Diamond is the hardest known natural substance, yet it is also highly brittle: cleavage can propagate along well-defined crystallographic planes with minimal resistance, and a single microscopic flaw may induce sudden, catastrophic failure. This intrinsic hardness–toughness trade-off originates from the strong covalent bonding network, which restricts dislocation-mediated plasticity and largely precludes the energy-dissipation mechanisms commonly available in metals and toughened ceramics.
In recent work, a joint research team led by Jilin University and Ningbo University has exploited this inherent brittleness as a design opportunity. Rather than attempting to arrest cracks after nucleation, the authors developed a diamond material that responds actively to the stress field at a crack tip, dissipating energy directly at the crack front. Their strategy, termed active fracture regulation, relies on a pre-programmed network of atomic-scale planar defects within the diamond lattice. Under crack-tip stresses, these defects undergo local reorganization, generating a multiphase bridging zone that shields the crack and forces it to propagate along a highly tortuous path.
Building a brick wall from graphite
The material was synthesized from flake graphite, which possesses a layered structure. Upon exposure to a pressure of 22 GPa and a temperature of 2600 K, the graphite transformed into diamond while retaining its flake-like morphology. The resulting microstructure resembles a brick wall and consists of elongated diamond grains separated by dense, aligned stacking faults across which the atomic stacking sequence is locally displaced.
Combined improvement in hardness and crack resistance
The optimized material, designated NPD8, exhibited a Knoop hardness of 140.1 GPa, approximately 17% higher than that of conventional polycrystalline diamond. More notably, its indentation fracture toughness reached 16.9 MPa·m 1/2 , corresponding to a 120% enhancement relative to reference ultrahard materials. This combination places NPD8 in a region of the hardness–toughness map that has rarely been attained by diamond-based materials, indicating that substantial simultaneous enhancement of hardness and fracture toughness is achievable in this system.
The material also displayed superior thermal stability. Its oxidation onset temperature in air was 1295 K, nearly 200 K higher than that of single-crystal diamond and approaching the value reported for nano-twinned diamond. This enhanced oxidation resistance is relevant for high-speed cutting and drilling applications, where frictional heating is severe.
How the defects fight cracks
High-resolution electron microscopy revealed the sequence of events near crack tips in NPD8. When a propagating crack approaches a bundle of stacking faults, the local stress field triggers a structural reconstruction: portions of the diamond transform into a strained cubic phase, localized regions become disordered, and nanoscale graphitic domains form. This reconstruction produces a three-phase bridging ligament that spans the crack wake and exerts closure tractions on the fracture surfaces. Energy is dissipated through the stress-driven conversion of sp 3 -hybridized carbon to sp 2 -hybridized carbon, thereby suppressing further crack propagation.
On larger length scales, the aligned stacking-fault arrays promote repeated crack deflection, branching, and the formation of crystalline bridges between the fracture surfaces. The resulting crack path is highly tortuous, which increases the effective fracture surface area and the total energy required for failure.
A new design philosophy for superhard materials
This study demonstrates that lattice defects should not be regarded merely as accidental imperfections; they can be deliberately engineered as functional elements that enable stress-responsive behavior. The same design principle may be extended to other covalent superhard materials, including cubic boron nitride, hexagonal diamond, and carbon nitrides, which exhibit similar brittleness.
National Science Review
Experimental study