Patent Description:
CVI is performed on porous fiber preforms to form CMC materials. The porous fiber preform can be made of carbon fibers, silicon carbide fibers, or any other suitable fibers. CVD is performed on materials with solid surfaces. Preforms used in CVD can be made of any material that can be coated through a CVD process, such as carbon, silicon carbide, metal oxides, and glass. During a CVI or CVD process, a preform is placed within a reaction vessel that is then depressurized and heated. The silicon carbide precursor materials that densify the preform flow into the reaction vessel, are vaporized inside of the reaction vessel, and slowly deposit silicon carbide at an atomic level on the surface of the preform. The silicon carbide precursor materials coat the exposed surfaces of the preform and densify the preform into a CMC material. Numerous precursor materials can be used during CVI and CVD processes to form various CMC materials.

Silicon carbide CMC materials are used in applications where end products are exposed to high-temperatures, such as in brakes or engines. When CVI and CVD processes are used to create a silicon carbide CMC material, a yellow or multi-colored surface develops on the CMC material due to a buildup of excess free silicon on the surface. This yellow or multi-colored surface negatively affects the performance of finished silicon carbide CMC materials because it chemically changes the silicon carbide composition, reducing the mechanical, thermal, and chemical properties of the CMC material.

<CIT> and <CIT> each disclose methods and apparatus for producing heat-resistant silicon carbide composite materials using chemical vapor deposition (CVD).

According to a first aspect, a method of depositing silicon carbide on a preform to form a ceramic matrix composite is provided according to claim <NUM>.

<FIG> shows a schematic of system <NUM> for conducting CVI or CVD that can be used to perform the steps shown in the flow chart of <FIG>. <FIG> includes system <NUM>, preform <NUM>, reaction vessel <NUM>, which includes exit <NUM> and entrance <NUM>, vacuum pump <NUM>, gas source tanks <NUM>, including first tank 22a, second tank 22b, third tank 22c, and fourth tank 22d. <FIG> also includes flow meter <NUM>, and heaters <NUM>, including first heater 26a and second heater 26b.

System <NUM> can be used to efficiently carry out and control a CVI process or a CVD process. To begin a CVI process or a CVD process, preform <NUM> is removably placed inside reaction vessel <NUM>. For a CVI process, preform <NUM> is a porous fabric woven from silicon carbide fibers, carbon fibers, or any other suitable fibers. Preform <NUM> densifies during the CVI process. For a CVD process, preform <NUM> is made of any material that can have a silicon carbide coating on its surface, such as carbon, silicon carbide, metals, metal alloys, and glass. Preform <NUM> has a roughened solid surface. Roughening the solid surface of preform <NUM> increases the porosity of preform <NUM> and the surface area that can react during the CVD process. Reaction vessel <NUM> is used in both the CVI process and the CVD process. Reaction vessel <NUM> is a vessel that can maintain the necessary temperature and pressure requirements for the CVI or CVD process. Reaction vessel <NUM> includes exit <NUM> and entrance <NUM> to allow gasses to flow through reaction vessel <NUM> during the CVI or CVD process.

Exit <NUM> is connected to vacuum pump <NUM>. After preform <NUM> is placed in reaction vessel <NUM>, vacuum pump <NUM> evacuates reaction vessel <NUM> by removing air and other unwanted gasses through exit <NUM>. Reaction vessel <NUM> is then backfilled through entrance <NUM> with a backfill gas from gas source tanks <NUM>. The backfill gas is an inert gas, such as argon or nitrogen. Reaction vessel <NUM> is evacuated and backfilled several times. Evacuating and backfilling reaction vessel <NUM> with an inert gas multiple times reduces the amount of air and other oxidizing agents in reaction vessel <NUM> to a negligible amount. Oxidizing agents reduce the efficacy of the CVI process or the CVD process. Vacuum pump <NUM> also lowers and maintains the pressure in reaction vessel <NUM> to an operating pressure. Throughout the CVI or CVD process, vacuum pump <NUM> removes unspent precursor materials and spent materials, like off gasses, and excess gas from reaction vessel <NUM>.

Reaction gasses, such as the backfill gas, precursor materials and carrier gas, are stored in gas source tanks <NUM>. Gas source tanks <NUM> allow for separate storage of the backfill gas, the precursor materials, and the carrier gas. First tank 22a, second tank 22b, third tank 22c, and fourth tank 22d are connected first to flow meter <NUM> and then to entrance <NUM>. The reaction gasses flow through flow meter <NUM> and into reaction vessel <NUM> through entrance <NUM>. Flow meter <NUM> helps control the relative ratio of the reaction gasses entering reaction vessel <NUM>. Controlling the relative ratio of the reaction gasses with flow meter <NUM> also helps control the pressure in reaction vessel <NUM>.

Heaters <NUM> are heat sources. First heater 26a is located near gas source tanks <NUM> to heat the reaction gasses to a preheat temperature. Heating the reaction gasses to the preheat temperature decreases the temperature change the reaction gasses will make as they enter reaction vessel <NUM>. This keeps the temperature of reaction vessel <NUM> more stable. Second heater 26b is located near reaction vessel <NUM> to heat reaction vessel <NUM> and preform <NUM>. Heater 26b heats reaction vessel <NUM> and preform <NUM> to an operating temperature necessary for the CVI process or the CVD process. The CVI process and the CVD process require very high temperatures to effectively densify preform <NUM>. First heater 26a and second heater 26b can be integrated with gas source tanks <NUM> and reaction vessel <NUM>, respectively, or can be separate.

After reaction vessel <NUM> is at the operating temperature and the operating pressure, the precursor materials flow from gas source tanks <NUM> through flow meter <NUM> and into reaction vessel <NUM>. The precursor materials are controlled by flow meter <NUM> so the precursor materials are introduced at a specified ratio into reaction vessel <NUM>. The precursor materials diffuse to the surface of preform <NUM>. The precursor materials will then absorb into preform <NUM>. In the CVI process, the precursor materials absorb into the pores of preform <NUM>. In the CVD process, the precursor materials absorb into the roughened surface of preform <NUM>. Absorption is important to ensure the precursor materials have sufficient contact with preform <NUM> to react. During the CVI process, the precursor materials adsorb onto and react with the fibers of preform <NUM>. This densifies preform <NUM> with silicon carbide to create the finished CMC product. During the CVD process, the precursor materials adsorb onto and react with the roughened surface of preform <NUM>. This creates a silicon carbide coating on preform <NUM>. Both the CVI process and the CVD process require desorption of extraneous gasses such as off gasses, reaction by-products, and unspent precursor materials to control the reactions. Vacuum pump <NUM> encourages desorption. Using vacuum pump <NUM> to help with desorption of the extraneous gasses increases the porosity of preform <NUM>. An increased porosity on preform <NUM> allows for diffusion, absorption, and reaction to continually occur on preform <NUM> throughout the CVI process or the CVD process. Desorption also keeps the concentrations of the precursor materials in the specified ration within reaction vessel <NUM>. Controlling the CVI process helps eliminate deposition of free silicon on the surface of preform <NUM>. Silicon carbide will deposit in a ratio of one silicon atom to one carbon atom. This increases the quality of the finished product during a controlled CVI or CVD reaction.

<FIG> is a flow chart showing the steps of process <NUM> to reduce colorization on the surface of CMC products formed using a CVI process or a CVD process. Process <NUM>, shown in <FIG>, includes step <NUM>, step <NUM>, step <NUM>, step <NUM>, step <NUM>, step <NUM>, and step <NUM>. Process <NUM> will be explained in relation to system <NUM> of <FIG>.

Step <NUM> requires placing preform <NUM> into reaction vessel <NUM>. For a CVI process, preform <NUM> is a porous silicon carbide fiber or carbon fiber material. One way to create preform <NUM> is by weaving silicon carbide fibers, carbon fibers, or a combination of the two fibers to create a fabric. Further, other suitable ceramic fibers can be used. For a CVD process, preform <NUM> is made of a material that can have a silicon carbide coating such as carbon, silicon carbide, metal, metal alloys and glass. Preform <NUM> will have a rough exterior surface to increase the surface area that can react during the CVD process. For either the CVI process or the CVD process, preform <NUM> is generally shaped like a finished product. For instance, if the finished product is a brake disk, preform <NUM> will be shaped like a brake disk. Reaction vessel <NUM> is a vessel suitable for process <NUM>. Reaction vessel <NUM> must be able to maintain the temperature and pressure required for process <NUM>. Reaction vessel <NUM> must also have exit and entrance points, such as exit <NUM> and entrance <NUM>, for gasses to flow out of and into reaction vessel <NUM>, respectively.

Step <NUM> requires removing air from reaction vessel <NUM> and backfilling reaction vessel <NUM> with an inert gas to an operating pressure. Removing the air from reaction vessel <NUM> can be accomplished by vacuum pump <NUM>. Ambient air enters reaction vessel <NUM> during step <NUM>. The air that enters reaction vessel <NUM> during step <NUM> includes oxidizing agents like oxygen, which reduce the effectiveness of chemical reactions that take place during process <NUM>. Removing air from reaction vessel <NUM> with vacuum pump <NUM> through exit <NUM> and backfilling reaction vessel <NUM> with a backfilling gas lowers the concentration of oxidizing agents in reaction vessel <NUM> to negligible levels and replaces the air with the backfilling gas. The backfilling gas used in step <NUM> is an inert gas which will not react with the silicon carbide of preform <NUM> or densifying layers created by process <NUM>. The backfilling gas is chosen from a group of gasses comprising nitrogen and argon. The backfilling gas can be stored in gas storage tanks <NUM>. For example, the backfilling gas could be stored in first tank 22a. To enter reaction vessel <NUM>, the backfilling gas flows from first tank 22a through flow meter <NUM>. Flow meter <NUM> controls the flow rate of the backfilling gas and the pressure of the backfilling gas entering reaction vessel <NUM>. From flow meter <NUM>, the inert gas can flow into entrance <NUM> of reaction vessel <NUM>. Step <NUM> creates a requisite operating pressure for process <NUM> within reaction vessel <NUM>. Process <NUM> requires the operating pressure to be lower than atmospheric pressure. The operating pressure for process <NUM> is between <NUM> torr and <NUM> torr. The operating pressure is optimally between <NUM> torr and <NUM> torr for process <NUM>.

Step <NUM> can be repeated multiple times to remove as much of the air as possible from reaction vessel <NUM>. Process <NUM> needs to be done without oxidizing agents because oxidizing agents will oxidize the surface of the fibers of preform <NUM> and the layers of densified silicon carbide that build during process <NUM>, reducing the quality of the finished CMC material. Oxidation on the surface of preform <NUM> and the layers of densified silicon carbide decreases the mechanical, thermal, and chemical properties of the finished product.

Step <NUM> requires heating reaction vessel <NUM> and preform <NUM> to an operating temperature. Process <NUM> requires an operating temperature between <NUM> and <NUM>. Heater 26b heats reaction vessel <NUM> and preform <NUM> to the operating temperature. Heating reaction vessel <NUM> to the operating temperature prepares preform <NUM> for the chemical reaction that takes place during process <NUM>. Heating reaction vessel <NUM> also allows for the precursor materials and the carrier gas to warm to the operating temperature when entering reaction vessel <NUM> through entrance <NUM>.

Step <NUM> requires heating precursor materials and a carrier gas to a preheat temperature. The precursor materials are those that will produce the silicon carbide layers in and on preform <NUM> during process <NUM>. The precursor materials include hydrogen gas and methyltrichlorosilane (MTS). The precursor materials can be stored in source tanks <NUM>. For example, a first precursor, such as hydrogen, could be in source tank 22b and a second precursor material, such as MTS, could be in source tank 22c. The carrier gas is a gas used to maintain the proper concentrations of precursor materials flowing into vessel <NUM> and help create the necessary operating pressure within reaction vessel <NUM> during process <NUM>. The carrier gas should be a nonreactive gas, such as nitrogen or argon. The carrier gas can also be stored in source tanks <NUM>. For example, the carrier gas could be stored in fourth tank 22d. The carrier gas can also be the same as the backfilling gas in step <NUM>. Heater 26a heats source tanks <NUM> to the preheat temperature. The preheat temperature is a temperature above room temperature but below the operating temperature, such as <NUM>. The preheat temperature needs to be less than the operating temperature to avoid the precursor materials reacting before entering reaction vessel <NUM> and contacting preform <NUM>. Increasing the temperature of the precursor materials and the carrier gas to the preheat temperature reduces the change in temperature undergone as the carrier gas and precursor materials enter reaction vessel <NUM>. This stabilizes the temperature of reaction vessel <NUM> and preform <NUM> throughout process <NUM>. Step <NUM>, step <NUM>, and step <NUM> may be performed in any order or at the same time.

Step <NUM> requires introducing the carrier gas and the precursor materials to reaction vessel <NUM> in a specified ratio for a first time interval, wherein the precursor materials are in a specified ratio. The precursor materials flow from second tank 22b and third tank 22c to flow meter <NUM>. Flow meter <NUM> regulates the relative amounts of the first precursor material (hydrogen) and the second precursor material (MTS) to mix the precursor materials in the specified ratio. A carrier gas from fourth tank 22d can also flow to flow meter <NUM> with the precursor materials. The precursor materials in the specified ratio and the carrier gas will then flow into reaction vessel <NUM>. Once in reaction vessel <NUM>, the precursor materials diffuse around and into preform <NUM>. During CVI, the reaction materials will diffuse between the fibers and around all of the surfaces of preform <NUM>. During CVD, the reaction materials will diffuse around all of the surfaces of preform <NUM>. The specified ratio is a volumetric ratio of hydrogen to MTS that is between <NUM> parts hydrogen to <NUM> part MTS (<NUM>:<NUM>) and <NUM> parts hydrogen to <NUM> part MTS (<NUM>:<NUM>), where the flow rate of hydrogen is <NUM>-<NUM> times the flow rate of MTS. Preferably, the specified ratio should be between <NUM> parts hydrogen to <NUM> part MTS (<NUM>:<NUM>) and <NUM> parts hydrogen to <NUM> part MTS (<NUM>:<NUM>), where the flow rate of hydrogen is <NUM>-<NUM> times the flow rate of MTS. More preferably, the specified ratio should be <NUM> parts hydrogen to one part MTS (<NUM>:<NUM>). The specified ratio of hydrogen to MTS densifies preform <NUM> by forming a silicon carbide layer that is stoichiometrically equivalent to the chemistry of silicon carbide: one atom of silicon is deposited for every atom of carbon deposited.

Step <NUM> lasts a first time interval. The first time interval is an amount of time that will allow for CVI or CVD to occur without an extra buildup of hydrogen gas. During the first time interval, the pores of preform <NUM> undergoing CVI will absorb the precursor materials. The roughened surface of preform <NUM> undergoing CVD will absorb the precursor materials. Absorption is important because it ensures the precursor materials interact with the fibers or surfaces of preform <NUM> to form silicon carbide. The first time interval is a window that allows time for the precursor materials to diffuse around, absorb into, and react with preform <NUM>. The first time interval is likely long because the reaction between preform <NUM> and the precursor materials to form silicon carbide is slow.

Step <NUM> requires removing off gasses, the precursor materials that are unspent, and the carrier gas from reaction vessel <NUM> to maintain the specified ratio of the precursor materials in reaction vessel <NUM> during the first time interval. As process <NUM> occurs, a silicon carbide layer will be formed on the surfaces of the fibers of preform <NUM> due to the reaction between preform <NUM> and the precursor materials. Off gasses are formed as byproducts of the reaction. The off gasses need to be removed from inside the pores of preform <NUM> and within reaction vessel <NUM> to continue the flow of the precursor materials into reaction vessel <NUM> and into and around preform <NUM>. Further, the off gasses, unspent precursor materials, and excess carrier gas remaining in reaction vessel <NUM> change the ratio of gasses inside reaction vessel <NUM> from the specified ratio. The off gasses, unspent precursor materials, and excess carrier gas are removed from reaction vessel <NUM> through exit <NUM> by vacuum pump <NUM>. This removal continues throughout the first time interval.

Step <NUM> requires stopping introduction of the precursor materials while removing excess hydrogen, off gasses, and unspent precursor materials from the reaction vessel for a second time interval. During process <NUM>, levels of hydrogen can rise in reaction vessel <NUM>. This creates conditions in reaction vessel <NUM> outside of the specified ratio. Using vacuum pump <NUM> to remove excess hydrogen, off gasses, and unspent precursor material, without introducing the reaction materials, helps maintain the specified ratio during process <NUM>. Step <NUM> may not be necessary for all CVI or CVD processes because the specified ratio may be maintained for shorter processes. Longer CVD processes and most CVI processes will likely require step <NUM>. Step <NUM> helps with desorption and increases the porosity of preform <NUM> so the precursor materials can continue reacting. After the second time interval, step <NUM>, step <NUM>, and step <NUM> may be repeated so process <NUM> is the appropriate time for the amount of deposition needed while also maintaining the specified ratio in reaction vessel <NUM>.

Process <NUM> minimizes colorization on the CMC material end products of silicon carbide CVI and CVD reactions. Colorization on silicon carbide CMC material end products is a yellow or multi-colored sheen on the surfaces of the end products. This yellow or multi-colored sheen is caused by excess free silicon buildup on fibers of preform <NUM> during a CVI reaction. Microscopically, this excess free silicon buildup creates pearlizations on the surfaces or fibers of preform <NUM>. The pearlizations are bumpy, flakey, or fluffy depositions on the surfaces or fibers of preform <NUM>. The excess free silicon buildup increases the chance of oxidation of the CMC material end product. This increased oxidation decreases the thermal shielding properties of the finished CVI or CVD process. A finished CVI product with pearlizations and excess free silicon buildup has reduced mechanical properties because the friction coefficient is lower than it would be without the excess silicon formations. Process <NUM> minimizes excess silicon buildup by keeping the precursor materials in the specified ratio in the localized area around preform <NUM> throughout the CVI reaction. Carefully controlling the specified ratio of the precursor materials creates densification in preform <NUM> close to the stoichiometric coefficient of silicon carbide where one atom of silicon is deposited for every atom of carbon. Depositing in the stoichiometric coefficient ratio of <NUM>:<NUM> carbon to silicon eliminates excess free carbon deposits. Densifying preform <NUM> with the correct stoichiometric ratio decreases pearlizations and the excess free silicon buildup thereby ridding the end product of colorization.

A method of depositing silicon carbide on a preform to form a ceramic matrix composite. The method includes placing the preform into a reaction vessel, removing air from the reaction vessel and backfilling the reaction vessel with an inert gas, heating the reaction vessel and the perform to an operating temperature, heating the carrier gas and precursor materials to a preheat temperature outside of the reaction vessel. The method further includes introducing the carrier gas and the precursor materials to the reaction vessel, wherein the precursor materials are introduced in a specific ratio and removing off gasses, the precursor materials that are unspent, and the carrier gas from the reaction vessel to maintain the specified ratio of the precursor materials in the reaction vessel.

The method of the preceding paragraph can optionally include, additionally and/or alternatively, any one or more of the following features, configurations and/or additional components:
The method further includes depositing a layer of silicon carbide on the preform with a chemical ratio of <NUM> atom of silicon to <NUM> atom of carbon.

The method further includes stopping the flow of the precursor materials to the reaction vessel while removing the precursor materials that are unspent from the reaction vessel.

The preform is made of a solid material that can have a silicon carbide coating selected from the group consisting of carbon, silicon carbide, metal, metal alloys, and glass.

The preform is a fiber preform made of fibers selected from the group consisting of silicon carbide fibers, carbon fibers, and combinations thereof.

The precursor materials comprise hydrogen and methyltrichlorosilane, and wherein the specified ratio of the hydrogen to the methyltrichlorosilane is a volumetric ratio between <NUM> parts hydrogen to <NUM> part methyltrichlorosilane (<NUM>:<NUM>) and <NUM> parts hydrogen to <NUM> part methyltrichlorosilane (<NUM>:<NUM>), is preferably between <NUM> parts hydrogen to <NUM> part methyltrichlorosilane (<NUM>:<NUM>) and <NUM> parts hydrogen to <NUM> part methyltrichlorosilane (<NUM>:<NUM>), and is preferably <NUM> parts hydrogen to <NUM> part methyltrichlorosilane (<NUM>:<NUM>).

The carrier gas and the inert gas are selected from the group consisting of nitrogen, argon, and combinations thereof.

The operating temperature is between <NUM> and <NUM>.

The operating pressure is between <NUM> torr and <NUM> torr and is preferably between <NUM> torr and <NUM> torr.

A method of depositing silicon carbide on a preform to form a ceramic matrix composite. The method includes placing the preform into a reaction vessel, filling the reaction vessel with an inert gas to reach an operating pressure, and heating the reaction vessel and the preform to an operating temperature. The method further includes introducing hydrogen and methyltrichlorosilane to the reaction vessel in a specified ratio. The specified ratio of the hydrogen to the methyltrichlorosilane is a volumetric ratio between <NUM> parts hydrogen to <NUM> part methyltrichlorosilane (<NUM>:<NUM>) and <NUM> parts hydrogen to <NUM> part methyltrichlorosilane (<NUM>:<NUM>).

The method further includes introducing a carrier gas to the reaction vessel with the hydrogen and the methyltrichlorosilane.

The method further includes removing off gasses, the hydrogen and the methyltrichlorosilane that are unspent, and the carrier gas from the reaction vessel to maintain the specified ratio of the hydrogen to the methyltrichlorosilane in the reaction vessel.

The method further includes stopping the flow of the hydrogen and the methyltrichlorosilane to the reaction vessel while removing the hydrogen and the methyltrichlorosilane that are unspent from the reaction vessel.

The specified ratio of the hydrogen to the methyltrichlorosilane is a volumetric ratio between <NUM> parts hydrogen to <NUM> part methyltrichlorosilane (<NUM>:<NUM>) and <NUM> parts hydrogen to <NUM> part methyltrichlorosilane (<NUM>:<NUM>), and is preferably <NUM> parts hydrogen to <NUM> part methyltrichlorosilane (<NUM>:<NUM>).

A method of depositing silicon carbide on a preform to form a ceramic matrix composite. The method includes placing the preform into a reaction vessel and then removing air from the reaction vessel and backfilling the reaction vessel with an inert gas to reach an operating pressure. Precursor materials are introduced to the reaction vessel in a specified ratio for a first time interval. The flow of the precursor materials to the reaction vessel is stopped while the precursor materials that are unspent are removed from the reaction vessel for a second time interval. The precursor materials are introduced to the reaction vessel for a third time interval.

The method further includes introducing a carrier gas to the reaction vessel with the precursor materials.

The method further includes removing off gasses, the precursor materials that are unspent, and the carrier gas from the reaction vessel to maintain the specified ratio of the precursor materials in the reaction vessel during the first time interval.

The method further includes removing off gasses, the precursor materials that are unspent, and the carrier gas from the reaction vessel to maintain the specified ratio of the precursor materials in the reaction vessel during the third time interval.

Claim 1:
A method of depositing silicon carbide on a preform to form a ceramic matrix composite, the method comprising:
placing (<NUM>) the preform (<NUM>) into a reaction vessel (<NUM>);
removing (<NUM>) air from the reaction vessel and backfilling the reaction vessel with an inert gas to an operating pressure;
heating (<NUM>) the reaction vessel and the preform to an operating temperature;
heating (<NUM>) a carrier gas and precursor materials to a preheat temperature outside of the reaction vessel;
introducing (<NUM>) the carrier gas and the precursor materials to the reaction vessel, wherein the precursor materials are introduced in a specified ratio; and
removing (<NUM>) off gasses, the precursor materials that are unspent, and the carrier gas from the reaction vessel to maintain the specified ratio of the precursor materials in the reaction vessel;
characterized by:
introducing hydrogen and methyltrichlorosilane to the reaction vessel in a specified ratio, wherein the specified ratio of the hydrogen to the methyltrichlorosilane is a volumetric ratio between <NUM> parts hydrogen to <NUM> part methyltrichlorosilane and <NUM> parts hydrogen to <NUM> part methyltrichlorosilane.