Composite article and method of making same

A method of making a composite article and a composite article specifically adapted for use in high temperature, corrosive and errosive environments comprising a carbon fibrous substrate, including a pyrolytic carbon sheath formed about each fiber of the substrate; a metallic carbide, oxide, or nitride compliant coating over the coated fibers of the substrate; and an impermeable metallic carbide, oxide or nitride outer protective layer formed about the entire periphery of the coated substrate. In accordance with the method of the invention, the compliant metallic coating is applied to the fibers in a manner such that any mechanical stresses built-up in the substrate due to a mismatch between the coefficient of thermal expansion of the fibrous substrate and the coating are effectively accommodated.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates generally to carbon or ceramic-metallic 
composite articles and the method of making same. More particularly the 
invention relates to unique carbon-silicon composite articles for use in 
high temperature, hostile fluid environments. 
2. Discussion of the Prior Art 
During recent years considerable effort has been directed toward the search 
for structural materials having increasingly high temperature 
capabilities, and superior dimensional stability, corrosion resistance, 
erosion resistance and tolerance to damage. In this connection substantial 
work has been done with metals, monolithic ceramics and carbon-graphite 
materials. Metals display toughness and tolerance to damage but are 
relatively limited in their temperature capability. Monolithic ceramics 
can withstand high temperatures and oxidizing environments, but are 
vulnerable to structural damage. Carbon-graphite composites, on the other 
hand, can withstand high temperature and structural damage but are subject 
to oxidative degredation. In view of these facts the development of new 
composite materials has commanded considerable attention. Since composites 
can combine many of the attractive features of metals while ameliorating 
various of the structural and degradation problems associated with carbons 
and ceramics, they are ideally suited for very high temperature, hostile 
environment applications. 
In pursuit of tough high temperature composites, various types of coating 
processes have been suggested. These processes generally involve 
contacting a molten metal and a carbon body under certain conditions, 
generally for the purpose of producing protective coatings for the carbon 
body in its environment of intended use. For example, Smiley U.S. Pat. No. 
3,019,128 discloses applying molten metal to a carbon body to form a metal 
carbide surface layer, which in combination with metal and metal oxide 
layers, produces a refractory and heat transfer coating desirable on 
rocket nozzles and the like. 
Similarly, Gurinsky U.S. Pat. No. 2,910,379 discloses a process in which 
molten metal is applied to a carbon liquid nuclear fuel container to 
prevent deleterious poisoning arising from graphite reaction with nuclear 
fuels and fission products. 
Other coating disclosures involving molten metal-carbon body contact are 
Steinburg U.S. Pat. No. 2,929,741, Winter U.S. Pat. No. 2,597,964 and 
Acheson U.S. Pat. No. 895,531. 
The U.S. Pat. to Fatzer et al, No. 3,925,577 describes a process for 
producing coated isotropic graphite members wherein a layer of silicon is 
first deposited on a graphite body and then the body is heated to a 
temperature to cause the silicon to melt and penetrate the pores of the 
graphite. Finally the article is coated with a layer of silicon carbide. 
In Hacke U.S. Pat. No. 3,348,967 a somewhat similar process is described 
in which graphite or charcoal bodies are impregnated with a molten metal 
which will react therewith to form carbide, thereby enabling the 
production of a wide variety of useful products. As will become clear from 
the discussion which follows, these prior art patents while generally 
related to the present invention are clearly distinguishable therefrom. 
A common thread running through many of the prior art disclosures 
concerning metallic coating of carbonaceous materials, and one which 
serves to clearly distinguish the present invention, has been the 
heretofore unquestioned acceptance of the basic premise that the substrate 
material should be isotropic and that it must have an expansion 
coefficient approximating that of the coating. This has been traditionally 
believed necessary to prevent cracking and spalling of the coating due to 
stresses induced by differences in the expansion coefficients when the 
article is subjected to thermal cycling. The U.S. Pat. No. to Howard et al 
3,393,085 discusses this premise in some detail. 
Another well established prior art premise was that the metallic coatings 
should have an adherent mechanical or chemical bond to the substrate 
material to assure proper load transfer and to guarantee the structural 
integrity of the coated composite system. As will be discussed in greater 
detail hereinafter, Applicant has also found this latter requirement to be 
not only unnecessary, but, in fact undesirable in the practice of his 
invention. 
By way of example and to illustrate the aforementioned prior art concepts, 
graphite susceptions have long been used to heat silicon wafers for 
semiconductor processing. Because graphite is very porous and provides a 
means of entrapping undesirable gases and other contaminants, the 
susceptors are typically coated with a chemical vapor deposited (CVD) 
silicon carbide to render them impermeable and non-reactive. Because 
silicon carbide has a high coefficient of thermal expansion (CTE) of on 
the order of 4.5 to 5.0 in/in/.degree.C., a high expansion 4.2 
in/in/.degree.C. nominal CTE graphite is used as the substrate material to 
assure an economic susceptor life. It is well known, however, that the 
actual characteristics exhibited by graphite materials can vary 10 to 15% 
from the nominal values described in the literature. Accordingly, given 
substrate characteristics, including CTE characteristics, vary widely from 
lot to lot. As a result of these variations in substrate expansion 
coefficients, the economic life of the susceptors is quite unpredictable. 
Compounding the problem is the fact that coating life is also highly 
variable and directly relates the matching of coating and substrate 
expansion coefficients. Thus, current practice by susceptor manufacturers 
is to guarantee susceptors for not more than four complete temperature 
cycles. 
In a similar vein, various prior patents U.S. Pat. including Nos. 2,512,230 
and 1,948,382 describe composites comprising coatings of silicon carbide 
on monolythic and composite carbonaceous substrates to provide erosion 
protection for the substrate as well as interlayer support and bonding 
between the substrate and coating. In such applications it has been 
uniformly taught that the thermal expansion coefficients of the substrate 
should approximate that of the interlayer of coating if cracking or 
spalling of the interlayer coating is to be prevented. 
For the reasons previously discussed, great difficulty has been experienced 
in satisfactorily and economically manufacturing composite articles 
suitable for very high temperature applications in which the coefficient 
of thermal expansion of the coating and the substrate is matched and in 
which the coating satisfactorily adheres to the base material. 
As will be appreciated from the discussion which follows, the process of 
the present invention totally overcomes the prior art problems of coating 
adherency and CTE matching and provides a unique CTE mismatched composite 
article which will maintain its dimensional stability and will effectively 
resist corrosion even in hostile environments at very high temperatures. 
In addition to the previously identified prior art patents, applicant is 
familiar with the following U.S. Pat. Nos. which serve to vividly 
illustrate the high degree of novelty of the present invention: 3,914,508, 
3,762,644, 3,759,353, 3,676,179, 3,673,051, 3,275,467, 2,614,947, 
1,948,382. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a structurally unique 
composite article consisting of a multiphase system comprising a carbon 
fiberous substrate, a metallic carbide, oxide or nitride compliant coating 
over the fibers of the substrate and an impermeable metallic carbide, 
oxide or nitride outer protective layer or seal coat formed about the 
periphery of the coated substrate. 
It is another object of the invention to provide a composite article of the 
aforementioned character which will substantially maintain its dimensional 
stability and strength even under severe high temperature oxidizing 
conditions. 
It is another object of the invention to provide a composite article of the 
class described which is substantially resistant to corrosion and erosion 
by high temperature, hostile gas, particulate and fluid environments. 
It is a particularly important object of the present invention to provide a 
composite article of the character described in the preceeding paragraphs 
in which the aforementioned metallic carbide, oxide or nitride compliant 
coating is controllably applied to the carbon fibrous substrate in a 
manner such that any mechanical stresses built up in the substrate due to 
mismatches in coefficients of thermal expansion between the fiberous 
substrate and the coatings are effectively accomodated or relieved. 
It is a further important object of the present invention to provide a 
process for making composite articles of the character described in the 
preceeding paragraphs in which the metallic carbide, oxide or nitride 
compliant coating is controllably applied to the carbon fibrous substrate 
in a manner such that the individual fibers of the substrate are free to 
move relative to the applied coating. 
More particularly, it is an object of the invention to provide a process as 
described in the previous paragraph in which a pyrolytic carbon coating is 
first deposited by chemical vapor deposition (CVD) about each of the 
fibers in such a manner that each fiber is substantially encased in a 
non-adherent pyrolytic carbon casing and then a metallic carbide, oxide or 
nitride coating is applied over the coated fibers in such a manner that 
the fibers remain freely movable relative to the applied coatings. 
It is another object of the invention to provide a composite article as 
described in the previous paragraph in which each fiber of the substrate 
is encased in a uniform CVD type carbon casing to promote superior load 
transfer from fiber to fiber when the article is stressed. This CVD carbon 
casing also provides a mechanical interface for increasing the surface 
fracture energy of the composite structure thus resulting substantial 
toughness and flaw resistance. 
It is still another object of the invention to provide a process of the 
aforementioned character in which, following the coating of the fibers, an 
impermeable carbide, nitride or oxide coating is controllably formed about 
the entire periphery of the substrate to seal it against hostile 
environments. 
In summary, these and other objects of the invention are realized by a 
composite article produced by a method comprising the steps of forming a 
starting substrate from a multiplicity of carbon fibers selected from a 
group consisting of pyrolyzed wool, rayon, polyacrylonitrile and pitch 
fibers; suspending the starting substrate within a first controlled 
environment; forming an intermediate substrate by heating the starting 
substrate to a temperature of between approximately 1500.degree. F. and 
approximately 2200.degree. F. while exposing the starting substrate to a 
hydrocarbon gas to form a uniform layer of pyrolytic carbon about each of 
the fibers in the starting substrate; removing the intermediate substrate 
from said first controlled environment and forming it into a shaped 
substrate having the approximate shape desired of the end product 
composite article; supporting the shaped substrate in a second controlled 
environment while heating it to a temperature of between approximately 
1800.degree. F. and approximately 3200.degree. F.; and forming a diffusion 
coated article by reacting the shaped substrate with a siliceous material 
for a period of time sufficient to permit the silicon to react with the 
pyrolytic carbon coating deposited on the fibers.

DISCUSSION OF THE PREFERRED EMBODIMENTS 
Before proceeding with a detailed discussion of the preferred embodiments 
of the present invention, the following definitions of the technical terms 
used herein are presented to facilitate a clear understanding of the 
nature and scope of the invention: 
1. Composite product--a product comprising a carbon, graphite or ceramic 
substrate and one or more metallic carbide, oxide or nitride coatings over 
the substrate material. 
2. Starting or basic substrate--as used herein, the starting substrate or 
interim product shape before the application of a metallic coating. 
3. Carbon fibrous substrate--a starting substrate comprising carbon 
material in fibrous form. 
4. Fiber volume--volume of carbon fibers present in the given substrate. 
5. Non-woven--coherent fibrous material formed without interlacing of 
threads, such as batting or felt. 
6. Woven--fabric formed by interlacing warp and filling threads on a loom, 
or the like. 
7. Ceramic--metallic or other inorganic oxides generally classed as 
glass-forming oxides. 
8. Pyrolytic or "CVD" material--a material made from the thermal 
decomposition of a gas containing the material. 
Stated in simple terms, the composite article of the invention consists of 
a three or more phase system comprising a basic substrate of carbon 
fibrous and/or ceramic materials, a metallic carbide, oxide or nitride 
compliant layer over the substrate and an impermeable carbide, oxide or 
nitride protective layer formed over the entire periphery of the coated 
substrate. An important feature of the article is the absence of a strong 
bond between the fibers and the matrix system to accommodate the mismatch 
in expansion coefficient between the fibrous substrate and the carbide, 
oxide or nitride compliant and protective layers. 
The articles of the invention are well suited for a variety of applications 
including turbine rotors, turbine augmentor divergent flaps and Diesel 
engine pre-combustion chambers made up of a carbon fiber, carbon (resin 
and/or pitch char and/or CVD deposit) matrix component which has been 
preformed to a selected component net geometry. Each of the substrate 
fibers is encased in a non-adherent, uniform, CVD type carbon case so as 
to promote good load transfer from fiber to fiber when the article is 
stressed. This also provides a mechanical interface for increasing the 
surface fracture energy of the composite structure, thus resulting in 
greater toughness and flaw resistance. 
Because carbon/carbon composites actively react with oxygen when heated to 
temperatures in excess of 300.degree. C., the outer portions of the 
article are reacted with silicon to form a continuous silicon carbide case 
around each of the fibers. This treatment effectively imparts a porous 
silicon carbide shell about the periphery of the selected carbon substrate 
geometry. The expansion coefficient of this shell is essentially that of 
monolithic silicon carbide, approximately two times that of the carbon 
preform. In practice, a mis-match in expansion coefficients of only a few 
percent between the substrate and the coating or outer shell is sufficient 
to induce mechanical stresses which cause cracking and ultimate failure of 
the protective layer or oxidation of the substrate. The lack of bond 
between the fiber and the carbon and silicon carbide case, however, allows 
the carbon fibers and silicon carbide case to move independent of one 
another on a microscopic scale, thus providing a compliance mechanism that 
effectively reduces thermally induced mechanical stresses. The appearance 
of the coated fibers is clearly illustrated in FIGS. 1 and 2 of the 
drawings. 
The article in this interim state with its compliant outer shell of silicon 
carbide remains quite porous. Therefore, to provide complete protection 
for use in high temperature oxidizing environments, a subsequent 
infiltration of the porous compliant layer with impermeable CVD silicon 
carbide is applied to seal the surface with a meterial whose expansion 
coefficient is compatible with that of the compliant layer. The final 
article thus formed is remarkably stable and highly corrosion resistant 
even in extreme environments. 
Referring breifly to FIGS. 4 and 5 of the drawings, one form of article 
made by the method of the invention is there illustrated. This article, 
which is a test section of a turbine augmentor divergent flap, comprises a 
central section 12 made up of carbon felt, chopped fiber or macerated 
material. At each end of the test section is a solid end portion 14 made 
from a carbon/carbon high strength material, such as is commercially 
available from the Hitco Division of Arco. Each end section 14 is provided 
with a bore 16 adapted to receive a pivot pin or rod. 
Surrounding the central section 12 and end portions 14 is a material layer 
18 comprising a woven carbon fibrous material such as carbon or graphite 
cloth commercially available from The Union Carbide Company and others. 
Formed about the article thus constituted, and in conjunction therewith 
comprising the starting substrate of this embodiment of the invention, is 
a carbon fibrous material generally designated by the numeral 20. 
In a manner which will be described in greater detail in the paragraphs 
which follow, the individual fibers of the material 20 are coated with a 
layer, or sheath, of CVD carbon and then with a compliant layer of silicon 
carbide. Finally, the entire surface of the test specimen is sealed with 
an outer layer 22 of CVD silicon carbide which extends about the entire 
periphery of the article. 
Referring to FIG. 1, the character of the coated fibrous material 20 is 
there vividly illustrated. FIG. 1 is a reproduction of a photomicrograph 
of the coated fibrous material at 70 times magnification. Turning to FIG. 
2, which is a reproduction of a photomicrograph at 1500 times 
magnification, the fiber, the CVD coating about the fiber and the silicon 
carbide coating superimposed thereupon, are clearly visible. In FIG. 3, 
which is a reproduction of a photomicrograph taken at 2200 times 
magnification, the ends of the individual fibers can clearly be seen 
protruding from the CVD carbon coating and the compliant coating of 
silicon carbide. As will be appreciated from the examples which follow, 
the compliant silicon carbide coating is applied to the fibers in a manner 
such that any mechanical stresses built up in the substrate due to a 
mismatch between the coefficient of thermal expansion of the fibrous 
substrate and the coating are effectively accommodated. This important 
feature of the present invention is clearly illustrated in FIGS. 2 and 3 
of the drawings. 
It is to be appreciated that the article shown in FIGS. 4 and 5 of the 
drawings is merely exemplary of the type of products which can be made in 
accordance with the method of the present invention. Other highly useful 
products of the invention include turbine rotors, deisel engine combustion 
chambers, and numerous specially designed products for nuclear and 
aerospace applications. 
As will be clearly illustrated by the examples which follow, the method of 
the invention stated in simple terms comprises the following steps: First, 
a multiplicity of carbon fibers such as rayon fibers are assembled into a 
basic or starting substrate. Next the starting substrate is placed in a 
controlled environment, heated to between about 1500.degree. F. and 
4200.degree. F. and exposed to a carbonaceous gas such as methane. During 
this step a uniform layer of CVD carbon is deposited about each of the 
fibers of the substrate. Following this step the interim substrate thus 
formed is machined or otherwise formed into the approximate shape of the 
end product. Next the shaped substrate formed in the previously described 
step is placed in a second controlled environment and heated to about 
1800.degree. F. to about 3200.degree. F. The heated shaped substrate is 
then reacted with a siliceous material for a period of time sufficient to 
permit the silicon to react with the CVD carbon coating deposited on the 
fibers. Finally, the article thus formed is once again heated in a 
controlled environment and exposed to a gas containing carbon and silicon 
such as trichlorosilane. This step forms a uniform CVD seal coating of 
silicon carbide about the entire periphery of the article. 
EXAMPLE NO. 1 
Using carbonized rayon felt as a starting material, a starting substrate 
was constructed. In this instance the starting substrate was approximately 
4 inches wide, 8 inches long and about 1 inch thick. The density of the 
substrate was on the order of 0.1 gm/cc and the fiber volume was about 
15%. Next, the starting substrate, along with several control specimens, 
was placed into a first controlled environment, which in this case was a 
vacuum chemical vapor deposition furnace of conventional design. The 
temperature of the substrate was then raised to about 1800.degree. F. 
while a vacuum of on the order of 15 mm Hg was maintained within the 
deposition chamber. A secondary, or intermediate, substrate was formed by 
controllably flowing methane gas interstitially of the substrate for a 
period of time of about 50 hours. Through this technique a uniform layer 
of pyrolytic carbon was deposited about each of the fibers within the 
substrate to form an intermediate substrate having a density of on the 
order of 0.75 gm/cc. 
Following the aforementioned infiltration step, the intermediate substrate 
was cooled, removed from the vacuum furnace and transferred to a machining 
area. In the machining area the intermediate substrate was machined in a 
conventional manner to form a shaped substrate. In this instance the 
shaped substrate was constructed in the configuration of a net dimension 
turbine augmentor divergent flap test section approximately 6 inches wide 
by 6 inches long by 1/2 inch thick (see FIG. 1). 
Following the machining step, the divergent flap section, or shaped 
substrate was supported in a vacuum chamber, or second controlled 
environment directly above a crucible containing molten elemental silicon. 
The shaped substrate was then heated to about 2700.degree. F. in a mild 
vacuum. With the substrate at this elevated temperature it was lowered 
into the molten silicon and maintained totally immersed for about four 
minutes. Following immersion the diffusion coated substrate was withdrawn 
from the molten silicon and maintained at temperature in a position 
directly above the silicon crucible for a period of time of about two 
minutes. 
Next, the diffusion coated article thus formed was cooled, removed from the 
vacuum chamber and transferred to an inspection area. Following removal of 
the excess silicon by means of an etching process, precision dimensional 
inspection was performed on the part. This inspection revealed that the 
diffusion coated article exhibited the dimensions of the shaped substrate 
within plus or minus 0.002 inches. Visual inspection of the control 
specimens which had been similarly processed showed the carbon fibers to 
be essentially unaffected by the methane or silicon treatment. Only the 
ends of the fibers exposed by the machining showed any signs of reaction 
with the silicon. Importantly, no bond was found to exist between the 
carbon fibers and the silicon carbide coating formed by reacting the 
shaped substrate with the molten silicon. On the other hand, a reaction 
with the CVD carbon deposited on the fibers during the preliminary methane 
treatment was most evident. This reaction, however, was confined to the 
CVD carbon sheath surrounding each of the fibers. In all cases no chemical 
or diffusion bonds were observed to exist betweed the fiber and/or matrix 
system. Accordingly, the fibers were free to move at a different rate from 
the carbon and/or silicon carbide and silicon matrix systems. This highly 
novel and important feature of the diffusion coated article of the present 
invention effectively minimizes any residual stresses tending to occur 
within the article. By comparison, a graphite body processed in a 
comparable fashion to that just described would exhibit significant 
internal residual stresses. These stresses result from an inherent 
mismatch in the thermal expansion coefficient (CTE) between the graphite 
article itself and the coatings applied thereto. The effect of these 
stresses are often catastrophic causing article cracking, crazing and/or 
spalling of the coatings. 
Following dimensional inspection of the diffusion coated article, it was 
returned to the first controlled environment, or vacuum chemical vapor 
deposition furnace. Once in place within the CVD apparatus, the article 
was heated to about 2200.degree. F. and a gas containing 
dimethyl-dichlorosilane was controllably passed over and about the 
article. Due to the porous nature of the diffusion coated article formed 
by the novel method of the present invention, a uniform coating of CVD 
silicon carbide was deposited over the coated fibers of the article and 
about the periphery thereof. This step provided an impermeable skin of 
silicon carbide over the entire diffusion coated article rendering it 
virtually impervious to corrosion and erosion caused by high temperatures 
and exposure to hostile gas and fluid environments. Subsequent testing and 
evaluation of the two phase compliant layer type coated article thus 
formed under extremely hostile environments showed it to be highly stable 
and remarkably resistant to thermally induced cracking, crazing or 
spalling. 
EXAMPLE NO. 2 
In constructing the starting substrate of this example chopped fibers of 
carbonized polyacrylonitrile were used. This substrate was also about 4 
inches wide, 8 inches long and about 1 inch think. The fiber volume of the 
substrate was on the order of 35%. 
The starting substrate was placed into a vacuum furnace and the fibers 
thereof coated with pyrolytic carbon in the manner described in Example 
No. 1. However, propane was used in lieu of methane as the carbonaceous 
gas. 
After infiltration the substrate was removed from the CVD furnace and was 
machined into a shaped substrate in the manner of Example No. 1. 
Following machining, the intermediate substrate was heated to about 
3200.degree. F. in a second controlled environment maintained at slightly 
greater than atmospheric pressure and a slurry of elemental silicon was 
deposited on the pyrolytic carbon coated fibers. 
The silicon coated article thus formed was dimensionally inspected and 
returned to the vacuum furnace wherein it was once again heated to about 
3200.degree. F. While being maintained at this elevated temperature a gas 
containing carbon and silicon, as for example trimethyl-chlorosilane was 
passed over and about the article to deposit a uniform coating of silicon 
nitride over the coated fibers and about the periphery of the article. 
Rigorous testing of the article showed it to be dimensionally stable and 
resistant to cracking, crazing and spalling even in hostile gas 
environments and at high temperatures. 
EXAMPLE NO. 3 
A carbonized rayon cloth made up of interwoven carbon fibers was cut into 
circular shaped pieces having a diameter of about 4 inches. A disc shaped 
starting substrate was constructed by stacking a plurality of the circular 
shaped pieces onto a base plate of a compression fixture. Each layer of 
cloth was rotated slightly with respect to the preceeding layer and a top 
plate was placed over the assembly and bolted to the base plate. The 
assembly was then compressed to bring the cloth layers into intimate 
contact. The starting substrate thus formed exhibited a fiber volume of 
about 35% and a fiber density of about 1.5 gm/cc. 
Next the starting substrate, along with the compression fixture, was placed 
into a CVD vacuum furnace and in the manner previously described, 
pyrolytic carbon was uniformly deposited over each of the fibers 
comprising the disc shaped starting substrate. 
The intermediate substrate thus formed was removed from the compression 
fixture and machined to form a disc about 31/2 inches in diameter and 
about 1 inch thick. 
Following machining, the substrate was returned to the CVD vacuum furnace 
and heated to a temperature of about 2200.degree. F. A gas containing 
dichlorosilane was passed over and about the shaped substrate for a period 
of time of about 3 hours to form a diffusion coated article in which a 
silicon coating was formed about each of the coated fibers of the 
intermediate substrate. The temperature of the substrate was then raised 
to about 2700.degree. F. to cause a reaction between the silicon and 
pyrolytic carbon to form silicon carbide. 
After undergoing another dimensional inspection, the still porous, 
diffusion coated article was returned to the vacuum furnace for final 
coating with silicon carbide in the manner described in Example No. 1. 
Once again the two phase, compliant layer coated article thus formed 
exhibited remarkable stability and durability during severe environmental 
testing. 
EXAMPLE NO. 4 
Using a tape material made up of closely woven, carbonized PAN fibers, a 
cylindrical shaped starting substrate was constructed by wrapping the tape 
about a mandrel. This substrate exhibited density of about 0.83 gm/cc and 
a fiber volume of about 40%. 
The substrate was removed from the mandrel and placed into a vacuum furnace 
wherein pyrolytic carbon was deposited on the fibers thereof in the manner 
described in Example No. 1, but using acetylene as the feed gas. 
Next the intermediate substrate was machined and then reacted with a 
siliceous material as in Example No. 1 to form a diffusion coated 
substrate. 
Finally the diffusion coated article thus formed was coated with a seal 
coat of silicon carbide by heating it under vacuum to a temperature of on 
the order of 1800.degree. F. and exposing it to a gas containing carbon 
and silicon as for example trimethyl-chlorosilane. This final step formed 
a uniform CVD coating of silicon carbide over the coated fibers and the 
periphery of the article rendering it virtually impervious to corrosion 
and erosion caused by high temperatures and exposure to hostile fluids. 
EXAMPLE NO. 5 
In constructing the starting substrate of this example the starting 
material used was a macerated material comprising a multiplicity of 
randomly oriented pyrolyzed wool fibers. This starting material was formed 
into a substrate which was approximately 4 inches wide, 8 inches long and 
about 1 inch thick, and exhibited a fiber density of on the order of 35%. 
The starting substrate was processed in the manner described in Example 
No. 1 except that silicon tetrachloride was used in applying the final 
coating to the diffusion coated article. 
EXAMPLE NO. 6 
Using a macerated material having a multiplicity of chopped pitch fibers a 
starting substrate was constructed as in Example No. 1. This starting 
substrate which exhibited a fiber volume of about 60% was also processed 
as described in Example No. 1 save that silicon dibromide was used in the 
final coating step. 
EXAMPLE NO. 7 
Using a starting substrate constructed and processed in the manner 
described in Example No. 3, a diffusion coating was applied by placing the 
substrate in a pack containing granular silicon carbide, aluminia and 
silicon. The pack and substrate were then slowly raised in temperature to 
about 3200.degree. F. over a five day period to form a diffusion coating 
on the substrate material. A final, or seal coating was applied about the 
periphery of the coated substrate in the manner described in Example No. 
1. 
Having now described the invention in detail in accordance with the 
requirements of the patent statutes, those skilled in this art will have 
no difficulty in making changes and modifications in the individual parts 
or their relative assembly in order to meet specific requirements or 
conditions. Such changes and modifications may be made without departing 
from the scope and spirit of the invention, as set forth in the following 
claims.