Description:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a method for making members of a fully dense carbide or mixed carbide of certain reactive metals.
More particularly, the invention pertains to carbides of metals of Groups IVB and VB of the Periodic Table, such as TiC, ZrC, VC, V 2 C, Nb 2 C, TaC, Ta 2 C, and mixed carbides of two or more, such as TiC-ZrC, NbC-ZrC, TaC-ZrC, and similar pseudo-binary alloys. These will be designated hereinafter as refractory metal carbides.
2. Description of the Prior Art
Heretofore, fully dense, single phase carbide shapes, such as TiC, WC, ZrC, and VC, having high purity and homogeneous compositions have been unavailable. The term "shapes" designates a member of specific physical structure or configuration as compared with powders or other minute pieces of irregular configuration. Likewise, fully dense pseudo-binary single phase alloys, such as TiC-ZrC, have not been available. Commerically, cemented carbides, hot-pressed carbides, and sintered carbides, all produced from powdered carbides, are characterized by porous, or heterogeneous structure and have been available in limited shapes because they are produced by powder metallurgy techniques, usually employing cobalt or other metal binder, and are limited to what can be produced by grinding or etching of such compacted powdered and sintered bodies.
The term "fully dense" means a carbide member which is substantially at 100 percent of the theoretical density of the carbide and is substantially free from voids or porosity based upon X-ray parameters. Carbides which are not fully dense, such as cemented or hot pressed carbides, have a duplex structure of a metal and metal carbide and have from 1 to 30 percent void volume present as porosity. Sintered metal carbide products produced from powdered metal particles are not sufficiently dense due to entrapped gas in the original charge. Such sintered products also usually contain varying amounts of two carbide bases, causing non-uniform hardness, such as disclosed in U.S. Pat. No. 2,286,672.
U.S. Pat. No. 1,840,457 discloses a metal carbide of the so-called "castable" type in which the stated amount of carbon in the tungsten carbide is only 4 1/2 to 5 percent. The carbides are prepared by melting alloying metals and casting immediately to avoid excessive pickup of carbon. Complete carburization is avoided so that the resulting material is duplex or multiplex in structure. This is in contrast with the single-phased, homogeneous carbides of the present invention in which there is a minimum of 6 % carbon in the tungsten carbide.
U.S. Pat. No. 3,254,955 discloses a method for producing single crystals of tantalum carbide which includes a step of zone melting in the tantalum carbide-carbide eutectic composition region of the tantalum-carbon system. Based upon the experience of the inventors of the present invention upon cooling, the product of that melting step is a carbide that contains up to 50 percent by volume of graphite flakes as an extraneous phase which greatly weakens the carbide.
One of the more basic problems limiting the operation of scientific, nuclear, space and other equipment to elevated temperatures not in excess of about 2,000°C. is the lack of suitably strong materials. The refractory-metal carbides would be of particular interest for such applications because of their high melting points and high physical strength at elevated temperatures and relative inertness in certain corrosive environments.
Fully dense and high purity carbides of such refractory and reactive metals would be preferred over prior art cemented carbides and the like which have porous characteristics and often are relatively impure, because the fully dense carbides have far superior high strength properties at room temperature and at elevated temperatures. For example, cold-pressed niobium carbide (NbC) and hot-pressed niobium carbide prepared by the best techniques presently known, have transverse rupture strengths of about 20,000 psi, respectively. By comparison, the fully dense members of niobium carbide, produced as will be disclosed hereinafter, have a transverse rupture strength of about 80,000 psi. In addition, the fully dense refractory metal carbides have greater wear resistance and a smooth surface, which differs from the pitted surfaces of the cold-pressed or porous carbides. For this latter reason the fully dense carbides are particularly useful for a variety of applications such as improved wire dies, lamp bulb filaments, phonograph record needle blanks, crucibles, and high temperature structural materials such as rocket nozzles. Moreover, the method of making fully dense carbides as disclosed herein is conducive to the preparation of large single crystals of the metal carbides.
Accordingly, it is an object of this invention to provide methods for making fully dense shapes of refractory carbides of metals having high strength and improved surfaces and structured properties.
It is another object of this invention to provide fully dense refractory metal carbide shapes that can be used as high strength, high temperature structural materials.
It is another object of this invention to provide fully dense refractory metal carbide shapes which are capable of service under conditions requiring high hardness and strength, refractoriness, unusual shapes and configurations and low vapor pressure.
Finally, it is an object of this invention to provide fully dense shapes of carbides of refractory metals of the Groups IVB and VB metals which accomplish the foregoing objects in a simple and effective manner.
SUMMARY OF THE INVENTION
Generally, the present invention comprises the steps of placing a quantity of the reactant Groups IVB and VB metal in a mold composed of a carbonaceous material, heating the metal and mold in an inert atmosphere to a temperature above the melting point of the metal, but below the metal carbide-carbon eutectic temperature, until a full carbide is formed by diffusion of carbon from the carbonaceous material into the metal, cooling to room temperature, and removing the resulting metal carbide member from the mold.
The metal to be converted into the single phase metal carbide is charged into the carbon mold cavity preferably as a solid body conforming to the shape of the mold cavity. Excellent results are obtained if the solid metal body is heated to a temperature above its melting point and depending upon solid state diffusion of carbon to convert it to the metal carbide. The metal may be introduced into the mold either as a solid body, or as an assembly of several loose particles or powders of such shape that the mold cavity is essentially completely filled, or as a molten mass, or a preformed compact (which may have been vacuum sintered) conforming to the shape of the mold cavity.
When metal powders are employed to fill the mold cavity, it is important to exclude fine particles. If any appreciable amount of the powder comprises particles less than 30 mils in diameter, it has been found that in the process of heating the mold and its contents to the melting point of the metal particles below 30 mils carburize substantially to a metal carbide while in the solid state and consequently will not melt at the metal melting temperature. Consequently, any metal particles that are uncarburized do melt and form a mushy mass with the appreciable amount of metal carbide powders, wherein contained gases are trapped and are unable to rise out of the heterogeneous mass. As a consequence the resulting carbide body will be full of voids and multiphase components.
According to the present invention fully dense carbide shapes are produced form the pure refractory metal or alloy of a mixture of two or more refractory metals by the following proecedure:
Placing a metallic charge of at least one constituent of the metals Ti, Zr, Hf, V, Nb, Ta, and alloys of two or more thereof in a mold of carbonaceous material, the charge being either in solid or molten forms of the constituent; heating the charge in an inert atmosphere, to a temperature in the range above the melting point of the metal charge and below the metal carbide-carbon eutectic temperature and maintaining the temperature in this range until sufficient carbon has been absorbed to raise the melting point and the charge is fully carburized but without melting the metal carbide; and thereafter cooling, for example, to room temperature, and removing the resulting metal carbide shape from the mold.
BRIEF DESCRIPTION OF THE DRAWING
For a better understanding of the invention reference is made to the drawings in which:
FIG. 1 is a vertical sectional view through an induction heating furnace for preparing fully dense carbides;
FIG. 2 is a zirconium-carbon phase diagram;
FIG. 3 is a chart showing a parabolic growth constant for carbides of the Groups IVB and VB metals as a function of the reciprocal of temperature (1/T°K);
FIG. 4 is a photomicrograph of fully dense and homogeneous NbC prepared by the method of this invention; and
FIG. 5 is a photomicrograph of hot pressed NbC prepared by a prior known method.
DESCRIPTION OF THE PREFERRED EMBODIMENT
The method of the present invention involves the direct reaction under controlled conditions of a refractory and reactive metal such for example, as titanium, zirconium, hafnium, vanadium, niobium, and tantalum, or mixtures or alloys of two or more with carbon or graphite within a critical temperature range to form a solid full mono-carbide body. It may be convenient or necessary to shape the metal charged to fit the graphite mold. For example, when preparing a carbide hollow cylinder, a tubular metal charge is conveniently used.
As shown in FIG. 1, a graphite mold or crucible 1 has an inner chamber 2 of any configuration in which a metal charge 3 is placed. A cover 4 for the mold is provided. The cover has an elongated stem 5 with a bore 6 extending therethrough to communicate with the chamber 2 of the mold. When assembled, the cover and crucible approximate black body condition within the chamber 2. The assembly is then placed within a quartz container 7 that is filled with a carbonaceous material such as lampblack which completely encloses the mold 1 and the cover 4 except the upper end of the stem 5. The lampblack provides insulation and thermal stability to the system. The container 7 is mounted on a support member 8 having a base 9 which rests upon a plate 10.
A plate 11 is disposed above the plate 10 where it is held by a quartz tube 12. A number of spaced tie bolts 13 extend between the plates 10 and 11 and are provided with tightening nut 14 at the upper end thereof for holding the plates against the opposite ends of the tube 12 in an air tight manner. Annular gaskets 15 are provided between ends of the tube 12 and the plates 10 and 11.
The assembly is heated by induction coils 16 of conventional construction to a high temperature at which carbon will react with the metal and rapidly diffuse into the metal to form the full carbide.
As shown in FIG. 1 an inert gas such as helium or argon is circulated through the tube 12 at a pressure slightly in excess of one atmosphere. For that purpose a gas inlet 17 is connected to a central aperture in the plate 10 and communicates with a gas passage 18 in the base member 9 of the support. The inert gas completely fills the interior of the tube 12, the container 7 and the crucible 1, and flushes out any existing oxygen or other undesirable gases. A gas seal 19 is provided in the plate 11 at the upper end of the quartz tube 12 which enables the inert gas to leave the interior of the tube 12 and prevents back-diffusion of air into the system.
The bore of the gas seal 19 and the bore 6 or the stem 5 of the cover 4 are aligned so that temperature readings may be taken of the metal charge 3 from time to time.
After a metal charge such as zirconium is placed in the graphite mold 1 the system is heated to the desired temperature. The temperature must be high enough to melt the metal, but must be below the carbide-carbon eutectic temperature. Critical temperatures in the various metal-carbon systems for the indicated metals are shown in the table as follows:
TABLE I
Critical Temperatures °Centigrade
Melting Metal-carbide Carbide-carbon Element Point Eutectic Temp. Eutectic Temp. Titanium 1668 1645 2760 Zirconium 1857 1850 2890 Hafnium 2130 .about.2200 Peritectic 3180 Vanadium 1710 1630 2760 Niobium 2500 2335 3300 Tantalum 2996 2880 3450
The metal-carbide eutectic temperature is for information purposes.
The charge is maintained within this temperature until the metal is completely carburized. The reaction time required for complete carburization is determined by the size of the sample and the growth rate of the carbide reaction product. Curves for the parabolic growth constants as a function of reciprocal temperature for some monocarbides are shown in FIG. 3.
In accordance with this invention the upper limit of the temperature at which carburization occurs is the temperature of the metal carbide-carbon eutectic. It has been found that where carburization of the metal occurs above that eutectic temperature the resultant carbide upon cooling to room temperature contains up to 50 percent by volume of graphite flakes as an extraneous phase, thereby greatly weakening the carbide.
The followinge example illustrates the practice of the invention:
EXAMPLE
The reaction between liquid zirconium and graphite at temperature substantially above 1,850°C. but below 2,890°C. and preferably, between 2,000°C. and 2,800°C. produced a fully dense reaction product, which formed between the graphite and the metal at such temperatures. In this process, a cylinder of zirconium metal (99.99 % Zr) was placed in a 1/4 inch inside diameter graphite crucible (99.9% C.) such as shown in the schematic drawing of the apparatus of FIG. 1. The sample was heated by induction with a 5 KW-450 kc power source. Temperature measurements were taken with a two-color pyrometer which was sighted into the crucible by reflection from an overhead surface reflecting mirror through the seal 19 and the bore 6 of the stem 5.
Temperatures inside the crucible were also compared with against the Zr-Z rC eutectic (1850°C.) and the Nb-Nb 2 C eutectic (2,335°C.) temperatures. The temperature was controlled by automatically adjusting the power input and variations of less than ± 5°C. were obtained during the carburization run.
After the system was purged with inert gas the sample was quickly heated to the melting point of zirconium (1,857°C.), held there for 1 minute, and then heated quickly (approximately 1 minute) to diffusion temperature of 2,700°C. and held at the latter temperature for 60 hours. Thereafter the cylinders were cooled at the rate of 1,400°C. per minute by cooling the furnace in an inert atmosphere. However, any other suitable cooling rate can be employed.
Under the microscope the fully dense materials of this invention are characterized by a grain structure typical of single-phase metals with sharply delineated grain boundaries and completely lacking in voids, as compared with hot pressed carbides having relatively large areas of voids.
The rate of formation of the full carbide, ZrC, from the surface at various temperatures is approximately as follows:
TEMPERATURE °C CENTIMETER PER MINUTE 2000 0.000075 2300 0.00015 2500 0.00055 2750 0.00094 2800 .00096
samples of the ZrC so produced were examined metallographically under the microscope. The products are homogeneous, fully dense materials characterized by a grain-structure typical of single-phase metals with sharply delineated grain boundaries and with no voids.
In a similar manner noibium carbide, hafnium carbide, and tantalum carbide members were produced. All were single phase bodies. All the other refractory metals and mixtures of two or more can be similarly converted to fully dense carbides.
Also an alloy of 90% Nb and 10% U (depleted) in the form of a cylinder 3/4 inch in diameter with a central aperture of about 1/8 inch was also fully carburized in this way. In the same manner disks of niobium-zirconium (50 percent by weight of each) are converted to the carbides.
In a similar manner a compact of particles of the capacitory metal, of over 30 microns is prepared and placed in a graphite mold and carburized according to this example.
A diamond polishing or grinding wheel may be used to provide a special surface finish or precise dimensions on the resulting metal carbide members.
The foregoing method is used to prepare fully dense high purity refractory carbides by the direct reaction of a carbonaceous material and metal from the group of titanium, vanadium, zirconium, hafium, niobium, and tantalum. Various advantages are obtained from the process including the provision of any desired castable shape, preparation of the carbides at high temperatures thus reducing the carburization time, and the provision of carbides of refractory metals which have high density as well as extra high strengths and exceptionally satisfactory moduli of elasticity. The hardness of these carbides is between that of sapphire and diamond. The purity is often better than that of the starting metal charge. Oxygen and nitrogen contents of 20 to 40 ppm have been obtained.
The densities and lattice parameters of six high stoichiometric fully dense carbides prepared in accordance with these examples are listed in Table II:
TABLE II
Density and Lattice Parameters of Carbides Prepared as "Fully-Dense" Carbides
Parameter A 0 , (A) Density (g./cm 3 ) TiC 4.330 4.90 ZrC 4.700 6.64 HfC 4.641 12.7 VC 4.160 5.81 NbC 4.469 7.80 TaC 4.450 4.57
the fully dense character of niobium carbide as prepared by the technique of this invention is shown in the photomicrograph of FIG. 4. The lack of porosity, the large homogeneous grains and sharp grain boundaries are evident. The vast improvement over the prior art is clearly apparent when compared with the photomicrograph of hot pressed NbC in which parts are shown by the dark areas of FIG. 5.
It will be understood that since the method of the Example is a carburization process involving formation of the full monocarbide from the surface inwardly at a predetermined rate, that in some cases the refractory metal body may purposely not be entirely converted to the full carbide by interrupting the process after a selected time interval so that there remains an uncarburized central core of the refractory metal. This central metal core can thereafter be etched out or mechanically ground, drilled or otherwise removed, leaving a shell or cylinder of the full carbide.
It is understood that the above specification and drawings are exemplary of technically and economically feasible methods for producing high density and high purity carbides of refractory metals which were heretofore unattainable. Moreover, it is understood that the specific examples disclosed pertains to both pure monocarbides and pseudo-binary alloy carbides.