Glass, an amorphous solid, has a disordered atomic arrangement unlike the neat, repeating structure of crystals. This unique structure gives glass its transparency but also makes it inherently susceptible to sudden, brittle failure from surface flaws. The search for the strongest glass involves finding materials that overcome intrinsic brittleness while maintaining the glassy state. Determining the “strongest” requires scientists to look beyond simple scratch resistance to understand how a material resists catastrophic breaking.
How Scientists Define Glass Strength
Scientists assess a material’s strength using a combination of distinct properties, moving beyond the public’s general concept of durability. Fracture toughness is considered the most reliable measure, quantifying a material’s ability to resist the propagation of a crack. This measure is expressed as the critical stress intensity factor (KIC), which for typical household silicate glass is quite low, often ranging from 0.7 to 0.9 MPa·√m.
Hardness, by contrast, refers only to the surface’s resistance to permanent deformation, such as scratching or abrasion. This is commonly measured using the Vickers hardness test, which involves pressing a diamond indenter into the material’s surface. High hardness is desirable for resisting everyday wear, but a very hard material can still shatter easily if it has low fracture toughness.
The third metric is tensile strength, which is the maximum stress a material can endure when being stretched or pulled before it breaks. Standard glass exhibits a relatively low working tensile strength, around 7 MPa, because microscopic surface flaws concentrate the pulling force. However, the theoretical strength of the silicon-oxygen bonds means the material’s upper limit is significantly higher, potentially reaching 17 GPa in a flawless state.
Identifying the Strongest Known Glass
The title of the strongest glass depends on whether one prioritizes extreme hardness or a combination of strength and toughness. For pure, unchallenged hardness, the current record holder is a synthetic material known as AM-III, a carbon-based glass synthesized by Chinese researchers in 2021. This ultrahard glass boasts an astonishing Vickers hardness value of 113 GPa.
This score places AM-III among the hardest materials known, capable of scratching a natural diamond, which typically measures between 45 and 70 GPa. The material is created by compressing and heating buckminsterfullerene (buckyballs) into an amorphous state. This process results in an ordered, non-crystalline arrangement of carbon atoms responsible for its resistance to surface indentation.
When the focus shifts to a material that is both exceptionally strong and resistant to fracture, a class of materials known as bulk metallic glasses (BMGs) becomes the top contender. A specific BMG alloy containing palladium (Pd79Ag3.5P6Si9.5Ge2) was identified for its combination of high yield strength and high fracture toughness. This material is stronger and tougher than high-grade steel and traditional Ni and Ti alloys.
Structural Secrets to Extreme Durability
The extreme durability of these materials stems from carefully engineered structural differences at the atomic level. Standard glasses are brittle because their amorphous structure offers no resistance to the propagation of a crack once it starts. The two most effective strengthening methods either prevent the crack from starting or stop it from spreading.
Chemical strengthening, used in many commercial applications, prevents cracks by inducing a high compressive stress on the surface layer. This process, called ion exchange, involves submerging glass in a molten salt bath containing larger ions, such as potassium (K+). These larger ions replace smaller sodium ions (Na+) embedded in the glass structure.
The larger ions are squeezed into the spaces, creating a highly compressed outer layer. Any external force, like an impact or scratch, must first overcome this compressive stress before the inner glass is put into tension, the state required for crack growth. This process dramatically increases the material’s resistance to surface flaws and impact.
In bulk metallic glasses, the structural secret lies in their unique, non-crystalline arrangement combined with specific chemical compositions. Unlike crystalline metals that deform easily through the movement of dislocations, BMGs resist deformation but tend to fail catastrophically once their yield strength is exceeded. The palladium-containing BMG combats this brittleness by promoting the formation of multiple shear bands when stressed. These shear bands are tiny, localized areas of plastic deformation that absorb significant amounts of energy, mimicking the toughness of metals.
Strongest Glass vs. Everyday Reinforced Glass
The current scientific record holders, like AM-III and the palladium BMG, exist in a league far beyond the types of reinforced glass the average person encounters. Everyday strong glasses are classified by the method used to build in surface compression to combat flaws. Tempered glass, for example, is thermally strengthened by heating it to over 600°C and then rapidly cooling the surface with air jets.
This rapid cooling causes the surface to solidify and contract before the interior, placing the surface permanently under compression. Tempered glass is typically four to five times stronger than regular annealed glass. Its stored energy causes it to break safely into small, dull pieces, making it a safety glass.
Chemically strengthened glass, such as those used in smartphone screens, is generally six to eight times stronger than standard float glass due to the ion-exchange process. Laminated glass, commonly used for car windshields, achieves its strength by bonding two or more layers of glass with a polymer interlayer, such as polyvinyl butyral. This construction does not necessarily make the glass harder, but the interlayer holds the fragments together upon impact.
In contrast, the record-holding AM-III’s 113 GPa hardness and the BMG’s ability to plastically deform represent an extreme level of mechanical performance. This level of performance is hundreds of times greater than the modest strengthening factors of everyday commercial products.