Advanced Ceramics in the Defense Industry
Ceramic composite armor materials are currently being investigated by researchers to take advantage of the potential for improved performance and reduced weight.
Within the defense industry, advanced ceramics are at the heart of modern armor systems due to their comparatively low weight and high performance during ballistic-scale impacts. Ceramic materials have been developed as an armor material since World War I, when enameling of steel plates was shown to improve their resistance to incoming bullets. Ceramic armor became more widespread during the Vietnam conflict, where boron carbide (B4C) was used in helicopter seats as protection against small-arms fire from the ground.
Over time, ceramic armor systems were found to be capable of stopping bullets and other projectiles while being more lightweight than other materials, such as metals. As we move into the 21st century, ceramic materials such as aluminum oxide (otherwise known as alumina, Al2O3), silicon carbide (SiC) and titanium diboride (TiB2) are used in aerospace, armored vehicles, and personal armor. The defense industry has been actively seeking novel armor materials that provide superior ballistic performance to existing materials while maintaining or improving the weight advantage.
The principal requirement of any armor material is its ability to resist high-energy ballistic impacts (i.e., to have high ballistic performance). In addition, a reduction in the weight of personal armor is essential to increase a soldier’s maneuverability in the field and alleviate long-term health problems associated with carrying heavy equipment. Furthermore, reducing the weight of armor also benefits vehicles by reducing their fuel consumption and structural strain, as well as helping with long-distance transport and other logistical issues. In addition to ballistic performance and low weight, these capabilities will ideally be features of a material that can be manufactured at low cost.
The development of armor materials has a very direct effect on human lives; optimization of their processing and the evolution of new materials is a never-ending challenge. Understanding test conditions is also essential if ceramic armor development is to continue.
State-of-the-Art Ceramic Armor Materials
Three of the most common ceramic armor materials are alumina, boron carbide and silicon carbide. Typical properties of these three materials are summarized in Table 1.1,2 Despite having the highest density of the three and a comparatively lower ballistic performance, alumina is the most widely used in the armor industry mainly due to inexpensive processing. Silicon carbide has lower density and higher ballistic performance, but at comparatively higher cost, and boron carbide has a significantly reduced density, but the higher cost restricts its use to weight-saving critical applications.
With the consistent threat of even higher energy projectiles and advances of hard-tipped armor-piercing rounds, interest in the development of ceramic armor materials continues to grow. Ceramic composite armor materials, which are currently being investigated by researchers to take advantage of the potential for improved performance and reduced weight, consist of a conventional ceramic matrix with a well-dispersed second phase; these have been shown to have superior mechanical properties when measured by conventional testing methodologies. The incorporation of SiC particles into an Al2O3 matrix, for example, reportedly reduces the density and increases the hardness and strength of pure Al2O3.3
Conventional powder-based processing of monolithic ceramics and/or ceramic composites involves:
- Milling of the ceramic powders into the required size range through ball milling, attrition milling, planetary milling, etc.
- Mixing of the matrix material and the secondary phase (for ceramic composites)
- Green forming through die pressing and/or isostatic pressing
- Sintering through pressureless sintering, hot pressing or hot isostatic pressing
Driven by the need for enhanced material performance at a reduced fabrication cost, the manufacturing of ceramic armor materials is confronted by several challenges. Control of the dispersion and size of the secondary phase has a drastic effect on the final properties of composites; dispersion can be improved through a slurry-based mixing process. By selecting a proper dispersant system, both the matrix and the secondary phase particles can be well-dispersed and stabilized in a slurry, enabling an optimal distribution of particles to be achieved.
Another issue is the preparation of homogenous, defect-free green bodies prior to sintering to minimize the pre-existing defects inside the material. A reduction in defect density can be achieved by green pressing granules that have been prepared using a spray freeze drying (SFD) technique. The improved flowability afforded by this technique facilitates the rearrangement of granules during pressing and crushing, resulting in fewer defects in the green body.
Ceramic armor materials can also benefit from developments in material processing throughout the industry. For example, the energy efficiency of ceramic manufacturing is an ongoing development that is not restricted to armor ceramics, and can be improved with emerging technologies such as field-enhanced sintering (FES). By applying a direct electric field during sintering, FES is able to densify a ceramic product at significantly lower temperatures and in a drastically reduced timeframe, compared to conventional sintering. In addition, it offers better microstructural control of the final ceramic product, which allows for more predictable mechanical properties.
Finally, increasing the complexity of ceramic shapes can be achieved using techniques such as viscous plastic processing (VPP), which involves the mixing of ceramic powders with a viscous polymer solution to produce “dough” that can be shaped. The extent of machining can be reduced further by using additive manufacturing (AM) techniques, otherwise known as 3D printing. Use of AM for metals and polymers is currently widespread; technologies to manufacture ceramic components using such techniques are on the rise.
In many cases, the mechanical property improvements offered by ceramic composites translate into superior armor performance; spark plasma sintered SiC-5 vol% B4C, for example, has been reported to perform better under ballistic impact conditions than monolithic B4C.4 A similar improvement in performance was also observed in 10% zirconia-toughened alumina (ZTA) over monolithic Al2O3.5 Furthermore, the introduction of Al into an Al2O3 matrix to produce an Al2O3-Al composite has also provided improved multi-hit capabilities.6 These results demonstrate that the future of ceramic armor is bright; the simple introduction of a secondary phase potentially provides huge benefits to the properties.
It is challenging, however, to develop new ceramic materials for armor that offer a confident improvement due to an incomplete understanding of how ceramic armor systems behave when subjected to ballistic impact. The properties that affect this are currently up for debate.
Understanding Ceramic Armor
Ceramic armor is used as a multi-layer composite armor system. The front plate consists of a hard ceramic strike face. An adhesive interlayer attaches an energy-absorbing ductile backing plate, which is often made from fiber-reinforced composite or metal, depending on the expected threats and weight-bearing capability of the wearer. Armored vehicles, for example, are expected to face more destructive threats and thus have heavier armor, whereas weight savings is more critical for aircraft. A diagram of a ceramic armor system is shown in Figure 1.
The purpose of the ceramic front plate is to break the incoming projectile; the fragments that are created (of both ceramic and projectile) are captured by the more ductile backing plate. The mechanisms by which these outcomes are achieved are not fully understood, but a simplification of the process follows.
Upon contact, the tip of the projectile is destroyed as it is crushed between the hard front of the ceramic and its own advancing rear. This effectively reduces the kinetic energy density at the point of contact by spreading the energy of the projectile across a larger contact area. The projectile may completely shatter itself against the armor if its kinetic energy is below a particular threshold; this phenomenon is known as interface defeat.
If the energy of the projectile is sufficient, the projectile will begin to penetrate into the ceramic plate. As it does so, after a brief initial period of inelastic deformation, the ceramic begins to fracture; this is also largely due to high-intensity stress waves induced throughout the system that are incurred by the impacting projectile. At first, compressive stress waves are induced in the ceramic. As they reach boundaries (e.g., at the edges of the tile or the adhesive interlayer), however, the compressive stress waves are reflected as more damaging tensile waves. Hence, engineering of the acoustic properties of the component materials in the armor system is an important consideration.
As the projectile advances into the fragmented ceramic, it is eroded by the hard, sharp shards, reducing the bulk mass of the projectile—many smaller bodies are easier to stop than one large body with the same sum of kinetic energy. Ceramic fragments directly ahead of the projectile are crushed and compacted (comminuted) into an area known as the Mescall Zone. This reduces the projectile velocity and is dependent on the fragments’ resistance to flow. The concentrated kinetic energy of the projectile is also distributed over a wider area, which allows the ductile backing to absorb what is left of the initial kinetic energy through elastic and plastic deformation.
As mentioned previously, this is a simplified description of a ballistic event and the possible outcomes that may take place. In reality, the response of ceramic armor systems under ballistic impact conditions is far more complicated, and it changes significantly based on the test conditions. For example, the combination of ceramic, adhesive and ductile backing layers alters the nature of the stress waves induced through the system, which affects the fragmentation and therefore the ability of the ceramic to resist penetration. Alternatively, in some outlying cases, armor materials may be penetrated by projectiles but able to withstand the same projectile fired at higher velocity, due to the time taken for certain energy dissipation mechanisms to come into play (a phenomenon known as “shatter gap”). Also, whether the projectile is a steel-core bullet, tungsten long-rod penetrator or an explosive affects how the armor system will respond.
It is generally acknowledged that hardness, toughness and fracture strength are the key material properties to consider when selecting a ceramic material for armor applications; the hardness, for example, influences the erosion/blunting of the projectile, and toughness improves multi-hit capabilities. However, consensus within the scientific community over what material properties have a direct and significant effect on the ballistic performance remains elusive; it is not possible to conclusively link ballistic performance to hardness, toughness, Young’s modulus, grain size, grain shape or any other easily-defined static material property.
As a result, the only widely accepted method of assessing the performance of a ceramic material or armor system is to conduct ballistic-scale testing. This state of events is not ideal; due to the variable nature of ceramic properties, many tests require at least 20-30 armor samples for a robust result, and up to and over 100 may be required for a complete statistical understanding of the material. This represents a significant investment to create a sufficient volume of material for testing.
Ballistic tests are still evolving to allow for more information than a pass/fail result to be extracted from them. For example, as part of a consortium of work with industrial and academic partners, a new novel ballistic test method* was developed that uses ballistic gel and a rubber sheet restraint (see Figure 2). This enables the collection of the fragments generated during a ballistic test, as shown in Figure 3. By evaluating these fragments as part of a comprehensive post-ballistic impact characterization process, new observations have been made on the behavior of ceramic materials subjected to ballistic testing. This information can be used in the development of new ceramic armor materials and test methods.1
*Developed by Lucideon and the University of Surrey.
For static armor materials, ceramic armor systems are currently state of the art. However, new materials are in development to improve on the key features of ballistic performance and low density (and ideally low cost). Ceramic composites represent a fertile avenue of investigation for the next generation of armor materials, but increased understanding of the factors involved in the ballistic event is required if the development cycle is to become more viable. New and novel test methods have recently come to light that can confer such knowledge; these findings can be used to overcome obstacles to growth in the armor industry and save lives in conflicts of the future.
1. Healey, A.; Cotton, J.; Maclachlan, S.; Smith, P.; and Yeomans, J., “Understanding the Ballistic Event: Methodology and Initial Observations,” J. Mat. Sci., 52, 2017, pp. 3074-3085.
2. Karandikar, P.G.; Evans, G.; Wong, W.; and Aghankanian, M.K., “A Review of Ceramics for Armor Applications,” Ceram. Eng. Sci. Proc., 29, 2009, pp. 163-178.
3. Sternitzke, M., “Review: Structural Ceramic Nanocomposites,” J. Euro. Ceram. Soc., 17(9), 1997, pp. 1061-1082.
4. Hallam, D.; Heaton, A.; James, B.; Smith, P.; and Yeomans, J., “The Correlation of Indentation Behavior with Ballistic Performance for Spark Plasma Sintered Armor Ceramics,” J. Euro. Ceram. Soc., 35(8), 2015, pp. 2243-2252.
5. Zhang, X.F. and Li, Y.C., “On the Comparison of the Ballistic Performance of 10% Zirconia Toughened Alumina and 95% Alumina Ceramic Target,” Mater. Des., 31, 2010, pp. 1945-1952.
6. Strassburger, E.; Lexow, B.; Beffort, O.; and Jeanquartier et al., “Ballistic Resistance and Impact Behavior of Al2O3-Al Ceramic Metal Composites,” 19th Int. Sump. Ballist., 2001, pp. 7-11.