Coating processes – Spray coating utilizing flame or plasma heat – Silicon containing coating
Reexamination Certificate
2000-05-11
2001-10-02
Bareford, Katherine A. (Department: 1762)
Coating processes
Spray coating utilizing flame or plasma heat
Silicon containing coating
C427S453000, C427S454000
Reexamination Certificate
active
06296909
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to coating systems suitable for protecting components exposed to high-temperature environments, such as the hostile thermal environment of a gas turbine engine. More particularly, this invention is directed to methods for thermally spraying mullite coatings that have use as bond coats for thermal/environmental barrier coating systems on silicon-containing ceramic substrates.
BACKGROUND OF THE INVENTION
Higher operating temperatures for gas turbine engines are continuously sought in order to increase their efficiency. However, as operating temperatures increase, the high temperature durability of the components of the engine must correspondingly increase. While significant advances in high temperature capabilities have been achieved through formulation of iron, nickel and cobalt-base superalloys, alternative materials have been proposed. For example, materials containing silicon, such as those with silicon carbide (SiC) as a matrix material or a reinforcing material, are currently being considered for high temperature applications, such as combustor and other hot section components of gas turbine engines. In many applications, a protective coating over the Si-containing material is beneficial, an example of which is a thermal-insulating layer to reduce the operating temperature and thermal gradient through the material. Additionally, such coatings can provide environmental protection by inhibiting the major mechanism for degradation of silicon carbide in a corrosive environment, namely, the formation of volatile silicon monoxide (SiO) and silicon hydroxide (Si(OH)
4
) products. On this basis, besides low thermal conductivity, a critical requirement of a coating system for a SiC-containing material is low activity of silica (SiO
2
) in its composition. Other important properties for the coating material include a coefficient of thermal expansion (CTE) compatible with the SiC-containing material, low permeability for oxidants, and chemical compatibility with SiC and silica scale. Consequently, the coating essentially has a dual function, serving as a thermal barrier and simultaneously providing protection from the environment. For this reason, such a coating system will be referred to herein as a thermal/environmental barrier coating system, or TEBC.
While various coating systems have been investigated, each has exhibited shortcomings relating to the above-noted requirements and properties for compatibility with a Si-containing material. For example, an yttria-stabilized zirconia (YSZ) coating serving as a thermal barrier layer exhibits excellent environmental resistance by itself, since it does not contain silica in its composition. However, YSZ exhibits high permeability to oxygen and other oxidants. In addition, YSZ cannot be adhered directly to silicon carbide because of a coefficient of thermal expansion mismatch. As a result, mullite (3Al
2
O
3
·2SiO
2
) has been proposed as a bond coat between SiC-containing substrate materials and ceramic coatings such as YSZ in order to compensate for differences in coefficients of thermal expansion.
Processibility by plasma spraying and good adhesion to silicon-based ceramics and ceramic composites have been shown for mullite. However, thermally-sprayed mullite bond coats have exhibited a large number of through-thickness cracks which serve as fast paths for the transport of corrosive species to the underlying substrate, and coating failures caused by extensive interfacial oxidation have been observed. The presence of the through-thickness cracks in mullite coatings was unexpected, because stoichiometric mullite (3Al
2
O
3
·2SiO
2
) has a coefficient of thermal expansion fairly close to that of silicon-based ceramic composite materials. Accordingly, there is a need for the prevention of through-thickness cracks in mullite bond coats for TEBCs on silicon-based materials.
SUMMARY OF THE INVENTION
The present invention generally provides a process for depositing a mullite layer on a silicon-based material, such as those used to form articles exposed to high temperatures and including the hostile thermal environment of a gas turbine engine. Examples of silicon-based materials of interest to this invention include those with a dispersion of silicon carbide particles as a reinforcement material in a metallic or nonmetallic matrix, as well as those having a silicon carbide matrix, and particularly composite materials that employ silicon carbide as both the reinforcement and matrix materials (Si/SiC composites). The invention is particularly useful for thermally spraying crack-free mullite bond coats for thermal/environmental barrier coatings (TEBCs) on silicon-based substrates.
According to this invention, it was determined that the presence of transverse through-thickness cracks in mullite coatings is attributed to large in-plane tensile stresses being generated during cooling from the elevated deposition temperatures required to deposit such coatings. Microstructural examinations showed that more than one distinct phase occurs within mullite coatings deposited by thermally spraying a mullite powder, and that these phases contain varying amounts of silicon and aluminum. One of the phases was determined to be alumina (Al
2
O
3
), leading to the conclusion that mullite phase decomposition can take place during thermal spraying. The presence of alumina and phases with high alumina content in a mullite coating was concluded to sufficiently increase the coefficient of thermal expansion and elastic modulus of the coating to cause the aforementioned through-thickness cracks. It was also determined that alumina and alumina-rich phases can be generated in a mullite coating if silica is lost during deposition of the mullite powder by means such as volatilization during deposition. Therefore, this invention provides processing techniques that maintain or reestablish phase equilibrium in a mullite layer deposited by thermal spraying.
The general method of the invention is to thermally spray a mullite powder to form a mullite layer on a substrate, in which the thermal spraying process is performed so that the mullite powder attains a temperature above its peritectic temperature (about 1830° C.) during deposition. In one form of the invention, the thermal spray parameters are adjusted to heat the mullite powder to a temperature above its liquidus temperature during deposition. With this technique, the deposition parameters must be controlled to prevent evaporation of silica from the mullite powder. The resulting mullite layer is then cooled at a rate sufficient to quench the molten powder and thereby prevent formation of alumina and silica-rich phases in the layer. According to this invention, the avoidance of such phases, and particularly alumina phases, in the mullite layer prevents the formation of through-thickness cracks in the mullite layer.
Alternatively, the mullite powder is heated during deposition to a temperature of between 1830° C. and the liquidus temperature of mullite (about 1950° C., depending on the exact composition of the mullite powder), such that the resulting mullite layer contains alumina and silica-rich phases in a mullite matrix. Thereafter, the mullite layer is heated before it has cooled sufficiently to crack. Heating is performed for a duration and at a temperature sufficient to convert the alumina and silica-rich phases to mullite, and thereby substantially achieves phase equilibrium within the mullite layer. As a result, following cooling the mullite layer is substantially free of the alumina phases and through-thickness cracks.
REFERENCES:
patent: 5391404 (1995-02-01), Lee et al.
patent: 5869146 (1999-02-01), McCluskey et al.
Heidorn Raymond W.
Spitsberg Irene T.
Wang Hongyu
Bareford Katherine A.
General Electric Company
Hess Andrew C.
Narciso David L.
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