Method for the chemical preparation of a ceramic article having good fracture toughness

Abstract: A ceramic article is described having good fracture toughness comprising substantially spherical ceramic particles, which may be a ceramic oxide such as zirconia or zirconia combined with yttrium oxide. The ceramic particles are compacted and sintered to a density greater than 90 percent of theoretical density. The article is characterized by a substantially uniform chemical composition and a substantially inclusion-free, homogeneous microstructure, which may be a metastable crystalline phase with less than 10 volume percent monoclinic crystalline phase. The article is produced by dissolving, such as in water, a soluble compound containing a metal, e.g. zirconium, to form a solution. A precipitation agent, such as sodium hydroxide or ammonium hydroxide, is combined with the solution to precipitate a hydrophilic colloid of the metal hydroxide, such as zirconium hydroxide. The precipitate is formed into particles which are rapidly solidified while preventing agglomeration thereof. The particles are dried, compressed to increase density and reduce size, calcined to convert the metal hydroxide to oxide, e.g. zirconium oxide, and then the particles are compacted and sintered to form a dense ceramic article.

Description: The present invention relates to a method for manufacturing zirconia ceramic articles. It is known to use zirconia ceramic articles for various applications requiring strength or wear resistance at elevated temperatures. Specifically, zirconia ceramics comprising mainly zirconia (ZrO₂) and yttrium oxide (Y₂0₃) are conventionally used for this purpose. In the use of zirconia ceramic articles of this type, it is important that the article be resistant to cracking, particularly in the presence of thermal shock to which these articles are typically subjected in high temperature applications. With conventional zirconia ceramic articles, a major source of crack initiation results from microstructural heterogeneities, which may be in the form of inclusions resulting during production of the particulate material from which the article is produced. Specifically, inclusions and impurities are produced in the particle mass during conventional ball milling. Incident to the manufacture of ceramic articles the material is ball milled to achieve fine particle size prior to compacting and sintering to achieve the desired dense article. In addition, crack initiation may result from chemical inhomogeneity resulting from incomplete diffusion and large crystal size caused by prolonged sintering times at elevated temperature required to achieve complete sintering. Resistance to crack propagation may be achieved in ceramic articles of this type by providing an increased content of metastable tetragonal crystalline phase. During conventional preparation of these ceramic articles, however, significant transformation of this desirable metastable phase to the monoclinic phase occurs. This results in decreased resistance to crack propagation. The present invention addresses the problem of how to provide a method for producing a ceramic article which is characterised by good fracture toughness. In particular, the article needs to have a microstructure that is resistant to crack initiation and crack propagation, particularly during thermal cycling. DE 3310924 A1 discloses subjecting intermediate precipitation products precipitated from a solution (such as hydroxides, carbonates, oxalates and mixtures thereof) to freeze drying to form a loose powder, free of hard agglomerates, which can then be annealed, followed by sintering, to form a high density, small grain size, ceramic article. In an example of the preparation of such powders, an aqueous solution of ammonia and ammonium carbaminate are added to an aqueous solution of zirconium oxychloride and calcium nitrate to co-precipitate a mixture of zirconium hydroxide and calcium carbonate, the zirconium hydroxide precipitate is freed of water by freeze-drying, the calcium carbonate precipitate is dehydrated at 120°C, and the co-precipitate is calcined at 1000°C to form calcium oxide-stabilised zirconium oxide. Sintered bodies produced from the zirconium hydroxide powder, after sinter-firing at 1250 o C have high density (95% of theoretical density) and small grain size (< 1 m). Ceram. Forum Int. vol 63, No. 7/8, 1986, (373 - 377) R. Ramine et al describes how agglomeration of hydroxide gel precipitated from solution in conventional manufacture of oxide powder can be avoided by freeze-drying the precipitated gel. Reference is also made to Advances in Ceramics, vol 12, 1984, (727 - 732), L. Rakotoson et al describing the preparation of freeze-dried Y₂O₃ - zirconia powders. Starting from a sulphate solution of yttrium and zirconium, the sulphate solution is sprayed into liquid nitrogen. After freezing, the sulphate solution is described as looking like a very low density sponge. After drying and removal of residual water, the sulfate is transformed by thermolysis into the oxide. According to the invention there is provided a method for producing a metallic oxide ceramic article having good fracture toughness, said method comprising forming a solution of a metal compound whose oxide is present in said metal oxide ceramic article, precipitating said metal from said solution in the form of a hydrophilic colloid of a hydroxide of said metal, forming spherical particles from said colloid by spray atomisation with a cryogenic liquid, calcining said particles to convert said metal hydroxide to a ceramic oxide, and compacting and sintering said calcined particles to form a ceramic article having good fracture toughness. In preferred ways of performing the invention, the starting compound is a soluble compound of a nitrate, chloride, oxynitride or oxychloride of metals such as zirconium, aluminum, magnesium and yttrium. The solution is combined with a precipitation agent, which may be an alkali metal hydroxide or other hydroxide, preferably ammonium hydroxide, to produce a precipitate of the hydroxide of the metal. Particles are formed from the precipitate, and the particles are rapidly solidified while preventing agglomeration thereof. This is achieved by spray atomisation into the cryogenic liquid, such as liquid nitrogen, as a quenching medium. After drying, the particles are compressed to increase the particle density and reduce particle size. The particles are calcined to convert the hydroxide to a ceramic oxide of the metal. The particles are then ready for compacting and sintering in the conventional manner to form the desired dense ceramic article. Preferably, the spray atomisation is performed using an inert gas, to form the substantially spherical particles. In the production of zirconia ceramics, the compound containing zirconium may be one of the soluble compounds zirconium chloride, zirconium nitrate, zirconium oxynitrate or zirconium oxychloride. The precipitate is zirconium hydroxide which during calcining is converted to zirconium oxide (zirconia). During calcining the particles are heated for a time at temperature to crystallize the zirconium oxide to a submicron crystal size. Specifically, the calcining is conducted at a temperature of 500 to 1000°C for 1 to 24 hours. Since the particles are of a homogeneous chemical composition with a mean crystal size not greater than 30 nm (300Å) with the powder particle size being not greater than about 5 µm, shorter than conventional times at temperature may be employed during the calcining operation to thereby maintain a finer than conventional crystal size with respect to the powder. This is also the case with respect to subsequent Sintering wherein the fine particle and crystal size and the chemical homogeneity of the particles permit shorter sintering times to achieve complete densification and thus it is possible to maintain during both calcining and subsequent sintering a desirable substantially homogeneous, tetragonal metastable crystalline phase having less than 10 volume percent monoclinic crystalline phase. Specifically, it has been determined that the article may be sintered within the temperature range of 1400-1600°C for periods of five minutes to 4 hours. The formation of spherical particles from the precipitate facilitates sintering by allowing shorter sintering times at temperature. The ceramic article comprise substantially spherical particles which are compacted and sintered to a density greater than 90 percent of theoretical density with a substantially uniform chemical composition and a substantially inclusion-free, homogeneous microstructure. The size and spherical configuration of the particles contribute to achieving the high density which may be as high as 99 percent of theoretical density. In the broader aspects of the invention, the particle crystal size may be within the range of 0.1 to less than 100 microns, but preferably the heating times and specifically the sintering times at temperature should be restricted to achieve a submicron grain size in the final ceramic article. In the production of ceramic articles including zirconia ceramic articles the article may typically include 1 to 8 percent by weight of yttrium oxide. This is achieved in accordance with a preferred way of performing the invention by providing a second soluble compound containing yttrium to result in precipitation of yttrium hydroxide. It is evident, therefore, that the invention may be used to provide multiphase ceramic articles, specifically articles containing zirconium oxide and yttrium oxide. By achieving the desired ceramic composition through chemical reaction to form a precipitate in the form of a hydrophilic colloid, it is possible to avoid the conventional practices of comminuting and blending of ceramic particles, as by ball milling, which results in the introduction of inclusions and non-uniformity with respect to the chemical composition. In addition, the particles so produced conventionally are of angular, rather than spherical, configuration which requires long times at temperature to achieve densification. The precipitate achieved may be formed into the desired spherical particles by spray atomization. In this operation the precipitate is atomized by an inert gas stream and the resulting atomized particles of the precipitate are collected, prior to solidification and agglomeration thereof, in a cryogenic liquid, such as liquid nitrogen. In this manner, the particles are rapidly quenched and solidified without agglomeration. The solidified particles which are coated with the quenching medium are then placed in a vacuum chamber which is refrigerated to maintain the particles and adhered cooling medium at or below the freezing temperature thereof. The particles are then heated in the vacuum chamber and the resulting water vapor is condensed for removal with the particles being maintained out-of-contact and with the spherical shape thereof being maintained. A sonic sifter is then used to compress the particles to increase the particle density and reduce the particle size. Although any conventional apparatus of this type may be used, one suitable embodiment is model number L3P manufactured by ATM Corporation. The sonic sifter conventionally includes a series of vertically spaced-apart sieves through which the particles pass and are collected in accordance with the size distribution thereof upon application of sound waves which serve to compress the particles. During the calcining operation the hydrated water is removed from the particles to convert the hydroxides to the final desired oxide constituents. In addition, the microstructure of the particles is changed from an amorphous to a finely crystalline structure. The amorphous structure is achieved by the combination of atomization and rapid cooling of the particles to prevent crystallization. The fine crystal structure, as discussed above, permits controlled growth of the crystal structure during subsequent sintering to achieve a desirable fine grain size which may be within submicron limits. In addition, since densification may be achieved by a shorter sintering cycle, sintering may be controlled to produce a desirable metastable, tetragonal crystal Structure which may have no greater than 10 volume percent monoclinic phase. It has been determined that the presence of the metastable, tetragonal crystal structure is beneficial from the standpoint of providing ceramic articles that are highly resistant to crack propagation Specifically, crack propagation is minimized in accordance with the present invention by a combination of fine crystalline size and a metastable, tetragonal crystal structure. This is demonstrated by the following specific example of the invention. Reference will now be made in detail to presently preferred embodiments of the invention, examples of which are described below. In the examples and throughout the specification and claims, all parts and percentages are in mole percent unless otherwise specified. EXAMPLE Sintered disks were made from zirconia compositions including 2 m/o, 3 m/o, and 4 m/o Y₂O₃. Powders for the disks were prepared in 10 gram batches by starting with Zr(NO₃)₄ 5H₂O from Johnson Matthey (Lot S92910) and Y(No₃)₃ 5H₂O from Rare Earth Products (Lot R2754). Pre-weighed amounts of zirconium nitrate and yttrium nitrate powders in the ratios given in Table 1 were dissolved into 500 ml portions of deionized water. Separately, a basic solution was prepared by adding NH₄OH to deionized water. The pH of the basic solution was adjusted to 10.0. A hydrophilic colloid of (Zr,Y)(OH) n · X H₂O wherein n is greater than 2 and X is greater than 1 then precipitated by slowly adding the (Zr,Y) nitrate solution to the base. The precipitate was allowed to settle overnight, and the supernatant fluid removed the following day. The pH of the colloid at this stage was 9.5. This was then adjusted to 7.0 by repeated washing. The washing was accomplished by adding warm deionized water to the precipitate, followed by centrifugation to separate the colloid from the excess liquid. After the pH was adjusted, the colloid was then diluted using a 3:1 ratio of warm water to reduce the viscosity for spraying. Employing an air atomization nozzle pressurized by N₂ gas to a pressure of 276 kpa (40 psi), the colloid was then sprayed into an insulated container of liquid N₂ to create particles consisting of the (Zr,Y)(OH) n · X H₂O precipitate and frozen water. The air atomization nozzle was a Spraying System nozzle 1/4JCOSS, made by Spraying System Co., Weaton, Ill. The frozen water surrounding the (Zr,Y)(OH) n · X H₂O precipitate was then removed by freeze-drying. Freeze-drying was carried out using a Virtis Freeze Dryer model console 12, made by Virtis Co., Gardner, N.Y.. The drying chamber was first pre-cooled to a temperature of -40°C. The slurry of frozen particles and liquid N₂ was transferred into the drying chamber, and the chamber evacuated to a pressure of 30 µm of mercury. The chamber was then slowly heated to 65°C in order to remove the frozen water by sublimation. The resulting product consisted of a highly friable white powder having a composition of (Zr,Y)(OH) n · X H₂O which was amorphous as determined by x-ray diffraction. The mean powder particle size was found by scanning electron microscopy to be approximately 20 µm. The powder particle size was then further refined by acoustic agitation. This was achieved by passing the powder through a column of sieves using sound vibrations in a sonic sifter. The sonic sifter was model L3P, made by ATM Corporation. The final mean powder particle size was less than 1 micron. Structural water in the (Zr,Y)(OH) n · X H₂O particles was then removed by calcination in air at a temperature of 800°C for 6 hours. The calcination was carried out in a Lindberg box furnace, model 51314, made by Lindberg Co., Watertown, Wi. Microprobe and x-ray diffraction analyses showed that the powder at this stage had converted to (Zr,Y)O₂, with the compositions given in Table 1. SEM and x-ray diffraction analyses further showed that the powder consisted of sub-micron particles containing crystallites of approximately 300 Å in size. Composition Fracture Toughness (MPA √m) ZrO₂: 2 m/o Y₂O₃ 8.6 ZrO₂: 2 m/o Y₂O₃ 10.1 ZrO₂: 2 m/o Y₂O₃ 10.4 ZrO₂: 2 m/o Y₂O₃ 11.0 ZrO₂: 2 m/o Y₂O₃ 11.0 ZrO₂: 3 m/o Y₂O₃ 11.5 ZrO₂: 3 m/o Y₂O₃ 11.6 ZrO₂: 4 m/o Y₂O₃ 9.4 ZrO₂: 4 m/o Y₂O₃ 8.1 From each batch, 3.5 grams of powder was then consolidated into disk shape by pressing in a double acting die at 1591 psi. The die employed was Buehler model 20-2120, made by Buehler Ltd., Evanston, Ill. The disk were then fired at 1494°C for the times listed in Table 1, employing the Lindberg furnace. The fired disks were characterized with respect to density, phase content, and grain size. Density was measured by the pycnometric method, employing deionized water as the reference fluid. Measurements were carried out on a Mettler balance, model AE100 made by the Mettler Co., Hightstown, NJ. All densities were found to be greater than 96% of theoretical, and typically 98-99% of theoretical. The phase content was analyzed by x-ray diffraction. X-ray diffraction scans were performed on a Rigaku DMAX II-A unit, made by Rigaku Denki, Japan. The ratio of the monoclinic (111) diffraction line (strongest monoclinic peak) to the tetragonal (200) diffraction line (strongest tetragonal peak) was used to determine the relative concentration of these phases. All samples were found to consist of the tetragonal metastable phase, except for those which were fired for times greater than 12 hours. The grain size was measured by transmission electron microscopy. The instrument employed was a Phillips 400, made by Phillips Co., Netherlands. All grain sizes were found to be sub-micron except for samples fired for times greater than 12 hours. The fracture toughness for additional disks of each composition sintered for 1 hour at 1494°C was also measured, using the method of diametral compression. Measurements were carried out on an MTS 20,000 lb hydraulic load frame, made by Mechanical Testing Systems Co., Minneapolis, Min. These data are reported in Table 2. A comparison of the fracture toughness and phase content of the chemically prepared disks to conventional mechanically prepared disks are given in Table 3. The mechanically prepared disks were made in accordance with the teachings of U.S. Patent 4,360,598. The chemically prepared disk showed no evidence of monoclinic phase formation. Furthermore, comparison of the mechanical properties shows that in accordance with the invention the chemically prepared disks have superior fracture toughness.

Claims: A method for producing a metallic oxide ceramic article having good fracture toughness, said method comprising forming a solution of a metal compound whose oxide is present in said metal oxide ceramic article, precipitating said metal from said solution in the form of a hydrophilic colloid of a hydroxide of said metal, forming spherical particles from said colloid by spray atomisation into a cryogenic liquid, calcining said particles to convert said metal hydroxide to a ceramic oxide, and compacting and sintering said calcined particles to form a ceramic article having good fracture toughness. A method as claimed in claim 1, wherein said metal compound (e.g. a compound of zirconium) is selected form the group consisting of the nitrates, chlorides, oxynitrates and oxychlorides of said metal (e.g., zirconium). A method as claimed in claim 1 or claim 2, wherein said metal is selected from the group consisting of zirconium, aluminium, magnesium and yttrium. A method as claimed in any one of claims 1 to 3, wherein said precipitation is affected with an alkali metal hydroxide or ammonium hydroxide. A method as claimed in claims 1 to 4, wherein said cryogenic liquid is liquid nitrogen. A method as claimed in any one of claims 1 to 5, wherein the colloid is spray atomised by an inert gas stream. A method of any one of claims 1 to 6, wherein said particles of hydrophilic colloid are rapidly solidified while preventing agglomeration thereof, followed by drying said particles and compressing said particles to increase the density and to reduce the size thereof, prior to calcining the particles. A method as claimed in any one of claims 1 to 7, wherein during said calcining, said particles are heated for a time (e.g., in the range from 1 to 24 hours) and at a temperature (e.g., in the range from 500 to 1000°C) effective to crystallize said ceramic oxide (e.g., zirconium oxide) to a submicron (i.e., less than 1 µm) crystal size. A method as claimed in any one of claims 1 to 8, wherein said article is sintered for a time and temperature to produce a substantially homogenous tetragonal metastable crystalline phase with less than 10 volume percent monoclinic crystalline phase. A method as claimed in claim 9, wherein said article is sintered for from 5 minutes to 4 hours at from 1400 to 1600°C.

Inventors: ANDERSON MICHAEL PAUL, ANDERSON, MICHAEL PAUL
Applicants: EXXON RESEARCH ENGINEERING CO, EXXON RESEARCH AND ENGINEERING COMPANY
Application year: 1988
Main classification: C04B35
IPC codes: C01G25, C04B35