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.
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.
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.
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
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.
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.
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.
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.
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.
Diamond’s Hardness Comes With Brittleness
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.
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.
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.
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.
A Separate Diamond Breakthrough
The July composite study followed another Chinese materials-science result published in Nature in March.
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.
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.
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.
Advanced structural measurements were used to identify the material as hexagonal diamond. The researchers reported that the bulk material had slightly higher hardness than cubic diamond and high thermal stability.
The study measured a Vickers hardness of about 114 GPa, according to reporting on the research and the team’s published results. The figure is above commonly cited hardness values for natural cubic diamond and should not be interpreted as evidence that hexagonal diamond is dramatically harder than all conventional diamond.
The Key Difference between the Studies
The two Chinese studies are related through the broader search for improved carbon materials, but they should not be treated as a single breakthrough.
The Nature Synthesis study focuses on toughness. Its diamond composite uses carbon nanotubes to make the material more resistant to fracture while maintaining high hardness.
The Nature study focuses on structure and hardness. It provides experimental evidence for millimeter-sized, phase-pure hexagonal diamond and reports hardness slightly above that of conventional cubic diamond.
I’m late, but I didn’t know China had the diamond game on lock with the lab joints
Does this basically mean mined diamonds will be worthless?
Together, the findings illustrate two different strategies for improving diamond-based materials: reinforcing conventional diamond to resist cracking and creating a different carbon crystal structure with potentially different mechanical and thermal properties.
Potential Industrial Applications
Neither study demonstrates that these materials are ready to replace conventional diamond at industrial scale.
For the nanotube-reinforced composite, further work would be needed to assess manufacturing scale, consistency, long-term durability and performance under real operating conditions. For hexagonal diamond, researchers will need to establish whether the material can be produced reliably in larger quantities and whether its measured properties translate into practical advantages.
The studies nevertheless show that researchers are pursuing different ways to address the limitations of conventional diamond, including the long-standing tension between hardness and resistance to fracture.
Full text: https://archive.ph/x0bxf
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