JPH0339029B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for manufacturing a sintered silicon nitride (Si ₃ N ₄ ) body that exhibits excellent mechanical properties not only at room temperature but also at high temperatures. Si ₃ N ₄ sintered bodies have excellent thermal shock resistance and other mechanical strength at high temperatures, so they are expected to be used as high-temperature structural materials such as gas turbine parts. However, since Si ₃ N ₄ has poor sinterability, it does not become densified even if it is sintered alone at a high temperature around 1900° C., which is close to the decomposition temperature. Therefore,
Conventionally, sintered bodies have been mainly manufactured by the following methods, but these methods also have drawbacks. () Reactive sintering method: This is a method in which a molded body of metal silicon powder is sintered in a nitrogen atmosphere. With this method, almost no sintering shrinkage occurs, so the dimensional accuracy of the sintered body is excellent, but the sintered body is sufficiently densified and has satisfactory high strength.
Obtaining Si ₃ N ₄ is difficult. () Ordinary pressure or pressure sintering method: This method combines Si ₃ N ₄ powder with metal oxides such as magnesium oxide (MgO), aluminum oxide (Al ₂ O ₃ ), and yttrium oxide (Y ₂ O ₃ ). is added as a sintering aid and sintered under normal pressure or pressure. According to this method, a dense sintered body can be obtained, but the sintered body
The disadvantage is that a relatively large amount of low melting point matrix is formed at the Si ₃ N ₄ grain boundaries, resulting in low mechanical strength and fracture toughness at high temperatures. Fine particles of zirconium oxide (ZrO ₂ ) are converted into Si ₃ N ₄
When uniformly dispersed in the grain boundaries, a Si ₃ N ₄ sintered body with excellent mechanical strength and fracture toughness at room temperature and high temperature can be obtained. However, according to the research of the present inventor, Si ₃ N ₄ and ZrO ₂ have significantly different specific gravities, so normal mixing methods such as ball milling cannot
It is extremely difficult to uniformly disperse ZrO ₂ powder in Si ₃ N ₄ powder, resulting in compositional heterogeneity and grain growth of ZrO ₂ during sintering, as well as monoclinic formation during the cooling process. It became difficult to avoid the transition to the ZrO ₂ system, and in the end, it was found that the improvement in the mechanical properties of the Si ₃ N ₄ sintered body was difficult to achieve satisfactorily. As a result of further experiments and research, the present inventor found that when adding ZrO ₂ powder in which a certain amount of Y ₂ O ₃ is dissolved in advance to Si ₃ N ₄ powder by normal pulverization mixing, ( a) Some of the Y ₂ O ₃ dissolved in ZrO ₂ is removed during the sintering process.
Diffuses into Si ₃ N ₄ or into Si ₃ N ₄ and a third component to form a new compound (e.g. Si ₃ N ₄ .Y ₂ O ₃ ),
As a result, sintering of Si ₃ N ₄ is promoted, and (b) ZrO ₂ fine particles containing remaining Y ₂ O ₃ as a solid solution are homogeneously dispersed in the matrix in a tetragonal system. (c) As a comprehensive effect of these results, the stress-induced phase transformation strengthening effect is exhibited, and high toughness and high strength _are obtained _.
It has been found that a Si ₃ N ₄ sintered body containing tetragonal ZrO ₂ in solid solution and having excellent mechanical properties at room temperature and high temperature can be obtained. The present invention was completed based on such new knowledge. The Si ₃ N ₄ powder used as the main component in the present invention is
It contains α phase in an amount of 50% by weight or more, more preferably 70% by weight or more, and has an average particle size of crystalline primary particles of 1.5 μm or less, more preferably 1.0 μm or less. However, the average particle of the crystalline primary particles is a calculated value obtained from the actually measured specific surface area of the powder, assuming that the shape of the particle is a true sphere. When the α-phase content is less than 50% by weight, the amount of transition from α-phase Si ₃ N ₄ to β-phase Si ₃ N ₄ during the firing process is small, and a structure in which columnar crystals are entangled does not form, resulting in mechanical failure. The physical characteristics are inferior. Moreover, if the average particle size of the Si ₃ N ₄ crystal primary particles exceeds 1.5 μm, the sinterability will be poor and the crystal grain size in the sintered body will be large and irregular, which is not preferable. Furthermore, Si ₃ N ₄ is
Even if oxide impurities such as Ca and Fe are contained up to about 1% by weight, the mechanical properties of the sintered body are not substantially impaired. The ZrO ₂ powder used as the second component in the present invention is
It is a powder containing 3 to 15 mol% of Y ₂ O ₃ as a solid solution, and the average particle size of primary crystal particles is 0.5 μm or less, preferably
Use one with a diameter of 0.2 μm or less. If the solid solution amount of Y ₂ O ₃ in ZrO ₂ is less than 3 mol %, the sintering progress rate of Si ₃ N ₄ will be very slow, and the ZrO ₂ particles will be in a tetragonal form and difficult to contain, which is preferable. but on the other hand 1.5
If it exceeds the mol%, the number of equiaxed _ZrO2 particles dispersed in _Si3N4 will increase, and no improvement in mechanical properties such as fracture toughness of the _{Si3N4 sintered} body will be observed _. . The average particle size of the crystalline primary particles of ZrO ₂ powder is 0.5μ
When it exceeds m, it becomes difficult to uniformly disperse Si ₃ N ₄ grain boundaries as fine particles. ZrO ₂ with solid solution of Y ₂ O ₃ used in the present invention
The fine powder is created, for example, as follows. First, ZrO ₂ /
After adding a yttrium chloride solution to a predetermined proportion of Y ₂ O ₃ , a precipitate of hydrated zirconia or zirconium hydroxide containing a predetermined amount of ZrO ₂ is produced by hydrolysis or ammonia neutralization. After drying the product, it is fired at about 800 to 1250℃ to form a Y ₂ O ₃ solid solution with a crystal primary particle size of 1 μm or less.
Obtain _ZrO2 . Next, this ZrO ₂ is ball milled,
The desired Y ₂ O ₃ solid solution fine powder is obtained by pulverizing and dispersing it with a fine pulverizer such as an attrition mill. However, the method for producing the Y ₂ O ₃ solid solution may be any method and is not limited to the above method. Note that when Y ₂ O ₃ is not dissolved in ZrO ₂ and both are added separately, the mechanical properties of the Si ₃ N ₄ sintered body are not significantly improved. Due to segregation due to differences in specific gravity and agglomeration, microscopic regions are divided into regions where a matrix is formed by Y ₂ O ₃ and Si ₃ N ₄ and regions where a matrix is formed by ZrO ₂ and Si ₃ N ₄ . As the ZrO 2 particles become clearly distinct, the structure becomes heterogeneous, the grain growth of ZrO ₂ particles, etc. occur, and the ZrO ₂
It is presumed that this is due to the fact that it is centered in the monoclinic system. The mixing ratio of Si ₃ N ₄ powder and ZrO ₂ powder varies depending on the particle size of the former, the amount of solid solution of Y ₂ O ₃ in the latter, firing conditions, etc. 30 parts by weight for a total of 100 parts by weight. If the proportion of ZrO ₂ powder is less than 3 parts by weight of the whole raw material,
It is insufficient to promote the sintering _{of Si3N4} , while when the proportion of _ZrO2 powder exceeds 30 parts by weight,
This is not preferable because the excellent properties of Si ₃ N ₄ itself cannot be maintained. In addition, in the method of the present invention, Si ₃ N ₄ powder and ZrO ₂
Al ₂ O ₃ and/or per 100 parts by weight of mixed powder with powder
Or the total amount of MgO is 10 parts by weight or less, preferably 2 to 2 parts by weight.
By adding 8 parts by weight, an even denser sintered body can be obtained. The method of the present invention is generally carried out as follows.
After pulverizing and mixing powders of Si ₃ N ₄ and ZrO ₂ or powders to which Al ₂ O ₃ and/or MgO have been added, they are molded into a predetermined shape and then molded in a nitrogen atmosphere under normal pressure or pressurization.
Sinter at 1650-1800℃. If necessary, it may be fired in a container filled with Si ₃ N ₄ powder. If the firing temperature is less than 1650°C, the densification sintering progress rate is extremely slow and it takes a long time to transition to β-type Si ₃ N ₄ , whereas if it exceeds 1800°C, Si ₃ N ₄ decomposes. This is undesirable because evaporation becomes active. Hereinafter, the features of the present invention will be explained in more detail with reference to Examples. Examples 1 to 4 ZrO ₂ powder containing Y ₂ O ₃ as a solid solution was added to Si ₃ N ₄ powder,
With Al ₂ O ₃ ball mill and Al ₂ O ₃ media,
Wet-grind and mix for 48 hours using denatured alcohol as a solvent. Average particle size of crystalline primary particles of Si ₃ N ₄ , α phase content in Si ₃ N ₄ , usage amount of Si ₃ N ₄ ,
Average particle size of crystalline primary particles of ZrO ₂ , Y ₂ O ₃ content,
Table 1 shows the amount of ZrO ₂ used. The obtained mixed powder is press-molded with a metal mold at 200 Kg/cm ² to obtain a preform, and then further pressurized with a hydrostatic press at 3000 Kg/cm ² to obtain a molded product. The obtained compact is placed in a container filled with Si ₃ N ₄ powder and held at a predetermined firing temperature in a nitrogen atmosphere for a predetermined time to obtain a sintered compact. Table 2 shows the firing temperature and time, as well as the physical properties of the sintered body. The three-point bending strength and K _1c were measured as follows. 3-point bending strength: Width 3.5mm, height 2.5mm from sintered body
After cutting out a square column with a total length of 28mm, the surface was polished with a 600 mesh diamond grindstone.
Measure the bending strength by applying a load to the center in the length direction with a span of 20 mm. K _1c ... A Bitkers indentation is made in the center bottom of a square column obtained in the same manner as in the case of three-point bending strength, and this is measured as a sample. Examples 5 to 8 ZrO ₂ powder containing Y ₂ O ₃ as a solid solution was added to Si ₃ N ₄ powder,
48 using denatured alcohol as a solvent using a plastic ball mill and Si ₃ N ₄ media.
Perform wet grinding and mixing for an hour. The composition of the mixed powder after pulverization is as shown in Table 1. Further, the physical properties of the sintered bodies obtained in the same manner as in Examples 1 to 4 are shown in Table 2. Example 9 ZrO ₂ powder containing Y ₂ O ₃ as a solid solution was added to Si ₃ N ₄ powder,
Further, Al ₂ O ₃ powder (purity 99.99%) and MgO powder (purity 99.9%) were added, and pulverization, mixing, molding, and firing were performed in the same manner as in Examples 5 to 8. The composition of the mixed powder and the physical properties of the sintered body are shown in Tables 1 and 2, respectively. Comparative Example 1 The powder was pulverized, mixed, molded, and fired in substantially the same manner as in Examples 5 to 8, except that ZrO ₃ powder containing an excess amount (25 mol %) of Y ₂ O ₃ was used. The results are shown in Tables 1 and 2. Comparative Example 2 The powder was pulverized, mixed, molded, and fired in substantially the same manner as in Examples 5 to 8, except that Y ₂ O ₃ and ZrO ₂ were separately added to the raw material powder. The results are shown in Tables 1 and 2. Comparative Example 3 Powder was pulverized, mixed, molded, and fired in substantially the same manner as in Examples 5 to 8, except that ZrO ₂ which did not contain Y ₂ O ₃ as a solid solution was used. The results are shown in Tables 1 and 2.

【table】

【table】

[Table] As is clear from the results shown in Table 2, Example 1
The mechanical properties of the sintered bodies obtained in Examples 1 to 9 are excellent. On the other hand, the sintered body of Comparative Example 1 uses ZrO ₂ in which more than 15 mol% of Y ₂ O ₃ is dissolved, so although it is dense, the ZrO ₂ particles have an increased equiaxed crystal system. ,
It is considered that the improvement in fracture toughness of the sintered body is not sufficient. In the sintered body obtained in Comparative Example 2, in which a total amount of 9.9 parts by weight of ZrO ₂ and Y ₂ O ₃ were added separately, in Example 7, in which 9.9 parts by weight of ZrO ₂ in which Y ₂ O ₃ was dissolved as a solid solution was used. It is clear that the bulk specific gravity and bending strength are significantly inferior to that of the sintered body. This is considered to be because, in the sintered body of Comparative Example 2, most of the ZrO ₂ particles present at the Si ₃ N ₄ grain boundaries are grown monoclinic crystals. Furthermore, in the sintered body of Comparative Example 3, which uses ZrO ₂ powder that does not dissolve Y ₂ O ₃ , the sintered body is not densified, and the ZrO ₂ particles present at the Si ₃ N ₄ grain boundaries are Since most of it is a monoclinic crystal, its bending strength is extremely low.

Claims (1)

[Scope of Claims] 1. 70 to 97 parts by weight of silicon nitride powder containing 50% by weight or more of α phase and having an average particle size of crystalline primary particles of 1.5 μm or less and 3 to 15 mol% of yttrium oxide, and crystals. By molding 100 parts by weight of a mixed powder consisting of primary particles and 3 to 30 parts by weight of zirconium oxide powder with an average particle size of 0.5 μm or less and firing it at 1650 to 1800°C in a nitrogen atmosphere, the main ZrO ₂ crystals are formed. A method for producing a silicon nitride sintered body, which comprises forming a tetragonal ZrO ₂ containing Y ₂ O ₃ as a solid solution. 2 70 to 97 parts by weight of silicon nitride powder containing 50% by weight or more of α phase and having an average particle size of crystalline primary particles of 1.5 μm or less and 3 to 15 mol% of yttrium oxide as a solid solution and having an average particle size of crystalline primary particles 100 parts by weight of a mixed powder consisting of 3 to 30 parts by weight of zirconium oxide powder of 0.5 μm or less is added with 10 parts by weight or less of aluminum and/or magnesium oxide, and molded in a nitrogen atmosphere.
Main _ZrO2 by firing at 1650-1800℃
1. A method for producing a silicon nitride sintered body, the method comprising forming a tetragonal ZrO ₂ crystal containing Y ₂ O ₃ as a solid solution.