Patent Application
20130071235
This disclosure relates to an air seal for a gas turbine engine.
In compressor and turbine sections of a gas turbine engine, air seals are used to seal the interface between rotating structure, such as a hub or a blade, and fixed structure, such as a housing or a stator. For example, typically, circumferentially arranged blade seal segments are fastened to a housing, for example, to provide the seal.
Relatively rotating components of a gas turbine engine are not perfectly cylindrical or coaxial with one another during engine operation. As a result, the relatively rotating components may occasionally rub against one another. To this end, an abradable material typically is adhered to the blade seal segments and/or the rotating component.
An air seal for use with rotating parts includes an abradable layer adhered to a substrate. The abradable layer comprises a matrix of agglomerated hexagonal boron nitride and an oxide ceramic. Another hexagonal boron nitride is interspersed with the matrix.
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.
Air seals 60 (
In one example shown in
The abradable layer consists of three ceramic materials, which have different material characteristics from one another, such as chemical composition and/or particle size. The abradable layer is a bimodal mix of a first ceramic material of an oxide ceramic (for example, stable up to at least 1200° F. (650° C.)) and second ceramic material of hexagonal boron nitride (“hBN”), and inclusions of a third ceramic material of larger hBN. No metallic material is used in the abradable layer, which greatly reduces its weight, for example, by around 30%. The abradable layer has a strength of at least 500 psi (3.5 MPa).
Feed stock used to provide the air seal 60 is made of oxide ceramic 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 matrix of oxide ceramic and hexagonal boron nitride (hBN) includes hBN particles in the range 1-10 micron particle sizes and the oxide ceramic in the range of 1-45 micron particle size. Polyvinyl alcohol or bentonite may be used as a binder to agglomerate the oxide ceramic and hBN before thermal 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 amount by volume of oxide ceramic in the abradable layer is about 25-45% with the matrix composite of oxide ceramic and hBN having a volume fraction of about 35-50% oxide ceramic. The amount by volume of porosity is about 5-15% of the abradable layer. The larger hBN particles make up the remainder of the coating, the total amount by volume of hBN in the abradable layer is 30-50% with up to 15% of the volume percent comprising the binder. In one example, the oxide ceramic is at least one of aluminum-, zirconium- and titanium-based.
In a first example, the oxide ceramic is a mix of aluminum oxide (Al2O3) and titanium dioxide (TiO2). This titanium dioxide improves cracking and spallation resistance of the oxide ceramic. The mix includes 0-15% by weight of titanium dioxide. One desired mix is 87 wt % aluminum oxide/13 wt % titanium dioxide, and another desired mix is 97 wt % aluminum oxide/3 wt % titanium dioxide.
In a second example, the oxide ceramic includes about 7% by weight yttrium stabilized zirconia (YSZ).
The powders are deposited by a known thermal spray process, such as high velocity oxygen fuel spraying (HVOF), combustion flame spray or air plasma spray (APS). Fine particle-sized hBN powders and the fine particle-sized oxide ceramic powders being pre-agglomerated as described, are deposited on the substrate by thermal spray. The larger particle-sized hBN particles may be added to the agglomerates as a particle blend and delivered to the spray apparatus pre-blended, or may be delivered to the spray apparatus through a separate delivery system. However, it is also possible to include the larger hBN particles in the agglomerates of matrix material.
Typically, the matrix of agglomerated hBN powder and oxide ceramic powder and the larger hBN powder are fed into the plasma plume from separate powder feeders. The abradable layer 70 is deposited onto the substrate 40 (or bond coat 65) to a desired thickness, for example, 15-150 mils (0.38-3.81 mm), and in one example, 80 mils (2.03 mm).
Although an example embodiment has been disclosed, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of the claims. For that reason, the following claims should be studied to determine their true scope and content.
