Coated insert

1. A coated cemented carbide cutting tool insert comprising

a substrate of cemented carbide having a composition comprising from about 5.0 to less than about 8.0 wt % Co, from about 3.0 to about 8.0 wt % cubic carbides of the metals Ti, Ta and Nb and possibly other carbides of the elements from group IVb, Vb or VIb of the periodic table, the addition of cubic carbide forming elements being such that the Ti/(Ti+Ta+Nb) ratio is from about 0.05 to about 0.3, from about 0.01 to about 0.2 wt % N and balance tungsten carbide (WC) having a grain size of from about 1.5 to about 3.5 μm in the as-sintered state and having a γ-phase depleted and binder phase enriched surface zone which has a depth of from about 5 to about 30 μm, and

a coating having a first, innermost layer system of at least two layers of TiC x N y O z with x+y+z≦1; and

the said coating having a second multilayer system comprising from about 5 to about 31 alternating layers of Al 2 O 3 and TiC x N y O z (x+y+z≦1), the Al 2 O 3 -layers having an individual layer thickness of less than about 0.6 μm and the TiC x N y O z -layers from about 0.01 to about 0.2 μm with a total thickness of the multilayer from about 1.0 to about 5.0 μm, said multilayer being exposed along the edge line and into the rake and flank face, such that the exposure on the rake face a is greater than about 0.03 and less than about 0.9 mm, the exposure on the flank face b is greater than about 0.02 and less than about 0.2 mm and a>b.

2. The insert of claim 1 wherein the said substrate of cemented carbide has a composition comprising from about 5.0 to less than about 7.0 wt % Co, from about 4.0 to about 7.0 wt % cubic carbides of the metals Ti, Ta and Nb and possibly other carbides of the elements from group IVb, Vb or VIb of the periodic table, the addition of cubic carbide forming elements being such that the Ti/(Ti+Ta+Nb) ratio is from about 0.1 to about 0.25, from about 0.01 to about 0.2 wt % N and balance tungsten carbide (WC) having a grain size of from about 2.0 to about 3.0 μm in the as-sintered state and having a γ-phase depleted and binder phase enriched surface zone which has a depth of from about 5 to about 25 μm.

3. The insert of claim 1 wherein the said second multilayer system comprises from about 11 to about 15 alternating layers of κ-Al 2 O 3 and TiN, the κ-Al 2 O 3 -layers having an individual layer thickness of from about 0.2 to about 0.5 μm and the TiN layers from about 0.02 to about 0.15 μm with a total thickness of the multilayer from about 1.5 to about 4.0 μm.

4. The insert of claim 1 wherein the innermost layer system of the coating comprises

a first layer TiC x N y O z with x+y+z≦1 with equiaxed grains with a size less than about 0.5 μm and a total thickness less than about 1.5 μm but greater than about 0.1 μm,

a second layer of TiC x N y O z with x+y+z≦1 with a thickness of from about 0.4 to about 4.9 μm.

5. The insert of claim 4 wherein

in said first layer, y>x and z<0.2 and said equiaxed grains have a total thickness of from about 0.1 to about 0.6 μm; and

in said second layer, z=0, x>0.3 and y>0.3 and said thickness is from about 1.5 to about 4.0 μm with columnar grains.

6. The insert of claim 5 wherein

in said first layer, y>0.8 and z=0; and

in said second layer, x>0.5.

7. The insert of claim 4 wherein the innermost layer system of the coating comprises

a third layer of TiC x N y O z with x+y+z≦1, with equiaxed grains with size less than about 0.5 μm and a total thickness less than about 1.5 μm but greater than about 0.1 μm.

8. The insert of claim 7 wherein

in said third layer, y>x and z<0.2, said total thickness of the equiaxed grains being from about 0.2 to about 0.8 μm.

9. The insert of claim 8 wherein

in said third layer, y>0.8 and z=0.

10. The insert of claim 7 , wherein the total thickness of the third layer is from about 0.2 to about 0.8 μm.

11. The insert of claim 4 , wherein the total thickness of the first layer is from about 0.1 to about 0.6 μm.

12. The insert of claim 1 wherein the total thickness of the innermost layer system of the coating is from about 0.7 to about 5.5 μm.

13. The insert of claim 12 wherein the said total thickness of the innermost layer system is from about 1.2 to about 5.0 μm.

14. The insert of claim 1 further comprising an outermost layer system on top of the multilayer of one or more layers of TiC x N y (x+y≦1).

15. The insert of claim 14 wherein the multilayer comprises three to five layers in sequence of TiN, TiC and TiN.

16. The insert of claim 1 wherein the total thickness of the first innermost layer and second multilayer system in the coating is from about 2.0 to about 9.0 μm.

17. The insert of claim 16 wherein the total thickness of the first innermost layer and second multilayer system in the coating is from about 4.0 to about 8.0 μm.

18. The insert of claim 1 wherein the multilayer and partly the innermost TiC x N y O z layer system is exposed along the edge line.