Glass, in its common form, is a brittle material whose strength is highly dependent on microscopic imperfections on its surface. These tiny flaws act as stress concentrators where fracture initiates under tension. The theoretical strength of glass is orders of magnitude higher than its practical strength. Modern manufacturing processes focus on mitigating the effect of these surface flaws to create products that withstand greater mechanical forces. Defining the “strongest” type of glass depends entirely on the specific type of stress it is designed to resist, such as impact, bending, or scratching.
How Strength in Glass is Measured
Quantifying the strength of glass involves multiple metrics because no single test captures all modes of failure. The most common measure of resistance to breaking under bending is the Modulus of Rupture (MOR), determined by standards like the ASTM C158 test. This flexural strength test measures the maximum stress a glass sample can withstand before it fractures, providing a reliable comparison. Since glass is notably weak under tensile stress, strengthening techniques primarily aim to counteract this vulnerability.
Impact resistance, a measure of how much energy the glass absorbs before shattering, is another crucial factor, often tested with a dropped steel ball or pendulum. Separately, the surface’s resistance to scratching and abrasion is measured as hardness using scales like Vickers or Mohs. Vickers testing involves pressing a diamond indenter into the surface to assess resistance to plastic deformation. Strengthening processes for glass work by placing the vulnerable surface layers into a state of compression.
Thermal Strengthening (Tempered Glass)
Thermal strengthening, commonly known as tempering, creates permanent compressive stress on the glass surface. The glass is first heated to temperatures exceeding \(600\) degrees Celsius, above its annealing point but below its softening point. It is then rapidly cooled, or quenched, by blasts of cold air. This rapid cooling causes the outer surfaces to solidify and contract immediately, while the inner core remains hot and fluid.
As the inner core eventually cools and attempts to contract, the rigid outer surface resists this movement. This differential contraction locks the outer layer into a high state of compressive stress, while the core remains in a compensating state of tension. This compressive layer must be breached before any external force can generate the tensile stress needed for fracture. The process typically makes the glass four to five times stronger than its original annealed state. A distinguishing feature of tempered glass is its failure mode, known as dicing, where the stored energy causes it to shatter completely into small, relatively blunt fragments.
Chemical Strengthening (Ion Exchange)
Chemical strengthening is a more advanced method that utilizes an ion exchange process to increase the inherent strength of glass. This technique is often applied to specialty compositions like alkali-aluminosilicate glass, which is thinner and lighter than standard glass. The glass is submerged into a molten salt bath, typically potassium nitrate, at temperatures below the glass’s annealing point. Smaller sodium ions (\(\text{Na}^{+}\)) migrate out of the surface structure.
Simultaneously, larger potassium ions (\(\text{K}^{+}\)) diffuse into the glass to take their place. Because the potassium ions occupy a greater volume than the sodium ions they replace, they become tightly wedged into the glass network. This crowding effect creates a layer of intense, permanent compressive stress on the surface without requiring extreme temperature cycling. The resulting surface compression can reach up to \(690\) megapascals (\(\text{MPa}\)), significantly higher than the \(\sim 69\text{ MPa}\) achieved through thermal tempering. This high compression makes the glass exceptionally resistant to surface damage and impact, making it ideal for high-wear applications like cover glass for portable electronics. The process is particularly effective for very thin sheets of glass that cannot be reliably strengthened using the thermal method.
Layered Safety Glass and Composites
A different approach to glass strength involves layering, which focuses on post-breakage performance and penetration resistance rather than inherent structural strength. Laminated glass is manufactured by bonding two or more panes of glass with a durable plastic interlayer, such as Polyvinyl Butyral (\(\text{PVB}\)) or the stiffer ionoplast interlayer, SentryGlas. This interlayer holds the glass fragments together if the pane is fractured, preventing shattering into dangerous shards. Laminated glass is defined by its safety characteristic: maintaining integrity after being broken.
The strength of these composites is achieved by energy absorption, where the plastic layer flexes and disperses the force of an impact across a wider area. More complex layered systems, such as bullet-resistant glass, combine multiple layers of glass, often including chemically strengthened or tempered glass, with thick interlayers of polycarbonate. This composite structure is designed so that the initial glass layers break and flatten the projectile, while the subsequent polymer layers absorb the remaining kinetic energy to prevent penetration. The layering technique creates a barrier that resists forced entry and maintains the building envelope.