Coating processes – Coating by vapor – gas – or smoke – Carbon or carbide coating
Reexamination Certificate
1999-12-10
2002-04-02
Chen, Bret (Department: 1762)
Coating processes
Coating by vapor, gas, or smoke
Carbon or carbide coating
C427S249110, C427S577000, C427S314000
Reexamination Certificate
active
06365230
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Fieid of the Invention
The present invention relates to a diamond film coated cutting tool and a manufacturing method thereof, and in particular to a diamond film coated cutting tool which provides and excellent adhesive force enabling machining of a hard workpiece.
2. Description of the Background Art
Generally, a workpiece which is not easily machined by conventional tools is a nonferrous metal such as Al—Si alloy or Cu, a composite material, and a semi-sintered material such as a graphite, a polymer composite material, ceramics, etc. In order to machine these kinds of hard workpieces, a diamond which has the highest hardness value among the existing materials is generally used. Heretofore, the polycrystalline diamond (PCD) cutting tools, which are obtained by brazing a blank (a sintered diamond) to a cemented carbide insert, are widely used for machining certain materials such as Al—Si alloy, etc, which are used as vehicle engine materials. However, a PCD is expensive, and the shape of the same is simple; therefore, it can only be used as an insert shaped tool. In addition, it is very difficult to manufacture a chip breaker which is used for adjusting the chip shape of a workpiece during a cutting operation, and it is difficult to manufacture a tool of complicated shapes like an end mill, drill, reamer, etc., so that the polycrystalline diamond cutting tool is not applicable widely.
In order to completely overcome the above-described problems of the polycrystalline diamond cutting tools, a method for vapor-depositing a diamond film on a substrate was disclosed. Namely, when depositing a diamond film on a certain shaped substrate (insert of a chip breaker type, grill, end mill, etc.) so that the deposited diamond film may have a diamond structure, it is possible to obtain a superior wear-resistance to the conventional polycrystalline diamond tool and to decrease the fabrication cost significantly.
A cemented carbide material, consisted of a tungsten carbide(WC) grain and a cobalt(Co) which is a binder material, is an ideal substrate material for a coated tool. As a binder, Co is added by 6~20 wt %, and other carbide materials such as TiC, TaC, NaC, VC, etc. are added by a few wt % to tens wt % in order to control a mechanical properties such as a toughness, a Near-resistance, etc. Since this material has a toughness adapted for a tool, it is generally used as a substrate material for a coated tool. Thus, it is very important to develop a cutting tool which is coated with diamond on the cemented carbide material having the above-mentioned properties and a fabrication method thereof.
The kinds of chemical vapor deposition (CVD) methods for coating the diamond film are a hot filament CVD, a microwave plasma CVD, a DC plasma CVD, a DC arc-jet CVD, etc, and the diamond film is coated in a state of dissociating a mixed gas of a hydrocarbon such as methane, etc and hydrogen by plasma or thermal energy. When depositing the diamond film on the cemented carbide substrate using the above-described method, the biggest problem is that an adhesive force between the diamond film and the substrate material is very weak irrespective of the deposition method of the diamond film. Subsequently, a premature flaking of the diamond film, i.e. the diamond film having a weak adhesive force is delaminated before a full wear of the diamond film is made, occurs. Therefore, a method to prevent delamination problems of the diamond film during the use of the tool must be developed priorly by enhancing the adhesive force between the diamond film and the cemented carbide substrate material.
In order to overcome the above-described problems, various methods were disclosed for enhancing the adhesive force between the diamond film and cemented carbide substrate material. These methods are based on eliminating any effects of the Co used as a binder material in the cemented carbide substrate material, and enhancing a surface roughness of the substrate material to increase the mechanical interlocking with the diamond film. Since Co acts as a catalyst in transforming a diamond to graphite, Co expedites the graphitization of the diamond during a deposition of the diamond, thereby decreasing the adhesive strength of the diamond film. Therefore, it is required to eliminate any effects of the Co for a diamond film coated tool.
Considering the above-described points, there is provided a method of etching the Co phase of a cemented carbide surface portion by a certain depth to eliminate any effects of the Co, and then etching the cemented carbide grain using a Murakami solution to protrude the cemented carbide grain from the surface, so as to enhance the mechanical interlocking between the diamond film and the substrate material. That is, the etching of the cemented carbide grains as well as the etching of the Co phase makes the surface of the substrate material protruded, and forms many fine windings so as to enhance the mechanical interlocking between the diamond film and the substrate material. The adhesive force of the diamond film can be increased from 60 kg to 100 kg as confirmed by the a Rockwell A indentation test. The etching thickness of the Co layer by 2~15 &mgr;m is required so as to completely eliminate any effects of the Co when the diamond film is coated. Therefore, the portion in which the Co phase acting as the binder material exists remains in a three-dimensionally connected holes (similarity to void in a sponge) by the etching depth from the surface. These voids may decrease the mechanical strength of the surface of the substrate material for thereby causing premature flaking of the diamond, therefore decreasing the life span of the tool. When checking the performance test of a commercial diamond film coated cutting tool, the cutting performance has a wide distribution. In particular, in the case of an interrupted cutting operation or cutting operation of Al alloy having a large amount of Si, the premature flaking of the diamond film is too high.
Also, in order to enhance a mechanical interlocking with the diamond film, there are provided a method of heat treating the substrate under a vacuum or an inert gas atmosphere to significantly decrease the concentration of Co phase on the substrate surface and a method of carburizing cemented carbide grains of the substrate surface to refine the cemented carbide grains or grow a cemented carbide grain for thereby increase the cemented carbide grain size. U.S. Pat. No. 5,623,256 discloses a method of easily evaporating Co of the cemented carbide grains surface by a heat treatment under a nitrogen or hydrogen atmosphere, and a method of forming Co carbides using a carbon atmosphere in a vacuum furnace and then passivating the carbides. In addition, the U.S. patent discloses that during the heat treatment, the grains are grown, which is used for increasing the adhesive force of the diamond film. U.S. Pat. No. 5,068,148 discloses that when the cemented carbide substrate is polished into a final shape, the cemented carbide grains of the substrate surface exist in a broken shape, and that since the diamond film is coated thereon thereafter, the adhesive force of the diamond film is decreased. Therefore, the U.S. patent provides a method of heat-treating the substrate materials at 1000° C. to 1600° C. under a vacuum or a non-oxidizaing atmosphere to melt the broken cemented grains for thereby forming the grown cemented carbide grains on the substrate surface. Thus, the adhesive force of the diamond film increases. However, in order to eliminate any effects of the Co, this patent employs an etching as well as heat treatment method to elimintate the Co layer of the substrate surface.
In a similar manner as U.S. Pat. Nos. 5,623,256 and 5,068,148, U.S. Pat. No. 5,585,176 discloses a method of heat-treating the cemented carbide substrate material at 1510° C. under a nitrogen atmosphere to increase the cemented carbide grain size of the substrate surface and increase the surface roughness by 25~40 pinches, and then coating the diamond film,
Baik Young Joon
Chae Ki Woong
Eun Kwang Yong
Lee Wook-Seong
Chen Bret
Korea Institute of Science and Technology
Scully Scott Murphy & Presser
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