Polycrystalline compacts include a polycrystalline superabrasive material comprising a first plurality of grains of superabrasive material having a first average grain size and a second plurality of grains of superabrasive material having a second average grain size smaller than the first average grain size. The first plurality of grains is dispersed within a substantially continuous matrix of the second plurality of grains. Earth-boring tools may include a body and at least one polycrystalline compact attached thereto. Methods of forming polycrystalline compacts may include coating relatively larger grains of superabrasive material with relatively smaller grains of superabrasive material, forming a green structure comprising the coated grains, and sintering the green structure. Other methods include mixing diamond grains with a catalyst and subjecting the mixture to a pressure greater than about five gigapascals (5.0 GPa) and a temperature greater than about 1,300° C. to form a polycrystalline diamond compact.

Polycrystalline compacts include a polycrystalline superabrasive material comprising a first plurality of grains of superabrasive material having a first average grain size and a second plurality of grains of superabrasive material having a second average grain size smaller than the first average grain size. The first plurality of grains is dispersed within a substantially continuous matrix of the second plurality of grains. Earth-boring tools may include a body and at least one polycrystalline compact attached thereto. Methods of forming polycrystalline compacts may include coating relatively larger grains of superabrasive material with relatively smaller grains of superabrasive material, forming a green structure comprising the coated grains, and sintering the green structure. Other methods include mixing diamond grains with a catalyst and subjecting the mixture to a pressure greater than about five gigapascals (5.0 GPa) and a temperature greater than about 1,300 C. to form a polycrystalline diamond compact.

Embodiments relate to polycrystalline diamond compacts (PDCs) that are less susceptible to liquid metal embrittlement damage due to the use of at least one transition layer between a polycrystalline diamond (PCD) layer and a substrate. In an embodiment, a PDC includes a PCD layer, a cemented carbide substrate, and at least one transition layer bonded to the substrate and the PCD layer. The at least one transition layer is formulated with a coefficient of thermal expansion (CTE) that is less than a CTE of the substrate and greater than a CTE of the PCD layer. At least a portion of the PCD layer includes diamond grains defining interstitial regions and a metal-solvent catalyst occupying at least a portion of the interstitial regions. The diamond grains and the catalyst collectively exhibit a coercivity of about 115 Oersteds or more and a specific magnetic saturation of about 15 Gauss.Math.cm.sup.3/grams or less.

An assembly for High Pressure High Temperature (HPHT) synthesis of a superhard material. The assembly comprises a container comprising a first metal. A closure also comprising the first metal is sealed to the container using a sealant material. The sealant material comprises a second metal, the seal comprising a composition of the first and second metals formable below the melting point of the second metal. The container contains superhard material.

A polycrystalline compact includes diamond, cubic boron nitride, and at least one hard material, which may be aluminum nitride, gallium nitride, silicon nitride, titanium nitride, silicon carbide, titanium carbide, titanium boride, titanium diboride, and/or aluminum boride. The diamond, the cubic boron nitride, and the hard material are intermixed and interbonded to form a polycrystalline material. An earth-boring tool includes a bit body and a polycrystalline diamond compact secured to the bit body. Methods of fabricating polycrystalline compacts include forming a mixture comprising diamond, non-cubic boron nitride, and a metal or semimetal; encapsulating the mixture in a container; and subjecting the encapsulated mixture to high-pressure and high-temperature conditions to form a polycrystalline material.

A polycrystalline compact includes diamond, cubic boron nitride, and at least one hard material, which may be aluminum nitride, gallium nitride, silicon nitride, titanium nitride, silicon carbide, titanium carbide, titanium boride, titanium diboride, and/or aluminum boride. The diamond, the cubic boron nitride, and the hard material are intermixed and interbonded to form a polycrystalline material. An earth-boring tool includes a bit body and a polycrystalline diamond compact secured to the bit body. Methods of fabricating polycrystalline compacts include forming a mixture comprising diamond, non-cubic boron nitride, and a metal or semimetal; encapsulating the mixture in a container; and subjecting the encapsulated mixture to high-pressure and high-temperature conditions to form a polycrystalline material.

A cutting element include a substrate and a diamond compact including at least two polycrystalline diamond portions separated by at least one metal carbide foil portion. The cutting element is made by placing diamond powder in a reaction container, placing a thin metal layer in the reaction container above or around the diamond powder and binder, placing additional diamond powder in the reaction container above or around the thin metal layer, and placing a pre-sintered substrate containing binder into the reaction container above all diamond powder and thin metal layer components. The assembled reaction container is put into a reactor and is subjected to a high-temperature high-pressure sintering process. The binder in the pre-sintered substrate sweeps through to sinter the first diamond portion, and then reacts with the thin metal layer to form a metal carbide, and then the binder continues to sweep through to sinter the second diamond portion.