JPH08301663A - Production of si3n4/sic-based nanocomposite material - Google Patents

Abstract

PURPOSE: To reduce the free carbon remaining when baking an Si3 N4 )/SiC-based nanocomposite material by using a ultrafine powdery SiC. CONSTITUTION: An Si3 N4 powder and an ultrafine powdery SiC in an amount of 3-45vol.% based on the Si3 N4 powder are heat-treated at a temperature within the range of 1388-1900 deg.C in a state of a pressure of a gas generated by reaction on or below a curve in the figure corresponding to the temperature for >=10min and then baked.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a method for producing a Si ₃ N ₄ / SiC nanocomposite material of ultrafine SiC composite material is produced by a plasma CVD method Stated in more detail and contain free carbon The present invention relates to a method for producing a Si ₃ N ₄ / SiC-based nanocomposite material by firing using ultra-fine powder SiC as a raw material of a composite element.

[0002]

BACKGROUND ART published below paper such as by indicating the room temperature and high temperature strength SiC is Si ₃ N ₄ / SiC nanocomposite material present in Si ₃ N ₄ grains in was very good is a composite element NIIHARA Have been.

[0003] 1) K. Izaki et al; Ultrastructure Pro
cessing of Advanced Ceramics, ed.JMMackenzie and
DRUlrich, John Wiley & Sons. New York, 1988, p
p.891-900. 2) K. Niihara et al; Proc. of MRS Int. Meeting on
Advanced Materials, Tokyo, 1988, pp. 107-12. 3) K. Niihara et al; J. Jpn. Soc. Powder Powder M
etall., 36 (1989) 243-46. 4) K. Niihara et al; J. Mater. Sci. Lett., 9 (199
0) 598-9. 5) K. Niihara et al; J. Mater. Sci. Lett., 10 (199)
0) 112-14. 6) K. Niihara et al; J. Jpn. Soc. PowderPowder Meta
ll., 37 (1990) 352-53 7) K. Izaki et al; J. Jpn. Soc. Powder Powder Meta
ll., 37 (1991) 357-60

[0004] In these reports, [S]
i (CH ₃ ) ₃ ] ₂ Si—C—N amorphous powder obtained by synthesis from NH—NH ₃ —N ₂ powder was used as the SiC raw material powder. However, since this SiC raw material powder contains a large amount of excess carbon, SiC and Si
₃ N ₄ that there is a problem that free carbon after crystallization inhibits densification of the remaining firing material has been found result of the inventors' studies.

On the other hand, the use of ultrafine SiC produced by a plasma CVD method as a composite element of the nanocomposite material is also known from the following article. In this paper,
Ultrafine SiC produced by the plasma CVD method is replaced with ordinary S
Mix with i ₃ N ₄ powder and sintering aid and sinter by hot pressing (sometimes called firing)
Wood. 8) J. Ceramic Society, Japan 101 (12) 1430-1431,
1993

Accordingly, the article 8) shows that a Si ₃ N ₄ / SiC nanocomposite material can be produced even when ultrafine SiC is used as a composite element.
However, in 8), the necessity and method of removing free carbon inevitably present in the ultrafine SiC powder are unknown.

Table 1 shows general analytical values and physical properties of ultrafine SiC produced by the plasma CVD method. What should be noted in this table is the total carbon content of 32.0% as an average value. Assuming that the remaining components in Table 1 excluding trace amounts of O and N are Si, Si is 67.55% on average. Assuming that all Si (67.55%) exists in the form of SiC, the carbon amount required for bonding with SiC is 28.8.
9%. That is, 32.0−28.89 = 3.11%
The carbon remains as free carbon in the raw material powder.

[0008]

[Table 1] General properties of ultrafine SiC ──────────────────────────────────── Chemical composition C (wt.%) 31.8-32.2 Total O (wt.%) 0.3-0.6 Total N (wt.%) <0.03 Na (ppm) <0.1 Mg (ppm) <0.1 Al (ppm) <0.1 Cr (ppm) <0.1 Mn (ppm) <0.1 Ni (ppm) <0.1 Cu (ppm) <0.1 Zn (ppm) <0 0.5 Fe (ppm) <0.2 Ca (ppm) <0.2 K (ppm) <0.2 Physical Properties of Powder Specific Surface Area 40 to 50 m ² / g Average Particle Size 0.03 μm True specific gravity 3.1 g / cm ³ Crystal structure 3C (β phase)

[0009]

As described above,
Free carbon inevitably contained in ultrafine SiC is Si
_The present inventors have found that they remain in the 3N ₄ / SiC-based nanocomposite material, impairing the compactness, and as a result, reducing the strength at room temperature and high temperature.

[0010]

Means for Solving the Problems The present invention has been made in view of the problems that the inhibition of densification due to the residual of such free carbon, Si ₃ N ₄ powder and, the Si ₃ N
₃ to 45% by volume of ultrafine SiC powder is
After the heat treatment for 10 minutes or more at a temperature in the range of 388 to 1900 ° C. and the pressure of the gas generated by the reaction on or below the curve in FIG. A method for producing a characteristic Si ₃ N ₄ / SiC nanocomposite material is provided.

First, the physicochemical aspects of the heat treatment method of the present invention will be described. In the present invention, most of the free carbon in the SiC in the ultrafine powder is converted to SiC as a composite element by reacting with the main material of the composite material. Si ₃ N ₄ + 3C (free carbon) = 3SiC + 2N ₂ ... In this reaction, the reaction initiation temperature when P _N2 = 1 atm can be calculated from the following equation. SiC = Si + C... The standard free energy of this reaction can be calculated by equation (1) in FIG. 3 / 2Si + N ₂ = 1 / 2Si ₃ N ₄ ... The standard free energy of this reaction can be calculated by equation (2) in FIG. The standard free energy of the equation is -3 ×-
Since it is obtained by 2 ×, it is represented by equation (3) in the figure. The Gibbs free energy of the formula is Si ₃ N ₄ , C and Si
Assuming that the activity (a) of C = 1, it is expressed by the equation (4) in FIG. If P _N2 (nitrogen gas pressure) = 1 atm, log
Equation (5) is the equilibrium condition of the equation from P _N2 = 0. From Expressions (3) and (4), Expression (6): 64700-2.73TlogT-30.
16T = 0 is obtained. That is, T that satisfies the equation (6) is a temperature at which the equation is balanced. Equation (6) cannot be solved algebraically, so if it is solved graphically, the temperature that satisfies Equation (6) is T
= 1388 ° C. That is, in an atmosphere of P _N2 = 1 atm, at 1388 ° C. or higher, free carbon reacts with Si ₃ N ₄ to form SiC.

By the way, in an experiment conducted by the present inventor, since 0.933% of free carbon was contained in the mixed powder,
The carbon sleeve having a diameter of 188 mm contained 450 g / times × 0.933 / 100 = 4.20 g / times of free carbon. This is 4.20 / 12 = 0.35 mol
Therefore, if this is completely reacted by the equation, the N ₂ gas becomes 0.35 mol × 2/3 = 0.233 mol.
1 occurs. Equation (7): P from the equation of state of an ideal gas
Since V = nRT, if the volume (V) occupied by the N ₂ gas in the carbon sleeve is calculated assuming that the filling rate of the raw material powder is 40%, V = π (18.8 / 2) ² × 3 × 0.6 ×
(1/1000) = 0.5 (liter) and P = 1
Atm, T = (273 + 1375) K = 1648K and substituting it into equation (7) (where R = 0.082 (liter) .atm / deg.mol), n = (1 × 0.
5) / (0.082 × 1648) = 3.7 × 10 ⁻³ (m
ol).

Assuming that the reaction of the formula occurs and the reaction gas is entirely confined in a carbon sleeve having a diameter of 188 mm × 30 mm, the pressure increase ΔP at that time becomes ΔP · V = △ nRT from equation (7). Equation (8): ΔP = (ΔnRT) / V Now Δn = 0.233 mol, T = 1684 ° K, V = 0.
Substituting 5 liters (each in the preceding paragraph) into equation (8) gives: ΔP = 0.233 × 0.082 × 1684 / 0.5 = 6
4.3atm

[0014] That is when changing the SiC in the reaction free carbon is, if the nitrogen gas pressure = 1 atm condition is that the N ₂ gas is generated with even a volume 64.3 fold if coercive be.

[0015] In this case, P _N2 is if the P _N2 is higher not kept the conditions of 1atm, to calculate whether the equilibrium temperature of expression is in many times. Since free energy = 0 represented by the equation (4) is an equilibrium condition, the equation (9) in FIG. 4 is obtained. When the equation (9) is written out using the equation (3), the equation (10) is obtained. FIG. 1 is a graph plotting values satisfying the temperature and the pressure of Expression (10).

The following can be seen from FIG. (A) The temperature at which the equation equilibrates when P _N2 = 1 atm is 1700
The nitrogen gas pressure (P _N2 ) is 5 at
When m is reached, no reaction of the formula occurs, and free carbon remains. (B) Temperature (T) corresponding to increase in nitrogen gas pressure (P _N2 )
When the nitrogen gas pressure (P _N2 ) increases, a very high temperature (T) is necessary for the reaction of the equation to proceed to the right. (C) This is because even if the raw material powder is charged into a die, a container, a jig, and the like and maintained at a temperature at which the reaction of the formula occurs, a large amount of N ₂ gas is generated by the reaction of the formula, and the gas is released. If the reaction is insufficient, the nitrogen gas pressure (P _N2 ) immediately rises and the reaction initiation temperature rises, which means that the reaction does not occur at the temperature at which the reaction of the formula first occurs. That is, free carbon remains.

Therefore, free carbon is converted into Si by the reaction of the formula
In the present invention to remove from C, it is important to maintain the reaction conditions in the region where the reaction of Si ₃ N ₄ + 3C → 3SiC + 2N ₂ occurs in FIG.

Having described the physicochemical aspects of the present invention, the constituent features of the present invention will now be described. First, ultrafine SiC is used. The ultrafine SiC in the present invention satisfies the following conditions (1) to (3). (1) The average particle size is 0.1 μm or less. (2) Having a crystalline structure rather than an amorphous state. (3) No metal Si exists.

In addition to the above conditions, the conditions required for ordinary ceramic raw material powders are as follows: (4) The amount of impurity elements is as small as possible. (5) The uniform particle size and sharp particle size distribution are satisfied. It is highly preferred that

Next, as the Si ₃ N ₄ powder, a powder produced by any of the direct nitriding method and the imide method can be used. However, in order to facilitate liquid phase sintering, the average particle size is desirably a nano size (nm) of 1 μm or less, and the particle size distribution is desirably sharp with a small deviation from the average value.
In order not to deteriorate the high-temperature strength, it is important that the Si ₃ N ₄ powder has high purity. At least the amount of impurities of the metal element and the halogen element is set to 100 pp for each element.
m or less, preferably 50 ppm or less.

Prior to firing, a heat treatment for fixing free carbon to SiC, which is the most significant feature of the present invention, is performed. Therefore, the temperature of the heat treatment applied to the mixed powder of the Si ₃ N ₄ powder and the ultrafine SiC powder (the specific mixing method will be described later) is set to 1388 ° C. or more and 1900 ° C. or less. If the temperature is lower than 1388 ° C., the reaction of the formula does not occur, and if the temperature is higher than 1900 ° C., Si ₃ N ₄ is decomposed and sublimated. The heat treatment time is 1
0 minutes or more. This is because the reaction of the formula starts everywhere in the powder if the temperature and the N ₂ gas pressure satisfy the reaction initiation conditions, but as a practical part, the N ₂ gas escapes from the powder and out of the heat treatment furnace. This is because a heat treatment time of about 10 minutes is required even for a fired body having a lower limit dimension.

It is very preferable to keep the pressure of the N ₂ gas generated by the reaction at about 1 atm. N ₂ gas pressure in the heat treatment furnace is decomposed and Si ₃ N at a significantly lowered when Si ₃ decomposition temperature of N ₄ drops annealing temperature than one atmosphere, also becomes higher than one atmosphere, since the reaction starting temperature is higher However, it is not preferable from the viewpoint of energy saving.

Hereinafter, a specific example of a method of firing the nanocomposite material after the heat treatment for converting free carbon to SiC will be described. However, the invention is not limited to this method,
Once disintegrated mixed powder heat treated, milled, followed by the addition of sintering aids further mixed as necessary, even dust and then firing method of, Si ₃ having the desired properties
An N ₄ / SiC nanocomposite material can be manufactured.

The sintering aid which may be added to the powder mixture subjected to the heat treatment may be of any kind as long as it is an oxide which forms a liquid phase with Si ₃ N ₄ and promotes sintering. Good. For example, Y ₂ O ₃ is generally used as a sintering aid to increase the high-temperature strength of a nanocomposite material, and Al ₂ O ₃ —Y ₂ O ₃ is used to achieve better sinterability.
The system is common. Sintering aid has an average primary particle diameter of 1 μm
It is very preferable that the particle size is the same as follows. In addition, it is highly desirable that the purity of the sintering aid itself be high,
It is highly desirable that each impurity be 100 ppm or less, especially 50 ppm or less.

Next, the mixing ratio of the ultrafine powder SiC, Si ₃ N ₄ and the sintering aid will be described. First, the mixing ratio of ultrafine SiC is 3 vol% with respect to the total amount of Si ₃ N _4.
In the range of 45 vol% or less, ultrafine SiC exhibits a preferable composite component effect. This compounding ratio is 3vol
% Or less, the effect of the ultrafine powder SiC as a composite element is small, and if it is 45 vol% or more, the amount of the ultrafine powder SiC is too large and the nanocomposite material is not densified.

To explain the compounding ratio of the sintering aid, it is preferable that the mixing ratio is 1 wt% or more and 10 wt% or less based on the total weight of the nanocomposite material. A particularly desirable addition range of the sintering aid is 3 wt% or more and 8 wt% or less. If it is 1 wt% or less, the amount of the liquid phase at the time of sintering is too small to exhibit an effect as a sintering aid. This is because calcination is hindered.

The method of mixing the raw material powder for heat treatment or firing is not limited at all. In consideration of the economics of mixing, the ball mill method is generally used. However, in the ball mill method, it is inevitable that the ball and the pot contaminate the raw material powder, so that a ball and a pot made of SiC, Si ₃ N ₄ or a sintering aid should be used. The mixing time should ensure that these three types of raw materials are sufficiently mixed. Specifically, the time is 24 hours or more, preferably 48 hours or more.

After the wet mixing, it is necessary to evaporate the mixed medium. However, if the medium is evaporated at an excessively high temperature, the mixed powder will agglomerate and hinder densification during firing. It is necessary to reduce the aggregation of the mixed powder as much as possible. For this purpose, one method is to evaporate the mixed medium while vacuum-sucking the container containing the mixed slurry. By performing vacuum suction, the evaporation point is lowered, so that the medium can be evaporated at a low temperature, and as a result, a dry mixed powder with very little aggregation can be obtained.

Next, the heat-treated mixed powder is formed into various shapes. It is assumed that the molding method may be anything such as press molding, injection molding, extrusion molding, tape molding, etc. which are common in ceramic technology. As the binder used for each of them, a general binder in the field of ceramics can be used. The binder removal method after molding may be a general method in the field of ceramics.

The firing may be normal pressure firing, gas pressure firing, hot pressing, or HIP firing. Preferred firing conditions in the subsequent firing step after the heat treatment differ depending on the firing method. First, in normal pressure and gas pressure firing, N ₂ gas should be used as an atmosphere gas to prevent sublimation of Si ₃ N ₄ . Firing temperature is 1500 ℃ or more and 2100 ℃
The following is assumed. 2100 ° C without densification below 1500 ° C
Above, the practical maximum gas pressure of gas pressure firing is about 1
This is because sublimation of Si ₃ N ₄ occurs at 0 kg / cm ² . However, if the N ₂ gas pressure is higher, firing at a higher gas pressure firing temperature becomes possible, which is more advantageous for densification. However, large costs are incurred for safety measures, maintenance and maintenance of the firing furnace. In addition, since there is little problem of sublimation in HIP baking, baking at an arbitrary temperature can be performed on a high temperature side.

On the other hand, in hot press firing, the mixed powder is filled in a mold such as a graphite mold, so that if it is filled as it is, the ultrafine powder SiC will react with graphite. For this reason, it is necessary to apply BN to the surface of the graphite mold so that the ultrafine powder SiC does not directly contact the graphite. The atmosphere gas may be Ar gas or N ₂ gas after the heat treatment of the mixed powder is completed. However, N ₂ gas is desirable in order not to cause sublimation of Si ₃ N ₄ . Firing temperature is 1500
The temperature is set to not less than 2000 ° C. This is because densification does not occur at 1500 ° C or lower, and at 2000 ° C or higher, a phenomenon occurs in which the structure becomes coarse and the strength of the reaction between the sintering aid and SiC is hindered. The hot pressing pressure is preferably 50 kg / cm ² or more and 450 kg / cm ² or less. 50kg / cm
^If it is less than ² , there is almost no pressure effect, and 450 kg /
Above cm ² , the graphite mold may break under pressure.
Desired pressure range in obtaining the Si ₃ N ₄ / SiC nanocomposite material close to the true density is 250 kg / cm ² or more ~400kg / cm ² or less.

[0032]

[Function] Si ₃ N ₄ -30 wt% ultrafine powder SiC-4 wt
The free carbon contained in the% Y ₂ O ₂ mixed powder is less than 1%, and the N ₂ gas generated by the reaction between the free carbon and Si ₃ N ₄ is 1 atm, which is 64 times the carbon die space. The reaction of the formula is very sensitive to the pressure of N ₂ gas and P
_{When N2} is slightly increased, the equilibrium temperature is significantly increased. For example, when P _N2 = 5 atm, the reaction initiation temperature is 1700 ° C. (ΔT
= 325 ° C). Therefore, it is important to improve the outgassing to keep the reaction conditions below the curve in FIG. Next, the present invention will be described with reference to specific examples.

[0034]

EXAMPLES Table 2 shows the powder data used in the examples for ultrafine SiC, Table 3 for Si ₃ N ₄ , and Table 4 for Y ₂ O ₃ used as a sintering aid.

[0035]

Table 2 Chemical analysis values Carbon (wt.%) 32.2 Oxygen (wt.%) 0.31 Nitrogen (wt.%) 0.012 Ca (ppm) 0.9 K (ppm) 0.1 Fe ( ppm) N.P. D. Cr (ppm) D. Ni (ppm) D. Al (ppm) 0.3 Na (ppm) 0.1 Zn (ppm) 0.1

In Table 2, carbon is LECO WR-1
Oxygen and H ₂ O were analyzed by LECO T
Nitrogen is LECO by C-436 oxygen / nitrogen analyzer
The TC-436 oxygen / nitrogen analyzer was used to analyze other elements by ICP emission spectroscopy flame method. The specific surface area of the ultrafine powder SiC is 48.0 m ² / g [BET
Method], the average particle diameter was 0.03 μm [TEM observation], and the crystal form was mainly β-SiC [powder X-ray diffraction method, electron beam diffraction method].

[0037]

Table 3 Powder data of Si ₃ N ₄ powder N (wt%)> 38.0 O (wt%) 1.48 Cl (ppm) <100 Fe (ppm) <100 Ca (ppm) <50 Al (ppm) ) <50 Crystallinity (%)> 99.5 β / (α + β) (wt%) <5 Specific surface area (m ² / g) 11.3

[0038]

Table 4 Inspection items of powder data of Y ₂ O ₃ powder Analysis value Y ₂ O ₃ (%)> 99.9 CeO ₂ (ppm) <10 Pr ₆ O ₁₁ (ppm) <10 Nd ₂ O ₃ (ppm) <10 Sm ₂ O ₃ (ppm) <30 Tb ₄ O ₇ (ppm) <50 Dy ₂ O ₃ (ppm) <50 CaO (ppm) <10 Fe ₂ O ₃ (ppm) <10 Ig loss (1000 ° C. for 1 hour) ) (%) 1.3 BET (m ^{2 /} g ) 19.3 Average particle size (μm) 0.4

60 g of ultrafine SiC powder and 1 g of Si ₃ N ₄ powder
32 g and 8 g of Y ₂ O ₃ powder were placed in a 2 liter pot made of Si ₃ N ₄ with 1 liter of ethanol and 50 balls of Si ₃ N _4.
The mixture was put together with 0 cc, and wet-mixed by a ball mill for 48 hours. After the wet mixing, evaporator drying (water temperature was 60 ° C.) was performed, and after drying, the mixture was sieved through a 60-mesh sieve, and only the lower part of the sieve was used as a mixed powder. The mixed powder was filled in a graphite die coated with BN and heat-treated under the conditions shown in FIG. Nitrogen pressure (P _N2 ) is 1 atm and heat treatment temperature is 130
0, 1400, 1500, 1600, 1650, 170
0 ° C. After that, the results of analysis of free carbon are shown in Table 5.
Shown in As a comparative example, the amount of free carbon in the mixed raw material powder was also analyzed at the same time. Table 5 shows the results.

[0040]

Table 5 Free carbon analysis results Heat treatment conditions Free carbon amount (wt%) Remarks 1300 ° C × 60 minutes 2.36 Comparative example 1400 ° C × 60 minutes 2.01 Example 1500 ° C × 60 minutes 1.83 Example 1600 ° C × 60 minutes 0.23 Example 1650 ° C. × 60 minutes 0.01 Example 1700 ° C. × 60 minutes 0.04 No Example 2.40 Comparative Example, Raw Material Powder

As can be seen from Table 5, the reaction of the formula is 140
The heat treatment started at around 0 ° C.
Since free carbon was almost completely fixed at 50 ° C., the temperature / time program for firing the mixed raw material powder was as shown in FIG. In FIG. 3, 1650 ° C. × 60
Min heat treatment is included. JIS from the fired body obtained
A test piece indicated by reference numeral 1601 was cut out, and the room temperature strength and the high temperature strength at 1300 ° C. were measured by a three-point bending test. The results were as follows. Room temperature strength 125 kg / mm ² (N = 10) Strength at 1300 ° C. 117 kg / mm ² (N = 5) As described above, a high strength exceeding 100 kg / mm ² was obtained at room temperature, and this was maintained up to 1300 ° C. Surprising performance.

[0042]

The nanocomposite produced by the method of the present invention has high room temperature strength and high temperature strength, and can be applied to cutting tools, engine parts, high temperature jigs, and the like.

[Brief description of the drawings]

FIG. 1 is a graph showing temperature and pressure conditions for converting free C into SiC. The left vertical axis is logP _N2 , and the right vertical axis is P _N2 (atm).

FIG. 2 is a graph showing a heat treatment pattern.

FIG. 3 is a graph showing a firing pattern.

FIG. 4 is a table showing equations (1) to (5), (9) and (10).

Claims (5)

[Claims] 1. A and Si ₃ N ₄ powder, a 3 to 45 volume% of the ultrafine powder SiC with respect to the Si ₃ N ₄ powder, 1388～
A heat treatment is performed at a temperature within a range of 1900 ° C. and a pressure of a gas generated by the reaction on the curve shown in FIG. 1 or below the curve corresponding to the temperature for 10 minutes or more, followed by baking. Of producing a Si ₃ N ₄ / SiN nanocomposite material. 2. The method according to claim 1, wherein a mixture obtained by mixing 1 to 10% by weight of a sintering aid with respect to the total amount of the Si ₃ N ₄ powder and the ultrafine powder SiC is fired. Si
_A method for producing a 3N ₄ / SiC-based nanocomposite material. 3. The sintering method is a gas pressure sintering method.
The production method of the described Si ₃ N ₄ / SiC nanocomposite material. 4. The sintering method is a HIP sintering method.
A method for producing the described Si ₃ N ₄ / SiC-based nanocomposite material. 5. The method for producing a Si ₃ N ₄ / SiC nanocomposite material according to claim 2, wherein the firing method is a hot pressing method.