Coated cutting insert

Abstract

There is disclosed a coated cutting insert particularly useful for cutting in cast iron materials. The insert is characterized by a straight WC-Co cemented carbide body having a highly w-alloyed Co binder phase, a well-defined surface content of Co and a coating including an innermost layer of tic_xN_yO_z with columnar grains, a layer of a fine-grained, textured Al₂O₃ layer and a top layer of tic_xN_yO_z that has been removed along the cutting edge line.

Description

• • where M_s is the measured saturation magnetization of the cemented carbide body in kA/m hAm²/kg and
  • weight % Co is the weight percentage of Co in the cemented carbide. The CW-value is a function of the W content in the Co binder phase. A low CW-value corresponds to a high W-content in the binder phase.

It has now been found according to the present invention that improved cutting performance is achieved, if the cemented carbide body has a CW-ratio of 0.75-0.93, preferably 0.80-0.90. The cemented carbide body may contain small amounts, <1 volume %, of eta phase (M₆C), without any detrimental effect.

The surface composition of the cemented carbide insert is well-defined and the amount of Co on the surface is within −4 weight % to +4 weight % of the nominal content.

Alternatively, the cemented carbide according to the present invention consists of WC and Co and has an about 100-350 μm wide, preferably 150-300 μm wide, binder phase depleted surface zone in which the binder phase content increases continuously and in a non-step-wise manner without maximum up to the nominal content of the binder phase in the inner portion of the cemented carbide body. The average binder phase content in a 50 μm surface zone is 25%-75%, preferably 40%-60%, of the nominal binder phase content.

The coating comprises

• • a first, innermost, layer of TiC_xN_yO_z with x+y+z=1, preferably y>x and z<0.1, most preferably y>0.8 and z=0, with a thickness of 0.1-2 μm, and with equiaxed grains with size <0.5 μm. In an alternative embodiment, the TiC_xN_yO_z layer preferably has the composition z<0.5 and y<0.1, most preferably 0.1<z<0.5 and y=0;
  • a layer of TiC_xN_yO_z with x+y+z=1, preferably with z=0, x>0.3 and y>0.3, most preferably x>0.5, with a thickness of 4-12 μm, preferably 5-10 μm, most preferably 6-9 μm with columnar grains and with a diameter of <5 μm, preferably <2 μm;

a layer of TiC_xN_yO_z, with x+y+z=1, with z<0.5, preferably x>y, most preferably x>0.5 and 0.1<z<0.4, with a thickness of 0.1-2 μm and with equiaxed or needle-like grains with size <0.5 μm, this layer being the same as or different from the innermost layer;

• • a layer of textured, fine-grained (with average grain size 0.5-2 μm) α-Al₂O₃ layer with a thickness of 3-8 μm, preferably 3-6 μm; and
  • an outer layer of TiC_xN_yO_z. This TiC_xN_yO_z layer comprises one or more layers with the composition x+y+z=1, z<0.05, preferably y>x. Alternatively, this outer layer can be a multilayer of TiN/TiC/TiN in one or several sequences and a total thickness of 0.5-3 μm, preferably 1-2 μm. This layer exhibits a grain size <1 μm.

In order to obtain a smooth cutting edge line suitable for machining, the edge of the coated insert is subjected to a brushing treatment giving a surface roughness R_max≦0.4 μm over a length of 10 μm according to the method described in Swedish Application No. 9402543-4 (which corresponds to U.S. Ser. No. 08/497,934, our reference 024444-144). This treatment removes the top layer of TiC_xN_yO_z along the cutting edge line. It is also within the scope of this invention that the surface might be smoothed by a wet blasting treatment.

Furthermore, as disclosed in U.S. Pat. No. 5,654,035 or Swedish Applications 9304283-6 (which corresponds to U.S. Ser. No. 08/348,084, our reference 024444-092) or 9400089-0 (which corresponds to U.S. Ser. No. 08/366,107, our reference 024444-093), the α-Al₂O₃ layer has a preferred crystal growth orientation in either the (104)-, (012)- or (110)-direction, preferably in the (012)-direction, as determined by X-ray Diffraction (XRD) measurements. A Texture Coefficient (TC) can be defined as: $\mathrm{TC}\left(\mathrm{hkl}\right)=\frac{I\left(\mathrm{hkl}\right)}{I_o\left(\mathrm{hkl}\right)}{\left\{\frac{1}{n}\sum \frac{I\left(\mathrm{hkl}\right)}{I_o\left(\mathrm{hkl}\right)}\right\}}^{-1}$
where

• • I(hkl)=measured intensity of the (hkl) reflection
  • I_o(hkl)=standard intensity of the ASTM standard powder pattern diffraction data
  • n=number of reflections used in the calculation, (hkl)
  • reflections used are: (012), (104), (110), (113), (024), (116)

TC for the set of (012), (104) or (110) crystal plans should be larger than 1.3, preferably larger than 1.5.

According to the method of the present invention, a WC-Co-based cemented carbide body having a highly W-alloyed binder phase with a CW-ratio of 0.75-0.93, preferably 0.8-0.9, is subjected to a conventional sintering process and removing the surface cobalt by etching as disclosed in U.S. Pat. No. 5,380,408. Alternatively, for cemented carbide consisting of WC and Co cooling at least to below 1200° C. may be performed in a hydrogen atmosphere of pressure 0.4-0.9 bar as disclosed in Swedish Application 9602750-3 (which corresponds to International Application No. PCT/SE97/01231).

The insert is coated with

• • a first, innermost, layer of TiC_xN_yO_z with x+y+z=1, preferably y>x and z<0.1, most preferably y>0.8 and z=0, with a thickness of 0.1-2 μm, and with equiaxed grains with size <0.5 μm. In an alternative embodiment, the TiC_xN_yO_z layer preferably has the composition z<0.5 and y<0.1, most preferably 0.1<z<0.5 and y=0;
  • a layer of TiC_xN_yO_z, with x+y+z=1, preferably with z0, x>0.3 and y>0.3, most preferably x>0.5, with a thickness of 4-12 μm, preferably 5-10 μm, with columnar grains and with a diameter of <5 μm, preferably <2 μm, deposited preferably by MTCVD technique (using acetonitrile as the carbon and nitrogen source for forming the layer in the temperature range of 700-900° C.). The exact conditions, however, depend to a certain extent on the design of the equipment used;
  • a layer of TiC_xN_yO_z, with x+y+z=1, with z<0.5, preferably x>y, most preferably x>0.5 and 0.1<z<0.4, with a thickness of 0.1-2 μm and with equiaxed or needle-like grains with size <0.5 μm, using known CVD methods, this layer being the same as or different from the innermost layer;
  • an intermediate layer of a smooth textured α-Al₂O₃ according to U.S. Pat. No. 5,654,035 or Swedish Applications 9304283-6 or 9400089-0 with a thickness of 3-8 μm, preferably 3-6 μm; and
  • an outer layer of TiC_xN_yO_z, comprising one or several individual layers each with composition x+y+z=1 and z<0.05, preferably y>x. Alternatively, this outer layer comprises a multilayer of TiN/TiC/TiN in one or several sequences. The total coating thickness of these outer layers is 0.5-3.0 μm, preferably 0.5-2 μm. The grain size in this outer layer is <1.0 μm.

The edge line of the inserts in smoothed, e.g., by brushing the edges based on, e.g., SiC, as disclosed in Swedish Application 9402543-4.

When a TiC_xN_yO_z layer with z>0 is desired, CO₂ and/or CO are/is added to the reaction gas mixture.

The invention is additionally illustrated in connection with the following Examples which are to be considered as illustrative of the present invention. It should be understood, however, that the invention is not limited to the specific details of the Examples.

Claims

1. A cutting tool insert comprising a coating and a cemented carbide body, said cemented carbide body comprising WC, 5-10 weight % Co and <0.5 weight % cubic carbides of metals from groups IVb, Vb, or VIb of the periodic table with a highly w-alloyed binder phase having a CW-ratio of 0.75-0.93 and a surface composition of the cemented carbide body being well-defined, the amount of Co on the surface being within −4 weight % to +4 weight % of the nominal Co content of the body and said coating comprising:

a first, innermost, layer of tic_xN_yO_z with x+y+z=1 and y>x and z<0.1, with a thickness of 0.1-2 μm, and with equiaxed grains having a size <0.5 μm;

a layer of tic_xN_yO_z where x+y+z=1, and z=0, x>0.3 and y>0.3, with a thickness of 5-10 μm with columnar grains having a diameter of <2 μm;

a layer of tic_xN_yO_z where x+y+z=1, z<0.5 and x>y with a thickness of 0.1-2 μm and with equiaxed or needle-like grains having a size <0.5 μm;

a layer of smooth, textured, fine-grained α-Al₂O₃ having a grain size of 0.5-2 μm with a thickness of 3-6 μm; and

an outer layer of tic_xN_yO_z where x+y+z=1, z<0.05 with a thickness of 0.5-3 μm and a grain size <1 μm, the outer coating layer having been removed in at least the edge line so that the Al₂O₃ layer is on top along the cutting edge line and the outer layer of tic_xN_yO_z is the top layer on the clearance side.

2. The cutting tool insert of claim 1 wherein the α-Al₂O₃ layer has a texture in (012)-direction and a texture coefficient TC(012) larger than 1.3.

3. A cutting tool insert comprising a coating and a cemented carbide body, said cemented carbide body comprising WC, 5-10 weight % Co and <0.5 weight % cubic carbides of metals from groups IVb, Vb, or VIb of the periodic table with a highly w-alloyed binder phase having a CW-ratio of 0.75-0.93 and a surface composition of the cemented carbide body being well-defined, the amount of Co on the surface being within −4 weight % to +4 weight % of the nominal Co content of the body and said coating comprising:

a first, innermost, layer of tic_xN_yO_z with x+y+z=1 and z<0.5 and y<0.1 with a thickness of 0.1-2 μm, and with equiaxed grains having a size <0.5 μm;

a layer of tic_xN_yO_z where x+y+z=1, and z=0, x>0.3 and y>0.3, with a thickness of 5-10 μm with columnar grains having a diameter of <2 μm;

a layer of tic_xN_yO_z where x+y+z=1, z<0.5 and x>y with a thickness of 0.1-2 μm and with equiaxed or needle-like grains having a size <0.5 μm;

a layer of smooth, textured, fine-grained α-Al₂O₃ having a grain size of 0.5-2 μm with a thickness of 3-6 μm; and

an outer layer of tic_xN_yO_z where x+y+z=1, z<0.05 with a thickness of 0.5-3 μm and a grain size <1 μm, the outer coating layer having been removed in at least the edge line so that the Al₂O₃ layer is on top along the cutting edge line and the outer layer of tic_xN_yO_z is the top layer on the clearance side.

4. The cutting tool insert of claim 1 wherein the outer tic_xN_yO_z layer comprises a multilayer of TiN/tic/TiN.

5. The cutting tool insert of claim 1 wherein the binder phase has a CW ratio of from 0.8-0.9.

6. The cutting tool insert of claim 1 wherein the cobalt content of the cemented carbide body is 5-8 weight %.

7. A method of making a cutting insert comprising a cemented carbide body and a coating wherein a WC-Co-based cemented carbide body is sintered, said sintering including a cooling step which at least to below 1200° C. is performed in a hydrogen atmosphere of pressure 0.4-0.9 bar and thereafter coating said sintered body with

a first, innermost, layer of tic_xN_yO_z with a thickness of 0.1-2 μm, with equiaxed grains with size <0.5 μm by CVD;

a layer of tic_xN_yO_z with a thickness of 4-12 μm with columnar grains and with a diameter of <5 μm deposited by MTCVD technique, using acetonitrile as the carbon and nitrogen source for forming the layer in a temperature range of 850-900° C.;

a layer of tic_xN_yO_z with a thickness of 0.1-2 μm with equiaxed or needle-like grains with size <0.5 μm, using CVD;

a layer of a smooth textured α-Al₂O₃ textured in the direction (012), (104) or (110) with a thickness of 3-8 μm using CVD; and

an outer layer of tic_xN_yO_z with a thickness of 0.5-3 μm, using CVD and thereafter removing the outer layer of tic_xN_yO_z on at least the cutting edge line so that the Al₂O₃ layer is on top along the cutting edge line and the outer layer of tic_xN_yO_z is the top layer on the clearance side of the cutting insert.

8. The cutting tool insert of claim 3 wherein the α-Al₂O₃ layer has a texture in (012)-direction and a texture coefficient TC(012) larger than 1.3.

9. The cutting tool insert of claim 3 wherein the outer tic_xN_yO_z layer comprises a multilayer of TiN/tic/TiN.

10. The cutting tool insert of claim 3 wherein the binder phase has a CW ratio of from 0.8-0.9.

11. The cutting tool insert of claim 3 wherein the cobalt content of the cemented carbide body is 5-8 weight %.

12. The cutting tool insert of claim 7 wherein the outer tic_xN_yO_z layer comprises a multilayer of TiN/tic/TiN.

13. The cutting tool insert of claim 7 wherein the binder phase has a CW ratio of from 0.8-0.9.

14. The cutting tool insert of claim 7 wherein the cobalt content of the cemented carbide body is 5-8 weight %.