Boron nitride

Boron nitride articles are made by heat treating turbostratic BN powder to reduce its oxygen content to 5-8% (mesographitic BN), washing this heat-treated powder, mixing it with 5-8% amorphous boron, shaping the mixture, explosively compacting the shape by a hydrodynamic method and reaction sintering the compacted shape, to form the article. The article is then buried in a powder mixture of BN and SiC and heat treated, to getter residual B.sub.2 O.sub.3. After this, the article is impregnated with low-viscosity oligomers (MW<1000) of methylsilanes selected to yield a high proportion of SiC on pyrolysis, and the article is heat-treated to fill the pores evenly throughout its thickness with SiC.

This invention relates to a method of making ceramic articles, especially 
from boron nitride (BN), and extends to the articles so made. Boron 
nitride articles are used in the electronic, electric, radio, 
metallurgical, atomic and rocket industries as high-temperature dielectric 
electrical insulation components, crucibles, boats for glass synthesis and 
smelting, metal alloys, monocrystal growing, evaporator-kettles, various 
devices for melting and pouring metals and alloys, refractory ceramics and 
structural components for gas turbine engines. 
Three main routes to consolidate boron nitride are currently practised: 
pyrolysis, hot-pressing and reaction-sintering of cold-pressed blanks. 
Pyrolytic boron nitride obtained by vapour deposition from boron halides 
has high purity and density and is of the highest quality, maintaining and 
in fact steadily increasing its strength when heated from 20.degree. C. to 
1400.degree. C. and beyond. However, pyrolysis is a complicated and 
energy-consuming process, involving expensive manipulations to filter out 
harmful effluent gases. Also, the maximum practical thickness of pyrolytic 
boron nitride articles which can be manufactured is about 5 mm. 
Hot-pressing--the process of simultaneous moulding and sintering in 
graphite press-moulds at 1800.degree.-2200.degree. C.--is a cheaper method 
of producing boron nitride articles. These show high density and strength 
but contain 10 to 15% of easy-melting phase of boric anhydride and lose 
virtually all of their structural strength at temperatures above 
1000.degree. C. Besides, hot-pressed boron nitride materials also have 
highly anisotropic properties, although less so than pyrolytic materials. 
Clearly it would be desirable to devise a boron nitride material having the 
strength at high temperatures of pyrolytic boron nitride while retaining 
the advantages of hot pressed boron nitride of cheapness, low pollution 
and fabricability of thick samples. 
The known method of sintering cold-pressed blanks of boron nitride, in 
other words through first shaping and then firing, as separate operations, 
is a step in this direction. The resulting material shows considerable 
porosity (up to 40%) and a strength of no more than 10-20 MPa, which is 
obviously insufficient for a whole range of uses. 
Thus Rusanova and Gorchakova have noted (Soviet Powder Metallurgy 1989 
28(2) 108) that the problem of sintering of boron nitride is directly 
related to its structure. Boron nitride has a basic structural element 
which is flat sheets of hexagonal rings with alternating B and N atoms 
having strong and primarily covalent bonds; cohesion between the sheets 
occurs primarily as the result of weak intermolecular interaction. The 
sheets stack to form flat, thin plates up to 50 nm thick with regular 
angles. In such a structure, only incomplete linking of the particles at 
the crystal faces is possible and linking at an angle to the hexagon does 
not in practice occur. 
Production of a self-bonded ceramic has become possible as the result of 
development of methods of synthesis of turbostratic-structure boron 
nitride, for example as disclosed in U.S. Pat. No. 3,241,919. Turbostratic 
boron nitride has a semi-amorphous structure in which groups of 
approximately parallel sheets are shifted at random or are rotated 
relative to the normal. The above researchers also established that the 
existence of packing defects in the stacking of the sheets is caused by 
the presence of oxygen between the sheets. This fact is of particular 
importance in the technology of shaping articles by explosive compaction 
of powder, since the article pressed from powder retains, after the 
explosion, all its oxygen and hence all its potential for sintering, which 
is realised in the following way: the oxygen chemically linked to the 
boron in turbostratic boron nitride powder causes a deficit of nitrogen, 
and when the temperature is increased sufficiently to rupture the B--O 
bonds, the oxygen leaves the structure, leaving uncompensated bonds with 
the help of which boron-to-boron "joining" of the hexagon sheets occurs. 
Proceeding from this insight into the mechanism of sintering articles 
pressed from turbostratic powders, Rusanova and Gorchakova (ibid) 
developed a method of reaction-sintering that is the technological 
solution closest to the present invention and is as follows. 
A turbostratic boron nitride powder is pre-mixed with 10-50% mass of 
amorphous boron; pressing is carried out under 1-2 tonne/cm.sup.2 
pressure. The pressed powder is fired at 1400.degree.-1800.degree. C. in 
nitrogen for the time necessary to complete the reaction of nitriding 
throughout the entire bulk of the article (USSR Inventor's Certificate No. 
635074 of Aug. 7th, 1978). The method is effective in producing refractory 
articles. 
Problems with this method include: the difficulty of completely nitriding 
the boron in a thick-walled article; 
the difficulty of removing the boric anhydride which is formed when 
sintering a turbostratic boron nitride/boron mixture, containing oxygen in 
the structure of the boron nitride; and 
the internal stresses generated by nitriding a large quantity of amorphous 
boron which, unevenly distributed in the matrix of turbostratic boron 
nitride, increases in volume by a factor of 2-3. The use of this method is 
thus limited to manufacturing articles of certain sizes only, it being 
unsuitable for making articles with less than 1% by mass of B.sub.2 
O.sub.3 and with walls thicker than 5-7 mm. 
UK Patent Application GB 2187477A discloses a hot-pressed sintered ceramic 
product of 5 mm wall thickness of boron nitride, titanium diboride and 
aluminium nitride. 
Cold-moulded (oxygen-containing) turbostratic boron nitride, sintered at 
1800.degree.-2000.degree. C. to drive off the oxygen and yield as pure an 
ordered hexagonal boron nitride as possible, has a reasonable strength 
(about half that of pyrolytic boron nitride), which it maintains from 
0.degree. C. to 1400.degree. C., significantly outperforming hot pressed 
boron nitride at all temperatures above 500.degree. C. 
The invention envisages that in order to activate the process of sintering 
of boron nitride, finely dispersed metal powders are added to the 
disordered structure BN powder. 
According to the invention, a method of making a boron nitride ceramic 
article from a mixture of mesographitic boron nitride powder and a 
reaction sintering agent other than elemental boron powder, comprises 
shaping the mixture into the form of the article, compacting the shaped 
mixture under pressure and reaction sintering the compacted mixture in a 
vacuum or inert or nitrogen atmosphere, characterised in that the 
mesographitic boron nitride contains from 5 to 10% by mass chemically 
bound oxygen, and that the reaction sintering agent is at least 
stoichiometrically equivalent to the chemically bound oxygen and can react 
chemically with the available B, N and/or O under the sintering conditions 
to form only refractory compounds plus, permissibly, a minor proportion of 
volatile products. It will be appreciated that "mesographitic" describes 
an intermediate range between the completely disordered turbostratic 
structure of BN and the completely ordered graphitic structure of BN. The 
metal should be chosen such that, upon interacting with the firing gas 
medium and the matrix of BN, it will produce refractory nitrides, oxides 
and borides, products with a higher molar volume than that of the 
component added to the mixture, and should also be such that its reaction 
is accompanied by a detectable exothermal effect. By exploiting the 
volume-increasing and thermal effects of the reaction, materials of 
satisfactory structural strength can be obtained under relatively mild 
firing conditions, that is, at temperatures up to 1800.degree. C. and 
nitrogen pressure no more than 0.06 MPa. The quantity of metal powders 
added to the mixture may be 5-50% by mass. Their specific surface is 
preferably not less than 10 m.sup.2 /g. These can be aluminium, titanium 
or silicon powders, or mixtures thereof, optionally also including boron, 
or compounds forming these metals in the course of firing. The method of 
reaction sintering can be used to obtain a number of various materials; 
from highly pure boron nitride, if boron is the added metal, to composite 
materials of complex structure containing up to five different compounds. 
Thus, also according to the invention, a method of making a boron nitride 
ceramic article from a mixture of mesographitic boron nitride powder and 
elemental boron powder, comprises shaping the mixture into the form of the 
article, compacting the shaped mixture under pressure and reaction 
sintering the compacted mixture in a vacuum or nitrogen atmosphere, 
wherein the mesographitic boron nitride contains from 5 to 10% by mass 
chemically bound oxygen, and the elemental boron is at least 
stoichiometrically equivalent to the chemically bound oxygen, further 
comprising the step of impregnating the resulting article with an 
organosilicon compound, which is itself liquid or which is in the form of 
a solution (preferably oxygen-free), the organosilicon compound having a 
molecular weight not exceeding 1000, and heat-treating the impregnated 
article at 600.degree.-1700.degree. C. in a vacuum or inert atmosphere. 
The compaction may be hydrodynamic. To make a complex shape such as a 
crucible, hydrodynamic compaction would be preferred, e.g. done under 
50-300 MPa, for example by an explosive shock wave in a hydraulic fluid 
surrounding the shaped mixture, but to fabricate a flat article, static 
compaction would be suitable. Also according to the invention, a method of 
making a boron nitride ceramic article from a mixture of mesographitic 
boron nitride powder and elemental boron powder, comprises shaping the 
mixture into the form of the article, hydrodynamically compacting the 
shaped mixture by an explosive shock wave in a hydraulic fluid surrounding 
the shaped mixture, and reaction sintering the compacted mixture in a 
vacuum or nitrogen atmosphere, wherein the mesographitic boron nitride 
contains from 5 to 10% by mass chemically bound oxygen, and the elemental 
boron is at least stoichiometrically equivalent to the chemically bound 
oxygen. 
A distinguishing feature of reaction sintering of boron nitride according 
to the invention is that the matrix of disordered-structure BN is active 
and the properties of the resulting ceramic are due less to the 
interaction between the added metal and the firing gas medium, as the 
definition of reaction-sintering may suggest, but are due more to the 
structural changes in the matrix of BN, accompanied by the removal of 
oxygen and, consequently, to the reaction of BN with the metal additives, 
resulting in the formation of oxides, borides, oxynitrides and other 
compounds. Thus, the composition and properties of the final ceramic are 
crucially influenced by the type of starting BN powder structure and the 
amount of oxygen it contains, and also by the amount of metal powders 
added, the gas medium and the firing temperature. 
According to the most general formulation of the present invention, a 
method of making a boron nitride ceramic article from a mixture of 
mesographitic boron nitride powder and elemental boron powder comprises 
shaping the mixture into the form of the article, compacting the shaped 
mixture under pressure and reaction sintering the compacted mixture in a 
vacuum or nitrogen atmosphere, and is characterised in that the 
mesographitic boron nitride contains from 5 to 10% by mass chemically 
bound oxygen, and in that the elemental boron is at least 
stoichiometrically equivalent to the chemically bound oxygen. The 
elemental boron may be amorphous or crystalline, preferably being in the 
form of powder with specific surface of at least 10 m.sup.2 /g, and may 
account for 5-8% by mass of the mixture. 
The starting boron nitride in each case is preferably turbostratic which, 
before mixing with the elemental boron or other reaction sintering agent, 
has been heat treated to equilibrium (e.g. 1 hour or more) at a 
temperature of from 1300.degree. to 1500.degree. C., preferably in an 
inert atmosphere or vacuum, reducing its oxygen content from preferably 
.about.15% to 5-10% thus partially converting the boron nitride from a 
turbostratic to a mesographitic structure. Its specific surface is 
preferably at least 100 m.sup.2 /g. The boron nitride is preferably washed 
(e.g. in water or alcohol or other suitable solvent) to remove boric 
anhydride B.sub.2 O.sub.3 to a level of preferably 11/2% by mass or less 
before mixing with boron. 
The invention is based generally on the principle of reaction sintering in 
an active matrix, that is, chemical interaction of the matrix with a 
reaction sintering agent, for example sintering of the BN--B system (as 
set forth above). Other examples would include the BN--Al (and BN--Al--Si) 
systems. Under heating, the reaction between chemically bound oxygen of BN 
and (in this example) aluminium, as well as between nitrogen and 
aluminium, will proceed and in the long run a refractory composite ceramic 
with BN--Al.sub.2 O.sub.3 --AlN will be formed. 
For each specific intended use of the resulting ceramic, the oxygen content 
in the BN powder and the amount of the addition of reaction sintering 
agent may be adjusted, and by carrying out the firing in vacuum, inert gas 
or nitrogen in separate stages, it is possible to obtain composite 
materials with the required composition, containing hexagonal boron 
nitride as the main component, and possibly nitrides, oxides, borides, 
oxynitrides and some more complex phases (such as sialon and mullite), in 
various proportions, as additions. 
According to the invention, a method of producing ceramic articles from 
boron nitride comprises semi-dry shaping the mixture of oxygen containing 
BN powders active for sintering, characterised in that with the aim to 
carry out effective low-temperature sintering of large dimensional 
intricate shapes without application of low-melting additives, to maintain 
isotropic properties and stable physical-technical characteristics at high 
temperatures, BN powder of disordered structure with chemically bound 
oxygen (containing up to 10% by weight of structurally bound oxygen) is 
heat treated at temperature 1300.degree.-1500.degree. C. in inert 
atmosphere, then washed in water or lower alcohol, mixed with a reaction 
sintering agent, the latter being in the form of chemical elements and/or 
compounds capable of forming refractory compounds out of nitrides, oxides, 
borides through chemical interaction with the said oxygen from the BN 
under the conditions of sintering in nitrogen, inert gas or vacuum. The 
reaction sintering agent is finely divided powder either of metallic boron 
as already mentioned, or of other raw materials such as aluminium, 
silicon, titanium or a mixture of such powders, powder hydrides of the 
above-mentioned metals, or their organic compounds. Examples of these raw 
materials are: 
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Raw materials Composition of ceramics produced: 
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BN + B BN 
BN + Al BN + Al.sub.2 O.sub.3 + AlN + 9Al.sub.2 O.sub.3.2B.sub.2 
O.sub.3 
BN + Ti BN + TiB.sub.2 + TiB + TiN 
BN + B + Al BN + Al.sub.2 O.sub.3 + AlN + AlB 
BN+ Al + Si BN + sialon 
BN+ Al + B + Si BN + sialon 
BN+ Ti + Al BN + AlN + Al.sub.2 O.sub.3 + TiB.sub.2 + TiN 
BN + Ti + B BN + TiB.sub.2 + TiB 
BN + TiH.sub.2 BN + TiB.sub.2 + TiB 
BN + B.sub.10 H.sub.1 2.2NH.sub.3 
BN 
BN + meta-B.sub.10 C.sub.2 H.sub.1 0(CH.sub.2 OH).sub.2 
BN 
(meta-carborandiol) 
BN + poly-carboranosilanes 
BN + SiC + Si.sub.3 N.sub.4 
or polycarbo-boro-siloxanes 
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The reaction sintering is preferably performed in nitrogen which at least 
occasionally exceeds 0.03 MPa (preferably exceeds 0.2 MPa), optionally 
with pressure cycling to allow, alternately, gaseous products (principally 
B.sub.2 O.sub.3) to escape and nitrogen to infiltrate the compacted 
mixture. 
Also according to the invention, a method of making a ceramic article 
comprises impregnating a porous ceramic article with an organosilicon 
compound which is liquid or in (preferably oxygen-free) solution and which 
has a molecular weight not exceeding 1000, and heat-treating the 
impregnated article at 600.degree.-1700.degree. C. in a vacuum or inert 
atmosphere. 
According to a further feature of the invention, after the reaction 
sintering a further (optionally repeated) step is carried out, comprising 
packing the reaction-sintered article in a particulate (preferably 
refractory) medium capable of absorbing volatile oxides evolved on heating 
the article, the particulate medium comprising a reducing agent or 
precursor thereof, and heating to at least 1400.degree. C. The reducing 
agent may be SiC or another refractory oxygen-free silicon-containing 
compound, the latter preferably containing nothing except Si, C, N and H, 
and the particulate medium ("burying meal") may also include a refractory, 
preferably BN. This getters residual B.sub.2 O.sub.3 evolved from the 
article. 
According to a yet further feature of the invention, after the reaction 
sintering and whether or not the "burying meal" step is used, the step may 
be performed of soaking, infiltrating or otherwise impregnating the 
article resulting from the method with an organosilicon compound, which is 
itself liquid or which is in the form of a solution (preferably 
oxgyen-free), the organosilicon compound having a molecular weight not 
exceeding 1000, and heat-treating the impregnated article at 
600.degree.-1700.degree. C. in a vacuum or inert atmosphere. The 
organosilicon compound will preferably yield an inorganic residue on 
pyrolysis at 1000.degree.-1200.degree. C. of at least 70% by mass and may 
be or may include an oligomethylhydridesilane; a mixture of in situ 
copolymerisable oligosilanes may be used, the MW of these preferably not 
exceeding 800, all to keep the viscosity low. 
The invention extends to ceramic articles made as set forth above, such as 
grommets, crucibles and thermal, electrical or structural components, 
which may be 20 mm thick or more without difficulty. 
The invention can be performed with reasonably inexpensive raw materials, 
using conventional equipment and without going to excessive temperatures. 
The invention will now be described by way of example. 
FEATURE 1 
Boron nitride powder is synthesised from boron-nitrogen and 
oxygen-containing compounds according to the process disclosed in U.S. 
Pat. No. 3,241,919, using urea and boric acid in an ammonia atmosphere in 
a reduced-pressure furnace for 30 hours, and has a disordered turbostratic 
structure and a chemically bound oxgyen content by mass of 15%, more if 
the structure is more disordered (not counting the oxygen present as 
B.sub.2 O.sub.3 which is later removed by alcohols or other suitable 
solvent). This powder is collected and heat-treated at 
1300.degree.-1500.degree. C. in an inert atmosphere, for sufficient time 
until no further change in structure occurs at the chosen temperature, to 
achieve partial graphitisation of the structure, i.e. a change from 
turbostratic to mesographitic structure with the oxygen content kept down 
to 5-10% by mass. This structural change is continuous, the 
`mesographitic` structure being a semiamorphous intermediate between the 
turbostratic and strictly hexagonal structures. Then the powder is washed 
with water or technical-grade alcohol to remove uncombined B.sub.2 O.sub.3 
to below 11/2% by mass. This washing cannot be performed earlier because 
the boron nitride before the heat treatment is very reactive and would 
react with the washing water/alcohol. 
A method of hydrodynamic semi-dry moulding (e.g. explosive compaction under 
30 to 500 MPa) on metal cores of a mixture of 92-95%, by mass, of this 
disordered-structure boron nitride and 5-50% reaction sintering agent 
including in this example 5-8%, by mass, of amorphous boron, with the 
addition of about 1 or 2%, in dry weight, of elastomer dissolved in 
`petroleum ether` hydrocarbon solvent, then follows. The moulded article 
is fired in a nitrogen atmosphere at 1400 or 1500.degree.-1800.degree. C. 
over the time necessary to nitride the boron throughout the entire bulk of 
the article and for the forming B.sub.2 O.sub.3 to vaporise away 
completely. 
The amount of additive, in this example boron, to be put into the mixture 
is fixed in accordance with the oxgyen content of the BN powder. The 
larger the oxygen content of the BN powder, the greater the quantity of 
boron that should be added to the mixture, since part of the boron goes 
into the formation of boric anhydride and only the leftover part is 
reacted with the matrix and/or gas, e.g. is nitrided, forming BN and 
compacting the ceramic. 
The addition of 5-8%, by mass, of amorphous boron permits the achieving of 
a complete nitriding throughout the entire bulk of thick-walled articles, 
to bring down the B.sub.2 O.sub.3 content to 0.2%, by mass, and lower, to 
avoid the development of stresses and, also, to reduce production costs. 
Less boron may be acceptable where mechanical strength is not a key 
characteristic, as in large-sized articles like crucibles, electric 
insulators or containers. In every case, however, the boron must be at 
least equal to the chemically bound oxygen in the boron nitride with which 
it is mixed. 
A lower strength as a result of using less boron can, to a large extent, be 
compensated for by an appropriate choice of the moulding method and 
parameters for yielding a higher density. Although isostatic pressing can 
be used, hydrodynamic pressing, through its even, allover compression of 
even a large intricately-shaped article with working fluid, achieves a 
high density and a high degree of uniformity in the density of moulded 
articles, and prevents the occurrence of internal stresses. The 
hydrodynamic pressure is chosen, within the range of 50-300 MPa, depending 
on the size of the load and so as to ensure a packing density of the 
particles that is best for the subsequent diffusion processes during 
reaction sintering. 
Reaction sintering is done in a nitrogen atmosphere under up to 1.0 MPa 
pressure and with the final temperature of 1500.degree.-1800.degree. C. 
Since the total heat evolved by the nitriding reaction is small when the 
boron content is as low as 5-8%, this heat cannot be relied upon to drive 
the sintering, and therefore the bottom limit of the furnace temperature 
for sintering cannot be below 1500.degree. C. This however allows a 
shorter sintering time. Raising the pressure of nitrogen in the working 
chamber of the oven to 0.03 MPa and above will intensify the sintering 
process and facilitate the nitriding of boron throughout the entire bulk 
of the article, and with 0.2 MPa pressure the speed of nitriding no 
longer, in practical terms, depends on the thickness of the article's 
walls. Because such a pressure could slow the evolution of B.sub.2 O.sub.3 
from within the thickness of the article, the pressure may be reduced to 
zero from time to time, e.g. in a two-hourly pressure/evacuation cycle.

EXAMPLE 1 OF FEATURE 1 
To shape a large-size thick-walled article, with walls up to 50 mm 
thick--say, a grommet--a mixture of 95%, by mass, of washed boron nitride 
powder and 5%, by mass, of elemental amorphous (or crystalline) boron 
powder of &gt;10 m.sup.2 /g is prepared and mixed with a small amount of 
binding agent, being a 2% solution of elastomer in `petroleum ether` 
hydrocarbon solvent. The boron nitride powder has previously been 
synthesised and heat-treated at 1500.degree. C. for 1 hour as described 
above. It now has a mesographitic structure and the oxygen content in it 
is 6-7% by mass, i.e. less than can react with all the elemental boron. 
The prepared mixture is packed around the sides of a powder metal core and 
an elastic sleeve is slipped over it so that the mixture is retained in 
the shape of a grommet. The shape of the core dictates the inner shape of 
the article; to obtain the desired parameters of the article, additional 
perforated packing can be added on the outer side. 
The construction (core, powder mixture and elastic sleeve) is then placed 
in the chamber of a hydrodynamic machine filled with working fluid (in 
this case, water), where the article is compacted with an explosion of 300 
MPa force. 
The formed article is taken out of the cover and dried at 100.degree. C. to 
remove the hydrocarbon solvent for the rubber. Then the article is buried 
in ground scrap boron nitride powder for reaction sintering. (This scrap 
BN powder does not sinter to the formed article because it contains no 
impurity or agent which could promote sintering. Other powders could in 
theory be used but none, so far, have proved as successful as BN, bearing 
in mind that the chosen powder must getter oxygen.) The reaction sintering 
is done by firing in an electric resistance vacuum furnace in nitrogen 
atmosphere under 0.05 MPa pressure with the final temperature of 
1600.degree. C. held for 10 hours. 
The resulting article has 30% porosity and high electric resistance 
.about.10.sup.12 .OMEGA..m; the B.sub.2 O.sub.3 content does not exceed 
0.5%, by mass, in the material; its flexural strength is 25-40 MPa. The 
article is stable against thermal shock and withstands repeated cyclical 
heating and cooling within the 20.degree.-2000.degree. C. temperature 
range, without losing its performance characteristics. 
EXAMPLE 2 OF FEATURE 1 
To shape an article with 10 mm-thick walls, a crucible, for example, a 
mixture of boron nitride, 92% by mass, and boron, 8% by mass, is used. The 
boron nitride powder is pre-heat-treated at 1300.degree. C.; it has a 
pronounced turbostratic structure tending to mesographitic and contains up 
to 10%, by mass, of oxygen. The process of moulding and compaction is 
analogous to the one described in Example 1 above, under 150 MPa pressure 
in the chamber of the hydrodynamic machine. Reaction sintering was carried 
out in a way analogous to that described in Example 1, but under 0.03 MPa 
nitrogen pressure and with the final temperature at 1500.degree. C., for 
10 hours. 
The overall content of metallic additions in the resulting ceramic does not 
exceed 10% by mass; the B.sub.2 O.sub.3 content is not more than 0.2% by 
mass; flexural strength is 30-50 MPa and does not decrease, at least up to 
1400.degree. C. The resulting crucible is stable against thermal shock and 
exposure to melted metals or glass; its high thermal conductivity and 
isotropic properties ensure a rapid and even heating through the entire 
bulk of the processed body. The crucible maintains its performance 
characteristics even after it has been used 10 times to melt titanium. 
EXAMPLE 3 OF FEATURE 1 (BN+Al) 
A disordered-structure BN powder containing .about.9% by mass of oxygen is 
mixed with 30% by mass of fine-grained aluminium powder (3-5 m.sup.2 /g). 
The mixture is compacted by semi-dry moulding into the shape of a cut-off 
ring for a horizontal steel casting machine (diameter up to 250 mm; 
thickness up to 25 mm), then fired in a vacuum to 1200.degree. C. and in a 
nitrogen atmosphere, to 1500.degree. C. 
Composition of the resulting ceramic article, in diminishing order of 
components: BN, Al.sub.2 O.sub.3, AlN, 9Al.sub.2 O.sub.3 *2B.sub.2 
O.sub.3. The BN content is about 60% by mass. The article has a mechanical 
strength of 40-60 MPa, high thermal stability and resistance to aggressive 
melts. It is fit for prolonged use in an inert or low-oxidising medium. 
EXAMPLE 4 OF FEATURE 1 (BN+Al) 
80% by mass of disordered-structure BN powder, containing .about.5% by mass 
of oxygen, is mixed with 20% by mass of fine-grained aluminium powder (3-5 
m.sup.2 /g). The mixture is compacted by hydrodynamic pressing as 
described above into the shape of an electrical insulation insert or a 
protective cover. The article is fired in a nitrogen atmosphere to 
1450.degree. C. 
Composition of the resulting ceramic article, in diminishing order of 
components: BN, AlN, Al.sub.2 O.sub.3. The BN content is about 70% by 
mass. The article has a mechanical strength of 40-80 MPa, electrical 
resistance .about.10.sup.n .OMEGA.m where n.about.10, high thermal 
stability and resistance to erosion and aggressive melts. It is fit for 
prolonged use in an inert or low-oxidising medium with sharp temperature 
drops and in contact with metal melts. 
EXAMPLE 5 OF FEATURE 1 (BN+Al+B) 
70% by mass of disordered-structure BN powder, containing .about.6% by mass 
of oxygen, is mixed with 25% by mass of fine-grained aluminium powder (3-5 
m.sup.2 /g) and 5% by mass of fine-grained boron powder (5-10 m.sup.2 /g). 
The mixture is compacted by hydrodynamic pressing into the shape of a 
grommet, ring or core. The article is fired in a nitrogen atmosphere to 
1550.degree. C. 
Composition of the resulting ceramic article, in diminishing order of 
components: BN, AlN, AlB.sub.2, Al.sub.2 O.sub.3. The BN content is about 
70% by mass. The article has a mechanical strength of 70-80 MPa, 
electrical resistance .about.10.sup.10 .OMEGA.m, density over 1.7 
g/cm.sup.3, high thermal stability and resistance to erosion and 
aggressive melts. It is fit for prolonged use as a structural component, 
e.g. a thermocouple protective cover, in an inert or low-acid medium with 
sharp temperature drops. 
EXAMPLE 6 OF FEATURE 1 (BN+Ti) 
A disordered-structure BN powder containing .about.8% by mass of oxygen is 
mixed with 15% by mass of fine-grained titanium powder (10-15 m.sup.2 /g). 
The mixture is compacted by hydrodynamic pressing into the shape of a 
boat, crucible or evaporator-kettle, then fired in a nitrogen atmosphere 
to 1600.degree. C. 
Composition of the resulting ceramic article, in diminishing order of 
components: BN, TiB.sub.2, TiN. The BN content is about 80% by mass. The 
article has a high thermal stability and resistance to aluminium melts. It 
is fit for prolonged use in an inert atmosphere or vacuum as a resistive 
element in an aluminium evaporator. 
EXAMPLE 7 OF FEATURE 1 (BN+Ti+Al) 
A disordered-structure BN powder containing .about.8% by mass of oxygen is 
mixed with 10% by mass of fine-grained titanium powder (10-15 m.sup.2 /g) 
and 10% by mass of fine-grained aluminium powder (3-5 m.sup.2 /g). The 
mixture is compacted by hydrodynamic pressing into the shape of a tube or 
plate, then fired in a nitrogen atmosphere to 1550.degree. C. 
Composition of the resulting ceramic article, in diminishing order of 
components: BN, AlN, Al.sub.2 O.sub.3, TiN, TiB.sub.2. The BN content is 
about 70% by mass. The article has a high thermal stability and a 
mechanical flexural strength of 20-30 MPa. The article is stable against 
thermal shock and aluminium melts. It is fit for prolonged use in an inert 
atmosphere or vacuum as the protective cover of a sensor for dipping into 
metal melts for metallurgical monitoring. 
EXAMPLE 8 OF FEATURE 1 (BN+TiH.sub.2) 
A disordered-structure BN powder containing .about.8% by mass of oxygen is 
mixed with 15% by mass of fine-grained titanium hydride powder (10-15 
m.sup.2 /g). The mixture is compacted by hydrodynamic pressing into the 
shape of a boat, crucible or evaporator-kettle, then fired in a nitrogen 
atmosphere to 1600.degree. C. 
Composition of the resulting article, in diminishing order of components: 
BN, TiB.sub.2, TiN. The BN content is about 80% by mass. The article has a 
high thermal stability and resistance to molten aluminium. It is fit for 
prolonged use in an inert atmosphere or vacuum as a protective element in 
an aluminium evaporator. 
EXAMPLE 9 OF FEATURE 1 (BN+Si+Al) 
A mixture of 4.2% by mass of silicon powder (3-5 m.sup.2 /g), 12.5% by mass 
of aluminium powder (5-10 m.sup.2 /g), 82.3% by mass of turbostratic boron 
nitride powder and 1% by mass of rubber binder is prepared for moulding. 
The boron nitride powder is pre-heat-treated at 1500.degree. C. and 
contains 6-7% by mass of oxygen. The prepared mixture is packed into a 
mould for cold semi-dry moulding and compacted under 0.5-1 tonne/cm.sup.2 
pressure into the shape of a 7.times.7.times.70 mm parallelepiped. 
The formed article is taken out of the mould and placed in a boron nitride 
particulate medium for reaction sintering. The firing is done in an 
electric resistance vacuum furnace in a nitrogen atmosphere under 0.05 MPa 
pressure at a final sintering temperature of 1600.degree. C. for 10 hours. 
The resulting article has a high electric resistance, .about.10.sup.11 
.OMEGA..m; the material, as shown by X-ray phase-by-phase and chemical 
analyses, is 75% hexagonal-structure boron nitride and 20-25% sialon, the 
B.sub.2 O.sub.3 content in the material not exceeding 0.5% by mass; its 
mechanical flexural strength is 20-30 MPa. The article is stable against 
thermal shock and can undergo repeated thermal cycling between 20.degree. 
C. and 2000.degree. C. in an inert medium without losing its performance 
characteristics. 
EXAMPLE 10 OF FEATURE 1 (BN+Si+Al+B) 
A mixture of 5.5% by mass of silicon powder (3-5 m.sup.2 /g), 16.6% by mass 
of aluminium powder (5-10 m.sup.2 /g), 16.6% by mass of amorphous boron 
powder (3-5 m.sup.2 /g), 60.2% by mass of turbostratic boron nitride 
powder and 1% by mass of rubber binder is prepared for moulding. The boron 
nitride powder is pre-heat-treated at 1500.degree. C. and contains 6-7% by 
mass of oxygen. The prepared mixture is packed into a mould for cold 
semi-dry moulding and compacted under 0.5-1 tonne/cm.sup.2 pressure into 
the shape of a 7.times.7.times.70 mm parallelepiped. 
The formed article is taken out of the mould and placed in a boron nitride 
particulate medium for reaction sintering. The firing is done in an 
electric resistance vacuum furnace in a nitrogen atmosphere under 0.05 MPa 
pressure at a final sintering temperature of 1600.degree. C. for 10 hours. 
The resulting article has high electric resistance, .about.10.sup.10 
.OMEGA..m; the material, as shown by an X-ray phase-by-phase and chemical 
analyses, consists of 70% hexagonal-structure boron nitride and 25-30% 
sialon, the B.sub.2 O.sub.3 content in the material not exceeding 0.5% by 
mass; its mechanical flexural strength is 30-45 MPa. The article is stable 
against thermal shock and can undergo repeated thermal cycling between 
20.degree. C. and 2000.degree. C. in an inert medium without losing its 
performance characteristics. 
EXAMPLE 11 OF FEATURE 1 (BN+B.sub.10 H.sub.12 *2NH.sub.3) 
85 parts of boron nitride, pre-heat-treated at 1500.degree. C. and 
containing 6-7% by mass of oxygen, are mixed with 15 parts by mass of 
decaborane diammine (formula: B.sub.10 H.sub.12 *2NH.sub.3, obtained from 
the reaction of decaborane B.sub.10 H.sub.12 with ammonia without 
additional purifying) in the form of a 5-10% acetonitrile solution. After 
0.5-1 hour of stirring at 60.degree.-70.degree. C., benzol (half of the 
volume of the decaborane diammine solution) is added to the mixture. After 
10 minutes' stirring, the solvent is boiled off in a vacuum at 100.degree. 
C. The resulting boron nitride powder with the added decaborane diammine 
is placed in a crucible and heat treated in an electric vacuum furnace in 
an argon or helium atmosphere to 700.degree. C. The resulting powder is 
mixed, in petrol, with 1% by mass of rubber binder and dried. The prepared 
mixture is packed into a mould for cold semi-dry moulding and compacted 
under 0.5-1 tonne/cm.sup.2 pressure into the shape of a 7.times.7.times.70 
parallelepiped. 
The formed article is taken out of the mould and placed in a boron nitride 
particulate medium for reaction sintering. The firing is done in an 
electric resistance vacuum furnace in a nitrogen atmosphere under 0.05 MPa 
pressure at a final sintering temperature of 1600.degree. C. for 10 hours. 
The resulting article has high electric resistance, .about.10.sup.13 
.OMEGA..m; the material, as shown by X-ray phase-by-phase and chemical 
analyses, is pure hexagonal-structure boron nitride, the B.sub.2 O.sub.3 
content in the material not exceeding 0.5% by mass; its mechanical 
flexural strength is 30-40 MPa. The article is stable against thermal 
shock and can undergo repeated thermal cycling between 20.degree. C. and 
2000.degree. C. in an inert medium without losing its performance 
characteristics. 
EXAMPLE 12 OF FEATURE 1 (BN+meta-B.sub.10 C.sub.2 H.sub.10 (CH.sub.2 
OH).sub.2) 
85 parts of boron nitride, pre-heat-treated at 1500.degree. C. and 
containing 6-7% by mass of oxygen, is mixed with 15 parts by mass of 
meta-carborandiol (meta B.sub.10 C.sub.2 H.sub.10 (CH.sub.2 OH).sub.2) in 
the form of a 10% solution in benzol. After 10 minutes' stirring, the 
solvent is boiled off in a vacuum at 100.degree. C. The resulting boron 
nitride powder with the added meta-carborandiol is put into a crucible and 
heat treated in an electric vacuum furnace in an argon or helium 
atmosphere to 900.degree. C. The resulting powder is mixed, in petrol, 
with 1% by mass of rubber binder and dried. The prepared mixture is packed 
into a mould for cold semi-dry moulding and compacted under 0.5-1 
tonne/cm.sup.2 pressure into the shape of a 7.times.7.times.70 mm 
parallelepiped. 
The formed article is taken out of the mould and placed in a boron nitride 
particulate medium for reaction sintering. The firing is done in an 
electric resistance vacuum furnace in a nitrogen atmosphere under 0.05 MPa 
pressure at a final sintering temperature of 1600.degree. C. for 10 hours. 
The resulting article has high electric resistance, .about.10.sup.13 
.OMEGA..m; the material, as shown by X-ray phase-by-phase and chemical 
analyses, is pure hexagonal-structure boron nitride, the B.sub.2 O.sub.3 
content in the material not exceeding 0.5% by mass; its mechanical 
flexural strength is 30-40 MPa. The article is stable against thermal 
shock and can undergo repeated thermal cycling between 20.degree. C. and 
2000.degree. C. in an inert medium without losing its performance 
characteristics. 
EXAMPLE 13 OF FEATURE 1 (BN+poly-carboranosilanes) 
(We use the word poly-carboranosilanes to mean polymers whose main chain 
comprises silane links --Si--NH--Si--NH-- alternating with inserted 
carborane fragments of the --B.sub.10 C.sub.2 H.sub.10 -- type.) 
One part by mass of meta-carborandiol B.sub.10 C.sub.2 H.sub.10 (CH.sub.2 
OH).sub.2 is dissolved in a 20% solution in benzol of 2 parts by mass of 
oligomethylhydridesilane (molecular mass about 800), and the mixture is 
stirred until the gas is completely vaporised away. The resulting solution 
of poly-carboranosilane is mixed with 20 parts by mass of boron nitride 
which has been pre-heat treated at 1500.degree. C. and contains 6-7% by 
mass of oxygen. After 10 minutes' stirring, the solvent is boiled off in a 
vacuum at 100.degree. C. The resulting boron nitride powder with the added 
poly-carboranosilane is packed into a mould for cold semi-dry moulding and 
compacted under 0.5-1 tonne/cm.sup.2 pressure into the shape of a 
7.times.7.times.70 mm parallelepiped. 
The shaped article is taken out of the mould and placed in a boron nitride 
particulate medium for reaction sintering. The firing is done in an 
electric resistance vacuum furnace in a nitrogen atmosphere under 0.05 MPa 
pressure at a final sintering temperature of 1600.degree. C. for 10 hours. 
The resulting article has high electric resistance, 10.sup.10 .OMEGA..m; 
the material, as shown by X-ray phase-by-phase and chemical analyses, is 
hexagonal-structure boron nitride with up to 5% in total of carbide and 
silicon nitride, the B.sub.2 O.sub.3 in the material not exceeding 0.5% by 
mass; its mechanical flexural strength is 30-40 MPa The article is stable 
against thermal shock and can undergo repeated thermal cycling between 
20.degree. C. and 2000.degree. C. in an inert medium without losing its 
performance characteristics. 
FEATURE 2 
As is known, and was mentioned in Example 1 of Feature 1, boron nitride 
ceramic articles may be buried in BN "meal" for firing. Such burying 
offers a number of technological advantages. Firstly, the high thermal 
conductivity of BN ensures good heat conduction and, therefore, low 
temperature gradients throughout the bulk of the articles. Secondly, the 
high absorption by the "meal" of oxygen, moisture and other impurities in 
the commercial-grade nitrogen atmosphere in the furnace prevents the 
ceramic article itself from oxidising. At the same time, it is important 
that the "meal" should be capable of absorbing the volatile elements 
released from the article undergoing reaction sintering. The firing 
improves the strength and the purity of the articles but does not affect 
their porosity. 
The process of boron nitride sintering is directly associated with the 
escape of residual oxygen in the form of B.sub.2 O.sub.3 vapour from the 
bodies of the individual BN particles to their surfaces, and then, to the 
surface of the article. The evacuation of boron oxides from ceramic 
articles proceeds rather slowly, especially in the case of thick-walled 
items. Therefore it is very important that the burying "meal" in which 
ceramic articles are sintered should promote the evacuation of the 
residual oxygen. 
The pure BN "meal" used in boron nitride sintering is good at absorbing 
B.sub.2 O.sub.3 vapours. However, the B.sub.2 O.sub.3 sorption-desorption 
in burying "meal" is a reversible process, therefore the "meal" does give 
rise to a positive B.sub.2 O.sub.3 vapour pressure, which opposes and 
therefore may slow down the evacuation of boron oxide from the ceramic 
articles. From this point of view, there is an advantage in subsequently 
using a second burying "meal" that creates a reducing atmosphere where 
B--O bonds are replaced with B--N bonds, whereby to remove any remaining 
B.sub.2 O.sub.3 in the interests of high purity and high strength. 
Thus, in addition to purging the previously reaction-sintered boron nitride 
ceramic article of boron oxide, the subsequently used second burying 
"meal" according to this Feature of the invention promotes its 
strengthening, as follows. Inter-particle contacts are the spots of higher 
concentration of residual oxygen, and it is here, to a large extent, that 
B.sub.2 O.sub.3 reduction reactions take place, the harder B--N bond 
replacing the B--O bond, which encourages further sintering and 
strengthening of the ceramic article. This sintering thus takes the form 
of increasing the strength of the adhesion bond between BN particles where 
they are already in contact, without however densifying the powder. 
The two "meals" cannot be combined since the elemental boron present at the 
stage of the first meal would react with any additives which might be used 
in the second meal. 
The process of further sintering in a reducing atmosphere according to this 
Feature of the invention may in practice be carried out by firing the 
articles buried in this second "meal", consisting of SiC or other 
refractory oxygen-free silicon compound ROSC, either alone or with another 
particulate medium. A mixture of a boron nitride "meal" and an ROSC (SiC) 
powder is preferred. This burying mixture has high thermal conductivity 
and good gas permeability and provides additional protection for the 
article surface from the impurities in the gaseous atmosphere of the 
furnace. 
Silicon carbide is a highly specific reducing agent suitable for this 
purpose, as are other ROSCs for use above 1400.degree. C. (i.e. in the 
range of SiO temperature stability). The reduction process can be summed 
up in the following equation (using SiC as an example): 
##EQU1## 
SiC gas! in this case denotes the gaseous phase that is present, spread 
evenly over the SiC surface, in the nitrogen atmosphere and which contains 
not only SiC vapour proper, but also its dissociation products (Si) and 
the products of its reaction with nitrogen (C.sub.2 N.sub.2, CN . . . ). 
Although SiC is the best ROSC for strength enhancement, an alternative 
version of this burying mixture is obtained by using a boron nitride 
"meal" impregnated with silicon compounds which, during pyrolysis, produce 
a more than 10%, by mass, output of SiC and other ROSC. Using such 
compounds ensures that there is no localised depletion in Si. The 
molecular weight of such compounds is of no consequence. 
It may again be noted that firing in a burying "meal" containing SiC (ROSC) 
or its precursors according to this Feature is a separate technological 
operation and that it cannot overlap with reaction sintering for the 
reason, already briefly noted above, that Si-containing vapours would 
enter into a reaction with the unnitrided boron of the charge, forming 
undesirable silicon borides. Silicon borides do not yield to nitriding 
under sintering conditions, and the strength of the ceramic, in such a 
case, would decline sharply and its dielectric properties would be 
irreversibly impaired. 
EXAMPLE 1 OF FEATURE 2 
The article made (including reaction-sintering) following Example 1 of 
Feature 1 is subjected to firing while buried in SiC powder (5-20 
.mu.m-diameter particles) at 1700.degree. C., for two hours in a nitrogen 
atmosphere. The resulting article has 50-60 MPa flexural strength at 
20.degree. C. and retains it at the same level all the way up to 
1500.degree. C. The B.sub.2 O.sub.3 content is 0.2-0.3% by mass. Other 
technical characteristics of the article, such as electric resistance and 
stability against thermal shock, remain at the same level as before. 
EXAMPLE 2 OF FEATURE 2 
The article made (including reaction-sintering) following Example 2 of 
Feature 1 is fired in boron nitride "meal" impregnated with a 5% 
oligomethylhydride-silane solution at 1700.degree. C. for two hours in a 
nitrogen atmosphere. Oligomethylhydride silane solutions are further 
described in Feature 3, although a wider range thereof can be used in 
this, Feature 2. 
The resulting article has 60-70 MPa flexural strength. The B.sub.2 O.sub.3 
content is 0.2-0.3% by mass. 
FEATURE 3 
This Feature is for the manufacture of fully dense structural ceramic 
composite materials. 
The platelet shape of boron nitride powder particles inevitably causes 
isotropic ceramics made of it to have a highly porous structure, even when 
sintering is done at extreme temperatures. The open porosity of such 
material can amount to 35-40% by volume, which results in relatively low 
strength. 
One of the most effective ways of improving the properties of sintered 
materials is to impregnate them with organoelemental compounds, then to 
subject them to pyrolysis until useful inorganic residues are formed. A 
wide range of impregnating compounds is conventionally used, depending on 
the requirements and purpose of the material; the selected compound 
preferably contains the highest possible proportion of the elements 
comprising the required inorganic residue, and may be for example an 
organosilicon compound (OSC) containing Si--O--Si, Si--N--Si and Si--C 
bonds. Due to the presence of these bonds, the compounds are pyrolysed 
down to SiO.sub.2, Si.sub.3 N.sub.4 and SiC, which residues are conducive 
to strengthening the sintered materials and protecting them against 
oxidising. 
It should be taken into account that, despite high porosity, by volume, in 
sintered BN ceramics, the pores are small in size, with typical "effective 
radius" of 1000-1500 A. Therefore, a full-depth impregnation of samples 
thicker than 10-12 mm with high-molecular-weight OSC solutions (MW=2000) 
is difficult in principle. The presence of high-molecular-weight 
components in the composition of tars greatly increases the viscosity of 
the solutions and creates a predisposition for an uneven impregnation of 
the articles, with low-molecular fragments penetrating through the pores 
of the ceramic to a great depth, but then being able to flow away equally 
easily, and high-molecular fragments concentrating in the surface layer. 
The use according to the present Feature of low-molecular oxygen-free 
oligomers (MW up to .about.1000) with a low molecular weight distribution 
which polymerise before they pyrolyse and which have a yield of at least 
70% in useful residue (e.g. SiC, Si.sub.3 N.sub.4) on pyrolysis, permits 
inhomogeneity to be avoided in the resulting ceramic materials. 
Oligomethylhydridesilane (OMHS) can serve as an example of such a 
compound. Its relatively low molecular weight (600-650) and the absence of 
high-molecular components in its composition make it possible to achieve 
an acceptably even impregnation of large-sized BN articles (=with walls up 
to 60 mm thick), and its pyrolysis results in a satisfactorily high yield 
of inorganic residues; over 80-85% by mass in the case of pyrolysis up to 
1000.degree.-1400.degree. C. (when the intermediate SiC or SiC.sub.x 
N.sub.y is formed) and 40-45% by mass when pyrolysed to 1600.degree. C. 
and beyond (when SiC predominates in the composition of the by-products). 
This high yield is necessary to ensure both high density of the product 
and minimisation of harmful internal evolution of gases in the article. 
An example of a compound which can achieve a high yield at heat treatment 
above 1600.degree. C. is a mixture of oligomers, the main ones of which 
are: 
EQU CH.sub.2 .dbd.CHSi(NH).sub.1.5 !.sub.3 CH.sub.3 HSi(NH).sub.1.5 !.sub.1 
(CH.sub.3).sub.3 Si(NH).sub.0.5 !.sub.3 
and 
EQU CH.sub.2 =CHSi(NH).sub.1.5 !.sub.6 CH.sub.3 HSi(NH).sub.1.5 !.sub.2 
(CH.sub.3).sub.3 Si(NH).sub.0.5 !.sub.4 
with structures typically based on 
##STR1## 
with average molecular weight .about.800. This is a compound of low 
viscosity and relatively stable properties. Impregnation with this can be 
done in a rather concentrated form--with 80-90% solutions in light 
hydrocarbons such as pentane, hexane etc. The yield of inorganic residues 
is high and approaching the theoretical maximum--up to 50-53%, by mass, of 
silicon carbide after pyrolysis at T.gtoreq.1600.degree. C., 
If the process of impregnation needs to be intensified to the maximum, 
mixtures of this or similar oligomers and "active diluents" can be used. 
Reactive oligomers with MW 300-600, capable of copolymerisation with the 
main component of the mixture, can be used as such active diluents. For 
example, this specific oligomer can be combined with 25% by mass of 
oligomethylhydridesilane oligomer: 
##STR2## 
A mixture like this can copolymerise in the pores of the ceramic at above 
100.degree. C., and the resulting copolymer has a high yield of desired 
inorganic residue. (Polymerisation is necessary so that, after the 
oligomers have been infiltrated throughout the article, they then remain 
in position (and do not drain away) during pyrolysis.) 
Analogously, low-molecular polycarbosilanes of the structure 
##STR3## 
where n.gtoreq.m and MW avg .about.400-600 can be used for the same 
purposes and with the same result. 
EXAMPLE 1 OF FEATURE 3 
A reaction-sintered article with 50 mm thick walls, made following Example 
1 of Feature 1, is impregnated with an 80% by mass solution in petrol of 
oligomethylhydridesilane with MW avg .about.650 for 72 hours. The 
impregnated article is dried at 100.degree. C., then heat-treated in 
nitrogen at a temperature rising slowly to 1400.degree. C. This procedure 
first polymerises the oligomethylhydridesilane, thus locking it in situ, 
and then pyrolyses it, thus filling the pores evenly throughout the 
thickness of the article with refractory residue of composition close to 
SiC. The duration of the heating will depend on the thickness of the 
article, since even the centre must reach 1400.degree. C. for an adequate 
time, and in this case was 48 hours, including holding at 1400.degree. C. 
for 4 hours. 
The resulting article has a flexural strength of 70-90 MPa at 20.degree. C. 
and of 80-100 MPa at 1500.degree. C. The specific volume electric 
resistance is 10.sup.13 .OMEGA..m. Oxidisability at 1300.degree. C. is 
five times lower than that of the material made following Example 1 of 
Feature 1. The life of the article under conditions of thermal erosion in 
a low-oxidising medium (viz, gaseous petrol combustion products) at 
1500.degree.-2000.degree. C. is 10 times longer than that of pure boron 
nitride. 
EXAMPLE 2 OF FEATURE 3 
An article--a crucible with 10 or 100 mm thick walls--made following 
Example 2 of Feature 1, is impregnated for 48 hours with a 90% by mass 
solution in a low-boiling "petroleum ether" hydrocarbon mixture of the 3:1 
mixture described in the third-last paragraph of Feature 3 of oligomer MW 
avg .about.800 and oligomethylhydridesilane, MW avg .about.450. After 
drying at 100.degree. C., the crucible is slowly heated in nitrogen up to 
1600.degree. C. taking 30 hours, including holding at 1600.degree. C. for 
10 hours. The resulting crucible has a flexural strength of 70-90 MPa at 
20.degree. C. and 80-100 MPa at 1500.degree. C. The open porosity has been 
reduced by 45-50% as against the material made following Example 2 of 
Feature 1. The crucible withstands 10-15 heats of aluminium (at boiling 
temperature), while a crucible of pure boron nitride can only be used for 
2-3 heats. 
Although it would not often be required, Feature 3 can be applied to a 
material made according to Features 1+2.