WO2009070107A1

WO2009070107A1 - Coated cutting tool insert

Info

Publication number: WO2009070107A1
Application number: PCT/SE2008/051354
Authority: WO
Inventors: Rolf Olofsson; Björn Ljungberg; Henrik Nordlund; Gunilla Anderson; Alexandra Kusoffsky
Original assignee: Sandvik Intellectual Property Ab
Priority date: 2007-11-28
Filing date: 2008-11-26
Publication date: 2009-06-04

Abstract

The present invention relates to a CVD-coated cutting tool insert with improved toughness properties having the ability to withstand high temperatures resulting from, e.g., extended times in cut or high metal removal. The insert has a cemented carbide body with composition of 4.0-5.3 wt-% Co, 6.4-8.1 wt-% cubic carbides, balance WC, a hardness HV3 of 1590-1650, a CW-ratio in the range 0.78-0.92 and having a surface zone of a thickness of 10 to 16 mm depleted from the cubic carbides TiC, TaC and/or NbC, the insert being at least partly coated with a 11-19 mm thick coating including at least one layer of TiC xN y and an a-Al 2O 3 layer being the outer layer on the rake face and on the cutting edge line, and that on said at least one rake face the TiC xN y-layer has a thickness of from 4 to 12 mm and a tensile stress level of 50-500 MPa and the a-Al 2O 3-layer having a thickness of from 8 mm to 12 mm and has a mean Ra value MRa < 0.12 mm, at least in the chip contact zone on the rake face, and on said at least one clearance face the TiC xN y-layer has a tensile stress in the range 500-700 MPa and the a-Al 2O 3-layer is covered with a thin 0.1-1 mm TiN, TiC xN y,ZrC xN y or TiC layer giving the insert a different colour on this face.

Description

COATED CUTTING TOOL INSERT

The present invention relates to a high performance coated cutting tool insert particularly useful for turning at stable conditions in unalloyed and low alloyed steels in wet and dry conditions, having the ability to withstand high temperatures, resulting from, e.g., extended times in cut or high metal removal.

The majority of today's cutting tools are based on a cemented carbide insert coated with several hard layers like TiC, TiC_xN_y, TiN, TiCχN_yO_z and Al₂O₃. 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) .

The CVD technique is conducted at a rather high temperature range, conventionally 950-1050 ⁰C. 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 productivity, wherein turning operations involving high metal removal have an important role. Such operations generally place high demands on the cutting insert in respect of ability to withstand high temperatures, often during extended times in cut, as well as high demands on wear resistance. Important improvements in this area have been achieved by combining inserts with a binder phase enriched surface zone and optimized, thicker coatings. However, with an increasing coating thickness, a 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. Further, thick coatings generally have a more uneven surface, negatively affecting the cutting performance. These problems can, however, largely be remedied by applying a post treatment of the coating.

Smoothing operation of the coating by brushing or by wet blasting are 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 outermost layer on the rake face leaving the TiN on the clearance side to be used as a wear detection layer.

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 influence the surface finish and the stress state (σ) 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. 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. As disclosed in, e.g., WO 2006/135330 and EP 1 734 155, a post treatment comprising intensive wet blasting with a sufficiently high energy can give the coating a favourable tensile stress level, i.e., create tensile stress relaxation, and give the Al₂θ₃-layer a top surface with an excellent surface finish.

It is an object of the present invention to provide a CVD- coated tool insert having the ability to withstand high temperatures, resulting from, e.g., extended times in cut or high metal removal .

It has surprisingly been found that by finely tuning the balance between the hardness of a WC-Co based substrate and the depth of a toughness inducing Co enriched surface zone of the substrate in combination with a thick and partly stress relaxed coating has a significant effect on an inserts ability to withstand high temperatures.

According to the present invention there is provided a coated cutting tool insert of cemented carbide comprising a body having at least one rake face and at least one clearance face wherein said insert has a composition of 4.0-5.3, preferably 4.4-5.25, most preferably 4.8-5.2, wt-% Co, 4.6-8.1 wt-% cubic carbides, balance WC and a hardness HV3 of 1590-1650.

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 = M₃/ (wt-%Co*0.0161) wherein M₃ = measured saturation magnetization in hAm²/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 CW-ratio is in the range 0.80-0.88, and the body has a surface zone of a thickness of 10-16 μm, preferably 10-15 μm, depleted from the cubic carbides TiC, TaC and/or NbC.

The insert is at least partly coated with a 10-25 μm thick coating including at least one layer of TiC_xN_y, where x≥O, y≥O and x+y=l, preferably MTCVD-TiC_xNy, and a well crystalline CC-Al₂O₃- layer, preferably 100 % α-Al₂θ3-layer, being the outermost visible layer on the rake face and on the cutting edge line, and on the at least one rake face of the insert

- the TiCχN_y-layer has a thickness of from 4 to 12 μm, preferably from 5 to 7 μm or from 9 to 11 μm, and a tensile stress level of 50-500, preferably 50-450, MPa and

- the α-Al₂θ3-layer has a thickness of from 8 to 12 μm, preferably from 9 to 11 μm and a mean Ra value

MRa < 0.12 μm, preferably < 10 μm, at least in the chip contact zone on the rake face, and on said at least one clearance face

- the TiCχN_y-layer has a tensile stress in the range 500-700 MPa and

- the α-Al₂O₃-layer is covered with a thin 0.1-1 μm TiN, TiC_xN^ ZrC_xNy or TiC layer giving the insert a different colour on this face .

The coating on the rake face and along the cutting edge line is intensively wet blasted with a sufficiently high energy to create tensile stress relaxation in both the A1₂C>3- and the TiC_xN_y- layers. The outermost A1₂C>3 layer has a very smooth surface at least in the chip contact zone on the rake face.

Preferably there is a thin 0.2-1 μm bonding layer of TiC_xN_yO_z, x≥O, z>0 and y≥O between the TiC_xN_y-layer and the CC-Al₂O₃-layer . 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.

According to the present invention the cutting tool insert is, thus, provided with a CVD-coating comprising a penultimate TiC_xN_y- layer and an outer α-Al₂θ₃-layer . The AI2O3 can be produced according to patent EP 603 144 giving the Al₂θ₃-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. However, in another embodiment, the AI2O3 is produced according to patent EP 738 336 giving the Al₂θ3-layer a crystallographic texture in 104-direction with a texture coefficient TC (104) > 2.0, preferably TC(104)>2.5.

The insert has a different colour on the clearance side than on the black rake face. Preferably, an outermost thin 0.1-1 μm colouring layer of TiN (yellow) , TiC_xN_y (grey or bronze) , ZrC_xN_y (reddish or bronze), where x≥O, y≥O and x+y=l, or TiC (grey) is deposited. The insert is then blasted removing the top layer exposing the black Al₂U₃-layer.

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 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.

Example 1 The following sample was prepared: A) Cemented carbide cutting inserts with the composition 5.0 wt-% Co, 2.9 wt-% TaC, 0.5 wt-% NbC, 1.9 wt-% TiC, 0.35 wt-% TiN, balance WC, a hardness HV3 of 1620, measured according to ISO 3878- 1983 (E), Hardmetals-Vickers test, and with a surface zone, 14 μm thick, depleted from cubic carbides.

The saturation magnetization, M₃, was measured to be 0.068 hAm²/kg giving a CW-ratio of 0.84. The inserts were coated with a 0.5 μm thick layer of TiN using conventional CVD-technique followed by a 10 μm TiC_xN_y-layer employing the MTCVD-technique . In subsequent process steps during the same coating cycle a layer of TiC_xO_z about 0.5 μm thick was deposited, and then the Al₂θ3-process was started up by flushing the reactor with a mixture of 2.0 CO₂ 3.0 HCl and 95 % N₂ for 2 min before a 10 μm thick layer of CC-Al₂θ3 was deposited. On top was a thin, approximately 0.5 μm, TiN layer deposited. The process conditions during the deposition steps were as below:

TiN TlC_xN_y TiC_xO_z Al₂O₃-start Al₂O₃

Step 1 and 6 2 3 4 5

TiCl₄ 1.5 % 1.4 % 2 %

N₂ 38 % 38 % balance

CO₂: 2.0 % 4.4 %

CO 6 %

AlCl₃: 3.2 %

H₂S - 0.3 %

HCl 3.0 % 3.0 %

H₂: balance balance balance - balance

CH₃CN - 0.6 %

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

Temp. : 930⁰C 885°C 1000⁰C 1000⁰C 1000⁰C

Time: 30 mm 6 h 20 mm 2 mm 10 h

Additional insert was:

B) Cemented carbide cutting inserts of the same type as in A) differing only in TiC_xN_y- and 0C-Al₂O₃-layer thickness, being 6 μm and 10 μm thick respectively, were manufactured using the same processing conditions except for the TiC_xN_y and Al₂O₃ depositing times being 4 h and 10 h, respectively. X-ray Diffraction Analysis (Bragg-Brentano diffractometer, Siemens D5000, Cu KCC-radiation) of the deposited Al₂O₃-layer of the inserts according to A) and B) showed that it consisted only of the α-phase with a texture coefficient TC (104) =2.6 defined as below:

- 1

TCdOD - ^I(104) I ¹ V ^{I (hkl )}

^{( }} I_o(104) ^{t n} 2^° ^{(hkl )} 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 the rake face. The resulting mean value from these ten Ra values, MRa, was 0.10 μm. The residual stress, σ, of the inner TiC_xN_y-layer was determined by XRD measurements using the well known sin²ψ method as described by I.C. Noyan, J. B. Cohen, Residual Stress Measurement by Diffraction and Interpretation, Springer-Verlag, New York, 1987 (pp 117-130), and the measurements were performed according to WO 2006/135330 with the following specifics:

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_α-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_xN_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 sin²ψ 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=O.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 the inserts according to A) and B) . A corresponding measurement on the rake face showed that a tensile stress of about 318 MPa was obtained for the inserts according to A) and a tensile stress of about 420 MPa was obtained for the inserts according to B) .

Example 2

Inserts A) from Example 1 were tested and compared with commercially available inserts (high performance inserts in the P05 and P15 area, respectively) in a roughing turning operation. Roughing of axle diameter, Φ=70 mm and length=300 mm, of tough- hardened steel.

Material: 42CrMoS4V (290 HB) . Cutting data: v_c = 180 m/min f_n = 0.35 mm/ rev a_p = 2 mm Coolant

Inserts style: CNMG120412-PR

Results: No. of axles before failure of insert

Examination of the inserts showed that the P15 grade had obtained plastic deformation/crater wear and the P05 grade had obtained chipping/breakage. The grade A) according to the invention showed only flank wear. It can be concluded from the test that insert A) according to the invention had a better edge line security than the P05 grade and a better resistance to plastic deformation than the P15 grade.

Example 3

Inserts B) from Example 1 were tested and compared with commercially available inserts (high performance inserts and competitive grade in the P05 area, respectively) in a roughing turning operation. Facing and turning of inner diameter of a CV- joint, Φ=70 mm, of unalloyed steel.

Material: Cf53 (220 HB) . Cutting data: v_c = 280 m/min f_n = 0.35 mm/rev a_p = 2 mm

Dry

Inserts style: WNMG080412-PR

Results: No. of CV-joints before failure of insert

Examination of the inserts showed that the high performance P05 grade had obtained chipping/breakage and the competitive P05 grade had obtained flank wear. The grade B) according to the invention showed only flank wear.

It can be concluded from the test that insert B) according to the invention had a better edge line security than the high performance P05 grade and a better wear resistance than the competitive P05 grade.

Example 4

Inserts A) from Example 1 were tested and compared with commercially available inserts (high performance inserts and competitive grade in the P15 area, respectively) in a roughing turning operation. Facing and turning of outer diameter of a ring, Φ=1250 mm, of ball bearing steel.

Material: 827B (190 HB) . Cutting data: v_c = 170 m/min f_n = 0.95 mm/rev a_p = 7 mm Coolant

Inserts style: SNMM 190624

Results: Time before failure of insert

Examination of the inserts showed that the high performance P15 grade had obtained flank wear and plastic deformation, and the competitive P15 grade had obtained flank wear, chipping and crater wear. The grade A) according to the invention showed only flank wear . It can be concluded from the test that insert A) according to the invention had a better deformation resistance and crater wear resistance than the high performance P15 grade and the competitive P15 grade.

From the tests in Example 2-4 it is evident that insert A) according to the invention has an improved wear resistance and a more secure performance compared to the commercially available grades .

Claims

1. A coated cutting tool insert of cemented carbide comprising a body having at least one rake face and at least one clearance face c h a r a c t e r i z e d in said insert having a composition of 4.0-5.3 wt-% Co, 4.6-8.1 wt-% cubic carbides, balance WC, a hardness HV3 of 1590-1650, a CW-ratio in the range 0.80-0.88 and having a surface zone of a thickness of 10 to 16 μm depleted from the cubic carbides TiC, TaC and/or NbC, said insert being at least partly coated with a 10-25 μm thick coating including at least one layer of TiC_xN_y, where x>0, y>0 and x+y=l, and an α-Al₂O₃-layer being the outer layer on the rake face and on the cutting edge line, and that on said at least one rake face

- the TiCχN_y-layer having a thickness of from 4 to 12 μm and a tensile stress level of 50-500 MPa and - the α-Al₂θ₃-layer having a thickness of from 8 to 12 μm and having a mean Ra value MRa < 0.12 μm, at least in the chip contact zone on the rake face, and on said at least one clearance face

- the TiCχN_y-layer having a tensile stress in the range 500-700 MPa and that

- the α-Al₂O₃-layer is covered with a thin 0.1-1 μm TiN, TiC_xN^ ZrC_xN_y or TiC layer giving the insert a different colour on this face.

2. A cutting tool insert according to claim 1 c h a r a c t e r i z e d in having 4.4-5.25, preferably 4.8-5.2, wt-% Co and a surface zone of a thickness of 10 to 15 μm, depleted from the cubic carbides TiC, TaC and/or NbC.

3. A cutting tool insert according to any of the preceding claims c h a r a c t e r i z e d in said at least one layer of TiC_xN_y is MTCVD-TiC_xN_y.

4. A cutting tool insert according to any of the preceding claims c h a r a c t e r i z e d in that on said at least one rake face the TiC_xN_y-layer having a thickness of from 5 to 7 μm, or from 9 to 11 μm, and having a tensile stress level of 50-450 MPa.

5. A cutting tool insert according to any of the preceding claims c h a r a c t e r i z e d in that on said at least one rake face the α-Al₂θ3-layer having a thickness of 9 to 11 μm and having a mean Ra value MRa ≤ 0.10 μm, at least in the chip contact zone on the rake face.

6. A cutting tool insert according to the preceding claim c h a r a c t e r i z e d in having a thin 0.2-1 μm TiC_xN_yO_z bonding layer, x≥O, z>0 and y≥O, between the TiC_xN_y- and the Al₂O₃-layer.

7. A cutting tool insert according to any of the preceding claims c h a r a c t e r i z e d in the 0C-Al₂θ₃-layer having a texture in the 104-direction with a texture coefficient TC(104)>2.0, preferably >2.5.

8. A cutting tool insert according to any of the preceding claims c h a r a c t e r i z e d in the α-Al₂θ₃-layer having a texture in the 012-direction with a texture coefficient TC(012)>1.3, preferably TC(012)> 1.5.

9. A cutting tool insert according to any of the preceding claims c h a r a c t e r i z e d in the 0C-Al₂θ₃-layer having a texture in the 110-direction with a texture coefficient TC(110)>1.5.