Doped silicon carbide ceramic matrix composite

1. A method for forming ceramic matrix composite (CMC) component, the method comprising:

forming a fiber preform;

positioning the fiber preform into a chemical vapor infiltration reactor chamber; and

densifying the fiber preform, wherein densification comprises:

infiltrating the fiber preform with a first gas comprising precursors of silicon carbide;

infiltrating the fiber preform with a second gas comprising a first rare earth element;

wherein the steps of infiltrating the fiber preform with the first gas and infiltrating the fiber preform with the second gas are conducted simultaneously to produce a first rare earth-doped silicon carbide matrix in a first region of the component;

discontinuing flow of the second gas to the reactor chamber;

infiltrating the fiber preform with only the first gas to form a first silicon carbide matrix in a second region outward from the first region;

infiltrating the fiber preform with the first gas and the second gas to form the first rare earth-doped silicon carbide matrix in a third region outward from the second region;

discontinuing flow of the second gas to the reactor chamber; and

infiltrating the fiber preform with the first gas to form a second silicon carbide matrix in a fourth region adjacent to and outward from the third region and forming an outermost layer of the matrix.

2. The method of claim 1 , wherein the second gas comprises an organometallic compound.

3. The method of claim 2 , wherein the organometallic compound is tris- cyclopentadienyl ytterbium.

4. The method of claim 1 , wherein the rare earth-doped matrix in the first region comprises between 1 and 25 percent of the first rare earth element by atomic percent.

5. The method of claim 4 , and further comprising varying a flow rate of the second gas into the reactor chamber to vary a concentration of the rare earth element in the doped silicon carbide matrix.

6. The method of claim 5 , wherein the flow rate of the second gas is varied to provide a doped silicon carbide matrix region comprising a continuous gradient of the first rare earth element.

7. The method of claim 6 , wherein the flow rate of the second gas is decreased as densification progresses outward from fibers of the fiber preform.

8. The method of claim 6 , wherein the flow rate of the second gas is increased as densification progresses toward an outer boundary of the fiber preform.

9. The method of claim 1 , and further comprising:

discontinuing flow of the second gas to the reactor chamber; and

infiltrating the fiber preform with the first gas and a third gas, the third gas comprising a second rare earth element different from the first rare earth element, to form a second rare earth-doped silicon carbide matrix in a fifth region outward from the first region.

10. The method of claim 1 , and further comprising providing an interface coating on the fibers of the fiber preform prior to densification.

11. A method for forming ceramic matrix composite (CMC) component, the method comprising:

forming a fiber preform;

positioning the fiber preform into a chemical vapor infiltration reactor chamber; and

densifying the fiber preform, wherein densification comprises:

infiltrating the fiber preform with a first gas comprising precursors of silicon carbide; and

infiltrating the fiber preform with a second gas comprising a first rare earth element;

wherein the steps of infiltrating the fiber preform with the first gas and infiltrating the fiber preform with the second gas are conducted simultaneously to produce a first rare earth-doped silicon carbide matrix in a first region of the component;

discontinuing flow of the second gas to the reactor chamber; and

infiltrating the fiber preform with the first gas and a third gas, the third gas comprising a second rare earth element different from the first rare earth element, to form a second rare earth-doped silicon carbide matrix in a second region outward from the first region.

12. The method of claim 11 , wherein the steps of infiltrating the fiber preform with the first gas and second gas and infiltrating the fiber preform with the first gas and third gas are separated by a step of infiltrating the fiber preform with only the first gas to form a first silicon carbide matrix in a third region that separates the first and second regions.

13. The method of claim 12 , and further comprising:

discontinuing flow of the third gas to the reactor chamber; and

infiltrating the fiber preform with the first gas to form a second silicon carbide matrix in a fourth region adjacent to and outward from the second region and forming an outermost layer of the matrix.

14. The method of claim 11 , wherein the first gas comprises a first organometallic compound and the second gas comprises a second organometallic compound.

15. A method for forming ceramic matrix composite (CMC) component, the method comprising:

forming a fiber preform;

positioning the fiber preform into a chemical vapor infiltration reactor chamber; and

densifying the fiber preform, wherein densification comprises:

infiltrating the fiber preform with a first gas comprising precursors of silicon carbide;

infiltrating the fiber preform with a second gas comprising a first rare earth element;

wherein the steps of infiltrating the fiber preform with the first gas and infiltrating the fiber preform with the second gas are conducted simultaneously to produce a first rare earth-doped silicon carbide matrix in a first region of the component;

decreasing a flow rate of the second gas as densification progresses outward from fibers of the fiber preform to form a matrix in a second region outward from the first region, wherein the first region adjacent to the fibers has a higher concentration of the first rare earth element than the second region; and

increasing the flow rate of the second gas as densification progresses toward an outer boundary of the fiber preform to form a matrix in a third region having a higher concentration of the first rare earth element than the second region, wherein the second region is disposed between the first and third regions.

16. The method of claim 15 , wherein the second gas comprises tris-cyclopentadienyl ytterbium.

17. The method of claim 15 , wherein the first rare earth-doped matrix in the first region comprises between 1 and 25 percent of the first rare earth element by atomic percent.