Patent Application
20150030826
This invention relates generally to the field of materials technology, and more particularly to a method for creating a textured surface in a bond coat of a thermal barrier coating system.
Ceramic thermal barrier coating systems are used on gas turbine engine hot gas path components to protect the underlying metal alloy substrate from combustion gas temperatures that exceed the safe operating temperature of the alloy. A typical thermal barrier coating system may include a bond coat, such as an MCrAlY material, deposited onto the substrate alloy and a ceramic topcoat, such as yttria stabilized zirconia, deposited onto the bond coat. It is known that strong adhesion between the layers of such systems is critical for proper functioning and long life of the coating system, and that a degree of surface roughness in the interface between the layers provides a beneficial mechanical interlock in that regard.
Bond coat material is often deposited by a spray process, such as High Velocity Oxy-Fuel (HVOF) or Air Plasma Spray (APS). It is known to control spray parameters when depositing a bond coat layer in order to achieve a degree of surface roughness in the deposited coating. However, the degree of roughness and the shape of the surface features in the deposited coating that are created by controlling the spray parameters are limited.
It is also known to texture the surface of a bond coat layer prior to the deposition of a ceramic insulating layer by using a material removal process, such as laser ablation, micromachining or photolithography, such as described in U.S. Pat. No. 5,723,078. As the firing temperatures of advance gas turbine engines continue to increase, further improvements in thermal barrier coating systems and methods of applying such coatings are desired.
The invention is explained in the following description in view of the drawings that show:
The present inventors have recognized that known bond coat texturing processes that rely upon material removal are inherently inefficient because unwanted material is first deposited and is then removed. The methods of the present invention cause material to be initially deposited in a way that creates the desired texture pattern. Furthermore, because the methods of the present invention create a textured surface by depositing a plurality of layers of material, a wide range of surface feature geometries, including undercuts, are made possible.
The steps of
The layer of powder 14 may be one to several millimeters in thickness in some embodiments rather than the fraction of a millimeter typical with known selective laser melting and sintering processes. Typical powdered prior art flux materials have particle sizes ranging from 0.5-2 mm, for example. However, the powdered alloy material 16 may have a particle size range (mesh size range) of from 0.02-0.04 mm or 0.02-0.08 mm or other sub-range therein. This difference in mesh size range may work well in the embodiment where the materials constitute separate layers; however, in the embodiment where the particles are mixed together before being applied to the surface 12, it may be advantageous for the powdered alloy material 38 and the powdered flux material 40 to have overlapping mesh size ranges, or to have the same mesh size range in order to facilitate mixing and feeding of the powders and to provide improved flux coverage during the melting process.
The flux material 18 and resultant layer of slag 22 provide a number of functions that are beneficial during the melting process. First, they function to shield both the region of molten material and the solidified (but still hot) alloy material 16′ from the atmosphere as the material cools. The slag floats to the surface to separate the molten or hot metal from the atmosphere, and the flux may be formulated to produce a shielding gas in some embodiments, thereby avoiding or minimizing the use of expensive inert gas. Second, the slag 22 acts as a blanket that allows the solidified alloy material 16′ to cool slowly and evenly, thereby reducing residual stresses that can contribute to cracking. Third, the slag 22 helps to shape the pool of molten alloy 16′. Fourth, the flux material 18 provides a cleansing effect for removing trace impurities such as sulfur and phosphorous that contribute to cracking. Such cleansing includes de-oxidation of the metal alloy powder 16. Because the flux powder 18 is in intimate contact with the alloy powder 16, it is especially effective in accomplishing this function. Furthermore, the flux material 18 may provide energy absorption and trapping functions to more effectively convert the beam energy 20 into heat energy, thus facilitating a precise control of heat input, such as within 1-2%, and a resultant tight control of material temperature during the process. Finally, the flux may be formulated to compensate for loss of volatized elements during processing or to actively contribute elements to the deposit that are not otherwise provided by the alloy powder itself.
The patterned energy beam 20 may be produced by a diode laser 30 having a generally rectangular cross-sectional shape, although other known types of energy beams may be used, such as electron beam, plasma beam, one or more circular laser beams, a scanned laser beam (scanned one, two or three dimensionally), an integrated laser beam, etc. The rectangular shape may be particularly advantageous for embodiments having a relatively large area to be textured. The broad area beam produced by a diode laser helps to reduce weld heat input, heat affected zone, dilution from the substrate and residual stresses, all of which reduce the tendency for the cracking effects normally associated with superalloy repair. The laser energy may be patterned by any known beam shaping optics, such as a cartridge filter 32 having pre-determine openings. The cartridge 32 used to deposit the first layer of material in
Flux materials which could be used include commercially available fluxes such as those sold under the names Lincolnweld P2007, Bohler Soudokay NiCrW-412, ESAB OK 10.16 or 10.90, Special Metals NT100, Oerlikon OP76, Sandvik 50SW or SAS1. The flux particles may be ground to a desired smaller mesh size range before use. Any available structural alloy, superalloy or bond coat material that is appropriate for thermal barrier coating systems may be used.
While various embodiments of the present invention have been shown and described herein, it will be obvious that such embodiments are provided by way of example only. Numerous variations, changes and substitutions may be made without departing from the invention herein. Accordingly, it is intended that the invention be limited only by the spirit and scope of the appended claims.
