Synthesis of H-Phase Products

BACKGROUND OF THE INVENTION The present invention relates to the production of ceramics, and in particular to ceramics of the formula M ₂ R ₁ X ₁ wherein M is transition metal, R is Si, Al, Ge, Pb, Sn, Ga, P, S, In, As, Tl and/or Cd, and X is B, C and/or N.

In general, metals are easily machined but do not retain their machined form at high temperatures. Ceramics retain their shape at extremely high temperatures, but are brittle and very difficult to machine into a desired shape. Materials scientists have directed a great deal of effort into finding compositions that are easily machined into a desired shape and are stable at extremely high temperatures. Certain H-phases of ternary systems have been described. See J.C. Schuster et al . , "The Ternary Systems: Cr-Al-C, V-Al-C, and Ti-Al-C and the Behavior of H-Phases (M ₂ A1C) " J. Solid Sta te Chem. , 32:213-219 (1980) and references cited therein as leading references. However, the manufacturing processes used to prepare these H-phases are not very amenable to economical large-scale production ( i . e . , multi-gram quantities) , and thus their physical and chemical properties have scarcely been examined.

The prior art refers to ternary compounds of the formula M ₂ RX as H-phases. The data available suggests that H-phases have a layered structure.

BRIEF SUMMARY OF THE INVENTION The present invention is directed to a process for forming a product comprising an M ₂ R ₁ X ₁ phase wherein M is at least one transition metal, R is a metal selected from the group consisting of Si, Al, Ge, Pb, Sn, Ga, P, S, In, As, Tl and Cd, and X is a non-metal selected from the group consisting of B, C and N, said process having the steps of

(a) forming a mixture of (i) transition metal species, (ii) co-metal species selected from the group consisting of silicon species, aluminum species, germanium species, lead species, tin species, gallium species, phosphorus species, sulfur species, indium species, arsenic species, thallium species and cadmium species, and (iii) non-metal species selected from the group consisting of boron species, carbon species and nitrogen species; and

(b) reactive hot pressing said mixture at a temperature of about 1000°C to about 1800°C under a pressure of about 5 MPa to about 200 MPa for a time sufficient to form MjRjX phase.

Another aspect of the invention is a process for forming a product comprising an phase, as defined above, in which a preform is formed from compaction under pressure of a mixture of either (i) the transition metal species and the non-metal species or (ii) the nonmetal species and the co-metal species, prior to combining the preform with the third component, i.e., the co-metal species or the transition metal species, respectively, and thereafter the resulting combination is heated at a temperature of about 1000°C to about 1800°C for a time sufficient to form the M ₂ R ₁ X ₁ phase. Still another aspect of the invention is a process for forming a product comprising an M ₂ [Al,R] ₁ [C,X] _x phase wherein M is at least one transition metal, R is a metal

selected from the group consisting of Si, Al, Ge, Pb, Sn, Ga, P, S, In, As, Tl and Cd, and X is a non-metal selected from the group consisting of B, C and N, said process having the steps of (a) forming a mixture of (i) transition metal species, (ii) aluminum carbide, (iii) one or more species selected from the group consisting of silicon species, germanium species, lead species, tin species, gallium species, phosphorus species, sulfur species, indium species, arsenic species, thallium species and cadmium species, and (iv) boron species and nitrogen species; and (b) heating the mixture of step (a) to a temperature of about 1000°C to about 1800°C for a time sufficient to form M ₂ [Al,R] ₁ [C,X] _x phase.

DETAILED DESCRIPTION OF THE INVENTION The present invention provides processes wherein a mixture of powders is exposed to high temperature and, preferably, pressure for a time sufficient to allow at least some of the powders to react with one another to form an HS- phase. As used herein, the term HS-phase encompasses ternary compounds and solid solutions, both of the formula M ₂ R ₁ X ₁ wherein M is one or more transition metals, R is one or more of silicon (Si) , aluminum (Al) , germanium (Ge) , gallium (Ga) , lead (Pb) , tin (Sn) , phosphorus (P) , sulfur (S) , indium (In) , arsenic (As) , thallium (Tl) and cadmium (Cd) , while X is one or more of boron (B) , carbon (C) and nitrogen (N) . For convenience, the elements denoted by "M" will be referred to as transition metals, the elements denoted by "R" will be referred to as co-metals (although could also be referred to as semi-metals or, more simply, as "metals"), and the elements denoted as "X" will be referred to as non-metals.

In general, a ternary compound consists essentially of three elements in a regular repeating array. The ternary compounds prepared by the inventive process are formed from a single transition metal (M) , a single co-metal (R) and a single non-metal (X) . The stoichiometry of the ternary compounds prepared by the invention is such that the M:R:X atomic ratio is substantially centered around 2:1:1. The prior art denotes such ternary compounds as H-phases. Preferred H-phases prepared by the invention include Ti ₂ AlC, Ti ₂ AlN, V ₂ A1C, Ta ₂ AlC, Cr ₂ AlC and Nb ₂ AlC.

Exemplary H-phase ternary compounds include, in addition to the preferred H-phases described above, the following: Ti ₂ GeC, Ti ₂ GaC, Ti ₂ PbC, Ti ₂ SnC, Ti ₂ SC, Ti ₂ InC, Ti ₂ TlC, Ti ₂ CdC, Zr ₂ PbC, Zr ₂ SnC, Zr ₂ SC, Zr ₂ InC, Zr ₂ TlC, Hf ₂ PbC, Hf ₂ SnC, Hf ₂ SC, Hf ₂ InC, Hf ₂ TlC, V ₂ GeC, V ₂ GaC, V ₂ PC, V ₂ AsC,

Nb ₂ GaC, Nb ₂ SnC, Nb ₂ PC, Nb ₂ SC, Nb ₂ InC, Ta ₂ GaC, Cr ₂ GeC, Cr ₂ GaC and Mo ₂ GaC.

In contrast to ternary compounds, the solid solutions prepared by the inventive process are formed from at least four and possibly many more elements, where each of the elements is either a transition metal, a co-metal or a non-metal as defined herein. Thus, M in a solid solution may be one or more transition metals, while R may be one or more of silicon (Si) , aluminum (Al) , germanium (Ge) , gallium (Ga) lead (Pb) , tin (Sn) , phosphorus (P) , sulfur (S) , indium (In) , arsenic (As) , thallium (Tl) and cadmium (Cd) , and X may be one or more of boron (B) , carbon (C) and nitrogen (N) . However, the molar ratio of the total of the transition metals (M) to the total of the co-metals (R) to the total of the non-metals (X), i . e . , M:R:X in a solid solution prepared by an inventive process is substantially centered around 2:1:1.

Thus, the solid solutions prepared by the invention are essentially H-phases wherein some of the transition metal is replaced with one or more different transition metals, and/or some of the co-metal is replaced with one or more different co-metals, and/or some of the non-metal is replaced with one or more different non-metals. Preferred solid solutions include Ti ₂ Al[C,N]; Ti ₂ [Al,Si]C; Ti ₂ [Al,Si] [C,N] ; [Ti,V] ₂ AlC; and [Ti,Zr] ₂ [Si,Al,Ge] [C,N] . Other exemplary compounds in this category include [Ti ₀₇₅ V ₀₂₅ ] ₂ SC and Nb ₀₂ Zr _{0 B} A1C. Taking the designation

[Ti,Zr] ₂ [Si,Al,Ge] [C,N] as an example, this term has its ordinary meaning as recognized in the art, and simply refers to a solid solution that consists of both titanium and zirconium as the transition metal, silicon, aluminum and germanium as co-metals and carbon and nitrogen as non- metals.

The ternary compounds (H-phases) prepared by the invention and the solid solutions thereof prepared by the invention will be referred to collectively herein as HS- phases. Some, but not all of the HS-phases and composites thereof that can be prepared according to the inventive processes have been previously described in the literature.

The HS-phases prepared by the invention are typically single phase and polycrystalline. In this instance, the term polycrystalline means that when viewed under a microscope, distinct grains can be seen wherein each distinct grain is formed of a single crystal of the ternary compound or solid solution. The grains can be distinguished from one another by their having unique crystal structure directionality. The term "single phase" is very well known in the art, and simply means that only one phase is present in the final microstructure.

The invention also provides for the preparation of composites of HS-phases, i.e., compositions wherein an HS- phase is in contact with at least one non-HS-phase. A non- HS-phase according to the invention is simply any phase which is not an HS-phase as defined herein. The non-HS- phase is a solid at room temperature and pressure. Preferably, although not necessarily, the non-HS-phase will be in thermal equilibrium with the HS-phase. Reference to a phase diagram will allow one of ordinary skill in the art to readily determine non-HS-phases that are in thermal equilibrium with an HS-phase. As prepared according to the invention, the composites will have the HS-phase preferably as the matrix.

The synthesis of HS-phases and composites thereof by the inventive process requires the preparation of a mixture of powders that has the same atomic constitution as the desired HS-phase or composite thereof. According to the inventive process, all or essentially all of the atoms present in the mixture of powders will also be present in the product HS-phase or composite thereof, and essentially all of the atoms present in the product HS-phase or composite will also have been present in the mixture of powders. Thus, it is essential to prepare the mixture of powders such that it has the same atomic ratio as is desirably present in the product HS-phase or composite.

The mixture of powders employed in the inventive process comprises (i) transition metal species, (ii) co- metal species such as aluminum species, germanium species, gallium species, silicon species, lead species, tin species phosphorus species, sulfur species, indium species, arsenic species, thallium species and/or cadmium species, and (iii) non-metal species such as boron species, carbon species and/or nitrogen species. The term "species" simply refers

to a chemical that contains the indicated element, where chemicals include molecules, salts, compounds, complexes, polymers, etc.

A transition metal species is a chemical that includes at least one transition metal. Exemplary transition metals are those of Group 3 (Sc, Y, La and Ac) , Group 4 (Ti, Zr and Hf) , Group 5 (V, Nb, Ta) and Group 6 (Cr, Mo and ) of the Periodic Table of the Elements (current IUPAC designations) . Other exemplary transition metals are first row transition metals, i.e., Mn, Fe, Co, Ni and Zn.

The transition metal species may be the transition metal per se, e . g. , titanium metal of greater than 99% purity, or it may be a transition metal compound, complex, molecule, salt, etc., such as a transition metal hydride ( e . g. , titanium hydride), transition metal boride (e.g., titanium boride), transition metal carbide (e.g., titanium carbide), transition metal silicide ( e . g. , titanium silicide) or a transition metal nitride ( e . g. , titanium nitride) .

Any transition metal species can be employed in preparing the mixture of powders according to the invention. However, since substantially all of the atoms present in the starting mixture of powders are also present in the final product composition, the transition metal species preferably contains only the elements that are desirably present in the final product composition. Transition metal hydrides are an exception to this general rule, and in fact transition metal per se and transition metal hydride are preferred transition metal species according to the invention.

Some transition metals are difficult to prepare in powdered form, and it is known in the art to react transition metal per se with hydrogen, to form transition

metal hydride that is substantially more brittle than transition metal per se . The transition metal hydride is then powdered and may be used in the inventive process, or may be converted back to transition metal per se by dehydriding before being used in the inventive process.

Titanium is a preferred transition metal for a transition metal species. In preparing products by the inventive process, titanium metal per se, including dehydrided titanium, as well as titanium hydride are preferred transition metal species. Thus, M is preferably Ti in the HS-phases and composites thereof prepared according to the invention. When H-phase solid solutions are prepared according to the invention, M is preferably predominantly titanium, more preferably M is at least about 80% of the transition metal component of the H-phase solid solution, and even more preferably is at least about 95% of the transition metal component of the H-phase solid solution.

Other preferred transition metals besides titanium include zirconium and hafnium.

A co-metal species of the invention is any chemical, e . g. , a compound, complex, molecule or salt, that contains an aluminum, germanium, gallium, silicon, lead, tin, phosphorus, sulfur, indium, arsenic, thallium and/or cadmium atom. Likewise, an aluminum, germanium, gallium, silicon, lead, tin, phosphorus, sulfur, indium, arsenic, thallium and/or cadmium species is any chemical that contains at least one aluminum, germanium, gallium, silicon, lead, tin, phosphorus, sulfur, indium, arsenic, thallium and/or cadmium atom, respectively. Thus, a co-metal species may be a co-metal per se ( e . g. , silicon metal), co-metal carbide ( e . g. , silicon carbide), co-metal nitride (e.g., silicon nitride) or a bimetallic transition metal/co-metal

species ( e . g. , titanium silicide). The co-metal species may be a metal per se of Si, Al, Ge, Pb, Sn, Ga, In, As, Tl and Cd or it may be elemental P and elemental S. Likewise, the co-metal may be a carbide or nitride of Si, Al, Ge, Pb, Sn, Ga, In, As, Tl and Cd; or a silicide of Al, Ge, Pb, Sn, Ga, In, As, Tl and Cd; a silicide of a transition metal, e . g. , TiSi; or a sulfide or phosphide of a transition metal,e.g., TiS.

Preferred aluminum species are aluminum metal (aluminum metal per se) , aluminum nitride and aluminum carbide.

Preferred germanium species are germanium metal (germanium metal per se) , germanium nitride and germanium carbide. Preferred gallium species are gallium metal

(gallium metal per se) , gallium nitride and gallium carbide.

Preferred silicon species are silicon metal (silicon metal per se) , silicon carbide, silicon nitride and transition metal suicides such as titanium silicide. Silicon carbide is a more preferred silicon species, where silicon carbide can also serve as a non-metal (carbon) species in the mixture, as discussed below.

A preferred lead species is lead metal (lead metal per se) . A preferred tin species is tin metal (tin metal per se) .

A preferred phosphorus species is elemental phosphorus powder. A preferred sulfur species is elemental sulfur powder.

A third component of the mixture of the invention is a non-metal species, which is any complex, compound, molecule, salt, etc., that contains at least one non-metal atom such as carbon, boron and nitrogen. Preferred non- metal species contain boron (denoted herein as boron

species) , carbon (denoted herein as carbon species) and/or nitrogen (denoted herein as nitrogen species) .

Carbon species are a preferred non-metal species, where preferred carbon species contain substantially exclusively carbon atoms, such as graphite, carbon black, charcoal and coke. However, carbon species containing atoms in addition to carbon, such as transition metal carbides ( e . g. , titanium carbide) and non-transition metal (i.e., co- metal) carbides ( e . g. , silicon carbide) may also be employed as the carbon species.

Boron species that are suitable non-metal species for the invention include boron, silicon boron, carborane and transition metal boride.

Nitrogen species that are suitable non-metal species for the invention include silicon nitride and transition metal nitride ( e . g. , titanium nitride) .

As seen from the above illustrations, a specific transition metal, co-metal or non-metal species may be elemental, i.e., formed of only the transition metal, co- metal or non-metal respectively. In addition, a specific transition metal, co-metal or non-metal species may be binary, i.e., formed from two elements, e . g. , SiC, although not necessarily in equimolar amounts, e . g. , Al ₄ C ₃ . Although not typically the case, the specific transition metal, co- metal or non-metal species may be ternary, quaternary, or even a higher order form.

The mixture of powders used as the starting material in the inventive process is formed from transition metal species, co-metal species and non-metal species. Thus, each of the transition metal species, co-metal species and non-metal species is preferably in powdered form prior to be mixed together to form the mixture of powders. A certain powder in the mixture of powders can serve in more

than one capacity, i.e., it can function as more than one of a transition metal species, co-metal species or non-metal species. For example, silicon carbide may be present in the mixture of powders, and serve as both a co-metal species and non-metal species.

The individual powders used to prepare the mixture of powders preferably have mean particle diameters (d of about 1 to about 200 microns and, more preferably, about 1 to about 50 microns. Another way to characterize the size of the powders is to specify the mesh size through which they will pass. By this convention, the powders used in the invention preferably have a mesh size of less than about 100, and more preferably have a mesh size of less than about 325 . The designation -325 mesh indicates that the powder will pass through a 325 mesh screen.

To prepare an HS-phase by the inventive process, a mixture of powders containing transition metal species, co- metal species and non-metal species is prepared such that the molar ratio of M:R:X in the mixture of powders is substantially centered around 2:1:1, and substantially only M, R and X are present in the mixture of powders. The exact amount by which the M:R:X molar ratio may vary from 2:1:1 and yet still form exclusively HS-phase depends on the identity of the HS-phase being prepared. Thus, reference to a phase diagram including the HS-phase of interest will reveal the stoichiometric boundaries of the HS-phase, and thus the stoichiometric boundaries within which the mixture of powders must stay if exclusively HS-phase is to be formed. As a rough estimate, the molar ratio of M:R:X should be within about 20% of the 2:1:1 ratio, i.e., about 2.4-1.6:1.2-0.8:1.2-0.8 for HS-phase to form exclusively.

If a composite of an HS-phase is to be prepared by the inventive process, then the molar ratio of M:R:X in the

mixture of powders can vary over a wider range than is the case when exclusively HS-phase is to be prepared. In those instances where the non-HS-phases are in thermal equilibrium with the HS-phase, then reference to a phase diagram will allow one of ordinary skill in the art to determine the molar ratio of M, R and X that may be present in the mixture of powders. In those instances where the non-HS-phase is not in thermal equilibrium with the HS-phase, then the mixture of powders should contain M:R:X in a ratio substantially centered around 2:1:1, along with powders which will form the non-HS-phase.

To prepare the mixture of powders, all of the individual powders of transition metal species, co-metal species and non-metal species are combined and then mixed thoroughly to provide a homogeneous mixture. Machines that can mix powders to homogeneity are well known in the art, and are suitably employed in the present invention. One such machine is known as a V-blender. A mixing time of from about 30 minutes to about 2 hours in a V-blender will typically provide a homogeneous mixture of powders suitable for use in the inventive process.

As a preferred but optional step, the mixture of powders is compacted to form what is known in the art as a "compact" or a "green body". Methods for forming compacts and green bodies from powders are well known in the art, and any such method may be employed in the inventive process. Green bodies for use in the inventive process may be formed by cold-pressing, i.e., no heat is applied while the mixture of powc s is placed under pressure. A binder may optionally be present in the mixture of powders when forming a green body, where the binder provides some cohesiveness to the powders that make up the green body. Appropriate binders are well known in the art.

A preferred process for forming the green body places the mixture of powders into a die, and then exerts a pressure of about 5 MPa to about 300 MPa, preferably about 180 MPa onto the mixture. A time of only a few minutes is typically sufficient to form the green body. For convenience, the following description will refer to the green body, however the (uncompacted) mixture of powders can just as easily be used in the following process.

According to the invention, the green body is exposed to high temperature, and optionally is simultaneously exposed to pressure. Under these conditions the components of the green body react with one another to form an HS-phase and, optionally, composites thereof. The term "hot pressing" is well-known in the ceramics art as referring to a number of specific processes wherein materials are heated under pressure. In the prior art hot pressing processes, a reaction may or may not occur between the components of the material being hot pressed. However, hot pressing according to the present invention necessarily provides for a reaction to occur between the components in the green body or mixture of powders, and thus a preferred embodiment of the inventive process will be referred to herein as reactive hot pressing. However, any hot pressing process as known in the art may be used to achieve reactive hot pressing according to the present invention.

The Concise Encyclopedia of Advanced Ceramic Materials, R. J. Brook, ed., Pergammon Press, Oxford, 1991 provides a description of hot pressing processes. Two hot pressing processes preferred according to the present invention are known as vacuum hot pressing and hot isostatic pressing (HIP) . While both of these techniques are widely used in the art and thus need not be described in detail, each will be briefly summarized.

In vacuum hot pressing, a sample is placed in a press, where the sample holder can be evacuated and heated. The sample is then steadily heated and the press is activated so that a steadily increasing load is applied to the sample. Samples can be exposed simultaneously to temperatures in excess of about 2000°C and pressures in excess of about 100 MPa by the vacuum hot pressing process. After the desired amount of time, the chamber is cooled and the pressure on the sample is released. In HIP, a sample is placed in a chamber, where the chamber can be quite large. The sample is encapsulated either before insertion into the chamber or becomes encapsulated during the HIP process. A convenient means to encapsulate the sample during the HIP process is to place the sample into a glass tube (e.g., Pyrex ^® ), place the glass tube into the HIP chamber, evacuate the chamber and heat the chamber to the softening point of the encapsulating glass (usually around 800-900°C, however this temperature can vary over a wide range) . After the encapsulated sample is in the chamber, an inert gas is pumped into the chamber to a pressure of about 40 MPa (again, this pressure can vary over a wide range) . Thereafter, the chamber is heated to a higher temperature resulting in a concomitant increase in pressure inside the chamber. After the desired amount of time, the chamber is cooled and the excess pressure is released.

Regardless of the details of the hot pressing process, a preferred embodiment of the invention provides that a green body is exposed to a temperature of about 1000°C to about 1800°C, preferably about 1200°C to about 1700°C, and more preferably about 1300°C to about 1600°C. Simultaneously, the green body is exposed to a pressure of about 5 MPa to about 200 MPa, and preferably about 15 MPa to

about 60 MPA. According to a preferred process, the green body is exposed to a temperature of about 1300°C to about 1600°C while being under a pressure of about 15 MPa to 50 MPa. Under these conditions, the powders preferably remain as solids, i.e., do not liquefy or volatilize. Thus, at all times during the hot pressing process, the materials being heated and pressed preferably remain in the solid state. However, with low melting components such as sulfur, phosphorus, lead and tin, the mixed powders in the green body may undergo at least some liquification during the reactive hot pressing process, prior to formation of the desired HS-phase product.

The green body is held under these conditions to form a HS-phase or composite thereof as described above. Preferably, the green body is held under these conditions until the reaction to form the HS-phase or composite thereof has gone to completion, i.e., until thermal equilibrium has been reached. A reaction time of about 5 hours is usually sufficient, and even shorter reaction times, for example about 1 hour or even about 10 minutes may be suitable.

However, shorter reaction times may be employed at higher reaction temperatures and/or pressures.

The HS-phases or composites thereof of the invention ("the products") preferably contain few if any voids, also called pores, between the grain boundaries of a product. Thus, a preferred product has less than 1 volume percent contributed by pores. Consequently, the preferred product will have a density equal to, or nearly equal to the theoretical density for the HS-phase or composite thereof. In another embodiment of the inventive process, the mixture of powders may be heated without applied pressure. Such heating is preferably accomplished in a controlled manner, i . e . , such that the powders do not

ignite. Another preferred pressureless reaction process is self-propagating high-temperature synthesis (SHS) , as described in, e . g. , H. Pampuch et al. , J. Materials Synthesis and Processing 1(2):93-100 (1993). In either event, some of the powder constituents may melt during the reaction that forms the HS-phase or composite thereof.

When the mixture of powders is heated without pressure, the final product will more likely than not be porous. For some applications, such a porous product can be used as is. For other applications, such a product may be ground or milled to form powder, and this powder can then be pressureless sintered or hot-pressed to form a final product. For example, Ti ₂ AlC powders can be fabricated by heating titanium (Ti) , aluminum carbide (Al ₄ C ₃ ) and carbon (as in Example 1, below) , followed by grinding or milling the reaction product to form powdered Ti ₂ AlC.

Tn yet another embodiment of the invention, a preform may be formed of (a) the co-metal and the non-metal or (b) the transition metal and non-metal, either as elemental powders or as compounds. The preform is prepared according to techniques known in the art, such as by simply pressing the powders, optionally with a binder. Then transition metal (in case (a)) or the co-metal (in case (b) ) is melted separately and poured or otherwise placed into contact with the preform to initiate the reaction and form the final body of HS-phase or composite thereof.

The following examples are set forth as a means of illustrating the present invention and are not to be construed as a limitation thereon. In the Examples that follow, the graphite (carbon component) was obtained from Aldrich Chemical Company, Inc., Milwaukee, WI . All other component powders (except as noted) were obtained from Alfa Aesar Johnson Matthey Co., Ward Hill, MA.

Example 1 A mixture was prepared from the following powders: 21.3 g of titanium (99% pure, -325 mesh), 0.7 g of graphite (99.0% pure, d., = 1-2 μm) and 8 g Al ₄ C ₃ (99% pure, -325 mesh) . The powders were dry-mixed in a V-blender for two hours, and then the mixture was cold pressed under 180 MPa in a 7.6 x 1.3 cm rectangular die to form a green body. The thickness of the green body was =15 mm.

The green body was wrapped in graphite foil, sprayed with boron nitride (which acts as a mold release) and then placed in a 7.6 x 1.3 cm split graphite die which was also sprayed with boron nitride on the inside. The die was then placed in a vacuum hot press, and subjected to the following temperature and pressure cycles (both cycles were started simultaneously): Temperature: heating rate was 600°C/hr to a temperature of 1600°C, held at 1600°C for 4 hours, then cooling at the same rate as it was heated. Pressure: loading rate was 41 kN/hr to a pressure of 42 MPa, held at 42 MPa for 5 hr and 40 min, then unloaded at the same rate as it was loaded.

After cooling, the resulting product was found to be fully dense, single phase Ti ₂ AlC. The hardness of the Ti ₂ AlC product was measured to be 5 ± 1 GPa. Density of the Ti ₂ AlC product was measured as 4.1 g/cm ³ , and electrical conductivity of the product was determined to be 2.8 x 10 ⁶ (ohm m)" ¹ . The Ti ₂ AlC product was easily machinable without lubrication using regular high speed tool steel, and was easily hand-tapped to form very well-defined threads. Scanning electron microscopy of the fractured surfaces unambiguously demonstrated the layered structure of the product.

Example 2 A mixture was prepared from the following powders: 11.98 g of titanium (99% pure, -325 mesh), 1.5 g of graphite (99.0% pure, d _m = 1-2 μm) and 9.07 g germanium metal (99.999% pure, -100 mesh).

The powders were dry-mixed and cold pressed as in Example 1. The resulting green body was placed in a vacuum hot press as in Example 1 and heated/pressed according to the following protocol: heating to 900°C at 300°C/hour for 2 hours, held for one hour at 900°C, then ramped at

300°C/hour to 1200°C, soak (held) at 1200°C for 4 hours, slowly cooled. Pressure: loading rate was 22 kN/hr to a pressure of 45 MPa, held at 45 MPa for 7 hr, then unloaded at the same loading rate as it was loaded. The resulting product was Ti ₂ GeC. The hardness of the Ti ₂ GeC product was measured to be 5 ± 1 GPa. Density of the Ti ₂ GeC product was measured as 5.3 g/cm ³ . Electrical conductivity of the product was determined to be 4.4 x 10 ⁶ (ohm m) ^"1 ; this electrical conductivity is believed to be a lower limit since the product was not fully single phase.

The Ti ₂ GeC product was easily machinable without lubrication using regular high speed tool steel, and was easily hand- tapped to form very well-defined threads. Scanning electron microscopy of the fractured surfaces unambiguously demonstrated the layered structure of the product.

Example 3 A mixture was prepared from the following powders : 47.9 g titanium metal (99% pure, -325 mesh) and 20.5 g aluminum nitride, AIN (98% pure, -325 mesh) . The mixture was formed into a green body and then hot pressed as described in Example 1.

After cooling, the resulting product was found to be fully dense, single phase Ti ₂ AlN. The hardness of the Ti ₂ AlN product was measured to be 3.5 ± 0.7 GPa. Density of the Ti ₂ AlN product was measured as 4.3 g/cm ³ ' and electrical conductivity of the product was determined to be 3.2 x 10 ⁶ (ohm m)" ¹ . The Ti ₂ AlN product was easily machinable without lubrication using regular high speed tool steel, and was easily hand-tapped to form very well-defined threads. Scanning electron microscopy of the fractured surfaces unambiguously demonstrated the layered structure of the product.

Example 4 A mixture was prepared from the following powders: 24.5 g of vanadium (99.5% pure, -325 mesh) , 0.7 g of graphite (99.0% pure, d,. = 1-2 μm) and 8.67 g Al ₄ C ₃ (99% pure, -325 mesh) . The powders were dry-mixed in a V-blender for two hours, and then the mixture was cold pressed under 180 MPa in a 7.6 x 1.3 cm rectangular die to form a green body. The thickness of the green body was =15 mm.

The green body was reacted using a hot isostatic pressing (HIP) process. According to this process, the green body was sealed in a Pyrex ^® glass tube under vacuum. The sealed glass tube was placed in the chamber of a hot isostatic press, the chamber was evacuated and subjected to the following temperature and pressure cycles. Starting

from room temperature, a heating rate of 30°C/min was used to attain a temperature of 850°C (the glass softening temperature) within the evacuated chamber, and the mixture held at that temperature for 1 hour. The system was then pressurized to 40 MPa and the heating continued at a rate of 10°C/min up to 1600°C which caused an increase in the pressure to 60 MPa. The sample was maintained under these conditions for four hours and then cooled.

After cooling, the sample was soaked in hydrofluoric acid to dissolve the encasing Pyrex ^® glass. The resulting product was found to be single phase V ₂ A1C. The V ₂ AlC product was easily machinable without lubrication using regular high speed tool steel. Scanning electron microscopy of the fractured surfaces unambiguously demonstrated the layered structure of the product.

Example 5 A mixture was prepared from the following powders: 48.24 g of tantalum (99.8% pure, -325 mesh), 0.4 g of graphite (99.0% pure, d _m = 1-2 μm) and 4.8 g A1 ₄ C ₃ (99% pure, -325 mesh) . The powders were dry-mixed and compacted as described in Example 4.

After treatment with hydrofluoric acid to remove the encasing glass, the resulting product was analyzed by x- ray diffraction and identified as Ta ₂ AlC. The Ta ₂ AlC product was a single phase material and was easily machinable without lubrication using regular high speed tool steel. Scanning electron microscopy of the fractured surfaces unambiguously demonstrated the layered structure of the produc .

Example 6 A mixture was prepared from the following powders: 49.5 g of niobium (99.8% pure, -325 mesh, obtained from Cabot Performance Materials, Boyertown, PA), 0.8 g of graphite (99.0% pure, d,- = 1-2 μm) and 9.6 g A1 ₄ C ₃ (99% pure, -325 mesh) . The powders were dry-mixed and compacted as described in Example 4.

After treatment with hydrofluoric acid to remove glass, the resulting product was analyzed by x-ray diffraction and found to be Nb ₂ AlC. The Nb ₂ AlC product was a single phase material and was easily machinable without lubrication using regular high speed tool steel. Scanning electron microscopy of the fractured surfaces unambiguously demonstrated the layered structure of the product.

Example 7

A mixture was prepared from a combination of the following powders: 47.9 g of titanium (99% pure, -325 mesh), 6 g of graphite (99% pure, d,. = 1-2 μm) and 59.34 g tin (99.8% pure, -325 mesh) . The powders were dry mixed for 30 minutes in a V-blender, and then a portion of the mixture was cold pressed in a steel die under 20,000 psi (140 MPa) at a rate of 200 psi/sec (1.4 MPa/sec) . The pressure on the thus-formed green body was removed at a rate of 1,000 psi/sec (6.9 MPa/sec) to form a green body in the shape of a rectangular block that was 32 x 12 x 9 mm in size and that weighed 18 g.

The green body was then sealed in an evacuated Pyrex ^® test tube under vacuum. The sealed glass test tube containing the green body was then placed in a furnace chamber of a Hot Isostatic Press (HIP) and was subjected to a temperature and pressure cycle as follows. Starting from

atmospheric pressure (about 760 mm Hg) and room temperature (about 20-25°C) , air was evacuated from the furnace chamber and the chamber was flushed with an inert gas, argon, to remove traces of air. A heating rate of 5°C/min was applied until a temperature of 850°C (glass softening temperature) was reached. The green body in the sealed glass test tube was then heat-soaked at 850°C for one hour. The furnace chamber was then pressurized to 40 MPa with argon and the heating cycle continued at a heating ramp rate of 10°C/min until a temperature of 1,325°C was reached. During the last heating ramp period, the pressure in the furnace chamber rose to 50 MPa since the HIP is a closed system. The furnace chamber was maintained at this pressure and at 1,325°C for four hours, after which the chamber was cooled to room temperature and its pressure was reduced to atmospheric pressure.

After cooling and removal from the HIP, the glass surrounding the processed sample was removed by mechanical means to recover the product. The resulting product was determined to be a fully dense, single-phase Ti ₂ SnC. The Vickers hardness of the Ti ₂ SnC was measured to be approximately 4 GPa. Density of the Ti ₂ SnC product was 6.12 g/cm ³ , and electrical conductivity of the product was 14 x 10 ⁶ (ohm m)" ¹ . The Ti ₂ SnC product was readily machinable without lubrication using regular high-speed tooled steel. Scanning electron microscopy of the fractured surfaces unambiguously demonstrated the layered structure of the product.

Example 8 The procedure of Example 7 was repeated except that zirconium was used in place of the titanium used in Example 7. A mixture was prepared from a combination of the following powders: 91.22 g zirconium (99.9% pure, -325 mesh, obtained from (Prochem Inc., Rockford, IL) , 6 g of graphite (99% pure, d., = 1-2 μm) and 59.34 g of tin (99.8% pure, -325 mesh) . A green body was formed using the same procedure described for Example 7, the green body being in the shape of a rectangular block of 32 x 12 x 9 mm and weighing 20 g.

The green body was then processed using the same HIP procedure as described for Example 7, except that the processing temperature was 1330°C. The resulting product was found to be fully dense, single-phase Zr ₂ SnC. The Vickers hardness of the Zr ₂ SnC was measured to be approximately 4 GPa. The density of the Zr ₂ SnC product was 6.9 g/cm ³ , and electrical conductivity of the product was 6.7 x 10 ⁶ (ohm m) ^"1 . The Zr ₂ SnC product was easily machinable and was demonstrated by scanning electron microscopy to be a layered structure.

Example 9 The procedure of Example 7 was repeated except that hafnium was substituted for the titanium used in Example 7. A mixture was prepared from the following powders: 178.5 g of hafnium (99.6% pure, -325 mesh) , 6 g of graphite (99% pure, d _m = 1-2 μm) and 59.34 g of tin (99.85 pure, -325 mesh) . A green body was formed from a mixture of these powders using the same procedure as described in Example 7 to form a green body having the shape of a rectangular block 32 x 12 x 9 mm and weighing 33 g.

The green body was processed using the same HIP procedure as described for Example 7, except that the processing temperature was 1300°C.

The resulting product was found to be a fully dense, single-phase Hf ₂ SnC. The Vickers hardness of the

Hf ₂ SnC was measured to be approximately 5 GPa. The density of the Hf ₂ SnC product was 11.8 g/cm ³ , and the electrical conductivity of the product was 2.4 x 10 ⁶ (ohm m)" ¹ . The Hf ₂ SnC product was easily machinable and was demonstrated by scanning electron microscopy to be a layered structure.

It will be appreciated by those skilled in the art that changes could be made to the embodiments described above without departing from the broad inventive concept thereof. It is understood, therefore, that this invention is not limited to the particular embodiments disclosed, but it is intended to cover modifications within the spirit and scope of the present invention as defined by the appended claims.