JPH08169764A - Production of silicon nitride-based sintered compact - Google Patents

Abstract

PURPOSE: To obtain high strength and high reliability silicon nitride structural parts. CONSTITUTION: When a silicon nitride-based sintered compact is produced, working stock for this sintered compact is heat-treated at a temp. within the range of (a temp. at which the max. value of specific peaks of the internal friction of the stock is shown)±150 deg.C. A typical example of the sintered compact is a sintered compact obtd. by compacting a mixture consisting of 5-15wt.% (expressed in terms of oxides) one or more kinds selected from among rare earth elements and Al, 0.5-5wt.% (expressed in terms of oxides) one or more kinds selected from along Mg, Ti and Ca and the balance Si3 N4 and sintering the resultant compact in an N2 -contg. atmosphere at 1,500-1,700 deg.C.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing a silicon nitride based sintered body, and provides a silicon nitride based structural component having high strength and high reliability.

[0002]

2. Description of the Related Art Since silicon nitride ceramics are excellent in light weight, wear resistance, high strength and high temperature strength, they are attracting attention as mechanical parts under extremely harsh operating environments. However, the quality and price requirements for these silicon nitride ceramics are becoming severer year by year. Therefore, various developments have been made on silicon nitride materials. By the way, since products used for such machine parts and the like are required to have dimensions and surface finishing accuracy, a method of finishing to a target shape by a grinding process using a diamond grindstone or the like is generally performed.

However, ceramics are generally brittle materials, and if defects are introduced into the surface by processing, the strength will be significantly reduced. This has become the biggest bottleneck for practical use of ceramics. For example, Imai et al. (Toyota Machinery Technical Report, Vol.26, No.3.4, P25, left 23
It has been reported that in the grinding process, cracks parallel to the grinding direction are introduced below the surface of the extremely machined work, thereby significantly reducing the original strength of the material. Therefore, when grinding ceramic parts, it is necessary to grind using diamond abrasive grains with a small grain size that suppresses damage to the material surface as much as possible. It is not an effective means. Therefore, various improvement methods have been proposed in order to solve these problems. For example, attempts have been made in various fields to repair cracks in the machined surface portion introduced during grinding by performing heat treatment under various temperatures and atmospheres. An example of disclosure is shown below.

JP-A-60-81076 (page 2, upper left 3
Rows to upper right row 11) hold silicon nitride ceramics in the air at 950 to 1400 ° C. for 30 minutes or more to react Si in Si ₃ N ₄ ceramics with oxygen (O ₂ ) in the air, A method of forming an oxide, eliminating defects such as cracks on the material surface by the oxide, and improving bending strength is disclosed. Japanese Unexamined Patent Publication No. 64-52679 (page 2, upper right line 9 to upper right line 14) discloses a heat treatment at 900 to 950 ° C. in the atmosphere as a method for improving the strength of sialon. As an effect in this case, It is said that a strength higher than that of a non-heat-treated defect-free material can be obtained. The reason is that the surface of the material is covered with an altered phase due to oxidation of the material surface and diffusion of Al or the like in the grain boundary phase, thereby blunting defects such as fine grinding scratches that reduce the strength.

Japanese Patent Publication No. 3-80755 (page 1, right line 17 to page 2, right line 22) grinds a silicon nitride ceramic sintered body using yttrium oxide, aluminum oxide and aluminum nitride as a sintering aid. After processing, when heat-treated at a temperature lower than the sintering temperature and higher than the softening temperature of the glass phase in the air, that is, 800 to 1100 ° C. for 1 to 24 hours in the air, a micro acute angle shape generated during grinding The notches are blunted, and the surface is made of silicon dioxide SiO 2.
It is disclosed that an oxide film layer composed of ₂ is formed and the mechanical strength is improved by the interaction thereof. Japanese Examined Patent Publication No. 4-50276 (Page 2, lines 3 to 41 on the left) shows that a sialon sintered body is heated to 875 to 950 ° C. in an atmosphere containing oxygen.
By heating for 30 minutes or more in the temperature range of, the glassy layer made of SiO ₂ , Al ₂ O ₃ , and Y ₂ O ₃ is formed on the outer surface of the sialon sintered body, the surface defects are closed, and the corrosion resistance and strength are improved. It is disclosed.

Japanese Unexamined Patent Publication No. 4-376579 (paragraph 0)
No. 008-0012) describes a method of forming an oxide film of 10,000 Å or less in which defects remaining on the surface of a silicon nitride sintered body are eliminated to improve fracture resistance and wear resistance. Specifically, the sintered body is α-Si ₃ N ₄ , β-Si.
_At least one of ₃ N ₄ , α'-sialon and β-sialon is contained, and this is contained in an oxidizing atmosphere at 800 to 1
It is heated to 100 ° C. to form an oxide film (oxide, oxynitride, oxycarbide, oxynitride carbide) to improve the strength.
In these disclosed examples, heat treatment is performed under a predetermined temperature, time and atmosphere, and an attempt is made to improve the reliability of the strength only by the healing action of the oxide film. Not really.

[0007]

In order to solve the above-mentioned conventional problems, a method for manufacturing a silicon nitride material is provided, thereby providing a method for dramatically improving strength, reliability and fatigue characteristics. Is the subject of the present invention.

[0008]

The present inventors have obtained the following findings as a result of earnest research on a method for producing a silicon nitride-based sintered body. That is, the present invention relates to a method for producing a silicon nitride-based sintered body, in which the material for processing the sintered body is heat-treated at a temperature within 150 ° C. above and below the temperature at which the maximum value of the peculiar peak of internal friction is exhibited. This is a characteristic method. Further, in that case, the predetermined temperature holding time of the heat treatment is 5 to 6
The production method is set to 00 minutes, and a production method in which the atmosphere is performed in an atmosphere other than the reducing gas atmosphere can be mentioned. Further, as a typical example of a composition of a processing material to be subjected to heat treatment composed of a silicon nitride sintered body and a processing method thereof,
(1) A step of mixing powders of Si ₃ N ₄ and a sintering aid to form a mixture, (2) a step of forming the mixture into a formed body, (3) the formed body in an N ₂ -containing atmosphere Sintered with
A step of forming a sintered body, (4) a step of processing the sintered body into a predetermined size to form a processing material, (5) a temperature of 150 ° C. above or below the temperature at which the processing material shows the maximum value of the peculiar peak of internal friction
Is a method for manufacturing a silicon nitride-based sintered body, which includes a step of performing heat treatment at an internal temperature.

The heat treatment in the step (5) is carried out at a predetermined temperature for 5 to 600 minutes and in an atmosphere other than the reducing gas atmosphere. INDUSTRIAL APPLICABILITY The manufacturing method of the present invention provides a manufacturing method suitable for imparting high strength, high fatigue resistance, and high reliability of sliding characteristics to automobile engine parts or materials used at medium and high temperatures (around 1000 ° C.). To do.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION The method for producing a silicon nitride-based sintered body of the present invention is a method of manufacturing a structural member made of a sintered body having the above-mentioned composition, which has been sintered and then processed into a predetermined shape. In order to improve the temperature, the heat treatment is performed in the temperature range of the peak temperature ± 150 ° C. of the peak that is specifically expressed by the internal friction of the structural member. Defects are repaired by the heat treatment, and a sintered body having stable strength is obtained. Internal friction is a phenomenon in which a part of the deformation energy applied to the material from the inside changes to thermal kinetic energy and is attenuated, and represents the damping characteristic of the material. In the case of a periodic external force such as vibration, internal friction is defined as the ratio of the energy lost in one cycle to the total elastic energy, and is considered to indicate the energy absorption rate when the vibration energy is applied to the material. A material having a larger internal friction value has a higher energy absorption rate.

This internal friction measuring method of the present invention is described in Journal of Material Science Letters (Jo
urinal of Material Science
Letters) No. 3, 349-351, 1984.
The resonance method described in. As can be seen from FIG. 1, which shows the temperature dependence of internal friction, it increases as the deformability of the bulk itself increases with increasing temperature. The peak of internal friction in the figure occurs in the rapidly increasing portion of internal friction that is specifically expressed. Generally, it is considered that these internal friction peaks are due to the portion softening with increasing temperature, that is, the grain boundary phase.

Here, when the heat treatment is performed at a temperature exceeding 150 ° C., which is the maximum peak of the singular point of the internal friction, crystallization of the grain boundary phase progresses, and the strength characteristics of the material body deteriorate, so that the strength is improved. It cannot be measured. Further, even when the temperature is lower than 150 ° C., the strength is not similarly improved. The reason for this is not clear, but it is considered that it is because the temperature at which the grain boundary phase portion of the material softens has not reached. Further, it is preferable to perform the heat treatment within a temperature range of ± 100 ° C. of the peak temperature. By performing the heat treatment in this temperature range, especially the sintering aid is efficiently diffused to the defective portion such as a crack, or the surface phase of the material is efficiently oxidized or healed, particularly the tip of the crack. Etc. are easily repaired and the strength is improved. Presumably this is because the grain boundary phase inside the material changes more. The heat treatment time is appropriately determined according to each composition (blending amount). Generally, 5 minutes to 600 minutes is desirable. If the time is less than 5 minutes, the strength cannot be sufficiently improved, and if the time exceeds 600 minutes, the treatment is performed for a long time, the production effect is reduced, and the strength is reduced.

The rate of increase in strength is high in the sintered body produced by the above manufacturing method. 100k especially in nitrogen
The strength of the material that was about g / mm ² is 200 kg / mm ² or more, and there is a strength recovery effect of about 2 times or more. There is no example of strength recovery at a high level like this example. In the present invention, the atmosphere for this heat treatment may be any atmosphere except an atmosphere containing hydrogen, but is preferably an inert gas such as nitrogen or argon and a mixed gas thereof, and a vacuum. The reason is that in an oxidizing atmosphere, the surface is oxidized and the surface roughness changes. Further, the surface layer of the material becomes brittle in the hydrogen atmosphere. However, other than that, strength can be expressed without causing surface roughness and surface alteration, which is advantageous when used in precision mechanical materials and the like.

Further, as a typical example of a processing material composition to be subjected to a heat treatment composed of a silicon nitride type sintered body, a rare earth element, A
5 to 15% by weight of at least one element of the 1 element in terms of oxide, 0.5 to 5% by weight of at least one of Mg, Ti and Ca elements in terms of the oxide, and the balance Si ₃ N _4. Can be mentioned. The manufacturing method in this case is as follows. First, a step of mixing the above powder composition, a step of molding the mixture to form a molded body, and the molded body of N ₂ at 1500 to 1700 ° C.
Processed material is made by the process of sintering in a contained atmosphere to produce a sintered body with a relative density of 97% or more, and the process of processing the sintered body to a predetermined size to make it into a processed material, and further the processed material It is preferable to perform the heat treatment at a temperature within 150 ° C. above and below the temperature at which the maximum value of the unique peak of friction occurs.

As a method of mixing, for example, Si ₃ N ₄ powder, Al ₂ O ₃ and at least one powder of rare earth oxide and M
At least one powder selected from gO, TiO ₂ , and CaO is mixed within a predetermined amount range. Here, it is desirable to select Y as the rare earth element. MgAl ₂ if needed
O _4, may be used MgTiO ₃ like composite oxide. Molding is performed by pressing, injection molding, casting molding, etc., and the method is not particularly limited. When the sintering temperature exceeds 1700 ° C., the auxiliary component is volatilized and the particle size of the material is increased, so that the required strength cannot be obtained and the strength varies. Also, 1
If it is less than 500 ° C, the relative density is low and desired strength cannot be obtained. The sintered body obtained by such a manufacturing method has a linear density of crystal particles per 50 μm length in an arbitrary two-dimensional cross section in the range of 120 to 250, a maximum area ratio maximum diameter of 1 to 2 μm, and a maximum diameter. The minimum diameter of the area ratio is 0.1
It is in the range of 0.5 μm.

This improves the strength of the sintered body,
The variation becomes smaller and the strength improving effect by the heat treatment increases. Here, the linear density is the number of particles cut by a 50 μm-long line segment when observing an arbitrary two-dimensional cross section of the sintered body. For example, the linear density of 120 means that 120 particles are present within a line segment of 50 μm. The maximum diameter of the maximum area ratio and the minimum diameter of the maximum area ratio are defined as follows. Here, the maximum diameter refers to the longest diameter (a1 to 5 in the figure) among the crystal grains observed on the two-dimensional cross section of the sintered body as shown in FIG. The minimum diameter is the shortest diameter (b1 to 5 in the figure). The area of each particle on the two-dimensional cross section is defined as "particle area", and the total area of all particles in the observed visual field is defined as "total particle area".

The particle area A observed at this time is obtained by approximating it to an ellipse having the maximum diameter b and the minimum diameter a as the major axis and the minor axis, respectively, and the sum thereof, that is, the total particle area ΣA.
At this time, the area ratio of the particle groups having the same maximum diameter (total particle area ΣA
The ratio of the area sum ΣAb of particle groups with the same maximum diameter to
The area ratio of the particle groups having the same minimum diameter (the ratio of the area sum ΣAa of the particle groups having the same minimum diameter to the total particle area ΣA) is set, and the maximum diameter value that maximizes the area ratio ΣAb / ΣA is set as the maximum diameter of the maximum area ratio. Similarly, the minimum diameter value that maximizes the area ratio ΣAa / ΣA is set as the minimum diameter of the maximum area ratio. These values can be obtained by obtaining the above b, a, A for each particle, rearranging the values of ΣAb, ΣAa for each b, a, and writing the distribution as shown in FIG. 3 and reading the peak value. . In the present invention, an auxiliary component other than the above-mentioned sintering auxiliary component may be selected as long as it satisfies the desired microstructure and mechanical properties. According to the manufacturing method of the present invention as described above, it is possible to efficiently obtain a high-strength and highly reliable silicon nitride-based sintered body that has never been obtained.

[0018]

Embodiments of the present invention will be described below. Example 1 (up to sintering step) The raw material powder constituting the present silicon nitride system has an average particle size of 0.5 μm.
m, α crystallization rate 96%, oxygen content (BET value) 1.4% by weight of silicon nitride, and an auxiliary agent consisting of Table 1, these powders were weighed according to Table 2, and then in ethanol for 72 hours. After wet-mixing with a nylon ball mill and drying, a mixed powder was obtained.

[0019]

[Table 1]

[0020]

[Table 2]

(* 1 All the values in the table are weight% in terms of oxide) Next, after pressing these powders, primary sintering was performed at 1500
℃ 4 hours, nitrogen 1 atmosphere, secondary sintering 1650 ℃,
It was carried out for 1 hour under 10 atm of nitrogen. Both heating rates are 40
It was set to 0 ° C./hr. The results of these densities and the temperature at which the internal friction specific peak appears are shown in Table 3. JIS of each sintered body after heat treatment in the air for 120 minutes at this peak temperature
Table 3 shows the results obtained by measuring the three-point bending strength according to R1601 in comparison with that before heat treatment.

[0022]

[Table 3]

From the above results, it has been clarified that a bending strength of 150 kg / mm ² or more can be easily obtained by performing heat treatment near the peak temperature of internal friction.

Example 2 (up to processing step) After pressing No. 2 sample of Table 2 of Example 1, 1
The secondary sintering was performed at 1500 ° C. and the secondary sintering was performed according to Table 4. In all cases, the atmospheric pressure was 1000 atmospheric pressure of nitrogen and the sintering was performed for 1 hour, and the temperature lowering time was 400 ° C./hr. Further, the material was processed in accordance with JISR1601 and subjected to a three-point bending bending test. The results are shown in Table 4. Then, after finishing a part of each test piece to Ra = 0.02 μm, HCl 5
Heat at 0 ° C. + 25% aqueous H ₂ O ₂ for 15 minutes at 60 ° C. After thoroughly washing, FAB (F
An Ar-neutral particle was irradiated using an ast-Atom Bombardment apparatus, and etching was performed for 10 minutes under the conditions of 4 to 5 KV and 2 mA (inclination angle of 60 °). Using a scanning electron microscope, the tissue thus obtained,
The range of 10 μm × 15 μm was observed with a scanning electron microscope at a magnification of 15,000, and the minor axis diameter and major axis diameter were evaluated. The linear density was measured by observing at 5000 times.

[0025]

[Table 4]

From the above results, the strength of the sintered body up to 1500 to 1700 ° C. is high, and the linear density is 1 within this range.
50 to 250 particles, the maximum diameter of the maximum area ratio is 1 to 2 μm, and the minimum diameter of the maximum area ratio is 0.1 to 0.5.
It can be seen that a sintered body having a bending strength of 150 kg / mm ² or more is easily obtained in the range of μm.

Example 3 (up to heat treatment step) The material of No. 2 in Table 2 of Example 1 was sintered in the same manner as in Example 1 and then finished to have Ra = 0.02 μm or less (lap material). Then, one-pass grinding was performed with a # 600 grindstone in the direction shown in FIG. 4 (one-pass abrasive). The temperature at which the peak maximum of these singular points of internal friction is 90
It was 0 ° C. Fifteen of these samples were held at each temperature shown in the left end of Table 5 for 4 hours and heat-treated in a nitrogen atmosphere to obtain a wiper coefficient having the same strength as the average strength.

[0028]

[Table 5]

From this result, the temperature 1 having the peak of internal friction was obtained.
Strength is improved by heat treatment within 50 ° C,
It is particularly preferable that the heat treatment within 100 ° C. further improves the strength and reliability.

Example 4 Strength was measured by changing the processing conditions of the material used in Example 3. Moreover, the surface roughness was measured [Ra (average line roughness), R _max (maximum surface roughness)].

[0031]

[Table 6]

In the present invention, it can be understood that the strength recovers even with such rough processing.

Example 5 (Heat Treatment Atmosphere) The heat treatment was carried out using the untreated one-pass ground article of Table 5 of Example 3 under the following conditions. The holding time of each temperature is 4 hours.

[0034]

[Table 7]

From these results, it can be seen that the strength characteristics are improved under the atmosphere other than hydrogen. In particular, it is understood that the heat treatment in an atmosphere other than the atmosphere containing no oxygen has a great effect of improving the strength.

Example 6 (Heat treatment in nitrogen stream) The untreated product of Table 5 was subjected to an experiment by changing the heat treatment temperature and time in nitrogen. The result is shown in FIG. From this result, the time is 15 to 600 in the temperature range of ± 150 ° C.
It is understood that a heat treatment of minutes is suitable. Example 7 (One-sided swing fatigue characteristics) The one-sided swing fatigue characteristics at room temperature and 800 ° C. were examined using a material heat-treated at 950 ° C. after one-pass grinding in Table 5 of Example 3. FIG. 6 shows the results. It was found that this sintered body is a material having excellent fatigue properties.

[0037]

According to the present invention, not only is the material excellent in static strength, but especially when a sintered body is processed and then heat-treated under the conditions according to the present invention, a material having extremely excellent characteristics. Can be provided. With this material, it is very effective for materials with severe requirements such as valve train components.

[Brief description of drawings]

FIG. 1 is a graph illustrating internal friction in the present invention.

FIG. 2 shows the minor axis diameter of particles for examining the test results of the present invention,
It is explanatory drawing of a major axis diameter.

FIG. 3 is a graph showing a particle size distribution.

FIG. 4 is an explanatory diagram of a processed material of the present invention.

FIG. 5 is a graph of test results showing the relationship between heat treatment temperature and time in a nitrogen stream and strength.

FIG. 6 is a graph showing the swing fatigue characteristics of the test piece of the present invention.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Akira Yamakawa 1-1-1 Kunyokita, Itami City, Hyogo Prefecture Sumitomo Electric Industries, Ltd. Itami Works

Claims (4)

[Claims] 1. A method of manufacturing a silicon nitride-based sintered body, comprising:
A method for producing a silicon nitride-based sintered body, which comprises subjecting the material for processing the sintered body to heat treatment at a temperature within 150 ° C. above and below the temperature at which the maximum value of the peculiar peak of internal friction is exhibited. 2. The predetermined temperature holding time of the heat treatment is 5 to 6
The method for producing a silicon nitride-based sintered body according to claim 1, which is 00 minutes. 3. The method for producing a silicon nitride-based sintered body according to claim 1, wherein the heat treatment is performed in an atmosphere other than a reducing gas atmosphere. 4. A method of manufacturing a material for processing a sintered body, comprising the steps of (1) mixing powders of silicon nitride and a sintering aid to form a mixture, (2) molding the mixture, and molding. A step of forming a body, (3) a step of sintering the formed body in an N ₂ -containing atmosphere to form a sintered body, and (4) a step of processing the sintered body into a predetermined dimension to form a processing material, A method for manufacturing a silicon nitride-based sintered body containing the same.