The invention relates to forming a thermal barrier coating having a porosity architecture. Specifically, the invention relates to an additive manufacturing processes that laser heats a fugitive material disposed in a ceramic material to build up a thermal barrier coating having the porosity architecture.
Additive manufacturing processes are widely used to produce three-dimensional parts from metal powders, polymer powders, and ceramic powders by fusing the powder to form a layer, and repeating the process to form additional layers until the part is completed. A powder bed is used to hold the component during processing and to supply powder for the additional layers. While this approach enables a layer-by-layer buildup of parts, the process is very slow and material characteristics cannot be tailored in a way possible with other processes such as when a melt pool is used. This is particularly so for ceramics such as those used in thermal barrier coatings (TBC).
Thermal barrier coatings have been employed on first and second row turbine blades and vanes as well as on combustor components exposed to the hot gas path of industrial gas turbines. In this environment TBCs are extensively applied to the hot sections and provide them protection against thermos-mechanical shock, high-temperature oxidation, and hot corrosion degradation, inter alia.
Thermal spraying (e.g. plasma spraying) is one of many methods used to produce overlay coatings (e.g. TBC) for the protection of materials from a wide range of adverse environmental, mechanical, and thermal conditions as well as for creating functional surfaces. In this process the deposit develops by successive impingement and inter-bonding among molten particles of feedstock material that are directed toward a surface. The particles of these coatings are prescribed by characteristics of the feedstock materials and the processing parameters. This enables the formation of coatings with a vast range of distinct microstructures which, in turn, alters the functionality and performance of the respective overlay coating. However, with the rapid solidification associated with this process, control of porosity of the coating depends on a multitude of parameters, including the spray ambient environment, plasma spray parameters (e.g. power level, gas flow features, spray distance etc.), and feedstock characteristics (e.g. morphology and size distribution).
Increasing firing temperatures and decreasing leakage path tolerances, both of which are enabled by TBCs, are causing a greater reliance on TBCs, and hence a demand for improved performance. Consequently, there remains room in the art for improvement.
The invention is explained in the following description in view of the drawings that show:
The present inventors have developed a unique and innovative way to create improved thermal barrier coatings (TBCs) improved functionality and performance. Many of the ceramic materials used in TBCs are transparent or translucent to lasers conventionally used in laser heating processes. This inherent characteristic has prevented TBC formation using conventional selective laser melting (SLM) and selective laser sintering (SLS) processes because the laser beam would simply pass though the ceramic material. The method disclosed herein takes advantage of the transparent and translucent nature of ceramics by placing a heat source material in the ceramic material. An energy beam (e.g. laser beam) is used to irradiate the heat source material and generate heat therein. The heat-source material absorbs the laser energy and is heated until sufficient heat is generated to sinter adjacent ceramic material. The heat-source material is dispersed in sufficient quantity and distribution that the heat generated in the heat source material is sufficient to sinter the entire volume in which the heat-source material is disposed.
An example volume of ceramic material is a layer of ceramic material. In such an exemplary embodiment, a layer of ceramic material with heat-source material therein may be processed to form a sintered layer. Other layers may be formed thereon iteratively in an additive manufacturing process to form a TBC having inconsistencies therein caused by the heat-source material. In an exemplary embodiment, the heat-source material is a fugitive material that may be partially or fully volatized during the laser processing of the layer. In this case the inconsistencies may include random or patterned voids where the fugitive material volatized. Alternately, some or all of the fugitive material may not be volatized during the laser processing of the layer, in which case the remaining fugitive material may serve another purpose in the interim or as part of a component in an operating gas turbine engine before fully volatizing.
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Innovatively, in the process disclosed herein, this characteristic is relied upon to permit the laser beam 12 to pass through the ceramic material 16 so that the laser beam 12 may reach a heat-source material 18. The heat-source material 18 is at least partly submerged in the ceramic material 16. As shown the heat-source material 18 is fully submerged. Either or both is acceptable in the layer 14. If the heat-source material 18 is fully submerged, a surface 20 of the layer 14 will be relatively smooth after final processing. If the heat-source material 18 is partly submerged then the surface 20 of the layer 14 may be relatively less smooth after final processing.
The laser beam 12 is directed at the heat-source material 18, heating the heat-source material 18. The heat-source material 18 is selected so that it may be heated by the laser beam 12 to a temperature and for a time sufficient to sinter adjacent ceramic material 30 into sintered ceramic 32. The heat-source material 18 is dispersed throughout the layer 14 in a density and volume sufficient to sinter the entire layer 14 of ceramic material 16. As can be seen here, the laser beam 12 has previously heated heat source material 18 to create the sintered ceramic 32 nearby the processed heat source material 18, while ceramic material 16 nearby unprocessed heat source material 18 (or heat source material 18 in the beginning stages of processing) remains unsintered.
Accordingly, once all of the heat-source material 18 is processed by the laser beam 12 all of the ceramic material 16 is sintered, thereby forming a sintered ceramic layer. In the case of transparent ceramic material 16 the ceramic material 16 absorbs a negligible amount of energy from the laser beam 12, and the heat-source material 18 is essentially the sole source of heat for the ceramic material 16. In the case of transparent material, some energy from the laser beam 12 may also be absorbed directly by the ceramic material 16.
The presence of the heat-source material 18 forms an inconsistency 40 in the morphology of the layer 14 when compared to a morphology of a layer of ceramic that is sintered without heat-source material 18 therein. The heat-source material 18 may a fugitive material 34 that at least partly volatizes during the laser processing. The fugitive material in particular can be any material that easily combusts and enables transfer of heat to surrounding ceramic particles. Example materials include polyester, graphite, or polymethyl methacrylate. In this exemplary embodiment the fugitive material 34 fully volatizes, leaving a void 42 in the sintered ceramic 32. The void 42 takes a shape generally consistent with a shape of the fugitive material 34. Accordingly, where the fugitive material 34 is a relatively large and discrete body when compared to the ceramic powder, the void 42 is likewise relatively large and discrete within the layer 14.
Alternately, the heat-source material 18 may not volatize at all, leaving remaining material 36 as indicated for one of the inconsistencies 40. In another alternate exemplary embodiment the fugitive material 34 may only partly volatize, leaving remaining material of reduced volume when compared to its pre-processed volume. In yet another exemplary embodiment, some heat-source material 18 may be fugitive, and some may not, and there may be composite heat-source material 18 having both fugitive material 34 and non-fugitive material. The remaining material 36 may be expected to volatize during operation in a gas turbine engine, or may be expected to survive. Any remaining material 36 may be relied upon to perform an additional function during handling and/or during operation in the gas turbine engine. For example, remaining material 36 may be a marker material and may be disposed in the sintered ceramic such that it is more densely packed deeper in the TBC. Exhaust from the gas turbine engine may be monitored for this marker material and an amount of wear of the TBC may be assessed.
Porosity affects thermal conductivity, strain tolerance, damping/internal friction, and, abradability, inter alia, and so the ability to control porosity within a layer 14, coupled with the ability to form a TBC in a layer-by layer manner through an additive manufacturing process as disclosed herein, enables the formation of TBCs having local variations in functionality.
From the foregoing it can be seen that the inventors have devised an innovative and unique method of creating a TBC in a layer-by-layer, additive manufacturing process. The TBC can be tailored locally within each layer as well as layer-by-layer to achieve a desired porosity architecture tailored to desired local functionality. The method disclosed enables this process using conventional equipment in an unconventional way, and thereby costs little to implement. Consequently, this represents an improvement in the art.
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.