U.S. patent application number 11/430013 was filed with the patent office on 2010-09-23 for compositions for improved ceramic armor.
This patent application is currently assigned to BAE Systems Advanced Ceramics, Inc.. Invention is credited to Daniel Ashkin, Richard Palicka.
Application Number | 20100240517 11/430013 |
Document ID | / |
Family ID | 42738157 |
Filed Date | 2010-09-23 |
United States Patent
Application |
20100240517 |
Kind Code |
A1 |
Ashkin; Daniel ; et
al. |
September 23, 2010 |
COMPOSITIONS FOR IMPROVED CERAMIC ARMOR
Abstract
The present invention contemplates the addition of zirconium
compounds to well known ceramic ballistic materials to increase
resistance to penetration by projectiles. In the preferred
embodiments of the present invention, the zirconium compound that
is employed consists of ZrO.sub.2 and is provided in the range of
about 0.1% to about 11%, by weight, of starting material before
densification. Preferred ranges of proportion of ZrO.sub.2 in the
finished ceramic material are in the ranges of about 0.30% to about
0.75%, by weight, or about 8-9%, by weight. The ballistic material
using the combination of SiC with low volume of sintering aid and
ZrO.sub.2 raises the theoretical density of the ceramic material to
between 3.225 and 3.40 g/cc, which is slightly higher than the
typical 3.22 g/cc theoretical density for hot pressed fully dense
SiC. The unexpected result that accrues through combining silicon
carbide and zirconia consists of the creation of controlled defects
in the finished ceramic that increase the surface area that is
fractured during a ballistic event, enhancing spreading of the
forces imposed on the ceramic material by a projectile and, as a
result, the ability of the ballistic material to withstand higher
forces while resisting penetration.
Inventors: |
Ashkin; Daniel; (San Marcos,
CA) ; Palicka; Richard; (San Clemente, CA) |
Correspondence
Address: |
H. JAY SPIEGEL - H. JAY SPIEGEL & ASSOCIATES
P.O. BOX 11
MOUNT VERNON
VA
22121
US
|
Assignee: |
BAE Systems Advanced Ceramics,
Inc.
|
Family ID: |
42738157 |
Appl. No.: |
11/430013 |
Filed: |
May 9, 2006 |
Current U.S.
Class: |
501/91 ;
501/88 |
Current CPC
Class: |
C04B 2235/80 20130101;
C04B 2235/77 20130101; C04B 35/575 20130101; C04B 2235/3244
20130101; C04B 2235/3225 20130101; C04B 2235/3865 20130101; C04B
2235/3246 20130101; C04B 2235/3834 20130101; C04B 2235/786
20130101; C04B 2235/383 20130101; C04B 2235/3839 20130101 |
Class at
Publication: |
501/91 ;
501/88 |
International
Class: |
C04B 35/565 20060101
C04B035/565; C04B 35/56 20060101 C04B035/56 |
Claims
1. A ceramic armor comprising a densified mixture of a silicon
carbide ceramic material and zirconia, said mixture comprising 0.1
to about 11%, by weight, of zirconia before densification, said
silicon carbide ceramic material being sintered with aluminum
nitride as a sintering aid, said armor having a density at least
99% of theoretical density.
2. The armor of claim 1, wherein said silicon carbide ceramic
material comprises a hot pressed dense SiC material made from
silicon carbide powder of which at least 92% is made up of alpha or
beta silicon carbide.
3. (canceled)
4. (canceled)
5. The armor of claim 1, wherein said zirconia reacts to zirconium
carbide during heat treatment.
6. The armor of claim 1, wherein said zirconia comprises about 0.3%
to about 0.75%, by weight, of said mixture.
7. The armor of claim 5, wherein said zirconia comprises about 0.3%
to about 0.75%, by weight, of said mixture.
8. The armor of claim 1, wherein said armor includes controlled
defects that enhance spread of forces when a projectile strikes
said armor.
9. The armor of claim 4, wherein said armor includes controlled
defects that enhance spread of forces when a projectile strikes
said armor.
10. The armor of claim 9, wherein said zirconia has a thermal
expansion greater than a thermal expansion of silicon carbide.
11. The armor of claim 10, wherein said armor has a density greater
than 3.22 g/cc.
12. The armor of claim 1, wherein said zirconia comprises about
8-9%, by weight, of said mixture.
13. A ceramic armor comprising a mixture of 89-99%, by weight, hot
pressed SiC material and 0.1 to 11%, by weight, of ZrO.sub.2
starting material before densification, said SiC material being
sintered with MN as a sintering aid, said armor having a density at
least 99% of theoretical density.
14. The armor of claim 13, wherein said ZrO.sub.2 comprises about
0.3% to about 0.75%, by weight, of said mixture.
15. The armor of claim 13, wherein said armor includes controlled
defects that enhance spread of forces when a projectile strikes
said armor.
16. The armor of claim 13, wherein said ZrO.sub.2 has a thermal
expansion greater than a thermal expansion of silicon carbide.
17. The armor of claim 13, wherein said armor has a density greater
than 3.22 g/cc.
18. The armor of claim 13, wherein said ZrO.sub.2 comprises about
8-9%, by weight, of said mixture.
19. (canceled)
20. The armor of claim 14, wherein said armor has a density greater
than 3.22 g/cc.
21. A ceramic armor comprising a densified mixture of a silicon
carbide ceramic material and a compound including a constituent
ingredient selected from the group consisting of Ti, V, Nb, Cr, W
and Mo, said compound comprising 0.05 to about 6%, by volume, of
material before densification.
22. The armor of claim 21, wherein said compound includes metal
oxide reacted to metal carbide during heat treatment.
23. The armor of claim 22, wherein said compound has a thermal
expansion greater than a thermal expansion of silicon carbide.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates to compositions for improved
ceramic armor. Dense silicon carbide ceramics have been shown to be
an effective means to protect against a wide variety of ballistic
threats because of their combination of high hardness, strength and
stiffness with low bulk density and favorable pulverization
characteristics upon impact. Dense silicon carbide is typically
produced using the following method steps: [0002] 1. Mixing fine
silicon carbide powders with a sintering aid. [0003] 2. Drying the
powder and if necessary adding processing aids to it. [0004] 3.
Green forming the powders into a shape so that individual powder
grains are in close contact with each other and the shape is
retained after forming. Common forming techniques are slip casting,
dry pressing, and extrusion. [0005] 4. Removing the processing aids
such as binders by heat treatment. [0006] 5. Densification of the
ceramic part by sintering, hot pressing, hot isostatic pressing
(HIP) or sinter+HIP in a controlled atmosphere.
[0007] Silicon carbide is a covalently bonded ceramic having low
self-diffusion coefficients. To increase diffusion and facilitate
densification, sintering aids are added in amounts generally less
than 5 volume percent. For silicon carbide, the first additive used
for densification was Al.sub.2O.sub.3 and the method for
densification was hot pressing. This was developed in the 1950s as
reported by the Journal of the American Ceramic Society, Volume 39,
pp. 386-89 (1956) in the article by Alliegro et al. titled
"Pressure-Sintered Silicon Carbide." Effective use of
Al.sub.2O.sub.3 was also reported by the Journal of Material
Science, Vol. 10, pp. 314-320 in the article by F. F. Lange titled
"Hot-Pressing Behavior of SiC Powders with Additions of Alumina,"
and was shown by Lange to be due to liquid phase sintering. In
1970s and 1980s, pressureless sintering was developed by using
compounds based on elements such as Boron (B), (S. Prochazka, "The
Role of Boron and Carbon in Sintering of Silicon Carbide," Special
Ceramics, Vol. 6, edited by P. Popper, Stoke-On-Trent, England,
1975, pp. 171-181), Carbon (C), Beryllium (Be), (U.S. Pat. No.
4,172,109), and Aluminum (Al), (D. H. Stutz, S. Prochazka, J.
Lorenz, "Sintering and Microstructure Formation of Beta-SiC," J.
Am. Ceram. Soc., 68[9], 479-82, (1985). These additives were added
for the purpose of promoting solid-state diffusion. Carbon was
added for the purpose of cleaning off the SiO.sub.2 layer from the
silicon carbide surfaces and allowing the surfaces to be activated
by B, Be and Al.
[0008] In the 1980s and 1990s, work was done on liquid phase
sintering of SiC using rare earth oxide additives. This was
disclosed in U.S. Pat. Nos. 4,564,490 and 4,569,921 and in an
article by L. Cordrey, et al. titled "Sintering of Silicon Carbide
with Rare-Earth Oxide Additions," in Sintering of Advanced
Ceramics, v. 7, (1990), pg. 618, edited by C. A. Handwerker, et al.
For these materials, the diffusion occurs through the liquid phase
instead of through the solid phase. For successful liquid phase
sintering, Negita stated that free energy of formation for the
metal oxide additives must be more negative than free energy of
oxidation for silicon carbide at sintering temperatures. See K.
Negita, "Effective Sintering Aids for Silicon Carbide Ceramics:
Reactivities of Silicon Carbide with Various Additives," J. Am.
Ceram. Soc., 69[12], C-308 C-310, (1986). By using oxide additives
that meet this criterion, the oxide additives remain stable and do
not result in oxidation and decomposition of the silicon carbide.
Oxidation and decomposition of the silicon carbide, besides
resulting in lost material, produces gas species that can inhibit
sintering. Shown below are the reactions for silicon carbide
decomposition along with the temperature in which they have the
lowest free energy. It is seen that reaction products change with
temperature.
SiC+O.sub.2.fwdarw.SiO.sub.2(s,l)+C (300 1800.degree. C.) 1)
2/3SiC+O.sub.2.fwdarw.2/3SiO.sub.2(s,l)+2/3CO (1800 2075.degree.
C.) 2)
SiC+O.sub.2.fwdarw.SiO(g)+CO (2075 2100.degree. C.) 3)
2 SiC+O.sub.2.fwdarw.2Si(s,l)+2CO (2100 2600.degree. C.) 4)
[0009] The free energy versus temperature is shown in FIG. 1. FIG.
2 shows the free energy of formation for rare earth oxides in
comparison to reaction c from 1800-2600.degree. K. Rare earth
oxides it is seen are stable versus silicon carbide. As such, much
emphasis was placed on the sintering behavior of these materials in
the 1990s.
[0010] In early 1990s, Andre Ezis at BAE Systems Advanced Ceramics
showed that the choice of sintering additive was important in
determining ballistic performance. See U.S. Pat. No. 5,354,536. The
use of AlN as a sintering additive was shown to result in clean
grain boundaries even in grades of SiC with metal impurities and
the optimum amount of sintering additive was found to be a function
of the surface area. The fracture of these materials was
intergranular. The superior ballistic performance of these
materials suggests the importance of fracture mechanism and
microstructure in determining ballistic behavior. It should be
noted that the ballistic event involves significant pulverization
of the ceramic. The pulverization characteristics of ceramics, in
which a significant mass of material is comminuted into fine
particles underneath the projectile, have not been related to
static mechanical properties. Static mechanical properties when
applied to ballistic properties have generally been applied to
cracks forming near the surface of the ceramic during impact.
[0011] In SiC for static applications, second phase additions have
been added to improve toughness. Specifically, M. Janney,
"Mechanical Properties and Oxidation Behavior of a Hot-Pressed
SiC-15 vol %-TiB.sub.2 Composite," Ceramic Bulletin, Vol. 66[2],
322-324, (1987), determined an increase in toughness from 3.1 to
4.3 MPa m 1/2 with 15 volume percent TiB.sub.2 additive (20 weight
percent) and G. C. Wei and P. Becher, "Improvement in Mechanical
Properties in SiC by Addition of TiC Particles," Journal American
Ceramic Society, 67[8] 571-74 (1984), determined an increase in
fracture toughness for different volume percent TiC additive. These
carbides and borides are stable versus SiC at high temperatures and
have been shown to toughen the material by crack deflection.
[0012] In further work, V. D. Krstic and M. Vlajic, U.S. Pat. No.
5,470,806, patented a powder bed technology to liquid phase sinter
SiC with transition metal oxide additives for the purpose of fusing
the oxides and converting them into carbides during the course of
sintering. The powder bed surrounding the part contained silicon
carbide, aluminum oxide and carbon and facilitated the conversion
to carbides in the sintered body and prevented excessive weight
loss during sintering. The part and the powder bed were contained
in a sealed graphite crucible. Aluminum oxide was used as the
sintering additive to promote rapid densification and minimize
reaction times. For an SiC composition containing 6.5 wt. %
Al.sub.2O.sub.3, 2.5 wt. % TiO.sub.2, 6.0 wt. % ZrO.sub.2 and 2.0
wt. % C, a fracture toughness of 6.3 Mpa m 1/2 could be achieved
while for an SiC composition containing 8.7 wt. % Al.sub.2O.sub.3,
20.0 wt. % TiO.sub.2, 6.3 wt. % ZrO.sub.2 and 5.0 wt. % C, a
fracture toughness of 7.2 MPa m 1/2 could be achieved. In these
materials, the TiO.sub.2 and ZrO.sub.2 reacted to TiC and ZrC
during sintering. Typical sintered SiC has a fracture toughness of
4.0 to 5.0 MPa m 1/2.
[0013] In an effort to increase the ballistic performance of SiC,
Applicants have looked at oxide additives that do not meet Negita's
criterion for use as sintering aid and result in carbide formation
below or at sintering temperatures. Oxide additives that do not
meet Negita's criterion cause the decomposition of SiC by either
simultaneous formation of metal carbide, silicon and CO or
simultaneous formation of silicon, metal and CO. The generalized
equations are shown below.
2SiC.sub.(s)+aM.sub.vO.sub.w
(s,l).sub.--2Si.sub.(s,l)+bM.sub.(s,l)+2CO.sub.(g)
SiC.sub.(s)+cM.sub.vO.sub.w
(s,l).sub.--dSiO.sub.(g)+eM.sub.XC.sub.y (s,l)
[0014] In the present invention, ZrO.sub.2 additions are added
which will result in decomposition of the SiC and formation of ZrC,
and some combination of Si, SiO and CO. The present invention
demonstrates that ZrO.sub.2 can be used to increase ballistic
performance of silicon carbide when added in small amounts and
using a furnace design in which the atmosphere can be controlled.
Both ZrO.sub.2 and stabilized ZrO.sub.2 such as 3 mole percent
Y.sub.2O.sub.3 stabilized ZrO.sub.2 (3TZY) can be used.
[0015] Along with decomposition of SiC and ZrO.sub.2 during
sintering of SiC, volatization reactions involving the SiO.sub.2
(native silica on the SiC grain) and ZrO.sub.2 to SiO, ZrO, and
O.sub.2 would be expected in SiC ceramics. The vapor pressure of
these species depends on temperature, the amount of SiO.sub.2,
ZrO.sub.2, free carbon, other phases and kinetic considerations.
The presence of these species however would not affect the
direction of the high temperature decomposition reactions at
sintering temperatures of SiC in inert gas or vacuum furnaces.
SUMMARY OF THE INVENTION
[0016] The present invention relates to compositions for improved
ceramic armor. The present invention includes the following
objects, aspects and features: [0017] (1) In a first aspect, the
present invention contemplates the addition of zirconium compounds
to well known ceramic ballistic materials to increase resistance to
penetration by projectiles. [0018] (2) In an effort to increase the
ballistic performance of SiC, Applicants have looked at hot press
technology and oxide additives that would decompose SiC and form
stable carbides to improve performance. These SiC mixtures were
loaded into a graphite hot press die and no powder bed was used.
The silicon carbide material is sintered with aluminum nitride as
the sintering aid as recited in original dependent Claim 3. The
compositions with improved performance included those with trace
amounts of oxides generally not considered sufficient for purposes
of toughening the materials. The trace quantities could be added by
various means including the use of attrition during ball milling.
For the case of the transition metal oxide, zirconium oxide,
zirconia (ZrO.sub.2) and yttria stabilized zirconia milling media,
could be used. Other elements with oxides that meet the condition
of decomposing SiC and forming stable carbides without forming
silicides are Ti, V, Nb, Cr, Mo, and W. In addition, metals of
these compounds can be used as reactants for carbide formation as
well as carbonates, nitrates, etc. that form oxides at elevated
temperatures. [0019] (3) In the preferred embodiments of the
present invention, the zirconium compound that is employed consists
of ZrO.sub.2 and is provided in the range of 0.1% to 11%, by
weight, to the starting ceramic powder. Preferred ranges of
proportion of ZrO.sub.2 in the powder are in the ranges of about
0.30% to about 0.75%, by weight, and about 8% to about 9%, by
weight and react to zirconium carbide during hot pressing. [0020]
(4) Applicants have found that creation of a ballistic material
using the combination of SiC--N and ZrO.sub.2 starting powders
raises the density of the ceramic material to about 3.23 to 3.40
g/cc, slightly higher than the theoretical density of SiC--N, 3.22
g/cc. SiC--N is a BAE Systems Advanced Ceramics (formerly Cercom)
product based on U.S. Pat. No. 5,354,536. [0021] (5) The unexpected
result that accrues through combining silicon carbide ceramics and
zirconia consists of the creation of controlled defects in the
microstructure of the finished ceramic that increases the fracture
surface energy during a ballistic event. These defects result in
enhanced spreading of the forces imposed on the ceramic material by
a projectile and, as a result, the ability of the ballistic
material to withstand higher forces while resisting
penetration.
[0022] As such, it is a first object of the present invention to
provide compositions for improved ceramic armor.
[0023] It is a further object of the present invention to provide
such a ballistic material in which a powder made up of a zirconium
compound is intermixed with a compound including silicon carbide in
a desired proportion.
[0024] It is a still further object of the present invention to
provide such a ballistic material that has controlled defects in
its microstructure that enhance resistance to projectile
penetration.
[0025] 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
[0026] FIG. 1 shows a graph of temperature dependence of the
standard free energies for the oxidation reaction of silicon
carbide.
[0027] FIG. 2 shows a graph of the formation of rare earth oxides
as compared to reactions "d" from FIG. 1, from 1800-2600 degrees
K.
[0028] FIG. 3 shows a graph from Negita (1986) of oxides that do
not decompose SiC at temperatures of 2400.degree. K. (hatched
area).
[0029] FIG. 4 comprises an SEM photomicrograph, backscatter
electron image, of the fracture surface of a PAD SiC--N with 0.75%,
by weight, starting ZrO.sub.2 powder. The ZrO.sub.2 in this
material has reacted to zirconium carbide.
[0030] FIG. 5 comprises an SEM photomicrograph, backscatter
electron image of the polished surface of a PAD SiC--N with 8.53%,
by weight, ZrO.sub.2 starting powder. The ZrO.sub.2 in this
material has reacted to zirconium carbide.
SPECIFIC DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0031] To improve the performance of Silicon Carbide for ballistic
applications, Applicants produced composites from starting
composition SiC+sintering additive+ZrO.sub.2 that reacts to
SiC+ZrC. ZrO.sub.2 is one of several metal or transition metal
oxides that form stable carbides when reacted with SiC at high
temperatures. ZrC has moderate strength, high hardness, and high
toughness. ZrC, though not used as a ballistic material, because of
high density and cost, shares some of the characteristics of an
armor material in showing good strength with high hardness. In
limited studies, ZrO.sub.2/ZrC has been added to SiC in amounts of
greater than 5 wt. % for the purposes of toughening. It should be
noted that the reaction of ZrO2 to ZrC reduces the molecular weight
of the zirconium containing compound from 123.2 g to 103.2 g and
the molar volume from 22 cc to 15.3 cc. As such, this reaction has
the effect of reducing the weight and volume percent of zirconium
containing compound.
[0032] In this work, it has been found that small additions, less
than 1 wt. % of ZrO.sub.2 to the starting powder can, in fact, be
beneficial for use in silicon carbide dwell armor. In dwell armor,
the ceramic is put into compression and remains intact as a long
rod tungsten projectile strikes the surface of the ceramic at
velocities up to or exceeding 1500 msec. See P. Lundberg, R.
Renstrom, L. Holmberg, "Impact of metallic projectiles on ceramic
targets: transition between interface defeat and penetration," Int.
J. Impact Engng., 24, 259-75, (2000). After the period of dwell in
which the projectile is ablated, it has been shown that the ceramic
cracks and then becomes comminuted. See D. A. Shockey, A. H.
Marchand, S. R. Skaggs, G. E. Cort, M. W. Burkett and R. Parker,
"Failure Phenomology of Confined Ceramic Targets and Impacting
Rods," Ceramic Armor Materials by Design, edited by J. W. McCauley
et al., (The American Ceramic Society, Westerville, Ohio 2002), pp.
385-402. The nature of this comminuted zone is not well understood
but ballistic tests have suggested that the fracture behavior of
the grains and grain boundaries are of utmost importance. With this
in mind, Applicants have introduced small additions of ZrO.sub.2 to
influence the fracture behavior of the grains and grain boundaries.
It was found by Applicants that ZrO.sub.2 additions, though not
thermodynamically stable as shown in FIG. 3, have a favorable
effect on the ballistic performance of SiC. These additives react
to ZrC and likely cause the decomposition of SiC to Si or SiO. This
decomposition reaction along with the residual stresses from the
thermal expansin mismatch between the SiC and ZrC leads to a
weakened grain boundary phase that acts as a controlled defect.
Since ZrC has a higher thermal expansion than SiC,
7.times.10.sup.-6 versus 4.5.times.10.sup.-6, the stresses on the
grain boundary are tensile. This means that upon pulverization
during a ballistic event, localized cracking occurs along these
grain boundaries. This cracking of the stressed and weakened grain
boundaries acts as an energy absorber as a result of a controlled
defect. For the purpose of having a weak grain boundary phase, it
is advantageous that the free energy of the decomposition reactions
for ZrO.sub.2 is only slightly less negative than that of the
oxidation reactions for silicon carbide. This implies that the
reaction of ZrO.sub.2 to ZrC occurs at near the final sintering
temperature of the SiC and defects from these decomposition
reactions are preserved in the microstructure. EDS analysis has
shown some residual oxygen (1-6%) in the zirconium carbide after
hot pressing, suggesting that reaction is still progressing during
final heat treatment. As such, compared to the other oxides outside
the hatched area in FIG. 3 that likely decompose SiC and form
carbides at lower temperatures, zirconium oxide is most preferable.
However other oxides that react more easily to carbides will also
have some beneficial effects on the ballistics of the SiC, both
from a weakened grain boundary and residual stress. The metals in
FIG. 3 that have associated oxides that form stable carbides at the
sintering/hot pressing temperature of SiC are Ti, V, Nb, Cr, W, and
Mo. Oxides of these metals could be substituted for zirconium
oxide.
[0033] The regions outside the hatched area will react with SiC and
decompose it. The y-axis of the plot correspond to the standard
free energy of formation for the reaction
bM.sub.(s,l)O.sub.2(g)aM.sub.vO.sub.w(s,l)
The x-axis of the plot correspond to the standard free energy of
formation for the reaction
dM.sub.xC.sub.y (s,l)+O.sub.2(g).fwdarw.aM.sub.vO.sub.w
(s,l)+fCO.sub.(g)
The horizontal and vertical lines correspond to reaction
D.(2SiC+O.sub.2.fwdarw.2Si(s,l)+2CO)
[0034] The thermal expansion of carbides associated with these
metals is similar to that of ZrC. The thermal expansions of
carbides for Mo, V, Nb, and Ti are between 7 and
8.5.times.10.sup.-6 in/in .degree. C. while Cr.sub.3C.sub.2 has a
thermal expansion of 11.5.times.10.sup.-6 in/in .degree. C. As
noted, silicon carbide has a thermal expansion of
4.5.times.10.sup.-6 in/in .degree. C. and the effect of these
differences in thermal expansion between SiC and carbides formed
from oxides such as zirconia is residual stresses and/or
microcracking that preferentially cause intergranular fracture
during a ballistic event. Intergranular fracture has been shown by
Ezis to be an important feature for ballistic grade SiC and it is
significant that these additions do not affect the grain boundary
chemistry in ballistic grade SiC and change the mode of fracture.
Instead, the formed carbides appear to act as a supplement that
effects crack propagation through the mass of SiC grains during
comminution. It should be noted that the improvement in ballistic
performance from zirconium carbide particles can not be related to
a simple improvement in fracture toughness from 2.sup.nd phase
particles since 2.sup.nd phase particles generally only have a
toughening effect when used in excess of 5 volume percent. It
should also be noted that a partial reaction of the oxide to
carbide has minimal effect on residual stresses of these materials
due to the oxides high thermal expansion. ZrO.sub.2 has a thermal
expansion of 12.times.10.sup.-6 in/in .degree. C. while TiO.sub.2
has a thermal expansion between 7 and 10.times.10.sup.-6 in/in
.degree. C. The other oxides associated with the metal have,
similarly, a higher thermal expansion than SiC.
[0035] In a first example, ZrO.sub.2 was added to BAE Systems
SiC--N powder by use of Yttria Stabilized Zirconia Grinding Media
(TZ-3Y) during ball milling in polypropylene jar. By measuring the
attrition of the grinding media, the amount added was found to be
0.72 weight % zirconia. This zirconia, because it is from the wear
of milling media, is fine grained in size and well dispersed
through the powder. After milling the powder and sintering
additive, the powder was dried and sieved as typical powder by BAE
Systems Advanced Ceramics PAD SiC--N processing. The material was
then hot pressed using typical BAE Systems Advanced Ceramics PAD
SiC--N cycle. The material after hot pressing was machined and
tested. The density of the hot pressed parts was 3.235 g/cc, which
is greater than PAD SiC--N, which has a theoretical density of 3.22
g/cc. The increase in density is due to the addition of zirconia
and/or zirconium compounds. As seen in FIG. 4, the microstructure
shows that the zirconium compounds, bright particles, are evenly
distributed and are fine grained (0.2 to 2 microns). XRD and
EDS/SEM analysis indicated that bright particles were zirconium
carbide indicating reaction of the starting zirconium oxide grains
with silicon carbide to zirconium carbide. The average grain size
of the final product was similar to PAD SiC--N, 3-5 microns.
[0036] The produced material with 0.72% zirconia starting additive
was shown by Army Research Laboratories to have better ballistic
performance than BAE Systems Advanced Ceramics PAD SiC--N for both
seam shots and center shots. At a thickness of 0.75'', tiles made
with zirconia additions could not be penetrated by threat typically
used to penetrate PAD SiC--N. This is despite the projectile being
tested at a higher velocity (maximum velocity). Tiles made at 0.5''
thick had an improved v.sub.50 compared to PAD SiC--N.
[0037] In a second example, ZrO.sub.2 was added to BAE Systems
Advanced Ceramics PAD SiC SC-1R powder by using Yttria Stabilized
Zirconia Grinding Media (TZ-3Y) during ball milling. By measuring
the attrition of the grinding media, the amount added was found to
be 0.75 weight % zirconia. This zirconia, because it is from the
wear of milling media, is on average 0.2 to 2 microns in size and
well dispersed through the powder. After milling the powder and
sintering additive, the powder was dried and sieved as typical
powder by BAE Systems Advanced Ceramics PAD SiC SC-1R processing.
The material was then hot pressed using typical BAE Systems
Advanced Ceramics PAD SiC SC-1R cycle. The material after hot
pressing was machined and tested. The density of the hot pressed
parts was 3.235 g/cc, which is greater than PAD SC-1R, which has a
theoretical density of 3.22 g/cc. The increase in density is due to
the addition of zirconia that reacts to zirconium carbide. PAD SiC
SC-1R has an average grain size of 1.5 microns.
[0038] The produced material with 0.75% zirconia was shown by Army
Research Laboratories to have better ballistic performance than BAE
Systems Advanced Ceramics PAD SiC--N and BAE Systems Advanced
Ceramics PAD SC-1R. At a thickness of 0.75'', tiles made with
zirconia could not be penetrated by a threat typically used to
penetrate BAE Systems Advanced Ceramics PAD SiC--N. This was the
case despite the projectile being tested at a higher velocity
(maximum velocity).
[0039] In a third example ZrO.sub.2 was added to BAE Systems
Advanced Ceramics SiC--N powder by use of Yttria Stabilized
Zirconia Grinding Media (TZ-3Y) during ball milling in
polypropylene jar. By measuring the attrition of the grinding
media, the amount added was found to be 0.3 weight % zirconia. This
zirconia, because it is from the wear of milling media, is fine
grained in size and well dispersed through the powder. After
milling the powder and sintering additive, the powder was dried and
sieved as typical powder by BAE Systems Advanced Ceramics PAD
SiC--N processing. The material was then hot pressed using typical
BAE Systems Advanced Ceramics PAD SiC--N cycle. The material after
hot pressing was machined and tested. EDS/SEM analysis indicated
that the zirconium carbide had been formed.
[0040] The produced material with 0.3% zirconia starting material
was shown by Army Research Laboratories to have better ballistic
performance than BAE Systems Advanced Ceramics PAD SiC--N in terms
of v.sub.50. The tests were performed on 0.500'' tiles.
[0041] In a fourth example, 8.53 weight percent TZO powder
(tetragonal ZrO.sub.2 powder with no yttria addition) from Tosoh
was added to PAD SiC--N powders by ball milling. These powders have
a typical surface area of 14 m.sup.2/g and are sub-micron in size.
The milling media was typical of what is used for PAD SiC--N. The
materials were milled for slightly shorter times and using a
different volatile organic solvent to maximize dispersion of the
powders. The material was dried and sieved as typical PAD SiC--N
powder. The powders were then hot pressed using a modified hot
pressing procedure. Compared to conventional PAD SiC--N, the
material was found to hot press at a lower temperature, suggesting
that the zirconia reacted with the sintering aids/oxides in the
system. Despite this interaction with sintering aids/oxides in the
system, the zirconia was found to react to zirconium carbide by
SEM/EDS analyses suggesting that zirconium carbide is the
thermodynamically stable phase at these temperatures. At this
concentration of zirconia addition, some zoning in the hot press
body was found. Areas of increased zirconium concentration were
found. Despite these microstructural inhomogeneities, ballistic
performance was found to increase for DOP tests (20% improvement in
performance) compared to PAD SiC--B.
[0042] In a fifth example, 8.53 weight percent TZ-3Y powder (yttria
stabilized ZrO.sub.2) from Tosoh was added to the PAD SiC--N
powders by ball milling. These powders have an average surface area
of 16 m.sup.2/g and are sub-micron in size. The milling media was
typical of what is used for PAD SiC--N. The material was dried and
sieved as typical PAD SiC--N powder and was hot pressed using the
same pressure and temperature schedule as used for the material
made from 8.53 weight percent TZ-0. This hot pressed material had
similar density, microstructural features, and ballistic
performance as the material made from 8.53 weight percent TZ-0
material. This suggests the effect of yttria in the TZ-3Y powders
is minimal. Both materials made from TZ-0 and TZ-3Y powders had a
density of between 3.32 and 3.34 g/cc. This corresponds to at least
99% of theoretical density. The zirconium containing particles in
these materials were visible using the backscatter mode of the SEM,
see FIG. 5. They were on the order 1 to 5 microns in size and could
be analyzed to be zirconium and carbon rich. Table I shows EDS
spectra of a zirconium rich particle in the fracture surface of
FIG. 5.
TABLE-US-00001 TABLE I EDS analysis of Zirconium Rich Particle in
FIG. 5 Element Weight % Atomic % Carbon 15.8% 56% Oxygen 2.2% 5.8%
Zirconium .sup. 82% 38.2%.sup.
[0043] The results from these examples show that the benefit of
adding zirconia to the starting material applies over a wide range
of weight percent additives and for different size additions.
Additions of 11 weight percent zirconia resulted in cracking of the
ceramic after densification. The densified material had a density
of between 3.38 and 3.40 g/cc, which corresponds to 9 to 10 weight
percent reached ZrC or 11 weight percent unreacted ZrO2. This is
the upper weight percent addition that is beneficial to ballistic
performance of the material. The benefit of zirconia additions at
even low concentrations of 0.30%, show that even small additions of
0.1% ZrO2/ZrC, are beneficial to ballistic performance.
[0044] The present invention contemplates a densified mixture of a
silicon carbide ceramic material and a zirconium compound, the
mixture consisting of 0.1 to about 11%, by weight, of zirconium
compound before densification as recited in original independent
Claim 1. The present invention also contemplates sintering the
silicon carbide ceramic material with aluminum nitride as the
sintering aid, as recited in original Claim 3 which was dependent
upon original independent Claim 1.
[0045] As such, an invention has been disclosed in terms of
preferred embodiments thereof which fulfill each and every one of
the objects of the invention as set forth hereinabove, and provide
new and useful compositions for improved ceramic armor of great
novelty and utility.
[0046] 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.
[0047] As such, it is intended that the present invention only be
limited by the terms of the appended claims.
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