Scientists make diamond five times tougher without losing any of its hardness
A team from the Chinese Academy of Sciences and Beihang University has solved one of materials science's longest-standing problems. They made diamond tougher without making it less hard. The answer is a 3D network of carbon nanotubes woven directly into the diamond structure. The resulting composite achieves a fracture toughness five times that of single-crystal diamond while preserving hardness at levels indistinguishable from the natural material.
The paper is titled "Multi-walled carbon nanotube network-toughened diamond composite via atomic interface continuity," with lead contributors Jiawei Zhang, Keliang Qiu, and Xiaohui Yu. The reported fracture toughness is approximately 31.9 MPa m1/2. Single-crystal diamond typically measures around 6 MPa m1/2. The hardness of the new material is approximately 91.6 GPa, which sits squarely within the range of natural and synthetic diamond .
A problem decades in the making
Diamond's weakness has always been its brittleness. A precise strike on a cleavage plane can shatter a stone that nothing else can scratch. That combination is not an accident of nature. The same carbon bonds that give diamond its hardness also make it inflexible in the face of fracture. Previous researchers used intrinsic toughening strategies: modifying diamond's internal microstructure through nanotwinning, stacking faults, or amorphous phases. These approaches improved toughness, but generally at some cost to hardness.
This work takes a different route, which the team classifies as extrinsic toughening. Rather than rearranging diamond's atomic architecture , the team introduced a reinforcing structure during synthesis: a three-dimensional, continuous network of multi-walled carbon nanotubes. The result is what they call atomic interface continuity between the nanotube network and the surrounding diamond.
How the nanotube network works
Multi-walled carbon nanotubes are extraordinarily small. At roughly one ten-thousandth the width of a human hair, a single nanotube is invisible under any optical microscope. Its tensile strength is dozens of times that of steel. Where the nanotubes contact the diamond matrix , the bonding is a mixture of sp3 (as in diamond) and sp2 (as in graphite and nanotubes) hybridizations. These mixed sp2-sp3 interfaces distribute mechanical stress across the composite rather than letting it concentrate at a single crack tip. That concentration is the mechanism by which diamond normally fractures.
The result is a material that absorbs and deflects crack propagation through the nanotube network, rather than failing catastrophically along a cleavage plane. The authors report that this toughness level surpasses that of the tungsten alloys used in armour-piercing ammunition, while hardness remains unchanged.
Where this changes the equation
Most industrial applications that rely on diamond — cutting tools, drilling equipment, precision grinding — currently accept brittleness as an unavoidable limitation and design around it. Aerospace components and high-performance wear surfaces face particularly demanding combinations of impact and abrasion that push conventional diamond to its limits. A material that retains diamond's hardness while achieving the fracture resistance of a structural metal alloy could expand the operational envelope for those applications significantly.
The work builds on prior research into diamond composite materials . A 2020 Nature paper by a separate team achieved high toughness through hierarchical structuring but could not fully preserve hardness. By establishing that an extrinsic nanotube-based architecture can resolve the trade-off cleanly, this paper introduces a new design strategy for superhard composites.
The papers were first published in the journal Nature Synthesis .
