JPH10158069A - Production of silicon nitride ceramic-base composite material - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a high strength silicon nitride ceramic-base composite material. SOLUTION: A mixture of α-silicon nitride having <=1μm average particle diameter with amorphous silicon nitride is used as starting material, 5-15wt.% sintering aid made of oxide of a rare earth metal is added to the starting material contg. 10-40 pts.wt. amorphous silicon nitride based on 100 pts.wt. of the starting material and the resultant powdery mixture is compacted in a prescribed shape. The resultant compact is heated at 1,550-1,650 deg.C in an inert atmosphere to form a porous sintered compact having 35-65vol.% open porosity and contg. columnar β-silicon nitride crystals each having <=1μm minor axis size and an aspect ratio of >=5. This porous sintered compact is preheated and a molten metal is impregnated into the sintered compact.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for producing a silicon nitride-based ceramic-based composite material, and more particularly, to a method for producing a silicon nitride-based ceramic-based composite material having both high strength and high toughness.

[0002]

2. Description of the Related Art Silicon nitride ceramics have excellent features such as high strength, excellent heat resistance, chemical stability, excellent corrosion resistance, high hardness, excellent wear resistance, low density and light weight. It has been put to practical use as a structural ceramic.

[0003] Since silicon nitride has a strong covalent bond, it is difficult to sinter. Therefore, in sintering silicon nitride, an appropriate oxide is added as a sintering aid for the purpose of promoting diffusion at the grain boundary and reducing interface energy. The sintering aid is silicon nitride and an oxide layer on its surface (SiO ₂ )
At the sintering temperature to form a liquid phase. Generally, silicon nitride ceramics practically used as structural ceramics are dense and high-strength sintered bodies composed of silicon nitride particles and a grain boundary phase in which the liquid phase is solidified.

[0004] However, silicon nitride ceramics exhibit excellent characteristics such as high strength and high hardness because the bonding state of the constituent atoms is a mixture of covalent bonds and ionic bonds, but it is difficult to move atoms due to dislocations. Since the stress concentration is not relaxed due to plastic deformation unlike metal, the fracture toughness is at least one digit smaller than that of metal. For this reason, there is a disadvantage that the strength tends to decrease due to external defects such as machining scratches. In order to solve such a defect, it has been attempted to increase the toughness by making the particles in the silicon nitride sintered body into columnar crystals. In the destruction of silicon nitride ceramics, cracks mainly advance at grain boundaries. Therefore, the fracture toughness of silicon nitride ceramics is affected by the particle shape,
The toughness was improved by growing the grains on the columnar crystals.
However, there is a problem that the grain boundary phase around the grown particles becomes a starting point of fracture and the strength is reduced.

In order to improve the toughness of ceramics, it has been proposed to impregnate porous ceramics with aluminum (see, for example, JP-A-63-40711).

[0006]

However, in such a conventional technique, it is necessary to enhance the impregnation of metal into ceramics in order to obtain a sufficient improvement in toughness. Conversely, when trying to increase the amount of metal impregnation, ceramics have many pores, in this case, the strength of the ceramic side is significantly reduced, and even if impregnated with metal, it is difficult to achieve both strength and toughness. I will. As described above, it has conventionally been difficult to achieve both high levels of strength and toughness in ceramics. An object of the present invention is to provide a method for producing a silicon nitride ceramic-based composite material having improved toughness while maintaining high strength of ceramics.

[0007]

According to the present invention, a starting material is a mixture of α-type silicon nitride and amorphous silicon nitride having an average particle size of 1 μm or less, and 100 parts by weight of the starting material. The mixing ratio of the amorphous silicon nitride in the mixture is 10 to 40 parts by weight, and a sintering aid composed of a rare earth metal oxide is added to the starting material in an amount of 5 to 15% by weight. Molding, (2) the obtained molded body in an inert atmosphere for 15 minutes.
Heating at a temperature of 50 to 1650 ° C. to form a porous sintered body having an open porosity of 35 to 65% by volume, a β-type silicon nitride columnar crystal having a minor axis of 1 μm or less and an aspect ratio of 5 or more;
(3) To provide a method for producing a silicon nitride ceramic-based composite material, comprising preheating the multi-porous sintered body and impregnating with a molten metal.

[0008] The silicon nitride ceramic-based composite material obtained by the method of the present invention has a structure in which the grain boundary phase of a silicon nitride sintered body obtained by a conventional method using a sintering aid is replaced by a metal. . Furthermore, in the method of the present invention, a predetermined amount of amorphous silicon nitride is mixed with α-type silicon nitride as a starting material, so that the columnar crystal of silicon nitride obtained is a silicon nitride sintered body manufactured by a conventional method. The aspect ratio is larger than that of β-type silicon nitride in the inside, that is, longer columnar crystals can be obtained even with the same diameter.

Since the metal has higher toughness than the grain boundary phase of a silicon nitride sintered body obtained by a conventional method using a sintering aid, the silicon nitride ceramic-based composite material obtained by the method of the present invention has Such a metal is continuously present around the columnar crystal of silicon nitride, thereby improving toughness. Further, since the silicon nitride crystal is a columnar crystal having a specific shape with a large aspect ratio and it is not necessary to grow the β-type silicon nitride particles as the strength decreases, the strength is greatly improved.

[0010]

DETAILED DESCRIPTION OF THE INVENTION The method according to the invention is characterized in that a mixture of α-type silicon nitride and amorphous silicon nitride is used as a starting material. In general, silicon nitride has a low-temperature α-type and a high-temperature β-type crystal structure. Amorphous silicon nitride does not have such a crystal structure and has an irregular atomic arrangement. Such amorphous silicon nitride is obtained in a predetermined ratio, that is, 10 to 40 parts by weight of amorphous silicon nitride per 100 parts by weight of the starting material (α-type silicon nitride + amorphous silicon nitride). The strength of the resulting silicon nitride ceramic-based composite material can be improved, and if the mixing ratio of amorphous silicon nitride is out of this range, the strength improving effect is poor.

The raw material silicon nitride powder has an average particle size of 1 μm.
m. If the average particle size of the powder exceeds 1 μm, the short diameter of the columnar silicon nitride crystal obtained by sintering cannot be reduced to 1 μm or less.

Next, a sintering aid is added to the starting material. As the sintering aid, a rare earth metal oxide, for example, yttria (Y ₂ O ₃ ), scandia (Sc ₂ O ₃ ), or the like can be used. The amount of the sintering aid used is 5
~ 15% by weight. If it is less than 5% by weight, the growth of columnar crystals of silicon nitride is not sufficient. I will.

After the mixture of the starting material and the sintering aid is formed into a predetermined shape, sintering is performed in an inert atmosphere. The heating temperature during this sintering is 1550 to 1650 ° C. 1550 ℃
If less than 1,650 g of columnar crystals of silicon nitride are insufficient,
When the temperature exceeds ℃, the columnar crystal of silicon nitride becomes coarse and becomes thicker than 1 μm, and the strength of the obtained composite material decreases.

By this sintering, a porous sintered body having an open porosity of 35 to 65% by volume, a β-type silicon nitride columnar crystal having a minor axis of 1 μm or less, and an aspect ratio of 5 or more is obtained. If the average minor axis exceeds 1 μm, the strength of the obtained composite material will decrease. Further, even if the aspect ratio (ratio of major axis / minor axis) is less than 5, the strength of the obtained composite material is reduced. By setting the open porosity to 10 to 65% by volume, a composite material having satisfactory strength and toughness can be obtained. In particular, the open porosity is preferably set to 50% by volume or more. that is,
This is because a highly reliable composite material having a large Weibull coefficient, that is, a small variation in strength, can be obtained.

It is preferable that the silicon nitride columnar crystals occupy 35 to 65% by volume of the porous silicon nitride sintered body. In the normal molding pressure range (50 to 3000 kgf / cm ² ), 35
It is difficult to form columnar crystals of less than volume%, while it is difficult to produce a sintered body of more than 65 volume% in the temperature range where the high strength of the present invention is exhibited.

The porous silicon nitride sintered body thus obtained is impregnated with a metal by a conventional method to occupy the metal in the open pores. This metal occupies the voids of the porous silicon nitride sintered body and gives the composite material toughness. Accordingly, the metal is not particularly limited, but is preferably a compound which does not form a low toughness compound at the interface thereof, for example, a silicide by reacting with silicon nitride, and examples thereof include pure aluminum, an aluminum alloy, and a copper alloy. You. As a method of impregnating a metal, a high pressure casting (squeeze casting) method is used.
It is preferable to carry out at a pressure of about ton. It is also possible to use the capsule HIP method. In this case, the capsule uses a metal foil having a higher melting point than the metal to be impregnated.

The metal to be impregnated preferably occupies 98% or more of the voids of the porous silicon nitride sintered body. If the proportion occupied by the metal is low, vacancy defects occur in the composite material, and the strength may be reduced.

As a composite material of a metal and a ceramic such as the composite material of the present invention, a metal matrix composite (MMC) has been conventionally known. Although this conventional composite material uses ceramic whiskers as a reinforcing material, the whisker content is limited to 30% in the production of ceramic fiber preforms. The composite material of the present invention uses a columnar crystal of silicon nitride as the equivalent of the whisker, and the content of the columnar crystal can be increased to about 35 to 65%, and therefore, the strength is higher than that of the conventional MMC. And a composite material having a high hardness can be provided.

[0019]

EXAMPLE silicon nitride powder (average particle size 0.2 [mu] m, alpha-conversion rate of almost 100%)
Was mixed with the amorphous silicon nitride powder at the ratio shown in Table 1 below to produce a starting material. Yttrium (Y ₂ O ₃ ) powder (average particle size 0.3 μm, purity
99.9%) were mixed in ethanol in the amounts shown in Table 1 below. This mixing is performed by mixing the silicon nitride powder used in the present example with a very small amount of Y ₂ O ₃ and Al ₂ O ₃ as a sintering aid.
This was performed using a silicon nitride ball mill and balls manufactured by sintering. After drying this mixture, 200k
It is molded under pressure at a pressure of gf / cm ² , and the obtained molded body is packed in a thin rubber bag, vacuum-sealed, molded at a pressure of 3000 kgf / cm ^{2 by} CIP, and a molded body of 50 × 50 × 10 mm is formed. Obtained.

The compact was heated in a furnace in a nitrogen atmosphere at atmospheric pressure under the conditions shown in Table 1. The heating rate was 5 ° C./min, and the holding time at the maximum temperature was 4 hours. The porosity of the porous silicon nitride based sintered body thus obtained and the average minor axis and aspect ratio of β silicon nitride columnar crystals were determined. The porosity is n
The relative density obtained by dividing the bulk density of the porous silicon nitride sintered body obtained by the Archimedes method using -butyl alcohol by the theoretical density was subtracted from 100%. The average minor axis and aspect ratio were determined by separating columnar crystals in the porous silicon nitride sintered body one by one in heated hydrogen fluoride and observing them with a microscope. Table 1 shows the results.

The above porous silicon nitride sintered body was put in a mold, and an aluminum alloy (JIS-AC1A) melted at 800 ° C.
The porous silicon nitride sintered body was impregnated with a pressure of 1000 kgf / cm ² . A test piece was cut out from the composite material obtained in this way, processed according to JIS R-1601, and then subjected to a four-point bending strength at room temperature.
Then, after processing according to JIS R-1607, the fracture toughness value was measured. Table 1 shows the results.

Comparative Example In the same manner as in the above example, molding, firing, and metal impregnation were performed under the conditions shown in Table 1, and the results of each evaluation are shown in Table 1.

[0023]

[Table 1]

FIG. 1 shows the results of the strength of a composite material manufactured using only a mixture of α-type silicon nitride and amorphous silicon nitride as a starting material and only α-type silicon nitride. As is clear from Table 1 and FIG. 1, the use of a material in which a predetermined amount of amorphous silicon nitride is mixed with α-type silicon nitride makes it possible to obtain a material in comparison with the case where only α-type silicon nitride is used. The strength of the composite material has greatly improved. The silicon nitride sintered body obtained by the method of the present invention is a polycrystalline body composed of columnar crystals of elongated silicon nitride, and there are open voids between the crystals. This silicon nitride columnar crystal has an average minor axis of 1 μm or less and an aspect ratio of 5 or more. Further, the composite material obtained by the method of the present invention has a structure in which aluminum alloy occupies continuous grain boundaries of the crystal.
The composite material obtained by the method of the present invention has both high strength and high toughness.

Conventionally, silicon nitride having a strong covalent bond and an aluminum alloy having a metal bond are hard to wet, and it is not considered that this aluminum alloy completely impregnates into fine pores such as a porous silicon nitride. Was. However, in the composite material obtained by the method of the present invention, the pores around the silicon nitride crystal were well impregnated with the aluminum alloy. The relative density measured by the Archimedes method was 99.8%, indicating that the aluminum alloy was densely impregnated. Furthermore, in the composite material obtained by the method of the present invention, the silicon nitride and the aluminum alloy were well wet and adhered.

[0026]

According to the method of the present invention, a composite having a structure in which the grain boundary phase (usually a sintering aid and vitreous silica) of a silicon nitride sintered body obtained by a conventional method is replaced with a metal. The material is obtained. The silicon nitride sintered body in this composite material is formed from a β-type silicon nitride columnar crystal having a large aspect ratio by using a mixture of α-type silicon nitride and amorphous silicon nitride as a starting material, and the strength is improved. . The composite material obtained by impregnating a metal into the silicon nitride sintered body has an improved toughness because a metal with high toughness is continuously present around the columnar crystal of silicon nitride. In addition, it is not necessary to grow silicon nitride particles more than necessary in order to increase toughness, and a decrease in strength can be prevented.

Further, the composite material obtained by the method of the present invention has a higher coefficient of thermal expansion than silicon nitride ceramics because it contains a continuous metal phase. Therefore, when used in combination with a metal material, it is possible to prevent rattling of the fitting due to the difference between the assembling temperature and the use temperature as in the case of using silicon nitride ceramics.

Further, the composite material obtained by the method of the present invention has a higher thermal conductivity than silicon nitride ceramics because it contains a continuous metal phase. Therefore, for example, when used in a reciprocating engine, it is possible to prevent the cooling effect from being lowered as in the case of using silicon nitride ceramics.

[Brief description of the drawings]

FIG. 1 is a graph showing the strength of a composite material manufactured using only a mixture of α-type silicon nitride and amorphous silicon nitride as a starting material and α-type silicon nitride.

Claims (1)

[Claims] The following steps (1) A mixture of α-type silicon nitride and amorphous silicon nitride having an average particle size of 1 μm or less is used as a starting material, and 100 parts by weight of the starting material is mixed with amorphous silicon nitride. The mixing ratio is 10 to 40 parts by weight, and a sintering aid comprising a rare earth metal oxide is added to the starting material in an amount of 5 to 15% by weight, and the mixed powder is formed into a predetermined shape. The formed body is heated in an inert atmosphere at a temperature of 1550 to 1650 ° C., and has an open porosity of 35 to 65% by volume, a β-type silicon nitride columnar crystal having a minor axis of 1 μm or less and an aspect ratio of 5 or more. (3) A method for producing a silicon nitride ceramic-based composite material, comprising: preheating the multi-porous sintered body and impregnating with a molten metal.