GB2227483A

GB2227483A - SiC fibres

Info

Publication number: GB2227483A
Application number: GB8917330A
Authority: GB
Inventors: Paulette Shafik Sidky; Michael Gwyn Hocking; David Godfrey
Original assignee: UK Secretary of State for Defence
Current assignee: UK Secretary of State for Defence
Priority date: 1988-08-19
Filing date: 1989-07-28
Publication date: 1990-08-01
Anticipated expiration: 2009-07-28
Also published as: GB8917330D0; GB8819760D0; GB2227483B

Abstract

Fibres are made by the process of infiltration of carbon fibres by liquid silicon. Silicon (12) is liquefied in a silicon nitride crucible (13) provided in its base with a fine hole 14 for passage of a carbon filament (10). The hole (14) is fine enough such that surface tension force retains the liquid silicon within the crucible. The crucible is heated by means of an induction furnace to a temperature of about 1575 DEG C, well above the melting point of silicon. The carbon fibre is then drawn through the melt, the speed of movement determining the amount of silicon loading of the filament. <IMAGE>

Description

Apparatus and Method 5N She invention relates to fibres for use as reinforcements in composite materials and in particular to the manufacture of silicon carbide fibres.

Modern fibre carosites provide today' s designer with an increasing number of solutions to materials problems. One of the main objectives is to achieve lighter and stiffer composites which in turn rely on reinforcement parameters. Silicon carbide is a good candidate and its properties became particulary useful at high temperatures where its ability to be incorporated in metallic or ceramic matrices becanes useful.

Up to the present, the production of silicon carbide fibres has focussed on two main methods. The e pyrolysis route uses an organometallic precursor fibre which is pyrolysed to give a coreless predominantly silicon carbide fibre. The e second method is concerned with the chemical vapour deposition of silicon carbide on a filament (usually tungsten, carbon or silicon carbide).

Another feasible method is the conversion of carbon to silicon carbide. This has usually taken the form of carbon cloth which is coated with silicon powder and then heated in an inert atmosphere to 1410 degC (i.e. just below the melting point of silicon). Rather more than the stoichoimetric proportion of silicon to carbon is used. Problems in handling the substrate due to the powder coating lead to a non-uniform deposit. In the case of a fibre with a low powder holding capability, problems of handling prior to heating make this process a non-practical method for production of silicon carbide fibres.

The methods of applying the silicon powder coat have taken several forms such as the immersion of the carbon substrate in a bath containing a suspension of silicon powder followed by drying or by immersion of the carbon structure in a bath of evaporative resin containing silicon powder and thinned by a solvent.

The viscosity of the bath has to be high enough to maintain suspension of the silicon for two hours.

Another method of converting carbon to silicon carbide is by impregnating an organic polymer such as rayon fibre by immersing in an aqueous solution containing the dissolved silicon compound (such as salt). This leads to swelling of the fibre and entrapment of the salt. Extreme care is needed to evaporate the solvent and several stages are needed to produce the final product. Shrinkage of the fibres occurs up to 60%. The process also suffers fran the possible degradation of the fibre due to the long soaking times which can exceed 3 days.

The e object of the present invention is to provide an easy and quick route for the partial or full conversion of carbon fibres to silicon carbide, overcoming problems encountered with silicon powder coating or fibre degradation.

In a first aspect the invention provides apparatus for the manufacture of silicon carbide fibres comprising: a) a crucible made of a material which is not wetted by molten silicon; b) means to heat the crucible so as to liquify silicon; and c) means to feed a yarn of carbon fibres through the crucible such that in use liquid silicon infiltrates into the carbon yarn.

Preferably the crucible is made of silicon nitride. Advantageously the crucible is provided with a hole in its base dimensioned so as to allow passage of a carbon yarn while containing molten silicon within the crucible.

In the preferred arrangement an induction furnace is used to heat the crucible. More than one hole may be provided in the base of the crucible to enable several silicon carbide fibres to be made simultaneously.

A further chamber, as described in GB Patent Application No 8811893, may be provided whereby the silicon carbide fibre issuing from the crucible may be heat treated or coated by chemical vapour deposition.

In another aspect the invention provides a method of manufacturing silicon carbide fibres comprising the step of infiltrating a carbon fibre by liquid silicon.

Preferably the method comprises the steps of: a) heating silicon above its melting point in a crucible; and b) passing a yarn of carbon fibres through the molten silicon at a controlled rate such that silicon infiltrates into the carbon yarn.

Preferably also the temperature of the silicon is maintained well above the melting point of silicon such that self propagating high temperature synthesis of silicon carbide occurs, giving a fast reaction rate.

Advantageously the depth of molten silicon and the speed of passage of the carbon fibre through the crucible are selected to optimise the reaction of silicon and carbon.

In one arrangement of the invention chopped silicon carbide fibres may be made by the additional steps of: a) passing a carbon yarn comprising a plurality of individual fibres through the molten silicon; b) heat treating the yarn to complete conversion of the silicon; c) chopping the yarn to the required length; and d) removing any excess silicon.

The e invention will now be described with reference to the accanpanying Figure which shows a carbon fibre 10 being drawn from a spool 11 through a quantity 12 of molten silica contained in a silicon nitride crucible 13. The crucible 13 has a No 76 drill hole 14 in its base through which the carbon fibre 10 is drawn. The crucible 13 is supported within a tubular silica vessel 15 by means of an alumina tube 16, the upper end of which engages inside a circular groove 17 provided therefor in the base of the crucible. The e alumina tube 16 is supported on a disc- shaped perforated stainless steel moveable pedestal 18 provided at its outer edge with a rebate for locating the alumina tube.

One edge of a 3.2mum internal diameter stainless steel tube 19, 76cms in length is fixed into a central hole through the pedestal 18. A length of capillary tube (not shown for clarity), 2mn internal diameter, is inserted into the lower part of the stainless steel tube 19 to restrict gaseous diffusion. The stainless steel tube 19 is connected by a gas-tight connection 20 to the lower domed end of the silica enclosure vessel 15. A B19 connical fitting 21 is provided on the lower end of the silica vessel 15 and this mates with the complementary surface 22 at one end of an SQ Quickfit tube 23 fitted to and coaxially surrounding the stainless steel tube 19. A lateral tube 24 is provided for entry of an inert gas through theQuickf it tube 23 into the silica vessel 15. Waste gas leaves the tubular vessel 15 via an exit pipe 25. The silica vessel 15 is formed of two portions 26, 27 with a flat flange joint 28 therebetween sealed by an "0" ring 29. The e upper portion 26 of the silica vessel 15 is domed and has an axially aligned 2mm glass capillary tube 30 fitted into a B10 ground glass joint. The capillary tube 30 at the upper end of vessel 15 and the similar capillary tube sealed into the stainless steel tube 19 at the lower end of the silica vessel 15 allow passage of the fibre 10 while containing the gas which flows through the vessel 15. A gas seal as described in GB Patent Application No 8909149.0 is preferably added to the top and bottom of the glass apparatus assembly to prevent or inhibit any diffusion of air.The e flange joint 28 is demwntable such that the upper portion 26 of the vessel 15 can be removed for access to the crucible 13.

A cylindrical graphite susceptor 31 surrounds the crucible 13 and a layer of graphite felt 32 is wrapped around the susceptor. Coaxially surrounding the susceptor 31 and external to the silica vessel 15 is an induction coil 33 from an induction furnace.

In use the silica vessel 15 is supported with its axis vertical and the carbon fibre 10 is slowly drawn from the spool 11 through the crucible 13, passing through the hole 14 in the base of the crucible. The e induction furnace maintains the temperature of the silicon at about 1550 - 1600degC. At these temperatures, well above the melting point of silicon, self-propagating high temperature synthesis of silicon carbide occurs, giving a fast reaction rate.

Infiltration of liquid silicon into the carbon fibre or yarn occurs rapidly as the carbon filament is drawn through the molten silicon. The e molten silicon does not wet the surface of the silicon nitride crucible 13 and the diameter of the hole 14 in the base of the crucible is chosen such that the surface tension forces of the molten silicon prevent liquid silicon from running down the fibre and out of the crucible. An inert gas such as argon or heliurtVneon is passed through the silica vessel 15 via pipes 24 and 25.

The silicon nitride crucible is machined from a compacted silicon powder rod.

Nitriding of the machined silicon crucible is carried out by heating in static nitrogen with a complex temperature schedule of 10 hours respectively at 1150 degC, 1200 degC and 1300 degC, followed by 60 hours at 1350 degC and 10 hours at 1450 degC to avoid the deleterious effects of over-vigorous reactions (eg.

melting of silicon). Once formed the silicon nitride crucible is not attacked by silicon and can be used repeatedly.

The very low thermal capacity of a monofilament carbon fibre, or a yarn comprising a plurality of short monofilaments, allows fast heating and this is conver.iently done by means of an induction furnace. The residence time for the fibre in the silicon melt is made a function of the fibre thickness.

For thicker fibres or yarn, speed can be achieved by having a deeper crucible with greater depth of molten silicon, thereby effectively increasing the residence time of the carbon fibre in the silicon without decreasing the speed of passage of the fibre through the furnace.

In case of monofilament carbon, silicon loading can be geared to avoid excess silicon on the fibre surface by slightly extending the hot zone above the melt. In case of carbon yarn some excess silicon can remain entrapped between the individual fibres forming the yarn. This should not have any deterimental effect as discussed below.

The invention can also be used in the production of chopped silicon carbide fibres with good average diameter and 200 - 1000 microns lengths. This is easily achieved by gentle crushing of the silicon carbide yarn produced by the above described process. The term crushing is used to mean the seperation of the original short length fibres which form the yarn. Any excess silicon that is entrapped in the yarn seperates into flakes and seperation of the short fibres can be easily achieved by conventional methods. In this application the invention produces short length fibres with no excess silicon.

In some cases excess silicon is desirable to reduce reaction between the fibre and the metal matrix for metal matrix composites. The e above described process can be varied to achieve the desired silicon profile across the fibre diameter.

The Figure 1 arrangement can be modified by adding another chairber (not shown) above the main chamber to allow for heat treating the fibre if desired or for chemical vapour deposition of any desirable coat on the issuing silicon carbide fibre. GB Patent Application No 8811893 (filed 19 May 1988) gives a method of heating the fibre without electrical contacts. This ensures that no deterioration of the fibre or yarn occurs.

Claims

C1iin

I1aims 1. Apparatus for the manufacture of silicon carbide fibres comprising: a) a crucible made of a material which is not wetted by molten silicon; b) means to heat the crucible so as to liquify silicon; and c) means to feed a yarn of carbon fibres through the crucible such that in use liquid silicon infiltrates into the carbon yarn and reacts with the carbon fibres.
2. Apparatus for the manufacture of silicon carbide fibres as claimed in claim 1 wherein the crucible is made of silicon nitride.
3. Apparatus for the manufacture of silicon carbide fibres as claimed in claim 1 or 2 wherein the crucible is provided with a hole in its base dimensioned so as to allow passage of a carbon yarn while containing molten silicon within the crucible.
4. Apparatus for the manufacture of silicon carbide fibres as claimed in any one of claims 1 to 3 wherein an induction furnace is used to heat the crucible.
5. Apparatus for the manufacture of silicon carbide fibres as claimed in any one of claims 1 to 4 wherein more than one hole may be provided in the base of the crucible to enable several silicon carbide fibres to be made simultaneously.
6. Apparatus for the manufacture of silicon carbide fibres as claiomed in any one of claims 1 to 5 wherein a further chamber is provided whereby the silicon carbide fibre issuing from the crucible may be heat treated or coated by chemical vapour deposition.
7. A method of manufacturing silicon carbide fibres comprising the step of infiltrating a carbon yarn by liquid silicon.
8. A method of manufacturing silicon carbide fibres as claimed in claim 7 comprising the steps of: a) heating silicon above its melting point in a crucible; and b) passing a carbon fibre through the molten silicon at a controlled rate such that silicon infiltrates into the carbon fibre.
9. A method of manufacturing silicon carbide fibres as claimed in claim7 or 8 wherein the temperature of the silicon is maintained well above the melting point of silicon such that self propagating high temperature synthesis of silicon carbide occurs, giving a fast reaction rate.
10. A method of manufacturing silicon carbide fibres as claimed in any one of claims 7 to 9 wherein the depth of molten silicon and the speed of passage of the carbon fibre through the crucible are selected to optimise the reaction of silicon and carbon.
11. A method of manufacturing silicon carbide fibres as claimed in any one of claims 7 to 10 comprising the additional steps of: a) passing a carbon yarn comprising a plurality of individual fibres through the molten silicon; b) heat treating the yarn to complete conversion of the silicon; c) chopping the yarn to the required length; and d) removing any excess silicon.
12. Apparatus for the manufacture of silicon carbide fibres substantially as described with reference to the accompanying Figure.
13. A method of manufacturing silicon carbide fibres substantially as described with reference to the accomp2nying Figure.