Materials which abrade readily in a controlled fashion are used in a number of applications, including as abradable seals. Very few thermal spray abradable coatings, however, are suitable for high-temperature applications. In general, contact between a rotating part and a fixed abradable seal causes the abradable material to erode in a configuration which closely mates with and conforms to the moving part at the region of contact. In other words, the moving part wears away a portion of the abradable seal so that the seal takes on a geometry which precisely fits the moving part, i.e., a close clearance gap. This effectively forms a seal having extremely close tolerances.
One particular application for abradable seals in high-temperature environments is their use in axial flow gas turbines. The rotating compressor or rotor of an axial flow gas turbine consists of a plurality of blades attached to a shaft which is mounted in a shroud. In operation, the shaft and blades rotate inside the shroud. The inner surface of the turbine shroud is most preferably coated with an abradable material. The initial placement of the shaft and blade assembly in the shroud is such that the blade tips are as close as possible to the abradable coating.
As will be appreciated by those skilled in the art, it is important to reduce back flow in axial flow gas turbines to maximize turbine efficiency. This is achieved by minimizing the clearance between the blade tips and the inner wall of the shroud. As the turbine blades rotate, however, they expand somewhat due to the heat which is generated. The tips of the rotating blades then contact the abradable material and carve precisely defined grooves in the coating without contacting the shroud itself. It will be understood that these grooves provide the exact clearance necessary to permit the blades to rotate at elevated temperatures and thus provide an essentially custom-fitted seal for the turbine.
In other gas turbines, the initial clearance is somewhat greater and the abradable coating is intended to protect the shroud and blade tips against wear during transient conditions (e.g., power surges).
In order for the turbine blades to cut grooves in the abradable coating, the material from which the coating is formed must abrade relatively easily without wearing down the blade tips. This requires a careful balance of materials in the coatings. In this environment, an abradable coating must also exhibit good resistance against particle erosion and other degradation at elevated temperatures. As known by those skilled in the art, however, few conventional thermal spray abradable coatings have the desired high-temperature performance characteristics.
A number of abradable coatings are known in the art. Limited success has been achieved by others with the use of ZrO.sub.2 based ceramic coatings in abradable applications. ZrO.sub.2 based powders have also been blended with plastic based powders, the blended mixture being plasma sprayed to form abradable coatings. These approaches, however, have produced coatings which exhibit limited abradability at high temperatures. In addition, the plastic powders tend to degrade during thermal spraying, producing inconsistent microstructures and inferior abradability.
Other conventional abradable coatings include such cellular or porous metallic structures as those illustrated in U.S. Pat. Nos. 3,689,971, 4,063,742, 4,526,509, 4,652,209, 4,664,973, and 4,671,735. Low melting point metallic coatings of indium, tin, cadmium, lead, zinc, and aluminum alloys have been suggested for use in providing "ablative" seals wherein heat generated by friction melts a clearance gap in the coating. This approached is exemplified in U.S. Pat. Nos. 2,742,224 and 3,836,156. Ceramics such as ZrO.sub.2 and MgO for use in forming abradable coatings are also shown in U.S. Pat. Nos. 4,405,284, 4,460,311, and 4,669,955.
In U.S. Pat. Nos. 3,508,955, a composite material is disclosed which comprises a porous metal impregnated with a fluoride of metals selected from Groups I and II of the Periodic Table of the Elements. The use of fluoride salts and a barium fluoride-calcium fluoride eutectic is specifically mentioned as is the use of the material in bearings and seals. It is also disclosed therein that the resultant material can be sprayed with a surface layer of fluoride eutectic slurry which is then dried and sintered.
In U.S. Pat. No. 4,867,639, abradable coatings for use in turbine or compressor shrouds are disclosed which are described as low melting fluoride compounds such as BaF.sub.2, CaF.sub.2 and MgF.sub.2 incorporated into a higher melting temperature ceramic or metallic matrix. It is disclosed that, alternatively, the soft ceramic phase may be used to fill or impregnate a honeycomb shroud lining made of the higher melting temperature ceramic or metal alloy, so that the soft ceramic is not eroded by hot gases in the turbine. Zirconia and/or alumina are disclosed as the preferred high melting temperature ceramic, and NiCr and NiCrAl are disclosed as preferred metals.
The use of metal matrix coatings having a plastic component such as a polyimide are also known for use in forming an abradable seal in high-efficiency compressors. Due to the lower temperatures generated in the compressor and the fact that the rotating blades are generally softer than those found in the turbine section, plastics have been used in lieu of solid lubricants such as CaF.sub.2. While the lower melting point of plastics is advantageous in such low temperature applications, the use of these coatings has not been successful in high temperature applications.
In U.S. Pat. No. 5,196,471, "Thermal Spray Powders for Abradable Coatings Containing Solid Lubricants and Methods of Fabricating Abradable Coatings," thermal spray powders are described which are characterized by the presence of a matrix-forming component, a solid lubricant component and a plastic component. Abradable coatings formed by thermal spraying the powders abrade readily to form abradable seals. The abradable coatings have a metal, metal alloy, or ceramic matrix with discrete inclusions of solid lubricant and plastic. Therein, the use of Zirconia is described as a preferred ceramic for use as the matrix-forming component.
Therefore, it would be desirable to provide a composite material which abrades readily at high temperatures without producing significant wear of rotating parts.
It would also be desirable to provide such a material which can be fabricated using conventional thermal spray techniques.
It would still further be desirable to provide a coating for forming abradable seals which can be custom formulated for a particular operating environment.
The present invention achieves these goals by providing thermal spray powders which are a two component blended mixture that forms high-temperature, abradable coatings by conventional thermal spray application.