The pursuit of ultra-durable materials has driven scientists to constantly redefine what is possible with glass, a substance often associated with fragility. The question of what constitutes the strongest glass does not yield a single, simple answer because “strength” is a complex scientific concept. Different applications require glass to resist different kinds of forces, meaning the strongest glass for a skyscraper window is not the same as the strongest glass for a smartphone screen. The material’s true champion status depends entirely on the specific metric being tested.
Defining the Metrics of Glass Strength
To understand glass strength, it is necessary to consider three separate mechanical properties, as no single number can fully describe a material’s resistance to all types of damage. Hardness measures a material’s resistance to permanent deformation, such as scratching or indentation. This property is often quantified using the Vickers scale, which involves pressing a diamond indenter into the surface and measuring the resulting impression size.
Another metric is toughness, which refers to the material’s ability to absorb energy and resist catastrophic failure once a crack has formed. Since glass is inherently brittle, its toughness measures how effectively it prevents a tiny surface flaw from rapidly propagating into a complete fracture. A material can be extremely hard but still have low toughness, meaning a microscopic scratch might not form, but a strong impact could still cause it to shatter immediately.
Flexural strength, sometimes called the modulus of rupture, measures a material’s ability to resist breaking under bending stress. This property is determined by tests that apply a load to a glass sample until it fails, usually by measuring the force required for a clean break in a three- or four-point bending setup. For common glass, this is the most practical measure of its ability to withstand everyday stresses like wind load or weight.
The World’s Strongest Known Glass
The current record-holder for the hardest and strongest amorphous material is a laboratory-created substance known as AM-III, essentially a super-hard glass. This remarkable material is not based on the common silicon dioxide structure but is a carbon-based fullerene glass. AM-III gains its extreme properties by starting with fullerene molecules—carbon atoms arranged in hollow spheres—and subjecting them to immense pressure and heat.
Researchers created this material by applying approximately 25 gigapascals of pressure, equivalent to 250,000 times the Earth’s atmospheric pressure, while heating the fullerene to over 1,200 degrees Celsius. This process causes the carbon “footballs” to crush and blend together, forming an exceptionally dense and robust glass-like network. The internal structure of AM-III is primarily amorphous, meaning it lacks the predictable, repeating lattice of a crystal.
The resulting material achieves a Vickers hardness rating of 113 gigapascals (GPa), a figure that allows it to scratch a natural diamond crystal. This places AM-III in the category of ultra-hard materials, demonstrating resistance to indentation orders of magnitude greater than standard glass. Its unique combination of an amorphous state with extreme hardness makes it the strongest glass-like substance known today, though it remains confined to specialized research environments.
How Common Glass is Made Strong
While AM-III represents the peak of laboratory strength, common glass products encountered daily are strengthened using two primary commercial processes. The first method is thermal tempering, often used for architectural glass and car windows. This involves heating the glass to between 670 and 720 degrees Celsius before rapidly cooling its surfaces with jets of air. This rapid cooling causes the surface to solidify and contract before the core, placing the outer layer under high compressive stress.
The interior of the glass remains in tensile stress, but the surface compression must be overcome before any surface flaw can propagate, making the glass approximately four to five times stronger than its untreated state. A safety feature of thermally tempered glass is that when it breaks, the stored energy causes it to shatter into small, relatively blunt fragments rather than large, dangerous shards.
The second, more advanced method is chemical strengthening, used for most modern thin-profile applications, such as smartphone screens. This technique involves submerging the glass in a hot bath of molten alkali salt, typically potassium nitrate, at temperatures around 400 degrees Celsius. Smaller alkali ions, like sodium ions naturally present in the glass, diffuse out of the surface and are replaced by larger alkali ions, such as potassium, from the salt bath.
Because the larger potassium ions are physically squeezed into the spaces previously occupied by smaller sodium ions, they create an extremely compact and dense surface layer. This results in a deep layer of high compressive stress that significantly boosts the glass’s flexural strength, often making it six to eight times stronger than the original glass. Chemical strengthening is preferred for thinner glass because it does not cause the optical distortion that can sometimes occur with the intense heat cycles of thermal tempering.