23. September 2013
I was discussing this summer’s blockbuster Superman movie with my son, including the renewed attention to one of the world’s most famous nicknames: “the man of steel.” Being a glass guy, I wasn’t surprised when my son asked me how glass stacks up to steel. As a man of steel, Superman possesses the strength of a material built to suspend bridges and frame skyscrapers, but isn’t glass strong too?
I thought it was an interesting question. Why is Superman the man of steel and not, say, the man of glass? In part, it may have been steel’s rise to prominence in the 20th century as part of buildings, ships, cars, and more. It’s also perception. Steel is commonly associated with strength, while glass, though just as strong in different ways, doesn’t have the same connotations due to its brittle nature.
But while both steel and glass have been used for thousands of years, we now know much more about metals than about glass. That means that scientists and engineers are more likely to discover truly transformational advances for the use of glass than for steel in the coming decades. Not to mention that glass is already one of the strongest, most enduring materials around, and offers additional qualities such as transparency. In short, Superman would be lucky to be a man of glass.
In applications like armored windshields or in electrical feedthroughs in nuclear power plants, glass’s strength offers the best features for the job. But it’s not necessarily a competition — some applications combine glass and steel working in harmony to create ideal structures that carry the strengths of both materials. If we look at tests of compressive and thermal strength, as well as overall longevity, we’ll begin to see why Superman could just as easily be called the man of glass.
Glass’s compressive strength
Glass is inherently strong in compression (pushing forces), while steel can yield and buckle under similar pressure. Steel, a more malleable material than glass, holds up under tension (pulling forces). That’s why in bridges, for example, the cables, which are under tension, are always made out of steel. The foundation of the bridge, which is under compression, is typically made from concrete, another material that, like glass, does well under compressive forces.
In places where you need transparency as well as compressive strength, such as the windows of a skyscraper, glass is the go-to material. You’ll see it in the Sears Tower’s glass observation deck, through which visitors can see the street 103 stories below their feet, or as part of the Grand Canyon Skywalk, a horseshoe-shaped glass platform extending out over the canyon.
Glass can get additional strength from thermal or chemical tempering. Quickly cooling the surface of the glass strengthens it by putting the surface in thermal compression, making it tougher and resistant to breaking. Chemical tempering involves dipping the glass in a salt bath at high temperatures to create a chemical exchange, swapping small sodium atoms in the glass for the larger potassium atoms. Similar to thermal toughening, this puts the glass surface in compression as it cools.
Glass’s thermal strength
Glass can be engineered to resist expansion as it changes temperature, a property that’s very difficult to reproduce in any other material. Metal expands and contracts all the time — you may hear the copper pipes in your house do so as hot water flows through them.
Glass ceramization, a process that creates nanocrystals that change the properties of the glass, allows one to control the thermal expansion of glass and can increase its strength and toughness. These glass-ceramics are designed to withstand heat in a number of applications. ROBAX transparent glass-ceramic, for example, resists the heat of the fireplaces it encloses, CERAN glass-cermic’s low heat conduction makes for safe cooktops, and ZERODUR extremely low expansion glass-ceramic helps telescope mirrors maintain their shape despite any swings in temperature.
One of Superman’s abilities is his eyes, which can send out beams that melt steel. As I explained to my son, his eyes have the power of glass, as many high-power lasers use glass as the material of choice. I showed him one of the world’s most powerful lasers, the National Ignition Facility at Lawrence Livermore National Laboratory, which uses glass. So the man of steel could very well have eyes of glass.
If you look into archeology, you’ll find more glass and ceramics than metal artifacts. Often the metal that’s discovered is gold or other metals that don’t rust. But it’s not uncommon to find glass, pots, and cups 2,000 years old. Scientists have even found meteorites made of glass that are millions of years old. Glass doesn’t rust or warp.
Glass can also be recycled forever, a process that’s very difficult to achieve with steel. Metals can pick up impurities, a side effect of the crystalline grains that make up the material. Too many impurities weaken the metal, meaning it can break more easily the more it’s recycled. But glass can be added to a melting tank with fresh raw materials, remelted, and formed again and again. So while the man of steel can fly around the world and turn back time if necessary, a man of glass can keep moving forward without sacrificing any of his strengths.
Of course glass, like any material, has its kryptonite. A brittle material, glass can take on flaws that can result in crack propagation. In fact, based on its chemistry, glass should show higher strength than steel, but these flaws tend to limit its performance. For example, glass fibers have higher strength than steel cables at lower weight because they’re manufactured to be free of surface flaws and defects. By then protecting the glass with a polymer, fiber glass composites can easily compete with steel in tension at lower cost and weight. But taking those flaws out, reliably, in large structures is certainly a challenge. But since glass and steel have different strengths and weaknesses, you can get the most out of both materials when they work together.
Skyscrapers, for example, use rigid, high-strength tempered glass to keep the building sturdy, but use more pliable steel so that the building can also absorb strong winds. The combination of compressive and tensile strength makes for a stronger overall structure. Metal and glass also team up in nuclear power plants, where electrical feedthroughs run electrical signals into the hot zone. These seals need to be highly reliable in a harsh environment, and glass-to-metal seals can withstand the pressure, temperatures, and radiation.
Glass deserves to be recognized for the strengths it brings to everything from skyscrapers to armored windshields to nuclear power plants, and researchers are working every day to find ways to help glass meet its phenomenal promise. Given its compressive strengths, thermal properties, and longevity, we wouldn’t be surprised if Superman would feel proud to be dubbed “the Man of Glass.”