was sputtered with negative oxygen ions having an energy of 14.5 keV. The primary ion current was about 30 nA, and the diameter of the focussed primary ion beam at the sample surface was about 30-40 μm. The generated positive secondary ions were accelerated to an energy of 4.5 keV and measured with a mass spectrometer at a mass resolution of m/Δm=1800 using a secondary ion multiplier in counting modus (for³²S) and with a Faraday cup (for ²⁷Al), respectively. The starting energy of the detected secondary ions was 55±20 eV to lower molecular interferences and increase the measurement accuracy (energy filtering). As the measurement results the average of six equal measurement cycles was calculated. The integration times per cycle were 25 sec for³²S and 3 sec for²⁷Al, respectively. For each sample the measurements have been repeated 5 times.

TC(0 0 12) X-Ray Diffraction Measurements

X-ray diffraction measurements were done on a diffraktometer XRD3003PTS of GE Sensing and Inspection Technologies using Cu K_α-radiation. The X-ray tube was run at 40 kV and 40 mA focussed to a point. 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 selected to avoid a spill over of the X-ray beam over the coated face of the sample. On the secondary side a Soller slit with a divergence of 0.4° and a 0.25 mm thick Ni K_β filter were used. θ-2θ scans within the angle range of 20°<2θ<100° with increments of 0.25° have been conducted. The measurements were done on a flat face of the coated insert, preferably on the flank face. The measurements were done directly on the alumina layer as the outermost layer. Any layer present in the coating above the alumina layer to be measured, if any, is removed by a method that does not substantially influence the XRD measurement results, e.g. etching. For the calculation of the texture coefficient TC(0 0 12) peak height intensities were used. Background subtraction and a parabolic peakfit with 5 measuring points were applied to the XRD raw data. No further corrections such as K_α2stripping or thin film correction were made.

CVD Coatings

All CVD coatings were prepared in a radial flow reactor, type Bernex BPX 325S.

EXAMPLES Example 1 α-Al₂O₃Coatings

Cemented carbide substrates for cutting inserts with a composition of 6.0 wt % Co and balance WC (hardness about 1600 HV) were coated with a Ti(C,N) layer by applying MT-CVD using 0.6 vol % CH₃CN, 3.8 vol % TiCl₄, 20 vol % N₂and balance H₂. The thickness of the Ti(C,N) MT-CVD layer was about 5 μm.

Onto this Ti(C,N) layer of separate substrate samples different layers consisting of about 8 μm α-Al₂O₃were deposited. The coating parameters are given in Table 1, and the texture coefficients, TC(0 0 12), measured by X-ray diffraction, and the sulphur concentrations in the α-Al₂O₃coatings, measured by SIMS, are given in Table 2.

The deposition of α-Al₂O₃was started by depositing a 0.05 μm to about 1 μm, preferably 0.5 μm to about 1 μm, thick bonding layer on top of the MTCVD layer from the system H₂—N₂—CO—TiCl₄—AlCl₃at a pressure of 50 to 100 mbar. For the preparation of the bonding layer the MTCVD layer was treated with a gas mixture of 3 vol % TiCl₄, 0.5 vol % AlCl₃, 4.5 vol % CO, 30 vol % N₂and balance H₂for about 30 min at a temperature of about 1000° C. The deposition was followed by a purge of 10 min using H₂before starting the next step.

α-Al₂O₃was nucleated on the (Ti,Al)(C,N,O) bonding layer by treating said layer with a gas mixture of 4 vol % CO₂, 9 vol % CO, 25 vol % N₂, balance H₂for 5-10 min at a temperature from about 750 to 1050° C., preferably at about 980 to 1020° C. and most preferably at 1000 to 1020° C. (P=80 to 100 mbar). The oxidation was followed by a purge of 10 min using Ar.

The alumina deposition was started with by introducing a gas mixture of AlCl₃, CO₂, Ar₂, N₂HCl and H₂, in the volume amounts as indicated in table 1, without precursor X for about 10 min at a temperature of about 1000° C. These precursors were shunted in simultaneously except HCl. HCl flow was shunted into the reactor 2 min after the start (8 min before X was introduced).

TABLE 1 α-Al₂O₃coatings H₂S SO₂ CO₂ AlCl₃ Ar₂ N₂ HCl H₂ Pressure CO₂/X Coating [vol %] [vol %] [vol %] [vol %] [vol %] [vol %] [vol %] [vol %] [mbar] ratio 1a 0.5 — 5.0 3.4 5.0 10.0 3.4 bal. 80 10 1b 0.5 — 2.5 3.2 5.0 10.0 3.3 bal. 80 5 2a 1.0 — 8.0 5.4 5.0 10.0 5.4 bal. 80 8 2b X 1.0 — 4.0 2.7 5.0 10.0 2.7 bal. 80 4 3a 1.8 — 14.4 9.6 5.0 10.0 9.6 bal. 80 8 3b X 1.8 — 9.0 6.0 5.0 10.0 6.0 bal. 80 5 3c X 1.8 — 3.6 2.4 5.0 10.0 2.4 bal. 80 2 4a 2.4 — 21.6 14.4 5.0 10.0 14.4 bal. 80 9 4b X 2.4 — 2.4 1.6 5.0 10.0 1.6 bal. 80 1 5 0.4 0.1 4.0 2.7 5.0 10.0 2.7 bal. 80 8 6 0.9 0.1 16.0 10.5 5.0 10.0 11.0 bal. 80 16 7 X 0.9 0.1 4.0 4.7 5.0 10.0 7.2 bal. 80 4 8 1.5 0.3 15.0 10.0 5.0 10.0 12.0 bal. 80 8.3 9 X 1.5 0.3 7.5 5.0 5.0 10.0 5.0 bal. 80 4.2 10 X 1.5 0.3 2.0 1.4 5.0 10.0 1.4 bal. 80 1.1 11 2.0 0.4 20.0 2.0 5.0 10.0 2.0 bal. 80 8.4 12 X 2.0 0.4 3.0 2.0 5.0 10.0 2.0 bal. 80 1.3 X = invention

TABLE 2 α-Al₂O₃coatings Sulphur Coating TC(0 0 12) [ppm] 1a — 15 1b 2.2 30 2a 2.6 40 2b X 4.8 101 3a 3.8 60 3b X 4.1 108 3c X 5.9 220 4a 3.2 80 4b X 6.8 390 5 4.2 51 6 0.8 32 7 X 5.7 102 8 3.8 40 9 X 6.1 119 10 X 6.7 222 11 2.3 72 12 X 6.9 385 X = invention

The inserts with coatings 4a and 4b were tested for friction coefficient using the pin-on-disc method. The coating 4a showed a friction coefficient of 0.56, whereas the coatings 4b and 12 showed a lower friction coefficient of 0.42 and 0.39, respectively. Thus, a high sulphur content in the alpha alumina coatings has been identified to be friction reducing.

Example 2 Edge Toughness Tests

The samples 1a to 4b of Example 1 were tested with respect to edge toughness (chipping resistance) in longitudinal turning of cast iron (GG25) using the following cutting parameters:

Work piece: GG25; cylindrical bar
Insert type: SNUN
Cutting speed: v_c=400 m/min
Feed (f)=0.4 mm/rev
Depth of cut: a_p=2.0 mm
Remarks: dry turning

The inserts were inspected after 2 and 4 minutes of cutting. As shown in Table 3, compared to the coating of the prior art, the edge toughness of the samples was considerably enhanced when the coating was produced according to this invention.

TABLE 3 Edge Toughness Flaking of the edge line (%) Flaking of the edge line (%) Coating after 2 minutes after 4 minutes 1a 22 34 1b 18 32 2a 18 41 2b 12 32 3a 19 29 3b X 2 8 3c X 0 4 4a 18 33 4b X 0 3 X = invention

Example 3 Turning Tests

The samples 6, 8, 10 and 12 of Example 1 were tested in carbon steel (C45) without coolant using the following cutting parameters:

Work piece: C45
Insert type: WNMG080412-NM4
Cutting speed: v_c=280 m/min
Feed (f)=0.32 mm/rev
Depth of cut: a_p=2.5 mm
Remarks: dry turning

The end of tool life criterion was flank wear >0.3 mm. Three edges of each variant were tested.

TABLE 4 Turning Test results Coating Tool Life (min) 6 13.2 8 15.5 10 X 21.3 12 X 32.2 X = invention

Claims

1. A cutting tool insert consisting of: TC   ( 0   0   12 ) = I  ( 0   0   12 ) I 0  ( 0   0   12 )  [ 1 n  ∑ n - 1 n   I  ( hkl ) I 0  ( hkl ) ] - 1

a substrate of cemented carbide, cermet, ceramics, steel or a superhard material; and

a coating with a total thickness of 5 to 40 μm, the coating consisting of one or more refractory layers of which at least one layer is an α-Al2O3 layer having a thickness of 1 to 25 μm,

wherein the at least one α-Al2O3 layer has a sulphur content of more than 100 ppm analysed by Secondary Ion Mass Spectroscopy (SIMS) and the at least one α-Al2O3 layer has a texture coefficient TC (0 0 12)>4 for the (0 0 12) growth direction, the TC (0 0 12) being defined as follows:

(hkl)=measured intensity of the (hkl) reflection

I0 (hkl)=standard intensity of the standard powder diffraction data according to JCPDF-card no. 42-1468

n=number of reflections used in the calculation, whereby the (hkl) reflections used are: (012), (104), (110), (113), (116), (300) and (0 0 12).

2. The cutting tool insert of claim 1 wherein the at least one α-Al2O3 layer has a sulphur content of more than 120 ppm analysed by SIMS.

3. The cutting tool insert of claim 1, wherein the coating comprises, in addition to the at least one α-Al2O3 layer, one or more refractory layers consisting of carbide, nitride, carbonitride, oxycarbonitride or borocarbonitride of one or more of Ti, Zr, V and Hf, or combinations thereof deposited using CVD or MT-CVD, having a thickness of from 0.5 to 20 μm.

4. The cutting tool insert of claim 1, wherein

a) the uppermost layer of the coating is the α-Al2O3 layer or

b) the uppermost layer of the coating is a layer of carbide, nitride, carbonitride or oxycarbnitride of one or more of Ti, Zr, V and Hf, or combinations thereof, having a thickness of from 0.5 to 3 μm and being deposited atop of the α-Al2O3 layer or

c) surface areas of the cutting tool insert comprise the α-Al2O3 layer as the uppermost layer whereas the remaining surface areas of the cutting tool insert comprise as the uppermost layer a layer of carbide, nitride, carbonitride or oxycarbnitride of one or more of Ti, Zr, V and Hf, or combinations thereof, having a thickness of from 0.5 to 3 μm and being deposited atop of the α-Al2O3 layer.

5. The cutting tool insert of claim 1 wherein the substrate consists of cemented carbide optionally 0.3-10 wt-% cubic carbides of the metals from groups IVb, Vb and VIb of the periodic table and balance WC.

6. The cutting tool insert of claim 1 wherein the substrate consists of cemented carbide comprising a binder phase enriched surface zone having a thickness of 5 to 30 μm from the substrate surface, the binder phase enriched surface zone having a Co content that is at least 1.5 times higher than in the core of the substrate and having a content of cubic carbides that is less than 0.5 times the content of cubic carbides in the core of the substrate.

7. The cutting tool insert of claim 1 wherein the at least one α-Al2O3 layer has a texture coefficient TC (0 0 12)>5, for the (0 0 12) growth direction.

8. A method of manufacturing a cutting tool insert of claim 1, comprising:

depositing said at least one α-Al2O3 layer by chemical vapour deposition (CVD),

wherein the reaction gas of the CVD process comprises H2, CO2, AlCl3 and X, with X being H2S, SO2, SF6, or combinations thereof, and optional additions of N2 and Ar,

wherein the X is present in the reaction gas mixture in an amount of at least 1.0 vol-% of the total volume of gases in the CVD reaction chamber, and

wherein the volume ratio of CO2 and X in the CVD reaction chamber lies within the range of 1≦CO2/X≦7 during deposition of the at least one α-Al2O3 layer.

9. The method of claim 8, wherein the volume proportion of the component X or the combination of components X is present in the reaction gas mixture during deposition of the at least one α-Al2O3 layer in an amount of at least 1.2 vol-% of the total volume of gases in the CVD reaction chamber.

10. The method of claim 8, wherein the volume ratio of CO2 and X in the CVD reaction chamber lies within the range of 2≦CO2/X≦6 during deposition of the at least one α-Al2O3 layer.

11. The method of any of claim 8, wherein the volume ratio of CO2/AlCl3 in the CVD reaction chamber is equal or smaller than 1.5 and/or the volume ratio of AlCl3/HCl in the CVD reaction chamber is equal or smaller than 1, during deposition of the at least one α-Al2O3 layer.

12. The method of claim 8, wherein the CVD process during deposition of the at least one α-Al2O3 layer is conducted at a temperature in the range of 850 to 1050° C. and/or the CVD process during deposition of the at least one α-Al2O3 layer is conducted at a reaction gas pressure in the range 50 to 120 mbar.

13. The method of claim 8, wherein the component X in the CVD process is H2S or SO2 or a combination of H2S and SO2, whereby, if the component X in the CVD process is a combination of H2S and SO2, the volume proportion of SO2 does not exceed 20% of the volume amount of H2S.

14. The method of any of claim 8, wherein the reaction gas of the CVD process comprises additions of N2 and/or Ar in a volume amount in the range of 4 to 20 vol % of the total volume of gases in the CVD reaction chamber.

15. The cutting tool insert of claim 2 wherein at least one α-Al2O3 layer has a sulphur content of more than 150 ppm analysed by SIMS.

16. The cutting tool insert of claim 7 wherein the at least one α-Al2O3 layer has a texture coefficient TC (0 0 12)>6 for the (0 0 12) growth direction.

17. The method of claim 9, wherein the volume proportion of the component X or the combination of components X is present in the reaction gas mixture during deposition of the at least one α-Al2O3 layer in an amount of at least 1.5 vol-% of the total volume of gases in the CVD reaction chamber.

18. The method of claim 12, wherein the temperature is in the range of 980 to 1050° C.

19. The method of claim 12, wherein the temperature is in the range of 1000 to 1020° C.

20. The method of claim 12, wherein the reaction gas pressure is in the range 50 to 150 mbar.