GB2169594A - Silicon nitride ceramics - Google Patents

Abstract

A sintered ceramic article of improved sintering property and high mechanical strength is produced by sintering a ceramic mixture consisting of not more than 10% by weight of oxides of rare earth elements (including 30-70 wt% of either yttrium oxide or cerium oxide), not more than 10% by weight aluminium oxide, not more than 10% by weight of aluminium nitride and the balance of silicon nitride.

Description

SPECIFICATION Sintered ceramic articles and method for production thereof The present application claims priority of Japanese patent application Serial No.59-280908 filed on December 29, 1984.

This invention relates to silicon nitride type sintered ceramic articles containing oxides of a plurality of rare earth elements as sintering assistants and excelling in mechanical strength and to a method for the production thereof.

Sintered ceramic articles having silicon nitride as a main ingredient possess a high heat-resisting property up to 1,900 C and a low thermal expansion coefficient and, therefore, are excellent also in resistance to thermal shock. In view of these beneficial properties, their utility in applications to various highstrength heat-resisting parts represented by gas turbine blades and nozzles is being tried. Since these sintered ceramic articles have a high ability to corrosionproof metals, their utility as refractory materials destined to contact molten metals has already been realized.

Heretofore, it has been customary to use yttrium oxide powder of high purity as the powdered raw material for yttrium oxide, one of the sintering assistants mentioned above. The reason for the high purity of the yttrium oxide powder is the generally accepted theory that when the powder of yttrium oxide as the raw material contains impurities such as oxides of other rare earth elements, sintered ceramic articles consequently produced exhibit strength at a room temperature and at elevated temperatures, resistance to oxidation, and resistance to thermal shock all at levels falling short of being satisfactory.

As the raw material, the yttrium oxide powder containing impurities such as oxides of other rare earth elements is far cheaper than the powdered yttrium oxide of high purity. It is, therefore, high desirable to develop a method which is capable of producing a sintered silicon nitride type ceramic article excelling in properties from the aforementioned cheaper powdered yttrium oxide.

The inventors studied the problem described above and have consequently found that a sintered silicon nitride type ceramic article produced by using yttrium oxide containing impurities such as oxides of other rare earth elements possesses outstanding properties when sintering assistants used as additives therein are selected in kind and content within prescribed ranges and the sintering conditions are controlled precisely as specified.

This invention has been produced in consequence of the knowledge acquired in the course of the inventors' study mentioned above. It aims to provide a sintered ceramic article of improved sintering property and high mechanical strength by using crude yttrium oxide powder as a sintering assistant.

Specifically, the sintered ceramic article of the present invention is characterized by being produced by sintering a ceramic mixture consisting of not more than 10% by weight of oxides of rare earth elements (excluding 0) including 30 to 70% by weight of either yttrium oxide or cerium oxide, not more than 10% by weight of aluminum oxide (excluding 0), not more than 10% by weight of aluminium nitride (excluding 0) optionally not more than 10% by weight of at least one compound selected from the group consisting of titanium oxide, zirconium oxide, magnesium oxide, and molybdenum carbide, and the balance of silicon nitride.The method of this invention for the production of this sintered ceramic article is characterized by molding in a desired shape the aforementioned ceramic mixture and sintering the molded ceramic mixture in a non-oxidative atmosphere at 1,650"C to 1,850"C.

The rare earth element oxide powder to be used as the sintering assistant in this invention may be a powdered crude raw material containing 30 to 70% by weight of either yttrium oxide or cerium oxide and the balance of oxides of such rare earth elements as lanthanum and scandium and other impurities.

Compared with the sintering by the use of yttrium oxide of high purity, the sintering by the use of the crude powder of this invention has the advantage that the sintering temperature is low and the amount of the raw material to be added is small.

As concrete examples of the powdered raw material answering the description given above, xenotime, monazite, and bastnaesite may be cited. Xenotime is rich in yttrium oxide and monazite and bastnaesite are rich in cerium oxide.

The oxides of rare earth elements containing either yttrium oxide or cerium oxide and the aluminum oxide component both function as sintering promotors. The contents of these components are each required to be not more than 10% by weight (excluding 0) based on the total amount of the ceramic mixture. The reason for these upper limits is that the sintered ceramic article excels in mechanical strength and resistance to thermal shock when the contents of these components in the ceramic mixture are not more than 10% by weight.

Aluminum nitride as another additive component in the ceramic mixture of this invention serves to enhance mechanical strength at elevated temperatures and also contributes to promoting the sintering.

The effect of this additive component in improving resistance to thermal shock and strength at a room temperature is conspicuous when the content of this additive component in the ceramic mixture is up to 10% by weight.

As concrete examples of other additive components usable in the ceramic mixture of this invention, titanium oxide, zirconium oxide, magnesium oxide, and molybdenum carbide may be cited. The ceramic mixture is desired to incorporate therein not more than 10% by weight of at least one of the compounds mentioned.

The total amount of additive components as the sintering assistants is desired to fall in the range of 10 to 15% by weight based on the amount of the ceramic mixture.

In the present invention, a mixture containing the aforementioned components in the prescribed ranges is first molded in a desired shape and the molded mixture is sintered in a non-oxidative atmosphere at a temperature in the range of 1,650" to 1,850"C. The atmosphere for this sintering is required to be formed of a non-oxidative gas such as nitrogen gas or argon. The reason for the non-oxidative atmosphere is that when the sintering is carried out in an atmosphere containing oxygen, silicon nitride is oxidized into SiO2 at elevated temperatures and the produced sintered ceramic article fails to manifest sufficiently high strength at elevated temperatures as expected.When this sintering is effected by the socalled atmospheric sintering technique or even by any of the other conventional sintering techniques such as, for example, the hot press technique, the ambient pressure technique, and the hot hydrostatic sintering (HIP) technique.

Example Now, the present invention will be described below with reference to a working example.

In a ball mill lined with rubber, a mixed powder consisting of 100 parts by weight of silicon nitride powder of an average particle size of 0.8 Fm, 5 parts by weight of yttrium oxide powder of an average particle size of 0.9 pkm and purity of 60% (xenotime), 4 parts by weight of aluminum oxide powder of an average particle size of 0.6 pbm, 3 parts by weight of aluminium nitride powder of an average particle size of 0.9 Fm, and 1.5 parts by weight of titanium oxide powder of an average particle size of 0.5 Wm was pulverized in the presence of n-butanol as a solvent for about 24 hours to prepare powdered raw material.

The powdered raw material was admixed with 7% by weight of stearic acid (binder) and the resultant mixture was molded in a prescribed shape under a pressure of 700 kg/cm2. The molded mixture so obtained was heated at 700"'C to expel the binder by volatilization. The preheated molded mixture was subjected to atmospheric sintering in an atmosphere of nitrogen gas at 1,700"C for two hours to obtain a sintered silicon nitride type ceramic article.

In a comparative experiment, a sintered silcon nitride type ceramic article was obtained by following the procedure of the working example, except that yttrium oxide powder of purity of 99.9% was used as the raw material and sintering temperature was changed to 1,750"C.

The sintered ceramic articles were tested for density and deflective strength (mechanical strength). The results are shown in the table.

TABLE Example Comparative Experiment Relative density (%) 99 99 Deflective strength (kg/mm2) Room temperature 94 100 1,200"C 50 50 Price of Y203 powder Low High Overall rating Excellent Good In the table, the density is the relative density (%) based on the theoretical density and the value of deflective strength represents the average of the results of four measurements of the magnitude by the three-point bending strength test conducted on test pieces 3 x 3 x 30 (mm) in size under the conditions of cross head speed of 0.5 mm/min and span of 20 mm, each at a room temperature and at 1,200"C.

As described above, the sintered ceramic articles according to the present invention are produced inexpensive and they excel in strength at a room temperature and at elevated temperatures.

Claims (14)

1. A sintered ceramic article, characterized by being produced by sintering a ceramic mixture consisting of not more than 10% by weight of oxides of rare earth elements (excluding 0) including 30 to 70% by weight of either yttrium oxide or cerium oxide, not more than 10% by weight of aluminum oxide (excluding 0), not more than 10% by weight of aluminum nitride (excluding 0), and the balance of silicon nitride.

2. A sintered ceramic article according to Claim 1, wherein a powdered raw material for said oxides of rare earth elements is xenotime, monazite or bastnaesite.

3. A sintered ceramic article according to Claim 1, wherein the total content of other compounds than silicon nitride is 10 to 15% by weight of said ceramic mixture.

4. A sintered ceramic article, characterized by being produced by sintering a ceramic mixture consisting of not more than 10% by weight of at least one compound selected from the group consisting of titanium oxide, zirconium oxide, magnesium oxide, and molybdenum carbide (excluding 0), and the balance of silicon nitride.

5. A sintered ceramic article according to Claim 4, wherein a powdered raw material for said oxides of rare earth elements is xenotime, monazite or bastnaesite.

6. A sintered ceramic article according to Claim 4, wherein the total content of other compounds than silicon nitride is 10 to 15% by weight of said ceramic mixture.

7. A method for the production of a sintered ceramic article, characterized by molding in a desired shape a ceramic mixture consisting of not more than 10% by weight of oxides of rare earth elements (excluding 0) including 30 to 70% by weight of either yttrium oxide or cerium oxide, not more than 10% by weight of aluminum oxide (excluding 0), not more than 10% by weight of aluminum nitride (excluding 0), and the balance of silicon nitride and sintering the ceramic mixture in a non-oxidative atmosphere at 1 ,650"C to 1 ,850 C.

8. A method according to Claim 7, wherein a powdered raw material for said oxides of rare earth elements is xenotime, monazite or bastnaesite.

9. A method according to Claim 7, wherein the total content of other compounds than silicon nitride is 10 to 15% by weight of said ceramic mixture.

10. A method for the production of a sintered ceramic article, characterized by molding in a desired shape a ceramic mixture consisting of not more than 10% by weight of at least one compound selected from the group consisting of titanium oxide, zirconium oxide, magnesium oxide, and molybdenum carbide (excluding 0), and the balance of silicon nitride, and sintering the ceramic mixture in a non-oxidative atmosphere at 1,650" to 1,850"C.

11. A method according to Claim 10, wherein a powdered raw material for said oxides of rare earth elements is xenotime, monazite or bastnaesite.

12. A method according to Claim 10, wherein the total content of other compounds than silicon nitride is 10 to 15% by weight of said ceramic mixture.

13. A sintered ceramic article substantially as hereinbefore described with reference to the Example.

14. A method for the production of sintered ceramic article substantially as hereinbefore described with reference to the Example.