JPH09227236A - Silicon nitride-base sintered compact and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a sintered compact having slight unevenness in mechanical strength, a cumulative breaking probability of 1/100,000 and high guaranteed strength and excellent in reliability by incorporating a specified amt. of a specified compd. SOLUTION: This sintered compact contains 0.5-9vol.% FeSi particles of 5-30μm average particle diameter. It is obtd. by compacting a powdery mixture consisting of 1-8wt.% one or more kinds of oxides of rare earth elements, 1-8wt.% Mg(OH)2 , 0.5-10wt.% Fe, 0.1-3wt.% TiSi2 and the balance Si3 N4 with inevitably contained SiO2 and sintering the resultant compact at 1,550-1,650 deg.C in an atmosphere contg. gaseous N2 . The Si3 N4 in the mixture is Si3 N4 powder having α- and/or β-crystal structure, preferably fine particles of <=5μm average particle diameter.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a silicon nitride sintered body having a small variation in mechanical strength and excellent reliability, and a method for producing the same.

[0002]

2. Description of the Related Art Silicon nitride has a strong covalent bond, heat resistance,
Since it has excellent strength, corrosion resistance, and abrasion resistance, and has a small coefficient of thermal expansion and excellent thermal shock resistance, it is being applied mainly to mechanical parts that require mechanical strength characteristics.

For the purpose of further enhancing such excellent characteristics of silicon nitride, compounding using silicon nitride as a matrix has been studied.

In Japanese Patent Application Laid-Open No. 56-32377, Ti is
The silicon nitride sintered material containing the above-mentioned carbide, nitride and carbonitride has been tried, and it is known that the wear resistance is improved in addition to the thermal shock resistance and the high temperature strength.
In the publication, in addition to the sintering aid of MgO + SiO ₂ ,
Attempts have been made to add carbides and nitrides of i, V, Cr, Zr, etc., and improvements in wear resistance have been reported.

Further, in Japanese Unexamined Patent Publication No. 3-199166, silicon nitride in which hard compound particles represented by nitrides, carbides, silicides and borides of IVa, Va and VIa group elements are dispersed in the sintered body. It has been reported that the toughness of the high quality sintered body is improved, and JP-A-3-290369 discloses that the strength is improved by adding carbides, oxides and silicides of W and Mo.

In Japanese Patent Laid-Open No. 7-267734, it has been reported that both the strength and the toughness of a silicon nitride sintered body in which Cr ₂ N particles are dispersed are improved.

Further, in Japanese Patent Laid-Open No. 2-157162, a system in which titanium silicide or / and zirconium silicide is added to rare earth oxide + MgO has been tried, and it is known that high strength and toughness can be obtained.

On the other hand, Japanese Unexamined Patent Publication (Kokai) No. 3-174364 examines the effect of the iron content in the raw material on the strength characteristics. In a silicon nitride sintered body having an iron content of 50 ppm or less, high strength and It discloses that toughness is obtained.

[0009]

However, with these techniques, although high average strength can be obtained, there is a large variation in strength, and for example, the strength at which the cumulative fracture probability is 1 / 100,000 is the guaranteed strength of the member. In this case (that is, when the probability that the member will be destroyed below the guaranteed strength is 1 / 100,000), the guaranteed strength is significantly lower than the average strength, resulting in a problem that the structural material lacks reliability.

When a material having a high average strength is compared with a material having a low average strength, and the variation in the strength of the former is larger than the variation of the latter, the above-mentioned guarantee strength is higher in the material having a lower average strength. It is also possible. Therefore, a material with high guaranteed strength that does not vary in strength characteristics is desired.

The present invention has been made to solve the above problems. It is an object of the present invention to provide a silicon nitride sintered body having a small variation in mechanical strength, a high guaranteed strength with a cumulative fracture probability of 1 / 100,000, and excellent reliability, and a method for manufacturing the same. .

[0012]

The silicon nitride-based sintered body of the present invention has an average particle size range of 5 to 30 μm.
eSi) particles are contained in an amount of 0.5 to 9% by volume.

The sintered body of the present invention contains FeSi particles, and the FeSi particles are present in the sintered body as spherical dispersed particles composed of a single crystal or a polycrystalline body.
Due to its low strength property, it behaves as a pseudo defect in the sintered body.

When the FeSi particles are uniformly dispersed in the sintered body, the destruction of the sintered body does not occur from the original structural defect as a monolithic material in the absence of FeSi particles, and the dispersed FeSi particles themselves. Serves as a starting point of fracture, and thus has an effect of substantially uniformly dispersing defects of which size is controlled in the sintered body, and has an effect of significantly reducing variation in strength.

In the sintered body of the present invention, the range of the average particle size of the FeSi particles is 5 to 30 μm. If the average particle size is less than 5 μm, the average strength increases, but the variation in strength increases, and if it exceeds 30 μm. This causes a decrease in average strength.

The volume fraction thereof is preferably in the range of 0.5-9% by volume. If it is less than 0.5% by volume, the difference between the strength in the vicinity of the particles and the strength in the region where no particles are present becomes large, resulting in a large variation in strength, and if it exceeds 9% by volume, the toughness deteriorates.

The FeSi grains of the present invention may contain a small amount of elements such as Cr, Ni and C which are inevitably present in the Fe raw material or in the sintered body.

The method for producing a silicon nitride sintered body of the present invention is as follows:
1-8% by weight of one or more kinds of rare earth oxides, 1-8% by weight of magnesium hydroxide (Mg (OH) ₂ ), iron (Fe)
0.5 to 10% by weight, titanium silicide (TiSi ₂ ) 0.1
Up to 3% by weight and the balance silicon nitride (Si ₃ N ₄ ) and Si
_A mixed powder of silicon oxide (SiO ₂ ) inevitably contained in ₃ N ₄ is molded, and the molded body is sintered in a temperature range of 1550 to 1650 ° C. in an atmosphere containing nitrogen gas.

Examples of the rare earth oxide used for producing the sintered body of the present invention include yttrium oxide (Y ₂ O ₃ ), cerium oxide (CeO ₂ ), neodymium oxide (Nd ₂ O ₃ ).
And the like.

The oxide of a rare earth element has a function of promoting the crystal phase transition from the α phase to the β phase in the melt during the sintering of silicon nitride, and further, by forming the columnar phase of silicon nitride, the strength and toughness are improved. Improve.

If the total content of these components exceeds 8% by weight, the mechanical strength of the obtained sintered body at high temperature decreases, so the content is preferably 8% by weight or less. 1% by weight
If it is less than the above range, the melt is insufficient and sufficient densification cannot be achieved, which is not preferable. Therefore, it is desirable that the amount added be in the range of 1 to 8% by weight.

Mg (OH) ₂ is 400 ° C. during the sintering temperature rising process.
Before and after, H ₂ O is released to become MgO, and MgO forms a Mg-containing glassy composite oxide at the time of sintering together with the above rare earth oxide. It has the effect of promoting densification.

Further, addition of Mg (OH) ₂ has the property of increasing the crystal grain size, so that the toughness can be improved.

For the production of the sintered body of the present invention, 1 to 8% by weight is used.
Mg (OH) ₂ is used, but if it is more than 8% by weight, the homogeneity of the structure is impaired, and if it is less than 1% by weight, sufficient densification cannot be obtained.

Fe is added to the S around the Fe particles during the sintering process.
It reacts with the i ₃ N ₄ phase to form FeSi single crystal grains or polycrystalline grains, and exists as a black stable compound in the sintered body.

FeSi grains do not interfere with the sinterability of Si ₃ N ₄ . When iron powder is used as a raw material for producing FeSi particles, carbonyl iron powder, atomized powder, plasma vapor-phase synthetic powder and the like can be used.

Further, when the Si ₃ N ₄ powder is manufactured or mixed with the sintering aid, an iron crushing machine may be used and added as a mixed powder from a pot or a crushing ball.

In the present invention, the raw material powder of Fe is 30
A fine powder smaller than μm is preferable. In the present invention, 0.5 to 10% by weight of Fe is used, but if it is less than 0.5% by weight, the variation in strength becomes large.
If it exceeds 0% by weight, toughness is deteriorated.

TiSi ₂ has the above-mentioned rare earth and Mg content during sintering.
In the compound oxide glass phase melt containing
It is thought that it acts as a nucleus when the phase transitions to the β phase, promotes the phase transition and contributes to homogenization of the structure, and particularly when firing a large sintered body, stable and homogeneous sintering is possible. The body is obtained. Further, the Ti element also has a function of blackening the sintered body.

In the production of the sintered body of the present invention, TiSi ₂ is used in an amount of 0.1 to 3% by weight, but if it is added in an amount of more than 3% by weight, the strength is lowered, and if it is less than 0.1% by weight, the structure is homogeneous. No contribution to the realization.

The silicon nitride powder used in the present invention is a silicon nitride powder having an α-type and / or β-type crystal structure, and in order to obtain a sufficiently high bulk density during sintering,
Fine particles having an average particle size of 5 μm or less are desirable.

The silicon nitride raw material may contain a small amount of SiO _{2 which} is unavoidably present.

Rare earth oxide, M added as a sintering aid
In order to obtain a homogeneous and high density sintered body, g (OH) ₂ and TiSi ₂ are also preferably fine particles having an average particle size of 2 μm or less.

In the method of the present invention, each of these components is mixed with water or an organic solvent using a mixer such as an attritor or a ball mill.

In addition to the sintering aid, an organic binder or the like may be added in order to improve the moldability and the strength of the molded body. The mixed powder thus adjusted is pressure-molded to obtain a molded product having a predetermined shape.

As the molding method, a known molding method such as a die press, a rubber press, a cast molding, an injection molding or the like is used. For example, in the case of a plate, the rubber press pressure is 100 to 7
Mold at 00 MPa.

This compact is heated and sintered at 1550 to 1650 ° C. to obtain a sintered body. As a sintering method, any of an atmospheric pressure sintering method, a gas pressure sintering method, a hot isostatic press sintering method, and a hot press sintering method can be used in an atmosphere containing nitrogen gas. It is also possible to combine one or more sintering methods.

Sintering in an atmosphere containing nitrogen gas is to suppress decomposition of Si ₃ N ₄ during sintering. 155
If the temperature is lower than 0 ° C, a sufficiently high density cannot be obtained. Also, 165
At a temperature higher than 0 ° C., FeSi particles react with the glass phase existing as the grain boundary phase of the sintered body to form a composite oxynitride melt containing Fe, and as a particle in the obtained sintered body. It no longer exists and cannot serve its intended purpose as a pseudo defect.

[0039]

The silicon nitride sintered body of the present invention has a structure in which FeSi particles, which are somewhat coarse and behave as pseudo defects in the sintered body, are dispersed in a mother phase composed of Si ₃ N ₄ grains and a grain boundary phase. Therefore, the Weibull coefficient, which is a guideline for variations in strength, is 22
As shown above, the strength characteristics show very little variation, and the guaranteed strength is 320M when the cumulative failure probability is 1 / 100,000.
Higher reliability as a structural material with Pa or more.

Next, examples of the present invention will be described together with comparative examples.

[0041]

EXAMPLES α-type Si ₃ N ₄ powder (average particle size 0.5 μm, α
Conversion rate of 97%) or β-type Si ₃ N ₄ powder (average particle size 5μ
m, β conversion rate 95%), rare earth oxide powder, Mg (OH)
₂ powder (average particle size 0.5 μm), TiSi ₂ powder (average particle size 2 μm) and Fe powder (average particle size 1 to 44 μm)
Was added in a predetermined amount (% by weight) shown in Table 1, 5% by weight of a PVA-based binder was added, and the mixture was kneaded for 4 hours with an attritor using water as a solvent and silicon nitride balls as grinding balls, and then spray dryer To obtain granulated powder.

The rare earth oxide powder used was Y ₂ O ₃
Powder (average particle size 1 μm), CeO ₂ powder (average particle size 0.
8 μm) and Nd ₂ O ₃ powder (average particle size 1.0 μm).

Next, the obtained granulated powder was sintered after molding. As molding conditions, pressurization by cold isostatic pressure 150MP
As a, a plate-like body having a size of 150 mm × 150 mm × 15 mm was obtained. The sintering was atmospheric pressure sintering in a nitrogen gas atmosphere at a temperature shown in Table 1 for 4 hours.

The strength, Weibull coefficient, and guaranteed strength of each sintered body obtained according to the present invention are determined by the addition amount of the sintering aid, the sintering conditions,
Table 1 shows the volume fraction of FeSi particles in the sintered body and the average particle diameter.

The particle diameter and volume fraction of the FeSi particles are 30 or more black particles and the black in the photographed surface from the optical microscope image (magnification of 400 times) of the mirror-polished surface of the sintered body. It was measured as a grain area fraction and expressed as an average value. The presence of FeSi grains was confirmed by JCPDS card 38-1397 using an X-ray diffraction method.

Regarding mechanical strength, JIS R160
4 according to 1 using 30 test pieces at room temperature
A point bending test was performed to measure the fold strength.

As a measure of the average strength, it is assumed that the distribution of the measured 4-point bending strength is in accordance with the single mode / two-parameter Weibull distribution, and the cumulative failure probability is 63.21%, that is, 2 Parameter Weibull distribution was calculated by the maximum likelihood method.

The Weibull coefficient m representing the degree of intensity variation was similarly estimated by applying the maximum likelihood method to the Weibull distribution function. If the Weibull coefficient is large, the variation in strength is small.

As the guaranteed strength, the strength at which the cumulative failure probability is 1 / 100,000 is the guaranteed strength of the member, and the obtained scale parameter and the estimated value of the Weibull coefficient are substituted into the above-mentioned Weibull distribution function. Calculated by Regarding toughness, the fracture toughness value K _IC was measured by the SEPB method of JIS R1607.

As shown in Table 1, the examples according to the present invention have a large Weibull coefficient of 22 or more and a guaranteed strength of 320 MPa or more at which the cumulative failure probability is 1 / 100,000, which corresponds to the comparative example. It was confirmed that it was superior to the sample.

[0051]

[Table 1]

[0052]

[Table 2]

[0053]

According to the present invention, in a silicon nitride sintered body, it is possible to significantly reduce variations in strength and to obtain a guaranteed strength of 320 MPa or more when the cumulative fracture probability is 1 / 100,000. It was This makes it possible to produce a silicon nitride-based sintered body having extremely excellent reliability, and its industrial utility is extremely large.

Claims (2)

[Claims] 1. A silicon nitride-based sintered body, characterized in that iron silicide (FeSi) particles having an average particle size range of 5 to 30 μm are contained in an amount of 0.5 to 9% by volume. 2. One or more rare earth oxides 1 to 8% by weight, magnesium hydroxide (Mg (OH) ₂ ) 1 to 8% by weight, iron (Fe) 0.5 to 10% by weight, titanium silicide (Ti).
Si ₂ ) 0.1 to 3% by weight, and the balance silicon nitride (S
i ₃ N ₄ ) and Si ₃ N ₄ inevitably contained silicon oxide (SiO ₂ ) mixed powder, and the compact is fired in a temperature range of 1550 to 1650 ° C. in an atmosphere containing nitrogen gas. A method for manufacturing a silicon nitride-based sintered body, comprising: