China Scientists Create 'Unbreakable' Crystal Stronger Than Di...
Science
China Scientists Create 'Unbreakable' Crystal Stronger Than Diamond: How It Works<br>Researchers used a carbon nanotube network to improve fracture resistance while retaining the material's extreme hardness.
By Lakshmi Prabha
August 11, 2026 21:42 +08
[This is a representational image] Researchers developed a diamond composite reinforced with a three-dimensional network of multi-walled carbon nanotubes to improve fracture toughness.<br>Wikimediacommons
Chinese researchers have developed a diamond composite designed to resist fracture while retaining high hardness, according to a study published July 9 in the peer-reviewed journal Nature Synthesis.
The material incorporates a three-dimensional network of multi-walled carbon nanotubes between diamond grains and recorded an average fracture toughness of 31.9 MPa m¹ᐟ², about five times that of single-crystal diamond.<br>The researchers, led by scientists affiliated with the Institute of Physics of the Chinese Academy of Sciences and Beihang University, reported a hardness of about 91.6 GPa for the composite. The study describes the result as an approach to improving diamond's fracture resistance without the conventional trade-off between toughness and hardness.<br>The finding is significant because diamond is exceptionally hard but can fracture under mechanical stress. Hardness and fracture toughness describe different properties: hardness measures resistance to deformation, while fracture toughness measures a material's ability to resist crack growth.<br>The study measured its mechanical properties under controlled laboratory tests rather than demonstrating that it cannot be broken by an impact such as a hammer blow.
Carbon Nanotubes Reinforce Diamond<br>The researchers introduced highly dispersed multi-walled carbon nanotubes, or MWCNTs, into the spaces between diamond grains. These nanotubes form a continuous three-dimensional network throughout the composite.<br>According to the study, interfaces between the nanotube network and the diamond matrix contain mixed sp²-sp³ carbon bonding. These interfaces help dissipate energy and impede the propagation of cracks through the material.<br>The researchers also built a three-dimensional diamond framework with strong diamond-to-diamond bonding. They reported that this structure helped prevent the nanotube addition from causing a substantial loss of hardness.<br>The material was prepared under high-pressure, high-temperature conditions. The paper's experimental figures include a composite prepared at 2,000°C and 15 GPa.<br>The study reported a maximum fracture-toughness measurement of 36.4 MPa m¹ᐟ², compared with an average of 31.9 MPa m¹ᐟ². The researchers said the average value was approximately five times that of single-crystal diamond and exceeded values reported for some tungsten alloys.<br>Diamond's Hardness Comes With Brittleness<br>Diamond's extreme hardness makes it valuable for cutting, drilling, polishing and other applications where resistance to wear is important. But hardness alone does not prevent a material from cracking.<br>The distinction has limited attempts to broaden diamond's use in applications involving repeated impact or mechanical loading. Increasing toughness can come at the cost of hardness, while preserving hardness can leave a material susceptible to fracture.<br>The Chinese team's approach instead places a reinforcing network inside the diamond structure. The researchers described this as an "extrinsic" toughening strategy, in contrast with approaches that modify diamond's internal microstructure.<br>The result could be relevant to advanced cutting tools and other components in which both wear resistance and resistance to cracking are important. However, the study does not establish that the material is ready for commercial production or large-scale industrial deployment.<br>A Separate Diamond Breakthrough<br>The July composite study followed another Chinese materials-science result published in Nature in March.<br>Researchers from Zhengzhou University, Nanjing University and Henan University of Science and Technology reported the synthesis of millimeter-sized, phase-pure hexagonal diamond, also known as lonsdaleite. The study was published March 4 in Nature.<br>Hexagonal diamond differs structurally from conventional cubic diamond. Its existence as a distinct carbon phase had been debated for decades because naturally occurring samples associated with meteorites were extremely limited and often contained other carbon structures.<br>The researchers produced hexagonal diamond from highly oriented pyrolytic graphite by compressing it along its crystal axis at elevated temperatures. The paper reports that the material was synthesized under pressures around 20 GPa, with one documented sample recovered after treatment at 20 GPa and 1,300°C.<br>Advanced structural measurements were used to identify the material as hexagonal...