Carbon seal assembly

1. A seal assembly comprising:

a housing at least partially defining a seal opening and at least partially surrounding a rotatable shaft; and

a carbon seal located at least partially in the seal opening and including a sealing surface wherein the housing forms a circumferential ring and the carbon seal is formed from circumferential segments each including a groove on a radially outer surface with a garter spring located in a corresponding groove of each the circumferential segments;

wherein the rotatable shaft includes a radially facing surface having a carbide based coating and a diamond-like carbon coating in engagement with the sealing surface on the carbon seal.

2. The assembly of claim 1 , wherein the carbide based coating is in direct contact with a radially outer surface of the rotatable shaft.

3. The assembly of claim 2 , wherein the diamond-like carbon coating is in direct contact with the carbide based coating and the carbon seal.

4. The assembly of claim 1 , wherein the carbide based coating and the diamond-like carbon coating extend circumferentially around the rotatable shaft.

5. The assembly of claim 1 , wherein the carbide based coating and the diamond-like carbon coating create a radial separation between the rotatable shaft and the carbon seal.

6. The assembly of claim 1 , wherein the carbon seal includes an electrocarbon grade carbon and the rotatable shaft is a spool for a gas turbine engine.

7. The assembly of claim 1 , wherein the diamond-like carbon coating is silicon doped.

8. The assembly of claim 1 , wherein the carbide based coating and the diamond-like carbon coating separate the rotatable shaft from contacting the carbon seal.

9. The assembly of claim 1 , further comprising a recess in the rotatable shaft for accepting the carbide based coating and the diamond-like carbon coating.

10. The assembly of claim 9 , wherein the sealing surface is located on a radially inner side of a projection of the carbon seal.

11. The assembly of claim 9 , wherein the carbon seal includes a first axial dimension and the sealing surface includes a second axial dimension less than the first axial dimension, wherein the sealing surface is a radially innermost surface of the carbon seal.

12. The assembly of claim 1 , wherein the carbide based coating is between 76 and 152 micrometers thick.

13. The assembly of claim 1 , wherein the diamond-like carbon coating is approximately 1 micrometer thick.

14. A seal assembly comprising:

a housing at least partially defining a seal opening and at least partially surrounding a rotatable shaft;

a carbon seal located at least partially in the seal opening and including a sealing surface, wherein the rotatable shaft includes a radially facing surface having a carbide based coating and a diamond-like carbon coating in engagement with the sealing surface on the carbon seal; and

a recess in the rotatable shaft for accepting the carbide based coating and the diamond-like carbon coating;

wherein the carbon seal includes a first axial dimension and the sealing surface includes a second axial dimension less than the first axial dimension, wherein the sealing surface is a radially innermost surface of the carbon seal.

15. A method of using a seal assembly, the method comprising:

locating a carbon seal adjacent a rotatable shaft, wherein the carbon seal includes a first axial dimension and a sealing surface in engagement with the rotatable shaft includes a second axial dimension that is less than the first axial dimension, wherein the sealing surface is a radially innermost surface of the carbon seal;

separating the carbon seal from the rotatable shaft with a carbide based coating and a diamond-like carbon coating; and

generating a lubricant between the carbon seal and the diamond-like carbon coating.

16. The method of claim 15 , wherein generating the lubricant includes forming a graphitic material between an interface of the carbon seal and the diamond-like carbon coating.

17. The method of claim 15 , further comprising rotating a shaft adjacent the carbon seal and the shaft includes a velocity of at least 152 m/s relative to the carbon seal and the shaft is a spool for a gas turbine engine.

18. The method of claim 15 , wherein the carbide based coating is applied through at least one of a high velocity oxygen fuel coating process or an APS coating process.

19. The method of claim 15 , wherein the diamond-like carbon coating is applied through at least one of a PVD or CVD process.

20. The method of claim 15 , including compressing multiple circumferential segments of the carbon seal with a garter spring located in a groove on a radially outer side of each of the multiple circumferential segments.