Thermal barrier coating systems and processes therefor
Coating systems and processes by which the coating systems can be deposited to be resistant to contaminants, and particularly resistant to infiltration and damage caused by CMAS. The coating systems include inner and outer ceramic layers. The inner ceramic layer consists essentially of zirconia stabilized by about 6 to about 9 weight percent yttria and optionally contains greater than 0.5 to 10 weight percent hafnium oxide. The outer ceramic layer overlies and contacts the inner ceramic layer to define the outermost surface of the coating system. The outer ceramic layer consists essentially of zirconia stabilized by about 25 to about 75 weight percent yttria, has a thickness that is less than the thickness of the inner ceramic layer and further contains greater than 0.5 to 10 weight percent hafnium oxide and optionally 1 to 10 weight percent tantalum oxide. The outer ceramic layer has a porosity level that is lower than that of the inner ceramic layer.
1. A coating system on a surface region of a component, the coating system comprising:
a bond coat; and
inner and outer ceramic layers on the bond coat, the inner ceramic layer overlying the bond coat, the inner ceramic layer consisting of zirconia stabilized by about 6 to about 9 weight percent yttria, the inner ceramic layer having a thickness and a porosity level, the outer ceramic layer overlying and contacting the inner ceramic layer and defining an outermost surface of the coating system, the outer ceramic layer consisting of zirconia stabilized by about 25 to about 75 weight percent yttria and greater than 0.5 to 10 weight percent hafnium oxide and optionally 1 to 10 weight percent tantalum oxide, the outer ceramic layer having a thickness that is less than the thickness of the inner ceramic layer and, having a porosity level that is lower than the porosity level of the inner ceramic layer, and
wherein the inner and outer ceramic layers are heat treated in a vacuum to a temperature and for a duration sufficient to relieve stresses therein.
2. The coating system according to claim 1 , wherein the outer ceramic layer contains greater than 1.0 to 10 weight percent hafnium oxide.
3. The coating system according to claim 1 , wherein the outer ceramic layer contains greater than 1.0 to 2.5 weight percent hafnium oxide.
4. The coating system according to claim 1 , wherein the inner ceramic layer has a thickness of at least 50 to about 500 micrometers and the outer ceramic layer has a thickness of up to 250 micrometers.
5. The coating system according to claim 1 , wherein the outer ceramic layer has a thickness of at least about 25 micrometers.
6. The coating system according to claim 1 , wherein the ratio of the thickness of the outer ceramic layer to that of the inner ceramic layer is not greater than 0.5.
7. The coating system according to claim 1 , wherein the porosity level of the inner ceramic layer is about 10 to about 25 volume percent and the porosity level of the outer ceramic layer is about 3 to about 15 volume percent.
8. The coating system according to claim 1 , wherein the zirconia of the outer ceramic layer is stabilized by about 36 to 42 weight percent yttria and the outer and inner ceramic layers define a thickness ratio of not greater than 0.5.
9. The coating system according to claim 1 , wherein the outer ceramic layer comprises a cubic crystal phase and the inner ceramic layer consists essentially of a tetragonal or modified tetragonal crystal phase.
10. The coating system according to claim 1 , wherein the outer ceramic layer reacts with a eutectic compound containing calcia, magnesia, alumina and silica to form calcium yttrium silicate at temperatures above 1200° C.
11. The coating system according to claim 10 , wherein the component is chosen from the group consisting of high and low pressure turbine vanes and blades, shrouds, combustor liners and augmentor hardware of a gas turbine engine.
12. The coating system according to claim 1 , wherein the bond coat is a metallic bond coat chosen from the group consisting of MCrAlX overlay coatings and/or diffusion aluminide coatings.
13. The coating system according to claim 1 , wherein the component is a gas turbine engine component formed of a nickel-base or cobalt-base superalloy.