U.S. patent application number 10/769788 was filed with the patent office on 2006-06-29 for ceramic armor and method of making by encapsulation including use of a stiffening plate.
This patent application is currently assigned to CERCOM, INC.. Invention is credited to Daniel Ashkin, Richard Palicka.
Application Number | 20060137517 10/769788 |
Document ID | / |
Family ID | 36609893 |
Filed Date | 2006-06-29 |
United States Patent
Application |
20060137517 |
Kind Code |
A1 |
Palicka; Richard ; et
al. |
June 29, 2006 |
CERAMIC ARMOR AND METHOD OF MAKING BY ENCAPSULATION INCLUDING USE
OF A STIFFENING PLATE
Abstract
A ceramic armor is disclosed in several embodiments. In a first
embodiment, a metal base plate has a metal frame assembled on it
having a central opening into which the ceramic material and
stiffening plate are placed. A cover plate is placed over the frame
to enclose the ceramic material on all sides. In a second
embodiment, the frame has an open central area that has two
crossing walls that define four sub-chambers. Four sets of ceramic
material and stiffening plate are placed in the respective
sub-chambers and a covering plate is placed over them. In a further
embodiment, the frame has a plurality of cavities mechanically
formed in it. A stiffening plate and a ceramic tile or plate are
placed in each cavity and a cover plate is placed over the frame.
The metal used to encapsulate the ceramic material may, if desired,
comprise a Titanium alloy such as Ti-6Al-4V, and the ceramic
material may comprise Silicon Carbide, Boron Carbide, Tungsten
Carbide, Titanium Diboride, Aluminum Oxide or Aluminum Nitride. The
stiffening plate is preferably made of a Ti--TiB cermet composite
but may also be comprised of an armor ceramic such as WC,
TiB.sub.2, Al.sub.2O.sub.3 or B.sub.4C. A hot pressing procedure is
carried out on the armor to cause the metal to plastically deform
about the encapsulated ceramic material.
Inventors: |
Palicka; Richard; (San
Clemente, CA) ; Ashkin; Daniel; (San Marcos,
CA) |
Correspondence
Address: |
H JAY SPIEGEL
P.O. BOX 444
MOUNT VERNON
VA
22121
US
|
Assignee: |
CERCOM, INC.
|
Family ID: |
36609893 |
Appl. No.: |
10/769788 |
Filed: |
February 3, 2004 |
Current U.S.
Class: |
89/36.02 |
Current CPC
Class: |
F41H 5/0421 20130101;
Y10T 428/12049 20150115 |
Class at
Publication: |
089/036.02 |
International
Class: |
F41H 5/02 20060101
F41H005/02 |
Claims
1. A ceramic armor, comprising: (a) a ceramic material body having
one side engaging a cermet composite or armor ceramic stiffening
plate, a metallic material surrounding said ceramic material body
and stiffening plate, with said stiffening plate being interposed
between said one side of said ceramic material and said metallic
material; and (b) said ceramic material body having other sides,
said metallic material being plastically deformed about said other
sides of said ceramic material body; and (c) said stiffening plate
being stiffer than said metallic material.
2. The ceramic armor of claim 1, wherein said metallic material has
a coefficient of thermal expansion (CTE) greater than a CTE of said
ceramic material body, and said stiffening plate has an elastic
modulus greater than that of the metallic material.
3. The ceramic armor of claim 1, wherein said metallic material
comprises a Titanium alloy.
4. The ceramic armor of claim 1, wherein said metallic material
comprises a Titanium alloy, and said stiffening plate comprises a
Ti--TiB composite.
5. The ceramic armor of claim 1, wherein said metallic material
comprises a Titanium alloy, and said stiffening plate comprises an
armor ceramic.
6. The ceramic armor of claim 1, wherein said metallic material
comprises a Titanium alloy, and said stiffening plate comprises an
armor ceramic chosen from the group consisting of WC, B.sub.4C,
Al.sub.2O.sub.3 and TiB.sub.2.
7. The ceramic armor of claim 6, wherein said Titanium alloy
comprises Ti-6Al-4V or Ti-6Al-4V ELI.
8. The ceramic armor of claim 7, wherein said ceramic material body
comprises a dense SiC ceramic material comprising pressure assisted
SiC--N.
9. The ceramic armor of claim 8, wherein elastic modulus of the
Titanium alloy is about 115 GPa, elastic modulus of the ceramic
material body is about 450 GPa, and elastic modulus of the
stiffening plate is greater than 130 GPa.
10. The ceramic armor of claim 1, wherein said metallic material
comprises a three piece assembly consisting of a backing plate, a
frame having an open center, and a cover plate, said assembly
defining an internal chamber designed to closely receive said
ceramic material body and stiffening plate being interposed between
said ceramic material body and said backing plate.
11. The ceramic armor of claim 10, wherein said frame includes a
plurality of cavities therein, each of said cavities being closely
filled with a ceramic material body and a stiffening plate.
12. The ceramic armor of claim 11, wherein said plurality of
cavities comprises four cavities, each filled with a ceramic tile
or plate and a stiffening plate.
13. The ceramic armor of claim 12, wherein said frame includes a
plurality of separate side pieces assembled together to form a
periphery, and a pair of cross members connected between opposed
side pieces to define said cavities.
14. The ceramic armor of claim 12, wherein said three piece
assembly comprises a first three piece assembly, said ceramic armor
further including at least one additional three piece assembly,
said three piece assemblies being stacked vertically.
15. A method of making ceramic armor, comprising: (a) providing a
backing plate, a frame having an open center, and a cover plate,
together defining an internal chamber; (b) inserting a metal
composite stiffening plate followed by a piece of ceramic material
into said chamber, said ceramic material and stiffening plate being
closely received within said chamber with said stiffening plate
interposed between said ceramic material and said backing plate,
said backing plate, frame, cover plate, stiffening plate, and
ceramic material together defining an assembly; (c) said metallic
material having a coefficient of thermal expansion greater than a
coefficient of thermal expansion of said ceramic material; (d)
placing said assembly with said ceramic material therein into a hot
press consisting of a furnace located within a sealed chamber; (e)
conducting a hot pressing procedure on said assembly under
controlled parameters of temperature, pressure and atmosphere until
said metallic material is plastically deformed around said ceramic
material.
16. The method of claim 15, wherein said metallic material
comprises a Titanium alloy, and said stiffening plate comprises a
Ti--TiB composite.
17. The method of claim 16, wherein said Titanium alloy comprises
Ti-6Al-4V or Ti-6Al-4V ELI, and a ratio by volume between Ti and
TiB in said composite is from 4:1 to 1:1.
18. The method of claim 17, wherein said ceramic material comprises
a dense SiC ceramic material such as PAD SiC--N.
19. The method of claim 18, wherein the coefficient of thermal
expansion (CTE) of the Titanium alloy is about 10.5.times.10.sup.-6
in/in .degree. C. from 0-600.degree. C., the CTE of the ceramic
material is about 4.1.times.10.sup.-6 in/in .degree. C. from
0-600.degree. C., and the CTE of the stiffening plate is about
8.5-10.5.times.10.sup.-6 in/in .degree. C. from 0-600.degree.
C.
20. The method of claim 15, wherein said hot pressing procedure
includes the following steps: (a) evacuating said sealed chamber to
a pressure of about 10 torr; (b) heating said sealed chamber to
about 800.degree. C. and, during said heating step, purging said
sealed chamber with an inert gas at least once followed by
evacuating said sealed chamber back to 1 to 1.5 torr; (c)
maintaining pressure in said sealed chamber to less than 1.5 torr
once temperature therein has risen to 800.degree. C.; (d)
increasing said temperature from 900.degree. C.-1300.degree. C.
21. The method of claim 20, wherein once said temperature reaches
900.degree. C., increasing physical pressure on said assembly in
said chamber to at least 250 psi and holding temperature and
physical pressure constant for at least two hours.
22. The method of claim 15, wherein said internal chamber of said
assembly includes four sub-chambers.
23. The method of claim 22, wherein said sub-chambers are created
by machining said frame using an EDM process.
24. The method of claim 15, wherein said coefficient of thermal
expansion of said ceramic material is no greater than
9.times.10.sup.-6 in/in .degree. C.
25. The method of claim 24, wherein said ceramic material is chosen
from the group consisting of Silicon Carbide, Boron Carbide,
Tungsten Carbide, Titanium Diboride, Aluminum Oxide, Silicon
Nitride, and Aluminum Nitride.
26. The method of claim 15, wherein said atmosphere comprises a
high purity Argon atmosphere.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates to ceramic armor and the
method of making it by encapsulation including use of a stiffening
plate adjacent to the metal backing plate. Ceramic containing armor
has been shown to be an effective means to protect against a wide
variety of ballistic threats because of its combination of high
hardness, strength and stiffness along with low bulk density and
favorable pulverization characteristics upon impact.
[0002] However, ceramic material alone has been found to be
ineffective against heavy ballistic threats such as Tungsten
Carbide projectiles, and long rod heavy metal penetrators. Long rod
projectiles can have a significant ratio of length to diameter, up
to 40, and can travel at velocities up to or exceeding 1 mile per
second. For the ceramic to effectively stop such threats, the
ceramic material must be supported or encapsulated with another
material such as metal or another composite capable of absorbing
energy and providing stiffness support for the ceramic. Applicants
have found that the use of a stiffening plate can also be
advantageous.
[0003] However, merely mechanically assembling an armor consisting
of ceramic material encapsulated by metal, and using a stiffening
plate, without further processing, fails to optimize the ballistic
performance of armor. As such, a need has developed for an
encapsulated ceramic armor material that optimizes ballistic
performance and may be manufactured in a repeatable, predictable
way. It is with this thought in mind that the present invention was
developed.
SUMMARY OF THE INVENTION
[0004] The present invention relates to a ceramic armor and the
method of making it by encapsulation including use of a stiffening
plate. The present invention includes the following interrelated
objects, aspects and features:
[0005] (1) The inventive armor is disclosed in several structural
embodiments which are considered to be exemplary of the teachings
of the present invention. In a first such embodiment, a metal
backing plate has a metal frame placed thereon having a central
opening into which a stiffening plate is placed followed by a
ceramic tile. A cover plate is placed over the frame to enclose the
ceramic tile and stiffening plate on all sides.
[0006] (2) In a second embodiment of the present invention, a metal
backing plate is covered by a frame having an open central area
that has two crossing walls therein to define four sub-chambers.
Four stiffening plates are placed in the respective sub-chambers
followed by four respective ceramic pieces or tiles, and a covering
plate is placed thereover.
[0007] (3) In a further embodiment, a flat backing plate is covered
by a second plate in which a plurality of cavities have been
mechanically formed. A stiffening plate is placed in each cavity,
followed by a ceramic tile, and a cover plate is placed
thereover.
[0008] (4) Concerning each of the embodiments described above, the
metal used to encapsulate the ceramic material may, if desired,
comprise a Titanium alloy such as Ti-6Al-4V. This material is
particularly effective as a ballistic material because it has a
relatively low density (4.5 g/cc), relatively high strength (900
MPa) and good ductility (yield strength of 830 MPa at 0.2% yield).
Thermal expansion of Ti-6Al-4V is approximately
10.5.times.10.sup.-6 in/in .degree. C. and deform about the
ceramic, from 0-600.degree. C. This coefficient of thermal
expansion is considerably higher than that of dense SiC which is a
common ceramic employed for armor applications. The thermal
expansion of SiC is 4.1.times.10.sup.-6 in/in .degree. C. from
0-600.degree. C. The SiC material described may comprise, for
example, PAD SiC--N ceramics.
[0009] (5) Concerning each of the embodiments described above, the
stiffening plate is preferably made of a Ti--TiB cermet composite
having an elastic modulus that is greater than the metal backing
plate and a coefficient of thermal expansion close to that of the
metal backing plate.
[0010] (6) In each of the physical embodiments of armor in
accordance with the teachings of the present invention, once the
armor is assembled with the ceramic material encapsulated within
the metallic material, and the stiffening plate interposed between
the ceramic material and the backing plate, the entire armor is
heated to a temperature sufficiently high enough to cause the metal
to be plastically deformed around the ceramic on the top and sides.
In order for the metal to plastically deform about the ceramic in a
controlled manner, the ceramic material must have dimensions so
that it is as close as possible to the dimensions of the chamber in
which the ceramic material is placed. The ceramic material must be
strongly confined on all sides during thermal cycling so that,
during the heating and cooling process, the ceramic is placed into
compression. The degree of compression to which the ceramic
material is exposed is a function of the thermal expansion mismatch
between the metal and the ceramic, the change in temperature during
the processing, the yield properties of the metal, the applied
pressure, and the dimensions of the device itself.
[0011] (7) The encapsulation of the ceramic by metal has been found
to allow for the phenomenon of interface defeat which increases the
ballistic performance of the ceramic armor. Interface defeat is a
phenomenon in which the projectile flows radially outwards on the
surface of ceramics without penetrating it significantly. In making
encapsulated parts for advanced armor systems, the relative
dimensions of the ceramic and metal plates and frames are of much
importance in determining the amount of compression on the ceramic,
the areal density of the part and the stiffness of the armor. All
of these variables are important for the phenomena of the dwell
along with the type of metal and ceramic. Typical ceramic armor
materials have densities nearly equal to or less than that of
Titanium and include Silicon Carbide, Aluminum Nitride, Aluminum
Oxide, and Titanium Diboride. The phenomenon of dwell has been
recognized to be of much importance in achieving success for
lightweight armor systems.
[0012] (8) The concept of the use of stiffening plates can be used
for all methods of encapsulation. However, an advantage of the use
of hot pressing a plate assembly is the simplicity of adding other
elements such as stiffening plates to the construction. For
encapsulation by methods using heat treatment, the stiffening plate
should not react with the ceramic being encapsulated unless the
difference in thermal expansion mismatch is minimal
(<1.times.10.sup.-6 in/in at 1000.degree. C.). The stiffening
plate should also not react with the metal used for encapsulation
unless the thermal expansion mismatch is less than
2.times.10.sup.-6 in/in at 1000.degree. C. One such element that
Applicants have developed for stiffening Ti--SiC assemblies is a
composite of Titanium and Titanium Boride. This material has a
density similar to that of Titanium but stiffness that is greater
than that of Titanium.
[0013] As such, it is a first object of the present invention to
provide ceramic armor and a method of making it by encapsulation
including use of a stiffening plate.
[0014] It is a further object of the present invention to provide
such an armor in various embodiments thereof including those in
which a single piece of ceramic is encapsulated within a single
cavity adjacent a stiffening plate.
[0015] It is a still further object of the present invention to
provide such a device in which a plurality of discrete ceramic
pieces are each encapsulated adjacent stiffening plates within a
sub-chamber within a metal portion.
[0016] It is a still further object of the present invention to
provide such a device in which the chambers that receive the
ceramic material and stiffening plate are formed through assembly
of separate parts in situ.
[0017] It is a yet further object of the present invention to
provide such a device in which the sub-chambers receiving the
ceramic pieces and stiffening plate are formed through an EDM or
conventionally milled process that mechanically forms the
sub-chambers or cavities.
[0018] It is a still further object of the present invention to
provide a method of creating ceramic armor in which the ceramic
material and stiffening plate encapsulated with the metal material
are subjected to a hot pressing process to cause the metal to be
plastically deformed around the ceramic.
[0019] These and other objects, aspects and features of the present
invention will be better understood from the following detailed
description of the preferred embodiments when read in conjunction
with the appended drawing figures.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] FIG. 1 shows a schematic cross-sectional representation of
an armor construction encapsulating a ceramic plate and stiffening
plate in accordance with the teachings of the present
invention.
[0021] FIG. 2 shows a top view of the construction of FIG. 1.
[0022] FIG. 3 shows an exploded perspective view of a second
embodiment of the present invention.
[0023] FIG. 4 shows a top view of a backing plate of a third
embodiment of the present invention.
[0024] FIG. 5 shows a side view of the backing plate of FIG. 4.
[0025] FIG. 6 shows a side view of a first cross beam to be
assembled to the backing plate of FIGS. 4-5.
[0026] FIG. 7 shows a side view of the cross beam of FIG. 6.
[0027] FIG. 8 shows a side view of a further cross beam to be
assembled to the backing plate of FIGS. 4-5.
[0028] FIG. 9 shows a top view of the cross beam of FIG. 8.
[0029] FIG. 10 shows a perspective view of the parts illustrated in
FIGS. 4-9 as assembled together.
[0030] FIG. 11 shows a perspective view of a plurality of
constructions of the embodiment of FIGS. 4-10 assembled together in
vertically spaced layers.
[0031] FIG. 12 shows a graph of temperature and pressure versus
time for the conducting of the hot pressing process for
encapsulating the metal alloy and ceramic material together.
[0032] FIG. 13 shows a graph of a portion of the hot pressing
process during the portion thereof when temperature is being
increased and showing several backfilling and evacuating steps.
[0033] FIG. 14 shows a graph of the elastic modulus for varying
volume fractions of TiB in a Ti--TiB composite.
[0034] FIG. 15 comprises a photomicrograph showing the
microstructure of an etched Ti--TiB composite used to create a
stiffening plate.
SPECIFIC DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0035] Reference is first made to FIGS. 1 and 2 which show a
schematic representation of a three layer construction in
accordance with the teachings of the present invention. The
inventive construction is generally designated by the reference
numeral 10 and is seen to include a backing plate 11, a metal frame
13, and a cover plate 15, which combine to define an internal
chamber 17. Within the chamber 17, a stiffening plate 3 and a
ceramic plate or tile 19 are encapsulated. The stiffening plate 3
is interposed between the backing plate 11 and the ceramic plate or
tile 19.
[0036] As shown in FIG. 2, the frame 13 may be generally
rectangular, having the internal chamber 17 sized to closely
receive the ceramic plate or tile 19 and the stiffening plate 3
therein.
[0037] With reference to FIG. 3, a second embodiment of the present
invention is generally designated by the reference numeral 20 and
is seen to include a backing plate 21, a middle plate 23, and a
cover plate 25. The middle plate 23 has a plurality of cavities 27,
29, 31 and 33 formed therein through any desired manner including
electrical discharge machining EDM processing or mechanical
processing.
[0038] Ceramic tiles 35, 36, 37 and 39 and stiffening plates 38 are
respectively received within the cavities 27, 29, 31 and 33,
whereupon the cover plate 25 is placed thereover to encapsulate the
ceramic tiles.
[0039] With reference, now, to FIGS. 4-10, a further embodiment of
the present invention is generally designated by the reference
numeral 40 (see FIG. 10). The embodiment 40 includes a backing
plate 41, a frame structure 43, and a cover plate 45. With
reference to FIGS. 4-9, the manner of assembly of the frame
structure 43 will be explained. With reference, first, to FIGS. 4
and 5, the frame structure 43 includes a backing plate 47 having a
top surface 49 into which crossing grooves 51 and 53 are formed, of
which the groove 51 is also seen in full lines in FIG. 5, and the
groove 53 is shown in phantom therein.
[0040] With reference to FIGS. 6 and 7, a cross beam 55 has a
bottom surface 57 inserted into the groove 51 and also includes an
upper slot 59. With reference to FIGS. 8-9, a further cross beam 61
includes a bottom surface 63 designed to rest within the groove 53
and a slot 65 that is placed over the slot 59 in the beam 55 when
assembled.
[0041] With reference to FIG. 10, the frame structure 43 is made up
of four legs 71, each of which has a rear slot 73 and a forward
protrusion 75 to form "tongue and groove" connections with adjacent
legs 71. Each of the legs has a vertical slot 77 therein which is
designed to receive one of the ends of either one of the cross
beams 55 or 61. As assembled, the frame structure 43 defines four
cavities 81, 82, 83 and 84. As before, each of these cavities
closely receives a ceramic plate or tile and a stiffening plate 76,
whereupon the cover plate 45 is placed thereover.
[0042] FIG. 11 shows a ceramic armor made up of a plurality of
armor constructions 40 stacked vertically with cover plates 90 and
a backing plate 92 shown.
[0043] In each of the embodiments of the present invention, it is
preferred that the stiffening plate(s) and ceramic plate or tile or
plates or tiles is/are machined to be, in combination, within 0.005
inches of the corresponding dimensions of the sub-chambers or cells
within which they are placed. In accordance with the teachings of
the present invention, it is preferred that the metal material used
to encapsulate the ceramic material consists of a material having
relatively low density, high strength and good ductility along with
a coefficient of thermal expansion higher than the coefficient of
expansion for the ceramic material encapsulated therewithin.
Applicants have found that an alloy of Titanium known as Ti-6Al-4V
or Ti-6Al-4V ELI (Extra Low Interstitials) is a suitable material
for this purpose. Ti-6Al-4V has a relatively low density (4.5
g/cc), relatively high strength (900 MPa), and good ductility
(yield strength of 830 MPa at 0.2% yield), and can be bought
already annealed according to Mil T 9046 spec. The thermal
expansion of Ti-6Al-4V is about 10.5.times.10.sup.-6 in/in .degree.
C. from 0-600.degree. C., a coefficient considerably higher than
that of dense SiC which has a thermal expansion coefficient of
4.1.times.10.sup.-6 in/in .degree. C. from 0-600.degree. C., a
difference in which the thermal expansion coefficient for the
Titanium alloy is over 2% times the thermal expansion coefficient
for the ceramic material.
[0044] In the preferred embodiment of the present invention, the
ceramic material employed may consist of PAD SiC--N, one of a
family of Cercom's dense hot pressed ceramics. Other grades and
types of armor ceramics such as Silicon Carbide, Boron Carbide,
Tungsten Carbide, Titanium Diboride, Aluminum Oxide, Silicon
Nitride and Aluminum Nitride or mixtures of the aforementioned
materials can be employed. Such armor ceramics have thermal
coefficients of expansion from about 3.0.times.10.sup.-6 to about
9.times.10.sup.-6 in/in .degree. C. and hardness greater than 1100
kg/mm.sup.2.
[0045] The concept of stiffening plates can be used for all methods
of encapsulation. However, an advantage of the use of hot pressing
a plate assembly is the simplicity of adding other elements such as
stiffening plates to the construction.
[0046] For encapsulation by methods using heat treatment, the
stiffening plate should not react with the ceramic being
encapsulated unless the difference in thermal expansion mismatch is
minimal (<1.times.10.sup.-6 in/in .degree. C. at 1000.degree.
C.). The stiffening plate should also not react with the metal used
for encapsulation unless the thermal expansion mismatch is less
than 2.times.10.sup.-6 in/in .degree. C. at 1000.degree. C.
However, such reaction does not reduce the effectiveness of the
stiffening plate, but merely adds a bond at the interface.
[0047] One element that Applicants have developed for stiffening
Ti--SiC assemblies is a composite of Titanium and Titanium Boride.
This material has densities similar to Titanium but stiffness that
is greater than Titanium. Table I shows the hot pressed density as
a function of TiB content, and FIG. 14 shows the elastic modulus
for different amounts of TiB. TABLE-US-00001 TABLE I Hot-Press
Densities DENSITY BY RULE OF MIXTURES VOLUME FRACTION (g/cc) 0.0
4.500 0.2 4.538 0.4 4.576 0.6 4.614 0.8 4.652 1.0 4.690
[0048] Table II shows the tensile strength as a function of TiB
content and Table III shows the Coefficient of Thermal Expansion
(CTE). TABLE-US-00002 TABLE II Ultimate Tensile Strength of Ti,
TiB, and Ti/TiB Composites Ultimate Tensile Strengths at Room
Temperatures Composition Tensile Strength Ti 720 MPa 80Ti/20TiB 550
MPa 60Ti/40TiB 260 MPa 40Ti/60TiB 270 MPa 20Ti/80TiB 360 MPa TiB
280 MPa
[0049] TABLE-US-00003 TABLE III Calculated Coefficient of Thermal
Expansion (CTE) from 20.degree. C. to 600.degree. C. of Ti, TiB and
Ti--TiB Composites Composition CTE Ti 10.5 .times. 10.sup.-6 in/in
.degree. C. 80Ti/20TiB 9.5 .times. 10.sup.-6 in/in .degree. C.
60Ti/40TiB 9.8 .times. 10.sup.-6 in/in .degree. C. 40Ti/60TiB 10.2
.times. 10.sup.-6 in/in .degree. C. 20Ti/80TiB 9.8 .times.
10.sup.-6 in/in .degree. C. TiB 9.0 .times. 10.sup.-6 in/in
.degree. C.
[0050] From Table III, it is seen that all graphed compositions
have a CTE similar to that of Titanium (within 2.times.10.sup.-6
in/in .degree. C.). A match in CTE is important to prevent cracking
when materials are pressed together and form a chemical bond. From
FIGS. 14-15 and Tables I, II and III, it can be seen that the
properties of the Ti--TiB composite can be tailored by changing the
ratio of Titanium and Titanium Boride. For instance, the stiffness
can be increased with increasing TiB content. The microstructure of
the material with intermediate amounts of TiB contains a
significant amount of whisker shaped grains (see FIG. 15). When the
Ti/TiB composite material is produced by hot pressing, the grains
can be oriented in particular planes as desired.
[0051] To illustrate this principle, Applicants have manufactured
encapsulates containing SiC tiles and Ti/TiB stiffening plates and
compared it to encapsulates with only SiC tiles. Encapsulates with
the Ti/TiB stiffening plate performed better than the encapsulates
with only Ti. An advantage of using the Titanium-Titanium Boride
composite for stiffening purposes is that it may bond with the
Titanium backing plate during hot pressing and provide a single
mechanical unit below the ceramic. Mechanical interfaces in armor
assemblies reflect shock waves and will stress the encapsulated
body. Minimizing these interfaces is therefore important and is an
advantage of using Ti--TiB composites for the stiffening plate. The
Ti--TiB composite also has a similar coefficient of thermal
expansion as that of Titanium and will therefore maintain similar
compressive stresses in the ceramic as would a single Titanium
backing plate.
[0052] Another advantage of adding a stiffening plate of Ti/TiB
composite material to a Ti encapsulated SiC is that the stiffness
of the backing plate can be increased while not changing the areal
density of the encapsulated assembly. The stiffness in the backing
plate of an encapsulated assembly is important to prevent the
premature bending/cracking of the ceramic. The stiffening of the
ceramic decreases the backside deformation and prolongs the time of
dwell. The use of Ti/TiB plate also does not significantly change
the amount of compression on the ceramic. The thermal expansions of
Ti/TiB and Ti are fairly similar.
[0053] Besides using cermets (ceramic metal composites) such as
Ti/TiB for the stiffening plates, other ceramic materials could be
used. Examples of these materials are WC, B.sub.4C, Al.sub.2O.sub.3
and TIB.sub.2. Compared to Silicon Carbide, which has a Young's
Modulus of 450 GPa, WC has a Young's Modulus of 695 GPa, TiB.sub.2
has a Young's Modulus of 555 GPa, B.sub.4C has a Young's Modulus of
450 GPa and Al.sub.2O.sub.3 has a Young's Modulus of 385 GPa. Thin
plates of these materials act to significantly stiffen the
assembly. Plates of B.sub.4C add stiffness at reduced weight.
B.sub.4C has a theoretical density of 2.52 g/cc while SiC has a
density of 3.22 g/cc.
[0054] The encapsulation of WC, B.sub.4C and TiB.sub.2 stiffening
plates is very similar to that of the bulk SiC ceramic. The SiC
material shows no significant reaction between it and the Titanium
at temperatures of up to 1000.degree. C. and has a lower thermal
expansion than Titanium. WC, B.sub.4C and TiB.sub.2 also show no
significant reaction with the Titanium at 900-1000.degree. C. and
they have a thermal expansion less than Titanium. For encapsulation
at higher temperatures, CVD or PVD coating could be used to prevent
reaction between the ceramic and the Titanium. In tests with SiC,
PVD and CVD, coatings of TiN and TiC were used to prevent reaction
between the SiC and the Ti at higher temperatures. In addition to
WC, B.sub.4C and TiB.sub.2' other armor ceramics could be used for
this application.
[0055] In practicing the method of hot pressing the ceramic armor
in accordance with any of the embodiments of the present invention,
after the ceramic material is completely encapsulated within the
metal material with the stiffening plate in place, the hot pressing
operation commences by placing the assembly within a furnace
contained within a chamber in which pressure can be controlled by a
mechanical or hydraulic press. The temperature is then increased
sufficiently such that the metal encapsulating the ceramic is
plastically deformed around the ceramic while contained within a
die of refractory material. The degree of compression of the
ceramic that is produced during hot pressing is a function of the
thermal expansion mismatch between the metal and ceramic, the rate
of temperature decrease during processing, the yield properties of
the metal, and the dimensions of the components. The stiffening
plate typically does not react with the ceramic material but may
form a bond at its interface with the backing plate.
[0056] Concerning each of the embodiments of the ceramic armor
described in detail hereinabove, the method of encapsulating the
ceramic material within the Titanium alloy is the same. The process
steps are as follows:
[0057] (1) First, all surfaces of the Titanium alloy must be
degreased and cleaned. Degreasing can be done by steam cleaning,
alkaline cleaning, vapor degreasing or solvent cleaning. Where the
surfaces are diamond machined and have a light oxide film,
mechanical cleaning by an abrasive pad such as that which is known
by the Trademark "SCOTCH BRITE," abrasive sand blasting, wire
brushing or draw filing is sufficient. Where the surfaces have been
machined, as is the case in the embodiment of FIG. 3, and have a
heavier oxide film, the alloy surfaces that have been so machined
should be cleaned by a combination of degreasing, molten salt
descaling, acid pickling, and abrasive grinding or polishing. In
the preferred process, acid cleaning should be carried out with a
mixture of 1-2% HF and 15-40% nitric acid for 1 to 5 minutes at
room temperature. The ratio of nitric acid to hydrofluoric acid
(HF) should be at least 15.
[0058] (2) The ceramic tiles or plates should be degreased using
suitable degreasing agents such as, for example, isopropanol
followed by acetone. If metal marks exist, an acid cleaning should
be performed.
[0059] (3) A refractory die such as one made of graphite is
prepared with the walls of the die and spacers thereof first coated
with mold release agents such as graphite foil. The graphite foil
besides acting as a mold release agent is provided to ensure a
tight fitting die. Examples of suitable thickness for the graphite
foil are 0.010 to 0.040'' depending upon the actual die and the
piece being hot pressed. The walls and surfaces of the spacers are
then coated with a Titanium foil having a suitable thickness. One
example of a suitable thickness for the Titanium foil is 0.008'',
although other thicknesses can be equally effective.
[0060] (4) The material is then loaded into the die with the bottom
of the die cavity having at least 1-2 graphite spacers. Depending
upon the complexity of the part, the order in which the part is
loaded into the die can vary. Where the ceramic armor consists of a
single piece of ceramic and a single stiffening plate encapsulated
by a Titanium alloy, the backing plate is loaded first followed by
the stiffening plate, the ceramic and then the other structures of
the Titanium alloy frame. For complex ceramic armor such as those
illustrated in FIGS. 3-11, the entire ceramic armor structure is
loaded into the die together with the Titanium alloy cover plate
put on top of the frame containing the ceramic plates or tiles. A
graphite spacer is then placed on top of the entire assembly. Where
multiple assemblies will be placed into the die simultaneously,
graphite spacers are placed between each separate assembly.
[0061] (5) The die with the assembly or assemblies therein is then
loaded into a vacuum hot press. The vacuum hot press consists of a
furnace in which the die may be received, with the furnace
contained within a sealed chamber in which the internal pressure
may be adjusted and inert gas such as Argon may be supplied and
exhausted. The atmosphere within the hot press is then preferably
lowered to an atmosphere of less than 1.5 torr. Of course, as known
to those skilled in the art, higher atmospheric pressures may also
be effectively employed if sufficient oxygen gettering material is
used in the furnace.
[0062] (6) Once the required vacuum atmosphere has been achieved,
the chamber is heated up to a temperature of about 800.degree. C.
and, depending on vacuum level, several optional purging and
evacuation cycles may be undertaken (FIG. 13) in which the chamber
is first purged with Argon and then evacuated. These cycles are not
essential to the process. Once the temperature reaches 800.degree.
C., the purging and evacuation steps, if they were employed, are no
longer undertaken and the atmosphere is maintained at a level of
less than 1.5 torr. Alternatively, the process at and above
800.degree. C. may be undertaken in an inert atmosphere such as
high purity Argon.
[0063] (7) As the temperature continues to increase, once it
reaches a temperature in which the metal can easily diffuse, the
physical pressure applied to the armor assembly is increased and
bonding is begun. For metals, the temperature at which diffusion
usually occurs at rates sufficient for diffusion bonding is equal
to, or greater than, one-half the melting temperature of the
material. For Titanium and its alloys, the melting temperature is
between 1575 and 1725.degree. C. For Ti-6Al-4V, the melting
temperature is 1650.degree. C. and, therefore, the minimum
temperature for hot pressing this alloy is 825.degree. C. After
achieving this temperature, the temperature is increased to its
final temperature of 900 to 1300.degree. C., and the necessary
physical pressure is applied. Of course, the necessary physical
pressure is a function of temperature and may fall within the range
of 250 psi to 5000 psi. With increased pressures and temperature,
significant plastic deformation of the Titanium alloy occurs
accompanied by increased diffusion rates. The bond formed between
the Titanium pieces is a diffusion bond and artifacts of the bond
are seen to cross individual grains at temperatures between 900 and
1000.degree. C. and hold times of 2.5 hours. For temperatures
greater than 1000.degree. C., artifacts of the bond are not visible
by microscopic analysis. Applicants have found that one may
conclude that diffusion and grain growth have occurred in the
material and that the bond is a "diffusion" bond. The significant
plastic deformation that occurs at this temperature and pressure
aids in grain-to-grain contact. The 900.degree. C. temperature and
increased pressure are held for up to 2% hours. For larger sized
ceramic armor pieces, the hold times are increased along with
reduction in heating rates. For lower temperature bonding,
additives or coatings can be added to the Titanium surfaces to
increase the local diffusion rate across the interface.
[0064] FIG. 12 shows a graph of temperature and pressure versus
time for the process as practiced in accordance with the teachings
of the present invention.
[0065] As such, an invention has been disclosed in terms of
preferred embodiments thereof that fulfill each and every one of
the objects of the invention as set forth hereinabove, and provide
a new and useful ceramic armor and method of making by
encapsulation including use of a stiffening plate of great novelty
and utility.
[0066] Of course, various changes, modifications and alterations in
the teachings of the present invention may be contemplated by those
skilled in the art without departing from the intended spirit and
scope thereof.
[0067] As such, it is intended that the present invention only be
limited by the terms of the appended claims.
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