Abstract:
A PVD process and apparatus ( 120 ) for depositing a coating ( 132 ) from multiple sources ( 110,111 ) of different materials. The process and apparatus ( 120 ) are particularly intended to deposit a beta-nickel aluminide coating ( 132 ) containing zirconium, hafnium, yttrium and/or cerium, whose vapor pressures are sufficiently lower than NiAl to require a different evaporation rate in order to achieve higher deposition rates and better control of the coating chemistry. The PVD process and apparatus ( 120 ) entail feeding at least two materials ( 110,111 ) into a coating chamber ( 122 ) and melting the materials ( 110,111 ) at different rates to form separate molten pools ( 114,115 ) thereof. Articles ( 130 ) to be coated are suspended within the coating chamber ( 122 ), and transported with a support apparatus ( 118 ) relative to the two molten pools ( 114,115 ) so as to deposit a coating ( 132 ) with a controlled composition that is a mixture of the first and second materials ( 110,111 ).

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application claims the benefit of U.S. Provisional Application No. 60/343,791, filed Oct. 19, 2001. 
     
    
     
       BACKGROUND OF INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention generally relates to coating apparatuses of the type used to deposit ceramic and metallic coatings. More particularly, this invention relates to a physical vapor deposition (PVD) apparatus and process for depositing a coating whose composition contains elements with relatively low and relatively high vapor pressures, such as a nickel aluminide alloyed with zirconium, hafnium, yttrium and/or cerium.  
           [0004]    2. Description of the Related Art  
           [0005]    Components within the turbine, combustor and augmentor sections of gas turbine engines are susceptible to oxidation and hot corrosion attack, in addition to high temperatures that can decrease their mechanical properties. Consequently, these components are often protected by an environmental coating alone or in combination with an outer thermal barrier coating (TBC), which in the latter case is termed a TBC system.  
           [0006]    Diffusion coatings, such as diffusion aluminides and particularly platinum aluminides (PtAl), and overlay coatings, particularly MCrAlX alloys (where M is iron, cobalt and/or nickel, and X is an active element such as yttrium or another rare earth or reactive element), are widely used as environmental coatings for gas turbine engine components. Ceramic materials such as zirconia (ZrO 2 ) partially or fully stabilized by yttria (Y 2 O 3 ), magnesia (MgO) or other oxides, are widely used as TBC materials. Used in combination with TBC, diffusion aluminide and MCrAlX overlay coatings serve as a bond coat to adhere the TBC to the underlying substrate. The aluminum content of these bond coat materials provides for the slow growth of a strong adherent continuous aluminum oxide layer (alumina scale) at elevated temperatures. This thermally grown oxide (TGO) protects the bond coat from oxidation and hot corrosion, and chemically bonds the TBC to the bond coat.  
           [0007]    Suitable processes for depositing MCrAlX coatings include thermal spraying such as plasma spraying and high velocity oxyfuel (HVOF) processes, and physical vapor deposition (PVD) processes such as electron beam physical vapor deposition (EBPVD), magnetron sputtering, cathodic arc, ion plasma, and pulsed laser deposition (PLD). PVD processes require the presence of a coating source material made essentially of the coating composition desired, and means for creating a vapor of the coating source material in the presence of a substrate that will accept the coating. FIG. 1 schematically represents a portion of an EBPVD coating apparatus  20 , including a coating chamber  22  in which a component  30  is suspended for coating. An overlay coating  32  is represented as being deposited on the component  30  by melting and vaporizing an ingot  10  of the desired coating material with an electron beam  26  produced by an electron beam gun  28 . The intensity of the beam  26  is sufficient to produce a stream  34  of vapor that condenses on the component  30  to form the overlay coating  32 . As shown, the vapor stream  34  evaporates from a pool  14  of molten coating material contained within a reservoir formed by a crucible  12  that surrounds the upper end of the ingot  10 . Water or another suitable cooling medium flows through cooling passages  16  defined within the crucible  12  to maintain the crucible  12  at an acceptable temperature. As it is gradually consumed by the deposition process, the ingot  10  is incrementally fed into the chamber  22  through an airlock  24 .  
           [0008]    More recently, overlay coatings (i.e., not a diffusion) of beta-phase nickel aluminide (β NiAl) intermetallic have been proposed as environmental and bond coat materials. The NiAl beta phase exists for nickel-aluminum compositions of about 30 to about 60 atomic percent aluminum, the balance of the nickel-aluminum composition being nickel. Notable examples of beta-phase NiAl coating materials include commonly-assigned U.S. Pat. No. 5,975,852 to Nagaraj et al., which discloses a NiAl overlay bond coat optionally containing one or more active elements, such as yttrium, cerium, zirconium or hafnium, and commonly-assigned U.S. Pat. No. 6,291,084 to Darolia et al., which discloses a NiAl overlay coating material containing chromium and zirconium. Commonly-assigned U.S. Pat. Nos. 6,153,313 and 6,255,001 to Rigney et al. and Darolia, respectively, also disclose beta-phase NiAl bond coat and environmental coating materials. The beta-phase NiAl alloy disclosed by Rigney et al. contains chromium, hafnium and/or titanium, and optionally tantalum, silicon, gallium, zirconium, calcium, iron and/or yttrium, while Darolia&#39;s beta-phase NiAl alloy contains zirconium. The beta-phase NiAl alloys of Nagaraj et al., Darolia et al., Rigney et al., and Darolia have been shown to improve the adhesion of a ceramic TBC layer, thereby increasing the service life of the TBC system.  
           [0009]    MCrAlY-type coatings have been deposited using PVD and thermal spray processes interchangeably to achieve acceptable levels of environmental protection. Although no advantages have been found for either deposition technique in terms of environmental performance, PVD processes have often been preferred for producing smooth as-coated surface finishes and coating thickness uniformity. In contrast, beta-NiAl coatings have been found to exhibit significantly improved spallation resistance and environmental properties if deposited by a PVD process. However, difficulties have been encountered when attempting to deposit beta-NiAl coatings alloyed with zirconium, hafnium, yttrium, cerium and/or another reactive element, as required by the NiAl compositions of Nagaraj et al., Darolia et al., Rigney et al., and Darolia. For example, when attempting to evaporate an ingot of NiAl+Zr or NiAl+Cr+Zr, deposition rates are low and the chemistry of the coating has been difficult to control.  
         SUMMARY OF INVENTION  
         [0010]    The present invention is a PVD process and apparatus for depositing a coating from multiple sources of different materials. The process and apparatus are particularly intended to deposit a nickel-base intermetallic coating, such as the gamma phase (γ[Ni(Co,Fe,Cr,Mo,W,Re)]) or beta phase (β NiAl), which is alloyed to contain zirconium, hafnium, yttrium, cerium and/or another reactive element whose vapor pressure is significantly lower than the constituents of the nickel-base intermetallic. According to the invention, a different evaporation rate for these reactive elements is necessary to achieve higher deposition rates and better control of the coating chemistry.  
           [0011]    The PVD apparatus and process of this invention entail providing at least two passages through which at least two materials are fed into a coating chamber, and means for melting each of the materials to form molten pools thereof. A first of the materials has a composition with a higher vapor pressure than a second of the materials, and the melting means is operable to melt the first and second materials at different rates. Finally, the apparatus includes means for suspending an article within the coating chamber. As a result of the presence of two separate molten pools, a nonhomogeneous vapor cloud is present within the chamber, necessitating that the article is transported relative to the two molten pools in order to deposit a coating with a controlled composition that is a mixture of the materials.  
           [0012]    The above coating process has been demonstrated by depositing beta-phase nickel aluminide coating materials containing zirconium by EBPVD and sputtering, though it is expected that the invention is also applicable to other PVD processes and to combinations of PVD and the use of gaseous sources of minor alloying elements. The demonstrated process produced coatings with controlled chemistries, and at deposition rates higher than possible if a single ingot containing NiAl and zirconium were evaporated.  
           [0013]    Other objects and advantages of this invention will be better appreciated from the following detailed description. 
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0014]    [0014]FIG. 1 shows a schematic representation of a portion of a prior art EBPVD apparatus used to deposit a coating of a nickel-base intermetallic material.  
         [0015]    [0015]FIGS. 2, 3 and  4  are schematic representations of portions of EBPVD apparatuses according to two embodiments of this invention.  
         [0016]    [0016]FIG. 5 is a graph comparing the performance potentials for coatings deposited by the EBPVD process of this invention with coatings deposited by a thermal spraying process. 
     
    
     DETAILED DESCRIPTION  
       [0017]    Three embodiments of a coating apparatus  120  are shown in FIGS. 2, 3 and  4  as being similar to the apparatus  20  of FIG. 1, and representative of a type suitable for depositing nickel-base intermetallic coatings. A particularly preferred coating material that can be deposited with the coating apparatus  120  is a beta-phase nickel aluminide, though it is also foreseeable that gamma-phase nickel intermetallics could be deposited.  
         [0018]    In FIG. 2, the apparatus  120  is shown as comprising a crucible  112  through which a pair of ingots  110  and  111  are fed into a coating chamber  122 , and an electron beam (EB) gun  128  generates an electron beam  126  that, in combination with a controlled beam jumping technique, melts the upper surfaces of both ingots  110  and  111  to produce separate molten pools  114  and  115  of the ingot materials within the crucible  112 . With such a technique, the beam  126  is briefly projected (in the millisecond range) on each ingot  110  and  111 , with the amount of time on each ingot  110  and  111  being adjusted so that the energy output achieves the energy balance required to obtain compositional control. As an alternative to the use of a single EB gun  128  shown in FIG. 2, two or more EB guns could be used to produce separate electron beams for maintaining both molten pools  114  and  115 .  
         [0019]    As with the apparatus  20  of FIG. 1, a component  130  is suspended within the coating chamber  122 , so that an overlay coating  132  is deposited on the component  130  as a result of the melting and vaporizing of the ingots  110  and  111 , which produces streams  134  and  135  of vapors that condense on the component  130 . The component  130  is supported on planetary tooling  118  that transports the component  130  over both pools  114  and  115 , so that the overlay coating  132  is a mixture of the ingot materials. As they are gradually consumed by the deposition process, the ingots  110  and  111  are incrementally fed into the chamber  122  through separate passages  124  and  125  in the crucible  112 . As known in the art, water or another suitable cooling medium preferably flows through cooling passages  116  defined within the crucible  112  to maintain the crucible  112  at an acceptable temperature.  
         [0020]    According to a preferred aspect of the invention, the compositions of the ingots  110  and  111  are chosen so that the coating  132  is a beta-phase NiAl intermetallic material. Notable examples of NiAl-based materials are those disclosed in Nagaraj et al., Rigney et al., Darolia, and Darolia et al., discussed previously, which are formulated as environmental coatings and bond coats for gas turbine engine applications. These NiAl-based coatings preferably contain about 20 to 32 weight percent (about 35-50 atomic percent) aluminum to achieve the beta phase, and may contain one or more of chromium, hafnium, titanium, tantalum, silicon, gallium, zirconium, calcium, iron, cerium and/or yttrium. These alloying ingredients have different vapor pressures than beta-NiAl, which this invention has determined leads to limited deposition rates and difficulties in achieving controlled chemistries for the coating  132 .  
         [0021]    With the present invention, those alloying ingredients with significantly lower vapor pressures, most notably zirconium, hafnium, yttrium and cerium (or another reactive element), are provided in one of the ingots (e.g.,  111 ), while those with higher vapor pressures, such as NiAl and chromium (if desired in the coating  132 ), are present in the other ingot (e.g.,  110 ). The electron beam  126  is then used to melt and vaporize the ingots  110  and  111  at controlled but different rates to produce a coating  132  with a predictable chemistry. In particular, the power of the beam  126  is jumped to an appropriate level to optimize the melting and vaporizing rates of the particular ingot material on which it is projected, so that the overall deposition rate can be increased and the uniformity of the coating chemistry improved. The power level inputs to the pools  114  and  115  are adjusted based in part on the coating chemistry, in addition to the relative sizes of the pools  114  and  115 . For ingots  110  and  111  of the same diameter, the required power level is lower for an ingot of zirconium and other reactive elements as compared to the NiAl ingot. Because of the different melting and vaporizing rates, the ingots  110  and  111  are fed into the chamber  122  at different rates as they are gradually consumed by the deposition process. As an example of the above, deposition of an NiCrAl overlay coating containing about 1 weight percent zirconium has been performed using separate ingots ( 110  and  111 ) of beta-phase NiAl+Cr and Zr, in which the molten pool size of the zirconium ingot is maintained to have a surface area of nearly fifty percent smaller than the pool size of the NiAl+Cr ingot. A lower power level input is able to maintain the molten zirconium pool because of its smaller size as well as the lower vapor pressure of zirconium compared to NiAl and chromium.  
         [0022]    While only two ingots  110  and  111  are shown in FIG. 2, it is foreseeable that multiple crucibles with multiple ingots of each type of material could be employed to increase the size of the coating zone within the chamber  122 . Also foreseeable is the use of ingots containing multiple constituents of the coating  132  (e.g., nickel, aluminum and chromium) that are present in separate regions of the ingot. Another alternative is to use a separate ingot for each of the elements desired for the coating  132 . For example, for a coating  132  of NiAl alloyed with chromium and zirconium, four separate nickel, aluminum, chromium and zirconium ingots could be used, each compositionally homogeneous and each individually melted to produce single-element pools that are co-evaporated. Again, the power levels of the electron beam used to evaporate the individual ingots would be controlled to optimize evaporation rates and promote the ability to obtain the desired chemistry for the coating  132 .  
         [0023]    In each of the different embodiments discussed above, the chemistry of the vapor cloud resulting from the vapor streams  134  and  135  will be quite different from one location to another within the cloud. Therefore, and particularly if multiple components or surfaces are to be coated at a single time, the location of the surfaces being coated within the vapor cloud will effect the coating chemistry. Therefore, the planetary tooling  118  shown in FIG. 2 is preferably employed to achieve more uniform chemistry control by transporting the component  130  above both pools  114  and  115 , so that the component  130  passes through those regions of the vapor cloud with different chemistries.  
         [0024]    The embodiment of FIG. 3 differs by modifying the crucible  112  to feed a wire  136  of the lower vapor pressure material into the chamber  122 , instead of the ingot  111 . The wire  136  is shown as being dispensed with a feed system  138  that includes a spool  140  from which the wire  136  is pulled by a wire feed device  142 , which feeds the wire  136  through a guide  144  to the molten pool  115 . Alternatively, the wire  136  could be fed by the wire feed device  142  through a passage  146  defined within the upper surface of the crucible  112 , as shown in FIG. 4. Other than appropriately modifying the control of the EB gun  128  to achieve a desired evaporation rate, the embodiments of FIGS. 3 and 4 are similar to that of FIG. 2. An advantage of feeding a smaller diameter wire is the capability of a more accurate measurement of the higher volume feed rate made possible with this invention, as compared to the accuracy possible with an ingot that is larger in diameter, and therefore requires lower and more accurate linear feed rates as compared to a wire to achieve the same volume feed rate.  
         [0025]    Various modifications could be made to the physical vapor deposition processes described above, including the use of one or more laser beams instead of the electron beams  126  to melt the ingots  110  and  111  or wire  136 . Other physical vapor deposition techniques could also be used, including magnetron sputtering, cathodic arc, ion plasma, pulsed laser deposition (PLD), and combinations thereof. Particular combinations of PVD techniques include EBPVD with cathodic arc, EBPVD with ion plasma, EBPVD with sputtering, EBPVD with PLD, sputtering with PLD, and cathodic arc with PLD. In addition, a PVD technique could be combined with the use of a gaseous source of a minor alloying element, such as the reactive element (zirconium, hafnium, yttrium and/or cerium).  
         [0026]    In an investigation leading to this invention, beta-phase NiAl-based coatings were deposited onto six sets of buttons made from the single-crystal superalloy known as Ren é NS. The targeted composition for the coatings was, in atomic percent, about 40 to 48% aluminum, about 5% chromium, and about 0.1 to 1.2% zirconium. The EBPVD process used to deposit the coatings used an ingot of NiAlCr that was continuously melted and evaporated by one electron beam, while an ingot of zirconium was continuously melted and evaporated by a second electron beam. In this manner, ingot chemistry was used to obtain the relative desired levels of nickel, aluminum and chromium, while the electron beam parameters were adjusted to vary the zirconium content from one button set to another. During the coating process, the buttons were transported through the vapor cloud in a manner similar to that represented in FIG. 2. The coatings were deposited to a thickness of about 2 mils (about 50 micrometers), after which the coated buttons were subjected to a vacuum diffusion heat treatment at about 1975° F. (about 1080° C.) for about two hours.  
         [0027]    On a second set of Ren é N 5  buttons, NiAlCr+Zr coatings with compositions of, in atomic percent, about 47.4% aluminum, about 5% chromium, and about 0.2-0.4% zirconium were deposited to thicknesses of about 2 mils (about 50 micrometers) using a high velocity oxyfuel (HVOF) thermal spray process. The buttons were diffusion heat treated at about 1975° F. (about 1080° C.) for about two hours in a vacuum, after which their NiAlCr+Zr coatings were subjected to surface grinding to improve their surface finishes to below 100 microinches (about 2.5 micrometers) Ra, such that the HVOF coatings had surface finishes and coating thickness uniformity comparable to the EBPVD coatings.  
         [0028]    All of the buttons were then coated by EBPVD with about 5 mils (about 125 micrometers) of a thermal barrier coating (TBC) of 7%YSZ (zirconia stabilized by about 7 weight percent yttria), such that the NiAlCr+Zr coatings served as bond coats for the TBC. The spallation life potentials of the TBC coatings were then evaluated by furnace cycle testing (FCT) at a maximum temperature of about 2125° F. (about 1160° C.) using one-hour cycles. The results of the experiments are plotted in FIG. 5. In addition to evidencing that FCT performance was strongly influenced by zirconium concentration, the plotted data show that the performance potentials for those coatings deposited by the PVD process of this invention were clearly superior those thermal sprayed coatings having the same bond coat chemistry.  
         [0029]    In addition to achieving greater FCT performance as compared to coatings deposited by HVOF, the coating process of this invention was found to proceed more rapidly than when a single ingot containing the same composition desired for the coatings was evaporated with a single electron beam, essentially as depicted in FIG. 1. Although the above results were obtained with NiAl-based coatings deposited by EBPVD, it is believed that similar results can also be achieved by depositing NiAl-based coatings by magnetron sputtering, as will as one or more of the other PVD processes noted above. Finally, it is believed that similar results would be obtained by intermittently (instead of continuously) melting the zirconium ingot with an electron beam, as discussed above.  
         [0030]    While the invention has been described in terms of a preferred embodiment, it is apparent that modifications could be adopted by one skilled in the art. Accordingly, the scope of the invention is to be limited only by the following claims.