WO1999055638A1 - Method for making shaped monolithic ceramics - Google Patents

Abstract

The process of the present invention comprises a method for fabricating shaped monolithic ceramics and ceramic composites through displacive compensation of porosity, and ceramics and composites made thereby. The method of the present invention includes three basic steps: 1) synthesis or other acquisition of a porous preform: a porous preform with an appropriate composition, pore fraction, and overall shape is prepared or obtained. The pore fraction of the preform is tailored so that the reaction-induced increase in solid volume can compensate partially or completely for such porosity. It will be understood that the porous preform need only be sufficiently dimensionally stable to resist the capillary action of the infiltrated liquid reactant; 2) infiltration: the porous preform is infiltrated with a liquid reactant; and 3) reaction: the liquid reactant is allowed to react partially or completely with the solid preform to produce a dense, shaped body containing desired ceramic phase(s). The reaction in step 3) above is a displacement reaction of the following general type between a liquid species, M(1), and a solid preform comprising the compound, NBXC(s): AM(1) + NBXC(s) = AMXC/A(s) + BN(1/g) (2) where MXC/A(s) is a solid reaction product (X is a metalloid element, such as, for example, oxygen, nitrogen, sulfur, etc.) and N(1/g) is a fluid (liquid or gas) reaction product. A, B and C are molar coefficients.

Description

Method for making Shaped Monolithic Ceramics

This application is a continuation-in-part of U.S. Patent Application, Serial No. 09/296,138, filed April 21, 1999 and claims the benefit of U.S. Provisional Application No. 60/083,534, filed April 29, 1998, both of which are incorporated herein by reference. Background

Refractory ceramics (e.g., aluminates, aluminosilicates) can exhibit several enhanced

properties relative to refractory metals and alloys, such as corrosion resistance, high temperature stability in oxidizing atmospheres, specific strength and stiffness, creep resistance, and wear resistance. However, the brittleness of ceramic bodies renders the fabrication of shaped

components tedious and expensive. Such brittleness also necessitates the use of reinforcements to enhance damage tolerance.

Despite extensive research and development over the past few decades, continuous- ceramic-fiber-reinforced, ceramic-matrix composites have not found widespread use in high- temperature structural applications involving oxidizing atmospheres (e.g., turbine engine

applications). The use of these materials has been hampered by the inability to produce low-cost, net-shaped composites that are capable of retaining both a high strength (i.e., owing to a high fiber strength) and a high toughness (i.e., owing to sufficiently weak fiber-matrix interface for

fiber-pullout toughening) under such high-temperature, oxidizing conditions (e.g., ~ 1200°C in

high-pressure air for turbine engine applications) [1-7]. Erosion/oxidation resistance are of

particular concern in the very high-temperature (« 2000°C) corrosive conditions of liquid-fueled

rocket engines [7]. Although continued research and development of high-strength oxide fibers and oxidation-resistant interface coatings may ultimately enable the low-cost fabrication of oxide

1 fiber-reinforced ceramic composites with satisfactory damage tolerance and erosion/oxidation resistance for jet and rocket engine applications, oxide-matrix composites with other types of reinforcements should be considered in the interim.

An alternate approach for reinforcing oxide matrices is to use metallic (or intermetallic) alloys that are oxidation resistant, such as Ni-based compositions, or very high-melting, such as Nb-based compositions (Nb, Nb-Al solid solutions, and Nb₃Al melt at 2468°C, 2060-2468°C,

and 1940-2060°C, respectively [8,9]). As reinforcement materials, Ni-rich or Nb-rich solid solutions, intermetallic compounds, or mixtures thereof offer a number of attractive features.

For example, owing to their oxidation resistance at elevated temperatures, damage tolerance (e.g., yield strength, ultimate tensile strength, toughness, fatigue resistance) over a range of temperatures, and capability for being fabricated into complex shapes (e.g., by casting), Ni- based superalloys are used extensively for high-temperature components in engine applications (e.g., combustor liners and ducts, disks and blades, and nozzle vanes in turbine engines; exhaust valves in reciprocating engines; pre-combustion chambers in diesel engines) [5-7,10-13]. Aluminum additions are used to enhance the high-temperature strength of such superalloys,

owing to the formation of the ordered compound γ'-Ni₃Al as a coherent precipitate dispersed

within the Ni-rich (FCC γ phase) solid solution matrix [1,5,11,14-19]. Pure γ'-Ni₃Al and its

alloys exhibit higher yield strengths at elevated temperatures than the γ phase [11,14-19]. The γ'

compound can also act as a ductile reinforcement material in composites (at room temperature or elevated temperatures), either as a polycrystalline material with proper alloying (e.g., Al concentration < 25 at% with dopants such as boron and chromium) or in single crystal form (e.g.,

as single crystal filaments) [11,14-18]. Stoichiometric β-NiAl, while less ductile at room

temperature than Ni-based superalloys and γ'-Ni₃Al alloys, possesses a higher elastic modulus, a higher thermal conductivity, and a higher melting point (1638°C) [15,19-23]. Niobium alloys and intermetallic compounds (e.g., Nb₃Al) are very high melting and exhibit better creep

resistance than Ni₃Al, possess high thermal conductivities at elevated temperatures, and are less

dense than other refractory metals (such as W, Ta, Mo) [8,24-26].

As indicated by the discussion above, these Ni-bearing and Nb-bearing alloys offer a tailorable range of thermo-mechanical properties that can be exploited in ceramic composites. In

addition, the negative features of these alloys (i.e., weight, creep) can be significantly reduced if such alloys are used as reinforcements within composites containing a stiff, continuous, lightweight ceramic phase.

Accordingly, there remains a need for a method for the fabrication of near net-shaped ceramic composites (e.g., containing an AEA^O^bearing compound where AE = Mg, Ca, Sr, or

Ba) reinforced with metallic solid solutions (e.g., M-Al where M = Ni, Nb) and/or intermetallic compounds (e.g., Nb, NiAl+Ni₃Al, or Ni₃Al+Ni-based solid solutions) [27].

Because the ionic species in a number of oxide compounds (e.g., in AEAI₂O₄-

compounds, where AE = Mg, Ca, Sr, or Ba) have essentially fixed valence states, these oxide

compounds are thermodynamically compatible with oxygen at high temperatures. On the other hand, metal elements and/or metallic solid solutions (e.g., Nb, Ni, (Nb,Al) or (Ni,Al) and/or

intermetallic compounds (e.g., γ'-Ni₃Al) can act as ductile reinforcements at ambient and

elevated temperatures (β-NiAl also becomes ductile above ~ 400°C) [15,17,19,23]. The thermal

conductivities of metal elements, metallic solid solutions, and intermetallic compounds are also usually much higher than for oxide compounds [25]. The coefficients of thermal expansion (CTE) of most oxide compounds (e.g., AE aluminates, where AE = Mg, Ca, Sr, Ba) are smaller than for most metal elements, metallic solid solutions, and intermetallic compounds, which should place the ceramic phase in compression upon cooling from the peak processing or use temperature [9,32]. For very high-temperature applications, composites of AE aluminate and Nb

or Nb₃Al are attractive in that these phases possess similar CTE values (8.2-9.2X10"⁶/°C [9,32]).

Although the properties of ceramic/metal alloy composites (e.g., AEAI2O₄/M-AI alloy

composites with AE = Mg, Ca, Sr or Ba and M = Ni or Nb) will depend on factors such as the

amounts, sizes, and distributions of phases, and the degree of interfacial bonding, the discussion above (and in the following sections) indicates that such composites can be:

♦ lighter and more resistant to high-temperature creep, oxidation, and erosion

relative to monolithic metallic alloys or intermetallics,

♦ more fracture resistant (tougher, higher fracture strengths) and thermally

conductive than monolithic aluminates,

♦ formed into near net shapes by the process of the present invention as described

herein.

Accordingly, and with the foregoing objectives and advantages in mind, the present invention is summarized below. In view of the present disclosure or through practice of the

present invention, other advantages may become apparent. Summary of the Invention

A variety of reaction-based techniques have been developed for the in-situ syntheses of

monolithic ceramics and ceramic composites, including the Co-Continuous Ceramic Composite

(C⁴) process [1,2], the reactive metal penetration process (RMP) [3-5], the Directed Metal

Oxidation (DIMOX) [6,7], the Reaction Bonded Metal Oxide (RBMO) process [8,9], Self- Propagating High-Temperature Syntheses (SHS) [10], Hillig or Hucke Process [11-13], and the

Solid Metal-Bearing Precursor (SMP) process [14-18]. Such ceramic/metal alloy composites should exhibit enhanced thermo-mechanical properties relative to monolithic ceramic or monolithic metal alloy bodies. Consider for example, AEAI₂O₄/M-AI composites where AE = Mg, Ca, Sr, or Ba and M = Ni or Nb.

Compared to Nb, Ni, and Ni-Al alloys, polycrystalline MgAl₂O4 and BaAl₂O possess higher

values of elastic moduli, lower densities, and, hence, higher values of specific stiffness (e.g., E/p

= 79, 40, 24, 23, and 12 MPa-cm³/g for MgAl₂O₄, NiAl, Ni₃Al, Ni, and Nb respectively, at room

temperature) [11,15,22,23,28]. These stiff, stoichiometric aluminates exhibit better creep

resistance at elevated temperatures than Nb, Ni, or Ni-Al-based solid solutions or intermetallic compounds [29-31].

The DCP process described in the present disclosure differs from several of these approaches, in that it does not rely upon oxidation reaction(s) involving externally-supplied, gaseous oxidants (e.g., the DIMOX, RBMO, SHS, and SMP processes involve oxidation reactions with externally-supplied 02(g)). The fabrication of large parts by processes that

involve the use of a fluid, external reactant (DIMOX, RBMO, SHS, SMP) can require relative long oxidation times, as the reactant must migrate inwards through the thickness of the precursor

from the outside surfaces.

Since the reactions involved in the DCP process do not involve an externally-supplied gaseous reactant, the rate of conversion does not scale with the size of the starting preform. The DCP and Hillig/Hucke processes both involve the reaction of a porous, solid preform with a liquid that has been infiltrated into the preform. However, unlike the Hillig/Hucke process, the porous, solid preform in the DCP route undergoes a displacement reaction with the liquid phase.

The C⁴ and RMP processes also involve a displacement reaction between a fluid metallic phase

and a solid oxidant. However, an important distinction between the DCP process and the C⁴ and RMP methods is the difference in volume of the solid products and reactants. In the C⁴ and

RMP processes, a liquid metal reacts with a solid oxidant to produce a ceramic-metal composite, such as by the following net reaction:

4Al(l) + 3Si0 = 2Al₂O3 + 3(Si) (1)

where (Si) refers to silicon dissolved in molten aluminum. The volume of 2 moles of alumina is

less than the volume of 3 moles of silica. The open space formed as a result of this reduction in solid oxide volume is filled by molten metal (Al-Si alloy) that penetrates into the solid preform during reaction. Because the molten metal accommodates this change in volume, near net-shaped

parts can be produced by the C⁴ and RMP processes after solidification of the molten claim. However, due to the reaction-induced reduction in solid volume, the ceramic content of the final

shaped part produced by the C⁴ and RMP processes will be less than that of the preform. The reaction(s) involved in the DCP process result in an increase in solid volume, so that ceramic content of the final shaped part is greater than that of a porous preform. Hence, shaped parts containing relatively high fractions of ceramic phase can be produced by the DCP process.

In accordance with the present invention, the following general displacement reaction

between a liquid species, M(l), and a solid preform comprising the compound, NgXc(s):

AM(1) + N_BXc(s) = AMX_C/A(S) + BN(l/g) (2)

where MXC/A(^S) is a solid reaction product (X is a metalloid element, such as, for example,

oxygen, nitrogen, sulfur, etc.) and N(l/g) is a fluid (liquid or gas) reaction product. A, B and C are

molar coefficients.

In the DCP process of the present invention, the reactants and reaction conditions are chosen such that the volume of "A" moles of the solid product, MXQ/ ^(S), is greater than the

volume of one mole of the solid reactant, NβXc(s). If the NβXc(^s) preform is porous, then this reaction-induced volume increase can be accommodated by such porosity, so that the external

dimensions of the final shaped, MXc/A(s)-bearing part can be close to those of the porous

preform. Reactions of the type (2) that involve an increase in solid volume are a key feature of

the DCP process of the present invention. Such volume-increasing, liquid/solid displacement reactions have not been used to date to produce near net-shaped, dense ceramic bodies.

Accordingly, the methods of the present invention includes a method for producing a material selected from the group consisting of ceramics and ceramic composites, the method comprising reacting: (1) a fluid comprising at least one displacing metal; and (2) a rigid, porous

ceramic-bearing (i.e., either ceramic or otherwise containing a ceramic) preform having a pore volume and comprising at least one component species (that may be any substance amenable to displacement; for instance, and preferably, derived from a non-alkaline earth metal such as aluminum, nickel and niobium), the at least one displacing metal capable of displacing the at

least one component species; and allowing the fluid to infiltrate the ceramic-bearing preform such that the at least one displacing metal at least partially replaces (and preferably substantially or

completely replaces) the at least one component species, such as non-alkaline earth ion(s), and so as to at least partially fill (and preferably substantially or completely fill) the pore volume, and so as to produce a ceramic or ceramic composite. Whether the resultant material is a ceramic or ceramic composite will depend upon whether the displaced metal remains in the product material

to form another phase. If so, the resultant product will be a composite; if not, the resultant

product will be a ceramic, only containing a ceramic phase.

As used herein, "oxidation" refers to the process by which metal elements are converted to a higher valence state (i.e., metal cations), and "reduction" refers to the process by which metal cations are converted to a lower valence state (i.e., metal elements). A "displacement reaction" shall refer to a process in which at least one first component of at least one first ceramic phase shall be exchanged with at least one second component of at least one first metallic phase, so as to convert the said first component of the at least one first ceramic phase into a third component of at least one second metallic phase and to convert the said second component of the at least one first metallic phase into a fourth component of at least a second ceramic phase. A displacement reaction may be an oxidation-reduction reaction in which the first component of the at least one

first ceramic phase undergoes reduction as a result of a reaction with the second component of the

at least one first metallic phase which undergoes oxidation. In the case of an oxidation-reduction reaction, the first and second ceramic phases are ionically-bonded compounds. A displacement reaction may also involve first and second ceramic phases that are predominantly covalently- bonded compounds. For the purposes of this specification, the alkaline earth metals are berylium, magnesium, calcium, strontium, barium and radium.

The displacing metal(s) may be any metal(s) adapted to replace at least one component of at least one ceramic phase. For instance, the displacing metal(s) may be selected from the group consisting of non-alkaline earth ions, such as ions derived from metals such as aluminum, nickel

and niobium. The displacing metal may also be selected from the group consisting of

magnesium, calcium, strontium, barium and mixtures thereof.

The rigid, porous ceramic-bearing material(s) may comprise any ionically-bonded or covalently-bonded material(s) or compound(s) adapted to contain the components to be displaced, such as ceramic components derived from non-alkaline earth metal(s), for instance

those materials that may be selected from the group consisting of oxides, sulfides, nitrides, halides, borides, oxynitrides, carbonitrides, and carbides. The rigid, porous ceramic material(s) also may comprise ionically-bonded or covalently-bonded materials or compounds selected from the group consisting of aluminates, aluminosilicates, silicates, titanates, zirconates, niobates,

8 chromites, ferrites, borates, germanates, and phosphates. The non-alkaline earth metal(s), for instance, may be selected from the group consisting of aluminum, nickel, niobium.

To make a net shaped article by the method of the present invention, the rigid, porous ceramic material is preformed into a shape, and, following the process of the present invention, the ceramic or ceramic composite maintains that shape, typically within a few percent of the original shape. This preforming may be done by a light sintering, for instance.

In a preferred embodiment, the fluid is a liquid, the liquid being supplied by a melting

solid comprising the displacing metal(s), such one or more alkaline earth metals. It is preferred that the two solid reactive components (1) and (2) be placed in contact while in a solid state for

ease of handling, then allowing the melting solid (i.e., a solid that would become melted at or before reaching the processing temperature) to melt so as to bring the liquid phase into contact with the rigid, porous ionic material. The same may also be accomplished by using a solid material that may sublime to produce such a fluid as reactant component (1) above.

The method of the present invention may also be described as a method for producing a

ceramic or ceramic composite, the method comprising reacting: (1) a fluid comprising at least one displacing metal; and (2) a rigid, porous ceramic-bearing material having a pore volume and

comprising at least one component, such as a non-alkaline earth ion, the at least one displacing metal capable of displacing the at least one component; and allowing the fluid to infiltrate the

ceramic-bearing material such that the at least one displacing metal at least partially replaces (and preferably substantially and even completely replaces) the at least one component, and so as to at

least partially fill (and preferably substantially and even completely fill) the pore volume and so as to undergo a general displacement reaction between reactants comprising a liquid species M(l) derived from the fluid, and the rigid, porous ceramic-bearing material of the general formula,

NβXc(s), as follows:

9 AM(1) + N_BX_C(s) = AMX_C/A(s) + BN(l/g)

wherein MXQ//XS) is a solid reaction product and wherein X is a metalloid (i.e., a metal or

metalloid element), N(l/g) is a fluid reaction product, and A, B and C are molar coefficients; and wherein the reactants are chosen such that the volume of A moles of the solid reaction product

MXC/A(S) is greater than the volume of one mole of the solid reactant, NβX ;(s), such that the

reaction-induced volume increase can be accommodated by such pore volume, and so as to produce a material selected from the group consisting of ceramics and ceramic composites.

The displacing metal(s), ceramic components, and rigid, porous ceramic-bearing material(s), as well as other preferred aspects of the method of the present invention, may be as described and exemplified above.

The method of the present invention may also be described as a method for producing a ceramic or ceramic composite, the method comprising the steps: (a) placing in contact:

(1) a solid adapted to produce a fluid material comprising at least one displacing metal;

(2) a rigid, porous ceramic-bearing material having a pore volume and comprising at least one ceramic component, such as those derived from non-alkaline earth metal(s), maintaining the solid at sufficient temperature such that the solid produces the fluid, the fluid infiltrating the ceramic-

bearing material so as to at least partially replace (and preferably substantially and even

completely replace) the at least one ceramic component, and so as to at least partially fill (and preferably substantially and even completely fill) the pore volume, and so as to produce a material selected from the group consisting of ceramics and ceramic composites.

The displacing metal(s), ion(s), and rigid, porous ionic material(s), as well as other preferred aspects of the method of the present invention, may be as described and exemplified

above. The present invention includes any ceramic or ceramic composite prepared in accordance with the inventive methods described herein.

The DCP process of the present invention comprises three basic steps:

1) Synthesis or other acquisition of a porous preform: A porous preform with an appropriate composition, pore fraction, and overall shape is prepared or obtained. The pore fraction of the preform is tailored so that the reaction-induced increase in solid volume can compensate partially or completely for such porosity. It will be understood that the porous preform need only be sufficiently dimensionally stable to resist the capillary action of the infiltrated liquid reactant.

2) Infiltration: The porous preform is infiltrated with a fluid (liquid or gas) reactant (which, if liquid, may come from the melting of a solid or otherwise from a substance introduced to the reaction in liquid form.

3) Reaction: The fluid reactant is allowed to react partially or completely with the solid preform to produce a dense, shaped body containing desired ceramic phase(s).

As the DCP process of the present invention involves the reaction of liquid and solid

phases throughout the infiltrated preform, the time required to form the desired ceramic phase(s) should depend on interfacial areas and/or the sizes of the reacting phases, and should not depend on the overall dimensions of the preform. As a result, the reaction time required for large parts can be relatively modest. By properly tailoring the pore fraction and composition of the preform, the shape and dimensions of the final transformed part can be close to those of the porous preform. Hence, costly ceramic machining of the final part can be minimized or avoided. Further, since porous ceramic preforms of desired shape can be produced by relatively low cost processes (e.g., slip casting, cold pressing), the DCP process is an inexpensive means of fabricating dense and shaped ceramic-bearing bodies.

1 1 By tailoring the composition of the liquid and the preform, the DCP process can be used to fabricate near net-shaped composites with a wide range of ceramic and metal contents. For example, the present invention may be used to synthesize co-continuous composites of refractory, alkaline-earth aluminates with high-melting metallic or intermetallic reinforcements (e.g., MgAl₂0₄/Nb, BaAl₂O₄/Ni₃Al+NiAl composites). Such AEAl₂O₄/M-Al composites should

exhibit enhanced stiffness/creep resistance, improved erosion/oxidation resistance, and lower

densities (p_tvig_Al₂o₄ < 0.5p_Ni or 0.5p_Nt_>) than pure metals or intermetallics. These composites

should also possess higher values of fracture strength, toughness, and thermal conductivity than

monolithic aluminates.

The method of the present invention may thus be carried out by the fabrication of a shaped, porous preform of solid oxide + metallic (or intermetallic) reactants, followed by the (preferably) pressureless infiltration of the preform with a low-melting, alkaline-earth-bearing liquid, followed by, or accompanied by, heat treatment to allow for an internal displacement reaction that converts the liquid/solid mixture into a dense, near net-shaped, high-melting

ceramic/metal alloy composite.

The present invention also includes the ceramic/metal alloy composites prepared by the

method of the present invention.

The present invention may be understood by reference to the processing/ microstructure/property correlations for DCP-derived, AEAl₂O₄/M-Al alloy composites (AE =

Mg or Ba; M = Ni or Nb; M-Al = metallic alloy and/or intermetallic compound) described herein. In accordance with the present invention, one skilled in the art may make additional adjustments to the processing parameters to affect the macrostructure (shape, dimensional retention) and

12 microstructure (phase fraction, size, chemistry; interface chemistry/morphology) of composites of the present invention.

The microstructural features of ceramic composites of the present invention are believed to improve mechanical properties (e.g., MOR, toughness/damage tolerance, and creep). Through the present invention, one may tailor the microstructure of ceramic composites without compromising the shape retention capability.

AEAI2O₄/M-AI alloy composites of the present invention may be fabricated by

infiltrating AE-bearing liquids into porous, oxide + metal preforms, and then conducting a displacement reaction of the following type:

{AE + wAl + xM}(l) + M_yAl_z(s) + MAl₂O₄(s) -> AEAl₂O₄(s) + M_1+x+yAl_w+z(s) where

AE is one or more alkaline earth metal, preferably Mg or Ba or Ca or Sr; M is a non-alkaline earth metal, such as Ni or Nb; bracketing "{}" refers to species dissolved in a liquid; M_yAl_z(s),

M_1+x+yAl_w+z refer to solid solutions, compounds, or mixtures of both. Such composites may

include, for example, i) MgAl₂O₄ or BaAl₂O₄ as the matrix phase, and ii) Nb or Ni-Al metallic

solid solutions and/or intermetallic compounds (e.g., Ni₃Al, NiAl) as the reinforcement phase(s).

Dense, shaped, aluminate-matrix composites with varied amounts of M-Al reinforcements may be produced by tailoring the composition of the liquid (AE, w, and x), and the pore fraction (P) and phase content (M, y, and z) of the preform. For example, a composite of 90.5 vol% BaAl₂0₄ and 9.5 vol% Ni_{0 94}A1_{0 06} solid solution can be produced if w=0.06, x=y=z=0, and

P=0.43. However, if w=0.95, y=6.00, x=z=0, and P=0.32, then a composite of 54 vol% BaAl₂0₄

and 46 vol% Ni_{0 88}A1_{0 12} solid solution can be synthesized.

1 Ceramic and reinforcement phase sizes in the transformed composites may be adjusted by varying the sizes of pores and reactant phases in the preform (i.e., by controlling particle sizes/distributions and compaction/sintering conditions).

Dopants optionally may be introduced into the liquid or the preform to alter the interface chemistry and morphology in the reacted composites.

By altering mechanical behavior through changes in microstructure/microchemistry, and tailoring the flexible DCP process of the present invention to obtain particular microstructural/microchemical features, one of ordinary skill may produce near net-shaped ceramic-matrix composites (e.g., for rocket or jet engine components), in accordance with desired enhanced performance.

The feasibility of fabricating shaped ceramic composites by the DCP process may be demonstrated by the syntheses of MgO/Mg-Al composites from porous AI2O3 preforms, as per

the following reaction:

3Mg(l) + Al₂O₃(s) = 3MgO(s) + 2(A1) (3)

where (Al) refers to aluminum dissolved in a magnesium-rich melt. Three moles of MgO possess a volume that is 32% greater than one mole of AI2O3, so that this reaction is of the type (2), with

A = 3, B = 2, C = 3, M = Mg, N = Al, and X = O. Disk-shaped AI2O3 preforms (10 mm dia., 2

mm thick) with 29% porosity were prepared by pressing and partial sintering (for 8 h at 1500 °C) of AI2O3 powder. The disks were sealed within a steel can along with solid pieces of

magnesium.. The sealed can was then placed in a reaction furnace at 1000°C for 15 h. Under these conditions, the magnesium melted, infiltrated, and reacted with the porous alumina preforms to yield 98% dense composites of 89.7 vol% MgO and 10.3 vol% Mg-Al alloy. The composites retained the shape and dimensions (to within 2.3%) of the porous alumina preforms. Hence, the porosity within the alumina preform was substantially compensated by the reaction- induced increase in solid volume.

The DCP method of the present invention has been used to produce near net-shaped MgO and MgAl₂0₄-bearing composites reinforced with lightweight Mg-Al or higher-melting (and

stronger) Fe-Ni-Al alloys. Composites reinforced with the latter alloys possessed fracture

strength and toughness values up to 470 MPa and 13 MPa-m^1/2, respectively.

Several process modifications can be introduced to the basic process discussed above to produce reinforced ceramic composites:

1). Instead of starting with a pure elemental reactant M(l) in reaction (2), an alloy liquid containing species M can be used. The other elements in the starting liquid alloy may be inert (and thereby wind up as additional metallic phases in the final part), may react with the ceramic preform (to yield other oxidized phases), or may alloy with or react with the species N liberated by reaction (2) (to yield a compound containing N). For example, if Mg is alloyed with Ni, then

reaction (3) may be modified as follows:

3(MgNi₂/3) + Al₂O₃(s) = 3MgO(s) + 2NiAl(s) (4)

where (MgNi2/3) refers to a molten Mg-Ni alloy, and NiAl(s) is an intermetallic

compound.

2). By limiting the amount of one of the reactants, reaction (2) may also not be allowed to go to completion. This would allow for the formation of a wider range of ceramic compositions in the final part. For example, if the ratio of magnesium to alumina in reaction (3) were reduced

from 3:1 to 3:4, then the following reaction can occur:

3Mg(l) + 4Al₂O₃(s) = 3MgAl₂O (s) + 2(A1) (5)

15 (Note: 3 moles of spinel, MgAl2θ_ι, possess a larger volume than 4 moles of AI2O3, so

that this reaction is still of the type (2).)

3). A gaseous reactant species (M in reaction (2)) could be used instead of a liquid species.

For example:

3Mg(g) + 4Al₂O₃(s) = 3MgAl2θ (s) + 2(A1) (6)

4). Additional phases may be added to the porous preform. Such phases may act as reinforcements in the final component. For example, SiC could be added to an alumina preform to produce MgO/SiC/(Al) composites via reaction (3).

It is important to note that, although reactions (3) - (6) involve the consumption and formation of oxide materials or compounds, the DCP process also may be applied to the fabrication of other ceramic materials or compounds (e.g., nitrides, sulfides, carbides, borides). The DCP process refers to the use of an in-situ displacement reaction involving an increase in solid volume to produce a ceramic-bearing body with a lower pore fraction than the starting preform (i.e., due to compensation of at least some of the porosity in a porous preform by the displacement reaction). Brief Description of the Drawings

Figure 1 is a schematic of one embodiment of the DCP method of the present invention.

Detailed Description of the Preferred Embodiments

In accordance with the foregoing summary of the invention, the following presents a detailed description of the preferred embodiment of the invention which is presently considered

to be one of the best modes.

The embodiment of the DCP method described herein includes a processing route comprising the following general steps (see Fig. 1 below) [27] :

16 1) Fabrication of a shaped, porous, rigid preform containing oxide and metallic or intermetallic reactants (or otherwise obtaining such a preform);

2) Pressureless infiltration of the porous preform with an AE-bearing liquid; and

3) Heat treatment to allow for an internal displacement reaction between the AE- bearing liquid and a component in the solid oxide.

(Note: steps 2) and 3) can be conducted with the same heat treatment; that is, infiltration and reaction preferably may be conducted simultaneously.)

By tailoring the phase content and porosity of the preform, and the composition and amount of liquid used, the DCP process can be tuned to yield a near net-shaped, dense composite containing particular amounts of AEAl O₄ and M-Al alloy phases of desired composition. The

porous preform can be produced in a desired shape by conventional ceramics forming operations such as: slip casting, powder injection molding, extrusion, tape casting, pressing, or tape

calendering [58,59]. The resulting green body can then be annealed to burnout organic binder material and to allow for neck formation between the solid particles (initial stage sintering), so as to obtain a rigid, yet porous, preform for subsequent infiltration. AE-bearing liquids are compatible with infiltration processing, in that such liquids melt at relatively low temperatures (see Table I below). AE elements also possess very high affinities for oxygen (i.e., higher even than aluminum, see Table II below), and tend to wet solid oxides, which allows for pressureless infiltration into porous oxide preforms (e.g., pressureless Mg(l) infiltration into porous preforms containing NiAl O₄ or Al₂O₃(s) has been demonstrated). Such high oxygen affinities also

provide a strong driving force for the DCP displacement reactions.

17 As an example of the process of the present invention and composite produced thereby, the following net displacement reaction between the magnesium in a liquid Mg-Al solution and the nickel oxide in solid NiAl₂O₄:

{Mg} + {Al} + NiAl₂O₄(s) -> MgAl₂O₄(s) + NiAl(s) (1)

where the bracketing "{}" refers to a species dissolved in a liquid phase.

Largely due to the high affinity of magnesium for oxygen, this reaction is strongly favored

at 1000°C. The Gibbs free energy change per mole of this reaction, ΔG, at 1000°C is -451.8 kJ/mole [60,61]. In this DCP reaction, the amount of magnesium present in the liquid can be set equal to the amount of nickel in the NiAl₂O₄(s) within the preform, so that: i) only the nickel

oxide (not the alumina) in NiAl₂O4 is reduced, and ii) essentially no residual metallic magnesium

is retained in the transformed body. The nickel oxide in NiAl₂O₄(s) is considerably easier to

reduce than the alumina in this compound, as indicated by comparing the magnitudes of the

standard Gibbs free energy changes, ΔG°, for the following reactions [60,61]:

Mg(l) + NiAl₂O₄(s) -> MgAl₂O₄(s) + Ni(s) (2a)

ΔG°(1000°C) = -349.68 kJ/mole

Mg(l) + NiAl₂O₄(s) -> 1/3 MgO(s) + 2/3 MgAl₂O₄(s) + 2/3 Al(l) + NiO(s) (2b)

ΔG°(1000°C) - -43.14 kJ/mole

Hence, by keeping the molar AE:NiAl₂O ratio < 1 :1, the nickel in NiAl₂O (s) should be

selectively reduced by the AE metal. Such selective reduction during the DCP process has been

demonstrated. Other authors have also shown that the Mg metal within Mg-Al alloys can be selectively oxidized by displacement reactions with oxides that are less stable than MgO or Al₂O₃ [63]. Further, from the thermodynamic data in references [60,61], reaction (1) should

18 proceed to the right at 1000°C as long as the Mg concentration in the Mg-Al melt exceeds

3.3X10"¹⁷ at%! That is, a negligible amount of magnesium should be retained in the melt, if the

Mg: NiAl₂0₄ ratio is maintained at < 1 :1. The latter prediction has been confirmed.

In the DCP process, displacement reactions are chosen that lead to an increase in solid volume (i.e., V_m(solid products) > V_m(solid reactants)). This volume increase is associated with

the conversion of the components in the low-melting AE-bearing liquid into solid oxide and metallic (or intermetallic) phases. As a result, the pore volume within the shaped preform can be filled with the solid reaction products, so that a dense body is produced. The molar volumes of

stoichiometric NiAl₂O₄(s), MgAl₂0₄(s), and NiAl(s) are 39.25, 39.76, and 14.51 cm³/mole [24],

respectively, so that the difference in volume between the solid products and reactants of reaction (1) is:

ΔV_pcnd₎ = V_f - V₀ = (39.76 + 14.51) - 39.25

= 15.02 cm³ /mole

Hence, the relative volume change is:

100ΔV/V_o = 100(15.02)/39.25

= 38.3% That is, a 38.3%) increase in solid, internal volume occurs as a result of reaction (1). This volume increase is due to the displacement reaction between magnesium and nickel oxide (in NiAl₂O₄)

and to the reaction of aluminum with the displaced nickel to yield NiAl(s). The net result of reaction (1) is that the magnesium and aluminum in the low-melting liquid are converted into higher-melting (solid) phases; that is, the DCP process involves a reaction-induced solidification. Since the reaction occurs at liquid/solid interfaces (as opposed to only at contacts between solid particles, such as for the case of a solid/solid reaction in a porous preform), and since the liquid

19 occupies the prior pore volume, this volume increase will lead to a filling of the prior pore spaces; that is, a reaction-induced densification will occur. If the reaction-induced increase in solid volume is greater than or equal to the pore volume in the starting preform, then a high- density body can be fabricated. Hence, for the present example, if the porosity within the porous NiAl₂0₄(s) preform is equal to:

P = [100(15.02)]/[15.02 + 39.25]

= 27.7%

then reaction (1) should lead to a dense composite comprised of 73.3 vol% MgAl₂O₄ and 26.7

vol% NiAl. For higher density preforms, some residual, unreacted NiAl₂0₄ (or an

(Mg,Ni)Al₂0₄ solid solution) will be retained within the specimen.

A more general form of reaction (1) can be written as follows:

{AE} + w{Al} + x{Ni} + Ni_yAl_z(s) + NiAl₂O₄(s) -> AEAl₂O₄(s) + Ni_1+x+yAl_w+z(s)

(3) where the bracketing "{}" refers to a species dissolved in a liquid, AE refers to Mg, Ca, Sr or Ba, and Ni_yAl_z(s) or Niι_+x+yAl_w+z refer to a (Ni,Al) solid solution, an intermetallic compound (e.g.,

Ni₃Al), or a mixture of both. (A similar type of reaction can be written for the syntheses of

AEA1₂0₄/Nb or AEAl O4/Nb-Al alloy composites, where NbAlU4 or other niobium aluminates

are used as solid reactants.) In this case, unlike for reaction (1), some of the nickel and aluminum required for the Ni_j+x+yAl_w+z(s) phase in the final composite are present within a solid Ni_yAl_z

phase in the preform. By tailoring the preform porosity, the liquid composition (i.e., w and x

values), and the preform metal content (i.e., y and z values), dense, shaped composites consisting of a wide range of ceramic, and metal or intermetallic phases can be fabricated. For example, if AE = Ba, w = 0.06, x = y - z = 0, and the pore fraction of the preform is 0.433, then completion

20 of the displacement reaction at 1000°C should yield a dense composite comprised of 90.5 vol% BaAl₂0₄ and 9.5 vol% Ni_{0 94}AI0.06 solid solution. However, if AE = Ba, w = 2, x = 0, y = 5, z =

0, and the pore fraction of the preform is 0.386, then a dense composite comprised of 53.3 vol% BaAl₂O₄ and 46.7 vol% Ni₃Al should be produced. Values of w, x, y, and z leading to a variety

of composites are shown in Table III below. By varying w, x, y, and z in a similar manner, MgAl 0 - bearing or BaAl O₄-bearing composites containing a range of ceramic, metallic

and/or intermetallic phases could also be produced.

The feasibility of using reactive infiltration processes to produce dense ceramic/metal composites (e.g., Al₂0₃/Al-Si, MgO/MgAl 0₄/Fe-Ni-Al) with flexural strengths and toughness

values of 300-500 MPa and 8-13 MPa-m^{1 2} has been demonstrated.

In accordance with the present invention, it is anticipated that the phase sizes in the final composites, for a given preform phase content, can be adjusted by varying the sizes of the oxide and metal phases (and, hence, pore sizes) in the preform and, further, that such adjustment will

lead to optimum values of strength and toughness. The strength and ductility of the

reinforcement phase are also important parameters.

Ni₃Al may act as a ductile reinforcement at both room temperature and elevated

temperatures in Al₂O₃ -bearing (or AEAl₂O₄-bearing) composites, provided that: i) the Ni₃Al

filament size in the composites is smaller than the Ni₃Al crystal size (i.e., so as to minimize the

number of grain boundaries within the Ni₃Al filaments) and/or ii) the Ni₃Al phase is of proper

stoichiometry and is properly alloyed (e.g., Al concentration < 25 at% with boron doping) [11,14- 18], By introducing dopants to the Ni_yAl_z(s) phase (e.g., boron, chromium) in the preform,

composites reinforced ductile Ni₃Al may also be produced. Such doping may also be used to

produce composites reinforced with Ni-Cr-Al-Y-based superalloys [5,35]. Similar approaches

21 may be used to fabricate AEA1₂0₄ composites reinforced with Nb-Al alloys of controlled

chemistry. Hence, AEAl₂O₄-bearing composites reinforced with Ni-bearing or Nb-bearing alloys

comprising a range of phase contents and phase compositions may be fabricated by the DCP method. Such flexibility in tailoring the phase content, phase size/distribution, and phase chemistry, without compromising the shape retention capability, is a unique feature of the DCP process.

Advantages Relative to Other Ceramic Processing Routes

The fabrication of AEAl₂O₄/Ni-Al alloy or AEA1 0₄/Nb-Al alloy composites in

accordance with one embodiment of the present invention is believed to be novel.

The closest related work is the processing of Al₂O₃/Ni-Al alloy composites. A variety of

synthesis methods have been used to produce Al₂O₃ composites reinforced with Ni (or a Ni solid

solution), Ni₃Al, or NiAl, including:

♦ hot pressing of mixtures of oxide + metal or intermetallic, where the mixtures

were prepared by milling or by sol-gel processing [39,50-52,62],

♦ gas-pressure infiltration of Ni-Al liquid into porous Al₂O preforms [40,64],

♦ selective reduction of NiO + Al₂O₃ mixtures or of NiAl₂U4 in a reducing

atmosphere [33,43-45,62],

♦ solid-state displacement reactions between NiO and NiAl [46], and

♦ displacement reactions between NiO(s) and Al(l) [41 ,42,62,65-67] .

The DCP method of the present invention differs from each of these methods and, for the following reasons, is a more attractive process:

22 1) Hot pressing and gas-pressure infiltration are not required to produce dense composites with the DCP process of the present invention. Indeed, hot pressing is not a viable method for fabricating ceramic bodies of complex shape (i.e., non-axial-symmetric geometries) and is a relatively expensive batch process (as is hot isostatic pressing). Gas pressure infiltration of Ni-Al liquids is also a relatively expensive batch process that requires the use of a specially-designed

system to allow for the infiltration of such high-melting Ni-Al liquids (T_m > 1360°C for Ni₃Al

and > 1385°C NiAl [8,9,20,53,54]).

2) The selective reduction of nickel oxide (as NiO or MAI₂O₄) in a reducing atmosphere

results in significant volume decrease that, in turn, leads to a loss of the original specimen dimensions upon sintering to a high density. Unlike the DCP method, the gaseous reduction process does not lead to near net-shaped composites. Furthermore, oxygen diffusion through a thick, dense NiO or MAI2O4 precursor body would require the use of relatively high

temperatures or long reduction times (e.g., anneals at 1600-1700°C were used to reduce 5 mm thick specimens in reference [62]). The reduction of a porous NiO or NiAl₂O₄ precursor body

(i.e., to allow for an enhanced rate of oxygen removal) would result in a further shrinkage upon subsequent sintering to a high density. Finally, the selective reduction of NiAl₂0₄ to yield A1₂0₃

composites reinforced with Ni-Al alloys (e.g., Ni₃Al or NiAl), as opposed to essentially pure Ni,

would require the use of very low and controlled oxygen partial pressures (e.g., po₂ ≤ 6.0X10^-29

atm at 1000°C [68]), which would be relatively difficult to scale up in a continuous fashion for

manufacturing.

3) Solid-state displacement reactions between NiO and Ni-Al alloys result in appreciable volume changes. For example, the reaction-induced volume change for the following reaction

[46]:

23 4.5 NiAl(s) + 3NiO(s) => 2.5 Ni₃Al(s) + Al₂0₃(s) (4)

is +44.3%). Since this reaction is initiated at contact points between NiAl and NiO particles, continued reaction will cause the separation distance between the particles to increase and, hence, a significant expansion of the body must occur. Such expansion could be reduced by reactive hot pressing [46], but the limitations of cost and specimen shape would again apply. 4) Prior work on displacement reactions between NiO(s) and Al(l) have been conducted by pressing mixtures of NiO and Al powders, and then annealing the pressed bodies well above the melting point of Al, with or without the application of an external pressure (reactive hot pressing) [41,42,62,66,67]. Appreciable shrinkages have been reported for displacement reaction processing conducted in this fashion [62,66,69]. Such shrinkages result from: i) the difference in porosity of the starting, as-pressed precursor (Al-oxide mixtures with the necessary oxide contents are difficult to press to a high relative density) and the final, dense product, and ii) the reaction-induced volume change [62,66,69].

In the DCP method of the present invention, an AE-bearing liquid preferably and most economically is infiltrated without the application of an external pressure into a porous oxide preform. AE-bearing liquids readily wet and infiltrate porous oxide preforms, even for oxides such as Al₂O₃ that tend not to be wet by Al(l), Ni(l), or most metallic liquids [39,70-72]. Such

pressureless infiltration is more amenable for scale up in manufacturing than gas-pressure- assisted infiltration. Unlike the methods discussed above, the DCP process may produce near net-shaped, dense composites at modest firing temperatures (e.g., at 900°C). The ability to retain the dimensions of the porous preform allows for the fabrication of complicated shapes, since porous preforms of complicated shape can be readily produced by conventional ceramic processing (slip casting, injection molding, etc.). The DCP approach is also well suited for the

24 fabrication of relatively large components, since the effective diffusion distance required for the transformation reaction is dependent on the internal phase sizes and not on the external component dimensions (i.e., the transport of gaseous oxygen over long distances is not required, as is the case for gas-phase reduction processing).

In short, the DCP method of the present invention is a relatively simple, low-cost, scaleable method for fabricating dense, high-temperature AE-oxide/metal alloy composites of complicated and near-net shape.

Industrial Applicability

The displacement-reaction-based process of the present invention may be used, for example in synthesizing near net-shaped, ceramic/metal alloy composites for high-temperature applications (e.g., static components in rocket or jet engines). Hence, DCP-derived AEA1₂0₄/M-

Al alloy composites may be used in liquid-fueled rocket engines (e.g., high-melting MgAl O₄/Nb

composites for nozzle liners) or in jet engines (e.g., oxidation-resistant BaAl₂θ₄/Ni₃Al+NiAl

composites for shrouds, vanes).

DCP-derived AEAl₂O₄/ i-Al alloy composites of the present invention typically would

be expected to exhibit comparable or better oxidation resistance and mechanical properties to that of monolithic Si₃N₄ prepared by conventional sintering or reaction bonding. Tuan and Brooks

have shown that the oxidation resistance of Al₂O₃/Ni composites at 1300°C in air is similar to

that for high-purity RBSN [33,34]. Since the oxidation kinetics of Ni-Al alloys can be several

orders of magnitude slower than that for pure Ni at 1300°C, the oxidation resistance of AEAl₂O₄/Ni-Al alloy composites should be much better than for Al₂O₃/Ni composites [35,36].

Typical values of flexural strength and toughness reported for sintered or reaction-bonded silicon

nitride (RBSN) at room temperature are 200-600 MPa and 2-5 MPa-m^{1 2}, respectively

25 [11,34,37,38]. Higher toughnesses with comparable fracture strengths have been achieved with displacement Al₂O₃/Al-Si and MgO/MgAl₂O /Fe-Ni-Al composites prepared in accordance with

the present invention. With further modification of the melt and preform chemistry (e.g., the addition of metallic nickel to the preform or the melt, the addition of niobium as metallic Nb or as NbAlO₄ to the preform), the DCP method of the present invention is thus capable of yielding

composites that are more oxidation resistant or higher melting (e.g., oxidation-resistant BaAl₂O₄/Ni₃Al+NiAl alloy composites or high-melting MgAl₂O /Nb composites). Higher bend

strength and toughness may also be achieved with such AEAl₂O₄/M-Al composites, by

correlating microstructural/microchemical features and mechanical behavior.

Certain microstructural features (e.g., the interconnectivity, size, strength, and ductility of the reinforcement phase, along with interface chemistry) of such Al O₃/Ni-Al alloy composites

can strongly influence the strength and toughness of ceramic/metal (or intermetallic) composites [39-52]. Further, because dense, near net-shaped AEAl₂O₄/Ni-Al alloy components can be

fabricated with the DCP process of the present invention (unlike conventionally-prepared Al₂O₃/Ni-Al alloy composites) without the use of costly hot pressing, such DCP-derived

composites are attractive alternatives to components based on monolithic Si₃N₄.

With respect to other DCP-derived MgAl₂U4/Nb (or Nb-Al alloy) composites, these may

be attractive materials for high temperature applications, such as combustor or nozzle linings in liquid-fueled rocket engines. The melting points of stoichiometric MgAl₂0₄, Nb, and

stoichiometric Nb₃Al are 2105°C, 2468°C, and 2060°C, respectively [8,9,20,53-56]. The melting

point of MgAl₂O₄ may be further raised by substituting some or all of the aluminum with

chromium (i.e., the melting point of MgCr₂O₄ is ~2400°C [57]). Furthermore, as spinel

26 possesses less than half the density of niobium [24], and since niobium is one of the lighter refractory metals (i.e., compared to W, Ta, and Mo), high-melting composites of spinel and niobium should be relatively light. For example, a composite comprised of an equimolar mixture of MgAl 0 and Nb has an overall specific density similar to that of titanium [24]. MgAl₂O /Nb

composites also exhibit attractive thermal properties. The average, linear CTE values of polycrystalline MgAl₂O₄, MgCr₂O₄, Nb, and Nb₃Al are similar (i.e., within the range of 7.2-

9.2X10"⁶/°C [9,32]), so that composites of these materials should be relatively resistant to

damage from repeated thermal cycling. Even more important is the high thermal conductivity exhibited by polycrystalline niobium at elevated temperatures (e.g., A68 W/mK at 1000°C and A82 W/mK at 2000°C [32]). At 1000-2000°C, Nb possesses a thermal conductivity that is greater than or equal to refractory carbides (e.g., SiC, B₄C, HfC, TaC) and several grades of

polycrystalline graphite [25]. Hence, cooling of co-continuous MgAl₂O₄/Nb composites can be

an effective method of reducing the composite temperature in service (i.e., by cooling one side of

a MgAl₂θ₄/Nb nozzle liner, the temperature near the hot face may be significantly reduce)

which, in turn, should enhance the erosion/corrosion resistance of such composites. These high- temperature properties, coupled with the capability for fabricating such composites in complex and near net-shapes by the DCP method, make DCP-derived, co-continuous MgAl₂O₄/Nb

composites very attractive for high-temperature applications in liquid-fueled rockets.

In view of the present disclosure or through practice of the present invention, it will be within the ability of one of ordinary skill to make modifications to the present invention, such as

through the use of equivalent process steps, process parameters and constituent compositions, in order to practice the invention without departing from the spirit of the invention as reflected in the appended claims, the teaching of which is hereby incorporated into this description.

27 Table I. Liquidus and eutectic temperatures for several binary AE-bearing compositions [9].

Amount (at%>) of Liquidus Amount (at%>) of Liquidus

AE Element Amount Liquidus AE Element Amount Liquidus

(at%) of Temperature (at%) of Temperature

Second Second

Element Element

Mg None 650 °C Sr None 769 °C

Mg 10at%Al 586 °C Sr 10at%Al 703 °C

Mg 25 at% Al 490 °C Sr 25 at% Al 652 °C

Mg 50 at% Al 456 °C Sr 50 at% Al 829 °C

Mg 10at%Ni 527 °C Sr 10at%Ni 680 °C

Mg 25 at% Ni 732 °C Sr 25 at% Ni 915 °C

Ca None 842 °C Ba None 727 °C

Ca 10at%Al 784 °C Ba 10at%Al 645 °C

Ca 25 at% Al 651 °C Ba 25 at% Al 547 °C

Ca 50 at% Al 864 °C Ba 50 at% Al 757 °C

Ca 10at%Ni 704 °C Ba 10at%Ni 702 °C

Ca 25 at% Ni 727 °C Ba 25 at% Ni 990 °C

Binary System Eutectic Temperature* Eutectic Liquid Composition*

Mg-Al 437 °C 31.0at%Al

Mg-Ni 506 °C 11.3 at%Ni

Ca-Al 545 °C 35.0 at% Al

Ca-Ni 605 °C 16.0at%Ni

Sr-Al 590 °C 18.3at%Al

Sr-Ni 660 °C 11 at% Ni

Ba-Al 528 °C 28.0 at% Al

Ba-Ni 660 °C 6.0 at% Ni

*For the most AE-rich eutectic point.

28 Table II. Oxygen partial pressures associated with metal/metal oxide equilibria

[60].

Equilibrium Oxygen

Element/Oxide Temperature Partial Pressure

Mg/MgO 1000°C 1.4xl0-³⁸atm

CaCaO 1000°C O xlO^atm

Sr/SrO 1000°C l.lxl0^"38atm

BaBaO 1000°C 5.5xl0^"36atm

Ni/NiO 1000°C 4.6 x 10^"πatm

Nb/Nb₂O₅ 1000°C 7.2xl0^"23atm

Si/SiO₂ 1000°C 9.3xl0^"29atm

Al/Al₂O₃ 1000°C 1.7xl0^"35atm

Table III. Melt and preform composition parameters for the DCP process and the resulting AEAl₂O₄Ni-Al Alloy composites (for AE = Ba).

Melt/Preform Composition Critical Phase Content of Final Composite

Parameters (as per reaction (3)) Preform (vol%)

Porosity

(vol%) w X y z AEAIO4 Ni or jA] NiAl (Ni,Al)

0 0 0 0 43.3 90.5 9.5 — —

0.12 0 0 0 43.3 90.4 9.6 ... ...

0 0.30 0 0 44.8 88.0 12.0 ... —

0 0.30 1.00 0 41.0 80.5 19.5 — —

0 0.30 3.00 0.41 35.8 66.7 33.3 — —

0.95 0 6.00 0 31.8 54.2 45.8 — ...

1.00 0 3.00 0 38.8 64.9 14.4 20.7 ...

0.50 0 0.50 0 44.2 82.0 — 18.0 —

2.00 0 5.00 0 38.6 53.3 — 46.7 —

2.00 0 6.00 2.00 32.8 44.7 — 29.4 25.9

1.00 0 0 0 49.1 81.2 — — 18.8

2.00 0 2.00 1.00 43.2 59.0 — — 41.0

2.00 0 4.00 3.00 34.0 46.3 — — 53.7

2.00 0 6.00 5.00 28.0 38.1 ... ... 61.9

29

Claims

What is claimed is:

1. A method for producing a material selected from the group consisting of ceramics and ceramic composites, said method comprising reacting:

(1) a fluid comprising at least one displacing metal; and

(2) a rigid, porous ionic material having a pore volume and comprising at least one ion,

said at least one displacing metal capable of displacing said at least one ion; and allowing said fluid to infiltrate said ionic material such that said at least one displacing metal at least partially replaces said at least one ion, and so as to at least partially fill said pore volume, and so as to produce a material selected from the group consisting of ceramics and ceramic composites.

2. A method according to claim 1 wherein said at least one displacing metal is selected from the group consisting of magnesium, calcium, strontium, barium and mixtures thereof.

3. A method according to claim 1 wherein said at least one ion is derived from a non-alkaline earth metal.

4. A method according to claim 3 wherein said at least one non-alkaline earth metal is selected from the group consisting of aluminum, nickel, and niobium.

5. A method according to claim 1 wherein said ionic material is selected from the group of ions derived from consisting of oxides, sulfides, nitrides and halides.

6. A method according to claim 1 wherein said ionic material is selected from the group consisting of aluminates, aluminosilicates, silicates, titanates, zirconates, and niobates.

7. A method according to claim 1 wherein said at least one displacing metal substantially replaces said at least one ion.

8. A method according to claim 1 wherein said said ionic material is preformed into a shape, and wherein said material selected from the group consisting of ceramics and ceramic composites maintains said shape.

9. A method according to claim 1 wherein said fluid is a liquid, said liquid being supplied by a melting solid comprising said at least one displacing metal.

10. A method for producing a material selected from the group consisting of ceramics and ceramic composites, said method comprising reacting:

(1) a fluid comprising at least one displacing metal; and

(2) a rigid, porous ionic material having a pore volume and comprising at least one ion,

said at least one displacing metal capable of displacing said at least one ion; and allowing said fluid to infiltrate said rigid, porous ionic material such that said at least one displacing metal at least partially replaces said at least one ion, and so as to at least partially fill said pore volume and so as to undergo a general displacement reaction between reactants comprising a liquid species M(l) derived from said fluid, and said rigid, porous ionic material of the general formula, NgXc(s), as follows:

AM(1) + N_BX_C(s) = AMX_C/A(S) + BN(l/g)

wherein MXQ/A(S) is a solid reaction product and wherein X is a metalloid element, N(l/g) is a fluid reaction product, and A, B and C are molar coefficients; and wherein said reactants are chosen such that the volume of A moles of the said solid reaction product MXQΆ(S) is greater than the volume of one mole of said solid reactant, NgXc(s), such that the reaction-induced volume increase can be accommodated by such pore volume, and so as to produce a material selected from the group consisting of ceramics and ceramic composites.

1

11. A method according to claim 10 wherein said at least one displacing metal is selected from the group consisting of magnesium, calcium, strontium, barium and mixtures thereof.

12. A method according to claim 10 wherein said at least one ion is derived from a non-alkaline earth metal.

13. A method according to claim 12 wherein said non-alkaline earth metal is selected from the groups consisting of aluminum, nickel, and niobium.

14. A method according to claim 10 wherein said ionic material is selected from the group of ions derived from consisting of oxides, sulfides, nitrides and halides.

15. A method according to claim 10 wherein said ionic material is selected from the group consisting of aluminates, aluminosilicates, silicates, titanates, zirconates, and niobates.

16. A method according to claim 10 wherein said at least one displacing metal substantially replaces said at least one non-alkaline earth ion.

17. A method according to claim 10 wherein said ionic material is preformed into a shape, and wherein said material selected from the group consisting of ceramics and ceramic composites maintains said shape.

18. A method according to claim 10 wherein said fluid is a liquid, said liquid being supplied by a melting solid comprising said at least one displacing metal.

19. A method for producing a ceramic composite, said method comprising the steps:

(a) placing in contact:

(1) a solid adapted to produce a fluid material comprising at least one displacing metal;

(2) a rigid, porous ionic material having a pore volume and comprising at least one ion,

32 maintaining said solid at sufficient temperature such that said solid produces said fluid, said fluid infiltrating said ionic material so as to at least partially replace said at least one ion, and so as to at least partially fill said pore volume, and so as to produce a material selected from the group consisting of ceramics and ceramic composites.

20. A method according to claim 19 wherein said at least one displacing metal is selected from the group consisting of magnesium, calcium, strontium, barium and mixtures thereof.

21. A method according to claim 19 wherein said at least one ion is derived from a non-alkaline earth metal.

22. A method according to claim 21 wherein said non-alkaline earth metal is selected from the groups consisting of aluminum, nickel, and niobium.

23. A method according to claim 19 wherein said ionic material is selected from the group consisting of aluminates, aluminosilicates, silicates, titanates, zirconates, and niobates.

24. A method according to claim 19 wherein said at least one displacing metal substantially replaces said at least one ion.

25. A method according to claim 19 wherein said ionic material is preformed into a shape, and wherein said material selected from the group consisting of ceramics and ceramic composites maintains said shape.

26. A method according to claim 19 wherein said fluid is a liquid, said liquid being supplied by a melting solid comprising said at least one displacing metal.

27. A ceramic composite prepared in accordance with any of the methods of claims 1-26.

28. A method for producing a material selected from the group consisting of ceramics and ceramic composites, said method comprising reacting:

33 (1) a fluid comprising at least one displacing metal; and

(2) a rigid, porous ceramic-bearing material having a pore volume and comprising at least one first component,

said at least one displacing metal capable of displacing said at least one first component; and allowing said fluid to infiltrate said rigid porous, ceramic-bearing material such that said at least one displacing metal at least partially replaces said at least one first component, and so as to at least partially fill said pore volume, and so as to produce a material selected from the group consisting of ceramics and ceramic composites.

29. A method according to claim 28 wherein said at least one displacing metal is selected from the group consisting of magnesium, calcium, strontium, barium and mixtures thereof.

30. A method according to claim 28 wherein said at least one first component is derived from a non-alkaline earth metal.

31. A method according to claim 30 wherein said at least one non-alkaline earth metal is selected from the group consisting of aluminum, nickel, and niobium.

32. A method according to claim 28 wherein said ceramic-bearing material is selected from the group of ions derived from consisting of oxides, sulfides, nitrides, halides carbides, borides, oxynitrides, and carbonitrides.

33. A method according to claim 28 wherein said ceramic-bearing material is selected from the group consisting of aluminates, aluminosilicates, silicates, titanates, zirconates, niobates, chromites, ferrites, borates, germanates, and phosphates.

34. A method according to claim 28 wherein said at least one displacing metal substantially replaces said at least one first component.

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35. A method according to claim 28 wherein said said ceramic-bearing material is preformed into a shape, and wherein said material selected from the group consisting of ceramics and ceramic composites maintains said shape.

36. A method according to claim 28 wherein said fluid is a liquid, said liquid being supplied by a melting solid comprising said at least one displacing metal.

37. A method for producing a material selected from the group consisting of ceramics and ceramic composites, said method comprising reacting:

(1) a fluid comprising at least one displacing metal; and

(2) a rigid, porous ceramic-bearing material having a pore volume and comprising at least one first component,

said at least one displacing metal capable of displacing said at least one first component; and allowing said fluid to infiltrate said rigid, porous ceramic-bearing material such that said at least one displacing metal at least partially replaces said at least one first component, and so as to at least partially fill said pore volume and so as to undergo a general displacement reaction between reactants comprising a liquid species M(l) derived from said fluid, and said rigid, porous ceramic- bearing material of the general formula, NgXc(s), as follows:

AM(1) + N_BX_c(s) = AMX_C/_A(s) + BN(l/g)

wherein MXQ/ (S) is a solid reaction product and wherein X is a metalloid element, N(l/g) is a fluid reaction product, and A, B and C are molar coefficients; and wherein said reactants are chosen such that the volume of A moles of the said solid reaction product MXQA( ) is greater than the volume of one mole of said solid reactant, NgXc(s), such that the reaction-induced volume increase can be accommodated by such pore volume, and so as to produce a material selected from the group consisting of ceramics and ceramic composites.

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38. A method according to claim 37 wherein said at least one displacing metal is selected from the group consisting of magnesium, calcium, strontium, barium and mixtures thereof.

39. A method according to claim 37 wherein said at least one first component is derived from a non-alkaline earth metal.

40. A method according to claim 39 wherein said non-alkaline earth metal is selected from the groups consisting of aluminum, nickel, and niobium.

41. A method according to claim 37 wherein said ceramic-bearing material is selected from the group consisting of oxides, sulfides, nitrides, halides, carbides, borides, oxynitrides, and carbonitrides.

42. A method according to claim 37 wherein said material is selected from the group consisting of aluminates, aluminosilicates, silicates, titanates, zirconates, niobates, chromites, ferrites, borates, germanates, and phosphates.

43. A method according to claim 37 wherein said at least one displacing metal substantially replaces said at least one non-alkaline earth ion.

44. A method according to claim 37 wherein said ceramic-bearing material is preformed into a shape, and wherein said material selected from the group consisting of ceramics and ceramic composites maintains said shape.

45. A method according to claim 37 wherein said fluid is a liquid, said liquid being supplied by a melting solid comprising said at least one displacing metal.

46. A method for producing a ceramic composite, said method comprising the steps:

(a) placing in contact:

(1) a solid adapted to produce a fluid material comprising at least one displacing metal;

36 (2) a rigid, porous ceramic-bearing material having a pore volume and comprising at least one first component, maintaining said solid at sufficient temperature such that said solid produces said fluid, said fluid infiltrating said ceramic-bearing material so as to at least partially replace said at least one first component, and so as to at least partially fill said pore volume, and so as to produce a material selected from the group consisting of ceramics and ceramic composites.

47. A method according to claim 46 wherein said at least one displacing metal is selected from the group consisting of magnesium, calcium, strontium, barium and mixtures thereof.

48. A method according to claim 46 wherein said at least one first component is derived from a non-alkaline earth metal.

49. A method according to claim 48 wherein said non-alkaline earth metal is selected from the groups consisting of aluminum, nickel, and niobium.

50. A method according to claim 46 wherein said ceramic-bearing material is selected from the group consisting of aluminates, aluminosilicates, silicates, titanates, zirconates, niobates, chromites, ferrites, borates, germanates, and phosphates.

51. A method according to claim 46 wherein said at least one displacing metal substantially replaces said at least one ion.

52. A method according to claim 46 wherein said ceramic-bearing material is preformed into a shape, and wherein said material selected from the group consisting of ceramics and ceramic composites maintains said shape.

53. A method according to claim 46 wherein said fluid is a liquid, said liquid being supplied by a melting solid comprising said at least one displacing metal.

54. A ceramic composite prepared in accordance with any of the methods of claims 28-53.

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