Method of making cemented carbide substrate

A carbide substrate including a binder prepared to receive a cutting material such as a diamond coating thereon. The substrate is immersed in an electrolyte solution with the substrate acting as the anode thereby providing for an electro-polished substrate surface. The electro-polished substrate surface is then etched to substantially remove the binder phase of the carbide substrate, the etching being to a depth of up to about 15 microns. The resulting surface is susceptible for receiving a coating of the diamond cutting material.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to diamond coated cemented carbide substrates 
used as inserts on cutting tools, which substrates have a surface 
susceptible to receive diamond coatings thereupon, and a method of 
preparing the surface of the cemented carbide substrate. 
2. The Prior Art 
For many years, the cutting tool art has struggled to find a repeatable way 
to utilize artificial diamond imbedded inserts on cutting tools to enhance 
the performance thereof. 
Generally, artificial diamond is of two varieties, polycrystalline diamond 
(PCD) and chemical vapor deposition (CVD) diamond film. PCD cutting tools, 
comprising a piece of polycrystalline diamond fastened to the tip of a 
tool insert are known in the art. However, these tools are expensive to 
manufacture and do not readily lend themselves to indexing for increased 
tool life. In addition, PCD inserts for tooling having complex shapes, 
i.e., taps, drill bits, router bits and the like, cannot be formed using 
known techniques. 
Consequently, numerous attempts have been made to provide diamond coated 
cutting tools using a chemical vapor deposition (CVD) process to deposit a 
coating or film upon a carbide substrate, such as tungsten carbide (WC), 
to provide a cutting tool with increased cutting performance. There has 
long been interest in inserts of CVD diamond-coated carbide substrate, 
because such an insert would be less costly to manufacture than a PCD 
insert, and because inserts of a CVD diamond-coated carbide substrate 
would be available in more complex shapes than with PCD tool inserts. 
Sintered tungsten carbide (WC) substrates without cobalt (Co) or other 
binders have been studied but can be too brittle to perform satisfactorily 
as cutting tool inserts. A good discussion of the prior art may be found 
in U.S. Pat. No. 5,236,740, which discloses a technique for applying CVD 
diamond film to unpolished cemented carbide substrates. As explained 
therein, a significant challenge to the developers of diamond-coated 
tooling is to optimize adhesion between the diamond film and the substrate 
to which it is applied, while retaining sufficient surface toughness in 
the finished product. Cemented tungsten carbide substrates incorporating a 
cobalt (Co) binder in concentrations equal to or less than 6% (WC/Co) have 
the requisite toughness and thus show the greatest long-term commercial 
promise for tooling applications. A cemented tungsten carbide substrate 
with up to 6% cobalt would provide adequate surface toughness for most 
machining and cutting tasks. Cemented tungsten carbide can be formed into 
a variety of geometries, making it a potential material for cutting tool 
inserts. To date, however, a repeatable, reliable CVD diamond coated 
cemented carbide substrate with repeatable cutting performance has escaped 
the art. 
It is therefore a goal to provide a cemented tungsten carbide substrate 
which may be effectively and reliably coated with a layer of CVD diamond 
film having adequate adhesion to the substrate for use as an insert on a 
machine or cutting tool. However, the solution to the problem of adequate, 
consistent and reliable adhesion of CVD diamond films to cemented carbide 
substrates has long eluded the art. 
It has been reported in the literature that the use of a cobalt binder in 
cemented carbides inhibits adhesion of the diamond film to the substrate. 
R. Haubner and B. Lux, Influence of the Cobalt Content in Hot-Pressed 
Cemented Carbides on the Deposition of Low-Pressure Diamond Layers, 
Journal De Physique, Colloque C5, supplement no 5, pp. C5-169-156, Toma 
50, May 1989. Indeed, conventional wisdom indicates that successful use of 
cemented rungsten carbide substrates may only be achieved by utilizing 
substrates containing no cobalt, as taught in U.S. Pat. No. 4,990,403; or 
no more than 4% Co binder, as taught in U.S. Pat. No. 4,731,296, or by 
deliberately depleting the cobalt concentration at the surface of the 
substrate. It is known to deplete the cobalt concentration at the surface 
of the substrate by selective etching or other methods, M. Yagi, Cutting 
Performance of Diamond Deposited Tool for Al 18 mass % Si Alloy, Abst. of 
1st Int. Conf. on the New Diamond Sci. & Technol, pp. 158-159, Japan New 
Diamond Forum 1988. However, this decreases the surface toughness of the 
substrate and can cause chipping of the substrate and applied diamond 
film. Increased adhesion of diamond to the substrate may be achieved by 
decarburizing the substrate prior to deposition, as taught in European 
Patent Application Publication No. 0 384 011, but use of this procedure 
does not optimize substrate toughness and does not lend itself well to 
manufacturing environments where repeatability and consistency are 
important issues. 
Some aspects of the prior art speculate that polishing or scratching the 
surface of a cemented tungsten carbide substrate prior to attempting 
diamond deposition may achieve improved adhesion of CVD diamond film, 
claiming there is an enhancement to the nucleation process caused by 
scratching and polishing. Haubner and Lux; Yagi; M. Murakawa et al., 
Chemical Vapour Deposition of A Diamond Coating Onto A Tungsten carbide 
Tool Using Ethanol, Surface Coatings Technology, Vol 36. pp. 303-310. 
1988; Kuo et al. Adhesion and Tribological Properties of Diamond Films on 
various substrates, J. Mol. Res. Vol. 5, No. 11, November 1990, pp. 
2515-2523. However, examination of these articles confirm that use of 
polished substrates yields poor results obtained by utilizing substrates 
whose surfaces have not been prepared by polishing or scratching. 
Another attempted solution to the adhesion problem has been to employ an 
interlayer between the diamond and a WC/Co substrate. This encapsulates 
the Co, optimizing adhesion while allowing the substrate to retain its 
toughness. It may also be possible to choose an effective interlayer 
material that bonds strongly to diamond further increasing adhesion. U.S. 
Pat. No. 4,707,384 discloses use of a titanium carbide interlayer. U.S. 
Pat. Nos. 4,998,421 and 4,992,082 disclose utilization of a plurality of 
layers of separated diamond or diamond like particles interposed with 
layers of a planarized bonding material. However, to date these 
technologies have not been demonstrated to be commercially repeatable or 
viable. 
Those skilled in the cutting tool art realize that the adhesion problem is 
particularly acute for cemented carbide cutting tools, and even more acute 
when the tool is intended to cut wood, especially man-made wood-like 
products such as particle board, fibre board and the like, because the 
carbide surface must be ground and/or polished to exacting dimensions to 
provide a keen or sharp (i.e., cutting) edge. Until now, no process or 
technique was effective or viable on a commercially repeatable basis for 
carbide substrates used for cutting tool inserts where the cutting edge of 
the carbide substrate was ground/polished. It has been discovered that the 
grinding/polishing process produces a film or "skin" on the surface of the 
cemented carbide substrate of undetermined morphology, which makes the 
adhesion of a CVD film, such as a diamond film, difficult and nonuniform, 
even when the substrate is acid-etched as the prior art teaches. It is 
believed that acid etching of cemented carbide surfaces is not effective 
on carbide substrates which have a "skin" because an etchant strong enough 
to penetrate/remove the "skin" also has the effect of corroding the 
carbide grains in a manner which renders the carbide grains ineffective as 
nucleating sites for the deposited CVD film. The present invention solves 
this long-standing problem, to provide a cemented carbide substrate very 
receptive to CVD diamond film, as shown by the cutting performance of 
inserts made according to the present invention. 
SUMMARY OF THE INVENTION 
According to the present invention, a cemented carbide substrate, such as 
tungsten carbide which uses a cobalt or nickel based binder at up to six 
percent by weight of the substrate, or a binder of a cobalt-based or a 
nickel-based alloy at up to six percent by weight, is prepared to receive 
a coating with a layer of CVD diamond film by subjecting the substrate 
surface to be coated to a process which first electro-polishes the 
substrate surface, which appears to remove the "skin" that is left by 
mechanical grinding/polishing of the substrate. Such mechanical 
grinding/polishing is necessary to impart a keen cutting edge to the 
substrate. The electro-polished substrate is then subjected to an acid 
etchant process which removes cobalt from a portion of the cemented 
carbide surface to a depth of up to about 15 microns, preferably about 1-5 
microns. The "skin"-free and cobalt-free surface of the cemented tungsten 
carbide substrate is thereby rendered particularly susceptible to an 
adherent CVD coating of diamond film. 
As presently preferred, an acceptable surface of the carbide body of the 
present invention is achieved by means of a two step process. The first 
step of the process is an electro-polishing step which is carried out by 
immersing the carbide body (substrate) in an electrolytic bath, connecting 
the carbide body (substrate) to an electrical circuit wherein the carbide 
body (substrate) acts as the anode connected via a power source to a 
cathode (e.g., stainless steel), and then subjecting the substrate to a 
selected current density for a predetermined time. The second step of the 
inventive process is etching the surface of the electro-polished cemented 
carbide with a selected etchant such as, for example, an inorganic acid or 
combination of inorganic acids, in concentrated or diluted form with 
distilled water, or oxidizing agents such as hydrogen peroxide (H.sub.2 
O.sub.2), or the like for a predetermined time. As presently preferred, 
the etching solution will include about one part hydrochloric acid (HCl), 
about one part solution containing 3% by weight hydrogen peroxide in 
distilled water, and two parts distilled water. Other mixtures may be used 
to control the rate and uniformity at which the cobalt is leached from the 
carbide surface. The effectiveness of this process may be enhanced by one 
or more cleaning steps with an anhydrous alcohol/acetone solution 
interposed before, between and/or after the above two steps. 
The electrolyte solution used in the electro polishing step is generally a 
sodium hydroxide (NaOH) solution wherein the concentration of sodium 
hydroxide is about 10%. Those skilled in the art will realize that other 
electrolyte solutions may be used, including but not limited to that 
described above. When using an anhydrous alcohol/acetone bath, the mixture 
is not critical; alcohol of about fifty percent (50%) and acetone of about 
fifty percent (50%) has proven satisfactory. 
Cemented tungsten carbide substrates made according to the present 
invention have been successfully and uniformly coated with CVD diamond 
film, such success being measured by the resistance of cutting tools 
containing CVD-diamond coated tungsten carbide inserts produced according 
to the present invention to the Rockwell "C" indentation hardness test, 
and further demonstrated by cutting performance of the cutting tools 
containing CVD-diamond coated tungsten carbide inserts produced according 
to the present invention on medium density particle board (MDPB).

DESCRIPTION OF A PREFERRED EMBODIMENT 
Those of ordinary skill in the art will realize that the following 
description of the present invention as set forth in the following 
examples is illustrative only and not in any way limiting. Other 
embodiments of the invention will readily suggest themselves to such 
skilled persons. 
EXAMPLE 1 
Referring to FIG. 1, a cross-section of the article of the present 
invention is shown as a carbide substrate, after being subjected to the 
novel surface preparation procedure of the present invention. In this 
example a substrate of Vermont American's OM3 grade tungsten 
carbide/cobalt (4.5% Co by weight) having a ground cutting edge was used. 
Reference to FIG. 3 shows that the grinding procedure, which is what gives 
the substrate its keen or cutting edge (and also provides relief in a 
known manner) leaves a characteristic series of grooves on the smoothly 
ground surface. As is further noted, the ground surface has a "skin" of 
complex morphology, wherein the WC carbide grains cannot be distinguished 
from the Co grains even though FIG. 3 is shown at 7500.times.. This "skin" 
is usually present to a depth of several microns, or about equal to the 
depth of the grinding grooves. The "skin" has been found quite difficult 
to remove through known etching techniques. 
FIG. 1 shows the article of the WC/Co substrate of the present invention 
after it has been made according to the novel process set forth herein, 
which both removes the "skin" and presents a multi-layered structure. 
Persons skilled in the art will recognize that the sub-surface layer of 
the substrate is comprised of sharp-edged WC grains 12 cemented together 
by a cobalt binder 14. FIG. 1 reveals the upper-most layer, about 10 
microns, of the substrate is free of cobalt binder 14, yet the angular 
edges of the WC grains have not been affected. Such a multi-layered WC/Co 
carbide substrate having an upper substantially cobalt-free layer of up to 
about 15 microns, presently preferably about 1-5 microns, wherein the 
carbide grains 12 exhibit angular sharp grain edges and a WC/Co layer 
adjacent and underneath the first layer, has been found particularly 
susceptible to receive a chemically-deposited coating such as a CVD 
diamond film. 
The surface of the inventive WC/Co substrate was made as follows: 
First the ground WC/Co substrate may be and preferably is cleaned with an 
anhydrous mixture of alcohol and acetone for a period of several minutes, 
presently preferably about five minutes, at ambient temperatures. Then the 
WC/Co substrate was electro-polished according to the following procedure 
(for ease of understanding reference may be made to FIG. 9, depicting the 
electro-polishing process), which has been found to remove disturbed metal 
(e.g., the "skin" of the ground surface) and sub-surface material from the 
WC/Co substrate as part of an electrolytic cell 13, as follows: the WC/Co 
carbide substrate 15 is fully or partially immersed in a suitable 
electrolyte 16, such as a 10% sodium hydroxide solution. The carbide 
substrate 15 is electrically connected to the negative terminal of, for 
example, a variable DC power supply 18 that acts as the anode of the cell. 
The positive power supply terminal is electrically connected to a cathode 
plate 20 placed between the electrolyte 16 and the carbide anode 15. The 
cathode plate 20 is typically stainless steel. A current is then applied 
to the cell 13 until a vigorous chemical reaction is observed at the anode 
surfaces of the carbide substrate 15. Current densities of 0.1A/cm.sup.2 
to 0.6 A/cm.sup.2 have been successfully employed with potentials not 
exceeding 20 volts. The vigorous electrochemical reaction produces an 
acceptable surface wherein the WC grains and the Co binder are 
distinguishable as they were prior to grinding. The impact of 
electro-polishing is quite rapid, as shown in FIGS. 3, 4 and 5. FIG. 3 
shows the surface of the WC/Co substrate after only 15 seconds of 
electro-polishing. It is seen that the WC grains and Co binder are now 
distinguishable. FIGS. 4 and 5 show that after 30 seconds and 60 seconds 
(one minute) respectively, the WC grains and cobalt binder are 
increasingly detectable, as more of the "skin" is removed. While an 
acceptable electro-polished surface can be produced in as short as 15 
seconds, at present an electro-polishing time of about 2 to 15 minutes at 
ambient temperature (about 25.degree. C.) produces the most desirable 
results. 
After electro-polishing the ground surface of the carbide substrate 15 as 
just described, the electro-polished substrate 15 may be and preferably is 
subjected to an intermediate anhydrous alcohol/acetone bath for several 
minutes, presently preferably about five minutes at ambient temperatures. 
The presently preferred anhydrous bath contains about a 50--50 mixture of 
alcohol (methanol or ethanol) and acetone. 
Then, the ground and electro-polished surface of the carbide substrate 15 
is subjected to an acid etchant for a period of about 30 seconds to four 
minutes, at ambient temperature. This etching step removes the cobalt 
binder 14 from the ground and electro-polished surface to a depth of up to 
about 15 microns, as shown in FIG. 1, with about 1 to 5 microns being 
presently preferred, without corroding the WC grains 12. The presently 
preferred etchant is one part hydrochloric acid, one part hydrogen 
peroxide (3% strength) and two parts distilled water. Such a mixture 
adequately removes cobalt to desired depth while at the same time 
retaining the integrity and angularity of WC grains, which is believed to 
present optimal nucleation sites to promote increased adherence of a CVD 
diamond film. Other etchants such as Murakami's reagent would likely 
produce acceptable results. 
Finally, the electro-polished and etched carbide substrate 15 may be and 
preferably is immersed in a final anhydrous alcohol/acetone bath for 
several minutes, presently preferably about five minutes, at ambient 
temperature. 
While the foregoing procedure has been described as preferably utilizing 
anhydrous alcohol/acetone baths before electro-polishing, between 
electro-polishing and etching and after etching, it has been observed 
WC/Co substrates subjected to only the electro-polishing and etching steps 
exhibit an acceptable surface to receive deposited coatings such as CVD 
diamond film. And, although the electro-polishing step has been described 
as taking place at ambient temperatures, the rate of polishing may be 
increased by raising the temperature of the electrolyte solution 16. In 
such a case, it may be necessary to subject the cell 13 to cooling means, 
such as shown in FIG. 9. 
A carbide substrate according to the present invention just described has 
proven susceptible to receiving CVD diamond film to a thickness of up to 
about 100 microns, with good integrity of the CVD diamond film. 
EXAMPLE 2. 
Referring to FIG. 2, there is shown a WC/Co carbide substrate, in this case 
also Vermont American's grade OM3 (4.5% Co by weight) having a ground 
(keen) edge which has been made according to the just-described process 
and then coated with a CVD diamond film, which in FIG. 2 is about 16 
microns thick. Reference to FIG. 2 reveals that the surface of the carbide 
substrate has been rendered substantially free of cobalt binder 14 to a 
depth of about 5 microns and that the WC grains 12 retained their 
angularity. 
Reference to FIGS. 10, 11 and 12 shows the difference between an 
electro-polished surface of a WC/Co substrate and the same surface which 
has been electro-polished and acid etched according to the present 
invention described above. FIG. 10 is an X-ray spectroscopic analysis of 
the keen (ground) surface of an electro-polished surface of WC/Co carbide 
substrate 10 prior to, and FIG. 11 shows an X-ray spectroscopic analysis 
of the same surface after, the acid etch step of the just-described 
procedure. FIG. 12 is a comparison of FIGS. 10 and 11 for the 
electro-polished surface of the same substrate before and after acid 
etching. Note that the electro-polished carbide surface (dotted graph) 
shows the presence of a cobalt K.sub..alpha. peak, but the 
electro-polished and etched surface (shaded area) show no activity of the 
K.sub..alpha. peak associated with the presence of cobalt. As can be seen, 
prior to the acid etch procedure there is a substantial amount of cobalt 
present, even after electro-polishing, as revealed in the presence of the 
distinctive K.sub..alpha. peak related to the presence of cobalt; and 
after being subjected to the acid etch step the substantial absence of 
cobalt at the electro-polished surface is indicated by virtually no 
spectroscopic activity at the known K.sub..alpha. peaks for cobalt. 
Reference to FIG. 7 reveals the surface of a WC/Co substrate of the present 
invention which has been subjected to only one minute each of 
electro-polishing and etching as described above. The surface is 
characterized by the presence of only angular WC carbide grains 15, and 
only traces of cobalt 14 remain. This will provide acceptable sites for 
nucleation with a chemically deposited film such as CVD diamond, the minor 
amount of cobalt 14 not being deleterious to effective deposition. 
Nevertheless, as shown in FIG. 1, the presently preferred article of the 
present invention is one which has been electro-polished in an electrolyte 
solution (10% NaOH+ distilled water) for about 10-12 minutes and then 
etched with a mixture of one part hydrochloric acid, one part hydrogen 
peroxide solution (3% strength) and two parts distilled water for about 
one minute, with an anhydrous alcohol/acetone bath before the WC/Co 
substrate is electro-polished (.about.five minutes), after 
electro-polishing (.about.five minutes), and after etching (.about.five 
minutes). FIG. 1 shows a substantial and virtual absence of cobalt to a 
depth of about 10 microns, rendering such surface particularly amenable to 
receiving a chemically deposited coating such as CVD diamond film. Thinner 
binder-free zones are preferable (about 1 to 5 microns), but in some cases 
thicker zones are acceptable. 
FIG. 2 shows the article of the present invention including a WC carbide 
substrate as in FIG. 1 which has been coated with about 16 microns of CVD 
diamond film 22 by a known CVD deposition technique. Note that the 
interface between the diamond film 22 and the angular WC carbide grains 12 
shows good and uniform bonding, even at 2250.times.. The effectiveness of 
the CVD diamond/WC carbide interface is further established by the 
resistance of the CVD-diamond coated surface to the Rockwell "C" 
indentation test. CVD-diamond coated carbide substrates as shown in FIGS. 
1 and 2 survive the 60 kg indentation test using a Rockwell "C" indentor 
with only minimal spalling of the CVD-diamond film. Further evidence of 
the effectiveness of the article of the present invention for receiving a 
chemically-deposited coating such as a CVD diamond film, is shown by 
cutting with tool inserts using carbide substrates of the present 
invention (FIG. 1) which have been coated with CVD-diamond film (FIG. 2). 
Inserts as described have survived more than 10,000 feet of cutting medium 
density particle board (MDPB) with the integrity of the interface of the 
CVD-diamond coating and the WC carbide grains still fully intact, and with 
virtually no wear of the CVD-diamond coating at the cutting surface. 
EXAMPLE 3 
FIG. 8 shows the surface of a WC/Co carbide substrate, Vermont American's 
grade OM3 (4.5% Co by weight), which has not been subjected to 
electro-polishing. This carbide substrate was subjected to acid etching 
with a solution of aqua regia for five minutes, the amount of time to 
sufficiently remove the "skin" and also render the surface cobalt-free in 
the manner according to the prior art. As can be seen, the WC grains are 
substantially corroded. While the surface is substantially depleted of 
cobalt, it has been found that a carbide surface of this type does not 
accept chemically deposited coatings, such as CVD diamond film, in an 
acceptable or repeatable manner. While the reason why a carbide surface 
such as shown in FIG. 8 does not receive CVD diamond film acceptably are 
not fully understood, it is believed that the corroded or "rounded" WC 
grains shown in FIG. 8 do not serve as effective nucleating sites for the 
chemically deposited film. Of course, appropriate initial nucleation is 
important to the strength and integrity of the carbide/CVD diamond 
interface. A carbide substrate as in FIG. 8 and coated with CVD diamond 
film exhibits poor resistance to the Rockwell C indentation test, with 
notable cracking and spalling of the CVD diamond coating. Also, cutting 
performance of CVD diamond coated cutting inserts made with carbide 
substrates as shown in FIG. 8 exhibit poor, and erratic, cutting 
performance of MDPB. The CVD diamond coating has a tendency to spall off, 
even without wear at the cutting edge. 
While embodiments and applications of this invention have been shown and 
described, it would be apparent and obvious to those skilled in the art 
that many modifications other than those mentioned are possible without 
departing from the inventive concept herein described. The invention, 
therefore, should not be restricted in scope except within the spirit of 
the appended claims.