WEAR-RESISTANT CERAMIC MATERIAL FOR CASTING RARE EARTH ALLOYS

Abstract: Refractory ceramic materials are provided which contain at least 70 mole percent yttrium oxide and from about 5 to 30 mole percent aluminium oxide, preferably about 11 to 30 mole percent aluminium oxide. The ceramic materials are particularly resistant to attack by molten rare earth alloys and are superior refractory materials for making jet-casting nozzles for use in the manufacture of permanent magnets from molten neodymium-iron alloys.

Description: This invention relates to rare earth oxide-aluminium oxide refractory materials which are particularly resistant to wear in the casting of molten rare earth-iron alloys (RE-Fe alloys). In particular it relates to refractory ceramic materials as specified in the preamble of claim 1, for example as disclosed in Chemical Abstracts 51693n, Vol 80, No.5, 1974, wherein the sintering of Y₂O₃ with 0.2 - 5 mole % Al₂O₃ yielded a new phase which was thought to be 2 Y₂O₃.Al₂O₃. One method of making rare earth-iron-based magnets is by jet-casting molten alloy through a small orifice onto a rapidly moving quench surface. This is disclosed, for example, in U.S. Patent 4,496,395 (Croat). Jet-casting results in a rapidly solidified alloy ribbon with a very, very fine-grained microstructure which is commensurate with the creation of permanently magnetic properties in the alloys. In production jet-casting operations, it is desirable to provide a jet-casting nozzle which is extremely resistant to wear by the flow therethrough of molten rare earth-iron alloys. In jet-casting nozzles, the size of the orifice increases as molten alloy flows through it. Before this invention, nozzles made of sintered and machined boron nitride exhibited the longest lives without excessive orifice wear. However, most nozzles did not last more than a few hours before orifice erosion resulted in underquenched ribbons, i.e., molten alloy flowed through the nozzle too fast and cooled too slowly to result in optimum permanently magnetic properties in the solidified alloy. Quartz nozzles have also been used to make rapidly solidified RE-Fe alloys but are much shorter-lived than boron nitride nozzles. This invention relates to novel ceramic compositions based on mixtures of yttrium oxide and aluminium oxide. Nozzles formed by pressing and sintering very fine powders of these compositions have proven to be exceptionally resistant to erosion by molten rare earth alloys. A refractory ceramic material according to the present invention is characterised by the features specified in the characterising portion of claim 1. In accordance with the invention, a mixture of yttrium oxide (Y₂O₃) powder and from about 11 to 30 mole percent alumina (Al₂O₃) powder is milled to result in an average particle size less than 10 micrometres and preferably in the micrometre to sub-micrometre size range. The particles are pressed, preferably by isostatic compaction, to at least 50% of the theoretical powder density. The compact is sintered to form a closed pore, opaque structure comprised predominantly of a very hard Y₂O₃.Al₂O₃ monoclinic phase. The invention will be better understood in view of the figures and detailed description which follows. Figure 1 shows a schematic view of a Y₂O₃.Al₂O₃ jet-casting nozzle in accordance with this invention. Figure 2 compares the characteristics of jet-caster nozzles made of boron nitride and 80:20 mole percent yttria:alumina ceramic material. EXAMPLE I Yttrium oxide (also referred to as yttria herein) was formed by mixing yttrium chloride hexahydrate (YCl₃.6H₂O, 303 grams) in a litre of water. 516 grams of polyacrylic acid were neutralized with 189 millilitres of concentrated ammonium hydroxide and diluted to 3 litres total volume with water. The yttrium chloride solution was added to the ammonium polyacrylate with rapid mixing. The resultant white precipitate was filtered and washed with water. It was dried at 100°C and the organic material was burned out at 400°C. The ash was calcined at 1100°C for one hour until no further weight loss occurred to yield Y₂O₃. Precipitation of the yttria in this manner results in a loose agglomeration of fine, relatively uniformly-sized particles which is readily ball-milled to form an even finer powder. 241.4 grams of aluminium trichloride hexahydrate were dissolved in one litre of water. 516 grams of polyacrylic acid were neutralized with 126 millilitres of concentrated ammonium hydroxide and diluted to a three-litre volume with water. The aluminium trichloride solution was added to the ammonium polyacrylate solution with rapid mixing and a white precipitate formed. The white precipitate was dried at 100°C and the organic material burned out at 400°C. The ash was calcined at 1050°C to yield Al₂O₃. Four moles of yttria (902.2 grams) and one mole of aluminium oxide (102 grams), prepared as set forth above, were mixed together. The mixture was ball-milled in water at room temperature for 30 hours using aluminium oxide balls. The product was dried and put through a 149 µm (100 mesh) screen. The powder was isostatically pressed into the shape of a jet-casting nozzle 2 of the type shown in Figure 1 in a room temperature vulcanized silicon rubber mould by application of an isostatic pressure of 303,369.44 kPa (44,000 p.s.i). A 0.76 mm (0.030 inch) diameter hole 4 was drilled in the green compact. The compact was sintered in air at 1650°C for about 1.3 hours and achieved greater than 90% theoretical density. The resultant opaque ceramic material had closed pores and a hardness of 94.6 on the Rockwell 15-N and 71.8 on the Rockwell C hardness scales. This is commensurate with the formation of a large proportion of an Al₂O₃.Y₂O₃ monoclinic phase in the sintered ceramic material. The nozzle was secured to the bottom of a conical yttria tundish with a threaded boron nitride sleeve. An alloy composition comprised, on an atomic percent basis, of about 12.3 percent neodymium, 7.1 percent boron and the balance iron was melted in the tundish. The alloy in the tundish was maintained in a molten state and the nozzle was heated to a temperature of about 1350°C by a surrounding heating coil. Figure 2 shows the normalized rate of flow of the molten alloy as a function of time through the yttria-aluminium nozzle and a similar HBC grade boron nitride nozzle operated under similar conditions. The original orifice diameter was about 0.71 mm (0.028 inches). (The hole shrinks with respect to that drilled into the green compact during firing.) Flow rate increases through a nozzle as the orifice is worn by the passage of molten metal through it. It is apparent from Figure 2 that the yttria-aluminium oxide nozzle is much more wear-resistant than boron nitride. It is believed that this is true, at least in part, because the boron nitride experiences both chemical and mechanical erosion in the presence of molten rare earth-iron alloys. The Y₂O₃.Al₂O₃ ceramic materials of the present invention are subject only to mechanical wear. However, pure yttria and yttria ceramic materials containing less than 5 atomic percent alumina showed no better wear properties than boron nitride -- even though they are not subject to chemical attack. Y₂O₃.Al₂O₃ ceramic materials containing more than 30 atomic percent aluminium oxide are more porous than those containing less Al₂O₃, and orifices in nozzles made from such compositions tended to plug in the presence of molten rare earth metals. The average life of jet-casting nozzles with 20 atomic percent alumina and the balance yttria is well over 8 hours or one complete manufacturing shift. In fact, the first two Y₂O₃.Al₂O₃ 80:20 nozzles evaluated in production lasted for 7.75 and 10 hours respectively, and then the jet-casting operation was shut down for purposes other than orifice erosion. After initial erosion of a yttria:alumina nozzle according to the present invention, later erosion is much slower than that of boron nitride nozzles: that is, the flow rate curve of Figure 2 levels off for yttria:alumina nozzles whereas the erosion rate of boron nitride remains fairly constant. While the invention has been described in terms of nozzles for jet-casting rare earth-iron-boron alloys, Y₂O₃.Al₂O₃ ceramic materials according to the present invention have use in many other high temperature and foundry applications. They could be used, for example, as crucible materials for reducing and alloying radioactive elements. They may also be useful for turbine engine parts or ceramic spark plug or sensor bodies. Accordingly, the invention is to be limited only in accordance with the following claims.

Claims: A refractory ceramic material formed from a mixture of yttrium oxide and aluminium oxide, in which the refractory ceramic material is an opaque material that contains a yttrium oxide/aluminium oxide phase, characterised in that the refractory ceramic material comprises at least 70 mole percent of yttrium oxide and from about 11 to 30 mole percent aluminium oxide, and is comprised predominantly of a Y₂O₃.Al₂O₃ monoclinic phase, which refractory ceramic material is resistant to wear by molten rare earth-iron alloys. A wear-resistant refractory nozzle, for casting molten metal, formed from a non-porous, opaque ceramic material according to claim 1.

Inventors: DUNGAN, DENNIS F., MICHELI, ADOLPH L.
Applicants: GENERAL MOTORS CORPORATION
Application year: 1988
Main classification: C04B35
IPC codes: C04B35, B22D41