Coated hard alloy

Details Coated hard alloy Coated hard alloy

2001-12-07

2004-06-29

Turner, Archene (Department: 1775)

Stock material or miscellaneous articles

Structurally defined web or sheet

Including components having same physical characteristic in...

C428S142000, C428S174000, C428S336000, C428S472000, C428S697000, C428S698000, C428S701000, C428S702000

Reexamination Certificate

active

06756111

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a coated hard metal, particularly one that is most suitable for cutting tools, that has high resistance against wear, chipping, welding, and flaking, and that can maintain the superior properties over an extended period of time.
DESCRIPTION OF THE BACKGROUND ART
Tools made of coated hard metal have become commercially practical and have come into wide use. The tool has a hard metal surface which is coated to improve the cutting property. The coating is composed of one or more layers of titanium carbide, titanium nitride, titanium carbonitride, or aluminum oxide, for example. The coating is deposited by chemical vapor deposition or physical vapor deposition.
Being non-oxide layers, such titanium-based coatings are useful in improving resistance to wear and to chipping. Oxide layers such as an aluminum oxide layer and a zirconium oxide layer are considered to be suitable for use in a temperature range exceeding 700° C. (high-speed cutting range) at the corner of a cutting tool, because they are excellent in chemical stability and resistance to heat.
Cutting work has improved, in recent years, in terms of speed and efficiency prompted by advancements in machine tools and in response to manufacturing-cost reduction demands and productivity improvement requirements. From the viewpoint of environmental protection, dry machining has been in urgent demand in order to reduce the consumption of cutting oil. In response to such a movement in the market, the use of the corner portion of a cutting tool at high-temperature range has been increasing.
In order to meet the market requirement and to lengthen the lifetime of a tool, an oxide layer excellent in chemical stability at high temperatures and in resistance to heat, such as an aluminum oxide layer and a zirconium oxide layer, has been used to coat the surface of a cutting tool, and the thickness of the oxide layer has been increased.
However, when the thickness of the oxide layer (especially the aluminum oxide layer) is increased to 1.5 &mgr;m or more, the crystal grains of the oxide layer constituting the coating become coarsened, developing an unevenness on the surface of the tool in response to the grain size. This unevenness allows the chips of the work material to apply local stresses to the surface of the tool, accelerating the wear and decreasing the toughness. In addition, the chip welds itself to the uneven portion. The welded portion in turn becomes a starting point of stress application, causing flaking of the layer or chipping. As a result, the lifetime of the cutting tool is shortened.
In order to resolve this problem, published Japanese patent application Tokukouhei 5-49750 offers a method for preventing the coarsening of the crystal grains by dividing the aluminum oxide layer into multiple layers. This method undoubtedly decreases the grain size of the aluminum oxide. On the other hand, this method increases the number of interfaces between an aluminum oxide layer and an layer made of another substance, causing flaking to occur easily at the interface. The flaking develops rapid damage, decreasing the lifetime of the tool.
Another published Japanese patent application, Tokukouhei 5-57507, lengthens the lifetime of a tool by removing the unevenness of the face of the oxide layer only at the cutting edge of the tool by polishing. Although this method can increase the lifetime of the cutting edge, the aforementioned face unevenness of the oxide layer remains as a concavity on the cutting face that cannot be processed by polishing. This remaining face unevenness accelerates crater wear and other types of wear, decreasing the lifetime of the tool.
Yet another published Japanese patent application, Tokukouhei 11-124672, uses an &agr;-type aluminum oxide for an outer layer as a high-strength layer and specifies the oriented texture. This method, however, produces on the surface of the tool an unevenness corresponding to the size of the crystal grains of the aluminum oxide in the outer layer, causing a decrease in the lifetime of the tool.
Yet another published Japanese patent application, Tokukouhei 8-158052, discloses a similar layer structure to that of the present invention. However, in Tokukouhei 8-158052, the face roughness of an intermediate layer is transferred to an outer layer without improvement. The present inventors have continued to study a further increase in the lifetime of a tool, especially in the case of high-speed cutting and dry cutting, and completed the present invention.
In view of the foregoing circumstances, the main object of the present invention is to offer a coated hard-metal cutting tool that has an outer layer in which the face unevenness specific to an oxide layer is reduced, that has improved resistance to wear, to chipping, to welding, and to flaking throughout the tool, and that can maintain the superior properties for a prolonged period of time.
DETAILED DESCRIPTION OF THE INVENTION
The present invention offers a coated hard metal having a coating on its surface. The coating comprises an inner layer, intermediate layer, and outer layer in that order from the hard-metal side. The individual layers have the following constitutions and the specified face roughness.
The inner layer includes a layer comprising at least one member selected from the group consisting of (a) the carbides, nitrides, borides, and oxides of the elements belonging to the IVa, Va, and VIa groups in the periodic table and (b) the solid solutions of these.
The intermediate layer includes a layer comprising at least one member selected from the group consisting of aluminum oxide, zirconium oxide, and their solid solution.
The outer layer includes a layer of titanium carbonitride having a columnar structure and a layer comprising at least one member selected from the group consisting of (a) the carbides, nitrides, borides, and oxides of the elements belonging to the IVa, Va, and VIa groups in the periodic table, (b) the solid solutions of these, and (c) aluminum oxide.
As for the face roughness obtained in a cross section showing the structure of the coated hard metal, the relation between “Amax,” which signifies the maximum roughness at the outer face (the interface) of the intermediate layer, and “Bmax,” which signifies the maximum roughness at the outer face (the interface) of the layer of titanium carbonitride having a columnar structure in the outer layer, satisfies equation 1. It is more desirable that the relation satisfy equation 2.
(Bmax/Amax)<1   equation 1,
where 0.5 &mgr;m<Amax<4.5 &mgr;m, and 0.5 &mgr;m≦Bmax 4.5 &mgr;m.
(Bmax/Amax)<0.8  equation 2.
The face roughness is measured by the following method: A cross section perpendicular to the flank of the tool is mirror-polished. A photograph such as shown in
FIG. 1
is taken under an optical microscope at 1,500 power. The unit evaluation length in a measuring area is 0.02 mm. The difference between the maximum peak height and the minimum peak height in the unit evaluation length is defined as the maximum roughness in the present invention. The maximum roughness at the outer face of the intermediate layer and the maximum roughness at the outer face of the layer of TiCN having a columnar structure in the outer layer are measured in five different fields of vision to obtain their respective average values. The average value is used as the value of the maximum roughness to calculate equation 1. Since the conventional probe method is unable to measure the face roughness at an interface, the above-described method is employed in the present invention.
When titanium carbonitride, which has high resistance to wear, is used as the outer layer, the temperature of the entire tool becomes high when the tool is used for cutting at a high-temperature range as in high-speed cutting, because titanium carbonitride has high thermal conductivity. As a result, the hard metal as the base material deforms plastically, raising the cutting resistance and thereby leading to fracture. In order to sol

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