Determining the hardest substance on Earth is a complex question with no single definitive answer. The hardness of a material depends on several factors, and different substances may be considered “hardest” based on the specific property being measured.
What does “hardness” mean?
Hardness refers to a material’s resistance to deformation, scratching, abrasion or cutting. There are a few key ways hardness is measured:
- Scratch hardness – Ability to resist scratching from another material
- Indentation hardness – Resistance to permanent shape change when a force is applied
- Rebound hardness – Ability to bounce back after being deformed. Also called elasticity.
Testing methods like the Mohs scale, Vickers hardness test and Brinell hardness test aim to quantify these hardness characteristics.
Hardest materials by scratch hardness
The Mohs scale ranks materials on their ability to resist scratching. It runs from 1 (softest) to 10 (hardest). On this basic scratch test, the hardest substances are:
- Diamond – 10
- Boron nitride – 9.5
- Corundum – 9
Diamond, an allotrope of carbon, occupies the top position with a perfect 10 rating. Its tightly bonded carbon atoms arranged in a crystalline lattice make it incredibly hard and resistant to scratching. Diamonds maintain their stiffness, strength and stability even at high temperatures.
Hardest materials by indentation hardness
Indentation hardness tests measure how well a material resists permanent shape change when a force is applied. Tungsten carbide and diamond again rank among the hardest.
| Material | Vickers hardness number |
|---|---|
| Reinforced carbon-carbon composite | 2850 |
| Tungsten carbide | 2400 |
| Titanium boride | 2400-2800 |
| Single crystal diamond | 7000-8000 |
Diamond has a Vickers hardness number between 7000-8000, much higher than all other materials. The Vickers test uses a pyramidal diamond indenting tool to make an impression, with more difficult impressions meaning a harder material.
Ultrahard materials like diamond demonstrate great stiffness and resistance even when a mechanical force is applied. However, diamond can still chip or fracture if struck a certain way. Tungsten carbide resists indentation almost as well, making it a frequent choice for industrial cutting and machining applications.
Hardest materials by elastic stiffness
Rebound or elastic hardness refers to a material’s “springiness” – how well it bounces back after being deformed. Materials with high elastic stiffness include:
- Diamond
- Carbyne
- Graphene
- Boron nitride nanotubes
Of these, the nanomaterial carbyne is considered the stiffest, with double the stiffness of graphene and diamond. Carbyne’s molecular chain of carbon atoms has an elastic modulus of 32.7 million psi, allowing it to withstand extremely high compression while retaining its shape. However, carbyne remains difficult to synthesize in usable quantities.
Hardest known mineral in nature
The hardest known naturally occurring substance by Mohs hardness is the rare mineral lonsdaleite. A polymorph of carbon with a hexagonal lattice structure, lonsdaleite forms when meteors containing graphite strike Earth. Its exceptional hardness results from its uncommon crystal symmetry, with ranks between 7-8 on the Mohs scale compared to 10 for diamond.
Only small amounts of lonsdaleite have been found on Earth. The mineral has been synthesised in laboratories, but so far only as microcrystalline fragments unsuitable for gems or tools. In its pure form, lonsdaleite may be 58% harder than diamond. However, confirmed naturally occurring specimens are likely not pure, reducing their relative hardness.
Hardest material created in a lab
In the lab, scientists can engineer even harder synthetic materials. Advances in nanotechnology and carbon nanomaterials have enabled creation of two ultrahard substances harder than any known natural material:
- Wurtzite boron nitride – In 2009, this hexagonal crystal form of boron nitride was produced via high pressure and temperature experiments. Simulation shows wurtzite boron nitride could be 18% harder than diamond.
- Q-carbon – Discovered in 2015, Q-carbon is created by allowing amorphous carbon to crystallize into diamond-like nanoscale crystals under extreme heat and pressure. It demonstrates exceptional hardness across multiple tests and is believed to be harder than diamond.
Both substances show unique physical and mechanical properties in testing. However, reproducing them in large, stable quantities remains a hurdle before varieties like Q-carbon can realize commercial potential.
Conclusion
Measuring hardness depends greatly on the test method and even the orientation or grain of a material’s crystal structure. Nonetheless, diamond stands out across almost all measures as the hardest naturally occurring substance. Only a few rare minerals like lonsdaleite come close. Engineered materials like wurtzite boron nitride and Q-carbon have proven harder still in small laboratory quantities. But difficulties in synthesis and scale-up mean diamond remains the benchmark for hardness under normal conditions on Earth.