Patent Application
20090274850
This invention relates to low cost, high temperature protective coatings in general and particularly to coatings on parts difficult to coat by line-of-sight techniques.
Components within the hot gas path of a gas turbine engine are necessarily protected by a thermal barrier coating (TBC) system. TBC systems on nickel based, cobalt based and iron based superalloy turbine components include a thermal insulating ceramic top coat, referred to as the TBC, typically bonded to the component with an environmentally protective bond coat. Bond coat materials widely used in TBC systems include overlay coatings such as MCrAlX (where M is nickel, cobalt and/or iron, and X is yttrium or another rare earth or reactive element such as hafnium, zirconium, etc.) and diffusion coatings such as diffusion aluminides, notable examples of which are NiAl and NiAl(Pt). Ceramic materials and particularly binary yttria stabilized zirconia are widely used as TBC materials because of their high temperature capability, low thermal conductivity, and relative ease of deposition by plasma spraying, flame spraying and physical vapor deposition (PVD) particularly electron beam physical vapor deposition (EBPVD) techniques.
Similar systems called environmental barrier coatings (EBCs) are also required on silicon based ceramic gas turbine components such as silicon nitride and silicon carbide in order to protect the base materials from water vapor. At high temperatures water vapor will cause the formation of volatile Si species such as Si(OH)x and SiO and result in material loss. EBC examples for ceramic turbine components include barium strontium alumino silicates (BSAS) and other alkaline earth alumino silicates as well as rare earth silicates and mullite. Bond coat materials widely used in EBC systems on silicon based substrates include layers of silicon, aluminosilicates, mullite, and rare earth silicates. Intermediate layers between the EBC and the bond coat are sometimes employed to moderate thermal expansion mismatch. EBC layers are typically deposited by air and vacuum plasma spraying (APS, VPS), high velocity oxyfuel (HVOF) spraying, chemical vapor deposition (CVD) and physical vapor deposition (PVD).
Many of the parts in gas turbine engine systems that require thermal and environmental protective coatings have complex shapes that are difficult to coat by conventional line-of-sight methods such as thermal spray and EBPVD. Examples of complex shaped parts in aircraft engines include vanes, rotors, blades, combustor liners, shrouds, transition ducts, airfoils, tubular gas turbine components, integral vane rings, and integrally bladed rotors. Coating complex shaped parts by line-of-sight processes is time consuming and expensive.
Non-line-of-sight coating processes are needed to increase throughput and decrease costs. Chemical vapor processes have been used but are expensive, slow and require a great deal of process development and operator skill, especially for coating large parts.
Consequently, there exists a need for a cost effective process for preparing and applying coatings that act as barriers to corrosive environments providing a thermal and environmental barrier function and extending the service life of complex shaped parts in all applicable industries. Additionally, the desire to decrease cost, improve versatility and throughput is also a need in the industries.
The present invention relates to processes suitable for the deposition of protective coatings on complex shaped substrates. The substrates may include, for example, nickel based, cobalt based or iron based superalloys and silicon based substrates which are used in articles and structures subjected to high temperature environments with water vapor present.
The processes of the present invention include lower cost non-line of sight processes and particularly dip coating, painting and spraying. Dip coating, painting and spraying can be used to deposit slurries containing a dispersion of particles of a protective coating or of precursors to a protective coating and other additives contributing to the properties of the final coating in a suitable liquid carrier. The dried, green deposited layers can be a bondcoat, a thermal barrier coat (TBC), an environmental barrier coat (EBC), a topcoat, and/or an intermediate layer coat.
In cases where the thermal treatment temperatures of the coating(s) would be high enough to be detrimental to metallic (e.g. superalloy) substrates, thermal treatment can be accomplished using alternate energy sources such as microwave energy or plasma arc energy.
The present invention relates to methods for applying a protective coating to complex shaped parts. The protective coating may serve as a thermal barrier coating on superalloy components exposed to high temperature combustion environments, or as an environmental barrier coating on silicon based components exposed to high temperature combustion environments.
Referring to
Generally, the substrate 10 may comprise a ceramic material, a metal based material, combinations comprising at least one of the foregoing and the like. For example, the substrate may comprise nickel based, cobalt based or iron based superalloys or silicon based ceramic turbine materials such as silicon nitride or silicon carbide. The substrates may also include austenitic and ferritic grade steels and ceramic matrix composites in need of environmental/thermal protection.
Referring again to
Referring to
The optional bondcoat layer 12 may be applied to the substrate 10 by any suitable manner known in the art such as, but not limited to, thermal spraying, slurry coating, vapor deposition (chemical and physical), combinations comprising at least one of the foregoing methods and the like. The optional intermediate layer 14 may also be applied to the substrate 10 or optional bondcoat layer 12 by the same methods and combinations as known in the art.
The final protective layer 16 is applied as shown at step 50 in
Following drying (step 60), the coating is thermally activated as shown in step 70 in
Slurry based coating processes are extremely versatile. Metal based and ceramic based bondcoats and intermediate coats as well as thermal barrier and environmental barrier coats can all be applied as slurries. The chemistry of the coatings can be controlled by the ingredients added to the slurry. The structures of the coatings can be controlled by the ingredients and the processing. For instance, density can be controlled through the addition of sacrificial pore formers or hollow ceramic spheres. By varying the drying conditions, coatings can be created with segmented and/or vertically cracked microstructures as needed. Examples of controlled microstructures and slurry based coatings will be shown below.
Slurry based thermal barrier coatings and environmental barrier coatings need to be processed to temperatures above at least about 2400° F. (1315° C.) to about 2600° F. (1426° C.) to generate microstructures with acceptable strength and erosion resistance. Since silicon based components can withstand these temperatures, the coatings on these components are routinely fired at these temperatures after application. With superalloy components, on the other hand, the firing temperatures are excessive. Heating superalloys above about 2100° F. (1148° C.) for extended periods of time results in loss of properties due to severe overaging of the alloys. Also, powder forms of most ceramics of interest for use as thermal barrier coatings and metals, when sintered at temperatures below about 2100° F. (1148° C.), do not possess acceptable densities to provide the function required of them. While the increased porosity in the coatings sintered at these temperatures is often desirable to reduce thermal conductivity, the highly porous structure has unacceptably poor erosion resistance. This invention proposes unique and non-traditional approaches for heating slurry-based, green thermal and environmental barrier coatings without significant heating of the substrate, therefore avoiding the associated risk. Layered approaches where porous inner layers for thermal insulation are capped using dense outer layers for erosion/corrosion protection are possible using the processes described herein.
Two approaches are proposed to ensure that the green, unfired coating applied as slurries can sinter and adhere to the substrate without causing any damage to the mechanical properties of the substrate. These approaches are microwave heating and plasma arc lamp heating. It should be emphasized that in addition to ceramic top coats, metallic bond coats may also be deposited using the slurry approaches described above and post processing approaches described herein.
Microwave processing is a relatively mature technology and its use in the heating and sintering of ceramics and powder metals has been recognized. While ceramic and metal powder processing using microwaves has been the subject of numerous publications (e.g., “Microwave Processing of Ceramic Composites and Metallic Materials”, D. Agrawal et al., in Microwave Solutions for Ceramic Engineers, Eds. D. D. E. Clark et al., Am. Ceram. Soc. Publ. pp. 205-228 (2005)), it has not yet been exploited for thermal or environmental barrier coating processing. The primary technical advantage of using microwave heating is that a dense (e.g. monolithic) metal substrate generally reflects microwave energy and therefore the metal part does not directly couple to the input energy. Metal and ceramic powders, on the other hand absorb microwave radiation and can be heated to temperatures in excess of about 2600° F. (1426° C.). It is therefore possible to heat the protective coating without detrimental overheating of the substrate.
Silicon nitride integral ceramic components with complex shapes are shown in
Microstructural control can be achieved in slurry based coatings.
Slurry based ceramic thermal barrier coatings have been shown to densify on superalloy substrates by microwave assisted heating without degrading the properties of the substrate.
In addition to microwave processing, high density infrared heating using a plasma arc lamp can also be used to heat slurry coated unfired surfaces. Plasma arc heating relies on electromagnetic radiation as a mechanism for rapidly and efficiently transferring radiant heat energy. Highly controllable heat fluxes can be delivered to a target surface and can be optimized to bring about selective surface heating to produce a desired temperature profile. Oak Ridge National Laboratories has a state-of-the-art facility and experience in plasma arc sintering and has demonstrated the technology for unidirectional heating with tremendous cost savings through reduced processing time, reduced operating cost, and environmental friendliness. T. R. Armstrong et al., “Economical Thermal Processing of Solid Oxide Membrane Materials”, Proceedings, 16th Annual Conference on Fossil Energy Materials, April 22-24, National Energy Technology Laboratory, Oak Ridge, Tenn. (2002). The primary drawback of the arc heating approach is the potential requirements of complex sample manipulation to access hidden (non-line-of-sight) areas. An example taken from the Armstrong et al. article illustrates the efficiency of plasma arc lamp thermal processing. The plasma arc lamp was used to sinter an yttria stabilized zirconia (YSZ) powder coating on a cast porous nickel oxide (NiO) powder substrate. The densified YSZ layer acts as a solid electrolyte and the NiO base is an electrode. The time temperature profile of the sintering process is shown in
In addition to the proposed non-conventional heating approaches, other processing variables can be adjusted. For example, slurry routes can be combined with chemical precursor methods such as sol-gel, to achieve improved coatings. Surface preparation, surface pretreatment, sintering atmosphere and other conditions can also be varied to further improve adhesion and densification characteristics. For example, the coatings may be processed in vacuum or an inert atmosphere with flowing N2 or Ar gases to reduce oxidation of the bond coat. If using air or inert gas it is also possible to induce chemical reactions between the layers to improve adhesion. The use of sol-gel precursors such as metal salts (nitrates, acetates, alkoxides etc.), metal organic compounds (e.g. metal alkoxides) in a slurry might also serve to improve the coating characteristics. Colloidal dispersions of metal oxides such as silica, alumina or zirconia may be used to improve reactivity of the slurry and densification characteristics of the resulting coatings.
Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.
