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Abstract

 

An increase
in the anti-armor threats has put forward the need for an increase in the
development of ballistic armor protection systems or mechanisms. The most
innovative idea or creation is nano-ceramics. Nano-ceramics has shown promise
in development of armors which has got significant ballistic properties. In
order to increase the effectiveness of the nano-ceramics in acting as an armor,
significant research has taken place. After conducting DOP tests on various
types of ceramic material it was concluded that the ballistic properties of the
ceramic material has increased with an increase in the macro hardness and by
using nano-ceramics with grain size greater than 0.5µm.

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1. Introduction

 

 Modern compact scenarios present itself with a
lot of situations that could be fatal for a soldier. It is necessary to equip a
soldier with body armors. These body armors should be capable of protecting the
soldier from various lethal weapons like bullets, body armor piercing rounds,
etc.

Armored
vehicles were introduced to protect the soldiers from bullets and safely reach
the battleground. But along with it came new technologies that made armored
vehicles like plastic. Kinetic 
energy  (KE)-penetrators or  armor 
piercing (AP)-bullets, 
shaped  charges,  explosively 
formed  penetrators  (EFP) 
and  fragments  from mines and improvised explosive device
(IED) in combination with blast waves are some of the threats faced by armored
vehicles nowadays 1.

Armor
used for resisting armor piercing rounds are made ceramic/composite “sandwich”
and is called as hybrid armor. The idea that hard
enamel coating on steel improved bullet resistance. Lack of lightweight armor

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

leads to loss of helicopters during
the Vietnam War. This in turn leads to a more dedicated and well-funded
research and development. It was soon concluded that ceramics was the best
choice for improved armor 2.

 

Further research and development on
ceramics has shown that ceramics alone was not sufficient to withstand incoming
projectiles. To overcome this obstacle an energy absorbing layer was
introduced. It was also found out that a combination of ceramic and composite
is lighter than a steel armor of the same size. This understanding lead to the
development of hybrid armor which is a layer formed by the bonding of a ceramic
and composite 2.

 

 

 

 

Fig 1: Hybrid Armor model

The effect of
adhesive was found to be negligible after conducting experiments on various
composites. One possible method to estimate the ballistic limited velocity of
the composite is by using an analytical model given by Florence. The model is
based on energy balance equations of the projectile and target. Kaufman et al.
conducted penetration depth tests on four different types of ceramics using a
12.7 mm projectile. The tests concluded that silicon carbide ceramics are
stronger than alumina ceramics 2.

 

Studies
conducted by various researchers reported that addition of nanoscale particles
in small percentage resulted in the improvement in the mechanical properties of
ceramics. Niihara was able to increase the strength of alumina by uniformly distributing
nano-particles inside the base structure of alumina. Sadough Vanini et al.
realized that addition of SiC (Silicon Carbide) particles changed the ceramic
fracture mode from inter-granular to trans-granular which in turn increased the
fracture toughness and strength of the ceramic.

 

2. Material Properties

 

Ceramics acts
as an effective material armor because of two main reasons: it reduces the
projectile into fragments on impact and distributes the impact load over a
larger area of the backing material.

 

Tile thickness

Shockey
et al. found out that the initial resistance is due to the compressive strength
of the ceramic. The projectile will be fractured or deformed upon impact. This
happens only when the strength of the ceramic exceeds the strength of the projectile 2. This implies that
the compressive strength of the ceramic plays a major role in fracturing the
projectile.

 

Hardness

 

From the
work done by Rosenberg and Yesherun, it can be noted that the ability of a
projectile to penetrate the backing material can be severely decreased by
blunting the projective. An observation by Den Reijer states that there is no
benefit in increasing the hardness of the projectile above the required value 2.

 

 

 

 

 

 

 

 

 

 

 

Density

 

Various
researchers have stated that low density is beneficial for target material.
Lower density contribute to lighter weight armor. At the same time it allows in
using a thicker ceramic without a substantial weight penalty 2.

 

Young’s, Bulk, Shear moduli

 

Young’s,
Bulk, Shear moduli are the main factors that determines the ability of a armor
to defeat a projectile. The values of these factors that result in an effective
armor has to be determined by conducting experimental trials on different types
of ceramics having different values of these factors 2.

 

Shear Strength

 

Tension
and large stress gradients exist next to the contact area and area directly
underneath the projectile core respectively. This implies that higher yield
strength helps in resisting the failure due to the shear stresses produced near
the impact site 2.

 

3. Experimental Procedures

 

The
method used for evaluating the ballistic protection capability is done by using
DOP (Depth of Penetration) test. The target is a composite made of a ceramic
under study and a backing material. The result is then compared with the
residual penetration in a semi-infinite reference target. This method of
experimentation helps in determining the maximal ballistic protection potential
and also to compare between different results used for experimentation. The
backing material used was an armor steel of medium tensile strength (1000 MPa)
and also as a reference material for comparison. Because of higher stiffness
steel was preferred over aluminum.

 

 

Erhardt
Lach et al. conducted ballistic tests using a tungsten heavy metal rod having a
geometry of 4 mm in diameter and 60 mm in length. The impact velocity added up
to 1660 m/s. Al2O3 ceramic tiles of 10 mm in thickness

 

 

 

 

 

 

 

 

 

 

 

 

and Si3N4
ceramic tiles of 20 mm in thickness were used for DOP tests 1. The parameters used
while performing DOP tests are schematically shown in the figure given below.
The data measurement will be done at the target and the results procured will
be used to evaluate the results using the equations 1.

 

 

 

Fm =

 

 

   Fs

 

      F =
Fm * Fs

 

 

 

Ali Asadi
et al. has conducted experiments using alumina powder which has a purity of
99.6% and has a grain size of 3 microns. SiC and MgO particles having a grain
size of 100 nanometers were added to improve the mechanical properties of
alumina. The process used to achieve this is explained using a flow chart 4.

 

 

 

 

 

 

Fig 2: The
process of preparing nano-composite tile ceramic

 

 

 

 

 

Six
different combinations of alumina powders combined with silicon carbides having
volume percentages of 0, 2.5, 5, 7.5, 10, 15 and 500 ppm MgO powder solute
where mixed in isopropyl alcohol in a planetary mill for a duration of 3 hours
inside a magnetic mixer at a temperature of 100°C and then kept for drying at a
temperature of 130°C. Hydraulic press of 20 to 30 bars is used to produce nano-composite
with dimensions 120x120x12 mm 4.

 

Various
factors like relative density, hardness, strength, elasticity modulus and fracture
were measured during the tests. The ballistic energy distribution coefficient
is calculated using

 

D=0.36(HCE)/ Kw
 2

 

Area-density of the panels
is evaluated using the given equation 4

Areal-density (

 =

 

Where n, di, ti
represents number of layers, density of separate layers and the number of
layers.

 

Andreas Krell
et al. tells about another important parameter that influence the transmission
measurements is specimen thickness. Due to the thickness effect only materials
with real-in-line transmission with a value close to the theoretical value can
be enlarged in thickness. Mustafa Beyila et al. performed several tests on
alumina/aluminum composite targets 5. The heat treatment
conditions that they investigated along with the thickness of aluminum and
alumina layers have shown that they have a major influence on the ballistic
behavior of the composite target.

 

M Bolduc et al.
have done DOP tests on Al2O3 ceramic composites. The
sintering of SiC + CNT was found out to be unsuccessful and also no samples
were available for the tests 3. The measured values
corresponds to straight lines from the point of penetration to the deepest
distance in the backing material as shown in the figure below:

 

 

 

Fig 3: DOP measurement

 

 

 

 

 

Fig 4: DOP measurement

 

 

4. Results

 

Erhardt Lach et al. after conducting DOP tests on Al2O3 and
concluded that the results obtained after the performing evaluation of the
equations with the values measured has shown that Fa values
increased with increase in hardness or with decrease in grain size 1. The best outcome
was obtained or seen for a grain size of 0.6 µm. It was also concluded that the
macro hardness decreased with decrease in grain size. This lead to a slight
increase in ballistic result. It was also observed that maximum in ballistic
protection was seen at the smallest grain size without massive amorphous
inter-grain phases. The grain size of Al2O3 with grain size of 0.6µm and 9.82µm
is shown below.

 

 

 

Fig 5: SEM micrographs of fractured Al2O3 with
grain size 0.6µm

 

 

 

 

 

 

 

 

Fig 6: SEM micrographs of fractured Al2O3 with
grain size 9.82µm

 

 

5. Conclusion

 

From the DOP tests conducted the authors mentioned in
the paper, it can be understood that the ballistic performance of a nano-ceramic
depends on the macro hardness and the type of the nano-ceramic used. Since
liquid phase sintering increases the amorphous phases between the crystalline
grains and thus leads to the prevention in the increase of the macro hardness,
it is always preferable to use ceramic material which could be sintered in
solid phase. It is always preferable to use nano-ceramic with a grain size more
than 0.5µm. Use of nano-ceramic with a grain size less than 0.5µm leads to the
decrease in the hardness of the nano-ceramic material which in turn decreases
the ballistic properties of the material. Also substituting the conventional nano-ceramic
with Al2O3 – SiC – MgO nano-ceramic leads to the reduction of
the areal-density of the samples by at least 30% 4.

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