Metallic coating for non-line of sight areas   

This disclosure generally relates to a process of applying a thermal and oxidative resistant coating. More particularly, this disclosure relates to a process of applying a thermal and oxidative resistant coating to non-line of sight regions of a component.

Components that operate in high temperature environments such as turbine vanes are coated to provide thermal and oxidative protection. Such coatings are often applied using a thermal spraying technique. Thermal spraying techniques use melted materials that are sprayed onto a desired surface. The coating material is heated by a plasma or arc torch and propelled onto the surfaces of the component part.

SUMMARY

A disclosed process for coating an aircraft component includes the steps of applying a coating to line of sight surfaces with a thermal spraying process and applying a slurry coating to non-line of sight surfaces to provide oxidative protection and a surface to which a ceramic material is bonded.

An example disclosed component is a turbine vane configured as a doublet. Turbine vanes operate in a high temperature environment and therefore are coated to provide oxidative resistance and to create a thermal barrier. The example process utilizes a thermal spraying process to apply a coating on line of sight surfaces and a slurry coating for applying a coating to surfaces not reachable with the thermal spraying process referred to as non-line of sight surfaces. The applied coating on both the line of sight and non-line of sight surfaces provides a bond layer for the application of a ceramic coating.

These and other features disclosed herein can be best understood from the following specification and drawings, the following of which is a brief description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of example process for coating a component.

FIG. 2 is a schematic representation of example layers of coating applied to the component.

DETAILED DESCRIPTION

Referring to FIG. 1, an example turbine vane 10 is configured as a doublet that includes two airfoils attached to common upper and lower platforms. Turbine vanes operate in a high temperature environment and therefore are coated to provide oxidative resistance and to create a thermal barrier. Moreover, turbine vanes The example process utilizes one process for applying a coating to surfaces reachable utilizing a thermal spraying process, referred to in this disclosure as line of sight surfaces, and a second process for applying a coating to surfaces not reachable with the thermal spraying process, referred to as non-line of sight surfaces in this disclosure. The coatings applied by thermal spraying and slurry application provide a bond layer for a ceramic layer. Although a turbine vane doublet 10 is disclosed by example, the disclosed process will benefit other components that require coatings to provide oxidation protection for the component improving the thermal durability.

The example coating process utilizes a line of sight thermal spraying process as indicated at 12. The example thermal spraying process utilizes a plasma torch 14 that directs heated materials 16 onto a surface of the turbine vane 10. The example thermal spraying process is a low-pressure plasma spraying process that utilizes heat generated by the plasma torch 14 to melt and propel materials on to line of sight surfaces 18. The example line of sight surfaces 18 are those surfaces that are reachable with the thermal spraying process such that desired coating coverages and thicknesses can be accomplished. As appreciated, there are some surfaces that are not feasibly reachable with the thermal spraying process where desired coating coverages and thicknesses cannot be reliable created. Although some material may be deposited in such non-line of sight surfaces schematically indicated at 20, the microstructure, coating thickness and physical properties achieved by thermal spraying may not be as desired.

The example process begins with the application of a coating 15 by the thermal spraying process 12. The disclosed example spraying process utilizes a plasma torch 14 to both melt and propel the coating material on to the line of sight surfaces 18. As appreciated, other thermal spaying processes may be utilized as desired to meet the final process parameters.

The example material that is coated onto the turbine vane 10 can include compounds of Nickel, Cobalt, Aluminum, Iron and any mixture of such materials that provide the desired thermal capability and oxidative protection. Moreover, the example material can also include compounds known in the art as MCrAlYs where the M denotes one of Nickel (Ni), Cobalt (Co) or NiCo materials and Cr, Al, and Y represent the elemental designations for Chromium, Aluminum, and Yttrium, respectively. The example material is also selected such that it serves as a bond coat for ceramic material applied subsequent to the metal coating.

Once the line of sight surfaces 18 have been coated using the thermal spraying process 12, the non-line of sight surfaces 20 may still require a coating. The non-line of sight surfaces include those surfaces of the turbine vane 10 that the thermal spraying process 12 cannot reliably deposit material required to form the bond coat for the ceramic material. Prior to any additional processing actions, the surfaces 18 and 20 are treated to remove any loose materials and to remove any oxidation that may have developed. The treatment of these surfaces is accomplished using a grit blast process, vapor blasting process, application of emery paper, or any other process known for removing an undesired oxidative coating. Moreover, the treatment of these surfaces may be bypassed depending on the amount, or lack of oxidation build up on the metallic coatings applied during the initial thermal spraying process.

The disclosed process includes the step of applying a slurry coating as is schematically indicated at 22. The example slurry coating indicated schematically at 24 is a suspension of Nickel or MCrAlY based alloy particles in an organic binder. The example particle sizes are less than about 25 microns. The prepared slurry is applied to the non-line of sight areas 20 through a process capable of applying the slurry 24 on to difficult to reach non-line of sight locations on the turbine vane 10.

In this example, the slurry coating 24 is brushed on. Different processes and methods can be utilized within the contemplation of the disclosed method for, filling cavities or spreading the slurry, such as other manual application processes utilizing various spreading and application tools including for example brushing, dipping and spraying processes. The viscosity of the slurry within the organic binder maintains the slurry coating on the non-line of sight surfaces 20. In other words, the slurry coating compositions provides a desired viscosity that temporarily adheres the slurry to the surface 20 of the turbine vane 10 until heat a heat treat diffusion process.

Once the slurry coating 24 is applied to the non-line of sight surfaces 20 of the turbine vane 10, a heat treat step indicated at 26 is performed. The heat treat step 26 provides for the elimination of the organic binder portion of the slurry coating 24. The heat treat step 26 also provides for the diffusion bonding of both the slurry coating 24 and the thermally sprayed coating 15. Diffusion bonding of the coatings 15 and 24 create the oxidative resistant layer as well as provides a layer onto which a thermal barrier, such as a ceramic coating, can adhere. This enables the desired operation in the high temperature environments in which the turbine vane 10 operates.

The example heat treat process 26 is accomplished by applying heat 28 to the turbine vane 10 to attain a temperature sufficient to burn out the organic binder of the slurry and to diffuse both the thermally sprayed coating 15 and the slurry coating 24 into the substrate of the turbine vane 10. The temperature and duration that the turbine vane 10 is maintained is dependent on the substrate, and the specific material composition of the coatings 15, 24. In the disclosed example, the coated turbine vane 10 is heat treated for 1 to 4 hours at a temperature range between 1600° F. and 2000° F. (871° C. and 1093° C.). Other durations and temperatures would be utilized depending on the material composition of the coatings. Diffusion bonding of the coatings 15 and 24 create an oxidative resistant layer that enables desired operation in the high temperature environment in which the example turbine vane 10 operates.

Once the turbine vane 10 is heat treated, the surface is mechanically treated and or prepared. The surface treatment as is schematically indicated at 30 comprises a cold working process such as peening or other surface blast processes that improve mechanical properties of the coating on the turbine vane 10. The example surface treatment process includes propelling shot 32 at the surface of the turbine vane 10. The mechanical working of the surface induces compressive stresses while reducing tensile stresses to provide an increased resistance to fatigue and corrosion. The surface treatment process also prepares the surface of the turbine vane 10 for the application of a ceramic coating 36 as is schematically indicated at 34. The preparation process smoothes the surface for ceramic coating and closes up pores in the coating on the turbine vane 10.

Referring to FIG. 2 with continued reference to FIG. 1, the turbine vane 10 includes the substrate 38 onto which the metallic coating layer 40 is applied. The layer 40 comprises the thermally sprayed coating 15 and the slurry coating 24. The layer 40 is heat treated such that it diffuses into the substrate 38. Surface treatment of the layer 40 improves the mechanical properties of the layer 40. Following surface heat treatment, turbine vane 10, which includes layer 40, is typically heat treated prior to application of the ceramic coating. This heat treatment forms a thermally grown oxide on the surface of the layer 40. In this example, the thermally grown oxide includes primarily aluminum oxide. Other thermally grown oxide compositions can form depending on the chemical composition of layer 40.

The ceramic layer 42 is then applied and bonds to the thermally grown oxide on layer 40. The ceramic layer 42 is applied using ceramic application techniques such as thermal spraying and vapor depositions processes. The layer 40 is present on both the line of sight 18 and non-line of sight 20 surfaces of the turbine vane 10 that form the gas path surfaces of the turbine vane 10. Therefore, the ceramic layer 42 can be applied and bonded to the gas path surfaces of the turbine vane 10. The ceramic layer 40 provides improved durability and thermal protection characteristics when applied not only to the line of sight surfaces 18, but also to the non-line of sight surfaces 20.

The example process enables ceramic coating of all gas path surfaces for components with difficult to coat geometries and configurations such as the example doublet turbine vane 10 because metal bond layer coatings can be applied, and controlled as desired on all gas path surfaces. Moreover, the example process provides a bond coat to which ceramic coatings can be applied on both line of sight and non-line of sight surfaces.

Although an example embodiment has been disclosed, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of this disclosure. For that reason, the following claims should be studied to determine the scope and content of this invention.