This invention is directed to a method for modifying a brittle environmental or bond coating applied to turbine airfoils by a thermal spray process, and specifically to a stoichiometric NiAl coating having key quality characteristics required to protect the underlying turbine airfoil in a high temperature, oxidative and corrosive atmosphere while permitting application of long life thermal barrier topcoats.
Many systems and improvements to these systems have been set forth in the prior art for providing protection to turbine airfoils used in the hot section of a gas turbine from the combined effects of high temperatures, an oxidizing environment and hot corrosive gases. These improvements include new formulations for the materials used in the airfoils and include exotic and expensive nickel-based superalloys. Other solutions have included application of coating systems. These coating systems include environmental coating systems and thermal barrier coating systems. The environmental coating systems include nickel aluminides, platinum aluminides and combinations thereof. A multitude of improvements in these coatings and in methods of applying these coatings has been set forth that increase the life of the system, and developments in these improvements continue. In certain systems, thermal barrier coatings (TBC""s) in the form of a ceramic are applied over the environmental coatings. In other systems, a bond coat such as a MCrAlX where M is an element selected from Ni, Co, Fe or combinations of these elements are applied as an intermediary between the airfoil and the applied ceramic. The bond coat desirably also is employed to improve the environmental performance of the system. These aluminides and MCrAlX alloys are substantially non-brittle alloys, being comprised substantially of gamma or gamma+gamma prime phases, although small amounts of higher Al content beta-phases may be present, particularly in the aluminides.
Although many of the solutions presented by the use of the nickel aluminides do provide improvements to the performance of the applied environmental coatings, one of the problems is that NiAl is a substantially stoichiometric composition, even when additions of rare earth material are made on a substitutional basis. These substantially stoichiometric compositions have increased Al content and exhibit outstanding oxidation resistance and act as stable bond coats that improve the system""s resistance to spallation of applied thermal barrier topcoats. However, substantially stoichiometric NiAl is an extremely brittle material at ambient temperatures, with very low tensile ductility.
Although the prior art methods for application of the nickel aluminides did not always achieve a uniform stoichiometric coating of NiAl on a substrate because of concerns with brittleness, these methods can be used to achieve a substantially stoichiometric composition. These processes and methods include thermal spray techniques including but not limited to low pressure plasma spray (LPPS), high velocity oxy-fuel (HVOF) and detonation gun (D-gun), that thermally spray a powder of a predetermined composition.
Another frequently used method is to apply a coating by placing the substrate in an elevated temperature atmosphere that has a high concentration of a preselected element or elements in a gaseous phase. Typically, the preselected elements include at least aluminum. These methods include vapor phase aluminiding (VPA) and CVD methods. The aluminide coating is formed as the preselected element or elements are incorporated into the substrate and then diffuse into near surface regions, combining with elements already present in the substrate, such as nickel.
A third method of applying the coating that is frequently used includes electroplating. Here the substrate is placed in an electrolytic bath that includes metallic ions, typically Ni or Pt, but also Al. A thin coating of the ions is applied to the substrate by passage of an electrical current through the substrate. The aluminide is then formed by exposing the plated substrate to Al by one of the above methods.
The inherent problem with all of these methods is that when a substantially uniform stoichiometric composition of coating across the surface of the substrate is achieved, very little can be done to modify the surface of the coated substrate due to the brittle nature of the substantially stoichiometric NiAl intermetallic. Thus, certain key quality characteristics may not be readily achievable by these prior art methods. These include the correct degree of coating density and the proper surface roughness as the brittle nature of the intermetallic NiAl precludes mechanical working the coated substrate in the same manner as has been done with nonstoichiometric compositions of NiAl or PtAl.
What is needed are cost effective methods that can be employed to modify surface roughness and, if possible, density, of a substantially stoichiometric composition of NiAl over the surface of a substrate such as a turbine airfoil without adversely affecting the brittle substrate. The method used to modify the surface of the stoichiometric composition of the coated substrate should control the final surface roughness of the coated article, and preferably if possible, the density of the applied coating by desirably acting on the substrate at or close to ambient temperatures without causing the brittle NiAl coating to be damaged.
Improvements in manufacturing technology and materials are the keys to increased performance and reduced costs for many articles. As an example, continuing and often interrelated improvements in processes and materials have resulted in major increases in the performance of aircraft gas turbine engines. Blade technology including the composition and manner in which coatings are applied can improve blade life and performance. Currently, most as-manufactured NiAl coatings are neither sufficiently smooth nor sufficiently dense to achieve the full benefits of the NiAl coating. One method of applying a nickel aluminide coating is by a thermal spray process, such as the high velocity oxy-fuel (HVOF) process. Spray processes such as HVOF produce a surface roughness typically in the range of 100-240 micro-inches, with a common roughness of 180xc2x130 microinches. If the surface formed by the HVOF spray is not stoichiometric, for example, if it is rich in Ni, then stoichiometry can be achieved by exposing the surface to an atmosphere rich in Al followed by a suitable heat treatment. However, these subsequent heat treatments will not affect the surface finish formed by the HVOF process. A smoother surface is desired as it will allow for better adhesion of a ceramic TBC, while a denser coating will improve the corrosion and oxidation performance of the coating over the operational life cycle of the part. In order to achieve the required surface finish and a desired density, the coated article is worked by one or a combination of controlled mechanical techniques that include impinging the surface of the article with particles of preselected size for a preselected time and intensity to provide a smoother surface finish and hopefully improved density of the parts without adversely affecting the brittle coating material. While the mechanical techniques have been used for other applications, they have not been used to improve the surface finish of stoichiometric NiAl coatings applied to turbine airfoils. In order to apply these techniques to brittle stoichiometric NiAl, certain process controls are required to prevent damage to the coating. The present invention utilizes steel balls of preselected size to peen the surface of the airfoil to achieve the desired surface finish of at least 120 micro-inches. Desirably, the peening also densifies the coating.
And while the present invention was developed for use with stoichiometric NiAl which is brittle, it may be used advantageously with any other coating with an unacceptably rough surface finish due to application techniques and that is inherently brittle, but which requires a smooth surface finish for proper performance. Typically, these coatings have a higher Al content than other, more ductile coatings and are identified as beta phases, and the coatings contain a substantial amount of the beta phases or are primarily beta phases.
An advantage of the present invention is the ability to tailor the surface roughness of a brittle, substantially stoichiometric NiAl coating. In this way, the inherent advantages of a substantially stoichiometric NiAl composition can be utilized, while the brittle nature of the stoichiometric composition can be overcome so that the surface finish of the article can be modified to achieve the same results currently achievable with non-stoichiometric compositions that are either low in Ni or low in Al.
Another advantage of the present invention is the ability to increase the density of the brittle coating without damaging it. Thus, the present invention can modify the as-sprayed coating to achieve the required surface finish and desired density in order to take advantage of the improved corrosion and oxidation capabilities of the smoother, denser coating without damaging the brittle coating. The airfoils that have had their surface finish modified in accordance with the present invention have a more aerodynamic gas flow path that serves to improve efficiency. Additionally, the furnace cycle testing (FCT) performance improves as the surface finish is improved, which is an indication of improved thermal performance, or alternatively, resistance to spalling.
Still another advantage of the methods of the present invention is that they can be applied to both new airfoils and to airfoils that have undergone repair. These methods provide a simple, effective technique for achieving substantially stoichiometric NiAl coatings that is cost effective and that can provide an adequate substitute for coatings that have a PtAl component.