CA2004406A1

CA2004406A1 - Ceramic/fiber composite and process for its production

Info

Publication number: CA2004406A1
Application number: CA002004406A
Authority: CA
Inventors: Marcellus Peuckert; Martin Bruck; Thomas Gerdau; Tilo Vaahs; Hans-Jerg Kleiner; Fritz Aldinger
Original assignee: Hoechst AG
Current assignee: Hoechst AG
Priority date: 1988-12-03
Filing date: 1989-12-01
Publication date: 1990-06-03
Also published as: EP0372381B1; CN1044273A; EP0372381A2; DE58909309D1; EP0372381A3; DE3840781A1; JPH02188471A; CN1027441C

Abstract

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.

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.

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) ~ 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 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~

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~

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 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 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~

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 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 ~0 3 Al2O3-SiOz 186 4 SiC 392 Si3N4 463

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) 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) 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 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.

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.