Differential needling of a carbon fiber preform

1. A preform for making a carbon-carbon composite comprising:

a plurality of layers comprising carbon fibers or carbon-precursor fibers, wherein the plurality of layers are stacked along a central axis to form a stack, wherein the plurality of layers comprise a first exterior layer defining a first major surface of the preform, a second exterior layer defining a second major surface of the preform opposite the first major surface, and at least one interior layer disposed between the first exterior layer and the second exterior layer, the at least one interior layer comprising a peripheral region that forms a portion of an outer surface of the preform; and

a plurality of needled fibers, wherein at least some needled fibers of the plurality of needled fibers extend through two or more layers of the plurality of layers;

wherein the preform has an exterior region and a core region, wherein the exterior region comprises the first exterior layer, the second exterior layer, and the peripheral region of the at least one interior layer;

wherein the core region includes a first composite diameter region that forms an outer diameter of the core region as measured from the central axis, a second composite diameter region that forms an inner diameter of the core region as measured from the central axis, a central region that lies radially between first composite diameter region and second composite diameter region, a first intermediate region that lies radially between the central region and the first composite diameter region, and a second intermediate region that lies radially between the central region and the second composite diameter region,

wherein the plurality of needled fibers define a first needled fiber number density (NFND) in the exterior region, a second NFND in the first composite diameter region, a third NFND in the second composite diameter region, a fourth NFND in the central region, a fifth NFND in the first intermediate region, and a sixth NFND in the second intermediate region,

wherein the second NFND, third NFND, fourth NFND, fifth NFND, and sixth NFND are each greater than the first NFND, and

wherein the fourth NFND is greater than at least one the fifth NFND or the sixth NFND.

2. The preform of claim 1 ,

wherein the fourth NFND is greater than the fifth NFND and the sixth NFND.

3. The preform of claim 1 , wherein the preform defines a disc shape with the first and second major surfaces are substantially orthogonal to the central axis, wherein the first and second major surfaces are separated by a thickness of the preform measured parallel to the central axis, wherein the preform comprises an aperture through the thickness of the preform along the central axis,

wherein the peripheral region of the at least one interior layer comprises a first peripheral region that forms an outer diameter of the disc shape; and

wherein the at least one interior layer comprises a second peripheral region that forms an inner diameter of the disc shape.

4. The preform of claim 3 , wherein the exterior region further comprises the second peripheral region.

5. A method comprising:

stacking a plurality of layers comprising carbon fibers or carbon-precursor fibers along a central axis to form a stack, wherein the plurality of layers comprises a first exterior layer, a second exterior layer, and at least one interior layer disposed between the first and second exterior layers, wherein the at least one interior layer comprises at least one peripheral region adjacent to an exterior surface of the stack; and

needling the stack substantially parallel to the central axis to from a preform comprising a plurality of needled fibers, wherein at least some needled fibers of the plurality of needled fibers extend through two of more layers of the plurality of layers;

wherein the preform comprises a core region and an exterior region, wherein the exterior region comprises the first exterior layer, the second exterior layer, and the at least one peripheral region of the at least one interior layer;

wherein the core region includes a first composite diameter region that forms an outer diameter of the core region as measured from the central axis, a second composite diameter region that forms an inner diameter of the core region as measured from the central axis, a central region that lies radially between first composite diameter region and second composite diameter region, a first intermediate region that lies radially between the central region and the first composite diameter region, and a second intermediate region that lies radially between the central region and the second composite diameter region,

wherein the plurality of needled fibers define a first needled fiber number density (NFND) in the exterior region, a second NFND in the first composite diameter region, a third NFND in the second composite diameter region, a fourth NFND in the central region, a fifth NFND in the first intermediate region, and a sixth NFND in the second intermediate region,

wherein the second NFND, third NFND, fourth NFND, fifth NFND, and sixth NFND are each greater than the first NFND, and

wherein the fourth NFND is greater than at least one the fifth NFND or the sixth NFND.

6. The method of claim 5 ,

wherein the fourth NFND, is greater than the fifth NFND and the sixth NFND.

7. The method of claim 6 , wherein the second NFND, the third NFND, and the fourth NFND are greater than the fifth NFND and the sixth NFND.

8. The method of claim 6 , wherein the fifth NFND and the sixth NFND are approximately equal.

9. The method of claim 5 , wherein the preform defines a disc shape with the first and second major surfaces are substantially orthogonal to the central axis, wherein the first and second major surfaces are separated by a thickness of the preform measured parallel to the central axis, wherein the preform comprises an aperture through the thickness of the preform along the central axis,

wherein the peripheral region of the at least one interior layer comprises a first peripheral region that forms an outer diameter of the disc shape; and

wherein the at least one interior layer comprises a second peripheral region that forms an inner diameter of the disc shape.

10. The method of claim 9 , wherein the exterior region further comprises the second peripheral region.

11. The method of claim 5 , further comprising densifying the preform to form a carbon-carbon composite.

12. The method of claim 5 , further comprising carbonizing the preform.

13. The method of claim 5 , wherein needling the stack comprises:

using a needling board comprising a plurality of needles; and

distributing the plurality of needles on the needling board to form the first needled fiber number density and the second needled fiber number density.

14. The method of claim 5 , wherein needling the stack comprises:

using a needling board comprising a plurality of needles comprising barbs; and

varying a number of barbs per needle to form the first needled fiber number density and the second needled fiber number density.

15. The method of claim 5 , wherein needling the stack comprises varying the needling rate to form the first needled fiber number density and the second needled fiber number density.

16. The preform of claim 1 , wherein the exterior region surrounds the core region.

17. The preform of claim 16 , wherein the second NFND, the third NFND, and the fourth NFND are greater than the fifth NFND and the sixth NFND.

18. The preform of claim 1 , wherein the second NFND, the third NFND, and the fourth NFND are greater than the fifth NFND and the sixth NFND.

19. The preform of claim 1 , wherein the fifth NFND and the sixth NFND are approximately equal.