Abstract:
A method of forming an article includes forming a silicon-containing layer on a silicon-containing region of a surface of a substrate of the article; forming a plurality of channels and ridges in the silicon-containing layer; and forming at least one outer layer overlying the surface of the silicon-containing region. The plurality of channels and ridges may be formed by adding silicon-containing material to the silicon-containing layer. The channels and ridges may be formed by subtracting material from the silicon-containing layer. The channels and ridges may be formed by forming channels or grooves in the silicon-containing region of the surface of the substrate prior to formation of the silicon-containing layer.

Description:
GOVERNMENT INTEREST 
       [0001]    The present technology was developed with Government support under Contract No. DE-FC26-05NT42643 awarded by the Department of Energy. The Government may have certain rights in the claimed inventions. 
     
    
     INCORPORATION BY REFERENCE 
       [0002]    The contents of commonly assigned U.S. application Ser. No. 13/711,250, filed Dec. 11, 2012 and titled ENVIRONMENTAL BARRIER COATINGS AND METHODS THEREFOR, and commonly assigned U.S. Application [Attorney Docket 262402-1], filed herewith and titled SILICA-FORMING ARTICLES HAVING ENGINEERED SURFACES TO ENHANCE RESISTANCE TO CREEP SLIDING UNDER HIGH-TEMPERATURE LOADING are incorporated herein by reference. 
       BACKGROUND OF THE TECHNOLOGY 
       [0003]    The present technology generally relates to coating systems and methods suitable for protecting components exposed to high-temperature environments, such as the hostile thermal environment of a turbine engine. More particularly, this technology is directed to an Environmental Barrier Coating (EBC) on a silicon-containing region of a component and to the incorporation of surface features in the silicon-containing region to inhibit creep displacement of the EBC when subjected to shear loading at elevated temperatures. 
         [0004]    Higher operating temperatures for 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. Ceramic composite materials are currently being considered for such high temperature applications as combustor liners, vanes, shrouds, blades, and other hot section components of turbine engines. Some examples of ceramic composite materials include silicon-based composites, for example, composite materials in which silicon, silicon carbide (SiC), silicon nitride (Si 3 N 4 ), and/or a silicide serves as a reinforcement phase and/or a matrix phase. 
         [0005]    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 water-containing environment, namely, the formation of volatile silicon hydroxide (for example, Si(OH) 4 ) products. A coating system having these functions will be referred to below as an environmental barrier coating (EBC) system. Desirable properties for the coating material include a coefficient of thermal expansion (CTE) compatible with the Si-containing substrate material, low permeability for oxidants, low thermal conductivity, stability and chemical compatibility with the Si-containing material. 
         [0006]    The silicon content of a silicon-containing bondcoat reacts with oxygen at high temperatures to form predominantly an amorphous silica (SiO 2 ) scale, though a fraction of the oxide product may be crystalline silica or oxides of other constituents of the bondcoat and/or EBC. The amorphous silica product exhibits low oxygen permeability. As a result, along with the silicon-containing bondcoat, the silica product that thermally grows on the bondcoat is able to form a protective barrier layer. 
         [0007]    The amorphous silica product that forms on a silicon-containing bondcoat in service has a relatively low viscosity and consequently a high creep rate under shear loading. High shear loads (e.g. from about 0.1 to 10 MPa) can be imposed by g forces (e.g. from about 10,000 to about 100,000 g&#39;s) resulting from high-frequency rotation of moving parts, such as blades (buckets) of turbine engines. Such shear loading may cause creep displacements of the EBC relative to the bondcoat and substrate which can result in severe EBC damage and loss of EBC protection of the underlying substrate. 
       BRIEF DESCRIPTION OF THE TECHNOLOGY 
       [0008]    The present technology provides an environmental barrier coating (EBC) system and a method of fabricating the EBC system on an article formed of a silicon-containing material, such as a ceramic matrix composite (CMC) in which a silicon-containing material serves as a reinforcement phase and/or a matrix phase. The EBC system and method are particularly well suited for protecting silicon-containing articles exposed to high temperatures, including the hostile thermal environment of a turbine engine. 
         [0009]    According to one example of the technology, a method of forming an article comprises forming a silicon-containing layer on a silicon-containing region of a surface of a substrate of the article; forming a plurality of channels and ridges in the silicon-containing layer; and forming at least one outer layer overlying the surface of the silicon-containing region, wherein the plurality of channels and ridges are formed by adding silicon-containing material to the silicon-containing layer. According to another example of the technology, the channels and ridges are formed by subtracting material from the silicon-containing layer. According to yet another example of the technology, the channels and ridges are formed by forming channels or grooves in the silicon-containing region of the surface of the substrate prior to formation of the silicon-containing layer. 
         [0010]    By interlocking the silicon-containing region with an initial layer of the environmental barrier coating system, displacement of the EBC attributable to creep of the constituent layer, for example, a silica layer that thermally grows on the silicon-containing layer or region, can be substantially inhibited, thereby promoting the structural integrity of the environmental barrier coating system and its ability to protect the article in high temperature applications. The technology is applicable to use with known environmental barrier coating materials and the interlocking features can be produced using various processes. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    These and other features, aspects, and advantages of the present technology will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein: 
           [0012]      FIG. 1  schematically depicts an article or component that may be coated with coatings of the present technology and according to methods of the present technology; 
           [0013]      FIG. 2  schematically depicts a section of the article or component of  FIG. 1  including a coating according to an example of the present technology; 
           [0014]      FIGS. 3-3B  schematically depict engineered surfaces of a bondcoat of the article or component according to examples of the present technology; 
           [0015]      FIG. 4  schematically depicts a process for forming engineered surfaces according to an example of the present technology; 
           [0016]      FIG. 5  schematically depicts a process for forming engineered surfaces according to an example of the present technology; 
           [0017]      FIG. 6  schematically depicts a process a forming engineered surfaces according to an example of the present technology; 
           [0018]      FIG. 7  schematically depicts an arrangement for forming engineered surfaces according to the present technology; 
           [0019]      FIG. 8  schematically depicts an arrangement for forming engineered surfaces according to the present technology; and 
           [0020]      FIGS. 9-20  are photographs of engineered surfaces formed according to the present technology. 
       
    
    
     DETAILED DESCRIPTION 
       [0021]    The present technology is generally applicable to components that operate within environments characterized by relatively high temperatures, stresses, and oxidation. Notable examples of such components include high and low pressure turbine vanes (nozzles) and blades (buckets), though the technology has application to other components. 
         [0022]    Referring to  FIGS. 1 and 2 , an article or component  2 , for example a turbine bucket or blade, may include an Environmental Barrier Coating (EBC) system  22  to protect the article or component when operated in a high-temperature, chemically reactive environment. The component  2  may include a substrate  4 , for example an airfoil section, extending from a platform  6 . The platform  6  may include a mounting and securing structure  8  configured to mount and secure the component to a rotating element, such as a rotor (not shown). The substrate  4  may include a silicon containing region. Examples of silicon-containing materials include those with a silicon carbide, silicon nitride, a silicide (for example, a refractory metal or transition metal silicide, including, but not limited to, for example Mo, Nb, or W silicides) and/or silicon as a matrix or second phase. Further examples include ceramic matrix composites (CMC) that contain silicon carbide as the reinforcement and/or matrix phase. 
         [0023]    The EBC system  22  of  FIG. 2  represents one of a variety of different EBC systems shown as being directly applied to a surface of the substrate  4 . A silicon-containing bondcoat is disclosed in, for example, U.S. Pat. No. 6,299,988. The bondcoat  10  is further represented as bonding a first, or initial, EBC layer  14  to the substrate  4 , and optionally at least one additional layer  16 ,  18 ,  20  of the EBC system  22 . The EBC system  22  provides environmental protection to the underlying substrate  4 . It may also reduce the operating temperature of the component  2 , thereby enabling the component  2  to operate at higher gas temperatures than otherwise possible. While  FIG. 2  represents the component  2  as including the silicon-containing bondcoat  10 , in which case the first EBC layer  14  is deposited directly on a silicon-containing surface region formed by the bondcoat  10 , the technology is also applicable to a component  2  that does not include a bondcoat  10  as described herein, in which case the first EBC layer  14  may be deposited directly on a silicon-containing surface region formed by the substrate  4 . It should be appreciated that a constituent layer  12 , or a portion of the constituent layer  12 , described in more detail below, may be present prior to application of the first EBC layer  14 . 
         [0024]    Degradation of a silicon-containing material in a combustion environment results in reaction with water vapor to form volatile silicon hydroxide (for example, Si(OH) 4 ) products. The EBC system  22  may serve to resist recession by chemical reaction of the bondcoat  10  and/or substrate  4  with water vapor, provide a temperature gradient to reduce the operating temperature of the component  2 , or both. Suitable EBC systems usable with the present technology include, but are not limited to, those disclosed in, for example, U.S. Pat. No. 6,296,941 and U.S. Pat. No. 6,410,148. The EBC system  22  may perform a multitude of sealing, reaction barrier, recession resistance, and/or thermal barrier functions. 
         [0025]    As noted above, each of the bondcoat  10  and substrate  4  may define a surface region of the component  2  that contains silicon. For example, the bondcoat  10  may comprise or consist essentially of elemental silicon. Alternatively, the bondcoat  10  may contain silicon carbide, silicon nitride, metal silicides, elemental silicon, silicon alloys, or mixtures thereof. Bondcoat  10  may further contain oxide phases, such as silica, rare earth silicates, rare earth aluminosilicates, and/or alkaline earth aluminosilicates. The use of silicon-containing compositions for the bondcoat  10  improves oxidation resistance of the substrate  4  and enhances bonding between the substrate  4  and first EBC layer  14 . The silicon of the bondcoat  10  reacts with oxygen at elevated temperatures to thermally grow the constituent layer  12  of predominantly amorphous silica (SiO 2 ) on its surface, as schematically represented in  FIG. 2 . The resulting thermally grown oxide of amorphous silica exhibits low oxygen permeability. As a result, along with the silicon-containing bondcoat  10 , the constituent layer  12  is able to deter permeation of oxygen into the bondcoat  10  and substrate  4 . During growth of the constituent layer  12 , some of the amorphous silica may crystallize into crystalline silica and additional impurity elements and second phases can be incorporated therein. 
         [0026]    In the absence of the silicon-containing bondcoat  10 , the first layer  14  of the EBC system  22  can be deposited directly on a silicon-containing surface region of the component  2  defined by the substrate  4 , in which case the substrate  4  is formed to have a composition whose silicon content is sufficient to react with oxygen at elevated temperatures and form a silica-rich constituent layer  12  described above. Furthermore, depending on the composition of the substrate  4 , this layer may be a predominantly amorphous silica product, a silica-rich glass, or a multi-phase mixture wherein at least one of the phases is silica-rich. As a matter of convenience, the remaining disclosure will make reference to embodiments that include the bondcoat  10  as represented in  FIG. 2 , though the disclosure should be understood to equally apply to a constituent layer  12  that forms on the surface of the substrate  4 . 
         [0027]    The constituent layer  12  that forms on the silicon-containing bondcoat  10  or another silicon-containing surface region, such as the substrate  4 , during high temperature service may grow to thicknesses of up to about 50 μm or more, depending on the application. The constituent layer  12  may have a relatively low viscosity and consequently a high creep rate under shear loading τ that can be imposed by g forces that occur during rotation of components, such as blades (buckets) of turbine engines. As a result of creep of the constituent layer  12 , displacements of the overlying EBC system  22  relative to the substrate  4  can exceed 100 mm over 25,000 hours service at about 1315° C. (about 2400° F.). Such large creep displacements can result in severe damage to the EBC system  22  and direct loss of environmental protection of the underlying substrate  4 . 
         [0028]    Referring to  FIG. 3 , creep of the constituent layer  12  that forms on the silicon-containing bondcoat  10  (or, in the absence of the bondcoat  10 , on the surface of the substrate  4 ) may be inhibited by providing the surface of the bondcoat  10  with engineered surfaces or features  24  configured to mitigate creep of the constituent layer  12 . As shown in  FIG. 3 , the surface features may take the form of ridges  24  as described in co-pending, commonly assigned U.S. application [Attorney Docket 262402-1], entitled “SILICA-FORMING ARTICLES HAVING ENGINEERED SURFACES TO ENHANCE RESISTANCE TO CREEP SLIDING UNDER HIGH-TEMPERATURE LOADING”. The ridges  24  may have a wavelength L and a span W that defines a ratio α (W/L that may be from about 0.1 to 0.9, for example about 0.2 to 0.8, for example about 0.4 to 0.6. Although the ridges  24  are shown as being generally square in cross section and extending substantially perpendicular to the shear loading direction (i.e. in a substantially chordwise direction), it should be appreciated that the engineered surfaces, i.e. ridges  24 , may have other cross sectional shapes, e.g. rectangular, trapezoidal, or any generally sinusoidal or wavy-shaped configuration. It should also be appreciated that although the examples show the surfaces  24  perpendicular to the shear stress, the surfaces  24  may be provided at an angle to the shear loading direction, e.g. up to about 45° to the shear loading direction. It should also be appreciated that although the engineered surfaces are shown as periodic and continuous, the surfaces may be non-periodic and/or non-continuous. It should further be appreciated that the engineered surfaces may be provided as sets of intersecting surfaces, e.g. diamond shapes formed, by example. Referring to  FIG. 3A , the engineered surfaces  24  may have a generally trapezoidal shape. Referring to  FIG. 3B , the engineered surfaces  24  may have a generally wavy or wave-like shape. 
         [0029]    Referring to  FIG. 4 , the engineered surfaces, e.g. ridges  24 , may be formed by an additive process to selectively add material to define the ridges  24  that are separated by groove valleys  25 . A thermal spray, e.g. an air plasma spray (APS) device  38 , is configured to spray material  43  (e.g. Si) for forming the bondcoat  10  through a patterned mask  36  having apertures or slots  44  that define the position of the ridges  24  on the substrate  4 . The APS device  38  is configured to move over the mask  36 , as shown by the arrows, to form the ridges on the bondcoat  10 . Alternatively, the ridges  24  may be formed by an additive process including spraying the material of the ridges  24  (e.g. Si) using a direct-write torch. It should be appreciated that any thermal spray process may be used, including for example, air plasma spray; plasma, including laser produced plasma, atmospheric or low pressure or vacuum plasma; HVOF; cold spray; combustion; or kinetic. 
         [0030]    Referring to  FIG. 5 , the engineered surfaces  24  may be formed by a subtractive process. A grit blasting device  40  may blast particles  46  through the apertures  44  of a patterned mask  36  to form groove valleys  25  thus forming the ridges  24 . The particles  46  may be, for example, SiC or alumina (Al 2 O 3 ) particles. The grit blast device  40  may move, for example as shown by the arrows, across the patterned mask  36  to form the ridges  24  on the bondcoat  10 . Alternatively, the groove valleys  25  may be formed by another subtractive process, for example laser machining or using a micro-waterjet to machine the grooves  25 . 
         [0031]    Referring to  FIG. 6 , the substrate  4  may be patterned to include engineered surfaces  27  so that upon application of the bondcoat  10 , the engineered surfaces  24  of the bondcoat  10  are formed corresponding to the engineered surfaces  27  of the substrate  4 . The engineered surfaces  27  of the substrate  4  may be provided by forming grooves  42  in the substrate. The grooves  42  may conform to the shape of the part and be continuous and substantially perpendicular to the shear loading direction, or at an angle up to about 45° to the shear loading direction. The engineered surfaces  24  of the bondcoat  10  may also be formed or partially formed by any of the processes described above with respect to  FIGS. 4 and 5 . The bondcoat  10  may be provided to the substrate by, for example, CVD, or any other suitable process. 
         [0032]    Referring to  FIGS. 7 and 8 , in the formation of the engineered surfaces by subtractive methods, e.g. grit blasting or micro-waterjet machining, or additive methods, e.g. APS, the mask  36  may be spaced a distance d from the substrate  4  and/or bondcoat  10  of about 5 mils (0.127 mm) or less and the mask  36  may have a thickness of between about 60 to 120 mils (1.5 to 3 mm). The slots  44  in the mask  36  may be tapered and have a nominal width of about 20 mils (0.5 mm). As shown in  FIG. 7  the mask  36  may be positioned so that the slots  44  converge toward the substrate  4  and bondcoat  10  for application of the engineered surfaces  24  through the additive process. Alternatively, as shown in  FIG. 8  the mask  36  may be positioned so that the slots  44  of the mask  36  diverge toward the substrate  4  and the bondcoat  10 . The openings of the slots  44  may be spaced a distance s1 from about 20 to 40 mils (0.5 to 1 mm) and the exits of the slots  44  may be spaced a distance s2 from about 20 to 40 mils. As disclosed above, the slots  44  provided in the mask  44  may be periodic and/or continuous, or may be non-periodic and/or non-continuous. As also discussed above, the slots  44  may intersect to provide the engineered surfaces as sets of intersecting surfaces. 
         [0033]    The masks were formed by scanning a micro waterjet across a mask substrate formed of, for example, metal (e.g. HASTALLOY®), having a thickness of about 60 mils (1.5 mm) or about 120 mils (about 3 mm), to form the slots  44 . The slots  44  formed by scanning the micro waterjet have a tapered profile, as shown for example in  FIGS. 7 and 8 . It should be appreciated, however, that slots  44  having generally straight (i.e. generally parallel) edges may be formed, by example by laser machining the mask substrate. The slots  44  may have a nominal width of about 20 mils (0.5 mm) at their narrowest portion. 
         [0034]    Referring to  FIGS. 9-18 , each sample shown was formed by applying a 4 to 5 mils (1 to 1.25 mm) bondcoat of SG-100 Si on a SiC—SiC ceramic matrix composite substrate. The mask was spaced about 5 mils (1.25 mm) from the sample and the engineered surfaces  24  were formed on the bondcoat by APS through the mask. The engineered surfaces applied were about 2 mils (0.05 mm). The mask was removed and additional layers  14 ,  16  of YbYDS and BSAS, respectively, of about 4 mils (0.1 mm) were applied. 
         [0035]    Referring to  FIG. 9 , the engineered surfaces  24  were formed in the bondcoat  10  by using a mask having converging slots, for example as shown in  FIG. 7 . The distance s2 between the slot exits was 20 mils and the mask had a thickness of 60 mils. Referring to  FIG. 10 , the engineered surfaces were formed in the bondcoat  10  using a mask having diverging slots, for example as shown in  FIG. 8 . The distance s2 between the slot exits was 20 mils and the mask thickness was 60 mils. As shown in  FIG. 9 , the engineered surfaces  24  formed through a mask having converging slots exhibit a generally rectangular profile whereas as shown in  FIG. 10  the engineered surfaces  24  formed through a mask having diverging slots exhibit a more rounded, wave-like profile. The generally rounded, wave-like engineered surfaces shown in  FIG. 10  exhibit lower stress concentrations than the generally rectangular engineered surfaces shown in  FIG. 9 , but may provide less creep resistance to the EBC system. 
         [0036]    Referring to  FIG. 11 , the engineered surfaces  24  were formed in the bondcoat  10  using a mask having converging slots, for example as shown in  FIG. 7 . The distance between the mask slot exits was about 40 mils (1 mm) and the mask had a thickness of about 60 mils (1.5 mm). As shown in  FIG. 11 , the engineered surfaces have a generally trapezoidal shape. The trapezoidal shape of  FIG. 11  is generally between the generally rectangular shape of  FIG. 9  and the generally rounded, wave-like shape of  FIG. 10 . The generally trapezoidal engineered surfaces thus exhibit lower stress concentration than the generally rectangular surfaces with improved creep resistance to the generally rounded, wave-like surfaces. 
         [0037]    Referring to  FIG. 12 , the engineered surfaces  24  were formed on the bondcoat  10  using a mask having diverging slots, for example as shown on  FIG. 8 , similar to the engineered surfaces shown in  FIG. 10 . The mask had a thickness of about 60 mils and the exits of the mask slots were spaced about 40 mils. The engineered surfaces of  FIG. 12  thus have a longer wavelength than the engineered surfaces of  FIG. 10 . 
         [0038]    Referring to  FIG. 13 , the engineered surfaces  24  were formed in the bondcoat  10  through a mask having converging slots, for example as shown in  FIG. 7 . The distance between the mask slot exits was 20 mils and the mask had a thickness of 120 mils (3 mm). The engineered surfaces  24  have a generally trapezoidal shape similar to the surfaces of  FIG. 11 . As the distance between the mask slot exits is the same as in  FIG. 9 , the engineered surfaces  24  in  FIG. 13  have a wavelength similar to the surfaces in  FIG. 9 . 
         [0039]    The engineered surfaces shown in  FIG. 14  were formed through a mask having diverging slots, for example as shown in  FIG. 8 . The mask slot exits were spaced 20 mils and the mask thickness was 120 mils. The surfaces  24  have a generally rounded, wave-like shape, similar to those shown in  FIGS. 10 and 12 , with a wavelength similar to those shown in  FIG. 10 . 
         [0040]    The engineered surfaces of  FIG. 15  were formed with a mask having converging slots, for example as shown in  FIG. 7 . The slot exits were spaced about 40 mils (1 mm) and the mask had a thickness of about 120 mils. The surfaces  24  have a generally trapezoidal shape with corners that are more rounded than, for example, the generally trapezoidal surfaces of  FIG. 11 . The engineered surfaces of  FIG. 16  were formed through a mask having diverging slots, for example as shown in  FIG. 8 . The slot spacing and mask thickness were the same as described with respect to  FIG. 15 . 
         [0041]    Referring to  FIG. 17 , the mask used to form the engineered surface had the same slot spacing and mask thickness as the mask used to form the engineered surfaces of  FIG. 15 . The micro-waterjet scan speed across the mask substrate used to produce mask for the engineered surfaces of  FIG. 17  was lower than the scan speed of the micro-waterjet used to form the mask used to form the engineered surfaces of  FIG. 15 . As shown in  FIG. 17 , the corners or edges of the generally trapezoidal surfaces  24  are less rounded using the lower scan speed of the micro-waterjet to form the mask slots. Similarly, the engineered surfaces of  FIG. 18  were formed through a mask of the same dimensions and diverging mask slots as that of  FIG. 16 , but with a lower scan speed of the micro-waterjet to form the mask slots. The surfaces of  FIG. 18  have a generally more rounded shape than the surfaces shown in  FIG. 16 . 
         [0042]    Referring to  FIG. 19 , the engineered surfaces  24  were formed by spraying silicon onto the bondcoat using a direct write torch. 
         [0043]    Referring to  FIG. 20 , the engineered surfaces  24  were formed by laser machining grooves into a ceramic composite matrix substrate. The bondcoat  10  and layers  14 ,  16  of an EBC system were provided over the substrate  4 . 
         [0044]    It is to be understood that not necessarily all such objects or advantages described above may be achieved in accordance with any particular example. Thus, for example, those skilled in the art will recognize that the systems and techniques described herein may be embodied or carried out in a manner that achieves or optimizes one advantage or group of advantages as taught herein without necessarily achieving other objects or advantages as may be taught or suggested herein. 
         [0045]    While only certain features of the present technology have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes.