SINTERED CERAMIC ARTICLES AND METHOD FOR PRODUCTION THEREOF

Abstract: When a shaped Si₃N₄ type ceramic article is sintered in the presence of SiO₂ or a SiO₂ -containing substance in an inactive gas atmosphere, there is obtained a sintered ceramic article having the surface thereof coated with a thin layer formed preponderantly of silicon oxynitride (Si₂N₂O). The sintered ceramic article of this invention excels in resistance to elevated temperatures, particularly tempera­ tures exceeding 1200°C, and possesses good surface prop­ erties.

Description: This invention relates to sintered ceramic articles and to a method for the production thereof. Since sintered ceramic articles formed preponderantly of Si₃N₄ excel in heat resistance and resistance to thermal shock, the feasibility of such articles in applications to structural parts such as gas turbine blades and nozzles which are required to offer high strength at elevated temperatures is being studied. It is known that these sintered Si₃N₄ type ceramic articles have their strength lowered by oxidation at elevated temperatures exceeding 1200°C. Thus, the desirability of improving these articles in resistance to oxidation is finding growing recognition. These sintered ceramic articles are generally desired, for the purpose of decreasing the cost of sintering, to be produced by the procedure of placing a plurality of shaped articles of Si₃N₄ in a container made of Al₂O₃ and sintering them in a furnace under an inactive gas atmosphere. This method of sintering however, has entailed a problem that during the course of sintering part of Si₃N₄ is decomposed into Si or SiO₂ and N₂ and expelled out of the shaped articles, leaving behind a coarsened surface. DE-A-2540015 discloses a product containing silicon nitride and silicon oxynitride. This invention, with a view to overcoming the aforementioned disadvantage of the prior art, aims to provide sintered ceramic articles excelling in resistance to oxidation at elevated temperatures exceeding 1200°C and enjoying good surface conditions and a method for the production of such articles. According to the invention, there is provided a method for the production of a sintered ceramic article, comprising the steps of molding a powdered raw material preponderantly of Si₃N₄ in a prescribed shape and sintering the resultant shaped ceramic article in the presence of SiO₂ or a mixture containing SiO₂ in a furnace under an inactive gas atmosphere and wherein said SiO₂ and SiO₂-containing mixture and said shaped ceramic articles are placed in said furnace in a total volume accounting for a filling ratio of not less than 50% of the total inner volume of said furnace. The drawing is a cross-sectional diagram illustrating in outline a sintering furnace to be used in the production of a sintered ceramic article of this invention. The term "sintered Si₃N₄ type ceramic article" as used herein means a sintered article which is obtained by using Si₃N₄ powder as a main component, adding thereto as a sintering aid at least one member or a mixture of two or more members selected from the group consisting of oxides of rare earth elements such as Y₂O₃, and Al₂O₃, AIN, and TiO₂, optionally further adding thereto an organic binder, molding the resultant mixture in a prescribed shape, degreasing the shaped mixture, and sintering it. In the present invention, the thin layer of Si₂N₂O manifests an effect of conferring upon the sintered ceramic article the resistance to oxidation at elevated temperatures exceeding 1200°C and consequently preventing the sintered ceramic article from lowering of strength. This layer is desired to have a thickness in the range of 5 to 500 µm, preferably 10 to 250 µm. The thin layer of Si₂N₂O is formed by the following method. First, Si₃N₄ powder and a sintering aid are mixed, optionally in conjunction with an organic binder. The mixture so produced is molded in a prescribed shape and heated to degrease. The shaped article so produced is placed in conjunction with SiO₂ or a mixture containing SiO₂ in a sintering furance and sintered under an inactive gas atmosphere such as nitrogen gas or forming gas at 1700 to 1800°C for 1 to 4 hours. The furnace for the sintering is desired to be a closed sintering container made of Al₂O₃ or Si₃N₄. The sintering container may be in a double-wall construction intended to ensure airtight closure. The atmosphere for the sintering may be under normal pressure. When desired, the pressure of the atmosphere can be increased up to about 10 kg/cm² (1MPa). The SiO₂ or the mixture containing SiO₂ is added to the furnace interior of the purpose of allowing the atmosphere of sintering to contain SiO₂ gas. Thus, SiO₂ can be used alone or as mixed with some other substance. It is used in a powdered, granulated, or pelletized form. For the convenience of the operation SiO₂ is desired to be used in the form of a mixture with some other substance. This mixture is desired to be obtained by mixing SiO₂ and Si₃N₄ both in powdered form and molding the resultant mixture in a desired shape, preferably in the shape of pellets. During the course of sintering, since SiO₂ are sublimited at 1550 to 1600°C, the Si₃N₄-SiO₂ pellets expel SiO₂ into the atmosphere to increase the partial pressure of SiO₂ in the atmosphere and consequently to control the otherwise possible decomposition of the sintered Si₃N₄ ceramic article. The sublimated SiO₂ is condensed in the form of frost on the inner wall of the closed container, on the surface of the shaped article being sintered, or on the inner wall of the sintering furnace. The condensed SiO₂ can be easily removed from the surface. On the other hand, the residual Si₃N₄ can be easily crushed. Thus, the two materials so recovered can be put to use repeatedly, promoting economization of resources. In the shaped mixture of Si₃N₄-SiO₂, such as pellets, the SiO₂ component is desired to account for 10 to 50% of the whole amount of the mixture. The amount of SiO₂ or the SiO₂-containing mixture to be placed in the furnace is selected to fall in the range of 0.1 to 20%, preferably 2 to 10%, as SiO₂ based on the weight of the shaped ceramic article. The volume to be occupied by the SiO₂ or SiO₂-containing mixture and the shaped ceramic article is required to be not less than 50% of the whole internal volume of the firing container. In the case of a tightly closed firing container, the ratio of SiO₂ to the shaped ceramic article can be decreased in proportion as the volume ratio mentioned above is increased. When the amount of SiO₂ falls in the range mentioned above, Si₃N₄ and SiO₂ react with each other to give rise to a thin layer of Si₂N₂O on the surface of the sintered Si₃N₄ type ceramic article, enabling the sintered article to acquire improved strength at elevated temperatures and improved surface condition. Now, the present invention will be described more specifically below with reference to working examples. It should be noted, however, that the present invention is not limited in any sense by these examples. Examples 1-3: A powdered mixture consisting of 5wt% of Y₂O₃, 3wt% of Al₂O₃, 3wt% of AIN, and 0.5% of TiO₂ collectively as a sintering aid and the balance of Si₃N₄ and 7% of an organic binder such as paraffin were blended and molded in the shape of a square block, 50 mmx50 mmx10 mm, followed by degreasing. Separately, 20 wt% of SiO₂ and 80 wt% of Si₃N₄ were mixed in powdered form and the resultant mixture was molded to obtain SiO₂ type pellets. A sintering furnace was used, as illustrated in the drawing, of a tightly closed cylindrical container 1 made of Al₂O₃ and measuring 200 mm in inside diameter and 60 mm in height, a tightly closed container 2 made of Si₃N₄ and adapted to enclose the cylindrical container 1, and a carbon container 3 adapted to accommodate the container 2. In the Al₂O₃ cylindrical container 1, 40 shaped ceramic articles (products) 4 prepared as described above were placed. In the cavity intervening between the cylindrical container and the products, the SiO₂ type pellets were placed in a varying filling ratio of 60% to 80% selected so that the amount of SiO₂ would account for 2%, 5%, or 10% of the weight of the products. The products were then subjected to sintering. The sintering was performed in the atmosphere of forming gas at 1750°C for four hours. The sintered ceramic articles consequently obtained were tested for density, ratio of weight loss during the conversion of shaped articles to sintered articles, flexural strength at room temperature and after 3000 hours, standing in air at 1200°C, surface condition, and thickness of Si₂N₂O layer. The results were as shown in the following table. The data given in the table under the heading "Comparative Experiment" represent the properties obtained of sintered ceramic articles produced in the absence of SiO₂. It is noted from the table that the sintered ceramic articles according to this invention possess high resistance to oxidation at elevated temperatures and, therefore, suffer from small loss of strength and, because of small loss of Si₃N₄ weight, enjoy good surface conditions.

Claims: 1. A method for the production of a sintered ceramic article, comprising the steps of molding a powdered raw material preponderantly of Si₃N₄ in a prescribed shape and sintering the resultant shaped ceramic article in the presence of SiO₂ or a mixture containing SiO₂ in a furnace under an inactive gas atmosphere and wherein said SiO₂ or SiO₂-containing mixture and said shaped ceramic articles are placed in said furnace in a total volume accounting for a filling ratio of not less than 50% of the total inner volume of said furnace, said sintering being carried out in a tightly closed sintering container, and said SiO₂ or SiO₂-containing mixture being present in a ratio of 1 to 20% as SiO₂ based on the weight of said shaped ceramic articles. 2. A method according to claim 1, wherein said SiO₂-containing mixture is a powdered mixture consisting of SiO₂ and Si₃N₄ and molded in a desired sahpe. 3. A method according to claim 2, wherein said mixed powder of SiO₂ and Si₃N₄ is molded in the shape of pellets. 4. A method according to claim 2, wherein SiO₂ is present in a ratio of 10 to 50% in said mixture of SiO₂ and Si₃N₄. 5. A sintered ceramic article formed by the method of any one of the preceding claims characterised by having a layer preponderantly of silicon oxynitride (Si₂N₂O) formed on the surface of a sintered Si₃N₄ type ceramic article. 6. A sintered ceramic article according to claim 5, wherein said silicon oxynitride (Si₂N₂O) layer has a thickness in the range of 5 to 500 µm.

Inventors: KOMATSU, MICHIYASU
Applicants: KABUSHIKI KAISHA TOSHIBA
Application year: 1985
Main classification: C04B35
IPC codes: C04B41, C04B35