Patent ID: 12246343

DETAILED DESCRIPTION

The disclosure relates to a method for controlled wetting during application of an aqueous slurry-based environmental barrier coating (EBC) to a silicon-containing substrate such as a ceramic matrix composite (CMC) substrate. The controlled wetting includes plasma treatment to remove organic surface contaminants from the surface of the substrate to be treated. Such contaminants can be hydrophobic, creating a high surface tension with the aqueous coating materials, and preventing wetting, which results in potentially low adherence of the resulting EBC.

An aqueous slurry as referred to herein generally has, as main constituent, an aqueous suspension of nanometer sized colloidal silica particles. This acts as an inorganic binder and holds other constituents of the slurry in place including but not limited to more water, gettering particles, and self-healing phase. Modifiers can be added including dispersants and surfactants.

Referring toFIG.1, there is illustrated an environmental barrier coating10formed over a substrate12of an article14. Coating10is configured to inhibit the formation of gaseous species of silicon when the article14is exposed to a high temperature, aqueous environment. The substrate12can be associated with articles14such as a turbine vane or a turbine blade, and/or any other component or components of a gas turbine engine, such as components in the hot section of the gas turbine engine, including rotating components and portions of combustors, and the like.

The substrate12can be constructed from materials containing silicon and can be a ceramic matrix composite material, a monolithic ceramic, a silicon-based or silicon containing ceramic substrate or a silicon containing metal alloy. In an exemplary embodiment, the substrate12can be silicon containing ceramic material such as, for example, silicon carbide, silicon nitride, silicon oxy-nitride and silicon aluminum oxy-nitride, alkaline earth or rare earth silicate glasses or glass ceramics and combinations thereof. Examples can include barium strontium aluminosilicate, strontium aluminosilicate, lithium aluminosilicate, aluminosilicate, mullite, yttrium silicate, ytterbium silicate, and the like. In accordance with a particular embodiment, the silicon containing ceramic substrate comprises a silicon containing matrix with reinforcing materials16such as fibers, particles and the like and, more particularly, a silicon based matrix which is fiber-reinforced. Particularly suitable ceramic substrates are a silicon carbide coated silicon carbide fiber-reinforced silicon carbide particle and silicon matrix, a carbon fiber-reinforced silicon carbide matrix and a silicon carbide fiber-reinforced silicon nitride matrix. Particularly useful silicon-metal alloys for use as substrates for the article14can include molybdenum-silicon alloys, niobium-silicon alloys, iron-silicon alloys, aluminum-silicon alloys, and silicon alloys with one or more of zirconium, hafnium, titanium, chromium, tungsten, boron, platinum and tantalum.

Referring also toFIG.2, an environmental barrier layer18can be applied to the substrate14on a surface20. A protective layer22can be applied on the environmental barrier layer18. The protective layer22can be configured to resist recession of the Si-containing volatile species when exposed to water vapor or steam. In an exemplary embodiment, the protective layer22can include binary or multicomponent oxides such as HfO2, ZrO2, or Gd2Hf2O7, Gd2Zr2O7, and refractory metal oxides. In other exemplary embodiments, the protective layer22can include silicates with lower SiO2activities. In another exemplary embodiment the protective layer22can include rare earth (RE) monosilicates, disilicates and alkaline earth (AE) aluminosilicates, silicates of Hafnium and zirconium.

The present disclosure is directed to enhancing adhesion of aqueous slurry-based coatings on surface20of substrate12. Such slurries can be applied by spraying, dipping, electrophoretic application, painting, and the like, and combinations thereof. In one non-limiting configuration, the resulting environmental barrier layer18can include an oxide matrix24and an oxidant getter phase26interspersed throughout the oxide matrix24. The oxide matrix24can include a multi-phase mixture, such as a SiO2rich phase and a self-healing phase28that can include a glass phase. In an exemplary embodiment, the composition of the oxide matrix24dictates the mole fraction of the glass and the SiO2. The self-healing phase28can include a material having properties that are in thermodynamic equilibrium with SiO2during operation at predetermined temperatures. The self-healing phase28comprises a material having properties including the ability to flow into cracks30formed in the matrix24during operation at those predetermined temperatures. The self-healing phase28can be sufficiently fluid at high temperatures to flow into the cracks30in the coating10, which imparts a self-healing functionality. The present disclosure is directed to aqueous slurry-based EBC application, and so the above composition or other compositions could be advantageously applied in this manner.

The aqueous-slurry based environmental barrier coating composition can be a mixture of water, binder, and oxidation protection phase, as would be known to a person having ordinary skill in the art, and can be as set forth above. These components can be provided in a broad range of ratios relative to each other, depending upon desired properties of the resultant coating.

The environmental barrier layer18can be applied to substrate12at a thickness of greater than or equal to about 0.5 mils (0.0005 inch), preferably between about 3 and about 30 mils and more preferably between about 3 and about 5 mils.

It is advantageous to apply the environmental barrier layer18to the surface20of the substrate14after the surface20has been treated and prepared to enhance wettability of the surface with an aqueous-slurry based fluid. Since surface20can have organic contaminants which can be hydrophobic, these contaminants can be problematic for a number of reasons, some related to the aqueous materials used in the method for application of the environmental barrier layer18. The organic contaminants can be removed from the surface20via techniques such as plasma treatment. In order to obtain good adhesion of the environmental barrier layer18to the surface20it can be advantageous to enhance the properties of the surface20.

In one aspect of the disclosure, the environmental barrier coating is applied in the form of an aqueous slurry. Application as a slurry allows relatively simple and uniform application of the EBC as desired, followed by curing into the final coating. However, good adherence to the surface of the substrate is important in providing a good bond of the coating to the substrate. Thus, treating the surface to enhance wettability can greatly improve overall adhesion of the resultant coating. Without such a treatment, the surface can be hydrophobic, resulting in poor dispersion or wetting of the aqueous slurry over the substrate to be coated, and poor quality coating.

FIG.3shows a schematic representation of a surface20of a substrate12which has organic contaminants32on surface20. As set forth above, such contaminants can typically be hydrophobic. Non-limiting examples of typical organic contaminants encountered during preparation of ceramic matrix component articles are absorbed hydrocarbons, oils from human handling (such as finger prints), lubricants from machining, residual hydrocarbon based contaminants from the process of making the substrate (for example through chemical vapor infiltration (CVI) or preceramic polymer pyrolosis). When such an article is to be coated with an environmental barrier coating10or layer18(FIGS.1,2), one application method is application as an aqueous slurry. Such processes produce good quality coatings when there is good adhesion of the slurry with the surface. Of course, hydrophobic contaminants on the surface can interfere with such adhesion. Thus, it is desirable to treat the surface to be coated to adjust the wettability of the surface to aqueous based materials.

In one aspect of such adjustment of wettability, the surface ofFIG.4can be treated, for example with a plasma treatment34, to remove the organic contaminants32(already removed inFIG.4) and produce a surface20which is more wettable to aqueous slurry-based materials.

FIG.4schematically illustrates a plasma treatment step34to remove organic contaminants32. This plasma treatment step can be carried out using any plasma bath, jet, spray or other treatment method which results in removal of organic contaminants to adjust wettability of the surface to be coated. In a typical plasma treatment process, the part or parts having a substrate to be coated can be placed in a plasma chamber, and a vacuum can then be drawn in the chamber. A gas can be backfilled into the chamber at a partial pressure. Composition of the gas can be changed as desired to change the effect on the surface. In addition, other methods can be used to introduce reactive gases into the plasma chamber including but not limited to the use of at least one of fluorine gas, Sulphur hexafluoride, water vapor, hydrogen, chlorine gas, hydrochloric acid, argon, iodine monobromide, iodine monochloride and the like. Plasma is then generated, for example using RF excitation, to expose the desired surface to plasma and thereby alter the surface by removing organic contaminants.

Chemical species of the gas can create a significant effect. For example, on a silicon carbide substrate, HF gas may remove oxides on the substrate and provide a fresh surface, oxidizing gas may remove contaminants and may create a very controlled oxidized layer that increases wetting. Power used to create the plasma can also be varied to control the degree of surface modification. Low-pressure (high vacuum) conditions enhance the plasma etch rates and the use of vacuum better than 10−2-10−6atmospheres is desirable. In one configuration, the plasma is achieved in the system by applying a strong radio frequency electromagnetic field to the chamber.

After treatment step34, wettability of surface20is now sufficiently adjusted that an aqueous slurry-based coating can be applied to surface20, resulting in an even and well wetted layer36of aqueous slurry-based coating materials on surface20such that subsequent curing and finishing steps result in an environmental barrier coating having high adhesion to the underlying surface as desired. The treatment step can be conducted to produce a hydrophilic surface, which of course enhances wetting with an aqueous-slurry based coating composition. It is well-suited to the overall process to conduct the treatment step to produce a surface which has a contact angle with water and/or with an aqueous-slurry based coating composition or fluid of less than 40 degrees.

Along with the disclosed process for removing organic contaminants using plasma and other treatment, it is also within the scope of this disclosure to alter the surface by changing the oxidative state of the surface, which can also alter the wettability of the surface. The disclosed method can be used to adjust the wettability of a surface to be coated with aqueous slurry-based materials such that the aqueous slurry-based materials fully wet the surface. This promotes strong adhesion of the thus-applied coating, and enables the capability of the environmental barrier coating system to endure higher thermal gradients, retain larger coating thickness, longer coating life, and higher resistance to spallation.

There has been provided a method of adjusting wettability of a surface for an environmental barrier coating. While the coating has been described in the context of specific embodiments thereof, other unforeseen alternatives, modifications, and variations may become apparent to those skilled in the art having read the foregoing description. Accordingly, it is intended to embrace those alternatives, modifications, and variations which fall within the broad scope of the appended claims.