FR2462404A1 - Sintered silicon carbide body - formed without sintering aid, prepd. from silicon carbide powder contg. boron component - Google Patents

Abstract

The prod. obtd. by sintering SiC powder contg. 0.2-10 wt.% uniformly dispersed B, calculated as B carbide and present as B carbide, opt. in solid soln. in SiC is prepd. by (a) mixing C powder, 20 micron or less particle size, Si metal powder and B203 powder such that molar % of each component falls in a given area of a specified C-Si-B203 ternary diagram, and (b) heating in oxidising atmos. contg. 0.3-35 vol.% 02 to induce a spontaneous continuous, instantaneously completed reaction at 800-1450 deg. C. The area is defined by points k, l, m, n, representing mol.\ C, Si, B203 of: 62.4, 37.4, 0.2; 34.9, 64.9, 0.2; 52, 39, 9; 69, 22, 9. The prods. are prepd. by compression moulding the SiC powder before or during heating at 1900-2250 deg.C. in an inert atmos. or under reduced pressure. The prods. are useful for heat and chemically resistant structural parts for automobile engines, crucibles, mortars, pestles and tools. They have bulk densities over 80%, e.g. 92%, and are pred. without use of sintering accelerator, opt. with sintering at atmos. pressure, from a simply prepd. SiC powder.

Description

 The present invention relates to sintered silicon carbide products and to a process for their preparation.

 Silicon carbide is used in applications that vary because of its excellent physical and chemical properties, for example in the form of heat and chemical resistant structural parts, for automobile engines, for the manufacture of mortar crucibles , pestles, tools of various types, and in similar applications. For these applications, it is essential to achieve dense and uniform sintered silicon carbide products by simple techniques.

Sintering of silicon carbide alone in the form of particles or powder is extremely difficult (see, for example,
J. Nadeau, Very High Pressure Rot Pressing of Silicon Carbide, Bull. Am.

Ceram, Soc. 52, 170-174 (1973). As a result, when a conventional technique such as hot pressing is used, the silicon carbide is mixed with a sintering accelerator such as Al, Fe, B or B4C and the hot pressing mixture is subjected to temperature of about 2000 to 24000C and a pressure of about 200 to 1000 kg / cm; this produces relatively dense sintered products (see, for example, Alliegro et al.,
Pressure-Sintered Silicon Carbide, J. Am. Ceram. Soc., 39 (Nov. 1956), pages 386-389.

This process has drawbacks: the additives necessary for the desired particle size and purity are not commercially available; mixing techniques and the like are complicated,
In addition, the silicon carbide used must itself be preferably in the form of fine particles; however, it is difficult to obtain fine particles of silicon carbide, and this difficulty is an obstacle to the practice of the process.

 A process for preparing a type 13 silicon carbide powder with a particle size of less than one micron has been described recently (see, for example, Japanese Patent No. 160 200/1975), which method incorporates uniformly, although small proportion (about 0.2 to 1.0% by weight), a component borated in silicon carbide. This method has made it possible to remedy the complications caused by the addition of sintering accelerators and to sinter the silicon carbide at a temperature of about 1900 to 21000.degree. C. at atmospheric pressure.

In this process, a gaseous mixture of a silicon halide, a boron halide and a hydrocarbon is used as the starting mixture and a thermal decomposition reaction in the gas phase is induced in a plasma jet. Therefore, this process involves restrictions on production techniques, requires complicated preparation of specific raw materials, and also requires a particular vapor phase reaction apparatus. On the other hand, this method of preparing sintered products is a prerequisite. object of restrictions in industrial practice because of the difficulties encountered in obtaining, under economic conditions, the powders used as starting material
Furthermore, Japanese Patent No. 28 317/1979 (corresponding to French Patent Application No. 78 23 167) describes a process which eliminates several restrictions with respect to production techniques and which allows the economical industrial production of a carbide powder. active silicon containing a boron compound in a much larger amount (about 0.2 to 10% by weight, expressed as boron carbide) than the products obtained by the process discussed above.The boron component is contained in the carbide of silicon in the state of boron carbide or in the form of solid solution of boron carbide or in a similar state, and in uniform dispersion. In this process for preparing silicon carbide powder, a carbon powder having a particle size of about 20 microns or less, silicon metal powder and a boron oxide powder such as According to this process, these raw materials are mixed in relative proportions such that the molar proportions% of each of the components in the ternary system of carbon (C), silicon (Si) and boron oxide (B 2 O 3) are within the area shown in the graph of the single figure of the attached drawing as being defined by the four points k (C = 62.4, Si = 35.4, 4> B203 = 0.2 ), (C = 34.9,
Si = 64.9, B203 = 0.2), m (C = 52, Si = 39, B203 = 9) and n (C = 69, Si = 22, B203 = 9), the mixture obtained is placed in a container of refractory material and is heated in an oxidizing atmosphere containing from about 0.3 to 35% by volume of oxygen, so as to cause, at a temperature of about 800 to 14500C, a spontaneous reaction that is virtually instantaneous.

 In the present invention, the starting material used is the active silicon carbide powder described in the French patent application No. 78 23 167. The invention provides advantages in that, contrary to what was the case in the process prior, it is not necessary to use a sintering auxiliary or other similar additive. This eliminates the problems posed by the addition of such an auxiliary product and the formation of a homogeneous mixture. The use of the active silicon carbide powder not only makes it possible to work by hot pressing, but also to sinter, in general, at atmospheric pressure, which, previously, was considered difficult. the starting powders are extremely simple to prepare, their use in practice is considerably facilitated and, compared with the previous process, a much larger quantity of a borated compound can be incorporated in the uniform dispersion state, can hope for much larger applications.

 The invention therefore relates firstly to a sintered silicon carbide product having a dense structure which is prepared from silicon carbide powder, itself obtained by the process described in Japanese Patent No. 28 317/1979 ( French Patent Application No. 78/23167).

 The invention also comprises a process for preparing a sintered silicon carbide product having a dense structure, characterized in that the active silicon carbide powder obtained by the process described in the application is subjected to compression molding. French Patent No. 78 23 167 and the molding is heated in a substantially inert atmosphere or under vacuum at a temperature of about 1900 to 22500C.

 Another object of the invention resides in a process for preparing a sintered silicon carbide product having a dense structure, which process is characterized in that the starting powder is molded by compression using a compression device. hot pressing by heating at a temperature of about 1900 to 2250C in a substantially inert atmosphere or under vacuum.

 Other features and advantages of the invention will emerge more clearly from the detailed description given hereinafter with reference to the graph of the accompanying drawing which is a triangular diagram showing the preferred composition (in mol%) of the starting mixture, in the carbon ternary system (C), silicon (Si), and boron oxide (B203) to achieve the active silicon carbide powder which is itself the raw material used for the preparation of sintered bodies according to the invention.

 With regard to the preparation of the raw material, reference may be made to French patent application No. 78 23 167, the teachings of which are considered as incorporated in the present application, however, in summary, the silicon carbide powder used as a raw material in the present invention is itself prepared from a carbon powder with a particle size of about 20 microns or less, a metal silicon powder and an oxide powder of Boron acid such as boric acid.These powders are mixed in relative proportions such that the% molar proportions for each of the components in the carbon ternary system (C), silicon (Si and boron oxide (B203) are in the area of the appended graph delimited by the points k (C = 62.4, Si = 37.4, B203 = 0.2), t (C = 34.9, Si = 64.9, B203 = 0.2), 2)> m (CP52), Si = 39, B203 = 9) and n (C = 69, Si = 22, B203 = 9)> the mixture is placed in a ref container The reaction medium is heated in an oxidizing atmosphere containing from about 0.3 to about 35 volumes of oxygen, causing a spontaneous reaction at a temperature of about 800 to about 145,000, which reaction is instantaneous.

 In the process according to the invention, the particle size of the carbon powder should be about 20 microns or less. If the particle size is greater than about 20 microns, the spontaneous reaction is not caused and most of the raw materials or part of the carbon powder remains unconverted. In practice, the particle size of the carbon powder can be determined, according to the specified conditions, depending on the final application of the product.

If one wants to obtain for example a fine raw material with high activity, it is necessary to choose a particle size carbon as fine as possible.

 When the spontaneous reaction is triggered, there is a release of heat and the temperature of the mixture rapidly increases a portion of the silicon, and most of the boron oxide, whose melting point is lower than that of the other materials raw materials, such as silicon or carbon, melt or vaporize and participate in the reaction with carbon. Thus, therefore, the particle size of the silicon and the particle size of the boron oxide may be coarser than those of the carbon. It is possible to work with silicon particles with a maximum dimension of 200 microns and with particles of boron oxide with a maximum dimension of about 500 microns.

 The carbon used in the invention must therefore be in the form of a powder which satisfies the requirements of dimensions indicated above.

In general, easy-to-use carbons such as natural graphite, artificial graphite, coke, crude coke, carbon black, coal or oil pitch, etc. are used. It is also possible to use a wide variety of silicon grades from the silicas suitable for
semiconductors up to silicas for general industrial applications with a purity of, for example, 90% by weight or more. Among the boron oxides which may be used, boric acid for plowing may be mentioned.
or for general industrial application. Boron oxide (B203)
also suitable. The purity of each of the powders used in the inven ~
tion does not have a great influence on the reaction leading to the formation of the raw material, but> on the other hand, the purity of the powders has a
influence to some extent on the purity and particle size of the raw material obtained. Therefore, we will choose correctly
the purity of the powders according to the intended final application for the
sintered product.

The molar proportions of the different powders depend on various factors such as the particle size of the starting products
the degree of mixing, the weight of the mixture, the speed and temperature of heating the concentration of oxygen in the atmosphere. It is difficult to calculate by stoichiometry the molar proportions of the powders to be used.

and, consequently, these molar proportions will be determined by experience. The powders are preferably mixed in such proportions that the molar proportions of the various constituents are in the area delimited by the points k,, m and n of the appended graph. It is thus possible to use in the invention mixtures whose compositions are in this area, but not in the four limit points or on the lines connecting these points. The powders are mixed correctly by
Conventional techniques and introduced into a suitable container refractory material, It is then heated in an oxidizing atmosphere. This causes a spontaneous reaction whatever the apparent density of the mixture. Heating is continued until the powder reaches a temperature sufficient for the spontaneous reaction to occur. This temperature varies according to the particle size, the nature of the starting powders and other factors, but in general
achieves good results between 800 and 1450 C approximately.

The silicon carbide product resulting from this
The reaction is easily reduced to powder without the need for any particular mechanical grinding technique.
gets a powder in which most of the particles have dimensions
about 500 microns or less. We can reach extremely easily
a powder having a maximum particle size of about 60 microns and a mean particle size of less than micron. X-ray and chemical analyzes have shown that the amount of edge component contained in these powders, expressed as boron carbide, is was in the range of about 0.2 to 10% by weight, and the purity of the sum of the silicon carbide component in the edge component could be up to 95% by weight or more. In the X-ray and chloride analyzes, the boron component appears to be distributed uniformly throughout the product in sili- tium carbide, in the form of boron carbide or in the solid solution with silicon carbide. Certain components are also observed. whose nature has not yet been clarified with precision.

 In accordance with the invention, a high density sintered product is prepared by sintering the silicon carbide above.

 When the boron component content is less than about 0.2 weight percent, expressed as boron carbide, the sintering is incomplete and the activity is insufficient to impart a dense structure to the product. As a result, the starting mixture should contain at least about 0.2% by weight of the borated component, but the maximum content of this component is about 10% by weight.

 With regard to the particle size of the powder which is subjected to sintering according to the invention, the particle size of the powder obtained by the process described in the French patent application No. 78 23 167 can be easily influenced. in a range of about 500 microns or less, with respect to the aggregate particle size, depending on the degree of powder reduction. These particles consist essentially of very fine and active primary particle aggregates having a surface area of about 3 m 2 / g (corresponding to a particle size of about 0.1 to 0.6 microns). the powder obtained by the process described in the aforementioned French Patent Application No. 78 23 167 is active irrespective of the aggregate particle size and can be readily reduced to a powder to a particle size of less than one micron.

However, coarse particles in the aggregate state are troublesome because they form a crosslinked structure during sintering, particularly during pressure-free sintering (ie atmospheric pressure), preventing the formation of a dense structure. . If sintered by hot pressing, sintering is possible even with aggregates having a particle size of 500 microns or less, but it is preferable to reduce sufficiently to powder. When sintering at ordinary pressure, it is recommended to reduce the powdered matira to a particle size of about 1 micron or less. In practice, the particle size can be selected according to the sintering technique. intended application for the sintered product and other similar factors,
The powder which serves as raw material is subjected to the sintering treatment according to conventional techniques. Thus, for example when working by hot pressing, the powder is introduced into a graphite mold and compressed by heat in an inert atmosphere or in a vacuum. If the applied pressure is less than about 100 kg / cm, it is difficult to achieve a sintered product having the desired density if, on the other hand, the applied pressure is greater than about 700 kg / cm, the density of the sintered product reaches substantially the saturation value and the pressure increase no longer have any practical effect. Therefore, the pressure range applicable is between 100 and 700 kg / cm and preferably between 100 and 300 kg / cm 2. cm. On the other hand, when sintered at ordinary pressure, the starting powder is first subjected to compression molding according to the conventional technique, and then the molded product is treated with heat in an inert atmosphere or under vacuum.

 With regard to the temperature of the heat treatment, and which is carried out by hot pressing or by sintering at ordinary pressure, if the temperature is below about 1900 ° C., the sintering is not complete, it is impossible to achieve to a sintered product at the desired density If, on the other hand, the temperature is greater than about 125 ° C., the desired density can be attained, but it is impossible to prevent an abnormal growth of the SiC particles, leading to a degradation of certain characteristics. As a result, the preferred temperature for the heat treatment is about 1900 to 22500.degree.

 The sintered silicon carbide product obtained by the process according to the invention has an apparent density of about 2.5 g / cm or more, which corresponds to about 80% or more of the theoretical density (3.21 g / cm 2). cm3) silicon carbide.

 The following examples illustrate the invention without, however, limiting its scope; in these examples, the indications of parts and percentages are by weight unless otherwise indicated.

EXAMPLE 1
2.92 kg of commercial carbon black (at 98.4% purity), average particle size 0.05 micron, 5.36 kg of commercial metal silicon powder (purity 94.6%) are mixed, average particle size 77 microns, and 2.06 kg commercial powdered boric acid (at 99.8% purity), average particle size 200 microns.

The molar proportion of the components of the mixture corresponds to point 1 of the appended graph, that is to say, in mole%, at C = 55, Si = 41 and B203 = 4.

Water is added to the mixture in an amount of 35 parts per 100 parts of the mixture and kneaded. The kneaded mixture is introduced into a cylindrical refractory container with an internal diameter of 260 mm, height 300 mm and, by simply placing a lid, it is heated.
I container in an electric furnace type siliconite air (02 = 20 Z by volume, N2 = 80 Z by volume) has a heating rate of about 3000 C / h. When the temperature reaches about 10800C, there is a clear release of smoke which indicates the beginning of a spontaneous reaction.

The phenomenon continues for about 1 to 2 minutes. The heating is continued and, when the temperature reaches 1120 ° C., the supply of current is cut off and allowed to cool.After 20 hours, the product is removed, which has a whitish appearance, but has a layer on its surface. oxidation of about 5 to 10 μm thick, while the interior of the product is yellowish gray, which clearly indicates the formation of a uniform reaction product. exclusion of the surface layer, indicates that the borated component content, expressed as boron carbide, is 6.1%.

 The reaction product is ground by passing it once between two rolls. The product obtained has the apparent particle size distribution shown in Table I below.

TABLE I
Particle size (microns)
> 500 500-250 250-125 125-74 74 44 less
> 500 500-250 250-125 125-74 74-44 of 44
Percentage 15 21.9 19.1 28.2 8.9 6.9
20 g of this powder are introduced into a square mold of 30 mm of artificial graphite and the temperature of the ambient temperature is raised to 21000 ° C in about 30 minutes by applying a pressure of about 500 kg / cm 2 to the using a high frequency induction heated press in a reducing atmosphere. This temperature level is maintained for 30 minutes, the pressure is depressurized, the current is turned off and the apparatus is allowed to cool. A dense sintered silicon carbide product is obtained (apparent density 2.95 g / cm 3, ie 92% the theoretical density of silicon carbide).

EXAMPLE 2
The starting material is the same as in Example 1, but subjected to a new dry milling. Its particle size distribution is then that shown in Table II below.

TABLE II
Particle size (microns)
74 74-44 less than 45
Percentage 3 4 93
On the other hand, the average particle size of this product, determined using a light transmission type device, is 1.5 microns.

 This starting material is used in an operation identical for the rest to that of Example 1.

 A sintered silicon carbide product is obtained which has a bulk density of 3.04 g / cm (95% of the theoretical density of silicon carbide).

EXAMPLE 3
The starting product here again is the same as in Example 1, but is further subjected to wet milling in a vibratory mill for 2 hours. Its average particle size, measured using a light transmission type device, is then 0.6 microns.

 A sintered silicon carbide product having an apparent density of 3.15 g / cm 2 (98% of the theoretical density of silicon carbide) is obtained.

EXAMPLE 4
The starting material is the same as in Example 3.

10 g of this product are placed in a metal mold and subjected to a pressure of 200 kg / cm; a cylindrical molding with a diameter of 20 mm, height approximately 20 mm is obtained.

The molding is placed in a Tammann electric oven and the temperature of the ambient temperature is brought to 200 ° C. in about 1 hour.
under a vacuum of about 5 nmnHg. This temperature is maintained for 30 minutes then the current is turned off and the oven is allowed to cool.

A sintered silicon carbide product is obtained
having an apparent density of 2.87 g / cm (89% of the density
theoretical silicon carbide).

EXAMPLE 5
The procedure is as in Example 4, but at a heating temperature of 2100 ° C.

A sintered silicon carbide product is obtained which
has an apparent density of 3.0 g / cm (92% of the theoretical density of silicon carbide).

The apparent density, apparent porosity and flexural strength of the products are reported in Table III below.
sintered examples 1 to 5.

TABLE III
Example Apparent density Apparent porosity Flexural strength
n0 g / cm3 Z kg / cm
-1 2.95 0.1 2600
2 3.04 0.00 4000
3 3.15 0.01 6000
4.87 0.2 2500
5 3.02 0.06 3600
The sintered product of Example 2 was also subjected to oxidation resistance and thermal shock resistance tests.

Oxidation resistance test
The weight increase in a 20 hour run at 120 ° C. was 0.05% (specimen size: 7 × 7 × 30 mm).

Thermal shock resistance test
Heating at 12000C and water cooling repeated at 3 repetitions, cracks are observed (dimension of the test piece 7 x 7 x 30 mm).

 The results reported in the preceding examples show that the process according to the invention makes it possible to easily obtain a sintered silicon carbide with a dense structure and high characteristics.

 It is clear that the invention is in no way limited to.

preferred embodiments described above as examples and that one skilled in the art can make various modifications without departing from its scope.

Claims (5)

1. Sintered silicon carbide product obtained by sintering a silicon carbide powder containing, in a uniform dispersion, a component borated in the state of boron carbide or in the form of solid solution of boron carbide in the carbide of silicon, in an amount of about 0.2 to 10% by weight, expressed as boron carbide, this sintered silicon carbide product being characterized in that the silicon carbide powder has been prepared by a process of mixing carbon powder having a particle size of about 20 microns or less, metal silicon powder and boron oxide powder in relative molar proportions corresponding to a point within the area delimited in the triangular diagram carbon (C), silicon (Si) and boron oxbTde (B203) by the points k, m and n at the coordinates of k (C = 62.4, Si = 37.4, 4, B203 = 0.2), e (C = 34.9, if = 64.9, 9> B203 = 0.2), m (C = 52, Si = 39, B203 = 9) and n (C = 69, Si = 22, B203 = 9), the mixture obtained is heated in an oxidizing atmosphere containing from about 0.3 to 35% by volume of oxygen, by triggering at a temperature of about 800 to 1450 C a spontaneous reaction and almost instantaneous. 2. Process for the preparation of a sintered silicon carbide product, the method being characterized in that a silicon carbide powder containing, in uniform dispersion, a component borated in the form of carbide boron or solid solution of boron carbide in silicon carbide in an amount of about 0.2 to 10% by weight, expressed as boron carbide, and the molding obtained is heated to a temperature of about 1900 to 22500C in a substantially inert atmosphere or under vacuum, the silicon carbide powder having been prepared by a process as defined in claim 1. 3. Process for preparing a sintered silicon carbide product, which process is characterized in that a silicon carbide powder containing a borax carbide borate component or a boron carbide borate component is of solid solution of boron carbide in silicon carbide, in an amount of about 0.2 to 10% by weight, expressed as boron carbide, by heating at a temperature of about 1900 to 22500C in a substantially inert atmosphere or under vacuum, said silicon carbide having been prepared by a process as defined in claim 1. 4. Method according to claim 2 or 3, characterized in that the compression molding is carried out at a pressure of about 100 to 700 kg / cm. Process according to claim 2 or 3, characterized in that the sintered silicon carbide product has a bulk density of about 2.5 g / cm3 or more.