Patent Application
20110219775
This disclosure generally relates process for creating a desired surface finish and dimension for a ceramic matrix composite component.
A ceramic matrix composite includes a fiber reinforcement layer saturated with ceramic material. The ceramic matrix composite is utilized to provide desirable mechanical and thermal properties. Typical ceramic matrix components provide thermal properties that are favorable for high temperature environments. The fiber reinforcement layer provides desired mechanical properties and improves durability. Including the fiber reinforcement layer further improves the durability properties of the ceramic matrix composite as compared to a purely ceramic component. The fiber reinforcement layer while improving the durability of the ceramic component contributes to the creation of rough surface finishes and inconsistent dimensional control.
A disclosed ceramic matrix composite (CMC) component includes a hardenable material applied to a surface of the CMC such that at least a portion of the CMC can be machined to provide a desired dimension and surface finish.
The example disclosed process includes the application of a hardenable material such as silicon to areas where a precise dimensional tolerance is desired. The hardenable material can then be machined to provide the desired geometry within acceptable dimensional tolerances.
Accordingly, the example process provides for the use of CMC components in an increased range of applications that require dimensional tolerances beyond those consistently obtained with CMC material fabrication processes.
These and other features disclosed herein can be best understood from the following specification and drawings, the following of which is a brief description.
Referring to
The example liner 18 is formed from a ceramic matrix composite material that provides the desired favorable thermal properties. As appreciated, the illustrated gas turbine engine 10 is one of many known gas turbine engine configurations that will benefit from the following disclosure. The example liner 18 includes different components exposed to the extreme temperatures generated during combustion.
Referring to
The reinforcement fibers 24 provide the increased strength and durability that is desired, but also inhibits significant machining or other secondary operations that could be implemented to accommodate limitations in dimensional control. Machining or any other cutting or material removal processes that are successfully utilized for other materials are of limited success for use with CMC materials. The example component includes a layer of hardenable material 38 that is bonded to a surface of the CMC 22. The example hardenable material 38 is comprised of silicon and can be machined to provide a desired dimension and surface finish.
The hardenable material includes at least one typical constituent of an environmental barrier coating (EBC). This includes at least one of silicon, refractory metal silicides, barium strontium aluminosilicate (BSAS), strontium aluminosilicate (SAS), yttrium silicates. Rare earth silicates, mullite, hafnium oxide, tantalum oxide, hafnium silicate, zirconium silicate. The hardenable material may be reinforced with chopped fibers or hard or soft ceramic particles. The reinforcements might include carbides, graphite, carbon, glass, silicon carbide, silicon nitride or boron nitride.
The hardenable material may be applied by any method of coating application known in the art. These include plasma spraying techniques such as vacuum plasma spray(VPS) and air plasma spray (APS); physical vapor deposition methods such as electron-beam physical vapor deposition (EBPVD), slurry approaches and pack-cementation methods, chemical vapor demosition etc.
The process and material utilized to produce the CMC portion 22 result in a surface deviation of approximately +/−0.004″ (+/−0.1016 mm) or greater. In this example, the silicon layer 38 is applied in areas where a more precise dimensional tolerance is desired. In this example the silicon layer 38 is applied to a portion of the component 20 where an overall thickness indicated at 32 is desired. Such a thickness may be required for areas of the component that must interface accurately with other members.
In this example, the component thickness 32 is formed from a first thickness 44 of the CMC 22 and a second thickness 42 that is formed from the silicon layer 38. The silicon layer 38 provides a surface that can be machined to desired tolerances. The silicon layer 38 provides a layer that is machinable without disturbing the matrix composition of the CMC 22.
The silicon layer 38 is applied in a non-solid form to a surface 30 of the CMC 22 and becomes solid upon cooling. The silicon layer 38 forms a bond to the surface 30 with a tensile strength substantially the same as the CMC 22. That is, the bond between the silicon layer 38 and the CMC 22 withstands tensile forces that are substantially the same as if the CMC 22 material were tested by itself. The bond 44 between the silicon layer 38 and the CMC 22 is therefore not a weak point in the component 20.
The silicon material layer 38 does not include continuous reinforcement fibers and therefore can be machined to provide a desired shape, thickness and surface finish. In the example component, a machined surface 40 of the silicon layer 38 includes a surface deviation that is much less than that of the surface deviation of the surface 30. Moreover, the example machined surface is machinable to a thickness within a desired tolerance range of +/−0.002″ (+/−0.0508 mm) or better.
Machining of the layer 38 can be performed using any known machining process. As appreciated, a desired tolerance of a desired dimension will govern the specific machining process utilized. In the example component 20, the silicon layer 38 is machined using a diamond grinding operation to provide the desired thickness 32. The example machining process provides for the creation of a machine surface within a tolerance of +/−0.002″ (+/−0.0508 mm) or better. Of course other machining processes and grinding operations are within the contemplation of this disclosure.
A CMC part is desirable for use in applications that encounter extreme temperatures. The thermal performance provided by CMC parts make it favorable for use shielding other less thermally resistant parts such as for example the combustor liner 18 (
Referring to
The inconsistent fabrication process of forming the CMC part 52 may not provide consistent results within desired tolerance limits. Therefore, in this example, a silicon layer 56 is applied at the interface 58. The silicon layer 56 is applied as a layer to form a complete overall dimension greater than a desired final dimension, such as for example a desired thickness 62. The silicon layer 56 is then machined to provide the thickness 62 within acceptable tolerance limits. In this example, the silicon layer 56 forms a substantial part of the interface surface 60 that abuts the metal support 54. Moreover, as shown the silicon layer 56 itself varies in thickness to accommodate inconsistencies in the CMC 52 part.
Referring to
The example CMC is melt infiltrated silicon carbide/silicon carbide (MI SiC/SiC) which consists of a silicon carbide (SiC) fiber, a boron nitride (BN) fiber/matrix interface, and a silicon-silicon carbide (Si—SiC) matrix. Chemical vapor infiltration (CVI) is used to apply the BN interface, along with a SiC overcoat. Final densification of the matrix is completed by slurry cast (SC) and melt infiltration (MI) processes that result in a Si—SiC matrix. It should be understood that other methods and material known for producing a ceramic matrix composite material are within the contemplation of this disclosure.
Once the CMC part is complete, the surface deviation for specific areas of the component may not be as desired. Therefore a layer of hardenable material, such as Silicon in this disclosed example, is applied to a surface of the CMC part as is indicated at 74. The layer of silicon can be applied to localized areas that comprise an interface with other components, or to a larger general area to provide a desired surface finish better than that produced by the CMC formation process. The layer of silicon is therefore applied to locations of the CMC where the desired final dimensions are not consistently obtainable with the CMC process alone. In the disclosed examples, the silicon layers 38 (
The example application process includes an air plasma spraying process as is indicated at 76. In an air plasma spraying process, silicon is applied in a non solid form in the presence of heat. Layers of silicon are applied sequentially to build up a sufficient thickness to provide sufficient material for machining to a desired completed dimension. Other application processes as are known could also be utilized for applying a layer of hardenable material of sufficient desired thickness.
Once the hardenable material is applied, the hardenable material will bond to the CMC material and harden as is indicated at 78. The desired bond between the silicon material and the CMC material is such that desired mechanical properties of the completed part are maintained.
With the silicon applied, bonded and hardened, the silicon layer can then be machined to provide the desired dimensions as is indicated at 80. In the disclosed examples, the silicon layers 38 (
The example machining process includes a diamond grinding operation that provides a desired surface finish and dimensional tolerance. As appreciated, the specific machining operation utilized can be varied as required to generate a desired tolerance and geometry. Once the machining processes are complete, the CMC component provides the desired thermal properties combined with the desired geometry within desired dimensional tolerance range. Other machining processes including ultrasonic machining and grinding may be used to achieve the desired dimensional control.
The example process provides for the use of CMC components in an increased range of applications that require dimensional tolerances beyond those consistently obtained with CMC material fabrication processes.
Although a preferred embodiment of this invention has been disclosed, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of this invention. For that reason, the following claims should be studied to determine the true scope and content of this invention.
The subject of this disclosure was made with government support under Contract No.: N00014-06-C-0585 awarded by the Navy. The government therefore may have certain rights in the disclosed subject matter.
