Patent Application
20110243716
With components of rotary machinery, such as a gas turbine engine, a consistent roundness (defined as a constant radius about a point or an axis) is difficult to obtain. A relatively inflexible cylindrical part, like a rotor, can be made very close to round but the part may be subject to material flaws and malformations, handling and assembly, and operating parameters that affect the constancy of its defining radii fairly constantly throughout the part.
Relatively flexible parts, like a blade or a casing complicate the issue because of their greater susceptibility to damage and motion during manufacture, assembly and use. For example, as blades rotate about a rotor, their rotating blade tips define a desired substantially cylindrical envelope in which the blades rotate. However, the blade lengths may not be equal, the blade radii (and their supports) lengthen and shorten as engine operating temperatures vary and the blades may flex under load.
Similarly, a thin, relatively flexible, stationary casing is disposed around the substantially cylindrical envelope. For efficiency, it is desired that this casing be closely aligned with the envelope to prevent air or other gasses from escaping around the blade tips. However, the casing may not react to temperature changes in the engine in the same manner as the blades and the rotors and is subject to other loads in the engine. Control systems may be used in the engine to keep the casing closely aligned with the cylindrical envelope. Such systems, however, may not be perfect and some blade tip-to-casing interference may occur.
During operation, especially when the engine is newer, the engine may define for itself its own definition of roundness and minimize out of roundness as parts interact and contact each other. Abradable coatings are used to protect the parts as interaction occurs. Some blades have coatings or tip treatments that affect the wear of the blades during operation.
According to an exemplar, an air seal for use with rotating parts in a gas turbine engine has a matrix of agglomerated fine hBN (hexagonal boron nitride) powder, the particles of which have a first dimension, and of a fine metallic alloy powder, the particles of which have a second dimension. A hBN (hexagonal boron nitride) powder, the particles of which have a third dimension that is greater than the first dimension, is mixed with the matrix.
According to a further exemplary, a gas turbine engine has an air seal disposed between relatively rotating parts. The air seal has a matrix of agglomerated fine hBN (hexagonal boron nitride) powder, the particles of which have a first dimension, and of a fine metallic alloy powder, the particles of which have a second dimension. A hBN powder, the particles of which have a third dimension that is greater than the first dimension, is mixed with the matrix.
According to a still further exemplar, a method of creating an air seal on a gas turbine engine part includes agglomerating a matrix of fine hBN (hexagonal boron nitride) powder, the particles of which having a first dimension and of a fine metallic alloy powder, the particles of which having a second dimension and mixing with the matrix an hBN (hexagonal boron nitride) powder, the particles of which having a third dimension that is greater than the first dimension.
These and other features of the present invention can be best understood from the following specification and drawings, the following of which is a brief description.
Prior art air seal materials (not shown) have either been designed for use with hard or abrasive blade tip treatments, or for use with bare Ti (Titanium), Ni (Nickel) or Fe (Iron) based blade tips. These arrangements typically exhibit wear ratios between the blade tips and air seal materials that are undesirable. With tipped blades, the wear is localized in the outer air seal, while with untipped blades, there is excessive wear in the blade tips, or blade material transfers to the seal thereby degrading the seal.
While engine dimensions and tolerances may vary, a balance of wear results between a blade and a seal with which it interacts resulting in a wear ratio. If the ratio is too high, e.g., the blade wears too much relative to the seal, the blade may need to be overhauled or replaced too early relative to other wear in the blade exposing an engine user to greater expense. Similarly if the ratio is too low, the seal may need to be replaced too often also causing additional expense to the engine user. Ideally, the blade 15 will wear an amount and the seal 55 will wear an amount to minimize expense and downtime to run the engine 10.
In the instant application, as an example, an optimum balance of wear between the blade 15 and seal 55 is about 0.25 for blade tip wear over seal wear. That is for about every 2 mils of linear blade 15 wear, the seal 55 will wear at a depth of about 8 mils. This ratio also reflects the relative amount of out of roundness that needs to be corrected by wear of blades 15 and seal 55. Depending on the shape of the blades 15, a volumetric (as opposed to a linear ratio as described hereinabove as ˜0.25) may also be used. While an ideal ratio for blades 15 and seal 55 is described for this engine 10, a user will understand that an ideal ratio is also desired and contemplated herein between a vane 35 and a seal 60 or other part rotating relative to the vane 35 or the like.
This linear wear ratio of ˜0.25 is a large ratio in the context of currently available coatings. Existing materials that do achieve wear ratios close to this level suffer from aerodynamic losses due to high gas permeability and high surface roughness in the air seals. Applicants have discovered that there is a need for an abradable blade outer air seal that can be used without costly hard coated or abrasive blade tip treatments while achieving optimal wear ratio with bare blade tips, has a smooth surface, low gas permeability and results in optimal efficiency.
An abradable air seal 55, 60 for use in conjunction with Ti, Fe or Ni based blades without abrasives added to their tips provides low blade tip wear, a smooth surface and low gas permeability for improved aerodynamic efficiency is described hereinbelow.
The material is a bimodal mix of a fine composite matrix of metallic based alloy (such as a Ni based alloy though others such as cobalt, copper and aluminum are also contemplated herein) and hexagonal boron nitride (“hBN”), and inclusions of hBN. Feed stock used to provide the air seals 55, 60 is made of composite powder particles of Ni alloy and hBN held together with a binder, plus hBN particles that are used at a variable ratio to the agglomerated composite powder to adjust and target the coating properties during manufacture. One of ordinary skill in the art will recognize that other compounds such as a relatively soft ceramic like bentonite clay may be substituted for the hBN.
The fine composite matrix, of Ni based alloy and hexagonal boron nitride (hBN) includes hBN particles in the range 1-10 micron particle sizes and the Ni based alloy in the range of 1-25 microns particle size. Polyvinyl alcohol may be used as a binder to agglomerate the particles of Ni based alloy and hBN before thermal spraying. Alternatively, the Ni based alloy may be coated upon the hBN before thermal spraying. If the particles are not agglomerated in some way, they may cake up, distort or react inappropriately during spraying.
Larger particles of hBN are added to the fine composite matrix prior to spraying or during spraying. The larger hBN particles are in the range of 15-100 microns particle size though 20-75 microns particle size may be typical. The ratio between the amount by volume of hBN to Ni alloy is about 40-60%.
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Low blade tip wear is achieved by reducing the volume fraction of metal in the mix of the coating relative to the prior art, while erosion resistance is maintained through strongly interconnected metallic particles. The strength of the mix is maintained through the use of a bi-modal distribution of hBN particles. As noted above, a first fine particle size composite is formed with about 40-60% by volume metallic Ni alloy that maintains good connectivity between metallic particles. This composite structure is then used as the matrix around larger dimension hBN particles. The result is that good connectivity is maintained between the metallic particles resulting in good erosion resistance, while being able to include an unprecedented volume fraction of hBN in the range of 75-80%. The desired low volumetric wear ratio of blade to seal material is achieved through this reduction in metal content of the seal.
Low gas permeability and roughness are achieved by creating a structure that is filled with hBN and takes advantage of a fine distribution of constituents.
Although a combination of features is shown in the illustrated examples, not all of them need to be combined to realize the benefits of various embodiments of this disclosure. In other words, a system designed according to an embodiment of this disclosure will not necessarily include all of the features shown in any one of the Figures or all of the portions schematically shown in the Figures. Moreover, selected features of one example embodiment may be combined with selected features of other example embodiments.
The preceding description is exemplary rather than limiting in nature. Variations and modifications to the disclosed examples may become apparent to those skilled in the art that do not necessarily depart from the essence of this disclosure. The scope of legal protection given to this disclosure can only be determined by studying the following claims.
