Are Diamonds the Hardest Substance on Earth? Exploring Ultrahard Carbon Nitrides

In materials science, the hardness of a material measures its resistance to deformation by indentation, scratching or other physical actions. By this metric, plastics and soft tissues are said to be very soft, whereas metals and hard minerals such as quartz are significantly harder. This raises an enduring question in both industry and academia: Are diamonds the hardest substance on Earth

Diamond has been widely considered to be the hardest material in existence – only being able to be scratched by other diamonds – followed by the synthetic material boron nitride. But, an international research team led by researchers from the University of Edinburgh, Scotland and experts from the University of Bayreuth, Germany and the University of Linköping, Sweden, report a material that could rival diamond.

Their synthesis of ultra-incompressible and superhard carbon nitride materials has been published in the journal Advanced Materials.

Understanding hardness in materials science

Hardness describes a material’s resistance to permanent deformation and is commonly assessed using several methods:

• Vickers hardness test: Applies a diamond pyramidal indenter under a known load to measure the size of the resulting indentation.
• Knoop hardness test: Uses an elongated indenter ideal for brittle or thin materials where minimal penetration is necessary.
• Mohs hardness scale: A qualitative mineralogical scale ranking materials from talc (1) to diamond (10) based on scratch resistance.
• Bulk modulus and compressibility: Hardness also correlates with the resistance to volume reduction under pressure, often measured during high-pressure X-ray diffraction.

Diamonds rank at the top of the Mohs scale and exhibit a Vickers hardness of 70–150 GPa depending on crystal orientation. Synthetic cubic boron nitride and tungsten carbide offer high degrees of hardness but still fall short of diamond.

However, theoretical materials such as carbon nitrides have long been predicted to potentially exceed this limit, provided that their atomic structures could be stabilized.

Credit: Tahlia Doyle/ Unsplash.

The search for ultrahard materials

In 1989, leading materials scientists published a paper in Science predicting that “hypothetical covalent solids formed between carbon and nitrogen” would be “good candidates for extreme hardness.”

Carbon nitride (C₃N₄) compounds featuring a three-dimensional network of corner-sharing CN₄ tetrahedra would then become one of the great aspirations in materials science, with hypothetical predictions expecting the compounds to have a hardness comparable to – or greater than – diamond.

In parallel, a growing demand for multifunctional materials in industry would drive scientists to more closely investigate these theoretical superhard compounds. Ultrahard materials, if successfully synthesized, could transform multiple industries. Potential applications include:

• High-endurance cutting tools and abrasive surfaces
• Protective coatings for aerospace, automotive and industrial equipment
• Components for high-pressure physics experiments
• Wear-resistant microelectromechanical systems
• High-performance optical or photonic components
• Solar panels, photodetectors and energy-storage technologies

However, after more than three decades of research and multiple synthesis attempts, nobody was able to bring about unambiguous evidence proving the synthesis of these hard carbon nitride compounds.

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Researchers have since reported the successful creation of three carbon nitride samples that exhibit extreme hardness and ultra-incompressibility.

Synthesized carbon nitride compounds rival diamond and boron nitride

To create the groundbreaking new materials, the researchers loaded various forms of carbon nitrogen precursors into laser-heated diamond anvil cells, subjecting them to extreme pressures of up to 1 million atmospheres and temperatures near 2500 Kelvin.

X-ray beam analysis on the compounds was carried out at three different particle accelerators – the European Synchrotron Research Facility in France, the Deutsches Elektronen-Synchrotron in Germany and the Advanced Photon Source based in the United States – to allow the researchers to study the crystal structure of the compounds formed under these conditions.

They found that the carbon nitride materials produced did indeed have the necessary building blocks for superhardness.

“Upon the discovery of the first of these new carbon nitride materials, we were incredulous to have produced materials researchers have been dreaming of for the last three decades,” said Dr. Dominque Laniel, UKRI Future Leaders Fellow in the University of Edinburgh’s Institute for Condensed Matter Physics and Complex Systems. “These materials provide [a] strong incentive to bridge the gap between high pressure materials synthesis and industrial applications.”

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When the materials were returned to ambient temperature and pressure, the researchers conducted physical properties testing using optical and scanning electron microscopes (SEM). The team found dents in the diamond anvil cells, suggesting that the new carbon nitrides were hard enough to deform diamond.

Table 1: A comparison of hardness between diamond, cubic boron nitride and carbon nitrides.

Additional testing and computational calculations suggested that the new materials could also display other interesting properties, such as photoluminescence and high energy density, where a large amount of energy can be stored in a small amount of mass.

“These materials are not only outstanding in their multi-functionality, but show that technologically-relevant phases can be recovered from a synthesis pressure equivalent to the conditions found thousands of kilometers in the Earth’s interior,” said Dr. Florian Trybel, assistant professor in the Linköping University Department of Physics, Chemistry and Biology. “We strongly believe this collaborative research will open up new possibilities for the field.”

Are diamonds still the hardest substance on Earth?

The long-standing belief that diamond is the hardest substance on Earth is being challenged by the synthesis of ultra-incompressible carbon nitrides featuring the long-theorized CN₄ tetrahedral network. Through the combined use of laser-heated diamond anvil cells, synchrotron X-ray diffraction and advanced microscopy, researchers have provided compelling evidence that these materials may equal or exceed diamond’s legendary hardness.

These developments signal a potential shift in the materials landscape, one in which synthetic carbon nitrides could become essential in cutting tools, protective coatings, photonic devices and high-pressure technologies. While further testing and scaling are required, the path toward next-generation ultrahard materials is now clearer than ever.

This article is a rework of a press release issued by the University of Edinburgh. Material has been edited for length and the content has been updated to provide additional context and details of related developments since the original press release was published on our website. This content includes text that has been created with the assistance of generative AI and has undergone editorial review before publishing. Technology Networks' AI policy can be found here.