GB2171092A

GB2171092A - Silicon nitride ceramics

Info

Publication number: GB2171092A
Application number: GB08531881A
Authority: GB
Inventors: Michiyasu Komatsu; Tadashi Miyano
Original assignee: Toshiba Corp
Current assignee: Toshiba Corp
Priority date: 1984-12-29
Filing date: 1985-12-30
Publication date: 1986-08-20
Also published as: JPH0535107B2; JPS61158868A; GB8531881D0

Abstract

A sintered silicon nitride type ceramic article of fine surface grain and high mechanical strength is produced with a small loss of weight during the sintering at 1650 DEG C- 1850 DEG C from a ceramic mixture consisting of not more than 10% by weight of rare earth elements, not more than 10% by weight aluminium oxide, not more than 10% by weight aluminium nitride and the balance of silicon nitride having a beta -phase content of at least 50%.

Description

SPECIFICATION Method for production of sintered ceramic article The present application claims priority of Japanese patent application Serial No. 59-280906 filed on December 29, 1984.

This invention relates to a method for inexpensive production of a sintered ceramic article consisting preponderantly of silicon nitride and excelling in mechanical properties.

Sintered ceramic articles having silicon nitride as a main ingredient possess a high heatresisting 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 high-strength heat-resisting parts represented by gas turbine blades and nozzles is being tried. Since these sintered ceramic aticles have a high ability to corrosion proof metals, their utility as refractory materials destined to contact molten metals has already been realized.

As the raw material for silicon nitride, the a-phase type silicon nitride is used in most cases. It has been held that a sintered ceramic article of high strength is not obtained unless this a-phase type silicon nitride is used.

The ss-phase type silicon nitride which is the other stable form of silicon nitride powder is far less expensive than the a-phase type. Further, it has the advantage that its loss of weight by sintering is small because the total oxygen content in its structure is small as compared with the a-phase type silicon nitride and because it undergoes no phase transition during the sintering. It is, therefore, advantageous to develop a method which is capable of producing a sintered silicon nitride type ceramic article excellent in properties by using this ss-phase type silicon nitride as the raw material.

The inventors have found that a sintered silicon nitride type ceramic article produced by using, as its raw material, silicon nitride powder consisting preponderantly of the ss-phase type silicon nitride possesses outstanding properties when the sintering assistants used as additives therein are selected in kind and content within prescribed ranges and the sintering conditions are controlled.

This invention has been produced in consequence of the knowledge acquired in the course of the inventor's study. It aims to provide a sintered silicon nitride type ceramic article of fine surface grain and high mechanical strength, produced with a small loss of weight during the sintering from the inexpensive ss-phase type silicon nitride powder.

Specifically, the method of this invention for the production of a sintered ceramic article is 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), not more than 10% by weight of aluminum oxide (excluding 0), not more than 10% by weight of aluminum 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 containing at least 50% of the ss-phase type silicon nitride, and by sintering the molded ceramic mixture in a non-oxidative atmosphere at 1,650 to 1,850"C.

The silicon nitride as the principal component of the ceramic mixture of this invention contains 50 to 100% of the ss-phase type silicon nitride. The ss-phase type silicon nitride content can be selected freely within this range.

The oxides of rare earth elements, aluminum oxide, and aluminum nitride which are essentially components for addition to silicon nitride in the present invention all function as sintering promotors. The contents of these components are each required to be not more then 10% by weight (excluding 0) based on the amount of the ceramic mixture. As concrete examples of the oxides of rare earth elements, yttrium oxide and cerium oxide may be cited. Among other oxides cited above, yttrium oxide proves particularly desirable. Optionally, these oxides may be used as suitably mixed.

As concrete examples of other additive components for the ceramic mixture, titanium dioxide, 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 these oxides.

The total of the contents of additive components for silicon nitride is desired to fall in the range of 10 to 15% by weight.

In this invention, a ceramic mixture containing the aforementioned components in a percentage composition specified above is molded in a given shape and the molded ceramic 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 oxided into SiO2 at elevated temperatures and the produced sintered ceramic article fails to manifest sufficiently high strength at elevated temperatures as expected.Even when this sintering is effected as described above by the so-called atmospheric sintering technique, it produces a silicon nitride type sintered ceramic article compact in texture and excellent in high-temperature mechanical strength and resistance to thermal shock. Of course, this sintered ceramic article is obtained with equal properties 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 working examples.

EXAMPLE 1 In a ball mill lined with rubber, a mixed powder consisting of 89% by weight of 50% ss-phase type silicon nitride powder of an average particle size of 1,um, 5% by weight of yttrium oxide powder of an average particle size of 0.8,us, 3% by weight of aluminum oxide powder of an average particle size of 0.6,um, and 3% by weight of aluminum nitride powder of an average particle size of 0.9m was pulverized in the presence of n-butanel 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 nitorogen gas at 1,750 C for two hours to obtain a sintered silicon nitride type ceramic article.

EXAMPLE 2 A sintered silicon nitride type ceramic article was obtained by following the procedure of Example 1, except that 100% ss-phase type silicon nitride powder was used in the place of 50% phase silicon nitrtide powder.

EXAMPLE 3-4 Sintered silicon nitride type ceramic articles were obtained by following the procedure of Example 1, except that 1.5% by weight of titanium oxide of an average particle size of O.5m (Example 3) and 3% by weight of molybdenum carbide of an average particle size of 0.9 ,um (Example 4) were added respectively to the composition of Example 1.

COMPARA TIVE EXPERIMENT I A sintered silicon carbide type ceramic article was obtained by following the procedure of Example 1, except that 95% a-phase type silicon nitride powder was used in the place of 50% ss-phase type silicon nitride powder.

The sintered ceramic articles obtained as described above were tested for weigt loss by sintering, deflective force (mechanical strength), and surface roughness. The results are shown in the table.

Table

Example Comparative Experiment 1 2 3 4 1 Weight loss by sintering 1.0 0.5 1.0 0.7 2.5 ( ~ Defective force (kg/mmZ) Room temperature 90 85 93 90 95 1,000it 83 80 85 80 85 1,200'C 60 58 50 50 60 Surface roughness 1.2 1.0 1.1 1.0 1.8 (lem) Price of powder Low Low Low Low High As described above, the sintered ceramic article obtained by this invention has a small loss of weight by sintering, possesses low surface roughness, and exhibits the same degree of mechanical strength as the sintered ceramic article using a-phase type silicon nitride powder.

Claims

1. 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), 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 having a ss-phase content of at least 50% and sintering the molded ceramic mixture in a nonoxidative atmosphere at 1,650 to 1,850"C.

2. A method according to Claim 1, wherein said oxide of rare earth element is yttrium oxide.

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

4. 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, and the balance of silicon nitride having a ss-phase content of at least 50%, and sintering the molded ceramic mixture in a non-oxidative atmosphere at 1,650 to 1,850"C.

5. A method according to Claim 4, wherein said oxide of rare earth element is yttrium oxide.

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

7. A method for the production of a sintered ceramic article substantially as hereinbefore described with reference to the Examples.