The present invention relates to a high performance coated cutting tool insert particularly useful for turning in low alloyed steel, carbon steel and tough hardened steels in the area from finishing to roughing in wet and dry conditions at high cutting speed, having the ability to withstand high temperatures without sacrificing edge security. The insert is based on WC, cubic carbides and a Co-binder phase with a cobalt enriched surface zone giving the cutting insert an excellent resistance to plastic deformation and a high toughness performance. Furthermore, the coating comprises a number of wear resistance layers which have been subjected to a surface post treatment giving the tool insert a surprisingly improved cutting performance. The majority of today's cutting tools are based on a cemented carbide insert coated with several hard layers like TiC, TiC _x Ny, TiN, TiC _x NyO ₂ and AI2O3 _# The sequence and the thickness of the individual layers are carefully chosen to suit different cutting application areas and work-piece materials. The most frequent employed coating techniques are Chemical Vapour Deposition (CVD) and Physical Vapour Deposition (PVD) . CVD-coated inserts in particular have a tremendous advantage in terms of flank and crater wear resistance over uncoated inserts.

The CVD technique is conducted at a rather high temperature range, 950-1050 ⁰ C. Due to this high deposition temperature and to a mismatch in the coefficients of thermal expansion between the deposited coating materials and the cemented carbide tool insert, CVD can lead to coatings with cooling cracks and high tensile stresses (sometimes up to 1000 MPa) . The high tensile stresses can under some cutting conditions be a disadvantage as it may cause the cooling cracks to propagate further into the cemented carbide body and cause breakage of the cutting edge.

In the metal cutting industry there is a constant striving to increase the cutting condition envelope, i.e., the ability to with- stand higher cutting speeds without sacrificing the ability to resist fracture or chipping during interrupted cutting at low speeds .

Important improvements in the application envelope have been achieved by combining inserts with a binder phase enriched surface zone and optimised, thicker coatings.

However, with an increasing coating thickness, the positive effect on wear resistance is out-balanced by an increasing negative effect in the form of an increased risk of coating delamination and reduced toughness making the cutting tool less reliable. This ap- plies in particular to softer work piece materials such as low carbon steels and stainless steels and when the coating thickness exceeds 5-10 μrn. Further, thick coatings generally have a more uneven surface, a negative feature when cutting smearing materials like low carbon steels and stainless steel. A remedy can be to apply a post smoothing operation of the coating by brushing or by wet blasting as disclosed in several patents, e.g., EP 0 298 729, EP 1 306 150 and EP 0 736 615. In US 5,861,210 the purpose has, e.g., been to achieve a smooth cutting edge and to expose the AI2O3 as the top layer on the rake face leaving the TiN on the clearance side to be used as a wear detection layer. A coating with high resistance to flaking is obtained.

Every post treatment technique that exposes a surface, e.g., a coating surface to a mechanical impact as, e.g., wet or dry blasting will have some influence on the surface finish and the stress state (σ) of the coating.

An intense blasting impact may lower the tensile stresses in a CVD-coating, but often this will be at the expense of lost coating surface finish by the creation of ditches along the cooling cracks or it can even lead to delamination of the coating. A very intensive treatment may even lead to a big change in the stress state, e.g., from highly tensile to highly compressive as disclosed in EP-A-I 311 712, in which a dry blasting technique is used.

It has now surprisingly been found that a cutting tool insert having a combination of a certain cemented carbide substrate composition and a certain coating structure and thickness, and being post treated by wet-blasting under controlled conditions obtains excellent cutting properties over a broader range of applications than prior art cutting tool inserts. The cobalt binder phase is highly alloyed with W. The content of W in the binder phase can be expressed as the CW-ratio: CW-ratio = M _s / (wt-%Co*0.0161) wherein M _s = measured saturation magnetization in hAm ² /kg and wt-% Co is the cobalt content in the cemented carbide. A low CW- ratio corresponds to a high W-content in the Co binder phase. The

' employed post treatment will give the coating a favourable tensile stress level, the AI2O3 layer a certain important crystallographic feature and a top surface with an excellent surface finish.

The mentioned combination with the blasting technique effec- tively expands the limitations of what coating thickness that can be applied without performance penalty. As a result of the invention application areas of unsurpassed width is now possible. The significant improvements achieved with respect to toughness behaviour and coating adhesion was surprising. To significantly change the stress state of a coating by blasting, the blasting media, e.g., AI2O3 grits have to strike the coating surface with a high impulse. The impact force can be controlled by, e.g., the blasting pulp pressure (wet blasting), the distance between blasting nozzle and coating surface, grain size of the blasting media, the concentration of the blasting media and the impact angle of the blasting jet.

It is an object of the present invention to provide CVD-coated tool inserts with improved toughness properties having the ability to withstand high temperatures without sacrificing edge security or toughness.

Fig. 1 shows a goniometer set-up for the evaluation of residual stress by X-ray measurements in which E = EuIer ^-cradle S = sample I = incident X-ray beam

D = diffracted X-ray beam θ = diffraction angle ω = θ ψ = tilt angle along the Euler ^-cradle Φ = rotation angle around the sample axis

The present invention thus relates to coated cutting tool inserts comprising a body of generally polygonal or round shape having at least one rake face and at least one clearance face, comprising a coating and a carbide substrate. The body has a composition of 4.4-6.6, preferably 5.0-6.0, most preferably 5.0-

5.8, wt-% Co, 4-8.5 wt-% cubic carbides, balance WC, preferably 85- 91 wt-% WC, most preferably 87-90 wt-% WC, preferably having an average grain size of 1-4 μm, a CW-ratio in the range 0.78-0.92 and a surface zone of a thickness of 15-40 μm, preferably 15-35 μm, most preferably 25-35 μm, depleted from the cubic carbides TiC, TaC

and/or NbC. The coating comprises at least one TiC _x Ny-layer and one well-crystalline layer of 100 % OC-AI2O3. One such CC-AI2O3 layer is the top visible layer on the rake face and along the cutting edge line and the layer can be intensively wet blasted with a sufficiently high energy to create tensile stress relaxation in both the AI2O3 and the TiC _x Ny-layers . The AI2O3 top layer has a very smooth surface at least in the chip contact zone on the rake face.

It has surprisingly been discovered that a significant im- proved toughness performance can be achieved if a coated cutting tool insert with a generally polygonal or round shape having at least one rake face and at least one clearance face, said insert being at least partly coated, produced to possess the following features : - a penultimate TiC _x Ny layer with a thickness of 5-15 μm, preferably 6-13 μm, most preferably 7-13 μm, where x>O, y>0 and x+y=l, preferably produced by MTCVD, with tensile stresses of 50- 500 MPa, preferably 50-400 MPa, most preferably 50-300 MPa and

- an outer α-Al2θ3~layer with a thickness of 3-12 μm, prefera- bly 3.5-8 μm, most preferably 4-8 μm, being the top layer on the rake face and along the edge line having a mean roughness Ra < 0.12 μm, preferably <_ 0.10 μm, at least in the chip contact zone of the rake face, measured over an area of 10 μmx 10 μmby Atomic Force Microscopy (AFM) and an XRD-diffraction intensity (peak height minus background) ratio of I (012) /I (024) > 1.3, preferably > 1.5.

Preferably there is a thin 0.2-2 μm bonding layer of TiC _x NyO ₂ , x>0, z>0 and y>0 between the TiC _x Ny-layer and the 0C-AI2O3-layer. The total thickness of the two layers is £25 μm.

Also according to the present invention, additional layers can be incorporated into the coating structure between the substrate and the layers, composed of metal nitrides and/or carbides and/or oxides with the metal elements selected from Ti, Nb, Hf, V, Ta, Mo, Zr, Cr, W and Al to a total coating thickness of <5 μm.

It is preferred to have some tensile stresses left in the TiC _x Ny layer since it was found that such induced compressive stresses were not as stable with respect to temperature increase, which occurs in a cutting operation, as compared to if the coating has some tensile stresses still present. It was also found, that if compressive stresses were to be induced by blasting, a very high

blasting impact force was required and under such conditions flaking of the coating frequently occurred along the cutting edge.

The residual stress, σ, of the inner TiC _x Ny layer is determined by XRD measurements using the well known sin ² ψ method as de- scribed by I.e. Noyan, J.B. Cohen, Residual Stress Measurement by Diffraction and Interpretation, Springer-Verlag, New York, 1987 (pp 117-130) . The measurements are performed using CuK _α -radiation on the TiC _x Ny (422) reflection with a goniometer setup as shown in Fig. 1. The measurements are carried out on an as flat surface as possible. It is recommended to use the side-inclination technique (ψ-geome- try) with six to eleven ψ-angles, equidistant within a sin ² ψ-range of 0 to 0.5 (ψ=45°) . An equidistant distribution of Φ-angles within a Φ-sector of 90° is also preferred. To confirm a biaxial stress state the sample shall be rotated for Φ=0° and 90° while tilted in ψ. It is recommended to investigate possible presence of shear stresses and therefore both negative and positive ψ-angles shall be measured. In the case of an Euler ^-cradle this is accomplished by measuring the sample also at Φ=180° and 270° for the different ψ- angles. The sin ² ψ method is used to evaluate the residual stress preferably using some commercially available software such as DIFFRAC ^plus Stress32 v. 1.04 from Bruker AXS with the constants Young's modulus, E=480 GPa and Poisson's ratio, V=O.20 in case of a MTCVD Ti (C,N) -layer and locating the reflection using the Pseudo- Voigt-Fit function. In the case of the following parameters are used: E-modulus=480 GPa and Poisson's ratio v=0.20. In case of a biaxial stress state the tensile stress is calculated as the average of the obtained biaxial stresses.

For the OC-AI2O3 it is in general not possible to use the sin^ψ technique since the required high 2Θ angle XRD-reflections are of- ten too weak. However, a useful alternative measure has been found which relates the state of the OC-AI2O3 to cutting performance.

For an OC-AI2O3 powder the diffraction intensity ratio 1(012) /1(024) is close to 1.5. Powder Diffraction File JCPDS No 43- 1484 states the intensities I ₀ (012) =72 and I ₀ (024) =48. For tensile stressed (with σ about > 350 MPa) CVD α-Al2O3-layers on cemented carbide, the intensity ratio I (012) /I (024) is surprisingly significantly less than the expected value 1.5, most often < 1. This may be due to some disorder in the crystal lattice caused by the tensile stresses. It has been found that when such a layer is stress released by, e.g., an intense blasting operation or if it

has been completely removed from the substrate and powdered, the ratio I (012) /I (024) becomes closer, equal or even higher than 1.5. The higher the applied blasting force the higher the ratio will be. Thus, this intensity ratio can be used as an important state feature of an OC-AI2O3 layer.

According to the present invention a cutting tool insert is provided with a CVD-coating comprising a penultimate TiC _x N _v -layer and an outer (X-AI2O3-layer . The AI2O3 can be produced according to patent EP 603 144 giving the AI2O3-layer a crystallographic texture in 012-direction with a texture coefficient TC(012)>1.3, preferably > 1.5 or produced according to patents US 5,851,687 and US 5,702,808 giving a texture in the 110-direction with texture coefficient TC (110) > 1.5. In order to obtain a high surface smoothness and low tensile stress level the coating is subjected to a wet blasting operation with a slurry consisting of F150 grits

(FEPA-standard) of AI2O3 in water at an air pressure of 2.2-2.6 bar for about 10-20 sec/insert. The spray guns are placed approximately 100 mm from the inserts with a 90° spray angle. The insert has a different colour on the clearance side than on the black rake face. An outermost thin 0.1-2 μm colouring layer of TiN (yellow), TiC _x Ny (grey or bronze) , ZrC _x Ny (reddish or bronze) , where x>_0, y>0 and x+y=l or TiC (grey) is preferably deposited. The inserts are then blasted removing the top layer exposing the black AI2O3 layer. The coating on the rake face will have the low desired tensile stress 50-500 MPa while the clearance side will have high tensile stresses in the range 500-700 MPa, the tensile stress on the rake face being lower than the tensile stress on the clearance face, dependent on the choice of coating and the coefficient of Thermal Expansion (CTE) of the used cemented carbide insert. In an other embodiment of the invention the coated insert is blasted both on the rake face and the clearance side and a coloured heat resistant paint is sprayed on the clearance side or a coloured PVD layer is deposited there in order to obtain a possibility to identify a used cutting edge.

Example 1

A) Cemented carbide cutting inserts with the composition 5.5 wt-% Co, 2.9 wt-% TaC, 0.5 wt-% NbC, 1.4 wt-% TiC, 0.9 wt-% TiN, balance WC, having an average grain size of about 2 μm, with a surface zone, 29 μm thick, depleted from cubic carbides. The

saturation magnetization, M _s , was measured to be 0.077 hAmVkg giving a CW-ratio of 0.87. The inserts were coated with a 0.5 μm thick layer of TiN using conventional CVD-technique at 930 ⁰ C followed by a 7 μm TiC _x Ny layer employing the MTCVD-technique using TiCl4, H2, N2 and CH3CN as process gases at a temperature of 885 ⁰ C. In subsequent process steps during the same coating cycle a layer of TiC _x O ₂ about 0.5 μm thick was deposited at 1000 °C using T1CI4, CO and H2, and then the Al2θ3~process was stared up by flushing the reactor with a mixture of 2 % CO2, 3.2 % HCl and 94.8 % H2 for 2 min before a 7 μm thick layer of α-Al2θ3 was deposited. On top was a thin approx. 0.5 μm TiN layer deposited. The process conditions during the deposition steps were as below:

TiN TiC _x Ny TiC _x O ₂ Al ₂ θ3-start ^Al 2θ ₃

Step 1 and 6 2 3 4 5

TiCl ₄ 1.5 % 1.4 % 2 %

N ₂ 38 % 38 %

CO ₂ : 2 % 4 %

CO 6 %

AlCl ₃ : 3.2 %

H ₂ S - 0.3 %

HCl 3.2 % 3.2 %

H ₂ : balance balance balance balance balance

CH ₃ CN - 0.6 %

Pressure: 160 iribar 60 mbar 60 mbar 60 mbar 70 mbar

Temp. : 930°C 885°C 1000 ⁰ C 1000 ⁰ C 1000 ⁰ C

Time: 30 min 4.5 h 20 min 2 min 7 h

B) Cemented carbide cutting inserts of the same type as in A) differing only in TiC _x Ny and OC-AI2O3 layer thickness, being 6 μm and 10 μm thick respectively, were manufactured using the same processing conditions except for the TiC _x Ny and AI2O3 depositing times being 4 h and 1O h, respectively.

XRD-analysis of the deposited AI2O3 layer of the inserts according to A) an B) showed that it consisted only of the α-phase with a texture coefficient TC (012) =1.4 defined as below:

where

I(hkl) = measured intensity of the (hkl) reflection I _o (hkl) = standard intensity of Powder Diffraction File JCPDS No 43-1484. n = number of reflections used in the calculation (hkl) reflections used are: (012), (104), (110), (113), (024), (116).

The coated inserts according to A) and B) were post treated by the earlier mentioned blasting method, blasting the rake face of the inserts, using a blasting pressure of 2.4 bar and an exposure time of 20 seconds.

The smoothness of the coating surface expressed as a well known roughness value Ra was measured by AFM on an equipment from Surface Imaging System AG (SIS) . The roughness was measured on ten randomly selected plane surface areas (lOμmxlOμm) in the chip contact zone on rake face. The resulting mean value from these ten Ra values, MRa, was 0.11 μm. X-ray Diffraction Analysis using a Bragg-Brentano diffractome- ter, Siemens D5000, was used to determine the I (012 ) /I (024) -ratio using Cu Kα-radiation.

The obtained I (012) /I (024) -ratio on the clearance side was about 1.4. A corresponding measurement on the rake face showed that the obtained I (012) /I (024) -ratio was about 2.2.

The residual stress was determined using ψ-geometry on an X- ray diffractometer Bruker D8 Discover-GADDS equipped with laser- video positioning, Euler 1/4-cradle, rotating anode as X-ray source (CuK _a -radiation) and an area detector (Hi-star) . A collimator of size 0.5 mm was used to focus the beam. The analysis was performed on the TiC _x N _y (422) reflection using the goniometer settings 2Θ=126°, ω=63° and Φ=0°, 90°, 180°, 270°, Eight ψ tilts between 0° and 70° were performed for each Φ-angle. The sin ² ψ method was used to evaluate the residual stress using the software DIFFRAC ^plus Stress32 v. 1.04 from Bruker AXS with the constants Young's modulus, E=480 GPa and Poisson's ratio, v=0.20 and locating the reflection using the Pseudo-Voigt-Fit function. A biaxial stress state was confirmed and the average value was used as the residual stress value. Measurements were carried out both on the rake face

and the clearance side. The obtained tensile stress on the clearance side was about 640 MPa for both the inserts according to A) and B) . A corresponding measurement on the rake face showed that a tensile stress of about 450 MPa was obtained for the inserts according to A) and a tensile stress of about 480 MPa was obtained for the inserts according to B) .

Example 2

Inserts A) from Example 1 were tested and compared with commercially available, nonblasted inserts (high performance inserts in the Pl5 area) with respect to toughness in a longitudinal turning operation with interrupted cuts. Material: Carbon steel SS1312. Cutting data: Cutting speed = 120 m/min Depth of cut = 1.5 mm

Feed = Starting with 0.15 mm and gradually increased by 0.08 mm/min until breakage of the edge

10 edges of each variant were tested Inserts style: CNMG120408-PM

Results:

Average feed at breakage

Commercially available inserts 0.244 mm/rev Inserts A) from Example 1 0.275 mm/rev

Example 3

Inserts B) from Example 1 were tested and compared with the same commercially available inserts as in Example 2 with respect to toughness in a longitudinal turning operation with interrupted cuts .

Material: Carbon steel SS1312. Cutting data:

Cutting speed = 140 m/min Depth of cut = 1.5 mm

Feed = Starting with 0.15 mm and gradually increased by 0.08 mm/min until breakage of the edge

10 edges of each variant were tested Inserts style: CNMG120408-PM

Results :

Average feed at breakage

Commercially available inserts 0.232 mm/rev Inserts B) from Example 1 0.315 mm/rev

Example 5

Inserts A) from Example 1 were tested with respect to resistance to gross plastic deformation in a facing operation of SS2541.

Cutting data:

Cutting speed= 220 m/min

Feed= 0.35 mm/rev. Depth of cut= 2 mm

Tool life criteria: flank wear >= 0.5 mm

Results : ^' Number of machining cycles needed to reach tool life Commercially available inserts 65 Inserts A) from Example 1 85

Example 6

Inserts A) from Example 1 were also tested with respect to resistance to plastic deformation close to the edge in turning of SS2244-05.

Cutting data:

Cutting speed: 200 m/min

Feed: 0.35 mm/rev.

Depth of cut: 2.5 mm

Tool life criteria: flank wear >= 0.4 mm

Results:

Number of machining cycles needed to reach tool life Commercially available inserts 19

Inserts A) from Example 1 27*

* The test for inserts A) was terminated prematurely, after 27 cycles, with the defined tool life criteria still not being reached.

Examples 3-6 show that the inserts A) and B) from Example 1 and according to the invention exhibit much better plastic deformation resistance in combination with better toughness behaviour in comparison to the inserts according to prior art.