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An article comprising a substrate formed of a silicon-comprising material, such as an article exposed to the hostile thermal environment of a gas turbine engine. The article further comprises an environmental barrier layer, e.g., an alkaline earth metal aluminosilicate, and a top coat comprising zir

An article comprising a substrate formed of a silicon-comprising material, such as an article exposed to the hostile thermal environment of a gas turbine engine. The article further comprises an environmental barrier layer, e.g., an alkaline earth metal aluminosilicate, and a top coat comprising zirconia or hafnia stabilized with up to about 10 mole % of an oxide of a metal selected from the group consisting of magnesium, calcium, scandium, yttrium, and lanthanide metals, and mixtures thereof. The article further comprises a transition layer between the environmental barrier layer and the top coat, the transition layer comprising zirconia or hafnia stabilized with up to about 10 mole % of an oxide of a metal selected from the group consisting of magnesium, calcium, scandium, yttrium, and lanthanide metals; and a low CTE oxide selected from the group consisting of niobia and tantala; and mixtures thereof. A method for preparing a thermal/environmental barrier coating system on a substrate formed of a silicon-comprising material is also disclosed.

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What is claimed is: 1. An article comprising: a) a substrate formed of a silicon-comprising material; b) an environmental barrier layer overlying the substrate; and c) a transition layer overlying the environmental barrier layer, the transition layer comprising hafnia stabilized with up to about 1

What is claimed is: 1. An article comprising: a) a substrate formed of a silicon-comprising material; b) an environmental barrier layer overlying the substrate; and c) a transition layer overlying the environmental barrier layer, the transition layer comprising hafnia stabilized with up to about 10 mole % of an oxide of a metal selected from the group consisting of magnesium, calcium, scandium, yttrium, and lanthanide metals; and a low CTE oxide selected from the group consisting of niobia and tantala; and mixtures thereof; and d) a top coat overlying the transition layer, the top coat comprising zirconia or hafnia stabilized with up to about 10 mole % of an oxide of a metal selected from the group consisting of magnesium, calcium, scandium, yttrium, and lanthanide metals, and mixtures thereof. 2. An article as recited in claim 1, wherein the substrate is formed of a material selected from the group consisting of silicon carbide; silicon nitride; composites having a matrix of at least one of silicon carbide, silicon nitride and silicon; and composites have at least one of a silicon carbide, silicon nitride and silicon matrix reinforced with at least one of silicon carbide, silicon nitride and silicon. 3. An article as recited in claim 1, wherein the environmental barrier layer comprises an alkaline earth metal aluminosilicate material. 4. An article as recited in claim 3, wherein the environmental barrier layer comprises barium strontium aluminosilicate. 5. An article as recited in claim 1, wherein the top coat comprises yttria-stabilized zirconia. 6. An article as recited in claim 1, wherein the transition layer and the top coat comprise yttria-stabilized hafnia. 7. An article as recited in claim 1, wherein the transition layer comprises from about 40% to about 80% of hafnia, or mixtures of hafnia and zirconia; from about 10% to about 60% of niobia or tantala, or mixtures thereof; and from about 2% to about 10% of yttria; all on a molar basis. 8. An article as recited in claim 7, wherein the environmental barrier layer has a substantially uniform composition of barium strontium aluminosilicate. 9. An article as recited in claim 1, wherein the environmental barrier layer has a thickness of from about 25 to about 500 micrometers. 10. An article as recited in claim 9, wherein the transition layer has a thickness of from about 25 to about 500 micrometers. 11. An article as recited in claim 10, wherein the transition layer comprises from about 40% to about 80% of hafnia, or mixtures of hafnia and zirconia; from about 10% to about 60% of niobia or tantala, or mixtures thereof; and from about 2% to about 10% of yttria; all on a molar basis. 12. An article as recited in claim 11, wherein the top coat has a thickness of from about 12.5 to about 1250 micrometers. 13. An article comprising: a) a substrate formed of a silicon-comprising material; b) an environmental barrier layer overlying the substrate; and c) a transition layer overlying the environmental barrier layer, the transition layer comprising zirconia or hafnia stabilized with up to about 10 mole % of an oxide of a metal selected from the group consisting of magnesium, calcium, scandium, yttrium, and lanthanide metals; and a low CTE oxide selected from the group consisting of niobia and tantala; and mixtures thereof; and d) a top coat overlying the transition layer, the top coat comprising zirconia or hafnia stabilized with up to about 10 mole % of an oxide of a metal selected from the group consisting of magnesium, calcium, scandium, yttrium, and lanthanide metals, and mixtures thereof; wherein the transition layer comprises sublayers, a first sublayer contacting the environmental barrier layer and comprising niobia and/or tantala, and a second sublayer contacting the topcoat and having a substantially uniform composition of yttria-stabilized zirconia and/or yttria-stabilized hafnia. 14. An article comprising: a) a substrate formed of a silicon-comprising material; b) an environmental barrier layer overlying the substrate; and c) a transition layer overlying the environmental barrier layer, the transition layer comprising zirconia or hafnia stabilized with up to about 10 mole % of an oxide of a metal selected from the group consisting of magnesium, calcium, scandium, yttrium, and lanthanide metals; and a low CTE oxide selected from the group consisting of niobia and tantala; and mixtures thereof; and d) a top coat overlying the transition layer, the top coat comprising zirconia or hafnia stabilized with up to about 10 mole % of an oxide of a metal selected from the group consisting of magnesium, calcium, scandium, yttrium, and lanthanide metals, and mixtures thereof; wherein the transition layer is compositionally graded and has a decreasing concentration of niobia and/or tantala and an increasing concentration of yttria-stabilized zirconia and/or yttria-stabilized hafnia in a direction away from the environmental barrier layer, and consists essentially of yttria-stabilized zirconia and/or yttria-stabilized hafnia at an interface of the transition layer with the top coat. 15. A gas turbine engine component comprising a substrate formed of a silicon-comprising material and having a thermal/environmental barrier coating system on a surface thereof, the thermal/environmental barrier coating system comprising: a) an environmental barrier layer overlying the substrate and having a thickness of from about 25 to about 500 micrometers; b) a transition layer overlying the environmental barrier layer, the transition layer having a thickness of from about 25 to about 500 micrometers and comprising hafnia stabilized with up to about 10 mole % of an oxide of a metal selected from the group consisting of magnesium, calcium, scandium, yttrium, and lanthanide metals; and a low CTE oxide selected from the group consisting of niobia and tantala; and mixtures thereof; and c) a top coat overlying the transition layer, the top coat having a thickness of from about 12.5 to about 1250 micrometers and comprising zirconia or hafnia stabilized with up to about 10 mole % of an oxide of a metal selected from the group consisting of magnesium, calcium, scandium, yttrium, and lanthanide metals, and mixtures thereof. 16. A gas turbine engine component as recited in claim 15, wherein the substrate is formed of a material selected from the group consisting of silicon carbide; silicon nitride; composites having a matrix of at least one of silicon carbide, silicon nitride and silicon; and composites with at least one of a silicon carbide, silicon nitride and silicon matrix reinforced with at least one of silicon carbide, silicon nitride and silicon. 17. A gas turbine engine component as recited in claim 16, wherein the environmental barrier layer comprises barium strontium aluminosilicate. 18. A gas turbine engine component as recited in claim 17, wherein the transition layer comprises from about 40% to about 80% of hafnia, or mixtures of hafnia and zirconia; from about 10% to about 60% of niobia or tantala, or mixtures; and from about 2% to about 10% of yttria; all on a molar basis. 19. A method for preparing a thermal/environmental barrier coating system a substrate formed of a silicon-comprising material, said method comprising: a) forming an environmental barrier layer overlying the substrate and having a thickness of from about 25 to about 500 micrometers; b) forming a transition layer overlying the environmental barrier layer, the transition layer having a thickness of from about 25 to about 500 micrometers and comprising hafnia stabilized with up to about 10 mole % of an oxide of a metal selected from the group consisting of magnesium, calcium, scandium, yttrium, and lanthanide metals; and a low CTE oxide selected from the group consisting of niobia and tantala; and mixtures thereof; and c) forming a top coat overlying the transition layer, the top coat having a thickness of from about 12.5 to about 1250 micrometers and comprising zirconia or hafnia stabilized with up to about 10 mole % of an oxide of a metal selected from the group consisting of magnesium, calcium, scandium, yttrium, and lanthanide metals, and mixtures thereof. 20. A method as recited in claim 19, wherein the substrate is formed of a material selected from the group consisting of silicon carbide; silicon nitride; composites having a matrix of at least one of silicon carbide, silicon nitride and silicon; and composites with at least one of a silicon carbide, silicon nitride and silicon matrix reinforced with at least one of silicon carbide, silicon nitride and silicon. 21. A method as recited in claim 20, wherein the environmental barrier layer comprises barium strontium aluminosilicate. 22. A method as recited in claim 21, wherein the transition layer comprises from about 40% to about 80% of hafnia, or mixtures thereof; from about 10% to about 60% of niobia or tantala, or mixtures thereof; and from about 2% to about 10% of yttria; all on a molar basis.