Materials science
by Chris Woodford. Last updated: March 12, 2023.
Why do plastic rulers shatter when you flex them while metal ones simply bend? How come you can twist a paperclip two or three times without it breaking but not twenty or thirty times? Why do some cars go rusty faster than others? How do astronauts survive in space without getting boiled by the Sun... or frozen by the lack of it?
These are the kinds of questions that orbit the minds of materials scientists—arguably some of the most important people on Earth. Why are they important? Because from the moment you get up (woken by an alarm clock built around a tiny crystal of ceramic quartz) to the time you go to bed (snoozing soundly on cotton or synthetic-fiber sheets), every single thing you do involves materials of one kind or another. Could we survive without materials? No! When you remember that materials provide everything from the clothes we wear and the food we eat to the energy we use for cooking and keeping warm, it's obvious that civilized human life is impossible without them.
Photo: Even the best materials don't last forever; testing and maintaining mechanical components is a key part of materials science. Here, an airplane engineer inspects a metal bolt for cracks using a dye and ultraviolet light. (There's another photo of this process further down the page.) Photo by Cecilio Ricardo courtesy of Defense Imagery Management Operations Center and DVIDS.
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Contents
Is it "materials science" or "materials technology"?
Before we go any further, it'll help to be clear what we mean by "materials science."
Science is all about making observations and doing experiments to form and test theories of how our world works; technology means using science in a practical way to solve everyday problems. Materials are technological almost by definition: who needs materials that have no obvious use? Yet there's almost no such thing as a "useless material"—and even some of the strangest materials invented in laboratories, which seemed pretty useless to begin with, have often turned out to have amazing applications later on. (Super-strong Corning® Gorilla® Glass, used in touchscreens, ever-sticky adhesive that lines the backs of Post-it® Notes, and nonstick Teflon® are three good examples of materials that found uses no-one imagined them having.)
Photo: Material difference: Ordinary glass is far too weak and brittle to use on a phone, while toughened glass is too thick and heavy. What if we could make light but tough glass that was strong enough to survive everyday knocks and scrapes? That's the basic idea behind the Gorilla® glass used in touchscreens. Originally called Chemcor, and developed by Corning in 1962 as a flat safety glass for windshields, it failed to achieve commercial success until cellphones with touchscreens started to use it over 40 years later. What was once something of a dud material has now become one of Corning's most lucrative products!
What, then, do we mean by materials science? From stone and bronze to steel and concrete, materials are useful for a particular purpose because they behave in a certain way under certain conditions: they have particular qualities, which we call their properties. Understanding these properties is what materials science is all about. Where our modern age differs from earlier periods of history is in having tools that allow us to turn materials inside out: unlike people who lived hundreds or thousands of years ago, we can explain the useful properties of different materials by peering inside them with microscopes, X rays, and all kinds of other neat techniques to inspect their atomic, molecular, or (in the case of once-living things) cellular structure. Just as science provides the foundations for technology, so materials science (understanding the inner structure of materials) helps us to advance materials technology (developing materials that are useful in different situations).
Why do we use one material instead of another?
Look round the room where you are now and quickly list the materials you can see. What do you notice? You can probably see metals, woods of various kinds, glass windows, and colorful plastics, as well as soft furnishings in rugs, cushions, and (of course) the clothes you're wearing. You might not spot some of the more subtle materials such as composites (generally ceramics or plastics combined with other materials), laminates (composites made from flat sheets bonded together), and alloys (metals mixed with other metals and nonmetals).
Photo: Everything in its right place: All these everyday objects are made from different materials, each chosen to suit its purpose. Sponge is great at picking up water, string is flexible to wrap around things, nylon makes hygienic toothbrushes that last a while, and batteries are packed with chemicals that can store energy. Now imagine a sponge made of metal or a toothbrush made of string—and you'll understand straight away what materials science if all about. Make any of these objects from some of the other materials and they wouldn't work as well—if at all.
The right material
What do you notice about the materials around you? For everything we need to do, we've found materials that are very well-suited to the job (and often very ill-suited to other jobs). So in buildings, we choose materials that are hard, strong, durable, waterproof, and good at retaining heat; when it comes to furnishings, we prefer quite different materials that are soft, flexible, colorful, and reasonably hard-wearing. Although some materials have many uses, and it's often possible to swap one material for another, it's hard to imagine a house built out of wool, or clothes made from glass!
Photo: Building materials like the steel (gray bars), concrete (white floor), and brick (red-brown blocks) you see here are designed to help a building balance forces, but they also have to withstand the weather, usually over a period of decades—or even longer—with as little maintenance as possible.
Leaving aside textiles, what sorts of properties do we look for in everyday materials? Think about the materials we use in buildings and structures. Glass, iron, wood, and plastics are generally solids, but in buildings they do different jobs and behave in very different ways. The most important consideration is the way a material responds to forces, known as its mechanical properties. Building materials need to withstand different types of forces that squeeze them (putting them into compression), stretch them (putting them into tension), or twist them (shearing them). Most buildings have to do little more than withstand gravity; some have to be designed to cope with more extreme forces such as earthquakes and hurricanes. Mechanical properties are part of the physical properties of a material. Other physical properties include whether materials conduct heat and electricity, whether they let light pass through them, and how they age or weather (do they rust like iron, rot like wood, or degrade in sunlight like certain plastics?).
The best compromise
Mechanical properties are also important in the materials we use for transportation: airplanes, space rockets, trucks and cars have to be made from strong materials both to withstand the forces they experience during acceleration and deceleration and to protect the occupants in case of an accident. But transportation materials illustrate another really important principle of how we choose and use materials: almost always we have to compromise. You could make an airplane out of super-strong steel, but then it might be too heavy to take off, or it might use too much fuel to be economical. That's why aerospace designers are just as likely to use strong but lightweight alloys made from aluminum or titanium, as well as composite materials. Although cars have traditionally been made from metals, some are made from composites such as fiberglass, which offer a compromise between strength, weight, and cost, and from lighter aluminum and titanium alloys.
Photo: Wood or metal? Diving boards (spring boards) used to be made from wood; now they tend to be made from aluminum metal instead. Why? It's lighter, cheaper, springier, and doesn't rot when it gets wet. Photo by Zachary Orr courtesy of US Marine Corps.
The same goes for every other use of materials: there's always an element of compromise. Gasoline is a brilliant way to power a car because, per unit of its weight, it holds more energy than almost any other widely available substance—but it makes air pollution and it's relatively expensive, so it's another case of "materials compromise." We could make everlasting shoes out of steel, but they'd be incredibly heavy and uncomfortable. Or we could make them out of amazingly comfortable cotton and wool, but then they wouldn't last very long. Instead, we use durable, flexible, and relatively comfortable materials such as leather and plastics—usually a good compromise between comfort, cost, and durability.
The local solution
In our modern age of jet planes and container ships, it's easy to transport materials anywhere in the world in a matter of days or weeks. In theory, that means we can use any material we want to use anywhere we might like to use it. Historically, that wasn't always the case. Although some materials (such as wood, iron, and coal) are common throughout the world, others (such as the oil that still powers much of the planet, the lithium we use in rechargeable batteries, and the uranium used to fuel nuclear energy plants) are available only in certain places. The huge diversity of building styles and architecture around the world is the best illustration of how people have used local materials to solve their problems. In Scandinavia, where wood is cheap and readily available, there are many timber-framed buildings. In Asian countries such as Vietnam, bamboo is widely used to make everything from bridges and houses to furniture and water pipes. Making things from plastics is the opposite approach to using local materials; we can make synthetic plastics in chemical plants anywhere on the planet. That's why modern plastic objects feel culturally and geographically neutral: they don't occur naturally anywhere on Earth and they have no obvious links to any particular country or region.
Photo: Dry-stone walls (rocks or stones stacked without mortar) are a timeless example of how people have always put the materials around them to good use.
