Carbon seal assembly

1. A seal assembly comprising:

a carbon seal having a sealing surface facing a first axial direction;

a seal seat having a sealing surface facing a second axial direction opposite the first axial direction, the seal seat is positioned for rotation relative to the carbon seal and the seal seat includes a recess extending in a circumferential direction forming a complete ring and defining an opening facing in the second axial direction with a radially inner portion of the sealing surface located radially inward of the recess and a radially outer portion of the sealing surface located radially outward from the recess;

an insert located in the recess; and

a diamond-like carbon coating at least partially forming the sealing surface on the insert, wherein the diamond-like carbon coating includes a sp2 content that is greater than a sp3 content.

2. The assembly of claim 1 , wherein the diamond-like carbon coating is located inward from a perimeter of the insert.

3. The assembly of claim 2 , wherein the diamond-like carbon coating is spaced from the seal seat.

4. The assembly of claim 3 , wherein the insert is comprised of at least one of a ceramic matrix composite, a silicon nitride, or a silicon carbide.

5. The assembly of claim 4 , wherein the recess is located in an axially facing surface of the seal seat.

6. The assembly of claim 4 , wherein the insert is press fit into the recess.

7. The assembly of claim 4 , including a fastener extending through a fastener opening in the seal seat and engaging the insert.

8. A seal assembly comprising:

a carbon seal having a sealing surface;

a shaft having a sealing surface and positioned for rotation relative to the carbon seal, wherein a radially outer surface of the shaft includes a recess that forms a complete circumferential recess around the shaft;

an insert located in the recess; and

a diamond-like carbon coating at least partially forming the sealing surface on the insert, wherein the diamond-like carbon coating includes a sp2 content that is greater than a sp3 content.

9. The assembly of claim 8 , wherein the diamond-like carbon coating is located inward from a perimeter of the insert.

10. The assembly of claim 9 , wherein the diamond-like carbon coating is spaced from the shaft.

11. The assembly of claim 10 , wherein the insert is comprised of at least one of a ceramic matrix composite, a silicon nitride, or a silicon carbide.

12. The assembly of claim 10 , wherein the recess is located in a radially outer surface of the shaft and the recess is spaced inward from distal ends of the shaft.

13. The assembly of claim 8 , wherein the insert is secured to the shaft by at least one fastener.

14. The assembly of claim 8 , wherein the insert is secured to the shaft by a press fit connection with the recess.

15. A seal assembly comprising:

a carbon seal;

a shaft having a shaft sealing surface and positioned for rotation relative to the carbon seal, wherein a radially outer surface of the shaft includes a shaft recess that forms a complete circumferential recess around the shaft;

a shaft insert located in the shaft recess;

a shaft diamond-like carbon coating at least partially forming the shaft sealing surface on the shaft insert, wherein the shaft diamond-like carbon coating includes a sp2 content that is greater than a sp3 content;

a static structure including a static structure recess facing radially inward towards the shaft;

a static insert located in the static structure recess; and

a static diamond-like carbon coating at least partially forming a carbon seal sealing surface on the static insert and in contact with the carbon seal, wherein the static diamond-like carbon coating includes a sp2 content that is greater than a sp3 content;

wherein the carbon seal includes an attachment surface located adjacent the static insert.

16. The assembly of claim 15 , wherein the shaft diamond-like carbon coating is located inward from a perimeter of the shaft insert.

17. The assembly of claim 16 , wherein the static diamond-like carbon coating is spaced from the static structure.

18. The assembly of claim 17 , wherein the shaft insert and the static insert are each comprised of at least one of a ceramic matrix composite, a silicon nitride, or a silicon carbide.

19. The assembly of claim 15 , wherein the shaft sealing surface is located on a radially inner side of the carbon seal and the attachment surface is located on a radially outer surface of the carbon seal.

20. The assembly of claim 19 , including a graphitic material between the attachment surface and the static diamond-like carbon coating.