Coated cemented carbide cutting tool

Details Coated cemented carbide cutting tool Coated cemented carbide cutting tool

1999-11-23

2001-09-25

Tsai, Henry (Department: 3722)

Cutters, for shaping

Comprising tool of specific chemical composition

C407S118000, C428S216000

Reexamination Certificate

active

06293739

ABSTRACT:

TECHNICAL FIELD
The present invention relates to a coated cemented-carbide cutting tool that has high toughness and superior wear resistance.
BACKGROUND ART
Prolongation of the tool life has been practiced by depositing titanium carbide, titanium nitride, titanium carbonitride, Al
2
O
3
, or another coating layer on the surface of a cemented-carbide cutting tool. Chemical vapor deposition (CVD), plasma CVD, and physical vapor deposition processes have been widely used for providing the coating layer.
However, the wear resistance of the coating layers has been insufficient, and the tool life has been shortened due to damage to or flaking of the coating layer when these coated cemented-carbide cutting tools are used particularly for the following machining: (1) machining, such as high-speed cutting of steel or high-speed machining of cast iron, that requires wear resistance and crater resistance in the coating layer at high temperatures, and (2) machining, such as small-parts machining, that has many machining processes and many leading parts on the workpiece.
In order to surmount these problems, controlling the structure and oriented texture of the coated layer has been studied on the multiple coated-layer structure in which the outer layer comprises Al
2
O
3
and the inner layer comprises titanium carbide or titanium carbonitride, for example, which is superior in hardness as well as in bonding with cemented carbides. For example, published Japanese patent application Tokuhyohei 9-507528 has disclosed a coating method in which Al
2
O
3
having an &agr;-type crystal structure, which is stable at high temperatures, is given a certain amount of oriented texture in order to improve the high-temperature properties. Although the Al
2
O
3
having an &agr;-type crystal structure is said to be superior in high-temperature properties, the material is well known to have difficulty in obtaining high bonding strength that prevents flaking at the time of cutting. In the above-mentioned prior technique also endeavor has been made to obtain high bonding strength by controlling the moisture content at the initial stage of the coating of Al
2
O
3
. However, it cannot be said that sufficient bonding strength is obtained by this technique.
DISCLOSURE OF INVENTION
Under these circumstances, an object of the present invention is to prolong the life time of tools extensively and stably by (1) considerably improving the flaking resistance of the coating layer at the time of cutting, (2) increasing the wear resistance and crater resistance of the coating layer itself, and (3) enabling the enhancement of the breakage strength of the coating layer in comparison with the conventional coated cutting tools.
In order to achieve the above-described object, the present invention offers the following structure:
The structure comprises:
a cemented-carbide substrate that comprises a hard phase comprising tungsten carbide as the main constituent and at least one member selected from the group consisting of carbide, nitride, and carbonitride of the metals in the I Va, Va, and V I a groups, and a bonding phase mainly consisting of Co; and
a ceramic coating layer on the cemented-carbide substrate, the ceramic coating layer comprising an inner layer and an outer layer.
The inner layer comprises at least one layer of Ti(CwBxNyOz), where w+x+y+z=1, and w, x, y, and z≧0. The outer layer has an Al
2
O
3
layer at the place where the outer layer is in contact with the inner layer. The Al
2
O
3
practically comprises &agr;-Al
2
O
3
. More specifically, the Al
2
O
3
has a region where grains having an &agr;-type crystal structure and grains having a &kgr;-type crystal structure coexist in the first row of the crystal grains that grow on the inner layer. The crystal grains of the &agr;-Al
2
O
3
in the region include practically no pores.
It is preferable that the outer layer include at least one layer of Ti(CwBxNyOz), where w+x+y+z=1, and w, x, y, and z≧0, in addition to the Al
2
O
3
.
The following effects are attained by the coexistence of the grains having an &agr;-type crystal structure and the grains having a &kgr;-type crystal structure in the first row of crystal grains that grow on the inner layer.
First, high bonding strength between the outer layer and inner layer can be obtained by providing a certain proportion of Al
2
O
3
having a &kgr;-type crystal structure, which is superior in bonding to the layer directly underneath, in the first row at the interface with the inner layer. In addition to that, the gradual dominance of the Al
2
O
3
having an &agr;-type crystal structure over the Al
2
O
3
having a &kgr;-type crystal structure during the growing process of the Al
2
O
3
enables the final growth, at the outermost layer, of the Al
2
O
3
having an &agr;-type crystal structure, which has superior mechanical and chemical wear resistance and breakage resistance under high-temperature cutting environments.
Second, the structure having practically no pores in the crystal grains of the &agr;-Al
2
O
3
in the region enables the suppression of the reduction in the bonding strength; this reduction has caused problems in the conventional coated cutting tools having &agr;-Al
2
O
3
. The low bonding strength of the conventional &agr;-Al
2
O
3
is attributable to the strength reduction in the coating layer caused by the pores; this strength reduction has generated the mechanism of breakage followed by flaking of the layer.
As described above, the structure of the present invention enables the formation of &agr;-A
2
O
3
, which is superior as a coating layer, on the inner layer with substantially high bonding strength, improving the cutting performance extensively.
It is desirable that the inner layer comprise two or more layers of Ti(CwBxNyOz), where w+x+y+z=1, and w, x, y, and z≧0, and that the layers mainly consist of titanium carbonitride having a columnar structure. This constitution enables the attainment of substantially high wear resistance through not only preventing the damage starting at the outer Al
2
O
3
layer during intermittent cutting and cutting for parts machining, for example, but also preventing coating-layer breakage in the inner layer and separation between the inner layer and the substrate, thus enabling dramatic improvement of the tool performance.
It is desirable that the Al
2
O
3
having an &agr;-type crystal structure in the structure of the present invention have a &kgr;/&agr; ratio of 0.25 to 0.75 in the first row lying on the inner layer, where the &kgr;/&agr; ratio means the existing ratio of the grains of the &kgr;-Al
2
O
3
to the grains of the &agr;-Al
2
O
3
. The &kgr;/&agr; ratio in this range enables easier concurrent attainment of the high bonding strength and the final coating of the Al
2
O
3
having an &agr;-type crystal structure at the outermost layer. It is preferable that the &kgr;/&agr; coexistence not be limited to the first row but extended to the following rows in a manner such that the &kgr;/&agr; ratio decreases in the upward direction from the first row and becomes zero within the coating layer. The reason being that if the &kgr;-type and the &agr;-type coexist only in the first row, strains caused by the abrupt change in the distribution of crystal structure may decrease the strength of the coating layer at this location. It is yet preferable that the coexisting region is limited within 1.5 &mgr;m of the interface with the inner layer because if the coexisting region extends beyond this limit, the existence of the Al
2
O
3
having a &kgr;-type crystal structure begins to worsen the quality of the coating layer.
In the structure of the present invention, the increase in the initial nucleation density in the Al
2
O
3
layer on the inner layer can increase the bonding strength. This increase in bonding strength is preeminent when the nucleation density has a level such that the majority of the grains in the first row, where &agr;-Al
2
O
3
and &kgr;-Al
2
O
3
coexist, on the inner layer have a grain

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