COMPOSITE CERAMIQUE/FIBRE ET PROCEDE DE FABRICATION

CERAMIC/FIBER COMPOSITE AND PROCESS FOR ITS PRODUCTION

Abrégé anglais

HOE 88/F 347
Abstract
Ceramic/fiber composite and process for its production
The invention relates to a ceramic/fiber composite and a
process for its production.
The process comprises impregnating fibers with a molten
polysilazane in a first step, converting the polysilazane
in the fibers into the infusible state in a second step
and, in a third step, heating the impregnated fibers to
800 to 2000°C in an atmosphere of nitrogen, noble gas, or
ammonia.

Revendications

- 14 - HOE 88/F 347
Patent Claims
1. A process for the production of a ceramic/fiber
composite, which comprises impregnating fibers with a
molten polysilazane in a first step, converting the
polysilazane in the fibers into the infusible state in a
second step, using NH3, Urotropin, an amine or a
chlorosilane of the general formula (CH3)nSiClm, in which
n + m - 4 and n = 1, 2 or 3, and, in a third step,
heating the impregnated fibers to 800 to 2000°C in an
atmosphere of nitrogen, noble gas or ammonia.
2. The process as claimed in claim 1, wherein a molten
polysilazane of the general formula (I)
<IMG>
is used, in which x and y represent the mole fractions
of the two structural units and where x + y = 1 and x =
0.7 to 0.95.
3. The process as claimed in claim 1, wherein a molten
polysilazane of the general formula (II)
<IMG>
is used, in which the free valencies of the nitrogen
atoms are saturated with H atoms or silyl radicals
R*SiXN< (X = H, Cl, N<, CH2CH2Si-) and in which R, R', R"
and R* denote alkyl or alkenyl groups having up to 6
carbon atoms and a, b and c denote the mole fractions of

- 15 -
the respective structural units.
4. The process as claimed in claim 3, wherein R, R', R"
and R* are alkyl or alkenyl groups having up to 3 carbon
atoms.
5. The process as claimed in claim 3, wherein
R=R'=R"=R*=CH3.
6. The process as claimed in one of claims l to 4,
wherein fibers of C, SiC, Si3N4 or Al2O3 or carbon fiber-
reinforced carbon are used.
7. The process as claimed in one of claims 1 to 5 t
wherein, a molten polysilazane is first spun into fibers,
these fibers are then converted by heat treatment at 800
to 1600°C into Si3N4 fibers, a two-dimensional structure
is produced from the latter and said structure is then
impregnated with the same or a different molten
polysilazane, the polysilazane is converted to the
infusible state and the product is heated to 800 to
2000°C.
8. The process as claimed in one of claims 1 to 7,
wherein the sequence of the three process steps is
carried out at least twice in succession on the same
fibers.
9. The process as claimed in one of claims 1 to 8,
wherein compounds of magnesium, aluminum, yttrium or of
a rare earth metal, singly or as a mixture, are dissolved
in the molten polysilazane as filler for the fibers and
the fibers are impregnated with this solution instead of
with pure polysilazane.
10. The process as claimed in claim 9, wherein the
nitrates, alcoholates, acetates or acetylacetonates of
the said metals are used, singly or as a mixture, as the
filler.

- 16 -
11. Ceramic/fiber composite obtainable in accordance with
the process as claimed in one of claims 1 to 10.
12. A ceramic/fiber composite obtainable in accordance
with the process as claimed in one of claims 1 to 11,
composed of fibers and a ceramic amorphous or partly
crystalline matrix, in which the matrix contains 45 to
60% by weight Si, 30 to 40% by weight N, 0 to 25% by
weight C and 0 to 20% by weight O, and is in the form of
crystalline Si3N4 to the extent of more than 30% by
weight.
13. A process for coating a ceramic/fiber composite,
wherein a ceramic/fiber composite according to claim 11
is mechanically processed and is then coated with molten
polysilazane in a first step, the polysilazane is
converted into the infusible state, in a second step
using NH3, Urotropin, an amine or a chlorosilane of the
general formula (CH3)nSiClm, in which n + m = 4 and n = 1,
2 or 3, and, in a third step the coated ceramic/fiber
composite is heated to 800 to 2000°C in an atmosphere of
nitrogen, noble gas or ammonia.
14. A coated ceramic/fiber composite obtainable in
accordance with the process as claimed in claim 13.

Description

4~)6
HOECHST AKTIENGESELLSCHAFT HOE 88/F 347 Dr.MA~je
DescriE~tion
Ceramic/fiber composite and process for its production
The present invention relates to a ceramic/fiber com-
posite and a process for its production.
Because of its high breaking strength, dimensional
stability, resistance to high temperature and to
corrosion, ceramic/fiber composite is gaining a
continuously growing Lmportance. The good properties of
the ceramic/fiber composite are based on the combination
of a matrix and built-in fibers.
A ceramic/fiber composite in which the fibers are first
impregnated with polysilazane and the polysilazane is
then thermally decomposed to silicon nitride is described
in EP-0,125,772 Al. A disadvantage with this process is
that, for the impregnation of the fibers, the
polysilazane must be dissolved in solvent.
After the impregnation the solvent must be removed.
Cavities form as a result of the removal of the solvent
from the fiber composite, resulting in a ceramic/fiber
composite pos~e~sing properties which are not always
satisfactory. It has been found that ceramic~fiber
compo~ites have an increasing breaking strength and
dimensional stability if there are only a few cavities in
the ceramic/fiber composite.
The object was, therefore, to provide a process for ~he
production of ceramic/fiber composites, with which a
ceramic/fiber composite having an increased breaking
strength and dimensional stability is obtained~ which in
the crude state is dimen~ionally stable and easily
workable and which remains dLmensionally stable during
heating.

06
-- 2 --
One suhject of the present invention is a process for the
production of a ceramic/fiber composite, which comprises
impregnating fibers with a molten polysila~ane in a first
step, converting the polysilazane in the fibers into the
infusible statQ, in a second step, using NH3, Urotropin,
an amine or a chlorosilane of the general formula
(CH3)nSiCl~, in which n + m = 4 and n = 1, 2 or 3, and, in
a third step, heating the impregnated fibers to 800 to
2000C in an atmosphere of nitrogen, noble gas or
ammonia. In this context the term "fibers" is to be
understood to mean both one-dLmensional structures and
also two-dimensional structures of all types formed
therefrom. Suitable molten polysilazanes are:
a) compounds of the general formula (I)
C2H5 C2H5
~ i t----~ Si-N<
(NH) 1/2 X (N~)1/2 Y
in which x and y denote the mole fractions of the two
structural units and in which x + y = 1 and x = 0.7-
0.95,
b) compounds of the general formula (II)
1 51-N~S1 N~{S1-N~
in which the free valencies of the nitrogen atoms are
satuc~ated with H atoms or ~ilyl radicals R*SiXN< (X
= H, Cl, N<, CH2CH2Si~) and in which R, R', R" and R*
are alkyl or alkenyl groups having up to 6 carbon
2S atoms, preferably up to 3 carbon atoms, and a, b and
c denote the mole fractions of the respective

~0~06
_ 3
structural units. R=R'=R =R*=CH3 is particularly
preferred.
The fibers used in the process according to the invention
could be composed of, for instance, C, SiC, Si3N4, Al2O3 or
carbon fiber - reinforced carbon. It is possible, for
oxample, first to spin molten polysilazane into fibres,
to convert them by heat treatment at 800 to 1600 C into
Si3N4 fibres, to produce a two-dimensional structure from
the latter and then to impregnate said structure
according to the invention with the same or a different
polysilazane, to convert the polysilazane into the
imfusible state and to heat the product to 800 to 2000 C.
If the polysilazane is to be rendered infusible by means
of an amine, in general methylamine or ethylamine is
used. However the preferred agent for converting to the
infusible state is NH3.
.
The combination of steps according to the invention can
also be used on the s~me fibers several times in
Il succesion.
~0 Furthermore, compounds of magnesium, aluminum, yttrium or
of a rare earth metal, singly or as a mixture can be
dissolved in the molten polysilazane as filler for the
fibers and the fibers impregnated with this solution
in~tead of with pure polysilazane; particularly suitable
compounds are the nitrates, alcoholates, acetates or
acetylacetonates, singly or as a mixture.
Of course, the impregnated fibers can also be shaped into
a shaped article before the heat treatment.
A further subject of the invention is a eeramic/fiber
composite, obtainable by means of the process ~ust
described, preferably carried out using compounds of the
formula (I) or (II).
A further sub~ect of the present invention is a ceramic/

Zl)~ fi
-- 4 --
fiher composite, obtainable by means of the said process,
in particular in its preferred embodiments, composed of
fibers and a ceramic amorphous or partly crystalline
matrix, in which the matrix contains 45-60% by weight Si,
30-40% by weight N, 0-25% by weight C and 0-20% by weight
O and crystalline Si3N4 is present to the extent of more
than 30% by weight.
In order ta increase the corrosion resistance it can be
advantageous if the finished and already mechanically
processed ceramic/fiber composite is subjected to a
further treatmant with molten polysilazane, namely that
it is coated with the latter, the coating is rendered
infusible and the product then heated to 800 to 2000C in
an atmosphere of nitrogen, noble ~as or ammonia.
The preparation of the compounds of formula (I) which are
suitable as starting materials is described in German
Patent Application P 37 37 921.6. Thi~ application
relates in general to the preparation o~ polymeric
silazanes by reacting one or more
dialkylaminoorganyldichlorosilanes of the formula RSiCl2-
NR'R', in which R = Cl-C4-alkyl, vinyl or phenyl and R' =
Cl-C4-alkyl, with at least 3.35 moles of ammonia per mole
of silane in a solvent at temperatures of -80C to +70C.
The dimethylaminoethyldichlorosilane CzH5SiCl2 N(CH3) 2
(also referred to as "aminochlorosilane" in the following
text) used as starting material for the polymeric ila-
zanes of the formula (I) can be obtained according to
S.5. Washburne, W.R. Peterson, J. Organometal. Chem. 21
(1970), page 59, by reacting ethyltrichlorosilane
C2HsSiCl3 with dimethylamine. The reaction i8 carried out
in aprotic solvents, preferably polar, such as ethers, in
particular in THF.
The molar ratio of ethyltrichlorosilane to dimethylamine
can assume values between 1:1 and 1-3; a ratio of about
1.2 is pre~erred.

O~
-- 5 --
The ammonium salts formed during the reaction precipitate
out of the reaction solution, whilst the aminochloro-
silane formed remains in solution.
The resulting aminochlorosilane of the formula
C2H5SiCl2-N(CH3)2 is reacted, per mole, with at least
3.35 moles, preferably with at least 3.5 moles of ammonia
in aprotic solvents, preferably polar, such as ethers, in
particular THF. This i8 effected at temperatures between
-80C and ~70C, preferably at -10C to 0C.
The resulting polymeric silazane of the formula (I) is
completely soluble in all common aprotic ~olvents.
In the formula (I~ Si is never bonded to Si directly, ~ut
always via a NH bridge. If, for example, x = O.9 (and
therefore y = 0.1), then 10% of the originally available
dimethylamino groups are still contained in the polymer
and 90~ of the silicon atoms are crosslinked three tLmes
via NH bridges. The controllable ratio of x to y
determines the degree of crosslinking and thus the
viscosity and the processability to ceramic.
In thi6 way value~ of x = 0.7-0.95 (y = 0.3-0.05) are
obtained if at least 3.35 moles of NH3 are used per mole
of aminochlorosilane. Preferably x = 0.85-0.95 (y = 0.15-
O.053; this is then the ca~e if at least 3.5 Moles of NH3
are u6ed per mole of aminochlorosilane. In general at
most 8 moles, preferably at most 6 moles of ~H3 are used
per mole of aminochlorosilane. Naturally a larger
relative quantity of NH3 than 8 moles will also be
successful, but thi~ higher expense is unnece~sary.
The preparation of compounds of the formula (II), which
are also suitable as ~tarting materials for the
ceramic/fiber composites according to the invention, has
already been described in part in the ~erman Patent
Application P 37 33 727.0; in this applicaiton the
compounds are referred to as polymeric
' ' ' ' '
,

X()~06
hydridochlorosilazanes. For their preparation
oligohydridoalkylsilazanes of the general formula
(R1SiHNH)n, in which n is about 3 to 12 and Rl denotes an
alkyl or alkenyl group having up to 6 carbon atoms, are
reacted with a dichlorohydridoalkylsilane of the general
formula R2SiHCl2, in which R2 denotes an alkyl or alkenyl
group having up to 6 carbon atoms, at 30 to 300C. During
this reaction highly volatile by-products are formed.
These by-products are removed during the reaction.
The oligohydridoalkylsilazanes (RlSiHNH)n, with n equal to
about 3 to about 12, used in this reaction can ~e
obtained by reacting a dichlorohydridoalkyl~ilane of the
formula R1SiHCl2, in which R1 has the above meaning, with
an excess of NH3 in a solvent, as described in US Patent
4,482,669 (see there in particular columns 4, 5, 7 and
8). In general, a mixture of linear and cyclic oligomers
of different chain lengths n forms in this process.
The radicals R1 and R2 in the oligohydridoalkylsilazanes
(RlSiHNH)n (also abbre~iated to "oligosilazanes" in the
following text) or in the dichlorohy~ridoalkylsilane
R2SiHCl2 (also abbreviated to "dichloroalkylsilane" in the
following text) can be identical or different; preferably
they have up to 3 carbon atoms.
It is particularly preferred that Rl = R2 = CH3. Prefer-
ably the molar ratio of the reactants in the above
reaction dichloroalkylsilane: R1SiHNH unit of the oligo-
silazane is about 0.2 : 1 to 1.5 : 1, in particular
0.3 : 1 ~o 1 ~ 1.
For the reaction of the reactants with each other the
oligosilazanes are preferably initially introduced and
the dichloroalkylsilane added. Since the reaction is
exothermic the temperature i5 preferably initially kept
at 30 to 50C during the mixing together of the
reactants. Subseguently the mi~ture is heated to
temperatures of 100 to 300C, preferably to 120 to 250C.

40~
-- 7 --
The low-boiling products formed as by-products, such as
RSiHC12, RSiClH2, RSiC13, HCl, H2, NH3 (in which R = R1 or
R2), partially escape during the reaction. On completion
o~ the reaction the residual low-boiling products are in
5 general removed from the reaction vessel ~y applying a
vacuum.
The NH4Cl also formed during the reaction largely sublimes
off from the reaction mixture in the course of the
reaction. Any remaining residue of NH4~1 can be separated
off from the prepared polymeric hydridochlorosilazane by
extraction with an inert organic solvent, such as n-
hexane, toluene or ether.
The reaction time depends on the speed of heating up and
on the reaction temperature. In general a reaction time
of 5 to 7 hours i8 sufficient.
It is also possible to carry out the reaction in an
organic solvent. Suitable solvents are tho~e which are
inert towards the reactants and have a sufficiently high
boiling point, such as, fox example, saturated aliphatic
or aromatic hydrocarbons, such as n-decane, Decalin,
xylene or toluene, chlorinated hydrocarbons such as
chlorobenzene, or ethers, such as dibenzyl ether or
diethylene glycol diethyl ether. When a solvent is u~ed
in which the NH4Cl formed is insoluble, the latter can be
separated off by filtration. The polymeric
hydridochlorosilazanes are then obtained by distilling
off the solvent under reduced pressure.
If appropriate, the process can also be carried out under
reduced pressure. It is also possible to oper~te at
pressuxes in the range of from 1 to 10 atmospheres. The
process can also be designed to run continuously.
The polysilazanes of the formula (II) prepared in this
manner have a net-like structure. The values of the mole
fractions b and c are the higher (and correspondingly the
,,

Z0()~406
-- 8 --
value of a the lower), the larger the ratio of dichloro-
alkylsilane : R1SiHNH unit of the oligosilazane. The
particular values of a, b and c in each case can be
determined by integration of the lH-NMR spectra and by
S elementary analysis. In general the values of a, b and c
are 0.1 to 0.8, where a + b + c = 1. Preferred values for
a and b are from 0.1 to 0.5, particularly from 0.2 to
0.4. The preferred value~ for c are 0.1 to 0.6,
particularly 0.3 to 0.6. As stated these values can be
adjusted by means of the relative proportion of the
dichloroalkylsilane in the reaction mixtuxe and monitored
by means of the methods of analysis mentioned.
Surprisingly, it has been found that in the production
of a ceramic/fiber composite according to the invention
a single impregnation with molten polysilazane, followed
by conversion to the infusible state and heating tthree-
step sequence) frequently already results in a completely
satisfactory breaking strength of the ceramic/fiber
composite. However, with repeated three-step sequences
l 20 carried out in ~uccession sometimPs a further increase
in the breaking strength and corrosion resistance of the
ceramic/fiber composite can be achieved.
1!'
; The process according to the invention is equally
applicable to one-dLmensional structures and two
1 25 dimensional struc~ures built up from these, that is
to say materials such as wovens, non-wovens, fleeces,
filaments, threads, fibers, cords or networks. As stated,
the term fibers shall be used to represent all of these
~tructures. The fibers can be dipped in molten
polysilazane, or the molten polysilazane is applied
dropwise to the fibers or poured onto them. It can be
advantageous to form thicker shaped articles from
individual impregnated, relatively thin material layers
by multistacking of one layer above another and
proce sing these thicker ~haped ar~icles further after
conversion of the polysilazane into the infusible state;
in other cases it can be better ~o ~tack the initially

~o~
- g -
non-impregnated material layers one above another and to
impregnate this stack as a whole with polysilazane.
If ~following the conversion of the polysilazane to the
infusible state) the heating of the impregnated ibers is
carried out in a nitrogen or noble gas atmosphere at 800
to 1200C, an amorphous silicon matrix is obtained, which
is composed of approximately 40 to 50~ by weight Si, 20
to 30% by weight N, 15 to 25~ by weight C, remainder O
and Cl.
If, on the other hand, the heating of the impregnated
fibers is carried out in an atmosphere of ammonia or an
inert gas containing ammonia at 800 to 1200C, an
amorphous silicon matxix is obtained which is composed of
approximately 50 to 60~ by weight Si, 30 to 40% by weight
N, less than 5~ by weight O, less than 1% by weight C and
less than 1% by weight Cl.
A matrix which is partly crystalline and composed of
Si3N4 is obtained on heating in N2, noble gas or NH3 to
temperatures from 1200C to about 1600C, particularly
from 1400C to about 1600C.
. A matrix composed of ~-Si3N4 i6 obtained on heating totemperature~ of about 1600 to 2000C. AboYe about 1800C
heating must then be carried out under an elevated
nitrogen pressure of about 10 to 50 bar, in order to
prevent a decomposition of the Si3N4.
A further subject of the present J n~ention is a process
for coating mechanically processed ceramic/fiber
composites, which compri~es enveloping the mechanically
processed ceramic/fiber composite with molten
polysilazane in a first step, converting the polysilazane
into the infusible state in a ~econd step, using NH3,
Urotropin, an amine or a chlorosilane of the general
formula (CH3)nSiClm, in which n + m - 4 and n = 1, 2 or 3,
and; in a third step, heating the enveloped ceramic/fiber

200~06
- 10 -
composite in an atmosphere of N2, noble gas or NH3 to 800
to 2000C. For this process compounds of the formulae (I)
and (II) are again par~icularly suitable as poly-
silazanes.
Using this process it is possible to envelop ceramics
which are not resistant to oxidation, such as, for
example, carbon fibers, with an Si3N4 layer and thus to
protect them against oxidation at high temperature or
against corrosion. NH3 is preferably used in order to
convert the polysilazane to the infusible state in the
process described above.
In the following examples the flexural strength of the
ceramic/fiber composites was measured as 4-point bending
strength according to USA Standard Mil.-STD 1942 using
the Instron 1326 universal testing machine:
4-point support with 40 mm/20 mm distance between the
supports and a constant increase in force of 500 N/s on
test pieces measuring 3.5 mm 4.5 mm 45 mm.
The following examples illustrate the invention. The
percentage figures are percentages by weight, unless
indicated otherwise.
Example 1
Polysilazane of the formula (I) with x = 0.9 and y = 0.1
and carbon fibers (~Sigrafil C from Sigri GmbH, Meitin-
gen, West Germany; with 40,000 individual filaments each
having a diameter of 7 ~m~ were introduced into a vessel.
The vessel was brought under a nitrogen atmosphere and
heated to 100C. The sizing had been removed previously
from the carbon fibers in an acetone bath. The
impregnated carbon fibers were removed from the poly-
silazane melt and cooled to 25C. 20 carbon fibers
impregnated in this manner were stacked at right angles
to each other to form a block and the block was compacted
in a press at a temperature of 50C under a pressure of

X0~)~4~6
50 bar. The shaped article obtained in this way was kept
at room temperature for 2 hours in an atmosphere of
CH3SiCl3 in order to convert the polysilazane to the
infusible state. Subsequently the shaped article was
heated in the course of a heating period of 15 hours to
a temperature of 100C in an atmosphere of nitrogen, left
at this temperature for 10 hours and then cooled. The
measured bending strength of the ceramic/fiber composite
obtained is given in the table following the examples.
Example 2
A ceramic/fiber composite was produced as in Example 1.
This was then subjected to two further three-step
sequences (Lmpregnation, conversion to the infusible
state, heating) using the same polysilaæane as in Example
1. The measured bending strength of the resultant
ceramic/fiber composite is again given in the table.
Example 3
!!ii
Z Polysilazane of the formula (II) with R = R'=R"=CH3 and
Al2O3-SiO2 fibers (85% Al2O3, 15% SiO2) with 1000 individual
filaments each having a diameter of 0.017 mm were heated
to 180C in a vessel under an atmosphere of nitrogen and
the fibers were then removed from the melt and cooled to
25C. The impregnated fibars were stacked in a crosswise
manner and the stack compressed to a shaped article in a
press at 110C under 40 bar pressure. The impregnated
material was held for 2 hour~ at room temperature under
an atmosphere of ammonia in a pressure vessel and then
heated in the course of 15 hours to a temperature of
1400C under an ammonia pressure of 10 bar, left for 10
hours at this temperature and then cooled. The matrix
consisted to the extent of 44~ by weight of Si3N4. The
measured bending strength of the cer~mic/fiber composite
obtained is again given in the table.

o~
- 12 -
Example 4
SiC fibers with 500 individual filaments each having adiameter of 0.015 mm were drawn, under nitrogen as a
blanketing gas, through a bath of molten polysilazane of
the fo~mula (I) with x = O.8 and y - O.2, to which of 5%
by weight yttrium acetate had been added. The SiC fibers
impregnated in this manner were stacked crosswise on top
of each other, compressed at a temperature of 50C in a
press under 40 bar pressure and held for 2 hours in an
atmosphere of ammonia. The shaped article was heated in
the course of 15 hours to 1700C under a nitrogen pres-
sure of 5 bar, left for 10 hours at this temperature and
then cooled.
The matrix consisted to the extent of 78% by weight of
~-Si3N4. The measured bending strength of the
ceramic/fiber composite obtained is again given in the
table.
Example 5
Polymeric hydridochlorosilazane of the formula (II) with
R=R'=R''=CH3 was introduced into a melt-spin apparatus
under N2 blanketing gas and heated to 140C and the melt
forced through a ~pinning ~et of 0.1 mm diameter using a
piston. The spun fiber was stretched to a fiber thickness
of 20 ~m under its own weight. The resulting fibers were
treated with ~H3 gas at room temperature and rendered
infusible by this means and were then subjected to
pyrolysis in a furnace under an atmosphere of NH3. For
thi~ purpose the temperature was increased in the co~rse
of 7 hours from 25C to 1200C, kept for 1 hour a 1200C
and then in the course of 4 hours reduc~d to room
temperature again. The resulting fiber was amorphous w~en
tested by X-rays and apart from the main constituents Si
and N also contained 0.1% by weight C, 0.6% by weight Cl
and 2.0% by weight O. The ceramic yield of the pyrolysis
was 64% by weight. The tensil strength of the fiber was
..

Z~ 40~i
- 13 -
2 GPa.
The fibers prepared in this manner were drawn in cords,
each of 500 individual filaments, under nitrogen
blanketing gas through a bath of molten polysilazane of
the formula (I) with x = 0.8 and y = 0.2. The fibers
impregnated in this manner were stacked crosswise one on
top of another and compressed to a shaped article at 50C
under a pressure of 40 bar. The shaped article was held
for 2 hours in an atmosphere of NH3. The shaped article
was heated in the course of 20 hours to 1200C under a
nitrogen pressure of 1 bar, left for 10 hours at this
temperature and then cooled. The measured bending
strength is given in the table.
Table
Example No. Fiber type Bending strength
MPa
C 110
2 C 262
~0 3 Al2O3-SiOz 186
4 SiC 392
Si3N4 463