Materials and their properties
by Chris Woodford. Last updated: February 6, 2018.
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: Miracle material? Look closely at this picture and you'll see
a ghostly blue square sitting just underneath the wax crayons: it's a small piece of aerogel, the world's lightest solid.
Aerogel is an amazing heat insulator, and here it's protecting the crayons from the blazing heat of the flame.
Although discovered decades ago, aerogel has proved very difficult to manufacture as a practical material, so
most people have never heard of it and it's yet to make a big impression on the world. Photo by courtesy of
NASA Jet Propulsion Laboratory (JPL).
Is it "materials science" or "materials technology"?
Before we go any further, it'll help to be clear what we mean by "materials
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).
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
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: Materials science on the move: "smart" cars are built from various different composite materials
and plastics, making them lighter and more energy efficient than steel cars but still safe and strong.
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.
Why do materials....?
Thinking scientifically about the inner structure of materials can help us understand why they behave as they do...
Why can you bend a paperclip only a few times before it breaks?
The repeated stresses and strains lead to what's called metal fatigue, which happens when tiny cracks inside a metal gradually grow into bigger ones, until the whole thing snaps in half. When you bend a paperclip first one way and then another, you're subjecting it to giant deformations (changes in shape), much bigger than the ones most everyday metal objects experience.
Why does rubber stretch even though it's a solid?
Rubber is made from long-chain molecules, called polymers, which are normally all scrunched up. When you stretch a rubber band, you straighten the molecules and pull them apart, so the rubber gets longer. When you let go, cross-link bonds between the polymers help to snap them back into place. This is what makes an elastic material such as rubber (one that returns to its original shape and size) different from a plastic material (one that changes shape but doesn't go back exactly to how it was), which includes most metals as well as most plastics.
Photo: Materials in motion: Elastic materials change shape but return to their original shape; plastic materials change shape but don't go back again. This is why you'll find metals often described as "plastic." Confusingly, some plastics are so hard and inflexible that they're not plastic at all!
Why do metals conduct electricity but plastics don't?
Metals are generally solids and inside they have a regular crystalline arrangement of atoms, like ping-pong balls packed into a box. Some of the electrons from the atoms are free to move throughout the whole crystal structure, carrying heat and electricity all the way through it. In plastics, there are no free electrons to move through the structure and carry electricity. This also explains why metals feel cold to the touch but plastics usually feel warm, even if they're the same temperature. A metal conducts heat in a similar way to electricity so, as you touch it, it draws heat from your body and makes your hand feel cold. Plastics don't have free electrons to conduct heat so they don't draw heat from your body as efficiently; that's why a piece of plastic feels warmer than a piece of metal even if it's at the same temperature.
Why can you snap a plank of wood but not stretch or squeeze it?
Wood is an example of what we call an anisotropic material, which means it has different mechanical properties in different directions. Why? Trees are much stronger if you apply a force in the same direction as the grain (vertically up and down the trunk) than across the grain (horizontally through the trunk)—simply because they grow that way. That's why wood works better in vertical poles (when it's in compression) than in horizontal beams and planks (when it's in tension). It's much easier to saw or chisel wood parallel to the grain than across (perpendicular to) it, which gives us the popular saying about tricky things "going against the grain." When you snap a plank, you're applying a force parallel to the grain where the wood is weakest; when you try to stretch or squeeze a plank, you're applying a force in the direction of the grain where the wood is strongest.
Photo: Testing, testing! Materials can fail at any time, which can have disastrous consequences if they're
part of something like an airplane jet engine. But how do you test materials for cracks you can't see? Here, an engineer is washing an airplane part in a fluorescent dye then shining ultraviolet light on it. The dye lodges inside any cracks, which then show up in the light. Photo by Dana Hill courtesy of US Air Force.
Why do laminates come apart?
A laminated material, such as a wooden floor, is made from flat sheets of different materials bonded together in layers with adhesives. A typical coaster (drip mat) on a coffee table might have a thin cellophane upper surface, then a thin piece of paper or card printed with a design of some kind, and a thicker cork base. Laminates come apart where one material joins another, for two common reasons. First, heat makes different materials expand by different amounts so, as a laminate warms up, one material might expand more than another, cracking the adhesive and making the layers come apart. Second, water might penetrate between the layers, weakening the adhesive or soaking into one of the layers and making it expand more than the others, again cracking it apart.
What are the different types of materials?
There are thousands of materials available to us. There are almost 100 naturally occurring
chemical elements, for example, and we can combine these in a virtually infinite number of ways to make compounds.
How do we make sense of all the materials in our world? Generally, we tend
to classify materials by the way they occur or are produced, by their
properties, or by their uses. However, there's no perfect way to
classify materials and it's worth remembering one thing: the only
reason to classify materials is to help us understand them better
(materials science) and use them more effectively (materials technology).
Natural and synthetic materials
The simplest way to classify materials is by splitting them into ones that occur naturally and
ones that are artificially made (synthetic). The natural materials
are the ones our human ancestors discovered and put to use first and
they include such things as stone, bone, wood, bamboo, and natural
textiles such as wool. Midway between natural and synthetic materials
are things that have to be taken from nature (typically dug from
the ground) and processed in various ways. That gives us ceramics
such as brick and clay, relatively easy-to-find metals such as iron and
copper, and also slightly more complex alloys like bronze (an alloy of copper and
tin) and steel (an alloy of iron and carbon, sometimes with other materials). We could also put vulcanized rubber, made from latex, into this category. Synthetic materials are much newer: plastics, for example, have only been widely available since the middle of the 20th century and composites (which often use plastics as one of their components) are also relatively new.
Photo: Natural or synthetic, materials are what we make of them. This computer memory chip is an example of an integrated circuit made from silicon. Although silicon is one of the world's most common chemical elements, found in sand, you have to do quite a lot of things to it before you can turn it into a
computer. So does it qualify as a natural material or a synthetic one? It's a bit of both!
Materials with different properties
It's also common to classify materials by their properties, such as when we divide things into
metals and nonmetals. Generally metals are hard, shiny solids that
conduct heat and electricity, and they're both malleable (they can be
shaped) and ductile (they can be pulled into wires). Although it
might seem useful to talk about metals and nonmetals, it's also very
misleading because metals are very different from one another.
Mercury, for example, is a liquid metal, while sodium is a metal so
soft you can cut it like butter! And while it's true that most metals
conduct electricity, some (such as silver and gold)
are much better than others (such as aluminum and lead)—and some
nonmetals (carbon, for example) conduct electricity very well too.
Materials with different uses
It makes much more sense to classify materials according to the way we use them. That gives us building
materials (such as iron and steel, glass, and wood), energy materials (from coal and gasoline to peat and uranium), materials for
transportation (including high-tech aerospace alloys and composites), electronics materials (metals and
semiconductors), medical materials
(such as plastics used in artificial limbs), and textiles we use for clothes, furnishings, and so on. One of the interesting things we see from classifying materials this way is the incredible versatility of certain materials that can be used in many different ways. Wood is probably the best example, used in everything from building boats and houses to fueling fires, making furniture, and even serving as early artificial legs! Plastics get their name from a word that means flexible—and their uses are just as diverse.
Our material future
Photo: What materials will we need in future? Before personal firearms became commonplace, no-one could possibly have imagined we'd need a material like bulletproof glass, shown here (a laminate of glass and plastic that slows bullets down by absorbing their energy); until plastics were developed in the 20th century, composite materials that could do things like this weren't really possible. Who knows what materials future humans will need? Picture courtesy of US Air Force.
Materials are the building blocks from which humans have fashioned civilization. Think of the
Stone Age, Bronze Age, and Iron Age (the three great periods of history based on tools
made from these amazing materials) and then consider our modern age of plastics, alloys,
and composites—a time when we've mastered every natural material Earth has to offer and
invented quite a few new materials all of our own. What could be more
important than materials and the things we do with them?
We're constantly facing new challenges that lead scientists and engineers to develop new materials.
Among the latest developments are biomimetic materials (ones that "mimic" the best features of biological materials, found in nature) and self-healing plastics and composites (materials that automatically repair themselves after damage). And how about "smart" materials that can change their properties as they need to, such
as energy-absorbing plastics (which instantly harden
to protect something fragile) and thermochromic materials
(which change color when you heat them)? Other exciting areas of future research include the development of new
semiconductors that will allow computers to keep on developing at the same furious pace (in line with a trend called Moore's law) and new prosthetic materials that
open up the possibility of entirely artificial human bodies!