The present invention generally relates to a process for producing single crystal metal carbide, nitride, or carbonitride whiskers and, in particular, to a process for producing metal carbide, nitride, or carbonitride whiskers by a chemical vapor deposition process and the products thereof. Description of the Prior Art

In recent years there has been an increasing need for materials having high fracture toughness, hardness, and wear resistance for use in cutting tools, wear parts, and structural applications. "Whiskers" are minute, high purity, single crystal fibers having strengths approaching interatomic bonding forces and improved resistance to high temperatures and greater toughness when compared to polycrystalline fibers. Due to their high modulus of elasticity, hardness, strength, and chemical stability, single crystal whiskers of such materials as carbides or nitrides of titanium, zirconium, hafnium, niobium, tantalum, and tungsten are candidate materials to reinforce and toughen metal, ceramic and glass matrix composites. Whiskers may be grown by a number of processes including chemical vapor deposition (CVD) . CVD is particularly attractive because the process allows close control of the growth and composition of

the whiskers. Typically methods for producing whiskers of metal carbides ^' or nitrides involve placing a substrate material suitable for whisker growth, e.g. alloy, graphite or ceramic plates, in a reaction chamber having a controlled atmosphere and heating the growth substrate to a temperature suitable for whisker growth. Typical temperatures in such reactors range from about 800 degrees to 1400 degrees C. The reactor is first flushed with hydrogen gas. Then, reactant gases, typically in a molar ratio of carbon or nitrogen to metal of about 1:1, are flowed through the heated reactor to form whiskers on the growth substrate. One such process for producing such whiskers is described in U.S. Patent No. 4,576,791, issued to D'Angelo et al. The choice of the growth substrate materials can be critical to the formation and type of whiskers (see Wokulski et al., J. Crystal Growth, 62, pp. 439- 446 (1983)) . For example, the use of graphite as a growth substrate has been shown to produce a variance in the C/Ti mole ratio. On the other hand, growth substrates of tungsten, molybdenum, and iron, while not affecting the growth of the whiskers, do not produce significant numbers of whiskers. In addition, iron- based growth substrates react vigorously with the reactants for producing the whiskers. Generally it has been found that growth substrates containing nickel, such as high nickel alloys and nickel impregnated graphite, have been most satisfactory in producing the highest numbers and most controllable whiskers. However, the prior art processes have presented the disadvantages of poor control over the amount and availability of nickel in the growth substrate. This can produce a lack of control of whisker morphology and dimensions and limitations on the efficiency of whisker production by such methods.

One unusual CVD process which has been used for growing beta silicon carbide whiskers is the vapor-

liguid-solid (VLS) process (see Milewski et al., J. Materials Science. 20, pp. 1160-1166 (1985)). In this process a liquid catalyst is used in place of a solid substrate. Thus, the catalyst must display the ability, when molten, to take into solution the elements and compounds necessary to produce the desired whiskers. For SiC whisker growth, transition metal and alloy powders have been satisfactory. Typically the reaction for the VLS process takes place at approximately 1400 degrees C or above the melting point of the catalyst.

One problem with the VLS process is that the whiskers which can be grown by this process have been limited to those having a reaction temperature equal to or greater than the melting point of the catalyst. In addition, residual catalyst may remain with the whiskers thereby adversely affecting the physical properties of the subsequent composite article. Attempts to remove the residual catalyst by chemical or physical means have resulted in damage to the whiskers.

U.S. Patent No. 4,686,197, issued to Elvin, discloses a process for demetallizing a petrochemical catalyst with chlorine gas at approximately 350 degrees C. According to Elvin, the process successfully removed Ni and/or V from a contaminated catalyst.

However, while generally teaching the removal of Ni by reacting with chlorine gas at elevated temperatures, Elvin was not directed to removing residual catalyst from single crystal whiskers. In addition, Elvin did not address the problem of reactions between the chlorine gas and the whiskers and reaction vessel.

U.S. Patent No. 4,492,767, issued to Fung, discloses a process for reactivating a coked petrochemical catalyst which includes a halide pretreatment step at approximately 300 to 540 degrees C and a halogen redispersion step with a mixture of elemental halogen and water vapor at approximately 500

to 540 degrees C. However,.the metal of interest in Fung is iridium and it is directed to redispersing the iridium rather than removing it from the catalyst.

U.S. Patent NOS. 3,234,154, 3,219,586, and 3,151,088, all assigned to Sinclair Research, Inc., are cited as generally teaching the removal of Ni from a petrochemical catalyst by either Cl ₂ , HC1, or CC1 ₄ . However, like Elvin, none of these references are directed to removing residual catalyst from single crystal whiskers or address the problem of reactions between the gases and the whiskers.

It has thus become desirable to develop an improved CVD process for the production of single crystal fibers which overcomes the disadvantages associated with the prior art of poor control over the amount and availability of catalyst which can produce a lack of control of whisker morphology and dimensions and limitations on the efficiency of whisker production by such methods and, at the same time, eliminates the prior art problems of residual catalyst which may remain with the whiskers, thereby adversely affecting the physical properties of composite articles incorporating such whiskers.

SUMMARY OF THE INVENTION The present invention solves the aforementioned problems associated with the prior art by providing an improved process for producing metal carbide, nitride, or carbonitride whiskers of controlled dimensions, morphology, and quality by a CVD/VLS process. The present invention is based on the surprising discovery that it is not necessary to be above the melting point of a powdered metal catalyst in order for whisker growth to occur. It is postulated that the catalyst combines with the reactant gases to produce a lower melting point eutectoid composition, thereby allowing whisker growth to occur.

In the preferred embodiment, the reaction chamber includes one or more growth substrate surfaces having nickel or high nickel alloy powder dispersed onto aluminum oxide plates to provide catalyzing and supporting substrates for nucleation and growth of the whiskers. The melting point of the powder is approximately 1455 degrees C. The growth substrate surfaces are maintained at an operating temperature of about 1120 to 1225 degrees C, preferably about 1190 to 1220 degrees C.

The process, according to the present invention, includes the steps of flushing the reaction chamber sealed from the ambient atmosphere with flowing hydrogen gas. The hydrogen flowing through the reaction chamber is then mixed, at about ambient pressure, with reactants comprising a metal halide gas and one or more gases selected from the group consisting of aliphatic hydrocarbon gases pyrolyzable at the operating- temperature to form free carbon. Nitrogen gas and/or ammonia may be substituted for or mixed with the hydrocarbon gases to produce nitride or carbonitride whiskers. The atomic ratios of carbon and nitrogen to metal in the incoming gases is about 0.7:1 and 20:1, respectively, and the volume ratios of hydrocarbon and nitrogen gases to hydrogen is 1:37 and 1:1.6, respectively.

The flowing mixture of gases is maintained at a suitable linear velocity, preferably about 2-4 cm/sec, for a time sufficient- to nucleate and grow metal carbide, nitride, or carbonitride whiskers on the growth substrate material surfaces. Suitable hydrocarbon gases are.compounds of the formulas ^c n ^H ₂ n+2' ^c n ^H 2n' ^{or c} n ^H ₂ n- ₂ ' ^wnere n is a positive integer of 1 to .4. The preferred hydrocarbon is methane.

In the preferred embodiment, the whiskers are subjected to a post-growth treatment to remove the

residual nickel metal catalyst used to grow the whiskers. The process is a two-step procedure. In the first step, the whiskers are treated at less than 427 degrees C, preferably between 350 to 400 degrees C, with 2.8 slm of HC1 in 11.4 slm of argon gas at one atmosphere pressure for approximately one hour to convert the elemental nickel residual catalyst to NiCl ₂ • In the second step, the whiskers are then heated to at least 973 degrees C, preferably greater than 1000 degrees C, and treated with 11.4 slm of argon for one hour to cause the NiCl ₂ to sublime. The nickel content of the treated whiskers is reduced from 1-2 wt% to 0.2 wt%, an order of magnitude reduction.

Accordingly, one aspect of the present invention is to provide a process for producing metal carbide, nitride, or carbonitride whiskers comprising the steps of flushing a reaction chamber sealed from the ambient atmosphere with flowing hydrogen gas, wherein the reaction chamber includes one or more growth substrate surfaces formed from one or more materials suitable for providing catalyzing and supporting substrates for nucleation and growth of the whiskers, and wherein the growth substrate surfaces are maintained at an operating temperature suitable for growing the whiskers, and, wherein the one or more substrate materials is a high temperature material having a metal powder deposited upon a portion of its surface; and mixing with the hydrogen flowing through the reaction chamber, at about ambient pressure, reactants including one or more metal halide gases selected from the group consisting of halides of titanium, zirconium, hafnium, niobium, tantalum and tungsten and one or more gases selected from the group consisting of nitrogen, ammonia, and aliphatic hydrocarbon gases pyrolyzable at the operating temperature to form free carbon, in an atomic ratio of carbon plus nitrogen to metal and a volume ratio of

hydrocarbon plus nitrogen or ammonia to hydrogen sufficient to form the whiskers, the flowing mixture of gases being maintained at a suitable linear velocity for a time sufficient to nucleate and grow the metal carbide, nitride, or carbonitride whiskers on the growth substrate material surfaces.

Another aspect of the present invention is to provide a process for producing metal carbide, nitride, or carbonitride whiskers comprising the steps of flushing a reaction chamber sealed from the ambient atmosphere with flowing hydrogen gas, wherein the reaction chamber includes one or more growth substrate surfaces formed from one or more materials suitable for providing catalyzing and supporting substrates for nucleation and growth of the whiskers, and wherein the growth substrate surfaces are maintained at an operating temperature of about 1120 to 1225 degrees C; and mixing with the hydrogen flowing through the reaction chamber, at about ambient pressure, reactants including one or more metal halide gases selected from the group consisting of halides of titanium, zirconium, hafnium, niobium, tantalum and tungsten and one or more gases selected from the group consisting of nitrogen, ammonia, and aliphatic hydrocarbon gases pyrolyzable at the operating temperature to form free carbon, in an atomic ratio of carbon plus nitrogen to metal of about 0.7:1 to 20:1 and a volume ratio of hydrocarbon plus nitrogen or ammonia to hydrogen of about 1:37 to 1:1.6, the flowing mixture of gases being maintained at a suitable linear velocity for a time sufficient to nucleate and grow the metal carbide, nitride, or carbonitride whiskers on the growth substrate material surfaces.

Still another aspect of the present invention is to provide a process for removing the residual metal catalyst from metal carbide, nitride, or carbonitride whiskers in a reaction chamber sealed from the ambient

atmosphere, the chamber including one or more substrate surfaces for supporting the whiskers, comprising the steps of supplying the reaction chamber with a reactant including one or more gases selected from the group consisting of halogens, hydrogen halides, or chlorocarbons at a first operating temperature for a first predetermined time, the first operating temperature being sufficient to form a halide with the metal catalyst, and flushing the reaction chamber at a second operating temperature for a second predetermined time to remove the metal halide from the reaction chamber, the second operating temperature being sufficient to cause the metal halide to sublime. Finally, another aspect of the present invention is to provide metal carbide, nitride, or carbonitride whiskers produced by chemical vapor deposition and treated to remove the residual catalyst, the treated whiskers having substantially the same physical properties as untreated whiskers. These and other aspects of the present invention will become apparent to those skilled in the art after a reading of the following description of the preferred embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the following description, it is to be understood that such terms as "forward", "rearward", "left", "right", "upwardly", "downwardly", and the like are words of convenience and are not to be construed as limiting terms. The present invention may be practiced using a conventional reactor. One particularly suitable reactor is a model TI-100-V reactor manufactured by TI Coatings, Inc., Mt. Clemens, Michigan. The total volume of the reactor is approximately 1400 in ³ . The reactor includes inlet and outlet means in the reaction vessel to permit the flowing gas to enter and exit the vessel in such a way that gas flow

is established permitting optimum contact of the flowing gases with the surface of the growth substrate.

The growth substrate is preferably nickel powder dispersed onto aluminum oxide plates or nickel electrolytically deposited onto TiC or TiN coated aluminum oxide plates. In the preferred embodiment, the growth substrate is prepared by ultrasonically suspending nickel powder having a particle size of approximately 2 microns in isopropyl alcohol and then spraying the solution onto 72 4"x4"xl/8" aluminum oxide plates with an air gun. The plates were weighed before and after spraying and it was calculated that the substrate surfaces received approximately 2.7 gms/meter ² of nickel powder. The use of aluminum oxide instead of graphite helps to eliminate an extra, uncontrollable source of carbon in the gas phase.

The growth plates are arranged radially and axially within a support structure. The reactor includes conventional heating means to heat the growth substrates to the operating temperature. Typically, the operating temperature may be monitored by thermocouples disposed adjacent to the surface of the support structure. Also, as is well known in the art, the entire reactor is sealed from the ambient atmosphere to prevent contamination of the whiskers grown therein.

According to the present invention, metal carbide whiskers are grown on an aluminum oxide growth substrate surface on which nickel powder has been dispersed or electrolytically deposited in a reactor having a controlled atmosphere. The growth substrate surface is heated to an operating temperature of about 1120 to 1225 degrees C, preferably 1190 to 1220 degrees C, while being flushed with hydrogen gas at ambient pressure. The pressure within the reaction chamber preferably is maintained at about 1 atmosphere throughout the operation of the reactor.

The flow of hydrogen is increased, with the addition of a suitable metal halide in an amount suitable for whisker growth. The atomic ratio of carbon to metal in the reactant gases is maintained at about 0.7:1 and the volume ratio of hydrocarbon to hydrogen at about 1:37. The flow of reactant gases is maintained, preferably at a linear velocity of about 2 cm/sec, for a time sufficient for whisker nucleation and growth, normally about 2 hours. Following the whisker growth, the flows of reactant gases are stopped, and the flow of hydrogen gas is maintained while the growth substrate surface is cooled to ambient temperature. The reaction chamber is flushed with an inert gas, such as argon, prior to removal of the whiskers.

Similarly, metal nitride whiskers also may be grown in the reactor, as described above, with respect to the metal carbide whiskers and the dispersed nickel on aluminum oxide growth substrate. The process is substantially the same as that for growing the metal carbide whiskers with the exception that the hydrocarbon for whisker growth is replaced by nitrogen gas. The atomic ratio of nitrogen with respect to metal is about 20:1 and the volume ratio of nitrogen to hydrogen is about 1:1.6.

In the preferred embodiment, the whiskers are subjected to a post-growth treatment to remove the residual nickel catalyst used to grow the whiskers. The process is a two-step procedure. In the first step, the whiskers are treated at between 350 to 400 degrees C at one atmosphere pressure with 2.8 slm of HC1 in 11.4 slm of argon gas, based on a total available amount of nickel catalyst of approximately 2 gms, for approximately one hour to convert the elemental nickel residual catalyst to NiCl ₂ . In the second step, the whiskers are then heated to 1000 degrees C and treated with 11.4 slm of argon for one

hour to cause the NiCl ₂ to sublime. The choice of reactant and process temperatures can be best be understood by examining Tables 1 and 2, shown below.

Table 1 Standard Gibb's Free Energy Calculations

For the NiX^ ¹ ) Formation (Kcal/mole Ni)

Reactant 227C 427C 627C 827C 1027C cι ₂ -55 -48 -41 -34 -27

HC1 -8 0 +7 +14 +22

HBr -14 -8 -2 +4 +10

CC1 ₄ w/H ₂ -56 -49 -43 -37 -31

C ₂ C1 ₄ w/H ₂ -68 -58 -49 -39 -30

(l) where X may be Cl, Br, I or F.

Examples of the equations for the above reactions are:

^Ni (s) ^{+ cl} ₂ (g) Yiel s NiCl _2(s)

Ni _(s) + 2 HCl _(g) yields NiCl _2(s) + H _{2 (g)}

Table 2

Standard Gibb's Free Energy Calculations For the TiC Decomposition (Kcal/mole TiC)

Reactant 227C 427C 627C 827C 1027C cι ₂ -133 -123 -113 -102 -92

HC1 -40 -29 -17 -6 +6

HBr -79 -67 -55 -44 -32

CC1 ₄ w/H ₂ -127 -124 -120 -115 -111

C ₂ C1 ₄ w/H ₂ -136 -136 -135 -135 -135

Examples of the equations for the above reactions are: ^iC (s) + ^2cl 2(g) yields TiCl _{4 (g)} ^{+ C} .(s) ^iC (s) + 4HC1 ₂ f -x yields ^iCl 4(g) ^{+ CH} 4(g)

As can be seen, any of the chlorine compounds will react with free nickel to produce NiCl ₂ over a wide range of temperatures. However, the hydrogen halides are particularly suitable for NiX ₂ formation since the free energy values for this family of

reactions become positive (i.e. non-reactive) above specific temperatures. This transition from a reactive to nonreactive region allows the reaction with nickel to be controlled by varying the temperature in the reactor.

In addition, the hydrogen halide HCl is particularly suitable since its free energy value with respect to Tic goes positive (i.e. non-reactive) above approximately 900 degrees C. In addition, the rate of reaction of HCl with TiC below 900 degrees C is apparently slow enough that the whiskers are not affected. Thus, the transitions from reactive to nonreactive regions allow both the reactions with nickel and TiC to be controlled by varying the temperature in the reactor.

The residual nickel catalyst content of the whiskers, produced according to the subject invention, and then treated, as discussed above, is reduced from 1-2 wt% to 0.2 wt%, an order of magnitude reduction. In operation, the heating and hydrogen flushing steps, and the introduction of the reactant gases are carried out as described above. The flowing gases enter the reaction chamber through inlet means, flow upwardly past the growth substrates, and exit the reactor through outlet means. The temperature in the reactor is monitored by a thermocouple.

After the reactant gases are allowed to flow past the growth substrate surface for a time sufficient to grow whiskers of a desired size, normally about 1 to 2 hours, the reactor is then shut down, cooled and opened.

The process and products according to the present invention will become more apparent upon reviewing the following detailed examples.

EXAMPLES 1-5 Growth of titanium carbide whiskers was carried out at about 1 atm pressure in a sealed reactor. The reactor was heated while being flushed with hydrogen at 10 slm. The hydrogen flow was then increased and methane was mixed with the hydrogen. Titanium tetrachloride liquid was flash evaporated at 150 to 240 degrees C and then mixed with the hydrogen- methane mixture for 2 hours for whisker growth. The atomic ratio of carbon to metal in the reactant gases was maintained at about 0.7:1 and the volume ratio of hydrocarbon to hydrogen at about 1:37. The flow of reactant gases was maintained at a linear velocity of about 2 cm/sec. After the whisker growth period, the flows of methane and titanium tetrachloride were shut off, and the hydrogen was allowed to flow at 19 slm until the reactor cooled to ambient temperature. Finally, the system was evacuated to about 30 torr and backfilled with argon before the reactor was opened.

The shape, and morphology of the whiskers were observed by a scanning electron microscope. The whiskers were found to vary in diameter from 1 to 5 micrometers and were straight to alternating surface morphologies on the faces parallel to the growth axis. Typical results are shown in Table 3.

Example Temp TiCl4 Total Flow Yield

No. (C) (g/min) (slm) (g/m ² )

1 1120 4.6 15.9 13.5

2 1.190 8.1 27.8 44.4

3 1190 13.8 47.7 39.0

4 1190 17.0 58.7 45.7

5 1210 17.0 58.7 45.7

1 . .-

-14-

As can be seen, the yield of TiC whiskers was relatively independent with respect to temperature and TiCl ₄ . However, lower total flow rates require an increase in the amount of TiCl ₄ in order to produce an equivalent yield as higher flow rates.

EXAMPLES 6-8 Growth of titanium nitride whiskers was carried out at about 1 atm pressure in a sealed reactor. The reactor was heated while being flushed with hydrogen at 10 slm. The hydrogen flow was then increased. Titanium tetrachloride liquid was flash evaporated at about 130 to 260 degrees C and then mixed with the hydrogen-nitrogen mixture for 2 hours for whisker growth. The atomic ratio of nitrogen to metal in the reactant gases was maintained at about 20:1 and the volume ratio of nitrogen to hydrogen at about 1:1.6. The flow of reactant gases was maintained, preferably at a linear velocity of about 2 cm/see.

After the whisker growth period, the flow of nitrogen and titanium tetrachloride was shut off, and the hydrogen was allowed to flow at 19 slm until the reactor cooled to ambient temperature. Finally, the system was evacuated to about 30 torr and backfilled with argon before removal of the TiN whiskers. The shape and morphology of the whiskers were observed by a scanning electron microscope. The whiskers were found to vary in diameter from 1 to 10 micrometers and were straight to alternating surface morphologies on the faces parallel to the growth axis. Typical results are shown in Table 4.

Table 4 - Growth of TiN Whiskers Example Temp TiCl ⁴ Total Flow Yield No. (C) (g/min) (slm) (g/m ² )

6 1195 34.0 117.3 4.6

7 1230 34.0 117.3 16.3

8 1175 34.0 117.3 11.4

As can be seen, unlike the TiC whiskers, the yield of TiN whiskers was relatively dependent on temperature. In addition, tests conducted at atomic ratios greater than about 30:1 or less than about 14:1 did not produce useful amounts of TiN whiskers.

Although the above examples deal with the growth of titanium carbide and titanium nitride whiskers, at specific temperatures and for specific growth periods, using titanium tetrachloride and methane or nitrogen as reactants, the invention is not limited to the specific examples. For example, the metal halide reactant gases may include halides of titanium, zirconium, hafnium, niobium, tantalum, or tungsten and be mixed with a hydrocarbon gas to produce carbide whiskers. Also, a mixture of metal halides may be used to produce whiskers comprising a solid solution of metal carbides, nitrides, or carbonitrides. Nitrogen gas and/or ammonia may be substituted for or mixed with the hydrocarbon gas to produce nitride or carbonitride whiskers of titanium, zirconium, hafnium, niobium or tantalum.

EXAMPLE 9 The whiskers produced according to the present invention were subjected to a post-growth treatment prior to removal of the whiskers from the reactor to remove residual nickel catalyst used to grow the whiskers. The process was a two-step procedure. First, the whiskers were treated at between 350 to 400 degrees C at one atmosphere pressure with 2.8 slm of HCl in 11.4 slm of argon gas for approximately one hour to convert the elemental nickel residual catalyst to NiCl ₂ . Second, the whiskers were then heated to 1000 degrees C and treated with 11.4 slm of argon for one hour to cause the NiCl ₂ to sublime. The nickel content of the treated whiskers was reduced from 1-2 wt% to 0.2 wt%, an order of magnitude reduction. • SEM examination of the treated

whiskers showed no change in their surface morphology. As a result, the treated whiskers should have substantially the same physical properties as untreated whiskers. In contrast, conventional physical and chemical methods, such as treatment with chlorine gas, attacked the surface of the whiskers.

Thus, the present invention provides an improved CVD process for producing high purity, single crystal metal carbide, nitride, or carbonitride whiskers useful for composite applications. For example, whiskers produced according to the present invention and incorporated in composite materials, are expected to provide composite materials having increased fracture toughness, hardness, and wear resistance for such applications as cutting tools, wear parts, dies, turbines, nozzles and the like.

See, for example, copending Mehrotra et al U. S. Patent Application Serial No. 056,091, filed on May 28, 1987, and assigned to the assignee herein. This patent application discloses compositions containing titanium carbide whiskers (e.g., 30 v/o) in an alumina matrix containing a small amount (l v/o) of a sintering aid, such as zirconia. These compositions are useful as indexable metalcutting inserts. All documents, patents and patent applications referred to herein are hereby incorporated by reference.

Certain modifications and improvements will occur to those skilled in the art upon reading of the foregoing description. It should be understood that all such modifications and improvements have been deleted herein for the sake of conciseness and readability but are properly within the scope of the following claims.