Fabrication method for diamond-coated cemented carbide cutting tool

A fabrication method for a diamond-coated cemented carbide cutting tool includes electrolytically etching the surface of a cemented carbide cutting tool with a NaOH or KOH aqueous solution or chemically etching the surface of the cemented carbide cutting tool with a KMnO.sub.4 +KOH aqueous solution, and depositing a diamond film on the cemented carbide cutting tool. A stronger etching effect than by using a Murakami solution can be achieved, and no poisonous material is included in the etchant. In addition, disposal of the waste etchant is made simpler, and the adhesion between the diamond film coating and the cemented carbide cutting tool can be strengthened.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to coating a diamond film on a cemented 
carbide cutting tool, and in particular, to an improved fabrication method 
for a diamond-coated cemented carbide cutting tool which is capable of 
enhancing the adhesion of a diamond film to a cemented carbide by 
electrolytically etching the surface of the cemented carbide with a NaOH 
or KOH aqueous solution before the deposition of the diamond film, or by 
chemically etching the same with a KMnO.sub.4 +KOH aqueous solution. 
2. Description of the Conventional Art 
Conventionally, a cemented carbide cutting tool having a surface which is 
coated with a diamond film is known to have a much superior cutting 
capability compared to a general cemented carbide cutting tool. 
However, a significant problem with such a cemented carbide cutting tool is 
that the diamond-coated film peels off from the cemented carbide cutting 
tool and as a result loses its function. 
The above problem occurs when the adhesion between the surface of the 
cemented carbide cutting tool and the diamond-coated film is not strong 
enough. Many methods to enhance the adhesion therebetween have been 
suggested, but the simplest method so far suggested is to etch the surface 
of the cemented carbide cutting tool with a chemical solution. 
Two etching methods are chiefly used: one is to etch away a binder phase of 
the surface of the cemented carbide cutting tool with an acid, and the 
other is to etch away the carbide phase. 
The former method involves removing cobalt down to a predetermined depth 
since the cobalt used as the binder phase forms a non-diamond phase 
between the diamond film and the cemented carbide cutting tool due to its 
catalytic action. But this method itself does not provide a satisfactory 
adhesion. 
On the other hand, the latter method involves etching the carbide phase by 
using a Murakami's reagent and then etch the cobalt phase as in the former 
method. U.S. Pat. No. 5,236,740, discloses a fabrication method for a 
diamond-coated cemented carbide cutting tool using a Murakami's reagent 
composed of 10 g of potassium ferricyanide +10 g of potassium hydroxide 
+100 g of water, in which the surface of the cemented carbide cutting tool 
is etched for more than two minutes, and then etched for more than five 
seconds with a mixed solution of sulfuric acid-hydrogen peroxide. 
According to this patent disclosure, the Murakami's reagent dissolves a 
predetermined amount of the carbide phase off the surface of the cemented 
carbide cutting tool, resulting in a considerable improvement in the 
adhesion of the diamond film, but there is no specific mention about the 
reason why such treatment increases the adhesion. In addition, the cost of 
potassium ferricyanide is about ten times higher than that of potassium 
hydroxide, wherein the two reagents comprise the Murakami reagent. Above 
all, when the potassium ferricyanide is heated or contacts with an acid or 
its vapor, extremely poisonous cyanide fumes are generated. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to provide an 
improved fabrication method for a diamond-coated cemented carbide cutting 
tool which is capable of enhancing the stability of the same and using a 
non-poisonous aqueous solution for etching the surface of the 
diamond-coated cemented carbide cutting tool. 
It is another object of the present invention to provide an improved 
fabrication method for a diamond-coated cemented carbide cutting tool 
which is capable of strengthening the adhesion of the diamond-coated film 
by enhancing the effect of a mechanical interlocking between the diamond 
film and the surface of the cemented carbide cutting tool by etching the 
carbide phase of the cemented carbide cutting tool and increasing the 
roughness of the surface of the same. 
When the potassium ferricyanide in the Murakami reagent which is used in 
the conventional art is reacted with KOH in the same reagent, it yields 
water and oxygen. 
EQU 2K.sub.2 Fe(CN).sub.6 +2KOH.fwdarw.2K.sub.4 Fe(CN).sub.6 +H.sub.2 
O+1/20.sub.2 
Thus, the pottassium ferrycianide acts as an oxidizer, and when an 
electrical potential is applied to a test piece to be etched instead of 
the oxidizer, the same effect can be achieved.(see: Metallography, 
Principles and practice, by George F. Vader Voort, MacGraw-Hill Book Co. 
1984) 
In a first method according to the present invention, the adhesion between 
the surface of the cemented carbide cutting tool and the diamond film can 
be enhanced by coating the diamond film on the surface of the cemented 
carbide after etching the same by applying a voltage to the objects to be 
etched instead of removing the poisonous potassium ferricyanide which is 
an oxidizer. 
And in a second method according to the present invention, a KMnO.sub.4 
+KOH aqueous solution is used as an etching solution for etching the 
surface of the cemented carbide cutting tool, with KMnO.sub.4 substituted 
for the poisonous pottassium ferricyanide, before coating the diamond film 
thereon to strengthen the adhesion between the surface of the cemented 
carbide cutting tool and the diamond film. 
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The fabrication method of a diamond-coated cemented carbide according to 
the present invention will now be described in more detail. 
The method of etching the surface of a cemented carbide cutting tool 
according to the present invention includes electrolytically etching the 
same using a NaOH or KOH aqueous solution as an electrolytic solution. 
Here, the preferable concentrations of the KOH or NaOH solution are 
1.about.60 wt % and 1.about.70 wt %, respectively, and the cemented 
carbide cutting tool is attached to the anode. The voltage and the current 
are controlled in accordance with the concentration of the KOH or NaOH 
solution. 
Another method according to the present invention is to chemically etch the 
surface of a cemented carbide cutting tool with a KMnO.sub.4 +KOH aqueous 
solution. The etching solution is preferably composed of 1.about.20 wt % 
KMnO.sub.4, 1.about.60 wt % KOH and balance H.sub.2 O, and the treatment 
proceeds for more than one minute as appropriate. 
The above mentioned etching methods are facilitated by first washing the 
cemented carbide cutting tool with distilled water, acetone or 
trichloroethylene(TCE). When the surface of the cemented carbide cutting 
tool is etched, the carbide phase of the cemented carbide is irregularly 
etched and the roughness of the surface of the cemented carbide is 
increased. Thus, the diamond coating and the surface of the cemented 
carbide cutting tool having the increased roughness come to have an 
increased mutual adhesion due to the effect of the mechanical 
interlocking. 
After the etched cemented carbide cutting tool is washed again with 
distilled water and acetone, the same is etched for 5.about.30 seconds 
using a mixed solution of H.sub.2 SO.sub.4 --H.sub.2 O.sub.2, resulting in 
the removal of the cobalt binder phase. 
Then, after washing the cemented carbide cutting tool with distilled water 
and acetone and seeding the same in an ultrasonic washer with a diamond 
powder slurry, it is ultrasonically washed. Then, the cemented carbide 
cutting tool is coated with a diamond film in accordance with a general 
known diamond coating process. 
Since the etching solution used in the method has a stronger etching effect 
than the Murakami solution and does not include poisonous potassium 
ferricyanide, the solution is not harmful to humans and the disposal of 
the waste of the etching solution is much simpler, and the fabrication 
cost is reduced because the price of KMnO.sub.4 is around 30% that of the 
pottassium ferrycianide.

The fabrication method of a diamond-coated cemented carbide cutting tool as 
described above will be understood more clearly with reference to the 
following examples: 
EXAMPLE 1 
A pair of K20 SPGN120308 cemented carbide cutting tool is ultrasonically 
washed with distilled water and acetone. A test piece A was etched for 30 
seconds with a solution comprised 20% sulfuric acid and 80% hydrogen 
peroxide, and after a test piece B was electrolytically etched for 15 
minutes under the condition of 8V and 10A, using a 10% KOH aqueous 
solution as an eletrolytic solution, the test piece B was etched with the 
same solution used for test piece A. 
After the test pieces A and B were washed with acetone again, the test 
pieces were ultrasonically treated for two minutes in a solution of 0.5 
.mu.m of diamond powder dispersed in ethyl alcohol and were then washed 
with distilled water and acetone again. 
A diamond film was deposited under the conditions of 5% methane, 110 torr 
of pressure and a 960.degree. C. substrate temperature, using a microwave 
chemical vapor deposition. The thickness of the diamond film deposited was 
about 50 .mu.m. 
After the deposition was finished and the temperature of the test pieces 
was cooled to an ambient temperature, diamond coating of the test piece A 
could be peeled off from the substrate, but the coating on test piece B 
could not be peeled off and remained well bonded. 
EXAMPLE 2 
The same procedures as as in the example 1 were performed except that a 10 
wt % NaOH aqueous solution was used as an electrolytic solution for 
treating the test piece B instead of the KOH aqueous solution, resulting 
in the same effect as in example 1. 
EXAMPLE 3 
Three K20 SPGN120308 cemented carbide cutting tools were ultrasonically 
washed with distilled water and an acetone. The first test piece A was 
left untreated, the second test piece B was etched for two hours with a 
Murakami reagent and the third test piece C was etched for two hours with 
a solution of 5 g of KMnO.sub.4 and 10 g of KOH dissolved in 100 g of 
distilled water. 
After the test pieces A and B were washed in distilled water, they were 
etched with a solution of 20% sulfuric acid 80% bydrogen peroxide for 30 
seconds. 
The surface roughnesses of the test pieces A, B and C were measured with a 
profilometer of which the tip radius of the stylus was 12 .mu.m. The 
surface roughnesses measured were Ra 0.08 .mu.m, Ra 0.4 .mu.m and Ra 2.0 
.mu.m, respectively. 
A diamond film having a thickness of about 50 .mu.m was deposited on the 
test pieces A, B and C under the same conditions as in example 1. 
Consequently, the coatings on test pieces A and B peeled off, but the 
coating on test piece C did not peeled off. 
EXAMPLE 4 
Two K20 SPGN120308 cemented carbide cutting tools were ultrasonically 
washed with distilled water and an acetone, and the first test piece A was 
treated identically to that in example 1. The test piece B was etched for 
one hour in a solution of 5 g of KMnO.sub.4 and 10 g of KOH dissolved in 
100 cc of distilled water. After the test pieces A and B were washed with 
distilled water and acetone, they were ultrasonically treated for two 
minutes in a solution of 0.5 .mu.m diamond powder particles dispersed in 
an ethyl alcohol and were then washed with distilled water and acetone 
again. A diamond film having a thickness of about 50 .mu.m was deposited 
on the test pieces A and B under the same conditions as in example 1, 
resulting in achieving the same effect as in example 1. 
EXAMPLE 5 
Two K20 SPGN120308 cemented carbide cutting tools were ultrasonically 
washed with distilled water and acetone, and the first test piece A was 
treated identically to that of example 1, while the second test piece B 
was etched for one hour in a solution of 5 g of KMnO.sub.4 and 10 g of KOH 
dissolved in 100 cc of the distilled water. After the test pieces A and B 
were washed with distilled water and acetone, they were ultrasonically 
treated for two minutes in a solution of 0.5 .mu.m diamond powder 
particles dispersed in ethyl alcohol and were then washed with distilled 
water and acetone again. A diamond film having a thickness of about 50 
.mu.m was deposited on the test pieces A and B under the same conditions 
as in example 1. 
Using the two test pieces, an experiment was conducted on Al-18% Si alloy 
used as the object to be cut. The cutting condition was that the cutting 
speed was 700 m/min, the feed was 0.25 mm/rev and the depth of cut is 1 
mm. And the size of the object to be cut was 120 mm in diameter and 120 mm 
in length, and the object was cut down to have a diameter of 60 mm. 
As a result, no peeling off of the diamond film occurred on test piece B, 
but instead the diamond film was gradually abraded. And after four or five 
objects were processed, the cemented carbide began to be revealed. But the 
test piece A was impossible to use for cutting due to the unexpected 
peeling off of the diamond film occurring while processing the first 
object to be cut. 
Although the preferred embodiments of the present invention have been 
disclosed for illustrative purposes, those skilled in the art will 
appreciate that various modifications, additions and substitutions are 
possible, without departing from the scope and spirit of the invention as 
recited in the accompanying claims.