Strongest glass in the world can scratch diamonds

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A piece of 1mm-wide AM-III glass left scratch marks on the surface of a natural diamond. Photo: National Science Review

Glass is associated with brittleness and fragility rather than strength. However, researchers in China were able to create a new transparent amorphous material that is so strong and hard that it can scratch diamonds. What’s more, this high-tech glass has a semiconductor bandgap, which makes it appealing for solar panels.

Strongest amorphous material in the world

Diamond, the hardest material known to date in the universe, is often used in tools for cutting glass. But the tables have turned.

“Comprehensive mechanical tests demonstrate that the synthesized AM-III carbon is the hardest and strongest amorphous material known so far, which can scratch diamond crystal and approach its strength. The produced AM carbon materials combine outstanding mechanical and electronic properties, and may potentially be used in photovoltaic applications that require ultrahigh strength and wear resistance,” the authors of the new study wrote.

The new material developed by scientists at Yanshan University in Hebei province, China, is tentatively named AM-III and was rated at 113 gigapascals (GPA) in the Vickers hardness test. Vickers hardness, a measure of the hardness of a material, is calculated from the size of an impression produced under load by a pyramid-shaped diamond indenter.

That’s more than many natural diamonds that have a Vickers score in the range of 70-100 GPa, but less than the hardest diamonds that can score up to 150 GPa.

It’s about ten times harder than mild steel and could be 20 to 100 times tougher than most bulletproof windows.

Shaped like diamonds, looks like glass

Like diamonds, AM-III is mostly made of carbon. But while carbon atoms in diamond are arranged in an orderly crystal lattice, glass has a chaotic internal structure typical of an amorphous material. This is why glass is typically weak, but AM-III has micro-structures in the material that appear orderly, just like crystals. So, AM-III is part glass, part crystal, which explains its strength.

In order to make AM-III, the Chinese researchers had to employ a process that is even more complicated than manufacturing artificial diamonds. The most common method for creating synthetic diamonds used in the industry is called high pressure, high temperature (HPHT). During HPHT, carbon is subjected to similarly high temperatures and pressure as those that led to the formation of natural diamonds deep in the Earth, around 1,300 degrees Celsius (1650 to 2370 degrees Fahrenheit) and a pressure 50,000 times greater on the surface.

Instead of graphite, the raw material of artificial diamonds, the Chinese researchers started off with fullerene, also called buckminsterfullerene. These molecules contain at least 60 atoms of carbon, commonly denoted as C60, arranged in a lattice that can either form a ball or sphere shape and are typically 1nm diameter.

These carbon “footballs” are typically soft and squishy. But after being subjected to great heat and pressure, the carbon balls are crushed and blended together.

The fullerene was subjected to about 25 GPa of pressure and 1,200 degrees Celsius (2,192 degrees Fahrenheit). However, the researchers were careful to reach these conditions very gradually, taking their time over the course of about 12 hours. Immediately subjecting the material to high pressure and heat may have turned the carbon balls into diamonds.

The resulting transparent material is not only hard but also a semiconductor, with a bandgap range almost as effective as silicon, the main semiconductor used in electronics. So besides bulletproof glass, it could prove useful in the solar panel industry where its properties can shine by allowing sunlight to reach photovoltaic cells, while also enhancing the lifespan of the product.

AM-III was described in a recent study published in the journal National Science Review

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