Composite polycrystalline diamond compact with improved impact and thermal stability

A compact blank for use in operations that require improved thermal stability, impact strength, and abrasion resistance is disclosed. The compact includes a substrate formed of tungsten carbide or other hard material with multiple abrasive diamond crystal layers bonded to the substrate. The abrasive diamond crystals are provided in successive layers of different size particles with the coarsest size particles being farthest away from the substrate. A catalyst is premixed with the diamond crystals in a weight percent which progressively decreases from the layer closest to the substrate through succeeding layers.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a sintered polycrystalline diamond 
composite for use in rock drilling, machining of wear resistant metals, 
and other operations which require the high abrasion resistance or wear 
resistance of a diamond surface. Specifically, this invention relates to 
such bodies that include a polycrystalline diamond layer attached to a 
cemented metal carbide substrate via processing at ultrahigh pressures and 
temperatures. 
2. Description of the Art 
Composite polycrystalline diamond compacts or PCD have been used for 
industrial applications including rock drilling and metal machining for 
many years. One of the factors limiting the success of PCD is the strength 
of the bond between the polycrystalline diamond layer and the sintered 
metal carbide substrate. For example, analyses of the failure mode for 
drill bits used for deep hole rock drilling show that in approximately 
thirty-three percent of the cases, bit failure or wear is caused by 
delamination of the diamond from the metal carbide substrate. 
Furthermore, when a precemented carbide mass is relied on to increase the 
impact resistance of PCD, the diamond layer is preferably relatively thin 
so that the diamond is never too far from its support. This restriction on 
the thickness of the diamond layer naturally limits both the life 
expectancy of the composite compact in use as well as the designs for PCD 
diamond tools. 
Yet another problem that has limited the thickness of the diamond layer in 
composite compacts is caused by the problem of "bridging". Bridging refers 
to the phenomenon that occurs when a fine powder is pressed from multiple 
directions. It is observed that the individual particles in a powder being 
pressed tend to stack up and form arches or "bridges" that block the full 
amount of pressure so that the pressure often does not reach the center of 
the powder being pressed. 
For optimal abrasion resistance of the compact product, very fine crystals 
of the abrasive are typically used, generally in particle size of less 
than 10 microns and preferably less than 5 microns. The fine abrasive 
crystals are crushed further under the high pressures applied during the 
compaction process resulting in a packing density of around 1.5 grams/cc 
increasing to greater than 2.5 grams/cc by crystal fracturing. The 
resulting abrasive mass is very dense and offers resistance to the 
catalyst metal or catalyst metal and carbide from sweeping through the 
crystal interstices. In practice, this resistance to sweep through by the 
dense, fractured abrasive crystals leads to soft spots of non-bonded 
abrasive. These soft spots are especially prevalent when the layer of 
abrasive crystals exceeds about 1 mm in thickness. Coarser abrasive 
crystals offer channels in the compacted mass that are less torturous for 
the bonding metal to sweep through; however, abrasion resistance 
considerations usually preclude the use of such coarse crystals as 
starting materials for the compact. 
One of the solutions to these problems is proposed in the teaching of U.S. 
Pat. No. 4,604,106. This patent utilizes one or more transitional layers 
incorporating powdered mixtures with various percentages of diamond, 
tungsten carbide, and cobalt to distribute the stress caused by the 
difference in thermal expansion over a larger area. A problem with this 
solution is that the cobalt cemented carbide in the mixture weakens that 
portion of the diamond layer because less diamond-to-diamond direct 
bonding occurs as a result of the carbide second phase. 
Other patents have discussed using grooved substrates in order to both 
increase the thickness of the diamond layer at certain locations and to 
increase the bond strength between the diamond layer and the substrate. 
For example, U.S. Pat. No. 4,784,023 teaches the grooving of 
polycrystalline diamond substrates; but does not teach the use of 
patterned substrates designed to uniformly reduce the stress between the 
polycrystalline diamond layer and the substrate support layer. In fact, 
this patent specifically mentions the use of undercut or dovetail portions 
of substrate grooves, which contributes to increased localized stress. 
FIG. 1 shows the region of highly concentrated stress that results from 
fabricating polycrystalline diamond composites with substrates that are 
grooved in a dovetail manner. Instead of reducing the stress between the 
polycrystalline diamond layer and the metallic substrate, the use of 
dovetail grooving actually makes the situation much worse. This is because 
the larger volume of metal at the top of the ridge will expand and 
contract during heating cycles to a greater extent than the 
polycrystalline diamond, forcing the composite to fracture at locations 1 
and 2 shown in FIG. 1. 
The disadvantage of using relatively few parallel grooves with planar side 
walls is that the stress again becomes concentrated along the top and, 
more importantly, the base of each groove and results in significant 
cracking of the metallic substrate along the edges 3 of the bottom of the 
groove as shown in FIG. 2. This cracking significantly weakens the 
substrate whose main purpose is to provide mechanical strength to the thin 
polycrystalline diamond layer. As a result, construction of a 
polycrystalline diamond cutter following the teachings provided by U.S. 
Pat. No. 4,784,023 is not suitable for cutting application where repeated 
high impact forces are encountered, such as in percussive drilling, nor in 
applications where extreme thermal shock is a consideration. 
Other configurations have been proposed in order to overcome problems of 
stress in the compact due to the mismatch in thermal expansion between the 
diamond layer and the tungsten carbide substrate. For example, U.S. Pat. 
No. 5,351,772 describes the use of radially extending raised lands on one 
side of the tungsten carbide substrate area on which a polycrystalline 
diamond table is formed and bonded. 
U.S. Pat. No. 5,011,515 describes a substrate with a surface topography 
formed by irregularities having non-planar side walls such that the 
concentration of substrate material continuously and gradually decreases 
at deeper penetrations into the diamond layer. U.S. Pat. No. 5,379,854 
describes a substrate with a hemispherical interface between the diamond 
layer and the substrate, the hemispherical interface containing ridges 
that penetrate into the diamond layer. U.S. Pat. No. 5,355,969 describes 
an interface between the substrate and polycrystalline layer defined by a 
surface topography with radially-spaced-apart protuberances and 
depressions. 
All of the above proposals show a diamond layer of varying thickness 
relative to the surface of the tungsten carbide substrate support. Thus, 
in areas where the diamond layer is thicker, the amount of cobalt 
available is less than in those areas where the diamond layer is thin. 
This results in a non-uniformly sintered diamond layer that substantially 
weakens the compact. Even when cobalt powder is premixed with the diamond 
prior to subjecting the compact to high pressure-high temperature 
conditions, the presence of cobalt in a substrate with a textured surface 
produces areas of varying concentration of cobalt within the diamond layer 
during the sintering process and causes soft spots or poorly sintered 
areas within the diamond layer. 
U.S. Pat. No. 4,311,490 teaches the use of coarse diamond particles next to 
the tungsten support with a layer of finer diamond particles placed on top 
as the exposed cutting surface. This is reported to reduce the occurrence 
of soft spots or poorly sintered areas in the diamond table since the 
coarser particles have larger channels between them making it easier for 
cobalt to sweep through the diamond nearest the tungsten carbide 
substrate, thus allowing thicker diamond layers to be sintered. For rock 
drilling applications, however, it has been found that although finer 
diamond results in higher abrasion resistance, it also results in 
significantly less impact resistance. The lower impact resistance produces 
compact cutter failure by way of fracturing and spalling of the diamond 
layer from the tungsten carbide support substrate. 
Thus, two problems remain: one of producing a compact with the advantages 
of using a substrate with a non-planar interface without the drawback of 
soft spots or otherwise poor, non-uniformly sintered areas and second, 
maintaining a higher abrasion resistant compact for rock drilling without 
loss of impact resistance. 
SUMMARY OF THE INVENTION 
The instant invention is a polycrystalline diamond compact that has at 
least two layers of abrasive crystals or diamond of different average 
crystal size in each separate layer, wherein the layer with the coarsest 
average particle size and the most impact resistance is disposed as the 
outermost layer and the layer with the finest average particle size 
between the outermost diamond layer and a disposed substrate. The fine 
layer offers the most abrasion resistance for rock drilling and is 
protected from impact by the outer layer of coarse diamond. In order to 
obtain uniform sintering, a catalyst, such as cobalt powder, is added at 
least to the layer of fine diamond. This addition of cobalt powder aids in 
the conventional sweep through process whereby the higher concentration of 
cobalt in the substrate migrates through the mass of diamond particles and 
sinters these crystals together and tightly bonds them to the substrate. 
The catalyst may be premixed in each layer with the weight percent of the 
catalyst varying from a highest weight percent in the crystal layer 
immediately adjacent to the substrate to progressively lower weight 
percents in successive layers further away from the substrate. 
Alternately, the crystal or diamond layers may have the same average 
particle size, with only the weight percent of catalyst in each layer 
decreasing in weight percent from the highest weight percent in the 
crystal layer immediately adjacent to the substrate, to lower weight 
percents through succeeding layers. 
This invention also greatly improves compacts made using textured 
substrates because prior art methods of directly placing a uniform layer 
of diamond or a single layer of diamond plus cobalt powder adjacent to the 
substrate results in a non-uniform sintering of the diamond layer due to 
the varying thickness of the substrate and the resultant varying 
concentration of cobalt readily available for the sintering process. By 
mixing a diamond layer with cobalt powder and placing this layer in the 
valleys of the textured substrate and then adding a second diamond layer 
with a lower weight-percent cobalt powder or without any cobalt, the 
amount of total cobalt available for sintering the outermost diamond layer 
is distributed more evenly over the area of interfaces between the diamond 
layers. This reduces the occurrence of soft spots where poor sintering has 
occurred due to a deficiency or non-uniform supply of catalyst metal and 
greatly improves the strength of the compact blank.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In the following description, it should be understood that the crystal 
layers described hereafter as formed of polycrystalline diamond, PCD, or 
sintered diamond as the material is often referred to in the art, can also 
be any of the superhard abrasive materials, including but not limited to, 
synthetic or natural diamond, cubic boron nitride, and wurzite boron 
nitride as well as combinations thereof. 
Also, the cemented metal carbide substrate refers to a carbide of one of 
the group IVB, VB, or VIB metals which is pressed and sintered in the 
presence of a binder of cobalt, nickel, iron and the alloys thereof. 
FIGS. 3 and 4 show two similar embodiments of this invention. These views 
show a plurality of layers 10, 12 of abrasive crystals, such as diamond, 
and the interface 14 between the crystal layers 10, 12 and a substrate 16 
in a polycrystalline compact or cutting element 18. 
The substrate 16 is preferably formed of a hard metal. In a specific 
example, the substrate 16 is formed of a metal carbide selected from the 
group consisting of tungsten carbide, titanium carbide, tantalum carbide, 
and mixtures thereof. The substrate 16 may also be formed of a carbide 
from the group of IVB, VB, or VIB metals which is pressed and sintered in 
the presence of a binder of cobalt, nickel, iron and alloys thereof. 
In FIG. 3, the interface 14 between one crystal layer 10 and the substrate 
16 has a planar or flat configuration. In FIG. 4, the substrate 16 is 
formed with a plurality of spaced, generally parallel, grooves. The 
grooves may be straight sided as shown in FIG. 4 or formed as dovetail 
groves with inward angled sidewalls. Other surface topographies known in 
the art may also be employed in the PCD compact 18. 
The plurality of layers 10, 12 of abrasive crystals, such as diamonds, are 
overlaid on each other. An important aspect of the first embodiment of 
this invention is that the at least two layers of diamond 10, 12 each have 
a different crystal coarseness whereby the layer 10 most immediately 
adjacent to the substrate 16 has a finer average particle size than the 
adjacent layer 12 which has a coarser average particle size. Although only 
two layers 10, 12 are illustrated in FIGS. 3 and 4, it will be understood 
that any number of layers may be used to form the polycrystalline compact 
18 with the outermost layer, i.e., farthest away from the substrate 16, 
having the coarsest or largest average crystal size. 
In another embodiment, a catalyst may be premixed with at least certain of 
the crystal layers 10 and 12. Preferably, the layer 10 immediately 
adjacent to the substrate 16 is premixed with a catalyst at a higher 
weight-percent of catalyst than the catalyst premixed with the adjacent 
second diamond layer 12 and has a finer distribution of particle size 
diamond. Preferably, the catalyst is a metal selected from the Group VIII 
metals. Cobalt can be used as a preferred catalyst metal, by example only. 
The addition of the catalyst, such as cobalt powder, aids in the 
conventional sweep through process whereby the higher concentration of 
cobalt in the substrate migrates through the mass of diamond particles and 
sinters these crystals together and tightly bonds the crystals to the 
substrate. 
Other methods of cobalt addition may be used in order to produce a 
cobalt-rich area near or within the layer of fine diamond particles. For 
example, a cobalt disc may be placed directly adjacent to the layer of 
fine diamond particles or a substrate may be used with a higher than 
normal concentration of cobalt. Either way, the end result is that the 
concentration of cobalt is higher in the finer diamond layers than in the 
coarser diamond layers. 
In the case of textured substrates wherein the topography of the interface 
between the diamond layer and the substrate varies as, for example, in 
FIG. 5, the diamond crystals in the layer 10 next to the substrate 16 may 
have the same amount of cobalt as the next outermost layer 12 prior to 
high pressure-high temperature processing provided an additional source of 
cobalt 20 is placed in the valleys of the topography at the interface 14 
between the substrate 16 and crystal layer 10. 
There may be more than two layers of diamond varying both in particle size 
and weight-percent of cobalt. The particle size of the diamond increases 
and the weight-percent of cobalt decreases with each successive layer 
proceeding from the substrate 16 toward the outermost diamond layer 12 
which has at least one exposed surface 22 engagable with a work material. 
Alternately, each diamond crystal layer 10, 12, etc. may have the same 
average particle size; but the catalyst in each layer 10, 12 decreases 
from layer to layer from a highest weight percent of catalyst in the layer 
10 immediately adjacent to the substrate 16. 
In a process for preparing the polycrystalline diamond compact 18 according 
to the present invention, the layers 10, 12 of abrasive or diamond 
crystals are successively placed in a protective shield enclosure in a 
reaction cell of a high pressure/high temperature apparatus, such as a 
conventional reaction vessel. The outermost layer 12 is placed in the 
enclosure first followed by the layer 10. The substrate 16 is then placed 
in the enclosure in contact with the layer 10 at the interface 14. High 
pressure and high temperature are then applied to the enclosure according 
to known techniques to sinter or join the diamond crystals in each layer 
10 and 12 to each other, to sinter the layers 10 and 12 together, and to 
sinter the layer 10 to the substrate 16 at the interface 14. 
EXAMPLES 
Example 1 
A 250-milligram sample of 25 micron diamond powder is placed in a 
molybdenum cup. Next, a 250-milligram sample of 10 micron diamond powder 
that has been thoroughly blended with 10 weight-percent cobalt powder is 
placed into the cup on top of the 25 micron diamond powder. Finally, a 
cobalt cemented tungsten carbide substrate is placed into the cup on top 
of the 10-micron diamond/cobalt powder mixture. This assembly is loaded 
into a high pressure cell and pressed to 45 K-bars for fifteen minutes at 
1450.degree. C. After cutting the power to the cell and allowing the cell 
to cool at high pressure for one minute, the pressure is released. The 
composite bodies are removed from the other cell components and then 
lapped and ground to final dimensions. 
Example 2 
A 250-milligram sample of 25 micron diamond powder is placed in a 
molybdenum cup. Next, a 250-milligram sample of 25 micron diamond powder 
that has been thoroughly blended with 10 weight-percent cobalt powder is 
placed into the cup on top of the 25 micron diamond powder. Finally, a 
cobalt cemented tungsten carbide substrate is placed into the cup on top 
of the 25-micron diamond/cobalt powder mixture. This assembly is loaded 
into a high pressure cell and pressed to 45 K-bars for ten minutes at 
1450.degree. C. After cutting the power to the cell and allowing the cell 
to cool at high pressure for one minutes, the pressure is released. The 
composite bodies are removed from the other cell components and then 
lapped and ground to final dimensions.