A Novel Method for Obtaining Polycrystalline Diamond Cutters Rômulo Crespo Tardima, Guerold Sergueevitch Bobrovnitchiib,

Sérgio Neves Monteiroc, Ana Lúcia Diegues Skuryd and Marcello Filgueirae

State University of Northern Rio de Janeiro - UENF, Av. Alberto Lamego, 2000, Campos dos Goytacazes, RJ – Brasil. CEP 28013-600

aromulo.tardim@gmail.com, bguerold@uenf.br, csergio.neves@ig.com.br, dlucia@uenf.br, emarcello@uenf.br

Keywords: Diamond composite; Polycrystalline diamond cutters; Properties; Wear.

Abstract. The present work comprises a study about the possibility of obtaining polycrystalline

diamond cutters through a novel method of sintering both layers at the same time. This possibility

was tested through the sintering of a diamond layer over a hard metal (WC+15wt%Co) support

under conditions of 5.0 and 6.5 GPa of pressure and 1400 to 1600º C of temperature. The sintering

conditions were imposed in two ways: directly, or with pre-sintering. The samples were tested by

measuring microhardness, wear resistance, densification, and SEM. The results of the tests have

shown the possibility of obtaining good quality inserts by sintering both layers of compacted

powder.

Introduction

A recent trend for the development of new high-efficiency polycrystalline diamond

materials points towards the optimization of the sintering technology of diamond bits with double

layers, also known as polycrystalline diamond cutters (PDC). The PDC emergence as a substitution

for natural diamonds, almost 30 years ago, represented a significant advance in the oil and gas

industry [1]. In other areas that also require especial tools for drilling rock formations, the

application of PDC has become the preferred standard [2].

The PDC consists of two layers sintered at high pressures from 6 to 8 GPa and high

temperatures from 1400 to 1800° C. Its configuration is associated with a thin diamond layer

adhered onto a substrate of cemented carbide. Most commonly tungsten carbide and cobalt, also

known as hard metal are used. The diamond layer is composed of diamond crystals connected by

the infiltration of a molten Co+W solution, coming from the substrate. This solution acts as a binder

in the process of sintering by high pressures and high temperatures [3]. The resulting composite is

endowed with unique properties since it combines high hardness and wear resistance of diamond

cutting in the edge, with high toughness of hard metal (WC + Co) which provides support to the

diamond layer.

Already published data [4,5] pointed out to the possibility of forming a composite by

impregnation of the diamond layer by a front of molten cobalt. This process was conducted at a

pressure of 5 GPa, and the relatively low temperature of 1250 ° C, leading to the establishment of

the diamond layer on the substrate.

Many scientific studies have noted that the compliance associated with the diamond-bonding

matrix is very important for the efficiency of use. In recent years, attention has been paid to the rate

of the instability of adhesion between the diamond layer and the substrate. As the diamond and the

carbide have different coefficients of thermal expansion, the shrinkage during cooling after the

sintering, generates high residual thermal stresses. Additionally, the cutting process also promotes

radial stresses on the cutter. Commercially, this weakness is mitigated by the machined profiles

convoluted interface between the layers, which would be a mechanical solution to increase the

adhesion between the layers.

The present study aims to examine the possibility of sintering PDCs using a new method

related to the simultaneous sintering of the two layers, having powders as starting materials. The

possibility of positive results for this innovative attempt is based on a number of advantages:

elimination of diamond crystals pulverization on the surface of hard substrate, increased adhesion

Materials Science Forum Vols. 660-661 (2010) pp 477-482Online available since 2010/Oct/25 at www.scientific.net© (2010) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/MSF.660-661.477

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between the layers, better infiltration of cobalt and light indentation of diamond crystals in the

substrate as well as the likely reduction in cost and time to obtain the cutters.

Experimental

Diamond powder [6] with particle size ranging from 28 to 40 µm was used in this

investigation. The substrate was composed of WC powder with particle size from 5 to 7.5 µm and

cobalt powder with particle size from 40 to 50 µm, in the proportion of WC+15wt%Co. The

homogenization was performed in a universal rotary mixer for 30 min. Mixtures were then weighed.

The samples were subject to a pre-compression pressure of 500 MPa. The pre-compacts were

obtained in cylindrical shape with 5 mm in diameter and 4.5 mm in length. The high pressure and

high temperature was performed in a high pressure device (HPD), corresponding to a 13.5 mm

central concavity-type anvil [7], shown in Fig. 1.

Figure 1. Schemes: a) High pressure device, toroydal anvil type, with 13.5-mm central concavity: 1

– deformable capsule; 2 – reactive cell; 3 – anvil; 4 – steel rings. b) Reactive cell arrangement: 5 –

WC+15wt%Co; 6 – diamond powder; 7 – calcite insulating cap; 8 – heating graphite tube; 9 –

heating graphite cap.

Before the sintering process occurs, the high pressure device together with the mounted

deformable capsule were calibrated at the 630 ton press as a function of pressure, p2 = f (p1), and

temperature T = f (I) or T = f (W), where p2 is the pressure in the compression chamber, p1 the

pressure in the main cylinder of the press, T the temperature in the reactive cell, and I or W the

electrical current or power of heating. The methodology used wass the same as for Bobrovnitchii et

al. [8]. Table 1 is shown to discriminate the samples in accordance with the parameters of sintering.

Table 1. Parameters used for the sintering of each sample.

N° Pre-sintering Sintering

p2 [GPa] T [°C] t [s] p2 [GPa] T [°C] t [s]

1 - - - 5,0 1400 60

2 - - - 6,5 1400 60

3 - - - 5,0 1600 60

4 - - - 6,5 1600 60

5 4,0 700 30 5,0 1400 60

6 4,0 700 30 6,5 1400 60

7 4,0 700 30 5,0 1600 60

8 4,0 700 30 6,5 1600 60

After sintering the samples were removed from the deformed capsule and mechanically

cleaned. The density was measured by means of the Archimedian method. The microhardness tests

were conducted in a model HMV Shimadzu apparatus, with a diamond Vickers indenter and a load

of 9.8 N. Wear resistance tests were conducted in a special Arotec apparatus, and consisted of

a) b)

Ø 13.5

25

1 2 4 3

Ø 150

9

8

5

6

7

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cutting a SiC-based rotatory abrasive wheel, and the adopted parameters were 50 rpm as the rotation

frequency, load of 10 N, and water as the coolant. The samples were fixed at a back rake angle of

11°. The test of each sample took 15 min long, and the relative wear index was determined by the

difference in mass before and after the test. Then, the sample with the worst performance got the 10

index, while the better ones were assigned lower proportional values. All samples were weighed

using an analytic BG200 Gehaka scales. The wear surface analysis was carried out by scanning

electron microscopy (SEM), in a model SSX-550 Shimadzu microscope.

Results and Discussion

The physical-mechanical properties of the samples were measured and are presented in

Table 2. Under the aspect of the densification of the samples, the use of an intermediate pressure

such as 6.5 GPa is favorable, as a sharp contraction tends to indicate a more dense sample.

Moreover, those with smaller yield of 5.0 GPa have less residual stress. These statements may or

may not be supported in the analysis of density. Table 2 presents the values.

Table 2. Density of the sintered samples.

Sample Density [g/cm3] Microhardness [HV] Wear index

1 12.15 360 ***

2 12.32 789 5.6

3 12.37 290 2.6

4 12.71 918 1.8

5 13.74 645 10

6 12.31 1046 3.9

7 12.84 1001 1.8

8 12.48 1177 1.7

The analysis of Table 2 shows that, in general, the use of higher pressures is more

advantageous. One must take into account two factors: the greater the degree of densification of the

substrate, the better the quality of PDC. Even if the samples show a low density, this does not

necessarily indicate poor quality, but that the sample has more embedded diamonds, as hard metal

is denser than diamond. It is known that higher pressures provide greater densification of hard metal

bodies, and the same can be observed for PDC. The lowest density of the sample 8 in comparison to

sample 7, for example, is a sign that there was adhesion of a substantial amount of diamond by the

substrate.

Due to the high hardness of diamond and the fragility of the diamond layer, microhardness

tests were conducted only on the hard metal tops of the samples. This was due to the fact that the

diamond could produce damage to the indenter and the fracture of the diamond layer would render

the measurement of the indentation diagonal. In Table 2, these results can also be observed.

The results presented show explicitly that the parameters of sintering play a fundamental

role in the value of microhardness of the samples produced. The application of 6.5 GPa instead of

5.0 GPa meant, for samples 3 and 4, an increase of 316% in the value of micro hardness, and the

two were subjected to the same temperature and same type of control diagram. Higher temperatures

and the application of pre-sintering also influence positively in this regard.

The wear test is extremely important, because it is the closest to the real practice associated

with the implementation of the cutters. The wear index is expressed by numbers ranging from near

zero to ten, where low scores refer to a relative low wear suffered by the sample, and 10 is the index

for the most damaged sample. Sample 1 had a tear so sharp that withstood only 4 minutes during

the test, which usually lasted 15 min for the other samples. Moreover, sample 1 had more than half

of its weight lost. As can be seen in Table 2, the pressure and temperature had great influence on the

wear index. As a general behavior, it can be observed that the application of p, T - parameters and

higher pre-sintering leads to better performance.

Materials Science Forum Vols. 660-661 479

Analysis of wear surfaces by SEM provided valuable information for understanding the

wear of the sample. In the case of sample 2, it can be observed that the diamond grains are not fully

adhered to the substrate. The edge discontinuities between the two layers (substrate and diamond

layer) indicate the premature detachment of diamond crystals during wear, as shown in Figure 2 (a).

Once they are exposed, the crystals come loose and even the whole groove generated by its friction

is evident. Fig. 2 (b) shows, with more details, that diamond crystals do not have the desirable

performance.

a) b)

Figure 2. Sample 2 (direct sintering, 6.5 GPa, 1400° C), for magnifications: a) 60x; b) 180x.

However, the sample 4 presents significantly higher adhesion. It can be observed that the

crystals at the diamond-edged remain there during the wear and do not detach prematurely. Hence

the better performance of the sample 4 vs. sample 2 can be attributed to a better adhesion of the

diamond to the cobalt matrix. In Fig. 3 it can be identified a partial destruction of a crystal in the

active edge.

Figure 4. Sample 4 (direct sintering, 6.5 GPa, 1600° C).

In the sample 7 a completely different situation can be seen, as depicted in Fig. 5. In this

figure, the substrate is sufficient to support the diamond layer. Thanks to this support, the diamond

crystals work intensively and are being consumed gradually. Furthermore, these crystals are not

coming off prematurely.

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Figure 5. Sample 7 (pre-sintering application, 5.0 GPa, 1600° C).

The sample 8 showed the best results, and this was no accident. In this sample it was

observed a behavior not shown by any other sample, which is the presence of diamond crystals to a

large depth, as can be seen in Fig. 6. This means that the sintering conditions of this sample were

favorable for the penetration of the liquid phase into the diamond powder.

Figura 6. Sample 8 (pre-sintering application, 6.5 GPa, 1600° C).

Conclusions

The conclusions related to the experimental investigation and the analysis of structure

formation and sintering of PDC are listed below:

• The attempt to form the structure of PDC at high pressures and high temperatures, using the hard metal substrate in the form of simple compact obtained by the cold compaction of the

powders at 800 MPa, explained that there is a possibility of obtaining PDC via this new

methodology.

• The new approach also includes a new method of applying high pressures and high temperatures, using the pre-sintering, in other words, pre-compression of 4.0 GPa and preheat

to 700 ° C, below temperature for graphitization of the diamond.

• The working pressures of 6.5 GPa and temperature of 1600° C along with the pre-sintering enables to obtain cutters with high wear resistance;

• Most likely, this new method, which was here developed and tested, can give the opportunity to reduce the manufacturing cost of PDC.

Materials Science Forum Vols. 660-661 481

Acknowledgments

The authors wish to thank the Brazilian agencies: CNPq, CAPES and FAPERJ for the support

provided to this investigation.

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