LOW TEMPERATURE METHOD FOR SYNTHESIZING MICROGRAIN TUNGSTEN CARBIDE

Abstract: A low temperature method for synthesizing micrograin tungsten carbide particles having an average diameter of 0.05 to 0.2 mu m includes the programmed heating of tungsten trioxide powder or other material containing tungsten in a flowing molecular hydrogen atmosphere having a constant proportion of molecular methane. The heating program is carried out at two different heating rates, the first being higher than the second. The first heating rate is 5 DEG C to 50 DEG C per minute from 25 DEG C to 535 DEG C. The second heating rate is 1 DEG C to 10 DEG C per minute from 535 DEG C to about 850 DEG C. The last temperature is then held until conversion of the tungsten trioxide or other tungsten-containing material to tungsten carbide is complete, typically from about 15 minutes to 3 hours.

Description: LOW TEMPERATURE METHOD FOR SYNTHESIZING MICROGRAIN TUNGSTEN CARBIDE Technical Field This invention relates generally to a method of converting a tungsten-containing material, tungsten trioxide (W0 3 ), to tungsten carbide (WC) without first reducing all of the WO3 to tungsten as a separate intermediate. Background Art Tungsten carbide, a ceramic material having good chemical stability and corrosion resistance, has been called the strongest of all structural materials. It has high hardness, wear resistance and temperature resistance. Tungsten carbide powders are converted by conventional processes to form cemented carbides, dies and cutting tools, wear resistant parts, cermets and electrical resistors. They are also used as abrasives in liquids. Cemented tungsten carbides are mixtures of 80 to 95 percent by weight WC and 5 to 20 percent by weight cobalt or other ductile metal as a binder phase. Cemented tungsten carbides are useful for forming tools and as abrasives for machining and grinding metals, rock, molded products, porcelain and glass. They are also useful in forming gauges, blast nozzles, knives and drill bits. The properties ultimately possessed by an article formed from WC powder depend, to a great extent, upon WC grain size and process conditions used to form the article. Certain tungsten carbides of fine grain or particle sizes (known as micrograin tungsten carbides) from 0.05 to 0.2 micrometers in diameter are especially useful for certain purposes, such as end milling and circuit board drilling applications. The particle size range suggests that micrograin tungsten carbides may also be useful in catalytic processes. Micrograin particulate materials preferably have a controlled morphology, a narrow size distribution, a well-defined stoichiometry and relatively high purity. Micrograin WC is not readily prepared from larger WC particles. The hardness for which WC is valued also impairs or prevents the mechanical reduction of large WC particles to a smaller size by conventional methods such as grinding or milling. Accordingly, it would be very desirable to directly manufacture micrograin WC particles. Some metal carbides can be produced by the carbothermal reduction of a metal oxide with solid carbon under a reducing atmosphere. Hydrogen is a conventional reducing gas. Conventional teachings do not, however, advocate the direct carbothermal reduction of W0 3 (or another solid, tungsten-containing material) with carbon in a molecular hydrogen atmosphere. For example, Funtai Oyobi Funmatsuyakin, v. 22(1), pp. 12-16 (1975) (Chemical Abstracts 88(24): 175199r) teaches thatthe reduction of WO3 by hydrogen occurs preferentially over the reduction of WO3 by carbon when a mixture of WO3 and carbon is heated in an atmosphere of molecular hydrogen. A variety of other processes have also been employed to make fine WC powders. Whilethey have each enjoyed relative success, they have also individually been subject to a variety of drawbacks. In general, the prior processes have been relatively expensive in requiring substantial capital investment in gas composition control systems and particle collection systems. Nonetheless, they yield products that have large variations in particle size and relatively low yields. While these drawbacks are particularly evident with gas-phase methods (entailing the reaction of at least two gaseous reactants heated in a furnace, or heated by RF induction or plasma) they are also present in other methods. U.S.-A ,115,526 is directed to a method for producing fine particle size tungsten or WC which entails a first step of reducing tungsten oxide in hydrogen under conditions which avoid the formation of tungsten oxide whiskers. The resulting powder ϊsthen subject to carburization in a methane atmosphere. This process is disadvantageous in requiring two different atmospheres and the isolation of an intermediate material. U.S.-A 4,460,697 discloses a method for synthesizing WC from tungsten oxide by heating tungsten oxide to greater than 800°C in the presence of reducing and carbonizing gases, either sequentially or at the same time. When both reducing and carbonizing gases are employed at the same time, the gases are placed in a plasma condition. One object of the present invention is to convert WO3 or another solid, tungsten- containing material to WC directly with a single reaction atmosphere of consistent composition. A second object of the present invention is to provide a relatively inexpensive method of producing WCthat is simplerthan prior methods. It should obviate the need for: isolating or cooling an intermediate product; adding carbon to an intermediate product; controlling the ratio of the partial pressures of carbon monoxide and/or carbon dioxide in the treatment atmosphere; and changing proportions of the atmosphere during treatment. Disclosure of Invention One aspect of the present invention is a method forforming a fine grained tungsten carbide that has a particle size within a range of from 0.05 to 0.2 μm in diameter, which comprises heating, a solid tungsten-containing material in a flowing atmosphere containing molecular hydrogen and molecular methane and having a fixed ratio of hydrogen to methane, for a time sufficient to convert substantially all of said material to tungsten carbide, wherein such heating is carried out at a first heating rate of 5 to 50°C per minute from 25°Cto 535°C, then ata second heating rate, lower than the first heating rate, of 1 to 10°C per minute from 535°Cto 850°C, and thereafter held at about 850°C for 15 minutes to 3 hours. The tungsten-containing material is preferably W0 3 . In a related aspect, the atmosphere can also contain argon, or have a ratio of 90 to 99 percent hydrogen to 1 to 10 percent methane, or both. In another related aspect, the heating is carried out in a space having a defined volume (such as a furnace). The flow rate of the atmosphere through such a space is at least about 1 , preferably about 10, and most preferably about 100 volumes per minute. In a second aspect, the present invention is directed to a method for forming a tungsten carbide having an average particle size between 0.05 and 0.2 μm, comprising heating powdered or particulate tungsten trioxide in a flowing atmosphere containing argon, molecular hydrogen and molecular methane, the ratio of hydrogen to methane in the atmosphere being fixed at 92 to 97 volume percent hydrogen to 3 to 8 volume percent methane, for a time sufficient to convert substantially all of the tungsten trioxide or other material to tungsten carbide, wherein the heating is carried out at a first heating rate of about 20°C per minute from 25°C to 535°C, and at a second heating rate of about 5°C per minute from 535°Cto850°C, and thereafter held at about 850°C for 30 to 90 minutes, such that the resultant product achieves a constant weight. The amounts of hydrogen and methane total 100 percent. Applicants have discovered that the growth of WC particles in the micrograin size range can be selectively controlled by controlling the presence of just one of the products arising from the various reactions that occur during the conversion of W0 3 or other tungsten- containing solid materials to WC. The reduction of WO. is representative of the reduction of othertungsten-containing materials. Accordingly, subsequent references to W0 3 include and exemplify such other materials, so long as they are subject to reduction to tungsten in hydrogen. Ammonium paratungstate is one such material. A review of the various reactions involved provides a basis for better appreciation of the nature of temperature control. The desired reduction reaction of W0 3 to tungsten metal in a hydrogen atmosphere is: (1) W0 3 + 3H 2 > W + 3H 2 0. Below about 535°C, however, this reaction is incomplete. For example, at 325°C, the solid product is not tungsten, but is instead a composition having the approximate formula W0 8- The formula changes at temperatures of 425°C and 450°C to, respectively, W0 2 72 and W0 2 _o- Only when the reaction temperature approaches 535°C is the solid product from the reaction substantially a tungsten metal powder. The desired reduction reaction istemperature sensitive above this point. When the reaction takes place at 537°C, the resultant tungsten powder has an average diameter of about 0.05 μm. At higher temperatures, larger particles are obtained. For example, at a reaction temperature of 900°C, the tungsten powder product has an average diameter of 2 μm. But temperature is not the only parameter affecting the particle size of the solid product. The tungsten particle size gradient (resulting from the reduction reaction at a given temperature) progressively increases with an increase in the partial pressure of water, the other product of the reaction. The water is capable of reacting with any unreacted W0 3 to yield ortho-tungstic acid, W0 2 (OH) 2 . Applicants observe that the relative volatility of the ortho-tungstic acid appears to encourage tungsten particle growth and ultimately determines the particle size of the WC product. Applicants' invention involves including in the hydrogen reduction atmosphere an agentthat consumes the water produced during the reduction . reaction. This limits particle growth by limiting the production of ortho-tungstic acid. The preferred consumption agent is methane, although carbonaceous gases such as carbon monoxide and carbon dioxide are contemplated as equivalents. The advantage of a carbonaceous consumption agent such as methane is that the agent not only provides the carbon necessary for carburization of the tungsten, through the equilibrium reactions: (2) CH 4 < > C + 2H 2 and (3) CH 4 + H 2 0 < > CO + 3H 2 , the latter in turn providing carbon through the equilibrium reaction: (4) CO + H 2 < > C + H 2 0; but simultaneously providing for the consumption of water by the equilibrium reaction: (5) C + 2H 2 0 < > CO + 2H 2 . These interrelated equilibria are of course reversible and temperature dependent. Additionally, reaction (5) is subject to the competing equilibrium reaction: (6) C0 2 ■-- C < > 2CO. Unexpectedly, it is not necessary to attempt to quantify these equilibria in order to achieve substantially complete, direct conversion of W0 3 to WC. Applicants have found that when an atmosphere of fixed proportions of molecular hydrogen and the consuming agent are continuously supplied to W0 3 ,the partial pressure of the water produced in the reaction (and thus the conversion of WO^to WC) can be completely regulated by a program of heating in which only the temperature, heating rates and residence or hold time need be controlled. The present invention thus entails heating WO j; or another sol id, tungsten- containing material, in a flowing atmosphere containing molecular hydrogen and molecular methane (or another carbonaceous, water-consuming agent), and having a fixed ratio of hydrogen to methane (or other carbonaceous, water-consuming agent). In this regard, "molecular" means merely that no unusual steps are taken to place the hydrogen or methane in a dissociated state, for example, by photolysis or creation of plasma conditions, except insofar as incidental to heating at relatively low temperatures. The flowing atmosphere desirably contains a ratio of 90 to 99 percent hydrogen to 1 to 10 percent methane, and preferably 92 to 97 percent hydrogen to 3 to 8 percent methane. The percentages can be determined on a molar (molecular) basis, a gas volume basis, or a gas partial pressure basis, as may be most convenient, since these measures appear to be essentially equivalent under the temperatures and pressures contemplated within the invention. The flowing atmosphere may include argon or another inert gas as a diluting agent, so long as the ratio of hydrogen to methane remains fixed. If the desired use of the micrograin WC requires minimal free carbon, the ratio of methane to hydrogen in the flowing atmosphere should be less than that dictated by the equilibrium decomposition reaction for methane at the peak temperature. The present invention also entails heating the W0 3 by a temperature ramping program. The temperature ramping program includes two continuously rising temperature steps followed by a holding period at the temperature achieved at the end of the second step. The first step is desirably carried out at a first heating rate of 5 to 50°C per minute, preferably at about 20°C per minute. The first step raises the temperature from 25°C (room temperature or ambient shop temperature) to 535°C. The second step immediately continues the heating ata second heating rate lower than the first heating rate. The second heating rate is desirably 1 to 10°C per minute, preferably about 5°C per minute. The second step raises the temperature from 535°C to a final temperature of about 850°C. The temperature is then held at the final temperature for 15 minutes to 3 hours, preferably for 30 minutes to 90 minutes. If trace amounts or larger, but minor, amounts of tungsten metal or ditungsten carbide are acceptable, the final temperature may be as low as about 800°C. For best results, however, the final temperature should be as close to 850°C as possi ble, with a range of 840°C to 855°C bei ng preferred. The temperature selected for the end of the first step and the beginning of the second step is about equal to the reported minimum temperature (537°C) at which the reduction of W0 3 to tungsten powderwould be just complete in a pure hydrogen atmosphere- It is not, however, essential to the success of the present invention that the first rate be slow enough to cause the reduction of the W0 3 to be complete at the end of the first step. In fact, at the end of the first step, the re a eta nt powder is likely to be a rather complex mixture of products. It is important that this complex mixture be fully reduced and carburized duringthe second step and the hold time after the second step. The hold time must be long enough to complete carburization and to lowerthe level of free carbon inthe resulting product to an acceptable amount. The acceptable level of free carbon or other contaminants (such as oxygen) depends upon the use to which the WCpowderwill be put. Typically, free carbon and oxygen levels of 1 and 2 percent or less by weight, respectively, are desired and can readily be achieved by the method of the present invention. The W0 3 (or other solid, tungsten-containing material) useful for conversion into WC by the method of the present invention requires no special pretreatmentand need not possess any particular characteristics. The W0 3 should be pure enough to give a WC product that has contaminants at or below any particular level desired or needed in view of the use to which the WCproductwill be put. The W0 3 particle size of the should be reasonably uniform and small enough that the amount to be treated is evenly heated during practice of the method of the present invention. AWO. particle diameter on the order of 25 μm is useful to yield WC particles having an average diameter between 0.05 and 0.2 μm. One such W0 3 powder is that sold by GTE Corporation underthe name "GTETO-3". During heating of the W0 3 powder, the flow rate of the atmosphere should be as great as possible without entraining or prematurely transporting the W0 3 reactant or the WC product. For example, if the method of the present invention is carried out by placing and heating W0 3 powder in a space having a defined volume, for example, in a furnace, the flow rate of the atmosphere of molecular hydrogen and molecular methane through the furnace be at least 1 furnace volume per minute. The atmosphere flow rate is preferably at least 10 furnace volumes per minute, most preferably about 100 furnace volumes per minute. The maximum flow rate depends upon the physical form of the W0 3 powder and the particle size of the product. The nature of the apparatus employed for heating inthe method of the present invention is not believed to be critical to the success of the method. The apparatus should be appropriate for the amount of WO. to be treated. Smaller quantities of W0 3 , on the order of 1 to 100 milligrams (mg), can be treated in a conventional thermal gravimetric analyzer (TGA). Larger quantities of W0 3 , on the order of several grams, can be treated in a conventional tube furnace. When a tube furnace is employed, it is convenient to charge the W0 3 to an inert carrier (such as a quartz boat) in order to facilitate handling. The carrier should be structured, and the quantity of W0 3 placed in it selected, so as to ensure a constant flow of the flowing atmosphere throughout the treatment and to permit even heating of the WO Stagnant areas or localized hot or cold spots are expected to adversely affect the uniformity of the resulting WC product. A better understanding of the invention will be had upon reference to the following examples: Example 1 A 20 mg sample of WO_ powder (GTE TO-3) was placed into a TGA and heated in an atmosphere containing 97 percent molecular hydrogen and 3 percent molecular methane. The total gas flow through the TGA was approximately 0.27 liters per minute (Ipm), corresponding to about 2 furnace volumes per minute. The sample was heated at a rate of 20°C per minute from 25°C to 535°C, then heated at a rate of 5°C per minute from 535°C to 850°C, and held for 90 minutes at 850 C C. The weight of the sample was monitored during treatment. The sample began to lose weight at 527 ± 5°Cand reached a maximum of 19.66 percent weight lost at a temperature of approximately 651 °C. After that point, the sample began to gain weight, and within 15 minutes had achieved a stable overall weight loss of approximately 17.83 percent by weight (wt-%). It is believed that this stabilized weight corresponds roughly to the conversion of W0 3 into ditungsten carbide (W 2 Q. Such a conversion has a theoretical weight loss of 18.12 percent. Upon further heating, at a temperature of approximately 567 ± 5°C the sample again began to gain weight, and eventually reached a steady state value of approximately 14.67 percent for the overall weight loss. It is believed that this weight loss corresponds roughly to the conversion of W0 3 into WC. The theoretical weight loss for such a conversion is 15.53 percent. The resulting product was cooled to room temperature and was found to be non-pyrophoric when exposed to air. Example 2 A 3g sample of the same W0 3 as used in Example 1 was placed in a quartz boat and loaded into a horizontal tube furnace of cylindrical construction. The furnace had an inside diameter of 2 inches (5.1 cm) and a length of 16 inches (40.6cm). The volume of the furnace was thus about 0.82 liters. The sample of WO. was subjected to the same temperature program as employed in Example 1 with the same atmosphere containing 97 percent molecular hydrogen and 3 percent molecular methane. The total gas flow for the atmosphere was 81.5 Ipm. Upon completion of the hold time, the product was cooled to room temperature and was found to be non-pyrophoric. X-ray diffraction analysis showed the sample to be composed solely of WC. The product was also subjected to scanning electron microscopy (SEM), which showed that the product retained the original shape of the WO. precursor material, but was composed of individual particles having an average diameter less than 0.1 μm. The product was also analyzed for total carbon content using a LECO carbon analyzer. The overall carbon content of the product was 6.13 wt-%. Replication of this example, save for reducing the temperature attained from 850°C to 660°C, yielded a product having more W.C than WC. Increasing the proportion of methane to 40% at the reduced temperature also yielded a product with more W 2 C than WC. An intermediate temperature of 750°C and a methane fraction of 7% also yielded a mixture of W 2 C and WC, but with an increased WC content. Example 3 Example 2 was duplicated save for reducing the temperature attained from 850°C to 800°C and increasing the methane proportion of the atmosphere from 3 percent to 4 percent molecular methane. Upon cooling, the product was analyzed by X-ray diffraction and was found to contain a major portion of WCand a minor portion of W 2 C. LECO carbon analysis showed the product to contain 5.18 wt-% total carbon. Example 4 Example 2 was duplicated save for reducing the hold time from 90 minutes to 30 minutes. Upon cooling, the product was analyzed by X-ray diffraction and found to contain a major portion of WC and only a trace of W 2 C. The product was found by LECO carbon analysis to contain 6.07 wt-% total carbon. Example 5 Example 2 was duplicated save for increasing the methane proportion from 3 percent to 8 percent molecular methane. The product was then cooled and analyzed by X-ray diffraction. The product was found to contain WC with no detectable trace of W 2 C present. The product contained 6.40 wt-% total carbon, as determined by a LECO carbon analyzer. Example 6 Example 5 was duplicated save for reducing the hold time at 850°C to 30 minutes. When cooled, the product was found by X-ray diffraction analysis to contain WCwith no detectable trace of W 2 C. LECO carbon analysis showed the product to contain 6.20 wt-% total carbon. None of the products of Examples 2-6 contained any detectable trace of tungsten metal that had not combined with carbon in one form or another. The WC or major portions of WC obtained in Examples 2-6 all possessed average particle diameters between 0.05 and 0.2 micrometers. The examples demonstrate that a WC powder of uniform, yet very small, particle size can be obtained by a method that is significantly simplerthan prior methods of obtaining comparable fine WC powders. The only physical parameter changed during the performance of the method of the present invention is temperature, a very easily and economically controlled parameter. Since a constant flowing atmosphere of fixed proportion is employed, the present invention economically eliminates the prior need for careful regulation of the partial pressures of carbon monoxide and carbon dioxide (or other reactant gases) during the conversion of W0 3 or another solid, tungsten-containing material into WC. The present invention also economically eliminates the need for mechanisms connected to the treatment furnace for supplying changing atmospheres during different steps in the treatment process. The present invention also enjoys cost savings in that no special pre-treatment of the WO3 or other material is required in order to achieve the benefits enjoyed by the invention. Similar results are expected with other combinations of heating rates and hold times, all of which are disclosed herein. While our invention has been described in terms of several specific embodiments, other embodiments could readily be adapted by one skilled in the art. Accordingly, the scope of our invention isto be limited only by the following claims.

Claims: Claims 1. A method for forming a fine grained tungsten carbide that has a particle size within a range of from 0.05 to 0.2 μm in diameter, which comprises heating a solid, tungsten- containing material in a flowing atmosphere containing molecular hydrogen and molecular methane and having a fixed ratio of hydrogen to methane of 90 to 99 percent hydrogen to 1 to 10 percent methane, for a time sufficient to convert substantially all of said material to tungsten carbide, wherein said heating is carried out at a first heating rate of 5 to 50°C per minute from 25°Cto 535°C, and at a second heating rate , lower than the first heating rate, of 1 to 10°C per minute from 535°Cto about 850°C, and thereafter held at about 850°C for 15 minutes to 3 hours. 2. A method as claimed in Claim 1, wherein said atmosphere contains a ratio of 92 to 97 percent hydrogen to 3 to 8 percent methane. 3. A method as claimed in Claim 1, wherein said atmosphere contains argon. 4. A method as claimed in any one of Claims 1-3, wherein said first heating rate is about 20°C per minute and said second heating rate is about 5°C per minute. 5. A method as claimed in Claim 4, wherein said temperature is held at about 850°Cfor 30 to 90 minutes. 6. A method as claimed in Claim 4, wherein said material is heated in a space having a defined volume, and wherein the flow rate of said atmosphere is at least about 1 volume per minute. 7. A method as claimed in Claim 6, wherein said flow rate of said atmosphere is at least about 10 volumes per minute. 8. A method as claimed in any one of Claims 5-7, wherein said material is powdered or particulate tungsten trioxide. 9. A method for forming a tungsten carbide having an average range size between O.OSand 0.2 micrometers, comprising heating powdered or particulate tungsten trioxide in a flowing atmosphere containing argon, molecular hydrogen and molecular methane, the ratio of hydrogen to methane being fixed at 92 to 97 percent hydrogen to 3 to 8 percent methane, for a time sufficient to convert substantially all of the tungsten trioxide to tungsten carbide, wherein the heating is carried out at a first heating rate of about 20°C per minute from 25°C to 535°C, then at a second heating rate of about 5°C per minute about 535°C to 850°C, and there-after held at about 850°C for 30 to 90 minutes, such that the resultant product achieves a constant weight.

Inventors: DUNMEAD STEPHEN D, GRINDAHL ERIC J, MOORE WILLIAM G, DUNMEAD, STEPHEN, D., GRINDAHL, ERIC, J., MOORE, WILLIAM, G.
Applicants: DOW CHEMICAL CO, THE DOW CHEMICAL COMPANY
Application year: 1992
Main classification: C01B31
IPC codes: C01B31