CONTROL OF DEFECTS AND SWEEP PATTERN IN PDC BY TREATING CARBIDE SUBSTRATE BEFORE SWEEP

A method for forming a polycrystalline diamond compact (PDC) includes treating a carbide substrate having a cobalt content therein with an acid, e.g., aqua regia, to remove cobalt from a surface portion of the carbide substrate; disposing diamond crystals on the treated carbide substrate; disposing a sweep material on the diamond crystals on a surface of the diamond opposite the carbide substrate; and applying high temperature and pressure to the carbide substrate, the diamond crystals and the sweep material such that the diamond crystals are sintered into a polycrystalline diamond attached to the carbide substrate to form the polycrystalline diamond compact.

DETAILED DESCRIPTION

Reference will now be made in detail to preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings. The detailed description of preferred embodiments can be read in connection with the accompanying drawings in which like numerals designate like elements.

FIG. 2is a cross-sectional view showing an exemplary polycrystalline diamond compact (PDC) in accordance with one exemplary embodiment of the present invention. As shown inFIG. 2, the PDC includes a substrate201, such as cemented carbide substrate, with a polycrystalline diamond table203bonded onto a top surface of the substrate201. The polycrystalline diamond table203does not have a sweep material from the substrate201. The sweep material may be cobalt, for example. The carbide substrate201may include tungsten carbide (WC) or other material and has a binder metal doped therein to improve integrity and strength of the carbide substrate201. The binder metal may be cobalt or other iron-group element. An exemplary PDC is cylindrical shaped tungsten carbide substrate with 11.5% cobalt therein having a diameter of 10-25 mm and a height of 5-20 mm where the polycrystalline diamond crystal is about 1-4 mm thick, for example. In one exemplary embodiment, the sweep material may be mixed with a plurality of diamonds before the plurality of diamonds are sintered onto the polycrystalline carbide substrate. In another exemplary embodiment, the sweep material may be from a cobalt disc on the plurality of diamond crystals on a surface of the diamond crystals at a distance from the cemented carbide substrate.

FIG. 3is a flowchart showing a method of forming a PDC in accordance with an exemplary embodiment of the present invention.FIG. 4is a schematic diagram showing cross-sectional views to illustrate an arrangement according to an exemplary sweep-through process according to the flowchart ofFIG. 3.

As shown in step301ofFIG. 3, a carbide substrate is treated with an aqua regia solution to remove the binder metal from a surface portion of the carbide substrate. For example, 400 ML of aqua regia including HCl and HNO3in a 1:3 ratio may be used. The carbide may be completely immersed in the aqua regia acid to leach all exposed surfaces of the carbide substrate. Alternatively, a single surface or a portion of one or more surfaces may be leached by protecting the rest of the surfaces using gaskets such as Viton® gaskets or acid-resistant paste, for example. The leach depth to leach time depends on the acid composition in the aqua regia, temperature, amount of cobalt being leached and quantity of acid. After a certain time (depending on the parameters mentioned above), the aqua regia solution may get saturated with the leaching byproducts, the tungsten carbide on the surface may separate and go into the solution and the leach depth may stay constant. However, it is desired that a sufficient amount of leaching occurs so that there is insufficient cobalt or other materials in the surface portion of the carbide substrate to function as a sweep catalyst during the sintering process. A leaching depth of 20 microns may be sufficient to prevent the remaining binder material in the substrate from melting and sweeping into the mass of diamond powder during sintering, but more desirably, a leaching depth of 40 microns substantially eliminates the carbide substrate as source of sweep material during the sintering process.

After removing the carbide substrate from the aqua regia and cleaning the carbide substrate, a bed of diamond crystals are packed on the top of the treated carbide substrate as shown in step303. The diamond crystals may be generally synthetic diamond in the form of a powder or grit.

As shown in step305, a sweep material is disposed on an upper surface of the diamond crystal bed. That is, the sweep material is on a surface of the diamond bed opposite the carbide substrate. The sweep material may be cobalt, nickel, iron or their alloys or other suitable sweep material. The sweep material may contain additives such as chromium or other metals such as cobalt, nickel and iron. Also, the binder metal in the carbide substrate need not be the same material as the sweep material. By contrast with the related art method, the aqua regia treatment of step301may have leached out any cobalt in the surface portion of the carbide substrate. Therefore, a separate sweep material is provided. The interface between the diamond bed and the sweep material may be planar to facilitate a uniform sweep front during the sintering process. A cup, such as a tantalum cup, may be disposed over the sweep material, diamond crystals, and carbide substrate to hold the materials in place, as shown for example inFIGS. 5 and 6. In addition, because the high pressure may be applied using a press with salt as the pressure transmitting medium, the cup also serves to prevent contamination.

Alternative to step305, the plurality of diamond crystals may be mixed with a sweep material, such as cobalt. Additive materials, such as chromium, nickel and iron may be added to the mixture of the plurality of diamond crystals with the sweep material. The mixture of the plurality of diamond crystals with the sweep material and optional additive materials may be disposed on the treated cemented carbide substrate.

In step307, the carbide substrate, the diamond crystals and the sweep material may be disposed in a press system so that high temperature and pressure may be applied. The particular parameters may vary by equipment, but exemplary parameters for pressure, temperature and time are: pressure may be greater than 50-55 Kbar for diamond to be stable and may be typically at 70-75 Kbar, 1400-1600° C., 5-10 min; another example at a lower pressure may be 60-65 Kbar, 1400° C.-1600 C, 20-30 min. In this manner, the diamond crystals are sintered into a polycrystalline diamond attached to the carbide substrate to form the polycrystalline diamond compact. Of course, additional materials as known in the art may be included in the sweep materials.

Because the interface between the sweep material and the diamond crystals is planar, a planar sweep front may be achieved. Thus, problems associated with the related art, such as the metal filled cracks/fingers and areas of poorly sintered diamond, may be prevented. Also, the sweep direction during the sintering process is toward the carbide substrate rather than away from the carbide substrate. Therefore, the described arrangement has the additional advantage of sweeping any contaminants away from the upper surface, i.e., the cutting edge. Here, the contaminants may be present due to a variety of sources, such as calcium from processing of the diamond crystals, contaminants within the diamond crystals that are exposed due to fracturing of the crystals at high pressure. Thus, the sweep direction improves the quality of the cutting edge. Moreover, a near net (final) shape is obtained, thereby reducing finishing costs. For example, in the disclosed process, contaminants are swept away from the cutting edge and into a region which does not take part in the cutting action. So the cutting edge contains well sintered diamond. In contrast, in prior art the contaminants end up at the cutting edge, and hence additional finishing steps are necessary such as lapping, grinding etc to remove a portion of the diamond and expose a well sintered cutting edge.

FIGS. 5 and 6are schematic views showing different arrangements of carbide substrates having non-planar surfaces in accordance with exemplary embodiments of the present invention. InFIG. 5, the carbide substrate has a raised central portion, and, inFIG. 6, the carbide substrate has a deep valley. Despite having non-planar interfaces between the carbide substrate and the diamond crystals, regular sweep patterns may be achieved using the exemplary arrangement of the sweep material and the exemplary HPHT processing as disclosed herein.FIGS. 5 and 6also illustrate the use of a cup, such as a tantalum cup, as described above. Exemplary embodiments may have various surfaces other than those shown inFIGS. 5 and 6.

FIGS. 7-11are SEM images showing the depth of the leach layers as a function of leach time using an aqua regia solution. For each image, 400 mL of aqua regia (300 mL HCl and 100 mL HNO3) was applied at 24° C. to a tungsten carbide (WC) substrate having 11.5% cobalt (Co) therein and an exposed surface area of 0.3 in2. For each leach time, two carbide substrates were used to check consistency. That is, ten carbide substrates were put in the aqua regia at the beginning and two substrates were taken out at each of the respective times.

FIG. 12is a graph summarizing the relationship between leach depth and leach time for the SEM images ofFIGS. 7-11. As shown inFIG. 12, the leach depth levels off after about 60 minutes of leach time under the conditions described above. It has been found that the sweep-through process resultant from cobalt in the carbide substrates is reduced but not eliminated when carbide substrates were leached for 20 minutes to achieve a leach depth of 28 microns. In this case, it was found that the sweep from the carbide substrate was sufficiently reduced so that the sweep from the cobalt disk on the diamond achieved before the sweep from the carbide substrate, thereby providing improved results as compared with the related art. It has also been found that the sweep-through process resultant from cobalt in the carbide substrates was substantially eliminated when the carbide substrates were leached for 60 minutes or more. Thus, under the specified conditions, it may be desired to leach the carbide substrates for at least 60 minutes. Moreover, it may be desired to achieve a leach depth of at least 40 microns.

By removing the cobalt in the layer of the carbide substrate closest to the diamond and by providing a different sweep source, the arrangement described in this application provides a way of altering and controlling the sweep pattern in the sweep-through process. As a result, PDCs may be obtained with improved cutting edges and reduced costs while also preventing defects and irregularities.