WE CLAIM:
1. An article (100) comprising:
a coating (110) disposed on a substrate (120), the coating (110) comprising a plurality of elongated surface-connected voids (130); and
a protective agent (150) disposed within at least some of the voids (130) of the coating (110);
wherein the protective agent (150) comprises a substance capable of chemically reacting with liquid nominal CMAS to form a solid crystalline product outside the crystallization field of said nominal CMAS,
wherein the solid crystalline product has a melting temperature greater than about 1200 degrees Celsius.
2. The article (100) as claimed in claim 1, wherein the protective agent (150) is present in the coating (110) in an effective concentration to substantially prevent incursion by said nominal CMAS into voids (130) in which the protective agent (150) is disposed.
3. The article (100) as claimed in claim 1, wherein the protective agent (150) comprises a nitride, an oxide, or combinations thereof.
4. The article (100) as claimed in claim 1, wherein the protective agent (150) comprises a rare earth oxide.
5. The article (100) as claimed in claim 1, wherein the substance has a weberite crystal structure.

6. The article (100) as claimed in claim 5, wherein the substance comprises a rare-earth element, oxygen, and an element selected from the group consisting of tantalum and niobium.
7. The article (100) as claimed in claim 1, wherein the protective agent (150) comprises an oxide having the nominal formula A1-xBxZr4-yDy(PO4)z ;
wherein A and B are selected from the group consisting of alkaline earth metals and rare earth metals, D is hafnium or titanium, x is a number from 0 to 1; y is a number from 0 to 4; and z is 3 or 6.
8. The article (100) as claimed in claim 7, wherein the protective agent (150) comprises MZr4(PO4)6, wherein M comprises at least one alkaline earth metal.
9. The article (100) as claimed in claim 1, wherein the substance has a perovskite crystal structure and comprises (a) a rare earth element, (b) tantalum, niobium, or a combination of tantalum and niobium, and (c) oxygen.
10. The article (100) as claimed in claim 1, wherein the substance comprises gadolinium, tantalum, and oxygen.
11. The article (100) as claimed in claim 1, wherein the substance comprises a compound having a scheelite, fergusonite, or wolframite crystal structure, and having a nominal formula ABX4, wherein A comprises a rare-earth element, B comprises tantalum, niobium, or a combination of tantalum and niobium, and X comprises oxygen, nitrogen, or a combination of nitrogen and oxygen.

12. The article (100) as claimed in claim 11, wherein A comprises yttrium, gadolinium, lanthanum, neodymium, ytterbium, or combinations of any of the foregoing.
13. The article (100) as claimed in claim 1, wherein the coating (110) comprises a plurality of layers (220).
14. The article (100) as claimed in claim 1, further comprising a barrier agent (160) interposed between the substrate (120) and the protective agent (150).
15. The article (100) as claimed in claim 14, wherein the barrier agent (160) comprises less than 34 atomic per cent rare earth elements.