COATED REFRACTORY COMPOSITIONS AND METHOD FOR PREPARING THE SAME

Abstract: A coated refractory composition for making high quality preforms comprising a refractory material selected from a metal refractory carbide, metal refractory nitride, metal refractory boride or diamond having a coating layer of hydrous alumina or a coating layer of anhydrous crystalline alumina, and to a method for enhancing dispersibility and improving oxidation resistance of such compositions. Further, a composition comprising an alpha alumina substrate having a first coating layer of calcium pyrophosphate and a second coating layer of hydrous alumina, and to a calcined composition comprising an alpha alumina substrate having a first coating layer of calcium pyrophosphate and a second coating layer of a densified anhydrous, crystalline form of alumina and to a method for preparing the same.

Description: TITLE COATED REFRACTORY COMPOSITIONS AND METHOD FOR PREPARING THE SAME BACKGROUND OF THE INVENTION The present invention relates to a coated refractory composition for making high quality preforms comprising a refractory material selected from a metal refractory carbide, metal refractory nitride, metal refractory boride or diamond having a coating layer of hydrous alumina, and, more particularly, to a method for enhancing the dispersibility and for improving the oxidation resistance and chemical inertness of such compositions. Another aspect of the present invention relates to a composition comprising a refractory substrate having a first coating layer of a calcium component and a second coating layer of an alumina component, and, more particularly, to a method for improving the strength of a ceramic composite by preparing an alpha alumina substrate having deposited thereon a first coating of calcium pyrophosphate and a second coating of hydrous alumina. Whereupon calcining said composition, said second coating converts to an anhydrous crytalline form of alumina. Ceramic composites start with a ceramic preform which is usually based on silicon carbide, silicon nitride, aluminum silicate, aluminum oxide or mixtures thereof. These preforms are imbedded in a ceramic matrix to form composites. Ceramic composites have been found to lose their strength at higher temperatures; a problem which has not yet been resolved. All the useful properties begin in the early stages of ceramic processing. The structural integrity of each material must be preserved through a variety of thermal and mechanical stress exposures. Thus, the need for a high-temperature, high-strength material exists. The unique properties of the present invention enable the composition to be used as a preform for a ceramic matrix composite having unexpected strength and toughness. These unexpected results are attributed to the coating that provides a debonding function between the preform and the matrix which results in limiting crack propagation in the composite body. U.S. Patent 4,249,913 describes a silicon carbide abrasive particles which is coated with alumina to inhibit dissolution in a bonding metal matrix during high temperature processing. The alumina coating, described as a dense, anhydrous and amorphous coating, is applied by sputtering or vapor deposition processes. U.S. Patent 4,801,510 relates to a composite article comprising 5 to 30 vol% silicon carbide whiskers and 70 to 95 vol% alumina, the articles having a thin coating of alumina. The silicon carbide and alumina in the composite are mixed together by ball milling, hot pressed to form a dense article, then coated with a thin layer of alumina using vapor deposition techniques. SUMMARY OF THE INVENTION The present invention is directed to a coated refractory composition for making high quality preforms comprising a refractory material selected from a metal refractory carbide, metal refractory nitride, metal refractory boride or diamond having a coating layer of hydrous alumina whereby the coating layer enhances the dispersibility of such compositions. The invention is further directed to a calcined composition comprising said refractory material having a coating of a crystalline anhydrous alumina whereby the coating layer further improves the oxidation resistance and chemical inertness of such compositions. Another aspect of the present invention is directed to a method for preparing the same comprising the steps of: (a) coating an aqueous suspension of a finely divided refractory material with hydrous alumina; (b) separating washing and drying said coated refractory; and (c) optionally calcining at a temperature in the range of from 400 to 1100°C for at least one hour. A further aspect of the present invention relates to a composition comprising a refractory substrate having deposited thereon a first coating layer of a calcium component and a second coating layer of an alumina component. The invention is further directed to a calcined composition wherein said first and second coating form a unique material, a portion of which is anhydrous crystalline alumina. Yet another aspect of the present invention is directed to a method for improving the strength and toughness of a ceramic composite according to the steps of: (a) preparing an aqueous admixture of a refractory substrate, a calcium component precursor and an aluminum salt; and (b) recovering, washing and drying the coated refractory composition. A further calcining step forms an anhydrous crystalline alumina coated refractory composition. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a transmission electron micrograph of a coated refractory composition prepared by the process of this invention. FIG. 2 is a transmission electron micrograph of a calcined coated refractory composition prepared by the process of this invention. FIG. 3 is a graphic representation illustrating the higher isoelectric points of the coated refractory compositions of the present invention. FIG. 4A and FIG. 4B are thermogravimetric analysis plots illustrating improved oxidation resistance of the coated refractory compositions of the present invention. DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a coated refractory composition demonstrating enhanced dispersibility and improved oxidation resistance and chemical inertness, wherein the composition comprises a refractory material selected from a metal refractory carbide, metal refractory nitride, metal refractory boride or diamond having a coating layer of hydrous alumina, i.e., a mixture of boehmite fine crystals and hydrous amorphous alumina. The coating increases the isoelectric point of the composition to that of alumina. The isoelectric point is defined herein to refer to the pH at which the zeta potential is zero. The increased isoelectric point and surface area relate to enhanced dispersibility of such compositions in water or solvent systems and result in stable aqueous dispersions useful, for example, in slip casting procedures in producing refractory articles. The refractory material, i.e., substrate is a finely divided solid having an average particle size between 1 to 2500 microns. Generally, the refractory substrate has a surface area in the range of from 0.05 to 20 m 2 /g. For best results, the refractory substrate has a surface area of 0.2 to 10 m 2 /g. In practicing the invention, useful refractory materials are metal refractory carbides, metal refractory nitrides, metal refractory borides or diamonds. For example, a refractory carbide such as silicon carbide is a suitable substrate. Other refractory carbides, that are stable in an aqueous medium, such as titanium carbide, tantalum carbide, niobium carbide, zirconium carbide, tungsten carbide, molybdenum carbide, vanadium carbide, hafnium carbide, thorium carbide and uranium carbide can be suitable substrates. In addition, diamonds and water insensitive refractory nitrides such as titanium, zirconium and silicon, and water insensitive refractory borides such as titanium and zirconium can be coated with hydrous alumina by the process of the invention. The shape of the refractory particles can vary such as equiaxial, acicular and platelet. Suitable substrates are equiaxial particles in the size range of from 1 to 2500 microns, acicular particles having an aspect ratio of from 2 to 200 and an average diameter of from 0.1 to 12 microns and platelets having as aspect ratio of from 5 to 200 and an average diameter of from 5 to 2500 microns. In addition, the refractory substrate should not be soluble in water or dilute acid or base. The coating on the refractory substrate comprises a layer of hydrous alumina, i.e., a mixture of boehmite alumina and hydrous amorphous alumina. The coating amounts to 0.2 to 12 wt% and typically 3 to 6 wt% of the total composition. The hydrous alumina coating is a mixture of fine crystals of boehmite and hydrous amorphous alumina and consequently the specific surface area is greater than that of the refractory particle and is in the range of from 0.5 to 40 m 2 /g, typically 2 to 15 m /g. The invention is further directed to a calcined composition in which the coating is a crystalline anhydrous alumina such as eta, theta, gamma or alpha phase or transition mixtures thereof, depending upon the calcination conditions. The uncalcined or calcined compositions are also useful for the preparation of high strength, toughened metal matrix and ceramic matrix composites. The coating increases the surface area and the isoelectric point of the compositions of that of alumina and result in enhanced dispersibility, which enable uniform preforms to be made. The enhanced dispersibility is due to the higher isoelectric point to that of alumina and the surface area of the coated refractory. The high quality preforms may be used in making high performance composites. The coating also has a protecting function, for example, the refractory carbides are protected from surface oxidation during high temperature processing under atmospheric conditions. The calcined coated compositions exhibit much improved oxidation resistance. Another aspect of the present invention is directed to a method for enhancing dispersibility of such compositions in water or solvent systems and improving oxidation resistance and chemical inertness of such compositions. According to the method, the steps for carrying out the present invention comprise: (a) forming an aqueous suspension of a refractory material, heating to a temperature in the range of between 40° and 95 β C and adjusting the pH in the range of from 5 to 9; (b) adding an aqueous solution of an aluminum salt selected from the group consisting essentially of alkali metal aluminates, ammonium aluminate, aluminum chloride, aluminum nitrate and the aluminum acetate with stirring and maintaining the temperature and pH for 5 to 60 minutes whereby hydrous alumina precipitates as a coating layer on the surface of said refractory material; (c) separating, washing, drying the coated refractory at 100" to 300 β C; and (d) optionally calcining at a temperature in the range of from 400" to 1100"C for at least one hour. In practicing the invention an aqueous suspension of a finely divided refractory material is prepared. The concentration of solids in the aqueous suspension is not especially critical and can range from 10 to 1000 grams per liter of water. Hydrous aluminum oxide is generated in the presence of the suspended particles at a carefully controlled rate so that all the hydrous aluminum oxide produced forms as a coating on the particles rather than forming as a separate precipitate. Either acidic or basic, water soluble aluminum salts can be used as sources of aluminum oxide, for example, alkali metal aluminates, ammonium aluminate, aluminum chloride, aluminum nitrate and aluminum acetate. Aqueous solutions of aluminum salt are prepared and contain aluminum equivalent to 5 to 40 wt% I2O3 and are added to the aqueous suspension. When acidic aluminum salts are used, alkali is added concurrently, again maintaining the pH in the range of 5 to 9 during the coating process. In the case of aluminates acid is added simultaneously maintaining the pH in the range of 5 to 9 throughout the process. The preferred source of aluminum oxide is sodium or potassium aluminate to which is added mineral acid, typically hydrochloric acid. The aqueous suspension of the refractory material is heated to a temperature in the range of from 50 to 95 β C and the pH is adjusted in the range of 5 to 9. Typically, the pH is within this range, if it is less than 5, then a few drops of dilute base, or if it is more than 9, then a few drops of dilute acid will bring it into the desired range. To the stirred suspension, the solution of alkali metal aluminate is added dropwise together with mineral acid, for example, 5 to 20% HCl at a rate which maintains the pH between 5 and 9. The thickness of the hydrous alumina coating layer is a function of the amount of alkali aluminate added to the aqueous slurry. At a temperature of from 60" to 90*C and a mixture pH in the range 7.5 to 9, the hydrous alumina will ordinarily be deposited on the surface of the suspended refractory particles at the rate of 2 to 3 wt%/hour. On completion of the addition, the suspension is stirred for a further 5 to 60 minutes maintaining the same temperature and pH range. This step stabilizes the coating layer of hydrous alumina on the slurried particles. The coated particles are then isolated, i.e., separated by filtration or centrifugation, washed with water until free from soluble ions. particularly sodium and chloride ions, and dried by heating at 100° to 300°C. A final calcining step is optional and whereupon calcining the composition additionally exhibits improved oxidation resistance in high temperature processing under atmospheric conditions. The coated refractory composition with the hydrous alumina coating may be calcined by heating in air or an inert atmosphere, e.g. N2 at 400" to 1100"C for at least one hour. Calcining densifies the coating layer and results in a decrease in specific surface area. The extent to which the surface area decreases depends on the time and temperature of the heating cycle. The coating is converted to an anhydrous crystalline transition alumina which may be in the gamma, eta, theta or alpha alumina phase or transition mixtures thereof, depending on the temperature to which the composition is heated. The calcined compositions may go through additional coating steps and the entire process repeated to further improve the oxidation resistance. The isoelectric point of the compositions of the present invention is a useful measure of surface charge. Stable aqueous dispersions of particles in water or solvent systems are obtained with particles which have an isoelectric points in the range of from about 6 to 9. The isoelectric point is determined by measuring the zeta potential of a suspended particle over a range of pH and ascertaining the pH where the zeta potential is zero. The coated refractory particles of the present invention exhibit an isoelectric point in the range of from about 6 to 9. In contrast, the isoelectric point of the uncoated refractory particle is usually less than 3. For example, a SiC surface is 2.0 to 2.5, reflecting the presence of silicon dioxide on the surface. Referring now to the drawings, by way of example, FIG. 3 is a graphic representation of zeta potential measurements on aqueous dispersions of 1000 grit silicon carbide (Plot A) ; the same silicon carbide having a coating of hydrous alumina (Plot B) ; and the latter after calcining to convert the coating to crystalline anhydrous gamma alumina (Plot C) . The isoelectric point occurs at 2.3 in Plot A, 8.6 in Plot B and 8.6 in Plot C, showing the coated products to have a considerably higher isoelectric point than the original silicon carbide. The 8.6 isoelectric point is characteristic of the alumina coating. The improved oxidation resistance of a refractory carbide particle is evident from FIG. 4A and FIG. 4B which show Thermogravimetric (TGA) analysis plots which were obtained for a 500 grit silicon carbide powder (FIG. 4A) ; and the same powder coated with hydrous alumina (FIG. 4B) . The rate of temperature increase was 10"C/minute up to 1000'C followed by a one hour hold at 1000 β C and the air flow was lOOcc/min. FIG. 4A shows a weight increase of 0.36%, attributable to surface oxidation of the silicon carbide particles. The hydrous alumina coated silicon carbide shows a weight decrease of about 2% which reflects dehydration of the hydrous alumina coating and conversion to a crystalline anhydrous transition alumina. A second composition is a refractory substrate having deposited thereon a first coating layer of a calcium component and a second coating layer of an alumina component. The refractory substrate can be a high temperature crystalline form of alumina such as alpha alumina. Other refractory substrates such as the oxides, carbides, borides, and nitrides, of aluminum, titanium, zirconium or silicon, are also suitable substrates provided that the surface of these substrates are first covered with a coating of alumina. The substrate is a finely divided powder having an average particle size which can vary over a wide range, i.e., 0.1 to 2500 microns depending upon the shape of the particles. The particle shape can also vary, i.e., the particle shape can be equiaxial, acicular and platelet. When the particles are of alpha alumina, calcium or sodium beta alumina may be present in the refractory substrate. In addition, the refractory substrate should not be soluble in water, dilute acid or base. The surface area of the particles ranges from 0.02 to 20 m 2 /g. Best results are achieved when the particles have a surface area of from 0.1 to 20 m 2 /g. The first coating comprises a calcium component, e.g., an amorphous refractory calcium compound such as calcium pyrophosphate, calcium phosphate, calcium silicate and calcium aluminate. The second outermost coating is an alumina component, e.g., hydrous alumina which can be amorphous, boehmite or mixtures thereof. The first and second coatings can amount to between 0.5 to 25 wt% and are typically 8 to 12 wt% of the total composition. The ratio of the first coating to the second coating is in the range of 0.01 to 0.95 on a weight basis. Referring now to the drawings, FIG. 1 is a transmission electron micrograph of a coated refractory composition obtained by the process of this invention. From this micrograph, the coating layers on the substrate is apparent. FIG. 2 is a transmission electron micrograph of a calcined coated refractory composition prepared by the process of present invention. The coating layers have densified and the hydrous formed alumina portion has been converted to an anhydrous crystalline alumina coating. Another aspect of the invention is the process for preparing the same. The first step in carrying out the invention is preparation is of an aqueous admixture of a refractory substrate, a calcium component precursor and a soluble aluminum salt. In practicing the invention, an aqueous suspension of a refractory substrate is prepared. Commercially available alpha alumina platelets can be used as the substrate and are preferred in terms of cost, convenience and operability. The concentration of particles in the aqueous suspension is not especially critical and can range from 100 to 1000 grams per liter of water. The aqueous substrate suspension is heated to a temperature in the range of from 40" to 95"C and the pH is adjusted in the range of from 5 to 9. Typically, the pH is within this range; if, however, the pH is less than 5 or more than 9, a few drops of dilute base or dilute acid, respectively, will bring the pH into the desired range. Next, aqueous solutions of a calcium salt and the desired alkali metal pyrophosphate, phosphate, silicate aluminate or mixtures thereof are prepared. The sodium salts are preferred and the concentration can range from 50 to 200 grams/liter. For example, sodium pyrophosphate crystals are suitable as a source of phosphate. If sodium silicate is selected as the soluble metal silicate, a clear aqueous solution having a Siθ2/Na2θ molar ratio of 3.24/1 can be used. In the case of sodium aluminate, an aqueous solution containing the equivalent of 5 to 40 wt% AI2O 3 may be used. Also, an aqueous solution of the desired concentration of a soluble calcium salt such as calcium chloride, calcium nitrate or mixtures thereof is prepared. With stirring, the desired alkali metal phosphate, silicate or aluminate solution and a solution of calcium salt are added concurrently to the aqueous suspension at about the same rate while maintaining the temperature in the same range. The pH is kept between 5 and 9 by the controlled addition of HCl (10 to 20%) . The process is carried out slowly, i.e., over a period of from 1 to 3 hours. After the addition is complete, stirring is continued and the same temperature range and pH are maintained for 5 to 60 minutes. Under these conditions, the substrate is coated with a layer of, for example, amorphous calcium pyrophosphate, calcium phosphate, calcium silicate or calcium aluminate or mixtures thereof depending upon the alkali metal salt selected. Then an aqueous solution of an aluminum salt is prepared. Either an acidic or basic water soluble aluminum salt can be used as a source of aluminum oxide, e.g., alkali metal aluminate, ammonium aluminate, aluminum chloride, aluminum nitrate, aluminum acetate and mixtures thereof. The solution used contains aluminum equivalent to 5 to 40 wt% AI2O3. When acidic aluminum salts are used, alkali is added concurrently and the pH is maintained in the range of 5 to 9 during the coating process. In the case of aluminates, acid is added simultaneously while maintaining the pH in the range of 5 to 9 throughout the process. The preferred source of AI2O 3 is sodium or potassium aluminate and mineral acid is added, typically HCl. The hydrated aluminum oxide is generated in the presence of the suspended first coated substrate at a carefully controlled rate so that all the hydrated aluminum oxide produced coats the first coated substrate rather than forming as a precipitate. The aqueous suspension of particles coated with the desired calcium component is maintained at a temperature in the range of from 50° to 95 β C and a pH in the range of from 5 and 9. To the stirred suspension, the aluminum salt solution is added dropwise and the pH is maintained between 5 and 9. The thickness of the hydrous alumina coating layer is a function of the concentration of the aluminum salt solution, the temperature, mixture pH and time. At a temperature of from 60° to 90°C and a mixture pH in the range of 7.5 to 9, the hydrous alumina will ordinarily be deposited on the surface of the suspended refractory particles at the rate of 2 to 3 wt%/hour. When the addition is complete, the suspension is stirred for an additional 5 to 60 minutes while maintaining the same range of temperature and pH. This curing step stabilizes the coating layer of hydrated alumina, which is a mixture of amorphous and boehmite alumina on the slurried particles. The coated particles are then isolated by filtration or centrifugation, washed with water, until free from soluble ions, particularly sodium and chloride ions, and dried by heating at 100° to 300°C . The particulate composition is a refractory substrate having a composite coating of an amorphous calcium compound and hydrous alumina which can be calcined by heating to 400° to 1100°C for at least one hour. The coating is densified, and the alumina forms a predominately anhydrous crystalline alumina. The following procedures were used to characterize the products of the invention. Specific surface area was measured by the BET nitrogen adsorption method. Isoelectric point measurements were made using an automated electrokinetics analyzer known as Pen Ken System 3000 and were manufactured by Pen Ken Inc., Bedford Hills, N.Y. This instrument measured the electrophoretic mobility of particles in a dilute suspension. Measurements were made at different pH levels and by graphically plotting the results, the isoelectric point, i.e., the pH at which the zeta potential was zero, was ascertained. Thermogravimetric analysis (TGA) was done using a Du Pont Model 951 Thermogravimetric Analyzer. Elemental analysis was performed by the EDAX procedure. X-ray diffraction was used to identify the crystalline phases present. The compositions of this invention and the method of preparation are illustrated in more detail in the following examples, but are not intended to limit the scope of the invention. The products from each of the EXAMPLES 5-9 were subsequently calcined at a temperature in the range of from 400° to 1100°C for at least 1 hour. EXAMPLE 1 This example describes the preparation of SiC refractory material coated with hydrous alumina. Four hundred and seventy five grams of 500 grit SiC powder, (Norton Company grade 100 GI) , having a surface area of 0.56 m 2 /g, was added to 3 1 of water with good agitation in a 4-liter beaker. The stirred aqueous suspension of SiC was heated to 65°C and the pH was adjusted to 8.5. An aqueous solution of sodium aluminate, [NaAl(0H)4, equivalent of 0.385 g AI2O3/CC; supplied by Vinings Corp.] was added to the SiC suspension and the pH was maintained at 8.5 by the concurrent addition of 20% HCl. After adding 50 ml of the NaAl(OH)4 solution over a period of about an hour, the suspension was stirred at pH 8.5 and a temperature of 65°C for an additional 30 minutes. All the AI2O 3 was precipitated onto the SiC, corresponding to 3.89 wt% AI2O 3 based on the product. The solids were recovered by filtering the suspension and washed with deionized water until free from sodium and chloride ions. The hydrous alumina coated SiC was dried overnight in an air oven at 120°C. The product was found to have a surface area of 10.6 m /g, measured by nitrogen adsorption, compared with 0.56 m 2 /g for the starting SiC. Isoelectric point (IEP) was measured on aqueous dispersions of the particles using an automated electrokinetics analyzer, (Pen Ken System 3000, manufactured by Pen Ken Inc. of Bedford Hills, N.Y.). The starting SiC had an IEP of 2.3 and that of the coated product was 8.6. Thermogravimetric analysis (TGA) was conducted using a Du Pont Model 951 Thermogravimetric Analyzer. The rate of temperature increase was 10 β C/minute up to 1000°C, followed by a one hour hold at 1000°C and the air flow was 100 cc/minute. The uncoated SiC showed a weight increase of 0.36% attributable to surface oxidation. The alumina coated SiC showed a weight decrease of about 2%, due to dehydration of the hydrous alumina coating. EXAMPLE 2 This example describes the preparation of a fine grit SiC refractory material coated with hydrous alumina. Using the procedure of Example 1, 500 g of Exalon ESK #1200 SiC, having a surface area of 2.3 m 2 /g was used. The period of the addition of the sodium aluminate solution was two hours. The product was found to have a surface area of 17.4 m 2 /g compared with 2.3 m 2 /g for the starting SiC. EXAMPLE 3 This example describes the preparation of TiB2 refractory material coated with hydrous alumina. Using the procedure of Example 1, 500 g of TiB2 powder, (Union Carbide Grade HCT-30) , having a surface area of 0.53m 2 /g was used. The period of the addition of the sodium aluminate solution was two hours. The product was found to have a surface area of 9.4m 2 /g compared with 0.53m 2 /g for the starting iB2. EXAMPLE 4 This example describes the preparation of a diamond refractory material coated with hydrous alumina. Placed 25 g of diamond powder (Beta Diamond Products, Inc. Grade SJK-5) in 800 cc deionized water in a 1 liter beaker on a hot plate. The beaker was equipped with a stirring paddle and pH probe. The slurry was heated to 65°C. The pH of the slurry was adjusted to 8.2 with 5 drops of 20% NaOH solution. Over a period of 2 hours, 0.9 ml of sodium aluminate solution (Vinings Corp.) that contained 0.385 g Al2θ3/ml was added dropwise to the stirred bath. The pH was maintained at 8.2 with HCl (0.1M) . After the alumina coating has been applied, the system was stirred 30 minutes at pH 8.2/65'C to cure the coating. The coated diamond powder was recovered by filtering, washing free of residual chlorides, and drying 2 hours at 120°C. The coated diamond powder has a surface area of 4.2m 2 /g. EXAMPLE 5 This example describes the preparation of a powder comprising platelets of alpha alumina having a composite coating of calcium pyrophosphate and hydrous alumina. 150 g sodium pyrophosphate crystals (Na4P2θ7.10H2θ) was dissolved in a liter of water to give a 0.336 M solution. 150 g anhydrous calcium chloride C Cl2) was dissolved in a liter of water to give a 1.35 M solution. 1000 g alumina powder, having a surface area of 1.4 m 2 /g (Microgrit Corp. grade WCA #3), was added to 3 liters of water with good agitation in a 4-liter beaker. The stirred aqueous suspension of alumina powder was heated to 75 β C and the pH was adjusted to 8.0 with a few drops of 6N NaOH. 297 ml of the 0.336 M solution of Na4P2θ7.IOH2O and 148 ml of the 1.35 M solution of CaCl2 were added concurrently over a two hour period and the pH was maintained at 8.0 by addition of 20% HCl. The suspension was cured by stirring at pH 8.0 and a temperature of 75"C for an additional 30 minutes. An aqueous solution of sodium aluminate, [NaAl(OH)4, equivalent 0.385 g AI2O3/CC; Vinings VSA #38 supplied by Vinings Corp.], was added dropwise to the suspension and the pH was maintained at 8.5 by the controlled addition of 20% HCl. After adding 130 ml of the NaAl(OH)4 solution over a period of two hours the suspension was cured by stirring at pH 8.5 and 75°C for an additional 30 minutes. All the Ca2P2 7 and AI2O 3 were precipitated onto the core alumina, corresponding to 1.59 wt% Ca2P2θ7 and 4.69 wt% AI2O3, based on the product. The Ca2P2θ7/Al2θ 3 ratio was 0.339. The solids were recovered by filtering the suspension and washed with deionized water until free from sodium and chloride ions. The solids were dried overnight in an air oven at 120°C. Determination of the elements, other than oxygen by EDAX analysis of the product gave Al 96.383, Ca 1.684 and P 1.933 on a wt% basis. The surface area was 23.6 m 2 /g. The product was subsequently calcined at a temperature in the range of from 400° to 1100°C for at least 1 hour. EXAMPLE 6 This example describes the preparation of a powder comprising platelets of alpha alumina having a composite coating of calcium silicate and hydrous alumina. 1000 g alumina powder, (alpha alumina platelets from Du Pont) having a surface area of 0.4 m 2 /g, was added to 2 liters of water with good agitation in a 4-liter beaker. The stirred aqueous suspension of AI2O 3 was heated to 80°C and the pH was adjusted to 9.0 with a few drops of 6N NaOH. 48 g of potassium silicate solution (K2Siθ 3 containing 0.25 g Siθ2 per gram of solution) and 148 ml of a 1.35 M solution of CaCl2 were added concurrently over a two hour period maintaining the pH at 9.0 by addition of 20% HCl. The suspension was cured by stirring at pH 9.0 and a temperature of 80°C for an additional 30 minutes. An aqueous solution of sodium aluminate, [NaAl(OH)4, equivalent 0.385 g AI2O3 CC; Vinings VSA #38 supplied by Vinings Corp.], was added dropwise to the suspension and the pH was maintained at 8.0 by the controlled addition of 20% HCl. After adding 130 ml of the NaAl(OH)4 solution over a period of two hours the suspension was cured by stirring at pH 8.0 and a temperature of 80°C for an additional 30 minutes. All the CaSiθ 3 and AI2O 3 were precipitated onto the core alumina corresponding to 0.726 wt% CaSiθ3 and 4.69 wt% AI2O 3 , based on the product. The CaSiθ3/Al2θ3 ratio was 0.155. The solids were recovered by filtering the suspension and washed with deionized water until free from potassium and chloride ions. The solids were dried by heating in an air oven at 120°C for 12 hours. The surface area of the dried product was 17.3 m 2 /g. The dried product was calcined at 1000°C for one hour to give a powder having a surface area of 8.4 m 2 /g. The product was subsequently calcined at a temperature in the range of from 400° to 1100°C for at least 1 hour. EXAMPLE 7 This example describes the preparation of a powder comprising alumina platelets having a composite coating amounting to three times as much as that described in EXAMPLE 5. The calcium pyrophosphate and alumina are in the same ratio. 100 g sodium pyrophosphate crystals was dissolved in a liter of water to give a 0.224 M solution. 150 g anhydrous calcium chloride was dissolved in a liter of water to give a 1.35 M solution. 200 g alumina powder, (alpha alumina platelets similar to Example 5) was added to 1 liter of water with good agitation in a 2-liter beaker. The stirred aqueous suspension of the alumina platelets was heated to 75°C and the pH was adjusted to 8.0 with a few drops of 6N NaOH. 267 ml of the 0.224 M solution of Na4P2θ7.10H2θ and 88.8 ml of the 1.35 M solution of CaCl2 were added concurrently over a two hour period and the pH was maintained at 8.0 by addition of 20% HCl. The suspension was cured by stirring at pH 8.0 and a temperature of 75°C for an additional 30 minutes. An aqueous solution of sodium aluminate, [NaAl(0H))4 equivalent of 0.385 g AI2O 3 /CC; Vinings VSA #38 supplied by Vinings Corp.] was added dropwise to the suspension and the pH was maintained at 8.5 by the controlled addition of 20% HCl. After adding 78 ml of the NaAl(OH)4 solution over a period of two hours the suspension was cured by stirring at pH 8.5 and a temperature of 75°C for an additional 30 minutes. All the Ca2P2θ7 and AI2O 3 were precipitated onto the core alumina, corresponding to 4.77 wt% Ca2P2θ7 and 14.07 wt% AI2O 3 based on the product. The Ca2P2θ7/A 2_>3 ratio was 0.339. The solids were recovered by filtering the suspension and washed with deionized water until free from sodium and chloride ions. The solids were dried overnight in an air oven at 120°C. The weight of dry solids recovered was 239.5 g, corresponding to a yield of 97.2%. The surface area of the product was 57.2 m 2 /g. The product was subsequently calcined at a temperature in the range of from 400° to 1100°C for at least 1 hour. EXAMPLE 8 This example describes the preparation of a powder comprising alumina platelets having a composite coating amounting to four times as much alumina and one quarter as much calcium pyrophosphate as that described in EXAMPLE 5. The solutions used and the procedure were the same as those described in EXAMPLE 7, the amounts being changed to 22.0 ml of sodium pyrophosphate, 7.4 ml of calcium chloride and 104.0 ml of sodium aluminate. All the Ca2P2θ7 and AI2O 3 were precipitated onto the core alumina, corresponding to 0.398 wt%, Ca2P2θ7 and 18.76 wt% AI2O3 based on the product. The Ca2P2θ7/Al2θ3 ratio was 0.0212. Dry solids recovered as in EXAMPLE 5 amounted to 234.0 g, corresponding to a yield of 94.6%. The surface area of the product was 43.0 m 2 /g. The product was subsequently calcined at a temperature in the range of from 400° to 1100°C for at least 1 hour. EXAMPLE 9 This example describes the preparation of a powder comprising crystalline alumina coated silicon carbide having a composite coating of calcium pyrophosphate and hydrous alumina. The solutions used were the same as those described in EXAMPLE 7. 1000 g of 500 grit SiC powder (Norton Company Grade 100 G.I.) having a surface area of 0.56 m 2 /g was added to 2500 ml of water with good agitation in a 4-liter beaker. The stirred aqueous suspension was heated to 75°C and the pH was adjusted to 8.5. An aqueous solution of sodium aluminate (Vinings VSA #38) was added to the SiC suspension and the pH was maintained at 8.5 by the concurrent addition of 20% HCl. After adding 100 ml of the NaAl(OH)4 solution over a period of two hours, the suspension was stirred at pH 8.5 and a temperature of 75°C for an additional 30 minutes. All the AI2O 3 was precipitated onto the SiC, corresponding to 3.71 wt% AI2O 3 based on the composition. The solids were recovered by filtering the suspension and washed with deionized water until free from sodium and chloride ions. The alumina coated SiC was dried overnight in an air oven at 120 β C and calcined at 1000"C for one hour to convert the coating to anhydrous crystalline alumina. The calcined product was added to 2500 ml of water with good agitation in a 4-liter beaker. The stirred aqueous suspension was heated to 75°C and the pH was adjusted to 8.5 with a few drops of 6N NaOH. 445 ml of a 0.224 M solution of Na4P2θ 7 .10H2θ and 148 ml of a 1.35 M solution of CaCl2 were added concurrently over a two hour period and the pH was maintained at 8.5 by addition of 20% HCl. The suspension was cured by stirring at pH 8.5 and a temperature of 75°C for an additional 30 minutes. Aqueous sodium aluminate (Vinings VSA #38) was added dropwise and the pH was maintained at 8.5 by the controlled addition of 20% HCl. After adding 130 ml of sodium aluminate solution over a period of two hours the suspension was cured by stirring at pH 8.5 and 75°C for an additional 30 minutes. All the Ca2P2θ7 and AI2O 3 were precipitated onto the alumina coated silicon carbide particles corresponding to 1.59 wt% Ca2 2°7 and 4.69 wt% AI2O 3 based on the product. The Ca2P2θ7/Al2θ 3 ratio was 0.339. The solids were recovered by filtering the suspension and washed with deionized water until free from sodium and chloride ions. The solids were dried overnight in an air oven at 120°C. The surface area was 25.2 m 2 /g. The product was calcined at 1000"C for two hours wherein the alumina portion of the coating formed an anhydrous crystalline alumina coating. The product was subsequently calcined at a temperature in the range of from 400° to 1100°C for at least 1 hour.

Claims: CLAIMS What is Claimed: 1. A coated refractory composition for making high quality preforms comprising a finely divided refractory material selected from diamond, metal refractory carbides, metal refractory nitrides, metal refractory borides or mixtures thereof having a hydrous alumina coating. 2. The composition of Claim 1 wherein the refractory material is selected from the group consisting essentially of silicon carbide, titanium carbide, tantalum carbide, niobium carbide, zirconium carbide, tungsten carbide, molybdenum carbide, vanadium carbide, hafnium carbide, thorium carbide, uranium carbide, titanium nitride, zirconium nitride, silicon nitride, titanium boride and zirconium boride. 3. The composition of Claim 1 wherein the refractory material is silicon carbide. 4. The composition of Claim 1 or Claim 2 or Claim 3 wherein the coating is a mixture of bohemite crystals and hydrous amorphous alumina. 5. A coated refractory composition having enhanced dispersibility and improved oxidation resistance comprising a refracting material selected from diamond, metal refractory carbides, metal refractory nitrides, metal refractory borides or mixtures thereof having an anhydrous crystalline alumina coating selected from an eta, theta, gamma, alpha phase or transition mixtures thereof. 6. The composition of Claim 1 or Claim 5 wherein said coating increases the isoelectric point of said refractory material. 7. The composition of Claim 6 wherein the coating reduces the chemical activity of the surface of the refractory material. 8. A method for enhancing the dispersibility of a refractory material comprising the steps of: (a) coating an aqueous suspension of a refractory material with hydrous alumina; and (b) separating, washing and drying and the coated refractory. 9. The method of Claim 8 further comprising the step of calcining at a temperature in the range of 400" to 1100°C for at least 1 hour. 10. A method for enhancing dispersibility and improving oxidation resistance in high temperature processing under atmospheric conditions of a refractory material comprising the steps: (a) coating an aqueous suspension of a refractory material with hydrous alumina; (b) separating, washing and drying the coated refractory; and (c) calcining at a temperature in the range of 400° to 1100°C for at least one hour. 11. The method of Claim 8 or Claim 9 or Claim 10 wherein the refractory material comprises diamond, metal refractory carbides, metal refractory nitrides, metal refractory borides or mixtures thereof. 12. The method of Claim 11 wherein the refractory material is selected from the group consisting essentially of silicon carbide, titanium carbide, tantalum carbide, niobium carbide, zirconium carbide, tungsten carbide, molybdenum carbide, vanadium carbide, hafnium carbide, thorium carbide, uranium carbide, titanium nitride, zirconium nitride, silicon nitride, titanium boride and zirconium boride. 13. The method of Claim 12 wherein the coating is a mixture of boehmite crystals and hydrous amorphous alumina. 14. A method for preparing a hydrous alumina coated refractory composition comprising the steps of: (a) forming an aqueous suspension of a refractory material, heating to a temperature in the range of between 40° and 95°C and adjusting the pH in the range of from 5 to 9; (b) adding an aqueous solution of an aluminum salt selected from the group consisting essentially of alkali metal aluminates, ammonium aluminate, aluminum chloride, aluminum nitrate and the aluminum acetate with stirring and maintaining the temperature and pH for 5 to 60 minutes whereby hydrous alumina precipitates as a coating layer on the surface of said refractory material; and (c) separating, washing, drying the coated refractory at 100° to 300°C. 15. The method of Claim 14 wherein the aluminum salt is an alkali metal aluminate and further comprising the step of adding concurrently a mineral acid. 16. The method of Claim 14 or Claim 15 further comprising the step of calcining at a temperature in the range of 400° to 1100°C for at least one hour. 17. A composition comprising a refractory ceramic substrate having a first coating of a calcium component and a second coating of an alumina component, wherein said first and second coating comprises about 0.5 to 25 wt% of the total composition and the ratio of first coating to second coating is about 0.01 to 0.095. 18. The composition of Claim 17 wherein the refractory substrate is alpha alumina. 19. The composition of Claim 17 wherein the refractory substrate is selected from the group consisting of oxides, carbides, borides, nitrides of aluminum, titanium, zirconium, silicon and mixtures thereof wherein said refractory substrate surface is covered with a crystalline form of alumina. 20. The composition of Claim 17 wherein the first coating is selected from the group consisting of calcium pyrophosphate, calcium phosphate, calcium silicate, calcium aluminate and mixtures thereof. 21. The composition of Claim 17 wherein the second coating is hydrous alumina. 22. The composition of Claim 21 wherein the hydrous alumina is predominately boehmite alumina. 23. A composition comprising an alpha alumina platelet having an average particle size in the range of from 0.1 to 2500 microns and a surface area in the range of from 0.1 to 50 m 2 /g, said alpha alumina platelet having deposited thereon a first coating of calcium pyrophosphate and a second coating of boehmite alumina. 24. A composition comprising a refractory component having a first coating of a calcium component and a second coating of an alumina component wherein said first and second coatings combine to provide a debonding function between a preform and a ceramic matrix. 25. A calcined composition comprising an alpha alumina substrate having a first coating of calcium pyrophosphate and a second coating of a densified, anhydrous, crystalline alumina coating. 26. A method for improving the strength and toughness of a ceramic composite comprising the steps of: (a) preparing an aqueous admixture of a refractory substrate, a calcium component precursor, and an alumina salt; and (b) recovering, washing and drying a coated refractory composition. 27. The method of Claim 26 wherein the aqueous admixture is prepared, in sequence, according to the steps of: (a) forming an aqueous suspension of a refractory substrate while heating to a temperature in the range of from 40 to 95 β C and adjusting the pH in the range of from 5 to 9; (b) simultaneously adding, with stirring, aqueous solutions of a calcium component precursor comprising a calcium salt and an alkali metal salts selected from the group consisting of pyrophosphate, phosphate, silicate and aluminate, then maintaining the temperature and pH for 5 to 60 minutes; and (c) adding an aqueous solution of an aluminum salt then maintaining the temperature and pH for about 5 to 60 minutes. 28. The method of Claim 26 or Claim 27 further comprising the step of calcining said coated refractory composition at a temperature in the range of from 400 to 1100"C for at least 1 hour. 29. The method of Claim 26 or Claim 27 wherein the refractory substrate is a high temperature crystalline form of alumina. 30. The method of Claim 26 or Claim 27 wherein the refractory substrate is selected from the group consisting of oxides, carbides, borides and nitrides of titanium, zirconium, silicon and mixtures thereof wherein said refractory substrate is covered with a crystalline form of alumina. 31. The method of Claim 29 wherein the calcium salt is selected from calcium chloride, calcium nitrate or mixtures thereof. 32. The method of Claim 31 wherein the aluminum salt is selected from the group consisting of alkali metal aluminate, ammonium aluminate, aluminum chloride, aluminum nitrate, aluminum acetate and mixtures thereof. 33. The method of Claim 32 wherein the aluminum salt is sodium aluminate. 34. A method for preparing an alpha alumina platelet having deposited thereon a first coating of calcium pyrophosphate and a second coating of boehmite alumina comprising the steps of: (a) preparing an aqueous suspension of AI2O 3 substrate platelets while heating to a temperature in the range of from 40° to 95"C and adjusting the pH in the range of from 5 to 9; (b) simultaneously adding solutions of a calcium salt and an alkali metal pyrophosphate then maintaining said temperature and said pH to form a first coating on said alpha alumina platelets; (c) adding a solution of an alkali metal aluminate with stirring then maintaining said temperature and pH to form a second coating on said first coated substrate; (d) recovering, washing, and drying the coated refractory composition; and (e) optionally calcining said coated refractory composition at a temperature in the range of from 400° to 1100 β C for at least 1 hour.

Inventors: JACOBSON HOWARD WAYNE, JACOBSON, HOWARD, WAYNE
Applicants: DU PONT, E.I. DU PONT DE NEMOURS AND COMPANY
Application year: 1991
Main classification: C04B
IPC codes: C04B35, C04B41