GB2220409A - Beta-silicon carbide powder for sintering and method of preparing same - Google Patents

Description

1 1.. 1.11 J1 ') i U 1 C 1. -:

i - 7 Z_ '. ' P-SiC POWDER FOR SINTERING AND METHOD OF PREPARING SAME This invention relates to a P-SiC powder useful as a sintering material and a method of preparing same.

Sintered bodies of SiC, in particular P-SiC (cubic system), are excellent in heat resistance and retain high mechanical strength at high temperatures. Of course sintered SiC is excellent in corrosion resistance and wear resistance like other ceramics. Therefore, much interest has been shown in the use of sintered SiC bodies as various machine parts and even as high temperature structural parts of gas turbines and other engines.

The mechanical characteristics of a sintered SiC body depend on the microstructure of the sintered body, and the microstructure is significantly affected by the physical properties of the SiC powder subjected to sintering. Besides, it is a matter of course that the mechanical and other physical characteristics of the sintered body are affected by the concentrations of impurities in the sintered body and, hence, impurities in the powder used for sintering. In view of such facts there are various proposals of methods for preparing SiC powder suitable as a sintering material.

In some of hitherto proposed methods for preparing P-SiC powder (e.g., JP-A 54-121298 and JP-A 61-168516) attention is paid to the content of the 2HQ phase of SiC.

However, in actually producing sintered bodies by using P-SiC powders obtained by the proposed methods it is difficult to desirably control the microstructure of the sintered bodies by controlling the sintering temperature and time. More particularly, it is often that abnormal growth of crystal grains occurs during sintering, and it is difficult to obtain sintered bodies constituted of uniformly sintered particles. Consequently the sintered bodies are liable to fail to fully exhibit excellent mechanical characteristics inherent to sintered j9-SiC. Presumably an important reason for such difficulties is that a relatively large amount of 2HQ phase is contained in the proposed SiC powders.

It is an object of the present invention to provide a method of preparing a 8-SiC powder which is suitable as a sintering material for producing sintered P-SiC bodies having a good and uniform microstructure.

In another aspect it is an object of the invention to provide a P-SiC powder which contains a limited amount of 2M phase and can easily be sintered to P-SiC bodies having a good and uniform microstructure and high mechanical strength.

According to the invention there is provided a method of preparing a PSiC powder which is not larger than 1,Am in mean particle size and contains less than 5 wt% and not less than 1 wt% of 2HQ phase of SiC, the P method comprising the steps of mixing a silica powder not larger than 50 Am in particle size and not more than 0.1 wt% in the total content of metal impurites with a carbon powder smaller than 1,Am in particle size and not more than 0.2 wt% in ash content such that in the obtained mixture the C/SiO 2 molar ratio is in the range from 2.8 to 4.0, and firing the mixture of the silica powder and the carbon powder in a nonoxidizing atmosphere for not more than 2 hr at an equalized temperature not lower than 1700 0 C and lower than 2000 0 C.

In this method the equalized temperature refers to the temperature in the center of a homogeneous temperature zone in which differences in temperature are not more than 500C. That is, in the case of performing this method batchwise, the silica/carbon powder mixture (in practice a vessel containing the mixture) is kept in a homogeneous zone preheated to a desired temperature for a predetermined length of time which is not longer than 2 hr. In the case of performing the same method in a continuous manner, the mixture is conveyed so as to stay in a homogeneous temperature zone for a predetermined length of time which is not longer than 2 hr.

Furthermore, the present invention provides a P-SiC powder which contains less than 5 wt% and not less than I wt% of 2M phase of SiC and is not more than 0.1 wt% in the total content of metal impurities, not larger, than 1,Km in mean particle size and in the range from 12 to 20 m 2 Ig in specific surface area measured by the BET adsorption method.

Regarding a sintered body of A-SiC, the mechanical strength of the sintered body is not so high as is expected if cystals of (X-SiC coexist in the sintered body. The principal reason for the lowering of the strength is that residual stresses exist in the sintered body because thermal expansion coefficients of different crystals are different according to directions. Besides, the phase transition from the P-phase to the Ck-phase is liable to cause abnormal growth of the grains under sintering and resultant degradation of the mechanical characteristics of the sintered body. From every point of view it is desirable to produce a sintered body which has a uniform structure and is close to a single-phase of t3-SiC, and accordingly it is desirable to use a powder of SiC close to purep-sic. Even from a practi cal point of view it is desired that that the content of Ck-SiC in the sintered body be not more than 5%.

We have discovered that when a P-SiC powder in which the content of 2HOk phase is less than 1 wt% is sintered at a temperature suited to sufficient densifi cation there occurs a considerable increase in the content of crystal grains of the Mi phase (Ck-SiC) resulting from the transition from the 3C phase to 6HM, so that the sintered body is liable to contain more than 5% of U-SiC. Furthermore, abnormal growth of grains is liable to-occur during sintering of such a P-SiC powder. A probable reason for the increase in 6HQ phase is that a very small amount of 6HO( phase is contained in the SiC powder and serves as seed crystals. We have experimentally confirmed that the addition of a very small amount of seed crystals of 6HO phase to a -SiC powder which does not suffer from the phase transition to 6HQ phase under usual conditions of sintering results in a considerable increase in the content of 6HO( phase during sintering.

On the other hand, it is unfavorable to use a P-SiC powder containing a large amount of 2HO( phase because the transition of 2HOkto P-SiC or other phases of 0(-SiC during sintering may cause abnormal growth of grains.

Furthermore, when it is intended to prepare a P-SiC powder containing more than 5 wt% of 2HQ phase it is almost inevitable that the particle size distribution of the obtained powder considerably expands toward the smaller particle size side. When a SiC powder having too fine particles is compacted into a green body and sintered both the green body and the sintered body are undesirably low in bulk density, and it is difficult to desirably control microstructure of the sintered body.

Considering the above matters, the present invention proposes to limit the content of 2HQ phase in a P-SiC powder within the above defined strict range-and control the particle size of the SiC powder in the above stated manner in terms of mean particle size and specific surface area. Besides, it is preferable to control the particle size distribution of the Sic powder as will be described hereinafter.

We have found that a P-SiC powder proposed herein can be prepared by controlling the properties of the raw materials and the firing conditions as stated as hereinbefore. By using a A-SiC powder according to the invention it is easy to produce sintered bodies of P-SiC which is less than 5 wt% in the content of Ck-SiC and free of abnormally grown grains and, hence, exhibits excellent mechanical characteristics.

When a powder mixture of silica and carbon is fired to obtain a Sic powder, the reduction of silica and carburization of silicon will proceed through reactions represented by the following equations.

c + 0 2 co 2 c + co 2 2C0 (1) (2) Sio 2 + co Sio + co 2 (3) SiO + 2C sic + co (4) Since the reaction of equation (1) does not directly participate in the formation of Sic, the total process of forming Sic can be taken as the summation of the equations (2), (3) and (4) and hence can be represented by the following equation (5).

Sio 2 + 3c - Sic + 2C0 (5) In firing a mixture of silica and carbon, CO 2 begins to evolve as the temperature reaches above 700 0 C because carbon in the mixture and/or carbon in the materials of the reaction vessel and firing apparatus reacts with oxygen unintentionally carried into the reaction system together with the raw materials. CO 2 formed by this reaction will not be able to stably exist at the high temperature to which the silica/carbon mixture is heated to form P-SiC and, rather, will react with coexisting carbon to form CO gas as represented by equation (2). Then, as represented by equation (3), the CO gas serves the function of reducing silica to evolve SiO gas. Next, the SiO gas attacks the carbon powder to result in the formation of -SiC in powder form by the reaction of equation (4).

Each of the above reactions is a gas-solid reaction, and accordingly the rate of reaction depends significantly on the physical form of the solid reactant. Furthermore, equation (4) of the final gas- solid reaction indicates that the particle size of the obtained A-SiC powder depends primarily on the particle size of the carbon powder used as one of the raw materials. Therefore, for preparing a A-SiC powder not larger than 1 Am in particle size it is necessary to use a carbon powder not larger than 1Km in particle size. In order that the SiC forming reactions may proceed smoothly it is necessary that the rate of the reaction of equation (3) be sufficiently high. For this reason it is necessary to use a silica powder not largern than 50 Am in particle size.

It is desirable to obtain a 13-SiC powder low in the total content of metal impurities. If a P-SiC powder containing considerable amounts of metal impurities is used for sintering, the impurities promote abnormal growth of grains and also transition of 3C and 2Ha phases of SiC to 6HOk and/or 4HOk phases during sintering, 10 and most of the impurities remain in the sintered body. In general, metals possibly present in SiC as unfavorable impurities are Al, Fe, Ca and Mg. To obviate such disadvantages and to obtain a p-SiC powder in which the total content of metal impurities is not more than 0.1 wt%, the present invention requires that the total content of metal impurities in the silica powder be not more than 1 wt% and that the ash content in the carbon powder be not more than 0.2 wt%.

In the powder mixture of silica and carbon the C/Sio 2 molar ratio is limited within the range from 2.8 to 4.0. If the C/SiO 2 molar ratio is below 2.8 a considerably portion of the silica powder will remain unreacted, and necking of the SiC particles with each other is liable to occur. It is uneconomical to make the C/SiO 2 molar ratio above 4.0 since it means using an unnecessarily large amount of carbon powder. Preferably the C/SiO 2 molar ratio in the powder mixture of silica and carbon is controlled to 3.0-3.2.

It is optional whether to fire the mixture of silica and carbon in powder form or to granulate the powder mixture in advance of firing.

For reaction to form P-SiC, the silica/carbon mixture is maintained at an equalized temperature lower than 2000 0 C and not lower than 1700 0 C for not more than 2 hr. If the firing temperature reaches or exceeds 2000 0 C or the firing time reaches or exceeds 2 hr the content of 2HO phase in the obtained SiC powder becomes lower than 1 wt%, and sintering of that SiC powder suffers from abnormal. growth of crystal grains and is accompanied by partial transition of the 3C and 2HCk phases to the 6HQ phase. That is, when the mixture of silica and carbon is.heated too intensely or for an excessively long time there appear nuclei of 6HC4 phase as a cause of the unwanted phase transition and abnormal growth of crystal grains. On the other hand, if the firing temperature is lower than 17000C the obtained SiC powder contains more than 5 wt% of M( phase, and the particle size distribution of the SiC powder expands toward the smaller particle size side. In sintering that SiC powder it is likely that the transition of the increased 2H0k phase to other phases causes abnormal growth of crystal grains as mentioned hereinbefore, and it is difficult to control the microstructure of the. sintered body by reason of the inclusion of excessively fine particles in the SiC powder.

The preparation of a P-SiC powder according to the invention can be carried out either batchwise or in a continuous manner.

Even though a P-SiC powder contains an adequate amount of 2HC( phase, it is difficult to produce sintered bodies of a uniform and good structure if the SiC powder is too coarse or too fine. A P-SiC powder according to the invention is not larger than 1,Am in mean particle 2 size and is in the range from 12 to 20 m Ig in specific surface area measured by the BET (Brunauer, Emmett and Teller's) adsorption method. Hereinafter specific surface area measure by this method will be called BET specific surface area.

The BET specific surface area of the P-SiC powder is limited within the aforementioned range because BET specific surface area is not merely an indication of particle size and does contain information as to the shape of the particles. As a sintering material, it is desirable that the particles of the SiC powder be spherical or near- spherical since spherical particles can be compacted into a green body high in bulk density. When the P-SiC powder is not larger than 1.,Am in mean particle size and is in the range from 12 to 20 m 2 /9 in BET specific surface area the particles of the powder have nearly spherical shapes necessary for very close packing.

In producing a sintered body by using a P-SiC powder according to the invention it is preferable to add at least one kind of sintering aid to the SiC powder. Known sintering aids for SiC are all useful in this invention, and it is preferred to use a combination of boron and carbon as sintering aids. Usually it is suitable to add 0.1-1 wt% of boron and 1-3 wt% of carbon to the A-SiC powder.

Usually the P-SiC powder and the sintering aid is mixed in a wet state by using a ball mill, and the mixture is dried to evaporate the liquid medium. The dried SiC powder containing the sintering aid is compacted in a metal mold into a green body of a desired shape. The green body is sintered at a temperature in the range from 1900 to 2100 0 C. If the sintering temperature is lower than 1900 0 C sintering remains incomplete, and hence the sintered body is not sufficiently high in bulk density. However, when the sintering temperature is 20 higher than 20000C there occurs phase transition from A- SiC to Ck-SiC to a considerable extent. Of course the sintering must be performed in a nonoxidizing atmosphere such as an argon gas atmosphere or a.nitrogen gas atmosphere. However, at the stage of raising the temperature it is preferable to keep the green body in vacuum until the temperature reaches about 1500 0 C. This is effective for promoting the reaction of carbon in the green body with silica and any other oxide or free silicon possibly existing in the green body and for dissipation of the resultant carbon compounds. When an organic compound is used as the source of carbon to serve as sintering aid, dissipation of the decomposition products of that organic compound is also promoted.

The invention is further illustrated by the following nonlimitative examples. EXAMPLE 1 As the raw materials a silica sand and a commercially available carbon black were empolyed. The silica sand had a mean particle size of 3 /Am and was less than 0.04 wt% in the content of metal impurities. The carbon black was smaller than 1 Am in particle size and 0. 1 wt% in ash content.

The silica sand and the carbon black were mixed so as to obtain a powder mixture in which the C/SiO 2 molar ratio was 3.0. In a graphite crucible the powder mixture was maintained at a constant temperature of 1800 0 C for 1.5 hr in a vertical Tamman furnace through which argon gas was passed. After that the crucible was left cooling, and the fired powder was taken out of the crucible.

By the above treatment the silica-carbon mixture turned into a SiC powder which had a mean particle size of 0.60,Am. The mean particle size was determined by measuring the particle size distribution of the SiC powder with a particle size distribution analyzer of the centrifugal settling type (SA-CP2 of Shimazu Seisakusho Co.) to obtain a cumulative distribution curve and taking the particle size at 50% of the cumulative curve as a mean particle size. For the measurement the SiC powder was dispersed in 0.2% aqueous solution of sodium hexametaphosphate to prepare a dilute slurry. After adding 1% aqueous solution of ammonia to adust the pH of the slurry to 10.5, the slurry was treated with an ultrasonic disperser for 15 sec and immediately subjected to measurement.

As to the polymorphism and crystallographic composition, X-ray diffraction analysis proved that the obtained powder was of P-SiC (3C phase) crystals containing 1.3 wt% of 2H phase. The polymorphism of SiC was determined from the peak intensities at 33.6 0 and 35.60 (20) by using the quantitative equation of Hase, Suzuki and Iseki (Yogyo-Kyokai- shi, 87, 576 (1979)).

As the source of carbon to serve as a sintering aid, a phenolic resin of resol type (residual carbon 52 wt%) was added to 100 parts by weight of the P-SiC powder such that carbon in the phenolic resin amounted to 2 parts by weight, and the phenolic resin in the mixture was cured. Then 0. 3 part by weight of boron was added as another sintering aid. Using nhexane as aliquid medium, wet mixing of the resultant mixture was carried out for 24 hr in a polyethylene ball mill.

After drying, the powdery mixture was press-shaped in a metal mold under a pressure of 200 kg/cm 2 and further compacted by application of a pressure of 1500 kg/cm 2 with a hydrostatic press.

The molded green body was placed in a highfrequency heating furnace, and the temperature in the heating zone was raised from room temperature up to 1500 0 C while the heating zone was continuously evacuated. After that argon gas was introduced into the furnace to keep an argon gas pressure of 1 atm in the heating zone, and the temperature was further raised up to 2000 0 C. The temperature of 2000 0 C was maintained for 60 min to thereby accomplish sintering of the green body.

The sintered body had a bulk density of 3.15 g/cm 3 measuring by the Archimedean method. The sintered body proved to be formed of 19-SiC crystals not containing OL-SiC by X-ray diffraction analTsis of powdered samples of the sintered body.

The surfaces of the sintered body were mirrorpolished and then treated with an etching liquid prepared by adding 10 g of NaOH and 10 g of K3[Fe(CN)6] to 100 ml of water. By observation of the thus treated surfaces with SEM it was evident that the sintered body was free of abnormally grown grains.

EXAMPLES 2-6

In these examples the 4-SiC powder preparing process and the sintering process of Example 1 were modified, as shown in Table 1, only in respect of the raw materials and their mixing ratio and the conditions of firing for reaction. In every example the obtained SiC powder and the sintered body were analyzed in the same manner as in Example 1. The results are shown in Table 1. In every example, the sintered body was free of abnormally grown grains.

COMPARATIVE EXAMPLES 1-10 For comparison,.the raw materials of SiC and/or the firing conditions in Example 1 were further modified as shown in Table 1. In every case the obtained SiC powder and the sintered body were analyzed in the same manner as in Example 1. The results are shown in Table 1. The sintered bodies of Comparative Examples 1, 2 and 3 were free of abnormally grown grains, but in Comparative Examples 4 to 10 abnormally grown grains were found in the sintered bodies.

TABLE 1

Raw Materials Firing SiC Powder Sintered Body 0--A44.4^- S10 2 Carbon Mean Metal Ash. C/Sio Parti- Impuri- 2 cle ties (wt%) ( ol:r (OC) (hr) Size r:ti 0m) (wt%) Mean Polymor- Bulk Polymorphism MSie Temp. Time Partiphism Density cle Size (wt%) (g/cm 3 (wt%) (JAM) Example 1 3 0.04 0.1 3.0 1800 1.5 0.60 3C, 2H 1.3 3.16 3C 100 Example 2 2 0.03 0.1 3.2 1800 1.0 0.58 3C, 2H 4.8 3.16 3C 100 Example 3 2 0.03 0.1 3.2 1750 2.0 0.56 3C, 2H 4.9 3.16 3C 100 Example 4 3 0.04 0.1 3.8 1780 1.0 0.57 3C, 2H 1.0 3.15 3C 100 I 0.

Example 6 26 0.04 0.1 3.0 1900 2.0 0.58 3C, 2H 3.0 3.14 3C 100 M Example 6 2 0.03 041 3.2 1950 0.6 0.60 3C, 2H 2.5 3.13 3C, 6H 98 Comp. Ex. 1 62 0.1 0.1 3.0 1950 1.5 0.45 3C, 2H.15.0 3.02 3C 100 Comp. Ex. 2 146 0.1 0.1 3.0 1800 2.0 0.36 3C, 2H 20.2 3.00 3C 100 Comp. Ex. 3 2. 0.03 0.1 3.2 2020 2.0 1.10 3C 0 2.95 3C, 6H 90 Comp. Ex. 4 3 0.04 0.1 3.0 1800 2.6 0.62 3C, 2H 0.5 3.16 3C, 6H 92 Comp. Ex. 5 3 0.04 0.1 3.0 2050 1.0 0.73 3C, 6H 0 3.10 3C, 6H, 16R 72 Comp. Ex. 6 3 0.15 0.1 3.0 1960 1.5 0.65 3C, 4H 0 3.08 3C, 4H, 6H 79 Comp. Ex. 7 3 0.32 0.1 3.0 1800 1.6 0.66 3C 0 3.07 3C, 4H, 6H 76 Comp. Ex. 8 3 0.04 0.3 3.2 1800 1.5 0.62 3C 0 3.07 3C, 4H, 6H 78 comp'. Ex. 9 3 0.04 0.1 2.6 1800 1.5 0.82 3C, 2H 25.0 3.04 3C, 6H 91 Comp. Ex.10 3 0.04 0.1 3.0 1750 3.0 0.76 3C, 2H 0.5 3.i4 3C, 4H, 15R 89 EXAMPLE 7

A A-SiC powder was prepared from a silica sand and a carbon black by fundamentally the same method as in Example 1. This P-SiC powder was 16.8 m 2 /9 in BET specific surface area and 0.4,Am in mean particle size and contained 1.5 wt% of 2HQ phase. The following impurities were detected in this SiC powder: 0.3 wt% of free carbon, 193 ppm of Al, 137 ppm of Fe and 60 ppm of Ca.

Using this SiC powder a green body was formed by the same process as in Example 1, and the green body was sintered by the same method conditions as in Example 1.

The sintered body was analyzed in the same manner as in Example 1 and proved to be free of O-SiC and free of abnormally grown grains. The sintered body had a 3 bulk density of 3.15 g/cm Besides, deflective strength of the sintered body was measured to be 71 kg/mm 2 by the three-point flexural method according to JIS R 1601.

EXAMPLES 8-11

These examples were modifications of Example 7 mainly in respect of the BET specific surface area of and 2Hr. phase content in the A-SiC powder and the sintering conditions. The particulars of the modifica tions are shown in Table 2 together with the character istics of the sintered bodies. In every example the and under the same sintered body was free of abnormally grown grains. COMPARATIVE EXAMPLES 11-17 For comparison, the particulars of the P-SiC powder and/or the sintering temperature in Example 7 were further modified as shown in Table 2. The sintered bodies of Comparative Examples 12, 13 and 14 were free of abnormally grown grains, but in Comparative Examples 11 and 15-17 abnormally grown grains were found in the sintered bodies. The sintered bodies of Comparative Examples 11 and 15-17 contained 4H, 6H and/or 15R phase of Ck-SiC besides A-SiC.

TABLE 2

13-SiC Powder Sintering Sintered Body Condition Bulk P-SiC Deflective BET Metal 2HQ Temp. Time Density Strength Specific Impuri- 0 3 2 Surface ties ( C) (hr) (g1CM) (wt%) (kg/mm) Area (m 2 /g) (wt%) (wt%) Example 7 16.8 0.04 1.5 2000 1.0 3.15 100 71 Example 8 18.2 0.06 4.5 1950 1.0 3.16 100 68 Example 9 13.2 0.02 2.0 2050 0.5 3.12 100 65 Example 10 19.8 0.05 4.8 2000 1.0 3.10 100 63 Example 11 13.9 0.08 2.5 2050 1.0 3.11 100 70 Comp. Ex. 11 16.8 0.04 1.5 2150 1.0 3.14 63 52 Comp. Ex. 12 16.8 0.04 1.5 1850 1.0 2.98 100 35 Comp. Ex. 13 26.2 0.04 10.4 2000 1.0 3.03 100 41 Comp. Ex. 14 10.9 0.03 3.2 2000 1.0 3.00 100 38 Comp. Ex. 15 15.3 0.20 2.0 2000 1.0 3.10 70 57 Comp. Ex. 16 16.5 0.06 0.9 2000 1.0 3.13 93 61 Comp. Ex. 17 15.6 0.10 0 2000 1.0 3.10 83 57 To use a sintered body of SiC as a machine part for which high precision of shape and dimensions is required, it is important to reduce the amount of shrinkage during sintering as much as possible to thereby reduce the cost. of machining of the sintered body. In this regard it is desirable to enhance the bulk density of the green body.

For shaping a -SiC powder according to the invention into a green body relatively high in bulk density the BET specific surface area of the powder is limited within the range from 12 to 20 m 2 1g as mentioned hereinbefore, and the powder is required to be not larger than 1,AAm in mean particle size. It is preferable that the mean particle size of the pSiC powder falls in the range from 0.4 to 0.8 ^m for is enhancing the bulk density of the green body and also for accomplishing good sintering at a relatively low temperature.

Furthermore, for enhancing the bulk density of the green body and thereby reducing the amount of shrinkage during sintering, it is preferable to control the particle size distribution of the -SiC powder such that the powder is constituted of 45-60 wt% of particles not larger than 0.7/Km, 20-35 wt% of particles larger than 1.4,Am and the balance of particles larger than 0.7 ^m and not larger than 1.4,Am. Such distribution of particle size means a good combination of fairly fine particles and a relatively coarse particles for enhancement of the density of a green body formed of that powder. When a P-SiC powder according to the invention has such distribution of particle size and is 0.4-0.8 ^m in mean particle size it is possible to form a green body having a bulk density above 1.95, so that it is easy to accomplish sintering of the green body with a small amount of shrinkage.

EXAMPLE 12

A,8-SiC powder was prepared from a silica sand and a carbon black by fundamentally the same method as in Example 1. The P-SiC powder had a mean particle size of 0.69,um and a BET specific surface area of 12.7 m 2 /g.

In this SiC powder, particles not larger than 0.7 /Am amounted to 51 wt%, particles larger than 0.7 ^m and not larger than 1.4 Am to 25 wt% and particles larger than 1.4 Am to 24 wt%.

Using this SiC powder a green body was formed by the same process as in Example 1. The green body had a bulk density of 2.04 g/cm 3. The green body was sintered by the same method as in Example 1, but in this case the sintering temperature was 1970 0 C.

The sintered body was analyzed in the same manner as in Example 1 and proved to be free of Ok-SiC and free of abnormally grown grains. The sintered body had a 3 bulk density of 3.12 g/cm In this case the amount of linear shrinkage of the SiC body during sintering was 15.2%.

EXAMPLES 13-21 As shown in Table 3, these examples were modifications of Example 12 in respect of the particle size characteristics of the P-SiC powder. In some examples the sintering conditions were varied.

TABLE 3

P-SiC Powder Green Sintering Sintered Body Body Condition Specific Mean Particle Size Bulk Temp. Time Bulk Linear Surface Particle Distribution Density Density Shrinkage Area Size 0. 7 Am >l. 4 (m 2 lg) (wt%) (wt%) (glcm 3) ( 0 C) (hr) (g/cm 3) (%) Example 12 12.7 0.69 51 24 2.04 1970 1.0 3.12 15.2 Example 13 13.3 0.71 49 28 2.05 1970 1.0 3.12 15.0 t113 Example 14 14.2 0.61 56 28 2.02 1960 1.0 3.12 15.6 Example 15 15.5 0.59 57 21 2.01 1960 1.0 3.14 16.0 Example 16 18.1 0.46 59 20 2.00 1960 1.0 3.13 16.1 Example 17 19.6 0.54 64 17 1.78 1950 1.0 3.12 20.6 Example 18 19.3 0.72 41 37 1.87 1950 1.0 3.12 18.6 Example 19 11.4 0.81 40 39 2.09 1970 1.0 2.99 12.7 Example 20 22.1 0.51 67 15 1.69 1970 0.5 3.13 22.8 Example 21 24.0 0.35 69 13 1.65 1970 0.5 3.13 23.8

Claims (11)

1. CLAIMS A method of preparing a P-SiC powder suitable as a sintering

   material, the powder being not larger than 1 Am in mean particle size and contains less than 5 wt% and not less than 1 wt% of 2HOL phase of SiC, the method comprising the steps of:

   mixing a silica powder not larger than 50,9m in particle size and not more than 0.1 wt% in the total content of metal impurites with a carbon powder smaller than 1,A(m in particle size and not more than 0.
2. 2 wt% in ash content such that in the obtained mixture the C/SiO 2 molar ratio is in the range from 2.8 to 4.0; and firing the mixture of said silica powder and said carbon powder in a nonoxidizing atmosphere for not more than 2 hr at an equalized temperature not lower than 1700 0 C and lower than 20000C. 2. A method according to Claim 1, wherein said C/SiO 2 molar ratia is in the range from
3. 3.0 to 3.2. 3. A P-SIC powder suitable as a sintering material, which comprises less than 5 wt% and not less than 1 wt% of 2M phase of SiC and is not more than 0.1 wt% in the total content of metal impurities, the P-SiC powder being not larger than 1,A(m in mean particle size and in the range from 12 to 20 m 2 lg in specific surface area measured by the BET adsorption method.
4. 4. A P-SiC powder according to Claim 3, wherein said mean particle size is in the range from 0.4 to 0.8, mm.
5. 5. A P-SiC powder according to Claim 4, wherein the particle size distribution of the powder is such that the powder is constituted of 45-60 wt% of particles not larger than 0.7,Am, 20-35 wt% of particles larger than 1.4,Am and the balance of particles larger than 0.7,Am and not larger than 1.4 9m.
6. 6. A method of producing a sintered body of P-SiC, comprising the steps of:

   compacting a 13-SiC powder according to Claim 3 into a green body of a desired shape; and sintering said green body in a nonoxidizing atmosphere at a temperature in the range from 1900 to 2100 0 c.
7. 7. A method according to Claim 6, further comprising the step of adding at least one sintering aid to said P-SiC powder before compacting said powder into said green body.
8. 8. A method according to Claim 7, wherein said sintering aid is a combination of boron and carbon.
9. 9. A method according to Claim 6, 7 or 8, wherein said P-SiC powder is constituted of 45-60 wt% of particles not larger than 0.7 ^m, 20-35 wt% of particles larger than 1.4,mm and the balance of particles larger than 0.7 ^m and not larger than 1.4(m, said mean particle 25 size being in the range from 0.4 to 0.8)Xm.
10. 10. A method of preparing a A-SiC powder, substantially as hereinbefore described in any of Examples 1 to 6.
11. 11. A method of producing a sintered body of j8-SiCp substantially as hereinbefore described in any of Examples 1 to 21.

    Published 1989 stThe PatentOfftee. State House.88.71 High Halborn. IA)ndanWClR4TP.Furt33Ler copies maybe obtsinedfrom The Patent Offtoe. Sales Branch, St Mary Cray, Orpington, Kent BR5 310. Printed by Multiplex techniques ltd, St Ma3L7 Cray, Kent, Gon- 1/87