Method and apparatus for selectively leaching portions of PDC cutters already mounted in drill bits

1. In a method for improving the performance of a PDC drill bit having a plurality of PDC cutters mounted therein, comprising the steps of:

coating the exterior surface of said drill bit and the exterior surface of at least one of said plurality of PDC cutters, each of said PDC cutters having a body of diamond crystals and a catalyzing material contained within said body, said coating comprising a material impervious to a given acid or a mixture of given acids;

selectively removing a segment of said coating from said at least one PDC cutter;

applying said given acid or mixture of given acids to said at least one PDC cutter to leach out at least some of the catalyzing material contained within said body of diamond crystals, thereby providing improved thermal stability to a first region of said at least one PDC cutter from which said coating has been removed while retaining improved impact strength within a second region of said at least one PDC cutter upon which said coating has been retained.

2. The method of claim 1 , wherein said coating comprises tetrafluroethylene.

3. The method of claim 1 , wherein said coating comprises polyethylene.

4. The method according to claim 1 , wherein said acid comprises hydrofluoric acid.

5. The method according to claim 1 , wherein said given mixture of acids comprises a mixture of hydrofluoric acid and nitric acid.

6. The method according to claim 1 , wherein said catalyzing material comprises cobalt.

7. The method according to claim 1 , wherein said catalyzing material consists essentially of either cobalt, nickel, iron, or alloys thereof.

8. The method of claim 1 , further comprising the step of heating the drill bit to facilitate leaching of said at least one PDC cutter, wherein the said given acid or mixture of given acids is heated by the drill bit when applied to said at least one PDC cutter.

9. The method of claim 1 , wherein the step of selectively removing a segment of said coating from said at least one PDC cutter comprises removing said coating from at least a portion of a cutting edge of said at least one PDC cuter while retaining said coating on the remainder of said at least one PDC cutter.

10. A PDC bit having a plurality of PDC cutters mounted therein, each of said PDC cutters having a first end comprising a layer of diamond crystals and catalyzing material contained within the interstices between said diamond crystals respectively, said first end of said bodies comprising a circular cutting edge for drilling through rock;

a coating covering the external surface of each of said layers of diamond crystals, said coating comprising a material which is impervious to a given acid or a mixture of given acids, at least one of the PDC cutters having a template through its coating exposing one or more segments of the cutting edge for such at least one PDC cutter, for introducing said given acid or mixture of given acids through said at least one template to thereby leach out some of the catalyzing materials contained within the interstices between said diamond crystals and thereby provide improved thermal stability to a first region of said at least one PDC cutter exposed through said template while retaining improved impact strength within a second region of said at least one PDC cutter covered by said coating.

11. The PDC drill bit of claim 10 , wherein said coating comprises tetrafluroethylene.

12. The PDC drill bit of claim 10 , wherein said coating comprises polyethylene.

13. The PDC drill bit according to claim 10 , wherein said given acid comprises hydrofluoric acid.

14. The PDC drill bit according to claim 10 , wherein said mixture of given acids comprises a mixture of hydrofluoric acid and nitric acid.

15. The PDC drill bit of claim 10 , wherein said catalyzing material comprises cobalt.

16. The PDC drill bit of claim 10 , wherein said catalyzing material consists essentially of cobalt, nickel, iron, or alloys thereof.