Compositions: ceramic – Ceramic compositions – Carbide or oxycarbide containing
Reexamination Certificate
1998-01-14
2001-04-17
Marcantoni, Paul (Department: 1755)
Compositions: ceramic
Ceramic compositions
Carbide or oxycarbide containing
C501S120000, C501S127000, C501S128000
Reexamination Certificate
active
06218324
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to the field of continuous fiber ceramic composites (CFCC) and, in particular, to new and useful ceramic composites with tungstate, molybdate, tantalate, and niobate interphases in the form ABO
4
providing weak interfaces with reinforcement and/or matrix phases so that the ceramic composites exhibit improved toughness and resistance to high temperature oxidizing environments. For tungstate and molybdate compounds of the general formula ABO
4
, B represents tungsten or molybdenum, and A represents a divalent cation. For tantalate and niobate compounds of the general formula ABO
4
, B represents tantalum or niobium, and A represents a trivalent lanthanide series cation.
BACKGROUND OF THE INVENTION
A known method for increasing the toughness of a ceramic body is to incorporate weak interphases or interfaces into the material. As an example, it is common to reinforce a ceramic matrix with continuous fibers which are coated with a thin layer of either boron nitride or carbon. The carbon or boron nitride creates a weak interface between the matrix and the fiber which allows sliding between the fiber and the matrix and/or causes propagating cracks to be deflected along the interface or within the interphase. These events allow the reinforcing fibers to remain intact and continue to reinforce the ceramic body and to arrest additional crack propagation. Similar interfacial failures can occur in particulate and whisker reinforced materials which contain weak interfaces. In these instances microcracking and crack deflection characteristics result in toughening of the ceramic body. In multilayered laminar composites, multiple parallel interfaces may fracture, thus increasing the work of fracture of the entire ceramic body.
Typical approaches to creating a weak interface between reinforcing phases and matrix phases incorporate interphase compounds which are characterized by layered crystal structures containing a crystal plane exhibiting weak shear properties. This characteristic promotes interfacial debonding and fiber pullout toughening (i.e., fiber frictional sliding) mechanisms. Materials commonly used include carbon, boron nitride, micaceous materials such as fluorphlogophite such as disclosed by U.S. Pat. No. 4,935,387 to Beall et al., and beta-alumina/magnetoplumbite compounds such as disclosed by U.S. Pat. No. 5,137,852 to Morgan et al.
Since many ceramic composites are used in high-temperature oxidizing environments (typically at temperatures exceeding 1100° C.), the reinforcement/matrix interphase must be oxidation resistant and stable thermodynamically with both the reinforcement and the matrix phase. Carbon and boron nitride materials oxidize readily and are unsuitable for use in composites requiring long-term service at temperatures greater than 600° C. In addition, boron nitride is rapidly degraded by water vapor making its use as an interphase material in combustion atmospheres, and other water vapor laden environments, very limited. Fluorphlogophite and beta-alumina/magnetoplumbite compounds have a tendency to react with many of the continuous fiber reinforcements currently commercially available.
Some non-layered oxide compounds have been studied as interphase in continuous fiber ceramic composites. These include tin oxide, zirconium oxide, and zirconium tin titanate. These compounds were considered for alumina reinforced composites because of the lack of reactivity between these interphase compounds and alumina. It was determined that these compounds work well as reaction barriers between matrix and reinforcement, but they do not provide the weak interphase or interface needed to cause crack deflection and fiber pullout toughening.
Monazite and xenotime compounds, phosphates with the general formula APO
4
, in which A represents trivalent rare earth elements of the lanthanide series, are non-layered crystal compounds which are disclosed for creating weak interfaces in ceramic composites by U.S. Pat. No. 5,514,474 to Morgan et al. These materials appear to provide a weak interface, rather than a weak interphase, by exhibiting high interfacial energies (low bonding) with possible reinforcements and matrices. Although some promising preliminary results have been demonstrated with monazite and xenotime in model composite systems, incorporation of these interfaces into actual continuous fiber reinforced ceramic composites has yielded mixed results.
SUMMARY OF THE INVENTION
There is a need for additional non-layered ceramic materials which create weak interfaces with typical composite reinforcing and matrix phases. It is desirable to have available other non-layered ceramics which can create even weaker interfaces than achievable with monazite and xenotime type of compounds. Since a greater number of candidate interphase materials are available to choose from to create weak interfaces within ceramic composites, the possible selection of reinforcing and matrix compositions which can be chosen from which to engineer high toughness composite systems is increased. Ceramic composites are often used in corrosive environments. Increasing the variety of ceramic composite systems which are available to operate in high temperature oxidizing environments, for example, gives the end users of these materials a greater chance of finding a composite system to withstand their specific corrosive environment.
It is thus an object of the present invention to provide an increased range of ceramic composite systems having improved toughness and resistance to stressful or corrosive environments.
It is a further object of the invention to provide new groups of fiber coatings for continuous fiber ceramic composites having weak interfaces or interphases for improving the durability characteristics of the composite.
Accordingly, one aspect of the present invention is drawn to a ceramic composite comprising a ceramic matrix and a material having the general formula ABO
4
, where A is a divalent cation and B is one of tungsten and molybdenum, dispersed within the ceramic matrix, to create a weak interface between the ceramic matrix and the material. This ceramic composite, if desired, may further comprise a reinforcement phase surrounded by the ABO
4
material imbedded in the ceramic matrix, and which can comprise one of continuous fibers, discrete particulates, laminae, and whiskers.
Another aspect of the present invention is drawn to a ceramic composite comprising: a ceramic matrix and a material having the general formula ABO
4
, where A is a trivalent lanthanide series cation and B is one of tantalum and niobium, dispersed within the ceramic matrix, to create a weak interface between the ceramic matrix and the material. Again, this ceramic composite, if desired, may further comprise a reinforcement phase surrounded by the ABO
4
material imbedded in the ceramic matrix, and which can comprise one of continuous fibers, discrete particulates, laminae, and whiskers.
Accordingly, ceramic composites are provided containing phases of tungstates and molybdates. The tungstate and molybdate compounds have the general formula ABO
4
, where A is a divalent cation and B is tungsten or molybdenum. The specific tungstates and molybdates are compounds from the Scheelite (CaWO
4
), Powellite (CaMoO
4
), Ferberite (FeWO
4
), Hueberite (MnWO
4
), Wulfenite (PbMoO
4
), Stolzite (PbWO
4
), and Sanmartinite (ZnWO
4
) mineral groups. Tungstate and molybdate compounds composed of the alkaline earth elements consisting of Mg, Ca, Sr, and Ba are all considered to be within the scope of this invention, as are also the molybdate analogs to Ferberite, Hueberite, and SanmartiNite. These composites are stable in oxidizing environments to temperatures which vary according to the melting temperature of the specific tungstate or molybdate. In general, the ABO
4
tungstates will yield ceramic composites with higher use temperatures than are achieved with composites containing ABO
4
molybdates. Composites containing CaWO
4
interphases yield the highest temperature compos
Baraona R. C.
Edwards R. J.
Marcantoni Paul
Marich Eric
McDermott Technology Inc.
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