Response to Ballistic Impact of Alumina-UHMWPE Composites

The response to ballistic impact of alumina-ultra high molecular weight polyethylene (UHMWPE) composites with different relative concentrations of alumina was investigated. The impact tests were carried out at subsonic speed using a compressed air system. The results showed that the depth of penetration (DOP) in a Medium Density Fiberboard (MDF) bulkhead protected by a disk of the composite decreased with increasing concentration of alumina in the composite. Scanning electron microscopy (SEM) images of composites with 80 %, 85 % and 95 % alumina showed transgranular, intergranular and ductile fracture mechanisms.


Introduction
In the beginning of the 21st century, local and regional conflicts are a sad reality.In this scenario, attack and defense equipment is a flourishing field of research.
The current bulletproof personal protection vests use a single layer of aramid fabric 1 .This protection, however, is limited to relatively low impact velocities.Protection against heavier armament requires a multilayered armor system (MAS) 2 .Conventional MAS have a ceramic front layer which erodes the projectile tip.This mechanism of energy dissipation involves not only fragmentation of the projectile but also fragmentation of the ceramic.
The main ceramic materials used in ballistic protection are alumina (Al 2 O 3 ), silicon carbide (SiC) and boron carbide (B 4 C).Alumina has been widely used for all kinds of protection because of its hardness, abrasion resistance and chemical inertia.However, its low flexural strength and low fracture toughness mean that the use of pure alumina for protection may lead to catastrophic failure.Moreover, its high density limits its use in applications where weight is crucial, such as bulletproof vests [3][4][5][6] .
The purpose of this work was to investigate the properties of alumina-UHMWPE composites in which UHMWPE is used to decrease density and increase flexural strength and fracture toughness, making the shield more suitable for personal protection and avoiding fracture after the first shot 7,8 .There is also an economic factor involved, since the composite is prepared at a relatively low temperature, 230 o C, while pure alumina must be prepared sintering alumina powder at high temperatures, of the order of 1400ºC, a more expensive procedure 9 .

Sample preparation
Composites with different alumina-UHMWPE mass ratios were prepared by mechanical mixing for 10 min and labeled A20, A40, A60, A80, A85, A90 and A95, where 20, 40, etc. is the alumina mass concentration in percent.Mixing was performed using a Britânia mechanical mixer.No solvent was used.
The samples were produced in the shape of discs 5 mm thick and 51 mm in diameter.The discs were pressed at 230ºC for 10 minutes under a force of 90 kN.The A00 and A100 samples are, respectively, pure UHMWPE and pure alumina sintered using a previously described route 9 .

Characterization
For the ballistic tests, a Gunpower SSS air rifle was used with a noise suppressor Padrão Armas.The projectile was a 22 gauge lead shot with an estimated mass of 3.3 g.The impact speed was measured using an Air Chrony ballistic chronograph model MK3 with a precision of 0.15 m/s.
After the ballistic tests, images of the composites with 80 %, 85 % and 95 % alumina were obtained in a FEI Quanta FEG 250 SEM.

Ballistic tests
In the ballistic tests, the air rifle was positioned 5 m away from the target, consisting of a composite disk attached with The noise suppressor was used to increase the stability of the projectile at the exit of the air rifle by reducing the swirling caused by the exhaust.
MDF plates were used as bulkheads because MDF is a homogeneous material, flat and dense, not having the grain of solid wood.
The ballistic chronograph was placed 10 cm from the exit of the noise suppressor.
Figure 1 shows the result of a typical shot.
All shots completely penetrated the disk.Two shots were made in each experiment and five experiments were performed for each composition.The results are shown in Fig. 2 for A00, A20, A40, A60 and A80 samples.
Table 1 shows the projectile mass, the average impact speed (v), the depth of penetration (DOP), the sample mass (M s ) and the figure of merit (FM) for each composition.Since the projectile mass and the impact speed were about the same for all tests, the results could be directly compared.Except for A100 (pure alumina), with is included only for comparison, the samples with the highest figure of merit were A00 and A90.This suggests that the adhesion between alumina particles and UHMWPE is very weak.
Figure 3 shows the dependence of DOP on alumina concentration.The composite with the lowest alumina concentration (A20) has a value of DOP larger than that of pure UHMWPE, probably due to the fact that a small addition of alumina introduces defects in UHMWPE without contributing significantly to increase its resistance to penetration.For larger concentrations of alumina, increasing the alumina concentration increases the shear thickening For ogival projectiles, the depth of penetration (DOP) in the bulkhead depend on the projectile mass and the impact speed [10][11][12][13][14][15] .Taking into account the fact that in the case of personal protection weight may be an important factor, one may define a figure of merit given by the following equation: where M s is the sample mass.effect with the appearance of hydroclusters during the collision 16,17 .For alumina concentrations above 60 %, one observes a significant decrease in DOP, which is probably due to the increased contribution of the UHMWPE crystallized region around the particles due to a decrease of the distance between the particles 16 .

SEM images of alumina powder and samples A80, A85 and A95
Figure 4 is a SEM Image of the alumina powder, showing the irregular shape of the particles, which improves the stiffness of the composites, since the aggregates formed by these particles have a low density and contain continuous pores that are filled with UHMWPE.The same phenomenon has been observed in fumed silica 17 .
Figure 5 shows the distal face of an A80 sample; the crater formed by the projectile impact is clearly seen, surrounded by a small concussion zone.
Figure 6a is a SEM image of the impact region of an A80 sample, showing a transgranular fragile fracture and a ductile region of UHMWPE.The surface of the sample has small voids, which are attributed to a low UHMWPE content.Figure 7 is a SEM image of an A95 sample, showing alumina particles with irregular shapes and high porosity.It shows transgranular fracture and reduced participation of ductile fracture in energy absorption.
Figure 8a is the SEM image of an A95 sample, showing that a good quality interface is produced, that is, a good contact (without voids) between matrix and reinforcement is observed; a transgranular fracture is clearly seen.Figure 8b shows a crack in another A95 sample, attributed to more than one mechanism.
Figure 9 shows the SEM image of an A95 sample with primarily a transgranular fracture and a ductile fracture of UHMWPE.Although samples A100 had the lowest penetration depth, they were shattered by the first shot.

Conclusions
The samples with the highest figure of merit were samples A00 and A90, suggesting the absence of interfacial adhesion between the alumina particles and the UHMWPE matrix.
Composition A90 (90 % alumina and 10 % UHMWPE) was the one that presented best performance in terms of weight and penetration depth.
SEM images of samples A80, A85 and A95 showed several modes of energy absorption of the projectile impact.The primary mechanism of energy absorption was transgranular fracture after hydroclusters form in the sample, followed by ductile fracture of partially crystallized UHMWPE.

a
Figueiredo et al. 2Materials Research

Figure 1 .
Figure 1.Result of a typical shot.(a) Depth of penetration (DOP) measured from the surface of the bulkhead to the tip the projectile; (b) projectile; (c) MDF bulkhead; (d) composite disk.

Figure 3 .
Figure 3. Dependence of DOP on alumina concentration.

Figure 4 .
Figure 4. SEM image of the alumina powder.

Figure 5 .
Figure 5. Optical image of the distal face of an A80 sample, showing the crater formed by impact.

Figure 7 .
Figure 7. SEM image of sample A95, showing the porosity of alumina particles.

Figure 8 .
Figure 8. SEM images of A95 samples, showing (a) a transgranular fracture and (b) a crack.

Figure
Figure6bis a SEM image of sample A85, showing a ductile fracture of UHMWPE that contributes to increase the tenacity7,8 .Figure7is a SEM image of an A95 sample, showing alumina particles with irregular shapes and high porosity.It shows transgranular fracture and reduced participation of ductile fracture in energy absorption.Figure8ais the SEM image of an A95 sample, showing that a good quality interface is produced, that is, a good contact (without voids) between matrix and reinforcement is observed; a transgranular fracture is clearly seen.Figure8bshows a crack in another A95 sample, attributed to more than one mechanism.Figure9shows the SEM image of an A95 sample with primarily a transgranular fracture and a ductile fracture of UHMWPE.

Figure 9 .
Figure 9. SEM image of a section of the crack of Figure 8b, showing participation of all mechanisms in the crack

Table 1 .
Average results of the ballistic tests.