Hexagonal diamond inspired by meteorites is the hardest yet

A plain lump of graphite has never looked so ambitious. By squeezing and heating it under brutal conditions, researchers in Beijing have coaxed carbon atoms to line up in an elusive hexagonal pattern that nature usually only forges during meteorite impacts.

The result is a millimeter-wide crystal that pushes the limits of hardness and could soon change how industry cuts, drills, and polishes the world.

The work comes from the Center for High Pressure Science and Technology Advanced Research (HPSTAR), where hexagonal diamond, also called lonsdaleite. has been the dream target for decades.

Lead investigator Ho-Kwang Mao says the new sample is the purest and largest yet, a claim supported by detailed X-ray and electron-microscope checks.

Why shape makes it stronger

Hexagonal diamond keeps the same strong sp³ bonds as ordinary cubic diamond, but its layers stack in a different order. That shift removes the uniform shear planes that let cracks start in the cubic form.

Materials theorists have long predicted that this rearrangement could raise indentation resistance by roughly 50 to 60 percent.

Hardness is usually reported with the Vickers hardness test, which presses a diamond pyramid into the surface and records how much it indents. Cubic diamond typically measures around 115 gigapascals on this scale.

Simulations suggested that hexagonal diamond could sail past 150 GPa, but earlier laboratory attempts produced only nanoscopic, impure grains that were impossible to test.

Meteorites hinted at what was possible. Tiny lonsdaleite inclusions found in the Canyon Diablo meteorite in 1967 revealed a form of carbon harder than any known material, but the fragments were so mixed with regular diamond that their true properties remained unclear for decades.

How hexagonal diamond was made

Mao’s team started with single-crystal graphite, then ramped the pressure to 20 GPa and the temperature to 2,552°F (1,400°C) inside a multianvil press. Previous studies suggested that window favored a direct graphite-to-hexagonal transition, and experiments on related samples at the same pressure have confirmed stable lonsdaleite formation.

The treated sliver, just under one millimeter (0.04 inches) across and seventy micrometers thick, survived decompression without reverting to softer forms.

“Once we know how to make it, anyone can produce it. It’s incredibly valuable,” said Mao after seeing the recovered crystal at the team’s Beijing lab.

High-resolution transmission-electron micrographs showed a tightly interwoven mosaic of 100-nanometer crystals with almost no cubic contamination. Such texturing, while good for toughness, may have slightly lowered the first hardness reading.

Testing toughness the hard way

Indentation experiments on the fresh sample gave a Vickers value of 120 GPa, edging past the lower end of natural diamond tests yet still shy of the theoretical maximum. Mao believes purer starting graphite and finer control of heating ramps will close that gap. 

“Obviously, the deeper you go, the hotter it gets, [and] it could enable them to go deeper underground,” said James Elliott, a materials engineer at the University of Cambridge who was not involved in the project, when asked about geothermal boreholes. 

Lab tests also suggest the new crystal rides comfortably above 2,012°F (1,100°C) before oxidation weakens it. This is far beyond the 1,292°F (700°C) ceiling that sends cubic diamond tooling to an early retirement.

That thermal edge stems from the absence of easy slip planes, which normally open pathways for oxygen and metal attack.

How hexagonal diamond helps drilling

Geothermal wells must pierce miles of abrasive, heat-soaked rock. Today’s polycrystalline diamond compact cutters dull quickly once temperatures climb past 1,202°F (650°C), forcing costly tripping of drill strings for replacement heads.

A cutter faced with hexagonal diamond could stay sharp for long enough to reach hotter reservoirs where supercritical water, and thus higher power output, awaits.

The same physics excites manufacturers of aerospace composites and semiconductor wafers. Silicon carbide, gallium nitride, and carbon-carbon brake disks chew through conventional diamond wheels because localized heating softens their edges.

A tougher, hotter-running abrasive could slice these materials cleanly, saving coolant, energy, and scrap.

Mass production is still a challenge

Scaling production remains the missing piece. The Beijing pressure cell yields a crystal roughly the diameter of a fine mechanical pencil lead after several hours of run time.

Mao’s group is filing patent paperwork while they hunt ways to pack bigger graphite seeds into arrays that can be transformed in a single press cycle.

Cost is another hurdle. High-pressure apparatus that can generate 20 GPa typically fills a room and draws kilowatts. Industry partners are experimenting with shock compression and laser-driven synthesis, but those routes currently create defective or mixed-phase outputs that fall short of tooling standards.

Material scientists also caution that Vickers hardness alone does not guarantee performance. Brittleness, fracture toughness, and chemical stability under lubricants all influence service life.

Early signs look promising, yet toolmakers will demand years of wear data before swapping from established cubic diamond composites.

The study is published in the journal Nature.

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