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Ballistic Performance of a Composite Metal Foam-ceramic Armor System

Abstract Composite Metal Foam is a low-weight, high-strength porous material capable of absorbing great amounts of energy under loading. In this report, Composite Metal Foam panels are manufactured using powder metallurgy technique and 2 mm steel hollow spheres in a steel matrix and used in conjunction with a ceramic plate to fabricate a new light-weight composite armor system. This armor system is tested under ballistic loading using 7.62x51 mm M80 and 7.62x63 mm M2 AP projectiles at varying impact velocities for single and multi-impact scenarios. The material behavior, failure mechanism, and ballistic performance of the armor system are studied for optimization.


Introduction
High performance ballistic protection systems are usually composed of different layers of materials with each layer providing a specific role in the attenuation of projectile energy. Typically, a hard material like ceramic or high hard steel is placed on the projectile strike face, and a softer para-aramid or polymer matrix composite is used as a back-plate. The role of the strike face is to decelerate and degrade the projectile by erosion, fracture, and tumble, while the softer back-plate absorbs the residual kinetic energy of the projectile and fragments, bringing it to rest (David (2009)). Ballistic panels composed of ceramic strike face and fiber reinforced back-plates have been widely studied and have proved to perform well in typical ballistic tests  Part 1, Medvedowvski (2010) Part 2, and Colombo (2006)). However, the complexity, high cost and high weight of these systems leaves much room for improvement.
Steel-Steel Composite Metal Foams (S-S CMF) have great potential for energy absorption under quasi-static and dynamic loading conditions (Rabiei (2006), Neville (2008), Rabiei (2009), Rabiei (2007, and Rabiei (2013)). Sintering hollow steel spheres in a compacted steel powder matrix gives CMF its low weight and high strength capabilities. Its foam structure, on the other hand, allows CMF to deform at high compressive stress (~136 MPa) to over 50% strains, thus absorbing great amounts of energy.
In this study, composite armor systems made of S-S CMF panels as backing plates in conjunction with strike face ceramic plates are tested under ballistic loading. The US National Institute of Justice (NIJ) ballistic test standard 0101.06 is used as a guideline to evaluate the ballistic performance of such armor system under impact from 7.62x51 mm M80 (Type III) and 7.62x63 mm M2 Armor Piercing (AP) (Type IV) projectiles for single and multi-shot scenarios.

Materials and Processing
Composite Metal Foam (CMF) panels were manufactured using 2 mm diameter steel hollow spheres embedded in a stainless steel powder matrix using powder metallurgy (PM) techniques previously developed (Rabiei (2006), Neville (2008), Rabiei (2009), Rabiei (2007, and Rabiei (2013)). The hollow spheres were fabricated by Hollomet GmbH in Dresden, Germany, while 316L stainless steel powder with a particle size of 44 µm produced by North American Hoganas High Alloys LLC was used as the matrix material.
Boron Carbide (B 4 C) ceramic tiles, 30cm x 30cm with different thicknesses were fabricated for use in the ballistic evaluation. The ceramic tiles were bonded to the CMF back-plate panels to create individual armor targets. All targets had a total thickness of less than 25 mm.
For ballistic testing, the armor system was placed against Roma Plastilina No. 1 clay. The ceramic -CMF armor system setup is shown in Figure 1A.

Ballistic Testing Procedure
Ballistic testing of the composite armor system was performed using the setup described in US government's National Institute of Justice (NIJ) 0101.06 standard (U.S. Department of Justice (2008)) for both Type III and IV armor systems using 7.62x51 mm M80 and 7.62x63 mm M2 AP projectiles respectively (ammunition shown in Figure 1B). A "Mann" type gun, mounted on a two axis adjustable rig was used to aim and fire the projectiles. Velocity chronograph, situated between the gun and the target, was used to calculate the speed of the bullet to ensure projectiles impacted the armor system at the precise speeds specified by the standard. Two high speed digital cameras were aimed at the armor system's front and back surfaces. The front high speed camera was used to observe projectile impact and its interaction with the ceramic strike face, while a second camera was used to obtain residual exit velocities at the rear of the sample, in the case of complete penetration of the bullet through the armor target. Figure 2 shows the notional setup for ballistic testing experiments.

Structural Properties
Digital images of the mesostructure of Steel-steel composite foam made with 2 mm hollow spheres are shown in Figure 3A-C. As can be observed in this figure, composite metal foam shows an even distribution of porosity throughout the entire surface of the panel with the powder matrix infiltrated between the spheres, bonding into a solid bulk material. Figure 3B shows the surface of CMF as processed, where the individual spheres can be seen arranged between the sintered metal-powder. Figure 3C shows a detail image of the cut section of the metal foam after Electrical Discharge Machining (EDM). This even distribution of same size porosities gives CMF its high strength and exceptional energy absorption capability.

Ballistic Performance
Images of the strike and rear face of the composite armor system after ballistic testing are shown in Figure 4A-B respectively for a Type III impact. The composite armor system formed by the combination of ceramic strike plate and S-S CMF backing plate showed superior ballistic performance against the Type III threat, completely stopping the projectile (Fig. 4A) in both single and multi-shot impacts at specification relevant impact velocities. Similarly, the armor system was able to completely degrade the hardened steel core of the Type IV armor piercing ammunition at velocities in excess of the NIJ Type IV standard. In Figure 4B, the composite metal foam shows compression at the center of the impact area, where the individual spheres are flatten. Some cracking is formed radially outward from the center of the impact area, at the bottom of the sample.
Typical deformation mechanism of an armor system is shown in Figure 5. Upon impact, the hard ceramic face plate shatters while deforming and eroding the tip of the projectile (Smith (1994)). The resulting damage pattern in the ceramic is conical in shape and serves to spread out the impact area so the kinetic energy of the bullet can be absorbed by the ductile backing plate over a larger area than the initial strike zone (Smith (1994)).
As penetration takes place, compressive waves reach the interface between the ceramic and the composite metal foam layer. Due to the mechanical impedance mismatch between the ceramic and the foam, residual tensile waves are formed and reflected back towards the impact point, forming the characteristic radial cracks on the ceramic material and an outward conical perforation on its cross section. The residual compressive waves, as well as the kinetic energy of the bullet, are absorbed by the CMF layer through the deformation of the foam, which then leaves an in-print signature on the ballistic clay ( Figure 5). Volumes of material loss for ceramic in compression and tension (V c ceramic and V t ceramic respectively) and deformed volumes of CMF (VCMF) and bullet (V c bullet ), along with the perforation diameter, back face signature (BFS), and depth of penetration (DOP) into the clay were then measured after each test for energy absorption evaluation.
It is notable that multiple energy absorption numbers on the chart that are related to the same speed are for samples with various thicknesses of ceramic face-plates. Nevertheless, an increase in the energy absorption per unit volume of the composite foams by a factor of 3, for impact speeds up to 870 m/s is observed.

Summary
In order to create a high performance ballistic armor system, a ceramic face-plate with high hardness was bonded to a steel-steel composite metal foam processed by powder metallurgy using 2 mm sphere. The armor system was tested under ballistic loading against the 7.62x51 mm M80 and 7.62x63 mm M2 AP projectiles (Type III and IV respectively) according to US NIJ 0101.06 standard. The results of the ballistic tests were successful at demonstrating the ability of this unique material to absorb ballistic impact energy. The armor system successfully stopped Type III threats, with the ceramic layer blunting and eroding the projectile, and the CMF layer absorbing over three quarters of initial kinetic energy of the projectile through plastic deformation. The armor system was also capable of defeating Type IV threats by completely eroding and fracturing the hardened steel core of the projectile. In this study, steel-steel composite metal foam showed high performance for high speed/high energy absorption applications, with over 3-times increase in energy absorption of the material at impact speeds up to 870 m/s.