CVD process for the manufacture of ceramic fibers

A process for depositing a coating on a filament, which comprises heating the filament and passing the heated filament through a deposition chamber containing gases which on contact with the hot filament deposit the coating; characterized in that said gases comprise chloroform and a hydrocarbon having I to 6 carbon atoms, a carbon coating being produced.

This invention relates to a process for the manufacture of ceramic fibres. 
It is well known to deposit ceramic coatings on filaments using chemical 
vapour deposition techniques. In a typical process, a filament is passed 
continuously through a deposition chamber containing gases which on 
contact with the hot filament deposit the desired coating. The filament is 
usually heated by passage of an electric current. 
It is known to pass a filament through a deposition chamber comprising 
halogenated hydrocarbons. Typically, EP-A-353934 discloses a process for 
depositing a carbon coating on a fibre in which the fibre is contacted 
with at least one halogenated aliphatic hydrocarbon. 
It is also known to pass a filament through a deposition chamber comprising 
hydrocarbons. Typically, JP-A-61219708 and JP-A-60145375 disclose the use 
of various gaseous hydrocarbons. 
Surprisingly, we have found that if the deposition chamber comprises both a 
halogenated hydrocarbon and a hydrocarbon, a carbon coated filament is 
obtained which has improved qualities over the coated filaments of the 
known prior art. 
Accordingly, the present invention provides a process for depositing a 
coating on a filament, which comprises heating the filament and passing 
the heated filament through a deposition chamber containing gases which on 
contact with the hot filament deposit the coating; characterised in that 
said gases comprise chloroform and a hydrocarbon having 1 to 6 carbon 
atoms, a carbon coating being produced. 
Any suitable C(1-6) hydrocarbon may be used, for example propane or, 
especially, propene. 
The gases in the deposition chamber may contain further components, for 
example an inert carrier gas such as argon or neon. Hydrogen may be 
present if desired, or the reaction may be carried out in the absence of 
hydrogen. 
The volume ratio of chloroform to C(1-6) hydrocarbon may vary widely, but 
is preferably in the range of from 3:1 to 1:8, especially 2:1 to 1:4. If 
an inert carrier gas is used, the volume ratio of C(1-6) hydrocarbon to 
carrier gas is preferably in the range of from 1:6 to 1:40, especially 
1:10 to 1:20. 
The deposition chamber is preferably a vertical tube. It has been found 
that especially good results are obtained when the gas inlet is at the 
lower end of the tube and the outlet at the upper end. 
The process according to the invention may be used for depositing a carbon 
coating on any desired filament. The filament may for example be tungsten, 
or carbon requiring a further carbon layer. Preferably however the 
filament is a ceramic filament, for example boron or, especially, silicon 
carbide. Such filaments are well known, and their manufacture described in 
many publications, for example U.S. Pat. No. 4,127,659 and U.S. Pat. No. 
3,622,369. 
In order to promote efficient deposition, the filament is preferably heated 
to a temperature in the range of from 800.degree. to 1300.degree. C., 
especially 900.degree. to 1100.degree. C. Most conveniently, the filament 
is heated by passage of an electric current supplied via two liquid metal 
electrodes through which the filament passes. These electrodes may contain 
pure mercury, or liquid metal mixtures selected from mercury/indium, 
mercury/cadmium or gallium/indium.

The filaments produced by the process of the invention are particularly 
useful for the preparation of titanium-based composites. Such composites 
may be prepared by embedding filaments in a matrix of titanium, titanium 
alloy or titanium intermetallic, under the action of heat. 
Commercially available chloroform may be used to carry out the present 
invention. The chloroform may contain levels of impurities, e.g. alcohols 
which function as stabilising agents to prevent dissociation of the 
molecule during storage. Typical stabilising agents include ethanol and/or 
amylene (tertiary amyl alcohol); these are typically present in 
concentrations of 1-3% volume and 20-40 ppm respectively. 
The present invention will now be described in greater detail: 
1. GENERAL METHOD 
FIG. 1, of the accompanying drawings, shows an apparatus which may be used 
to carry out the invention. A filament 1, for example silicon carbide with 
a tungsten core, is fed from a supply 2 via a tube 3 to a store 4. The 
filament 1 passes through mercury electrodes 5 and 6 at the ends of the 
tube 3. The electrodes 5 and 6 form part of an electric circuit (not 
shown) which supplies an electric heating current to the filament, raising 
it to a temperature typically of from 800.degree. to 1100.degree. C., e.g. 
900.degree. to 1100.degree. C. Argon (flow rate 1000 to 2000 standard 
cubic centimeters per minute (sccm)), propene (flow rate 5 to 125 sccm) 
and commercial chloroform ex BDH containing 1-3% ethanol, (40 to 160 sccm 
e.g. 30 to 70 sccm) are fed into the tube 2 via inlet 7, and spent gases 
removed via outlet 8. Filament entering the store 4 has a high-quality 
carbon coating. 
2. TESTS 
Fibre Strength Test 
The coated fibres were tensile tested over a 25 mm gauge length. Aluminium 
grips were used to clamp and protect the fibres during testing. Tension 
was applied and the resulting force which caused the fibre to break was 
recorded. The mean of ten tests was used to calculate the fibre strength. 
3. EXAMPLES 
Comparative Example 1. Propene Only 
The aforementioned procedure was carried out using propene only. Propene 
was fed into the deposition chamber under a flow rate of 200 cm.sup.3 
min.sup.-1 and an argon flow rate of 1400 cm.sup.3 min.sup.-1. The 
filament was heated to 800.degree.-1100.degree. C. It was not possible to 
identify a coating using scanning electron microscopy (SEM). A thin 
coating of 50 nm was identified using secondary ion mass spectroscopy 
(SIMS). 
COMATIVE EXAMPLE 2. CHLOROFORM ONLY 
The procedure of Comparative Example 1 was repeated in the absence of 
propene but under a flow rate 80 cm.sup.3 min.sup.-1 of chloroform. The 
resulting coating is 0.8 .mu.m in thickness and is shown in FIG. 2. The 
coating is nodular in appearance, exhibits poor adhesion and gives a fibre 
strength of 3.33 GPa. 
EXAMPLE 1. PROPENE AND CHLOROFORM 
The procedure of Comparative Example 2 was repeated in the presence of 
propene (flow rate of 75 cm.sup.3 min.sup.-1). The resulting coating is 
0.5 .mu.m in thickness and is shown in FIG. 3. The coating is visibly 
smoother than that obtained from using only chloroform, exhibits good 
adhesion and gives a fibre strength of 4.02 GPa.