Stock material or miscellaneous articles – Sheet – web – or layer weakened to permit separation through...
Reexamination Certificate
1997-10-27
2001-05-08
Le, H. Thi (Department: 1773)
Stock material or miscellaneous articles
Sheet, web, or layer weakened to permit separation through...
C428S403000, C428S549000, C428S552000, C428S610000, C428S698000, C428S699000, C428S701000, C428S702000, C428S704000
Reexamination Certificate
active
06228453
ABSTRACT:
TECHNICAL FIELD
The present invention generally relates to filler materials which are adapted for use as the reinforcement phases(s) in composite bodies. Coated ceramic filler materials comprised of ceramic particles, fibers, whiskers, etc. having at least two substantially continuous coatings thereon are provided. The coatings are selected so that the interfacial shear strength between the ceramic filler material and the first coating, between coatings, or between the outer coating and the surrounding matrix material, are not equal so as to permit debonding and pull-out when fracture occurs. The resultant, multicoated ceramic filler materials may be employed to provide composites, especially ceramic matrix composites with increased fracture toughness. The ceramic filler materials are designed to be particularly compatible with ceramic matrices formed by directed oxidation of precursor metals, but such ceramic filler materials are also adaptable for use in many other composite material systems.
The present invention also relates to techniques for increasing the corrosion resistance of composite materials, particularly of ceramic fiber reinforced composites exposed to oxygen and water vapor at elevated temperatures. One approach to inhibiting corrosion in a ceramic matrix composite body is to reduce the number and/or size of microcracks in the body, thereby reducing access of corrodants to the interior of the body. Another broad approach is to provide chemical additives to the body which are capable of gettering a corrodant or interfering with its corrosion mechanisms.
BACKGROUND ART
A ceramic composite is a heterogeneous material or article comprising a ceramic matrix and filler such as ceramic particles, fibers or whiskers, which are intimately combined to achieve desired properties. These composites are produced by such conventional methods as hot pressing, cold pressing and firing, hot isostatic pressing, and the like. However, these composites typically do not exhibit a sufficiently high fracture toughness to allow for use in very high stress environments such as those encountered by gas turbine engine blades.
A novel and useful method for producing self-supporting ceramic composites by the directed oxidation of a molten precursor metal is disclosed in Commonly Owned U.S. Pat. No. 4,851,375, which issued on Jul. 25, 1989, described below in greater detail. However, the processing environment is relatively severe, and there is a need, therefore, to protect certain fillers from the strong oxidation environment. Also, certain fillers may be reduced at least partially by molten metal, and therefore, it may be desirable to protect the filler from this local reducing environment. Further, the protective means should be conducive to the metal oxidation process, yet not degrade the properties of the resulting composite, and even more desirably provide enhancement to the properties. Still further, in some instances it may be desirable for the means or mechanisms for protecting the filler during matrix or composite formation to also protect the fillers against undesirable attack of oxidants diffusing through the matrix during actual service of the composite.
It is known in the art that certain types of ceramic fillers serve as reinforcing materials for ceramic composites, and the selection or choice of fillers can influence the mechanical properties of the composite. For example, the fracture toughness of the composite can be increased by incorporating certain high strength filler materials, such as fibers or whiskers, into the ceramic matrix. When a fracture initiates in the matrix, the filler at least partially debonds from the matrix and spans the fracture, thereby resisting or impeding the progress of the fracture through the matrix. Upon the application of additional stress, the fracture propagates through the matrix, and the filler begins to fracture in a plane different from that of the matrix, pulling out of the matrix and absorbing energy in the process. Pull-out is believed to increase certain mechanical properties such as work-of-fracture by releasing the stored elastic strain energy in a controlled manner through friction generated between the material and the surrounding matrix.
Debonding and pull-out have been achieved in the prior art by applying a suitable coating to the ceramic filler material. The coating is selected so as to have a lower bonding strength with the surrounding matrix than the filler, per se, would have with the matrix. For example, a boron nitride coating on silicon carbide fibers has been found to be useful to enhance pull-out of the fibers. Representative boron nitride coatings on fibers are disclosed in U.S. Pat. No. 4,642,271, which issued on Feb. 10, 1987, in the name of Roy W. Rice, and are further disclosed in U.S. Pat. No. 5,026,604, which issued on Jun. 25, 1991, in the name of Jacques Thebault. However, the use of boron nitride coated fibers in composites may present significant processing disadvantages. For example, the production of ceramic matrix composites containing boron nitride coated materials requires the use of reducing atmospheres since a thin layer of boron nitride readily oxidizes (e.g., converts to boron oxide in an oxygen-containing atmosphere) at temperatures above 800-900° C. A reducing atmosphere, however, may often times not be compatible with the directed oxidation of molten parent metal for fabricating ceramic composites. Further, in the directed oxidation process the coating desirably is compatible with the molten metal in that the molten metal wets the coated filler under the process conditions, for otherwise the oxidation process and matrix growth may be impeded by the filler.
Another drawback of boron nitride is that, upon oxidation, the boria reaction product can dissolve or further react with water to form boric acid, which can be a vapor under the local oxidizing conditions. Thus, the boria is not a passive layer, but can be continually removed through volatilization. U.S. Pat. No. 5,593,728 to Moore et al. addresses this shortcoming of boron nitride. Specifically, by producing a pyrolytic BN coating containing from 2 to 42 wt % silicon, with substantially no free silicon present, Moore et al. observe greatly reduced rates of oxidative weight loss. The coating is formed by CVD using reactant vapors of ammonia and a gaseous source of both boron and silicon. The gases are flowed into a reaction chamber between a temperature of 1300° C. and 1750° C. and within a pressure range of 0.1 Torr to 1.5 Torr.
It is not clear, however, whether the modified BN layer of Moore et al. permits molten parent metal to wet the coating (for infiltration) and yet resist any adverse reaction therewith. Further, the modified BN coatings of Moore et al. were deposited onto single filaments. Due to the high deposition rates resulting from the deposition conditions, it is unclear whether the Moore et al. technique could be applied to coat a plurality of fibers, e.g.a stack of fabrics making up a preform.
Also, in order to prevent or minimize filler degradation, certain limits may be imposed on the conventional fabrication processes, such as using low processing temperatures or short times at processing temperature. For example, certain fillers may react with the matrix of the composite above a certain temperature. Coatings have been utilized to overcome degradation, but as explained above, the coating can limit the choice of processing conditions. In addition, the coating should be compatible with the filler and with the ceramic matrix.
A need therefore exists to provide coated ceramic filler materials which are capable of debonding and pull-out from a surrounding ceramic matrix. A further need exists to provide coated ceramic filler materials which may be incorporated into the ceramic matrix at elevated temperatures under oxidizing conditions to provide composites exhibiting improved mechanical properties such as increased fracture toughness.
In order to meet one or more of these needs, the prior art shows filler materials bearing one o
Fareed Ali Syed
Garnier John Edward
Kennedy Christopher Robin
Schiroky Gerhard Hans
Sonuparlak Birol
Lanxide Technology Company, LP
Le H. Thi
Palguta Larry J.
Ramberg Jeffrey A.
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