Abrasive tool making process – material – or composition – Impregnating or coating an abrasive tool
Reexamination Certificate
1997-09-25
2001-01-09
Speer, Timothy M. (Department: 1775)
Abrasive tool making process, material, or composition
Impregnating or coating an abrasive tool
C051S307000, C051S309000
Reexamination Certificate
active
06171351
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to strip coatings for metal components of drive units. Such strip coatings can be categorized as abrasive or abradable coatings and the invention is particularly concerned with abrasive coatings which act on the abradable coatings.
The invention relates particularly to the construction of such abrasive strip coatings for metal components of drive units and to their manufacture.
BACKGROUND AND PRIOR ART
Abradable strip coatings, of relatively complex structure for drive units are described in U.S. Pat. No. 3,042,365. Therein, the blade tips of moving blades abrade these abradable coatings, and the blades have, as a rule, only the hardness of the basic material of the blade or a blade coating and no specific application on the blade tip of an abrasive coating. Since the efficiency of compressors and turbines depends to a great extent on the size of the gap between the stator and the rotor, when there is increasing wear of the blade tips in a stripping process, this efficiency is reduced. The wear of the blade tips or of sealing tips on labyrinth seals is still further aggravated, if the strength and hardness of the abradable coatings is increased for increasing its resistance to erosion and/or for increasing its temperature stability. In this case, the blade tips or the sealing tips of the labyrinth seals must be coated with an abrasive.
Such an abrasive coating for blade tips is disclosed in U.S. Pat. No. 4,169,020. This abrasive coating comprises a metal matrix with particles of mechanically resistant material embedded in the matrix. Due to the high heat conductivity of the metal matrix material, there is a disadvantage that the structural part, namely the blade tip, can be overheated during the stripping process. Another disadvantage is that the particles of mechanically resistant material have no orientation and are randomly arranged in the matrix, so that the abrasion of the abradable coating by the abrasive coating is deficient as only a disordered scratching is produced on the abradable coating by the tips of the particles of mechanically resistant material. A determined reduction in the heat of friction is not provided with the abrasive coatings known in the art.
SUMMARY OF THE INVENTION
An object of the invention is to provide a strip coating of the above type, which overcomes the disadvantages of the prior art and is suitable as abrasive coatings with high strength and hardness for blade tips or sealing tips such that in the stripping process, a uniform, minimal gap is formed between the abraded coating and the abrasive coating. The abrasive coating will reduce any drop in efficiency with a high service life of the power unit.
This object is achieved by forming the abrasive coating as a thermally sprayed ceramic coating, and by providing the ceramic coating with a profile having cutting edges and free spaces arranged between the cutting edges, which take up and remove the abraded material of the abradable coating. The abrasive coating has the advantage that it produces a smooth surface on the abradable coating during the stripping process due to its profiled cutting edges and assures a minimum uniform gap between the rotating and stationary structural parts of the power unit. It simultaneously protects the coated structural part from overheating, since it comprises throughout a heat-insulating ceramic material with intermediate spaces, which are free of a heat-conducting metal matrix. Further, the intermediate spaces provide for immediate removal of the hot abraded material of the abradable coating, so that heating due to friction can be reduced. A further advantage is that the profiling can be oriented to provide optimal stripping results, taking into consideration the direction of the relative motion between the structural part with or without an abradable coating and the structural part with the abrasive coating.
Preferably, ZrO
2
7
Y
2
O
3
is used as the ceramic material for the abrasive coating. This material possesses not only an essentially higher hardness than the metal base material of the coated structural part and the material of the abradable coating, but it also has a lower heat conductivity.
Another preferred ceramic material for the abrasive coating is Al
2
O
3
, which is known as conundrum, and can be utilized appropriately in a cost-favorable manner. In addition, mixed oxides can be used for the abrasive coating of the invention.
The abrasive coating preferably covers the blade tip of a blade of a drive unit such as a turbine or compressor, and the gap between a stationary abradable coating on a shroud and the rotating blade tip essentially determines the efficiency of the drive unit.
In another preferred application of the abrasive coating of the invention, sealing tips of labyrinth seals are coated, said seals being used in drive units between the drive shaft and the housing for sealing bearing blocks. In addition, sealing tips on a blade tip cover strip are preferably protected with an abrasive strip coating according to the invention. These sealing tips on the blade tip covering strips also abrade a stationary abradable coating on a shroud during the stripping process.
A preferred process for producing an abrasive strip coating for a metal component of a drive unit, which is adapted to abrade an abradable coating during a stripping operation, includes the following steps:
a) applying a perforated mask onto the structural surface to be coated, and
b) thermal spraying a ceramic material through the perforated mask onto the surface of the structural part to be coated at a spraying angle of 0-50°, preferably 5-30°, to form a succession of cutting edges and free spaces therebetween on the surface of the structural part.
The surface of the structural part can be roughened for better adherence of the ceramic spray layer or the surface is coated with an adhesive layer.
An advantage of this process is that a profiling of the surface of the structural part ready for cutting can be obtained with a spray process, without expensive pre-profiling treatment of the surface of the structural part or expensive post-processing machining of a cutting profile into the ceramic layer.
The perforated mask preferably comprises a wire grid, in which the ratio of the open mesh width and the wire diameter is between 2 and 6, and the wire diameter preferably is between 0.1 and 0.5 mm. Perforated masks in the form of a wire grid have the additional advantage that they comprise round wires and thus promote the formation of cutting-capable edges, since only a fraction of the wire surface lies orthogonal to the spray jet and a high fraction of the spray material is deflected from the wire in the direction onto the surface of the structural part, so that accumulations of sprayed material are found on the surface of the structural part as pyramidal deposits having cutting edges. Another advantage in the use of wire grids as perforated masks is that the mesh openings form squares and consequently sharp edges are formed at the bases of the deposits at an angle of 90° to each other. These deposits can be optimized to provide acute triangularshaped tips as on the surface of fine files. For this purpose, the wire grid is arranged in such a way that it is impacted diagonally by the angled spray jet. The disposition of the cutting edges may be changed by the position of the wire grid and by the angle of the spray jet. In this way, the process of the invention makes possible an optimal orientation of the cutting edges with respect to the relative motion between a structural part with or without an abradable coating and a structural part with the abrasive coating.
Another preferred process for the production of an abrasive strip coating for metal components of drive units, has the following process steps:
a) profile etching the surface of a structural part of a component of a drive unit to be coated to form a profiled surface of cutting edges and intermediate free spaces on said surface, and
b) thermal spraying a ceramic material onto said profi
Fischer Herbert
Fischhaber Erwin
Legrand Norbert
Schroder Johannes
Uihlein Thomas
Ladas and Parry
MTU Motoren-und Turbinen Union M{umlaut over (u)}nchen GmbH
Speer Timothy M.
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