Cutting elements, bearings, and earth-boring tools including multiple substrates attached to one another

1. A bearing for an earth-boring tool, comprising:

a first bearing member comprising a first bearing pad having a first contact surface; and

a second bearing member comprising a second bearing pad having a second contact surface, at least a portion of the first contact surface being in sliding contact with at least a portion of the second contact surface;

wherein at least one of the first bearing member and the second bearing member comprises:

a polycrystalline table attached to a portion of a first substrate on which the polycrystalline table was formed, the portion of the first substrate comprising first hard, carbide, nitride, oxide, and/or boride particles within a first metal or metal alloy matrix material, the first hard, carbide, nitride, oxide, and/or boride particles having a first average particle size; and

another substrate attached to the portion of the first substrate, the other substrate comprising second hard, carbide, nitride, oxide, and/or boride particles within a second metal or metal alloy matrix material, the portion of the first substrate being interposed between the polycrystalline table and the other substrate, the second hard, carbide, nitride, oxide, and/or boride particles having a second, smaller average particle size;

wherein the portion of the first substrate comprises a first volume percentage of the first metal or metal alloy matrix material and the other substrate comprises a second, different volume percentage of the second metal or metal alloy matrix material.

2. The bearing of claim 1 , wherein the first volume percentage is greater than the second volume percentage.

3. The bearing of claim 1 , wherein the first average particle size is at least two times greater than the second average particle size.

4. The bearing of claim 1 , wherein a thickness of the portion of the first substrate is less than 1 mm.

5. The bearing of claim 1 , wherein the portion of the first substrate comprises an annular member bounding a central recess attached at a periphery of the polycrystalline table and the another substrate comprises a protrusion located in the central recess of the portion of the first substrate.

6. The bearing of claim 1 , wherein at least one of an end of the portion of the first substrate and a facing end of the another substrate comprises a rough- ground surface.

7. The bearing of claim 6 , wherein a surface roughness of the rough-ground surface is between 0 . 01 μin and 10 pin.

8. The bearing of claim 1 , wherein at least a portion of the polycrystalline table is free of catalyst material.

9. The bearing of claim 1 , further comprising at least one of a catalyst material, a braze material, an inert reinfiltrant material, an organic adhesive, and an organo-metallic adhesive attaching the portion of the first substrate to the other substrate.

10. An earth-boring tool, comprising:

a first member;

a second member rotatable about the first member; and

a bearing located between the first member and the second member, the bearing comprising:

a first bearing member comprising a first bearing pad having a first contact surface; and

a second bearing member comprising a second bearing pad having a second contact surface, at least a portion of the first contact surface being in sliding contact with at least a portion of the second contact surface;

wherein at least one of the first bearing member and the second bearing member comprises:

a polycrystalline table attached to a portion of a first substrate on which the polycrystalline table was formed, the portion of the first substrate comprising first hard, carbide, nitride, oxide, and/or boride particles within a first metal or metal alloy matrix material, the first hard, carbide, nitride, oxide, and/or boride particles having a first average particle size; and

another substrate attached to the portion of the first substrate, the other substrate comprising second hard, carbide, nitride, oxide, and/or boride particles within a second metal or metal alloy matrix material, the portion of the first substrate being interposed between the polycrystalline table and the other substrate, the second hard, carbide, nitride, oxide, and/or boride particles having a second, smaller average particle size;

wherein the portion of the first substrate comprises a first volume percentage of the first metal or metal alloy matrix material and the other substrate comprises a second, different volume percentage of the second metal or metal alloy matrix material.

11. The earth-boring tool of claim 10 , wherein the first volume percentage is greater than the second volume percentage.

12. The earth-boring tool of claim 10 , wherein the first average particle size is at least two times greater than the second average particle size.

13. The earth-boring tool of claim 10 , wherein a thickness of the portion of the first substrate is less than 1 mm.

14. The earth-boring tool of claim 10 , wherein the portion of the first substrate comprises an annular member bounding a central recess attached at a periphery of the polycrystalline table and the another substrate comprises a protrusion located in the central recess of the portion of the first substrate.

15. The earth-boring tool of claim 10 , wherein at least one of an end of the portion of the first substrate and a facing end of the another substrate comprises a rough-ground surface.

16. The earth-boring tool of claim 15 , wherein a surface roughness of the rough-ground surface is between 0.01 μin and 10 μin.

17. The earth-boring tool of claim 10 , wherein at least a portion of the polycrystalline table is free of catalyst material.

18. The earth-boring tool of claim 10 , further comprising at least one of a catalyst material, a braze material, an inert reinfiltrant material, an organic adhesive, and an organo-metallic adhesive attaching the portion of the first substrate to the other substrate.