Compliant composite tubular liners of fiber reinforced glass/glass-ceramic having utility as gun barrel liners

A compliant tubular liner particularly adapted to withstand brisant forces is disclosed. These liners comprising graphite or silicon carbide fiber reinforced glass or glass-ceramic have a high fracture toughness, a relatively low elastic modulus and a high temperature resistance. These articles are particularly useful as gun barrel liners.

DESCRIPTION 
TECHNICAL FIELD 
The field of art to which this invention pertains is fiber reinforced 
matrix composite structures and particularly compliant tubular liners 
capable of withstanding brisant forces. 
BACKGROUND ART 
When a ballistic round is fired, an explosion takes place just behind the 
projectile, creating hot gases which expand and force the projectile out 
of the barrel. In order to insure the greatest power from this explosion, 
(efficiency), it is necessary to prevent these gases from passing between 
the internal diameter of the barrel (bore) and the outer diameter of the 
projectile. This is accomplished by making the tolerances between the bore 
and projectile very close. However, these close tolerances mean that some 
physical contact between the projectile and the wall is inevitable as the 
projectile makes its way through the barrel. This contact creates local 
stresses in the barrel, which in turn create cracks and fissures along the 
bore. This physical contact also erodes some of the material thereby 
enlarging the bore diameter. In addition, the explosion of the powder 
creates a brisance or shattering effect which places sharp and sudden 
stresses on the gun barrel. These stresses can create additional 
fractures, cracks and fissures in low fracture toughness materials which 
further destroy the close tolerances required for maximum performance. The 
hot gases which are created during this explosion also add to the 
deterioration of the bore dimensions. These gases are both erosive and 
corrosive and have a strong oxidizing effect on the metal alloys presently 
being used for gun barrel application. These materials do not possess the 
optimum in corrosion and erosion resistance necessary for a long life. The 
current state of the art gun barrels are fabricated from stellite lined, 
chrome-plated steel tubes. Due to the high percentage of the critical 
elements cobalt and chromium, barrels of this type are becoming 
increasingly expensive to manufacture. 
In light of these limitations a liner made of ceramic materials has been 
sought for this application because in general, they can provide a greater 
resistance to erosion and corrosion forms of deterioration. However, 
ceramics have a low fracture toughness which makes them susceptible to 
chipping, flaking and cracking from the stresses created during firing and 
place a serious limitation on their use. Similarly, these ceramics 
typically have elastic modulii greater than that of the surrounding steel 
main gun barrel. A typical elastic modulus for steel is 30.times.10.sup.6 
psi (206 GPa) while that for high performance ceramics such as silicon 
carbide, silicon nitride and alumina is approximately 50.times.10.sup.6 
psi (345 GPa) or greater. This relatively higher stiffness of the ceramic 
liner limits the ability of the surrounding steel to provide structural 
reinforcement and causes high stresses to occur in the ceramic liner 
during gun barrel firing. A tough, compliant non-metallic tubular liner 
capable of overcoming the disadvantages present in the prior art could 
find utility in many areas. 
DISCLOSURE OF INVENTION 
The disclosure is directed to compliant composite liners for tubular 
structures comprising either graphite or silicon carbide fiber reinforced 
glass or glass-ceramic. These liners, when used in combination with high 
strength outer sleeves are capable of withstanding the sudden sharp 
stresses associated with brisant effects. These liners are characterized 
by a relatively low modulus of elasticity and high fracture toughness and 
a maintenance of this performance at elevated temperatures. 
Another aspect of the invention includes a gun barrel comprising a 
composite liner of the above material surrounded by a conventional metal 
barrel. 
The foregoing and other features and advantages of the present invention 
will become more apparent in light of the following description and 
accompanying drawings.

BEST MODE FOR CARRYING OUT THE INVENTION 
Two fiber materials which may be employed in the present invention are 
graphite and silicon carbide. 
The fiber should have a high elastic modulus, high strength and a preferred 
length of 1 cm to about 3 cm. A preferred graphite fiber is Celion 
(Celanese Corporation) having a modulus of elasticity of about 
34.times.10.sup.6 psi (234 GPa) and an average strength of about 
400.times.10.sup.3 psi (2760 MPa). The preferred silicon carbide fiber is 
available from Nippon Carbon Corporation of Japan and has a modulus of 
elasticity of about 28.times.10.sup.6 psi (193 GPa) and then average 
strength of about 300 ksi (2000 MPa). 
In a fiber reinforced glass composite, many glass matrices may be used as 
long as the coefficients of thermal expansion of the glass and the fibers 
are compatible. This will help to ensure structural integrity. A glass 
which is particularly suitable is a borosilicate glass designated Corning 
7740 (available from Corning Glass Works). This particular glass has an 
anneal point of 560.degree. C., a softening point of 821.degree. C., a 
liquidus temperature of 1017.degree. C., a density of 2.23 gm/cm.sup.3, an 
index of refraction of 1.474, a dielectric constant of 4.6, a coefficient 
of linear expansion of 32.5 cm/cm .degree.C..times.10.sup.-7 and a modulus 
of elasticity of 9.1.times.10.sup.6 psi (63 GPa). 
When silicon carbide fibers are employed an added strict limitation is the 
amount and activity of the titanium present in the glass or glass-ceramic. 
Accordingly, if titanium nucleating agents are present, they must be 
inactivated or kept below 1% by weight. While conventional lithium 
aluminosilicate is the preferred glass-ceramic, other conventional 
glass-ceramics such as aluminosilicate, magnesium aluminosilicate and 
combinations of the above can be used as long as the matrix material is 
titanium free. It is preferable that at least 90% of the glass-ceramic 
used with either the graphite or silicon carbide fibers be capable of 
passing through a -325 mesh sieve. 
In general the fibers of the present invention are discontinuous and laid 
in a controlled manner, for example substantially equal numbers of fibers 
in-plane in every direction. The fiber orientation can be favored in a 
particular in-plane direction when it is known that a particular article 
will undergo stress primarily in one direction. These fibers are generally 
arranged in paper-like sheets wherein the fibers are temporarily bound 
together by an organic binder. These sheets are then combined with glass 
powder and laid up in a desired form with the fibers in the desired 
direction. The composites of the present invention are preferably formed 
by cutting the fiber paper to the desired shape followed by removal of the 
binder, for example, by solvent immersion or passing the paper through a 
Bunsen burner flame to burn off the binder. The fiber paper is next passed 
through a glass powder containing slurry or stacking them with layers of 
glass dispersed between them. The glass must be of sufficient quantity to 
fill the spaces between the fibers. The formed articles are then hot 
pressed and densified at elevated temperatures (1000.degree.-1400.degree. 
C.) and under pressure (around 6.9 MPa) to form the composite. The optimum 
fiber contents for these structures is about 15% to about 30% by volume. 
These compliant tubular liners may be formed using different combinations 
of materials and techniques. Which technique and what materials are 
employed are a function of, among other things, the properties of the 
final product and the shape desired. Although these compliant tubular 
structures are well suited as gun barrel liners, which are traditionally 
cylindrical in shape, there is no reason to believe that any other 
cross-sectional form would not work just as satisfactorily. 
EXAMPLE 
Celion graphite fiber paper (International Paper Company) containing about 
5 to about 10% by weight of a polyester binder was cut into individual 
pieces about 71/2 cm.sup.2. The plies were then dipped into a slurry of 
Corning 7740 borosilicate glass in propanol. A preferred slurry 
composition comprised 40 gm of powdered glass in 780 ml of propanol. 
Preferably the glass is ground so that at least 90% of it passes through a 
-325 mesh sieve. The plies are then dried, by either exposure to air or a 
radiant heat source, i.e. a heat blower, to remove the propanol. After 
drying, about 60 pieces of the glass impregnated plies were laid up in a 
graphite mold for consolidation at elevated temperatures. For this 
particular structure the plies were laid such that all the fibers were in 
the radial and tangential direction only, with no fibers in the direction 
along the axis of the cylinder. The mold with the plies was baked out at 
400.degree. C. to remove the binder on the graphite paper; and then hot 
pressed at 1200.degree. C. for 15 minutes in a 10.sup.-5 torr vacuum under 
about 6.9 MPa pressure. Approximately a 0.500 inch thick plate was 
produced by this method. A total of 8 plates were prepared using the above 
method. The plates were then stacked in graphite dies and repressed again, 
under the same conditions as before, to form the final 
3".times.3".times.4" (71/2 cm.times.71/2 cm.times.10.2 cm) thick block of 
graphite fiber/glass composite material. There were no discontinuities 
introduced at the bond line between the plates when this method was used. 
Four cylinders, four inches long with nominal ID's of 0.475 inch and 
nominal OD's of 0.925 inch (indicated as character 2 in FIG. 1) were 
diamond core drilled from this block (indicated as character 1 in FIG. 1). 
These cylinders were then machined to the proper dimensions for insertion 
into the metal sleeves (4) which are then inserted into the gun barrel 
jackets (5) shown in FIG. 2. The machined cylinders are shown as character 
3 in FIG. 2. These barrels were then subjected to ten shots single fire. 
Inspection showed that the graphite/glass composite liners were capable of 
withstanding the pressure stresses resulting from firing without failure. 
Even when pre-existing cracks were present, no tendency for crack growth 
or catastrophic failure was exhibited. The Table presents the physical 
properties of the discontinuous graphite fiber reinforced glass gun barrel 
liner described above. 
TABLE 
______________________________________ 
discontinuous graphite/glass 
______________________________________ 
Modulus of Elasticity 
8 .times. 10.sup.6 psi (55.8 GPa) 
Critical Stress Intensity 
10 .times. 10.sup.3 psi (inch).sup.1/2 
Factor 
Tensile Strength 19.4 .times. 10.sup.3 lbs/in.sup.2 
Flexural Strength 
45.5 .times. 10.sup.3 lbs/in.sup.2 
Failure Strain .79% 
______________________________________ 
The present compliant non-metallic tubular liners are superior over the 
current state of the art in several ways. The tubular liners of the 
present invention have a relatively low elastic modulus which gives them 
greater compliancy. When placed within a steel outer barrel, this 
compliancy allows them to withstand deforming stresses without cracking. 
The elastic modulus for a discontinuous graphite fiber/glass liner is less 
than about 12.times.10.sup.6 psi (83 GPa) while a discontinuous silicon 
carbide fiber/glass-ceramic structure would be less than about 
15.times.10.sup.6 psi (103 GPa). These tubular liners also have a high 
fracture toughness, exemplified by their high critical stress intensity 
factors making them capable of withstanding the sudden sharp and repeated 
shocks associated with brisant forces without fracturing even in the 
presence of cracks and flaws. This fracture toughness is exemplified in 
the graphite/glass composite as a critical stress intensity factor greater 
than 8.times.10.sup.3 psi (inch).sup.1/2 and in the silicon 
carbide/glass-ceramic as a critical stress intensity factor greater than 
5.times.10.sup.3 psi (inch).sup.1/2. These compliant tubes also retain 
their properties at elevated temperatures associated with gun barrel 
firing. 
Compliant fiber reinforced composite tubular structures such as described 
will find application not only as gun barrel liners or as part of an all 
composite gun barrel but where any tubular structure is required to 
withstand brisant forces in erosive or corrosive environments. Exhaust 
structures for rockets or for missiles are two alternative possible 
applications. 
Although the invention has been shown and described with respect to a 
preferred embodiment, it will be understood by those skilled in this art 
that various changes in form and detail thereof may be made without 
departing from the spirit and scope of the claimed invention.