GB2031027A - Sintered silicon carbide product and process for producing the same - Google Patents

Abstract

Sintered silicon carbide with a density of more than 95% is produced by hot pressing silicon carbide powder doped with 1 to 10% by weight of at least one element selected from elements of the first to the fourth periods, groups IIa, IIIb, VIa and VIII of the periodic table as an additive at 1,900 DEG to 2,050 DEG C under a pressure of 100 kg/cm<2> or higher in vacuum or in an inert gas atmosphere. The dopant is incorporated in the silicon carbide by addition to the raw materials used in the production of the silicon carbide by electric heating.

Description

SPECIFICATION Sintered silicon carbide product and process for producing the same BACKGROUND TO THE INVENTION Silicon carbide is a chemically stable material having~ a very high strength even at a high temperature, and thus is a suitable structure material for use in a high temperature atmosphere and particularly a promising blade material for a high temperature gas turbine.

Heretofore, sintered silicon carbide product has been produced according to a hot pressing method, reaction sintering method, or gas phase reaction method.

The gas phase reaction method comprises leading compounds of carbon and silicon (mainly organic compounds or halogen compounds) together with a carrier gas into a reaction chamber, and thermally decomposing these compounds, thereby forming silicon carbide on an appropriate substrate.

A dense sintered silicon carbide product can be obtained according to said method, but the thickness of the resulting film is about 1 mm at best, and a film of large thickness is hardly obtainable.

According to the reaction sintering method, usually carbon powder and silicon or silicon dioxide powder are mixed together, and sintered to obtain sintered silicon carbide product, where products of relatively large size can be obtained, but dense sintered silicon carbide products are hardly obtainable.

That is, dense products of large size can be obtained only according to the hot pressing method.

It was known that silicon carbide is a less sinterable material, but since Alliegro et al reported that a sintered product having a density near the theoretical density of 3.21 g/cm2 was obtained by adding aluminum, iron, or the like thereto, and hot pressing the mixture [j. Am. Ce ram. Soc. 39 386-389 (1956)], various studies have been made of additives.

For example, Svante Prochazka of General Electric Company, USA, discloses in US Patent No.

3,853,566 that 0.5-3.0 parts of boron or boron carbide in terms of boron is added to 100 parts by weight of silicon carbide, and the resulting mixture is hot pressed, whereby a ceramic having 98% theoretical density is obtained. Furthermore, Gerald a Weaver et al of Norton Company, USA, discloses in US Patent No. 3,836,673 the silicon carbide containing 0.55.0% by weight of aluminum or aluminum oxide as an additive can have 99% or higher theoretical density.

According to these prior arts, fine silicon carbide powder having particle sizes of a few microns or submicrons is used to obtain a dense sintered silicon carbide product, and the boron or aluminum powder as the additive must also be similar fine powder. To obtain a uniformly dispersed mixture of fine silicon carbide powder and fine additive powder, a milling treatment in a ball mill using, for example, tungsten carbide balls has been required. Particularly, sufficient milling treatment is required, because the mechanical strength of sintered product depends upon the uniformity of the mixture.

SUMMARY OF THE INVENTION An object of the present invention is to provide a high density, sintered silicon carbide product having a less fluctuation in mechanical strength.

Another object of the present invention is to provide a process for producing a high density, sintered silicon carbide product especially without the milling treatment as so far required.

Other objects will be apparent from the following description.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a diagram showing relations between a sintering temperature and a density of sintered silicon carbide product.

Figure 2 is a diagram showing relations between a sintering time and a density of sintered silicon carbide product.

Figure 3 is a diagram showing relations between a sintering pressure and a density of sintered silicon carbide product.

Figure 4 is a diagram showing relations between an average particle size of silicon carbide powder and a density of sintered silicon carbide product.

DETAILED DESCRIPTION OF THE INVENTION As described above, silicon carbide is a less sinterable material, and thus requires an additive.

According to the prior art, powder of an additive is made ready, independently from silicon carbide powder, and uniformly mixed with the silicon carbide powder in a milling treatment using, for example, a ball mill, whereas according to the present invention, such a milling step is not required.

In the present invention, the fine powder of silicon carbide, when prepared, is doped in advance with said element. Such silicon carbide can be readily synthesized according to the ordinary process.

In the production of such silicon carbide powder in an industrial scale, an electric resistance furnace, the so-called Acheson process (Phillips Research Report 18 No. 3, 171), has been so far employed. According to the Acheson process, a mixture of 50 parts by weight of quartz sand, 40 parts by weight of coke, 7 parts by weight of saw dusts, 3 parts by weight of sodium chloride, and a predetermined amount of additive is filled in the furnace, and metallurgical coke powder is filled at the center of the furnace in a longitudinal direction, and is connected, at both ends, to carbon electrodes. An electric current is passed between the electrodes, and once the temperature reaches maximum 2,7000C, the temperature is lowered, and kept at about 2,000 C for about 30 minutes.Then, the electric current is turned off to lower the furnace temperature, and an ingot of silicon carbide formed in the furnace is taken out.

The silicon carbide ingot doped with these additives has a higher hardness than that not doped with the additives, and thus it has been used heretofore mainly as a polishing material, grinding stone, etc.

According to the present invention, silicon carbide powder doped with 1 to 10% by weight of at least one element selected from elements of the first to the fourth periods, Groups I Ia, I lIb, Vla and VIII of the periodic table is hot pressed in vacuum or in an inert gas atmosphere at a temperature of 1,900 to 2,0500C under a pressure of 100 to 700 kg/cm2 for 5 to 60 minutes.

The elements of the first to the fourth periods, Groups Ila, Illb, Vla and VIII include Be, Mg, Ca, Cr, Fe, Co, Ni, B, Al, Ga, etc.

The effect of an additive in the production of sintered silicon carbide products is not exactly clarified yet, but can be roughly interpreted as follows: it seems that, when a mixture of silicon carbide and an additive is heated at about 2,0000 C, atoms of-the additive is diffused into a crystal lattice of silicon carbide, and, as a result, a bonding of silicon carbide crystal grains themselves is promoted, resulting in formation of a dense sintered product. In that case, it is essential that the additive is uniformly dispersed, and a poor dispersion produces partially unsintered locations. It is said that, when such sintered product is exposed to a high temperature, a deterioration takes place at such partially unsintered locations.

Since silicon carbide doped with the additive component in advance is used in the present invention, such non-uniform dispersion of the additive, as described above, never appears, or there is no fear of partial unsintered locations even after the sintering. Furthermore, no milling treatment is especially required for the dispersion.

There are several important conditions for hot pressing silicon carbide to obtain a dense sintered product.

As to the silicon carbide powder as the starting material, firstly, the amount of the additive component doping the silicon carbide powder is important. When the amount of the additive component is less than 1% by weight, a dense sintered silicon carbide product is hardly obtainable, whereas when more than 10% by weight of the additive component is contained, grain growth considerably proceeds, lowering the strength and oxidation resistance, though a high density can be readily obtained. A preferable amount of the additive component is 2 to 5% by weight.

Secondly, an average particle size of the silicon carbide is also important. The smaller the particle size, the better, but a high density can be obtained so long as the particle size of the silicon carbide is less than 10 microns. However, the powder having particle sizes of 20 to 70 microns can be contained, so far as the amount of such powder is less than 20% by weight.

As to the hot pressing conditions, temperature, pressure and time are important, and are related to one another. To obtain a dense sintered silicon carbide product, firstly, the temperature must be 1 ,9000C or higher When the temperature is lower than 1 ,9000C, it seems that a dense sintered product cannot be obtained owing to an insufficient diffusion of the additive component. When the temperature exceeds 2,0500C on the other hand, an excessive grain growth is liable to take place.

Secondly, the pressure is important, but is greatly dependent upon the material of a die to be used.

When a graphite die is used, a pressure of 700 kg/cm2 is almost an upper limit, but a dense sintered product can be obtained even under 100 kg/cm2 by adjusting the temperature and time. Thirdly, the sintering time is also important, and an optimum time is determined in view of relations between the temperature and the pressure. Generally when the temperature is for example, 2,0000C or higher, and the pressure is 400 kg/cm2 or higher, the sintering must be carried out for 5 to 1 5 minutes, and when the temperature and the pressure are lower than said values, respectively, the sintering must be carried out for 30 to 60 minutes. Even ifthe sintering time is prolonged, the sintering density is not so much improved, and to the contrary an excessive grain growth is liable to take place. This is not preferable.

A further important condition is an atmosphere for sintering, and an oxidative atmosphere promotes oxidation of silicon carbide to produce silicon dioxide. Thus, such atmosphere canno be used.

Thus, the sintering must be carried out in vacuum or in an inert gas atmosphere.

A sintering silicon carbide product having a density of more than 95% of theoretical density (3.21 g/cm3), that is, at least 3.05 g/cm3, can be obtained by hot pressing the silicon carbide powder under said conditions. The resulting sintered product also has a less fluctuation in mechanical strength.

EXAMPLE 1 Silicon carbide powder having an average particle size of 3 microns, doped with aluminum, was subjected to chemical analysis. Results of the analysis are given in Table 1.

TABLE 1

Element j Fe Ca Al Ni Cr Ti B Content (wt %) 0.16 0.02 0.81 0.01 0.03 0.04 0.03 1-00 Parts by weight of said silicon carbide powder was admixed with 10 parts by volume of silicon oil and 10 parts by volume of xylene, and kneaded. The resulting mixture was placed in a mold, shaped under a pressure of 1 ton/cm2 and then placed in a graphite die. The density of the shaped article resulting from the mold shaping was 1.71 g/cm3. Hot pressing was carried out in a reduced pressure of less than 1 x 10-4 torr in an induction heater. A pressure of 200 kg/cm2 was applied and maintained until the hot pressing was completed.Temperature was elevated from room temperature to specific temperatures for hot pressing over a period of about 2 hours, and the specific temperatures were kept for 5 to 100 minutes. Then, the article, was left standing for cooling down to room temperature. Results are shown in Figures 1 and 2.

EXAMPLE 2 Sintered silicon carbide products were produced from silicon carbide powder having an average particle size of 3 microns in the same manner as in Example 1 ,while charging the kind and concentration of the element doping the silicon carbide to various degrees. Conditions for the hot pressing are as follows: Temperature 2,0000C Pressure : 200 kg/cm2 Time : 30 minutes The kind and 96 by weight of the element added, and the density of the resulting sintered products are given in Table 2. In the case of sample No. 6 in which the sum total of the elements doping the silicon carbide is less than 1% by weight, the resulting sintered product has a considerable low density.

TABLE 2

Doping element (wt. %) Sintered product Sample - . . Fresco density No. Al B Cr Be +Ni Mg+Ca (g/cm3) 1 0.02 0.98 0.02 .< 0.01 0.22 0.03 3.19 2 0.03 0.03 1.01 < 0.01 0.19 0.02 3.16 3 0.04 0.03 0.01 1.18 0.14 0.06 3.14 4 0.04 0.02 0.08 < 0.01 1.53 0.11 3.16 5 0.22 0.03 0.04 < 0.01 0.22 1.88 3.11 6 0.02 0.10 0.02 < 0.01 0.42 0.08 1.95 7 5.36 0.03 0.03 < 0.01 0.55 0.04 3.20 8 0.28 3.19 0.16 < 0.01 0.33 0.12 3.20 9 2.11 3.55 1.58 < 0.01 2,24 1.66 3.21 10 5.66 3.77 1.84 < 0.01 2.69 2.01 3.21 EXAMPLE 3 Sintered silicon carbide products were produced from silicon carbide powder having an average particle size of 3 microns, doped with the elements whose contents are shown in Table 3, in the same manner as in Example 1. To investigate an influence of hot pressing pressure, the pressure was changed while keeping the sintering temperature at 20000C and the sintering time for 30 minutes.

TABLE 3

Fe+Co Element Al B Cr Be +Ni Mg+Ca Content (wt. %) 0.88 0.77 0.16 > 0.01 0.24 0.04 The density of the sintered products when the pressure was changed is shown in Figure 3. A density of more than 3.10 g/cm3 can be obtained under a pressure of more than about 100 kg/cm2.

EXAMPLE 4 Silicon carbide powder having various ranges of particle size was prepared from the same sample as used in Example 3. Hot pressing was carried out in the same manner as in Example 1 at 2,0000Cand 200 kg/cm2 for 30 minutes. Relations between the average particle size and the density are shown in Figure 4. It is seen that, when the average particle size becomes less than 10 microns, the density of sintered product is increased. Particle size distribution of silicon carbide having an average particle size shown in Figure 4 is shown in Table 4. When the particles having particle sizes of 20-70 microns take more than about 20% by weight the density of sintered product becomes 2.8 g/cm3.

TABLE 4

Particle size less range (m) than 1 1-3 3-5 5-10 10-20 20-70 Proportion (wt. %) 2 13 16 33 18 18 EXAMPLE 5 Sample No. 6 of the silicon carbide powder used in Example 2 was mixed with other silicon carbide powder in various mixing ratios, and hot pressed in the same manner as in Example 1 , whereby sintered silicon carbide products were obtained.The hot pressing conditions are as follows: Temperature 2,0000C Pressure : 200 kg/cm2 Time : 30 minutes Relations between the mixing ratio of the silicon carbide powder and the density of sintered silicon carbide products are given in Table 5. When the mixing ratio of Sample No. 6 is 70%, a sintered product having uneven portions is obtained, and the density of the sintered product is lowered.

TABLE 5

Mixing ratio Sintered product density Other component glcm3 10 No. 4: 90 3.13 30 : 70 3.07 50 ,, : 50 3.05 70 ,, : 30 2.88 30 No. 1: 70 3.03 50 ,, : 50 2.91 70 No. 8: 30 3.12 50 ,, : 50 3.14 EXAMPLE 6 Among said sintered silicon carbide products, test pieces (2 mm x 2 mm x 5 cm) were prepared from high density sintered products (density: 3.20 g/cm3 or higher), polished with a diamond paste containing particles having a particle size of 1 micron, and subjected to a three-point bending test by means of a device having a span length of 3 cm.

Simple average flexural strength: 50 test pieces measured room temperature : 75--109 kg/mm2 1,3000C : 70-105kg/mm2 EXAMPLE 7 Sample No. 7 of Table 2 was admixed with 1.5% by weight of Al in a ball mill to prepare a Sample No. 11. Sample No. 8 of Table 2 was admixed with 5% by weight of Al in a ball mill to prepare Sample No. 12. Sample No. 6 of Table 2 was admixed with 1 % by weight and 3% by weight of B in a ball mill to prepare Samples No. 13 and 14, respectively.

Said Samples No. 7, 8, 11-14 were prepared into sintered products by hot pressing in the same manner as in Example 1. Hot pressing conditions were 2,0000C, 200 kg/cm2, and 30 minutes. The resulting sintered products were subjected to bending test (room temperature) in the same manner as in Example 6. Tests were conducted each of 1 00 pieces to obtain a simple average strength and calculate a Weibull modulus as an index of fluctuation, and the results are shown in Table 6.

Breaking strength of brittle materials such as a sintered product, etc. has a proper fluctuation, which is presumed to be in accordance with a Weibull distribution. A Weibull distribution function as given by the following equation is valid for statistical calculation: P(a) = 1 - Q(a) = exp[-#m/#o] where mis a Weibull modulus, P(a) is a non-destruction probability under a stress a, and a0 is a scale parameter. The larger the Weibull modulus, the smaller the fluctuation, that is, the higher the reliability.

TABLE 6

Average flexural Sample strength Wei bul l No. (Kg/mm2) modulus 7 108 20.3 8 77 19.1 11 95 9.8 12 108 11.1 13 38 7.6 14 56 12.7 In Table 6, Sample Nos. 11-14 are the same as those containing the additive according to the conventional art and have a smaller Weibull modulus, that is, a larger fluctuation than Samples Nos. 7 and 8 according to the present invention.

According to the present invention, a sintered silicon carbide product having a density quite near the theoretical density and a large Weibull modulus so far never obtained can be produced. The larger the Weibull modulus, the better, but it is difficult to set a lower limit to the Weibull modulus.

In the present invention, it is essential to dope silicon carbide powder with 1 to 10% by weight of at least one element of the first to the fourth period, Groups Ila, Illb, Vlb and Vlli, of the periodic table, but it is not objectionable that silicon carbide powder is doped with the other elements than the above in addition.

Claims (13)

1. A sintered silicon carbide product having a density of more than 95% of theoretical density, prepared by hot pressing fine silicon carbide powder doped with 1 to 10% by weight of at least one element selected from elements of the first to the fourth periods, Groups Ila, Illb, Vla and VI II, of the periodic table.

2. A sintered silicon carbide product according to Claim 1 , wherein the fine silicon carbide powder is doped with 2 to 5% by weight of one or more of said elements.

3. A sintered silicon carbide product according to either of Claims 1 and 2, wherein the fine silicon carbide powder has an average particle size of not more than 10 ym.

4. A sintered silicon carbide product according to any one of Claims 1 to 3, wherein the fine silicon carbide powder is doped with at least one of Al, B and Be.

5. A sintered silicon carbide product according to any one of claims 1 to 4, wherein the sintered silicon carbide product has a density of more than 98% of theoretical density.

6. A sintered silicon carbide product as claimed in Claim 1, substantially as hereinbefore described.

7. A process for producing a sintered silicon carbide product, which comprises hot pressing fine silicon carbide powder doped with 1 to 10% by weight of at least one element selected from elements of the first to the fourth periods, Groups Ila, glib, Vla and VIII, of the periodic table in vacuum or in an inert gas atmosphere at 1,9000 to 2,0500C under 100 kg/cm2 or higher.

8. A process according to Claim 7, wherein the fine silicon carbide po'wder is doped with 2 to 5% by weight of at least one of said elements.

9. A process according to either of Claims 7 and 8, wherein the fine silicon carbide powder has an average particle size of less than 10 Mm.

10. A process according to any one of Claims 7 to 9, wherein the fine silicon carbide powder is doped with at least one of Al, B and Be.

11. A process according to any one of Claims 7 to 10, wherein the sintered silicon carbide product has a density of more than 98% of theoretical density.

1 2. A process according to any one of Claims 1 to 11, wherein the hot pressing is effected after preliminary shaping without milling treatment.

13. A process as claimed in Claim 1, for producing a sintered silicon carbide, substantially as hereinbefore described.

1 4. A sintered silicon carbide when produced by a process as claimed in any one of Claims 7 to 13.