Method of producing ceramic parts

Abstract: Ceramic parts of complicated configurations with differ­ ent thicknesses portions can be mass produced economically with high production yield and excellent quality by mixing a ceramic powder or metal powder with a molding additive, injection molding the mixture to form a green body of a desired shape, processing the green body to a substantially final configuration, removing the molding additive from the processed body to yield a processed and degreased body, and firing the processed and degreased body to sinter the same.

Description: The present invention relates to a method of producing ceramic parts, particularly to a suitable method of economically producing ceramic parts of complicated configurations which hitherto can not be produced directly to their final configurations using injection molding process. Recently, ceramic parts made of a material with superior high temperature characteristics, such as Si₃N₄ or SiC, have increasing application fields in high temperature structural materials by virtue of their superior high temperature strength, heat resistance, thermal shock property and corrosion resistant property, and have shown remarkable technical progresses. Ceramic parts of complicatd configurations with a thin blade portion and comparatively thick axis portion, such as turbo charger rotors for Diesel engines or radial type turbine rotors for gas turbine engines, must have strict dimensional precision as well as superior mechanical and thermal characteristics. Heretofore, for producing such ceramic parts as the abovementioned rotors of complicated configurations consisting of two portions of different thicknesses, there have been attempted such methods as the slip casting method and the injection molding method using superior heat-resistant Si₃N₄ or SiC powder. However, the slip casting method has a drawback in that it could not produce distal ends of blade portions of such rotors with sufficient precision of shape and dimension, which distal ends have a great influence on efficiency of the rotors. The heretofore used injection molding method has a drawback in that it could not produce directly an integrally molded part of final shape having a thin blade portion of complicated configuration and a thick columnar axis portion, since a die for filling the molding material sufficiently into its cavity is hardly fabricated. Therefore, in conventional injection molding methods, the ceramic part is produced at first by molding a green body using a material powder and a resin as a molding additive, removing (degreasing) the resin from the molded green body, and machining the degreased body before or after the final firing. This DE-A-2 910 932 teaches a process for producing a turbocharger rotor by mixing a ceramic powder such as Si₃N₄ or SiC with a molding additive, injection molding the mixture to form a green body, removing the molding additive to yield a degreased body and sintering the degreased body. However, it has drawbacks in that the degreased molded body is liable to have a crack therein or deformation thereof owing to different degreasing rates at different thicknesses portions of the body in the degreasing process so that the yield of product is low, and that the chips produced by machining the degreased body can never be reused owing to the disappearance of the molding additive so that the production cost is extremely expensive. Moreover the earlier European application EP-A-116 300, published on 22.8.84 and having the priority date of 14.1.83 teaches the production of ceramic bodies by molding a mixture of ceramic powder and a molding additive to form a green body, machining said green body so as to form a desired shape and finally sintering the green body. Injection molding for forming the green body is not mentioned in this document. The present invention seeks to obviate the aforementioned drawbacks of the prior art by providing a method of producing ceramic parts of complicated configurations having portions of different thicknesses using an injection molding process. The present invention provides a method of producing ceramic parts by mixing a ceramic powder or metal powder with a molding additive, injection molding the mixture to form a green body, removing the molding additive to yield a degreased body, and firing the degreased body to sinter the same, which is characterized in that a green pre-form is formed by injection molding and the green pre-form is processed to a substantially final configuration. After the firing, the sintered body can, if necessary, be finished by further machining. As the ceramic powder use is made of a powder of a ceramic material such as Si₃N₄ a SiC or a mixture thereof. If Si₃N₄ powder is used for improving the density and strength of the final product, preferably 1-20 parts by weight per 100 parts by weight of the Si₃N₄ powder of a powder of at least one of MgO, Al₂O₃, Y₂O₃, ZrO₂, CeO₂ and SrO is added as an additive for sintering (to be referred as "sintering additive" hereinafter) to the Si₃N₄ powder. If SiC powder is used, preferably (i) each 0.1-5 parts by weight of B or B₄C and C or carbonizable organic substance in powdery states are added to the SiC powder, or (ii) 0.1-10 parts by weight of Al₂O₃ powder is added to the SiC powder, the amounts being per 100 parts by weight of the SiC powder. As the metal powder, use can be made of a powder of, for example, Si and/or Al that can be converted to a ceramic substance such as Si₃N₄ or AIN by reacting with a nitriding environmental gas such as N₂ in the firing process. As the molding additive, use can be made of at least one thermoplastic resin selected from polystyrene, polypropylene, polyethylene and ethylene-vinyl acetate copolymer, wax, or a lubricant such as stearic acid and/or oleic acid. In a preferred embodiment of the present invention, the injection molding is performed as follows. A material powder selected from the ceramic powder and the metal powder is mixed completely with the molding additive, and then the mixture is pelletized to pellets of an average diameter of 1-5 mm by a pelletizer, and thereafter injection molded to a green molded body of a desired shape of complicated configurations having different thicknesses portions. In a preferred embodiment of the present invention, the degreasing process is conducted by a moderate heating of the green body at a temperature of 300-550°C, preferably 350-500°C, for about 3-8 hrs. For a better understanding of the present invention, reference is made to the accompanying drawings, in which: Fig. 1 is a block diagram of the method according to the present invention; Fig. 2A is a schematic longitudinal side view of a molded green body of ceramic part produced by the method of the present invention; and Fig. 2B is a schematic longitudinal side view of a processed body produced by the method of the present invention; Throughout different views of the drawings, reference numeral 21 is a rotor for a turbo charger, 22 is a blades portion, 23 is an axis portion before the processing, 24 is an axis portion after the processing, and 25 is a groove for sealing. The steps of the method of the present invention is schematically shown in block diagram as shown in Fig. 1. Condition for the injection molding is properly selected depending on the types and amounts of the material powder and the molding additive. For Example, if 100 parts by weight of material Si₃N₄ powder and a molding additive consisting of 15 parts by weight of polystyrene, 3 parts by weight of polyethylene and 2 parts by weight of stearic acid are used for the injection molding, an injection pressure of 500-1,500 kg/cm² (49 to 147 MPa), and a molding temperature of about 150-250°C are preferably used. Injection molded green body of complicated configurations is subsequently processed to a substantially final shape. The term "molded green body" or "green body" used herein means an injection molded material not yet treated by a treatment such as degreasing, firing, processing or machining, except for a drying treatment. Processing of the green body is usually effected by a ceramic bit such as WC or Si₃N₄. It can be effected also by using a diamond or boron nitride grinding stone. The green body processed to a substantially final shape is degreased or removed from the molding additive. The removing process varies depending on the types and amounts of the used molding additive. If polystyrene, polyethylene and stearic acid were used in combination as the molding additive, the molding additive can be removed by raising the temperature from a room temperature to about 400°C at a temperature raising rate of about 4°C/hr, and holding the temperature of about 400°C for about 3-5 hrs. If non-oxidized material e.g. a ceramic powder such as Si₃N₄ or SiC or a powder of metal such as Si or Al was used as a material powder, environmental atmospheres in the degreasing process and the final firing process are preferably a non-oxidizing atmosphere such as N₂ or Ar for preventing oxidation of the non-oxidized material. According to the method of the present invention, the molded green body of complicated configuration is processed to a substantially final configuration before it is subjected to the degreasing process, so that even when a structural part of a complicated configuration is to be produced, the final machining process which is sometimes used for finishing the sintered body can be dispensed with or substantially decreased and the raw materials are effectively used far better than conventional methods. In other words, the green body processed to a sub stantially final configuration before degreasing thereof has almost the same shape and dimension as those of the final product after firing. Thus, final machining of the sintered body after the firing is completely or substantially unnecessary, i.e. only a little bit of machining is sufficient if such is necessitated. In addition, the chips of the raw materials containing the molding additive resulting from the processing of the green body can be reused for the injection molding by mixing it with a fresh raw materials because it still contains the molding additive, so that necessary amount of the raw materials for producing a certain yield of product can be remarkably reduced as compared with conventional methods. Thus, material efficiency is remarkably improved. Besides, the time required for the green body-processing process is far shorter than that required for machining or grinding the sintered body by a diamond grinding stone in conventional methods, and it can be effected with the use of a ceramic bit which is cheap as a grinding stone, so that it is economically much advantageous in labor time and costs. When strict dimensional precision of the ceramic parts is required, a finishing machining is necessary for the product after the final firing process. However, even in such a case, only a slight finishing machining of the product is sufficient to meet the requirement, because the final configuration of the product has substantially been attained already by the processing of the green body, so that the finishing machining can be effected much shorter and cheaper than conventional methods. Hereinafter, the present invention will be explained in more detail with reference to preferred embodiments. In the following Examples, all parts are on a weight basis, unless otherwise specified. Example 1 100 parts of Si₃N₄ powder are mixed with a sintering additive consisting of 4 parts of MgO, 1 part of SrO and 5 parts of CeO₂, and further added with a molding additive consisting of 15 parts of polystyrene, 3 parts of polyethylene, 2 parts of stearic acid and 2 parts of ethylene-vinyl acetate copolymer. The mixture is kneaded sufficiently by a pressure type kneader, and then granulated to pellets of diameters of about 2-3 mm by a pelletizer. The pellets are heated to 200°C and fed in a die of an injection pressure of 1 t/cm² and a temperature of 50°C to mold five rotors for turbo chargers, as shown in Fig. 2A. Each rotor has a blades portion 22 of a diameter of 60 mm, and an axis portion 23 of a diameter of 25 mm. The blades portion 22 has a thickness of 0.7 mm at its most thin point. Two of the five rotors are immediately processed or ground at the axis portion 23 by means of a ceramic bit such that the axis portion 24 formed by grinding has a diameter of 12 mm, and the axis portion 24 is further processed to form a sealing groove 25, as shown in Fig. 2B. The two processed bodies and the remaining three unprocessed green bodies are put in a hot air circulation type electric furnace, and heated therein in such a manner that the temperature of the furnace is raised from room temperature to 400°C at a temperature raising rate of 4°C/hr, and held at 400°C for 5 hrs. to remove the molding additive. The two processed and degreased bodies according to the present invention are found to have no crack or deformation at both the blades portion and the axis portion. While, one of the three unprocessed and degreased bodies is found to have a crack at the boundary between the blades portion and the axis portion. The two processed and degreased bodies according to the present invention and the two unprocessed and uncracked degreased bodies are fired in N₂ atmosphere at 1,700°C for 30 min. to sinter the same, and thereafter machined at their axis portiohs to a final configuration with the aid of a diamond grinding stone. The time required for the machining is only 10 min. for each of the processed, degreased and sintered bodies, whereas the time is 1 hr for each of the unprocessed, degreased and sintered body. Example 2 100 parts of SiC powder were mixed with a sintering additive consisting of 2 parts of B₄C and 2 parts of carbon black and a molding additive consisting of 15 parts of polystyrene, 5 parts of paraffin wax and 2 parts of oleic acid, and mixed, kneaded and pelletized as in Example 1. The pellets are heated to 150°C, and fed in a die of an injection pressure of 500 kg/cm² (49 mPa) and a die temperature of 50°C to injection mold three radial type turbine rotors for gas turbines. The rotors have a blades portion of which the minimum thickness is 0.8 mm, and an axis portion of a diameter of 30 mm. One of the three molded green bodies is processed to a substantially final configuration by using a ceramic bit, taking the dimensional change due to shrinkage by firing into consideration. The processed body and the other two unprocessed green bodies are heated in Ar atmosphere in a hot air circulation type electric furnace from room temperature to 500°C at a temperature raising rate of 5°C/hr, and held at 500°C for 3 hrs, to remove or degrease the molding additive. The processed body is found to have no crack or deformation at both the blades portion and the axis portion after removal of the molding additive. While, one of the two unprocessed bodies is found to have a minor crack at the boundary between the blades portion and the axis portion after removal of the molding additive. The non-cracked and non-deformed processed body according to the invention and the non-cracked unprocessed body both after the degreasing are fired and sintered in Ar atmosphere at 2,200°C for 1 hr, and thereafter machined at their axis portions to a final configuration by the use of a diamond grinding stone. The time required for the machining is only 15 min. for the processed, degreased and sintered body, whereas the time is 1.5 hrs for the unprocessed, degreased and sintered body. Grinding chips produced by processing the molded green body are recovered, mixed with a fresh raw materials and tested whether they can be again utilized as raw materials, for the turbine rotors or not. As a result, it is found that they are sufficiently usable as raw materials for the ceramic part without any problems. Example 3 100 parts of Si powder are mixed with 1.5 parts of Fe₂O₃ as a nitriding additive (sintering additive) and further with a molding additive consisting of 4 parts of polyethylene and each 5 parts of three waxes, each wax respectively having a melting point of 76°C, 90°C and 137°C, and, kneaded and pelletized as in Example 1. The pellets are injection molded to a rotor for a turbo charger, as shown in Fig. 2A. The molded green body is processed to a final configuration using a ceramic bit, and the processed body is heated in N₂ atmosphere from room temperature to 350°C at a temperature raising rate of 3°C/hr and held at 350°C for 8 hrs. to remove the molding additive completely. Then, the degreased body is fired in N₂ atmosphere at 1,400°C for 10 hrs. so as to become sintered and nitrided. Thus obtained sintered turbo charger rotor made of silicon nitride is found to have no crack or deformation, and has an appreciably good dimensional precision, and only needs a surface finishing at the axis portion. As clearly apparent from the foregoing detailed explanation, the method according to the present invention produces, in an economical way, non-cracked or non-deformed ceramic parts of complicated configurations with different thicknesses portions that could not be produced directly to final configurations by conventional injection molding methods, so that it is eminently useful industrially.

Claims: A method of producing ceramic parts by mixing a ceramic powder or metal powder with a molding additive, injection molding the mixture to form a green body, removing the molding additive to yield a degreased body and firing the degreased body to sinter the same, characterized in that a green pre-form is formed by injection molding and the green pre-form is processed to a substantially final configuration, before it is subjected to the degreasing process. A method according to claim 1, wherein the molding additive is at least one thermoplastic resin, wax or lubricant. A method according to claim 2, wherein the thermoplastic resin is at least one of polystyrene, polypropylene, polyethylene and ethylene-vinyl acetate copolymer. A method according to claim 1, 2 or 3, wherein the mixture further contains, as sintering additive, at least one of MgO, Al₂O₃, Y₂O₃, ZrO₂, CeO₂ and SrO. A method according to any one of claims 1 to 4, wherein the ceramic part is a rotor for a turbo charger. A method according to any one of claims 1 to 4, wherein the ceramic part is a rotor for a gas turbine.

Inventors: ITO SHIGENORI, MIZUNO TAKEYUKI, ITO, SHIGENORI, MIZUNO, TAKEYUKI
Applicants: NGK INSULATORS LTD, NGK INSULATORS, LTD.
Application year: 1984
Main classification: C04B35
IPC codes: B28B11, B28B1, B22F3, C04B35, B22F5, F01D5