Coated tool of cemented carbide

Details Coated tool of cemented carbide Coated tool of cemented carbide

1999-06-28

2001-02-13

Turner, A. A. (Department: 1775)

Stock material or miscellaneous articles

Structurally defined web or sheet

Including components having same physical characteristic in...

C051S307000, C051S309000, C407S119000, C428S336000, C428S698000, C428S701000, C428S702000

Reexamination Certificate

active

06187421

ABSTRACT:

TECHNICAL FIELD
This invention relates to a cutting tool, in particular, which is most suitable as a coated cemented carbide cutting tool used for cutting steels and cast irons and which is excellent in wear resistance as well as breakage resistance.
BACKGROUND TECHNIQUE
Hitherto, cemented carbides (WC-Co alloys or WC-Co alloys to which carbonitrides of Ti, Ta or Nb are added) have been used as a tool material for cutting metallic materials. However, as cutting speeds have lately been increased, a tendency of using cemented carbide tools comprising cemented carbide substrates coated with coated films consisting of carbides, nitrides, carbonitrides, carboxides, boronitrides or oxides of Group IVa, Va and VIa elements of the Periodic Table or Al or their solid solutions by CVD or PVD methods in a thickness of 3 to 15 &mgr;m is enhancing. The thickness of the coated films tends to further increase and CVD coated cemented carbides with a coating thickness of at least 20 &mgr;m have been proposed. In such CVD coated cemented carbide tools, there arises a problem that a tensile residual stress occurs in the coated film during cooling after the coating due to difference in coefficient of thermal expansion between the coated film and substrate, and the breakage resistance of the tool is thus lowered.
For a coated cemented carbide tool, on the other hand, it has been proposed in order to improve its breakage resistance, to introduce cracks into a coated film to be penetrated therethrough to a substrate by applying mechanical impact to a surface of a cemented carbide, for example, by blasting (JP-B-7-6066). In this proposed method, it is confirmed that the breakage resistance can be improved to some extent, but because of previously introducing cracks into the coated film to be penetrated therethrough to the substrate, Griffith' precrack length is increased, thus resulting in lowering of the breakage resistance, wear fluctuation of the coated film and deterioration of the wear resistance from the longer cracks.
As described above, the coated cemented carbide tools of the prior art have the problems that when the thickness of a coated film is increased to improve the wear resistance, the breakage resistance of the tool is decreased and even when cracks are previously introduced into a coated film with a relatively large thickness, the wear resistance is rather lowered depending on the cracked state. These problems have not been solved yet.
Under the situation, the present invention aims at providing a coated cemented carbide tool whose both properties of a breakage resistance and wear resistance are improved and service life as a tool is lengthened.
DISCLOSURE OF INVENTION
In order to achieve the above described object, the inventors have made various studies and consequently, have found that using a cemented carbide alloy consisting of a matrix of WC and a binder phase of an iron group metal, a ceramic film having a specified film quality and structure is coated onto its surface and the lengths and intervals of cracks introduced into the coated film are precisely controlled by a thermal or mechanical procedure, whereby to improve both the properties of a breakage resistance and wear resistance and to lengthen the tool life to a great extent. That is, the present invention comprises specified inventions or embodiments summarized below:
(1) A coated cemented carbide cutting tool comprising a substrate consisting of a matrix of WC and a binder phase of an iron group metal and a plurality of coated layers provided on a surface of the substrate, in which (a) an innermost layer, adjacent to the substrate, of the coated layers consists of titanium nitride having a thickness of 0.1 to 3 &mgr;m, preferably 0.3 to 1 &mgr;m, (b) on a mirror-polished cross-sectional microstructure of the said tool, an average crack interval in the coated film on a ridge of a cutting edge and/or rake face is smaller than an average crack interval in the coated layer on a flank face, (c) at least 50%, preferably at least 80% of the cracks in the coated film on the said ridge of the cutting edge and/or rake face have ends of the cracks in the said innermost titanium nitride layer, in a layer above the titanium nitride or in an interface between these layers and (d) an average crack length in the coated film on the said ridge of the cutting edge is shorter than an average coated film thickness on the flank face.
(2) The coated cemented carbide cutting tool as described in the above (1), wherein the interface between these layers is a interface between the innermost titanium nitride layer and the layer directly above the titanium nitride.
(3) The coated cemented carbide cutting tool as described in the above (1) or (2), wherein the said innermost titanium nitride layer is coated with titanium carbonitride layer of columnar structure with an aspect ratio of at least 5, preferably 10 to 50, having a thickness of 3 to 30 &mgr;m, preferably 5 to 15 &mgr;m, and further coated with at least one alumina layer of 0.5 to 10 &mgr;m, preferably 1 to 8 &mgr;m.
(4) The coated cemented carbide cutting tool as described in the above (3), wherein at least 50%, preferably 80 to 100% of the cracks in the coated film on the said ridge of the cutting edge and/or rake face have ends of the cracks, at the substrate side, in the said innermost titanium nitride layer, in the said titanium carbonitride layer of columnar structure or in an interface between the said titanium nitride layer and the said titanium carbonitride layer of columnar structure. (The existing amount of the ends of the cracks at the substrate side herein means the total mounts.)
(5) The coated cemented carbide cutting tool as described in the above (1) or (2), wherein the said innermost titanium nitride layer is coated with alumina layer of 3 to 20 &mgr;m, further coated with titanium carbonitride layer of columnar structure having a thickness of 3 to 30 &mgr;m with an aspect ratio of at least 5 and further coated with alumina layer of 0.5 to 10 &mgr;m.
(6) The coated cemented carbide cutting tool as described in any one of the above (1) to (5), wherein the average crack interval in the coated film on the said ridge of the cutting edge and/or rake face is at most 10 &mgr;m.
(7) The coated cemented carbide cutting tool as described in any one of the above (1) to (6), wherein when a narrower average crack interval in the coated film of the ridge of the cutting edge or rake face on the said cross-sectional microstructure is X and an average value of the crack intervals in the coated film on the flank face is Y, a value of Y/X satisfies at least 2.
(8) The coated cemented carbide cutting tool as described in any one of the above (1) to (7), wherein at least 50%, preferably 75 to 100% of the ends of the cracks at the surface side in the coated film on the said ridge of the cutting edge and/or rake face are not penetrated to the surface of the coated film.
(9) The coated cemented carbide cutting tool as described in any one of the above (2) to (8), wherein at least 50%, preferably 70 to 100% of the cracks in the coated film on the said ridge of the cutting edge and/or rake face exist in only the said titanium carbonitride film of columnar structure and are not penetrated to the upper and lower layers thereof.
(10) The coated cemented carbide cutting tool as described in any one of the above (1) to (9), wherein the surface of the said cemented carbide substrate has a &bgr;-free layer.
(11) The coated cemented carbide cutting tool as described in any one of the above (1) to (10), wherein the cracks in the coated film on the said ridge of the cutting edge are mechanically introduced after coating.
(12) The coated cemented carbide cutting tool as described in any one of the above (3) to (11), wherein the said titanium carbonitride layer of columnar structure is coated at 800° C. to 1000° C., preferably, 850° C. to 950° C. by a CVD method comprising using an organo CN compound as a reactant gas.
(13) The coated cemented carbide cutting tool as described in any one of the above (

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