Methods for fabricating YAG barrier coatings

1. A method of forming a coated alloy comprising:

providing a superalloy substrate;

applying a bond coat onto the superalloy substrate;

depositing an yttrium oxide film and an aluminum oxide film onto the bond coat;

reacting the yttrium and aluminum oxide films at a temperature effective to form a single phase yttrium aluminum garnet (YAG) oxidation barrier coating directly on the surface of the bond coat; and

depositing a top coat on the single phase YAG oxidation barrier coating.

2. A method according to claim 1 wherein the bond coat comprises MCrAlY, MAl, M 3 Al or combinations thereof wherein M comprises Ni, Pt, Co, NiCo or combinations thereof.

3. A method according to claim 1 wherein the bond coat comprises at least about 10% by wt aluminum.

4. A method according to claim 1 wherein the top coat comprises lanthanum phosphate.

5. A method according to claim 4 wherein the lanthanum phosphate has a thermal conductivity of about 1.5 to about 2.0 w/m·K at about 600 to about 700° C. and a density of about 4.0 to about 6.0 g/cm 3 .

6. A method according to claim 4 wherein the lanthanum phosphate is produced by

reacting aqueous mixtures of lanthanum nitrate and alkyl phosphates at temperatures below about 130° C. to form a lanthanum phosphate powder; and

densifying the lanthanum phosphate powder by sintering at temperatures of from about 1400 to about 1550° C. and/or hot-pressing at temperatures of from about 1300 to about 1450° C.

7. A method according to claim 1 wherein the oxidation barrier coating comprises a thickness of about 0.5 to about 2 μm.

8. A method according to claim 1 wherein the oxidation barrier coating comprises nano-sized, bonded primary grains of YAG comprising a thickness of from about 500 nm to about 1000 nm.

9. A method according to claim 1 wherein the yttrium oxide and/or the aluminum oxide comprise a thickness of about 0.5 to 1 μm.

10. A method according to claim 1 wherein the yttrium oxide and aluminum oxide films are deposited as alternating layers onto the bond coat.

11. A method according to claim 1 further comprising heating at a temperature of from about 1000° C. to about 1200° C. prior to depositing the top coat.

12. A method according to claim 1 wherein the superalloy substrate comprises nickel.

13. A method of forming a coated alloy comprising:

providing a superalloy substrate;

applying a bond coat onto the superalloy substrate, wherein the bond coat comprises a surface layer comprising a preformed aluminum oxide film;

depositing an yttrium oxide film onto the surface layer of the bond coat;

reacting the yttrium oxide film and the preformed aluminum oxide film at a temperature effective to form a single phase yttrium aluminum garnet (YAG) oxidation barrier coating directly on the surface of the bond coat; and

depositing a top coat onto the single phase YAG oxidation barrier coating, wherein the top coat comprises rare earth phosphates.

14. A method according to claim 13 wherein the yttrium oxide film comprises yttria (Y 2 O 3 ), YAM (Y 4 Al 2 O 9 ), YAP (YAlO 3 ), yttrium aluminates, or combinations thereof.

15. A method according to claim 13 wherein the preformed aluminum oxide film comprises a thickness of from about 0.1 to about 1 μm.

16. A method according to claim 13 further comprising heating at a temperature of from about 1000° C. to about 1200° C. prior to depositing the top coat.

17. A method according to claim 13 wherein the superalloy substrate comprises nickel.

18. A method according to claim 13 wherein the top coat comprises lanthanum phosphate.

19. A method according to claim 18 wherein the lanthanum phosphate is produced by

reacting aqueous mixtures of lanthanum nitrate and alkyl phosphates at temperatures below about 130° C. to form a lanthanum phosphate powder; and

densifying the lanthanum phosphate powder by sintering at temperatures of from about 1400 to about 1550° C. and/or hot-pressing at temperatures of from about 1300 to about 1450° C.

20. A method of forming a coated alloy comprising:

providing a superalloy substrate;

applying a bond coat comprising aluminum onto the superalloy substrate;

depositing an yttrium oxide film on the surface of the bond coat;

reacting the yttrium oxide film and the aluminum in the bond coat in an oxidizing atmosphere at a temperature effective to form a single phase yttrium aluminum garnet (YAG) oxidation barrier coating directly on the surface of the bond coat; and

depositing a top coat on the single phase YAG oxidation barrier coating, wherein the top coat comprises rare earth phosphates.

21. A method according to claim 20 further comprising heating at a temperature of from about 1000° C. to about 1200° C. prior to depositing the top coat.

22. A method according to claim 20 wherein the superalloy substrate comprises nickel.

23. A method of claim 20 wherein the top coat comprises lanthanum phosphate.

24. A method according to claim 23 wherein the lanthanum phosphate is produced by

reacting aqueous mixtures of lanthanum nitrate and alkyl phosphates at temperatures below about 130° C. to form a lanthanum phosphate powder; and

densifying the lanthanum phosphate powder by sintering at temperatures of from about 1400 to about 1550° C. and/or hot-pressing at temperatures of from about 1300 to about 1450° C.