Abstract:
An environmental barrier coating system, a method of application and an article formed thereby suitable for reducing creep by incorporation of doping materials in grain boundaries of a bond coat layer to inhibit creep displacement of the EBC system when subjected to shear loading at elevated temperatures. The EBC system includes the bond coat layer on a silicon-containing substrate and at least one ceramic layer on the bond coat layer. The bond coat layer includes silicon and at least one doping material that includes a creep-resistant element. The doping material is located at grain boundaries within the bond coat layer in sufficient size and quantity to improve the creep resistance of the bond coat layer.

Description:
BACKGROUND OF THE INVENTION  
       [0001]    The present invention generally 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 a bond coat layer on a silicon-containing region of a component such as a ceramic matrix composite (CMC) and to the incorporation of one or more doping materials in the bond coat layer to inhibit creep displacement of the environmental barrier coating (EBC) when subjected to shear loading at elevated temperatures. 
         [0002]    Higher operating temperatures for gas turbine engines are continuously sought in order to increase their efficiency. Though significant advances in high temperature capabilities have been achieved through formulation of iron, nickel and cobalt-base superalloys, alternative materials have been investigated. For example, composite materials are currently being considered for such high temperature applications as combustor liners, vanes, shrouds, airfoils, and other hot section components of gas turbine engines. Of particular interest are silicon-based composites, such as silicon carbide (SiC) as a matrix and/or reinforcing material. 
         [0003]    In many high temperature applications, a protective coating is beneficial or required for a Si-containing material. Such coatings should provide environmental protection by inhibiting the major mechanism for degradation of Si-containing materials in a corrosive water-containing environment, namely, the formation of volatile silicon monoxide (SiO) and silicon hydroxide (Si(OH) 4 ) products. A coating system having these functions will be referred to below as an environmental barrier coating (EBC) system. Important properties for the coating material include a coefficient of thermal expansion (CTE) compatible with the SiC-containing material, low permeability for oxidants, low thermal conductivity, stability and chemical compatibility with the Si-containing material and silica scale formed from oxidation. 
         [0004]    Various single-layer and multilayer EBC systems have been investigated for use on Si-containing substrates. Coatings of zirconia partially or fully stabilized with yttria (YSZ) as a thermal barrier layer exhibit excellent environmental resistance. However, YSZ does not adhere well to Si-containing materials (SiC or silicon) because of a CTE mismatch (about 10 ppm/° C. for YSZ as compared to about 4.9 ppm/° C. for SiC/SiC composites). Mullite (3Al 2 O 3 .2SiO 2 ), barium-strontium-aluminosilicate (BSAS; (Bal-xSrx)O—Al 2 O 3 —SiO 2 ) and other alkaline earth aluminosilicates have been proposed as protective coatings for Si-containing materials. For example, U.S. Pat. No. 5,496,644 to Lee et al. and U.S. Pat. No. 5,869,146 to McCluskey et al. disclose the use of mullite and U.S. Pat. Nos. 6,254,935, 6,365,288, 6,387,456 and 6,410,148 to Eaton et al. disclose the use of BSAS as outer protective barrier coatings for silicon-containing substrates. In the Eaton et al. patents, BSAS barrier coatings are described as being bonded to a silicon-containing substrate with an intermediate layer (bond coat layer) that may be, among other possible materials, mullite or a mixture of mullite and BSAS. 
         [0005]    Eaton et al. further teach that when the bond coat layer comprises silicon, the silicon preferentially reacts with oxygen to form a non-gaseous product to reduce the formation of voids that would otherwise deteriorate the bond between the silicon containing substrate and the EBC. Additionally, the resulting silicon oxide (SiO 2 ) exhibits a low oxygen permeability. Hence, the bond coat layer acts as a protective barrier that deters permeation of oxygen into the substrate layer by at least two mechanisms. The source of gas generation is eliminated and voids are prevented that would otherwise accumulate at the interface between the external coating and the silicon containing substrate. 
         [0006]    The desired amorphous SiO 2  oxide product formed on the bond coat layer in service has a relatively low viscosity and consequently a high creep rate under shear loading. Shear loading can be imposed by the g forces attendant on high-frequency rotation of moving parts such as blades (buckets) of gas turbine engines. Displacements of EBC oxide layers with respect to the bond coat layer can result in severe EBC damage and even direct loss of EBC protection of the underlying substrate. 
         [0007]    Prior attempts to improve the creep resistance of bond coat layers have focused on changing the composition of the bond coat layer mechanically or chemically inside the matrix. Although these methods have shown improved creep resistance in EBC systems, improved methods are still under development. 
         [0008]    In view of the above, it can be appreciated that there are certain problems, shortcomings or disadvantages associated with the prior art, and that it would be desirable if creep could be reduced in EBC systems to at least partly overcome or avoid these problems, shortcomings or disadvantages. 
       BRIEF DESCRIPTION OF THE INVENTION  
       [0009]    The present invention provides an EBC system, a method of application and an article formed thereby suitable for reducing creep in EBC systems by incorporation of one or more doping materials in the grain boundaries of the bond coat layer to inhibit creep displacement of the environmental barrier coating (EBC) when subjected to shear loading at elevated temperatures. 
         [0010]    According to a first aspect of the invention, an environmental barrier coating system for a silicon-containing substrate includes a bond coat layer on the silicon-containing substrate and at least one ceramic environmental barrier layer on the bond coat layer. The bond coat layer includes silicon and at least one doping material that includes a creep-resistant element. The doping material is located at grain boundaries within the bond coat layer in sufficient size and quantity to improve the creep resistance of the bond coat layer. 
         [0011]    According to a second aspect of the invention, an article includes a silicon-containing substrate comprising and an environmental barrier coating system on the silicon-containing substrate. The environmental barrier coating system includes a bond coat layer on the silicon-containing substrate and at least one ceramic layer on the bond coat layer. The bond coat layer includes silicon and at least one doping material that includes a creep-resistant element. The doping material is located at grain boundaries within the bond coat layer in sufficient size and quantity to improve the creep resistance of the bond coat layer. 
         [0012]    According to a third aspect of the invention, a method of applying an environmental barrier coating system on a silicon-containing substrate includes forming a bond coat layer that includes silicon on the silicon-containing substrate, doping the bond coat layer with a doping material that includes a creep-resistant element in a manner so that the doping material is located at grain boundaries within the bond coat layer in sufficient size and quantity to improve the creep resistance of the bond coat layer, and applying at least one ceramic layer on the bond coat layer. 
         [0013]    A technical effect of the invention is the ability to produce an EBC system with improved creep resistance which allows for higher operating temperatures and longer article operating lifetimes. 
         [0014]    Other aspects and advantages of this invention will be better appreciated from the following detailed description. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]      FIG. 1  is a schematic illustration of a cross-sectional view of a ceramic article having an environmental barrier coating system in accordance with an embodiment of this invention. 
           [0016]      FIG. 2  is a schematic illustration of a microstructure of a bond coat layer of the environmental barrier coating system of  FIG. 1  in accordance with an embodiment of this invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0017]    Embodiments described herein generally relate to environmental barrier coating (EBC) systems for high temperature ceramic components. 
         [0018]    The EBC systems described herein may be suitable for use in conjunction with CMCs or monolithic ceramics. As used herein, CMC refers to silicon-containing matrix and reinforcing materials. Some examples of CMC systems acceptable for use herein can include, but should not be limited to, materials having a matrix and reinforcing fibers comprising silicon carbide, silicon nitride, and mixtures thereof. As used herein, “monolithic ceramics refers to materials comprising silicon carbide, silicon nitride, and mixtures thereof. CMC systems and monolithic ceramics are collectively referred to herein as ceramics. 
         [0019]    The EBC systems herein may be suitable for application to ceramic components, or simply components, found in high temperature environments (e.g. operating temperatures of about 2500° F. (1370° C.)), such as those present in turbomachinery, including but not limited to turbine engines such as those used in the power generating industry. 
         [0020]    More specifically,  FIG. 1  represents an article  10  as comprising an EBC system  14  on a silicon-containing substrate  12 , which can include a bond coat layer  16  adjacent to the silicon-containing substrate  12 , an optional silica layer  18  adjacent to the bond coat layer  16 , at least one optional transition layer  20  adjacent to the bond coat layer  16  (or the silica layer  18  if present), and an outer layer  22  adjacent to the transition layer  20 . The outer layer  22  forms an outermost surface  24  of the component  10 . Although not all of the layers  18 ,  20  and  22  on the bond coat layer  16  may be necessary for individual applications, the EBC system  14  has at least one ceramic layer  18 ,  20 , and/or  22  on the bond coat layer  16 . 
         [0021]    If the article  10  is one of the aforementioned Si-based CMC materials, a preferred composition for the bond coat layer  16  comprises elemental silicon or a silicon-containing composition, such as SiC, Si3N4, etc. Suitable materials for the transition layer  20  include, but are not limited to, silicates, alkaline-earth metal aluminosilicates and/or rare-earth metal silicates, and particularly compounds of rare-earth oxides and silicates such as barium-strontium-aluminosilicates (BSAS) and other alkaline-earth aluminosilicates. Suitable materials for the outer layer  22  include, but are not limited to, YSZ alone or with additions of rare-earth oxides capable of promoting properties of the outer layer  22 . Outer layers formed of other ceramic materials are also foreseeable, for example, zirconate or perovskite materials. Examples of the above suitable materials are further described in U.S. Pat. No. 5,496,644 to Lee et al., U.S. Pat. No. 5,869,146 to McCluskey et al., and U.S. Pat. Nos. 6,254,935, 6,365,288, 6,387,456 and 6,410,148 to Eaton et al., the contents of which relating to the composition of the EBC system  14  are herein incorporated by reference. 
         [0022]    The bond coat layer  16  may be applied by plasma spray processes, chemical vapor deposition (CVD) processes, electron beam physical vapor deposition (EBPVD) processes, dipping in molten silicon, sputtering processes, and other conventional application processes known to those skilled in the art. The forming of the bond coat layer  16  may be followed by a conventional heat treatment process known to those skilled in the art. 
         [0023]    In accordance with an embodiment of the present invention, strategic proportional doping of the bond coat layer  16  is performed after the heat treatment. As used herein, the term strategic proportional doping refers to any doping technique that allows for one or more doping materials to be directly applied to grain boundaries within at least a surface region of the bond coat layer  16 . For example, suitable doping processes may include ion implantation, impregnation, or liquid infiltration. The bond coat layer  16  is doped with one or more doping materials consisting entirely of one or more creep-resistant elements, comprising one or more creep-resistant elements, or comprising a compound of one or more creep-resistant elements.  FIG. 2  schematically represents a microstructure of the bond coat layer  16  having doping materials  28  located on or near the grain boundaries  26  of the bond coat layer  16 . It will be appreciated that  FIG. 2  is for illustrative purposes only and is not to scale. 
         [0024]    The doping material  28  is deposited over a region or the entirety of the outermost surface  24  of the bond coat layer  16 . The doping material  28  will likely preferentially diffuse to the grain boundaries  26  within the bond coat layer  16  as the grain boundary energy is reduced by the doping material  28 . The doping material  28  accumulates at the grain boundaries  26  within the bond coat layer  16  in sufficient size and quantity to improve the creep resistance of the bond coat layer  16 . A sufficient quantity of the doping material  28  located at the grain boundaries  26  is dependent on the region covered and the sticking coefficient (that is, the ratio of the average number of gas particles sticking to the surface to the average number gas particles incident on the surface) of the doping material  28  used as well as other parameters such as the temperature of the bond coat layer  16 . Preferably, at least five percent by weight doping material  28  is located at the grain boundaries  26 , and more preferably between about five and about fifteen percent by weight. 
         [0025]    As used herein, creep-resistant elements of the doping material  28  are large elements, the inclusion of which has the effect of increasing the creep resistance of the bond coat layer  16 . Since creep is more severe in materials that are near their melting point (or range), a preferred aspect of the invention is that elements preferred for the doping material  28  improve creep resistance by increasing the melting temperature of the bond coat layer  16 . Furthermore, elements that decrease the grain boundary energy are preferable. These elements favor diffusing to the grain boundaries within the bond coat layer  16  and further improve creep resistance by pinning down dislocation clouds. Suitable elements include, but are not limited to, antimony (Sb), arsenic (As), titanium (Ti), hafnium (Hf), indium (In) and bismuth (Bi). The element should be present in the bond coat layer  16  in an amount of at least five weight percent to have the desired effect on the melting temperature of the bond coat layer  16 , but not exceed about fifteen weight percent in order to avoid any negative impact on the physical properties of the bond coat layer  16 . Preferably, the element is present in the bond coat layer  16  in an amount of about five to about ten weight percent, and more preferably in an amount of about seven to about ten weight percent. 
         [0026]    Preferably, the melting temperature of the bond coat layer  16  is increased by a minimum of 20° C., and more preferably increased by a minimum of 30° C. This effect is due to steric hindrance wherein the larger metal atoms occupy more space in the matrix than the silicon atoms. The atoms of the matrix are brought closer together resulting in an increased associated cost in energy due to overlapping electron clouds. The increased energy cost thereby increases the energy required to cause melting in the bond coat layer  16 . It is believed that the melting temperature of the bond coat layer  16  will generally be increased by about 30° C. for every ten weight percent of the element added to the bond coat layer  16 . 
         [0027]    Additionally, the doping material  28  preferably acts as a barrier to grain boundary sliding by pinning down dislocation clouds around the grain boundaries  26 . The doping material  28  acts as the obstacles to the grain boundary movements and reduces the motions of dislocations, thereby increasing the grain boundary sliding activation energy in the bond coat layer  16 . The system then acts similar to the classic Cottrell Atmosphere, where the doping material  28  gets caught by the dislocations and consequently strengthens the matrix of the bond coat layer  16 . This increase in grain boundary sliding activation energy substantially improves the overall creep resistance of the bond coat layer  16 . 
         [0028]    While the invention has been described in terms of preferred embodiments, it is apparent that other forms could be adopted by one skilled in the art. For example, the doping processes could differ from that described and materials other than those noted could be used. Therefore, the scope of the invention is to be limited only by the following claims.