Patent Application
20070075455
This invention relates in general to materials technology and more particularly to a method of sealing an edge of exposed material such as a 2-dimensional laminated composite structure.
Ceramic materials generally have excellent hardness, heat resistance, abrasion resistance, and corrosion resistance and are desirable for high temperature machine applications such as gas turbines and the like. However, ceramic materials are easily fractured by tensile stresses and exhibit a high degree of brittleness. To improve upon the fracture toughness of a ceramic material, it is known to provide a ceramic matrix composite (CMC) material wherein a plurality of inorganic fibers are disposed in a matrix of ceramic material.
A CMC material may be formed by impregnating a preform of fiber containing fabric material with ceramic material powder using a known wet method such as slip casting or slurry infiltration. Alternate methods include impregnating a fiber tow and winding the tow on a mandrel to form a desired shape, or removing the wound tow from the mandrel, cutting it and using it to lay-up shapes. The cast, laid-up or wound part is then dried using low pressures and temperatures to form a green body. The green body is then sintered by known techniques such as atmospheric pressure sintering or reaction sintering to sinter the matrix to its final density to form the CMC material.
The material properties of CMC material are typically anisotropic. This is because the properties of the matrix binder material may differ from the properties of the continuous fiber phase within the woven fabric layer. A fundamental problem with this type of material system is its susceptibility to free edge stresses under thermal and mechanical loads. This may result in delaminating or effective separation of the laminated structure, which may lead to failure of not only the CMC material but also a component formed from the CMC material. Further other components that support the CMC material may be susceptible to failure if the CMC material delaminates.
Laminate layers 20 may be, for example, woven fabric such as oxide based ceramic fibers selected from the group of refractory oxide fibers including Al2O3, Al2O3—SiO2, mullite, YAG, and Al2O3-YAG eutectics. Matrix material layers 22 may be, for example, an oxide based ceramic material and may be one selected from the group of Al2O3, Al2O3—SiO2, mullite, YAG, and LaPO4. The present invention may be applied to any such reinforced matrix composite material including non-oxide based, and may be applied for use in high temperature environments, such as gas turbine engines.
2-dimensional composite layers or laminated oxide CMC materials 10 typically exhibit anisotropic properties in that they provide excellent in-plane mechanical properties but by comparison exhibit poorer properties in the transverse direction. It will be appreciated that CMC materials 10 may have “free” edges such as the lateral edges or ends of the material forming the perimeter of a piece of CMC material 10 when fabricated, or cut to shape or length. For example, as appreciated from
CMC materials 10 having free edges are susceptible to stresses arising from both thermal and mechanical loads, which may result in the initiation of a delaminating crack that may grow to the point of causing failure in the composite structure. Methods of the present invention allow for using lasers to create a dense edge surface that will both improve the resistance of the CMC material 10 to edge stresses and increase the wear and/or hardness properties of the material, which is a long-term concern with respect to surfaces that make contact during use.
It is known to cut and shape CMC material 10 by using water jet cutting and fine grinding operations to finish the CMC material 10. These techniques typically result in edge surfaces having exposed fabric laminate layers 20 and matrix material layers 22, i.e., free edges. This can be appreciated from
Exposed or free edge surface 30 has exposed fabric laminate layers 20 and matrix material layers 22 similar in arrangement to those shown in the cross-sectional view of
Layer or coating 32 is formed as a result of local deposition of retained molten CMC material 10 that is generated as a function of the laser cutting action. Coating 32 may be formed as a consistent homogenous, dense coating on edge surface 30 created during laser cutting of CMC material 10. The thickness of coating 32 may be controlled to avoid spallation of coating 32 off the free edge surface 30. The thickness of coating 32 may be optimized as a function of various parameters such as material composition or the addition of powder to edge surface 30 before or during melting, as well as lasing process parameters. During cool down, the glassy/non-crystalline type melt of CMC material 10 (which can be at temperatures in excess of 2000° C.) solidifies to form a monolayer consisting of the constituents of the laser-processed material, which shrinks onto the newly formed edge surface 30 upon cooling.
One beneficial aspect with respect to a free edge surface 30 of a 2-dimensional laminated CMC material 10 underlying coating 32 is that the melt layer or coating asserts a state of compression on the immediate surface areas of edge surface 30 to which it is contacted. Melt layer or coating 32 will be in residual tension but is denser and stronger than the CMC matrix material 22 and has higher strength margin to accommodate these tensile stresses. The result on the free edge properties of the 2-dimensional CMC material 10 is to increase its resistance to interlaminar separation. Another beneficial aspect is that the wear resistance of the CMC material 10 is increased along free edge surface 30, which is now covered with coating 32. This improved wear resistance increases the useful life and improves performance of CMC materials 10 used in situations where a free edge at least partially covered with coating 32 is continuously or periodically contacting a neighboring structure or component.
Embodiments of the invention allow for various laser-cutting processes to be optimized to deposit melt layer or coating 32 on a newly formed edge surface 30 of the CMC material 10 being cut. Known techniques for cutting CMC material 10 using a laser would typically include applying a gas under pressure to “wash out” the waste from the laser cutting process. The inventors of the present invention have determined that the melted CMC material 10 may be retained during the cutting process and the laser parameters controlled so the retained material may be deposited onto the newly created edge surface 30 to form coating 32.
Aspects of the invention allow for using laser processing to shape the CMC material 10 and to seal edge surface 30 with coating 32. Traditional water jet cutting and fine grinding operations to finish CMC material 10 typically results in exposed free edges susceptible to damaging the CMC material. Laser finishing may be optimized to deposit coating 32, which produces a seal of higher density than the base CMC material 10 exposed along edge surface 30. The base CMC material 10 is a heterogeneous system including two different material phases: a) a monolithic ceramic matrix 22 and a ceramic fiber 20, together having anisotropic properties. Embodiments of the invention allow for applying a laser to the base CMC material 10 to melt the material and form a single phase isotropic, i.e., homogenous material that solidifies to form seal 32. The properties of seal 32 prevent or minimize the likelihood of crack initiation and propagation in the matrix material layers 22 that lie between fabric layers 20.
The laser melt seal formed by coating 32 is harder than the materials of layers 20, 22 and is more resistant to crack initiation and contact stresses. This improves the structure of CMC material 10 proximate edge surface 30 in terms of resistance to interlaminar stresses and handle ability. In this respect, coating 32 prevents loose edges of fabric layer 20 or matrix layer 22 from catching on an object or wearing away due contacting other surfaces. Laser induced coating 32 may be a single, homogenous material that doesn't have any separation or de-lamination points such as ones that might occur between fabric layer 20 and matrix layer 22. Thus, coating 32 operates as a seal along edge surface 30 that may function as an interface to any other external system, such as a component within a gas turbine.
Embodiments of the present invention may be implemented using the laser processing techniques disclosed in U.S. Pat. No. 6,617,013, which is specifically incorporated herein by reference. Those laser processing techniques and others disclosed herein may be used to create a continuous melt phase of molten CMC material 10 along free edge surfaces 30 exposed during the laser cutting process. Laser cutting parameters may be controlled to control the thickness of melt layer or coating 32 to avoid spalling of the layer when CMC material 10 is in use. Laser cutting parameters may be optimized depending on the composition of CMC material 10, desired thickness of coating 32 and other process parameters. For instance, the laser type, nozzle type, focal length, pulse frequency, pulse width and gas pressure, for example, may be controlled to obtain the desired thickness of coating 32.
In an embodiment, melt layer or coating 32 may be optimized to create a continuous coating on the free edge surface 30 of the CMC material 10.
Other process parameters that may be used in conjunction with this equipment are an above surface focal point of 18 mm with a standard nozzle and a 125 mm focal length lens. Average power may be about 200 watts with a pulse frequency of 30 hertz, pulse width of 1 millisecond and a gas, such as nitrogen (N2), carrier pressure of 1 psi. Pulsed beam or continuous wave lasers may be used such as those producing wavelengths between 249 nm and 10,600 nm, for example, to produce similar effects in the CMC material 10 and coating 32. The process parameters with respect to such lasers may be adjusted to provide molten CMC material 10 that forms melt layer 32 for sealing the edges of the material. Particular material conditions and the need to achieve reasonable production rates may be considered to select the optimal laser settings for a particular application.
Another exemplary embodiment allows for depositing a material such as a powder material onto a free edge surface 30 having exposed material from fabric layer 20 and matrix layer 22. Free edge surface 30 is a relatively porous composition. The powder material may compensate for the porosity in the CMC material 10 to help form a homogenous coating layer 32 during laser melting/processing. The powder material may be deposited or placed in intimate contact with the 2-dimensional CMC exposed along surfaces 30. The deposited powder material may form a layer that provides additional material for forming homogenous sealing layer 32. The powder material may be the same composition and/or may use the same constituents as the CMC material 10, or the powder material may be of a different composition and/or constituents.
In an embodiment, a laser beam may be applied to the deposited powder and CMC material 10 to form coating 32. Melting the powder material and proximate CMC material 10 on edge surface 30 creates a dense homogenous coating 32. Coating 32 may have a density of greater than about 90% formed from the powder, portions of porous fabric material layer 20 and portions of matrix material layer 22. In alternate embodiments, a laser-assisted thermal spray process may be used for depositing the same, or an alternative composition coating to the free edge 30 of CMC material 10. This provides the benefit of a single stage operation and may be directly applicable to thicker edges of CMC material 10 where surface deposition and local heating aspects are more critical to forming a dense homogeneous seal coating. In this aspect, a powder material may be introduced into the laser beam that melts and is subsequently deposited onto an edge surface 30 of the CMC material 10. An exemplary coating 34 formed using a powder material 35 is illustrated in
Powder material 35 may be chosen based on its general hardness or wear resistance properties and/or coefficient of thermal expansion, for example, or for other performance properties of a coating 32, 34. Depositing powder material 35 on edge surface 30 prior to or contemporaneously with laser treatment may enhance the sealing effect of coating 32, 34 and/or achieve other performance objectives of a finished component, such as a protective coating and/or improved wear resistance of a coating.
This may be accomplished by tailoring the chemistry of powder material 35 to the CMC material 10. In an embodiment, powder material 35 may be similar in chemical composition to the CMC material 10 and may be, for example, mullite, alumina and/or silicate-based material compositions. Powders such as alumina, zircon, zirconia, mullite (crystalline), hafnia, yttrium aluminum garnet (YAG), yttria, spinelle or other compositions may be deposited to improve the hardness or wear resistance of coating 32, 34. These powders as well as others may be selected because they are denser than CMC material 10 and have a linear thermal expansion coefficient within a range, such as (±3), for example, of the CMC material 10. Other powder material compositions may be used to achieve specific performance objectives of a coating 32, 34 as appreciated by those skilled in the art.
For example, powder material 35 may be deposited having a lower coefficient of thermal expansion or a higher thermal conductivity relative to that of the CMC material 10 to achieve performance objectives. Powder material 35 may be used to control a desired height or thickness of coating 32, 34. In one aspect, powder material 35 may be deposited onto free edge surface 30 of the CMC material 10 with a portion deposited marginally onto the surfaces of the adjacent sides of edge surface 30 to create a clamping type effect. In this respect, end portions 36, for example, of an exemplary coating 34 (
Depositing powder material 35 having a lower coefficient of thermal expansion than CMC material 10 may create a differential in the coefficients of thermal expansion between coating 34 and CMC material 10 shown in
While the preferred embodiments of the present invention have been shown and described herein, it will be obvious that such embodiments are provided by way of example only. Numerous variations, changes and substitutions will occur to those of skill in the art without departing from the invention herein. Accordingly, it is intended that the invention be limited only by the spirit and scope of the appended claims.
