Patent Description:
Ceramic matrix composite (CMC) articles are generally considered well suited for structural components of a turbine engine due to, for example, their thermal resistance, high temperature strength, and chemical stability. However, CMC articles in a turbine engine can suffer from damage due to overstressing, damage (e.g., fracture), improper formation, etc. One technique for addressing the damage-prone nature of the CMC articles is densification. However, the success of conventional techniques for densifying a melt infiltrated CMC is limited due to the myriad of defects that can occur as a result of infiltration of a densifier (e.g., infiltration of silicon). Such defects in silicon infiltration can cause, for example, obstructions, cracks, and/or areas of reduced material infiltration during the densification process and therefore weaknesses within the resulting densified CMC article. These drawbacks with conventional densified CMC articles tend to be exacerbated the thicker the CMC article.

Conventional techniques for casting parts (e.g., metal casting) include injection molding, high pressure die casting, and low pressure die casting. Injection molding may inject the material to form the part at a velocity between <NUM> to <NUM> millimeters per second (mm/s), and under an injection pressure of <NUM> to <NUM> Bars. High pressure die casting may inject the metal material (e.g., aluminum (Al) alloy) to form the part at a velocity above <NUM>/s, and under an injection pressure of <NUM> Bars or more. Low pressure die casting is performed using lower operational parameters for example, injection velocity <NUM>/s, injection pressure <NUM>-<NUM> Bars. However, none of these conventional techniques for casting have been utilized to perform the densification process on CMC articles because the operational parameters for performing the techniques may increase the likelihood and/or may cause damage to the CMC articles when performing the densification process. <CIT> discloses the infiltration of a diamond ceramic matrix with molten silicon. <CIT> discloses a porous CMC body that is pressure infiltrated with molten Si.

Methods for densifying a melt infiltrated ceramic matrix composite (CMC) article are disclosed. In a first aspect of the invention, a method for densifying a melt infiltrated CMC article is provided, the method according to claim <NUM>.

The present disclosure generally relates to densification of ceramic matrix composite (CMC) articles, and more particularly to densification of melt infiltrated CMC components of a gas turbine engine. As noted above, the success of conventional techniques for densifying a melt infiltrated CMC is limited due to the myriad of defects that can occur as a result of infiltration of a densifier (e.g., infiltration of silicon) and which in turn results in obstructions (lacks of infiltration) and/or cracks during the densification process. Accordingly, such conventional techniques provide densified CMC articles with a non-uniform density (e.g., dry spots, porosity) and locations of weakness therein, both of which can be worsened as the thickness of the CMC article increases (e.g., extreme non-uniformity of density and/or increased locations of weakness). In contrast to convention, various aspects of the disclosure include methods for densifying a melt infiltrated CMC article by creating a pressure head of a molten silicon source during infiltration and using a casting apparatus to effect infiltration. Densified melt infiltrated CMC articles produced according to the methods of the disclosure can have a substantially uniform density, even as the thickness of the CMC article is increased. The methods of the disclosure for densifying a melt infiltrated CMC article will be discussed below with reference to <FIG> for ease of comprehension.

In an embodiment, the method of the disclosure for densifying a melt infiltrated CMC article includes first providing a porous CMC preform. "CMC" as used herein refers to ceramic matrix composite wherein ceramic fibers are embedded in a ceramic matrix. A CMC article may include a ceramic matrix continuously or discontinuously reinforced with ceramic fibers. The ceramic matrix may include carbon (C), silicon carbide (SiC), silicon nitride (Si<NUM>N<NUM>), aluminum oxide (Al<NUM>O<NUM>) and/or aluminum silicate (Al<NUM>O<NUM>-SiO<NUM>). The ceramic fibers may include carbon (C), silicon carbide (SiC), silicon nitride (Si<NUM>N<NUM>), aluminum oxide (Al<NUM>O<NUM>) and/or aluminum silicate (Al<NUM>O<NUM>-SiO<NUM>). The ceramic matrix and the ceramic fibers may be composed of the same material or they may be different. For instance, when the ceramic matrix and the ceramic fibers are composed of the same material, the CMC article may be, for example, a silicon carbide fiber reinforced silicon carbide (SiC/SiC) article. When the ceramic matrix and the ceramic fibers are composed of different materials, the CMC article may be, for example, a carbon fiber reinforced silicon carbide (C/SiC) article. Typically, CMC articles may be made from any suitable manufacturing process known in the art such as, for example, injection molding, slip casting, tape casting, infiltration methods (e.g., chemical vapor infiltration, melt infiltration, etc.) and various other suitable methods and/or processes. The present disclosure relates to densifying melt infiltrated CMC articles. As such, a "CMC preform" as used herein refers to a CMC article prior to melt infiltration with a densifier.

Referring to <FIG>, a CMC preform <NUM> is provided within a first region <NUM> of a casting apparatus <NUM>. CMC preform <NUM> is a pre-infiltration CMC article as explained above. <FIG> includes both a depiction of a pre-infiltration arrangement (left side of arrow) and a post-infiltration arrangement (right side of arrow) of casting apparatus <NUM>.

The method of the disclosure further includes providing a molten densifier <NUM> within a pressure head area of a second region <NUM> of casting apparatus <NUM>. Providing molten densifier <NUM> in second region <NUM> may occur before, after or contemporaneously with providing porous CMC preform <NUM> in first region <NUM>. First and second regions <NUM>, <NUM> of apparatus <NUM> are operably connected to one another such that molten densifier <NUM> may be applied to CMC preform <NUM>. Molten densifier <NUM> may include any suitable molten source of a compound or composition capable of increasing the density of porous CMC preform <NUM>. Molten densifier <NUM> includes one or more molten sources of silicon.

After providing both CMC preform <NUM> and molten densifier <NUM>, the method of the invention further includes applying a first pressure to molten densifier <NUM>. The first pressure can include atmospheric pressure (e.g., <NUM> Bar) or a pressure slightly greater than atmospheric pressure (e.g., less than <NUM> Bars). In another non-limiting example, applying the first pressure to molten densifier <NUM> may be performed in or under a vacuum at a pressure lower than atmospheric pressure, for example, approximately <NUM> x <NUM>-<NUM> Bars. In other non-limiting examples, the first pressure may be applied during the densification process to CMC preform <NUM> within a range of approximately <NUM> Millibar to <NUM> Bar. The first pressure allows for infiltration of voids (pores) within porous CMC preform <NUM> by molten densifier <NUM>. As depicted in <FIG> (right side of arrow), a densified melt infiltrated CMC article <NUM> is formed by this infiltration.

<FIG> also depicts an embodiment where the first pressure can be atmospheric pressure. As shown, first and second regions <NUM>, <NUM> of apparatus <NUM> are oriented with one another such that the weight of molten densifier <NUM> itself can cause infiltration of CMC preform <NUM> with the molten densifier <NUM>. This type of atmospheric pressure based arrangement may also be referred to as a gravity fed system.

Even in a gravity fed system where increased pressures are not of concern, the material characteristics of CMC preform <NUM> may need to be accounted for by apparatus <NUM>. First region <NUM> of apparatus <NUM> is a die set having at least two portions <NUM>, <NUM>. Portions <NUM>, <NUM> are capable of moving relative to one another such that a die opening <NUM> can be created there between. Thus, prior to infiltration (left side of arrow - <FIG>), die opening <NUM> may be set (adjusted) to reduce or eliminate pressure on CMC preform <NUM> in order to avoid damaging (e.g., fracturing) the CMC preform's <NUM> porous matrix. The width of die opening <NUM> can be adjusted, for example decreased, during infiltration as the density of CMC preform <NUM> increases. Upon infiltration completion (right side of arrow - <FIG>), the width of die opening <NUM> may be approximately zero. Additionally, or alternatively, the performance of this process and/or the closing of the width of die opening <NUM> to approximately zero may improve the debulking of the CMC preform and/or part-to-part dimensional/size/feature consistencies.

In the example where apparatus <NUM> is formed as a gravity fed system (e.g., <FIG>) molten densifier <NUM> may be delivered at a controlled speed. For example, molten densifier <NUM> may be delivered at a controlled velocity of less than or equal to <NUM> millimeter per second (mm/s). In delivering molten densifier <NUM> at this speed (as well as under the pressure identified herein), molten densifier <NUM> may infiltrate CMC preform <NUM> without causing undesirable stress/strain, damage (e.g., fracture), and/or improper formation during the densification process.

In addition to a gravity fed system, apparatus <NUM> of <FIG> can be used in a pressure based system as well. In other words, the first pressure applied to molten densifier <NUM> may be greater than atmospheric pressure. In such a pressure based system, controlling and adjusting the width of die opening <NUM> may be critical for avoiding damage to the CMC preform <NUM>.

<FIG> depicts another embodiment where the first pressure may be greater than atmospheric pressure. In this embodiment, a porous CMC preform <NUM> is provided within a first region <NUM> of a casting apparatus <NUM>. Before, after or contemporaneously with providing porous CMC preform <NUM> in first region <NUM>, a molten densifier <NUM> is provided within a pressure head area of a second region <NUM> of casting apparatus <NUM>. Preform <NUM> and molten densifier <NUM> may be the same as described above (<NUM>, <NUM>).

First and second regions <NUM>, <NUM> of casting apparatus <NUM> are operably connected to one another such that molten densifier <NUM> may be applied to CMC preform <NUM>. Taking into consideration the increased pressure of this embodiment (i.e., greater than atmospheric), first and second regions <NUM>, <NUM> of casting apparatus <NUM> may be operably coupled with one another such that the infiltrating of preform <NUM> with molten densifier <NUM> at the increased pressure is performed without damaging the porous matrix of preform <NUM>. For example, one or both of first and second regions <NUM>, <NUM> of apparatus <NUM> may include an inlet <NUM> for passing molten densifier <NUM> to preform <NUM>. Inlet <NUM> can be configured to reduce a velocity of molten densifier <NUM> upon contact with preform <NUM>. One inlet <NUM> is depicted, however any number and any configuration of inlet(s) <NUM> may be utilized. In this non-limiting example, and similar to <FIG>, molten densifier <NUM> may be delivered at a controlled velocity of less than or equal to <NUM>/s by controlling and/or adjusting the pressure in which molten densifier <NUM> is delivered in apparatus <NUM>.

The methods of the disclosure can be performed with reliance on a first pressure applied to the system (e.g., atmospheric for a gravity fed system and greater than atmospheric for a pressure based system), as discussed above. The methods of the disclosure may also be performed utilizing multiple pressures.

Returning to <FIG>, first region <NUM> of casting apparatus <NUM> that holds preform <NUM> therein may have a second pressure associated therewith. For example, the second pressure may be less than the first pressure applied to molten densifier <NUM>. For instance, the second pressure may be less than atmospheric pressure. In an instance where first pressure is atmospheric pressure, as is the case in a gravity fed system, the second pressure being less than atmospheric pressure may aid in the infiltration process by pulling (or wicking) molten densifier <NUM> into preform <NUM>. This pulling of molten densifier <NUM> into preform <NUM> can be in addition to the weight of molten densifier <NUM> pushing into preform <NUM>.

While the second pressure may be less than atmospheric pressure, the second pressure may also be greater than a minimum pressure at which damage to the porous matrix of preform <NUM> occurs. For instance, a second pressure below the minimum pressure may cause collapse of the voids (pores) within the CMC preform <NUM>. As such, a pressure setting device (not shown) may be added to the system for the benefit of controlling the second pressure of first region <NUM> of casting apparatus <NUM>.

<FIG> depicts another exemplary embodiment wherein methods of the disclosure may be performed utilizing multiple pressures. In this embodiment, a porous CMC preform <NUM> is provided within a first region <NUM> of a casting apparatus <NUM>. Before, after or contemporaneously with providing porous CMC preform <NUM> in first region <NUM>, a molten densifier <NUM> can be provided within a pressure head area of a second region <NUM> of casting apparatus <NUM>. Preform <NUM> and molten densifier <NUM> may be the same as described above (<NUM>, <NUM> and/or <NUM>, <NUM>). This embodiment differs from those discussed above in that a third region <NUM> of casting apparatus <NUM> is utilized.

Third region <NUM> has its own pressure, different from that of the first pressure of second region <NUM>. More specifically, the first pressure of second region <NUM> may be greater than the pressure of third region <NUM> and the porous CMC preform <NUM> may be located between second and third regions <NUM>, <NUM> (e.g., spanning there between). It is the difference between the first pressure of second region <NUM> and the pressure of third region <NUM> that allows for infiltration of preform <NUM> by molten densifier <NUM>. In the above noted instance where the first pressure of second region <NUM> is greater than the pressure of third region <NUM>, the first pressure exerts force (see arrows) on molten densifier <NUM> and forces it into porous CMC preform <NUM>.

<FIG> depicts relative velocities and depths associated with one or more embodiments of the disclosure wherein the methods of the disclosure are performed utilizing multiple pressures applied to the molten densifier in a plurality of stages. More specifically, the methods of the disclosure can include applying a stage-one pressure to the molten densifier for a first period of time and then applying a stage-two pressure to the molten densifier for a second period of time. As depicted in <FIG>, the stage-one pressure can establish a first infiltration velocity 410v of the molten densifier across a first depth 410d of porous CMC preform <NUM>. Similarly, the stage-two pressure can establish a second infiltration velocity 420v of the molten densifier across a second depth 420d of porous CMC preform <NUM>. The methods of the disclosure may be performed utilizing any number of pressure stages. <FIG> depicts three stages, i.e., a third infiltration velocity 430v across a third depth 430d, however the disclosure is not so limited. Additionally in a non-limiting example the infiltration velocities 410v, 420v, 430v may be equal to or less than <NUM>/s, as discussed herein.

As depicted in <FIG>, preform <NUM> includes ceramic fibers <NUM>, <NUM>. Ceramic fibers <NUM> are those located within first depth 410d and ceramic fibers <NUM> are those located within second depth 420d. Ceramic fibers <NUM> may be less compacted than ceramic fibers <NUM>. Accordingly, to ensure sufficient infiltration of the molten densifier into the entirety of preform <NUM>, first infiltration velocity 410v may be less than second infiltration velocity 420v. In another embodiment, ceramic fibers <NUM> and ceramic fibers <NUM> may be compacted to substantially the same degree. Accordingly, any difference between first and second infiltration velocities 410v, 420v may be lessened. As also depicted in <FIG>, ceramic fibers <NUM>, <NUM> may be oriented in substantially the same direction during infiltration. This is sometimes referred to as in-plane infiltration.

<FIG> depicts a cross-plane infiltration wherein ceramic fibers of a CMC preform are oriented substantially orthogonal to one another. As depicted in <FIG>, the stage-one pressure can establish a first infiltration velocity 510v of the molten densifier across a first depth 510d of porous CMC preform <NUM>. Similarly, the stage-two pressure can establish a second infiltration velocity 520v of the molten densifier across a second depth 520d of porous CMC preform <NUM>. The methods of the disclosure may be performed utilizing any number of pressure stages. <FIG> depicts two stages, however the disclosure is not so limited. Additionally in a non-limiting example the infiltration velocities 510v, 520v may be equal to or less than <NUM>/s, as discussed herein.

Preform <NUM> includes ceramic fibers <NUM>, <NUM>. Ceramic fibers <NUM> are those located within a first depth 510d of preform <NUM> and ceramic fibers <NUM> are those located within a second depth 520d of preform <NUM>. Ceramic fibers <NUM> are oriented substantially orthogonal to ceramic fibers <NUM>, fibers <NUM> being in the direction of infiltration (see arrow) and fibers <NUM> being substantially orthogonal thereto. Accordingly, to ensure sufficient infiltration of the molten densifier into the entirety of preform <NUM>, first infiltration velocity 510v may be less than second infiltration velocity 520v.

Utilizing any one or more of the above discussed methods, a densified melt infiltrated CMC article is obtained. The densified CMC article can include a porous ceramic matrix having a densifier (e.g., at least silicon) infiltrated therein, and ceramic fibers embedded in the ceramic matrix. The densified CMC article of the disclosure can have a substantially uniform density. The substantially uniform density of the CMC article of the disclosure can be obtained and/or maintained even when the CMC article has an increased thickness and/or includes portions having a greater thickness than other portions (e.g., non-uniform thickness). As noted at the outset of the disclosure, such densified CMC articles of the disclosure can be used as components of a gas turbine engine due to their low density, thermal resistance, high temperature strength and creep properties, and chemical stability.

Accordingly, a value modified by a term or terms, such as "about," "approximately" and "substantially," are not to be limited to the precise value specified. "Approximately" as applied to a particular value of a range applies to both values, and unless otherwise dependent on the precision of the instrument measuring the value, may indicate +/-<NUM>% of the stated value(s). "Substantially" refers to largely, for the most part, entirely specified or any slight deviation which provides the same technical benefits of the disclosure.

Claim 1:
A method for densifying a melt infiltrated ceramic matrix composite (CMC) article, comprising:
providing a porous CMC preform within a first region of a casting apparatus;
providing a molten densifier within a pressure head area of a second region of the casting apparatus, the first and second regions being operably connected and the molten densifier including at least one source of silicon; and
applying a first pressure to the molten densifier within the pressure head, thereby infiltrating voids within the porous CMC preform with the molten densifier and forming a densified melt infiltrated CMC article,
wherein the first region of the casting apparatus includes a die set containing the porous CMC preform, the die set having at least two portions that are capable of moving relative to one another and the die set having a die opening between the at least two portions.