JPH1067561A - Silicon nitride fired product - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a silicon nitride fired product which has reduced surface roughness and alteration layers and is suitable for sliding members by placing a sintering additive of the same components as those in the molded products in a sintering furnace, when a molded product composed of silicon nitride and the sintering additive is fired. SOLUTION: When a molded product comprising a mixed powder of silicon nitride and a firing additive is fired, a firing additive having the same formulation as that of the firing additive included in the molded product is placed in the sintering furnace and fired in an atmosphere containing the sintering additive of the same components at 1,600-1,950 deg.C for 4-15 hours thereby producing sintered silicon nitride ceramic. The silicon nitride to be used in sintering may be produced from any process, but has preferably fine particle sizes, high α-phase content, and is prepared by the thermal decomposition of imide. As a firing additive, oxides and/or nitrides of Y, Al, Mg, Sc, La, Ce, Be, Zr are used, but a combination of Y2 O3 with AL2 O3 is particularly preferred. In the production of the molded products, the addition of a polyhydric alcohol fatty acid ester increases the fluidity of the green mix and is advantageous in the molding. The powders of the raw materials are preferably granulated by using a binder such as polvinyl alcohol or the like.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to the reduction of a surface altered layer in a ceramic sintering step, and more particularly, to an extremely small surface altered layer in a sintering step. Firing conditions,
Regarding firing atmosphere.

[0002]

2. Description of the Related Art Since sintering generally proceeds from the surface layer of a molded body, sintering is completed in the surface layer of the molded body while sintering is still in progress inside the molded body, and pores remain in the molded body. The deteriorated layer or the fired body may be decomposed in the surface layer portion of the formed body. For this reason, conventionally, by firing in a powder having the same composition as the compact and firing in a crucible made of silicon nitride, decomposition of the fired body and evaporation of the sintering aid have been performed. (See FIG. 16).

However, in a ceramic sliding member, the sliding contact surface is important, and the contact surface has pores (poor surface roughness) or a deteriorated layer, which causes peeling,
It is considered to cause vibration, seizure, and destruction.
Therefore, in the production of a sliding member, it is necessary to avoid the occurrence of surface roughness and a deteriorated layer on the contact surface as much as possible. However, the conventional method does not provide a sliding member having sufficient sliding characteristics. . For this reason, it is necessary to perform a step of removing the altered layer after firing, and the thickness of the molded body must be designed in consideration of the removed portion.

[0004] On the other hand, even in dry pressure molding, if densification due to deformation and destruction of the silicon nitride forming granules is not sufficiently performed, pores between the granules remain in the formed body, and the fired body has strength, Physical properties such as density may be inferior to physical properties of a fired body obtained by injection molding or casting. Further, in the case of dry pressure molding, there is also a problem that cracks are liable to occur in the molded article due to a large friction between the molded article and the mold at the time of release. Therefore, as a binder for molding silicon nitride, a binder obtained by using a binder such as polyvinyl alcohol (PVA), an acrylic resin, or a wax in combination with a plasticizer such as polyethylene glycol (PEG) or dibutyl phthalic acid is used. It has been attempted to improve the moldability. However, even if such a binder is used, a high molding pressure is required in order to obtain a molded article having a uniform density, and there is a certain limit in the reduction of pores. In addition, in the silicon nitride sliding member, not only the presence of the altered layer on the surface of the member described above, but also the remaining fine intergranular pores have a large adverse effect on the sliding characteristics, so that the intergranular pores are reduced. This is also an important issue.

[0005]

SUMMARY OF THE INVENTION An object of the present invention is to provide firing conditions under which a fired body having excellent sliding characteristics can be obtained.

[0006]

SUMMARY OF THE INVENTION The present invention relates to a fired silicon nitride product obtained by firing a molded product comprising a mixed powder composed of silicon nitride and a sintering aid. In an atmosphere containing a sintering aid of the same component as the sintering aid,
The present invention provides a silicon nitride fired body fired at 600 to 1,950 ° C. for 4 to 15 hours.

[0007]

BEST MODE FOR CARRYING OUT THE INVENTION A compact to which the method for producing a sintered compact according to the present invention can be applied is a compact obtained by molding a mixed powder comprising silicon nitride powder and a sintering aid using an organic binder. Silicon nitride (Si ₃ N ₄ ) used in the present invention
May be produced by any method such as imide pyrolysis method, direct siliconizing method of metal silicon, reducing siliconizing method of silica, gas phase reaction method, etc.
In view of the high phase content, silicon nitride by imide pyrolysis is preferred.

A preferred specific surface area of the nitride silicon powder used in the present invention is preferably 5.0~15.0m ² / g, especially 9.0~13.0m ² / g is more preferable. If the specific surface area is less than 5.0 m ² / g, the sinterability may be reduced or the filling property of the powder may be deteriorated depending on the particle size distribution. On the other hand, if the specific surface area exceeds 15.0 m ² / g, the powder In some cases, the moldability and the filling property of the molded article are reduced, the handling of the molded article is deteriorated, and the cost may increase. Examples of an apparatus for obtaining a preferable specific surface area include a ball mill, a stirring mill, and a jet mill, and any of wet grinding and dry grinding may be used. Ethyl alcohol, methyl alcohol, and other organic solvents are used as a medium for wet grinding.

In the present invention, a mixed powder obtained by adding a sintering aid to a silicon nitride powder is used as a sintering raw material. As sintering aids, Y, Al, Mg, Sc, La, Ce, B
An oxide or nitride of e and Zr can be used. Among them, Y ₂ O ₃ , Al ₂ O ₃ , SiO ₂ , MgO
And these can be used alone or in combination. In the present invention, Y ₂ O ₃ and Al ₂ O
The combination of ₃ is most preferred.

The specific surface area of the sintering aid is 3 to 50 m ² /
g, particularly preferably 7 to ²⁰ m ² / g. The pulverization for obtaining such an average particle size is performed by a ball mill, a stirring mill, a jet mill, or the like, and may be any of wet pulverization and dry pulverization.

The proportion of the sintering aid in the mixed powder is preferably 0.5 to 13.0% by weight, more preferably 3.5 to 13.0% by weight.
8.0% by weight is more preferred. If the proportion of the sintering aid is less than 0.5% by weight, the amount of liquid phase generated is small and liquid phase sintering is not performed, so that the discharge of pores between particles does not proceed.
On the other hand, if it exceeds 13.0% by weight, sintering tends to proceed, but since the amount of the liquid phase is large, high-temperature characteristics are reduced and voids are more likely to be generated than in the liquid phase, resulting in impaired mechanical characteristics. . In particular, when Y ₂ O ₃ is used as a sintering aid, _{2.5 to} 5.0% by weight of the mixed powder is preferable.
When Al ₂ O ₃ is used, 0.5 to 5.0% by weight
Is preferred. When Y ₂ O ₃ and Al ₂ O ₃ are used in combination, the ratio of the added amount of Y ₂ O ₃ to Al ₂ O ₃ is preferably 5: 1 to 1: 1 by weight.

A powder mixture of silicon nitride and a sintering aid (hereinafter, also simply referred to as a "mixed powder") is mixed with an organic binder to form granules for molding (hereinafter, "granules for molding silicon nitride"). Granulated. The organic binder to be used can be appropriately selected, for example, polyvinyl alcohol, polyacrylate, liquid paraffin,
Polyvinyl butyral, carboxymethyl cellulose,
Examples include methyl cellulose, polyvinyl acetate, and polyacrylamide copolymer.

[0013] The amount of the binder to be added is 0.5 to 100 parts by weight of the mixed powder in terms of solid content or active ingredient.
The amount is preferably 20.0 parts by weight, more preferably 2.0 to 11.0 parts by weight. When the binder is polyvinyl alcohol, 0.1 parts by weight based on 100 parts by weight of the mixed powder.
To 4 parts by weight, and more preferably 0.4 to 2 parts by weight. When the amount of polyvinyl alcohol is less than 0.1 part by weight, the strength of the green molded body is reduced and the handling property is deteriorated. On the other hand, when the amount exceeds 4 parts by weight, the granules become too hard and a large amount of granules is contained in the molded body. Because the pores remain,
The physical properties of the fired body decrease.

At the time of granulation, a plasticizer, a lubricant, a dispersant, a wetting agent, an antifoaming agent and the like can be added to the mixed powder in addition to the binder.

[0015] Examples of the plasticizer include polyethylene glycol, glycerin, dibutyl phthalate and the like. Among them, polyethylene glycol is preferable. The amount of the plasticizer added is 0.01 to 100 parts by weight of the mixed powder.
~ 15.0 parts by weight are preferred. Among them, when the plasticizer is polyethylene glycol, the amount is preferably 0.1 to 9 parts by weight, particularly preferably 0.6 to 7 parts by weight. If the polyethylene glycol is less than 0.1 part by weight, the granules become too hard and a large amount of intergranular pores remain in the molded body, so that the physical properties of the fired body are reduced. , The fluidity of the granules decreases, the packing density becomes uneven,
Further, the strength of the green molded body is reduced, and the handling property is deteriorated.

As the lubricant, any of a hydrocarbon-based lubricant and a fatty acid-based lubricant can be used. That is, stearic acid, lauric acid, metal salts thereof, fatty acid amides, fatty acid esters, and liquid lubricants such as liquid paraffin, paraffin wax, and chlorinated hydrocarbons may be used as fatty acid-based lubricants. it can. Among them, polyhydric alcohol fatty acid esters are most suitable as the lubricant. The polyhydric alcohol fatty acid ester used in the present invention is preferably such that the polyhydric alcohol is an anhydrosorbite such as sorbitan, sorbite, and sorbite, and particularly preferably sorbitan. As the fatty acid, higher fatty acids such as lauric acid, oleic acid, palmitic acid and stearic acid are preferable, and lauric acid and oleic acid are more preferable.

When the polyhydric alcohol fatty acid ester is added to the mixed powder, the fluidity of the obtained granules is improved, and a molded article without pores between granules can be obtained even at a relatively low molding pressure. In addition, when the mold is released after molding, the frictional force between the molded body and the mold is reduced, so that the influence of springback is small, and as a result, surface distortion that causes cracks does not occur.

The amount of the lubricant is preferably 0.1 to 15.0 parts by weight based on 100 parts by weight of the mixed powder. When the lubricant is a polyhydric alcohol fatty acid ester, the amount is preferably 0.5 to 10 parts by weight, more preferably 1 to 6 parts by weight, based on 100 parts by weight of the mixed powder. When the polyhydric alcohol fatty acid ester is less than 0.5 parts by weight,
The frictional force between the ceramic molding granules and the mold becomes large, cracks are easily generated in the molded body at the time of mold release, and a large amount of intergranular pores remain in the molded body, resulting in low physical properties of the fired body. On the other hand, if it exceeds 10 parts by weight, the fluidity of the granules for ceramic molding decreases, the packing density becomes non-uniform, the strength of the green molding decreases, and the handling of the molding deteriorates.

Examples of the granulation method include a spray granulation method, a stirring granulation method, a tumbling granulation method, a fluidized granulation method and the like, and may be appropriately selected according to the purpose.

The average particle size of the silicon nitride granules used in the present invention is preferably from 10 to 100 μm, particularly preferably from 40 to 100 μm.
80 μm is more preferred. If the average particle size is less than 10 μm, it becomes difficult to obtain desired fluidity, while
When m exceeds m, defects such as depression are likely to occur in the granules.

The silicon nitride forming granules can be formed into a compact by molding by pressure molding. The method of pressure molding is not limited, and uniaxial press molding, cold isostatic press molding (dry, wet), warm isostatic press molding, and the like are applied, but are appropriately selected according to the purpose. be able to. Further, after preforming by a uniaxial press forming method, secondary forming may be performed by a cold isostatic press, a warm isostatic press or the like.

The molding pressure is preferably 3~5t / cm ^2, particularly 3~4t / cm ² is more preferable. If the molding pressure is less than 3 t / cm ² , pores between granules are not sufficiently eliminated, while if it exceeds 5 t / cm ² , springback increases and cracks are liable to occur. When performing the preforming, 0.1~3t / cm ^2, then preferably subjected to molding at ^{0.5~1t / cm 2, 3~5t / cm} 2 the secondary molding, preferably 3 to It is preferably performed at 4 t / cm ² .

The degreasing step before firing is not particularly limited, and may be performed according to a conventional method. The degreasing temperature and the degreasing time may be appropriately selected.
The temperature is preferably from 00 to 700C, more preferably from 600 to 700C. The degreasing time is preferably 0.5 to 10 hours, particularly preferably 1 to 5 hours.

The subsequent firing step is performed in an atmosphere containing the same sintering aid as the sintering aid used for the compact.
Here, the atmosphere containing the sintering aid means that the sintering aid is
An atmosphere that exists in the firing furnace in any of a gas phase, a liquid phase, and a solid phase. In addition, the sintering aid referred to herein is a sintering aid derived from a source other than the molded body, and does not include the sintering aid originally included in the molded body. Therefore,
In the present invention, in order to make the inside of the firing furnace an atmosphere containing a sintering aid, a sintering aid having the same component as that used for the molded article (hereinafter referred to as a “sintering aid for the firing atmosphere” is provided in the firing furnace. ") And fire. For example, when sintering a compact made of a mixed powder using Y ₂ O ₃ and Al ₂ O ₃ as a sintering aid, Y ₂ O ₃ and / or Al ₂ O ₃ are placed in a furnace.
Place O ₃ and fire. Among them, Y ₂ O ₃ and Al
It is most preferable to provide ₂ O ₃ at a ratio contained in the molded body. At this time, as the firing crucible, a crucible made of silicon nitride, which is the same material as the raw material powder, is preferable from the viewpoint of preventing carbon from being mixed in from the heater.

The total installation of the firing atmosphere for the sintering aid, 0.01 g / cm ³ or more preferably against crucible volume, particularly preferably 0.04~0.50g / cm ^3. The shape of the sintering aid for the sintering atmosphere is not particularly limited, but is preferably powder because it is easily released by the sintering atmosphere.

By the way, in the step of firing a molded body using a mixed powder comprising silicon nitride and a sintering aid, rearrangement of silicon nitride particles occurs because the sintering aid generates a liquid phase, and The silicon nitride particles dissolve-precipitate in the liquid phase and change from α-silicon nitride to β-silicon nitride,
This involves the surrounding α-silicon nitride during grain growth and further grain growth to cause secondary rearrangement, and secondary arrangement does not progress due to entanglement of β-silicon nitride, so that the final A structure is formed and firing is completed. The pores existing in the molded body move in the course of such sintering, in particular, in the process of rearrangement / dissolution-precipitation of silicon nitride, and move out of the liquid phase of the sintering aid and are discharged to the outside. (See FIG. 15).

The significance of firing in an atmosphere containing a sintering aid in the present invention is that during firing, the sintering aid is constantly supplied to the compact from outside the compact.
Rearrangement and dissolution of silicon nitride in the surface layer of the compact
Prolong the time of the precipitation process, as a result, increase the amount of α-silicon nitride converted to β-silicon nitride and precipitate, and keep the time for the pores inside the molded body to be discharged sufficiently long. Another object of the present invention is to further advance the densification of a molded body.

Therefore, the sintering temperature is not only the temperature at which the sintering aid originally contained in the compact produces a liquid phase, but also the temperature at which the sintering aid installed in the furnace is supplied to the compact, for example, It must be above the vaporization temperature of the auxiliaries. For this reason, in this invention, baking of a molded object is performed at 1,600-1950 degreeC. Among them, sintering aids are Y ₂ O ₃ and Al
_In the case of ₂ O ₃ , 1,750 to 1,850 ° C. is preferable.

The sintering time is preferably 2 to 15 hours, and more preferably 4 to 1 hour, in order to perform sintering completely because the time required for the rearrangement / dissolution-precipitation process of silicon nitride is long.
0 hours is more preferred. If the sintering time is less than 2 hours, the sintering is insufficient and the density of the sintering body is reduced, resulting in a decrease in strength. If the sintering time exceeds 15 hours, grain growth proceeds more than necessary, and the strength of the sintering body is reduced. And the fired body surface layer may be decomposed. Further, when the maximum thickness of the molded body is large, it is preferable to select the firing time in consideration of the maximum thickness, such as setting the firing time longer.

The firing is preferably performed in an atmosphere of an inert gas such as nitrogen gas or argon gas in order to prevent the decomposition reaction of silicon nitride.

The atmospheric pressure is preferably 9 to 10 kg / cm ² when the firing furnace is a GPS furnace, and is 1 when the firing furnace is a HIP furnace.
It is more preferably from 1,000 to 1,000 kg / cm ² . The atmosphere pressure here refers to the atmosphere pressure when the inside of the furnace reaches the firing temperature. If the density of the molded body before degreasing is high, the furnace temperature is increased to some extent,
It is preferable to start pressurizing after reaching 00 ° C.

Since the silicon nitride fired body thus obtained is fired for a long time while the sintering aid is constantly supplied, the pores inside the molded body are sufficiently discharged, and the pore area ratio is reduced. It is small, and the thickness of the altered layer on the surface of the fired body is considerably small. For example, in the fired silicon nitride body of the present invention, the thickness of the surface altered layer can be set to 0.05 mm or less. In addition, since the sintering aid generates a liquid phase and the silicon nitride powder rearranges and prolongs the time for dissolution and precipitation, the amount of silicon nitride that undergoes a phase transition from the α phase to the β phase increases, and β- Densification by entanglement of silicon nitride has progressed further, resulting in a high-strength fired body.

Further, when the molded body is composed of granules containing a lubricant, the number of pores originally contained in the molded body is small, so that the supply of the sintering aid during firing further reduces the pores. Particularly, it is a silicon nitride fired body having excellent strength and hardness.

[0034]

The present invention will be described below with reference to examples. Examples 1 to 3 (1) Preparation of granules As raw material powder, Si ₃ having a specific surface area of 10 ± 1 m ² / g
92 parts by weight of N ₄ [imide decomposition method, manufactured by Ube Industries, Ltd., E-10], 5 parts by weight of Y ₂ O ₃ [manufactured by Nippon Yttrium Co., Ltd.], Al ₂ O ₃ [Sumitomo Chemical Industries, Ltd. ), AKP
-30] was weighed at 3 parts by weight, and 51 parts by weight of water was added thereto.
0.4 part by weight of a dispersant was added and mixed by a ball mill for 64 hours. To this mixture, 2 parts by weight of polyvinyl alcohol and 3 parts by weight of oleic acid ester of sorbitan were added in terms of active ingredients to obtain a slurry-like turbid liquid. It was then spray-dried with a spray drier, subjected to mesh pass (150 [mu] m), an average particle size of 80 [mu] m Si ₃
It was obtained N ₄ molding granules.

[0035] (2) a uniaxial press molding a Si ₃ N ₄ sintered bodies created Si ₃ N ₄ forming a granulate 1t / cm ² and (Kawada formula), after preformed into a cylindrical diameter 5 × height 15 mm, Cold isostatic pressing was performed at 4 t / cm ² , secondary molding was performed, and this was held in the air at 600 ° C. for 1 hour to degrease the molded body. 160 g of powdery yttrium oxide and aluminum oxide 160
g was placed in a crucible having a capacity of 1,800 cm ³ (FIG. 1), and fired in nitrogen gas at a firing rate of 6 ° C./min.
Firing at 00 ° C. for 4 hours (Example 1), 1,800 ° C. for 10 hours (Example 2), and 1,850 ° C. for 4 hours (Example 3) (hereinafter referred to as “auxiliary system”) Atmosphere)) to obtain a sintered body of Si ₃ N ₄ . At this time, the pressure of the atmosphere was started when the temperature in the furnace reached 800 ° C., and was maintained at 9 kg / cm ² during firing. Example 1 and Example 2
FIG. 2 shows the firing pattern of.

As shown in FIG. 3, the obtained fired body was cut in the vertical and horizontal directions and mirror-finished, and from the cross section, the pore area ratio near the surface layer and the pore area ratio at the center were evaluated. Further, the fired bodies of Examples 1 and 2 were cut out in the vertical direction to prepare test pieces for measuring Vickers hardness, and Vickers hardness was measured. The results are shown in FIGS. In addition, an enlarged photograph of a cross section near the surface layer of Example 2 is shown in Reference FIG. 1, and scanning electron microscope photographs of a cross section near the surface layer and in the center are shown in FIGS.

The pore area ratio and Vickers hardness were measured as follows. It was calculated by analyzing an image of the cut surface of the pore area ratio by an optical microscope image. Vickers hardness (Hv) With respect to the Vickers hardness measurement TP, the Vickers hardness was measured at each measurement point shown in FIG. 4, and the average value was calculated. The load was 500 gf and the pushing time was 15
Seconds.

[0038] As Comparative Example 1 to 3 Si ₃ N ₄ powder, the specific surface area; Si ₃ N ₄ [imide decomposition method 10 ± 1m ² / g, Ube Industries, Ltd., E-1
0], and baked in a silicon nitride crucible without Y ₂ O ₃ and Al ₂ O ₃ (hereinafter referred to as “standard atmosphere”) at 1,800 ° C. in the same manner as in Example 1. 4
Time (Comparative Example 1), fired at 1,800 ° C. for 10 hours (Comparative Example 2), and fired at 1,850 ° C. for 4 hours (Comparative Example 3) to produce a silicon nitride fired body, pore area ratio and Vickers The hardness was measured. The results are shown in FIGS. In addition, FIG. 2 shows an enlarged photograph of a cross section near the surface layer of Comparative Example 1, and FIGS. 9 to 10 show scanning electron micrographs of a cross section near the surface layer and the center of the fired body of Comparative Example 1.

[0039]

[Table 1]

According to FIGS. 1 and 2, the thickness of the deteriorated layer of the fired body of Comparative Example 1 is 0.4 to 0.5 mm, whereas the thickness of the deteriorated layer of the fired body of Example 2 is slightly 0.02-
It turned out that it is very thin, about 0.03 mm.

According to FIGS. 5 and 6, the silicon nitride fired bodies of Examples 1 and 2 were fired in an auxiliary agent atmosphere in any of the cutting direction (vertical and horizontal) and the measurement position (surface layer, center). It was found that the fired body obtained had a smaller pore area ratio than that fired in a standard atmosphere. Among them, it was found that those fired at 1,800 ° C. for 10 hours in an auxiliary atmosphere had the smallest pore area ratio. In addition, it was found that there was a tendency that the pore area ratio was larger in the central portion than in the surface layer at the measurement site.

Further, according to Table 1, it was found that the Vickers hardness did not decrease even when firing in an auxiliary system atmosphere. No difference in hardness was observed between the surface layer and the center.

7 to 10, it was found that the fired body of Example 2 (FIGS. 7 and 8) having the minimum pore area ratio had a fired body structure in which β-type columnar particles were entangled. This structure has a dense liquid phase due to the prolonged sintering time of 10 hours, while the auxiliary agent is constantly supplied in the auxiliary system atmosphere, and the liquid phase generation time has been prolonged. It is thought that it became. On the other hand, in the fired body of Comparative Example 1 (FIGS. 9 to 10), it was found that the β phase transition of α-silicon nitride did not progress.

Examples 4 to 7 Except that the primary molding was performed by hand pressing at 350 kg / cm ² , the fired body produced in the same manner as in Example 1 was subjected to the same procedure as in Example 1 in terms of Vickers hardness and bending strength. Was measured, and the composition was analyzed by EPMA analysis and fluorescent X-ray quantitative analysis. The results are shown in Tables 2 and 3. In addition,
In Examples 6 and 7, granules for molding silicon nitride without adding oleic acid ester of sorbitan were used. Calcination was performed at 1,800 ° C. for 4 hours (Examples 4, 6),
This was performed at 00 ° C. for 10 hours (Examples 5 and 7). FIG. 11 is a scanning electron micrograph of a cross section at the center of Examples 6 and 7.
~ 12. The Weibull coefficient is JIS-R16
The three-point bending strength was measured in accordance with No. 01, and determined from its Weibull distribution.

Comparative Examples 4 to 7 In the same manner as in Example 6, except that the sintering was performed in a crucible in a standard atmosphere without Y ₂ O ₃ and Al ₂ O ₃ , 1,80.
By firing at 0 ° C. for 4 hours (Comparative Examples 4 and 6) and at 1,800 ° C. for 10 hours (Comparative Examples 5 and 7), a silicon nitride fired body was prepared, and Vickers hardness and Weibull coefficient were measured. The composition was analyzed by EPMA analysis and quantitative X-ray fluorescence analysis. The results are shown in Tables 2 and 3. Comparative Examples 6 to
7 to 14 show scanning electron micrographs of a central section of FIG.
Shown in

[0046]

[Table 2]

[0047]

[Table 3]

According to Table 2, the fired body fired in the atmosphere containing the sintering aid has a higher sintering aid than the fired body fired in the standard atmosphere containing no sintering aid. It was found that the agent did not decrease much. Therefore,
It has been found that the constant supply of the sintering aid from the atmosphere is effective in preventing the reduction of the sintering aid and making the sintering agent dense.

From Table 3, it was found that the addition of the lubricant was superior to the addition of no lubricant in both the strength and the Weibull value. Above all, the firing time was 4 at 1,800 ° C.
It was found that the baking at 1,800 ° C. for 10 hours and the baking atmosphere improved the strength and Weibull value more in the auxiliary atmosphere than in the standard atmosphere.

According to FIGS. 11 to 14, the fired body of Comparative Example 6 (FIG. 11) which was fired at 1,800 ° C. for 4 hours in a standard atmosphere without adding a lubricant was added without adding a lubricant. Compared with the fired body of Example 7 (FIG. 12) fired at 1,800 ° C. for 10 hours in an agent-based atmosphere, the phase transition to β-silicon nitride did not progress, and thus the powder was not densified. The strength was found to be low. Further, the fired body of Example 5 (FIG. 13)
Compared with the fired body of Comparative Example 4 (FIG. 14), it was found that the phase transition to β-silicon nitride was advanced, the amount of columnar particles was increased, the density was increased, and the strength was high.

[0051]

The fired silicon nitride of the present invention is fired for a long time while continuously supplying the sintering aid, so that pores inside the formed body are sufficiently discharged to the outside through the liquid phase. As a result, the pore area ratio is reduced, and the number of altered layers on the surface of the fired body can be reduced. Also, the sintering aid produces a liquid phase,
Silicon nitride powder, rearrangement-because the time to dissolve and precipitate was extended, the silicon nitride that phase transition from the α phase to the β phase increased, the densification by the entanglement of β-silicon nitride further advanced, It is a high strength fired body.

Further, when the compact is composed of granules containing a lubricant, the number of pores originally contained in the compact is small. Therefore, the supply of the sintering aid during firing further reduces the pores. In particular, a fired silicon nitride body having excellent strength and hardness can be obtained.

[Brief description of the drawings]

FIG. 1 is a schematic sectional view showing a state in a crucible at the time of firing in Example 1.

FIGS. 2A and 2B are charts showing firing patterns according to an example of the present invention, in which FIG. 2A shows the firing pattern of Example 1, and FIG. 2B shows the firing pattern of Example 2. FIG.

FIG. 3 is a perspective view showing a cut surface of the fired body of Example 1 in a vertical direction and a horizontal direction.

4A and 4B are diagrams showing measurement points of Vickers hardness in the test piece of Example 1, (a) is a perspective view of a measurement cross section, and (b) is a front view.

FIG. 5 is a chart showing a pore area ratio in a longitudinal section of the fired bodies of Examples 1 to 3.

FIG. 6 is a chart showing a pore area ratio in a cross section in a lateral direction of the fired bodies of Examples 1 to 3.

FIG. 7 is a scanning electron micrograph of a cross section near the surface layer of the fired body of Example 2.

FIG. 8 is a scanning electron micrograph of a cross section of a central portion of a fired body of Example 2.

FIG. 9 is a scanning electron micrograph of a cross section near the surface layer of the fired body of Comparative Example 1.

FIG. 10 is a scanning electron micrograph of a cross section of a central portion of a fired body of Comparative Example 1.

FIG. 11 is a scanning electron micrograph of a cross section of a central portion of a fired body of Comparative Example 6.

FIG. 12 is a scanning electron micrograph of a cross section of a central portion of a fired body of Example 7.

FIG. 13 is a scanning electron micrograph of a cross section of a central portion of a fired body of Example 5.

FIG. 14 is a scanning electron micrograph of a cross section of a central portion of a fired body of Comparative Example 4.

FIG. 15 is a schematic diagram of a pore release model in a dissolution-precipitation process of silicon nitride during firing.

FIG. 16 is a schematic cross-sectional view showing a state in a crucible at the time of baking with a conventional filler.

Claims (2)

[Claims] 1. A fired silicon nitride body obtained by firing a molded body comprising a mixed powder composed of silicon nitride and a sintering aid, wherein the sintered body has the same components as the sintering aid used in the molded body. In an atmosphere containing a sintering aid, 4 at 1,600 to 1,950 ° C
A silicon nitride fired body fired for up to 15 hours. 2. The silicon nitride fired body according to claim 1, wherein the molded body contains a polyhydric alcohol fatty acid ester as a lubricant.