Coating processes – Vacuum utilized prior to or during coating – Metal base
Reexamination Certificate
1999-03-05
2002-06-11
Bareford, Katherine A. (Department: 1762)
Coating processes
Vacuum utilized prior to or during coating
Metal base
C427S294000, C427S397700, C427S427000, C427S443200, C264S101000, C264S102000, C264S279000, C264S682000
Reexamination Certificate
active
06403158
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a method for infiltrating a porous body of a material with silicon.
Lamellar and fibrillar materials are commonly introduced by melt infiltration to act as fillers and reinforcements in various composite materials. Silicon melt infiltration into a porous body containing silicon carbide (SiC) and/or carbon (C) is a common method for fabricating reaction bonded silicon carbide, or “siliconized” silicon carbide composite products. Examples of such composite products are silicon/silicon carbide (Si/SiC) ceramics and toughened ceramic matrix composites. An advantage of the silicon melt infiltration method is that it produces a product in near net shape, meaning that little or no change in preform dimension occurs during the infiltration/densification method. A near net shape method permits fabrication of products that require minimum machining.
An important aspect of the melt infiltration method is the process by which the silicon is brought into contact with the porous preform body. The silicon can be placed directly into contact with the preform. In another process, a carbon fiber wick is used to transport liquid silicon by capillary action from a resevoir to the material being infiltrated. These processes have several drawbacks. In the case of direct contact of preform and silicon, achieving complete infiltration of the body without leaving excess silicon on the body surface requires very precise metering of the amount of silicon. Bodies prepared by different techniques or with different ratios of starting materials require different levels of silicon for full infiltration. Each process change requires adjustment by trial and error of the amount of silicon. If a silicon-boron alloy is used for the infiltration, boron silicides (such as B
3
Si, B
4
Si and B
6
Si) are formed during melting. These boron silicides do not fully dissolve in the alloy during the infiltration process. Consequently an adherent residue of these borides (and other impurities) is left on the surface of the body that must be removed by machining. If a wick is used, a residue is left on the wick rather than on the infiltrated body. However, the wicks are strongly adherent to the infiltrated body and require machining for removal. Also, problems associated with adjusting the silicon level are made worse since now any variations in the size of the wick itself need to be taken into account. The use of wicks also limits the number of locations where the silicon can be introduced to the body. If the body is large or complex in shape, the silicon may be infiltrated in a non-uniform manner that results in composition and property variations.
Another problem with silicon melt infiltration is caused by the fact that silicon expands during freezing. As the silicon expands, excess silicon is pushed out to the body surface to form bumps of silicon metal. The bumps of silicon cause the body to fall out of tolerance, again requiring expensive machining after infiltration.
Thus, there is a need to provide a silicon melt filtration method that does not result in composition and property variations and that does not require expensive machining of the infiltrated composite product.
SUMMARY OF THE INVENTION
The invention is a method for infiltrating a body with silicon. A mixture is formed that comprises silicon and at least some to about 10 weight % hexagonal boron nitride. A body comprising a component that is wetted by or reacts with silicon is contacted with the powder mixture and the contacted body is infiltrated with silicon from the mixture. The phrase “at least some to about 10 weight %” includes as little as about 0.1 weight % hexagonal boron nitride, further includes about 1.0 weight % to about 10 weight % hexagonal boron nitride, and yet includes about 4 weight % to about 10 weight % hexagonal boron nitride.
In another aspect, the invention relates to a method for infiltrating a body with silicon, comprising coating a body comprising a component that is wetted by or reacts with silicon with a slurry of hexagonal boron nitride and silicon. The coated body is then infiltrated with silicon from the slurry.
DETAILED DESCRIPTION OF THE INVENTION
A porous body can be infiltrated with silicon by embedding the body in a mixture of silicon powder and a controlled amount of hexagonal boron nitride powder. Morelock U.S. Pat. No. 4,737,328 discloses a process for producing a composite by embedding a porous body comprising a substance which reacts with silicon in a powder mixture composed of silicon and hexagonal boron nitride powder. The Morelock patent teaches limits for the silicon-boron nitride (Si—BN) mixture of 10% to 90% of silicon by volume (equivalent to 90.6 wt % to 9.6 wt % boron nitride (BN). Presently, it has been found that compacted the silicon-boron nitride Si—BN mixtures containing 50% to 10% by weight of boron nitride (BN) do not result in infiltration of silicon (Si) into silicon carbide/carbon (SiC/C) bodies. The silicon infiltrates only when the boron nitride (BN) level is controlled at levels below about 10%.
Additionally, the Morelock patent specifies a method of contacting the porous preform that uses a dry compaction within a mold or die. According to the present invention, an aqueous slurry of the silicon-boron nitride (Si—BN) can be used to coat a preform to effect infiltration.
The present invention provides an improved method of performing silicon melt infiltration, which provides a clean body surface free of excess silicon metal. The silicon infiltrant in the form of a powder, is mixed with at least some to less than about 10 wt % boron nitride powder. The hexagonal boron nitride (BN) can be present in an amount less than about 9.6 wt %. In a preferred embodiment, the boron nitride powder is present in an amount from about 1 wt % to about 9.5 wt % and most preferred in an amount from about 5 wt % to about 9.0 wt % of the mixture. The silicon can include other alloying additives such as boron (B) but should be present in the powder mixture at least in an amount sufficient to produce the desired composite. Preferably the silicon (Si) is present in a range from about 99 wt % to about 80 wt % of the total mixture.
The mixture can be formed by a number of techniques. For example, the two powders can be simply mixed together. At least a significantly uniform mixture of the two powders is formed and preferably a uniform or substantially uniform mixture is formed. The mixture is then used as the source of silicon for melt infiltration. In another aspect of the invention, the mixture is mixed with water to form a slurry, which is dip coated or spray coated onto a preform body. The body is then subjected to an infiltration heat treatment wherein the silicon melts and infiltrates into the porous body. The vacuum pressure, furnace temperature, and time at temperature for infiltration are the same, or substantially the same, as used in the techniques of the Morelock patent, which is incorporated herein by reference in its entirety.
In all aspects of the invention, excess silicon in the infiltrant mix or silicon that is pushed out to the body surface during freezing, remains on the surface mixed with the boron nitride (BN) powder. After infiltration, any material remaining on the surface of the body is a friable boron nitride (BN) mixture that can be easily scraped away. Additionally, the spray and dip coating embodiments of coating the preform body facilitate introduction of silicon over an entire surface of the preform to provide a uniform and faster infiltration.
The silicon powder can range widely in size but preferably should not be greater than about 100 mesh, i.e. no greater than about 150 microns. Mesh herein means U.S. Sieve Size. Larger particles have a tendency to coalesce and not infiltrate the body. Preferably, the silicon powder has a particle size of about 200 mesh, i.e. no greater than about 75 microns. The hexagonal boron nitride (BN) powder can range in size but preferably is not greater than about 100 mesh, i.e. no greater than about 150 micr
Bareford Katherine A.
Cabou Christian G.
General Electric Company
Johnson Noreen C.
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