Rice University researchers have developed a method to keep diamond stable during high-temperature, low-pressure processing, enabling the creation of a durable bulk composite material. Their research also revealed that high-speed impacts can quickly transform diamond into graphite, providing new insight into how diamond absorbs energy and changes under extreme stress. Because diamond combines exceptional hardness with high thermal conductivity, these findings could help scientists design stronger, more resilient materials for aerospace, defense and other high-performance applications.
Small diamond particles are relatively inexpensive and easy to produce, but turning them into larger diamond structures has proved difficult, Ajayan said.
One way to join those particles into a larger structure is through sintering, a process that uses heat and pressure to form a solid. With diamond, however, the high temperatures can turn it into graphite. High-pressure, high-temperature methods can produce polycrystalline diamond, but they require extreme pressure and limit the size of the samples produced.
To keep diamond stable during processing, the researchers mixed microscopic diamond grains with cubic boron nitride, a material with properties similar to diamond, and cobalt to bind and stabilize the mixture.
They used spark plasma sintering, a rapid process that applies heat and pressure to turn powders into a solid. The process produced an extremely strong composite with diamond particles embedded in the boron nitride matrix and cobalt distributed throughout.
“The composite made by this process is almost nonmachinable and tough due to the presence of dispersed diamond particles and could help researchers design tougher materials for aerospace, defense and other technologies that face extreme conditions,” said Abhijit Biswas, first author and research scientist in Rice’s materials science and nanoengineering department.
The researchers then subjected the composite to high-speed collisions to study how it behaved under extreme force.
Researchers fired tiny metal projectiles measuring 1-4 millimeters across at the composite at hypersonic speeds, and the material held together when struck by one of the projectiles traveling at more than seven times the speed of sound. A larger projectile traveling even faster caused the composite to break apart.
During the collision, nearly all the diamonds involved in the transformation became graphite within microseconds.
Diamond and graphite are both made of carbon, but their atoms are arranged differently. That difference makes diamond hard and graphite much softer.
“We found that extreme impact can drive diamond to graphite within microseconds, rather than through the slower heat-driven process we normally associate with this transformation,” Biswas said. “That gives us a new view of how diamonds behave under some of the most demanding mechanical conditions.”
The researchers examined the fractured composite and used molecular dynamics simulations, computer models that track how atoms move and rearrange. Their analysis revealed areas where diamond and graphite met, providing clues to how the transformation occurred.
The study attributes the change to energy from the shock and structural rearrangements that turn diamond into graphite.
The researchers also found that the conversion to graphite helped absorb some of the energy from the collision as the diamond’s atomic structure changed.
The findings offer a closer look at how internal transformations can help toughen materials under severe conditions.
“Understanding how materials change their structure and phase under force, along with their strength and hardness, could help guide the design of future protective materials,” Biswas said.
For more information: Materials Today
Image: Pulickel Ajayan, left, and Abhijit Biswas, right. Photo by Jorge Vidal.
