Alloyed powder and method of preparing same

Abstract: A powder comprising an alloy of a refractory material and carbothermic reaction product produced by carbothermically reacting a blend of at least two metal oxides.

Description: Field of the Invention The present invention relates to a method of preparing alloyed powders which are hot-pressed or sintered to form refractory bodies. Background of the Invention Certain refractory powders, used to form refractory shapes by compressing the powders at an elevated temperature, can be produced by reaction of metal oxides with carbon to form carbides, or borides if a boron source is also added. Such processes are disclosed in, for example United States Patents 3,351,428, 2,906,605, 2,973,247, 3,004,830, 3,013,862, 3,246,956, and 3,328,127. US Patent 3,048,493 teaches compositions which can consist of silicon, boron, zirconium and/or titanium. These are alloys of ternary or quaternary component compositions. Moreover, beside blending carbon with e.g. titanium dioxide and boron trioxide in step A, the present invention also allows the production of boron carbide (B₄C) in step B, and that in addition to titanium boride (TiB₂). Surprisingly enough, a uniform distribution of the components of the alloy is achieved which is made possible with the aid of the carbothermic reaction and a higher carbon content. For some application, an alloyed refractory shape is desired, e.g. a shape of titanium boride and chromium diboride. Typically these alloyed refractory shapes are formed by blending different powders together, with a powder for each component, and then hot-pressing the blended mixture. A problem with these processes, is that, even with extensive blending, the uniform distribution of the different powders is difficult to achieve. In addition, each powder must be prepared separately, which results in a substantial increase in the number of process steps required to form the refractory shape. In addition to blended powders for alloys, mixed powder compositions are often produced to introduce sintering aids, and other process aids, into the powders. Certain refractory powders, such as those of titanium diboride and boron carbide, cannot be hot-pressed or sintered in their pure form to form a product of sufficient density and strength. In order to form suitably dense and strong refractory shapes from these powders, a small amount of sintering aid is required. For example, titanium diboride powders used commercially usually contain iron, nickel, and/or cobalt, which act as sintering aids. Generally, these sintering aids are introduced into the powder by ball milling the powders with stainless-steel or tungsten carbide balls (which generally contain 6 to 12 wt.% cobalt) through attrition of the balls. Alternately, sintering aids, such as the above metals, nickel diboride, or chromium diboride, can be added to the refractory powder and the mixture blended before hot-pressing of the powder. A problem with these methods, is that it is difficult to obtain a uniform distribution of the sintering aid in the powder. In addition, in methods which rely upon attrition of the milling balls to add the sintering aid, it is difficult to control the amount of sintering aid added to the powder. OBJECTS OF THE INVENTION It is, therefore, an object of the invention to provide a process for producing alloyed powders of refractory materials powders, wherein the components are substantially uniformly distributed. It requires fewer process steps than in prior art processes. It is also an object of the invention to produce a truly alloyed powder of two or more components, rather than a mechanical mixture of different powders such that the components do not segregate or become non-uniformly distributed when mixed. It is further an object of the invention to provide a process for producing powders of refractory materials containing a controlled amount of an additive, such as a sintering aids. Additional objects of the invention will become evident in the description that follows. SUMMARY OF THE INVENTION The present invention provides a method for preparing an alloyed powder of carbothermic reaction products of metal oxides and carbon, which method comprises: a) blending carbon with at least a first metal oxide selected from an oxide of titanium or of zirconium and at least one second metal oxide selected from an oxide of boron alone or in combination with an oxide of silicon for forming, in situ, at least two separate carbothermic reactions in a reaction mixture of said first and second metal oxide and carbon; b) heating the reaction mixture from step a) to an elevated temperature sufficient to simultaneously form an alloy of (1) a refractory boride of titanium or of zirconium from reaction of carbon and said first and second metal oxides and of (2) a carbide of the second metal from the reaction product of carbon and the second metal oxide independent of the first metal oxide; and c) comminuting the alloy of step b) to form an alloy powder. Carbothermic reactions are reactions with metal oxides and carbon at elevated temperatures to form a reduction product of the metal oxide. The first metal oxide, which can be one metal oxide or a combination of metal oxides defined above, forms a refractory compound in a carbothermic reaction of the first metal oxide include boron carbide, silicon carbide and the carbides and borides of tungsten. Refractory compounds of most commercial interest are silicon carbide, boron carbide, and the carbides and borides of tungsten (wolfram), zirconium, and titanium. Representative carbothermic reactions forming refractory compounds include, for example; WO₃ + 4C → WC + 3CO, 2B₂O₃ + 7C → B₄C + 6CO, TiO₂ + B₂O₃ + 5C → TiB₂ + 5CO, TiO₂ + 3C → TiC + 2CO, ZrO₂ + B₂O₃ + 5C → ZrB₂ + 5CO, SiO₂ + 3C → SiC + 2CO. The carbothermic reaction product of the second metal oxide is different from the first metal oxide and can be any of the refractory reaction products discussed above as well as other carbothermic reaction products, such as preferably those which are additives to improve the properties of the final ceramic shape. These include, for example, grain growth inhibitors, such as chromium diboride (CrB₂), and various sintering aids, such as, for example, borides of iron, nickel tungsten, and cobalt. The iron, nickel, tungsten or cobalt boride reaction products can, for example, be produced by the reaction of carbon with the corresponding oxide compounds, e.g. Fe₂O₃, NiO, CoO, Co₃O₄, and Co₂O₃, in the presence of a boron source. Other sintering aids which can be produced as the reaction product of the second metal oxide are, for example, ZrC, B₄C, SiB₄, and SiB₆. The carbon source in the method of the invention includes any finely divided form of carbon used for formation of refractory materials by carbothermic reaction, such as carbon sources currently used in TiB₂ production. Suitable forms of carbon include carbon black, lampblack, and other similar forms of carbon. A suitable carbon black is available under the Trademark "Thermax," from Thermatomic Carbon Co., Division of Commercial Solvents Co., Sterlington, LA. The first metal oxide, the second metal oxide and carbon are blended or mixed together by any suitable solid-solid mixing method. Suitable methods of blending include the use of tumblers, Vee-blenders, vertical screw blenders, and the like. The particle size of the first and second metal oxides, and carbon reactants is not critical, although the reactants should be sufficiently fine to allow intimate mixture to assist in the carbothermic reaction. Suitable average particle sizes for the reactants are 325 Tyler mesh (0.044 mm) and less. For the formation of borides, any suitable boron source is added to the reaction mixture, such as boron or a boron oxide. Preferably the boron source is B₂O₃. The amounts of the first and second metal oxides, and the carbon used in the reaction mixture are determined by the desired composition of the final product. In general the final product can be determined by application of known principles. Typically the nature of the final product is dependent upon the amount of carbon reactant, and the thermodynamic stability of the final product. For example, in a reaction to form TiB₂ from carbon, TiO₂, and B₂O₃, if more than a stoichiometric amount of carbon is present, and B₂O₃ is not in excess, the product is predominately a mixture of TiB₂ and TiC. If an excess of B₂O₃ is used, and a stoichiometric amount of the carbon is used, essentially only TiB₂ is formed, since TiB₂ is more thermally stable than TiC. Other considerations in choosing the amount of reactants are material handling, material vapor pressure, stability at reaction temperature and residence time. As a specific example, in a reaction mixture of TiO₂, B₂O₃, SiO₂, and C, to form TiB₂ and SiC, the reactants in stoichiometric excess are usually B₂O₃, and SiO₂. For another example, in a reaction mixture of TiO₂, B₂O₃, Fe₂O₃, and C, to form TiB₂, with FeB sintering aid, B₂O₃ and Fe₂O₃ are usually in a stoichiometric excess. After blending the first metal oxide, the second metal oxide, and the carbon to form the reaction mixture, the reaction mixture is heated to induce a carbothermic reaction. The reaction conditions are generally the same as those known in the art to produce the pure products by the carbothermic reaction of oxides. Generally this requires a temperature between 1600°C and 2000°C in a nonoxidizing atmosphere. The reaction product is then comminuted to form a powder, such as by conventional milling methods, e.g. micromilling, ball milling, and attrition milling. Generally, for powders used in the production of hot-pressed refractory articles, an average particle size less than 20 µm (microns), preferably about 5 µm (microns), or less, is suitable. The powders produced by the method of the invention may be used to form sintered shapes. Typically the powders are formed into shapes by a sintering or hot-pressing technique. Generally the temperature and pressure conditions typically used to form hot-pressed shapes from mixed powders are suitable. In the case where the alloyed powders include a sintering agent, the temperature of the hot-pressing can be somewhat less than in the prior art powders with blended sintering aid. Because the powders prepared in accordance with the invention are a true alloy, the sintering aid is more evenly dispersed and its effect in promoting sintering is thereby improved. Therefore, by practice of the invention, preformed ceramic shapes can be sintered or hot-pressed at lower temperatures. This in turn leads to a long mold life and the tendency toward grain growth in the ceramic shape during sintering is reduced. The reduced grain growth results in finer-grained shapes which have improved toughness. By practice of the invention the amount of additive, such as a sintering aid, may be more exactly controlled. In addition, since the sintering aid is more effectively dispersed, thus increasing the effect of the sintering aid, the amount of sintering aid required is less than in prior art methods. Typically, when the reaction product of the second metal oxide is a the sintering aid, it is present in an amount of from 0.1 wt.% to about 3 wt.%. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a flow-chart illustrating a prior method for forming a mixed powder of WC and cobalt. Figure 2 is a flow-chart illustrating a method of the invention for forming an alloyed powder of titanium diboride and silicon carbide. Figure 3 is a flow-chart illustrating a prior method for forming a mixed powder of titanium diboride and silicon carbide. DESCRIPTION OF THE INVENTION The invention will now be illustrated by examples. Example I This is a comparative example showing a prior art method for producing blended powders. Referring to Figure 1, which is a flow chart which illustrates the method commonly used in commercial practice for producing a mixed blend of WC and Co powders. As shown by the flow chart, WC powder is first produced by mixing and reacting tungsten and carbon, and milling the reaction product to form a WC powder. Separately Co powder is formed by reducing Co₃O₄ in hydrogen at 700°C, and milling the product to form Co powder. The WC, and Co powders are then blended together in a ball mill, filtered, dried, and screened to form a mixed powder of WC and Co. Since the prior art method require separate production of two different powders, the capital and labor costs for the prior art method is significantly higher than that of the invention, which requires only one reaction step. An additional advantage is that the powder of the invention is a true alloy of WC and Co, rather than a mechanical mixture. As such the powder of the invention is not subject to separation and segregation of the powder components, and has a more uniform distribution of the components. Example II This example illustrates production of an alloyed powder according to the invention comprising approximately 70 wt.% SiC and 30 wt.% TiB₂. A blend of metal oxides and carbon is prepared by mixing 1049.1 g SiO₂, 345 g TiO₂, 331 g B₂O₃, and 887.4 g carbon (Thermax) in a v-blender for 60 minutes. The blended material is pressed at room temperature and a pressure of 827.4 bar (12000 psi) to form plugs in the form of rectangular solids 1.27 cm X 1.27 cm X 2.54 cm (0.5 inch X 0.5 inch X 1.0 inch). Each plug weighs approximately 5 g with a density of about 1.22 g/cm³. The plugs are placed into a graphite capsule with an inside diameter of about 8.89 cm (3.5 inches), and about 30.48 cm (12 inches) long. The capsules containing the plugs are placed into a 10.16 cm (4 inch) inside diameter resistance heated furnace. After purging the furnace with argon for one hour, the power to the furnace is turned on, and the capsule is heated at a rate of 300°C/hour to a temperature of 1800°C. The carbothermic reaction is carried out at 1800°C for one hour while the argon flow rate is maintained at 113.3 l/h (4 ft³/hr). The power to the furnace is then turned off, and the capsule allowed to cool. After cooling, the reaction product is removed from the capsule, weighed, and micromilled to a particle size less than 0.208 mm (65 mesh). If the carbon and the metal oxides in the plugs are fully converted, the reaction product weighs about 1000 g. Figure 2 is a flow-chart summarizing the above described process for producing alloyed TiB₂ and SiC powders. Figure 3 illustrates a comparable prior-art process for forming a mixed powder. By comparing the processes of Figures 3 and 4, the significant reduction in process steps provided by practice of the invention is clearly seen. Example III This example illustrates production of an titanium diboride powder containing an iron sintering aid according to the method of the invention. A blend of metal oxides and carbon is prepared by mixing 2 grams Fe₂O₃, 368 g TiO₂, 353 g B₂O₃, and 277 g carbon (Thermax) in a v-blender for 15 minutes. The blended material is pressed at room temperature and a pressure of 2068.5 bar (30,000 psi) to form plugs about 2.54 cm (1 inch) in diameter and 3.81 cm (1.5 inches) long. Each plug weighs approximately 23 g with a density of about 1.2 g/cm³. The plugs are placed into a graphite capsule with an inside diameter of about 8.89 cm (3.5 inches), and about 30.48 cm (12 inches) long. The capsules containing the plugs are placed into a 10.16 cm (4 inch) inside diameter resistance heated furnace. After purging the furnace with argon for one hour, the power to the furnace is turned on, and the capsule is heated at a rate of 300°C/hour to a temperature of 1800°C. The carbothermic reaction is carried out at 1800°C for one hour while the argon flow rate is maintained at 113.3 l/h 4 ft³/hr. The power to the furnace is then turned off, and the capsule allowed to cool. After cooling, the reaction product is removed from the capsule, weighed, and micromilled to a particle size less than 0.208 mm (65 mesh). If the carbon and the metal oxides in the plugs are fully converted, the plugs weigh 320 g or less. Example IV A sample of TiB₂ powder with a iron sintering aid was made according to the method of invention described above in Example III. The properties of the powder are summarized below in Table A. For comparison, shown also in Table A are the properties of the following powders; (1) a pure as-produced TiB₂ powder without any sintering additives, (2) as-produced TiB₂ powder after ball-milling with stainless steel balls for 2 hours, (3) as-produced TiB₂ powder after ball-milling for 2 hours in stainless steel mill with tungsten carbide balls containing 6 wt.% cobalt, (4) as-produced TiB₂ powder after ball-milling for 1 hour in stainless steel mill with tungsten carbide balls containing 12 wt.% cobalt, (5) as-produced TiB₂ powder after dry-blending with 1 wt.% iron for 30 minutes. Each powder was hot-pressed at a temperature of 1950°C, under 172.3 bar (2500 psi), for a period of 2 hours. The density of the resulting refractory molded shape is also shown in Table A. As seen by the data in Table A, the sintered shape of the powder of the invention is generally superior to the comparative shapes. The sintered shape from the comparative pure as-produced powder (1) had significantly inferior density due to the absence of a sintering aid. The shapes from comparative ball-milled powders (2, 3 & 4) have satisfactory properties, but the amount of the iron and/or cobalt sintering aids in the powder could not be controlled, and as compared to the invention, an excess of sintering aid was unavoidably introduced into the sintered shape. In addition, these comparative powders generally require more sintering aid than the invention to compensate for a poorer distribution of sintering aid in the powder. In addition, the comparative powders that were ball-milled with tungsten carbide balls containing cobalt (3 & 4) were unavoidably contaminated with tungsten. The comparative sample, wherein iron was blended into the powder (5), required an excess of iron, as compared to the invention, to compensate for the poorer distribution of iron through the powder. The poorer distribution was due largely to a settling effect wherein the heavier iron particles separate and settle out from the mixture. The tungsten contaminant in 3 & 4, and the iron contaminant in 5, both of which are dense materials, also resulted in higher density than the shapes without contaminant. Although higher densities for sintered shapes often imply increase strength of the sintered shape, the density increase caused by dense impurities generally does not reflect such improved properties.

Claims: A method for preparing an alloyed powder of carbothermic reaction products of metal oxides and carbon, which method comprises: a) blending carbon with at least a first metal oxide selected from an oxide of titanium or of zirconium and at least one second metal oxide selected from an oxide of boron alone or in combination with an oxide of silicon for forming, in situ, at least two separate carbothermic reactions in a reaction mixture of said first and second metal oxide and carbon; b) heating the reaction mixture from step a) to an elevated temperature sufficient to simultaneously form an alloy of (1) a refractory boride of titanium or of zirconium from reaction of carbon and said first and second metal oxides and of (2) a carbide of the second metal from the reaction product of carbon and the second metal oxide independent of the first metal oxide; and c) comminuting the alloy of step b) to form an alloy powder. The method of claim 1 wherein the oxide of boron is B₂O₃. The method of claim 2 wherein when the first metal oxide is TiO₂, and the second metal oxide is B₂O₃ and SiO₂, the refractory boride is TiB₂ and the reaction product of carbon and the second metal oxide is SiC. The method of claim 2 wherein the second metal oxide further comprises metal oxides selected from the group comprising Fe₂O₃, Co₃O₄, NiO, WO₃, CoO, Co₂O₃, and mixtures thereof such that an additional boride reaction product is formed selected from the group consisting of FeB, CoB, NiB, WB, and mixtures thereof. The method of claim 2 wherein when the first metal oxide is TiO₂ and the second metal oxide is B₂O₃, the refractory boride is TiB₂ and the reaction product of carbon and the second metal oxide is B₄C.

Inventors: MONTGOMERY LIONEL CLAYTON, PARRISH LIEN NGUYEN, MONTGOMERY, LIONEL CLAYTON, PARRISH, LIEN NGUYEN
Applicants: UNION CARBIDE CORP, UNION CARBIDE CORPORATION
Application year: 1989
Main classification: C04B35
IPC codes: B22F9, C04B35, C01B31