The question of the hardest glass to break does not have a single answer, as “hardest” refers to two different physical properties. Glass is inherently brittle, lacking the ability to deform plastically and being highly susceptible to failure from surface imperfections. The strength of modern glass relies on modifications to its composition or structure to overcome this natural brittleness. Durability depends on whether the goal is to prevent scratching or to resist catastrophic impact failure.
Defining Durability and Fracture Resistance
In material science, resistance to damage is categorized into two distinct metrics: hardness and toughness. Hardness refers to a material’s resistance to permanent deformation, such as scratching or abrasion, often measured on scales like Mohs or Vickers. High hardness resists scratches from everyday objects like keys or sand particles.
Toughness, or fracture resistance, is the ability of a material to absorb energy before failing. Standard glass has low fracture toughness, explaining its tendency to shatter upon impact. Glass typically fails when a microscopic surface flaw, known as a micro-crack, encounters tensile stress and rapidly propagates.
The fundamental goal of creating hard-to-break glass is to counteract micro-cracks and manage the stresses that cause them to grow. By introducing a layer of permanent compression onto the glass surface, scientists effectively “close” these surface flaws. Any external force must first overcome this engineered compressive stress before it can put the glass surface into tension and cause the micro-crack to open and spread.
Chemically Strengthened Glass
Chemically strengthened glass is a high-performance material widely used in consumer electronics, where thinness and high impact resistance are paramount. Strengthening is achieved through ion exchange, which modifies the glass composition after it has been formed. The glass is submerged in a molten salt bath, typically potassium nitrate, at high temperatures.
During this immersion, smaller sodium ions present in the glass surface migrate out and are replaced by larger potassium ions from the bath. Because the potassium ion has a larger atomic radius, it wedges into the space left by the smaller ion. This forced crowding creates a deep, permanent layer of compressive stress on the glass surface.
This engineered layer of compression acts as a shield, closing existing micro-cracks and preventing new ones from propagating. The compressive layer must be overcome by an external impact before the inner core of the glass, which is under compensating tension, can be exposed to fracture. This process increases the glass’s flexural strength and resistance to impact and scratching.
Thermally Treated and Laminated Safety Glass
For structural applications in architecture and vehicles, two other common processes are used to create durable glass: thermal tempering and lamination. These methods focus less on scratch resistance and more on safety and structural integrity.
Tempered glass is produced by heating standard glass in a furnace and then rapidly cooling the surfaces with high-pressure air jets. This thermal quenching causes the outer surfaces to cool and solidify quickly while the core remains hot. As the core cools and contracts later, the rigid outer surfaces are pulled inward, inducing a permanent state of high compressive stress on the exterior and tensile stress in the center.
This internal stress makes tempered glass four to five times stronger than regular glass of the same thickness. When it breaks, the stored internal energy is released, causing the pane to fracture instantly into thousands of small, relatively blunt granular pieces, a process known as dicing. This breakage pattern prevents the formation of large, jagged shards, classifying it as a safety glass.
Laminated glass focuses on fragment retention rather than increased initial strength. It is a composite material made by bonding two or more layers of glass with a polymer interlayer, most commonly Polyvinyl Butyral (PVB). The glass-polymer-glass sandwich is fused together under heat and pressure. The PVB interlayer is highly resilient and flexible, designed to absorb energy from impact. When laminated glass breaks, the fragments adhere firmly to the plastic interlayer, preventing the glass from shattering and falling out of its frame.
Extreme Resistance Materials and Composite Glass
Beyond the common commercial methods, some specialized materials push the boundaries of glass durability, often at a higher cost. Sapphire glass, a synthetic crystalline form of aluminum oxide, offers the highest level of hardness. Measuring 9 on the Mohs scale, second only to diamond, it is virtually impervious to scratching.
However, this extreme hardness comes with a trade-off, as crystalline structures can be more brittle under high impact than chemically strengthened amorphous glass. While it resists abrasion better, a sharp, forceful blow can cause catastrophic failure more easily than in a toughened material designed for energy absorption.
Glass-ceramic composites are produced by the controlled crystallization of a base glass. These materials are structurally homogeneous and combine the properties of glass with the strength and durability of ceramics. By incorporating a reinforcing phase, such as fibers, they can be tailored for high fracture toughness, high-temperature stability, and exceptional impact resistance. These specialized materials are used in applications demanding extreme performance, such as aerospace components and high-end cookware.