In lab, Rice builds diamond composite without extreme pressure

A tiny metal projectile hits a block of engineered material at more than seven times the speed of sound. The block holds together. In Rice University experiments, the team also found that the impact can turn diamond into graphite within microseconds, revealing a way the material absorbs some of the collision's energy.

The result begins with a manufacturing problem. Small diamond particles are relatively cheap and simple to fabricate, but binding them into large solid shapes is difficult: at high temperatures, diamond naturally breaks down into softer graphite unless extreme pressure is applied. Conventional high-pressure, high-temperature processing can make polycrystalline diamond, but the pressure limits the size of the parts.

Rice's team combined fine diamond powder with cubic boron nitride, a material with similar physical properties, and added cobalt as a binder. Using spark plasma sintering—a rapid process that applies heat and pressure at the same time—the researchers made a dense composite in which diamond grains are embedded in a continuous cubic boron nitride structure, with cobalt spread throughout.

The composite is tough and nearly impossible to machine because of the embedded diamond particles. During testing, researchers fired projectiles measuring 1-4 millimeters across at hypersonic speeds. The material resisted destruction when the projectile traveled faster than seven times the speed of sound, but fractured when a larger projectile moved even faster.

So what changes in practice? The work gives aerospace and defense researchers a route to investigate larger diamond-based protective parts without relying on the extreme pressures used in standard processing. The immediate result is still a laboratory composite, and the study does not establish a finished product or field deployment. Its most useful finding may be the mechanism: under extreme shock, diamond can rapidly rearrange into graphite, absorbing kinetic energy while the surrounding composite holds together.