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Preparation and Impact Resistance Properties of Hybrid Silicone-Ceramics Composites

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This article presents the method of preparation a new type of ballistic armor based on hybrid silicone-ceramic (HSC) composites with considerable flexibility. An experimental study on the ballistic behavior of HSC composites connected with soft body armor is presented against FSP.22 fragments. The effect of Al2O3 ceramics on the ballistic performance of HSC composite was investigated, and the fragmentation resistance process of the composite armor combining the HSC composite and soft aramid insert is clarified. Furthermore, impact resistance tests made with a drop tower which allows for a gravity drop of a mass along vertical guides onto a sample placed with an energy of 5 J were performed. The results presented in this paper show that the HSC composites can be successfully used as a hard body armor. However, they do not exhibit the properties of absorbing the impact energy generated during the drop tower tests. The test results show that the ballistic performance of composite armors is influenced by the hardness and Young modulus of ceramics and soft body armor panel. Additionally, in the article, the results of mechanical properties of silicones used for preparation of composites were presented and compiled to determine their role in the performance of impact protection.
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applied
sciences
Article
Preparation and Impact Resistance Properties of
Hybrid Silicone-Ceramics Composites
Katarzyna Ko´sla *, Paweł Kubiak , Marzena Fejdy´s, Karolina Olszewska, Marcin Łandwijt and
Edyta Chmal-Fudali
Institute of Security Technologies “MORATEX”, Marii Sklodowskiej-Curie 3 Street, 90-505 Lodz, Poland;
pkubiak@moratex.eu (P.K.); mfejdys@moratex.eu (M.F.); itb@moratex.eu (K.O.); mlandwijt@moratex.eu (M.Ł.);
efudali@moratex.eu (E.C.-F.)
*Correspondence: kkosla@moratex.eu; Tel.: +48-42-637-37-10
Received: 25 November 2020; Accepted: 16 December 2020; Published: 19 December 2020


Featured Application: The developed hybrid silicone-ceramic composite can be used as a ballistic
inserts in bulletproof vests, which was confirmed by the conducted research. Obtained results
indicates that the developed hybrid silicone-ceramic composites are characterized by higher
values of the V50 parameter for the FSP.22 fragment compared to the currently used, traditional,
ballistic hard plates with a comparable surface mass.
Abstract:
This article presents the method of preparation a new type of ballistic armor based on
hybrid silicone-ceramic (HSC) composites with considerable flexibility. An experimental study
on the ballistic behavior of HSC composites connected with soft body armor is presented against
FSP.22 fragments. The eect of Al
2
O
3
ceramics on the ballistic performance of HSC composite was
investigated, and the fragmentation resistance process of the composite armor combining the HSC
composite and soft aramid insert is clarified. Furthermore, impact resistance tests made with a drop
tower which allows for a gravity drop of a mass along vertical guides onto a sample placed with an
energy of 5 J were performed. The results presented in this paper show that the HSC composites
can be successfully used as a hard body armor. However, they do not exhibit the properties of
absorbing the impact energy generated during the drop tower tests. The test results show that the
ballistic performance of composite armors is influenced by the hardness and Young modulus of
ceramics and soft body armor panel. Additionally, in the article, the results of mechanical properties
of silicones used for preparation of composites were presented and compiled to determine their role
in the performance of impact protection.
Keywords:
ballistic protection; impact absorption; body armor; hybrid silicone-ceramic composites;
ballistic inserts
1. Introduction
The development of personal protection systems with improved ballistic performance and reduced
weight has received a great interest in the last decade with the unfortunate ever-increasing threats
and conflicts. Nowadays, ballistic protection systems are being increased in their functionality and
technological aspects. Many eorts have been taken to improve their ballistic resistance alongside the
weight reduction. Armor systems development works concern many sectors of the economy, including
defense and security, where the key role is played by personal protective equipment (e.g., bulletproof
vests, ballistic inserts, etc.). Their main objective is to provide adequate impact (ballistic) protection for
the user who is exposed to the hazards of loss of life or health due to the nature of his or her work.
While applying dierent ballistic and high performance materials, like ceramic tiles composites and
Appl. Sci. 2020,10, 9098; doi:10.3390/app10249098 www.mdpi.com/journal/applsci
Appl. Sci. 2020,10, 9098 2 of 20
Twaron/Kevlar based composites, integrated into one armor (ballistic) system, it is also possible to use
the personal ballistic protection system, or enforcement for dierent valuable goods or properties like
vehicles, airplanes or military equipment.
Among the systems intended for the implementation of personal protective equipment, such as
ballistic vests, body armor plays a leading role. Ballistic body armor is broadly classified into two
categories, namely hard body armor and soft body armor. Soft armor should protect the wearer against
most common and low to medium energy projectiles which could go up to a velocity of 500 m/s.
These types of body armor are mostly made from high performance fibers, such as aramid or Ultra
High Molecular Weight Polyethylene (UHMWPE) fibers and are widely used in personnel ballistic
protective clothing for military and law enforcement application thanks to typical flexibility and light
weight [
1
4
]. On the other hand, hard body armor was designed to resist projectile velocity of NIJ
Standard level IIIA [
5
], or more than 500 m/s velocity when worn in conjunction with soft armor.
Hard body armor is made mainly of metal, composite, or ceramic plates [
6
8
]. Ceramic materials
such as aluminum oxide (A1
2
O
3
), silicon carbide (SiC), or boron carbine (B
4
C) have been widely
applied in armor design due to their high compressive strength, high hardness, and low density. High
hardness and high compressive strength of ceramic materials contribute to projectile destruction and/or
spreading of the impact load on a larger area of the armor during ballistic impact [912].
The shape and dimensions of the ceramic elements have a significant influence on the ballistic
resistance of the hard body armor. According to Su and Chen’s studies [
13
], the ballistic simulation
tests show dierences between regular hexagonal and square shapes of ceramic panels. During the
research, it was shown that the hexagon units have better performance in the intersection points of
panels while the ballistic performance of square unit and regular hexagonal unit is basically the same
in central and eccentric areas of panels. Moreover, the results of the simulation were confirmed to
the real ballistic test. The ballistic eciency of alumina tiles with various sizes, shapes, and target
configurations was also measured by Song et al. [
14
]. The ballistic eciency of square tiles roughly
8 mm thick struck by 12.7 mm diameter bullets rapidly increased with tile size up to about 100 mm.
Circular shape tiles had lower ballistic eciencies than those of square shape tiles for the same width
and thickness. The authors explained the obtained results by the eect of reflected waves at edges
and the propagation of resulting cracks on the projectile penetration process. In turn, Hazell and
co-workers [
15
] performed tests with dierent areal geometries silicon carbide square tiles. The tiles
were manufactured via two dierent processes, and have been bonded to polycarbonate layers to
evaluate their ballistic performance. Four ceramic tile sizes were tested: 85 mm, 60 mm, 50 mm,
and 33 mm,
and obtained results showed that there is a crucial dimension of tile that should be
used in a silicon carbide-based ceramic-faced mosaic armor system design to ensure its optimum
performance. Another comparison of the ballistic performance was made over a series of additively
manufactured alumina tiles placed over the back plate, which were investigated using both forward-
and reverse-ballistic experiments. The results show some major dierences between various types of
tiles, increasing its ballistic performance along with the thickness [16].
In general, ceramic hard body armor systems consist of a hard brittle ceramic facing the projectile
and a soft deformable backing material. The ceramic destroys the projectile tip, slows it down,
and distributes the load over a large area of the backing. The backing supports the ceramics and brings
the comminuted ceramics and the projectile to rest. The backing material is selected in structural,
ballistic, and weight terms [
15
,
17
]. Body armor still has a lot of problems related to weight, ergonomics
(especially adjustment to the user’s body and air permeability), scope, and area of protection and better
energy absorption. For this reason, new solutions in the field of construction and materials are constantly
sought to eliminate the above-mentioned disadvantages. In recent years, many solutions have been
proved to be very ecient in providing superior ballistic performances and weight reduction [
18
23
].
Among dierent hybrid composites, ceramic-polymer composite armors are particularly
interesting for their high strength and light weight with high energy absorption capability. While the
function of ceramics is to retard ballistic impact penetration, a polymer panel is to absorb high energy
Appl. Sci. 2020,10, 9098 3 of 20
generation from the propagation of elastic/stress waves [
24
]. Colombo et al. [
25
] presented studies of
composite layered systems based on monolithic armor ceramic tiles joined with polymer infiltrated
ceramic foams which have been designed and evaluated for lightweight ballistic protection. Open cell
silicon carbide foams of various cell sizes infiltrated with thermosetting or elastomeric polyurethane
were used for this design.
In addition, the state of the art research carried out in the field of flexible ballistic shields
manufactured based on ceramic and/or elastomeric elements made it possible to indicate a patent
application describing a method of making a product in the form of a composite ballistic plate absorbing
and dissipating kinetic energy after impact with a high velocity projectile, intended for covers for
vehicles or fixed objects. The plate comprises a layer made of individual ceramics pellets or a sintered
refractory material. The pellets form regular rows and columns and are bound and held in the form
of a plate by the solidified elastic material. A thermoplastic polymer such as polycarbonate or a
thermosetting material such as epoxy resin is used as the bonding material. Moreover, the plate may
contain layers of woven or non-woven textile material. However, due to its significant surface weight,
thickness, and stiness, this solution is not suitable for use in personal protective clothing [26].
Furthermore, based on the patent description No. PL 224825 [
27
], the possibility of producing
a flexible armor intended for use mainly as an insert for bullet-proof and fragment-proof vests is
specified. The armor consists of layers of ballistic textile material between which there is a layer
composed of closed pouches, partially overlapping each other. The pouches are filled with STF (Shear
Thickening Fluid) or MRF (Magneto Rheological Fluid). The armor provides resistance to penetration
by 5.56–14.5 mm small and medium caliber armor-piercing shells and to penetration by fragments or
an explosion.
Gamache et al. [
28
] developed a body armor composite material which include a flexible liner,
a polymer binder disposed on the liner, and ceramic solids embedded in the binder. The flexible liner
conforms to a portion of the wearer and elastically deforms in response to application of mechanical
force. The binder can be a polyurea foam. The solids can be spheres arranged in a single-layer pattern,
substantially parallel to liner. Martin and co-workers [
29
] presented a similar solution—composite
material system of armor comprises a strike stratum and a backing stratum. The strike stratum includes
elastomeric matrix material and inventive ceramic-inclusive elements embedded therein and arranged
(e.g., in one or more rows and one or more columns) along a geometric plane corresponding to the
front (initial strike) surface of the strike stratum. More rigid than the strike stratum, the backing
stratum is constituted by, e.g., metallic (metal or metal alloy) material or fiber-reinforced polymeric
matrix material. Some inventive embodiments also comprise a spall-containment stratum fronting the
strike stratum.
Despite the presence of several patent applications and publications, where solutions based on
ceramic-polymer composites were discussed, the results of tests of resistance to fragments and impact
energy absorption made with a drop tower by silicone-ceramic composites have not been presented
so far. The results presented in this paper show that the hybrid silicone-ceramic composites can be
successfully used as ballistic hard body armor. However, they do not exhibit the properties related to
absorption of the impact energy generated during the drop tower tests. The test results show that the
ballistic performance of composite armors is influenced by hardness and Young modulus of Al
2
O
3
ceramics and the soft body armor panel. The obtained HSC composites were aimed at improving
ergonomic properties by having increased flexibility compared to standard hard body armor made of
metal or pressed aramids or UHMWPE sheets.
This work is part of a project aimed at developing a next generation anti-blast and fragment-proof
protective suit, designed to provide the personal protection to bomb-disposal experts during their
direct operations linked to the explosive devices neutralization. For this reason, the HSC composite was
developed for its use in explosion-proof and fragment-proof clothing, which should meet requirements
of NIJ Standard 0117.01 [
30
]. The results obtained may be a reference point enabling the assessment of
Appl. Sci. 2020,10, 9098 4 of 20
the use of hybrid silicone-ceramics composites in armor, personal protective system, and applications
related to the ballistic protection of vehicles and other objects.
2. Materials and Methods
2.1. Materials
2.1.1. Silicone Materials
Three types of silicone elastomers were used to produce the hybrid silicone-ceramic composites:
Za 22 Mould (Zhermack, Badia Polesine, Italy), MM 228 silicone (ACC Silicones, Bridgwater, England),
MM 922 silicone (ACC Silicones, Bridgwater, England). The properties of applied silicone materials
are presented in Table 1. Density, hardness, and tear strength under static stretching parameters have
been determined in accordance with the test methodology described in points Sections 2.3.12.3.3.
Table 1.
Technical specification of silicone elastomers applied for the hybrid silicone-ceramic composites.
No. Parameter Unit Results Test Method
1. Silicone name Za 22 Mould MM 228 MM 922 manufacturer’s
technical specification
2. Density
g/cm
31.09 ±0.04 1.11 ±0.01 1.23 ±0.01 PBCH-09/2017 [31]
3. Hardness ShA 20 ±1 28 ±2 22 ±1 PN-EN ISO 868:2005 [32]
4. Tear strength under
static stretching MPa 3.35 ±0.16 1.54 ±0.32 3.64 ±0.18 PN-ISO–37:2007 [33]
5. Breaking strength MPa 4.0 5.06 3.6
manufacturer’s
technical specification
6. Elongation at break % 380 746 497
7. Tensile strength
kN/m
20.0 31.0 26.2
8. Viscosity mPas 4000 13,000 19,000
9. Lifetime min 15 55 45–120
10. Demolding time h 1 5 8–12
2.1.2. Ceramic Materials
To produce the hybrid silicone-ceramic composites, hexagonal ceramics made from aluminum
oxide (Al
2
O
3
content 98%, CeramTec, Plochingen, Germany) were used. Hexagonal ceramic elements
had the following dimensions: side of the hexagon—(11.5
±
0.2) mm, longer diagonal of the ceramic
tile—(23.0
±
0.2) mm, shorter diagonal of the ceramic tile—(19.9
±
0.2) mm. The technical parameters
of ceramic materials are presented in Table 2. Parameters have been determined in accordance with
the test methodology described in Section 2.3.5.
Table 2. Physical and mechanical parameters of Al2O3ceramics (CeramTec, Plochingen, Germany).
Parameter Density
[g/cm3]
Young’s
Modulus
[GPa]
Acoustic
Impedance
[105g/cm2s]
Vickers
Hardness
[GPa]
Resistance to
Brittle Fracturing,
[MPa m1/2]
Test method PN-EN 993-1:1998 [34] ASTM C 1419-99a [35] PN-EN-ISO 6507-1:2007 [36]
Al2O33.0 mm thickness 3.81 ±0.1 464.0 ±8.0 40.0 ±0.2 15.4 ±0.4 4.29 ±0.4
Al2O33.5 mm thickness 3.81 ±0.1 472.6 ±10.0 40.0 ±0.2 18.9 ±0.3 4.32 ±0.3
2.1.3. Additional Materials
The following materials were used as additional reinforcement layers: Poron
®
XRDMA (Polting
Foam Sp. zo.o, Gliwice, Poland) and Twaron
®
CT612 (Teijin, Wuppertal, Germany) with the
physico-mechanical properties given in Tables 3and 4. In addition, there were ceramic elements placed
on a carrier film which was a self-adhesive film 4622/WS 40 (Bochemia, Radom, Poland) in order to
stabilize them and improve the eciency of the production process of silicone-ceramic composites.
Appl. Sci. 2020,10, 9098 5 of 20
Table 3.
Physical andmechanical parameters of the aramid fabric Twaron
®
CT 612 (Teijin, Wuppertal, Germany).
No. Parameter Unit Results of
Metrological Tests Test Method
1 Width m 1.31 ±0.01 PN-EN 1773:2003 [37]
2 Areal density g/m2123 ±1 PN-ISO 3801:1993 [38]
3 Number of threads -warp
cm/dm
112 ±2PN-EN 1049-2:2000 [39]
-weft 108 ±2
4 Thickness mm 0.18 ±0.02 PN-EN ISO 5084:1999 [40]
5 Maximum force -warp N5700 ±205 PN-EN ISO
13934-1:2013-07 [41]
-weft 5800 ±211
6
Elongation at rupture
-warp %4.2 PN-EN ISO
13934-1:2013-07 [41]
-weft 5.0
Table 4.
Physical and mechanical parameters of the Poron
®
XRDMA (Polting Foam Sp. zo.o, Gliwice, Poland).
No. Parameter Unit Results of Metrological Tests Test Method
1 Density g/cm30.19 ±0.01 PBCH-09/2017 [31]
2. Thickness mm 3.0 ±0.2 PN-EN ISO 5084:1999 [40]
3. Areal density g/m2555 ±12 PN-EN ISO 2286-2:2016 [42]
4.
Tensile strength
kPa 446 ±29 PN-EN ISO 1798:2001 [43]
5. Tear strength N/m 297 ±44 PN-EN ISO 8067:2009 [44]
2.1.4. The Tested System in a Form of a Hybride Silicone-Ceramic Composite and Soft Ballistic Inserts
Hybrid silicone-ceramic composites (HSC) (Figure 1) consist of 5 layers, including:
outer layers made of elastomer;
inner layer made of ceramic elements placed on the carrier film;
reinforcement layers made of aramid fabrics and/or foamed polymer materials.
Appl. Sci. 2020, 10, x FOR PEER REVIEW 5 of 20
The following materials were used as additional reinforcement layers: Poron®XRDMA (Polting
Foam Sp. zo.o, Gliwice, Poland) and Twaron®CT612 (Teijin, Wuppertal, Germany) with the physico-
mechanical properties given in Tables 3 and 4. In addition, there were ceramic elements placed on a
carrier film which was a self-adhesive film 4622/WS 40 (Bochemia, Radom, Poland) in order to
stabilize them and improve the efficiency of the production process of silicone-ceramic composites.
Table 3. Physical and mechanical parameters of the aramid fabric TwaronCT 612 (Teijin, Wuppertal,
Germany).
No. Parameter Unit Results of Metrological Tests Test Method
1 Width m 1.31 ± 0.01 PN-EN 1773:2003 [37]
2 Areal density g/m2 123 ± 1 PN-ISO 3801:1993 [38]
3 Number of threads -warp cm/dm 112 ± 2 PN-EN 1049-2:2000 [39]
-weft 108 ± 2
4 Thickness mm 0.18 ± 0.02 PN-EN ISO 5084:1999
[40]
5 Maximum force -warp N 5700 ± 205 PN-EN ISO 13934-
1:2013-07 [41] -weft 5800 ± 211
6 Elongation at rupture -warp % 4.2 PN-EN ISO 13934-
1:2013-07 [41]
-weft 5.0
Table 4. Physical and mechanical parameters of the PoronXRDMA (Polting Foam Sp. zo.o, Gliwice,
Poland).
No. Parameter Unit Results of Metrological Tests Test Method
1 Density g/cm3 0.19 ± 0.01 PBCH-09/2017 [31]
2. Thickness mm 3.0 ± 0.2 PN-EN ISO 5084:1999 [40]
3. Areal density g/m2 555 ± 12 PN-EN ISO 2286-2:2016 [42]
4. Tensile strength kPa 446 ± 29 PN-EN ISO 1798:2001 [43]
5. Tear strength N/m 297 ± 44 PN-EN ISO 8067:2009 [44]
2.1.4. The Tested System in a Form of a Hybride Silicone-Ceramic Composite and Soft Ballistic
Inserts
Hybrid silicone-ceramic composites (HSC) (Figure 1) consist of 5 layers, including:
outer layers made of elastomer;
inner layer made of ceramic elements placed on the carrier film;
reinforcement layers made of aramid fabrics and/or foamed polymer materials.
Figure 1. Hybrid silicone-ceramic composite—general view.
The scheme showing the structure of the composite is presented in Figure 2.
Figure 1. Hybrid silicone-ceramic composite—general view.
The scheme showing the structure of the composite is presented in Figure 2.
In the research conducted, the outer layer consisted of the following silicone elastomers: Za
22 Mold (Zhermack, Badia Polesine, Italy), MM 228 silicone (ACC Silicones, Bridgwater, England)
and MM 922 silicone (ACC Silicones, Bridgwater, England). The inner layer consisted of hexagonal
ceramic elements based on Al
2
O
3
ceramics with a thickness of (3.0
±
0.2) mm and (3.5
±
0.2) mm.
These elements were placed on a carrier which was a self-adhesive film 4622/WS 40 in order to stabilize
them and improve the eciency of the production process. The following materials were used as
reinforcing layers: Poron
®
XRDMA (Polting Foam Sp. zo.o, Gliwice, Poland) and Twaron
®
CT 612
(Teijin, Wuppertal, Germany).
Appl. Sci. 2020,10, 9098 6 of 20
Appl. Sci. 2020, 10, x FOR PEER REVIEW 6 of 20
Figure 2. Diagram of the construction of a hybrid silicone-ceramic composite.
In the research conducted, the outer layer consisted of the following silicone elastomers: Za 22
Mold (Zhermack, Badia Polesine, Italy), MM 228 silicone (ACC Silicones, Bridgwater, England) and
MM 922 silicone (ACC Silicones, Bridgwater, England). The inner layer consisted of hexagonal
ceramic elements based on Al2O3 ceramics with a thickness of (3.0 ± 0.2) mm and (3.5 ± 0.2) mm. These
elements were placed on a carrier which was a self-adhesive film 4622/WS 40 in order to stabilize
them and improve the efficiency of the production process. The following materials were used as
reinforcing layers: Poron®XRDMA (Polting Foam Sp. zo.o, Gliwice, Poland) and Twaron®CT 612
(Teijin, Wuppertal, Germany).
Soft ballistic inserts were obtained from combining the sheets of Twaron®CT 612 (Teijin,
Wuppertal, Germany), with a total areal density of (5 ± 0.5) kg/m2.
2.2. Manufacture of Hybrid Silicone-Ceramic Composites
The outer layers in the form of the Za 22 Mold silicone elastomer were obtained by mixing
silicone components A and B (A-base, B-catalyst) in a 1: 1 weight ratio. The MM 228 silicone layer
was prepared by mixing the base and standard catalyst in a 10:1 weight ratio. The MM 922 silicone
was prepared by mixing the silicone base and the catalyst in a 100:5 weight ratio. The amount of
silicone mass to produce a single outer layer containing the base with the catalyst was approximately
300 g.
After the components of the silicone elastomer were mixed in the weight ratio given above, the
silicone mass was poured into a mold. So that the silicone pre-cross-links and the remaining layers
after their application on its surface do not fall to the bottom of the mold, the mass was left for the
period:
up to 120 min in the case of MM 922 silicone,
up to 30 min for Za 22 Mold silicone,
up to 50 min for MM 228silicone.
Then, a single reinforcing layer—Poron®XRDMA with a thickness of (3.0 ± 0.2) mm—was
applied to the surface of the silicone layer. Hexagonal ceramic tiles previously stacked on the carrier
film were placed on the reinforcing layer. Then another reinforcing layer in the form of a
Twaron®CT612 sheet was applied. The whole was poured with silicone elastomer and allowed to
cross-link. The demolding time of the composite was the minimum: 12 h for MM 922 silicone, 5 h for
MM 228 silicone, and 1 h for Za 22 Mold silicone.
2.3. Methods
2.3.1. Density
The basic physical and mechanical parameters of silicone elastomers such as: density, hardness,
and tear strength with static tensile were tested according to the methodology described below.
The density of elastomers was determined by the hydrostatic method on the basis of PBCH-
09/2017 test procedure “Determination of density by the hydrostatic method”, 2nd edition of
01/12/2017 [31]. The density was calculated from the following formula:
Figure 2. Diagram of the construction of a hybrid silicone-ceramic composite.
Soft ballistic inserts were obtained from combining the sheets of Twaron
®
CT 612 (Teijin, Wuppertal,
Germany), with a total areal density of (5 ±0.5) kg/m2.
2.2. Manufacture of Hybrid Silicone-Ceramic Composites
The outer layers in the form of the Za 22 Mold silicone elastomer were obtained by mixing silicone
components A and B (A-base, B-catalyst) in a 1: 1 weight ratio. The MM 228 silicone layer was prepared
by mixing the base and standard catalyst in a 10:1 weight ratio. The MM 922 silicone was prepared
by mixing the silicone base and the catalyst in a 100:5 weight ratio. The amount of silicone mass to
produce a single outer layer containing the base with the catalyst was approximately 300 g.
After the components of the silicone elastomer were mixed in the weight ratio given above,
the silicone mass was poured into a mold. So that the silicone pre-cross-links and the remaining
layers after their application on its surface do not fall to the bottom of the mold, the mass was left for
the period:
up to 120 min in the case of MM 922 silicone,
up to 30 min for Za 22 Mold silicone,
up to 50 min for MM 228 silicone.
Then, a single reinforcing layer—Poron
®
XRDMA with a thickness of (3.0
±
0.2) mm—was applied
to the surface of the silicone layer. Hexagonal ceramic tiles previously stacked on the carrier film were
placed on the reinforcing layer. Then another reinforcing layer in the form of a Twaron
®
CT612 sheet
was applied. The whole was poured with silicone elastomer and allowed to cross-link. The demolding
time of the composite was the minimum: 12 h for MM 922 silicone, 5 h for MM 228 silicone, and 1 h for
Za 22 Mold silicone.
2.3. Methods
2.3.1. Density
The basic physical and mechanical parameters of silicone elastomers such as: density, hardness,
and tear strength with static tensile were tested according to the methodology described below.
The density of elastomers was determined by the hydrostatic method on the basis of PBCH-09/2017
test procedure “Determination of density by the hydrostatic method”, 2nd edition of 01/12/2017 [
31
].
The density was calculated from the following formula:
ρ=(A/AB) ×ρ0(1)
ρ—sample density (g/cm3),
A—weight of sample in air (g),
B—weight of sample in liquid (g),
ρ0—density of liquid (g/cm3).
Appl. Sci. 2020,10, 9098 7 of 20
2.3.2. Tear Strength with Static Tensile of Silicones
Determination of mechanical properties of silicone elastomers at static tension was performed
in accordance with PN-ISO-37:2007 [
33
]. The tensile test was carried out on standard paddle or ring
shaped samples in a testing machine (Instron) at a constant tensile speed. The tensile stress was
calculated as the force related to the unit of area of the initial cross-sectional area of the measuring
section of the tested elastomer sample. Tensile strength is defined as the maximum recorded tensile
stress and elongation at break is defined as the deformation of the measuring section at break. In such
a method of determining the tensile strength of elastomers, the eect of transverse deformation of the
sample during the test is not taken into account.
2.3.3. Silicone Elastomers Hardness
Silicone elastomers were characterized in terms of their hardness. Hardness measurements were
carried out in accordance with PN-EN ISO 868:2005 standard “Plastics and Ebonite—Determination
of Identification Hardness by Means of Durometer (Shore Hardness)” [
32
] using a Shore A hardness
tester mounted on a tripod. The hardness result was read after 15 s. Five hardness measurements were
made for each sample and the average of the five measurements was given as the result.
2.3.4. Structural Testing of Elastomers Using FT-IR
The research was performed in accordance with the research procedure PBCH-02/2014
“Spectrophotometric analysis of spectra in the aspect of determining the raw material composition
in selected textile materials”, 2nd edition of 01/12/2017 [
45
] using the FTIR-Nicolet iS10 single beam
spectrophotometer—THERMO Scientific. In order to perform a correct IR analysis, two measurements
were taken: the crystal background spectrum and the sample being tested spectrum. The FTIR
spectrum was determined as the ratio of the sample spectrum to the background spectrum (when
performing the background spectrum, the response of the spectrometer itself is measured, without the
sample). The division of the sample spectrum by the background spectrum (so-called “Rationing”)
removes the adverse eects caused by the instrument and weather conditions, so that the signals
present in the final spectrum only come from the sample. The measurements were carried out with the
use of DTGS KBr detector, with resolution equal to 4.
2.3.5. Methodology of Physical and Mechanical Testing of Al2O3Ceramic Elements
Tests of physico-mechanical properties were conducted for the ceramic materials. Apparent
density (
ρ
), Young’s modulus (E), acoustic impedance (Z), Vickers hardness (HV20), and resistance
to brittle fracturing (K
1c
) were determined. Apparent density of ceramic was determined by the
hydrostatic method according to BS EN 993-1:1995 [
34
], and resistance to brittle fracturing and Vickers
hardness were determined based on the PBS 1-4 procedure according to references [
36
,
46
] under a
load of 9.8 N. In turn, the velocity of sound propagation, Young’s modulus, and acoustic impedance
were determined based on the measurement of the time of a passage of an ultrasound wave through
the material tested according to the PBS 5-1 procedure developed under thestandards [
35
]. Tests of
physico-mechanical properties of the ceramic materials were conducted at the Department of Ceramics
and Refractories, AGH University of Science and Technology, Poland.
2.3.6. Determination of the HSC Composite Mass Per Unit Area
The test was carried out in accordance with PN-EN ISO 4674-1:2017-02 [
47
], using an analytical
balance allowing for weighing with an accuracy of 0.001 g. The mass per unit area was calculated
according to the formula:
mp=(mi/d×s) ×106(2)
where:
Appl. Sci. 2020,10, 9098 8 of 20
mi—material mass, [g];
d—material length calculated as an arithmetic mean of 3 length measurements, [mm];
s—material width calculated as an arithmetic mean of 3 width measurements, [mm].
2.3.7. Fragmentation Resistance Test
Samples intended for the assessment of fragmentation resistance were hybrid silicone ceramic
composites (HSC) combined with “soft” ballistic inserts sewn into the cover and with mass per unit
area of (5.0
±
0.5) kg/m
2
. Soft ballistic inserts were created by cutting out and assembling layers of
Twaron
®
CT612 para-aramid ballistic material. The inserts were then sewn on the corners. The distance
between seams (stitches) was (1.0
±
0.1) cm. The distance from the stitch to the edge was
(2.0 ±0.1) cm.
The stitches were fixed at the ends with a return stitching (about 1 cm). The PBB-09 2nd edition
of 12.2013 Test Procedure [
48
] based on the requirements and testing methodology contained in
Stanag 2920 [
49
] has been used to determine the fragmentation resistance. For HSC fragmentation,
resistance tests combined with soft ballistic inserts, a standard FSP.22 fragment of mass of (1.10
±
0.03)
g, diameter (5.46
±
0.05) mm and length of 6.35 mm, made of steel with hardness of (27
±
3) HRC was
used. The test of fragmentation resistance was conducted in a “dry” state. The sample dimensions
were
250 ×300 mm.
The test was conducted at room temperature (20
±
5)
C at relative air humidity
(65 ±10)%.
At least six shots were fired to each sample: three causing partial puncture and three
causing total puncture. The limit of ballistic protection V50 was determined as an average of the equal
number of the highest measured speeds of the fragment causing only partial puncture and the lowest
measured speeds causing total puncture within the velocity spread 40 m/s.
2.3.8. Impact Tests
Impact tests were performed according to test procedure PBB-07 ed. II: 12.2008 “Impact tests.
Determining the level of impact energy attenuation for body protectors” [
50
], developed based on BS
7971-1:2002 “Protective clothing and equipment for use in violent situations and in training. General
requirements” [
51
], in an accredited Ballistic Testing Laboratory ITB, “MORATEX”. The tests were
conducted for hybrid silicone ceramic composites and silicone elastomers using a flat impactor and
R 150 cylindrical die. For the impact tests, a Drop Tower was used to drop the mass (“gravitational
drop”) along vertical guides with energy (5
±
1) J on the sample placed on the test die. The principle of
operation of the device is as follows: on a sample placed on a die attached to the base, an impactor of a
certain mass and energy drops along the vertical guides. During the test, the force transferred to the
die under the tested guard is recorded using a piezoelectric force sensor mounted in the die base.
During the above mentioned tests, a drop impactor of weight (5000
±
10) g, made of polished
steel with a flat surface of 40
×
80 mm and rounded edges (50
±
5) mm. A semi-circular die with a
50 mm radius and height of (180
±
20) mm, made of polished steel, was also used. The tests were
carried out under ambient conditions, but previously the samples were acclimatized for (48.0
±
0.5) h
at
(23 ±2) C
and relative humidity (50
±
5)%. The tests were performed for hybrid silicon-ceramic
composites, samples obtained from silicones previously used to produce HSC and material samples
obtained from polymers used commercially as impact energy absorbers.
3. Results and Discussion
3.1. Analysis of the Results of Physical, Mechanical and Structural Tests of Silicones Used for the HSC
Composites Production
In the first phase of the work, the physico-mechanical parameters of silicones used to produce
HSC composites were determined and their chemical structure was compared by performing and
analyzing infrared spectroscopy spectrums. Physical-mechanical parameters of silicones used to
prepare composites are presented in Table 1.
Appl. Sci. 2020,10, 9098 9 of 20
The lowest values of parameters, i.e., hardness, elongation at tear, tensile strength, density,
and viscosity, are provided by Za 22 Mould silicone (Zhermack, Badia Polesine, Italy). This silicone
is also characterized by the shortest lifetime and demolding time, which is an important element in
semi-technical and industrial processing. The highest parameters of hardness, tear strength, elongation
at tear, and tensile strength are characterized by MM 228 silicone (ACC Silicones, Bridgwater, England).
Later on, FT-IR structural tests of silicones used to produce hybrid silicon-ceramic composites were
also conducted. The obtained FT-IR spectrums are shown in Figure 3.
Figure 3. FT-IR spectra of silicone elastomers used to make hybrid silicone-ceramic composites.
The spectrums obtained for each of the three silicones used to obtain hybrid silicon-ceramic
composites are characterized by the occurrence of bands at wavelengths of 660–690 cm
1
, corresponding
to bending oscillation of single carbon bonds. The presence of a low intensity band in the wavelength
range 761 cm
1
corresponding to bending vibrations of C–H bonds was also observed. Moreover,
the presence of bands at wavelengths of 801 cm
1
and 865 cm
1
was observed from deformed pendulous
oscillation of CH
3
–Si–CH
3
group and 1021.3 cm
1
and 1093–1094 cm
1
—characteristic for asymmetric
tensile oscillation of Si–O–Si group. Based on the analysis of the FT-IR spectrums obtained, the presence
of bands characteristic for symmetrical and asymmetrical deformation oscillations of Si–CH
3
groups
can also be noted, with wavelengths of 1261 cm
1
and 1411 cm
1
respectively. The bands in the range of
wave numbers 2905 cm
1
and 2963 cm
1
characteristic for tensile oscillations of C–H bonds indicate the
presence of methyl groups in the silicone molecule. However, no bands in the range of 2160–2170 cm
1
were recorded, which indicates the absence of –SiH groups in molecules of the compounds used.
There were also no dierences in the occurrence of individual bands, their displacement towards
higher or lower wavelengths or significant changes in intensity of individual bands. The obtained
FT-IR spectrums indicate that all silicones used are aliphatic compounds containing methyl groups,
which can be classified as polydimethylsiloxanes.
Appl. Sci. 2020,10, 9098 10 of 20
3.2. Analysis of the Properties of Ceramic Elements Used to Produce Hybrid Silicone-Ceramic Composites
Ballistic properties of ceramic composites depend on several factors. These include density,
porosity, hardness, tear strength, Young’s modulus, acoustic impedance, mechanical strength of
ceramic elements, and several other factors. A number of works [
17
,
52
54
] have shown that single
properties of ceramic elements do not have a direct correlation with ballistic properties because
the mechanism of tearing during a real bullet impact is very complicated. In addition, it was also
determined that microstructural features aect physical and ballistic properties, causing dierences in
the mechanisms of tear propagation and energy dissipation and ultimately aecting ballistic properties.
In order to determine which of the physico-mechanical properties are essential for the ballistic resistance
of hybrid silicone ceramic composites, tests were performed to compare the properties, i.e., density,
Young’s modulus, hardness, acoustic impedance, and brittle fracturing resistance coecient for two
dierent thicknesses of ceramic elements made of alumina (Table 2), and they were correlated with the
obtained for the FSP.22 fragment limit values of ballistic protection V50.
The results of the physical and mechanical tests of Al
2
O
3
ceramic elements showed that there were
practically no dierences in the obtained values of density, fracture toughness coecient, and acoustic
impedance. In the case of the other two parameters, Young’s modulus and Vickers microhardness,
lower values were obtained for ceramic elements with a thickness of (3.0
±
0.2) mm. These parameters
will play a significant role in shaping the cracking mechanism during the actual impact of the projectile
against the HSC composite and, consequently, aect its ballistic properties.
3.3. Fragmentation Resistance Test Results
Fragmentation resistance tests were carried out for the following groups of ballistic inserts:
soft ballistic inserts containing 42 layers of Twaron®CT612;
silicone elastomers used in conjunction with soft ballistic insert containing 42 layers of
Twaron®CT612;
HSC composites containing Al
2
O
3
ceramics with a thickness of (3.0
±
0.2) mm used in conjunction
with soft ballistic insert containing 42 layers of Twaron®CT612;
HSC composites containing Al
2
O
3
ceramics with a thickness of (3.5
±
0.2) mm used in conjunction
with soft ballistic insert containing 42 layers of Twaron®CT612;
HSC composites containing Al
2
O
3
ceramics with a thickness of (3.5
±
0.2) mm, without reinforcing
layers in the form of Poron
®
XRDMA and Twaron
®
CT612 sheets, used in conjunction with soft
ballistic insert containing 42 layers of Twaron®CT612.
The test results are presented in Table 5and Figure 4.
Table 5. Fragmentation resistance test results.
No. Sample
Composition
V50 [m/s]
Silicone Used in
Conjunction with
Soft Ballistic Armor
Hybrid Silicone-Ceramic Composites (HSC) Composite Used in
Conjunction with Soft Ballistic Armor, Consisting of:
Al2O3Ceramics
3.0 mm Thickness and
Reinforcing Layer
Al2O3Ceramics
3.5 mm Thickness and
Reinforcing Layer
Al2O3Ceramics 3.0 mm
Thickness without
Reinforcing Layer
1. MM 922 712.6 ±20.0 1288.8 ±20.1 1526.8 ±21.9 1530.7 ±24.3
2. MM 228 709.9 ±20.2 1324.0 ±24.4 1538.9 ±23.2 1554.0 ±23.8
3. Za 22 Mould 718.4 ±23.2 1258.6 ±20.6 1543.6 ±21.8 1546.3 ±19.8
For the soft ballistic insert, the value of the V50 ballistic protection limit was obtained equal to
(619
±
15) m/s. The use of a hybrid silicon-ceramic composite containing ceramic elements with a
thickness of (3.0
±
0.2) mm resulted in more than 2-fold increase of resistance to FSP.22 fragment. When
comparing composites containing ceramic elements of dierent thicknesses, it can be determined that
increasing the thickness of the ceramic element by about (0.5
±
0.2) mm causes an increase in the
Appl. Sci. 2020,10, 9098 11 of 20
value of the V50 ballistic protection limit in the range of 14–18%. Changes in ballistic resistance of
polyethylene composites containing ceramic elements based on SiC and Al
2
O
3
ceramics, depending on
the thickness of the ceramic elements used, were discussed in detail in the study of
Fejdy´s et al. [53].
On the basis of the results contained in this paper, it was indicated that the main parameters influencing
the changes in ballistic protection values along with the change in thickness of the ceramic element
are acoustic impedance and fracture toughness of ceramic elements. The acoustic impedance should
be as close as possible to the acoustic impedance of the base material in the ballistic armor, and the
fracture toughness (K
1c
) should be as high as possible. The combination of these two properties
gives the best ballistic armor protection when tested with 7.62
×
39 mm MSC and 5.56
×
45 mm SS
109 ammunition. In the case of tests carried out herein for silicone ceramic composites, the applied
ceramics with thicknesses (3.0
±
0.2) mm and (3.5
±
0.2) mm show no dierences in the obtained values
of density, resistance to brittle fracturing and acoustic impedance. The changes of resistance to FSP.22
fragment between the developed HSC composites with dierent thicknesses of the ceramic element
are therefore related to dierences in parameters such as Young’s modulus and Vickers microhardness
of ceramic fittings. It can be clearly stated that with the increase of values obtained for these two
parameters, the ballistic properties of the tested composite system are improved. Similar relationships
resulted also from studies conducted by Kaufmann et al. [52], Krell et al. [55], or Cegła et al. [56].
Appl. Sci. 2020, 10, x FOR PEER REVIEW 11 of 20
For the soft ballistic insert, the value of the V50 ballistic protection limit was obtained equal to
(619 ± 15) m/s. The use of a hybrid silicon-ceramic composite containing ceramic elements with a
thickness of (3.0 ± 0.2) mm resulted in more than 2-fold increase of resistance to FSP.22 fragment.
When comparing composites containing ceramic elements of different thicknesses, it can be
determined that increasing the thickness of the ceramic element by about (0.5 ± 0.2) mm causes an
increase in the value of the V50 ballistic protection limit in the range of 14–18%. Changes in ballistic
resistance of polyethylene composites containing ceramic elements based on SiC and Al2O3 ceramics,
depending on the thickness of the ceramic elements used, were discussed in detail in the study of
Fejdyś et al. [53]. On the basis of the results contained in this paper, it was indicated that the main
parameters influencing the changes in ballistic protection values along with the change in thickness
of the ceramic element are acoustic impedance and fracture toughness of ceramic elements. The
acoustic impedance should be as close as possible to the acoustic impedance of the base material in
the ballistic armor, and the fracture toughness (K1c) should be as high as possible. The combination
of these two properties gives the best ballistic armor protection when tested with 7.62 × 39 mm MSC
and 5.56 × 45 mm SS 109 ammunition. In the case of tests carried out herein for silicone ceramic
composites, the applied ceramics with thicknesses (3.0 ± 0.2) mm and (3.5 ± 0.2) mm show no
differences in the obtained values of density, resistance to brittle fracturing and acoustic impedance.
The changes of resistance to FSP.22 fragment between the developed HSC composites with different
thicknesses of the ceramic element are therefore related to differences in parameters such as Young’s
modulus and Vickers microhardness of ceramic fittings. It can be clearly stated that with the increase
of values obtained for these two parameters, the ballistic properties of the tested composite system
are improved. Similar relationships resulted also from studies conducted by Kaufmann et al. [52],
Krell et al. [55], or Cegła et al. [56].
In addition, it was determined that there are no differences in the obtained V50 ballistic
protection limit values between composites containing ceramic elements of identical thickness (3.5 ±
0.2) mm differing in the presence of reinforcement layers in the form of Poron®XRDMA (Polting Foam
Sp. zo.o, Gliwice, Poland) and Twaron®CT612 (Teijin, Wuppertal, Germany) sheets. Therefore, it can
be concluded that the reinforcement layers do not affect the fragmentation resistance properties of
the developed composite. These layers are intended to limit the generation of secondary fragments
from the ceramic fragments formed during the impact of the fragment or bullet on the sample and to
contribute to the “stabilization” of the ceramic layer, understood as limiting the process of
detachment of adjacent ceramic fragments under the impact of the fragment. At the same time, there
is no correlation between the type of silicone elastomer used and its physical and mechanical
properties (Table 1) and the obtained value of the V50 ballistic protection limit due to too minor
Figure 4. Hybrid ceramic-silicone composites after ballistic tests.
In addition, it was determined that there are no dierences in the obtained V50 ballistic protection
limit values between composites containing ceramic elements of identical thickness (3.5
±
0.2) mm
diering in the presence of reinforcement layers in the form of Poron
®
XRDMA (Polting Foam Sp.
zo.o, Gliwice, Poland) and Twaron
®
CT612 (Teijin, Wuppertal, Germany) sheets. Therefore, it can
be concluded that the reinforcement layers do not aect the fragmentation resistance properties of
the developed composite. These layers are intended to limit the generation of secondary fragments
from the ceramic fragments formed during the impact of the fragment or bullet on the sample and to
contribute to the “stabilization” of the ceramic layer, understood as limiting the process of detachment
of adjacent ceramic fragments under the impact of the fragment. At the same time, there is no
correlation between the type of silicone elastomer used and its physical and mechanical properties
(Table 1) and the obtained value of the V50 ballistic protection limit due to too minor dierences
between individual values of this parameter. Therefore, the lifetime as well as decomposition, viscosity,
and price of the silicone elastomer for the sample preparation processes of hybrid silicone-ceramic
Appl. Sci. 2020,10, 9098 12 of 20
composites will play a role in the selection of the silicone elastomer, which may determine the speed
and profitability of future production methods. On the basis of the results obtained, it can be indicated
that in the case of HSC composites, the elements responsible for the absorption of impact energy at high
velocities (in the range of 600–1500 m/s) are ceramics and soft ballistic inserts made of Twaron
®
CT612
(Teijin, Wuppertal, Germany). The use of silicone increases the value of the ballistic protection limit by
about 100 m/s in relation to the V50 values obtained for the Twaron
®
CT612-based (Teijin, Wuppertal,
Germany) soft ballistic inserts, which is about 14%.
The analysis of the literature data allowed to determine that in the case of soft ballistic inserts,
four categories of factors influencing the performance and ballistic resistance of the package can be
distinguished. These are: material parameters, constructional parameters, parameters of the fragment
and/or bullet used for testing, and parameters of the ballistic test carried out [
57
,
58
]. The material
parameters in the case of soft ballistic inserts are mainly: density of the fibers, their tensile strength,
and friction occurring between the fibers. Structural parameters include: twist of the yarn, weave,
thread density, number of layers, etc.
The parameters associated with the bullets, such as weight, shape, and speed, and the parameters
associated with testing, such as the location of the shot, the number of shots, the angle at which the
bullet hits the target, and boundary conditions can significantly aect ballistic performance. However,
an in-depth discussion on bullet parameters and testing method is not the subject of this study.
In the case of material and design parameters of soft ballistic inserts, it has been observed that
during an impact of a fragment or high speed bullet, its energy is absorbed by mechanisms such as yarn
decrimping, fiber and yarn extension, yarn and fiberpull-out, and yarn rupture [
4
,
59
]. The relationships
between the above mentioned mechanisms and the ballistic performance of the materials used for soft
ballistic inserts are not yet fully described as they depend on a number of variables related to the type of
materials used to produce the ballistic package and the bullet and/or fragment used. With reference to
the research carried out herein, it is possible to confirm the occurrence of mechanisms, i.e., the tearing
and extension of fibers and yarn of soft ballistic insert (Figure 5c,d).
Appl. Sci. 2020, 10, x FOR PEER REVIEW 12 of 20
differences between individual values of this parameter. Therefore, the lifetime as well as
decomposition, viscosity, and price of the silicone elastomer for the sample preparation processes of
hybrid silicone-ceramic composites will play a role in the selection of the silicone elastomer, which
may determine the speed and profitability of future production methods. On the basis of the results
obtained, it can be indicated that in the case of HSC composites, the elements responsible for the
absorption of impact energy at high velocities (in the range of 600–1500 m/s) are ceramics and soft
ballistic inserts made of Twaron®CT612 (Teijin, Wuppertal, Germany). The use of silicone increases
the value of the ballistic protection limit by about 100 m/s in relation to the V50 values obtained for
the Twaron®CT612-based (Teijin, Wuppertal, Germany) soft ballistic inserts, which is about 14%.
The analysis of the literature data allowed to determine that in the case of soft ballistic inserts,
four categories of factors influencing the performance and ballistic resistance of the package can be
distinguished. These are: material parameters, constructional parameters, parameters of the fragment
and/or bullet used for testing, and parameters of the ballistic test carried out [57,58]. The material
parameters in the case of soft ballistic inserts are mainly: density of the fibers, their tensile strength,
and friction occurring between the fibers. Structural parameters include: twist of the yarn, weave,
thread density, number of layers, etc.
The parameters associated with the bullets, such as weight, shape, and speed, and the
parameters associated with testing, such as the location of the shot, the number of shots, the angle at
which the bullet hits the target, and boundary conditions can significantly affect ballistic
performance. However, an in-depth discussion on bullet parameters and testing method is not the
subject of this study.
In the case of material and design parameters of soft ballistic inserts, it has been observed that
during an impact of a fragment or high speed bullet, its energy is absorbed by mechanisms such as
yarn decrimping, fiber and yarn extension, yarn and fiberpull-out, and yarn rupture [4,59]. The
relationships between the above mentioned mechanisms and the ballistic performance of the
materials used for soft ballistic inserts are not yet fully described as they depend on a number of
variables related to the type of materials used to produce the ballistic package and the bullet and/or
fragment used. With reference to the research carried out herein, it is possible to confirm the
occurrence of mechanisms, i.e., the tearing and extension of fibers and yarn of soft ballistic insert
(Figure 4c,d).
Figure 4. Components of the HSC composite after the ballistic test: (a) ceramics tiles—view from the
impact side of the fragment; (b) silicone outer layer—view from the outlet side of the fragment; (c)
Twaron®CT612 soft ballistic armor—view from the impact side of the fragment; (d) Twaron®CT612
soft ballistic armor—view from the outlet side of the fragment.
In turn, crushing of ceramic elements and partial delamination of the external elastomeric
coating were observed for HSC composites. However, no processes related to melting or rupture of
the elastomer have been reported during the fragmentation resistance tests, which is probably related
to the significant thermal stability of the silicones used (Figure 4a,b).
Figure 5.
Components of the HSC composite after the ballistic test: (
a
) ceramics tiles—view from
the impact side of the fragment; (
b
) silicone outer layer—view from the outlet side of the fragment;
(
c
) Twaron
®
CT612 soft ballistic armor—view from the impact side of the fragment; (
d
) Twaron
®
CT612
soft ballistic armor—view from the outlet side of the fragment.
In turn, crushing of ceramic elements and partial delamination of the external elastomeric coating
were observed for HSC composites. However, no processes related to melting or rupture of the
elastomer have been reported during the fragmentation resistance tests, which is probably related to
the significant thermal stability of the silicones used (Figure 5a,b).
The mechanism of rupture of ceramic elements and their influence on ballistic resistance was
analyzed in previous studies. Cegła et al. [
60
] pointed out that the role of ceramics is to blunt the bullet’s
blade, break it into smaller fragments, and absorb some of its energy through fracture. He also stated
Appl. Sci. 2020,10, 9098 13 of 20
that the role of a composite made of fibrous material such as UHMWPE or para-aramid pre-impregnate,
to which a ceramic layer is attached, is to stop fragments of the bullet core by elastic deformation and
absorption of kinetic energy. Energy absorption takes place through combinations of deformation,
fiber extension, and composite delamination [
61
]. When a bullet hits, the ceramic layer is exposed
to very strong compressive stress. A stress wave travels through the material and when it reaches
the adhesive layer, it is reflected as a tensile stress wave. The interaction of these two waves leads to
cracking and destruction of the armor frontal ceramic cover. However, before this happens, the tip
of the bullet is blunted or broken into smaller pieces. Therefore, it is important that the front layer
of ceramics is as hard as possible [
61
,
62
]. Moreover, in the construction of the layered armor, a very
important role is played by the adhesive layer between the ceramics and the fibrous substrate, which
must be strong enough to ensure that after a hit, the undamaged areas of the ceramics layer and the
substrate remain bound together [60].
The process of bullet penetration into the ceramic armor was also presented by Magier [
63
].
He described this process as a four-stage one with the individual stages:
1. bullet and armor collision;
2. initial penetration of the bullet into the armor at a constant speed;
3. braking the bullet by the inertial and strength forces of the armor material;
4. final crater formation.
In the first stage of the bullet’s collision with the armor, the wave that occurs at the moment of
impact propagates from the tip of the bullet to its end, generating stresses exceeding the static limit of
material strength. At this stage, the tip of the bullet is plastically deformed and the shock wave moving
to the end of the bullet creates a stress that causes axial cracking at its edges. As the deformation
wave moves along the bullet, cracks appear. On the other hand, due to the impact, the armor creates
tension and pressure, which causes the bullet’s armor materials to become liquid and create a crater.
At the second stage, the bullet penetrates the armor at constant speed. The crater is enlarged by the
flow of the liquid phases of the bullet and armor on the sides. The back of the bullet moves faster
than its tip, which then erodes. At the third stage, the high pressure field disappears and the speed
of the bullet penetrating the armor is gradually lost. At the fourth stage, the crater shrinks under
the influence of recrystallisation and annealing of the armor material. In addition, the mechanism of
destruction of ceramic elements due to the impact of small arms ammunition was the subject of the
work of Reddy et al. [64] and Hogan et al. [65].
An analogous role as presented in the above mentioned publications is played by the ceramic
layer of the obtained silicon-ceramic composite. By crushing the hexagonal elements of Al
2
O
3
ceramics,
the fragment is blunted and some of its energy is absorbed due to the fracture of the hexagonal Al
2
O
3
fittings. The outer silicone layer makes a small contribution to the fragment retention mechanism,
as shown by the tests. It acts as a matrix in which the ceramic elements are placed and enables the
composite system to fit the user’s body, which increases the ergonomic properties of the cover.
The research also compared the value of V50 ballistic protection limit obtained for FSP.22 fragment,
for HSC composites with ballistic plates manufactured from “traditional” materials, i.e., hard ballistic
armor manufactured in the process of thermal-pressure pressing from Twaron
®
CT736 (Teijin, Germany),
metal plates (steel and Ti-6Al-4V alloy) and ballistic hybrid plates manufactured by combining ballistic
plates manufactured in the process of thermal-pressure pressing from Twaron
®
CT736 with ceramic
elements or Ti-6Al-4V alloy. It should be noted that each of the above mentioned ballistic systems was
placed on a soft ballistic insert made of Twaron
®
CT612 aramid with a surface mass of (5.0
±
0.5) kg/m
2
.
The results are presented in Figure 6.
It has been shown that soft ballistic packets with a surface mass (5.0
±
0.5) kg/m
2
obtained
from aramid materials such as Twaron
®
CT709, CT612, or CT608 exhibit a fragmentation resistance
of
(600 ±25)
m/s. Retrofitting soft inserts with plates obtained from Twaron
®
CT736 aramid
pre-impregnate in the process of thermo-pressure pressing increases resistance to FSP.22 fragment
Appl. Sci. 2020,10, 9098 14 of 20
from (600
±
25) m/s to 800–1100 m/s depending on the surface mass of the tested system. Ballistic
systems obtained on the basis of hybrid composites produced by combining plates obtained in the
process of thermal-pressure pressing with Twaron
®
CT736 with ceramic elements or Ti-6Al-4V alloy
were characterized by the highest V50 values and at the same time the highest surface mass. In this
case, the resistance to FSP.22 fragment was 1580–1850 m/s. At the same time, it can be pointed out that
the developed hybrid silicone-ceramic composites containing Al
2
O
3
ceramic elements of thickness
(3.5 ±0.2) mm
have mass per unit area comparable to a ballistic system containing steel or titanium
ballistic plates at a V50 value higher by about 10%. On the basis of Figure 6, it can also be determined
that the obtained HSC composites containing Al
2
O
3
ceramic elements with thickness (3.0
±
0.2) mm
and mass per unit area within the limits (21.0
±
0.5) kg/m
2
are characterized by higher values of ballistic
protection limit V50 by about 17% in comparison to systems based on pressed Twaron®CT736 plates
with comparable mass per unit area.
Appl. Sci. 2020, 10, x FOR PEER REVIEW 14 of 20
Figure 5. Dependence of V50 with the FSP.22 fragment of mass (1.10±0.03) g on the surface mass for
traditional ballistic systems and hybrid silicone-ceramic composites.
It has been shown that soft ballistic packets with a surface mass (5.0 ± 0.5) kg/m2 obtained from
aramid materials such as Twaron®CT709, CT612, or CT608 exhibit a fragmentation resistance of (600
± 25) m/s. Retrofitting soft inserts with plates obtained from Twaron®CT736 aramid pre-impregnate
in the process of thermo-pressure pressing increases resistance to FSP.22 fragment from (600 ± 25)
m/s to 800–1100 m/s depending on the surface mass of the tested system. Ballistic systems obtained
on the basis of hybrid composites produced by combining plates obtained in the process of thermal-
pressure pressing with Twaron®CT736 with ceramic elements or Ti-6Al-4V alloy were characterized
by the highest V50 values and at the same time the highest surface mass. In this case, the resistance
to FSP.22 fragment was 1580–1850 m/s. At the same time, it can be pointed out that the developed
hybrid silicone-ceramic composites containing Al2O3 ceramic elements of thickness (3.5 ± 0.2) mm
have mass per unit area comparable to a ballistic system containing steel or titanium ballistic plates
at a V50 value higher by about 10%. On the basis of Figure5, it can also be determined that the
obtained HSC composites containing Al2O3 ceramic elements with thickness (3.0 ± 0.2) mm and mass
per unit area within the limits (21.0 ± 0.5) kg/m2 are characterized by higher values of ballistic
protection limit V50 by about 17% in comparison to systems based on pressed Twaron®CT736 plates
with comparable mass per unit area.
Thus, the conducted tests indicate that the developed hybrid silicone-ceramic composites are
characterized by higher V50 parameter values for the FSP.22 fragment compared to the currently
used, traditional, ballistic hard plates of comparable mass per unit area.
In addition, the obtained experimental data were also compared with the literature. For example,
fragmentation tests performed by Iremonger and Went’s [66] showed that laminates made using
nylon 6,6 plain woven fabric and laminated with a matrix of ethylene vinyl acetate results in V50
values for 1.1 g FSPs fragment in the range of 322 to 486 m/s (depending on the number of layers of
the laminate). Additionally, literature data presents polymer ceramic composites which were
evaluated against fragment simulating projectiles of various calibers to investigate their ballistic
impact response. Samples were prepared by mechanically mixing boron carbide (B4C) and cubic
boron nitride (cBN) over a range of ratios and combinations with either thermosetting phenolic or
epoxy resin and aramid pulp. Ballistic tests performed with a 1.1 g FSP fragment for cBN-based armor
results in a V50 value of approximately 784 m/s. The obtained value was almost expected as cBN is
one of the hardest commercially available materials, second only to industrial diamond. Polymer
ceramic composites containing B4C tested for a V50 parameter results in a 702 m/s and Kevlar target
of the same areal density results with a 680 m/s V50 parameter value [67].
Figure 6.
Dependence of V50 with the FSP.22 fragment of mass (1.10
±
0.03) g on the surface mass for
traditional ballistic systems and hybrid silicone-ceramic composites.
Thus, the conducted tests indicate that the developed hybrid silicone-ceramic composites are
characterized by higher V50 parameter values for the FSP.22 fragment compared to the currently used,
traditional, ballistic hard plates of comparable mass per unit area.
In addition, the obtained experimental data were also compared with the literature. For example,
fragmentation tests performed by Iremonger and Went’s [
66
] showed that laminates made using nylon
6.6 plain woven fabric and laminated with a matrix of ethylene vinyl acetate results in V50 values
for 1.1 g FSPs fragment in the range of 322 to 486 m/s (depending on the number of layers of the
laminate). Additionally, literature data presents polymer ceramic composites which were evaluated
against fragment simulating projectiles of various calibers to investigate their ballistic impact response.
Samples were prepared by mechanically mixing boron carbide (B
4
C) and cubic boron nitride (cBN)
over a range of ratios and combinations with either thermosetting phenolic or epoxy resin and aramid
pulp. Ballistic tests performed with a 1.1 g FSP fragment for cBN-based armor results in a V50 value
of approximately 784 m/s. The obtained value was almost expected as cBN is one of the hardest
commercially available materials, second only to industrial diamond. Polymer ceramic composites
containing B
4
C tested for a V50 parameter results in a 702 m/s and Kevlar target of the same areal
density results with a 680 m/s V50 parameter value [67].
Appl. Sci. 2020,10, 9098 15 of 20
The ballistic limit of the aluminum plate was investigated by the Aziz’s et al. [
68
]. In this study,
there were dierent types of plate arrangement. The first category was a single plate with 3 mm
thickness. Meanwhile, the second category was two aluminum plates with distance of 100 mm between
them. Those plates were impacted by the cylindrical FSP fragments with 6 mm long, 5 mm in diameter,
and a weight of 1.1 g. The ballistic limit V50 for the single plate was equal to 276 m/s and for the double
plate about 388 m/s.Much lower V50 ballistic limits were obtained for ballistic materials such as Kevlar
®
(DuPont, Wilmington, United States) and Twaron
®
aramids and Dyneema
®
HPPE (DSM, Heerlen,
Netherlands), which were tested in a single-layer configuration. Impact velocity of carbon steel cylinder
FSP fragment with 4.5 mm diameter and 0.78 g weight at 50% probability of perforation (V50) through
the fabric ranged from about 120 to 250 m/s depending on the type of used material [
69
]. Another type
of hard body armor obtained on the basis of poly (methyl methacrylate) (PMMA) and polycarbonate
(PC) laminates was investigated by Hsieh and co-workers [
70
]. The ballistic measurements were
carried out using the 1.1 g weight 0.22 caliber FSP fragments. Authors showed that the PC-PMMA-PC
laminates exhibit a 37% higher value of the V50 (846 m/s) when compared with the PC-PC-PC laminates
of equivalent overall layer configuration. The ballistic limit against the FSP.22 increased with increasing
plate thickness of PMMA.
Comparing the literature data with results obtained for the developed HSC composites concludes
that these composites provide higher ballistic protection than various types of laminates, aluminum
plates, or polymer ceramic composites with B4C or cBN ceramics.
3.4. Impact Test Results
The obtained silicon-ceramic composites were also subjected to impact forces generated at much
lower velocities (about 1–10 m/s) during falls that occurred during various types of physical activities
(roller sports, cycling, horse riding). The obtained results are summarized and presented in Figure 7
and Table 6.
Appl. Sci. 2020, 10, x FOR PEER REVIEW 15 of 20
The ballistic limit of the aluminum plate was investigated by the Aziz’s et al. [68]. In this study,
there were different types of plate arrangement. The first category was a single plate with 3mm
thickness. Meanwhile, the second category was two aluminum plates with distance of 100 mm
between them. Those plates were impacted by the cylindrical FSP fragments with 6 mm long, 5 mm
in diameter, and a weight of 1.1 g. The ballistic limit V50 for the single plate was equal to 276 m/s and
for the double plate about 388 m/s.Much lower V50 ballistic limits were obtained for ballistic
materials such as Kevlar® (DuPont, Wilmington, United States) and Twaron® aramids and
Dyneema®HPPE (DSM, Heerlen, Netherlands), which were tested in a single-layer configuration.
Impact velocity of carbon steel cylinder FSP fragment with 4.5 mm diameter and 0.78 g weight at 50%
probability of perforation (V50) through the fabric ranged from about 120 to 250 m/s depending on
the type of used material [69]. Another type of hard body armor obtained on the basis of poly (methyl
methacrylate) (PMMA) and polycarbonate (PC) laminates was investigated by Hsieh and co-workers
[70]. The ballistic measurements were carried out using the 1.1 g weight 0.22 caliber FSP fragments.
Authors showed that the PC-PMMA-PC laminates exhibit a 37% higher value of the V50 (846 m/s)
when compared with the PC-PC-PC laminates of equivalent overall layer configuration. The ballistic
limit against the FSP.22 increased with increasing plate thickness of PMMA.
Comparing the literature data with results obtained for the developed HSC composites
concludes that these composites provide higher ballistic protection than various types of laminates,
aluminum plates, or polymer ceramic composites with B4C or cBN ceramics.
3.4. Impact Test Results
The obtained silicon-ceramic composites were also subjected to impact forces generated at much
lower velocities (about 1–10 m/s) during falls that occurred during various types of physical activities
(roller sports, cycling, horse riding). The obtained results are summarized and presented in Figure 6
and Table 7.
Figure 6. Dependence of the maximum force transferred to the sample on the surface mass of the
tested composite systems.
Table7. Impact test results for the impact energy of 5 J.
No. Composition of Samples Surface Mass
(kg/m2) Maximum Force Transmitted under the Sample (kN)
Silicone samples:
1. MM 922 silicon 11.7 ± 0.4 5.27 ± 0.22
2. MM 228 silicon 12.0 ± 0.5 4.00 ± 0.12
Figure 7.
Dependence of the maximum force transferred to the sample on the surface mass of the tested
composite systems.
Slightly higher values of the maximum force transmitted under the sample were obtained for
hybrid silicone-ceramic composites compared to samples consisting only of silicone elastomers of
the same thickness. It should be noted that HSC composites show a much higher mass per unit area
(increase by about 50%) compared to samples based only on silicone elastomers (Table 6). In view of
Appl. Sci. 2020,10, 9098 16 of 20
the above, it can be clearly indicated that the presence of ceramic elements does not improve the shock
absorption capacity during impact with 5 J aecting the composite. However, in the case of impact tests,
it is possible to indicate the relationship between the type of silicone elastomer used and the maximum
value of energy transferred to the sample. The lowest values for both the composite and the elastomer
sample were obtained for MM 228 silicone, whereas the highest for MM 922. Analyzing the obtained
test results in terms of correlation between the ability to absorb impact energy and the physical and
mechanical parameters, it can be indicated that only in the case of tear strength, the following relation
was noted, i.e., the increase in tear strength was accompanied by a decrease in the value of maximum
force transmitted to the sample. For the remaining parameters, i.e., hardness, density, extension at tear,
and tensile strength, no dependence on the ability to absorb impact energy was determined.
Table 6. Impact test results for the impact energy of 5 J.
No. Composition of Samples Surface Mass
(kg/m2)
Maximum Force Transmitted
under the Sample (kN)
Silicone samples:
1. MM 922 silicon 11.7 ±0.4 5.27 ±0.22
2. MM 228 silicon 12.0 ±0.5 4.00 ±0.12
3. Za 22 Mould silicon 11.8 ±0.5 4.33 ±0.28
Hybrid silicone-ceramic composite containing Al2O3ceramics (3.0 mm thick) and silicone:
4. MM 922 18.1 ±0.5 5.58 ±0.10
5. MM 228 18.4 ±0.6 4.54 ±0.16
6. Za 22 Mould 17.7 ±0.6 4.72 ±0.19
Samples of commercial systems containing polymer:
7. EVA 3.2 ±0.3 1.77 ±0.17
8. Poron®XRDMA 3.3 ±0.3 <4.0
9. EVA+EPDM 0.9 ±0.2 2.40 ±0.15
10. PE 1.0 ±0.4 1.64 ±0.13
The results obtained for silicone samples and HSC composites were also compared with
commercially available materials such as Poron
®
XRDMA, EVA copolymer, polyethylene, used
in market products intended for the production of protectors reducing the risk of human body injury.
The tests were performed for an impact energy of 5 J, for samples with thicknesses corresponding to
the remaining composite samples (Table 6). In this case, it has been determined that both the silicone
elastomers and hybrid silicone ceramic composites used have a poorer ability to absorb impact energy
with a significantly higher mass per unit area, which excludes them from use on an industrial scale in
the construction of protectors. However, the obtained results did not allow to indicate the reasons for
poor ability to absorb impact energy generated at low speeds of 1–10 m/s for the developed composites,
as they depend on many factors, such as: properties of the composite used, its physical-mechanical
parameters, shape and geometry of the sample or the value of the applied load, as well as on the
processes occurring during impact, i.e., deformations, plastic and/or elastic deformations, cracks,
delamination, thermal eects, and others. This may be a challenge for further research work.
4. Conclusions
As a part of the work, hybrid silicone-ceramic composites were obtained. These composites in
combination with a soft ballistic armor with an area weight (5.0
±
0.5) kg/m
2
provide V50 ballistic
protection limit in the range of 1200–1500 m/s. The developed HSC composite may be an alternative
to the currently used ballistic composites produced in the thermal-pressure pressing process based
on aramid or UHMWPE materials, as well as steel or titanium alloy ballistic armor. The research
showed that the ballistic resistance of the HSC composite is the result of the destruction of ceramic
elements by the fragment and the pulling, rupture, decrimbing, and extension of yarns and fibers
occurring in the soft ballistic insert. It was determined that the V50 value increases as the thickness of
Appl. Sci. 2020,10, 9098 17 of 20
the ceramic element increases. An increase in the thickness of the Al
2
O
3
ceramic element by about
(0.5
±
0.2) mm resulted in an increase in the V50 value in the range of 14–18%. The decisive factors
here were the physical and mechanical parameters of ceramic elements, i.e., hardness and Young’s
modulus. The outer silicone layer makes a slight contribution to the fragments’ stopping mechanism,
as demonstrated in the research. It acts as a matrix in which ceramic elements are placed and allows
the composite system to adjust to the user’s body, which increases the ergonomic properties of the
armor. The conducted research allowed to determine that there are no correlations between the type
of silicone elastomer used and its physico-mechanical properties, and the obtained value of the V50
ballistic protection limit.
Comparison of the results for the impact energy equal to 5 J generated during the drop tower
tests, obtained for samples of HSC composites with commercially available materials used in market
products intended for the production of protectors reducing the risk of human body damage, indicated
that the hybrid silicone-ceramic composites had lower ability to shock absorption with a significantly
higher surface weight. Therefore, they should be excluded from use on an industrial scale in the
construction of these type of protectors.
Designing of energy absorbing materials is challenging since various mechanisms, such as: wave
propagation, dynamic cracks, delamination, thermal eects, dislocation generation, elastic and shear
deformations, yarn and fiber pull-out and rupture, etc. are acting concurrently, at dierent material
scales, and intertwined during impact. Therefore, new research is needed, especially in producing
materials with strictly defined characteristics, modeling, and simulation of impact to be able to reach
a point where the designed composites will meet all user’s requirements related to low weight,
ergonomics, and ballistic resistance.
5. Patents
The work reported in this manuscript based on the Patent no. PL 424443(A1)—Method for
producing flexible ballistic armor and the armor produced by this method.
Author Contributions: Conceptualization, K.K. and K.O.; methodology, K.K., M.Ł., and E.C.-F.; formal analysis,
K.K., M.Ł.; investigation, K.K., M.Ł., P.K., and E.C.-F.; resources, K.K. and P.K.; writing—original draft preparation,
K.K.; visualization, K.K.; writing—review, M.F.; supervision, M.F. and K.K.; project administration, M.F. All authors
have read and agreed to the published version of the manuscript.
Funding:
This research was funded by the National Centre for Research and Development, Research Grant No.
POIR.04.01.04-00-0007/18-00 implemented as part of the Smart Growth Operational Programme 2014–2020.
Conflicts of Interest: The authors declare no conflict of interest.
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