Environmental barrier coating material and coating structure and ceramic structure using the same
An environmental barrier coating material comprising one or more constituents selected from a group consisting of hafnia; hafnia stabilized by one or more rare-earth oxides and/or silica; zirconia-containing hafnia; and zirconia-containing hafnia stabilized by one or more rare-earth oxides and/or silica, which when formed as a coating structure for covering a substrate which has a low thermal expansion coefficient, has hafnon (HfSiO 4 ) serving as a first layer directly formed on the substrate, and hafnia with which the first layer is coated as a second layer.
1. An environmental barrier coating material comprising zirconia-containing hafnia stabilized by silica, and one or more rare-earth oxides, wherein
the silica content in the coating material gradually decreases toward the surface of the coating material; and
the zirconia content in the coating material gradually increases toward the surface of the coating material.
2. The environmental barrier coating material of claim 1 , wherein the surface of the coating material does not contain any silica.
3. An environmental barrier coating structure for covering a substrate that has a low thermal expansion coefficient, comprising hafnon (HfSiO 4 ) serving as a first layer directly formed on the substrate, and hafnia with which the first layer is coated as a second layer.
4. The environmental barrier coating structure according to claim 3 , wherein the hafnia contains zirconia (ZrO 2 ).
5. The environmental barrier coating structure according to claim 4 , wherein in the second layer the zirconia content is higher in a portion near a coating surface than a portion near the first layer.
6. The environmental barrier coating structure according to claim 5 , wherein in the second layer the zirconia content gradually increases as receding from a portion facing the first layer.
7. The environmental barrier coating structure according to claim 3 , wherein the hafnia is stabilized.
8. The environmental barrier coating structure according to claim 7 , wherein stabilization of the hafnia is performed by one or more constituents selected from a group consisting of rare-earth oxides and silica.
9. The environmental barrier coating structure according to claim 8 , wherein a layer of hafnia which contains zirconia and is stabilized by one or more rare-earth oxides and silica is formed as the second layer, and the silica constituent of the second layer has a gradient in content so as to gradually decrease from a portion facing the first layer toward the coating surface.
10. The environmental barrier coating structure according to claim 9 , wherein in the second layer a portion closest to the first layer comprises hafnia stabilized by only silica and a coating surface portion comprises hafnia stabilized by only one or more rare-earth oxides.
11. The environmental barrier coating structure according to claim 8 , wherein the rare-earth oxide comprises one or more hafnia stabilizers selected from a group consisting of Y 2 O 3 , La 2 O 3 , Nd 2 O 3 , Sm 2 O 3 , Gd 2 O 3 , Dy 2 O 3 Ho 2 O 3 , Er 2 O 3 , Yb 2 O 3 and Lu 2 O 3 .
12. The environmental barrier coating structure according to claim 7 , wherein the stabilized hafnia layer as the second layer comprises a small amount of one or more of SiO 2 , Ln 2 Si 2 O 7 (Ln representing a rare-earth), Ln 2 SiO 5 , HfSiO 4 and ZrSiO 4 phases as a subphase.
13. The environmental barrier coating structure according to claim 12 , wherein a thickness of the stabilized hafnia layer is from 1 μm to 600 μm.
14. The environmental barrier coating structure according to claim 7 , wherein the stabilized hafnia is stabilized in a cubic system or a tetragonal system by one or more rare-earth oxides and has a mole ratio with respect to hafnia of AO 2 (A representing hafnia):Ln 2 O 3 (Ln representing a rare-earth) in a range from about 97:3 to 50:50.
15. The environmental barrier coating structure according to claim 7 , wherein the stabilized hafnia is stabilized in a cubic system or a tetragonal system by one or more rare-earth oxides and silica, and has a mole ratio AO 2 :Ln 2 O 3 and SiO 2 in a range from about 9:1 to 6:4.
16. The environmental barrier coating structure according to claim 3 , wherein the first layer is a hafnia-silica oxide layer comprising hafnon (HfSiO 4 ) as a main constituent and at least one of hafnia (HfO 2 ), zirconia (ZrO 2 ) or silica (SiO 2 ).
17. The environmental barrier coating structure according to claim 16 , wherein the hafnia-silica oxide layer as the first layer has a hafnium and silicon composition ratio by atomic ratio in a range from about 3:7 to 7:3.
18. The environmental barrier coating structure according to claim 17 , wherein a thickness of the hafnia-silica oxide layer is from 0.1 μm to 600 μm.
19. The environmental barrier coating structure according to claim 3 , wherein the substrate is formed of silicon nitride ceramics, silicon carbide ceramics or a composite material thereof.