FR2723082A1 - Sintered boron carbide ceramic bodies - Google Patents

Abstract

Sintered boron carbide ceramic bodies are produced by (a) mixing boron carbide powder with an organosilicon polymer ceramic precursor (I) chosen from polysiloxanes, polysilazanes, polysilanes, metallopolysiloxanes and metallopolysilanes to form a homogeneous mixt. in which the (I) is present in such an amt. that the free carbon index of the mixt. is greater than 0.2 wt.% w.r.t. the total wt. of the boron carbide powder and the carbonisation prod. derived from the (I); (b) forming the homogeneous mixt. into the desired shape under press. and at a temp. below ca. 500 deg C to form a green body that can be further manipulated and (c) sintering the green body in an inert atmos. at a temp. above ca. 2200 deg C to form a sintered body with a density of above ca. 2.0.

Description

 The present invention relates to the manufacture of a highly densified boron carbide ceramic body, by pyrolysis of a mixture comprising boron carbide powder and an organosilicon polymer precursor of ceramic material selected from polysiloxanes, polysilazanes, polysilanes, metallopolysiloxanes and metallopolysilanes.

These highly densified ceramic bodies can be manufactured by pressure sintering or by using a pressureless process.

 The products and methods of the present invention offer several distinct advantages over the methods of the prior art. (1) The bodies in green (raw) state have great strengths, which facilitates handling and machining before sintering. (2) The composition of the ceramic precursor mixture can be varied to suit various molding techniques such as pressing and sintering or transfer / injection molding and sintering applications. (3) The green bodies of the invention are denser and thus give ceramic products which exhibit less pyrolytic shrinkage and greater sinteredness in the sintered state.

 The present invention discloses for the first time that high strength and high density boron carbide ceramic products can be unexpectedly obtained by sintering a mixture comprising an organosilicon polymer precursor of ceramic material and boron carbide powder. .

 The present invention relates to a method of manufacturing a green body of boron carbide that can be manipulated. The method comprises mixing boron carbide powder and a ceramic organosilicon polymer precursor selected from polysiloxanes, polysilazanes, polysilanes, metallopolysiloxanes and metallopolysilanes, to obtain a homogeneous mixture. The organosilicon polymer precursor of ceramic material is present in this mixture in an amount such that the free carbon number of the mixture is greater than 0.2 percent by weight relative to the total weight of the boron carbide powder and the product of carbonization derived from the organosilicon polymer precursor of ceramic material. The homogeneous mixture is then shaped to the desired shape, under pressure at a temperature below about 500 ° C, to produce the manipulable green body.

 The invention also relates to a method of manufacturing a sintered ceramic body comprising boron carbide. The process comprises sintering the manipulable green body prepared above in an inert atmosphere at a temperature above about 22000C. The sintered bodies thus formed have densities greater than about 2.0.

 The present invention relates to the manufacture of highly densified sintered bodies from organosilicon precursor polymers of ceramic material and boron carbide powder. The sintered bodies produced by the practice of the present invention have densities greater than about 2.0, which corresponds to about 80% of the theoretical density of boron carbide.

As is admitted here, the theoretical density of the body is assumed to be equal to that of boron carbide, namely 2.52, even if SiC is also generated by the ceramic precursor polymer. These highly densified bodies are useful as wear parts, shielding and in the nuclear industry.

 The invention also relates to a process for preparing green bodies that can be handled. By the term "green bodies that can be handled" is meant green bodies that have green strength sufficient to be handled or machined to a desired shape prior to sintering. In general, in the practice of the present invention, green resistances of 2.1 MPa or more can be obtained. This green resistance is achieved mainly because the precursor mixture of ceramic material comprises an organosilicon polymer which behaves as a matrix for the boron carbide powder. The greater green resistance obtained in the practice of the present invention mitigates the problems associated with the handling of fragile objects and allows the production of more complex shapes by machining, milling, etc.

 Green bodies can be formed by conventional techniques known in the art. These techniques include pressure casting, uniaxial pressing, isostatic pressing, extrusion, transfer molding, injection molding, and the like.

The present invention is particularly advantageous in this respect since the composition of the precursor mixture of ceramic material can be easily modified (report
B4C: polymer) to adapt it to multiple molding techniques without affecting the quality of the sintered product.

 The shaped green bodies are then fired at an elevated temperature in an inert atmosphere to be converted into ceramic articles having densities greater than about 80% of the theory. It is preferable that the density of the ceramic article be greater than about 90% of theory (2.27). It is particularly preferable that the density be greater than about 2.4 (95% of theory).

 During pyrolysis, the organosilicon polymers of the present invention give SiC and free carbon.

This phenomenon tends to decrease the rate of shrinkage that occurs when the mixture is sintered, because the SiC is formed in the interstices between the grains of boron carbide, thus limiting the shrinkage caused by densification. Since shrinkage is less, sintered articles can be formed with closer tolerances.

The resulting ceramic bodies formed by the above pyrolysis comprise a mixture of boron carbide and silicon carbide. In general, the amount of
SiC is less than about 10% of the total weight of the ceramic body. The term "boron carbide body" is used herein to refer to these ceramic bodies.

 The compositions of the present invention can be sintered under pressure or using a pressureless process to produce a highly densified ceramic article. Since the sintering process using pressure generally gives ceramic articles having a higher density, such a method is preferred if a maximum density is desired. In general, however, the pressureless sintering technique is preferred because of the simpler operations involved.

 For sintering, inert atmospheres are used to avoid oxygen incorporation and silica formation. The sintering process and the density of the sintered product are thus optimized. In the sense of the present invention, an inert atmosphere comprises an inert gas, a vacuum, or both. If an inert gas is used, it may be for example argon, helium or nitrogen. If a vacuum is used, it may for example be in the range of 13.3 Pa to 26.7 kPa, preferably 13.3 to 40.0 Pa. An example of a combined process may include cooking of the composition in argon up to 1200 ° C, baking at 1200-1500 ° C under vacuum and baking at 1500-2775 ° C under argon.

 The sintering can be carried out in any conventional high temperature oven equipped with a furnace atmosphere control device. Temperatures of about 22,000 ° C or more are generally employed, with the preferred range being about 2250 ° to 2300 ° C. A most preferred sintering temperature is about 2275 ° C. Although lower temperatures may be used, the ceramic product may not have the desired density.

 The temperature program for sintering depends on both the volume of the pieces to be cooked and the composition of the mixture. In the case of small objects, the temperature can be raised fairly quickly. In the case of larger objects or which contain high concentrations of the organosilicon polymer, however, longer programs are needed to produce uniform ceramic bodies.

 The organosilicon polymers useful in the present invention are generally well known in the art. Organosilicon polymers having a high yield of ceramic char is preferred because the rate of binder shrinkage that occurs on pyrolysis decreases as the yield of char increases. It is therefore preferred that the yield of ceramic char is greater than about 20 percent by weight. It is particularly preferred to use organosilicon polymers having ceramic char yields greater than about 35 percent by weight.

 The organosilicon polymer must also give a ceramic carbonization product containing free carbon. According to the blending rule, the ceramic char must contain more than about 30 percent by weight of total carbon for free carbon to be present. Organosilicon polymers having more than 40 percent by weight of total carbon are preferred because they contain about 86 percent by weight of SiC and 14 percent by weight of free carbon (i.e., in 100 g of the carbonization product). there are 60 g of Si / 28 (MW of Si) = 2.14 moles of Si, 2.14 moles x 12 (PM of C) = 26 g of C as SiC, and 40 g of total C 26 g of C in the form of SiC = 14 g of free carbon). Most preferred are organosilicon polymers which provide ceramic chars containing greater than about 50 percent of total carbon, since such ceramic chars contain about 28 percent by weight of free carbon. It is generally preferable for the ceramic char to contain at least 10 weight percent free carbon. More preferably, the ceramic char contains at least 25 weight percent free carbon.

 The organosilicon polymers used in the context of the present invention include polysiloxanes, polysilazanes, polysilanes, polymetallosiloxanes and polymetallosilanes. If the organosilicon polymer is an organopolysiloxane, it may contain units of general structure [R3SiO0,5], (R2SiO], [RSiO1,5] and [SiO2] where each R is independently selected from hydrogen, the alkyl radicals containing 1 to 20 carbon atoms such as methyl, ethyl, propyl, etc., aryl radicals such as phenyl and unsaturated alkyl radicals such as vinyl.

Preferred organopolysiloxanes contain various amounts of [PhSiO1.5], [MeSiO1.5], [MePhSiO], [Ph2SiO] and [PhViSiO] units. Particularly preferred organopolysiloxanes are described by the pattern formulas
[PhSiO1.5] [MeSiO1.5] [PhViSiO]
and [MeSiO1,5] [MePhSiO] [PhSiO1,5] [Ph2SiO] where the various mole fractions of each unit are suitable for providing resinous polymers with convenient molding characteristics. Organopolysiloxanes useful in the present invention may contain other siloxane units in addition to or in place of the siloxane units just mentioned.

Examples of such siloxane units include [ViSiO1,5], (MeHSiOj, (MeViSiOj, (Me2SiOJ, (Me3 SiOo.5), etc. Mixtures of organopolysiloxanes can also be used.

The organopolysiloxanes of the present invention can be prepared by techniques well known in the art. The process actually employed to prepare the organopolysiloxanes is not critical. Most commonly, the organopolysiloxanes are prepared by hydrolysis of organochlorosilanes. These and other methods are described by Noll in Chemistry and
Technology of Silicones, Chapter 5 (English translation of the 2nd German edition, Academic Press, 1968).

où chaque R est choisi indépendamment parmi l'hydrogène, les radicaux alkyles contenant 1 à 20 atomes de carbone tels que méthyle, éthyle, propyle, etc., des radicaux aryles tels que phényle et des radicaux hydrocarbonés insaturés tels que vinyle, et chaque R', R'' et R''' est choisi indépendamment parmi l'hydrogène, les radicaux alkyles ayant 1 à 4 atomes de carbone, des radicaux aryles tels que phényle et des radicaux hydrocarbonés insaturés tels que vinyle. En général, on préfère les polysilazanes qui contiennent des motifs [Ph2SiNH], [PhSi(NH)15J et/ou

If the organosilicon polymer precursor of ceramic material is a polysilazane, it may contain units of the type tR2SiNH1, (RSi (NH) 1,5J and / or

where each R is independently selected from hydrogen, alkyl radicals containing 1 to 20 carbon atoms such as methyl, ethyl, propyl, etc., aryl radicals such as phenyl and unsaturated hydrocarbon radicals such as vinyl, and each R R '' and R '''are independently selected from hydrogen, alkyl radicals having 1 to 4 carbon atoms, aryl radicals such as phenyl and unsaturated hydrocarbon radicals such as vinyl. In general, polysilazanes which contain [Ph2SiNH], [PhSi (NH) 15J and / or

The polysilazanes useful in the present invention may however contain other silazane units such as [MeSi (NH) 1.5J, [Me2SiNH], (ViSi (NH) 1.5J, tVi2SiNH], [PhMeSiNHJ, tHSi (NH) l 5], (PhViSiNHJ, (MeViSiNHJ, etc.

 The polysilazanes of the present invention can be prepared by techniques well known in the art. The process actually employed to prepare the polysilazanes is not critical. Suitable ceramic precursor silazane or polysilazane polymers can be prepared by the Gaul methods disclosed in U.S.

Nos. 4,312,970, 4,340,619, 4,395,460, and 4,404,153. Suitable polysilazanes also include those prepared by the methods described by Haluska in U.S. Pat. Nos. 4,482,689 and Seyferth et al in U.S. Patent No. 4,397,828. Other polysilazanes useful in the present invention can be prepared by the methods described by Cannady in U.S. Nos. 4,540,803 and 4,543,344, and Burns et al., In J. Mater Sci., 22 (1987), pp. 2609-2614. Particularly preferred polysilazanes are polysilacyclobutasilazanes, polyclisacylclobutasilazanes and and polysilacyclobutasilazanes modified by US Pat. A silane For the purposes of the present description, the term "silacyclobutasilazane M polymer" covers polysilacyclobutasilazanes, polydisi lacyclobutasilazanes and silane-modified polysilacyclobutasilazanes. The silacyclobutasilazane polymers can be crosslinked thermally or catalytically. As a result, the green bodies prepared from these silacyclobutasilazane polymers can be cured before the sintering step. Such green cured bodies generally have greater green strengths than similar uncured green bodies.

 The polysilacyclobutasilazanes of the present invention may be prepared, for example, by the Burns method described in U.S.

No. 4,835,238; polydisilacyclobutasilazanes can be prepared, for example, by the Burns method described in US Pat. No. 4,774,312; and the preferred silane-modified polysilacyclobutasilazanes can be prepared, for example, by the method of
Burns described in the US patent application

07 / 277,080 or U.S. Patent Application 07/213 380.

 If the organosilicon polymer precursor of ceramic material is a polysilane, it may contain units of general structure [R, Si], [R2 Si] and [RSi] where each R is independently selected from hydrogen, the alkyl radicals containing 1 to 20 carbon atoms such as methyl, ethyl, propyl, etc., aryl radicals such as phenyl and unsaturated hydrocarbon radicals such as vinyl. Preferred polysilanes contain 5 to 25 mole percent (Me 2 Si) units and 75 to 95 mole percent (PhMeSi) units. Polysilanes useful in the present invention may contain other silane units in addition to or in place of the silane units just mentioned. Examples of these silane units include [MeSi), [PhSi], (ViSi), [PhMeSi), tMeHSi, tMeViSi], [Ph2 Si), (Me2 Si), (Me3 Si), and the like.

The polysilanes of the present invention can be prepared by techniques well known in the art. The process actually employed to prepare the polysilanes is not critical. Suitable polysilanes can be prepared by the reaction of organohalosilanes with alkali metals, as described by Noll in Chemistry and Technology of
Silicones, 347-49 (English translation of the 2nd German edition, Academic Press, 1968). More particularly, suitable polysilanes can be prepared by sodium metal reduction of organically substituted chlorosilanes, as described by
West in US Pat. No. 4,260,780 and West et al., In Polym Preprints 4 (1984). Other suitable polysilanes can be prepared by the general procedures described by Baney et al., US Pat. US 4 298 559.

If the organosilicon polymer precursor of ceramic material is a polymetallosiloxane, it may be any suitable polymer containing repeating units of the metal-O-Si type. Examples of suitable compounds include borosiloxanes and aluminosiloxanes, both of which are well known in the art. For example, Noll, in Chemistry and
Technology of Silicones, Chapter 7 (English translation of the 2nd German edition, Academic Press, 1968) describes many polymers of this type as well as their manufacturing process. In addition, the Japanese patent
Kokai N "Sho 54 [1979] -134744 granted to Tamamizu et al.

and the U.S. Patent. No. 4,455,414 to Yajima et al also describe the preparation and use of various polymetallosiloxanes as binders for powdered SiC.

 If the organosilicon polymer precursor of ceramic material is a polymetallosilane, it may be any suitable polymer containing repeating units of the metal-Si type. Examples of metals suitable for inclusion include boron, aluminum, chromium and titanium. The process used to prepare said polymetallosilanes is not critical. It can be, for example, the method of Candra et al. described in U.S. Patent Nos. 4,762,895 or Burns et al in U.S. Patent Application 07 / 266,561.

 Those of the above organosilicon polymers which contain vinyl groups may be preferred because the silicon-bonded vinyl groups provide a mechanism by which the organosilicon polymer may be cured prior to sintering. In addition, units containing silacyclobutyl groups may also be preferred because they allow curing without the addition of catalysts.

Finally, mixtures of any of the above organosilicon compounds are also contemplated in the present invention.

 The examples included in the present description illustrate particular processes for the preparation of suitable organosilicon polymers.

 The organosilicon polymers used as binders for boron carbide powder are particularly advantageous compared to the binders of the prior art, since a polymer can be chosen which provides the desired amount of free carbon in the carbonization product and provides an appropriate yield of the product. of carbonization. In this manner, the polymer can be adapted to obtain a polymer / B4C ratio in the ceramic precursor mixture which is suitable for the molding technique employed and yet provides the appropriate amount of free carbon in the char. If the polymeric carbon sources which form a carbonization product consisting of 100% free carbon were used, for example, increasing the polymer / B4C ratio in the precursor mixture of ceramic material for the purposes of This particular molding would lead to significant excess carbon, which would hinder the final densification of the ceramic body.

 The organosilicon polymer precursor of ceramic material is present in the compositions of the present invention in an amount such that the free carbon number of the mixture is greater than 0.2 percent by weight, based on the total weight of the carbide powder boron and the carbonization product derived from the organosilicon polymer precursor of ceramic material.

 By the expression "free carbon number of the mixture" is meant in the present invention the amount of free carbon or excess carbon derived from the organosilicon polymer during the pyrolysis, expressed as a percentage by weight relative to the total weight of the carbide powder. of boron and the carbonization product derived from the organosilicon polymer The total amount of carbon in the ceramic carbonization product is equal to the sum of the amount of free or excess carbon and the amount of carbon in the form of silicon carbide or of boron carbide.

 The amount of free carbon derived from the organosilicon polymer is determined by pyrolyzing the polymer, in the absence of any boron carbide powder, at a high temperature in an inert atmosphere until a stable ceramic carbonization product is obtained.

For the purposes of the present invention, a "stable ceramic carbonization product" is defined as the ceramic carbonization product formed at a high temperature whose weight does not appreciably decrease upon re-exposure to elevated temperature.

Normally a stable ceramic carbonization product is obtained by pyrolysis at 1800 ° C for about 30 minutes under argon Other high temperatures may be used to form the stable ceramic char, but the duration of the temperature exposure elevated temperature should be increased for temperatures below 1800 ° C.

The ceramic yield and the silicon and carbon content of the stable ceramic carbonization product are then determined. By applying the mixing rule, the quantities of
SiC and free carbon ceramic product of stable carbonization. The amount of free carbon thus calculated is normally expressed as the amount produced per gram of organosilicon polymer precursor of ceramic material. Knowing the amount of free carbon produced by pyrolysis, it can be determined how much polymer is needed to obtain a polymer mixture: boron carbide having the desired free carbon number.

Obviously, if the same or nearly the same organosilicon polymer is used to produce a sintered body, it is not necessary to determine the amount of free carbon each time.

 This method can probably be better explained by an example. Consider an organosilicon polymer (100 g) which provides, by pyrolysis at 1800 ° C, a yield of 50 percent by weight of a char that contains 45 percent by weight of carbon and 55 percent by weight of silicon. Such a carbonization product contains 27.5 g (0.98 moles) of silicon and 22.5 g of carbon, and by applying the rule of mixtures the carbonization product contains 0.98 moles (11.8 g) of carbon. In the form of SiC, since the char contains 22.5 g of carbon, the amount of free carbon in the char is 10.7 g (22.5 g minus 11.8 g). each gram of the organosilicon polymer precursor of ceramic material provides 0.107 g of free carbon.

If the precursor mixture of ceramic material must contain 100 g of boron carbide powder, then the following equation is used to calculate the amount of organosilicon polymer (x) to add to obtain a given free carbon index (ICL).
ICL = (0.107x)
(100 + 0.5x)
The numerator of the equation above represents the total amount of free carbon produced for x grams of the organosilicon polymer. The denominator of the above equation represents the total amount of boron carbide (100 g) and the carbonization product derived from x grams of organosilicon polymer.

 By applying this method, the amount of organosilicon polymer that is required to prepare the compositions of the present invention can be determined. This method avoids the long and costly empirical procedure that would otherwise be necessary.

This seemingly complex process can be summarized by the following steps
1) pyrolyzing a known weight of the organosilicon polymer until a stable carbonization product is obtained
2) weighing the char and expressing the result as a percentage by weight of the starting compound, i.e. the "yield of char" of the polymer
3) analyze the resulting char to determine its element content;
(4) applying the blending rule, calculate the amount of "free carbon" by subtracting the amount of silicon-bonded carbon from the total carbon present, the resulting value being expressed as the free carbon produced per gram of the starting material; and
5) calculate the amount of organosilicon polymer to be added for a given free carbon index, using the following equation
ICL = (BC) + (CLP x P)
(B4 C) + (PCPR x P) where CLP = grams of free carbon produced per gram of
Organosilicon polymer; P = grams of organosilicon polymer; and RPCP = Return in Product of
Carbonization of organosilicon polymer.

 The free carbon number of the mixture must be greater than 0.2 percent by weight relative to the total weight of the boron carbide powder and the carbonization product derived from the organosilicon polymer. With free carbon numbers of less than about 0.2 weight percent, the density of the sintered body generally falls below about 2.0 (80% of theoretical density). It is preferred that the free carbon number of the mixture is between 0.5 and 3.0 percent by weight, with a further preferable range being from 1.0 to 2.0 percent by weight. The optimum density is generally obtained when the free carbon number of the mixture is about 1.5 percent by weight.

If the desired amount of free carbon can not be incorporated into the polymer, an additional source of carbon may be added. The process employed to effect this incorporation is described and claimed in copending U.S. Patent Application Serial No. 07 / 458,451 entitled "Multicomponent Binders for SiC Powders" by the inventors Gary Thomas Burns.
Ronald Keller, Willard Hauth and Chandan Kumar Saha.

The compositions of the invention also contain boron carbide powders. Many of these materials are commercially available and well known in the art. For example, Callery Chemical
Co. provides a boron carbide powder having an average particle size of about 0.03 microns and Elektroschmeltzwerk, Kempten Gmblt (ESK) provides a boron carbide powder having an average particle size of about 2.25 microns. In general, the preferred boron carbide powders have an average particle size of less than 5 micrometers; powders having an average particle size of less than 1.0 micron are still preferable; and those with an average particle size of less than 0.1 micrometer are the most preferred.

 The compositions of the present invention may also contain curing agents that are used to crosslink the organosilicon polymer prior to sintering. The green bodies thus produced generally have greater mechanical strength than the uncured articles and they can therefore better withstand any handling or machining operations before sintering. These curing agents are generally activated by heating the green body containing the curing agent to a level sufficient to cause crosslinking in the polymer. Therefore, the actual amount of the curing agent depends on the activity of the particular agent used and the amount of polymer present. However, when a peroxide is used as a curing agent, it is normally present in an amount of about 0.1 to 5.0 percent by weight based on the weight of the compound to be cured, the preferred amount being about 2.0 percent by weight. When platinum-containing curing agents are used, their amount is normally such that the platinum is present at a concentration of about 1 to 1000 millionths of the weight of the compound to be cured, the preferred amount being about 50 to 150 millionths of platinum. .

 In addition to the above curing agent, a crosslinking agent may also be included in the mixture to modify the curing characteristics. These agents may include, for example, polyfunctional silanes or siloxanes. The preferred crosslinking agents are siloxanes having functional Si-H bonds, such as Ph 2 Si (OSiMe 2 H) 2 or PhSi (OSiMe 2 H) 3.

The addition of other processing aids such as lubricants, deflocculants and dispersants is also contemplated within the scope of the present invention. Examples of these compounds include stearic acid, mineral oil, paraffin, calcium stearate, aluminum stearate, succinic acid, succinimide, succinic anhydride or various commercial products such as Oloa 1200tu
Once the amounts of the various ingredients have been determined, they are combined in a manner that ensures an intimate and homogeneous blend to avoid the appearance of areas of varying density in the mass of the sintered product. Intimate and homogeneous mixtures can be prepared using conventional mixing techniques such as grinding the various powder, dry or wet, and ultrasonic dispersion. Wet milling is generally preferred, in which the various powders are mixed and milled with organic solvents, and the solvents are then removed. Other mixing and grinding techniques will be obvious to those skilled in the art.

 The intimate and homogeneous blend can then be shaped to the desired shape. Preferably, the desired shape is made under pressure using techniques such as injection molding, uniaxial pressing, isostatic pressing, extrusion, transfer molding, and the like.

 The composition is preferably cured before final shaping. Hardening techniques are well known in the art. In general, this curing can be carried out by heating the article to a temperature between about 50 and 500 C, preferably in an inert atmosphere such as an argon or nitrogen atmosphere.

 The green body obtained by the above process has a green resistance sufficient to allow its handling and / or a modification of its shape by operations such as machining, milling, etc. In this way, products having precise shapes can be obtained.

 Once the final shape has been obtained, the article is sintered in an inert atmosphere and / or under vacuum to a temperature of 2200 ° C or higher.The preferred sintering temperature is about 2250 ° C to 2300 ° C. C, and most preferably about 2275 ° C.

 While not wishing to be bound by theory, it is believed that the free carbon donated by the organosilicon polymer performs three different functions in the formation of highly densified sintered bodies.

First, it helps remove the oxygen present in the boron carbide powder; secondly, it clearly works by transforming any potential phases of low-melting boron carbide (8C, x> 4) into B, C and third, it inhibits the growth of large grains of B4C and thereby improves the properties to the sintered state. Boron carbide powders often contain what is known as "free carbon". However, the "free carbon" that is present in the boron carbide powder is not as active or efficient as the free carbon generated "in situ" by the organosilicon polymer. It has not been clearly established whether free carbon produced "in situ" is more chemically active or simply more uniformly dispersed. Nevertheless, when the free carbon number of the mixture (as defined above) is about 1.5 percent by weight, sintered bodies having maximum densities are obtained.

 The following examples are given so that those skilled in the art can better appreciate and understand the invention.

Unless otherwise indicated, all percentages are by weight. Throughout the present description, "Me" represents a methyl group, "Ph" represents a phenyl group, "Bu" represents a butyl group, "Vi" represents a vinyl group, "AIP" represents isopropyl alcohol and "ICL" represents the free carbon index.

In the following examples, the analytical methods employed are as follows
Proton NMR spectra were recorded on a Varian EM360 or EM390 spectrometer and the results are shown here in ppm; Fourier transform IR spectra were recorded on a Nicolet DX spectrometer. Gel permeation chromatography (GPC) results were obtained on a Waters gel permeation chromatograph equipped with a model 600E system regulator, and model 410 and W model 490 differential diffractometer detectors; all values are relative to polystyrene. The results of
ATG and ATM were recorded on a Du Pont 940 thermomechanical analyzer (ATM) and an Omnitherm thermogravimetric analyzer (ATG) interfaced to an Omnitherm 2066 computer.

 Determination of carbon, hydrogen and nitrogen was performed on a Control-Equipment Corporation 240-XA Elemental Analyzer. Oxygen determination was performed on a Leco oxygen analyzer equipped with an oxygen meter 316 (model 783700) and an EF100 electrode oven. Silicon was determined by a fusion technique which consists in converting the material into soluble silicon forms and analyzing the solute for the determination of total silicon by atomic absorption spectrometry.

The melt mixture was carried out on a
Brabender plasticorder (model PL-V151) equipped with roller pallets. For a larger scale mixing operation, a 1.78 liter (1 gallon) Day mixer with sigmoid blades was used. This mixer was generally used at two-thirds of its capacity, using 400-500 grams of binder and enough powder to reach the desired level of charge. High shear cylinder milling was performed on a 6 x 6 x 13 laboratory Bolling mill used with a hot cylinder (70-800C) and the other cooled with water. The test bars were formed in a laboratory Carver press (Fred S.

Carver Inc., Summit, N.J., E.U.A.). The molding operation was performed in a 12.5-ton Hull (model 359E) console molding machine for transfer molding and a 221E / 221P model from Arburg for injection molding. The pyrolysis was carried out in a tubular furnace with graphite element, Astro, model 1000-3060 FP12, equipped with a controller / programmer model 822.

The oven was equipped with an Ircon Modeline optical pyrometer
More to control the temperature above 900 ° C. The flexural strengths (using the four-point bending technique) were determined with an Instron device, either the TTC model or the 8562 model. cooked state were measured by water immersion techniques according to ASTM
C373-72. The machined test bars were prepared according to the Mil 1942 (MR) standard.

The boron carbide powder used was either from Callery Chemical Co. (Cal) or from
Elektroschmeltzwerk, Kempten Gmblt (ESK). Resin
Phenolic was provided by Union Carbide. A partially inhibited platinum catalyst ("Pt catalyst") was prepared by dissolving 1.0 g (1.49 mmol) of (Bu 3 P) 2 PtCl 2 and 0.182 g (2.98 mmol) of HOCH 2 CH 2 NH 2 in 100 g of toluene.

Example I - Sintering with hot powder press
of boron carbide using
a siloxane as a binder
A. Synthesis of the polymer
A mixture of 3960 g of PhSi (OMe) 3 and 620 g of (ViMe 2 Si) 2 O is added to a solution of 3 g of trifluoromethanesulfonic acid in 800 g of water. After about 20 minutes, the solution is refluxed for 5 hours. The solution is cooled and neutralized with 2.73 g of potassium carbonate. Volatiles are distilled off until an internal temperature of 1200C is reached. The reaction mixture is cooled and 1500 g of toluene and 125.7 g of a 3% KOH solution in water are added. The solution is heated to reflux and the water is removed in a Dean-Stark separator.

After removal of all the water, the mixture is cooled and 20 ml of Me 2 ViSiCl are added. After stirring at room temperature for 2 hours, the mixture is filtered through a 0.2 micron membrane filter and the filtrate is concentrated by rotary evaporation. The residue is dried for about 1 to 2 hours at 100 ° C. under 133 Pa. The amount obtained is 3053.3 g.

B. Pyrolysis of the polymer and composition calculations
carbonization product
A mixture of 14.85 g of the resin formed in Part A, 5.16 g of Ph 2 Si (OSiMe 2 H) 2 and 0.01 g of Pt catalyst is prepared. An aliquot of the mixture is crosslinked at 1200 ° C. for 1 hour. hour. An aliquot of the crosslinked polymer is placed and weighed in a graphite crucible. The crucible is transferred to a tubular furnace
Astro. The oven is evacuated to less than 2.67 kPa and refilled with argon. This procedure is repeated twice. Under an argon purge, the sample is heated to 1900 ° C (from room temperature to 1200 ° C at 13 ° C / minute, 1200 to 1900 ° C at 5 C / minute) and held at this temperature for 2 hours before cooling to room temperature. The mass retention of the sample is 44.9%. The elemental composition of the carbonization product is 53.4% carbon and 46.6% silicon (by difference). The following calculation is performed: 100 g of hardened polymer gives 44.9 g of a ceramic carbonization product consisting of 20.9 g of silicon (0.75 mole) and 24.0 g of carbon. The char was 29.9 g SiC (66.6%) and 15.0 g C (33.4%). Therefore, each gram of polymer gives 0.299 g of SiC and 0.15 g of excess C.

C. Preparation of the mixture and hot pressing
A mixture is prepared according to the following procedure: 2.62 g of a mixture comprising the resin prepared in Part A and Ph 2 Si (OSiMe 2 H) 2 in a weight ratio of 3: 1 are dissolved in 250 ml of hexane and they are mixed in a beaker with 0.042 g of Pt catalyst and 17.52 g of Callery boron carbide powder. The mixture is sonicated for 5 minutes and transferred to a round bottom flask. The solvent is removed under vacuum and the residue is dried further. The dried powder is ground with mortar and pestle and sieved through a sieve of 90 microns mesh. An aliquot of the above mixture was cooked at 2275 in argon by applying the following program: from room temperature to 2275 ° C. at 10 ° C. / min with holding for 1 hour at this temperature. MPa is applied to the sample during the firing cycle, resulting in a density of 2.44 (97% of theoretical density).

Example II Powder hot sintering
of boron carbide using
a siloxane as a binder
A. Preparation of the mixture and hot pressing
A mixture is prepared according to the following procedure: 3.82 g of a mixture comprising the resin prepared in Part A of Example I and Ph 2 Si (OSiMe 2 H) 2 in a ratio by weight are dissolved in 250 ml of hexane. of 3: 1 and mixed in a beaker with 0.074 g of Pt catalyst and 26.11 g of boron carbide powder.
ESK. The mixture is sonicated for 5 minutes and transferred to a round bottom flask. The solvent is removed under vacuum and the residue is dried further. The dried powder is ground with mortar and pestle and sieved through a sieve of 90 microns mesh.

An aliquot of the above mixture was cooked at 2275 ° C under argon by applying the following program: from room temperature to 2275 ° C at 10 C / min with holding for 1 hour at this temperature. A pressure of 27.6 MPa is applied to the sample during the cooking cycle. The density of the resulting part is 2.46 (97.7% of the theoretical density).

EXAMPLE III Hot Powder Sintering
of boron carbide using
Phenolic Resin (for comparison)
A. Preparation of the mixture and hot pressing
A mixture is prepared according to the following procedure: 2.98 g of Phenolic Resin are dissolved in 250 ml of acetone and mixed in a beaker with 25.61 g of Callery boron carbide powder. The mixture is sonicated for 5 minutes and transferred to a round bottom flask. The solvent is removed under vacuum and the residue is dried further. The dried powder is ground with mortar and pestle and sieved through a sieve of 90 microns mesh. An aliquot of the above mixture was cooked at 2275 ° C under argon by applying the following program: from room temperature to 227SiC at 10 C / min with holding for 1 hour at this temperature. 6
MPa is applied to the sample during the cooking cycle. The density of the resulting part is 2.51 (99.7% of the theoretical density).

EXAMPLE IV Sintering with hot powder press
of boron carbide using
Phenolic Resin (for comparison)
A. Preparation of the mixture and hot pressing
A mixture is prepared according to the following procedure: 0.88 g of Phenolic Resin is dissolved in 250 ml of acetone and mixed in a beaker with 19.78 g of Callery boron carbide powder. The mixture is sonicated for 5 minutes and transferred to a round bottom flask. The solvent is removed under vacuum and the residue is dried further. The dried powder is ground with mortar and pestle and sieved through a sieve of 90 microns mesh. An aliquot of the above mixture was cooked at 2275 ° C under argon by applying the following program: from room temperature to 2275 ° C at 10 C / min with holding for 1 hour at this temperature. A pressure of 27.6
MPa is applied to the sample during the cooking cycle. The density of the resulting part is 2.51 (99.7% of the theoretical density).

Example V Sintering with hot powder press
of boron carbide using
Phenolic Resin (for comparison)
A. Preparation of the mixture and hot pressing
A mixture is prepared according to the following procedure: 1.59 g of Phenolic Resin are dissolved in 250 ml of acetone and mixed in a beaker with 18.42 g of boron carbide powder ESK. The mixture is sonicated for 5 minutes and transferred to a round bottom flask. The solvent is removed under vacuum and the residue is dried further. The dried powder is ground with mortar and pestle and sieved through a sieve of 90 microns mesh. An aliquot of the above mixture was cooked at 2275 ° C under argon by applying the following program: from room temperature to 2275 ° C at a rate of 10 ° C / minute with holding for 1 hour at this temperature. 6 MPa is applied to the sample during the firing cycle, resulting in a density of 2.50 (99.2% of the theoretical density).

Table 1 - Hot Pressing Results at 2275 ° C
under a pressure of 27.6 MPa
Example - Density Powder% of at the% state of the
N "of B4C Bonding Binder% C /% SiC Cooked Theory
1 Cal Siloxane 13.0 1.95 / 3.88 2.44 97.0
2 ESK Siloxane 12.75 1.91 / 3.81 2.46 97.7
3 Phenolic Cal 10.1 5.05 / 0.0 2.51 99.7
4 Cal Phenolic 4.25 2.12 / 0.0 2.51 99.1
5 Phenolic ESK 8.0 4.0 / 0.0 2.5 99.2
Cal = Callery Chemical Co.

Example VI Sintering without pressure of carbide powder
boron using a siloxane as a binder
A. Preparation of the mixture and hot pressing
A mixture is prepared according to the following procedure: 5.88 g of the resin prepared in Part A of Example I and 0.1 g of the Pt catalyst are dissolved in 250 ml of hexane and mixed into a beaker with 40.78 g of boron carbide powder ESK. The mixture is sonicated for 5 minutes and transferred to a round bottom flask. The solvent is removed under vacuum and the residue is dried further. The dried powder is ground with mortar and pestle and sieved through a sieve of 90 microns mesh. The sieved powder is dry pressed to 35 x 8 x 2 mm test bars in a lined WC matrix using a 317 MPa Carver laboratory press. The test bars are heated at 250 ° C. for 24 hours to crosslink the polymer. The test bars have a mean hardness of 1.48 and a resistance (flexion of 4 points) of 11.58
MPa. The test bars were fired at 2275 ° C under argon by applying the following program: from room temperature to 1200 ° C at 10 C / minute hold for 30 minutes, from 1200 to 2275 ° C at 10 C / minute minute with hold for 30 minutes at this temperature. The average density of the test bars is 2.39 (94.8% of the theoretical density). The test bars have a mean strength (flexion of 4 points) of 107.9 MPa before machining and 102.7 MPa after machining.

Example VII Unpressurized Sintering of Carbide Powder
of boron using Phenolic Resin
as a binder (for comparison)
A. Preparation of the mixture and hot pressing
A mixture is prepared according to the following procedure: 2.016 g of Phenolic Resin is dissolved in 250 ml of acetone and mixed in a beaker with 48.98 g of boron carbide powder ESK. The mixture is sonicated for 5 minutes and transferred to a round bottom flask. The solvent is removed under vacuum and the residue is dried further. The dried powder is ground with mortar and pestle and sieved through a sieve of 90 microns mesh. The sieved powder is dry pressed to 35 x 8 x 2 mm test bars in a lined WC matrix using a 317 MPa Carver laboratory press. The test bars have a cured average density of 1.35 and a resistance (4-point bending) of 1.21 MPa. The test bars were fired at 2275 ° C under argon by applying the following program: from room temperature to 1200 ° C at 10 C / minute hold for 30 minutes, from 1200 to 2275 ° C at 10 C / minute minute with hold for 30 minutes at this temperature. The average density of the test bars is 2.15 (85.4% of the theoretical density). The test bars have average strength (4-point bending) of 166.8 MPa before machining and 167.3 MPa after machining.

EXAMPLE VIII Sintering without pressure of carbide powder
of boron using Phenolic Resin
and a polycarbosilane (for comparison)
A. Preparation of the mixture and hot pressing
A mixture was prepared according to the following procedure: 1.39 g of Phenolic Resin and 3.79 g of polycarbosilane were dissolved in 250 ml of tetrahydrofuran and mixed in a beaker with 45.11 g of boron carbide powder. ESK. The mixture is sonicated for 5 minutes and transferred to a round bottom flask. The solvent is removed under vacuum and the residue is dried further. The dried powder is ground with mortar and pestle and sieved through a sieve of 90 microns mesh. The sieved powder is dry pressed to 35 x 8 x 2 film test bars in a lined WC matrix using a 317 MPa Carver laboratory press. The test bars have a mean hardness of 1.45 and a resistance (4-point bending) of 1.45 MPa. The test bars were fired at 2275 ° C under argon by applying the following program: from room temperature to 1200 ° C at 100C / minute; hold for 30 minutes; from 1200 to 2275 ° C at a rate of 100C / minute with hold for 30 minutes at this temperature The average density of the test bars is 2.41 (95.4% of the theoretical density). have an average resistance (flexion of 4 points) of 99.3 MPa before machining and 107.2 MPa after machining.

Example IX Sintering without pressure of carbide powder
boron using a siloxane as a binder
A. Synthesis of the polymer
In a 5-liter three-neck flask equipped with a drain valve, a thermometer, a condenser and an addition funnel, 1790 g of water and 324 g of AIP are placed. A mixture of 134.6 g (0.90 mole) MeSiCl 3, 116.1 g (0.90 mole) Me 2 SiCl 2, 285 is added below the surface of the water over a period of six minutes. 5 g (1.35 mol) of PhSiCl3, 151.8 g (0.60 mol) of Ph2 SiCl2 and 176 g (1.25 g) of MeViSiCl2 dissolved in 782 g of toluene. After stirring for 30 minutes, the water is removed. The resin is washed twice with warm tap water and transferred to a 3 liter flask where it is azeotropically dried and thickened for 3 hours with 0.1% zinc octoate. The solvent was removed and the resin dried at 125 ° C at 2.67 kPa.

B. Pyrolysis of the polymer and composition calculations
carbonization product
A sample of the resin prepared in the part
A, mixed with 1% Lupersolt M (bis (t-butylperoxy) -2,5-dimethylhexane), is cured at 200 ° C. for 1 hour An aliquot of the crosslinked polymer is placed and weighed in a graphite crucible The crucible is transferred to an Astro tube furnace, the furnace is evacuated to less than 2.67 kPa and refilled with argon, and this procedure is repeated twice. The sample is heated up to 19,000 C (from room temperature to 1900 ° C at 15 ° C / minute) and maintained at that temperature for 2 hours before cooling to room temperature. sample is 43.7% The elemental composition of the carbonization product is 40.6% carbon and 59.4% silicon (by difference).

The following calculation is performed: 100 g of cured polymer gives 43.7 g of a ceramic carbonization product consisting of 26 g of silicon (0.93 mole) and 17.7 g of carbon. The char was 37.1 g SiC (84.9%) and 6.6 g C (15.1%). Therefore, each gram of polymer gives 0.371 g of SiC and 0.066 g of excess C.

C. Preparation of the mixture and testing
A mixture is prepared according to the following procedure: 7.56 g of the resin prepared in Part A and 0.10 g of Lupersoltn dissolved in 250 ml of hexane are mixed in a beaker with 46.78 g of carbide powder. of boron ESK. The mixture is sonicated for 5 minutes and transferred to a round bottom flask. The solvent is removed under vacuum and the residue is dried further. The dried powder is ground with mortar and pestle and sieved through a sieve of 90 microns mesh. The sieved powder is dry pressed to 35 x 8 x 2 mm test bars in a lined WC matrix using a 317 MPa Carver laboratory press. The test bars are heated at 250 ° C for 24 hours to cure the polymer The test bars have a mean hardness of 1.51 and a resistance (flexion of 4 points) of 11.00 MPa The test bars were fired at 2275 ° C under argon by applying the following program: from room temperature to 1200 ° C at 10 ° C / minute; hold for 30 minutes; from 1200 to 2275 ° C at a rate of 10 C / minute with 30 minutes hold at this temperature The average density of the test bars is 2.39 (94.8% of the theoretical density). test have a mean strength (bending 4 points) of 133 MPa before machining and 134.9 MPa after machining.

EXAMPLE X Unpressurized Sintering of Carbide Powder
boron using a siloxane as a binder
A. Synthesis of the polymer
In a 5-liter three-neck flask equipped with a drain valve, a thermometer, a condenser and an addition funnel, 895 g of water and 162 g of AIP are placed. A mixture of 67.2 g (0.45 mole) MeSiCl 3, 9.55 g (0.05 mole) PhMeSiCl 2, 84 is added below the surface of the water over a period of six minutes. , 5 g (0.40 mol) of SiCl 3 and 25.29 g (0.10 mol) of Ph 2 SiCl 2 dissolved in 390 g of toluene. After stirring for 30 minutes, the water is removed. The resin is washed twice with two 1-liter portions of warm tap water. The resin phase is dried and concentrated under vacuum to give a brittle resin.

B. Pyrolysis of the polymer and composition calculations
carbonization product
An aliquot of the resin prepared in Part A is placed and weighed in a graphite crucible.

The crucible is transferred to an Astro tube furnace. The oven is evacuated to less than 2.67 kPa and refilled with argon. This procedure is repeated twice. Under an argon purge, the sample is heated to 1800 ° C (from room temperature to 1800 ° C at 10 C / minute) and held at that temperature for 2 hours before cooling to room temperature. room. The mass retention of the sample is 35.9%. The elemental composition of the carbonization product is 36.8% carbon and 63.2% silicon (by difference). The following calculation is carried out: 100 g of cured polymer gives 35.9 g of a ceramic carbonization product consisting of 22.7 g of silicon (0.81 mole) and 13.2 g of carbon. The carbonization product consists of 32.4 g of SiC (90.3%) and 3.5 g of C (9.7%). Therefore, each gram of polymer gives 0.324 g of SiC and 0.035 g of excess C.

C. Preparation of the mixture and testing
A mixture is prepared according to the following procedure: 5.01 g of the micrometer resin are mixed in a beaker. The sieved powder is dry pressed to 35 x 8 x 2 mm test bars in a lined WC matrix using a 317 MPa Carver laboratory press. The test bars have a mean hardness of 1.41 and a resistance (flexure of 4 points) of 2.11 MPa. The test bars were fired at 2275 ° C under argon by applying the following program: from room temperature to 1200 ° C at 10 C / min, hold for 30 minutes, from 1200 to 2275 ° C at 10 C / minute with holding for 30 minutes at this temperature. The average density of the test bars is 2.34 (92.9% of the theoretical density). The test bars have an average strength (flexion of 4 points) of 154.1 MPa before machining and 209.3 MPa after machining.

EXAMPLE XI Sintering without pressure of powder
boron carbide treated using
a siloxane as a binder
A. Powder treatment
An aliquot of boron carbide powder
ESK is stirred with H2 S04 1M for 16 hours to try to eliminate any metal impurities.

The powder is filtered and dried.

C. Bar Preparation and Testing
A mixture is prepared according to the following procedure: 5.89 g of the resin prepared in Part A of Example I and 0.11 g of Pt catalyst dissolved in 250 ml of hexane are mixed in a beaker. 79 g of the boron carbide powder ESK treated. The mixture is sonicated for 5 minutes and transferred to a round bottom flask. The solvent is removed under vacuum and the residue is dried further. The dried powder is ground with mortar and pestle and sieved through a sieve of 90 microns mesh. The sieved powder is dry pressed to 35 x 8 x 2 mm test bars in a jacketed WC matrix using a press
Laboratory carver at 317 MPa. The test bars are heated for 250 hours for 24 hours to cure the polymer. The test bars have a mean hardness of 1.49. The test bars were fired at 22750C under argon by applying the following program: from room temperature to 1200 C at 10 C / minute hold for 30 minutes; from 1200 to 22750C at a rate of 10C / minute with holding for 30 minutes at this temperature. The average density of the test bars is 2.37 (93.9% of the theoretical density). The test bars have a mean strength (flexion of 4 points) of 90.18 MPa before machining and 91.84 MPa after machining.

Example XII Sintering without pressure of boron carbide
using a silazane as a binder
A. Synthesis of the polymer
A mixture of 112.9 g (0.80 mol) of 1,1-dichloro-1-silacyclobutane, 101.3 g (0.40 mol) of diphenyldichlorosilane and 84.6 g (0.degree. 40 moles) of phenyltrichlorosilane dissolved in 1.2 liters of anhydrous toluene. Ammonia is rapidly bubbled through the solution for 3 hours. The solution is allowed to warm to room temperature and the excess ammonia is distilled off. The solution is filtered through a medium-sized glass frit and the filtrate is concentrated in vacuo. The residue is stripped of volatiles at 150-170 ° C for 3 hours under 133 Pa.

B. Pyrolysis of the polymer and composition calculations
carbonization product
An aliquot of the resin prepared in Part A is introduced and weighed in a graphite crucible. The crucible is transferred to a tubular furnace
Astro. The oven is evacuated to less than 2.67 kPa and refilled with argon. This procedure is repeated twice. Under an argon purge, the sample is heated to 1800 ° C (from room temperature to 1800 ° C at 10 C / minute) and held at that temperature for 2 hours before cooling to room temperature. room. The mass retention of the sample is 52.2%. The elemental composition of the char is 57.7% carbon, 40.1% silicon and 0.6% oxygen. The following calculation is carried out: 100 g of cured polymer gives 52.5 g of a ceramic carbonization product consisting of 22.2 g of silicon (0.79 mole) and 30.3 g of carbon. The char was 31.5 g of SiC (60.4%) and 20.7 g of C (39.6%). Therefore, each gram of polymer gives 0.315 g of SiC and 0.207 g of excess C.

C. Preparation of the mixture and testing
A mixture is prepared according to the following procedure: 4.88 g of the resin prepared in Part A dissolved in 250 ml of toluene are mixed in a beaker with 47.64 g of boron carbide powder ESK. The mixture is sonicated for 5 minutes and transferred to a round bottom flask. The solvent is removed under vacuum and the residue is dried further. The dried powder is ground with mortar and pestle and sieved through a sieve of 90 microns mesh. The sieved powder is dry pressed to 35 x 8 x 2 mm test bars in a jacketed WC matrix using a press
Laboratory carver at 317 MPa. The test bars are heated at 250 ° C for 24 hours to cure the polymer, the test bars have a mean hardness of 1.48 and a resistance (flexure of 4 points) of 9.025 MPa. The test bars were fired at 2275 ° C under argon by applying the following program: from room temperature to 12000C at 10 C / minute; hold for 30 minutes; from 1200 to 2275 ° C. at a rate of 10 C / minute with 30 minutes holding at this temperature The average density of the test bars is 2.36 (93.7% of the theoretical density). test have an average strength (bending on 4 points) of 145.6 MPa before machining and 145.6 MPa after machining.

Example XIII Sintering without pressure of boron carbide
using a silazane as a binder
A. Synthesis of the polymer
Ammonia is rapidly bubbled into a solution of 90.27 g (0.427 mol) of PhSiCl 3, 43.3 g (0.268 mol) of SiCl 3 and 19.05 g (0.128 mol) of MeSiCl 3 in about 700 ml of toluene. cooled to -78 ° C. After 45 minutes, the introduction of ammonia is stopped and the reaction mixture is allowed to warm to room temperature. The resulting suspension is filtered through a medium-sized glass frit and the filtrate is concentrated in vacuo. 61.9 g (74% yield) of a brittle resin are obtained. Tg = 96.80C.

B. Pyrolysis of the polymer and composition calculations
carbonization product
An aliquot portion of the resin formed in Part A is placed and weighed in a graphite crucible.

The crucible is transferred to an Astro tube furnace. The oven is evacuated to less than 2.67 kPa and refilled with argon. This procedure is repeated twice. Under an argon purge, the sample is heated to 1900 ° C (from room temperature to 1900 ° C at 15 C / min) and maintained at that temperature for 2 hours before cooling to room temperature. room. The mass retention of the sample is 53.8%. The elemental composition of the carbonization product is 60.1% carbon and 39.9% silicon (by difference). The following calculation is carried out: 100 g of cured polymer gives 53.8 g of a ceramic carbonization product consisting of 21.5 g of silicon (0.77 mole) and 32.3 g of carbon. The carbonization product consists of 30.7 g of SiC (57.1%) and 23.1 g of C (42.9%). Therefore, each gram of polymer gives 0.307 g of SiC and 0.231 g of excess C.

C. Preparation of the mixture and testing
A mixture is prepared according to the following procedure: 2.01 g of the resin prepared in Part A and 0.035 g of LupersolTH dissolved in 250 ml of toluene are mixed in a beaker with 22.6 g of boron carbide powder ESK . The mixture is sonicated for 5 minutes and transferred to a round bottom flask. The solvent is removed under vacuum and the residue is dried further. The dried powder is ground with mortar and pestle and sieved through a sieve of 90 microns mesh. The sieved powder is dry pressed to 35 x 8 x 2 mm test bars in a lined WC matrix using a 317 MPa Carver laboratory press. The test bars are heated at 200 ° C for 16 hours to cure the polymer The test bars have a mean hardness of 1.39.

The test bars were fired at 2275 ° C under argon by applying the following program: from room temperature to 1200 ° C at 100C / minute; hold for 30 minutes; from 1200 to 2275 ° C. at a rate of 10 C / minute with holding for 30 minutes at this temperature The average density of the test bars is 2.31 (91.5% of the theoretical density). Machined test have an average resistance of 148.2 MPa.

EXAMPLE XIV Sintering without pressure of boron carbide
using a silazane as a binder
A. Synthesis of the polymer
Ammonia was rapidly sparged in a solution of 185.3 g (0.877 moles) of PhSiCl31 in 500 ml of toluene cooled at -78 ° C for 1 to 2 hours. The solution is allowed to warm to room temperature and the excess ammonia is distilled off. The resulting suspension is filtered through a medium-sized glass frit and the filtrate is concentrated in vacuo. 80.6 g (72.1%) of a brittle resin are obtained. T = 123.90C.

B. Pyrolysis of the polymer and composition calculations
carbonization product
An aliquot portion of the resin formed in Part A is placed and weighed in a graphite crucible.

The crucible is transferred to an Astro tube furnace. The oven is evacuated to less than 2.67 kPa and refilled with argon. This procedure is repeated twice. Under an argon purge, the sample is heated to 1900 ° C (from room temperature to 1900 ° C at 15 ° C / min) and maintained at that temperature for 2 hours before cooling to room temperature. The mass retention of the sample is 49.5% The elemental composition of the carbonization product is 50.8% carbon and 49.2% silicon (by difference) The following calculation is carried out: 100 g cured polymer gave 49.5 g of a ceramic carbonization product consisting of 24.3 g of silicon (0.87 mole) and 25.2 g of carbon, and the char was 34.7 g of SiC ( 70.2 t) and 14.8 g of C (29.8%) Therefore, each gram of polymer gives 0.347 g of SiC and 0.148 g of excess C.

C. Preparation of the mixture and testing
A mixture is prepared according to the following procedure: 2.5 g of the resin prepared in Part A dissolved in 250 ml of toluene are mixed in a beaker with 22.5 g of boron carbide powder ESK. The mixture is sonicated for 5 minutes and transferred to a round bottom flask. The solvent is removed under vacuum and the residue is dried further. The dried powder is ground with mortar and pestle and sieved through a sieve of 90 microns mesh. The sieved powder is dry pressed to 35 x 8 x 2 mm test bars in a jacketed WC matrix using a press
Laboratory carver at 317 MPa. The test bars have an average density of 1.45 and a resistance (flexion of 4 points) of 2.33 MPa. The test bars were fired at 2275 ° C under argon by applying the following program of room temperature at 1200 ° C at 10 C / minute; hold for 30 minutes; from 1200 to 2275 ° C. at a rate of 10 C / minute with holding for 30 minutes at this temperature The average density of the test bars is 2.33 (92.5% of the theoretical density). Machined test have an average resistance of 156.8 MPa.

EXAMPLE XV Sintering without pressure of boron carbide
using a borosiloxane as a binder
A. Synthesis of the polymer
41.52 g (0.40 mol) of B (OMe) 3 are added to a stirred mixture of 39.6 g (0.20 mol) of Ph (OMe) 3, 23.25 g (0.125 mol) of (Me2 SiVi) 2 O, 48.8 g (0.20 mol) of
Ph2Si (OMe) 2, 46.8 g (2.6 moles) distilled water and 0.20 ml CF3 SQH. The reaction mixture is refluxed for 2 hours and then stirred at room temperature for 12 hours. The methanol and water are distilled off until the distillate temperature reaches more than 90 ° C. The reaction mixture is cooled and 0.56 g of NaHCO 3 and 115 g of toluene are added. The toluene is heated to reflux and the water is removed in a Dean-Stark separator.When the distillate is clear (about 3 to 4 hours), the reaction mixture is cooled and 1 ml of
Me2SiViCl. After stirring for 0.5 hours, the reaction mixture is filtered and the filtrate concentrated in vacuo.

The amount obtained is 90.75 g (92.7%).

B. Pyrolysis of the polymer and composition calculations
carbonization product
A mixture of 6.07 g of the resin obtained in Part A and 0.23 g of Lupersol ™ is prepared and an aliquot is crosslinked at 200 ° C for 3 hours under an argon atmosphere An aliquot of the crosslinked polymer is placed and weighed in a graphite crucible.

The crucible is transferred to an Astro tube furnace. The oven is evacuated to less than 2.67 kPa and refilled with argon. This procedure is repeated twice. Under an argon purge, the sample is heated up to 1900 C (from room temperature to 1900 C at 15 ° C / minute) and maintained at this temperature for 2 hours before cooling to room temperature The mass retention of the sample is 26.3% The elemental composition of the char is 48.7% carbon, 5.0% boron and 41.2% silicon (by difference). The following calculation is carried out: 100 g of cured polymer gives 26.3 g of a ceramic carbonization product consisting of 46.3% by weight of silicon (by difference), 48.7% by weight of carbon and 5.0 % by weight of boron The carbonization product consists of 17.4 g of
SiC (66.1%), 7.6 g of C (28.9%) and 1.5 g of B (4.9%).

Therefore, each gram of polymer gives 0.174 g of SiC, 0.013 g of B and 0.076 g of excess C.

C. Preparation of the mixture and testing
A mixture is prepared according to the following procedure: 2.50 g of the resin prepared in Part A and 0.08 g of LupersolTn dissolved in 250 ml of toluene are mixed in a beaker with 22.50 g of carbide powder. boron ESK. The mixture is sonicated for 5 minutes and transferred to a round bottom flask. The solvent is removed under vacuum and the residue is dried further. The dried powder is ground with mortar and pestle and sieved through a 140 micron mesh screen. The sieved powder is dry pressed to 35 x 8 x 2 mm test bars in a lined WC matrix using a 317 MPa Carver laboratory press. The test bars are then cured in an argon atmosphere by heating to 200 ° C at 3 ° C / minute and held for 1 hour. The test bars are fired at 2250 ° C. under argon by applying the following program: from room temperature to 1200 ° C. at a rate of 10 ° C. / min, hold for 30 minutes, from 1200 to 2250 ° C. at a rate of 100 ° C. with holding for 2 hours at this temperature The average density of the test bars is 2.275 (90.03% of the theoretical density) The machined test bars have a mean strength (flexion of 4 points) of 180.99 MPa .

Table 2 - Pressure Free Sintering
MDR to
Exem- Conditions Density state full of de in hardened state
N Bonding Binder% C / * SiC hardening cured (MPa)
6 Siloxane 12.79 2.06 / 4.11 250 C / 24 h 1.483 11.58
7 Phenolic 4.03 2.06 / 0.0 nil 1.352 1.21
8 + PN + PCS 7.5 / 2.5 2.04 / 4.14 nil 1.445 1.45
Siloxane 14.07 1.01 / 5.67 250 C / 24 h 1.505 11.00 Siloxane 9.5 0.35 / 3.27 Nil 1.406 2.11 11 * Siloxane 12.82 2.06 / 4, 11,250 C / 24 h 1,487 ND

12 Silazane 10.24 2.00 / 3.05 250 C / 24 h 1.476 9.03 13 Silazane 8.0 1.85 / 2.46 200 C / 16 h 1.39 14 silazane 10.0 1.47 / 3.48 nil 1.45 2.33 Borosiloxane 9.0 0.76 / 1.74 200 C / 3 hrs
Properties of Ceramic Products after Pyrolysis at 2275 C
% of the MCR after X-ray analysis
Example density MDR machining (% by weight)
N Theoretical Density (MPa) (MPa) B, C SiC C
6 2.39 94.8 108.2 102.7 95.9 4.1 1.0
7 2.15 85.4 166.9 167.5 99.4 frames
8 2.41 95.5 99.3 106.9 92.2 7.8 2-3
9 2.39 94.8 133.1 135.1 93.8 6.2 traces
10 2.34 92.9 153.8 209.6 95.9 4.1 nil
11 2.37 93.9 90.3 91.7 95.0 5.0 traces
12 2.36 93.7 145.5 145.5 96.0 4.0 <1
13 2.31 91.5 148.2 97.8 2.2 traces
14 2.33 92.5 156.5 95.9 4.1 traces
15 2,275 90,03 180,99 PN = Phenolic Resin * Boron carbide was washed with acid
sulfuric, 1M before use.

EXAMPLE XVI Transfer Molding of Boron Carbide
with a siloxane as a binder
A. Test bar manufacture and testing
In a Sigma double-armed mixer at 110 ° C., 658 g of boron carbide powder ESK, 435.5 g of the resin produced in Part A of Example X and 3.7 g of Lupersol ™ are placed. The mixer is operated at 33 rpm for about 1 hour, after which the heating is stopped and the material is removed.The mixture is transfer molded into a 12-cavity test bar mold (each cavity-6, 2 x 37.8 x 2.5 mm) at 195 ° C with a plunger pressure of 8.62 MPa and a clamping pressure of 12.75 MPa. After having been removed from the hot mold in 5-minute cycles, the test bars are further cured at 250 ° C. for 16 hours, the test bars have a mean hardness of 1.63 and a resistance in the cured state of 17.51 MPa The test bars are fired at 2225 ° C in an argon atmosphere using the following program: from room temperature to 200 ° C at 3 ° C / minute from 200 to 1400 ° C. at a rate of 1 ° C./minute, maintained for 30 minutes, from 1400 to 2225 ° C. at a rate of 1 ° C./minute, with a hold for 30 minutes at this temperature. from 100C / minute to 900 ° C. The average density of the cooked test bars is 2.33 (88.14% of the theoretical density). Unmachined test bars have an average strength (flexion of 4 points) of 75.29
MPa.

EXAMPLE XVII Transfer Molding of Boron Carbide
using a silazane as a binder
A. Synthesis of the polymer
A mixture of 610 g (4.4 moles) of 1,1-dichloro-1-silacyclobutane, 278 g (1.10 moles) of diphenyldichlorosilane and 375 g (2.40 moles) of dissolved methyltrichlorosilane is cooled to -78 ° C. in 8 liters of anhydrous toluene. Ammonia is bubbled rapidly into the solution for 3 hours. The solution is allowed to warm to room temperature and the excess ammonia is distilled off. The solution is filtered through a medium-sized glass frit and the filtrate is concentrated in vacuo. The residue is stripped of volatiles at 150-170 ° C at 133 Pa for 3 hours.

B. Pyrolysis of the polymer and composition calculations
carbonization product
An aliquot portion of the resin formed in the portion
A is placed and weighed in a graphite crucible. The crucible is transferred to an Astro tube furnace. The oven is evacuated to less than 2.67 kPa and refilled with argon. This procedure is repeated twice. Under an argon purge, the sample is heated to 1800 ° C (from room temperature to 1800 ° C at 10 ° C / min) and maintained at that temperature for 2 hours before cooling to room temperature. room. The mass retention of the sample is 45.0%. The elemental composition of the carbonization product is 50.6% carbon, 48.6% silicon and 0.23% oxygen. The following calculation is performed: 100 g of cured polymer gives 45.0 g of a ceramic carbonization product consisting of 21.9 g of silicon (0.78 mole) and 22.8 g of carbon. The char was 31.3 g SiC (70.0%) and 13.4 g C (30.0%). Therefore, each gram of polymer gives 0.313 g of SiC and 0.134 g of excess C.

A. Test bar manufacture and testing
In a Brabender mixer at 140 ° C., 50.1 g of boron carbide powder ESK, 0.3 g of zinc stearate and 25.8 g of the resin produced in part A are placed. operation at 60 rpm for about 0.5 hours, after which the heating is stopped and the material is removed The mixture is transfer molded into a 12-cavity test bar mold (each cavity = 6.2 x 37.8 x 2.5 mm) at 240 ° C with a piston pressure of 5.56 MPa and a clamping pressure of 12.75
MPa. After having been removed from the hot mold in 10-minute cycles, the test bars are further cured at 250 ° C. for 16 hours, the test bars have a mean hardness of 1.64 and a resistance in the cured state of 12.90 MPa The test bars are fired at 2225 ° C. in an argon atmosphere by applying the following program: from room temperature to 200 ° C. at a rate of 3 ° C. per minute, from 200 ° C. at 1400 C at a rate of 1 C / minute, maintained for 30 minutes, from 1400 to 2225 C at a rate of 1C / minute with a hold for 30 minutes at this temperature. The oven is cooled at a rate of 10C / minute up to 900 C. The average density of the cooked test bars is 2.37 (91.3% of the theoretical density). The unmachined test bars have an average strength (flexion of 4 points) of 91.56 MPa.

Table 3 - Molding Examples
Do? at
Density state Example% of iUlBt in hardened state
N Bonding Binder% C /% SiC Hardened Spiral (MPa)
16 Silonane 40 3.41 / 19.15 9.5 1.63 17.51
17 Silazarxes 34 5.6 / 13.1 28.5 1.64 12.90 Properties of C6ranic Productions after Pyrolysis at 2275 C
% of X-ray Analysis
Example MCR density (% by weight)
N Theoretical Density (MPa) B, C SiC C
16 2.33 88.1 75.2 75.0 24.8 tees
17 2.37 91.3 91.7 76.0 24.4 traces
Example XVIII Effect of sintering temperature
on the properties in the cooked state
A. Test bar manufacture and testing
Specimens are made by uniaxial pressing from the mixtures of Examples VI, VII, VIII and X. These specimens are fired by applying the following temperature program: from room temperature to 1200 ° C. at a rate of 100 ° C. per minute, hold for 30 minutes. minutes, 1200 C at the final temperature at a rate of 10 C / minute with holding for 30 minutes at this temperature. The final temperatures are 2225 C, 2250 C and 2275 C. The results are summarized in the following table (Table 4). These results clearly show that temperatures exceeding 2250 ° C. are preferable for obtaining high-density sintered boron carbide with these binder systems.

Table 4 - Effect of Sintering Temperature Lu: '. a11wn at 2275 C cooking at 225OC bush at 22250C full MDR MDR MDR
N Density (%) (MPa) Density (%) (MPa) Density (%) (MPa)
6 2.15 (85.4) 167.2 2.06 (81.9) 194.1 1.95 (77.8) 123.2
7 2.41 (95.5) 107.2 2.26 (89.6) 224.4 2.24 (88.9) 243.3
8 2.39 (94.8) 102.7 2.27 (89.9) 218.0 2.24 (88.9) 241.7
10 2.34 (92.9) 209.3 2.27 (90.1) 218.4 2.13 (84.6) 192.2
Note: The density value given in parentheses is the
% of the theoretical density of boron carbide.

Example XIX Isostatic pressing of large bars
and records
A. Parts manufacturing and testing
A mixture is prepared according to the following procedure: 47.0 g of the resin prepared in Part A of Example 10 and 0.50 g of
Lupersol ™ dissolved in 1000 ml of toluene with 450 g of boron carbide powder ESK. The mixture is sonicated for 10 minutes and transferred to a round bottom flask. The solvent is removed under vacuum and the residue is dried further. The dried powder is ground with mortar and pestle and sieved through a sieve of 90 microns mesh. The sieved powder is dry-pressed into test bars of 53.25 x 12.7 x 4.75 mm and 75.0 mm diameter discs in lined WC dies using a Frecon automatic press. laboratory at 69 MPa. The test pieces are then placed in a rubber bag, evacuated and subjected to isostatic pressing at 207 MPa, followed by hardening at 250 ° C. for 4 hours under argon. The test bars are fired at 23000C under argon by applying the following program: from room temperature to 1200 C at 5 C / minute; hold for 30 minutes; from 1200 to 1500 C at a rate of 2.5 C / minute under vacuum; and 1500 to 2300 C at 5 C / minute with holding for 1 hour at this temperature. The results of this example are shown in Tables 5 and 6.

Example XX Isostatic pressing of large bars
and records
A. A mixture is prepared according to the following procedure: 78 g of the resin prepared in Part A of Example 1 and 0.50 g of Lupersol ™ dissolved in 1000 ml of hexane are mixed in a beaker with 450 g of boron carbide powder ESK. The mixture is sonicated for 10 minutes and transferred to a round bottom flask. The solvent is removed under vacuum and the residue is dried further. The dried powder is ground with mortar and pestle and sieved through a sieve of 90 microns mesh. The sieved powder is dry pressed into test bars 53.25 x 12.7 x 4.75 mm and discs 75 mm in diameter.

The solvent is removed under vacuum and the residue is dried further. The dried powder is ground with mortar and pestle and sieved through a sieve of 90 microns mesh. The sieved powder is dry pressed into test bars of 53.25 x 12.7 x 4.75 mm in jacketed WC dies by means of an automatic press.
Laboratory Frecon at 69 MPa. The test pieces are then placed in a rubber bag, evacuated and subjected to isostatic pressing at 207 MPa, followed by hardening at 250 ° C. for 4 hours under argon. The test bars are fired at 23000C under argon by applying the following program: from room temperature to 1200 C at 5C / minute; hold for 30 minutes; from 1200 to 1500 ° C at 2.5 ° C / min under vacuum; and from 1500 to 2300 ° C at 5 C / min with hold for 1 hour at this temperature The results for the products of this example are shown in Table 5.

Example XXII Isostatic pressing of large bars
with Phenolic Resin (for comparison)
A. A mixture is prepared according to the following procedure: 2.016 g of resin are mixed in a beaker
Phenolic dissolved in 250 ml of acetone with 48.98 g of boron carbide powder ESK. The mixture is sonicated for 5 minutes and transferred to a round bottom flask. The solvent is removed under vacuum and the residue is dried further. The dried powder is ground with mortar and pestle and sieved through a sieve of 90 microns mesh. The sieved powder is dry pressed into test bars of 53.25 x 12.7 x 4.75 mm in jacketed WC dies by means of an automatic press.
Laboratory Frecon at 69 MPa. The test pieces are then placed in a rubber bag, evacuated and subjected to isostatic pressing at 207 MPa and cured at 250 ° C for 4 hours under argon.The test bars are fired at 2300 ° C. argon by applying the following program: from room temperature to 1200 C at 5 C / minute; hold for 30 minutes; from 1200 to 1500 C at a rate of 2.5ec / min under vacuum; and 1500 to 2300 C at 5 C / minute with holding for 1 hour at this temperature. The results for the products of this example are shown in Table 5.

Table 5 - Bars produced by isostatic pressing
Density Temperature Density in the baked ME state Example in the baking state (% of machining)
N hardened (C) theory (MPa)
19 1.45 2300 2.33 92.5 157.8 + 5.0
20 1.59 2275 2.38 94.4 289.2 + 32.7
21 1.42 2275 2.28 90.5 170.2 + 10.6
22 1.32 2275 2.06 81.7 168 5 + 14.1
Table 6 - Disks produced by isostatic pressing
% of the
Example density density
N the state cooked theoretical Baking conditions
19 2.19 86.9 2300 C - hold 2 h
2.14 84.9 2300 C - hold 2 h
2.13 84.5 2300 C - hold 2 h
20 2.34 92.9 2275 C - hold 2 h
2.34 92.9 2275 C - hold 2 h
2,37 93,9 2275 C - holding 2 h

Claims (7)

 1. A method of manufacturing a sintered boron carbide body, characterized in that it consists of  (a) mixing components comprising boron carbide powder and a ceramic precursor organosilicon polymer selected from polysiloxanes, polysilazanes, polysilanes, metallopolysiloxanes and metallopolysilanes to form a homogeneous mixture in which the precursor organosilicon polymer of ceramic material is present in an amount such that the free carbon number of the mixture is greater than 0.2 percent by weight relative to the total weight of the boron carbide powder and the carbonization product derived from the precursor organosilicon polymer of ceramic material  (b) shaping the homogeneous mixture to the desired shape under pressure at a temperature below about 500 ° C to obtain a manipulable green body;  (c) sintering the manipulable green body in an inert atmosphere at a temperature above about 2200 C to obtain a sintered body having a density greater than about 2.0.  2. Method according to claim 1, characterized in that the free carbon content of the carbonization product derived from the organosilicon polymer precursor of ceramic material is determined before the formation of a green body that can be handled, by heating a known amount of the polymer organosilicon precursor of ceramic material under an inert atmosphere at an elevated temperature for a time sufficient to convert the organosilicon precursor polymer of ceramic material into a stable ceramic carbonization product, by determining the yield of stable ceramic carbonization product and the contents of silicon and in carbon of the stable ceramic carbonization product, and then calculating the amount of free carbon in the stable ceramic carbonization product by part of the organosilicon polymer precursor of ceramic material.  3. Method according to claim 2, characterized in that the homogeneous mixture contains, in addition, at least one agent which promotes crosslinking, selected from hardening agents and crosslinking agents, in an amount effective to crosslink the precursor organosilicon polymer of ceramic material.  4. A method of forming a green body that can be handled, characterized in that it consists of  (a) mixing components comprising boron carbide powder and a ceramic precursor organosilicon polymer selected from polysiloxanes, polysilazanes, polysilanes, metallopolysiloxanes and metallopolysilanes to form a homogeneous mixture in which the precursor organosilicon polymer of ceramic material is present in an amount such that the free carbon number of the mixture is greater than 0.2 percent by weight relative to the total weight of the boron carbide powder and the carbonization product derived from the precursor organosilicon polymer of ceramic material  (b) shaping the homogeneous mixture to the desired shape, under pressure, at a temperature below about 500 ° C.  5. homogeneous mixture, characterized in that it comprises boron carbide powder and an organosilicon polymer precursor of ceramic material selected from polysilazanes, polysilanes, metallopolysiloxanes and metallopolysilanes, the organosilicon polymer precursor of ceramic material being present in an amount such that the free carbon number of the mixture is greater than 0.2 percent by weight relative to the total weight of the boron carbide powder and the carbonization product derived from the organosilicon polymer precursor of ceramic material.  6. Mixture according to claim 5, characterized in that it additionally contains at least one agent which promotes crosslinking, chosen from hardening agents and crosslinking agents, in an amount effective to cross-link the organosilicon polymer precursor of ceramic material .  7. homogeneous mixture, characterized in that it consists essentially of boron carbide powder and organopolysiloxane precursor of ceramic material, the organopolysiloxane precursor of ceramic material being present in an amount such that the free carbon index of the mixing is greater than 0.2 percent by weight relative to the total weight of the boron carbide powder and the carbonization product derived from the organopolysiloxane precursor of ceramic material.