Polycrystalline compacts having differing regions therein, cutting elements and earth-boring tools including such compacts, and methods of forming such compacts

A polycrystalline compact, comprising:

• a volume of polycrystalline diamond, comprising:
• a first region adjacent a substrate and, comprising:
• a first plurality of diamond grains;
• a second plurality of diamond grains having an average grain size of about five hundred nanometers (500 nm) or less disposed and interspersed between the grains of the first plurality of diamond grains, the first plurality of diamond grains and the second plurality of diamond grains being interspersed and inter-bonded;
• and
• a catalyst material for catalyzing the formation of inter-granular diamond bonds disposed in interstitial spaces between the inter-bonded grains of the first plurality of diamond grains and the second plurality of diamond grains;
• and
• a leached second region adjacent and directly bonded to the first region, the leached second region having a greater volume percentage of interstitial spaces than the first region and comprising inter-bonded diamond grains, the inter-bonded diamond grains of the leached second region comprising between about eighty percent (80%) and about ninety-two percent (92%) of a volume of the leached second region, voids in the interstitial spaces between the inter-bonded diamond grains of the leached second region at least substantially comprising a remainder of the volume of the leached second region.

A cutting element, comprising:

• a cutting element substrate;
• and
• a polycrystalline compact bonded to the cutting element substrate, the polycrystalline compact comprising:
• a hard polycrystalline material, comprising:
• a first region adjacent the cutting element substrate and comprising:
• a first plurality of grains of hard material having a first average grain size;
• a second plurality of grains of hard material having a second average grain size smaller than the first average grain size, the grains of the first plurality of grains of hard material and of the second plurality of grains of hard material being interspersed and inter-bonded;
• and
• catalyst material for catalyzing the formation of inter-granular bonds between the grains of the first plurality of grains of hard material and of the second plurality of grains of hard material, the catalyst material disposed in interstitial spaces between the inter-bonded grains of hard material of the first plurality of grains of hard material and of the second plurality of grains of hard material;
• and
• a second region adjacent and directly bonded to the first region along an interface between the first region and the second region, the second region having a smaller volume percentage of hard material than the first region and comprising a third plurality of grains of hard material having a third average grain size, the grains of the third plurality of grains of hard material being interspersed and inter-bonded, wherein interstitial spaces between the inter-bonded grains of the third plurality of grains of hard material are at least substantially free of catalyst material for catalyzing the formation of inter-granular bonds between the grains of the third plurality of grains of hard material.

An earth-boring tool, comprising:

• a tool body;
• and
• at least one cutting element attached to the tool body, the at least one cutting element comprising:
• a polycrystalline compact comprising:
• a hard polycrystalline material, comprising:
• a first, region adjacent the tool body and comprising:
• a first plurality of grains of hard material having a first average grain size;
• a second plurality of grains of hard material having a second average grain size smaller than the first average grain size, the grains of the first plurality of grains of hard material and of the second plurality of grains of hard material being interspersed and inter-bonded;
• and
• catalyst material for catalyzing the formation of inter-granular bonds between the grains of the first plurality of grains of hard material and of the second plurality of grains of hard material, the catalyst material disposed in interstitial spaces between the inter-bonded grains of hard material of the first plurality of grains of hard material and of the second plurality of grains of hard material;
• and
• a second region adjacent and directly bonded to the first region along an interface between the first region and the second region, the second region having a smaller volume percentage of hard material than the first region and comprising a third plurality of grains of hard material having a third average grain size, the grains of the third plurality of grains of hard material being interspersed and inter-bonded, wherein interstitial spaces between the inter-bonded grains of the third plurality of grains of hard material are at least substantially free of catalyst material for catalyzing the formation of inter-granular bonds between the grains of the third plurality of grains of hard material.

A method of forming a polycrystalline compact, comprising:

• forming an unsintered compact preform, comprising:
• mixing a first plurality of grains of hard material having a first average grain size with a second plurality of grains of hard material having a second average grain size smaller than the first average grain size to form a first particulate mixture;
• and
• positioning a third plurality of grains of hard material having a third average grain size adjacent the first particulate mixture within a container;
• sintering the compact preform at a pressure greater than about five (5.0 GPa) and a temperature greater than about 1,300° C. in the presence of a catalyst material for catalyzing the formation of inter-granular bonds between the grains of hard material of the first plurality of grains of hard material, the second plurality of grains of hard material, and the third plurality of grains of hard material, sintering the unsintered compact comprising forming a hard polycrystalline material having a first region comprising inter-bonded grains of hard material from the first plurality of grains of hard material and the second plurality of grains of hard material, and a second region having a smaller volume percentage of hard material than the first region and comprising inter-bonded grains of hard material formed from the third plurality of grains of hard material, the first region having a first density of the hard material higher than a second density of the hard material in the second region;
• and
• removing catalyst material from interstitial spaces within the second region of the hard polycrystalline material without entirely removing catalyst material from interstitial spaces within the first region of the hard polycrystalline material.