Abstract:
Systems for applying a thermal barrier coating to a superalloy substrate including at least one target for supplying a material for making the thermal barrier coating; at least one laser operably directed toward the target for liberating atomic particles from the target; and a plasma torch for generating a plasma for accelerating and depositing the atomic particles onto the superalloy substrate as the thermal barrier coating where the superalloy substrate is a nickel based superalloy or a cobalt based superalloy.

Description:
TECHNICAL FIELD 
       [0001]    Embodiments described herein generally relate to systems for applying a thermal barrier coating to a superalloy substrate. More particularly, embodiments herein generally relate to systems for carrying out laser assisted plasma coating at atmospheric pressure for applying a thermal barrier coating to a superalloy substrate. 
       BACKGROUND OF THE INVENTION 
       [0002]    Increasingly stringent demands are being imposed on the efficacy of gas turbine engines employed in the aerospace and power generation industries. This demand is driven by the requirement to reduce the consumption of fossil fuels, and in turn, operating costs. One way to improve turbine efficiency is to increase the operating temperature in the turbine section of the engine. However, with increased operating temperatures comes an increased demand on materials used in the turbine section. Not only must these materials be able to withstand the higher operating temperatures (from about 800° C. to about 1500° C.), but they must also endure increased mechanical stresses, corrosion, erosion, and other severe operating conditions, while continuing to fulfill lifetime requirements expected by the industry. This can be accomplished through the use of thermal barrier coatings (TBCs) applied to the high temperature component. 
         [0003]    Conventional practices often utilize plasma spray or electron beam physical vapor deposition (EBPVD) to apply the high temperature TBCs, both of which can be problematic. For example, plasma spray can produce highly porous coatings having lower erosion and impact resistance than EBPVD. Such plasma sprayed coatings can be susceptible to plugging up the cooling holes of turbine components to which they are applied. While EBPVD can produce more desirable coatings, it is an expensive process because it is carried out under a high vacuum and has higher equipment costs. 
         [0004]    Accordingly, there remains a need for systems that are capable of producing coatings that are structurally similar to those resulting from EBPVD, without the costly vacuum and equipment requirements set forth previously. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0005]    Embodiments herein generally relate to systems for applying a thermal barrier coating to a superalloy substrate comprising: at least one target for supplying a material for making the thermal barrier coating; at least one laser operably directed toward the target for liberating atomic particles from the target; and a plasma torch for generating a plasma for accelerating and depositing the atomic particles onto the superalloy substrate as the thermal barrier coating wherein the superalloy substrate is a nickel based superalloy or a cobalt based superalloy. 
         [0006]    Embodiments herein also generally relate to systems for applying a thermal barrier coating to a superalloy substrate comprising: two targets for supplying a material for making the thermal barrier coating; two lasers, one laser operably directed toward each of the targets for liberating atomic particles from the targets; and a plasma torch for generating a plasma for accelerating and depositing the atomic particles onto the superalloy substrate as the thermal barrier coating wherein the superalloy substrate is a nickel based superalloy or a cobalt based superalloy. 
         [0007]    Embodiments herein also generally relate to systems for applying a thermal barrier coating to a superalloy substrate comprising: two targets for supplying a material for making the thermal barrier coating, a first target comprising zirconium oxide and a second target comprising yttrium oxide; two Nd:YAG lasers, one laser operably directed toward each of the targets for liberating atomic particles from the targets; and a plasma torch for generating a plasma for accelerating and depositing the atomic particles onto the superalloy substrate as the thermal barrier coating comprising about 92% by weight zirconium oxide and about 8% by weight yttrium oxide. 
         [0008]    These and other features, aspects and advantages will become evident to those skilled in the art from the following disclosure. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    While the specification concludes with claims particularly pointing out and distinctly claiming the invention, it is believed that the embodiments set forth herein will be better understood from the following description in conjunction with the accompanying figures, in which like reference numerals identify like elements. 
           [0010]      FIG. 1  is a schematic cross-sectional representation of one embodiment of a laser assisted plasma coating at atmospheric pressure (LAPCAP) system in accordance with the description herein; and 
           [0011]      FIG. 2  is a schematic cross-sectional representation of an alternate embodiment of a LAPCAP system in accordance with the description herein. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0012]    Embodiments described herein generally relate to systems for carrying out laser assisted plasma coating at atmospheric pressure (LAPCAP) for applying a thermal barrier coating to a superalloy substrate. While the systems herein are designated “at atmospheric pressure,” they should not be limited to such. More specifically, the LAPCAP system may be utilized at near atmospheric pressure (e.g. about 0.5 Atm to about 3 Atm). 
         [0013]    In general, the LAPCAP system involves using at least one pulsed laser to liberate atomic particles from at least one target, and then feeding those atomic particles into a plasma for deposition onto a substrate to form a thermal barrier coating. As used herein, “liberate” can refer to any of ablating, vaporizing, melting, or some combination thereof. While the coatings described herein may be used on any substrate exposed to high temperature environments (from about 800° C. to about 1500° C.), such coatings are particularly suited for use on components in the turbine section of a gas turbine engine. 
         [0014]    In one embodiment, and as shown in  FIG. 1 , LAPCAP system  10  can generally comprise a plasma torch  16 , at least one target  12 , and at least one laser  14 . Plasma torch  16  can include a gas stream  18  that feeds into a discharge tube  20  having a plurality of inductively coupled plasma (ICP) coils  22  that can serve as a radio frequency generator, as set forth below. 
         [0015]    More particularly, gas stream  18  can feed into discharge tube  20  to help generate plasma  24  and can comprise any gas suitable for carrying out conventional plasma spray processes, which in one embodiment, can be selected from argon, nitrogen, hydrogen, helium, oxygen, and combinations thereof. In particular, as gas stream  18  feeds into discharge tube  20 , the radio frequency field generated by ICP coils  22  can be activated. As gas stream  18  passes through discharge tube  20 , adjacent to ICP coils, gas stream  18  can become electrically conductive, and form plasma  24 . At low flow rates (e.g. about 0.5 L/minute, for example) the plasma can be more stationary, whereas at higher gas flow rates (e.g. about 30 L/minute, for example) the plasma can take the form of a jet. It will be understood that a variety of flow rates, both above and below those provided herein, can also be utilized to alter the surface morphology and of the thermal barrier coatings. In an alternate embodiment, plasma  24  can be created by a microwave discharge (not shown) instead of, or in conjunction with, ICP coils  22 . 
         [0016]    Target  12  may comprise any material capable of being atomized by laser  14  and suitable for use as a thermal barrier coating, such as for example, ceramic materials and metallic materials. As used herein, “ceramic materials” can include zirconium oxide, yttrium oxide, alumina and pre-alloyed combinations thereof, while “metallic materials” may include zirconium, yttrium, aluminum, and combinations thereof. 
         [0017]    In the embodiment shown in  FIG. 1 , target  12  can be positioned below discharge tube  20  of plasma torch  16 , adjacent to plasma  24 , and secured in place using any suitable means. In an alternate embodiment, target  12  can be placed inside of plasma torch  16 , or be positioned to permit target  12  to replace and function as discharge tube  20 . Laser  14  can be operably directed toward target  12  such that during operation, laser  14  can strike target  12  to liberate atomic particles  26 , which can combine in the proper proportion in plasma  24  needed to make the desired TBC. Plasma  24  can then be used to accelerate and deposit atomic particles  26  onto substrate  28  as set forth below. When metallic materials are used as target  12 , reactive gases, such as oxygen and nitrogen, can be used to oxidize or nitrodize the atomic particles to obtain the desired coating composition and properties. Such gases can be added to the LAPCAP system or come from the atmosphere. 
         [0018]    Several varieties of solid state pulsed lasers having sufficient energy to liberate the atomic particles from the target can be utilized, including, but not limited to, neodymium-doped yttrium aluminum garnet (Nd:YAG) lasers. Because of the adjacency of target  12  to plasma  24 , atomic particles  26  are fed into plasma  24  as they are liberated. Plasma  24  can then accelerate the atomic particles, forcing them onto substrate  28 , where they can deposit as TBC  30 . Variation in the combination of laser operating parameters, including laser pulse length, laser pulse energy, laser intensity, and laser spot size can allow the atomic particle flux and distribution to be tailored to achieve the desired coating composition and properties. Generally, pulsed laser  14  can have a pulse length of from about 5 femtoseconds to about 100 microseconds, a pulse energy of from about 0.001 mJ to about 10 J, an intensity of from about 10 4  W/cm 2  to about 10 15  W/cm 2 , and a laser spot size ranging from about 1 micrometer to about 5 millimeters. 
         [0019]    While a variety of substrates  28  can be used in conjunction with the embodiments herein, in one embodiment, substrate  28  may be selected from superalloys suitable for use in high temperature (from about 800° C. to about 1500° C.) environments, such as those present in the turbine section of a gas turbine engine. Some examples of such superalloys can include, but should not be limited to, nickel based superalloys, and cobalt based superalloys. In order to achieve the desired TBC  30  thickness, which can range from about 50 microns to about 750 microns, substrate  28  can be moved beneath a stationary LAPCAP system  10  to build up layers of TBC  30 . In an alternate embodiment, substrate  28  can be stationary while the system  10  moves as needed using a pre-programmed robotic armature (not shown). The embodiments herein can result in the deposition of a TBC that has a columnar microstructure similar to that of coatings obtained using EBPVD. More specifically, the TBCs herein can have a column width of from about 0.5 microns to about 60 microns, and an intra column porosity of from about 0% to about 9%. In one embodiment, the TBC can comprise smaller diameter columns and about 0% porosity. 
         [0020]    In an alternate embodiment, more than one target and more than one laser can be used. As used here, “lasers” can refer to either multiple independent lasers, or alternately, one laser split into multiple beams. In such instances, each target may comprise the same or different materials (such as in the exemplary embodiment below). It will be understood that one laser, i.e. either an independent laser, or a split laser beam, can be operably directed toward each target to liberate atomic particles therefrom. 
         [0021]    By way of example and not limitation, and as shown in  FIG. 2 , LAPCAP system  110  can comprise a gas stream  18 , which in one embodiment can be argon, two pulsed Nd:YAG lasers  14 , two targets comprising ceramic materials, a first target  112  comprising ZrO 2 , and a second target  212  comprising Y 2 O 3 . Gas stream  18  can comprise a gas flow of from about 0.05 L/minute to about 0.6 L/minute. In this embodiment, pulsed lasers  14  can have a pulse length of from about 5 ns to about 10 ns, a pulse energy of about 10 mJ, an intensity of from about 10 7  to about 10 8  W/cm 2 , and a laser spot size ranging from about 1 micrometer to about 2 millimeters. This particular combination of laser operating parameters can liberate atomic particles  126  of zirconium, oxygen, and yttria, which can be deposited onto a combination nickel based, and cobalt based, superalloy substrate  28  as a thermal barrier coating  130  comprising about 92% by weight ZrO 2  and about 8% by weight Y 2 O 3  and having a thickness of from about 50 microns to about 750 microns. Those skilled in the art will understand that this is an example of one possible system and that other systems of varying parameters are within the scope of the present embodiments. 
         [0022]    The embodiments described herein differ from conventional processes. Particularly, unlike EBPVD, LAPCAP does not require the use of costly vacuum pumps, and particle generation, acceleration, and deposition can be accomplished using a single apparatus. However, in spite of these differences, LAPCAP can produce coatings having a columnar microstructure that is similar to coatings made using EBPVD. This is possible since LAPCAP deposition occurs on an atomic level. The result is a TBC that can be less susceptible to impact and erosion damage than coatings produced using conventional plasma spray processes. Additionally, the clogging of cooling holes that can occur with plasma spray can be greatly reduced or eliminated using LAPCAP. 
         [0023]    This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to make and use the invention. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal language of the claims.