Continuous atmospheric pressure CVD coating of fibers

Barrier coatings are deposited onto fibrous materials at atmospheric pressure by a continuous CVD process. A relatively short furnace tube with an unrestricted outlet is used. Thus, the supply and takeup reels do not need to be contiguous with the coating part of the apparatus. This allows the periodic removal of fuzz and soot without the interruption of the coating process.

FIELD OF THE INVENTION 
This invention relates to the coating of fibers to make them more suitable 
for use in fiber-reinforced composites. More specifically, the invention 
concerns a process for coating reinforcing fibers, and apparatus for 
continuously coating such fibers. 
BACKGROUND OF THE INVENTION 
High-performance fibers are being increasingly used as the reinforcement of 
plastic, metal, ceramic, and carbon matrix composites. When the composite 
has a ceramic matrix, the main role of the fibers is to toughen the 
composite to prevent brittle failure. The degree of toughness attained is 
greatly affected by the bond strength between the fibers and matrix. If 
the bond strength is too high, cracks propagate through the fibers; if too 
low, the load is not transferred to them. 
The most demanding of these applications are those involving high operating 
temperatures. In such environments, the matrix may chemically react with, 
or dissolve the fiber. Although chemical reaction may in some cases be 
beneficial, it usually leads to drastic reductions in strength and 
toughness. In many cases, these high-temperature problems can be solved by 
applying barrier coatings on the fibers by chemical vapor deposition 
(CVD). As the name implies, CVD involves the deposition of coatings onto 
substrates by chemical reaction from the vapor phase. The technique is 
widely known and a number of review articles exist on the subject. See, 
for example, Blocher, J. M., Jr., "Deposition Technologies for Films and 
Coatings", Noyes Publications, page 335-364 (1982). Blocher discusses the 
roles of thermodynamics in predicting the possibility of deposition with 
various reactants under given temperatures and reactant partial pressures, 
of kinetics on the rate of deposition, and of transport processes such as 
diffusion and heat transfer in CVD. This reference also describes the 
effects of these variables and their interactions on coating properties. 
The application of a coating by CVD to a monofilament is a fairly simple 
procedure. See, for example, EPO Pat. Publ. No. 0,222,960 (Schachner) 
where a monofilament is drawn and coated by CVD in-line. When the 
monofilament is electrically conductive, it can be heated resistively 
during CVD, thereby activating the chemical deposition reaction primarily 
on the monofilament and not in the gas phase or on the unheated wall of 
the tube that confines the gaseous mixture. There is a considerable amount 
of art describing such "cold wall" deposition systems by which CVD 
coatings are applied to monofilaments by continuous processes. See, for 
example, U.S. Pat. Nos. 3,549,424 and 4,068,037. 
When coating by CVD a fiber that has multiplicity of filaments, e.g., a 
tow, it is necessary no diffuse the reactant(s) between the filaments. A 
translation of French Patent No. 2,607,840 states that: 
"This vapor phase deposition, designated by the American acronym CVD, does 
not allow one to obtain good protection for all of the individual 
filaments that constitute the tow of carbon. Actually it is very 
difficult, if not impossible, to avoid preferential deposition, 
particularly in the peripheral zones of the tow, and equally, to avoid the 
cementing together of the filaments. These drawbacks make the process 
unsuitable for industrial utilization. In other words, the CVD technique, 
even though very attractive in theory, does not allow the control of 
coating thickness and homogeneity of the carbide deposit specially when, 
as is the case in a tow, the gaseous reaction medium diffuses poorly into 
the center. It follows that the individual filaments of carbon are not 
coated homogeneously, regularly, and with controlled thickness." 
To alleviate the difficulties mentioned in the French patent, it is common 
to coat multiple-filament fibers by CVD at low pressures (LPCVD). This 
increases the mean free path of the reactants, thereby decreasing 
homogeneous nucleation and the growth of soot particles in the gas phase. 
It also facilitates the diffusion of the gases between the filaments, thus 
reducing the variability in coating thickness. However, such a reduced 
pressure coating process requires building and maintaining of 
sophisticated, expensive equipment. See U.S. Pat. No. 4,343,836 (Newkirk 
et al.) In many cases, such equipment is limited to batch processes. See, 
for example, U.S. Pat. No. 3,212,926 (Morelock) and U.S. Pat. No. 
4,214,037 (Galasso et al.), the latter of which suggests that similar 
coating results can be obtained in a continuous process. 
Difficulties are encountered when LPCVD processes for coating a 
multi-filament fiber are made continuous. Deposits on the inner wall of 
the coating chamber gradually diminish its volume and eventually require 
it to be replaced. Fuzz (tangles of broken off filaments) and soot 
(homogeneously nucleated and grown particles) that form in the system 
during coating interfere with fiber movement. To prevent fiber breakage, 
the fuzz and soot must be periodically removed. When cleaning is required 
or when the fiber breaks during the coating process, the vacuum has to be 
broken, and after repair the system has to be pumped down. If the coating 
is performed at high temperatures, partial cool-down and subsequent 
reheating of the coating system is also required. 
These difficulties demonstrate a need for a system by which a 
multi-filament fiber can be continuously coated by CVD at atmospheric 
pressure (APCVD), and such systems have been reported. See, for example, 
Honjo et al., Composite Interfaces, Proc. Int. Conf. 1st, pp. 101-107 
[1986]; Aggour et al., Carbon, Vol. 12, pp. 358-362 [1974]; and Amateau, 
J. Compos. Mater., Vol. 10, pp. 279-296 [1976]). However, the apparatus 
illustrated in each of those publications would need to be disassembled to 
be cleaned and so would offer little advantage over a LPCVD system such as 
that shown in the Newkirk patent. 
U.S. Pat. No. 4,373,006 (Galasso et al.) says that "even when upwards of 
10,000 fibers are bundled together to form a strand of yarn the chemical 
vapor deposition of silicon carbide produces an essentially uniform 
coating of silicon carbide over the surface each fiber even on those 
fibers in the center of the yarn and even on those areas of fibers which 
are in close proximity to one another" (col. 2, lines 57-64). It also says 
that carbon fibers were coated with silicon carbide "by holding the fibers 
in a chamber . . . maintained at a temperature of between 1100.degree. and 
1200.degree. C. by passing them through an R.F. heated graphite susceptor" 
(col. 3, lines 41-49). However, there is no disclosure of the nature of 
the fiber-handling apparatus and no drawing. 
For a detailed discussion of the advantage of coating ceramic fibers with 
BN for use in composites, see U.S. Pat. No. 4,642,271 (Rice). However, 
coating conditions are not given. It is not even stated whether APCVD or 
LPCVD is used, and no apparatus is illustrated. 
U.S. Pat. No. 4,731,298 (Shindo et al.) concerns coating carbon fibers 
first with a layer of carbon and then with a metal carbide. "The carbon 
fibers may be in the form of yarns, tows or webs of continuous filaments. 
The carbon fibers may be used in the form of yarn and webs of short fibers 
or the like" (col. 2, lines 65-68). However, no method is disclosed; 
neither is any apparatus disclosed. 
SUMMARY OF THE INVENTION 
The present invention provides what is believed to be the first apparatus 
by which barrier coatings can be continuously applied by CVD to a 
multiplicity of filaments or fibers such as a tow of filaments or a yarn 
or a strip of woven fabric. Such a multiplicity of filaments or fibers is 
sometimes hereinafter referred to as "the fibrous material." 
Briefly, the novel coating apparatus includes a furnace, 
a straight, elongated furnace tube extending through the furnace, which 
furnace tube is formed with 
a uniform inside diameter of sufficiently large size to receive a tool for 
periodic cleaning while the apparatus is in use, 
an unconstricted outlet, 
an ambient atmosphere-excluding (preferably constricted) inlet through 
which said fibrous material enters, and 
intake means for receiving a gaseous mixture comprising one or more 
reagents that can coat said fibrous material by CVD. 
The apparatus permits a long length of inorganic fibrous material to be 
carried continuously through the gaseous mixture, the residue of which is 
freely exhausted through the unconstricted outlet. At the unconstricted 
outlet, back-diffusion of the air is kept at acceptable levels by the 
flux. The furnace tube can be fitted with liner that is easily replaced 
simply by pulling it through the outlet. This allows deposits of fuzz and 
soot to be periodically eliminated without interruption of the coating 
process and without disturbing either the inlet for the fibrous material 
or the intake means. 
Preferably the novel coating apparatus is operated at atmospheric pressure, 
thus eliminating the need for the supply and takeup reels to be contiguous 
with the furnace tube and making it easier to remove a liner. The 
in-diffusion of air through the fiber inlet can be kept at acceptable 
levels by using a long small-diameter inlet tubing. When that inlet-tubing 
has a uniform, slot-like cross-sectional area, a plurality of tows, 
rovings or yarns can be carried side-by-side through the inlet tubing 
without crowding. A slot-like interior is also useful for coating strips 
of woven fabric. 
The invention also provides a method of continuously applying CVD coatings 
onto a long moving length of inorganic fibrous material. The method 
comprises the steps of: 
a. while excluding the atmosphere, continuously carrying the fibrous 
material lengthwise at atmospheric pressure through a heated gaseous 
mixture comprising one or more reagents that deposit CVD coatings, 
b. freely exhausting the residue of the gaseous mixture along the path of 
the fibrous material in the direction of its movement, 
c. maintaining the fibrous material within the gaseous mixture for a time 
to deposit a CVD coating onto the moving fibrous material, and 
d. removing the coated fibrous material from the gaseous mixture. 
To enhance the uniformity of the coating thickness distribution, one 
operates at low temperatures and low partial pressures of the reactants. 
These conditions favor surface reaction (heterogeneous growth) as opposed 
to reactions that involve ready nucleation and growth of particles in the 
gas phase (homogeneous reactions). Such conditions entail relatively low 
deposition rates and thus limit the speed at which the fibrous material 
may be pulled through the reactor while still attaining the desired 
coating thickness. There is thus a compromise between economics and 
coating thickness uniformity. 
For identical deposition conditions, the coating thickness distribution is 
more uniform on tows than on fabrics, because the filaments are more 
constrained in the latter. Nevertheless, as will be demonstrated in 
Example 36, even in fabrics one can completely cover each filament with a 
CVD deposit. 
When the CVD employs two reagents that react readily with each other and so 
should be separated until they are in position to be deposited on the 
fibrous material, there preferably is a separate intake for each of those 
reagents and an additional port between those two intakes for receiving 
inert gas. This keeps the reagents separated until they have been heated 
in the furnace tube to the deposition temperature. 
For easy cleaning, the inner wall of the furnace tube preferably is 
cylindrical. To receive a cleaning tool without undue danger of damaging 
the fibrous material, the inside diameter of a cylindrical furnace tube or 
its liner preferably is not less than one cm, and more preferably its 
inside diameter is at least 2 cm. On the other hand, its inside diameter 
preferably does not exceed 5 cm, because substantially larger diameters 
may require a wasteful increase in flux. Instead of being cylindrical, 
tubes with different cross-sectional shapes, e.g., rectangular, may be 
used. 
The length of the furnace tube preferably is from 25 to 50 cm. 
Substantially longer furnace tubes would make the removal of fuzz and soot 
difficult. In shorter furnace tubes, some CVD reactions may not yet have 
reached equilibrium at the outlet, and thus the maximum possible deposit 
may not yet have formed. 
Fibrous materials that can be coated in the novel apparatus are inorganic 
and include ceramic, carbon, and other high-performance fibers. CVD 
barrier coatings that can be applied to fibrous materials in the novel 
apparatus include ceramics, carbon and metals. 
Barrier-coated fibrous materials of the invention have utility in 
toughening inorganic and organic matrices derived from metals, ceramics, 
glasses, carbon, and polymers. The barrier coatings can improve wetting of 
the fibrous materials by the matrix, establish a favorable bond strength 
between the fibrous materials and matrix to further toughen the composite, 
prevent chemical reaction between fibrous materials and matrix, and 
prevent dissolution of the fibrous materials in the matrix.

DETAILED DESCRIPTION OF THE INVENTION 
FIG. 1 shows a simple APCVD apparatus 10 that includes a cylindrical 
furnace 11 and a straight, elongated cylindrical quartz or mullite furnace 
tube 12 extending beyond the ends of the furnace. The furnace tube 12 has 
a uniform diameter throughout its length and is fitted with a cap 14 at 
its fiber-receiving inlet. Fitted into the cap is a long, cylindrical, 
small-diameter inlet tubing 16, and aligned with the inlet tubing is a 
supply roll 18 of a continuous tow 20. The inlet tubing 16 has a uniform 
inside diameter throughout its length and (except being flared outwardly 
at its entrance 22) is barely large enough to receive the tow. The furnace 
tube is unconstricted at its outlet 26. Upon exiting from the furnace 
tube, the tow 20 is drawn across a thread guide 23 and to a take-up roll 
24. 
The cap 14 is formed with a conduit 28 that serves as an intake for a 
gaseous mixture including a reagent. The cap also is formed with a second 
conduit 30 that serves as a port to feed a carrier gas such as argon into 
the furnace tube 12, which carrier gas may include additional reagent. The 
total flux, when subjected to the high temperature of the furnace 11, 
deposits a continuous coating by CVD onto the filaments of the moving tow 
20. The flux exiting through the unconstricted outlet 26 should be 
sufficient to minimize the back diffusion of air such that non-oxide 
coatings can be attained with acceptable levels of oxygen contamination. 
Deposition takes place not only on the tow 20 but also on the walls of the 
furnace tube 12. As the cross-sectional area of the furnace tube 12 
gradually decreases, the tow rubs more-and-more against the deposit, 
causing breakage of ever increasing numbers of the filaments until finally 
the tow breaks entirely, necessitating replacement of the furnace tube. 
However, to maintain acceptable reproducibility in the properties of the 
coatings, reproducible gas flow characteristics must also be maintained. 
It is therefore desirable to change the furnace tube before it becomes 
clogged and the fibrous material breaks. To avoid having to reattach the 
inlets, a removable liner 58 may be used as shown in FIG. 4. 
Depending on the fibrous material used and on the coating conditions 
(temperature, gas composition, contact with fuzz, and the deposit on the 
wall of the reactor), some filaments break off the tow and form a 
"fuzzball". Also accumulating in the furnace tube are soot particles that 
form by homogeneous nucleation and growth. Many of these soot particles 
are carried out by the flux, but others deposit on the wall of the furnace 
tube and on the fuzzball, densifying the latter. If left in the furnace 
tube, the fuzzball and soot deplete the gaseous reactants, become cemented 
together by CVD, and increasingly rub against the fibrous material. This 
causes breakage of additional filaments and eventually of the whole 
fibrous material. The elongated tools of FIGS. 2 and 3 are effective for 
removal of the fuzzballs and soot. 
FIG. 2 shows an elongated tool 34 that can be used to clean the interior of 
the furnace tube 12. The cleaning tool 34 includes a long rod 35, to the 
end of which is fixed a semi-circular flange 36, the diameter of which is 
smaller than the inside diameter of the furnace tube. The flange 36 is 
semi-circular so that the tow 20 can continue to be drawn through the 
furnace tube while it is being cleaned. The cleaning tool 34 preferably is 
formed of fused silica. 
Another useful cleaning tool 37, as shown in FIG. 3, is formed from a 
quartz rod to have a flag-shaped projection 38 which is advantageous to 
use when constriction of the cross-sectional area of the of the furnace 
tube (or its liner) precludes the use of the tool 34. 
For more demanding reactions, the equipment shown in FIG. 4 is substituted 
into the apparatus of FIG. 1. Fitted over the furnace tube 12 is a cap 40 
into which are fitted three coaxial sleeves 42, 43 and 44. The outer and 
inner sleeves 42 and 44 are formed with conduits 46 and 48, respectively, 
each for receiving a gaseous mixture of a reactant. The central sleeve 43 
has a conduit 47 for receiving inert gas. 
Fitted within the inner sleeve 44 is a long, small-diameter cylindrical 
inlet tubing 50 that is flared outwardly at its entrance 52. The inlet 
tubing 50 has a uniform inside diameter barely larger than the tow 20 
which is being carried through the APCVD apparatus as modified in FIG. 4. 
Each of the cap 40 and the inlet tubing 50 is formed with a conduit 56 and 
57, respectively, for receiving inert gas. 
In a modified version of the apparatus shown in FIG. 4 that has been used 
experimentally, the sleeve 42 was omitted and the second reagent was added 
through the conduit 56. 
The apparatus of FIG. 4 also differs from that of FIG. 1 by a liner 58 that 
is fitted into the furnace tube 12. The liner is easily replaced and 
deposits are thus eliminated. 
As illustrated in FIG. 5, one may introduce a low-vapor-pressure liquid, by 
the use of a circulating pump, directly into the furnace tube 12 of FIG. 1 
through an arm 60 of a tube 62 that is fitted into a cap 63. The addition 
of a carrier gas through an inlet 64 of the tube 62 prevents 
discontinuous, dropwise addition of the reactant. To prevent the buildup 
of a "puddle" below the exit 66 of a fiber-receiving tubing 68, it is 
advantageous to tilt the apparatus a few degrees as illustrated. 
It is evident to those skilled in the art that a solid reagent may be 
introduced by an auger. 
FIG. 6 shows heat-treating or APCVD coating apparatus 70 that include a 
furnace 71 containing a furnace tube 72 in-line with the furnace tube 12 
and liner 58 of FIG. 4. When exposure of the fibrous material to the 
atmosphere between the two furnaces is undesirable, a long transfer tubing 
75 is used, and it has gas inlet arms 78 and 79 that serve to establish 
the desired atmosphere. The uniform cross-sectional area of the transfer 
tubing is barely large enough to receive the tow 20 (the path of which is 
indicated by a phantom line). The length of the transfer tubing 75 is 
selected so that there is sufficient space to allow cleaning. 
The gap between the liner 58 and the transfer tubing 75 is kept small to 
restrict exposure of the fibrous material to the atmosphere. To further 
minimize the exposure and to ensure against carrying gases and soot from 
the liner 58 into the transfer tubing, a conduit 80 directs a neutral gas 
into the gap. For the same reason, the arm 79 near the inlet 80 to the 
transfer tubing preferably directs its gas toward the inlet as shown. 
The inlet of furnace tube 72 is fitted with cap 74, that is formed with gas 
inlet conduit 76, through which the desired heat treatment atmosphere is 
established. To use the second furnace for CVD, conduit 76 would be 
replaced with the type illustrated as 28 in FIG. 1, thus allowing the 
discharge of one of the reagents within the furnace 71. The cap 74 is also 
fitted with a second conduit 77 (corresponding to conduit 30 in FIG. 1). 
FIGS. 7a and 7b show two views of equipment for modifying apparatus 10 of 
FIG. 1 for the coating of a continuous narrow strip of fabric or a 
plurality of side-by-side tows (not shown). Fitting through a stopper 81 
in the inlet of the furnace tube 12 is a long inlet tubing 82 having a 
uniform slot-like cross-sectional area, except being flared outwardly at 
its entrance 83. The slot-like area is barely large enough to receive 
fibrous material to be coated. Also fitting through the stopper 81 are a 
pair of conduits 84 and 85, through each of which a gaseous mixture of one 
or more CVD reagents is fed. To keep out air, a stream of inert gas is 
passed through each of a pair of arms 87 and 88 formed in the inlet tubing 
82. 
FIGS. 8-11 are discussed in connection with the working examples below. 
In most cases, fibrous material to be coated is sized. The sizing is easily 
removed by passing the fibrous material, prior to entering the CVD 
apparatus, through an open-tube furnace at a temperature sufficient to 
burn off the sizing. When it is desirable to size fibrous material after 
it has been coated by CVD, this can be done continuously between the 
coater and the takeup spool. 
In some cases, the fibrous material to be coated should be protected from 
the CVD atmosphere. In case of NEXTEL.RTM. 480, this can be accomplished 
by depositing a carbon coating during the preparation of the fiber. 
Alternatively, sizing may be pyrolized in an inert atmosphere. If a 
thicker carbon subcoat is desired, a gaseous carbon source may be added to 
the inert gas stream. A carbon subcoat may also have the additional 
advantage of establishing a favorable bond strength between the fibrous 
material and the CVD coating. Subcoats other than carbon (e.g., BN) can 
provide the same advantages. 
Penetration of the reagents between the filaments can be beneficially 
affected by pulsing the gas flow. This is accomplished by passing at least 
one of the gas streams through a valve that can be set, e.g., at 
one-second-on/one-second-off, such as valve No. 52C19T34-8 available from 
Valcor Engineering Corp., of Springfield, N.J. 
It is evident to those skilled in the art that the properties of the 
coatings can be varied by changing the composition of the reactants, their 
partial pressures and flow rates, and the deposition temperature. 
Materials with narrow homogeneity regions, such as SiC, can be deposited 
with or without excess silicon or carbon by the suitable choice of 
temperature and of the Si/C/H ratio in the reactant gas stream. See, for 
example, H. J. Kim & R. F. Davis, J. Appl. Phys., Vol. 60, p. 2897 (1986). 
In the case of coatings with a wide homogeneity range, such as Ti.sub.x 
C.sub.1-x, the value of x can be predetermined by the suitable choice of 
temperature and of the partial pressures of the reactants. See F. 
Teyssandier, et al., J. Electrochem. Soc., Vol. 165, p. 225 (1988). 
The temperature range of the process can be lowered by using highly 
reactive gas(es) for example, SiH.sub.4 or BH.sub.3 as the sources for 
silicon and boron respectively. 
The coatings deposited at low temperatures tend to be amorphous. If highly 
crystalline coatings are desired, higher temperatures should be employed. 
In the case of highly reactive gases, this leads to homogeneous nucleation 
and growth, i.e. to poor quality coatings and low yield. Hence it is 
advantageous to use less-reactive gases. For example, in the case of 
carbon, CH.sub.4 may be used instead of unsaturated or higher molecular 
weight hydrocarbons. However, if too high a temperature is used, grain 
growth can take place in both the fiber and the coating, and this can 
adversely affect the mechanical properties. Also, other variables being 
constant, the number of filaments that break during the coating process 
increases with increasing temperature. 
Using the process taught in this invention, coatings can be deposited on 
any fibrous material that is stable in the CVD gas stream at the 
deposition temperature. In the case of corrosive reactants, a thin precoat 
of a non-reactive material as a protective barrier allows the deposition 
of the desired coating. 
As examples of the practice of the present invention, the following fibers 
were coated: aluminum borosilicate and mullite fibers, specifically the 
family of NEXTEL.RTM. fibers available from 3M Company, alumina fiber 
SV-01-lK available from Sumitomo; an alumina-zirconia fiber PRD166 
available from E. I. duPont; and a carbon fiber AS4 G-12K available from 
Hercules. 
Any suitable inert gas such as nitrogen, helium, argon, and neon can be 
used as the carrier gas. 
Useful classes of refractory barrier coatings for the present invention 
include oxides, carbides, borides, nitrides, silicides, carbon and metals. 
From the above discussion it can be seen by one skilled in the art that the 
properties of the barrier coatings (thickness, composition, crystallinity) 
can be varied at will, and that for a given set of coating properties, 
deposition conditions can be selected to minimize materials and process 
costs. 
The techniques that were used for the evaluation of the barrier coatings 
are illustrated below: 
Characterization Methods 
To evaluate the usefulness of the present invention, it is necessary to 
determine the coating thickness distribution, the grain structures of the 
coatings, and the strengths of the coated fibers. The following 
characterization techniques were used: 
Optical 
Coating quality on transparent fiber was qualitatively determined by 
optical microscopy. The coated filaments were examined in cross-polarized 
light, preferably in the index oil corresponding to their refractive 
index. The coatings caused bright lines to appear at the two edges of the 
filaments. These lines were brightest at 45.degree. orientations to the 
planes of polarization. The continuity and intensity of the lines is a 
qualitative indication of coating quality. Color of the lines, being due 
to interference between light reflected at the filament-coating interface 
and at the surface of the coating, allowed the qualitative calculation of 
the coating thickness. 
A measure of coating thickness uniformity was obtained in the case of 
colored coatings on transparent fibers by measuring the light transmission 
through a number of filaments in an index oil matched to their refractive 
index. This approach assumes the absorption is proportional to the coating 
thickness. The measurements were made using the Zeiss PMl photometer with 
a circular aperture of 630 micrometers. The 100% transmission was set in a 
filament-free area. The percent transmission was plotted as a histogram. 
It shows the degree of coating thickness control obtainable by the present 
invention. 
Mechanical Test Methods 
The mechanical test results were determined using a Sintech Inc. 
(Stoughton, Mass.) computer-controlled load frame. A 1000-lb load cell, 
equipped with Instron Model No. 2712-003 pneumatic grips with rubber 
coated faces (Instron catalog #2702-015), was used for the tensile testing 
of the fibrous material. The gauge length for "Strand Strength" 
measurements was 15.24 cm. "Bend Strength" determinations were made using 
fiber samples which were 7.6 cm in length, bending them around a 1.27 mm 
diameter rod, and then applying tension. 
Other Methods 
SEM was used for the determination of the morphology of the coatings. 
Examination of fracture cross-sections allowed estimation of coating 
thickness. Observation of the degree of spalling, if any, adjacent to the 
fracture gave a qualitative indication of bond strength between fiber and 
coating. 
TEM allowed the determination of crystal and grain structure. 
Depth profiles obtained by Auger electron spectroscopy revealed the 
composition and, if any, the change in composition through the coating. 
ESCA was used for the determination of composition and bonding in and near 
the surface. Due to the inability of the technique of focusing at a single 
filament, it cannot give depth profiles. 
Elements in the coating that do not form part of the fiber were determined 
by standard techniques, e.g., by ICP or for carbon and nitrogen by the 
LECO method. 
The electrical resistance was roughly determined by contacting the coated 
fibers with two probes. 
Comparisons of results obtained on the same sample by various methods could 
in many cases be used for the estimation of their accuracy. 
Various modifications and alterations cf this invention will become 
apparent to those skilled in the art without departing from the scope and 
spirit of this invention. Therefore, it should be understood that this 
invention is not to be limited to the following examples in which parts 
are given by weight. 
EXAMPLES 
Example 1 
In this example, the apparatus was like that of FIG. 1 having 
length of furnace 11: 30 cm 
inside diameter of furnace tube 12: 2.5 cm 
inside diameter of inlet tubing 16: 0.4 cm 
Hydrogen (190 cc/min) was bubbled through boiling (CH.sub.3).sub.2 
SiCl.sub.2 to produce a gaseous mixture that was passed through a 
20.degree. C. reflux condenser and then through conduit 28 of FIG. 1 
(called "Flux 1" in Table I). Argon (2100 cc/min) was passed through 
conduit 30 (called "Flux 2" in Table I). The apparatus was used to coat a 
2000-denier, 760-filament tow (mullite with a 2% B.sub.2 O.sub.3 content 
available as NEXTEL.RTM. 480) that had a 0.2% carbon coating. This tow was 
pulled through the furnace tube (kept at 1050.degree. C.) at 37 cm/min to 
provide a CVD coating about 100 nm in thickness as measured by electron 
microscopy and verified by calculations based on elemental analysis of 
carbon. 
The resulting filaments of the tow had infinite electrical resistance. 
Their barrier coatings of SiC afforded a golden color to the filaments. 
Uniform orange-colored lines observed under cross-polarized light 
indicated a uniform coating thickness of about 130 nm. The coating 
thickness distribution, as shown by a histogram (FIG. 8) based on light 
absorption, was reasonably narrow. The composition of the CVD coating, as 
indicated by an Auger depth profile, was stoichiometric. A TEM micrograph 
(FIG. 9) showed most of the grains to be 1 to 3 nm in size. 
Etching away the mullite cores with aqueous HF allowed the hulls to be 
observed (FIG. 10). 
Examples 2-23 
Lengths of the same carbon-coated tow used in Example 1 were provided with 
different CVD barrier coatings by the procedure of Example 1 except as 
indicated in Table I and except that Examples 10, 19 and 22 were coated 
using a concentric sleeve apparatus similar to that of FIG. 4; Examples 6 
and 23 were coated using apparatus as shown in FIG. 5, with the xylene and 
Zr(BuO).sub.4 added as liquids. In Examples 14-16 and 31, TFAA denotes 
trifluoroacetic anhydride. About half of the filaments of Examples 3 and 4 
that were coated with BN had white edges, thus indicating a thickness less 
than 170 nm. The other half had yellow edges, indicating a thickness 
exceeding 170 nm. 
The last column in TABLE I gives the weight precent carbon that was 
deposited onto the fiber during its manufacture. 
Examples 24-33 
Most of the barrier coatings of Examples 1-23 caused deterioration in 
strength, especially in bend strength. However, undercoating with BN 
and/or with carbon counteracted the deterioration. Hence, lengths of the 
tow used in Example 1 were provided with CVD barrier coatings in the same 
manner as in Example 1 except that they were passed through the apparatus 
more than once. The deposition conditions of the multiple coatings are 
listed in Table II, with the outer coating listed first. 
The effects of BN and carbon undercoats on the bend strengths of SiC-coated 
NEXTEL.RTM. 480 fibers were charted in FIG. 11 as curves 90 and 92, 
respectively. To generate the curve 90, the thickness of the BN layer was 
adjusted by changing the pulling speed between 0.73 and 3.54 m/min. It may 
be assumed that the coating thickness is an inverse function of the 
pulling speed. The pulling speed was likewise changed to provide fibers 
with varying weight percents of carbon as indicated by the curve 92. 
Examples 34-35 
Again proceeding as in Example 1, the simultaneous deposition of more than 
one phase yielded composite coatings as indicated in Table II. In Example 
35 a 4000 denier NEXTEL.RTM. 480 was coated. 
Heat-Treatment 
When each of the tows of Examples 2, 24 and 34 was coated, it was carried 
in-line through a second furnace like that of FIG. 6 (80 cm long, 
1150.degree. C.) through which a stream of nitrogen was flowing. The 
resulting tows had improved resistance to hydrolysis as compared to tows 
that had been CVD coated in the same way except without heat-treatment. By 
performing heat treatments in oxidizing atmospheres, e.g., in air, 
oxynitrides were prepared. There was variation in the strength values of 
the various batches of fibers that were used as substrates. 
Properties of the coated fibers of Examples 1-35 are listed in Table III. 
Most of the strength figures are averages of five measurements. An entry 
0.0 indicates that the fiber was too weak to allow meaningful measurements 
to be made. The deviations in strand strength and bend strength refer to 
the change in strength relative to the uncoated tow, except in case of the 
multiple coatings where the values prior to the deposition of the 
outermost layer were the reference (in Examples 32 and 33, the changes 
from the uncoated tow are listed). 
TABLE I 
__________________________________________________________________________ 
CARBON 
FLUX 1 FLUX 2 SUBCOAT 
EXPL. 
COATING cm.sup.3 /min 
cm.sup.3 /min 
cm.sup.3 /min 
cm.sup.3 /min 
.degree.C. 
cm/min 
% 
__________________________________________________________________________ 
1 SiC (CH.sub.3).sub.2 SiCl.sub.2 
30 H.sub.2 
190 Ar 2100 1050 
37 0.2 
2 AlN AlCl.sub.3 
40 H.sub.2 
190 NH.sub.3 
1060 N.sub.2 
1900 700 91 0.2 
3 BN Et.sub.3 B 
100 N.sub.2 
1670 NH.sub.3 
1650 N.sub.2 
425 1050 
73 None 
4 BN Et.sub.3 B 
100 N.sub.2 
1670 NH.sub.3 
1650 N.sub.2 
425 1050 
354 None 
5 B.sub.4 C 
BCl.sub.3 
116 Ar 650 Et.sub.3 B 
1212 H.sub.2 
1350 1100 
91 0.1 
6 C xylene 60 N.sub.2 
1100 N.sub.2 
400 1000 
107 None 
7 Mo MoCl.sub. 5 
24 Ar 590 H.sub.2 
1000 Ar 280 700 30 0.2 
8 MoSi.sub.2 
MoCl.sub.5 
20 Ar 500 SiCl.sub.4 
200 Ar 810 1000 
30 0.2 
9 SiO.sub.2 
Si(OEt).sub.4 
23 N.sub.2 
670 700 30 None 
10 Si.sub.3 N.sub.4 
SiCl 83 N.sub.2 
2200 NH.sub.3 
760 1100 
137 None 
11 SnO.sub.2 
(CH.sub.3).sub.4 Sn 
42 Ar 420 air 1670 500 61 None 
12 SnO.sub.2 
(CH.sub.3).sub.4 Sn 
42 Ar 420 air 1670 500 122 None 
13 SnO.sub.2 
(CH.sub.3).sub.4 Sn 
42 Ar 420 air 1670 500 30 0.5 
14 SnO.sub.2 + F 
(CH.sub.3).sub.4 Sn 
42 Ar 470 air 1670 TFAA 
6 500 61 None 
15 SnO.sub.2 + F 
(CH.sub.3).sub.4 Sn 
42 Ar 470 air 1670 TFAA 
6 500 122 None 
16 SnO.sub.2 + F 
(CH.sub.3).sub.4 Sn 
42 Ar 470 air 1670 TFAA 
6 500 30 0.5 
17 TaN.sub.2 
TaCl.sub.5 
7 N.sub.2 
720 H.sub.2 
1000 800 85 0.2 
18 TiB.sub.2 
TiCl.sub.4 
40 H.sub.2 
480 Et.sub.3 B 
80 Ar 1350 900 91 0.1 
19 TiB.sub.2 
TiCl.sub.4 
180 Ar 1600 BCl.sub.3 
140 H.sub.2 
1400 1000 
354 0.5 
20 TiN TiCl.sub.4 
32 H.sub.2 
3200 N.sub.2 
1200 1000 
183 0.2 
21 TiN TiCl.sub.4 
32 H.sub.2 
3200 N.sub.2 
1200 NH.sub.3 
40 1000 
183 0.2 
22 ZrN ZrCl.sub.4 
126 N.sub.2 
740 NH.sub.3 
490 N.sub.2 
820 1000 
91 0.1 
23 ZrO.sub.2 
Zr(BuO).sub.4 
30 N.sub.2 
1420 CO.sub.2 
490 500 61 0.1 
__________________________________________________________________________ 
TABLE II 
__________________________________________________________________________ 
CARBON 
FLUX 1 FLUX 2 SUBCOAT 
EXPL. cm.sup.3 /min cm.sup.3 /min 
cm.sup.3 /min 
cm.sup.3 /min 
.degree.C. 
cm/min 
% 
__________________________________________________________________________ 
MULTIPLE COATINGS 
24 AlN AlCl.sub.3 
40 H.sub.2 
190 NH.sub.3 
1060 N.sub.2 
1900 700 
91 
BN Et.sub.3 B 
100 N.sub.2 
1670 NH.sub.3 
1650 N.sub.2 
425 1050 
150 0.1 
25 C xylene 60 N.sub.2 
1100 1000 
122 
BN Et.sub.3 B 
100 N.sub.2 
1670 NH.sub.3 
1650 N.sub.2 
425 1050 
150 0.1 
26 SiC (CH.sub.3).sub.2 SiCl.sub.2 
30 H.sub.2 
190 Ar 2100 1050 
37 
BN Et.sub.3 B 
100 N.sub.2 
1670 NH.sub.3 
1650 N.sub.2 
425 1050 
73 0.0 
27 SiC (CH.sub.3).sub.2 SiCl.sub.2 
30 H.sub.2 
190 Ar 2100 1050 
259 
BN Et.sub.3 B 
100 N.sub.2 
1670 NH.sub.3 
1650 N.sub.2 
425 1050 
107 0.0 
SiC (CH.sub.3).sub.2 SiCl.sub.2 
30 H.sub.2 
190 Ar 2100 1050 
37 
28 SiC (CH.sub.3).sub.2 SiCl.sub.2 
30 H.sub.2 
190 Ar 2100 1050 
37 0.2 
C xylene 25 N.sub.2 
1100 N.sub.2 
380 1000 
107 
29 Si.sub.3 N.sub.4 
SiCl.sub.4 
83 N.sub.2 
2200 NH.sub.3 
260 1000 
137 
BN NH.sub.3 
490 BCl.sub.3 
50 N.sub.2 
1900 1000 
150 None 
30 SnO.sub.2 
(CH.sub.3).sub.4 Sn 
42 Ar 420 air 1670 500 
30 
BN Et.sub.3 B 
100 N.sub.2 
1670 NH.sub.3 
1650 N.sub.2 
425 1050 
90 None 
31 SnO.sub.2 + F 
(CH.sub.3).sub.4 Sn 
42 Ar 470 air 1670 TFAA 
20 500 
30 
BN Et.sub.3 B 
100 N.sub.2 
1670 NH.sub.3 
1650 N.sub.2 
425 1050 
150 None 
32 TiB.sub.2 
TiCl.sub.4 
180 Ar 1600 BCl.sub.3 
140 1000 
354 
C xylene 29 N.sub.2 
1100 N.sub.2 
380 1000 
305 
BN Et.sub.3 B 
100 N.sub.2 
1670 NH.sub.3 
1650 N.sub.2 
425 1050 
150 0.1 
33 TiC TiCl.sub.4 
36 H.sub.2 
680 CH.sub.4 
46 Ar 2270 1100 
91 
BN Et.sub.3 B 
100 N.sub.2 
1670 NH.sub.3 
1650 N.sub.2 
425 1050 
90 0.1 
COMPOSITE COATINGS 
34 AlN--BN 
Et.sub.3 B 
126 N.sub.2 
2100 AlCl.sub.3 
18 N.sub.2 
420 800 
61 0.1 
NH.sub.3 
1060 
35 SiC--TiC 
CH.sub.3 SiCl.sub.3 
30 N.sub.2 
1700 TiCl.sub.4 
50 CH.sub.4 
40 1150 
73 0.1 
__________________________________________________________________________ 
TABLE III 
__________________________________________________________________________ 
ELECTRICAL 
STRAND BEND 
RESISTANCE 
STRENGTH 
DEV. STRENGTH 
DEV. 
EXPL COLOR 
(k .OMEGA./m) 
(kg) (%) (kg) (%) 
__________________________________________________________________________ 
SINGLE COATING 
1 SiC golden 
.infin. 2.4 -33 0.3 -92 
2 AlN *grey 
9000 3.9 -30 1.6 -48 
3 BN straw 
.infin. 4.2 -4 3.2 -4 
4 BN straw 
.infin. 3.2 -27 3.0 -12 
5 B.sub.4 C 
black 
3 .times. 10.sup.4 
1.7 -64 1.0 -74 
6 C black 
76 4.2 -5 3.6 7 
7 Mo black 
2.4 4.7 -16 2.7 45 
8 MoSi.sub.2 
black 
400 4.1 -28 1.9 2 
9 SiO.sub.2 
white 
.infin. 2.9 2.2 -30 
10 Si.sub.3 N.sub.4 
white 
.infin. 1.2 1.2 
11 SnO.sub.2 
*golden 
32 1.9 -56 0.0 
12 SnO.sub.2 
white 
10 2.2 -48 0.5 -86 
13 SnO.sub.2 
black 
32 3.1 -36 0.5 -91 
14 SnO.sub.2 + F 
*olive 
2.4 1.8 -57 0.0 
15 SnO.sub.2 + F 
white 
10 2.1 -51 0.5 -81 
16 SnO.sub.2 + F 
black 
1.4 4.2 -15 0.5 -86 
17 Ta.sub.2 N 
black 
5.6 5.9 0 1.7 -7 
18 TiB.sub.2 
black 
8 5.9 28 2.8 -30 
19 TiB.sub.2 
black 
4 4.9 0 0.7 -70 
20 TiN black 
170-6700 
2.6 -52 1.0 -75 
21 TiN golden 
1.1 0.0 0.0 
22 ZrN olive 
.infin. 5.1 0 1.8 
23 ZrO.sub.2 
white 
.infin. 3.8 -20 3.2 10 
MULTIPLE COATINGS 
24 AlN/BN grey .infin. 4.5 -3 2.4 -34 
25 C/BN black 
28 5.6 21 2.7 -32 
26 SiC/BN golden 
.infin. 4.2 0 1.3 -59 
27 SiC/BN/SiC 
golden 
.infin. 4.2 -21 2.4 -11 
28 SiC/C black 
160 3.2 -27 0.8 -75 
29 Si.sub.3 N.sub.4 /BN 
white 
.infin. 1.2 1.2 
30 SnO.sub.2 /BN 
*golden 
36 2.0 -25 0.5 -84 
31 SnO.sub.2 + F/BN 
olive 
1.4 2.3 -7 0.5 -84 
32 TiB.sub.2 /C/BN 
black 
4 4.6 0 3.0 0 
33 TiC/BN black 
6 5.4 -10 4.4 76 
COMPOSITE COATINGS 
34 AlN--BN *grey 
.infin. 4.5 -12 3.1 35 
35 SiC--TiC 
grey 1 .times. 10.sup.4 
7.6 -25 1.7 -68 
__________________________________________________________________________ 
*light color 
Example 36 
With the apparatus of FIG. 1 modified as in FIG. 7a and 7b, the procedure 
of Example 1 (with the changes noted) was used to coat a 2.5 cm wide woven 
borosilicate woven fabric (NEXTEL.RTM. 440) at a speed of 30 cm/min. Argon 
was passed through a solenoid value (one-second-on, one-second-off) and 
then through the conduit 84 at a rate of 2140 ml/min. Into the conduit 85 
was fed a gaseous mixture of (CH.sub.3).sub.2 SiCl.sub.2 (120 ml/min) and 
hydrogen (720 ml/min). The temperature in the reaction zone was maintained 
at 1050.degree. C. The tape inlet tubing 82 was purged with streams of 
argon to keep air out of the reactor. 
The SiC-coated fabric was golden colored. In cross-polarized light, all 
filaments of the fabric showed orange lines. They were all completely 
coated.