Hierarchical Diamond-Based Ceramic Composites

An innovative combination of experimental synthesis and testing and multiscale simulation techniques explored the effects of hierarchical microstructure (mesoscale diamond packing and nanoscale interfaces) on the mechanical and ballistic performance of diamond–silicon carbide (SiC) composite ceramics.

December 1, 2022

Army Research Laboratory, Aberdeen Proving Ground, Maryland

High-level schematic linking the synthesis, characterization, testing, and evaluation of diamond ceramic composites in this program. Primary goals were to parameterize the microstructure from the atomistic through mesoscale hierarchy using an array of modeling and targeted experimental techniques that validate the outputs and allow iterative developments that improve material properties.

This research developed and utilized advanced processing, modeling, and micromechanical tools to discover and demonstrate hierarchically structured diamond-based composites with exceptional mechanical and ballistic behavior. Understanding how nanoscale and mesoscale microstructural features in diamond–silicon carbide (SiC) composites influenced the physics of failure was critical in uncovering ways to improve performance for soldier protection and discover potential defeat mechanisms.

Emphasis was placed on the fundamental understanding of the deformation and failure mechanisms, which enabled the design and development of robust materials to support Army core functions. Development of new materials was the focus with specific emphasis on fundamental knowledge of microstructural grain boundaries in diamond-SiC composites. Novel processing routes to selectively tailor the nano-mesoscale microstructure in heterogeneous ceramic armors were explored via conventional hot-pressing, reactive hot-pressing, and spark plasma sintering (SPS). Correlations between the nano-mesoscale hierarchical microstructure, deformation mechanisms, and mechanical response were explored using advanced characterization methods and small-scale mechanical testing. Higher fidelity mesoscale mechanics models were sought by using experimentally obtained microstructural information coupled with atomistic models of relevant grain boundary interfaces.

SiC and boron carbide (B4C) have long been investigated as materials critical to the performance of armor ceramic and related systems. Their combination of high hardness, low density, and strength have long been identified among the key differentiating characteristics for ceramic materials. Over roughly the last two decades, diamond-SiC composites have been developed for commercial applications requiring high abrasion resistance, thermal stability, and thermal conductivity. Thus, it follows that composites made from the hardest known bulk material, diamond, could be integrated and optimized for armor ceramic systems.

While extensive work at the U.S. Army Combat Capabilities Development Command (DEVCOM) Army Research Laboratory (ARL) has improved the knowledge and performance of SiC- and B4C-based armors, the inclusion of diamond has received comparatively little attention. Globally, the diamond grit industry is driving down costs such that diamond-based composite armor solutions are more tractable. These factors combined to inspire this basic research study in diamond-based ceramic composites.

The most common low-pressure processing route is a liquid silicon (Si) infiltration method where Si reacts with a carbon (C) source to form SiC under ambient or low pressure. Diamond-SiC composites made by this technique require a minimum of large open pathways between the diamond particulates for Si migration, which constrains the amount of diamond solids loading. Low C diffusivity in SiC can also limit the reaction leaving residual Si metal within the matrix if the initial distribution of carbon and infiltration pathways are not carefully optimized.

Research studies investigating diamond ceramic composites currently produced using these methods have shown that their performance is linked to hierarchical microstructure features. In diamond-SiC these hierarchical microstructural features can be divided between the mesoscale distribution of super hard diamond particulates and the nanoscale interfaces between the diamond-SiC and SiC grain boundaries. Prior research studies have shown there are likely correlations between these hierarchical features on the mechanical response. For example, the overall concentration of diamond particulates plays a major role in the hardness as described by composite theory.

Prior work in this area noted that the performance of diamond-SiC is also influenced strongly by the interfacial strength between diamond and SiC¹² and that decreasing the grain size of the SiC matrix can improve the fracture toughness from 8 to 12 MPa·m^1/2 (a dramatic increase over bulk diamond at 3–5 MPa·m^1/2). Because the initial intended application of these composites was never for armor, prior research on the terminal mechanics of diamond-SiC under high strain rates is extremely limited. The only known ballistic data on high-solids content diamond materials demonstrated a 45 percent improvement over B4C in the transition impact velocity against tungsten heavy alloy penetrators. Extensive studies have not examined the ballistic response of polycrystalline or single crystalline diamond due to high material costs and unavailability in sizes of interest.

This work was performed by Anthony DiGiovanni, Shawn Coleman, Matthew Guziewski, Jerry LaSalvia, Mathew Ivill, Samuel G. Hirsch, William T Shoulders, Raymond Brennan, Philip Goins, Thomas Scharf, Jonathan Ligda, Daniel Magagnosc, Brian Powers, Richard B. Leavy, John Clayton, Debjoy Mallick, Jennifer Dunn, S. Vennila Raju, Michael Kornecki, Scott Walck, and Brian Schuster for the Army Research Laboratory. For more information, download the Technical Support Package (free white paper) below.