Direct bonded environmental barrier coatings for sic/sic composites and methods for preparing the same
A method of preparing a ceramic matrix composite (CMC) article is disclosed. The method includes depositing a first layer of a coating composition directly onto a surface of a silicon carbide fiber-reinforced silicon carbide matrix (SiC/SiC) composite substrate, with the coating composition comprising a rare earth silicate and a sintering aid. The method also includes heating the first layer to sinter the coating composition to form an environmental barrier coating (EBC) adjacent the SiC/SiC composite and a transition layer integrally bonded to and between the substrate and the EBC. CMC articles prepared according to the method, including coated turbomachine components, are also disclosed.
1 . A method of preparing a ceramic matrix composite (CMC) article, the method comprising:
depositing a first layer of a coating composition directly onto a surface of a silicon carbide fiber-reinforced silicon carbide matrix (SiC/SiC) composite substrate, the coating composition comprising a rare earth silicate and a sintering aid; and
heating the first layer to sinter the coating composition to form an environmental barrier coating (EBC) adjacent the SiC/SiC composite substrate and a transition layer integrally bonded to and between the SiC/SiC composite substrate and the EBC, thereby preparing the CMC article, wherein the transition layer comprises a reaction product of the rare earth silicate of the coating composition and the SiC/SiC composite substrate, wherein the transition layer has a thickness of from 3 to 10 mils, and the EBC has a thickness of from 1.5 to 10 mils.
2 . The method of claim 1 , wherein the surface of the SiC/SiC composite substrate on which the first layer of the coating composition is deposited is free from a bond coating.
3 . The method of claim 1 , wherein the first layer of the coating composition is deposited via direct application to the SiC/SiC composite substrate by doctor blading or airbrushing.
4 . The method of claim 3 , wherein the coating composition is in the form of a slurry or paste, and wherein the method further comprises preparing the slurry or paste prior to depositing the first layer thereof by combining particles of the rare earth silicate, particles of the sintering aid, a carrier vehicle, and optionally a binder.
5 . The method of claim 1 , wherein heating the first layer of the coating composition comprises:
a burnout phase in which the first layer of the coating composition is heated to a first temperature (T 1 ) within a first time period, and subsequently
a sintering phase in which the first layer of the coating composition is heated to a second temperature (T 2 ) higher than T 1 within a second time period.
6 . The method of claim 5 , wherein during the sintering phase, the first layer of the coating composition is substantially free from: (i) carbon; (ii) free silicon; or (iii) both (i) and (ii).
7 . The method of claim 5 , wherein the second temperature (T 2 ) and the second time period are selected to prepare the transition layer comprising the reaction product of the rare earth silicate of the coating composition and the SiC/SiC composite substrate.
8 . The method of claim 1 , wherein the rare earth silicate is further defined as a rare earth disilicate (RE 2 Si 2 O 7 ), and wherein the first coating composition comprises from about 80 to about 99 wt. % of the rare earth disilicate, based on the total weight solids of the first coating composition.
9 . The method of claim 8 , wherein the rare earth silicate comprises ytterbium disilicate (Yb 2 Si 2 O 7 ).
10 . The method of claim 9 , wherein the coating composition comprises from about 1 to about 20 wt. % of the sintering aid, based on the total weight solids of the first coating composition.
11 . The method of claim 1 , wherein the sintering aid comprises alumina (Al 2 O 3 ).
12 . The method of claim 1 , wherein the sintering aid is substantially free from: (i) carbon; (ii) silicon; or (iii) both (i) and (ii).
13 . The method of claim 1 , further comprising depositing a layer of a second coating composition over the EBC; and heating the second layer to form a second EBC bonded to the EBC prepared from the first coating composition.
14 . The method of claim 13 , wherein the first and second coating compositions are substantially the same.
15 . The method of claim 1 , wherein the SiC/SiC composite substrate is a part body, and wherein depositing the first layer of the coating composition comprises encapsulating at least a portion of the part body with the coating composition.
16 . The method of claim 1 , further comprising disposing a thermal barrier coating (TBC) over the EBC.
17 . The method of claim 1 , wherein:
the EBC has a first coefficient of thermal expansion (CTE);
the transition layer has a second CTE; and
the SiC/SiC composite substrate has a third CTE,
wherein the second CTE more closely matches the third CTE than the first CTE matches the third CTE, and this closer match is achieved because the transition layer includes a reaction product formed from the rare earth silicate of the coating composition and the SiC/SiC composite substrate.