JPH0788257B2 - Method for producing zirconia-dispersed silicon nitride sintered body - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of application] The present invention is a method for producing a zirconia-dispersed silicon nitride sintered body which is applied to mechanical structural parts required to have strength and toughness under a high temperature, corrosive or abrasive environment. It is about.

[Prior Art] Silicon nitride (Si ₃ N ₄ ) is known to have relatively high strength and excellent heat resistance among ceramics.

However, the toughness is low, which is a characteristic of ceramics in general.

However, among ceramics, zirconia (ZrO ₂ ) is known to exhibit high toughness due to a unique phase transition mechanism.

Therefore, a method of increasing the toughness of silicon nitride by dispersing zirconia in a silicon nitride matrix has been attempted.

Conventionally, the second method for producing a zirconia-dispersed silicon nitride sintered body is the second.
It was done as shown in the figure. As shown in the drawing, the silicon nitride powder 1, the zirconia powder 2 and the sintering aid 3 were mechanically mixed 4 by a ball mill or the like to be molded 5 into a desired shape 5, followed by sintering 6.

[Problems to be Solved by the Invention] By the way, this type of manufacturing method has the following problems.

Direct contact between silicon nitride and zirconia reacts during sintering to produce zirconium nitride and zirconium oxynitride. Therefore, there is a problem that it is difficult to leave zirconia having high toughness in the sintered body.

Also, since this zirconium nitride has low oxidation resistance,
There is a problem that the oxidation resistance of the sintered body is also impaired.

Further, the sintering aid of silicon nitride generally has a function of suppressing the phase transition of zirconia. Therefore, when the zirconia and the sintering aid are in direct contact with each other, there is a problem that the aid diffuses into the zirconia during sintering, and the phase transition thereof is suppressed later to hinder the toughening.

In view of the problems as described above, the present invention aims to provide a manufacturing method capable of manufacturing a zirconia-dispersed silicon nitride sintered body having high strength and high toughness under a high temperature, corrosive or abradable environment. To do.

[Means for Solving Problems] In order to solve the problems in the prior art, the present invention mixes a silicon nitride powder coated with a sintering aid layer on the surface for promoting sintering and a zirconia powder. Then, in a method of molding this into a desired shape and sintering, the reaction of silicon nitride and zirconia is prevented on the surface of the zirconia powder at the time of performing the above-mentioned sintering, and a sintering aid zirconia is formed. Is coated with an alumina layer for preventing the diffusion of Al.

[Operation] As described above, by coating the surface of the zirconia powder with the alumina layer at the time of sintering, silicon nitride and zirconia do not come into direct contact with each other, so that they are prevented from reacting during sintering. , Suppresses the formation of zirconium nitride, which has low oxidation resistance, and since alumina has good oxidation resistance,
The oxidation resistance of the sintered body does not deteriorate.

Further, by coating this alumina layer, since the zirconia and the sintering aid do not come into direct contact with each other, the above aid is prevented from diffusing into zirconia during sintering, and the tetragonal crystal of zirconia is changed to a monoclinic crystal. The toughness of the sintered body is increased by smoothly performing the phase transition of.

Furthermore, as described above, almost all of the zirconia dispersed in the silicon nitride matrix is capable of phase transition and does not become zirconium nitride, so the toughness of the sintered body is remarkable.

Furthermore, by coating the surface of the silicon nitride powder with a sintering aid layer, the above aid acts evenly on silicon nitride and improves the material strength of the sintered body. The amount of the auxiliary agent is reduced because the auxiliary agent directly contacts the surface of each particle of silicon nitride.

[Example] An example of the method for producing a zirconia-dispersed silicon nitride sintered body of the present invention will be described below for each step with reference to FIG.

A. Preparation of zirconia powder The zirconia powder 2 as a raw material has a strength of 1 μm due to the strength of the sintered body.
The following particle size, preferably 0.5 μm or less, is used. This may be a solid solution of hafnia, which is a family of zirconia. It is known that the toughness is exhibited by the phase transition of zirconia from the tetragonal system to the monoclinic system, and the easiness of this phase transition is known to be influenced by the grain size and the addition of the metal oxide. Therefore, metal oxides such as Y ₂ O ₃ , CeO ₂ , CaO and MgO may be solid-dissolved in zirconia to control this phase transition. The amount of these solid solutions is about 0 to 10 mol%, and the optimum amount depends on the relationship between the particle size of the metal oxide and the particle size of zirconia.

The reaction preventing layer 8 is coated with the reaction preventing layer precursor 7 on the surface of the particles of the zirconia powder 2 as described above. The first function of the reaction prevention layer 8 is to prevent direct contact between zirconia and silicon nitride to prevent these reactions during sintering. The second function is to prevent the sintering aid element of silicon nitride, which will be described later, from diffusing into zirconia. Sintering aid elements generally include Y ₂ O ₃ , CeO ₂ , and MgO.
Etc. are adopted. When these compounds diffuse into zirconia, the phase transition is less likely to occur, and thus the toughness due to this phase transition is suppressed.

Reaction-preventing layer precursor 7 for forming the reaction-preventing phase 8
Must be one that does not form a solid solution in zirconia, for example, Al ₂ O ₃ is adopted. As a method for forming the coating of the reaction preventive layer 8 on the particle surface of the zirconia powder 2, CVD and P
There are gas phase methods such as VD and liquid phase methods such as alkoxide method and precipitation method. Among these, the liquid phase method is promising in view of mass productivity and economical efficiency. Specifically, when the alkoxide method is adopted, after the zirconia powder 2 is dispersed in the non-aqueous solution of the alkoxide, the non-aqueous solution containing water is gradually added to the surface of the zirconia powder 2 for oxidation resistance. Alumina (Al ₂ O ₃ ) or aluminum hydroxide having good characteristics is deposited for coating. When the precipitation method is adopted, for example, one in which zirconia powder 2 is dispersed in an aqueous solution of Al ₂ (NO ₃ ) ₃ and then ammonia, urea, formamide or the like is added to generate a base by thermal decomposition In addition, aluminum hydroxide is deposited on the particle surface of the zirconia powder 2 for coating. Further, a sol other than those using these solution reactions may be used.

The amount of the reaction-preventing layer precursor 7 to be added is appropriately 1 to 15 Wt%. If the amount is less than this range, it is insufficient to form the reaction preventive layer 8. If the amount exceeds this range, the strength of the sintered body, especially the high temperature strength, is not desirable.

B. Preparation of Silicon Nitride Powder Silicon nitride (Si ₃ N ₄ ) powder 1 used as a raw material has an average particle size of 1 μm.
Adopt m or less. A sintering aid layer 10 is formed from the sintering aid precursor 9 on the particle surface of such silicon nitride powder 1. The sintering aid precursor 9 includes, for example, Al ₂ O ₃ , Y ₂ O ₃ ,
Use one or more of alkaline earth oxides and rare earth oxides. The blending amount, that is, the coating amount is set to 3 to 15 Wt%, and if the amount is less than this, it is difficult to densify the sintered body, and if it exceeds this amount, characteristics such as high temperature strength of the sintered body deteriorate. This sintering aid is silicon nitride powder 1
It promotes sintering by forming a liquid phase with silica existing on the surface of the particles. Therefore, by uniformly coating the particle surfaces of the silicon nitride segment 1 with each other, the uniformity of the sintered body as a material can be improved and the compounding amount thereof can be reduced. The reduction of the blending amount of the sintering aid is effective from the viewpoint of improving the high temperature characteristics of the sintered body and also from the viewpoint of preventing the diffusion of the sintering aid into the zirconia.

As a method of forming the coating of the sintering aid phase 10 on the particle surface of the silicon nitride powder 1, as described in Preparation A of Zirconia Powder, the alkosiside method, the precipitation method, the sol and the like can be used.

In general, the step of coating the reaction preventive layer 8 on the particle surface of the zirconia powder 2 and the step of coating the sintering aid layer 10 on the particle surface of the silicon nitride powder are performed. However, when the reaction preventive layer precursor 7 also serves as the sintering aid precursor 9, such as Al ₂ O ₃ , for example, the zirconia powder 2 and the silicon nitride powder 1 are mixed in advance, The coating treatment may be applied simultaneously to both.

C. Mixing / Molding / Sintering Next, the zirconia powder 2 coated with the reaction preventing layer 8 and the silicon nitride powder 1 coated with the sintering aid layer 10 are mixed 4 together.
Specifically, each is dispersed in water or a non-aqueous solvent to form a slurry, which is mixed in a slurry state. Then, this mixed slurry is dried by spray drying or the like to be dry-molded 5. Alternatively, the slurry content may be adjusted, and then the slurry may be directly cast.

After that, the molded body was placed in an inert gas such as nitrogen gas at 1650-
Sinter 6 at 1850 ° C. Sintering is difficult at temperatures lower than this temperature range, and silicon nitride is decomposed at high temperatures. Further, pressure sintering may be performed by a pot press, HIP, or the like.

In order to confirm the effect of the method for manufacturing a zirconia-dispersed silicon nitride sintered body of the present invention as described above, a sintered body was actually manufactured in the following three specific examples and the characteristics thereof were examined.

Specific Example 1. First, a volume ratio of zirconia powder having a specific surface area of 18 m ² / g and an impurity amount of 0.1% or less and silicon nitride powder having an average particle size of 0.5 μm in propanol in which aluminum butoxide is dissolved is 20: Dispersed to 80. Then, a mixed liquid of water and propanol was gradually added thereto, and aluminum hydroxide was used as a reaction-preventing layer and a sintering aid layer on the particle surfaces of the zirconia powder and the silicon nitride powder particles. Each was deposited. The reaction preventive layer and the sintering aid layer are 10 Wt% in terms of alumina. Next, this mixed liquid slurry was filtered and washed, and then dispersed in water. Then, ceria sol was added to this water slurry so as to be 5 Wt% with respect to silicon nitride in terms of ceria, and the mixture was stirred. After that, this slurry was dried with a spray dryer and then CIP molding was performed at 5000 kg / cm.
A pressure of ² was applied to form a plate. This molded body is calcined at 500 ° C in the atmosphere, and alumina and ceria are used as the reaction prevention layer and the sintering aid layer, and then at 1750 ° C in nitrogen gas.
And sintered for 1 hour.

The sintered body thus obtained has a density of 99% or more,
There was no formation of zirconium nitride. The bending strength at room temperature was 800MPa and the fracture toughness was 9MN / m ^3/2, which showed high mechanical properties.

Further, after mixing method of the present invention the _{Si 3 N 4 -ZrO 2 -Al 2} O 3 -CeO 2 sintered body having the same composition in order to compare with the conventional method, the powder raw material in a ball mill, molding, sintering It was manufactured by the conventional method. As a result, almost all of the zirconia became zirconium nitride. And bending strength at room temperature 400MP
a, Fracture toughness showed low mechanical properties of 3.5MN / m ^3/2 .

Specific Example 2. Specific surface area of 18 m ² / g in an aqueous solution of aluminum sulfate and urea
The zirconia powder of is dispersed and heated to 90 ° C. to coat the reaction surface of aluminum hydroxide on the particle surface of the zirconia powder.

Further, silicon nitride having an average particle size of 0.5 μm was dispersed in a magnesium sulfate, yttrium sulfate and urea solution. The amounts of magnesium sulfate and yttrium sulfate were 4 Wt% with respect to silicon nitride in terms of oxide. And
This slurry was heated to 90 ° C. to coat the particles of the above silicon nitride powder with a sintering aid layer of magnesia and yttrium hydroxide.

Next, the zirconia slurry and the silicon nitride slurry were mixed in a volume ratio of 20:80.

Further, this mixed slurry was molded into a plate by sludge casting using a plaster mold.

After that, it was calcined at 500 ° C. in the atmosphere and then sintered at 1750 ° C. for 1 hour in a nitrogen atmosphere.

The sintered body thus obtained had a density of 99% or more, and showed high mechanical properties such as a bending strength at room temperature of 730 MPa and a fracture toughness of 8.5 MN / m ^3/2 .

Further, after mixing method of the present invention the _{Si 3 N 4 -ZrO 2 -Al 2} O 3 -Y 2 O 3 sintered body having the same composition in order to compare with the conventional method, the powder raw material in a ball mill, molding, It was manufactured by the conventional method of sintering. At the time of this sintering, the sintering was performed in a SiO atmosphere in order to prevent the formation of zirconium nitride. as a result,
Although no zirconium nitride was formed, the zirconia did not undergo a phase transition and remained all stabilized in the tetragonal system.
It also showed low mechanical properties such as bending strength at room temperature of 600 MPa and fracture toughness of 4 MN / m ^3/2 .

Specific Example 3. 75:25 by volume of silicon nitride powder and zirconia powder were dispersed in prepanol, and aluminum butoxide corresponding to 10 Wt% of the total in terms of Al ₂ O ₃ was added. A mixed liquid of water and ethanol was added to this to deposit Al ₂ O ₃ on the particle surfaces of the silicon nitride powder and the zirconia powder particles. Then, after it was dried, it was molded into a cylinder by CIP molding. After that, after calcining at 500 ° C. in the atmosphere, this was vacuum-sealed in a Vycor glass tube and HIP sintering was performed at 1750 ° C. for 1 hour.

The sintered body thus obtained had a substantially theoretical density and exhibited high mechanical properties such as a bending strength at room temperature of 850 MPa and a fracture toughness of 7.3 MN / m ^3/2 .

[Effects of the Invention] In summary, according to the present invention, the following excellent effects are exhibited.

(1) A surface of a particle of silicon nitride powder is coated with a sintering aid layer and a surface of a particle of zirconia is coated with an alumina layer, and these are mixed, and then molded into a desired shape and sintered. As a result, in order to prevent the reaction of zirconia and silicon nitride, which adversely affects the material properties of the sintered body, and the diffusion of the sintering aid into the zirconia, the high strength and high strength can be obtained without deteriorating the oxidation resistance. It is possible to obtain a zirconia dispersion strengthened silicon nitride sintered body having high toughness.

(2) Since it is such a tough ceramic material, it can be applied to bearings, wear resistant parts, machine parts and the like used under high temperature, corrosive or wear environments.

[Brief description of drawings]

FIG. 1 is an explanatory view showing a method for manufacturing a zirconia-dispersed silicon nitride sintered body of the present invention, and FIG. 2 is an explanatory view showing a conventional method for manufacturing a zirconia-dispersed silicon nitride sintered body. In the figure, 1 is silicon nitride powder, 2 is zirconia powder, 4 is mixture, 5 is molding, 6 is sintering, 8 is a reaction preventive layer, and 10 is a sintering aid layer.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Hideo Ohashi, Inventor Hideo Ohashi, 3-1-15-1 Toyosu, Koto-ku, Tokyo Ishikawajima Harima Heavy Industries, Ltd. Technical Research Institute (56) Reference JP-A-63-40768 (JP, A) Kai 58-64280 (JP, A) JP 61-91066 (JP, A) JP 62-70262 (JP, A)

Claims (3)

[Claims] 1. A silicon nitride powder having a surface coated with a sintering aid layer for promoting sintering is mixed with zirconia powder,
In a method of shaping this into a desired shape and sintering, the reaction of silicon nitride and zirconia is prevented on the surface of the zirconia powder at the time of performing the above-mentioned sintering, and the diffusion of the sintering aid into zirconia is performed. A method for manufacturing a zirconia-dispersed silicon nitride sintered body, which is characterized by being coated with an alumina layer for preventing 2. The amount of zirconium nitride and zirconium oxynitride produced by the reaction of silicon nitride and zirconia is suppressed to 0.5% by volume or less while dispersing 5 to 40% by volume of zirconia in the silicon nitride matrix. The method for producing a zirconia-dispersed silicon nitride sintered body according to claim 1, wherein the method is as described above. 3. The method for producing a zirconia-dispersed silicon nitride sintered body according to claim 1, wherein the content of the alumina layer is 1 to 15 Wt%.