Polycrystalline compacts, earth-boring tools including such compacts, and methods of fabricating polycrystalline compacts

1. A polycrystalline compact, comprising:

diamond grains;

cubic boron nitride grains; and

grains of an additional nitride, carbide, or boride selected from the group consisting of aluminum nitride, gallium nitride, silicon nitride, titanium nitride, silicon carbide, titanium carbide, titanium boride, titanium diboride, and aluminum boride;

wherein the diamond grains, the cubic boron nitride grains, and the grains of the additional nitride, carbide, or boride are intermixed and interbonded to form a polycrystalline material, and

wherein the diamond grains consist of diamond grains having a grain size from about 1 μm to about 40 μm.

2. The polycrystalline compact of claim 1 , wherein the polycrystalline compact has a density of at least about 3.40 g/cm 3 .

3. The polycrystalline compact of claim 1 , wherein the polycrystalline compact has a Young's modulus of at least about 700 GPa.

4. The polycrystalline compact of claim 1 , wherein the polycrystalline compact comprises from about 70 atomic percent to about 90 atomic percent carbon.

5. The polycrystalline compact of claim 1 , wherein the polycrystalline compact comprises from about 5 atomic percent to about 15 atomic percent boron.

6. The polycrystalline compact of claim 1 , wherein the polycrystalline compact comprises from about 5 atomic percent to about 15 atomic percent nitrogen.

7. The polycrystalline compact of claim 1 , wherein the polycrystalline compact comprises from about 0.1 atomic percent to about 5.0 atomic percent of an element selected from the group consisting of aluminum, gallium, silicon, and titanium.

8. The polycrystalline compact of claim 1 , wherein the grains of the additional nitride, carbide, or boride comprise aluminum nitride grains.

9. The polycrystalline compact of claim 1 , wherein the polycrystalline compact is substantially free of metallic phases comprising cobalt, nickel, iron, and alloys thereof.

10. An earth-boring tool, comprising: a bit body; and

at least one polycrystalline diamond compact secured to the bit body, the polycrystalline diamond compact comprising:

diamond grains;

cubic boron nitride grains; and

grains of an additional nitride, carbide, or boride selected from the group consisting of

aluminum nitride, gallium nitride, silicon nitride, titanium nitride, silicon carbide, titanium carbide, titanium boride, titanium diboride, and aluminum boride;

wherein the diamond grains, the cubic boron nitride grains, and the grains of the additional nitride, carbide, or boride are intermixed and interbonded to form a polycrystalline material, and

wherein the at least one polycrystalline diamond compact has a density of at least about 3.40 g/cm 3 .

11. The earth-boring tool of claim 10 , wherein the diamond grains consist of diamond grains having a grain size from about 1 μm to about 40 μm.

12. A method of fabricating a polycrystalline compact, comprising:

forming a mixture comprising diamond grains, non-cubic boron nitride grains, and a metal or semimetal, the metal or semimetal selected from the group consisting of aluminum, gallium, silicon, and titanium;

encapsulating the mixture in a container; and

subjecting the encapsulated mixture to a pressure of at least 5.0 GPa and a temperature of at least 1,100° C. to form a polycrystalline material from the mixture, the polycrystalline material comprising the diamond grains, cubic boron nitride grains formed from the non-cubic boron nitride grains, and grains of an additional nitride, carbide, or boride selected from the group consisting of aluminum nitride, gallium nitride, silicon nitride, titanium nitride, silicon carbide, titanium carbide, titanium boride, titanium diboride, and aluminum boride;

wherein the diamond grains, the cubic boron nitride grains, and the grains of the additional nitride, carbide, or boride are intermixed and interbonded within the polycrystalline material.

13. The method of claim 12 , wherein subjecting the encapsulated mixture to the pressure of at least 5.0 GPa and the temperature of at least 1,100° C. comprises converting at least a portion of the boron nitride of the non-cubic boron nitride grains from a wurtzitic phase to a cubic phase.

14. The method of claim 12 , wherein forming the mixture comprising diamond grains, non-cubic boron nitride grains, and the metal or semimetal comprises mixing the diamond grains with the non-cubic boron nitride grains and an aluminum powder.

15. The method of claim 12 , wherein subjecting the encapsulated mixture to the pressure of at least 5.0 GPa and the temperature of at least 1,100° C. comprises maintaining the encapsulated mixture at the pressure of at least 5.0 GPa and the temperature of at least 1,100° C. for a period of time from about 1 second to about 5 minutes.

16. The method of claim 12 , wherein forming the mixture comprising diamond grains, non-cubic boron nitride grains, and the metal or semimetal comprises mixing metal or semimetal particles having an average particle size from about 50 nm to about 1 μm with the diamond grains and the non-cubic boron nitride grains.

17. The method of claim 12 , wherein forming the mixture comprising diamond grains, non-cubic boron nitride grains, and the metal or semimetal comprises mixing diamond grains having an average particle size from about 50 nm to about 40 μm with the metal or semimetal and the non-cubic boron nitride grains.

18. The method of claim 12 , wherein forming the mixture comprising diamond grains, non-cubic boron nitride grains, and the metal or semimetal comprises mixing non-cubic boron nitride grains having an average particle size from about 1 μm to about 40 μm with the diamond grains and the metal or semimetal.

19. The method of claim 12 , wherein subjecting the encapsulated mixture to the pressure of at least 5.0 GPa and the temperature of at least 1,100° C. comprises subjecting the encapsulated mixture to a pressure of at least about 7.5 GPa.

20. The method of claim 12 , wherein subjecting the encapsulated mixture to the pressure of at least 5.0 GPa and the temperature of at least 1,100° C. comprises subjecting the encapsulated mixture to a temperature of at least about 1,400° C.