HIGH-STRENGTH ALUMINIUM OXIDE

Abstract: An extruded and sintered Al₂O₃ body having high flexural strength is produced by extruding and sintering an extrusion mixture of alpha-Al₂O₃ powder, an organic dispersing agent, an optional deaeration agent, and a zirconia sol. The zirconia sol is present in a small amount such that upon firing the final ZrO₂ content of the final body is less than 1 weight precent. The sintered alumina body has a uniform distribution of ZrO₂ particles in the size range of 0.1-1.0 microns.

Description: This invention relates to a method to produce extruded alumina (Al₂O₃) bodies containing at least 99 weight % (w/o) Al₂O₃ with less than 1 w/o ZrO₂ having high flexural strength. Zirconia has been added to alumina to improve its properties. In US-A- 4 316 964 ZrO₂ was added in an amount from 5 to 95% of the volume of the alumina ceramic to increase the fracture toughness and strength of the Al₂O₃/ZrO₂ ceramic. The zirconia had dissolved in it a rare earth oxide such as Y₂O₃, CeO₂, La₂O₃ and/or Er₂O₃ to promote retention of the metastable tetragonal ZrO₂. The 5 volume % zirconia in the above composition corresponds to 7.3% on a weight basis. US-A- 4 419 311 discloses the production of high-strength ceramic bodies of alumina with unstabilized zirconia for the formation of controlled microfissures. An unstabilized zirconia dioxide and/or hafnium dioxide with a particle size of 0.3 to 1.25 microns was added as a ceramic embedment material to generate tensile stresses for the formation of controlled microfissures. The amount of the ceramic embedment was from 4 to 25% of the volume of the body to be formed. In this composition, 4 volume % corresponds to 5.9 weight %. US-A- 4 218 253 discloses improving the ductility of sintered alumina bodies by uniformly distributing individual particles of tetragonal zirconia or hafnia in amounts of 1% - 50% by volume. The dispersed particles are to have an average size of 0.05 to 2 microns. In this composition 1 volume % ZrO₂ is equal to 1.5 weight % ZrO₂. In this reference, and in the above described reference, there is no mention of extruded articles. Only articles prepared by pressing techniques are disclosed. S. Hori et al in their article "Influence with Small ZrO₂ Additions on the Microstructure and Mechanical Properties of Al₂O₃" in Advances in Ceramics , Vol. 24A: Science and Technology of Zirconia III, 1988 p. 423 (published by American Ceramic Society) disclose that the addition of ZrO₂ in an amount of up to 1 wt % increased the strength but decreased the toughness of Al₂O₃ due to the strong dependence of these properties on the Al₂O₃ grain size. The articles produced have a three point flexural strength of 399910 kPa (58 000 psi) at 1 w/o ZrO₂ levels. This value is substantially below our average value of 503335 kPa (73 000 psi) (obtained without organic binders). Furthermore, the products of this reference were prepared by isostatically pressing mixtures of alumina and alumina/zirconia powders, whereas we are mixing alumina powder with a zirconia sol, thereby introducing the zirconia at a much finer particle size (5-10 nanometers). The zirconia, with particles this small, permit more intimate mixing of the zirconia with the alumina. P. F. Becher in "Transient Thermal Stress Behavior in ZrO₂-Toughened Al₂O₃" in the Journal of the American Ceramic Society, 64 , No. 1, January 1981, pp. 37-39, discloses characteristics of hot-pressed Al₂O₃-ZrO₂ composites. The article discusses how the resistance of ceramics to transient thermal stresses can be improved by modifying the thermal-mechanical properties of the matrix. The articles produced have 420595 kPa (61 000 psi) (3 point flexural strength) at 0.75 w/o ZrO₂ levels. These strengths are again lower than our values (without binder) . The composites were prepared by mixing Al₂O₃ and ZrO₂ obtained from mixtures of gelled alkoxide derived sols. The powders were vacuum hot pressed in order to obtain dense products. This procedure is typical for gel derived powders, which are typically difficult to densify. Our extruded product, densifies very readily. It is an object of this invention to extrude Al₂O₃ bodies containing at least 99 w/o Al₂O₃ and less than 1 w/o ZrO₂ having high flexural strength. It is a further object of this invention to form extruded Al₂O₃ bodies by combining an organic dispersing agent and a zirconia sol to produce high-strength bodies. It is a further object of this invention to form extruded Al₂O₃ bodies by combining an organic dispersing agent, a zirconia sol and an organic deaerating agent to produce high-strength bodies. It is a further object of this invention to combine an organic dispersing agent, a zirconia sol and an organic binder with Al₂O₃ to produce large size, high-strength bodies. It is a further object of this invention to form extruded Al₂O₃ having less than 1 w/o ZrO₂ bodies having no organic binder and having 3 point flexural strengths greater than 448175 kPa (65 000 psi). It is a further object of this invention to form extruded Al₂O₃ bodies having less than 1 w/o ZrO₂ using organic binders with 3 point flexural strengths greater than 379225 kPa (55 000 psi). These and further objects will become apparent as the description of the invention proceeds. Alumina extruded bodies containing at least 99 w/o Al₂O₃ can be formed with high flexural strength by combining with the Al₂O₃ prior to extrusion an organic dispersing agent (preferably in an amount of 1 to 2 weight % on an alumina basis), a zirconia sol and an optional deaerating agent. When making small size objects no organic binder is needed. However, in a preferred embodiment for making larger size objects, small amounts of a further organic binder can be added. The mixture is extruded and upon firing the organic materials are burned off leaving a final body having less than 1 w/o ZrO₂ (preferably uniformly distributed as particles of a size of 0.1-1 micron). These products have flexural strengths of greater than 448175 kPa (65 000 psi) or when using organic binders the flexural strengths can be greater than 379225 kPa (55 000 psi). They are suitable for use as wear tiles or grinding media. Alpha-Al₂O₃ powder is mixed with an ammonium salt of a low molecular weight polyacrylic acid such as Darvan® 821A, sold by R. T. Vanderbilt Co., followed by the addition of a zirconia sol. The sol is preferably a commercial product obtained from Nyacol Products, Inc. an affiliate of the PQ Corporation. The ZrO₂ particle size in the sol is 5-10 nanometers. The resulting mix is then extruded to form an alumina body. A small amount of n-octanol can be added prior to extrusion to remove residual entrapped air. The extrusions are fired in a furnace at about 1650°C to produce dense Al₂O₃ bodies. The Al₂O₃ content of the fired body is greater than 99 w/o. Flexural strengths (3-point) average 503335 kPa (73 000 psi) and can range as high as 620550 kPa (90 000 psi). It is believed that the Darvan® 821A first acts as a dispersant for alpha-Al₂O₃, breaking up agglomerates, and then causes gelation of the sol. The gelled sol provides good rheological properties for extrusion. The n-octanol removes trapped air pockets and bubbles. The ZrO₂ also functions as a grain growth inhibitor for alpha-Al₂O₃, which also leads to higher strengths. Typical dispersing agents for this invention are the ammonium salts of polyacrylic or polymethacrylic acids. Darvan® C and Darvan® 821A, sold by R. T. Vanderbilt, Co., Inc. are typical examples. Darvan® 821A is preferred. The flexural strength tests were performed on an Instron Model TM Dynamic Materials Tester. The tests were 3-point, using a 1.27 cm (1/2 inch) span at cross-head speed of 0.051 to 0.254 cm/min (0.02 to 0.1 inch/min). If large extruded pieces are desired, a small amount, such as 0.5-2.0 w/o on an alumina basis, of an organic binder can be added to provide green strength. Typical binders are any of the methylcellulose derivatives such as Methocels® sold by Dow Chemical, hydroxypropylcelluloses or polyvinyl alcohols. Methocels® are preferred. The ZrO₂ sol can either be prepared in the laboratory by peptization, ion exchange or solvent extraction or obtained commercially. The ZrO₂ sol marketed by Nyacol is preferred. The amount of ZrO₂ in the final product is preferably less than 1.0 w/o on an alumina basis and more preferably it is present in an amount to give a final ZrO₂ concentration of 0.25 to 1.0 w/o on an alumina basis. The extruded bodies are then air dried or oven dried to remove residual excess water. The dried bodies are then heated to about 500-600°C to burn off any organics. The bodies are then heated to 1500-1650°C to sinter and densify. Heating rates can vary, and are not critical. N-octanol has been found to be an effective deaeration agent. It is believed to act by forming a surface film which breaks up air bubbles. Generally it is used in an amount of 0.1-0.5 w/o on an alumina basis. Other de-foaming agents can also be used. N-octanol is preferred. Having described the basic aspects of our invention, the following examples are given to illustrate specific embodiments thereof. Example 1 This example illustrates the unexpected high strengths obtained by using the components of this invention. To 100 parts by weight of alpha-alumina (Alcoa A-16 SG) was added 1.25 parts Darvan® 821A (an ammonium polyelectrolyte dispersion agent from R. T. Vanderbilt Co., Inc., Norwalk, CT), 27.5 parts of a diluted zirconia sol (1 part ZrO₂ sol, acetate based, 20% ZrO₂, obtained from Nyacol Products, Inc., Ashland, MA, per 4.8 parts of H₂O) and 0.3 part of n-octanol. The ingredients were mixed together and then extruded into rods 2 mm in diameter by approximately 150 mm in length. The extrusions were air dried and then heated in a furnace at 10°C/min to 600°C and held at this temperature for 1 hour to remove organics. After the 1-hour hold, the material was then heated to 1650°C at 17.5°C/min, with a 2-hour hold at 1650°C to sinter and densify. Flexural strengths (3 point) were obtained and the average of 6 values was 504025 kPa (73 100 psi). The average of the four highest values was 571596 kPa (82 900 psi). These values are significantly higher than what has been reported in the literature such as the Hori et al and Becher references cited supra where there are values of 399910 kPa (58 000 psi) and 420595 kPa (61 000 psi), respectively. Example 2 This example shows that substitution of the ZrO₂ sol by a conventional organic binder results in lower strengths. The experiment of Example 1 was repeated with the exception that a 1% solution of polyvinyl alcohol was substituted for the diluted ZrO₂ sol. The average strength obtained (6 values) was 256494 kPa (37 200 psi). Example 3 This example shows the effect of omitting the n-octanol deaeration agent. The experiment of Example 1 was repeated, with the exception that the n-octanol was omitted. The average flexural strength obtained was 362677 kPa (52 600 psi) (5 values). Example 4 This example shows the effect of omitting the Darvan® 821A dispersion agent. The experiment of Example 1 was repeated, with the exception that the Darvan® 821A was omitted. The average flexural strength obtained was 360609 kPa (52 300 psi) (6 values). Example 5 This example shows the effect of reduced ZrO₂ levels. The experiment of Example 1 was repeated, with the exception that a 1.725 w/o ZrO₂ sol was used instead of a 3.45 w/o ZrO₂ sol, thus reducing the ZrO₂ level from 0.95 w/o to 0.475 w/o on an alumina basis. The average flexural strength obtained was 391636 kPa (56 800 psi) (4 values). Example 6 This example shows the effect of the addition of an organic binder used to provide green strength for large size bodies. Alcan AM-2 alumina (4000 g) was mixed with 50 g Darvan® 821A dispersant and 800 g 2-propanol. 2-Propanol was substituted for water, because it resulted in a more fluffy powder. Water could also be used. The mixture was dried overnight at 75°C. The dried powder was then mixed in a sigma blade mixer with 40 g Methocel® 20-213 and 190 g ZrO₂ sol (Nyacol - 20% ZrO₂) and 685 g deionized H₂O. The resulting paste was extruded into bars using a Loomis ram extruder. The bars were fired at 10°C/min to 600°C, held for 1 hour and then fired at 17.5°C per min. to 1610°C and held for 2 hours followed by cooling to room temperature. The average flexural strength for these bars was 386120 kPa (56 000 psi), compared to 303380 kPa (44 000 psi) for similar bars prepared without the ZrO₂ sol.

Claims: A method of producing an extruded Al₂O₃ body having high flexural strength comprising the steps of: a) forming an extrusion mixture of alpha-Al₂O₃ powder, an organic dispersing agent, a deaeration agent, and a zirconia sol, the amount of said sol being such that upon firing, the final ZrO₂ content of the final body is less than 1 w/o; b) extruding the mixture of step (a) to form an extruded body; c) drying the extruded body; d) heating the dried body; e) sintering the extruded body of step (d) to form a final Al₂O₃ body containing less than 1 w/o ZrO₂ having high flexural strength. A method according to Claim 1, wherein the extrusion mixture of step (a) further contains an organic binder. A method according to Claim 2, wherein the organic binder is present in an amount of about 0.5 to 2.0 weight %. A method according to Claim 1, wherein the extruded body has a 3 point flexural strength greater than 448175 kPa (65 000 psi). A method according to Claim 4, wherein the extruded body has a 3 point flexural strength greater than 503335 kPa (73 000 psi). A method according to Claim 2, wherein the extruded body has a 3 point flexural strength greater than 379225 kPa (55 000 psi). A method according to Claim 1, wherein the zirconia sol has particles in the size range of about 5-10 nanometers. A method according to Claim 1, wherein the organic dispersing agent in step (a) is present in an amount of 1 to 2 weight % on an alumina basis. A method according to Claim 1, wherein the zirconia sol in step (a) is present in an amount sufficient to give a final ZrO₂ concentration of 0.25 to 1.0 w/o on an alumina basis. A method according to Claim 1, wherein the deaeration agent in step (a) is present in an amount of 0.1-0.5 w/o on an alumina basis. A method according to Claim 10, wherein the deaeration agent is n-octanol. A method according to Claim 1, wherein no deaeration agent is added. A method according to Claim 2, wherein no deaeration agent is added. An extruded and sintered Al₂O₃ body having less than 1 w/o ZrO₂ uniformly distributed as particles of a size of 0.1-1 micrometers and having a 3 point flexural strength of at least 379225 kPa (55 000 psi). An extruded and sintered Al₂O₃ body according to Claim 14, having a 3 point flexural strength of at least 448175 kPa (65 000 psi). An extruded and sintered Al₂O₃ body made by the process of Claim 1 and having less than 1.0 w/o ZrO₂ where the zirconia is sol derived. An extruded and sintered Al₂O₃ body according to Claim 16, wherein the extruded body has a 3 point flexural strength greater than 448175 kPa (65 000 psi). An extruded and sintered Al₂O₃ body made by the process of Claim 2 and having less than 1.0 w/o ZrO₂ where the zirconia is sol derived and having a 3 point flexural strength of at least 379225 kPa (55 000 psi).

Inventors: BLOCK, JACOB
Applicants: W.R. GRACE & CO.-CONN.
Application year: 1990
Main classification: C04B35
IPC codes: C04B35